No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Comparison of Biomechanical Function at Ideal and Varied Surgical Placement for Two Lumbar Artificial Disc Implant Designs: Mobile-core vs. Fixed-core
Abstract
Study design:
Finite element model.
Objectives:
To estimate the effect of lumbar mobile-core and fixed-core artificial disc design and placement on the loading of the facet joints, and stresses on the polyethylene core.
Summary of background data:
Although both mobile-core and fixed-core lumbar artificial disc designs have been used clinically, the effect of their design and the effect of placement within the disc space on the structural element loading, and in particular the facets and the implant itself, have not been investigated.
Methods:
A 3D nonlinear finite element model of an intact ligamentous L4-L5 motion segment was developed and validated in all 6 df based on previous experiments conducted on human cadavers. Facet loading of a mobile-core TDR and a fixed-core TDR were estimated with 4 different prosthesis placements for 3 different ranges of motion.
Results:
Placing the mobile-core TDR anywhere within the disc space reduced facet loading by more than 50%, while the fixed-core TDR increased facet loading by more than 10% when compared with the intact disc in axial rotation. For central (ideal) placement, the mobile- and fixed-core implants were subjected to compressive stresses on the order of 3 MPa and 24 MPa, respectively. The mobile-core stresses were not affected by implant placement, while the fixed-core stresses increased by up to 40%.
Conclusion:
A mobile-core artificial disc design is less sensitive to placement, and unloads the facet joints, compared with a fixed-core design. The decreased core stress may result in a reduced potential for wear in a mobile-core prosthesis compared with a fixed-core prosthesis, which may increase the functional longevity of the device.
... Homogeneous 3D isotropic solid elements (C3D8) were used to define the vertebral bone. Cortical bone was defined by a thin layer of 0.5 mm of thickness and the endplate was modeled as a thin layer of 0.635 mm of thickness [34], [35]. The IVD contains two extremely particular structures: Annulus Fibrosus AF, which was simulated as a typical composite-like structure comprising a solid matrix, its hyperelastic structure was modeled using hybrid formulation (C3D8H). ...
... The IVD contains two extremely particular structures: Annulus Fibrosus AF, which was simulated as a typical composite-like structure comprising a solid matrix, its hyperelastic structure was modeled using hybrid formulation (C3D8H). Collagen-reinforced crisscross fibers were embedded in concentric rings and defined by truss elements (T3D2) (see Fig. 2) [35], [36]. These lamellar elements were arranged in 7 crossed layers to mimic the realistic inclined fibers [35]. ...
... Collagen-reinforced crisscross fibers were embedded in concentric rings and defined by truss elements (T3D2) (see Fig. 2) [35], [36]. These lamellar elements were arranged in 7 crossed layers to mimic the realistic inclined fibers [35]. Nucleus Pulposus NP is assumed to occupy 43% of the total volume of the IVD. the NP was supposed as an incompressible body, define as a fluid-filled cavity using hydrostatic fluid elements F3D4 [22], [23], [31], [32], [37]. ...
The finite element modelling (FE) of human tissue plays a significant role in the biomechanics by offering a new prominent quantification and simulation of the functional behaviour of the human body in various physiology conditions. This study focused on the development of an effective process for generating a simplified anatomical FE model of the lumbar spine. The proposed model of the lumbosacral spine L1-S1 was subjected to pure and combined loading modes and validated compared to in vitro and in vivo measurements. This validation was carried out by comparing the results with previous FE models and literature for combined loading: moment + axial follower load of 7.5 Nm + 150 N to predict the behaviours of the functional spinal unit FSU L4-L5. The validation of the whole lumbar spine was performed by determining the range of motion ROM under only pure moment of 7.5 Nm. The presented results provide a good agreement with FE median of previous FE models in different loading cases. The predicted L1-L5 rotations demonstrate a fair agreement with in vitro ranges except smaller values for flexion-extension, 0.3~3° comparing to FE median, 0.3~1.5 ° for lateral bending and 0.3~1.6° for axial rotation for all levels comparing to in vitro median. The facet joint forces fit the reported median in vivo and median FE values, 36N in extension and 63N in lateral bending. The Intradiscal pressures are in range of in vivo and in vitro values under compressive loads. Although, the nucleus pressures were smaller than those of reported in vivo results for lateral bending. The incorporation of simplification techniques will allow us to generate more thorough models of the lumbar spine and boost model applicability.
... The radiographic apparent assumed that the asymptomatic adjacent segment disease ASD is common after lumbar fusion, but this does not correlate with the functional outcomes while compensatory increased motion and stresses at the adjacent level of fusion is well-known to be associated to ASD [4,5,8]. Newly developed, hybrid and flexible stabilizations are allocated to substitute for mostly the superior level of the fusion in an attempt to reduce the number of fusion levels and likelihood of degeneration process at the adjacent levels during the fusion with pedicle screws. ...
... Lumbar fusion is considered as the "gold standard" surgical treatment indicated for a wide range of well-recognized painful conditions [1,8,11]. Which aimed to alleviate and treat the pain from disc/facet loading in the instable degenerated levels, probably cause of the abnormal load share and stresses in lumbar spine [8,12], while the surgical intervention is based on the topping off fusion stabilization, we used a validated Finite Element (FE) Model to determine the biomechanical basis exists for believing that the reduced stiffness and increased axial motion conferred by dynamic instrumentation using mobile and fixed screw implants to alter the stresses in adjacent-level discs [8,11]. Conventionally, the Posterior Dynamic Stabilization PDS constructs has been promoted to be used as a total fixation system [5] or joined to spinal fusion (topping off or bottoming off fusion) of various rigid or semi rigid fixators assumed to diminish the load on adjacent disc and the facet joint [3,6,9]. ...
... Lumbar fusion is considered as the "gold standard" surgical treatment indicated for a wide range of well-recognized painful conditions [1,8,11]. Which aimed to alleviate and treat the pain from disc/facet loading in the instable degenerated levels, probably cause of the abnormal load share and stresses in lumbar spine [8,12], while the surgical intervention is based on the topping off fusion stabilization, we used a validated Finite Element (FE) Model to determine the biomechanical basis exists for believing that the reduced stiffness and increased axial motion conferred by dynamic instrumentation using mobile and fixed screw implants to alter the stresses in adjacent-level discs [8,11]. Conventionally, the Posterior Dynamic Stabilization PDS constructs has been promoted to be used as a total fixation system [5] or joined to spinal fusion (topping off or bottoming off fusion) of various rigid or semi rigid fixators assumed to diminish the load on adjacent disc and the facet joint [3,6,9]. ...
... The finite element model was tested for its accuracy in predicting mechanical response to external loads. The in vitro experimental and finite element results (8,9) were selected for the finite element model validation, as this work includes the response of the lumbar motion segment to all physiologic loading modes. Six forces of 150 N (±Fx, ±Fy and ±Fz) were applied to the model. ...
... The anterior and posterior longitudinal ligaments were transected and each artificial disc (disc height of 10 mm and PE thickness of 6 mm) was simulated to implant posteriorly at 4 mm offset from the intact nucleus center (Fig. 3). Material properties of the various tissues were selected from the literature and are summarized in Tables I and II (9,10). A perfect bond was assumed between the metallic endplates and adjacent vertebrae. ...
... A perfect bond was assumed between the metallic endplates and adjacent vertebrae. The friction coefficient was assigned to be 0.02 for sliding contact between the polyethylene core and the cobalt chrome alloy endplate (9). ...
... The finite element model was tested for its accuracy in predicting mechanical response to external loads. The in vitro experimental and finite element results (8,9) were selected for the finite element model validation, as this work includes the response of the lumbar motion segment to all physiologic loading modes. Six forces of 150 N (±Fx, ±Fy and ±Fz) were applied to the model. ...
... The anterior and posterior longitudinal ligaments were transected and each artificial disc (disc height of 10 mm and PE thickness of 6 mm) was simulated to implant posteriorly at 4 mm offset from the intact nucleus center (Fig. 3). Material properties of the various tissues were selected from the literature and are summarized in Tables I and II (9,10). A perfect bond was assumed between the metallic endplates and adjacent vertebrae. ...
... A perfect bond was assumed between the metallic endplates and adjacent vertebrae. The friction coefficient was assigned to be 0.02 for sliding contact between the polyethylene core and the cobalt chrome alloy endplate (9). ...
Purpose:
Deeper insights into the mechanical behavior of lumbar disc prostheses are required. Prior studies on the biomechanical performance of artificial discs were mostly performed with finite element analyses, but this has never been analyzed with altering articulate curvature. This study aimed to ascertain the influence of the geometry of a ball-and-socket disc prosthesis for the lumbar spine.
Materials and methods:
Three-dimensional finite element model of human L4-L5 was reconstructed. Convex, concave, and elliptic artificial disc models were also established with Computer-Aided-Design software. Simulations included: (1) three articulate types of polyethylene (PE) insert were implanted inferiorly and (2) concave and convex PE inserts were implanted on the superior or inferior sides in flexion/extension, lateral bending, and axial rotation in the lumbar spine. Shear stresses and von Mises stresses on PE insert were assessed for their loading distributions.
Results:
High shear stresses of all articulate types occurred in flexion, and convex PE insert performed the maximum stress of 23.81 MPa. Under all conditions, stresses on concave PE inserts were distributed more evenly and lower than those on the convex type. Elliptic geometry enabled confining the rotation of the motion unit. The shear force on the convex PE insert on the inferior side could induce transverse crack because the shear stress exceeded yielding shear stress.
Conclusions:
The concave PE insert on the inferior side not only decreased loading concentration but had relatively low stress. Such a design may be applicable for artificial discs.
... The NP was modeled as hydrostatic fluid elements [23]. The facet joint was simulated by a cartilaginous layer of a thickness of 0.4 mm using frictionless surfaces-to surface contact with softened contact in the normal direction and contact stiffness of 200 Nm/ • [22][23][24]. The seven ligaments, including Anterior Longitudinal Ligament (ALL); Posterior Longitudinal Ligament (PLL); Intertransverse Ligament (ITL); Ligamentum Flavum (LF); Capsular Ligament (CL); Supraspinous Ligament (SSL); and Interspinous Ligament (ISL) were modeled as three dimensional 2 noded truss elements (T3D2) and allocated nonlinear hypoelastic behavior [21,24,25]. ...
... The facet joint was simulated by a cartilaginous layer of a thickness of 0.4 mm using frictionless surfaces-to surface contact with softened contact in the normal direction and contact stiffness of 200 Nm/ • [22][23][24]. The seven ligaments, including Anterior Longitudinal Ligament (ALL); Posterior Longitudinal Ligament (PLL); Intertransverse Ligament (ITL); Ligamentum Flavum (LF); Capsular Ligament (CL); Supraspinous Ligament (SSL); and Interspinous Ligament (ISL) were modeled as three dimensional 2 noded truss elements (T3D2) and allocated nonlinear hypoelastic behavior [21,24,25]. The entire model consists of 113,742 C3D8 element, 324 T3D2 element and 11,200 SFM3D4 element. ...
Introduction: Interbody fusion is a very common surgical treatment for degenerative disc diseases. It is necessary to explain the effect of pedicle based hybrid stabilization systems (PBHS) on the lumbar spine, as there is no consensus in the literature about their performance.
Hypothesis: Topping off a fusion with a PBHS may provide some protection against adjacent segment failure.
Material and Methods: The biomechanical effect PBHS on fused and adjacent to fusion levels were investigated, including range of motion, bending stiffness, Von Mises stress A 3D Finite Element model of the L2-S1 spine was used and modified to simulate pre and postoperative changes during combined loading. Five models instrumented with different systems Titanium and PEEK fusion; Dynesys hybrid system; NFlex hybrid stabilization and PEEK topping off fusion were compared to those of healthy model.
Results: After hybrid instrumentation, the L4-L5 level did not lose its motion completely, NFlex hybrid stabilization system maintained 82% of flexion at the adjacent to fusion level, reduced bending stiffness by 40% in axial rotation. Dynesys hybrid system represented more restricted motion than NFlex. PEEK topping off fusion system was the most rigid one among all three systems. It increased bending stiffness at the adjacent level and increased the axial motion by 25%. High risk of rod breakage was computed for PEEK topping off system as 48.8 MPa in lateral bending.
Conclusion: Hybrid stabilization can delay adjacent segment failure and compensate lumbar spine mobility. However, it is clear that PBHS need to be further tested before being considered for clinical use
... Concerning the parametric meshing, the developed spinal mesh was symmetrically modeled across the midsagittal plane as adopted in the literature [15], the mesh density and geometry was defined to represent the key passive anatomical features: cortical and cancellous bone; endplates; posterior elements of the vertebrae; annulus fibrosis and nucleus pulposus; articular cartilage; synovial fluid of the facet joint [11,12]. 8-noded solid element (C3D8) were used to define The cancellous bone and posterior elements of the vertebrae and articular cartilage of the facet joints ,the cortical bone and endplates of the vertebrae and elsewhere using hexahedral element based on hybrid formulation for hyperelastic structure for the Annulus Pulposus (AP). ...
... Cortical bone and endplates of the vertebrae modeled using 8-noded brick elements with 0.635 mm thickness. [14,15] .The commercial FE package Abaqus, version 14.1 was used to formulate the FE-model. ...
... Tying restrictions are often used when orthopaedic device models are inserted into pre-existing spine models, due to device geometrical characteristics and/or positioning that makes it difficult to obtain congruent mesh interfaces with the vertebral models (Lacroix et al., 2006;Moumene and Geisler, 2007;chen et al., 2009;H-J Kim et al., 2011) (Fig. 5.13(b)). In such context, the restrictions of the tied nodes also simulate a perfect integration between the implant and the surrounding modelled tissues. ...
... Patient-specific full spine model with rigid body vertebrae and pelvis used to predict the outcomes of different intraoperative sequences of manoeuvre for corrective surgery in scoliotic patients (adapted from Lafon et al., 2010aLafon et al., , 2010b sliding contact articulation of some artificial intervertebral discs (Moumene and Geisler, 2007;Schmidt et al., 2009), and annular lesions within the intervertebral disc (Little et al., 2007;Galbusera et al., 2011a). On one hand, different numerical studies presented in the literature demonstrated that facet contact force predictions are particularly sensitive to variations in modelling hypotheses . ...
This chapter focuses on the finite element method applied to spine modelling. First, the functional biomechanics of the healthy spine are presented in terms of load transfers among the different spinal structures and further related to the pathologic and treated spine. This overview naturally justifies the finite element method as a particularly convenient tool to explore the spine as an integrated organ where tissue maintenance and degeneration are strongly influenced by complex mechanical factors. The different approximations usually used in spine finite element analysis are presented and discussed in relation to the exploitability of the predictions and the computational cost. On one hand, spine models require thorough verification and improved validation protocols. On the other hand, the rapid mechanistic developments proposed are continuously increasing the reliability of both model calibration and predictions on a clinical basis, suggesting that spine finite element models will probably become highly valuable clinical tools in the near future.
... Semi-constrained TDRs are divided in designs with and without anterior–posterior (AP) transla- tion [5, 6]. The Mobidisc is an unconstrained TDR and the Activ-L TDR is a semi-constrained design with absence of lateral translation and limited AP translation [7,91011. In a clinical case series on the Mobidisc TDR, the back pain decreased from 6.8 to 2.6 on the visual analog scale (VAS) 2 years after implantation [7, 12]. ...
... While, others state that a mobile COR leads to a lower facet contact force by an equilibrium between facet loading and ligament tension [5, 9]. Also malpositioning of the TDR causes higher stresses on the facets [11, 42]. Nevertheless, facet joint degeneration was present at L4–L5 and L5–S1 in both cases with both TDR designs. ...
This study evaluates the short-term clinical outcome, radiological, histological and device retrieval findings of two patients with second generation lumbar total disc replacement (TDR).
The first patient had a single level L4-L5 Activ-L TDR, the second patient a L4-L5 Mobidisc and L5-S1 Activ-L TDR. The TDRs were implanted elsewhere and had implantation times between 1.3 and 2.8 years.
Plain radiographs and CT-scanning showed slight subsidence of the Activ-L TDR in both patients and facet joint degeneration. The patients underwent revision surgery because of recurrent back and leg pain. After removal of the TDR and posterolateral fusion, the pain improved. Histological examination revealed large ultrahigh molecular weight polyethylene (UHMWPE) particles and giant cells in the retrieved tissue surrounding the Mobidisc. The particles in the tissue samples of the Activ-L TDR were smaller and contained in macrophages. Retrieval analysis of the UHMWPE cores revealed evidence of minor adhesive and abrasive wear with signs of impingement in both TDR designs.
Although wear was unrelated to the reason for revision, this study demonstrates the presence of UHMWPE particles and inflammatory cells in second generation TDR. Long-term follow-up after TDR is indicated for monitoring wear and implant status.
... Fixed-core TDRs increase it by>10% while mobile-core reduce it by>50% [50], 79% smaller for semi-constrained than non-constrained TDRs6. In flexion, facet joint load is similar to the intact disc for semi-constrained [35] but increases for non-constrained TDRs [51]. ...
The intact intervertebral disc is a six-freedom degree elastic deformation structure with shock absorption. “Ball-and-socket” TDR do not reproduce these properties inducing zygapophyseal joint overload. Elastomeric TDRs reproduce better normal disc kinematics, but repeated core deformation causes its degeneration. We aimed to create a new TDR (ADDISC) reproducing healthy disc features. We designed TDR, analyzed (Finite Element Analysis), and measured every 500,000 cycles for 10 million cycles of the flexion-extension, lateral bending, and axial rotation cyclic compression bench-testing. In the inlay case, we weighted it and measured its deformation.ADDISC has two semi-spherical articular surfaces, one rotation centre for flexion, another for extension, the third for lateral bending, and a polycarbonate urethane inlay providing shock absorption. The first contact is between PCU and metal surfaces. There is no metal-metal contact up to 2000 N, and CoCr28Mo6 absorbs the load. After 10 million cycles at 1.2–2.0 kN loads, wear 140.96 mg (35.50 mm3), but no implant failures. Our TDR has a physiological motion range due to its articular surfaces' shape and the PCU inlay bumpers, minimizing the facet joint overload. ADDISC mimics healthy disc biomechanics and Instantaneous Rotation Center, absorbs shock, reduces wear, and has excellent long-term endurance.
... Lou et al. [16] investigated IDP at adjacent levels in mobilecore and fixed-core prostheses, indicating higher values in the distal level (C6-C7) with a fixed-core prosthesis. Theoretically, inappropriate placement of a constrained prosthesis can increase the pressure at the prosthesis-bone interface and the risk of ASD [126,127]. On the other hand, a mobile-core prosthesis can disperse pressure at the bone-prosthesis interface due to the capability of anteroposterior translation of the prosthesis core [128]. A novel prosthesis with an anatomically accurate surface can increase the contact area between the cervical endplate and the prosthesis as well as further distribute the axial load [129]. ...
In-vitro testing has been conducted to provide a comprehensive understanding of the biomechanics of the cervical spine. This has allowed a characterization of the stability of the spine as influenced by the intrinsic properties of its tissue constituents and the severity of degeneration or injury. This also enables the pre-clinical estimation of spinal implant functionality and the success of operative procedures. The purpose of this review paper was to compile methodologies and results from various studies addressing spinal kinematics in pre- and post-operative conditions so that they could be compared. The reviewed literature was evaluated to provide suggestions for a better approach for future studies, to reduce the uncertainties and facilitate comparisons among various results. The overview is presented in a way to inform various disciplines, such as experimental testing, design development, and clinical treatment. The biomechanical characteristics of the cervical spine, mainly the segmental range of motion (ROM), intradiscal pressure (IDP), and facet joint load (FJL), have been assessed by testing functional spinal units (FSUs). The relative effects of pathologies including disc degeneration, muscle dysfunction, and ligamentous transection have been studied by imposing on the specimen complex load scenarios imitating physiological conditions. The biomechanical response is strongly influenced by specimen type, test condition, and the different types of implants utilized in the different experimental groups.
... In contrast, the best-trade-off AD design was achieved without excessively compromising any metrics. Therefore, careful selection of Pareto-optimal AD designs has the potential to simultaneously reduce the incidences of multiple post-operative complications, such as abnormal segmental kinematics, facet joint overloading, and UHMWPE component failures, as have been reported in clinical results [35][36][37][38] and retrieval studies [5,19,39,40] following treatments using contemporary lumbar TDAs. However, as with any model-based decision making, such optimized designs still require preclinical and clinical testing to observe their behavior when subjected to non-idealized conditions. ...
Total disc arthroplasty (TDA) using an artificial disc (AD) is an attractive surgical technique for the treatment of spinal disorders, since it can maintain or restore spinal motion (unlike interbody fusion). However, adverse surgical outcomes of contemporary lumbar TDAs have been reported. We previously proposed a new mobile-bearing AD design concept featuring a biconcave ultra-high-molecular-weight polyethylene mobile core. The objective of this study was to develop an artificial neural network based multiobjective optimization framework to refine the biconcave-core AD design considering multiple TDA performance metrics, simultaneously. We hypothesized that there is a trade-off relationship between the performance metrics in terms of range of motion, facet joint force, and polyethylene contact pressure. By searching the resulting three-dimensional Pareto frontier after multiobjective optimization, it was found that there was a "best-trade-off" AD design, which could balance all the three metrics, without excessively sacrificing each metric. However, for each single-objective optimum AD design, only one metric was optimal, and distinct sacrifices were observed in the other two metrics. For a commercially available biconvex-core AD design, the metrics were even worse than the poorest outcomes of the single-objective optimum AD designs. Therefore, multiobjective design optimization could be useful for achieving native lumbar segment biomechanics and minimal polyethylene contact pressures, as well as for improving existing lumbar motion-preserving surgical treatments.
... After multiobjective optimization, a TDA design (the best-tradeoff TDA design) was achieved without excessively compromising any one of ROM, FJF, and PCP performance metrics. It indicates that the best-tradeoff TDA design may result in a lower incidence of common complications as reported in the clinic results [71,[120][121][122] and retrieval studies [6,10,76,77] following the surgical treatments using commercial available lumbar TDAs. Furthermore, we demonstrated that the responses of the TDA-treated lumbar segment were very sensitive to TDA designs. ...
Total disc arthroplasty (TDA) is a motion-preserving surgical technique used to treat spinal disorders, when more conservative medical therapies fail. Unfortunately, a high incidence of revision surgery exists due to postoperative complications including abnormal kinematics, facet joint arthritis, and implant failures. However, TDA is still an attractive option, since an optimally designed artificial disc is expected to reproduce native segmental biomechanics. Correspondingly, it would mitigate the development of adjacent segment diseases (a major concern of spinal fusion) caused by altered segmental biomechanics.
Design optimization is a process of finding the best design parameters for a component/system to satisfy one/multiple design requirements using optimization algorithms. The shape of a candidate design is parametrized using computer-aided design, such that design parameters are manipulated to minimize one/multiple objective functions subject to performance constraints and design space bounds. Optimization algorithms typically require the gradients of the objective/constraint functions with respect to each design variable. In the traditional design optimization, due to the high computational cost to calculate the gradients by performing finite element analysis in each optimization iteration, it often results in a slow process to seek the optimal solution. To address the problem, an artificial neural network (ANN) was implemented to derive the analytical expressions of the objective/constraint function and their gradients. By incorporating analytical gradients, we successfully developed a multiobjective optimization (MOO) framework considering three performance metrics simultaneously.
Furthermore, a new mobile-bearing TDA design concept featuring a biconcave polyethylene (PE) core was proposed, to strengthen the PE rim, where a high risk of fracture exists. It was hypothesized that there is a trade-off relationship among postoperative performance metrics in terms of spinal kinematics, facet joint loading, and metal-on-polyethylene contact mechanics. We tested this hypothesis by refining the new TDA to match normal segmental biomechanics and alleviate PE core stress. After performing MOO, the best-trade-off TDA design was determined by the solved three-dimensional Pareto frontier. The novel MOO framework can be also used to improve existing TDA designs, as well as to push the cutting edge of surgical techniques for the treatment of spinal disorders.
URL: https://orb.binghamton.edu/dissertation_and_theses/99/
... When its location misses the center, inappropriate placement of a fixed-core prosthesis can theoretically increase the pressure at the prosthesis-bone interface, and even increase the risk of accelerated adjacent segment changes. [20,21] In addition, facet loads were not measured in this study, as there has always been a controversy about the influence of CDR with a mobile-or fixed-core prosthesis on the facet loads. Some previous studies indicated that CDR with a fixed-core prosthesis has lower pressure on the facet joint than CDR with a mobile-core prosthesis. ...
In vitro biomechanical analysis after cervical disc replacement (CDR) with a novel artificial disc prosthesis (mobile core) was conducted and compared with the intact model, simulated fusion, and CDR with a fixed-core prosthesis. The purpose of this experimental study was to analyze the biomechanical changes after CDR with a novel prosthesis and the differences between fixed- and mobile-core prostheses.
Six human cadaveric C2–C7 specimens were biomechanically tested sequentially in 4 different spinal models: intact specimens, simulated fusion, CDR with a fixed-core prosthesis (Discover, DePuy), and CDR with a mobile-core prosthesis (Pretic-I, Trauson). Moments up to 2 Nm with a 75 N follower load were applied in flexion–extension, left and right lateral bending, and left and right axial rotation. The total range of motion (ROM), segmental ROM, and adjacent intradiscal pressure (IDP) were calculated and analyzed in 4 different spinal models, as well as the differences between 2 disc prostheses.
Compared with the intact specimens, the total ROM, segmental ROM, and IDP at the adjacent segments showed no significant difference after arthroplasty. Moreover, CDR with a mobile-core prosthesis presented a little higher values of target segment (C5/6) and total ROM than CDR with a fixed-core prosthesis (P > .05). Besides, the difference in IDP at C4/5 after CDR with 2 prostheses was without statistical significance in all the directions of motion. However, the IDP at C6/7 after CDR with a mobile-core prosthesis was lower than CDR with a fixed-core prosthesis in flexion, extension, and lateral bending, with significant difference (P < .05), but not under axial rotation.
CDR with a novel prosthesis was effective to maintain the ROM at the target segment and did not affect the ROM and IDP at the adjacent segments. Moreover, CDR with a mobile-core prosthesis presented a little higher values of target segment and total ROM, but lower IDP at the inferior adjacent segment than CDR with a fixed-core prosthesis.
... The fixed centre of rotation spinal disc replacements is likely to increase the volume of wear debris if not accurately placed. 28 The effects of pseudotumors have been found to be locally destructive, requiring revision surgery in a high proportion of patients. 29 The extent of the encapsulation may be significant enough to appear on computed tomography scans. ...
BACKGROUND: Spinal disc arthroplasty implants are primarily manufactured from metal/polymer materials. Biological reaction to wear debris ultimately requires clinical studies for assessment. Research into biological reaction of metal-on-polyethylene and metal-on-metal wear debris of knee and hip arthroplasties is well progressed as opposed to similar research on spinal arthroplasties. MATERIALS AND METHOD: The Swedish Spine Register provides a resource for the evaluation of adverse events and clinical outcome to lumbar metal-on-metal total disc replacements. The resource will be used for a retrospective analysis of the cases in this study. The material reviewed consists of a total of 378 Swedish patients treated between October 2003 and May 2009 (181 male, 197 female); average age was 39.2 years. By means of a questionnaire, 94% of the patients were followed up after two years and 88% after five years RESULTS: No reported cases were found of suspected or confirmed metal hypersensitivity or pseudotumors. This may be due to symptom-producing pseudotumors being extremely rare and the difficulty to form questions which would be able to indicate the presence of the adverse outcome CONCLUSION: Based on the results from this study, it can be concluded that the results do not exclude the possibility that patients might have non-symptomatic pseudotumors, but being non-symptomatic, the authors doubt the importance and relevance of further investigating those isolated cases
... Many of the cited complications such as implant extrusion and vascular injury can be linked to inadequate training, improper sizing and lack of confirmation of satisfactory placement on imaging [107] with implant impingement linked to the aforementioned factors [108]. Design variables relating to constraint and core mobility have also been shown to alter wear and force transmission in in vitro studies [109,110]. ...
Lumbar degenerative disc disease is extremely common. Current evidence supports surgery in carefully selected patients who have failed non-operative treatment and do not exhibit any substantial psychosocial overlay. Fusion surgery employing the correct grafting and stabilization techniques has long-term results demonstrating successful clinical outcomes. However, the best approach for fusion remains debatable. There is some evidence supporting the more complex, technically demanding and higher risk interbody fusion techniques for the younger, active patients or patients with a higher risk of non-union. Lumbar disc arthroplasty and hybrid techniques are still relatively novel procedures despite promising short-term and mid-term outcomes. Long-term studies demonstrating superiority over fusion are required before these techniques may be recommended to replace fusion as the gold standard. Novel stem cell approaches combined with tissue engineering therapies continue to be developed in expectation of improving clinical outcomes. Results with appropriate follow-up are not yet available to indicate if such techniques are safe, cost-effective and reliable in the long-term.
... The fixed centre of rotation spinal disc replacements is likely to increase the volume of wear debris if not accurately placed. 28 The effects of pseudotumors have been found to be locally destructive, requiring revision surgery in a high proportion of patients. 29 The extent of the encapsulation may be significant enough to appear on computed tomography scans. ...
Abstract
Background: Spinal disc arthroplasty implants are primarily manufactured from metal/polymer materials.
Biological reaction to wear debris ultimately requires clinical studies for assessment. Research into biological
reaction of metal-on-polyethylene and metal-on-metal wear debris of knee and hip arthroplasties is well
progressed as opposed to similar research on spinal arthroplasties.
Materials and method: The Swedish Spine Register provides a resource for the evaluation of adverse events and
clinical outcome to lumbar metal-on-metal total disc replacements. The resource will be used for a retrospective
analysis of the cases in this study. The material reviewed consists of a total of 378 Swedish patients treated
between October 2003 and May 2009 (181 male, 197 female); average age was 39.2 years. By means of a questionnaire,
94% of the patients were followed up after two years and 88% after five years.
Results: No reported cases were found of suspected or confirmed metal hypersensitivity or pseudotumors. This
may be due to symptom-producing pseudotumors being extremely rare and the difficulty to form questions
which would be able to indicate the presence of the adverse outcome.
Conclusion: Based on the results from this study, it can be concluded that the results do not exclude the possibility
that patients might have non-symptomatic pseudotumors, but being non-symptomatic, the authors doubt
the importance and relevance of further investigating those isolated cases.
Key words: spinal arthroplasty, metal-on-metal, pseudotumors, hypersensitivity, metallosis, SweSpine
... The buckling stress-strain curve of the physiological discs was incorporated; however, metals were not used in the upper and lower plates, due to the shaping and workmanship required, which subsequently may affect stability as it hinders bone fusion with the prosthesis. Furthermore, it is not possible to examine the prosthesis by X-ray, which is inconvenient during operative installation and postoperative follow-up (32,33). ...
This study aimed to optimize the design and application of semi-constrained integrated artificial discs (SIADs) using a finite element (FE) analysis following implantation, wherein the zygapophysial joints of the segment were biomechanically reconstructed. An FE model of the L4-L5 segment was constructed. Variations in the stresses on the discs and zygapophysial joints were observed during 5° anteflexion, 5° extension and 5° rotation under the 400-N applied axial load. Stresses and load translation analyses of the discs and zygapophysial joints were conducted during anteflexion, extension and rotation under the 400-N applied axial load. Following implantation of the lumbar segments, the stresses on the SIAD zygapophysial joints were not significantly different from those of physiological discs during anteflexion, and these were both marginally greater compared with those of non-constrained artificial discs (NADs). During extension, the increase in the stress on the SIAD zygapophysial joints was less than that on NAD zygapophysial joints. Stresses on the NAD zygapophysial joints were higher than those on SIAD and physiological discs during rotation. The stress on the SIAD zygapophysial joints was not significantly different from that on physiological discs during rotation. For SIADs and NADs, the stresses on the zygapophysial joints and the displacements of the discs were greater compared with those of the physiological discs during extension. The SIADs affected the variations in the stresses on the implanted segment more than the NADs, and the SIADs protected the zygapophysial joints of the implanted segment to a higher degree than the NADs.
... Analyzing the effects of load and force transmis- sion as well as the motion changes resulting from fusion or arthroplasty represents a significant research effort that investigation with the CHARITÉ artificial disc and the ProDisc-L have initiated clinically and in finite element models. 3 Our study represents an initial attempt at charac- terizing motion distribution and changes over time. It was not designed to review effects of load and forces or to pro- vide conclusive, definitive statements on the biomechanical dynamics of fusion compared with arthroplasty and its clin- ical consequences. ...
As the authors of the articles under discus- sion, we thank Dr. McCormick for a very thorough review of both papers and the resulting Editorial allowing further discussions of multiple subtleties of the CHARITÉ FDA IDE.
... A previous finite element study evaluated the FCF between a fixed and mobile core TDR at varying positions and documented FCFs equivalent to 175% of the intact state depending on positioning and TDR type. 49 This study attributed increases in FCF to a fixed center of rotation of a fixed-core TDR. However, this study applied known physiologic rotations to all of the models instead of loads, thereby negating the potential for increased mobility that arises after TDR, 48 even when one is using a hybrid loading approach. ...
Ulf Fernström implanted stainless steel ball bearings following discectomy, or for painful disc disease, and termed this procedure disc arthroplasty. Today, spherical interbody spacers are clinically available, but there is a paucity of associated biomechanical testing. The primary objective of the current study was to evaluate the biomechanics of a spherical interbody implant. It was hypothesized that implantation of a spherical interbody implant, with combined subsidence into the vertebral bodies, would result in similar ranges of motion (RoM) and facet contact forces (FCFs) when compared with an intact condition. A secondary objective of this study was to determine the effect of using a polyetheretherketone (PEEK) versus a cobalt chrome (CoCr) implant on vertebral body strains. We hypothesized that the material selection would have a negligible effect on vertebral body strains since both materials have elastic moduli substantially greater than the annulus.
A finite element model of L3-L4 was created and validated by use of ROM, disc pressure, and bony strain from previously published data. Virtual implantation of a spherical interbody device was performed with 0, 2, and 4 mm of subsidence. The model was exercised in compression, flexion, extension, axial rotation, and lateral bending. The ROM, vertebral body effective (von Mises) strain, and FCFs were reported.
Implantation of a PEEK implant resulted in slightly lower strain maxima when compared with a CoCr implant. For both materials, the peak strain experienced by the underlying bone was reduced with increasing subsidence. All levels of subsidence resulted in ROM and FCFs similar to the intact model.
The results suggest that a simple spherical implant design is able to maintain segmental ROM and provide minimal differences in FCFs. Large areas of von Mises strain maxima were generated in the bone adjacent to the implant regardless of whether the implant was PEEK or CoCr.
... Oswestry disability indexes suggested that FJ pain is the most recurrent negative outcome in BoS implant patients [39]. Accordingly, numerical finite element (FE) analyses pointed out that the constrained or semi-constrained transversal deformability of the segments treated with BoS devices could strongly affect FJ biomechanics [21, 37]. An IVD substitute made of fully deformable materials instead of rigid bodies with predefined relative motions would give instantaneous centres of rotation that better respect the interactions among spinal components including FJ contacts . ...
When the intervertebral disc is removed to relieve chronic pain, subsequent segment stabilization should restore the functional
mechanics of the native disc. Because of partially constrained motions and the lack of intrinsic rotational stiffness ball-on-socket
implants present many disadvantages. Composite disc substitutes mimicking healthy disc structures should be able to assume
the role expected for a disc substitute with fewer restrictions than ball-on-socket implants. A biomimetic composite disc
prototype including artificial nucleus fibre-reinforced annulus and endplates was modelled as an L4–L5 disc substitute within
a L3–L5 lumbar spine finite element model. Different device updates, i.e. changes of material properties fibre distributions
and volume fractions and nucleus placements were proposed. Load- and displacement-controlled rotations were simulated with
and without body weight applied. The original prototype reduced greatly the flexibility of the treated segment with significant
adjacent level effects under displacement-controlled or hybrid rotations. Device updates allowed restoring large part of the
global axial and sagittal rotational flexibility predicted with the intact model. Material properties played a major role,
but some other updates were identified to potentially tune the device behaviour against specific motions. All device versions
altered the coupled intersegmental shear deformations affecting facet joint contact through contact area displacements. Loads
in the bony endplates adjacent to the implants increased as the implant stiffness decreased but did not appear to be a strong
limitation for the implant biomechanical and mechanobiological functionality. In conclusion, numerical results given by biomimetic
composite disc substitutes were encouraging with greater potential than that offered by ball-on-socket implants.
... 6,7 Moumene et al. reported in an experimental work that a mobile-core prosthesis reduced facet loading compared with a fixed-core design. 17 This study showed that the majority of the core was effectively mobile. A mean amount of motion equal to 1.12 Ϯ 0.8 mm was observed for a design that allows 2.5 mm multidirectional translation between flat surfaces of the inferior part of the core and the superior part of the inferior prosthesis endplate. ...
Background
An artificial disc prosthesis is thought to restore segmental motion in the lumbar spine. However, it is reported that disc prosthesis can increase the intervertebral translation (VT). The concept of the mobile-core prosthesis is to mimic the kinematic effects of the migration of the natural nucleus and therefore core mobility should minimize the VT. This study explored the hypothesis that core translation should influence VT and that a mobile core prosthesis may facilitate physiological motion.
... Oswestry disability indexes suggested that FJ pain is the most recurrent negative outcome in BoS implant patients [39]. Accordingly, numerical finite element (FE) analyses pointed out that the constrained or semi-constrained transversal deformability of the segments treated with BoS devices could strongly affect FJ biomechanics [21,37]. An IVD substitute made of fully deformable materials instead of rigid bodies with predefined relative motions would give instantaneous centres of rotation that better respect the interactions among spinal components including FJ contacts. ...
... The FE-Model included all the critical components of the spine: cortical and cancellous bone; endplates; annulus fibrosus and nucleus pulposus of the intervertebral discs; articular cartilage; synovial fluid of the facet joints; anterior longitudinal ligament; posterior longitudinal ligament; intertransverse ligament; ligamentum flavum; capsular ligament; supraspinous ligament; and interspinous ligament. Material properties of these various tissues were selected from the literature and are summarized elsewhere [7]. The FE-Model was validated based on in-vivo study by Wood et al [8]. ...
In this study we conducted a finite element analysis to investigate changes in load-sharing characteristics and the screw-bone
loading of a PEEK rod system compared to a traditional titanium rod system when used as posterior instrumentation in conjunction
with an interbody fusion. A 400N follower load, 14o Flexion, 8o extension was applied to an L1-S1 FE-model with fusion at
L4-L5. Under compression, the load sharing between the anterior column support and posterior instrumentation is 70%-30% for
the titanium rod and 85%-15% for the PEEK rod. The screw-bone loading for the PEEK rod is reduced by 85% and 71% in flexion
and extension respectively when compared to a titanium rod.
... There is a subgroup of TDR patients with excellent early results, but who develop new LBP later with positive facet joint block [21]. This could be a matter of patient selection, or an opportunity to prevent secondary facet joint degradation [20]. Whether or not TDR reduces the frequency of adjacent level disease remains uncertain. ...
Study design:
Prospective randomized controlled study.
Objective:
To compare the outcome of indirect decompression by means of the X-Stop (Medtronics Inc., Minneapolis, MN) implant with conventional decompression in patients with neurogenic intermittent claudication due to lumbar spinal stenosis.
Summary of background data:
Decompression is currently the "gold standard" for lumbar spinal stenosis but is afflicted with complications and a certain number of dissatisfied patients. Interspinous implants have been on the market for more than 10 years, but no prospective study comparing its outcome with decompression has been performed.
Methods:
After power calculation, 100 patients were included: 50 in the X-Stop group and 50 in the decompression group. Patients with symptomatic 1- or 2-level lumbar spinal stenosis and neurogenic claudication relieved on flexion were included. X-Stop operations were performed under local anesthesia.The mean patient age was 69 (49-89) years, and the male/female distribution was 56/44. Minimal dural sac area was in all cases except two 80 mm or less.The noninferiority hypothesis included 6, 12, and 24 months of follow-up, and included. intention-to-treat as well as as-treated analyses.The primary outcome meansure was the Zürich Claudication Questionnaire, and the secondary outcome measures was the visual analogue scale pain, Short-Form 36 (SF-36), complications, and reoperations.
Results:
The primary and secondary outcome measures of patients in both groups improved significantly. The results were similar at 6, 12, and 24 months and at no time point could any statistical difference between the 2 types of surgery be identified. Three patients (6%) in the decompression group underwent further surgery, compared with 13 patients (26%) in the X-Stop group (P = 0.04). Results were identical in intention-to-treat and as-treated analyses.
Conclusion:
For spinal stenosis with neurogenic claudication, decompressive surgery as well as X-Stop are appropriate procedures. Similar results were achieved in both groups, however, with a higher number of reoperations in the X-Stop group. Patients having X-Stop removal and decompression experienced results similar to those randomized to primary decompression.
Level of evidence:
1.
... Moumene and Geisler [70] performed a study to evaluate the loading on the facet joints and stress on the polyethylene core after implantation of Charité (unconstrained) and Prodisc (Semiconstrained) TDA. The unconstrained TDA unloads the facet joints and presents decreased core stress as compared to the fixed-core Semiconstrained TDA. ...
Disc degeneration and associated disorders are among the most debated topics in the orthopedic literature over the past few decades. These may be attributed to interrelated mechanical, biochemical, and environmental factors. The treatment options vary from conservative approaches to surgery, depending on the severity of degeneration and response to conservative therapies. Spinal fusion is considered to be the "gold standard" in surgical methods till date. However, the association of adjacent level degeneration has led to the evolution of motion preservation technologies like spinal arthroplasty and posterior dynamic stabilization systems. These new technologies are aimed to address pain and preserve motion while maintaining a proper load sharing among various spinal elements. This paper provides an elaborative biomechanical review of the technologies aimed to address the disc degeneration and reiterates the point that biomechanical efficacy followed by long-term clinical success will allow these nonfusion technologies as alternatives to fusion, at least in certain patient population.
... Many spine arthroplasty technologies are currently being tested with the intent of relieving pain and maintaining motion while preserving the health of adjacent segments. 43 Arthroplasty options include total disc replacement (TDR) and subtotal disc replacement or nucleoplasty. 13,44 TDR, while permitting some motion preservation, is destructive to the native joint and requires highly invasive implantation techniques. ...
Investigation of injectable nucleus pulposus (NP) implant.
To assess the ability of a recently developed injectable hydrogel implant to restore nondegenerative disc mechanics through support of NP functional mechanics.
Although surgical intervention for low back pain is effective for some patients, treated discs undergo altered biomechanics and adjacent levels are at increased risk for accelerated degeneration. One potential treatment as an alternative to surgery for degenerated disc includes the percutaneous delivery of agents to support NP functional mechanics. The implants are delivered in a minimally invasive fashion, potentially on an outpatient basis, and do not preclude later surgical options. One of the challenges in designing such implants includes the need to match key NP mechanical behavior and mimic the role of native nondegenerate NP in spinal motion.
The oxidized hyaluronic acid gelatin implant material was prepared. In vitro mechanical testing was performed in mature ovine bone-disc-bone units in 3 stages: intact, discectomy, and implantation versus sham. Tested samples were cut axially for qualitative structural observations.
Discectomy increased axial range of motion (ROM) significantly compared with intact. Hydrogel implantation reduced ROM 17% (P < 0.05) compared with discectomy and returned ROM to intact levels (ROM intact 0.71 mm, discectomy 0.87 mm, postimplantation 0.72 mm). Although ROM for the hydrogel implant group was statistically unchanged compared with the intact disc, ROM for sham discs, which received a discectomy and no implant, was significantly increased compared with intact. The compression and tension stiffness were decreased with discectomy and remained unchanged for both implant and sham groups as expected because the annulus fibrosus was not repaired. Gross morphology images confirmed no ejection of NP implant.
An injectable implant that mimics nondegenerate NP has the potential to return motion segment ROM to normal subsequent to injury.
... The motion they provide is therefore different from the IVD. Current TDRs have been found to lead to changes in ROM Moumene and Geisler, 2007;Panjabi et al., 2007a;Panjabi et al., 2007b), to an altered motion pattern (O'Leary et al., 2005), and to a different segmental lordosis (Cakir et al., 2005). These changes may cause overloading of the facet joints (Chung et al., 2009;Denoziere and Ku, 2006;Lemaire et al., 1997), and increase the risk of ASD (Shim et al., 2007). ...
To improve the treatments for low back pain, new designs of total disk replacement have been proposed. The question is how well these designs can act as a functional replacement of the intervertebral disk. Four finite element models were made, for four different design concepts, to determine how well they can mimic the physiological intervertebral disk mechanical function. The four designs were a homogenous elastomer, a multi-stiffness elastomer, an elastomer with fiber jacket, and a hydrogel with fiber jacket. The best material properties of the four models were determined by optimizing the model behavior to match the behavior of the intervertebral disk in flexion-extension, axial rotation, and lateral bending. It was shown that neither a homogeneous elastomer nor a multi-stiffness elastomer could mimic the non-linear behavior within the physiological range of motion. Including a fiber jacket around an elastomer allowed for physiological motion in all degrees of freedom. Replacing the elastomer by a hydrogel yielded similar good behavior. Mimicking the non-linear behavior of the intervertebral disk, in the physiological range of motion is essential in maintaining and restoring spinal motion and in protecting surrounding tissues like the facet joints or adjacent segments. This was accomplished with designs mimicking the function of the annulus fibrosus.
Spinal fusion and screw fixation techniques have been used by surgeons for a long time to promote structural stabilization in the treatment of degenerative spinal diseases. However, the fusion technique replaces a mobile segment with a fixed state, which leads to an increase in physical stress and tension in adjacent segments. This factor is thought to accelerate degenerative changes in adjacent segments. To overcome these shortcomings of fusion surgery, various dynamic stabilization concepts with motion preservation and related devices have been developed by surgeons and industries. Cervical artificial disc replacement (CADR), lumbar artificial disc replacement (LADR), interspinous process devices (IPD), and screw-dynamic rod systems have been proposed as alternatives to fusion surgery. This has great significance in preventing problems that occur during fusion and motion preservation. However, they cannot fully replace the fusion technique because of the limitations of this concept. In this chapter, a detailed explanation of the motion preservation technique is described and the direction to be developed in the future is discussed.KeywordsMotion preservation techniqueArtificial discInterspinous process deviceScrew-dynamic rod system
Background:
Cervical disc arthroplasty replacement (CDA) was developed to avoid specific disadvantages of cervical fusion. The purpose of this paper is to provide 10-year follow-up results of an ongoing prospective study after CDA.
Methods:
384 patients treated using the Mobi-C (ZimVie, Troyes, France) were included in a prospective multicenter study. Routine clinical and radiological examinations were reported pre and postoperatively with up to 10-year follow-up. Complications and revision surgeries were also documented.
Results:
at 10 years showed significant improvement in all clinical outcomes (NDI, VAS for arm and neck pain, SF-36 PCS and MCS). Motion at the index level increased significantly over baseline (mean ROM=7.6° vs. 8° at 5 years and 6.0° preoperatively; P<0.001) and 71.3% of the implanted segments remained mobile (ROM>3°). Adjacent discs were also mobile at 10 years with the same mobility as preoperatively. At 10 years, 20.9% of the implanted segments demonstrated no heterotopic ossification (HO). Thirty-four patients (8.9%) experienced 41 adverse events, with or without reoperation during the 5 first years. We found only two additional surgeries after 5 years. We observed an increased percentage of working patients and a decrease in medication consumption. Regarding the overall outcome, 94% of patients were satisfied.
Conclusions:
Our 10-year results showed significant improvement in all clinical outcomes, with low rates of revision or failure. This experience in patients with long-term follow-up after CDA endorses durable, favorable outcomes in properly selected patients.
Degenerative diseases and injuries of the spine are one of the main reasons for the disability of the population. Total discs arthroplasty is the main treatment for severe spinal diseases. Hence, an overview of the methods used for studying the mechanical behavior of spinal tissues during endoprosthesis replacement becomes topical. Herewith we describe the methods of an experimental and numerical study of the mechanical characteristics and functional properties of tissue and the spine elements that endoprosthesis must meet. The need is shown to develop methods for the numerical study of the mechanical behavior of the lumbar vertebrae during arthroplasty, taking into account degenerative changes in biological tissues.
There is an increased interest in studying the biomechanics of the facet joints. For in silico studies, it is therefore important to understand the level of reliability of models for outputs of interest related to the facet joints. In this work, a systematic review of finite element models of multi-level spinal section with facet joints output of interest was performed. The review focused on the methodology used to model the facet joints and its associated validation. From the 110 papers analysed, 18 presented some validation of the facet joints outputs. Validation was done by comparing outputs to literature data, either computational or experimental values; with the major drawback that, when comparing to computational values, the baseline data was rarely validated. Analysis of the modelling methodology showed that there seems to be a compromise made between accuracy of the geometry and nonlinearity of the cartilage behaviour in compression. Most models either used a soft contact representation of the cartilage layer at the joint or included a cartilage layer which was linear elastic. Most concerning, soft contact models usually did not contain much information on the pressure-overclosure law. This review shows that to increase the reliability of in silico model of the spine for facet joints outputs, more needs to be done regarding the description of the methods used to model the facet joints, and the validation for specific outputs of interest needs to be more thorough, with recommendation to systematically share input and output data of validation studies.
Hybrid stabilization is widely performed for the surgical treatment of degenerative disc diseases (DDD). Pedicle-Based Hybrid Stabilization (PBHS) intends to reduce fusion-associated drawbacks of adjacent segment degeneration (ASD), construct failure and pseudoarthrosis. Recently, many types of PBHS systems have been developed and optimized, using polymeric devices as an adjunct for lumbar fusion procedures. Therefore, the purpose of this study was to evaluate the effect of new PBHS on bending stiffness and center of rotation (COR) at an operated and adjacent levels in comparison with established semirigid and rigid devices in lumbar fusion procedures. A validated 3D Finite Element (FE) model of the L3-S1 segments was modified to simulate postoperative changes during combined loading (Moment of 7.5 Nm + Follower Load of 400 N). Two models instrumented with PBHS (Dynesys Transition Optima [DTO], NFlex), semirigid system (polyetheretherketone [PEEK]) and rigid fixation system (Titanium rod [Ti]) were compared with those of the healthy and degenerated models. Contact force on the facet joint during extension increased in fusion (40N) with an increase of bending stiffness in Dynesys and NFlex; The COR shifted in posterior and cranial directions of the fused level. The CORs in the lower lumbar spine is segment-dependent and altered with the adopted construct. The bending stiffness was varied from 1.47 N.m/deg in lateral bending for the healthy model to 5.75 N.m/deg for the NFlex stabilization, which had the closest COR, compared to the healthy COR. Locations of COR, stress and strain distribution varied according to construct design and materials used. These data could help understand the biomechanical effects of current PBHS on the behaviour of the lower lumbar spine.
Objective:
The purpose of this study is to compare the heterotopic ossification (HO) between cervical disc arthroplasty (CDA) with fixed- and mobile-core prosthesis, and simultaneously compare the clinical and radiographic outcomes.
Methods:
Retrospectively analysis the patients underwent CDA in our institute. Patients were divided into fixed-core group (Prodisc-C and Discover disc), and mobile-core group (Bryan and Prestige-LP disc). HO was assessed based on McAfee classification. The Visual analogue scale (VAS), neck disability index (NDI), and Japanese orthopaedic association (JOA) scores were used to evaluate the clinical outcome. In addition, radiographic assessments included: cervical sagittal alignment (C2-7 angle), segmental angle (SA), and range of motion (ROM) of the index level.
Results:
A total of 218 patients were included with a minimal follow-up of 24 months. There were 137 patients in mobile-core group (Prestige-LP, 102; Bryan, 35) and 81 patients in the fixed-core group (Discover, 43; and ProDisc-C, 38). The mean follow-up time was 46.8 months. At the final follow-up, the overall incidence of HO was 39.9%. The incidence of HO in fixed-core group was significantly higher than those in the mobile-core group (50.6% vs 33.6%, P = 0.013). However, there were no significant differences in VAS, NDI or JOA scores, C2-7 angle, SA, or ROM of the index level between the two groups.
Conclusions:
The fixed-core prosthesis had higher incidence of HO than mobile-core prosthesis after CDA. However, both CDA with fixed- and mobile-core prostheses obtained good clinical outcome, and maintained cervical sagittal alignment.
Study design:
A three-dimensional finite element model of intact lumbar spine was constructed and four surgical finite element models implanted with ball-and-socket artificial discs with four different radii of curvature were compared.
Objective:
To investigate biomechanical effects of the curvature of ball-and-socket artificial disc using finite element analysis.
Summary of background data:
Total disc replacement (TDR) has been accepted as an alternative treatment because of its advantages over spinal fusion methods in degenerative disc disease. However, the influence of the curvature of artificial ball-and-socket discs has not been fully understood.
Methods:
Four surgical finite element models with different radii of curvature of ball-and-socket artificial discs were constructed.
Results:
The range of motion (ROM) increased with decreasing radius of curvature in extension, flexion, and lateral bending, whereas it increased with increasing radius of curvature in axial torsion. The facet contact force was minimum with the largest radius of curvature in extension, flexion, and lateral bending, whereas it was maximum with the largest radius in axial torsion. It was also affected by the disc placement, more with posterior placement than anterior placement. The stress in L4 cancellous bone increased when the radius of curvature was too large or small.
Conclusion:
The geometry of ball-and-socket artificial disc significantly affects the ROM, facet contact force, and stress in the cancellous bone at the surgical level. The implication is that in performing TDR, the ball-and-socket design may not be ideal, as ROM and facet contact force are sensitive to the disc design, which may be exaggerated by the individual difference of anatomical geometry.
Level of evidence:
N/A.
Conceptually, nucleus replacement offers the promise of a novel technology to add to the spectrum of surgical techniques to treat a variety of degenerative spine pathologies. Specifically, nucleus replacement has the potential to address degenerative pathologies more complex than simple disc herniation but less advanced than severe degenerative disc disease (DDD). Therefore, nucleus replacement may fill a surgical niche between simple discectomy and total disc replacement or spinal fusion. However, the potential success of nucleus replacement must also be tempered by the lack of long-term clinical results. Today, new technologies for spine surgery are judged on the basis of their safety and efficacy as well as cost-effectiveness. Several nucleus replacement devices are entering or completing pilot feasibility studies. Further clinical investigation with well-designed prospective, randomized pivotal trials is needed to ultimately determine the ideal indications and efficacy of nucleus replacement in the treatment of lumbar DDD.
Study design:
A biomechanical comparison among the intact C2-C7 segments, the C5-C6 segments implanted with fusion cage and three different artificial disc replacements (ADR) by finite element (FE) model creation reflecting the entire cervical spine below C2.
Objective:
To analyze the biomechanical changes in subaxial cervical spine after ADR and to verify the efficacy of a new mobile core artificial disc Baguera C which is designed to absorb shock.
Summary of background data:
Scarce references could be found and compared regarding the cervical ADR devices' biomechanical differences that are consequently related to their different clinical results.
Methods:
One fusion device (CJ cage system, WINNOVA) and three different cervical artificial discs (Prodisc-C Nova (DePuy Synthes), Discocerv (Scient'x/Alphatec), Baguera C (Spineart)) were inserted at C5-6 disc space inside the FE model and analyzed. Hybrid loading conditions, under bending moments of 1 Nm along flexion, extension, lateral bending and axial rotation with a compressive force of 50 N along the follower loading direction, were used in this study. Biomechanical behaviors such as segmental mobility, facet joint forces, and possible wear debris phenomenon inside the core were investigated.
Results:
The segmental motions as well as facet joint forces were exaggerated after ADR regardless of type of the devices. The Baguera C mimicked the intact cervical spine regarding the location of the center of rotation (COR) only during the flexion moment. It also showed a relatively wider distribution of the contact area and significantly lower contact pressure distribution on the core compared to the other two devices. A 'lift off' phenomenon was noted for other two devices according to the specific loading condition.
Conclusion:
The mobile core artificial disc Baguera C can be considered biomechanically superior to other devices by demonstrating no 'lift off' phenomenon, and significantly lower contact pressure distribution on core.
Level of evidence:
N/A.
Currently available standardized methods for evaluating the long-term wear of total disk replacements do not incorporate the effects of potential device impingement. Creation of a standard that incorporates device impingement is difficult without a thorough understanding of the associated biomechanical environment. Arbitrary modification of the currently available wear-test protocols to account for device impingement may add unnecessary cost, and potentially inaccurate, unrealistic results. Finite element models provide the ability to control variation and test for a wide range of parameters without the excessive time and monetary costs associated with cadaveric testing or wear simulations. However, careful validation and verification of these models is required in order to ensure predictability. Retrieved implants can be used to validate the clinical predictability of finite element models (FEMs). The objective of the current study was to quantify the ability of a previously developed FEM of the lumbar spine to predict polyethylene damage modes and impingement in actual clinical scenarios, and extract the loading and boundary conditions for implementation into a new lumbar TDR wear simulation standard. In order to achieve this objective, actual clinical scenarios, associated with retrieved implants, were modeledand simulated. We hypothesized that clinical damage modes, including both impingement and non-impingement scenarios, can be predicted using a FEM that incorporates case-specific clinical factors, anterior-posterior shear forces, coupled translations, and facet contact.
Study design:
Measurement of zygapophyseal joint pressure and displacement was performed after placement of a semiconstrained integrated artificial disc (SIAD) in a cadaver model.
Objective:
To understand the likelihood of accelerated zygapophyseal joints degeneration as a result of the implant.
Summary of background data:
A SIAD has been developed to treat lumbar spondylosis secondary to segmental disc degeneration and spinal stenosis. The SIAD replaces the stenotic segment's disc. Previous studies have demonstrated that nonconstrained artificial disc (NAD) replacements fail to achieve their optimal long-term outcomes likely because of significantly increased zygapophyseal joint pressure and displacement at the implanted level. Moreover, clinical studies have reported an increased incidence of zygapophyseal joint degeneration after lumbar disc replacement.
Methods:
Eight cadaver lumbar specimens (L2-L5) were loaded in flexion, neutral, extension, left bend, and right rotation. Zygapophyseal joint pressure and displacement were measured during each of the 5 positions at each of the 3 levels with the ratio of deformation calculated under the different loads. An artificial disc was placed at the L3-L4 level, and the measurements were repeated.
Results:
After L3-L4 segment implantation, the pressure in the zygapophyseal joint at operative segment was not significantly changed by SIAD and NAD implantation in axial compression and flexion, compared with physiological disc. Notable differences in zygapophyseal joint pressure between the SIAD and NAD were identified at the operative level in extension, left bend, and right rotation. The adjacent-level effect of NAD was significantly greater than that of SIAD. The ratio of deformation difference between the 2 discs was increased by load experienced in extension, flexion, left bend, and right rotation.
Conclusion:
The SIAD provided a superior biomechanical milieu for zygapophyseal joints at the implanted and adjacent levels compared with NAD, which may avoid the acceleration of postoperative zygapophyseal joint degeneration.
Level of evidence:
1.
Introduction
The role of fusion of lumbar motion segments for the treatment of intractable low back pain (LBP) from degenerative disc disease (DDD) remains controversially debated. Total lumbar disc replacement (TDR) has been used as an alternative in a highly selected patient cohort. However, the amount of long-term follow-up (FU) data on TDR is limited.
Materials and Methods
Visual analog scale (VAS), Oswestry disability index (ODI) and patient satisfaction rates were acquired within the framework of an ongoing prospective clinical trial. The patient's professional activity/employment status, complications, and reoperations were recorded. A comparison was performed between mono- and bisegmental cases.
Results
The initial cohort consisted of 201 patients; 181 patients were available for final FU (90.0% FU rate) after a mean FU of 7.4 years (range, 5.0-10.8 years). VAS and ODI scores revealed a highly significant improvement from baseline levels at all postoperative stages ( p < 0.0001). VAS scores demonstrated a slight (VAS 2.6-3.3) but statistically significant deterioration from 48 months onward ( p < 0.05). Patient satisfaction rates remained stable throughout the entire postoperative course, with 63.6% of patients reporting a “highly satisfactory” or a “satisfactory outcome” (22.7%), while 13.7% of patients were not satisfied. The overall complication rate was 14.4% ( n = 26/181). The incidence of revision surgeries for general and/or device related complications was 7.2% ( n = 13/181). Results for two-level TDR were significantly inferior in comparison with one-level cases and were associated with higher complication (11.9 vs. 27.6%; p = 0.03) and inferior satisfaction rates ( p < 0.003).
Conclusion
Despite the fact that the current data comprise the early experiences and learning curve associated with a new surgical technique, the results demonstrate satisfactory and maintained mid- to long-term clinical results after a mean FU of 7.4 years. Patient safety was proven with acceptable complication and reoperation rates. Fear of excessive late complications or reoperations following the primary TDR procedure cannot be substantiated with the present data. In carefully selected cases, TDR can be considered a viable treatment alternative to lumbar fusion for which spine communities around the world seem to have accepted mediocre clinical results as well as obvious and significant drawbacks. Disclosure of Interest None declared
Ultra-high molecular weight polyethylene (UHMWPE) and Co-Cr alloy are still commonly used bearing materials for total disc replacement (TDR) as well as total joint replacement (TJR). Compared to TJR, in TDR, there are different articulating conditions such as higher contact pressure, shorter sliding distance, and less lubricant, unlike hip and knee joints. In this study, friction and wear phenomena between UHMWPE and Co-Cr alloy were characterized under the wide range of contact pressures including the extremely high contact pressure with limited amount of lubricant in lumbar TDR. All friction and wear tests were conducted by using a Pin-on-Disk type tribo-tester under normal contact pressures of 5, 10, 20, 40, 60 and 80MPa in the repeat pass rotational and the linear reciprocal sliding motions, respectively. UHMWPE cylindrical pins slid against Co-Cr alloy disks in three different kinds of dry, rarely and fully immersed lubricated conditions for friction tests and in a fully immersed lubricated condition for wear tests. For all lubricated conditions, mean coefficients of friction decreased as contact pressure increased. There were statistically significant differences in the mean coefficients of friction among six levels of contact pressures and among three kinds of lubricated conditions. The amount of wear linearly increased as number of sliding cycles increased for all contact pressures, and also it increased as the contact pressure increased. There were statistically significant differences in the wear rates of UHMWPE among six levels of contact pressures. Coefficients of friction of UHMWPE against Co-Cr alloy under very high contact pressure such as 40–80MPa in the lubricated condition were so low as to induce no clinical problem at the bearing surface. For reducing wear of UHMWPE component in TDR, it is necessary to decrease the contact pressure in design as much as possible.
The study is to develop a three-dimensional finite element model to study the disease related to lumbo-pelvic-femoral complex. The CT images from the first lumbar vertebra to the upper third of femurs of a 25-years-old healthy male were obtained. Triangle mesh model was generated using these images by Simpleware. Then the whole solid model were set up after repair and optimization in Geomagic. The finite element model was established and analyzed using ABAQUS. The three-dimensional finite element model of pelvis-lumbar-femur (3D- FEMLPFC) had 106557 nodes and 435736 elements. The difference of anatomic shape between 3D-FEMLPFC and CT images was between -0.74 mm and 1.64 mm in absolute error, and between -1.39% and 3.16% in relative error. It was high similar with specimen in geometric shape. The stress distribution of 3D-FEMLPFC was the same as clinical observation and published literatures. It was valid and reasonable and could be used for biomechanical analysis.
The placement of artificial disks can alter the center of rotation and kinematic pattern; therefore, forces in the spine during the motion will be affected as a result. The relationship between the location of joint center of artificial disks and forces in the spinal components is not investigated. A musculoskeletal model of the spine was developed, and three location cases of center of rotation were investigated varying 5 mm anteriorly and posteriorly from the default center. Resultant joint forces, ligament forces, facet forces, and muscle forces for each case were predicted during sagittal motion. No considerable difference was observed for joint force (maximum 14%). Anterior shift of center of rotation induced the most ligament forces (200 N) and facet forces (130 N) among the three cases. Posterior and anterior shifts of centers of rotation from the default location caused considerable changes in muscle forces, respectively: 108% and 70% of increase in multifidi muscle and 157% and 187% of increase in short segmental muscle. This study showed that the centers of rotation due to the design and the surgical placement of artificial disk can affect the kinetic results in the spine.
In the search for better surgical treatment of chronic low-back pain (LBP) in the presence of disc degeneration, total disc replacement has received increasing attention in recent years. A possible advantage of total disc replacement compared with fusion is maintained mobility at the operated level, which has been suggested to reduce the chance of adjacent segment degeneration.
The aim of this systematic review was to assess the effect of total disc replacement for chronic low-back pain in the presence of lumbar disc degeneration compared with other treatment options in terms of patient-centred improvement, motion preservation and adjacent segment degeneration.
A comprehensive search in Cochrane Back Review Group (CBRG) trials register, CENTRAL, MEDLINE, EMBASE, BIOSIS, ISI, and the FDA register was conducted. We also checked the reference lists and performed citation tracking of included studies.
We included randomised controlled trials (RCTs) comparing total disc replacement with any other intervention for degenerative disc disease.
We assessed risk of bias per study using the criteria of the CBRG. Quality of evidence was graded according to the GRADE approach. Two review authors independently selected studies and assessed risk of bias of the studies. Results and upper bounds of confidence intervals were compared against predefined clinically relevant differences.
We included 40 publications, describing seven unique RCT's. The follow-up of the studies was 24 months, with only one extended to five years. Five studies had a low risk of bias, although there is a risk of bias in the included studies due to sponsoring and absence of any kind of blinding. One study compared disc replacement against rehabilitation and found a statistically significant advantage in favour of surgery, which, however, did not reach the predefined threshold for clinical relevance. Six studies compared disc replacement against fusion and found that the mean improvement in VAS back pain was 5.2 mm (of 100 mm) higher (two studies, 676 patients; 95% confidence interval (CI) 0.18 to 10.26) with a low quality of evidence while from the same studies leg pain showed no difference. The improvement of Oswestry score at 24 months in the disc replacement group was 4.27 points more than in the fusion group (five studies; 1207 patients; 95% CI 1.85 to 6.68) with a low quality of evidence. Both upper bounds of the confidence intervals for VAS back pain and Oswestry score were below the predefined clinically relevant difference. Choice of control group (circumferential or anterior fusion) did not appear to result in different outcomes.
Although statistically significant, the differences between disc replacement and conventional fusion surgery for degenerative disc disease were not beyond the generally accepted clinical important differences with respect to short-term pain relief, disability and Quality of Life. Moreover, these analyses only represent a highly selected population. The primary goal of prevention of adjacent level disease and facet joint degeneration by using total disc replacement, as noted by the manufacturers and distributors, was not properly assessed and not a research question at all. Unfortunately, evidence from observational studies could not be used because of the high risk of bias, while these could have improved external validity assessment of complications in less selected patient groups. Non-randomised studies should however be very clear about patient selection and should incorporate independent, blinded outcome assessment, which was not the case in the excluded studies. Therefore, because we believe that harm and complications may occur after years, we believe that the spine surgery community should be prudent about adopting this technology on a large scale, despite the fact that total disc replacement seems to be effective in treating low-back pain in selected patients, and in the short term is at least equivalent to fusion surgery.
Although most of the patients suffering from discogenic low back pain (LBP) can be successfully treated conservatively, patients
failing to respond to the conservative treatments often seek surgery. Both fusion and total disc arthroplasty (TDA) are considered
as common surgical treatments. Clinically, randomized controlled studies compared TDA with fusion and showed good results.
However, the invasiveness of the surgery, revision risk and accelerated facet degeneration may be the issues which require
careful follow-up. Efforts have been made to develop a device to replace only the diseased nucleus, while maintaining disc
height and being less invasive than TDA. The NuBac™ Disc Arthroplasty System is a technology incorporating these characteristics
in combination with preservation of motion. This is an ongoing, prospective, non-randomized, multi-centre cohort study evaluating
the clinical results with the NuBac disc arthroplasty system. Patients who have discogenic LBP with or without leg pain and
failed 6 months conservative treatment are included in this longitudinal study. Patient pathology and surgeon preference determine
which surgical approach, anterolateral, lateral or posterior, is used to implant NuBac. Patient demographics, VAS and ODI
are collected pre-operatively and at 6 weeks, 3, 6, 12 and 24 months post-operatively. The data demonstrate that NUBAC is
a less invasive treatment option which is safe and effective in relieving pain and improving function and is clinically viable
for all three major surgical approaches.
Degenerative disk disease (DDD) at the level adjacent to an immobilized (fused) spinal segment is a well-recognized condition
which is believed to result from the supraphysiological biomechanical stress and increased range of motion (ROM) created by
the rigid fixation of the lumbar spine. It is hypothesized that topping a fusion with a posterior dynamic stabilization (PDS)
system at the level adjacent to a fusion may diminish these increased stress and ROM, and subsequently, reduce the incidence
of adjacent-level disk disease. The objective of this work was therefore to determine the biomechanical effect of PDS by measuring
the stress and the ROM on a segment adjacent to a fusion prior and after PDS instrumentation. A three-dimensional nonlinear
finite element model (FEM) of the human osseo-ligamentous lumbar spine was developed and validated based on experiments conducted
on human cadavers. PDS topping off a fusion was shown in this model to relieve facet loading and reduce intradiscal pressure
on adjacent segments. Assuming that these stresses contribute to adjacent-level disk disease, these data may indicate that
PDS topping of a fusion may provide some protection against fusion-induced adjacent-level degeneration. In addition, reduced
bone screw interface loading further suggests that PDS may be an option to avoid adjacent segment decompensation.
A 3-dimensional finite element model of 2 lumbar motion segments (L4-L5 and L5-S1) was used to evaluate the sensitivity of lumbar total disc replacement (TDR) impingement to disc height distraction, spinal sagittal orientation, implant position, and implant lordosis. The models were implanted with a mobile-bearing TDR and exposed to simulated sagittally balanced erect posture.
The objective of this study was to determine the sensitivity of TDR impingement to disc height distraction, implant lordotic angle, implant anterior-posterior position, and spinal orientation relative to the horizon.
TDR has the potential to replace fusion as the "gold standard" for treatment of painful degenerative disc disease. However, complications after TDR have been associated with device impingement and accelerated polyethylene wear.
A previously developed finite element model of the lumbar spine was altered to include implantation of a mobile-bearing TDR. A series of sensitivity analyses was performed to determine impingement risk. Specifically, spinal orientation, disc height distraction, footplate lordotic angle, and anterior-posterior position were evaluated.
Generally, TDR tended to result in an increase in extension rotation and facet contact force during simulated erect posture when compared with the intact models. Impingement risk was sensitive to all of the tested parameters.
The data from this study indicate that lumbar mobile-bearing TDR impingement is sensitive to disc height distraction, anterior-posterior position, implant lordosis, and spinal sagittal orientation. TDR impingement risk can be minimized by choosing an implant with an appropriate amount of lordosis, not overdistracting the disc space, and taking care not to place the implant too far anterior or posterior.
To determine the frequency of different patterns of centralization and their association with outcomes and MRI findings in patients experiencing sciatica.
A prospective longitudinal cohort study of 176 patients with radicular pain below the knee, who all had an MDT clinical assessment. Based on their pain response, patients were divided into five groups: abolition centralization, reduction centralization, unstable centralization, peripheralization, and "no effect". Patients had an MRI.
Overall, 84.8% of patients reported experiencing centralization, 7.3% peripheralized and 7.9% reported "no effect". The median reduction in RMQ scores across all the three centralization groups was 9.5 points at 3 months, and 12.0 points at 12 months. The peripheralization group improved similarly. The 'no effect' group improved significantly lower (p < 0.001), by 3.0 at both time points. Patients who centralized, and peripheralized had a significantly reduction in leg pain, the "no effect" group demonstrated a less favorable outcome (p < 0.02). There was no association between pain responses and the type of disc lesion.
In patients with sciatica, centralization was common and associated with improvement in activity limitation and leg pain. Centralization was very common in ruptured disc therefore the study does not support the theory, that centralization only occurs if the intra-discal hydrostatic mechanism is functional.
The collagen content, proportion of Types I and II collagen, and the relative concentrations of the reducible crosslinks of human lumbar intervertebral discs have been found to vary with age and location and to be highly dependent on the topography of the tissue. From adolescence to mature adulthood, the most significant change is an increase in the content of Type I collagen at the expense of genetically distinct Type II collagen in the outer lamella of the posterior quadrant, while just the reverse is true of the anterior quadrant. These changes are accompanied by similar but smaller alterations in the total collagen content and in the crosslink hydroxylysinohydroxynorleucine. The same differences in the distribution of Types I and II collagens occur in the annull on the concave and convex sides of scollotic curves. Together, these data establish that active cellular activity and tissue remodelling occur in the annull fibrosl and suggest that these specific changes are initiated in response to overall increases in compressive loading on the concave side and tensile loading on the convex side of the spine and the subsequent changes they induce in the magnitude and distribution of internal stresses within the annull. In its most general formulation, the biological behavior of annull fibrosl to mechanical forces appears to follow Wolff's Law.
It has been argued that a clarification of the mechanical causes of low-back pain requires a knowledge of the states of stress and strain throughout the lumbo-sacral spine. Since a purely experimental approach cannot provide this information, analytical model studies, to supplement measurements, are called for. In the present study, a general three-dimensional finite element program has been developed and applied for the analysis of the lumbar L2-3 disc-body unit. The analysis accounts for both the material and the geometric nonlinearities and is based on a representation of the annulus as a composite of collagenous fibers embedded in a matrix of ground substance. The geometry of the model analyzed is based on in vitro measurements. The validity of the model and the analysis procedure has been established by a comparison of those predictions that are also amenable to direct measurements, eg, the response of the disc-body unit to compressive load in terms of axial displacement, disc bulge, end-plate bulge, and intradiscal pressure. The states of stress and strain have then been computed in the cancellous bone, cortical shell, and the subchondral end-plate of the intervertebral body and in the annulus fibers and ground substance of the disc when the unit is subjected to a compressive load. The results indicate that for a normal disc with an incompressible nucleus, the most vulnerable elements under compressive load are the cancellous bone and the end-plate adjacent to the nucleus space. On the other hand, for a degenerated disc, simulated in an extreme fashion by assuming it to be void of the nucleus, the analysis predicts the annulus bulk material to be also susceptible to failure. The annulus fibers do not appear to be vulnerable to rupture when the disc-body unit is subjected to pure compressive force.
This second part of a three-part report considers the mechanical behavior of 42 fresh human cadaver lumbar motion segments in compression, and in anterior, posterior, and lateral shear. It reports the motions and the intradiskal pressure increases that occurred with posterior elements both intact and destroyed. Also examined is to what extent intervertebral disk gross morphology can explain the large scatter in the results presented here, as well as in the results reported in Part I concerning mechanical behavior in flexion, extension, lateral bending, and torsion.
Study Design. The literature on diagnostic tests available to the spine clinician for the evaluation of chronic low back pain was reviewed. Objectives. To review critically the available information and data on invasive diagnostic tests used for evaluation of chronic low back pain. Summary of Background Information. Numerous published studies have described the technique and clinical results of diagnostic blocks for chronic low back pain. There are various methodologies, but most lack of an adequate “gold standard” with which to compare the results of the diagnostic test. Methods. The available published studies of diagnostic tests commonly used in the evaluation of chronic low back pain were reviewed, with a focus on invasive techniques. The techniques were evaluated on the basis of the data available to support the conclusions that could be drawn for each of these techniques. The principles of diagnostic testing, including specificity and sensitivity, were reviewed and applied in the context of the data available for each of these invasive tests. Results. The essential features the clinician seeks in a diagnostic test are accuracy, safety, and reproducibility. It is essential to have a gold standard with which to compare the accuracy of a given diagnostic test. There is no completely reliable gold standard with which to compare a diagnostic test (or injection) when the absence of pain is the end point. The clinical setting in which the test is used directly affects the test results. The prevalence of the disease therefore affects the meaningfulness of the test results. Imaging studies have their greatest value in the exclusion of other conditions. These studies alone were not adequate for predicting the patients who would respond to controlled diagnostic blocks of the facet joint. Facet joint diagnostic blockade probably is most accurately performed by median nerve branch block. The greatest specificity for a positive response to a facet denervation procedure is achieved when the diagnosis is established via highly controlled anesthetic blocks. Over the past few decades, the sacroiliac joint has received varying degrees of interest as an important pain generator of low back pain. Despite testimonials to the contrary, no diagnostic physical examination has correlated with sufficient specificity to diagnose this condition reliably from a clinical standpoint. Lumbar discography has been one of the single most controversial subjects in the management of degenerative, painful lumbar spine conditions. The specificity and sensitivity are high for the diagnosis of disc degeneration. The question that revolves around discography concerns the accuracy of this test for the diagnosis of discogenic pain. An integral part of the problem is the lack of an adequate gold standard. In a comparison of nerve root blockade, sciatic nerve block, posterior ramus block, and subcutaneous injection in a cohort of patients with sciatica, the sensitivity of nerve root block was very high, with only a moderate level of specificity. In the case of diagnostic selective nerve blocks used for evaluation of complex or protean nerve compression, surgical confirmation and clinical results should be a reliable gold standard. Conflicting results have been presented depending on the target lesion and method of study. Conclusions. There are inherent limitations in the accuracy of all diagnostic tests. The tests used to diagnose the source of a patient’s chronic low back pain require accurate determination of the abolition or reproduction of the patient’s painful symptoms.
Typescript (photocopy). Thesis (Ph. D.)--University of Iowa, 1984. Bibliography: leaves 167-177.
The influences of age, sex, disc level, and degree of degenration on the mechanical behavior of 42 fresh cadaver lumbar motion segments are reported. The motions and intradiscal pressure changes that result from the application of flexion, extension, lateral bending, and torsional moments; compression; and anterior, posterior, and lateral shears are described. The authors find that the mean behaviors of the different segment classes sometimes differ, but these differences are seldom pronounced. Scatter in the behavior of individual motion segments is pronounced, and very often overshadows any class differences.
The mechanical properties of machined cortical bone specimens from human femora and tibiae were determined in tension, torsion, and compression for a population ranging in age from twenty-one to eighty-six years. No significant differences were found between the mechanical properties of male and female specimens. Tibial specimens had greater ultimate strength, stiffness, and ultimate strain than femoral specimens. Consistent decreases with age for all mechanical properties except plastic modulus were found in the femoral but not in the tibial specimens. No consistent significant differences in tension properties were found in specimens from normal, osteoporotic, and corticosteroid-treated individuals.
The purpose of this in vitro experimental study was to determine the role of alar ligaments in providing flexion, extension, and lateral bending stability to the upper cervical spine. Ten fresh human cadaver specimens occiput-C3 were studied in a complete unconstrained and three-dimensional manner, first intact and then after sequential cutting of the left and right alar ligaments. At the C0-C1 joint, there were increases in flexion motion with sequential cutting of the alar ligaments but none in extension. For the same joint, cutting of the left alar ligament resulted in a significant increase in neutral zone in right lateral bending but not in left lateral bending, whereas there were no significant increases in the ranges of motion. At the C1-C2 joint, there were significant increases both in flexion and extension due to cutting of the left alar ligament, but subsequent cutting of the right alar ligament resulted in a small increase for flexion only. At this joint, right lateral bending increased due to cutting of the left alar ligament, but the same was not true for the left lateral bending. Subsequent cutting of the right alar ligament resulted in significant increases for both the right and left lateral bending.
The three-dimensional (3-D) morphology of lumbar spinal ligaments was studied using 22 functional spinal units. A specially constructed 3-D morphometer instrument was used for quantitative measurements of each ligament's attachment point locations, origins and insertions, and attachments to the lower and upper vertebral bodies, respectively. Lengths and orientations of the ligaments according to the vertebral level were then computed and presented in reference to a local anatomic coordinate system. Associated cross-sectional areas are also obtained.
The problems presented by unsuccessful posterior lumbar interbody fusion (PLIF) have not been addressed. The cases of 12 patients who presented with failed PLIF were reviewed. Five patients were male and seven female (age range, 24-50 years; mean age, 40 years). All patients underwent at least one PLIF; many had undergone other procedures. A total of 37 procedures had been performed on the 12 patients. Chronic radiculopathy was present in all patients, as detected with electromyographic or nerve conduction velocity examination. At the time of reconstructive surgery, in the 11 patients in whom the canal was explored, all had extensive epidural fibrosis. Nine of the 12 patients had pseudarthrosis of the previous PLIF. Four patients had evidence of motion segment dysfunction at nearby levels: two had positive discograms adjacent to the PLIFs; one developed a facet syndrome at L5-S1, caudal to an L4-5 PLIF; and one demonstrated frank segmental instability at L2-3, cranial to a previous PLIF at L3-4. Twelve patients underwent a total of 22 procedures after referral. Eleven patients initially underwent decompression and fusion, and one patient underwent a sympathectomy. Seven patients underwent an additional 10 procedures, including repeat decompression, repair of pseudarthrosis, and implantation of an epidural analgesic pump system. After all surgical treatment, five patients rated their pain as improved. Seven patients were thought to have a solid fusion. The presence of a solid fusion did not correlate with satisfactory relief of pain (chi-square). Continued extremity pain was the predominant complaint of all the patients. Two shortcomings of the PLIF were evident.(ABSTRACT TRUNCATED AT 250 WORDS)
To determine the motion characteristic of the normal lumbar spine, 61 of 612 optimal radiographs of healthy young Japanese adults in the neutral erect, active maximum flexion and maximum extension positions were carefully selected. The coordinates of the instantaneous axis of rotation (IAR) and vertebral center from L1 to L5 vertebra were analyzed. The directional shifts in IAR in the x-axis and the y-axis closely reflected translational and rotational movements of the vertebra. The IAR showed qualitative values with very large, 95%, probability ellipses in the normal lumbar spine. Analysis of coordinates of the vertebral center provided quantitative data of the extension-to-flexion motion of each vertebra. In extension-to-flexion motion, the L4 vertebra showed a translation-predominant motion characteristic. The L5 vertebra had a rotation-predominant motion characteristic. The patterns of motion of the L1-L4 vertebrae were correlated to the intervertebral disc height and the inclination angle of the facet joint. The L5 vertebra alone showed some specific patterns suggestive of the involvement of a separate ligamentous stabilizer, the iliolumbar ligaments, which reduces translation.
Specially developed intervertebral and intradiscal pressure transducers are used to map the pressures in the nucleus pulposus and within the annulus fibrosus of the human spinal discs, to the point of fracture of the segmental units. The results show that for normal discs, pressures in the nucleus and in the annulus are linearly related to each other and to the applied compressive loads. Mechanical strains which develop around the periphery ofthe vertebral body are also linear with the applied compressive loads, when the partial vertebral column is loaded to the point of bony fracture.
Failure of the intervertebral discs at the point of fracture occurred through the vertebral body endplates and the annulus fibrosus remained intact.
Three-dimensional nonlinear finite element models of the intact L4-5 one motion segment/two-vertebrae and L3-5 two motion segments/three-vertebrae were developed using computed tomography (CT) films. The finite element mesh of the L4-5 motion segment model was modified to simulate bilateral decompression surgery. The mesh was further altered to achieve stabilization, using an interbody bone graft and a set of Steffee plates and screws. The model behavior of the intact specimen in all loading modes and of the stabilized model in compression, flexion, and extension modes were studied. The stresses in the cancellous bone region were found to decrease. The interbody bone graft, due to an overall decrease in stresses in the bone below the screw, transmits about 80% of the axial load as compared with 96% transmitted by an intact disc in an intact model. Thus, the use of a fixation device induces a stress shielding effect in the vertebral body. The results indicate that although the bone graft transmits lesser loads than the intact disc, it is active in transmitting loads. The presence of low stresses in the cancellous bone region and high localized stresses in the cortical pedicle region surrounding the screw, compared with the intact case, suggests that the screws are likely to become loose over time. The use of an interbody bone graft alone or in combination with any existing fixation device also induces higher stresses at the adjacent levels. This may be responsible for the adverse iatrogenic effects seen clinically.
Lateral radiographs of ten normal individuals were studied to determine the location of the instantaneous axis of rotation (IAR) of every lumbar vertebra for the movements of flexion and extension from the upright position and flexion from the fully extended position; and errors involved in the technique were quantified to establish confidence limits for the results of the calculations. The distribution of the IARs was found to fall within a small range from the mean location at each level, particularly for the movement of flexion from the extended position. Within-observer and between-observer errors occurred in tracing and superimposing radiographs and marking x and y coordinates. Unacceptably large errors occur when the movement of the joint is less than 5 degrees, and only the IAR for flexion from extension can be plotted with acceptable confidence. This result invalidates the notion that plotting centrodes may be of diagnostic value in recognizing mechanical disorders. The determination of a single extension to flexion IAR may be of more value clinically, to which end this study provides essential normative data.
Previously reported biomechanical studies on the effects of various types of spinal fusion procedures upon the adjacent segment indicate a significant degree of increased stress at that segment. This study presents 18 patients in whom new symptoms developed from the segment adjacent to a fusion after an average symptom-free interval of 8.5 years (1-38 years). The most common pathologic condition at the adjacent segment was hypertrophic degenerative arthritis of the facet joints. Spinal stenosis was found there in eight cases; severe disc degeneration in five; degenerative spondylolisthesis in two; and spondylolysis acquisita in one.
At the Department of Orthopedics, Humboldt University School of Medicine, Charité Hospital, the three-piece disk endoprosthesis SB Charité of the modular type, which works according to the low-friction principle, has been developed combining the advantages of a beneficial effect on the patient's complaints by simultaneous distraction in the vertebral segment, and of achieving movement within the physiological range. So far, this disk endoprosthesis consisting of two cover plates made of implantat steel or titanium sheet, which are fixed without cement, as well as a moveable polyethylene sliding core being vertically adjustable, has been implanted in sixteen patients. The therapeutic results show that it is possible with this method to replace the nucleus pulposus, thus providing, in addition to conventional nucleotomy and segment fusion, another therapeutic alternative for lumbar disk lesions.
The posterior ligaments: ligamentum flavum, articular, interspinous and supraspinous ligaments of twenty five fresh cadaveric intervertebral segments, from T11-T12 to L4-L5, extracted from fourteen spines were tested in tension. A progressive dissection method was used, that is, each segment was tested after first resecting the disk with the ligaments intact and a force-elongation curve obtained. Then one ligament was cut and the test repeated, and so on. The most restrictive ligament was found to be the ligamentum flavum followed by the articular, interspinous, and supraspinous ligaments.
Cylindrical trabecular bone samples from the central part of the first lumbar vertebral body were obtained from 42 normal people (27 females and 15 males) aged 15-87 years and analyzed by a compression test in either vertical or horizontal direction. Maximum stress, maximum stiffness, energy absorption capacity, and strain at maximum stress (compressibility) were calculated from the load-deformation curves. The ash density of the samples was measured after incineration. From age 20 to 80 years, the decline (P less than 0.01) in ash density was 48-50% compared to a 75-80% decrease (P less than 0.001) in vertical stress, stiffness, and energy absorption capacity. Qualitatively similar age-related changes (P less than 0.01) were observed by horizontal compression, but the absolute values were smaller. In both directions, highly significant positive correlations (P less than 0.01) were observed between the biomechanical properties and ash density stress, stiffness and energy absorption capacity still showed significant decreases with age (P less than 0.01). The vertical maximum strain values, which increased (P less than 0.05) with age, were inversely related to the other biomechanical variables (P less than 0.05) and to the ash density (P less than 0.05). It is concluded from the study that the biomechanical competence of vertebral trabecular bone depends not only on bone mass (ash density) but also on the continuity of the trabecular lattice, which changes with increasing age.
Torsion as a cause of failure in the lumbar intervertebral joint was studied using a three-dimensional nonlinear finite element model. The role of facets and ligaments as well as the stress distributions in the posterior elements, the disk, the ligaments, and the vertebral body were examined. For physiological range of torsion, the facets carried 10 to 40 percent of the torque. The fiber stresses in the disk were the highest at the lateral margin of the outer layer of the annulus. Therefore, torsion itself is unlikely to cause posterior or posterolateral disk prolapse.
This paper presents a technique and discusses the results of treating failed posterior lumbar interbody fusions (PLIFs) of the spine with pulsing electromagnetic fields (PEMFs). Thirteen male patients suffering from failed PLIFs, with an average time of 40 months since the last surgical fusion attempt, were the subjects of this study. PEMFs were applied by the patient according to strict criteria but in the comfort of their home. Initial and subsequent medical evaluations closely monitored the patient's condition and progress. PEMFs promoted a significant increase in bone formation in 85% (11 of 13) of the patient pool and achieved body-to-body fusion throughout the intervertebral disc space in 77% (ten of 13) over the treatment period. The treatment required no hospitalization, reduced morbidity, and avoided the risks associated with surgical intervention. The results suggest that this surgically noninvasive outpatient therapy may become a successful alternative treatment of failed PLIF.
This paper reports the results of a biomechanical study of 43 human spinal ligaments from fresh cadavers and living subjects. Tensile tests were performed with an original testing machine. The tension load and relaxation were applied at the same constant slow rate (1 mm min-1) on entire ligaments. In order to avoid ligament injuries, fixation in the apparatus was on the bone held in clamps specifically designed for each bone, never the ligament itself or its bony attachments. All the load-deformation curves had a sigmoid shape and during load-unload cycles ligaments exhibited elastic properties. This was particularly evident for the ligamentum flavum. The intertransverse posterior longitudinal ligament and the ligamentum flavum were the most resistant. The elongation-tension curve pattern may be explained by the microscopic architectural change that was studied in supraspinous ligament.
The anterior annulus from patients undergoing surgery for low-back pain was compared with the same region removed at autopsy. Collagen, proteoglycan, and water content were measured in different regions across the annulus. Water and proteoglycan contents increased from the outer to the inner annulus, whereas the collagen contents declined. Quantitative differences were found between agematched surgical and autopsy specimens. Discs from most patients showed lowered proteoglycan contents, and a few of these showed high water contents. Loss of collagen in isolated areas of the annulus was demonstrated in some patients. Collagen loss occurred most frequently in the lumbosacral disc, whereas changes in proteoglycan and water were more frequent higher in the lumbar region. No qualitative changes in collagen types were detected immunochemically. In many patients, the levels of proteoglycan which aggregated in vitro were extremely low. Changes in hexosamine molar ratio of disc proteoglycan and in the length of the chondroitin sulfate side chains were observed with aging in this group of patients.
It has been argued that a clarification of the mechanical causes of low-back pain requires a knowledge of the states of stress and strain throughout the lumbo-sacral spine. Since a purely experimental approach cannot provide this information, analytical model studies, to supplement measurements, are called for. In the present study, a general three-dimensional finite element program has been developed and applied for the analysis of the lumbar L2-3 disc-body unit. The analysis accounts for both the material and the geometric nonlinearities and is based on a representation of the annulus as a composite of collagenous fibers embedded in a matrix of ground substance. The geometry of the model analyzed is based on in vitro measurements. The validity of the model and the analysis procedure has been established by a comparison of those predictions that are also amenable to direct measurements, eg, the response of the disc-body unit to compressive load in terms of axial displacement, disc bulge, end-plate bulge, and intradiscal pressure. The states of stress and strain have then been computed in the cancellous bone, cortical shell, and the subchondral endplate of the intervertebral body and in the annulus fibers and ground substance of the disc when the unit is subjected to a compressive load. The results indicate that for a normal disc with an incompressible nucleus, the most vulnerable elements under compressive load are the cancellous bone and the end-plate adjacent to the nucleus space. On the other hand, for a degenerated disc, simulated in an extreme fashion by assuming it to be void of the nucleus, the analysis predicts the annulus bulk material to be also susceptible to failure. The annulus fibers do not appear to be vulnerable to rupture when the disc-body unit is subjected to pure compressive force.
The aim of this study was to define the motion of the L4-5 spinal segment moving from full extension to full flexion. A new technique using moiré fringes was applied to measure angles of 3-degree change, a technique proven to be highly accurate for small angular changes. This investigation has identified a locus of centers of rotation (centrode) that is located in the posterior half of the intervertebral disc. The direction of the locus passes from posterior to anterior and back to posterior. The technique can be adapted for evaluation of in vivo radiographs. This technique may prove useful in identifying aberrations in movement associated with spinal segment instability in its earliest stages.
The effects of injury to the intervertebral disc were investigated using three-dimensional flexibility and creep measurements of functional spinal units from fresh cadaver lumbar spines. The techniques utilized were accurate and the data had a high degree of reproducibility. An injury to the annulus and a removal of the nucleus significantly altered the mechanical properties of the spinal unit. Not only were the main motions affected but also the coupled motions. Sagittal plane symmetry was disturbed, resulting in asymmetric facet joint movements. These effects of injury could be measured because of the three-dimensionality of the experiments. Previous studies, utilizing only axial compression loading, claimed to observe no changes due to the disc injuries and are, therefore, in conflict with the present findings.
The collagen content, proportion of Types I and II collagen, and the relative concentrations of the reducible crosslinks of human lumbar intervertebral discs have been found to vary with age and location and to be highly dependent on the topography of the tissue. From adolescence to mature adulthood, the most significant change is an increase in the content of Type I collagen at the expense of genetically distinct Type II collagen in the outer lamella of the posterior quadrant, while just the reverse is true of the anterior quadrant. These changes are accompanied by similar but smaller alterations in the total collagen content and in the crosslink hydroxylysinohydroxynorleucine . The same differences in the distribution of Types I and II collagens occur in the annuli on the concave and convex sides of scoliotic curves. Together, these data establish that active cellular activity and tissue remodelling occur in the annuli fibrosi and suggest that these specific changes are initiated in response to overall increases in compressive loading on the concave side and tensile loading on the convex side of the spine and the subsequent changes they induce in the magnitude and distribution of internal stresses within the annuli. In its most general formulation, the biological behavior of annuli fibrosi to mechanical forces appears to follow Wolff's Law.
Unlabelled:
We studied the validity of predictions of compressive loads on the lumbar spine and contraction forces in lumbar trunk muscles based on a biomechanical model. The predictions were validated by quantitative measurements of myoelectric activities at twelve locations on the trunk and of the pressure in the third lumbar disc. Twenty-five tasks were performed isometrically by four healthy volunteers. The model predicted that the tasks imposed mean compressive loads on the spine of as much as 2400 newtons and required contraction forces of the posterior muscles of the back of as much as 1800 newtons. Intradiscal pressures of as much as 1600 kilopascals were measured. The predicted and measured quantities were well correlated. It appears that the model adequately predicted the compressive loads on the lumbar spine and the tensions in the back muscles.
Clinical relevance:
Patients with low-back disorders limit their physical activities, which indicates that loading on the spine must be a factor in those disorders. This study shows that the loads imposed on the spine by physical activities need not be measured. They can easily be calculated. This will significantly accelerate biomechanics research on low-back disorders. The calculation techniques that we validated for predicting loads on the spine can be used to calculate the loads on any skeletal structure. Those loads are largely determined not by the externally applied loads, but by the moments of those applied loads and by the moments of the weights of the body segments that the structure must support.
The issue of segmental stability using finite element analysis was studied. Effect of ligament and facet (total and partial) removal and their geometry on segment response were studied from the viewpoint of stability.
To predict factors that may be linked to the cause of rotational instabilities, spondylolisthesis, retrospondylolisthesis, and stenosis.
The study provides a comprehensive study on the role of facets and ligaments and their geometry in preserving segmental stability. No previous biomechanical study has explored these issues in detail.
Three-dimensional nonlinear finite element analysis was performed on L3-L4 motion segments, with and without posterior elements (ligaments and facets), subjected to sagittal moments. Effects of ligament and facet (partial and total) removal and their orientations on segment response are examined from the viewpoint of stability.
Ligaments play an important role in resisting flexion rotation and posterior shear whereas facets are mainly responsible for preventing large extension rotation and anterior displacement. Facet loads and stresses are high under large extension and anterior shear loading. Unlike total facetectomy, selective removal of facets does not compromise segmental stability. Facet loads are dependent on spatial orientation.
Rotational instability in flexion or posterior displacement (retrospondylolisthesis) is unlikely without prior damage of ligaments, whereas instability in extension rotation or forward displacement (spondylolisthesis) is unlikely before facet degeneration or removal. The facet stress and displacement distribution predicts that facet osteoarthritis or hypertrophy leading to spinal stenosis is most likely under flexion-anterior shear loading. Selective facetectomy may restore spinal canal size without compromising the stability of the segment. A facet that is more sagittally oriented may be linked to the cause of spondylolisthesis, whereas a less transversely oriented facet joint may be linked to rotational instabilities in extension.
The lumbar region is a frequent site of spinal disorders, including low-back pain, and of spinal trauma. Clinical studies have established that abnormal intervertebral motions occur in some patients who have low-back pain. A knowledge of normal spinal movements, with all of the inherent complexities, is needed as a baseline. The present study documents the complete three-dimensional elastic physical properties of each lumbar intervertebral level from the level between the first and second lumbar vertebrae through the level between the fifth lumbar and first sacral vertebrae. Nine whole fresh-frozen human cadaveric lumbar-spine specimens were used. Pure moments of flexion-extension, bilateral axial torque, and bilateral lateral bending were applied, and three-dimensional intervertebral motions were determined with use of stereophotogrammetry. The motions were presented in the form of a set of six load-displacement curves, quantitating intervertebral rotations and translations. The curves were found to be non-linear, and the motions were coupled. The ranges of motion were found to compare favorably with reported values from in vivo studies.
This study provides the quantitative three-dimensional surface anatomy of the articular facets for the entire human vertebral column based on a study of 276 vertebrae. Means and standard errors of the means for linear, angular, and area dimensions of the superior and inferior articular facets were measured for all vertebrae from C2 to L5. Facet orientations were described as angles with respect to the sagittal and transverse planes and also as card angles. The plane angles are similar to the angles seen on traditional radiographic views--radiographs and computed tomographic scans. The card angles, a new concept, are better at helping visualize the three-dimensional orientations of the facets. Excluding the superior C2 facet, the following minimum and maximum dimensions were found for the facets from C3 to L5: width = 9.6-16.3 mm; height = 10.2-18.4; surface area = 72.3-211.9 mm2; interfacet width = 20.8-40.6; interfacet height = 12.2-33.0 mm; transverse plane angle = 41.0-86.0; sagittal plane angle = 67.4-154.8; X-card angle = 41.0-86.0; and Y-card angle = 5.8-66.1. The quantitative anatomy of the facets may improve the understanding of the spinal anatomy, help improve the clinical diagnosis and treatment, and provide the necessary data for constructing more realistic mathematical models of the spine.
This study examined the role of facet joint morphology in the etiology of both degenerative spondylolisthesis and isthmic spondylolysis. To this end, the axial facet joint morphology of the lower lumbar spine in a normal population and in populations of patients with spinal stenosis or degenerative spondylolisthesis at L4-5 and in patients with isthmic spondylolysis at the L5 level were characterized. Computed tomographic scans were digitized, defining the axial morphology of the normal facet joint at five stations from proximal to distal within the joint. Assessments were made of facet joint orientation, transverse articular dimension, depth of the articular surface, and shape of the articular surface at levels L3-4, L4-5, and L5-S1. There was a gradually more coronal orientation from proximal to distal among the stations at each level, and a maximal transverse articular dimension at the level of the superior endplate of the caudad vertebra. Minimal error in the recording process at this level. In addition to the maximal joint dimension, made this level the most representative of the overall morphology and most useful for further studies. At the L4-5 level, a significantly more sagittal facet orientation was found in the degenerative spondylolisthesis group when compared to both the normal population and spinal stenosis groups (P < 0.01). At L5-S1, the only significant morphologic difference between the normal population and the patients with isthmic spondylolysis was reduced transverse articular dimension. These results support the hypothesis that patients developing degenerative spondylolisthesis are predisposed to this by a developmental sagittal orientation of the L4-5 facet joints.
Our study was an attempt to salvage severely unstable posterior lumbar interbody fusions (PLIFs) by using the lateral bilateral intertransverse single-level (floating) fusion. A retrospective review of six patients with failed L4-L5 PLIFs was carried out. These patients underwent reoperation between 2 weeks and 2 years after the initial surgery. All patients had persistent symptoms, including low back pain and sciatica, interfering with activities of daily living. Computed tomography (CT) scans revealed extrusion of the bone plug into the spinal canal in five patients. All six had a nonunion of the PLIF, and three patients had grade 3 or 4 iatrogenic spondylolisthesis caused by the PLIF. The operation involved decompression followed by lateral intertransverse process fusion with autogenous bone graft. All patients were placed in external immobilization after the procedure for 3 months. Follow-up averaged 38.5 months (range, 21-55), at which time five had substantial pain improvement. Five were radiographically fused. One had a second nonunion, which subsequently healed after reoperation with repeat lateral fusion and Luque plate hardware. Our fusion rate was 83% without the use of instrumentation. In conclusion, we recommend the simplistic procedure of lateral bilateral intertransverse process single-level fusion along with liberal external immobilization for the difficult problem of salvage of failed PLIFs.
Most episodes of low back pain are mechanical in origin and resolve within a 12-week period. These acute episodes of back pain are associated with muscle strain and intervertebral disc herniation with radiculopathy. A smaller proportion of individuals have back pain with a duration greater than 12 weeks. These patients have back pain secondary to a wide variety of mechanical and nonmechanical disorders. The mechanical disorders associated with chronic low back pain include osteoarthritis and lumbar spinal stenosis; the nonmechanical disorders include infectious, neoplastic, rheumatologic, endocrinologic, vascular, and gynecologic. The clinical symptoms associated with each variety of disorder helps guide the appropriate diagnostic evaluation. Plain roentgenograms are useful in documenting the presence of spinal stenosis, benign or malignant tumors, osteoporosis, sacroiliitis, and spondylitis. CT scan is helpful in defining the bony alterations associated with malignant tumors and the vascular abnormalities associated with aneurysms. MR imaging is the technique of choice to document the extent of malignant processes and the presence of endometriosis in the pelvis. The therapy of these entities are specific for the disease entity causing the chronic low back pain. Although most of the disorders that cause chronic low back pain cannot be cured, therapy can decrease pain and improve function of the symptomatic patient.
Lumbar fusion is commonly performed to relieve pain from degenerative conditions, including spinal stenosis and spondylolisthesis. While clinical studies have reported favorable fusion rates with limited complications, few have investigated the effect of fusion on the adjacent motion segment. A solid fusion alters the biomechanics at the adjacent level, resulting in increased mechanical demands. There have been reports of increased rates of adjacent-level pathologic lesions after fusion, but these have not accounted for the natural history of degenerative changes. Biomechanical and radiographic studies have shown increased forces, mobility, and intradiscal pressure in adjacent segments after fusion. It has been hypothesized that these changes lead to an acceleration in pathologic changes.
The influence of lumbar disc height and cross-sectional area on the mechanical response of the disc to physiologic loading was determined using a finite element model.
To identify which geometric characteristics are potentially related to motion segment mechanical response to applied load, such as flexibility, fiber stress, disc bulge, and nucleus pressure.
The height and area of the lumbar disc varies within the disc itself, between disc levels, between people, between men and women, with aging, and during the day. Mechanical theory dictates that the height and area influence the mechanical response of the disc to loading. This could have important consequences in risk of injury.
Three-dimensional finite-element models representing three disc heights (5.5 mm, 8.5 mm, and 10.5 mm) and three disc areas (1060 mm2, 1512 mm2, and 1885 mm2) were generated. The effect of disc geometry on the mechanical properties of the disc were studied for four moment loads (magnitude, 7.5 Nm) with compressive preload (400 N) and for three different direct forces. Commercially available finite-element software was used.
Discs with a ratio of small disc area to disc height were more prone to larger motion, higher anular fiber stresses, and larger disc bulge. When the disc height alone was increased by a factor, its flexibility also increased, either by the same amount or by a much larger ratio.
Discs with the most height and smallest area are exposed to much higher risk of failure than other combinations of disc height and geometry.
A finite element parametric study was performed to investigate the structural roles of the vertebral cortical shell and the trabecular centrum.
To address the debated issue of the relative load-carrying role of the vertebral cortical shell.
Several experimental and computational studies have been aimed at quantifying the load-carrying roles of the human vertebral cortical shell and trabecular centrum. These studies, however, have supported no consensus.
A finite element model of three lumbar vertebral bodies was developed to predict the fraction of the total compressive load acting on the lumbar vertebral body, under two different loading conditions, that was supported by the cortical shell. Parametric variations in vertebral material and geometric properties were examined to determine how this fraction was influenced by such changes.
The fraction of the compressive load supported by the cortical shell was found to be strongly dependent on the distance from the endplate, increasing from about 34% at either endplate to approximately 63% at the midtransverse plane. This fraction was independent of the loading characteristics, proportional to the properties of the cortex, and inversely proportional to the modulus of the centrum. Additionally, the cortical shell force fraction was affected significantly by changes in the overall vertebral geometry.
Our findings indicate that the structural dominance of the cortical shell and centrum alternate depending on the location within the vertebral body. However, as age-related bone loss progresses, the load-carrying role of the cortical shell could increase significantly.
The literature on diagnostic tests available to the spine clinician for the evaluation of chronic low back pain was reviewed.
To review critically the available information and data on invasive diagnostic tests used for evaluation of chronic low back pain.
Numerous published studies have described the technique and clinical results of diagnostic blocks for chronic low back pain. There are various methodologies, but most lack of an adequate "gold standard" with which to compare the results of the diagnostic test.
The available published studies of diagnostic tests commonly used in the evaluation of chronic low back pain were reviewed, with a focus on invasive techniques. The techniques were evaluated on the basis of the data available to support the conclusions that could be drawn for each of these techniques. The principles of diagnostic testing, including specificity and sensitivity, were reviewed and applied in the context of the data available for each of these invasive tests.
The essential features the clinician seeks in a diagnostic test are accuracy, safety, and reproducibility. It is essential to have a gold standard with which to compare the accuracy of a given diagnostic test. There is no completely reliable gold standard with which to compare a diagnostic test (or injection) when the absence of pain is the end point. The clinical setting in which the test is used directly affects the test results. The prevalence of the disease therefore affects the meaningfulness of the test results. Imaging studies have their greatest value in the exclusion of other conditions. These studies alone were not adequate for predicting the patients who would respond to controlled diagnostic blocks of the facet joint. Facet joint diagnostic blockade probably is most accurately performed by median nerve branch block. The greatest specificity for a positive response to a facet denervation procedure is achieved when the diagnosis is established via highly controlled anesthetic blocks. Over the past few decades, the sacroiliac joint has received varying degrees of interest as an important pain generator of low back pain. Despite testimonials to the contrary, no diagnostic physical examination has correlated with sufficient specificity to diagnose this condition reliably from a clinical standpoint. Lumbar discography has been one of the single most controversial subjects in the management of degenerative, painful lumbar spine conditions. The specificity and sensitivity are high for the diagnosis of disc degeneration. The question that revolves around discography concerns the accuracy of this test for the diagnosis of discogenic pain. An integral part of the problem is the lack of an adequate gold standard. In a comparison of nerve root blockade, sciatic nerve block, posterior ramus block, and subcutaneous injection in a cohort of patients with sciatica, the sensitivity of nerve root block was very high, with only a moderate level of specificity. In the case of diagnostic selective nerve blocks used for evaluation of complex or protean nerve compression, surgical confirmation and clinical results should be a reliable gold standard. Conflicting results have been presented depending on the target lesion and method of study.
There are inherent limitations in the accuracy of all diagnostic tests. The tests used to diagnose the source of a patient's chronic low back pain require accurate determination of the abolition or reproduction of the patient's painful symptoms.
Lumbar spine fusion is a commonly performed procedure in various pathologic conditions of the spine. Its role remains debated, and moreover, delayed complications may occur, among which is transitional segment alteration leading to recurrence of back pain, gross instability, and neurologic symptoms. Little is known about the long-term prevalence of this complication because of a lack of specific studies. We analyzed the fate of the transitional segments in a homogeneous group of patients operated on during a 14-year period for degenerative conditions of the lumbar spine resistant to conservative treatment. Follow-up ranged from 2 to 15 years. Seventy-five percent of our study group had a minimal 5-year follow-up. In this subgroup, 41% of the patients developed transitional segment alterations, and 20% needed a secondary operation for extension of the fusion. Potential risk factors such as postoperative delay, length of fusion, and spine imbalance were recognized. The frequency of delayed alterations of the adjacent segment and the severity of symptoms related to this complication in this study raise questions about the justification of fusion procedures in degenerative conditions of the spine without threatening instability. Data from the literature confirm the severity of the problem, but many uncertainties remain because of the lack of homogeneous and complete data on both the normal evolution of motion segments of the lumbar spine with age and the fate of the same segments when transformed in transitional segments. Future prospective studies on the subject are needed and must deal with homogeneous groups of patients. More reconstructive surgical procedures need to be developed to lessen the need for fusion procedures.
This in vitro biomechanical study was undertaken to quantify the multidirectional intervertebral kinematics following total disc replacement arthroplasty compared to conventional stabilization techniques.
Using an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties and map the center of intervertebral rotation of total disc arthroplasty versus conventional threaded fusion cages and cages augmented with transpedicular fixation for single-level spinal instrumentation.
The utilization of motion-preserving implants versus instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties.
A total of eight human cadaveric lumbosacral spines (L2 to sacrum) were utilized in this investigation and biomechanically evaluated under the following L4-L5 reconstruction conditions: 1) intact spine; 2) SB Charitè disc prosthesis; 3) BAK cages; and 4) BAK cages + ISOLA pedicle screw/rod fixation (anteroposterior). The superior (L3-L4) and inferior (L5-S1) intervertebral levels remained uninstrumented to quantify adjacent level properties. Multidirectional flexibility included pure, unconstrained moments (+/-8 Nm) in axial rotation, flexion-extension, and lateral bending, with quantification of the operative and adjacent level range of motion and neutral zone, which were normalized to the intact spine condition.
The SB Charitè prosthesis indicated an average percentage increase in axial rotation range of motion by 44% compared to the intact condition (P < 0.05), whereas the BAK and anteroposterior reconstructions decreased range of motion by 29% and 80%, respectively (P < 0.05). The SB Charitè was significantly different from BAK and combined anteroposterior reconstructions (P < 0.05). Flexion-extension indicated a minor increase in range of motion for the SB Charitè (3%) versus the intact disc (P > 0.05), whereas the BAK and anteroposterior stabilization groups resulted in significant decreases in range of motion (BAK = 57%, anteroposterior = 93%) (P < 0.05) when compared to the intact and SB Charitè conditions. Based on flexion-extension radiographs, the intervertebral centers of rotation were in the posterior one-third of the operative intervertebral disc only for the SB Charitè reconstruction and intact spine condition, with definitive evidence of physiologic intervertebral translation (intact 2.06 +/- 77 mm; SB III = 1.9 +/- 0.98 mm).
Total disc arthroplasty serves as the next frontier in the surgical management of discogenic spinal pathology. The SB Charitè restored motion to the level of the intact segment in flexion-extension and lateral bending and increased motion in axial rotation. The anterior annular resection necessary for device implantation and unconstrained design of the prosthesis account for this change in rotation. The normal lumbar flexion-extension axis of rotation is an ellipse rather than a single point. Only disc replacement rather than pedicle instrumentation or BAK interbody instrumentation preserves the kinematic properties and normal mapping of segmental motion at the operative and adjacent intervertebral disc levels.
A retrospective review of patient outcomes after lumbar spinal fusion.
To determine whether patients with a fusion ending adjacent to a "degenerated disc" (DDD group) had worse clinical outcomes than patients with fusions ending adjacent to "normal" discs (NL group).
Although it has been suggested that creating a rigid motion segment adjacent to a degenerated segment may negatively impact clinical outcomes after lumbar fusion, this question has not been addressed to our knowledge in the English literature.
Twenty-five consecutive patients treated with lumbar fusion for degenerative instability who had preoperative lumbar spine magnetic resonance imaging, who completed health status questionnaire Short Form 36 (SF-36), and were seen in the office for radiographic follow-up at least 2 years following surgical treatment formed the study group. The magnetic resonance images were reviewed independently by two spine surgeons and rated for the presence of any degenerative changes. Statistical analysis of the SF-36 data was performed with chi2 and Mann-Whitney U testing.
Of the 25 patients, 20 were fused adjacent to at least one degenerated level (DDD group), whereas 5 were fused adjacent to a normal level (NL group). At follow-up, SF-36 scores were higher for the DDD group in all eight subgroups, contrary to the research hypothesis. A power analysis demonstrated with at least 98% certainty that if patients in the DDD group had even a 10% lower score in any of the 8 SF-36 subgroups, this study would have detected it.
This retrospective review of patients who underwent lumbar fusion for degenerative instability demonstrated no adverse impact on clinical outcomes when the lumbar fusion ended adjacent to a degenerative motion segment. Although a power analysis validated these results with 98% certainty, larger prospective studies are needed to confirm that there is no benefit to include degenerated adjacent segments in a lumbar fusion for degenerative instability.
Although interbody cages are widely used, there is little histological documentation of the tissue within cages in the human spine. The purpose of this study was to describe the contents of retrieved, clinically failed, interbody cages from human patients, with special reference to the influence of graft type on the viability of bone in the cages.
Seventy-eight cages that had been retrieved from forty-eight patients were analyzed. There were eight carbon-fiber cages and seventy threaded metal cages. Of the sixty-seven cages for which information about grafting was available, fifty-six had been packed with autograft only, six had local autograft mixed with demineralized bone matrix, four had allograft, and one had demineralized bone matrix only. The indications for cage retrieval included a failed fusion, malposition or migration of the cage, trauma (a compression fracture at the fusion site), low-back pain, progressive spondylosis, nerve-root impingement, and/or infection. The cages had been in situ for an average of twenty-two months. Undecalcified sections through the center of each plastic embedded cage were reviewed, and the approximate areas occupied by viable bone, necrotic bone, fibrocartilage, hyaline cartilage, fibrous tissue, and graft substitute were visually estimated. Debris particles were estimated by a semiquantitative scoring system.
Seventy-one of the seventy-eight cages showed evidence of vascular ingrowth and areas of histologically viable bone, representing incorporating bone graft. The average area occupied by viable bone was 44% (range, 0% to 80%). In some cages, relatively large fragments of cortical bone graft were associated with only minimal new-bone formation. Fibrocartilage occupied up to 50% of the available area in these failed cages. Some cages also contained small fibrocartilage seams connecting segments of bone in a pattern that suggested motion in vivo. In thirty-one of the seventy-eight cages, > or = 5% of the available area was occupied by hyaline cartilage, probably from vertebral end plates or facet joints.
While this study was not designed to test the efficacy of cages or of bone graft, the prevalence of hyaline and fibrocartilage in these failed cages illustrates the importance of graft and graft-site preparation to maximize bone-graft incorporation. Level of Evidence: Therapeutic study, Level IV (case series [no, or historical, control group]). See Instructions to Authors for a complete description of levels of evidence.
A primary concern after posterior lumbar spine arthrodesis is the potential for adjacent segment degeneration cephalad or caudad to the fusion segment. There is controversy regarding the subsequent degeneration of adjacent segments, and we are aware of no long-term studies that have analyzed both cephalad and caudad degeneration following posterior arthrodesis. A retrospective investigation was performed to determine the rates of degeneration and survival of the motion segments adjacent to the site of a posterior lumbar fusion.
Two hundred and fifteen patients who had undergone posterior lumbar arthrodesis were included in this study. The study group included 126 female patients and eighty-nine male patients. The average duration of follow-up was 6.7 years. Radiographs were analyzed with regard to arthritic degeneration at the adjacent levels both preoperatively and at the time of the last follow-up visit. Disc spaces were graded on a 4-point arthritic degeneration scale. Correlation analysis was used to determine the contribution of independent variables to the rate of degeneration. Survivorship analysis was performed to describe the degeneration of the adjacent motion segments.
Fifty-nine (27.4%) of the 215 patients had evidence of degeneration at the adjacent levels and elected to have an additional decompression (fifteen patients) or arthrodesis (forty-four patients). Kaplan-Meier analysis predicted a disease-free survival rate of 83.5% (95% confidence interval, 77.5% to 89.5%) at five years and of 63.9% (95% confidence interval, 54.0% to 73.8%) at ten years after the index operation. Although there was a trend toward progression of the arthritic grade at the adjacent disc levels, there was no significant correlation, with the numbers available, between the preoperative arthritic grade and the need for additional surgery.
The rate of symptomatic degeneration at an adjacent segment warranting either decompression or arthrodesis was predicted to be 16.5% at five years and 36.1% at ten years. There appeared to be no correlation with the length of fusion or the preoperative arthritic degeneration of the adjacent segment.
To evaluate the safety profile of platelet glycoprotein IIb/IIIa inhibitors administered as adjunctive therapy to patients with large-vessel occlusion and acute ischemic stroke refractory to pharmacological thrombolysis with recombinant tissue plasminogen activator (rtPA) and mechanical disruption, balloon angioplasty, or both.
Twenty-one patients (mean age, 62 yr; range, 29-88 yr) met the following criteria: 1) large-vessel occlusion and acute ischemic stroke syndrome at presentation, 2) failure to recanalize after administration of rtPA (intra-arterial and/or intravenous) with or without mechanical thrombolysis, and 3) subsequent treatment with IIb/IIIa inhibitors (intra-arterial or intravenous).
Eleven patients had ischemia in the dominant hemisphere, 8 in the vertebrobasilar system, and 2 in the nondominant hemisphere. Twelve patients received intravenous rtPA without significant improvement; 9 patients were not candidates for intravenous rtPA. All patients received intra-arterial rtPA. The IIb/IIIa inhibitors were administered intravenously in 3 patients, intra-arterially in 16, and both intravenously and intra-arterially in 2. Balloon angioplasty was performed in 18 patients. Complete or partial recanalization was achieved in 17 of the 21 patients. After thrombolysis, 15 improved clinically. Three patients (14%) sustained an asymptomatic intracerebral hemorrhage after thrombolytic therapy. No patient was clinically worse after intervention. At last follow-up (mean, 8.5 mo), 13 patients were functionally independent (modified Rankin score, 0-3) and 8 were disabled or dead.
IIb/IIIa inhibitors are an alternative for achieving recanalization. The risk of hemorrhage may be low. As part of an escalating protocol that includes pharmacological and mechanical thrombolysis, IIb/IIIa inhibitors may improve clinical outcomes.
Lumbar artificial disc technology has been commercially available outside the United States for nearly 2 decades. With the Food and Drug Administration approval of the Charité Artificial Disc in October 2004, an entirely new spinal surgeon population will be able to offer this technology to their patients as a treatment option. As with other techniques in spinal surgery, indications for lumbar total disc replacement are paramount to the success of the procedure. The correct surgical technique is also important to a successful outcome. This article describes the technique for placement of the Charité Artificial Disc in indicated patients. The technique is similar to that of an anterior lumbar interbody fusion procedure, but many differences between the techniques make lumbar total disc replacement a unique procedure in the spinal surgeon's armamentarium. Although this article is thorough in its description of the surgical technique for total disc replacement with the Charité Artificial Disc, it should not be used as a substitute for company-sponsored training.
A prospective, randomized, multicenter, Food and Drug Administration-regulated, investigational device exemption clinical trial.
To compare the safety and effectiveness of lumbar total disc replacement (TDR) with the CHARITE artificial disc (DePuy Spine, Raynham, MA) to anterior lumbar interbody fusion for the treatment of single-level degenerative disc disease from L4-S1 unresponsive to nonoperative treatment. In addition, to evaluate the radiographic outcomes of lumbar artificial disc replacement at either L4-L5 or L5-S1 with the CHARITE artificial disc as compared to anterior lumbar interbody fusion with cylindrical cages and iliac crest bone graft; and to determine if a correlation exists between clinical outcomes and surgical accuracy of TDR placement within the disc space.
Prior investigators have reported excellent radiographic results with the CHARITE artificial disc for the treatment of lumbar degenerative disc disease. These encouraging results are the product of retrospective reviews without a control. Very few studies have reported on the segmental motion of an intervertebral level implanted with an artificial disc, and no studies have reported a correlation of radiographic and clinical outcomes.
A prospective, randomized, multicenter, US Food and Drug Administration, investigational device exemption study with 24-month follow-up was performed at 14 centers throughout the United States. A total of 304 subjects were randomized in a 2:1 ratio, with 205 in the investigational group (TDR with the CHARITE artificial disc) and 99 in the control group (anterior lumbar interbody fusion with BAK cages and iliac crest bone graft). A total of 71 TDR training cases were performed (up to 5 at each site) before randomization beginning at each site. Plain radiographs were analyzed for each subject in both groups regarding range of motion (ROM) in flexion/extension, restoration of disc space height, and subsidence. Prosthesis placement in the coronal and midsagittal planes was analyzed for the 276 patients with TDR. Correlations were performed between prosthesis placement and clinical outcomes.
Patients in the investigational group had a 13.6% mean increase, and those in the control group an 82.5% decrease in mean flexion/extension ROM at 24 months postoperatively compared to baseline. Patients in the investigational group had significantly better restoration of disc height than the control group (P < 0.05). There was significantly less subsidence in the investigational group compared to the control group (P < 0.05). The surgical technical accuracy of CHARITE artificial disc placement was divided into 3 groups: I, ideal (83%); II, suboptimal (11%); and III, poor (6%), and correlated with clinical outcomes. The flexion/extension ROM and prosthesis function improved with the surgical technical accuracy of radiographic placement (P = 0.003).
Preoperative ROM in flexion/extension was restored and maintained in patients receiving a TDR. TDR with the CHARITE artificial disc resulted in significantly better restoration of disc space height, and significantly less subsidence than anterior interbody fusion with BAK cages. Clinical outcomes and flexion/extension ROM correlated with surgical technical accuracy of CHARITE artificial disc placement. In the majority of cases, placement of the CHARITE artificial disc was ideal.
In situ mechanical behavior of posterior spinal ligaments in the lumbar region: an in vitro study 1850 Spine @BULLET Volume 32 @BULLET Number 17 @BULLET 2007 29 An analytical investigation of the mechanics of spinal instrumentation
- Ga Dumas
- L Beaudoin
- Drouin
- Vk Goel
- Ye Kim
- Lim
- Th
Dumas GA, Beaudoin L, Drouin G. In situ mechanical behavior of posterior spinal ligaments in the lumbar region: an in vitro study. J Biomech 1987;20: 301–10. 1850 Spine @BULLET Volume 32 @BULLET Number 17 @BULLET 2007 29. Goel VK, Kim YE, Lim TH, et al. An analytical investigation of the mechanics of spinal instrumentation. Spine 1988;13:1003–11.
Measurement of pressures in the nucleus and within the annulus of the human spinal disc: due to extreme loading Loads on the lumbar spine: validation of a biomechanical analysis by measurements of intradiscal pressures and myoelectric signals
- Ranu
- A Schultz
- G Andersson
- R Ortengren
Ranu HS. Measurement of pressures in the nucleus and within the annulus of the human spinal disc: due to extreme loading. Proc Inst Mech Eng H 1990; 204:141– 6. 36. Schultz A, Andersson G, Ortengren R, et al. Loads on the lumbar spine: validation of a biomechanical analysis by measurements of intradiscal pressures and myoelectric signals. J Bone Joint Surg Am 1982;64: 713–20.
- Aa White
- Mm Panjabi
White AA, Panjabi MM. Clinical Biomechanics of the Spine. Philadelphia:
Lippincott; 1990.