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Abstract
Mechanical testing of cadaveric lumbar spines and dual energy radiograph absorptiometry scanning were performed.
To devise a technique to measure the strength of lumbar spinous processes and to determine the bone mineral density of the vertebrae used.
The spinous process has been identified as the weakest part of the anatomy to which a flexible fixation device can be attached. It was unknown if the spinous processes could withstand the forces applied by the device.
A hook was fitted to the spinous process of 32 lumbar vertebrae. A custom-built rig was designed to secure a vertebra to a materials testing machine. A loop of cord was passed over a bar mounted on the crosshead of the machine and around the two bollards of the hook. As the crosshead was raised, a tension was applied to the cord. Each vertebra was tested to failure. The bone mineral density of each vertebra was then measured using dual energy radiograph absorptiometry.
Failure of the specimens occurred by failure of the spinous process, pedicles, or vertebral body. The logarithm (base 10) of the load (N) at which failure occurred was 2.53 +/- 0.3, which corresponded to a mean failure load of 339 N. The bone mineral density of each vertebral body varied between 0.263 and 0.997 g/cm2. A significant linear correlation was found between bone strength and bone mineral density (P < 0.0001).
Specimens with a bone mineral density in the range of 0.263-0.997 g/cm2 failed at a mean load of 339 N when the load was applied through the spinous process hook of a flexible fixation device.
... 6,11 Consequently, this treatment is contra-indicated in patients with osteoporosis. [4][5][6]9,10,12 Additionally, due to the frequent bone mass loss, advanced age (older than 75 years) can be considered a risk factor as well. ...
... Rigid interspinous spacers were developed and validated for treatment of symptoms of INC. [4][5][6] Use of these devices is contraindicated in patients with osteoporosis, as defined by a t-score of Ͻ2.5, or with a history of prior fragility fracture. [4][5][6]9,10,12 Elderly individuals are often affected by bone fragility even without a definite diagnosis of osteoporosis. Hence, they should be considered at risk for this procedure. ...
Interspinous spacers are implanted to treat symptomatic lumbar stenosis. Posterior vertebral element fractures can occur during or after interspinous spacer implants, especially in patients with osteopenia. The purpose of our study was to assess the biomechanical rationale, safety, feasibility, and effectiveness of posterior vertebral arch cement augmentation (spinoplasty) in preventing delayed spinous process fractures after interspinous spacer implants in patients with risk factors for fragility fractures.
We performed a nonrandomized historically controlled clinical trial. From June 2007 to March 2010, we implanted interspinous spacers in 35 eligible patients with fragility-fracture risk factors. In 19/35 patients treated after April 2009, after we assessed the theoretic biomechanical effects of cement augmentation of the spinous process and laminae by FEM, a percutaneous spinoplasty was also performed. Clinical and radiologic follow-up ranged between 12 and 36 months after the intervention.
No intraprocedural spinous process fractures were observed in either group, and no patients in the 24-hour postoperative period had complications that were procedure-related. Symptomatic delayed spinous process fractures were diagnosed in 4/16 patients who did not undergo spinoplasty (25.0%), while no fractures were diagnosed in the 19 treated patients (P = .035).
Spinoplasty is feasible and safe. It has a biomechanical rationale, as demonstrated by an FEM. In our preliminary experience, it seems effective in preventing delayed fractures of the posterior arch post-interspinous spacer placement in patients at risk for fragility fractures. These patients have a significant risk of developing a symptomatic delayed spinous process fracture if not treated with spinoplasty.
... This is because the placement of the IPD during flexion and extension shifts the stress mechanism on the spinous process from tension to pressure, resulting in elevated spinous stresses and an increased risk of spinous fracture and fatigue fracture of the implant. A cadaveric study by Shepherd and others [28] documented an average breaking load of 339 N on the intact spinous process under upwards loading of the spinous device. Some authors have also optimized the structure of Coflex by means of topological optimization to reduce the concentration of spinous stress and reduce the incidence of complications [29,30]. ...
Background
Lumbar spinal stenosis (LSS) is a common disease among elderly individuals, and surgery is an effective treatment. The development of minimally invasive surgical techniques, such as the lumbar interspinous process device (IPD), has provided patients with more surgical options.
Objective
To investigate the biomechanical properties of different IPDs, including BacFuse, X-Stop and Coflex, in the treatment of LSS.
Methods
Based on the computed tomography images of a patient with LSS, four finite element (FE) models of L3-S5 were created in this study. The FE models included a surgical model of the intact lumbar spine and surgical models of the lumbar IPDs BacFuse, X-Stop, and Coflex. After validating the models, they were simulated for four physiological motions: flexion, extension, lateral bending and axial rotation, and range of motion (ROM). Stress distribution of discs and facet joints in each segment, stress distribution of the spinous process in the operated section, and stress distribution of the internal fixation were compared and analysed.
Results
Compared to the model of the intact lumbar spine, the other three models showed a decrease in ROM and disc and facet joint stresses in the surgical segment during movement and an increase in ROM and disc and facet joint stresses in the adjacent segments. These effects were greater for the proximal adjacent segment with BacFuse and more pronounced for the distal adjacent segment with Coflex, while X-Stop had the greatest stress effect on the spinous process in the surgical segment.
Conclusion
BacFuse, Coflex and X-Stop could all be implemented to effectively reduce extension and disc and facet joint stresses, but they also increase the ROM and disc and facet joint stresses in adjacent segments, which may cause degeneration.
... Fracture of the spinous process is the primary complication of interspinous devices [11,30,31]. A cadaveric study by Shepherd et al. [32] recorded the average failure load of an intact spinous process as 339 N under a superiorly-directed load from a spinous process device. In this study, the maximum contact force on the spinous processes occurred in the L3 process in all instrumented models and is safe within the strength of the bone. ...
Recently, various designs and material manufactured interspinous process devices (IPDs) are on the market in managing symptomatic lumbar spinal stenosis (LSS). However, atraumatic fracture of the intervening spinous process has been reported in patients, particularly, double or multiple level lumbar decompression surgery with IPDs. This study aimed to biomechanically investigate the effects of few commercial IPDs, namely DIAM TM , Coflex TM , and M-PEEK, which were implanted into the L2-3, L3-4 double-level lumbar spinal processes. A validated finite element model of musculoskeletal intact lumbar spinal column was modified to accommodate the numerical analysis of different implants. The range of motion (ROM) between each vertebra, stiffness of the implanted level, intra stress on the intervertebral discs and facet joints, and the contact forces on spinous processes were compared. Among the three implants, the Coflex system showed the largest ROM restriction in extension and caused the highest stress over the disc annulus at the adjacent levels, as well as the sandwich phenomenon on the spinous process at the instrumented levels. Further, the DIAM device provided a superior loading-sharing between the two bridge supports, and the M-PEEK system offered a superior load-sharing from the superior spinous process to the lower pedicle screw. The limited motion at the instrumented segments were compensated by the upper and lower adjacent functional units, however, this increasing ROM and stress would accelerate the degeneration of un-instrumented segments.
... The estimated mean load on the spinous processes was 292 N under 8 mm of ISPF device compression and 212 N under 4 mm of distraction. Although the failure strength depends strongly on the bone size and quality of each individual, the applied loads were well below the range of mean failure strength values from previous studies (339-493 N) [16,28]. Accordingly, no spinous process fractures were observed during this study. ...
Background
Rigid interspinous process fixation (ISPF) may serve as a minimally disruptive adjunct to lumbar interbody fusion. Previous biomechanical assessments of ISPF have demonstrated particularly advantageous outcomes in stabilizing the sagittal plane. However, ISPF has not been well characterized in regard to its impact on interbody load, which has implications for the risk of cage migration or subsidence, and sagittal alignment. The purpose of this study was to biomechanically assess in vitro the interbody load (IBL), focal lordosis (FL), and spinous process loading generated by in situ compression/distraction with a novel ISPF device capable of incremental in situ shortening/extension. Bilateral pedicle screw fixation (BPSF) was used as a control.
Methods
Two fresh frozen human lumbar spines were thawed and musculature was removed, leaving ligaments intact. Seven functional spinal units were iteratively tested, which involved a standard lateral discectomy, placement of a modified lateral cage possessing two load cells, and posterior fixation. BPSF and ISPF were performed at each level, with order of fixation was randomized. BPSF was first performed with maximum compressive exertion followed by 75% exertion to represent clinical application. The ISPF device was implanted at a neutral height and incrementally shortened/extended in situ in 1-mm increments. IBL and FL were measured under each condition. Loads on the spinous processes were estimated through bench-top mechanical calibration.
Results
No significant differences in IBL were observed, but the ISPF device produced a significantly greater change in FL compared to the clinically relevant BPSF compression. IBL, as a function of ISPF device height, expressed linear behavior during compression and exponential behavior during distraction.
Conclusions
The novel ISPF device produced clinically effective IBL and FL, performing well in comparison to BPSF. Additionally, incremental ISPF device manipulation demonstrated predictable and clinically safe trends regarding loading of the interbody space and spinous processes.
... In the design of the tension band interspinous device, Golish et al. [21] measured the carrying capacity of the L4 spinous process as 453 ± 16 N when the center line was not reduced, and after depressing, the carrying capacity decreased to 264 ± 99 N; the carrying capacity of the L5 spinous process was 517 ± 190 N. When the midline was not reduced, the carrying capacity decreased to 269 ± 184 N after depressing. When the spinous process of the lumbar spine was loaded with clamps and hooks, Shepherd et al. [22] found that the carrying capacity of L3 and L4 spinous processes was 339 N. ...
Background
Each part of the rear bone structure can become an anchor point for an attachment device. The objective of this study was to evaluate the stiffness and strength of different parts of the rear lumbar bone structure by axial compression damage experiments.
Methods
Five adult male lumbar bone structures from L2 to L5 were exposed. The superior and inferior articular processes, upper and lower edges of the lamina, and upper and lower edges of the spinous process were observed and isolated and then divided into six groups (n = 10). The specimens were placed between the compaction disc and the load platform in a universal testing machine, which was first preloaded to 5.0 N tension to eliminate water on the surface and then loaded to the specimen curve decline at a constant tension loading rate of 0.01 mm/s, until the specimens had been destroyed.
Results
Significant differences in mechanical properties were found among different parts of the rear lumbar bone structure. Compared with other parts, the lower edge of the lamina has good mechanical properties, which have a high modulus of elasticity; the superior and inferior articular processes have greater ultimate strength, which can withstand greater compressive loads; and the mechanical properties of the spinous process are poor, and it is significantly stiffer and weaker than the lamina and articular processes.
Conclusion
These data can be useful in future spinal biomechanics research leading to better biomechanical compatibility and provide theoretical references for spinal implant materials.
... 7 Sheperd et al measured the mechanical force required to fracture the spinous process of 32 specimens with average to below-average bone mineral density. 17 They found a significant linear correlation between bone mineral density and bone strength. A mean load of 339 N was required to cause a spinous process failure with a 95% confidence interval of 257-447 Newtons. ...
Early interspinous process fixation constructs utilize rigid fixation plates with immobile spikes which increase the difficulty of device implantation when anatomic variations are encountered. Second generation systems have been designed with polyaxial properties with the goal of accommodating natural osseous anatomic variations to achieve optimal implant placement and fixation integrity. The purpose of this study was to evaluate clinical outcomes in patients treated with this device to supplement the biomechanical data from previous studies.
A retrospective, non-randomized, single-center chart review at or beyond the one year postoperative time point was conducted to collect preoperative and perioperative data on patients treated with a polyaxial intraspinous fixation system. A postoperative numerical pain rating scale and modified MacNab classification score were obtained from each patient in the cohort via phone survey.
A total of 53 patients were included in the study. Median hospital stay was 2 days (range 1-7 days). There were no reported perioperative blood transfusions or cases of radiographic fracture/migration of the device at the 6 week post-operative time point. There was a significant improvement in pain index score in the overall patient study group and a satisfactory (excellent or good) MacNab result was obtained in 48% of all patients. Patients with preoperative pain scores greater than 8/10 reported more pain improvement than patients with preoperative pain scores less than 5 (0 points, p = 0.96, n = 8). Patients with a BMI less than 30 had significantly better MacNab outcome classifications than patients with a BMI greater than 30.
The polyaxial interspinous fusion system produces significant clinical improvement when employed to treat patients with stenosis, herniated disc, or low grade spondylolisthesis. This device can be implanted with a low complication rate and short postoperative hospital admission time. Patients with high pre-operative pain score and BMI under 30 can be predictors of better clinical outcome and should be considered prior to implantation.
... In recent years, relatively good clinical results of lumbar spine fixation have been reported with the Lumbar Alligator Spinal System, CD HORIZON SPIRE spinous process plate and S-plate, as spinal instrumentation using a spinous process as an anchor [12,13], but the Tadpole system is a unique spinal instrumentation hook and rod system. Shepherd et al. [14] demonstrated that the spinous process had sufficient grasping capacity when a hook was placed in the spinous process, suggesting the validity of fixing our Tadpole system to the spinous process with a hook. ...
Background
Failure of pedicle screw fixation is often seen in patients with severe osteoporosis. We developed new lumbar spinal instrumentation (Tadpole system) for elderly patients who have osteoporotic bone and poor general health status. The objective of this study was to document the long-term clinical outcomes after Tadpole system fixation, the rate of spinal fusion, the incidence of adjacent segment degeneration, the rate of instrumentation failure, and the overall complications.
Methods
Sixty patients who underwent posterolateral spinal fusion using the Tadpole system, in whom a radiograph of the lumbar spine was taken at more than 5 years after operation, were involved in this study. The improvement rate of the Japanese Orthopaedic Association (JOA) score, rate of spinal fusion, presence or absence of adjacent segment degeneration, rate of instrumentation failure, and postoperative complications of each patient were assessed at 5 years postoperatively.
Results
The mean JOA score improvement was 72.5%, and the posterolateral spinal fusion rate was 93.3% (56 of 60 patients) at the last follow-up. Adjacent segment degeneration occurred in only two patients who showed decreased intervertebral disc height, and instrumentation failure (hook deviation) was observed in one patient. No other complications were observed in any patients.
Conclusion
Tadpole system fixation shows favorable long-term clinical outcomes.
... The lateral force required to fracture a human lumbar spinous process with varying bone densities ranges between 95-786 N with a load of average 317 N [41]. The distraction force necessary to break the lumbar spinous process ranged between 242-1 and 300 N with an average load of 339 N [54]. ...
A large number of interspinous process devices (IPD) have been recently introduced to the lumbar spine market as an alternative to conventional decompressive surgery in managing symptomatic lumbar spinal pathology, especially in the older population. Despite the fact that they are composed of a wide range of different materials including titanium, polyetheretherketone, and elastomeric compounds, the aim of these devices is to unload spine, restoring foraminal height, and stabilize the spine by distracting the spinous processes. Although the initial reports represented the IPD as a safe, effective, and minimally invasive surgical alternative for relief of neurological symptoms in patients with low back degenerative diseases, recent studies have demonstrated less impressive clinical results and higher rate of failure than initially reported. The purpose of this paper is to provide a comprehensive overview on interspinous implants, their mechanisms of action, safety, cost, and effectiveness in the treatment of lumbar stenosis and degenerative disc diseases.
... In some cases of mechanical testing it is beneficial to use human cadaveric material [41]. For example, cadavers have been used to investigate the attachment of a hook device to the spinous process of the lumbar spine in terms of strength and slippage [42,43]. In addition, cadavers can be used to trial surgical instruments [44]. ...
The design process for medical devices is highly regulated to ensure the safety of patients. This paper will present a review of the design process for implantable orthopedic medical devices. It will cover the main stages of feasibility, design reviews, design, design verification, manufacture, design validation, design transfer and design changes.
... Because the biomechanical strength of the spinous process is not high, there is limited flexibility in spinal instrumentation using the spinous process as an anchor, but recent studies of lumbar fusion surgery using the Lumbar Alligator Spinal system® [9] and CD Horizon Spire spinous process plate® [4,10] have shown relatively favorable clinical outcomes. In a biomechanical study carried out by Shepherd et al. [11], holding the spinous process with a hook provided sufficient holding ability. Thus, the available evidence indicates that the Tadpole system® has advantages over other systems. ...
There have been reports of serious complications associated with pedicle screw fixation, including nerve root injuries caused by accidental screw insertion. We have developed a new system of lumbar spinal instrumentation that we call Tadpole system. The purposes of this report were to show the results of a biomechanical study and the short-term outcome of a clinical study, as well as to determine the usefulness of this system.
The Tadpole system lumbar spinal fusion is a hook-and-rod system according to which the spine is stabilized using 2 sets of 2 spinous processes each that are held in place by 4 hooks tandemly connected to a rod. The biomechanical study was done using 5 human lumbar cadaveric spines, and the range of motion (ROM) was examined in a non-treatment model, an injured model, a pedicle screw fixation model and a Tadpole system model. For the short-term clinical study the Tadpole system was used in 31 patients, and the factors analyzed were operation time, time required for spinal instrumentation, amount of intraoperative bleeding, postoperative improvement rate of the Japanese Orthopaedic Association (JOA) score for lumbar spinal disorders, instrumentation failure, spinous process fracture, spinal fluid leakage, nerve root injury, postoperative infection, and bone fusion 2 years after the operation.
The ROM in the Tadpole system model was slightly bigger than that in the pedicle screw fixation model, but smaller than that in the normal control model. These biomechanical data indicated that the Tadpole system provided fairly good stability. The mean operation time was 79 min, the mean time required for spinal instrumentation was 8 min, and the mean amount of intraoperative bleeding was 340 mL. The mean postoperative improvement rate of JOA score was 70.9 +/- 24.8%. Instrumentation failure (dislocation of a hook) occurred in one patient, and none of the patients developed spinous process fracture, spinal fluid leakage, nerve root injury, or postoperative infection. Two years after the operation, bone union was confirmed in 29 of the 31 patients (93.5%).
We conclude that this system is a useful, easy-to-use and safe spinal instrumentation technique for lumbar fusion surgery.
... Spinous process strength has been shown to decrease with BMD and patients with low BMD are at a greater risk of fracture either during or after the X STOP surgery than those with higher BMD. [7][8][9] This principle has been applied to the X STOP, which is implanted into the interspinous ligament, between 2 spinous processes of the lumbar region. The principle behind the new injection concept is to increase the strength of the spinous processes by injecting the relevant spinous processes with PMMA bone cement. ...
Biomechanical.
To determine if cement injection into the spinous process will improve compression strength.
The X STOP (St. Francis Medical Technologies) has been shown to be a safe and effective means for decompressing 1- or 2-level lumbar spinal stenosis (LSS). The X STOP is indicated for LSS patients with osteoporosis, but contraindicated for patients with severe osteoporosis. In an attempt to address these LSS patients with demonstrably weaker bone, a technique to strengthen the spinous process with polymethylmethacrylate (PMMA) injection is presented.
Nine pairs of adjacent fresh frozen cadaveric lumbar vertebrae were DEXA scanned before testing. They were randomly assigned to the PMMA group and a control group. Nine of the specimens were injected with PMMA. Each spinous process was then compressed between 2 X STOPs. The testing model was designed to simulate the loading of a 2-level X STOP placement. The mean load to failure and stiffness values of the treated and untreated groups were calculated. The specimens were inspected carefully for PMMA infiltration and extrusion.
The mean bone mineral density (BMD) values of the control and PMMA treatment groups were 0.99 +/- 0.13 g/cm and 0.98 +/- 0.10 g/cm, respectively; P > 0.616. The mean volume of cement injected was 2.2 +/- 0.3 cc. The mean failure load values of the control and PMMA treatment groups were 1250 +/- 627 N and 2386 +/- 1034 N, respectively; P < 0.001. The mean stiffness values of the control and PMMA treatment groups were 296 +/- 139 N/mm and 381 +/- 131 N/mm, respectively; P > 0.059. Most specimens had flow of the cement into the laminae and some into the facet and pedicle. No PMMA was found within the spinal canal.
This first reported technique of posterior element vertebroplasty may increase the indications and success for patients with decreased BMD who seek an interspinous implant such as the X STOP. A possible role may exist in increasing the effectiveness of such devices. However, clinical trials have yet been performed. These results demonstrate that PMMA injection in the spinous processes is effective in increasing resistance to compressive forces in an X STOP model.
A systematic literature review was conducted on studies comparing interspinous process (ISP) devices to traditional methods of posterior spinal instrumentation (pedicle screw-rod construct), in terms of indications of use, complications, pain assessment, estimated blood loss, length of hospital stay, reoperation rates, and return to work. The objective was to analyze, evaluate and summarize the current published literature on the proposed efficacy and clinical and surgical long-term outcomes of the ISP device in comparison to the traditional posterior spinal instrumentation (pedicle screw-rod construct). The ISP device is a minimally invasive and less disruptive alternative to traditional methods of posterior spinal instrumentation (pedicle screw-rod construct). However, very few published literature studies to date have reported the comparison of ISPs in terms of efficacy and clinical and surgical outcomes, to traditional posterior spinal instrumentation.
A systematic literature review was performed in PubMed and Google Scholar to evaluate the results of published research that meet the defined inclusion and exclusion criteria and to analyze clinical indications and surgical outcomes of the ISP device compared to traditional methods of posterior spinal instrumentation (pedicle screw-rod construct). Inclusion criteria included keywords such as “ISP device, ISP, posterior spinal instrumentation, pedicle screw fixation, bilateral pedicle screws, interbody fusion with posterior spinal instrumentation, lumbar spinal stenosis, and posterior lumbar stability.” No exclusion criteria keywords were included in this literature review.
ISPs provide a high degree of spinal stability in multiple planes, including a decreased range of motion restriction in flexion-extension, and comparable results to bilateral pedicle screw (BPS) in axial rotation. The use of the ISP device in adjunct with an interbody fusion, ensures less estimated operative blood loss (EBL), shorter operative time, less bony exposure without the need for extensive soft tissue or muscle retraction, a decrease in the rate of pseudoarthrosis, and a shorter length of hospital stay (LOHS) when compared to the traditional posterior instrumentation (pedicle screw-rod construct).
Based on the various published literature reviews noted throughout this research paper, it is safe to conclude, that an ISP device that is accompanied by interbody fusion, including posterior approaches posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF); anterior approaches such as anterior interbody fusion (ALIF), and lateral approaches including direct lateral interbody fusion (DLIF), lateral lumbar interbody fusion (LLIF), extreme lateral interbody fusion (XLIF), is considered a credible and an effective minimally invasive option for the treatment of mild to moderate lumbar stenosis and stable low-grade spondylolisthesis (less than two) when compared to the traditional posterior spinal instrumentation of a pedicle screw-rod construct. Surgeons that are relatively new to the ISP technologies for spinal instrumentation would likely benefit from more clinical and surgical evidence of safety and efficacy in published peer-reviewed medical literature. Further clinical trials are needed to manifest the efficacy of ISPs regarding postoperative outcomes when compared to traditional posterior instrumentation techniques (pedicle screw-rod construct) with adjunct interbody fusions.
Study design:
In-vitro cadaveric biomechanical study STUDY OBJECTIVE: The objective of this study was to characterize the biomechanical implications of spinous process compression, via in-situ shortening of a next-generation interspinous process fixation (ISPF) device, in the context of segmental fusion.
Methods:
Seven lumbar cadaveric spines (L1-L4) were tested. Specimens were first tested in an intact state, followed by iterative instrumentation at L2/3 and subsequent testing. Order: 1) stand-alone ISPF (neutral height); 2) stand-alone ISPF (shortened in-situ from neutral height; 'shortened'); 3) lateral interbody cage ('LLIF')+ISPF (neutral); 4) LLIF+ISPF (shortened); 5) LLIF+unilateral pedicle screw fixation (PSF); 6) LLIF+bilateral PSF. A 7.5Nm moment was applied in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) via a kinematic test frame. Segmental range-of-motion (ROM) and lordosis were measured for all constructs. Comparative analysis was performed.
Results:
Statistically significant FE ROM reductions: all constructs vs. intact condition (p<0.01); LLIF+ISPF (neutral & shortened) vs. stand-alone ISPF (neutral & shortened) (p<0.01); LLIF+USPF vs. ISPF (neutral) (p=0.049); BPSF vs. stand-alone ISPF (neutral & shortened) (p<0.01); LLIF+BPSF vs. LLIF+UPSF (p<0.01). Significant LB ROM reductions: LLIF+ISPF (neutral & shortened) vs. intact condition & stand-alone ISPF (neutral) (p<0.01); LLIF+UPSF vs. intact condition & stand-alone ISPF (neutral & shortened) (p<0.01); LLIF+BPSF vs. intact condition & all constructs (p<0.01). Significant AR ROM reductions: LLIF+ISPF (shortened) & LLIF+UPSF vs. intact condition & stand-alone ISPF (neutral) (p≤0.01); LLIF+BPSF vs. intact condition & all constructs (p≤0.04).
Conclusion:
In-situ shortening of an adjustable ISPF device may support increased segmental stabilization in comparison to static ISPF.
Interspinous devices are in the class of medical devices that can be implanted in the lumbosacral spine using a minimal and often mini-invasive approach. Because their use has boomed over the last decade, we can state with confidence that this technological sector attracts a great deal of interest in a quest for techniques and materials able to reduce the invasiveness of the surgical procedure and increase its general bio-compatibility. An initial classification of interspinous devices from a biomechanical viewpoint may be carried out by assessing the rigidity of the distraction element (Table 1, Appendix). This identifies devices that are inaccurately categorized as nondeformable, involving the insertion of material with high mechanical rigidity into the space between the spinous process, where the distraction between the spinous processes may be considered constant. In other devices, a material with a shock-absorbing function is inserted between the spinous processes, which then undergoes appreciable elastic or visco-elastic deformation under physiological loads to increase bone implant compliance. In parallel with this sub-category, there are also devices that work by rigid stabilization of the interspinous space where stable posterior interspinous fusion is brought about by applying autologous or homologous bone and cruentation of the spinous processes. Clinical trials on interspinous devices available in the literature show a good relationship between the benefits for the patient and the use of resources in the disease treatment. Nevertheless, there is still margin for clinical investigation and for the establishment of verification and validation procedures of these devices in order to clearly define the relationship between the effects on the biomechanics of the functional unit and the clinical indications of such devices.
Medication and physical therapy exhibit minimal efficacy in treating degenerative lumbar spinal stenosis. Laminectomy for compression has great trauma, which is no accepted by the patients. According to symptom changes of patients with nerve-derived intermittent claudication, i.e. aggravated with lumbar extension and relieved with flexion, Coflex system, a kind of dynamic spinous process implants, was designed by GENERAL CARE, Germany. It is non-fusion implant. Coflex system is a minimally invasive method, used to treat lumbar spinal stenosis and lumbar instability. The intrer-spinous device stabilizes the compression-induced lumbar instability and prevents the instability after a long period of time. Coflex system is a promising implantation system for clinical application, which provides a safe and effective therapy for degenerative lumbar spinal stenosis.
ASTM F2624-07 prescribes applying flexion/extension, lateral bending, and axial rotation moments to test and evaluate extra discal motion preserving implants (EDMPI). The standard provides a suggested laterally oriented construct (LOC) for conducting flexion/extension (FE) testing. The suggested construct does not have a fixed center of rotation (COR). An alternate set of fixtures to conduct FE testing using a vertically oriented construct (VOC) with a fixed COR is presented. Testing was conducted to quantify the loads applied to an EDMPI using both types of fixtures, quantify shear loading applied to the actuator, determine whether results are frequency dependent, and evaluate whether the fixtures can differentiate between EDMPI using a measure of displacement amplitude. A miniature load cell was mounted to both fixtures to quantify load applied to an EDMPI during testing at several different applied loads or torques and at two testing frequencies (2 Hz and 5 Hz). Lateral actuator shear was measured using a dial indicator. The load cell was replaced with cylindrical test specimens made of stainless steel, titanium, polyether ether ketone, or a polyethylene cable slipped inside a polyurethane tube, and the testing repeated. Results indicated that both fixtures generated similar tensile/compressive loads at an EDMPI. The LOC fixture generated more actuator shear than the VOC. Using a VOC fixture rather than the LOC suggested in ASTM F2624-07 in FE testing may be a reasonable alternative for certain types of devices.
Background:
There has been increasing interest in using the lumbosacral spinous processes for fixation as a less invasive alternative to transpedicular instrumentation. Though prior studies have described the appearance and biomechanics of lumbar spinous processes, few have evaluated the dimensions, morphology, or strength of the sacral spinous processes.
Purpose:
Goals of this study were to characterize the morphology of the S1 spinous process and biomechanical strength of the S1 spinous process when loaded in a cranial direction.
Study design:
This study was performed as both an analysis of radiographic data and biomechanical testing of cadaveric specimens.
Methods:
Lumbosacral spine radiographs and CT scans of twenty patients were evaluated for visibility and morphology of the S1 spinous process. S1 spinous process length, height, and size of the L5-S1 segment were measured. Additionally, thirteen cadaveric lumbosacral spinal segments were obtained for biomechanical testing and morphologic analysis. Specimens were loaded at the S1 spinous process in a cranial direction via a strap, simulating resistance to a flexion moment applied across the L5-S1 segment. Peak load to failure, displacement, and mode of failure were recorded.
Results:
The S1 spinous process was clearly visible on lateral radiographs in only 10% of patients. Mean spinous process length (anterior-posterior) was 11.6mm while mean spinous process height (cranial-caudal) was 23.1mm. We identified six different morphologic subtypes of the S1 spinous process: Fin, Lumbar, Fenestrated, Fused, Tubercle, and Spina Bifida Occulta. During tension loading of the S1 spinous process in the cephalad direction, mean peak load to failure was 439N, with 92% of specimens failing by fracture through the spinous process.
Conclusions:
This is the first study evaluating sacral spinous process morphology, visibility, and biomechanical strength for potential instrumentation. Compared to lumbar spinous processes, sacral spinous processes are smaller with more variable morphology but have similar peak load to failure. For ideal visualization of morphology and suitability for interspinous fixation, preoperative three-dimensional imaging may be a valuable tool over plain radiographs.
OBJECT
The authors evaluated the biomechanical effects of an interspinous process (ISP) device on kinematics and load sharing at the implanted and adjacent segments.
METHODS
A 3D finite-element (FE) model of the lumbar spine (L1–5) was developed and validated through comparison with published in vitro study data. Specifically, validation was achieved by a flexible (load-control) approach in 3 main planes under a pure moment of 10 Nm and a compressive follower load of 400 N. The ISP device was inserted between the L-3 and L-4 processes. Intact and implanted cases were simulated using the hybrid protocol in all motion directions. The resultant motion, facet load, and intradiscal pressure after implantation were investigated at the index and adjacent levels. In addition, stress at the bone-implant interface was predicted.
RESULTS
The hybrid approach, shown to be appropriate for adjacent-level investigations, predicted that the ISP device would decrease the range of motion, facet load, and intradiscal pressure at the index level relative to the corresponding values for the intact spine in extension. Specifically, the intradiscal pressure induced after implantation at adjacent segments increased by 39.7% and by 6.6% at L2–3 and L4–5, respectively. Similarly, facet loads at adjacent segments after implantation increased up to 60% relative to the loads in the intact case. Further, the stress at the bone-implant interface increased significantly. The influence of the ISP device on load sharing parameters in motion directions other than extension was negligible.
CONCLUSIONS
Although ISP devices apply a distraction force on the processes and prevent further extension of the index segment, their implantation may cause changes in biomechanical parameters such as facet load, intradiscal pressure, and range of motion at adjacent levels in extension.
Despite interest in lumbar spinous process-based surgical innovation, there are no large published studies that have characterized morphometry of lumbar spinous processes.
To provide accurate level-specific morphometric data with respect to human lumbar spinous processes using a human cadaveric lumbar spine model and to describe morphometric variation of lumbar spinous processes with respect to gender, race and age.
Anatomic observational METHODS: This study utilized 2,955 cadaveric lumbar vertebrae from 591 adult spines at the Hamann-Todd Human Osteological Collection. Specimens were ages 20-79 years. Each vertebra was photographed in standardized positions and measured using ImageJ software. Direct measurements were made for spinous process (SP) length, width, height, slope and caudal morphology. Gender, race and age were recorded and analyzed.
SP length was 24.8±4.6 mm (L5) to 33.9±3.9 mm (L3). Effective length varied from 19.5±2.6 mm (L1) to 24.6±3.3 mm (L4). Height was shortest at L5 (18.2±2.7 mm). Caudal width was greater than cranial width. Slope, caudal morphology and radius measures showed large inter-specimen variation. Slope at L5 was steeper than other levels (23.7±10.5 degrees, p<0.0001). Most specimens demonstrated convex caudal morphology. L4 had the highest proportion of convexity (80.7%). L1 was the only level with predominantly concave morphology. Measurements for female SPs were smaller, but slope was steeper. Anatomic and effective SP lengths were longer for specimens from white individuals. Specimens from black individuals had larger width and height, as well as steeper slope. Black specimens had more convex morphology at L4 and L5. With increasing age, SP length, effective length and width increased. Height increased with age only at L4 and L5. Slope and caudal radius of curvature decreased with age and increasingly convex morphology was noted at most levels.
This large cadaveric study provides level-specific morphometric data regarding the osseous dimensions of lumbar spinous processes relevant to techniques and devices targeting lumbar spinous processes or the interspinous space. Of particular importance is the recognition that L5 has relatively different morphology when compared to more cranial levels. Potentially important differences were noted comparing female to male, black to white and aging populations.
Copyright © 2015 Elsevier Inc. All rights reserved.
This paper presents a high fidelity computational approach to be used in validation of biomechanics experimental measurements. As a demonstration, a case study involving a spinous process implant to correct scoliosis is presented. The biomechanical behavior of the spinous process and implant under tensile loading is investigated using experiments and computations. The experimental study examined the ultimate strength of calf thoracic and lumbar spinous processes in three pullout directions. A statistical analysis was performed on the experimental results to reveal relationships and variations between pullout direction and vertebral type. The finite element high fidelity computational analysis was performed to validate the experimental results. In the process, the material properties of cortical and trabecular bone were elucidated for calf spinous processes. Good comparisons are obtained. The high fidelity computational approach detailed here should serve useful in validation of experimental values from spine biomechanics experimental.
Background Context
While multiple mechanisms of device attachment to the spinous processes exist, there is a paucity of data regarding lumbar spinous process morphology and peak failure loads.
Purpose
Using an in-vitro human cadaveric spine model, the primary objective of the current study was to compare the peak load and mechanisms of lumbar spinous process failure with variation in spinous process hole location and pullout direction. A secondary objective was to provide an in-depth characterization of spinous process morphology.
Study Design
Biomechanical and anatomical considerations in lumbar spinous process fixation using an in-vitro human cadaveric model.
Methods
A total of 12 intact lumbar spines were utilized in the current investigation. The vertebral segments (L1-L5) were randomly assigned to 1 of 5 treatment groups with variation in spinous process hole placement and pullout direction: 1) Central hole placement with superior pullout (n=10), 2) Central hole placement with inferior pullout (n=10), 3) Inferior hole placement with inferior pullout (n=10), 4) Superior hole placement with superior pullout (n=10) and 5) Intact spinous process with superior pullout (n=14). A 4mm diameter pin was placed through the hole followed by pullout testing using a material testing system (MTS). As well, the bone mineral density (BMD) (g/cm3) was measured for each segment. Data were quantified in terms of anatomic dimensions (mm), peak failure loads (Newtons) and fracture mechanisms, with linear regression analysis would identify relationships between anatomic and biomechanical data.
Results
Based on anatomic comparisons, there were significant differences between the anteroposterior and cephalocaudal dimensions of the L5 spinous process versus L1 through L4 (p<0.05). Statistical analysis of peak load at failure of the 4 reconstruction treatments and intact condition demonstrated no significant differences between treatments (range 350 to 500 Newtons) (p>0.05). However, a significant linear correlation was observed between peak failure load and anteroposterior and cephalocaudal dimensions (p<0.05). Correlation between BMD and peak spinous processes failure load was approaching statistical significance (p=0.08). 30/54 specimens failed via direct pullout (plow through), while 8/54 specimens demonstrated spinous process fracture. The remaining cases failed via plow through followed by fracture of the spinous process 16/54 (29%).
Conclutions
The current study demonstrated that variation in spinous process hole placement did not significantly influence failure load. However, there was a strong linear correlation between peak failure load and the anteroposterior and cephalocaudal anatomic dimensions. From a clinical standpoint, the findings of the current study indicate that attachment through the spinous process provides a viable alternative to attachment around the spinous processes. In addition, the anatomic dimensions of the lumbar spinous processes have a greater influence on biomechanical fixation than either hole location or bone mineral density.
Zusammenfassung Das interspinöse DIAM™- („device for interspinous assisted motion“) Implantat ermöglicht mit einer kleinen, reversiblen Operation
eine viskoelastische Unterstützung des lumbalen Bewegungssegments. Es wurde 2004 in Deutschland eingeführt und soll nach einer
Bandscheibenoperation restabilisieren, die Überlastung des Facettengelenks vermeiden und das Nachbarsegment nach einer Fusion
mechanisch entlasten. Biomechanische Arbeiten weisen eine Wirkung in Extension und Flexion, kaum jedoch in Rotation und Seitneigung
nach. Obwohl häufig in der Klinik eingesetzt, gibt es nur eine vergleichsweise geringe Zahl an klinischen, hauptsächlich retrospektiven
Arbeiten, die den Erfolg des Implantats belegen. Sichere wissenschaftliche Daten fehlen bisher. Mehrere prospektive, randomisierte
und kontrollierte Studien werden zur Zeit durchgeführt, um diese Lücke zu schließen. Letztlich wird der Wert des Implantats
in der Behandlung der degenerativen Kaskade der Lendenwirbelsäule erst nach deren Fertigstellung zu beurteilen sein.
Interspinous process distraction devices represent a relatively new option for minimally invasive surgical treatment of lumbar spinal stenosis. The X STOP device is the first such implant to receive FDA approval for this indication, and promising results from initial clinical studies have encouraged development of multiple alternative implant designs. These devices are designed for placement between adjacent spinous processes and function to create relative segmental flexion at the treated level, indirectly increasing the cross-sectional area of the spinal canal as well as the neuroforamina. This technology is an option for patients with neurogenic claudication aggravated by standing or walking and relieved by sitting. Short-term clinical results appear comparable to laminectomy and superior to nonoperative treatment. Long-term clinical follow-up data are not yet available.
Lumbar dynamic fixation is a subspecialty of joint surgery as practiced for years in peripheral articulations. It is used
to surgically treat pain caused by lumbar degenerative lesions while preserving anatomical integrity and function. Technically,
the concept is based on two options: improvement of intervertebral kinematics and unloading of the discs and facet joints.
Its justification lies in finding an alternative to arthrodesis 1 and total disc replacement 2, especially for young and active
adults.
There has been increasing interest in spinous process tension band devices, as distinct from spinous process spacers and plates. The purpose of this study was to load spinous processes caudally at L-4 and cranially at L-5 parallel to the long axis of the spine in a biomechanical model of tension band loading. The goal was to provide normative data for the design of a spinous process tension band device after varying degrees of surgical decompression and across varying bone mineral densities (BMDs).
Fresh-frozen L4-5 lumbar vertebrae pairs were divided into 3 surgical groups: intact, midline-sparing decompression (laminotomy and medial facetectomy), and midline decompression with foraminotomy (one-half of spinous process resected, laminotomy, and medial facetectomy). After decompression, specimens were disarticulated into isolated L-4 and L-5 vertebrae. Each vertebra was loaded to failure in a caudal (L-4) or cranial (L-5) direction parallel to the long axis of the spine via a 6-mm-wide strap looped around the spinous process. Failure strength and mode were recorded.
Seventeen L-4 and L-5 lumbar vertebrae were tested from 17 cadavers. There were 10 male (59%) and 7 female (41%) cadavers, with a mean age of 66.6 ± 16.5 years (range 41-100 years) and a mean BMD of 1 ± 0.23 g/cm(2) (range 0.66-1.34 g/cm(2)); the mean is expressed ± SD throughout. For data analysis, specimens were grouped into those with no or midline-sparing decompression (Group 1: 11 of 17) and those with midline decompression (Group 2: 6 of 17). At L-4, the mean failure strength for Group 1 was 453 ± 162 N, and for Group 2 it was 264 ± 99 N (p = 0.02; Cohen's d = 1.4). At L-5, the mean failure strength for Group 1 was 517 ± 190 N, and for Group 2 it was 269 ± 184 N (p = 0.02; Cohen's d = 1.3). There was no significant difference in failure strength between the intact and midline-sparing decompression groups at L-4 (p = 0.91) or L-5 (p = 0.41).
Across specimens with a wide range of BMDs, midline-sparing decompression was not found to decrease the mean failure strength of the L-4 and L-5 spinous processes (453 and 517 N, respectively), whereas midline surgical decompression decreased the failure strength of these processes (264 and 269 N, respectively) in a biomechanical model of tension band loading relevant to the design of a tension band device.
The interspinous distraction devices are used to treat variable pathologies ranging from facet syndrome, diskogenic low back pain, degenerative spinal stenosis, diskopathy, spondylolisthesis, and instability. The insertion of a posterior element with an interspinous device (ISD) is commonly judged responsive to a relative kyphosis of a lumbar segment with a moderate but persistent increase of the spinal canal and of the foraminal width and area, and without influence on low-grade spondylolisthesis. The consequence is the need of shared specific biomechanical concepts to give for each degenerative problem the right indication through a critical analysis of all available experimental and clinical biomechanical data. We reviewed systematically the available clinical and experimental data about kyphosis, enlargement of the spinal canal, distraction of the interspinous distance, increase of the neural foramina, ligamentous structures, load of the posterior annulus, intradiskal pressure, strength of the spinous processes, degeneration of the adjacent segment, complications, and cost-effectiveness of the ISD. The existing literature does not provide actual scientific evidence over the superiority of the ISD strategy, but most of the experimental and clinical data show a challenging potential. These considerations are applicable with different types of ISD with only few differences between the different categories. Despite-or because of-the low invasiveness of the surgical implantation of the ISD, this technique promises to play a major role in the future degenerative lumbar microsurgery. The main indications for ISD remain lumbar spinal stenoses and painful facet arthroses. A clear documented contraindication is the presence of an anterolisthesis. Nevertheless, the existing literature does not provide evidence of superiority of outcome and cost-effectiveness of the ISD strategy over laminectomy or other surgical procedures. At this time, the devices should be used in clinical randomized independent trials in order to obtain more information concerning the most advantageous optimal indication or, in selected cases, to treat tailored indications.
Two three-dimensional models of the fourth and fifth vertebrae were developed to clarify the mechanical causes of low back pain. The lumbar structures produced allowed the simulation, by the performance of finite element analysis, of different situations not normally achievable by experimentation. The simulations yielded data on the stress distribution inside the vertebrae and the amount of deformation that takes place.Models of the vertebrae were reconstructed by transferring data points to the software package I-DEAS Master Series™. The results show large stress concentrations were found in the superior and inferior facet region and on the central surfaces of the vertebral body. Higher stress concentrations were also found in the cortical shell of the vertebrae. Whilst it was observed that the cancellous core was absorbing some of the compressive loading. The study indicated that the vertebrae act similar to man-made sandwich materials, where the outer hard cortical bone has the ability to resist indentation and abrasion, while the cancellous core is tough and has the ability to absorb energy. With mechanical loading playing a central role in many low back disorders, even when there is no history of trauma, and when degeneration is evident. This present study provides a strong rationale for use of this modelling method as a research tool to assist the clinician in many ways: by indicating how to avoid overloading spinal structures, by assisting diagnosis and the development of surgical techniques.
Posterior spinal plating devices have recently made a re-emergence as both stand-alone devices and for use in conjunction with anterior fusion. Yet, the structural integrity of the posterior elements to support loads throughout the spine and the impact of plating on posterior element strength has not been well characterized. This study aims to quantify the mechanical strength of the posterior elements (spinous processes/laminae) throughout the spine and to determine the effect of attaching posterior element plating systems on their ultimate load to failure. Vertebral levels from six cadaveric spines were grouped in pairs to account for varying geometries and sizes of the human posterior elements (a total of 59 levels in 5 groups). One sample from each pair was tested in its native state, and the complementary vertebra was tested via posterior plating. Posterior element plating caused moderate reductions in posterior element failure strength (15-24 percent) throughout the cervical, thoracic, and lumbar spine. Bone mineral density of the posterior elements had the most significant impact on ultimate load to failure (a decrease of 0.1 g/cm3, yields a 189N reduction in). The modest structural impact of posterior element plating motivates continued investigation into potential use of less invasive plating devices for posterior spinal fusion.
The patient with neurogenic claudication resulting from lumbar spinal stenosis who fails to experience satisfactory relief from nonsurgical measures has limited treatment options. Lumbar epidural steroid injections and surgical laminectomy are generally accepted alternatives for the patient with moderate to severe symptoms. Interspinous process spacers, a relatively new class of technology, are proposed for use in the patient who prefers less invasive surgery or in whom medical comorbidities preclude a major surgical procedure. Early data from biomechanical and clinical studies support the short-term efficacy of interspinous process spacers in treating claudication related to spinal stenosis. Sufficient medium- and long-term data are lacking, however, particularly with respect to durability of symptomatic relief and the risk of device migration or dislocation. Although interspinous process spacers are a promising new technology, the results of longer-term clinical follow-up studies are needed to more clearly define their role in the management of lumbar spinal stenosis.
Prospective observational study.
To provide a more accurate estimate of the rate of acute spinous process fractures associated with IPS surgery.
Biomechanical cadaveric studies have suggested adequate spinous process strength to support placement of interspinous process spacers (IPS). Postoperative spinous process fractures have been reported in one%-to 5.8% of patients in previous series based on routine biplanar radiographic evaluation. However, most fractures occur between the base and midportion of the spinous process in an area that is typically difficult to visualize on plain radiographs due to device design.
All patients underwent preoperative biplanar plain radiographs and computed tomography (CT) of the lumbar spine to confirm anatomy favorable for IPS placement and rule out fracture or spondylolysis. Postoperatively, all patients underwent repeat CT imaging within six months of surgery, biplanar radiographs at two weeks, six weeks, three months, six months, and one year. All studies were reviewed independently by a neuroradiologist and two orthopedic spine surgeons.
Fifty implants (38 L4-5, 12 L3-4) were placed in 38 patients who completed follow-up and were included in final analysis. Three IPS designs were included (34 Medtronic X-STOP titanium, 8 X-STOP PEEK, 8 Lanx Aspen). Postoperative CT revealed 11 nondisplaced spinous process fractures in 11 patients (28.9% of patients, 22% of levels). Five fractures were associated with mild to moderate lumbar back pain and six fractures were asymptomatic. No patient reported a traumatic incident. No fracture was identifiable on plain radiographs. One fracture displaced during follow-up evaluation. Three patients underwent IPS removal and laminectomy. Three fractures healed by CT in one year. Overall, patients with fractures tended toward poorer outcomes by Zurich Claudication Questionnaire (ZCQ) (28.5% vs. 34.8% improvement in symptom severity, P = 0.496; 21.4% vs. 30.7% improvement in physical function, P = 0.199) and tended toward lower satisfaction rates (50% vs. 73.7%, P = 0.24) at one year compared to patients without fracture.
Interspinous process spacer surgery appears associated with a higher rate of early postoperative spinous process fracture than previously reported. In all cases, in this series, plain radiographs were inadequate to identify fractures because all fractures were initially minimal or nondisplaced, many patients were osteopenic, and the metallic wings of the devices often obscured fractures. Moreover, in most patients, fractures were associated with mild or no acute localized pain. This study suggests that unrecognized spinous process fracture may be responsible for a significant number of patients who experience unsatisfactory outcome after IPS surgery. CT imaging is required to identify the vast majority of such fractures.
This study examines the anatomic proportions of the interspinous space and the spinous processes, considering the optimal placement of an interspinous spacer.
Between January 2008 and December 2009, 565 patients undergoing computed tomography (CT) scans of the abdomen for various reasons were collected retrospectively for the study. Using the CT scan data, spinous processes of the lumbar spine L1-5 and the interspinous spaces T12-L5 were measured.
The average measured interspinous space was 9.1 ± 2.5 mm. This space became significantly (p < 0.001) smaller from anterior to posterior. Average cortical thickness of all lumbar spinous processes was 2.5 ± 0.5 mm. Cortical thickness decreased significantly (p < 0.001) from anterior to posterior. The cortex of the spinous processes from L2 (2.67 ± 0.45 mm) and L3 (2.66 ± 0.94 mm) was significantly thicker (p < 0.001) than that of the others. The spinous process of L5 had the thinnest (p < 0.001) cortex (2.10 ± 0.41 mm), as well as the smallest (p < 0.001) volume (3.0 ± 1.0 ml) and the shortest (p < 0.001) height (16.6 ± 3.6 mm).
The spinous processes of L2 and L3 are the largest and sturdiest, and that of L5 is the weakest. The L4/5 segment features the smallest interspinous space with the thinnest cortex of all lumbar spinous processes. Because the interspinous space narrows posteriorly and the cortex is thicker anteriorly, it seems that the best anatomic position for a stand alone interspinous spacers is anterior.
The DIAM (Device for Interspinous Assisted Motion) interspinous device offers the possibility of a viscoelastic supplementation of the lumbar motion segment with a small, reversible operation. It is intended as a restabilisation procedure after a discectomy to avoid facet joint overload and as a topping of procedure for the segment adjacent to a fusion. The device has been on the German market since 2004. There are several biomechanical studies available showing an effect mainly in extension and flexion, but hardly any in rotation and lateral inclination. Despite frequent clinical use, there have only been a few, mainly retrospective clinical studies indicating the success of the implant; however, sound scientific data are missing. Several prospective, randomised, controlled studies are now underway to fill that gap. Only then will it be possible to assess whether this implant is of true value to improve clinical results and to slow down the degenerative cascade or not.
In an attempt to fill a gap between simple decompression alone and fusion in the spectrum of surgical treatment for degenerative lumbar spinal stenosis (DLSS), the authors sought to demonstrate the efficacy and reproducibility of soft stabilization with interspinous artificial ligament after microsurgical fenestration to prevent post-decompression segmental instability for mildly unstable DLSS.
Clinical outcomes from 556 patients treated with soft stabilization with artificial ligament following microdecompression for mildly unstable DLSS from March 1998 to June 2006 were retrospectively obtained from three institutions in three countries. Outcomes were measured at a mean of 48.9 months after surgery using MacNab criteria. Peri- and postoperative complications and revision surgery cases were also analyzed.
Follow-up was achieved in 391 (70.3%) of the 556 patients. Clinical outcomes were excellent in 43.7%, good in 36.7%, fair in 12.2%, and poor in 7.4% of the patients. The overall clinical success rate was 80.4%. No major complications except postoperative hematoma and wound infection (6/391 cases, 1.5%) were observed, and few revision surgeries (2.3%) were done during follow-up.
We found favorable and reproducible results with soft stabilization with artificial ligament after microdecompression for mildly unstable DLSS from three institutions in three countries. Soft stabilization with artificial ligament may be concluded to represent an effective prevention of increased post-decompression instability with reliable reproducibility for mildly unstable DLSS in carefully selected patients.
The in vivo loading environment of load-bearing implants is generally largely unknown. Loads are typically approximated from cadaver tests or biomechanical calculations for the preclinical assessment of a device's safety and efficacy.
To determine the actual in vivo loading environment of an elastic interlaminar-interspinous implant (Coflex).
A retrospective radiographic study to noninvasively measure the in vivo implant loads of 176 patients.
For this study, neutral, flexion, and extension radiographs were quantitatively analyzed using validated image analysis technology. The angle between the Coflex arms was measured for each radiograph and statistically evaluated. Separately, the Coflex implant was characterized using mechanical test data and finite element analysis, which resulted in a load-deformation formula that describes the implant load as a function of its size and elastic deformation. Using the formula and the elastic implant deformation data obtained from the radiographic analysis, the exact implant load was calculated for each patient and each posture. For statistical analysis, the patients were grouped by indication and procedure, which resulted in 12 different groups. The determined loads were compared with the strength of the posterior lumbar spinal elements obtained from the literature and with the static and dynamic mechanical limits of the Coflex interlaminar-interspinous implant.
The force data were independent of implant size, diagnosis (with one exception), number of levels of the decompression procedure, number of levels of implantations (one or two), and follow-up time. The median compressive force acting on the Coflex implant was found to be 45.8 N. The maximum load change between flexion and extension was 140 N; the maximum overall load exceeded 239 N in extension.
The average loads exerted by the Coflex implant on the spinous process and lamina are 11.3% and 7.0% of their respective static failure load. The implant fatigue strength is significantly higher than the measured median force, which explains the extremely rare observation of a Coflex fatigue failure.
A prototype flexible fixation system for the lumbar spine was subjected to tensile testing to failure and cyclic tensile testing in order to determine any regions of weakness. The system consisted of a spinous process hook and two laminar hooks made of stainless steel (316L). Each laminar hook was attached to the spinous process hook by a loop of polyester braid secured by a crimped metal sleeve. In five tensile tests, the system failed by irreversible deformation of the spinous process hook at 2.5 +/- 0.3 kN (mean +/- standard deviation). In three cyclic tests, in which the applied tension varied sinusoidally between 0.04 and 0.4 kN at a frequency of 5 Hz, failure occurred after less than 400,000 loading cycles. This occurred as a result of fatigue crack initiation and propagation in the spinous process hook. A finite element model showed a stress concentration in the region where the crack occurred, which raised the applied stress above the tensile fatigue strength of this stainless steel. The spinous process hook was redesigned for manufacture in a titanium alloy (Ti-6AI-4V ELI) to minimize artefacts in magnetic resonance imaging. Further finite element models showed no unacceptable stress concentrations.
For lesions involving the anterior and/or middle column of the spine, an anterior approach is adequate for curetting the lesion and restoring spinal stability. Materials such as autogenous bone grafts, cages with bone chips, some artificial materials, or allografts are used as strut materials. Rib material is usually removed when the anterior approach is conducted for thoracic or thoracolumbar lesions. A rib itself is not rigid enough to support the load, and a bone union is not easily obtained. The purpose of this paper is to describe a method of grafting vascularized rib in folded form to fill the defects left after removal of a spinal lesion.
The rib, with the artery and vein at two levels cranial to the involved vertebral body, was isolated from surrounding tissues such as the intercostal nerve, muscles, and pleura. After curetting the lesion, the rib was folded into three or four pieces to a length adequate to fill the defect and inserted as a pedicled vascularized graft.
A total of 23 cases, including 14 men and nine women, underwent surgery in which this grafting technique was used. The pathological conditions requiring anterior decompression and fusion were spinal trauma in nine cases, spinal infection in six cases, osteoporotic fracture in seven cases, and spinal metastasis in one case. In all cases a solid bone union was obtained and all infections resolved.
With vascularized rib graft folded into three to four pieces, solid bone union can be obtained without use of any other grafted materials even in cases of infection and osteoporosis.
This study was undertaken to measure the amount of slippage of a spinous process hook (that forms part of a flexible fixation system) during flexion. Human cadaveric lumbar spines (10) were fitted with the device. A rig was designed to apply flexural displacements to a spine using a materials testing machine. Spherical markers were attached to the spine and hook. As a spine was flexed a digital video camera was used to record the positions of the markers. The movements of the markers were measured using interactive computer software to assess any slippage of the spinous process hook. During flexion the overall mean hook slippage was measured to be 0.10mm (standard deviation 0.04mm). The mean hook slippage, for each of the 10 specimens, was in the range of 0.05-0.14mm. The results imply that slippage of a spinous process hook during flexion is small.
Wire sutures, cerclage constructs, and tension bands have been used for many years in orthopedic surgery. Spinous process and sublaminar wires and other strands or cables are used in the spine to re-establish stability of the posterior spinal ligament complex. Rigid monofilament wires often fail due to weakening created during twisting or wrapping. Stronger metal cables do not conform well to bony surfaces. Polyethylene cables have higher fatigue strength than metal cables. The Loop cable is a pliable, radiolucent, polyethylene braid. Creep of the Loop/locking clip construct is similar to metal cable constructs using crimps. Both systems have less creep than knotted polyethylene cable constructs.
Biomechanical studies of the thoracic spine often scan cadaveric segments by dual energy X-ray absorptiometry (DXA) to obtain measures of bone mass. Only one study has reported the accuracy of lateral scans of thoracic vertebral bodies. The accuracy of DXA scans of thoracic spine segments and of anterior-posterior (AP) thoracic scans has not been investigated. We have examined the accuracy of AP and lateral thoracic DXA scans by comparison with ash weight, the gold-standard for measuring bone mineral content (BMC). We have also compared three methods of estimating volumetric bone mineral density (vBMD) with a novel standard-ash weight (g)/bone volume (cm3) as measured by computed tomography (CT). Twelve T5-T8 spine segments were scanned with DXA (AP and lateral) and CT. The T6 vertebrae were excised, the posterior elements removed and then the vertebral bodies were ashed in a muffle furnace. We proposed a new method of estimating vBMD and compared it with two previously published methods. BMC values from lateral DXA scans displayed the strongest correlation with ash weight (r=0.99) and were on average 12.8% higher (p<0.001). As expected, BMC (AP or lateral) was more strongly correlated with ash weight than areal bone mineral density (aBMD; AP: r=0.54, or lateral: r=0.71) or estimated vBMD. Estimates of vBMD with either of the three methods were strongly and similarly correlated with volumetric BMD calculated by dividing ash weight by CT-derived volume. These data suggest that readily available DXA scanning is an appropriate surrogate measure for thoracic spine bone mineral and that the lateral scan might be the scan method of choice.
A growing number of interspinous process devices have been introduced to the lumbar spine implant market. Implant designs vary from static spacers to dynamized devices. Furthermore, they are composed of a range of different materials including bone allograft, titanium, polyetheretherketone, and elastomeric compounds. The common link between them is the mechanical goal of distracting the spinous processes to affect the intervertebral relationship. In contrast, the purported clinical goals are more variable, ranging from treatment of degenerative spinal stenosis, discogenic low back pain, facet syndrome, disk herniations, and instability. Though some clinical data exist for some of these devices, defining the indications for these minimally invasive procedures will be crucial. Indications should emerge from thoughtful consideration of data from randomized controlled studies.
The results of the first 50 consecutive patients using the Graf stabilisation system are presented. The average age of the patients was 41 years; there were 32 women and 18 men in the group. All patients suffered from intractable symptomatic degenerative disc disease which could be localised to one or more levels. All patients gave a history of chronic back pain, but the mean period of severe disability was 24 months. The mean preoperative disability score (Oswestry questionnaire) was 59%. The average period of follow-up was 24 months (range 19-36 months). At the latest review, the mean disability score was 31%. The clinical results were classified as "excellent" or "good" in 72% of patients, "fair" in 10%, "the same" in 16% and "worse" in 2%. All but three patients felt that surgery was worthwhile. The results have not deteriorated over the period of follow-up.
The complications of 648 consecutively inserted Universal AO pedicle screws (140 in the thoracic spine and 508 in the lumbar spine) performed by one surgical team to treat 91 patients with spinal problems, were reviewed. The spinal pathology consisted of: scoliosis (34 patients), degenerative lower lumbar spinal disease (25 patients), neoplastic spinal disease (11 patients), thoracic kyphosis (8 patients), spinal fractures (7 patients), lumbo-sacral spondylolisthesis (3 patients), and osteomyelitis (3 patients). Intraoperative complications were: screw misplacement (n = 3), nerve root impingement (n = 1), cerebrospinal fluid leak (n = 2) and pedicle fracture (n = 2). Postoperative complications were; deep wound infection (n = 4), screw loosening (n = 2) and rod-screw disconnection (n = 1). The conclusion was that pedicle screw fixation has an acceptable complication rate and neurological injury during this procedure is unlikely.
The authors present preliminary results in a series of 27 patients with a lumbar and/or lumbosacral facet syndrome operated on by use of the soft-system-Stabilization (SSS) according to Graf. Attention is paid to the correct selection of patients for this surgical technique. Excellent, good, satisfactory, moderate, and poor results were obtained in 19 (70%), 2 (7.5%), 5 (18.5%), 1 (3.5%), and 0 instances, respectively.
The authors think that a Graf-procedure may be indicated in young to middle-aged patients with a lumbar and/or lumbosacral facet syndrome with 1.) no arthrotic changes of the facet joints, 2.) a still intact disc and/or only mild loss of intervertebral distance, 3.) well trained low-back muscles, and 4.) a clear-cut pain relief on test-anaesthesia of articular nerves and trial immobilization in a plastic jacket.
The bone mineral content of 34 lumbar vertebrae obtained from ten cadavers (three men, seven women; age 61-88 years) was measured using a pulsed source dual-energy X-ray absorptiometry (DEXA) apparatus. Scanning was performed in the frontal projection and was repeated on the vertebral bodies obtained after removal of the posterior elements of the vertebrae. Subsequently a nondestructive neutron activation analysis (NAA) was performed. The mineral content of the vertebral bodies was found to represent (mean, SEM) 53.0% (1.9%) of the content of the whole vertebrae. The mineral content of the vertebral bodies assessed with NAA (BMC NAA) and with DEXA (BMC DEXA) showed a high correlation: BMC NAAA = (1.016 x BMC DEXA) + 0.990 r = 0.949 (p less than 0.001). We conclude that the mineral content of lumbar vertebral bodies can be accurately measured in vitro in a water environment by DEXA, and that the mean contribution of the posterior elements of the vertebra to the calcium hydroxyapatite content of whole vertebrae measured in the frontal projection is as high as 47.0%.
Posteriorly directed load to failure testing of four different types of spinal implants was performed in individual T5 to S1 vertebra harvested from seven fresh-frozen human cadaveric spines. The implants tested were: 1) Drummond spinous process wires, 2) Harrington laminar hooks, 3) Cotrel-Dubousset transpedicular screws, and 4) Steffee VSP transpedicular screws. The ultimate failure of each implant was compared with the bone mineral density of each vertebra to determine which implants, if any, were particularly advantageous in osteoporotic vertebrae. Before biomechanical testing, the spines were analyzed in vitro by dual photon absorptiometry to determine the bone mineral densities (gm/cm2) of each vertebra. The mean tensile loads to failure for each of the implants tested were as follows: Cotrel-Dubousset transpedicular screws: 345 Newtons; spinous process wire/button: 382 Newtons; Steffee transpedicular screws: 430 Newtons; and laminar hooks: 646 Newtons. The difference between the loads to failure for laminar hooks and the other implants was significant (P less than 0.05) using one-way analysis of variance. The overall correlation coefficient for bone mineral density with ultimate load to failure was 0.30 (P less than 0.001). The correlation coefficients were 0.47 (P less than 0.001) for spinous process wires alone; 0.096 (not significant) for laminar hooks alone; 0.37 (P less than 0.001) for Cotrel-Dubousset pedicle screws; and 0.48 (P less than 0.001) for Steffee pedicle screws. Of the four different implants tested, laminar hooks were most resistant to failure from posteriorly directed forces.(ABSTRACT TRUNCATED AT 250 WORDS)
The authors present preliminary results in a series of 27 patients with a lumbar and/or lumbosacral facet syndrome operated on by use of the soft-system-Stabilization (SSS) according to Graf. Attention is paid to the correct selection of patients for this surgical technique. Excellent, good, satisfactory, moderate, and poor results were obtained in 19 (70%), 2 (7.5%), 5 (18.5%), 1 (3.5%), and 0 instances, respectively. The authors think that a Graf-procedure may be indicated in young to middle-aged patients with a lumbar and/or lumbosacral facet syndrome with 1.) no arthrotic changes of the facet joints, 2.) a still intact disc and/or only mild loss of intervertebral distance, 3.) well trained low-back muscles, and 4.) a clear-cut pain relief on test-anaesthesia of articular nerves and trial immobilization in a plastic jacket.
New developments in dual x-ray absorptiometry (DXA) allow the performance of high precision anteroposterior (AP) and lateral scans of spinal bone mineral density (BMD, units: g/cm2) without the patient moving from the supine position. Data from both projections may be combined to give an estimate of the true volumetric bone mineral density (VBMD, units: g/cm3) of the lumbar vertebral bodies. This report presents a cadaver study designed to validate DXA measurements of volumetric bone density. Sections of whole lumbar spine were scanned in AP and lateral projections in a water tank to simulate soft tissue. Individual vertebrae were then divided to separate the vertebral body from the neural arch, and vertebral body volume was measured using the displacement of sand. The bone mineral content (BMC) of vertebral bodies and neural arches was measured by ashing at 250 degrees C for 60 hours followed by 500 degrees C for a further 24 hours. The results showed that DXA scanning systematically underestimated ashing data by 14% for AP BMC, 33% for vertebral body BMC, 23% for vertebral body volume, and 12% for VBMD. Despite these significant systematic errors, the DXA measurements and ashing values were highly correlated (r = 0.979-0.992). The results suggested that after allowing for the systematic errors, lateral DXA parameters related closely to true BMC, volume, and VBMD.
Fractures of the thoracic spine account for a large portion of vertebral fractures in the elderly, yet noninvasive measurements of bone mineral properties are limited to the L2-L4 vertebral bodies. The purpose of this investigation was to determine whether bone mineral properties of the lumbar spine correlate with the failure properties of thoracic vertebrae. Cadaveric lumbar segments were scanned using dual-energy x-ray absorptiometry (DXA) from both the lateral and anteroposterior projections. Three-body segments L1-L3 and T10-T12 were then compressed to create crush fractures in the L2 and T11 vertebral bodies, and linear correlation analyses were performed to compare each DXA measure with the failure properties of L2 and T11. Lumbar BMD from the lateral view correlated significantly with T11 ultimate load (r = 0.94, P < 0.001), as did lumbar BMD from the anteroposterior projection (r = 0.83, P = 0.001). Significant correlations were also found between both lumbar BMD and BMC and the stiffness and energy to failure of T11. Furthermore, BMD and BMC measured at L2 correlated significantly with L2 ultimate load, stiffness, and energy to failure. We conclude that bone mineral properties measured at the lumbar spine provide a valid assessment of the compressive strength of both thoracic and lumbar vertebrae. Lumbar BMD may therefore be used to derive an index for the prediction of thoracolumbar fractures to aid in the early intervention of vertebral fractures.
The authors present their diagnostic and therapeutic protocol as well as the surgical outcome in a series of 119 patients with the lumbar facet syndrome.
By use of different surgical techniques such as translaminar screw fixation (n=56), Louis plate fixation (n=36), Cotrel-Dubousset instrumentation (n=11) and soft system stabilization according to Graf (n=14) excellent, good, satisfactory, moderate and poor results were obtained in 78 (67%), 20 (17%), 14 (12%), 4 (3%), and 1 (1%) instances, respectively.
The authors report their clinical experience with Graf's supple intervertebral stabilization system. The results are discussed with respect to the biomechanical and clinical definitions of vertebral instability and the natural evolution of degenerative disease of the lumbar spine. The result are favorable for the cases with clinical instability, but are not as good for the degenerative cases or after discectomy. In spite of the short-term clinical benefits for the cases with pathological articular hypermobility, the authors are cautious regarding the use of this technique. They emphasize the unknown long-term clinical and biomechanical evolution and the difficulties encountered for secondary arthrodesis caused by the volume of the pedicular screws.
This was a retrospective review of one surgeon's results using three different lumbosacral arthrodesis techniques: Group 1, no instrumentation; Group 2, Luque Rod and sublaminar wire technique; and Group 3, AO intrapedicular screw and plate technique.
To determine whether the use of metal implants results in a higher fusion rate. Once a solid arthrodesis is achieved, is this correlated with a good clinical result?
Controversy persists regarding the value of the use of intrapedicular fixation to augment arthrodesis of the lumbosacral junction. Controversy also exists regarding the correlation of solid arthrodesis with relief of preoperative symptoms.
Three serial sequential populations (50 subjects each) undergoing varied primary multiple-level lumbosacral arthrodesis procedures were studied retrospectively. The ultimate clinical results of these three different surgical populations were studied after prolonged follow-up.
Group one had a 14% fusion rate and a 4% complication rate. Group two had a 36% fusion rate and an 8% complication rate. Group three had a 64% fusion rate and an 18% complication rate. Complications were intraoperative dural tears and nerve root injuries. Patient satisfaction with each operative procedure to relieve preoperative low back pain was statistically correlated with whether a solid arthrodesis was obtained.
Intrapedicular fixation technique is the most reliable method for obtaining a solid multiple-level lumbosacral arthrodesis. Solid arthrodesis is correlated with a successful clinical result. Complications associated with the use of intrapedicular fixation were frequent but their occurrence demonstrated a "learning curve pattern."
Reference ranges used in dual energy X-ray absorptiometry (DXA) have previously used piecewise linear fits to the whole data set for spine or femur bone mineral density (BMD) as a function of age. In a study of 329 Caucasian normal women we present a refinement to the normal range by fitting straight lines between quinquennial mean values of BMD for each site measured (lumbar spine, femoral neck and Ward's triangle). From the age of 40 years onwards the premenopausal women demonstrated minimal loss of BMD whereas postmenopausal women showed a rapid loss amounting to 27% in the lumbar spine, 27% in the femoral neck and 38% in the Ward's triangle region in the age range under examination. Comparison of quinquennial means for pre and postmenopausal women in age bands 45-49 years and 50-54 years shows that at these ages postmenopausal BMD is significantly lower than premenopausal BMD (P < 0.05). This finding suggests that separate normal ranges should be used for pre and postmenopausal women. As reduction in the production of oestrogen is a major factor in postmenopausal bone loss and oestrogen function is related to years since menopause (YSM), a more logical way of displaying postmenopausal normal BMD ranges would be in terms of YSM rather than chronological age. Such data are given in this paper.
The bone density of thoracolumbar vertebral columns (T1 to L5) from 18 individuals was measured using quantitative computed tomography and dual energy x-ray absorptiometry. Three hundred six isolated vertebral bodies were tested in a materials test device to determine their compressive strength. Between T1 and L5 the mean segmental increase in bone mineral content was 0.3 g, while the corresponding mean decrease in trabecular density was 4.7 HU. Midvertebral body cross-sectional area increased by an average of 46 mm2 per segment and the mean segmental increase in compressive strength was 0.17 kN. Compressive strength was significantly correlated with bone mineral density measured with dual energy x-ray absorptiometry (r = 0.86). Vertebral trabecular density samples measured with quantitative computed tomography were poorly correlated with compressive strength (r = 0.28); however, this was improved when the trabecular density was multiplied by the midvertebral body cross-sectional area (r = 0.83). This study provides information concerning the relationships between density and mechanical properties of all thoracic and lumbar vertebral bodies across a wide age range. While the load-bearing capacity of the vertebral bodies is largely dependent on their geometry and bone density, this relationship has been only extensively tested for the lumbar spine. This study extends these observations over the lumbar and thoracic regions to provide a comprehensive analysis of the strength characteristics of each vertebral body. This is particularly important given the paucity of data on the thoracic spine where age-related vertebral fractures predominate. These data provide a basis for the development of models to predict the potential for thoracolumbar fractures in the elderly vertebral column.
This study determined the predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, pedicular geometry, and mechanical testing in assessing the strength of pedicle screw fixation in an in vitro mechanical test of intra-pedicular screw fixation in the human cadaveric lumbar spine.
To test several hypotheses regarding the relative predictive value of densitometry, pedicular geometry, and mechanical testing in describing pedicle screw pull-out.
Previous investigations have suggested that mechanical testing, geometry, and densitometry, determined by quantitative computed tomography or dual energy x-ray absorptiometry, predict the strength of the screw-bone system. However, no study has compared the relative predictive value of these techniques.
Forty-nine pedicle screw cyclic-combined flexion-extension moment-axial pull-out tests were performed on human cadaveric lumbar vertebrae. The predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, insertional torque, in situ stiffness, and pedicular geometry was assessed using multiple regression.
Several variables correlated to force at failure. However, multiple regression analysis showed that bone mineral density of the pedicle determined by quantitative computed tomography, insertional torque, and in situ stiffness when used in combination resulted in the strongest prediction of pull-out force. No other measures provided additional predictive ability in the presence of these measures.
Pedicle density determined by quantitative computed tomography when used with insertional torque and in situ stiffness provides the strongest predictive ability of screw pull-out. Geometric measures of the pedicle and density determined by dual energy x-ray absorptiometry do not provide additional predictive ability in the presence of these measures.
Internal fixation techniques are in common used to stabilize vertebral fractures and correct severe scoliosis. Consolidation of injured vertebrae with neighbouring intact vertebra is the goal in the former case whilst fusion of the vertebrae in a corrected position is aimed at in the latter case. Degenerative spine diseases are not considered in this paper. Classical instrumentation consists of rods (e.g., Cotrel-Dubousset, Harrington, Luque-Galveston) attached to the bone by means of hooks or wires. More recently, transpedicular screws are introduced as an alternative bone/implant interface. Comparing the results of several studies, the posterior pedicle screw based devices demonstrate the ability to produce the most rigid constructs. However, the insertion of pedicle screws implicates a relatively high complication risk and its success strongly depends on the experience of the surgeon. Incorrect drilled holes or malplacement of the screws can result in nerve root injuries and fracture of the pedicle. Studies reported complication ratios up to 30% with substantial neurological implications. A certain degree of automation of the critical actions may be necessary to enhance the safety of pedicle screw insertion. Two techniques of computer assisted spine surgery are compared. Both techniques permit a computer assisted surgical planning based on CT images. During operation the first system permanently observes the position of the drill relative to the spine and informs the surgeon on the deviation of the actual drill path to the planned drill path. The second system uses a pre-operative surgical planning to design and construct a mechanical drill guide, fitting perfectly on the patient's spine.
Lumbar motion segments were tested in vitro to examine biomechanical changes after posterior fixation by a flexible device.
To assess changes in load distribution and conformation of vertebral structures after a flexible stabilization. This should provide the foundations for a scientific understanding of the immediate effects of this surgical procedure.
Hooks were placed over the proximal spinous process and the distal laminas of a motion segment and connected by a polyester braid. Tension applied to the braid then generated a compression of the posterior elements. The force between the articular facets, the displacement of the posterior anulus fibrosus of the intervertebral disc, and the change in the relative position of the adjacent vertebrae were measured as the applied tension was increased.
Facet joint force, disc bulge, and vertebral angulation increased with applied tension until a position of "locking" was achieved, apparently when the bony margin of the superior half of the facet joint contacted the inferior pars interarticularis. A tension of between 50 to 100 N in the braid was required for this. Facet joint force was less than 40% of this, and disc bulge was only 0.15 mm. The extension of the motion segment was between 2 degrees and 8 degrees.
The results suggest that if such a system is applied surgically, stabilization is produced by compaction of the bony margins of the facet joints. Only a relatively small proportion of the posteriorly applied load is carried by the facet joints themselves, and little angulatory change is expected with minimal disc bulge.
The estimation of vertebral fracture risk in individuals with suspected osteopenia is commonly based on measurements of lumbar spine bone density. The efficacy of vertebral size and deformity, as assessed by vertebral morphometry, in the prediction of fractures has been less studied. In an ex vivo investigation the regional relationships between vertebral size, vertebral deformity, bone density and compressive strength throughout the thoracolumbar spine were examined. In 16 vertebral columns (T1-L5) the bone mineral content (BMC) and bone mineral density (BMD) of each segment were measured using lateral projection dual-energy X-ray absorptiometry, and the vertebral cancellous density (VCD) and mid-vertebral cross-sectional area (CSA) measured using quantitative computed tomography. Vertebral body heights were determined from mid-sagittal CT scans, and vertical height ratios calculated for each segment. The failure load and failure stress of the isolated vertebral bodies were determined using a material testing device. Separate analyses were performed for the upper (T1-4), middle (T5-8) and lower (T9-12) thoracic, and lumbar (L1-5) segments. In all regions, failure load was strongly correlated with BMD (r = 0.82-0.86), moderately correlated with VCD (r = 0.60-0.71) and vertebral height (r = 0.22-0.49), and poorly correlated with the height ratios (r = 0.04-0.33). Failure stress was best predicted by BMD (r = 0.73-0.78) and VCD (r = 0.70-0.78) but was poorly correlated with all morphometric variables (r = 0.01-0.33). The segmental correlations between BMD and VCD ranged form r = 0.49 to r = 0.79. For all regions, BMD and VCD were included in the stepwise regression models for predicting failure load and failure stress. Either the mid-vertebral height or CSA were included in all the failure load models, while mid-vertebral height was included in only one of the failure stress models. The results suggest that vertebral deformity and size (as assessed by vertebral morphometry) make only a minor contribution to the prediction of vertebral strength additional to that provided by bone densitometry alone. The consistent regional relationships between variables appear to support the practice of global fracture risk assessment based on lumbar spine densitometry.
Spinal bone mineral density (BMD) measurements and calcaneal ultrasound were compared in terms of their ability to predict the strength of the third lumbar vertebral body using specimens from 62 adult cadavers (28 females, 34 males). BMD was measured using dual X-ray absorptiometry (DXA) in both vertebra and calcaneus. Quantitative computed tomography (QCT) was used to determine trabecular BMD, cortical BMD, cortical area, and total cross-sectional area (CSA) of the vertebral body. Bone velocity (BV) and broadband ultrasonic attenuation (BUA) were measured in the right calcaneus. Vertebral strength was determined by uniaxial compressive testing. Vertebral ultimate load was best correlated with DXA-determined vertebral BMD (r2 = 0.64). Of the QCT parameters, the best correlation with strength was obtained using the product of trabecular BMD and CSA (r2 = 0.61). For vertebral ultimate stress, however, the best correlation was observed with QCT-measured trabecular BMD (r2 = 0.51); the correlation with DXA-determined BMD was slightly poorer (r2 = 0.44). Calcaneal ultrasound correlated only weakly with both ultimate load and stress with correlation coefficients (r2) of 0.10-0.17, as did calcaneal BMD (r2 = 0.18). Both spinal DXA and spinal QCT were significantly (p < 0.001) better predictors of L3 ultimate load and stress than were either calcaneal ultrasound or calcaneal DXA. Multiple regression analysis revealed that calcaneal ultrasound did not significantly improve the predictive ability of either DXA or QCT for L3 ultimate load or stress. Calcaneal DXA BMD, bone velocity, and BUA correlated well with each other (r2 = 0.67-0.76), but were only modestly correlated with the DXA and QCT measurements of the vertebra. These data indicate that spinal DXA and spinal QCT provide comparable prediction of vertebral strength, but that a substantial proportion (typically 40%) of the variability in vertebral strength is unaccounted for by BMD measurements. Ultrasonic measurements at the calcaneus are poor predictors of vertebral strength in vitro, and ultrasound does not add predictive information independently of BMD. These findings contrast with emerging clinical data, suggesting that calcaneal ultrasound may be a valuable predictor of vertebral fracture risk in vivo. A possible explanation for this apparent discrepancy between in vivo and in vitro findings could be that current clinical ultrasound measurements at the calcaneus reflect factors that are related to fracture risk but not associated with bone fragility.
The strength of bone is determined not only by bone density but also by structure. Therefore, quantification of the structure in radiographs by texture parameters may result in a better prediction of fracture risk. Since in radiographs density and structure are strongly correlated, the predictive power of texture parameters should be corrected for the influence of BMD to determine the additional information conveyed by these parameters. In this study, we evaluated the predictive power of various texture parameters based on the Grey-Level Dependence Method and the Morphological Gradient Method. This study was performed on 67 vertebrae obtained from 20 male and 12 female human cadaver thoracolumbar spines. BMD and area of the vertebral body were determined from QCT images and texture parameters were derived from direct magnification (DIMA) radiographs. The fracture force, measured under conditions simulating the in vivo situation, was corrected with the area of the vertebra to yield the fracture stress (FS). Results of the study indicate that BMD correlates significantly with FS r = 0.82 (P < 0. 001, n = 24) and r = 0.94 (P < 0.001, n = 43) for female and male vertebrae, respectively. Correlation coefficients of the investigated texture parameters were as high as 0.80 (P < 0.001) and 0.67 (P < 0.001) for the female and male vertebrae, respectively. Multiple regression analysis showed that in female vertebrae, the addition of one texture parameter to BMD results in a better prediction of strength. The multiple correlation coefficient was 0. 87 (P < 0.001) in this case. In male vertebrae, BMD was the best predictor of fracture stress. These results suggest that texture parameters, as measured in magnification radiographs, can predict bone strength. Whereas in all cases BMD is the best single predictor of bone strength, for women texture parameters contain useful additional information.
Measurements were made on transverse-plan, computed tomography scans from three different patient groups.
To describe the correlation between two previously described pedicle screw entry points to the pedicle axis and the predicted frequency of pedicle breakthrough from the use of a 6.5-mm screw placed parallel to the pedicle axis.
Fluoroscopic assistance improves the accuracy of pedicle screw placement. Whether this is a result of improved accuracy of the starting point or correct directional guidance is unclear. No morphologic studies have been done to assess the accuracy of previously described entry points.
Computerized digitizing and mathematic superimposition of the images from computed tomography scans of the low lumbar spine were used to quantify facet and pedicle anatomy and the correlation between two previously described entry points and the pedicle axis.
The two previously described entry points are significantly medial to the pedicle axis. They are most medial at the L5 pedicle in patients with L4 degenerative spondylolisthesis.
The two previously described entry points for pedicle screws in the low lumbar spine that were studied here are not reliable and tend to direct screw placement medial to the pedicle axis enough to lead to a substantial frequency of pedicle breakthrough for screws parallel to this axis. Surgeons implanting screws should take this tendency into account and use alternative methods to obtain accurate entry to the pedicle.
Bone mineral density and bone cross-sectional area of human cadaveric vertebral bodies were investigated radiologically and histologically, respectively. After ventral instrumentation with ventral derotation spondylodesis screws, axial pullout force was measured and compared with radiologic and histologic data.
To elucidate how well ventral derotation spondylodesis screw fixation strength can be estimated before surgery by specified applications of dual-energy x-ray absorptiometry, quantitative computed tomography, T2*-relaxation time in magnetic resonance imaging, and histomorphometry.
It is postulated that bone quality plays a crucial role in initial strength of the instrumented spine. Bone quality is even more important in anterior fixation because of the prevalence of spongy bone in the vertebral body.
Bone mineral density of human cadaveric lumbar-vertebral bodies was assessed by dual-energy x-ray absorptiometry and quantitative computed tomography (cancellous and cortical bone separately). Cancellous bone was also characterized by T2*-relaxation time, measured by magnetic resonance imaging and histomorphometric study. Vertebral bodies were instrumented ventrally with VDS screws, and screw axial pullout force was measured and correlated with each of the nonmechanical measures. Patients with manifest osteoporosis, osteomalacia and tumors were excluded. For statistical analysis, the Mann-Whitney rank sum test was used with a significance value of P < 0.05.
The highest correlation with pullout force was for density of cancellous bone determined by quantitative computed tomography (r = 0.72; P < 0.001), immediately followed that determined by dual-energy x-ray absorptiometry (r = 0.70; P < 0.001). Results of measurement of T2*-relaxation time and those of histomorphometric study correlated moderately (r = 0.55; r = 0.50), whereas cortical bone density determined by quantitative computed tomography showed negligible correlation (r = 0.2).
The absorptiometric techniques, quantitative computed tomographic scan of cancellous bone and dual-energy x-ray absorptiometric study, provide more accurate readings for preoperative estimation of initial VDS screw fixation strength than do the other methods studied.