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A finite element study of the initiation of failure of fixation of cemented total hip replacements
Abstract
In order to study initial mechanisms of failure in cemented femoral total hip components, an anatomically accurate three-dimensional linear finite element model was constructed and verified against experimental strain measurements in the cement mantle. Good agreement was found between predicted and measured strains. The likelihood of failure initiation due to cement-prosthesis debonding and crack initiation at voids was studied for loading conditions simulating both one-legged stance and stair climbing. The "out of plane" forces involved in stair climbing appear to be the greatest threat to the fixation of total hip replacements. In stair climbing, cement-prosthesis debonding and pore crack initiation were probable in the proximal anteromedial region of the cement mantle, and near the distal tip of the implant. The proximal stresses in stair climbing were higher than the distal stresses in either stair climbing or one-legged stance.
... Finally, the in vivo loading situation of acrylic bone cement involves complex multiaxial stresses; however, there is no data in the literature to describe the fatigue behaviour of acrylic bone cement under these stresses. Stresses predicted in the cement mantle have been shown to be below the fatigue limit of bone cement by a factor of three (Prendergast et al., 1989, Harrigan et al., 1992, although a number of retrieval studies implicate fatigue failure as the predominant cause of failure in the in vivo situation (Jasty et al., 1991, Topoleski et al., 1990. The question therefore arises, "Do multiaxial stresses significantly influence the fatigue life of acrylic bone cement?" ...
... Stresses within the cement in the in vivo loading situation have been predicted to be low, i.e. one third of the fatigue strength of bone cement (Prendergast et al., 1989, Harrigan et al., 1992. Nonetheless failure occurs. ...
... Bone cement is subjected to multiaxial loading conditions in vivo (Prendergast et al., 1989, Harrigan et al., 1992. Failure due to multiaxial loading has been observed in the form of secondary cracks propagating in the plane perpendicular to the main crack (Topoleski et al., 1990). ...
... Although the tension applied to the cables was sufficient to deform the filaments at the tension edge, the cement at the compression edge was not damaged. The minor degree of debonding seen on SEM at the tension edge of two of the cables raises the concern that such defects produced at the time of tensioning could act as a starting point for crack propagation (6,8). However in the current experiment we discovered only one crack adjacent to a wire, and none associated with the cables. ...
... -Miles cables passing through Palacos cement does not appear to damage the mantle. (Hip International 2003; 13: 29-31) KEY WORDS: Arthroplasty, Bone cement, Bone wires, Microscopy, Electron scanning sociated with crack propagation and loosening(1,6,8), and so we conducted a study to investigate their effects. ...
Dall-Miles cables are widely used for trochanteric re-attachment in hip arthroplasty, but their effects on the cement mantle have not been reported. We have carried out an ex vivo study to investigate the influence of cables on the mantle. Charnley femoral components were implanted in eight proximal human femora using Palacos cement. Wires (control group, n=4) or 2mm stainless steel Dall-Miles cables (n=4) were passed through holes drilled in the proximal femur before the cement was introduced. The wires or cables were tightened using the appropriate instruments after the cement had hardened. The specimens were sectioned using a diamond saw, and examined by scanning electron microscopy and light microscopy before and after staining with penetrant dye. Deformation of the strands at the tension edge of each cable, with debonding from the cement in two specimens, was observed. There was no damage to cement at the compression edge. Tensioning of Dall-Miles cables passing through Palacos cement does not appear to damage the mantle.
... Several investigators [2][3][4][5][6][7][8] used FE modelling to analyze the human body. Agsari et al [2] compared the FE modelling of a generic stem-less hip implant with conventional implants and showed that the stem-less implant had lesser deviation than the stemmed ones. ...
... Rohlmann et al [3] used FE modelling technique to determine the stress distribution in a human femur with an endoprosthesis. Harrigan et al [4] observed that the proximal stresses in stair climbing are higher than the distal stresses in either stair climbing or one-legged stance by using a FE model of hip. Stolk et al [5] studied the composite hip reconstructions with two different implants using FE simulations and have validated the FE model by the experimental bone and cement strains. ...
Motorcycle riders are subjected to hand arm and whole body vibrations. Both these vibrations affect the health of the rider. This paper attempts to analyze the effects of these vibrations on heart using finite element simulation and experiments. Firstly, the hand arm and whole body vibration forces are measured by conducting experiments on different motorcycles and for various road conditions. These measured forces are applied to the finite element model of a motorcycle rider to predict the distribution of Von mises stress around heart. Secondly, the average relative heart rate, the hand arm and whole body vibration accelerations are measured with by conducting experiments on different riders with typical body mass index and for various road conditions. The variations in the vibrations magnitude and the Von mises stress around heart correlate well. The Von mises stress and heart rate are more sensitive to the hand arm vibrations than the whole body vibrations. This effect on the heart may cause a cardiac problem in the long run. © 2010. MechAero Foundation for Technical Research & Education Excellence.
... All sections were assigned isotropic material properties with an elastic modulus of 16.7 GPa for cortical bone [15], 0.155 GPa for cancellous bone [16], 2.8 GPa for polymethylmethacrylate cement [17], 195 GPa for Orthinox stainless steel (Stryker, Kalamazoo, MI) [18], and 210 GPa for Co-Cr [19]. A Poisson's ratio of 0.3 was used for all materials [18]. ...
Background:
Total hip arthroplasty with femoral shortening is frequently recommended for patients with high hip dislocation. However, the possibility of postoperative rotational deviation of the stem presents a challenge for surgeons. We aimed to determine the optimal position for osteotomy in total hip arthroplasty under full weight-bearing and turning torque by using finite element analysis.
Methods:
Four models of femoral osteotomy with 30-mm transverse shortening at 30% (model 30), 40% (model 40), 50% (model 50), and 60% (model 60) from the proximal end of the full length of the Exeter stem were constructed. Using finite element analysis, the constructs were first analyzed under an axial load of 1500 N and then with an added torsional load of 10°.
Results:
The analyses under torsional loading conditions revealed that the maximum von Mises stress on the stem in each model occurred at the proximal end of the distal fragment and the distal side of the stem. The maximum stress values at the stem were 819 MPa (model 30), 825 MPa (model 40), 916 MPa (model 50), and 944 MPa (model 60). The maximum stress values at the osteotomy site of the medullary cavity side of the distal bone fragment were 761 MPa (model 30), 165 MPa (model 40), 187 MPa (model 50), and 414 MPa (model 60).
Conclusions:
The osteotomy level should be around the proximal 40% of the full length of the Exeter stem, which is most suitable for rotation stability in the early postoperative period.
... All sections were assigned isotropic material properties with an elastic modulus of 16.3 GPa for cortical bone [23], 0.15 GPa for cancellous bone [24], 2.8 GPa for polymethylmethacrylate (PMMA) cement [25], 195 GPa for Orthinox stainless steel [26], and 110 GPa for Titanium [27]. A Poisson's ratio of 0.3 was used for all materials [26]. ...
Background
Internal fixation is recommended for treating Vancouver B1 periprosthetic femoral fractures. Although several fixation procedures have been developed with high fixation stability and union rates, long-term weight-bearing constructs are still lacking. Therefore, the aim of the present study was to evaluate the stability of a double-plate procedure using reversed contralateral locking compression-distal femoral plates for fixation of Vancouver B1 periprosthetic femoral fractures under full weight-bearing.
Methods
Single- and double-plate fixation procedures for locking compression-distal femoral plates were analysed under an axial load of 1,500 N by finite element analysis and biomechanical loading tests. A vertical loading test was performed to the prosthetic head, and the displacements and strains were calculated based on load-displacement and load-strain curves generated by the static compression tests.
Results
The finite element analysis revealed that double-plate fixation significantly reduced stress concentration at the lateral plate place on the fracture site. Under full weight-bearing, the maximum von Mises stress in the lateral plate was 268 MPa. On the other hand, the maximum stress in the single-plating method occurred at the defect level of the femur with a maximum stress value of 1,303 MPa. The principal strains of single- and double-plate fixation were 0.63 % and 0.058 %, respectively. Consistently, in the axial loading test, the strain values at a 1,500 N loading of the single- and double-plate fixation methods were 1,274.60 ± 11.53 and 317.33 ± 8.03 (× 10 − 6 ), respectively.
Conclusions
The present study suggests that dual-plate fixation with reversed locking compression-distal femoral plates may be an excellent treatment procedure for patients with Vancouver B1 fractures, allowing for full weight-bearing in the early postoperative period.
... All sections were assigned isotropic material properties with an elastic modulus of 16.3 GPa for cortical bone [23], 0.15 GPa for cancellous bone [24], 2.8 GPa for polymethylmethacrylate (PMMA) cement [25], 195 GPa for Orthinox stainless steel [26], and 110 GPa for Titanium [27]. A Poisson's ratio of 0.3 was used for all materials [26]. ...
Background: Internal fixation is recommended for treating Vancouver B1 periprosthetic femoral fractures. Although several fixation procedures have been developed with high fixation stability and union rates, long-term weight-bearing constructs are still lacking. Therefore, the aim of the present study was to evaluate the stability of a double-plate procedure using reversed contralateral locking compression-distal femoral plates for fixation of Vancouver B1 periprosthetic femoral fractures under full weight-bearing.
Methods: Single- and double-plate fixation procedures for locking compression-distal femoral plates were analysed under an axial load of 1,500 N by finite element analysis and biomechanical loading tests. A vertical loading test was performed to the prosthetic head, and the displacements and strains were calculated based on load-displacement and load-strain curves generated by the static compression tests.
Results: The finite element analysis revealed that double-plate fixation significantly reduced stress concentration at the lateral plate place on the fracture site. Under full weight-bearing, the maximum von Mises stress in the lateral plate was 268 MPa. On the other hand, the maximum stress in the single-plating method occurred at the defect level of the femur with a maximum stress value of 1,303 MPa. The principal strains of single- and double-plate fixation were 0.63% and 0.058%, respectively. Consistently, in the axial loading test, the strain values at a 1,500 N loading of the single- and double-plate fixation methods were 1,274.60 ± 11.53 and 317.33 ± 8.03 (× 10⁻⁶), respectively.
Conclusions: The present study suggests that dual-plate fixation with reversed locking compression-distal femoral plates may be an excellent treatment procedure for patients with Vancouver B1 fractures, allowing for full weight-bearing in the early postoperative period.
... All sections were assigned isotropic material properties with an elastic modulus of 16 [25], 195 GPa for Orthinox stainless steel [26], and 110 GPa for Titanium [27]. A Poisson's ratio of 0.3 was used for all materials [26]. ...
Background: Internal fixation is recommended for treating Vancouver B1 periprosthetic femoral fractures. Although several fixation procedures have been developed with high fixation stability and union rates, long-term weight-bearing constructs are still desired. Therefore, the aim of the present study was to evaluate the stability of a double-plate procedure using reversed contralateral locking compression-distal femoral plates for fixation of Vancouver B1 periprosthetic femoral fractures under full weight-bearing conditions.
Methods: Single and double fixation procedures for locking compression-distal femoral plates were analysed under an axial load of 1,500 N by finite element analysis and biomechanical loading tests. A vertical loading test was performed to the prosthetic head in biomechanical testing, and the displacements and strains were calculated based on load-displacement and load-strain curves generated by the static compression tests.
Results: Double-plate fixation significantly reduced stress concentration at the fracture site of the lateral plate by finite element analysis. Under full weight-bearing conditions, the maximum von Mises stress in the lateral plate was 268 MPa. On the other hand, maximum stress in the single-plating method occurred at the defect level of the femur with a maximum stress value of 1,303 MPa. The principal strains of single- and double-plate fixation were0.63% and 0.058%, respectively. Consistently, in the axial loading test, the strain values at 1,500 N loading of the single- and double-plate fixation methods were 1,274.60 ± 11.53 and 317.33 ± 8.03 (X 10⁻⁶), respectively.
Conclusions: The present study suggests that dual-plate fixation with reversed locking compression-distal femoral plates may be an excellent treatment procedure for patients with Vancouver B1 fractures, providing full weight-bearing in the early postoperative period.
... To better understand the problems of loosening of femoral prostheses, it was developed a digital mechanical model of the "femur, cement, implant" system [11,12], it represents the prosthesis in its anatomical environment. While a significant challenge, the incentive to carry out 3D modeling of crack behavior and the compute stress intensity factors of three dimension crack in the cement is the ability to predict the cement mantle reliability from nondestructive inspection of implants such as high resolution micro tomography that can reveal the presence of cracks in the cement [13,14,15]. In this study, the existence of a crack emanating from a cavity was assumed; its assessment takes into account two parameters, the position of the crack in the cement and the stress intensity factor (SIF) that was calculated in the proximal part of orthopedic cement. ...
In orthopedic surgery and particularly in the total hip arthroplasty (THP), the stem fixation is performed in general using surgical cement which consists essentially of poly (methyl methacrylate) (PMMA). Fracture of cement and prosthesis loosening appears after high concentrations of stress. This phenomenon origin is due to the presence of micro-cavities in the cement volume due to patient movements. The focus of this study is the modeling using the finite element method of a crack emanating from a cavity. It was assumed several positions and orientations of the crack in the cement to calculate the stress intensity factor (SIF). Results show that the presence of a crack emanating from a cavity in the cement increases the risk of fracture of cement.
... The material of SHS was considered to be titanium. The modulus of elasticity was based on the study by Harrigan et al. [19] (Table 1). ...
Background:
Femoral trochanteric fractures are common among older adults. In the reduction of trochanteric fractures, acquiring the support of the anterior cortex at the fracture site on lateral view immediately after surgery is important. However, even if the cortical support is acquired, postoperative displacement due to the loss of this support often occurs. This study aimed to investigate local stress distribution in several trochanteric fracture models and to evaluate risk factors for postoperative displacement using the finite element (FE) method.
Methods:
Displaced two-fragment fracture models with an angulation deformity at the fracture site and a non-displaced two-fragment fracture model were constructed. The models with an angulation deformity were of two types, one with the proximal fragment directed backward (type A) and the other with the proximal fragment rotated forward from the femoral neck axis (type B). Thereafter, FE models of the femur and a sliding hip screw mounted on a 135° three-hole side-plate were constructed. A 2010-N load was applied to the femoral head, and a 1086-N load was applied to the greater trochanter. Under this condition, the maximum value of the von Mises stress distribution and the amount of displacement of the femoral head vertex in the distal direction were investigated.
Results:
A larger maximum stress value at the medial femoral neck cortex and a higher amount of displacement in the distal direction were particularly recognized in type A models. These results indicate that microstructural damage was larger in type A models and that type A fracture alignment may be particularly related to fracture collapse and subsequent postoperative displacement.
Conclusion:
Even if support of the anterior cortex at the fracture site on lateral view is acquired immediately after surgery, caution is necessary for cases in which the proximal fragment is directed backward in the postoperative displacement from the viewpoint of the biomechanics of the FE method.
... 32,36,51 In fact, such regions are associated with higher cement stress. 58,59 The stem length of cemented designs does not seem to be a significant factor with regards to the long-term implant stability. Prins et al. 60 presented excellent long-term results with a stem length of 130 mm (Lubinus SP II) while Williams et al. 7 reported similar results with a different design and an implant length starting with 103 mm (Exeter). ...
The current trend is toward shorter hip stems. While there is a general agreement on the need for a cement mantle thicker than 2 mm, some surgeons prefer line‐to‐line cementation, where the mantle has only the thickness provided by the cement‐bone interdigitation. The aim of this study was to assess if a relatively short, polished hip stem designed for a standard cementation can also be cemented line‐to‐line without increasing the risk of long‐term loosening. Composite femurs with specific open‐cell foam to allow cement‐bone interdigitation were used. A validated in‐vitro biomechanical cyclic test replicating long‐term physiological loading was applied to femurs where the same stem was implanted with the Standard‐mantle (optimal stem size) and Line‐to‐line (same rasp, one‐size larger stem). Implant‐bone motions were measured during the test. Inducible micromotions never exceeded 10 μm for both implant types (differences statistically not‐significant). Permanent migrations ranged 50–300 μm for both implant types (differences statistically not‐significant). While in the standard‐mantle specimens there was a pronounced trend toward stabilization, line‐to‐line had less tendency to stabilize. The cement cracks were observed after the test by means of dye penetrants: The line‐to‐line specimens included the same cracks of the standard‐mantle (but in the line‐to‐line specimens they were longer), and some additional cracks. The micromotions and cement damage were consistent with those observed in‐vitro and clinically for stable stems, confirming that none of the specimens became dramatically loose. However, it seems that for this relatively short polished stem, standard‐mantle cementation is preferable, as it results in less micromotion and less cement cracking. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res
... This is relevant to THR and TKR, where the cement mantle experi- ences both tension and compression. [39][40][41][42][43][44][45][46] However, this condition is not relevant to the human vertebra because: ...
There is a growing interest towards bone cements for use in vertebroplasty and kyphoplasty, as such spine procedures are becoming more and more common. Such cements feature different compositions, including both traditional acrylic cements and resorbable and bioactive materials. Due to the different compositions and intended use, the mechanical requirements of cements for spinal applications differ from those of traditional cements used in joint replacement. Because of the great clinical implications, it is very important to assess their long-term mechanical competence in terms of fatigue strength and creep. This paper aims at offering a critical overview of the methods currently adopted for such mechanical tests. The existing international standards and guidelines and the literature were searched for publications relevant to fatigue and creep of cements for vertebroplasty and kyphoplasty. While standard methods are available for traditional bone cements in general, no standard indicates specific methods or acceptance criteria for fatigue and creep of cements for vertebroplasty and kyphoplasty. Similarly, a large number of papers were published on cements for joint replacements, but only few cover fatigue and creep of cements for vertebroplasty and kyphoplasty. Furthermore, the literature was analyzed to provide some indications of tests parameters and acceptance criteria (number of cycles, duration in time, stress levels, acceptable amount of creep) for possible tests specifically relevant to cements for spinal applications.
... First, PMMA does not have intrinsic adhesive properties, and only acts as a space filler to closely hold the implant against the bone [14]. Such a weak interfacial link between the cement and bone (or implant) results in implant failure [15]. Second, PMMA is a brittle, notch-sensitive material. ...
In biomedicine, adhesives for hard and soft tissues are crucial for various clinical purposes. However, compared with that under dry conditions, adhesion performance in the presence of water or moisture is dramatically reduced. In this review, representative types of medical adhesives and the challenging aspects of wet adhesion are introduced. The adhesion mechanisms of marine mussels, sandcastle worms, and endoparasitic worms are described, and stemming from the insights gained, designs based on the chemistry of molecules like catechol and on coacervation and mechanical interlocking platforms are introduced in the viewpoint of translating these natural adhesion mechanisms into synthetic approaches.
... The mechanical properties of the total hip prosthesis components are given in Table 1. The cortical bone, the spongious bone, the orthopedic cement and the stem are all considered as elastic isotropic materials [25][26][27]. ...
The total hip replacement is an operation that replaces a diseased hip with a mechanical articulation. Both components of the mechanical articulation (stem and the cup) are bonded to bone using orthopedic cement, whose reliability determines the longevity of the implant. The cement around the metallic stem forms a mantle whose strength and toughness determine its resistance to fatigue and failure by fracture. Typical cements are acrylic polymers that often suffer from internal cracks and other defects created during polymerization. This study is a systematic analysis of preexisting 3D crack behavior in the orthopedic cement mantle when subjected to external body forces. Different crack orientations and angular positions around the mantle are studied to identify which locations will propagate the crack. This is accomplished by a global stress analysis of the mantle followed by a failure analysis. Amongst others, the existence of a crack in the proximal region of the orthopedic cement is identified as a critical area, especially in the lateral sides of the stem in the radial direction.
... The interfaces between the femoral component and bone cement were known to be a weak area of bone-bone cement prosthesis complex [16]. Previously, Harris and Jasty found that the main mechanism of aseptic loosening on the femoral side was the debonding of the femoral component-bone cement by analyzing the prosthesis removed. ...
Objective. Aim to study how the content of alendronate affected shear strengths at bone-bone cement-metal interfaces. Methods. All samples were divided into 6 groups, G0-G5. On the 1st and 60th day after surgery, bone-bone cement interface shear strengths and bone densities were examined. Interface strengths of metal-bone cement specimens were studied before immersion and 4 weeks after immersion. Results. On the 60th day, bone-bone cement interface shear strengths and bone densities showed significant differences (P < 0.05), and compared with G0, G2-G5 values increased significantly (P < 0.05), and the peak value was met in G3. Compared with the 1st day, on the 60th postoperative day both factors decreased significantly in G0 and G1 (P < 0.05). Four weeks after immersion, with the increasing dose of alendronate, the shear strengths decreased gradually and in G5 decreased significantly (P < 0.05). Compared with before immersion, the metal-bone cement interface strengths decreased significantly 4 weeks after immersion (P < 0.05). Conclusions. 50-500 mg alendronate in 50 g cement powders could prevent the decrease of shear strengths at bone-bone cement interfaces and had no effect on metal-bone cement interface strengths. While the addition dose was 100 mg, bone cement showed the best strengths.
... Mechanical integrity can only be maintained if the overall stress is kept below some threshold over time [30]. Another practical problem is that the influence of cement porosity may dominate the effect of the stress [31]. These stresses may occur as tensile, compressive, shear, or a stress combination known as equivalent von Mises stresses. ...
Dynamic loads from routine activities applied to the stem create dynamic stresses varying in time and resulting in the fatigue failure of the prosthesis components. Therefore, a finite element model can be used to predict mechanical failure. The purpose of this study was to develop a three-dimensional model of the cemented hip femoral prosthesis and to carry out finite element analysis to evaluate stress distributions in the bone, the cement and the implant compounds under dynamic loads from different human activities. Linear elastic analysis is adapted; von Mises stress, normal stress and shear stress are the values that are of concern. Results show that the stresses distribution in the femoral arthroplasty components depends on the human activity. The analysis also showed that the stresses are high in the proximal and distal parts of the cement mantle.
... The implants were loosening with large areas of osteolytic (bone) resorption, so much so that at revision surgery the implants could be removed with minimal effort. The rationale for these failures was considered in detail and the concepts of 'cement disease' and 'stress shielding' were developed (Engh et al. 1987;Harrigan et al. 1992). ...
Composite implants have been used in fracture fixation since the 1980s. Now filled polymer composites have been developed for applications from ear implants through to tissue engineering. In this article, both degradable and nondegradable composites are considered. The progress of development and some of the test considerations are discussed.Keywords:polymers;calcium phosphate;nondegradable;degradable;tissue engineering;fracture fixation;joint replacement
... The mechanical properties of the total hip prosthesis components are given in Table 1. The cortical bone, the spongious bone, the orthopedic cement and the stem are all considered as elastic isotropic materials [25][26][27]. ...
The total hip replacement is an operation that replaces a diseased hip with a mechanical articulation. Both components of the mechanical articulation (stem and the cup) are bonded to bone using orthopedic cement, whose reliability determines the longevity of the implant. The cement around the metallic stem forms a mantle whose strength and toughness determine its resistance to fatigue and failure by fracture. Typical cements are acrylic polymers that often suffer from internal cracks and other defects created during polymerization. This study is a systematic analysis of preexisting 3D crack behavior in the orthopedic cement mantle when subjected to external body forces. Different crack orientations and angular positions around the mantle are studied to identify which locations will propagate the crack. This is accomplished by a global stress analysis of the mantle followed by a failure analysis. Amongst others, the existence of a crack in the proximal region of the orthopedic cement is identified as a critical area, especially in the lateral sides of the stem in the radial direction.
... 14,15) . 유한요소해석상(finite element analysis) 상 대 퇴근위부와 대퇴스템 원위부에 peak stress가 발생하여 이것이 시멘트 맨틀의 crack의 시작이 될 수 있음을 보여 주고 있다 16,17) ...
Cemented total hip arthroplasty was first introduced by Sir John Charnley in 1961 and it has become one of the principal fixation techniques for fixation of an implant. The surgical technique has since been modified and this has resulted in improved longevity and reliability.
... At the boundary of the implant, the titanium mesh was completely fixed to the bone, the implant surfaces were polished to ensure that no bone was attached to other bone-implant interfaces, and the surface of the femur model's inferior extremity was completely immobilized. Table 1 18,19) were used. Maximum principal stress was quantified at regions where the BV/TV and BAp orientation were measured. ...
The present work was aimed at clarifying the stress-shielding effect caused by hip-joint implantation into a femur by using a human cadaver with a cementless hip implant. In particular, bone quality was assessed from the standpoint of preferential c-axis orientation of biological apatite (BAp). Comparing the implanted side to the non-implanted side, a finite element analysis (FEA) indicated that artificial hip-joint implantation had a significant stress-shielding effect on the femur. The results also showed a marked decrease in the degree of preferential BAp orientation as well as bone loss in the medial-proximal femur. This is the first report showing a reduction in the degree of preferential BAp orientation due to a stress-shielding effect after artificial hip-joint implantation. Since preferential BAp orientation is an important index for determining bone mechanical function, these findings should be taken into account in future artificial hip-joint designs, especially those involving the stem component.
... Assuming a neck angle of 135° and a torsion angle of 15° the reported loads can be converted to the current coordinate system through rotation of 45° about the y-axis and 15° about the z-axis. Muscle load data for singlelegged stance and stair climb was obtained from Harrigan et al. (1992). They used joint load directions that corresponded to those found by Davy et al. (1988) but the magnitude corresponded to a subject with a body weight of 641 N. ...
... Stolk et al. [5] compared the results from strain gauge measurements to those of FE analyses and developed a preclinical test to assess the two existing stem designs against cement fracture. Other studies have also validated FE analyses by measuring cement strain values [6,7] by stem subsidence values [8,9] and by strain values on the cortical bone with cemented THA. Although their procedures are useful for highlighting differences in stress patterns around different stems, the methods did not include a new ideal stem design. ...
This study proposes novel optimized stem geometry with low stress values in the cement using a finite element (FE) analysis combined with an optimization procedure and experimental measurements of cement stress in vitro. We first optimized an existing stem geometry using a three-dimensional FE analysis combined with a shape optimization technique. One of the most important factors in the cemented stem design is to reduce stress in the cement. Hence, in the optimization study, we minimized the largest tensile principal stress in the cement mantle under a physiological loading condition by changing the stem geometry. As the next step, the optimized stem and the existing stem were manufactured to validate the usefulness of the numerical models and the results of the optimization in vitro. In the experimental study, strain gauges were embedded in the cement mantle to measure the strain in the cement mantle adjacent to the stems. The overall trend of the experimental study was in good agreement with the results of the numerical study, and we were able to reduce the largest stress by more than 50 % in both shape optimization and strain gauge measurements. Thus, we could validate the usefulness of the numerical models and the results of the optimization using the experimental models. The optimization employed in this study is a useful approach for developing new stem designs.
... This requires the validation of FE models with diierent implants relative to experimental measurements. There are a number of papers describing the validation of FE models of hip reconstructions relative to bone surface strains11121314 and in case of cemented implants, internal cement strains are measured to validate the FE models in few papers151617. In all these validation studies, the femurs were subjected to bending in the frontal plane. ...
In this study, the finite element modeling and comparison of the stress and strain analyses were carried out for three different structures that are intact bone, stemless implant and stemmed one. Currently proposed stemless design studied here is the generic concept of stemless implant. This generic stemless implant reconstruction was numerically compared to the conventional stemmed implant and also to the intact bone as control solution. Two loading conditions were applied to the most proximal part of the models, while the most distal part was fixed for all degrees of freedom. The models were divided into two regions and studied along two paths of medial and lateral aspect. The results of this study showed that the stemless implant had less deviation from the control solution of the bone in all regions and in both loading conditions, comparing to the large deviation of the stemmed implant from the intact bone. However, it was shown that the fixation of this type of implant and its effect on sub-trochanter region must be carefully considered for designing the final product of any specific design of stemless implant.
... The true bone microstructure and also the cement composition (e.g. influence of cement porosity) will affect the localised stress concentrations (Harrigan et al. 1992). In this respect, monitoring the peak stress (as in this study) may give an incorrect picture of the potential durability of the cemented fixation. ...
Patient-specific finite element models of the implanted proximal femur can be built from pre-operative computed tomography scans and post-operative X-rays. However, estimating three-dimensional positioning from two-dimensional radiographs introduces uncertainty in the implant position. Further, accurately measuring the thin cement mantle and the degree of cement-bone interdigitation from imaging data is challenging. To quantify the effect of these uncertainties in stem position and cement thickness, a sensitivity study was performed. A design-of-experiment study was implemented, simulating both gait and stair ascent. Cement mantle stresses and bone-implant interface strains were monitored. The results show that small variations in alignment affect the implant biomechanics, especially around the most proximal and most distal ends of the stem. The results suggest that implant position is more influential than cement thickness. Rotation around the medial-lateral axis is the dominant factor in the proximal zones and stem translations are the dominant factors around the distal tip.
... The 3-D damage approach is essential when applied to FE models of hip joint reconstructions. The stress situation in the cement mantle is essentially of a 3-D nature [25]. Hence, the orientation of the microcracks will vary throughout the entire cement mantle. ...
Acrylic bone cement is used to fixate hip replacement implants into the bone. Creep and fatigue failure of the cement promote failure of the implant. For the purpose of implant testing, we derived a finite element algorithm that simulates creep and damage accumulation in acrylic bone cement. The simulation combines a Maxwell creep model, with a 3-D continuum damage mechanics approach modeling anisotropic damage accumulation. The technical details of the simulation are described. In a first application tensile fatigue tests on tubular cement specimens are simulated. The creep elongation and fatigue life of the specimens, as predicted by the simulations, are successfully correlated to the experi-mental results. In a second application, the simulation is used to predict creep and fatigue failure of the cement mantle around two hip implants with different clinical outcomes. It is shown how the simulation is able to predict the locations of cement damage around the implants, and the amounts of implant migration attributable to creep.
The ability of extremely low-amplitude mechanical
strains to promote bony ingrowth was evaluated
in an in vivo animal model, the functionally
isolated turkey ulna. A cylindrical, porous-coated
titanium implant was placed across the dorsal and
ventral cortices of the left ulna diaphysis of 12 animals.
Back scatter electron microscopy was used
to quantify the relative bony ingrowth after eight
weeks of: ( I ) disuse alone, (2) disuse plus 100 seconds
per day of a 1-Hz, 150-microstrain (pt) mechanical
stimulus, or (3) disuse plus 100 seconds
per day of a 20-Hz stimulus of similar strain magnitude.
Disuse alone caused a mean 8.3% (-+5.5%)
loss of bone away from the implant, with the area
between implant and bone actively filling with a
fibrous membrane. A daily 100-second regimen of
low-magnitude, 1-Hz mechanical stimulation
caused 28% (26.2%) of the implant area available
for ingrowth to be filled with bone. At 20 Hz, the
amount of bony ingrowth increased to 69%
(23.0%). These data demonstrate that brief exposure
to extremely low-amplitude mechanical
strains can enhance the biologic fixation of cementless
implants. Moreover, the degree of ingrowth
is dependent on the frequency of the applied
strain.
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Charley Davidson’ s pal Sanza Time had a similar accident, but, instead of fracturing his tibia, he sustained a closed fracture of the femur. This fracture was treated by closed intramedullary rodding, and Sanza, against the advice of his surgeon, was back on his bike as soon as the repair shop pronounced it ready. Also against the advice of his doctor, he went back to his job in a steel mill, where he was on his feet all day, and even played a little Softball on weekends. His doctor was hardly surprised, then, that, about 11 weeks after his operation, Sanza had a sudden pain in the thigh. A roentgenogram showed a broken rod associated with an ununited femur. How could such a thing happen?
“Cement”, a word comes from the domain of architecture construction. It consists of a system of powder/liquid materials which, when mixed to a paste, set to a hard mass. “Bone cement” is to benefit this system for an application in medicine, for example: filling of bone defects and fixation of surgical prosthesis etc. The history of the application of bone cement dates to more than 100 years. In 1890, Dr. Gluck described the use of the ivory ball-and-socket joints which were especially useful in the treatment of diseases of the hip joint. These joints were stabilised in the bone with a cement composed of colophony, pumice powder and plaster. He stated that the cement remained walled off in the marrow cavity in the same way as a bullet, the marrow cavity appearing to have almost unlimited tolerance to aseptic implantation (Gluck Arch Klin Chir 41:187, 1891). In 1951, Dr. Haboush used self-curing acrylic dental cement to secure a total hip replacement (Haboush Bull Hosp Joint Dis 14:242, 1953). Also at this time similar resins were being used to repair defects in the skull after brain surgery. Polymethylmethacrylate (PMMA) cement was used primarily in dentistry to fabricate partial dentures, orthodontic retainers, artificial teeth, denture repair resins, and an all-acrylic dental restorative. Dr. Charnely had used a cold-cured acrylic as a possible luting cement to retain the femoral shaft in total hip arthroplasty (Charnely J Bone Joint Surg [Br] 46:518, 1964).
One hundred nineteen consecutive primary hybrid total hip arthroplasties with a precoated femoral component were performed by one surgeon in 100 patients and followed up prospectively, Ninety-eight hips in 82 patients (mean age, 67 years) were evaluated clinically and radiographically at a mean of 6.5 years (range, 5-9 years). The hips were evaluated clinically using the Harris hip score, and radiographs were evaluated for femoral cement grade, loosening, and osteolysis. Ninety-five hips remained in place at the most recent followup, Two femoral components were revised for definite loosening, and one well fixed femoral component was removed because of late hematogenous infection. Excluding the three hips that were revised, the clinical result was excellent or good in 79 hips (83%), fair in 12 hips (13%), and poor in four hips (4%), All other femoral components were well fixed. There were defects of the cement mantles (C1 and C2) in 90 hips. No femoral component had a stem and cement radiolucent line. Focal femoral osteolysis was seen in only two hips. One acetabular component was removed at 5 years because of late hematogenous infection. One acetabular component had asymptomatic migration. The remaining 96 acetabular components were well fixed. Focal acetabular osteolysis was present in four hips. The mean linear polyethylene wear rate was 0.06 (+/- 0.05) mm per year. In contrast to other reports of early failure and osteolysis, the use of a precoated femoral component in this study did not adversely affect the fixation of hybrid total hip arthroplasty, with definite failure of only 2% (two of 98) of the femoral components.
The word “cement” comes from the domain of architecture construction. It consists of a system of powder/liquid materials which, when mixed to a paste, set to a hard mass. “Bone cement” uses this system for application in medicine, for example: filling of bone defects and fixation of surgical prostheses, etc.
This chapter considers the principles behind the design of cemented femoral stems and concentrates on the issues that are important to the surgeon when selecting a stem for use in clinical practice. Two main issues are central to a surgeon's choice of implant in hip replacement surgery: long-term function of the prosthesis and the versatility of the hip replacement system. The first part of the chapter examines how stem design may affect fixation of the stem within the femur and the long-term performance of a hip replacement. The second part of the chapter considers the needs of the surgeon in the operating room and how design of a cemented stem system may help the surgeon recreate each patient's anatomy and thereby achieve the optimum outcome.
Total hip arthroplasty (THA) is widely regarded as one of the most successful procedures in orthopedic surgery. It signifi cantly reduces pain, increases mobility, and restores function to patients who are otherwise incapacitated by degenerative joint disease. In addition, THA has a cost/utility ratio that rivals treatments for hypertension and coronary artery disease making it one of the most cost-effective medical interventions known [1, 2]. Despite this, an ever-increasing life expectancy and greater patient expectations for post-surgical activity have spurred advances in design and surgical technique which seek to increase the longevity of the prosthesis while minimizing morbidity. Such developments are crucial to reducing revision rates in THA patients who are younger and may require multiple revisions in their lifetime.
The hip arthroplasty with cementless hip prosthesis is a solution usually used for the patients suffering the problems of the musculoskeletal system. However, such a solution has a major disadvantage, pointed by all users : the lack of primary stability of the prosthesis. This weakness can cause serious complications or failure of the surgery. Therefore, to achieve a good primary fixation is a crucial point of this type of surgery to ensure a short and a long term clinical satisfaction. In order to better understand this central issue, a preoperative track is adopted. A finite element model to describe the mechanical behavior of the coupled system " femur-cementless prosthesis : DePuy Corail® "has been created and validated by the experiments in vitro. Then, in order to take into account the high variability of model parameters, inherent to the nature of the problem, the stochastic modeling of random input parameters has been introduced and a mechanical-probabilistic strategy has been proposed, on the one hand to quantify, in probabilistic terms, the effect, on the response, of the uncertainties affecting the input parameters of the model, and on the other hand to evaluate the primary stability of the bone-prosthesis system in reliability context. The practical implementation of this approach is realized by using the numerical tools based on the standard Monte Carlo method and the stochastic collocation procedure. The originality of the work presented is primarily in the proposition of a probabilistic methodology capable of taking into account the uncertainties in the problem of primary stability of cementless hip prostheses. It also lies in the potentiality of this methodology to be transplantable easily in industrial context.
抄録
The present work was aimed at understanding the stress-shielding caused by hip-joint implantation into a femur by using a human cadaver with a cementless hip implant. In particular, bone quality was assessed from the standpoint of preferential c-axis orientation of biological apatite (BAp). Comparing the implanted side to the non-implanted side, a finite element analysis (FEA) indicated that artificial hip-joint implantation had a significant stress-shielding on the femur. The results also showed a remarkable decrease in the degree of preferential BAp orientation as well as bone loss in Haversial canal in the medial-proximal femur. This is the first report showing a reduction in the degree of preferential BAp orientation due to a stress-shielding after artificial hip-joint implantation. Since preferential BAp orientation is an important parameter for determining bone mechanical function, these findings should be taken into account in future artificial hip-joint designs, especially those involving the stem component.
In this investigation, we propose a new concept to embed cohesive zone into the continuum structure of bone cement, an example of brittle material, in investigating the mechanical behavior and fracture mechanism and to predict the fracture which elastic fracture mechanics (EFM) is unable to. Four finite element (FE) models with embedded cohesive zones for the simulations of tensile, compression, double shear and 3-point bending tests have been implemented. Cohesive zones (CZ) are embedded at high risks of fracture with orientations determined by fracture mode. A bilinear cohesive traction-separation law (TSL) is applied. The fracture parameters in traction-separation curve are validated and justified in the simulations to agree well with the force-displacement curves in the four practical tests. Apart from the maximum load, the perpetual safe working load (SWL) in theory also can be predicted by tracing the history of the stiffness degradation of fractured cohesive zone by means of simulation. A distinct advantage of our numerical model is that it is able to extend to investigate the mechanical behavior and fracture mechanism of other brittle materials. The proposed method with embedded cohesive zones in FE models can be introduced to predict the fracture and to forecast the maximum load and safe working load (SWL) of the continuum structure in more complicated loading conditions.
The total hip replacement (THR) has been used as the most effective way to restore the function of damaged hip joint. However, various factors have caused some side effects after the THR. Unfortunately, the success of the THR have been decided only by the proficiency of surgeons so far. Hence, It is necessary to find the way to minimize the side effect caused by those factors. The purpose of this study was to suggest the definite data, which can be used to design and choose the optimal hip implant. Using finite element analysis (FEA), the biomechanical condition of bone cement was evaluated. Stress patterns were analyzed in three conditions: cement mantle, procimal femur and stem-cement contact surface. Additionally, micro-motion was analyzed in the stem-cement contact surface. The 3-D femur model was reconstructed from 2-D computerized tomography (CT) images. Raw CT images were preprocessed by image processing technique (i.e. edge detection). In this study, automated edge detection system was created by MATLAB coding for effective and rapid image processing. The 3-D femur model was reconstructed based on anatomical parameters. The stem shape was designed using that parameters. The analysis of the finite element models was performed with the variation of parameters. The biomechanical influence of each parameter was analyzed and derived optimal parameters. Moreover, the results of FE A using commercial stem model (Zimmer's V erSys) were similar to the results of stem model that was used in this study. Through the study, the improved designs and optimal factors for clinical application were suggested. We expect that the results can suggest solutions to minimize various side effects.
This article describes the simple step by step system, to teach object recognition and tracking in computer vision systems. Methodology is based on object recognition system complexity incrementation. Student doesn't need any knowledge about mathematical principles of computer based object detection. This is achieved using OpenCV and other high level libraries. Article consists from explanation of simple detection methods based on OpenCv, to complex pattern-based detection methods provided by OpenTLD libraries. All library recognition methods are explained in C/C++ language and results are illustrated as graphical output images.
The purpose of this study was to study the mechanical properties of poly(methyl methacrylate) (PMMA)-based bone cement incorporated with hydroxyapatite (HA) nanoparticles after surface modification by poly(methyl methacrylate-co-γ-methacryloxypropyl timethoxysilane) [P(MMA-co-MPS)]. PMMA and P(MMA-co-MPS) were synthesized via free-radical polymerization. P(MMA-co-MPS)-modified hydroxyapatite (m-HA) was prepared via a dehydration process between silane and HA; the bone cement was then prepared via the in situ free-radical polymerization of methyl methacrylate in the presence of PMMA and P(MMA-co-MPS)–m-HA. Fourier transform infrared (FTIR) spectroscopy, 1H-NMR, and gel permeation chromatography were used to characterize the P(MMA-co-MPS). Thermogravimetric analysis and FTIR were used as quantitative analysis methods to measure the content of P(MMA-co-MPS) on the surface of HA. The effect of the proportion of m-HA in the PMMA-based bone cement on the mechanical properties was studied with a universal material testing machine. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay was also carried out to determine the cytotoxicity of the composite bone cement. The results showed that the surface modification of HA greatly improved the interaction between the inorganic and organic interfaces; this enhanced the mechanical properties of bone cement for potential clinical applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40587.
Experimental pre-clinical tests associated with numeric models of cemented implants are important for screening of new implants in the market. The aim of this study was to measure strain profiles and maximum temperature polymerization inside a cement mantle of an in vitro cemented hip reconstruction using optical fiber Bragg grating (FBG) sensors. For this purpose, a hip femoral prosthesis was instrumented with 12 FBG sensors, three in each aspect of the femur, anterior, posterior, medial and lateral. These were positioned at the proximal, middle and distal part of the cement mantle relatively to the stem. Another sensor was placed in the lateral-proximal region of the mantle to measure the maximum temperature of cement polymerization. The strains measured were compared with those obtained with a Finite Element model, both for quaistatic mechanical loading. The results show that the experimental technique used can measure strains inside the cement mantle with good correlation, R2 = 0.970, with the numerical model results. The results present a maximum temperature of polymerization around 110°C inside of cement at proximal region. It was also observed strain concentration in lateral aspect of the femur in polymerization process. The procedure hereby explained can be used to improve experimental pre-clinical tests to measure the strain distribution inside the cement mantle as well as residual strain and temperature variation along with time, as a result of the curing process of cement.
Strontium (Sr) has become more attractive for orthopaedic applications as they can simultaneously stimulate bone formation and prevent bone loss. A Sr-containing bioactive bone cement (Sr-BC) has been designed to fix osteoporotic bone fracture. Sr is a trace element, so the safety of containing Sr is concerned when Sr-BC is implanted in human body. The preclinical assessment of biocompatibility of Sr-BC was conducted according to ISO 10993 standards. MTT assay showed that this bioactive bone cement was non-toxic to mouse fibroblasts, and it met the basic requirement for the orthopaedic implant. The three independent genetic toxicity studies including Ames, chromosome aberration and bone marrow micronucleus assays demonstrated absence of genotoxic components in Sr-BC, which reassured the safety concerns of this novel bone cement. The muscle implantation results in present study were also encouraging. The acute inflammation around the cement was observed at 1week post-implantation; however, no significant difference was observed between control and Sr-BC groups. These responses may be attributed to the presence of the foreign body, but the tissue healed after 12weeks implantation. In summary, the above preclinical results provide additional assurance for the safety of this implant. Sr-BC can be used as a potential alternative to the traditional bone cement.
Owing to the large number of total hip replacements (THR) included (over 90,000), the Swedish multi-center studies have demonstrated the possibility to investigate the factors which influence the success rate of total hip replacements [15]. In that study it was found that the failure rates of cemented THR are significantly affected by prosthetic design factors. They could also demonstrate that improved surgical and cementing techniques improved the prospects of THR. These findings can be considered as the fruits of the efforts of the orthopaedic community. However, as the incidence of primary THR is increasing dramatically, and the success rates for (re)revised THR’s are significantly lower than for the primary ones, major problems for the orthopaedic surgeon lay ahead.
There are well over 100,000 total hip replacements done per year in the U.S., and over 250,000 per year worldwide. Extending the service life of these implants is a high priority due to the large number of patients and the increasing use of joint replacement in younger patients. In an older population, total hip replacements usually outlast the patient, due to advanced age and the relatively low physical demands placed on the prosthesis. Younger, more active, and heavier patients often require revision of their total hip due to progressive implant loosening and the pain which results. Revised total hips do not last as long as the original, and a younger patient can sometimes require more than three revisions.
Clinical studies have attributed considerable importance to the torsional loading of prosthetic femoral implants; however, the effect of this load has frequently been neglected in load simulations of the hip. The objective of this study, therefore, is to investigate the effects of the transverse joint load component in a model analysis of the proximal femur.
Cement mantle stresses in a three-times full-size model of an Exeter™ total hip replacement were investigated using three-dimensional embedded strain transducers. Six sites were analysed for two separate loading configurations, namely the two-dimensionally loaded single-legged stance and the ‘toe off’ phase of gait which represents a three-dimensional hip reaction. Results showed a considerable variation in stem/cavity contact conditions due to the application of the transverse load component. Furthermore, large distal bending stresses are induced in the sagittal plane, with considerable shearing stresses due to torsion evident at all sites.
The study highlights the significance of the transverse load and emphasizes the considerable limitations of finite element studies in modelling realistic load-dependent interface conditions.
There are many instances where it is necessary to measure strains inside polymethyl methacrylate. For instance, most cemented prosthetic devices fail owing to cement failure. Thus, it is important to measure the cement stresses in vitro, in order to optimize the design. Measuring internal strains with an embedded gauge is extremely difficult. The techniques proposed in the literature either are inaccurate or require large-scale models. In the present work a technique is proposed where a single strain gauge rosette is used. The strain gauge is calibrated to correct the errors deriving from the pressure acting on the grid. Thus accurate measurements can be obtained with a small sensor. The errors associated with this technique are estimated. A pilot study is presented (a) to verify whether the procedure can be applied to the actual geometry of the prosthesis, (b) to verify whether the gauges could safely be inserted into the femur, (c) to estimate the variability of this preparation and (d) to test the robustness of the preparation to repeated loading. Promising results were found in terms of accuracy, precision and robustness of the technique.
Stem-cement and cement-bone interfacial failures as well as cement fractures have been noted in cemented total hip arthroplasty (THA) as the cause of aseptic loosening. Attempts to reduce the risk of femoral component loosening include improving the stem-cement interface by various coatings, using a textured or porous coated stem surfaces or by using a tapered stem having a highly-polished surface. The latter approach, often referred to as "force-closed" femoral stem design, would theoretically result in stem stabilization subsequent to debonding and 'taper-lock'. Previous work using three-dimensional finite element analysis has shown a state of stress at the stem-cement interface indicative of 'taper-lock' for the debonded stem and indicated that stem-cement interface friction and bone cement creep played a significant role in the magnitudes of stresses and subsidence of the stem. However, the previous analysis did not include the viscoelastic properties of bone, which has been hypothesized to permit additional expansion of the bone canal and allow additional stem subsidence (Lu and McKellop, 1997). The goal of this study was to investigate the effect of bone viscoelastic behavior on stem subsidence using a 3D finite element analysis. It was hypothesized that the viscoelastic behavior of bone in the hoop direction would allow expansion of the bone reducing the constraint on bone over time and permit additional stem subsidence, which may account for the discrepancies between predicted and clinical subsidence measurements. Analyses were conducted using physiological loads, 'average peak loads' and 'high peak loads' for 'normal patient' and 'active patient' (Bergmann et al., 2010) from which short and long term subsidence was predicted. Results indicated that bone creep does contribute to higher stem subsidence initially and after 10 years of simulated loading. However, it was concluded that the "constraint" upon the cement mantle is not mitigated enough to result in stem subsidence equivalent to that observed clinically.
Acrylic bone cement is weakened by its porosity, which promotes the formation of microcracks, which contribute to major crack propagation and ultimately failure of the cement mantle. Bone cement mixing techniques play a significant role in determining the quality of bone cement produced. A high degree of porosity is found to exist in cement that is inadequately mixed. Current commercial bone cement mixing systems allow for the preparation of the bone cement under the application of a vacuum in a closed, sealed chamber by means of a repeatable mixing action. These mixing systems are perceived to be repeatable and reliable by orthopaedic community. In this paper, the quality of bone cement mixed using an operator independent bone cement mixing system was compared with that of cement prepared using commercially available devices. The results of the investigation highlighted that cement prepared using the automated, repeatable mixing regime that is operator independent demonstrated consistently better physical and mechanical properties in comparison with cement mixed using proprietary cement mixing devices. Furthermore, Design of Experiments software established the optimal factors that influenced the physical and mechanical properties of PMMA bone cement.
The mechanical properties of the three cement preparations most widely used in the United States were compared by conducting tensile and fatigue tests on Simplex P, LVC, and Zimmer Regular bone cements. Specimens of all three cement preparations were prepared for mechanical testing with and without centrifugation of the cement immediately after mixing. Although the results of the tensile testing revealed a few specific instances of significant differences in the tensile properties of the three cement preparations, there was no consistent evidence that one cement was superior in tension to the others. However, the fatigue properties of Simplex P were consistently and significantly superior to the fatigue properties of both LVC and Zimmer Regular bone cements. Centrifugation of the cement immediately after mixing significantly improved both the tensile and fatigue properties of all three bone cements. However, the fatigue strength of centrifuged Simplex P was substantially and significantly superior to the fatigue strength of the centrifuged LVC and Zimmer Regular bone cements. Since in total joint replacements bone cement is subjected to cyclic loading, these data suggest that centrifuged Simplex P is a preferable bone cement to LVC and to Zimmer Regular cement with or without centrifugation.
We evaluated the initial stability of cemented and uncemented femoral components within the femoral canals of cadaver femurs during simulated single limb stance and stair climbing. Both types were very stable in simulated single limb stance (maximum micromotion of 42 microns for cemented and 30 microns for uncemented components). However, in simulated stair climbing, the cemented components were much more stable than the uncemented components (76 microns as against 280 microns). There was also greater variation in the stability of uncemented components in simulated stair climbing, with two of the seven components moving 200 microns or more. Future implant designs should aim to improve the initial stability of cementless femoral components under torsional loads; this should improve the chances of bony ingrowth.
The usefulness of the push-out test as an indicator of interface strength was evaluated using finite element models of intact and partially failed cylindrical push-out specimens loaded against a rigid annular support. The irregular stress distributions that were found in intact specimens depended more on interface conditions at the loading fixture than on a 35% increase in interface area. The maximum stress at the interface was a tensile stress. Critical energy release rates for interface failure were calculated for flawed specimens in which flaw size was either 10 or 100 microns, and for boundary conditions at the loading fixture that were either fixed or slipping in the radial direction. The critical energy release rates depended heavily on the support boundary conditions. Thus, the results of parametric push-out tests can be reasonably compared only for specimens that are very similar in geometry and that are loaded in very carefully controlled fixtures.
Eleven whole anatomic specimens of the femur were retrieved at autopsy from patients who previously had cemented total hip arthroplasty. Implant duration ranged from 0.5 to 210 months. Clinically and roentgenographically the implants were stable. A detailed biomechanical analysis evaluated bone strains and implant stability in both the single-limb stance and stair-climbing positions using a 100-pound spinal load. The stability offered by cement in these well-fixed prostheses was remarkable, with the maximum axial micromotion being 40 mu. This is a reflection of intimate osseointegration at the bone-cement interface with only rare intervening fibrous tissue. The strain gauge and photoelastic strain-coating studies revealed that marked stress shielding in the proximal medial femoral cortex persists long after a cemented femoral component is inserted. Even 17 years after surgery, the strain in the calcar region did not normalize.
A telemeterized total hip prosthesis was implanted in one patient and force-data were obtained. Thirty-one days postoperatively, the magnitude of the joint-contact force during double-limb stance was 1.0 times body weight. During ipsilateral single-limb stance the joint-contact force was 2.1 times body weight, and during the stance phase of gait the peak force typically was 2.6 to 2.8 times body weight, with the resultant force located on the anterosuperior portion of the ball. During stair-climbing, the force was 2.6 times body weight. At peak loads, the angle between the resultant force and the axis of the neck was 30 to 35 degrees and that between the resultant force and the plane of the prosthesis was 20 degrees. During stair-climbing or straight-leg raising, the out-of-plane orientation of the resultant force increased substantially. These data provide information concerning the forces that must be sustained by prosthetic hip joints during a number of common activities of daily living within the first month after implantation. The results also provide insight into the progression of early recovery and demonstrate the variety of forces that are generated during this period.
With the aid of a three-dimensional finite element stress analysis a parameteric study of the essential aspects of the femoral component of hip endoprostheses was carried out. Various designs were investigated; these included the prosthetic stem length, the existence or non-existence of the prosthetic collar and the elastic modulus of stem and cement. Variations in the stem length have only minor effects on the stress distribution for a stem length greater than 100 mm, but the elastic modulus of the prosthetic stem has a considerable influence on the stresses. A high elastic modulus of the stem increases the stresses in the stem but it decreases stresses in the cement. The cement stresses increase with an increase in the modulus of elasticity of the cement.
In this study centrifugation dramatically reduced the porosity and substantially increased the mechanical properties of bone cement. Monotonic tensile tests to failure of centrifuged specimens of cement demonstrated an increase of 24 per cent in the mean ultimate tensile strength compared with the control value. Mean ultimate tensile strain was improved by 54 per cent. In fully reversed tension-compression fatigue-testing, centrifugation resulted in a mean increase in fatigue life of 136 per cent. These strong advantages in mechanical properties were obtained without any detrimental changes. There was no change in elastic modulus, setting time, or peak temperature. Handling properties were improved. There was no increase in systemic toxicity as demonstrated in dogs by assessment of arterial blood-pressure response and peak levels of monomer in the serum during simulated total hip arthroplasty. We also present a practical system of cement centrifugation and delivery that is suitable for use in the operating room.
It was the object of the studies described in this work to provide general, fundamental concepts on some aspects of human joint replacement and the performance of artificial joints in the body. The work is divided into four sections of which the first discusses joint replacement and bone-prosthesis structures in general. In section II analyses of the process of heat generation and conduction in self-curing acrylic cement, as used for implant fixation, are presented with the object of establishing the chances of thermal bone-tissue necrosis and evaluating precautions that can be taken to prevent it. Section III is devoted to stress analyses of intra-medullary fixation structures under loading, with the object of determining the characteristic mechanical parameters of such structures, evaluating their influences on the mechanical performance and providing guidelines for prosthesis designs and implantation procedures. In section IV some conclusions and recommendations, as reached on the basis of the analyses, are once more briefly discussed in general terms.
Loosening is a major cause of failure in total arthroplasties. The efficacy of the fixation systems depends not only on the bulk properties of the components but also on the interfaces through which they interact. This study was initiated to examine the implant/bone-cement interface for four of the most commonly used implant materials, CoCrMo, Ti6Al-4V, 316SSLVM and ultrahigh molecular weight polyethylene (UHMWPE). The surface preparation, specimen design, joining, and testing techniques were studied and then standardized in a manner which accurately represents current clinical procedures. The interfaces were tested for both their quasistatic and fatigue properties. Finite-element and fracture toughness analyses of the quasistatic shear specimens were performed in order to provide results of an absolute nature which could be subsequently compared to bulk material properties of bone cement. The interfaces were tested “dry” (i.e., at room temperature and 50% R.H.) and in physiological saline at 37°C. The interfaces demonstrated both fracture toughness and fatigue properties far inferior to those of bone cement. A predominantly interfacial type failure was observed using SEM fractography. The ultimate compressive strength (U.C.S.) of bone cement was measured after prolonged exposure to saline at 37°C and showed no decrease in U.C.S. suggesting that the reasons for the interface strength reductions were interfacial rather than bulk in nature. The “wetting'” ability of bone cement was measured using contact angles at various cure times on the four implant materials. These measurements showed that intimate interfacial contact is impossible with current clinical methods. This study indicates that failure of the implant/bonecement interface is likely only a short period after implantation and therefore may be a major contributor to implant loosening.
Unlabelled:
A comparative study of the various aspects of the design of the femoral components of total hip replacements was done using three-dimensional finite-element stress analysis. The aspects of deisgn that were considered included: length, cross-sectional size, and material properties of the stem; presence or absence of a medial collar; and material properties of the cement. We found that increasing the length of the stem generally increased the stress present in the stem while decreasing the stress present in the cement. Increasing the cross-sectional size of the stem decreased the stress in both the stem and the cement. Decreasing the modulus of elasticity of the stem material decreased the stress in the stem but increased the stress in the cement. Increasing the modulus of elasticity of the cement decreased the stress in the stem and increased the stress in the cement. Contact of the collar of a femoral prosthesis with the calcar femorale increased the longitudinal component of stress within the region of the calcar femorale.
Clinical relevance:
The mechanical longevity of a total joint reconstruction is related to the stress distribution throughout the prosthesis, cement, and bone. The stress distribution is related to a number of factors, including the design of the prosthetic components (for example, stem size, stem length, stem modulus of elasticity, and cement modulus of elasticity). Reducing the stresses in prosthetic components to minimize the risk of failure can be accomplished only through systematic analysis of all components of the reconstruction.
Fracture toughness of acrylic bone cements: elastic-plastic analysis of miniature short rod specimens
- Wang
Experimental strain analysis in the femoral cement mantle of simulated total hip arthroplasties
- Burke