Standardized Loads Acting in Knee Implants

The loads acting in knee joints must be known for improving joint replacement, surgical procedures, physiotherapy, biomechanical computer simulations, and to advise patients with osteoarthritis or fractures about what activities to avoid. Such data would also allow verification of test standards for knee implants. This work analyzes data from 8 subjects with instrumented knee implants, which allowed measuring the contact forces and moments acting in the joint. The implants were powered inductively and the loads transmitted at radio frequency. The time courses of forces and moments during walking, stair climbing, and 6 more activities were averaged for subjects with I) average body weight and average load levels and II) high body weight and high load levels. During all investigated activities except jogging, the high force levels reached 3,372–4,218N. During slow jogging, they were up to 5,165N. The peak torque around the implant stem during walking was 10.5 Nm, which was higher than during all other activities including jogging. The transverse forces and the moments varied greatly between the subjects, especially during non-cyclic activities. The high load levels measured were mostly above those defined in the wear test ISO 14243. The loads defined in the ISO test standard should be adapted to the levels reported here. The new data will allow realistic investigations and improvements of joint replacement, surgical procedures for tendon repair, treatment of fractures, and others. Computer models of the load conditions in the lower extremities will become more realistic if the new data is used as a gold standard. However, due to the extreme individual variations of some load components, even the reported average load profiles can most likely not explain every failure of an implant or a surgical procedure.     

Experimental model of tibial plateau fracture for biomechanical testing

Although adequate reduction and stable fixation have been recognized to be the prime goals in the treatment of displaced tibial plateau fractures, the optimal fixation technique remains controversial. The lack of a reliable model and a standard methodology contribute to this situation. The purpose of this study is to develop an experimental model of a tibial plateau fracture and a testing methodology that reproduces the failure mode commonly seen in the clinical setting. Using solid-foam and composite Sawbones tibiae, three different models of bi-condylar tibial plateau fracture (solid-foam, reinforced solid-foam and composite), six specimens for each model, were created and stabilized with double plating. The specimens were subjected to cyclic axial compression with increasing maximum load until failure. A femoral component of a total knee replacement of similar size and shape to the synthetic tibial surface was used as a load applicator. The experiment was repeated on six specimens of human cadaver tibiae. Among the Sawbones specimens, only the reinforced solid-foam model was found to produce a consistent failure mode (collapse in the medial plateau) comparable to that reported clinically in the literature. This mode of failure was also confirmed by the cadaver experiments. The failure load of the reinforced solid-foam model ranged from 4150 to 4260 N with a mean +/- SD of 4201 +/- 44 N and a coefficient of variance of 0.01, whereas for the cadaver model the failure load ranged from 1675 to 6096 N with a mean +/- SD of 3768 +/- 1482 N and a coefficient of variance of 0.39. We recommend the reinforced-foam model for future mechanical tests to compare different fixation methods for tibial plateau fractures.     

In vitro response of the natural cadaver knee to the loading profiles specified in a standard for knee implant wear testing

The purpose of this study was to examine how a natural knee responds to the inputs of a total knee replacement testing standard developed by the International Organization for Standardization (ISO). This load control standard prescribes forces to be used for wear testing of knee replacements independent of implant size or design. A parallel ISO standard provides wear testing inputs that are displacement based instead of force based. Eight fresh frozen cadaveric knees were potted and tested in a 6 degree of freedom knee simulator using the load-control standard. The resulting displacements during load-control testing were compared to the prescribed displacements of the ISO displacement standard. At half the tibial torque prescribed by the load standard there was three times more average internal tibial rotation (20.3°) than is prescribed by the displacement standard (5.7°). The AP motion resulting from load testing was much different than is specified by the displacement standard. All eight knees had anterior tibial translation with respect to the femur during swing phase while the displacement standard specifies posterior tibial displacement. The variation in these motions among knees and their difference from the ISO displacement standard may be one factor that explains why wear results of total knee replacements based on ISO load or displacement testing frequently do not agree with each other or with clinical retrievals.     

In-Vivo Assessment of Femoral Bone Strength Using Finite Element Analysis (FEA) Based on Routine MDCT Imaging: A Preliminary Study on Patients with Vertebral Fractures

PurposeTo experimentally validate a non-linear finite element analysis (FEA) modeling approach assessing in-vitro fracture risk at the proximal femur and to transfer the method to standard in-vivo multi-detector computed tomography (MDCT) data of the hip aiming to predict additional hip fracture risk in subjects with and without osteoporosis associated vertebral fractures using bone mineral density (BMD) measurements as gold standard.MethodsOne fresh-frozen human femur specimen was mechanically tested and fractured simulating stance and clinically relevant fall loading configurations to the hip. After experimental in-vitro validation, the FEA simulation protocol was transferred to standard contrast-enhanced in-vivo MDCT images to calculate individual hip fracture risk each for 4 subjects with and without a history of osteoporotic vertebral fractures matched by age and gender. In addition, FEA based risk factor calculations were compared to manual femoral BMD measurements of all subjects.ResultsIn-vitro simulations showed good correlation with the experimentally measured strains both in stance (R² = 0.963) and fall configuration (R² = 0.976). The simulated maximum stress overestimated the experimental failure load (4743 N) by 14.7% (5440 N) while the simulated maximum strain overestimated by 4.7% (4968 N). The simulated failed elements coincided precisely with the experimentally determined fracture locations. BMD measurements in subjects with a history of osteoporotic vertebral fractures did not differ significantly from subjects without fragility fractures (femoral head: p = 0.989; femoral neck: p = 0.366), but showed higher FEA based risk factors for additional incident hip fractures (p = 0.028).ConclusionFEA simulations were successfully validated by elastic and destructive in-vitro experiments. In the subsequent in-vivo analyses, MDCT based FEA based risk factor differences for additional hip fractures were not mirrored by according BMD measurements. Our data suggests, that MDCT derived FEA models may assess bone strength more accurately than BMD measurements alone, providing a valuable in-vivo fracture risk assessment tool.     

During sideways falls proximal femur fractures initiate in the superolateral cortex: Evidence from high-speed video of simulated fractures

Results of recent imaging studies and theoretical models suggest that the superior femoral neck is a location of local weakness due to an age-related thinning of the cortex, and thus the site of hip fracture initiation. The purpose of this study was to experimentally determine the spatial and temporal characteristics of the macroscopic failure process during a simulated hip fracture that would occur as a result of a sideways fall. Twelve fresh frozen human cadaveric femora were used in this study. The femora were fractured in an apparatus designed to simulate a fall on the greater trochanter. Image sequences of the surface events related to the fractures were captured using two high-speed video cameras at 9111 Hz. The videos were analyzed with respect to time and load to determine the location and sequence of these events occurring in the proximal femur. The mean failure load was 4032 N (SD 370 N). The first surface events were identified in the superior femoral neck in eleven of the twelve specimens. Nine of these specimens fractured in a clear two-step process that initiated with a failure in the superior femoral neck, followed by a failure in the inferior femoral neck. This cadaveric model of hip fracture empirically confirms hypotheses that suggested that hip fractures initiate with a failure in the superior femoral neck where stresses are primarily compressive during a sideways fall impact, followed by a failure in the inferior neck where stresses are primarily tensile. Our results confirm the superolateral neck of the femur as an important region of interest for future hip fracture screening, prevention and treatment research.     

Off-axis loads cause failure of the distal radius at lower magnitudes than axial loads: a finite element analysis

Distal radius (Colles') fractures are a common fall-related injury in older adults and frequently result in long-term pain and reduced ability to perform activities of daily living. Because the occurrence of a fracture during a fall depends on both the strength of the bone and upon the kinematics and kinetics of the impact itself, we sought to understand how changes in bone mineral density (BMD) and loading direction affect the fracture strength and fracture initiation location in the distal radius. A three-dimensional finite element model of the radius, scaphoid, and lunate was used to examine changes of +/-2% and +/-4% BMD, and both axial and physiologically relevant off-axis loads on the radius. Changes in BMD resulted in similar percent changes in fracture strength. However, modifying the applied load to include dorsal and lateral components (assuming a dorsal view of the wrist, rather than an anatomic view) resulted in a 47% decrease in fracture strength (axial failure load: 2752N, off-axis: 1448N). Loading direction also influenced the fracture initiation site. Axially loaded radii failed on the medial surface immediately proximal to the styloid process. In contrast, off-axis loads, containing dorsal and lateral components, caused failure on the dorsal-lateral surface. Because the radius appears to be very sensitive to loading direction, the results suggest that much of the variability in fracture strength seen in cadaver studies may be attributed to varying boundary conditions. The results further suggest that interventions focused on reducing the incidence of Colles' fractures when falls onto the upper extremities are unavoidable may benefit from increasing the extent to which the radius is loaded along its axis.     

Effect of loading rate on endplate and vertebral body strength in human lumbar vertebrae

Previous studies have implied that increases in loading rate resulted in changes in vertebral mechanical properties and these changes were causative factors in the different fracture types seen with high-speed events. Thus many researchers have explored the vertebral body response under various loading rate conditions. No other study has investigated the role of the endplate in high-speed vertebral injuries. The current study determined changes in the endplate and vertebral body strength with increases in displacement rate. The endplate and vertebral body failure loads in individual lumbar vertebrae were documented for two displacement rates: 10 and 2500 mm/s. Using cross-sectional areas from the endplate and vertebral body, failure stresses for both components were calculated and compared. Both the endplate and vertebral body failure loads increased significantly with increased loading rate (p<0.005). Although the vertebral body failure stress increased significantly with loading rate as well (p<0.01), the endplate stresses did not (p>0.35). In addition, the endplate and vertebral strengths were not significantly different under high-speed loading (p>0.60), which inhibits possible predictions as to which bony component would fail initially during a high-speed injury event. It is possible that load distribution may contribute more to the fracture patterns seen at high speeds over vertebral component strength.     

Fracture mechanics of the femoral neck in a composite bone model: Effects of platen geometry

Load applicator (platen) geometry used for axial load to failure testing of the femoral neck varies between studies and the biomechanical consequences are unknown. The purpose of this study was to determine if load application with a flat versus a conical platen results in differing fracture mechanics. Femurs were aligned in 25° of adduction and an axial compressive force was applied to the femoral heads at a rate of 6 mm/min until failure. Load application with the conical platen resulted in an average ultimate failure load, stiffness, and energy to failure of 9067 N, 4033 N/mm, and 12.12 J, respectively. Load application with the flat platen resulted in a significant (p<0.05) reduction in ultimate failure load (7620 N) and stiffness (2924 N/mm). Energy to failure (12.30 J) was not significantly different (p=0.893). Different fracture patterns were observed for the two platens and the conical platen produced fractures more similar to clinical observations. Use of a flat platen underestimates the strength and stiffness of the femoral neck and inaccurately predicts the associated fracture pattern. These findings must be considered when interpreting the results of prior biomechanical studies on femoral neck fracture and for the development of future femoral neck fracture models.     

Acute changes in foot strike pattern and cadence affect running parameters associated with tibial stress fractures

Tibial stress fractures are a common and debilitating injury that occur in distance runners. Runners may be able to decrease tibial stress fracture risk by adopting a running pattern that reduces biomechanical parameters associated with a history of tibial stress fracture. The purpose of this study was to test the hypothesis that converting to a forefoot striking pattern or increasing cadence without focusing on changing foot strike type would reduce injury risk parameters in recreational runners. Running kinematics, ground reaction forces and tibial accelerations were recorded from seventeen healthy, habitual rearfoot striking runners while running in their natural running pattern and after two acute retraining conditions: (1) converting to forefoot striking without focusing on cadence and (2) increasing cadence without focusing on foot strike. We found that converting to forefoot striking decreased two risk factors for tibial stress fracture: average and peak loading rates. Increasing cadence decreased one risk factor: peak hip adduction angle. Our results demonstrate that acute adaptation to forefoot striking reduces different injury risk parameters than acute adaptation to increased cadence and suggest that both modifications may reduce the risk of tibial stress fractures.     

Contact stresses and fatigue life in a knee prosthesis: comparison between in vitro measurements and computational simulations

The evaluation of contact areas and pressures in total knee prosthesis is a key issue to prevent early failure. The first part of this study is based on the hypothesis that the patterns of contact stresses on the tibial insert of a knee prosthesis at different stages of the gait cycle could be an indicator of the wear performances of a knee prosthesis. Contact stresses were calculated for a mobile bearing knee prosthesis by means of finite element method (FEM). Contact areas and stresses were also measured through in vitro tests using Fuji Prescale film in order to support the FEM findings.The second part of this study addresses the long-term structural integrity of metal tibial components in terms of fatigue life by means of experimental tests and FEM simulations. Fatigue experimental evaluations were performed on Cr-Co alloy tibial tray, based on ISO standards. FEM models were used to calculate the stress patterns. The failure risk was estimated with a standard fatigue criterion on the basis of the results obtained from the FEM calculations. Experimental and computational results showed a positive matching.     

The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL

This study investigated the effect of hamstring co-contraction with quadriceps on the kinematics of the human knee joint and the in-situ forces in the anterior cruciate ligament (ACL) during a simulated isometric extension motion of the knee. Cadaveric human knee specimens (n = 10) were tested using the robotic universal force moment sensor (UFS) system and measurements of knee kinematics and in-situ forces in the ACL were based on reference positions on the path of passive flexion/extension motion of the knee. With an isolated 200 N quadriceps load, the knee underwent anterior and lateral tibial translation as well as internal tibial rotation with respect to the femur. Both translation and rotation increased when the knee was flexed from full extension to 30 of flexion; with further flexion, these motion decreased. The addition of 80 N antagonistic hamstrings load significantly reduced both anterior and lateral tibial translation as well as internal tibial rotation at knee flexion angles tested except at full extension. At 30 of flexion, the anterior tibial translation, lateral tibial translation, and internal tibial rotation were significantly reduced by 18, 46, and 30%, respectively (p<0.05). The in-situ forces in the ACL under the quadriceps load were found to increase from 27.8+/-9.3 N at full extension to a maximum of 44.9+/-13.8 N at 15 of flexion and then decrease to 10 N beyond 60 of flexion. The in-situ force at 15 was significantly higher than that at other flexion angles (p<0.05). The addition of the hamstring load of 80 N significantly reduced the in-situ forces in the ACL at 15, 30 and 60 of flexion by 30, 43, and 44%, respectively (p<0.05). These data demonstrate that maximum knee motion may not necessarily correspond to the highest in-situ forces in the ACL. The data also suggest that hamstring co-contraction with quadriceps is effective in reducing excessive forces in the ACL particularly between 15 and 60 of knee flexion.     

A finite element inverse analysis to assess functional improvement during the fracture healing process

Assessment of the restoration of load-bearing function is the central goal in the study of fracture healing process. During the fracture healing, two critical aspects affect its analysis: (1) material properties of the callus components, and (2) the spatio-temporal architecture of the callus with respect to cartilage and new bone formation. In this study, an inverse problem methodology is used which takes into account both features and yields material property estimates that can analyze the healing changes. Six stabilized fractured mouse tibias are obtained at two time points during the most active phase of the healing process, respectively 10 days (n=3), and 14 days (n=3) after fracture. Under the same displacement conditions, the inverse procedure estimations of the callus material properties are generated and compared to other fracture healing metrics. The FEA estimated property is the only metric shown to be statistically significant (p=0.0194) in detecting the changes in the stiffness that occur during the healing time points. In addition, simulation studies regarding sensitivity to initial guess and noise are presented; as well as the influence of callus architecture on the FEA estimated material property metric. The finite element model inverse analysis developed can be used to determine the effects of genetics or therapeutic manipulations on fracture healing in rodents.     

The LSP test for determining oscillation resistance of spinal implants]

QUESTION: New spinal implants need to be tested for primary stability in vitro under standardized laboratory conditions. To ensure the reliability of the test procedures, quality assurance standards in accordance with ISO 9000 were introduced to standardize testing including experimental set ups, loading and test frequency. These standards, however, require a relatively long time to implement. METHOD: The LSP test was used to compare various surface treatments by different shot peening processes applied to spinal rods for dorsal spine implant systems. 6 rods made of two different cp-Titanium materials (Ti-2 and Ti-4) were tested. Dynamic tests were performed with the MTS 810 mono-axial servo-hydraulic test equipment. Beginning with a load of 200 N the rods were subjected to tensile and compressive loads, which were increased in steps of 100 N after every 50.000 cycles until rod failure. RESULTS: Results were available after one to two weeks. The best results (LSP 167 million) were achieved with Ti-4 rods shot peened with steel balls and glass beads. In comparison, the lowest LSP value of 81 million was found with Ti-2 rods shot peened with glass beads only. CONCLUSION: This high speed testing method has reduced the development time from two years to 6 months.     

Surface pretreatment for prolonged survival of cemented tibial prosthesis components: full- vs. surface-cementation technique]

Aseptic loosening of tibial components due to degradation of the interface between bone cement and metallic tibial shaft component is still a persistent problem, particularly for surface-cemented tibial components. The surface cementation technique has important clinical meaning in case of revision and for avoidance of stress shielding. This study was done to prove crack formation in the bone cement near the metallic surface when this is not coated. We propose a newly developed coating process by SiOx-PVD layering to avoid crack formation. A biomechanical model for a vibration fatigue test was done to prove that crack formation can be significantly reduced in the case of coated surfaces. It was found that coated tibial components showed a highly significant reduction of cement cracking near the metal/bone cement interface (p < 0.01) and a significant reduction of gap formation in the metal-to-bone cement interface (p < 0.05). Coating dramatically reduces hydrolytic- and stress-related crack formation at the prosthesis metal/bone cement interface. This leads to a more homogenous load transfer into the cement mantle which should reduce the frequency of loosening in the metal/bone cement/bone interfaces. With surface coating of the tibial component it should become possible that surface-cemented TKAs reveal similar loosening rates as TKAs both surface- and stem-cemented. This would be an important clinical advantage since it is believed that surface cementing reduces metaphyseal bone loss in the case of revision and stress shielding for a better bone health.     

Stretched polyethylene (UHMWPE)--a new substance modification for minimizing wear in artificial knee joint replacement]

Polyethylene wear is a major cause of aseptic loosening of knee endoprostheses. With the aim of minimizing this mechanical wear, we stretch-modified the structure of polyethylene. From plates of UHMWPE, stretched PE samples were produced under defined conditions, and subsequently submitted to friction tests in a fluid environment (cycles 5 x 10(5), frequency 1.5 Hz, load 500 N, contact stress 10 MPa). After load testing, the stretched samples revealed a 70% reduced wear rate in comparison with non-stretched samples. Microscopic examination (light microscopy, scanning electron microscopy) showed abrasive scratches in all test samples, but only in the non-stretched specimens were signs of pitting found. Translating these results to unidirectional tractive rolling on the tibial plateau in the knee joint suggests that a significant reduction in polyethylene wear can be expected. However, further experimental investigations need to be carried out to confirm this highly promising possibility.     

Primary stability of tibial plateaus under dynamic compression-shear loading in human tibiae – Influence of keel length,cementation area and tibial stem

The objective of our study was to evaluate the impact of the tibial keel & stem length in surface cementation, of a full cemented keel and of an additional tibial stem on the primary stability of a posterior stabilised tibial plateau (VEGA® System Aesculap Tuttlingen, Germany) under dynamic compression-shear loading conditions in human tibiae.We performed the cemented tibial plateau implantations on 24 fresh-frozen human tibiae of a mean donor age of 70.7 years (range 47–97). The tibiae were divided into four groups of matched pairs based on comparable trabecular bone mineral density. To assess the primary stability under dynamic compression shear conditions, a 3D migration analysis of the tibial component relative to the bone based on displacements and deformations and an evaluation of the cement layer including penetration was performed by CT-based 3D segmentation.Within the tested implant fixation principles the mean load to failure of a 28 mm keel and a 12 mm stem (40 mm) was 4700 ± 1149 N and of a 28 mm keel length was 4560 ± 1429 N (p = 0.996), whereas the mean load to failure was 4920 ± 691 N in full cementation (p = 0.986) and 5580 ± 502 N with additional stem (p = 0.537), with no significant differences regarding the dynamic primary stability under dynamic compression-shear test conditions.From our observations, we conclude that there is no significant difference between a 40 mm and a 28 mm tibial keel & stem length and also between a surface and a full cementation in the effect on the primary stability of a posterior stabilised tibial plateau, in terms of failure load, migration characteristics and cement layer thickness including the penetration into the trabecular bone.     

Effect of varus/valgus malalignment on bone strains in the proximal tibia after TKR: an explicit finite element study

Malalignment is the main cause of tibial component loosening. Implants that migrate rapidly in the first two post-operative years are likely to present aseptic loosening. It has been suggested that cancellous bone stresses can be correlated with tibial component migration. A recent study has shown that patient-specific finite element (FE) models have the power to predict the short-term behavior of tibial trays. The stresses generated within the implanted tibia are dependent on the kinematics of the joint; however, previous studies have ignored the kinematics and only applied static loads. Using explicit FE, it is possible to simultaneously predict the kinematics and stresses during a gait cycle. The aim of this study was to examine the cancellous bone strains during the stance phase of the gait cycle, for varying degrees of varus/valgus eccentric loading using explicit FE. A patient-specific model of a proximal tibia was created from CT scan images, including heterogeneous bone properties. The proximal tibia was implanted with a commercial total knee replacement (TKR) model. The stance phase of gait was simulated and the applied loads and boundary conditions were based on those used for the Stanmore knee simulator. Eccentric loading was simulated. As well as examining the tibial bone strains (minimum and maximum principal strain), the kinematics of the bone-implant construct are also reported. The maximum anterior-posterior displacements and internal-external rotations were produced by the model with 20 mm offset. The peak minimum and maximum principal strain values increased as the load was shifted laterally, reaching a maximum magnitude for -20 mm offset. This suggests that when in varus, the load transferred to the bone is shifted medially, and as the bone supporting this load is stiffer, the resulting peak bone strains are lower than when the load is shifted laterally (valgus). For this particular patient, the TKR design analyzed produced the highest cancellous bone strains when in valgus. This study has provided an insight in the variations produced in bone strain distribution when the axial load is applied eccentrically. To the authors' knowledge, this is the first time that the bone strain distribution of a proximal implanted tibia has been examined, also accounting for the kinematics of the tibio-femoral joint as part of the simulation. This approach gives greater insight into the overall performance of TKR.     

Calibration of hyperelastic material properties of the human lumbar intervertebral disc under fast dynamic compressive loads

Under fast dynamic loading conditions (e.g. high-energy impact), the load rate dependency of the intervertebral disc (IVD) material properties may play a crucial role in the biomechanics of spinal trauma. However, most finite element models (FEM) of dynamic spinal trauma uses material properties derived from quasi-static experiments, thus neglecting this load rate dependency. The aim of this study was to identify hyperelastic material properties that ensure a more biofidelic simulation of the IVD under a fast dynamic compressive load. A hyperelastic material law based on a first-order Mooney-Rivlin formulation was implemented in a detailed FEM of a L2-L3 functional spinal unit (FSU) to represent the mechanical behavior of the IVD. Bony structures were modeled using an elasto-plastic Johnson-Cook material law that simulates bone fracture while ligaments were governed by a viscoelastic material law. To mimic experimental studies performed in fast dynamic compression, a compressive loading velocity of 1 m/s was applied to the superior half of L2, while the inferior half of L3 was fixed. An exploratory technique was used to simulate dynamic compression of the FSU using 34 sets of hyperelastic material constants randomly selected using an optimal Latin hypercube algorithm and a set of material constants derived from quasi-static experiments. Selection or rejection of the sets of material constants was based on compressive stiffness and failure parameters criteria measured experimentally. The two simulations performed with calibrated hyperelastic constants resulted in nonlinear load-displacement curves with compressive stiffness (7335 and 7079 N/mm), load (12,488 and 12,473 N), displacement (1.95 and 2.09 mm) and energy at failure (13.5 and 14.7 J) in agreement with experimental results (6551 ± 2017 N/mm, 12,411 ± 829 N, 2.1 ± 0.2 mm and 13.0 ± 1.5 J respectively). The fracture pattern and location also agreed with experimental results. The simulation performed with constants derived from quasi-static experiments showed a failure energy (13.2 J) and a fracture pattern and location in agreement with experimental results, but a compressive stiffness (1580 N/mm), a failure load (5976 N) and a displacement to failure (4.8 mm) outside the experimental corridors. The proposed method offers an innovative way to calibrate the hyperelastic material properties of the IVD and to offer a more realistic simulation of the FSU in fast dynamic compression.     

Numerical investigations on the strain-adaptive bone remodelling in the periprosthetic femur: Influence of the boundary conditions

Background  

There are several numerical investigations on bone remodelling after total hip arthroplasty (THA) on the basis of the finite element analysis (FEA). For such computations certain boundary conditions have to be defined. The authors chose a maximum of three static load situations, usually taken from the gait cycle because this is the most frequent dynamic activity of a patient after THA.     

Experimental investigation of failure load and fracture patterns of C2 (axis).

The injury mechanism and magnitude of failure load of C2 fractures are important in the clinical treatment of its fixation. The magnitudes of the failure load of C2 and the mechanism of injury in vivo are uncertain. Accordingly, nine C2 vertebrae obtained from cadaver spines, ranging in age from 51 to 80 years, were used for the study. Special restraint conditions were applied to yield specific fracture of C2. With the posterior element potted postero-anteriorly up to one-quarter of the inferior facet, posterior shear force ranging from 840 to 1220N was required to cause fracture across the pars interarticularis. For odontoid fracture study, a special rig was fabricated to encapsulate the body of C2 in a cell using ISOPON, and a thin layer of ISOPON sandwiched between the inferior facets and two lateral plates. The assembled rig permits slight sagittal movement of C2 about the cup lateral pivot supports. Failure load of between 900 and 1500N was recorded for odontoid fracture. These values are in agreement with published data. The experiment carried out under these two different restraint conditions had specifically resulted in different fractures of C2. In reality, depending on factors such as the inclination of this force vector applied to the head, the precise posture at the time of trauma, the spinal geometry, and the physical properties, different types of fracture patterns of C2 may be produced. This additional data will be useful in the biomechanical study of C2 vertebra using analytical approaches, and in surgical anterior/posterior fixation using screws.