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To evaluate a potential tool for assessing the risk of a pathologic fracture of the femoral shaft, we examined whether fracture loads computed by our computed tomography scan-based finite element models are predictive of measured fracture loads. We also evaluated whether the precision of the computed fracture loads for shafts with metastases is altered if models are generated using mechanical property-density relationships for bone without metastases. We investigated whether femoral shafts with a hemispheric defect and shafts with metastases have qualitatively similar structural behavior. Using identical four-point bending loading conditions, we computed and measured fracture loads of femoral shafts with and without metastases and with a burred hemispheric defect to simulate a tumor. Finite element model fracture loads were strongly predictive of the measured fracture loads (range, 0.92-0.98) even when the models of bones with metastases used mechanical property relationships for bone without metastases. Specimens with hemispheric defects behaved structurally differently than specimens with metastases, indicating that these defects do not accurately simulate the effects of metastases. Results of our study show that these computed tomography scan-based finite element models can be used to estimate the strength of femoral shafts with and without metastases. These models may be useful for assessing the risk of pathologic fractures of femoral shafts.

Finite element (FE) models are often used to model bone failure. However, no failure theory for bone has been validated at this time. In this study, we examined the performance of nine stress- and strain-based failure theories, six of which could account for differences in tensile and compressive material strengths. The distortion energy, Hoffman and a strain-based Hoffman analog, maximum normal stress, maximum normal strain, maximum shear strain, maximum shear stress (tau(max)), Coulomb-Mohr, and modified Mohr failure theories were evaluated using automatically generated, computed tomographic scan-based FE models of the femur. Eighteen matched pairs of proximal femora were examined in two load configurations, one approximating joint loading during single-limb stance and one simulating impact from a fall. Mechanical testing was performed to assess model and failure theory performance in the context of predicting femoral fracture load. Measured and FE-computed fracture load were significantly correlated for both loading conditions and all failure criteria (p < or = 0.001). The distortion energy and tau(max) failure theories were the most robust of those examined, providing the most consistently strong FE model performance for two very different loading conditions. The more complex failure theories and the strain-based theories examined did not improve performance over the simpler distortion energy and tau(max) theories, and often degraded performance, even when differences between tensile and compressive failure properties were represented. The relatively strong performance of the distortion energy and tau(max) theories supports the hypothesis that shear/distortion is an important failure mode during femoral fracture.

Hip fracture is an important cause of morbidity and mortality among the elderly. Current methods of assessing a patient's risk of hip fracture involve local estimates of bone density (densitometry), and are limited by their inability to account for the complex structural features of the femur. In an effort to improve clinical and research tools for assessing hip fracture risk, this study investigated whether automatically generated, computed tomographic (CT) scan-based finite element (FE) models can be used to estimate femoral fracture load in vitro. Eighteen pairs of femora were examined under two loading conditions one similar to loading during the stance phase of gait, and one simulating impact from a fall. The femora were then mechanically tested to failure and regression analyses between measured fracture load and FE-predicted fracture load were performed. For comparison, densitometry measures were also examined. Significant relationships were found between measured fracture load and FE-predicted fracture load (r = 0.87, stance; r = 0.95, fall; r = 0.97, stance and fall data pooled) and between measured fracture load and densitometry data (r = 0.78, stance; r = 0.91, fall). These results indicate that this sophisticated technique, which is still early in its development, can achieve precision comparable to that of densitometry and can predict femoral fracture load to within -40% to +60% with 95% confidence. Therefore, clinical use of this approach, which would require additional X-ray exposure and expenditure for a CT scan, is not justified at this point. Even so, the potential advantages of this CT/FE technique support further research in this area.