Evaluation of finite element analysis for prediction of the strength reduction due to metastatic lesions in the femoral neck.

Between 30 and 70% of almost one million new cancer patients diagnosed each year will develop osseous metastases. Clinicians are faced with the difficult task of determining which patients require prophylactic stabilization to prevent pathologic fracture. The objective of this study was to test the ability of macroscopic finite element models to predict the fracture strength of the proximal femur with a lesion in the femoral neck. Drill hole defects in human cadaver femora were used to simulate lesions that penetrate one cortex of the femoral neck. Based on the first of two series of in vitro experiments, the fracture strength of a femur with a lesion that penetrates either the inferior-medial or superior-lateral cortex of the neck is approximately 45% less than the fracture strength of the paired intact femur; based on the second series, the fracture strength with the inferior-medial lesion is approximately 20% less than the fracture strength with the superior-lateral lesion. A series of three-dimensional finite element models were used to predict the fracture strength for anterior and posterior lesions, as well as the inferior-medial and posterior-lateral lesions tested in vitro. Based on a direct comparison of the strengths predicted by the finite element models to the measured in vitro fracture strengths, the finite element models performed poorly. In particular, the application of an anisotropic strength criterion to the stresses predicted by the models resulted in a considerable underestimation of the percentage reduction in the in vitro fracture strength. This may reflect a fundamental inability of a linear, macroscopic continuum-based analysis to predict accurately the fracture strength of a bone structure as complex as the proximal femur. However, despite this lack of agreement in absolute fracture strength, the general trends for gait and stair ascent loading for the inferior-medial and superior-lateral lesions were consistent with the in vitro data. The greatest reduction in strength was predicted for the inferior-medial lesion, followed by the anterior lesion and then the superior-lateral lesion, and the least reduction in strength was predicted for the posterior lesion. Most importantly, the predicted strength ratio varied considerably as a function of the applied loads. Any metastatic lesions of the femoral neck may be especially sensitive to some particular activity, making it difficult to determine precisely the risk of fracture.