The development of a computational stress analysis of the femoral head. Mapping tensile, compressive, and shear stress for the varus and valgus positions.

Using a computer-based, two-dimensional finite-element analysis which has wide application to problems involving the hip joint, alterations in the distribution of stress in the femoral head consequent to varus and valgus osteotomy were studied. The mathematical model used in this analysis incorporated experimentally measured spatial variations in the stiffness of the bone of the femoral head and neck. These variations led to patterns of load transmission that were strikingly different from those in a homogeneous material. Because of their lower stiffness, the central region of the head and the medullary region of the neck make very little contribution to weight-bearing, regardless of the orientation of the femoral head. In the neutral configuration (normal neck-shaft angle), the lateral cortex of the neck is in slight tension, while the medial cortex is under strong compression and provides the support for the over-all load on the joint. With increasing valgus angulation, the bending component of the joint load disappears progressively, and when the valgus angulation is 30 degrees in excess of normal essentially equal compressive stresses prevail in both cortices. Varus osteotomy exaggerates the bending component relative to the compressive component of the load. The computed stress patterns in the femoral head and neck for the normal neck-shaft angle show elevations of shear stress where the lateral epiphyseal artery enters and branches within the femoral head. This finding may be significant since this region is the area at risk of infarction in both Legg-Perthes disease in children and idiopathic aseptic necrosis in adults. Since a 30-degree varus angulation induces tensile stresses in the lateral cortex of the neck that are increased fourfold above those for the neutral configuration, it is postulated that force transmitted through the femur when the hip is in abduction could produce shear fractures of the bone in the region of the central branch of the lateral epiphyseal artery and thus occlude this vessel and initiate aseptic necrosis of the femoral head.