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.