Relation of coxarthrosis to stresses and morphogenesis. A finite element analysis.

We calculated subchondral deformations and stresses in the femoral head and acetabulum during weight bearing using finite element models. Areas of high joint contact pressures on the femoral head were shown to correspond to high hydrostatic compression in subchondral bone. The magnitude of the subchondral bone compressive hydrostatic stress correlated with cartilage thickness and was highest in the superior femoral head and moderate at the acetabular roof. The seldom contacting surfaces of the medial-inferior and peripheral areas of the femoral head and the roof of the acetabulum had lower hydrostatic compression and significant subchondral bone tensile strains tangential to the joint surface. Initial cartilage fibrillation and osteophyte formation are often found in these areas. These findings suggest that fluctuating hydrostatic pressure inhibits vascular invasion and the degeneration and ossification of articular cartilage. The generation of tensile strain may promote the degenerative process by direct mechanical mechanisms. Additionally, since tensile strains are associated with a reduction in the compressive hydrostatic stresses in the cartilage and an increase in shear stresses, their presence may permit or promote vascular invasion, cartilage degeneration, and osteophyte formation. These mechanical principles in arthrosis are the same as those that have been previously demonstrated to guide the degeneration and ossification of the cartilage primordium during skeletal morphogenesis. In this sense, arthrosis may be viewed as the final stage in the degeneration and ossification of the cartilage anlage.