The pathogenesis of femoral head deformity in congenital dislocation of the hip. Experimental study of the effects of articular interpositions in pigs.

An experimental study was conducted based on the hypothesis that articular interpositions such as inverted limbi are major factors influencing deformities of the femoral head and neck that complicate treatment of congenital dislocation of the hip. In 24 neonatal pigs, the hip was fixed in a cast for one to three hours in the frog-leg position after insertion of an allogeneic meniscus into the hip joint. No macroscopic changes were observed immediately and one day after the above procedure, but electron microscopy showed degeneration of the cells in the physis. After 29-39 days, varus deformity and flattening of the femoral head were observed in all animals. After five months, marked deformity of the femoral head and replacement of the physis by fibrous tissue were noted. Angiograms obtained during cast fixation and 30 minutes after removal of the cast showed no circulatory occlusion. A short period of immobilization in the frog-leg position combined with articular interposition plays a major role in degeneration of the physis and predisposes to deformities of the femoral head and neck.