Containment of the hip: a theoretical comparison of osteotomies.

Using a computerized model derived from anatomic specimens, coverage of the femoral head by the acetabulum during the gain cycle was studied and compared for normal hips, varus-derotation osteotomy, and innominate osteotomy (Salter). Theoretical estimates of joint reaction force were also studied during the gain cycle for the three conditions. The entire proximal femoral epiphysis was never covered by the acetabulum with any of these procedures. Innominate osteotomy gave better anterior coverage (with resultant poorer posterior coverage) than the femoral procedure. Varus-derotation osteotomy increased medial shear force at the epiphyseal plate to twice body weight, but in all cases the major component of force was normal to the epiphyseal plate (compression) rather than parallel to it (shear).