The biomechanics of the pediatric hip.

Containment refers to the relationship of the most lateral part of the femoral epiphysis to the lip of the acetabulum. Congruence refers to the relation between articular surfaces, but is not as useful a concept as sphericity. Coverage refers to the relationship of the lateral lip of the acetabulum to the resultant force across the hip joint and in most cases is best estimated by the center-edge angle. Femoral heads can be well contained but poorly covered and vice versa. Appreciation of these three concepts is important, not only in considering treatment of the pediatric hip but also in communicating with one's colleagues. Varus osteotomy redirects muscle forces about the hip and improves containment. The innominate osteotomy has these effects also but in addition improves coverage.