Biomechanics of hip dysplasia.

When walking, each hip alternately carries the body mass minus the supporting leg. This mass exerts a force K acting on the hip with a lever arm h'; it is counter-balanced by a force M exerted by the abductor muscles, which acts on the hip with a lever arm h. The hip joint transmits the resultant R of forces K and M. Force R evokes compressive stresses in the joint. In a normal hip, force R is exerted at the center of the force transmitting surface of the joint, and the stresses are evenly distributed over this surface. This is reflected by the narrow ribbon of subchondral sclerosis in the roof of the socket. The normal femoral neck is stressed in bending with shear. It has a medial bundle of cancellous trabeculae, stressed in compression, intersecting with a lateral bundle, stressed in tension. Dysplasia and an imbalance of the muscles may have similar mechanical consequences: a shortening of the lever arm h and a more vertical orientation of force M. Shortening of h results in an increase of the force M necessary to counterbalance the moment of force K; the change in direction of force M displaces the resultant R towards the edge of the socket. The result is an increased resultant force R which becomes unevenly distributed over a smaller surface. Subluxation of the femoral head due to a shallow socket will also result in an uneven distribution of force R on a smaller surface. The abnormal distribution of compressive stresses is reflected by the development of a triangular sclerosis at the edge of the socket. This is the beginning of subluxating osteoarthritis of the hip. In a coxa valga the femoral neck may be stressed in a pure compression. This appears in the structure of the cancellous bone of the neck. Coxa vara has opposite consequences: lengthening of the lever arm h of the abductor muscles and a more horizontal orientation of force M. This results in a decrease of the resultant force R, which is displaced medially over a larger surface, resulting into smaller compressive stresses in the joint. If the acetabular cartilage of the socket does not develop further medially, the distribution of compressive stresses may become uneven with a maximum medially, where a dense triangular sclerosis develops. This is the beginning of protrusive osteoarthritis of the hip. In coxa vara, the femoral neck is stressed in bending more than in a normal hip, which is reflected by more marked cancellous trabeculae which intersect at right angles in the femoral head. The medial bundle is stressed in compression, the lateral bundle in tension.