FUNCTIONAL ASPECTS OF THE ABDUCTOR MUSCLES OF THE HIP

The study of the forces acting in and around the hip joint explains numerous, apparently unrelated, clinical observations. The reacting force in the head of the femur resists not merely the pressure of the superincumbent body weight, but, in addition, the force of the abductor muscles and the tension on the iliotibial tract which is necessary to hold the pelvis in equilibrium. This latter force exceeds the weight of the body by a ratio of from 1.4:1 to 1.9:1. The minimum static pressure on the head of the femur is approximately 2.4 to 2.6 times the body weight. Normally this force is not borne vertically, but at an angle of approximately 165 to 170 degrees from the vertical and in line with the medial trabeculae of the femoral neck. This static force remains constant in direction, irrespective of whether the pelvis is levated or depressed on the non-weight-bearing side. With the knowledge of the direction of the reacting force through the head and neck of the femur against the pelvis, the changing angle of the capital epiphysis during growth becomes partially explicable. Close inspection of roentgenograms of the upper end of the femur in individuals of various ages reveals that the epiphyseal line always lies at a right angle to the direction of the medial trabeculae of the femoral neck (Fig.9). Since the reacting force in the femur follows these trabeculae, there is no sheer on the epiphyseal cartilage. The only force to which it is subjected normally is a compressional one. As growth occurs, with increasing length of the femoral neck and increasing angle of pull of the abductor muscles and the fascia, the epiphyseal cartilaginous plate rotates so as to lie perpendicular to the equilibrant force in the femoral neck. The interrelationship of these forces becomes altered in cases of paralysis of the abductor muscles and in congenital dislocation of the hip. In the first instance, the loss of muscle power prevents the attainment of equilibrium with the body in a normal position. To prevent the pelvis from rotating toward the non-weight-bearing side, the individual shifts his center of gravity over the affected hip by bending to that side. The resultant forces, acting through the hip, become more nearly vertical. The equilibrant or reacting force in the femoral neck likewise shifts toward the vertical; the epiphyseal cartilaginous plate, remaining perpendicular to these forces, continues to be relatively horizontal; and coxa valga results. This is a constant finding in individuals who have had paralysis early in life, and the degree of coxa valga is in proportion to the loss of muscle power. Coxa valga occurs in individuals suffering from congenital dislocation of the hip, for the same factors are at work. In these cases, the loss of fulcrum prevents the normal development of the abducting forces. The load is borne vertically on the femur, and the capital epiphysis reacts by remaining horizontal. [see pdf for figure] The so-called "antalgic" gait of the individual who has a painful disease of the hip joint is of interest. On the surface,it appears rather peculiar that a patient with a painful hip will walk by throwing himself over the painful joint, giving rise to a gait which may be compared with that of a glutaeus medius weakness. By so doing, however, the individual shifts his center of gravity over the hip, thus decreasing the required pull of the abductor muscles. This, in turn, decreases the pressure upon the femoral head from a force of 2.4 to 2.6 times the body weight to simply the superincumbent body weight. This expediency of gait, while decreasing the total load on the femoral head, results in a change in the direction of the reacting force in the femur. Instead of the body weight being borne at an angle which thrusts the head into the acetabulum, the load is carried vertically upon the femur. This alteration in direction of the forces causes changes in the femoral head, which are depicted in the roentgenograms. In malum coxae senilis, the bone changes affect predominantly the superior aspect of the femoral head, because the forces are concentrated there. In epiphysiolysis the assumption of an antalgic gait, while leading to a decrease in the total load on the femur, causes that load to be borne vertically, with the result that a sheer is applied to the epiphyseal plate, which normally is not present. This sheer will facilitate further slipping. The importance of the abductor muscles and the iliotibial tract in altering the direction of the reacting forces in the femoral neck should be emphasized. Any surgical approach to the hip joint which injures these structures and weakens the forces exerted by them will not only modify the magnitude, but also the direction, of the reacting force through the femoral neck.