[The medius gluteus with 1-legged stance. Geometric variations on the theme of femoral and pelvic osteotomies].

The work of Pauwels has supplied evidence of the pressure applied to the hip by the contraction of the gluteus medius as its acts to maintain the pelvis in equilibrium when weight is born on one limb. The force exerted by the gluteus medius is greater the larger the distance separating the gluteus medius from the centre of the femoral head compared with the distance separating the femoral head from the centre of gravity. It is this force that mechanical procedures such as osteotomy of the femur or pelvis attempt to diminish. A study was made in depth of the variations in the activity of the gluteus medius with regard to various parameters including not only the gluteus medius distance (parameter of Pauwels) but also of the direction of the muscle and the angle formed by the cephalo-trochanteric line below and the iliac wing above the ilio-trochanteric angle. Using routine radiographs to act as a geometric model, a method was devised based on a combination of geometric construction, an experimental study and trigonometric calculation. Curves have been established determining the value of the force exerted by the gluteus medius in relation to the ilio-trochanteric angle. The smallest force exerted by the gluteus medius corresponds to the lower pole of the curve and arises when the cephalo-trochanteric line is perpendicular to the direction of the muscle; the force is greatest in the highest and lowest positions of the tip of the great trochanter, that is to say on either side of the lower pole of the curve. Since the position of the greater trochanter is, in practice, always lower, it can be said that, as a general rule, angulation into varus diminishes the tension of the gluteus medius whilst angulation into valgus increases it. However, contrary to current opinion, the effect produced on the force applied by the gluteus medius is not proportional to the ammount of varisization. The effectiveness of a varisization is greater the more the hip is in valgus and there is a critical zone with increasing varus where the effect becomes minimal. Displacement of the femoral shaft (McMurray's operation) has little effect. In Chiari's operation, diminution of the ilio-trochanteric angle and shortening of the medial lever with lengthening of the lateral lever by medial transposition of the point of application of the femoral head diminishes the force required of the gluteus medius very effectively. In particular, the effect of Chiari's operation is greater when the hip is displaced or there is coxa valga. The method described allows a precise calculation to be made on a tracing of the patient's radiograph of the effect to the proposed operation on the force required by the gluteus medius. The different parameters could be applied to a computer and thus allows statistical definition of anthropometric date concerning the force of contraction of the gluteus medius.