The biomechanics of the normal and dysplastic hip

The author describes a simple method to highlight forces, by extrapolating on x-rays of normal and dysplastic hips the direction of the skeletal lamellae of the pelvis and that of the proximal femoral metaepiphysis. In the pelvis is described a tridimensional structure, made of two basic lamellar systems: one that begins from the "lamina quadrilatera" and one from the lateral border of the "sourcil." They move in the opposite direction: the first towards the Waldayer tuberosity, the anterosuperior and inferior iliac spine; the second towards the sacroiliac joint. They form a structure similar in shape to an hourglass, the inferior cone of which underlines the "sourcil" and looks like a Gothic arch. In the normal hip the "sourcil" is slightly oblique (3-4 degrees) in bipodalic support. It becomes horizontal in monopodalic support. From the direction of the lamellae of the Gothic arch the author obtains the direction of the forces acting in the static and dynamic phase of the normal hip. In the dysplastic hip the "sourcil" may be oblique in an anterocraniolateral or posterocraniomedial direction. Consequently, the lamellae of the hour-glass and of the Gothic arch undergo a different orientation. From these altered orientations the author visualizes the direction of the pathologic forces. Based on the shape of the Gothic arch and of the angle of inclination of the femoral neck the author describes the forces acting on the pelvis and on the femoral head when there is either valgus or varus in the neck; according to the degrees of inclination of the femoral neck he describes the variation of magnitude of loading on the support surfaces.