[Spherical calculation of the contact surface between the acetabulum and femur head]

A template for calculating the contact area between the femoral head and the acetabulum is described. The projected image of a sphere is divided into segments of equal surface areas, each segment comprising 0.5% of the total surface area of the sphere. By adding the segments and multiplying the resulting percentage by the total surface area resulting from the radius of the sphere, any partial area of the sphere can be calculated. It is thus possible, without any special mathematical knowledge, to calculate the contact area between the femoral head and the acetabulum, as well as any partial surface area relevant to loading of the joint, on the a.p. roentgenogram. A prerequisite for this is a sufficient area of spherical femoral head in the region in question. In addition, the template can also be used to calculate the biomechanically relevant angle between the frontal plane, set through the center of the femoral head, and the anterior limit of the acetabulum, on the a.p. roentgenogram of the hip.