Spherical assessment of the hip on standard AP radiographs: a simple method for the measurement of the contact area between acetabulum and femoral head and of acetabular orientation.

A template that allows a simple three-dimensional assessment of the hip joint is presented. The total surface of a sphere is subdivided into spherical quadrangles and triangles that cover areas of exactly identical size. On the template, these quadrangles and triangles are projected onto a plane. Each segment represents 0.5% of the total surface of the sphere. Calculation of any surface area is achieved by adding the number of segments involved of a template of appropriate radius. In addition, various angles can be measured. The template is drawn in various sizes on transparent foil. The template of the appropriate size is laid on the anteroposterior radiograph of the hip joint. It allows measurement of the contact area between the acetabulum and the femoral head and also acetabular orientation. A cadaver study showed good reliability of the measurements.