Radiographic calculation of anteversion in acetabular prostheses.

The position of the acetabular prosthesis is critical for preventing dislocation following total hip arthroplasty. The reliability of a mathematical model for radiographically calculated acetabular cup version was examined. A porous-coated anatomic acetabular prosthesis was mounted in a mold. Anteroposterior radiographs were taken with the cup in five different positions of anteversion. These were reviewed by five orthopaedic surgeons, and measurements were taken from each radiograph. From these measurements, the mathematically derived degree of version was calculated. The results were examined for accuracy and intraobserver reliability. It was concluded that intraobserver reliability was very good and that the accuracy was within a clinically acceptable range. This technique could be useful in studying the "safe zone" for acetabular prostheses.