The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios

Background: Prosthetic impingement due to poor positioning can limit the ra nge of motion of the hip after total hip arthroplasty, In this study, a com puter model was used to determine the effects of the positions of the aceta bular and femoral components and of varying head-neck ratios on impingement and range of motion. Methods: A three-dimensional generic hip prosthesis with a hemispherical cu p, a neck diameter of 12.25 millimeters, and a head size ranging from twent y-two to thirty-two millimeters was simulated on a computer. The maximum ra nge of motion of the hip was measured, before the neck impinged on the line r of the cup, for acetabular abduction angles ranging from 35 to 55 degrees and acetabular and femoral anteversion ranging from 0 to 30 degrees. Stabi lity of the hip was estimated as the maximum possible flexion coupled with 10 degrees of adduction and 10 degrees of internal rotation and also as the maximum possible extension coupled with 10 degrees of external rotation. T he effects of prosthetic orientation on activities of daily living were ana lyzed as well. Results: Acetabular abduction angles of less than 45 degrees decreased flex ion and abduction of the hip, whereas higher angles decreased adduction and rotation. Femoral and acetabular anteversion increased flexion but decreas ed extension. Acetabular abduction angles of between 45 and 55 degrees perm itted a better overall range of motion and stability when combined with app ropriate acetabular and femoral anteversion, Lower head-neck ratios decreas ed the range of motion that was possible without prosthetic impingement. Th e addition of a modular sleeve that increased the diameter of the femoral n eck by two millimeters decreased the range of motion by 1.5 to 8.5 degrees, depending on the direction of motion that was studied. Conclusions There is a complex interplay between the angles of orientation of the femoral and acetabular components. Acetabular abduction angles betwe en 45 and 55 degrees, when combined with appropriate acetabular and femoral anteversion, resulted in a maximum overall range of motion and stability w ith respect to prosthetic impingement. Clinical Relevance: During total hip arthroplasty, acetabular abduction is often constrained by available bone coverage, while femoral anteversion may be dictated by the geometry of the femoral shaft, For each combination of acetabular abduction and femoral anteversion, there is an optimum range of acetabular anteversion that allows the potential for a maximum range of mot ion without prosthetic impingement after total hip arthroplasty, These data can be used intraoperatively to determine optimum position.