Acetabular Alignment and Primary Arc of Motion for Minus, Skirtless, and Skirted 28-, 32-, 36-, and 40-mm Femoral Heads

We examined the influence of acetabular cup alignment on the primary arc of motion (PAOM) for minus (−3.5), skirtless (+3.5), and skirted (+10.5) neck lengths with 28-, 32-, 36-, and 40-mm femoral heads. Total hip arthroplasty was performed in a pelvic saw bone model. The acetabulum was positioned in 30°/45°/60° abduction and 0°/15°/30° anteversion, respectively. The PAOM was determined using a digital inclinometer for each of the acetabular positions and 12 femoral heads mentioned above. Best PAOM was found in 45° abduction and 15° to 30° anteversion as well as 60° abduction and 0° anteversion. Combinations of excessive abduction and anteversion as well as horizontalization and retroversion resulted in the worst PAOM. Primary arc of motion is mainly determined by cup alignment even when using larger femoral heads.