Differences in range of motion with the same combined anteversion after total hip arthroplasty

Purpose We investigated the various impingement angles (including both bony and prosthetic impingement) and impingement types that can occur after THA, even when the same combined anteversion parameter is used. We also investigated the relationship between impingement angle and acetabular morphology or femoral anteversion. Methods We evaluated 83 patients with no hip arthritis. We divided them into six groups according to acetabular CE angle (<= 15 degrees, >15-<= 25 degrees, and >25 degrees) and femoral anteversion (<= 20 degrees and >20 degrees). Usingthree-dimensional templating software, we changed stem and cup anteversion to satisfy a combined anteversion (CA) of 50 degrees in each hip (stem anteversion + cup anteversion = 50 degrees) and investigated the resulting impingement angles. Results Even with the same CA, differences in impingement angle occurred: 18.3 degrees +/- 7.2 degrees with flexion, 30.2 degrees +/- 9.7 degrees with internal rotation at 90 degrees flexion, 20.2 degrees +/- 12.5 degrees with extension, and 26.2 degrees +/- 7.8 degrees with external rotation. As stem anteversion increased, the impingement type changed from prosthetic impingement to bony impingement in flexion and internal rotation and from bony impingement to prosthetic impingement in extension and external rotation. The flexion angle and internal rotation angle at 90 degrees flexion increased (p < 0.016) as CE angle decreased. There were no significant differences between high and low femoral anteversion. Conclusions Combined anteversion theory should be used with care because of large differences in impingement angles. A stem anteversion of 30 degrees and cup anteversion of 20 degrees appear to be ideal for obtaining a larger impingement angle under this condition.