Increased range of motion to impingement with large head total hip arthroplasty: point of diminishing returns.

Large head total hip arthroplasty (THA) is known to increase range of motion to impingement and decrease risk of dislocation, however, this is dependent on accurate component positioning and patient anatomy. In this study, a computer-aided design model was used to determine the effects of component positioning on range of motion to impingement with increasing head size. Three-dimensional models were made of 7 cadaver CT scans and virtual THA was performed with a conventional implant system. Theoretical range of motion to impingement was tested before and after the components were implanted in flexion, extension, internal/external rotation, abduction, adduction, and flexed internal/external rotation. Range of motion increased non-linearly in every motion except for external rotation and adduction with increasing head size, with gains in ROM limited by osseous impingement. Use of large head THA leads to increased ROM to impingement, but with larger head sizes, benefits are limited by bony anatomy.