Minor Deviations in Cup Center of Rotation Can Reduce Impingement-Free Range of Motion in Total Hip Arthroplasty

Background Precise restoration of the acetabular center of rotation (COR) is important for hip biomechanics, particularly in patients with stiff spinopelvic mobility. This study quantified the effect of small COR shifts on impingement-free virtual range of motion (ROM). Methods We retrospectively reviewed the final acetabular component position achieved during robotically assisted total hip arthroplasty in 100 patients. Impingement-free ROM was measured for the final plan, then reassessed after translating the COR 2 mm anteriorly, superiorly, or medially. Patients were stratified by spinopelvic flexibility. Results A 2 mm COR shift altered virtual ROM in 93% of patients. Medial translation increased failure to meet any functional ROM threshold from 16% to 30% ( P < .001). Stiff-spined patients showed significantly higher failure rates after all COR shifts ( P < .05). Conclusions Small deviations from planned COR meaningfully reduce virtual ROM and have the greatest impact in patients with stiff spinopelvic mechanics.