Evaluation of optimal implant alignment in total hip arthroplasty based on postoperative range of motion simulation

Background: Dislocation after total hip arthroplasty is a frequent cause of revision surgery. This study was performed to determine the optimal implant alignment in total hip arthroplasty by simulating the postoperative range of motion. Methods: All operations were performed via posterolateral approach using combined anteversion of the stem and cup technique. Maximum range of motion without implant impingement was simulated in 79 replaced hips using postoperative computed tomography and the achievement of the required range of motion defined by previous studies was assessed. Optimal cup and stem alignment for impingement-free range of motion were statistically determined using the receiver operator coefficient curve. Findings: Cup inclination and anteversion, stem anteversion, and combined anteversion were 37.6°, 20.1°, 26.2°, and 46.3°, respectively. Maximum range of motion in flexion, extension, internal rotation at 90° of flexion, and external rotation were 131.8°, 42.3°, 56.4°, and 64.5°, respectively. Flexion >110°, extension >30°, internal rotation >30° at 90° of flexion, and external rotation >30° were fulfilled by 96%, 86%, 92%, and 96% of all replaced hips, respectively. Optimal implant alignment for impingement-free range of motion was 34°–43° of cup inclination, 18°–26° of cup anteversion, 17°–29° of stem anteversion, and 35°–56° of combined anteversion. Both cup and stem anteversion showed significant relationship with postoperative range of motion. Interpretation: Surgeons could gain valuable insights into optimal cup and stem alignment to perform postoperative range of motion simulations.