Hip Stability After Total Hip Arthroplasty: Quantifying Capsule and Passive Muscle Contributions.

This study developed a computational model that incorporated the hip capsule and local muscles, as well as hip stem alignment variability, to quantify contributions to hip passive stability following total hip arthroplasty (THA). Soft tissue properties were calibrated to minimize the root mean square error between the model and experimentally measured torque responses across various hip flexion angles. Our results demonstrated that the capsule contributed an average of 31.0% to the resistance against hip internal-external rotation, while local passive muscle constraint accounted for 69.0%. Notably, the capsule's contribution was significantly higher during hip extension, accounting for 81.4% of the resistance to femur external rotation at 20° hyperextension and 63.1% at 0° flexion. Specifically, the iliofemoral ligament and gluteus medius were identified as the primary structures restricting anterior dislocation, with the former contributing up to 80.8% resistance to femur external rotation in hyperextension and the latter providing approximately 48.3% resistance during hip flexion. In contrast, the ischiofemoral ligament and external rotators, particularly the quadratus femoris, played a critical role in resisting posterior dislocation, collectively providing approximately 78.9% resistance to femur internal rotation across all tested hip flexion angles. These structures should be preserved or carefully repaired during THA whenever possible. Optimizing implant alignment can influence hip stability by improving the implant-bone relative position and altering soft tissue tension. For patients at higher risk of dislocation, a slightly superior stem alignment increased dislocation resistance by 18.3%. In contrast, excessive external positioning resulted in early impingement during anterior dislocation simulations, suggesting a potential risk of impingement-related instability.