Flatback Deformity and Stiff Pelvic Mobility Alter the Optimal Cup Anteversion After Total Hip Arthroplasty

Background: Flatback deformity and stiff spinopelvic mobility (SPM) are recognized risk factors for dislocation after total hip arthroplasty. This study aimed to determine patient-specific optimal cup anteversion ranges associated with spinopelvic abnormalities. Methods: We analyzed postoperative sit-to-stand motion in 108 hips using three-dimensional-to-two-dimensional model-image registration. Implanted components were virtually positioned at their actual positions with inclination at 40°. Using pelvic tilt at maximal anterior tilt and standing, the range of combined anteversion (CA) satisfying required range of motion was evaluated. CA midpoint (CA_mid) and range (CA_range) were compared across spinopelvic alignment groups. Results: Flatback hips demonstrated a significantly lower CA_mid than normal (41.1° vs. 46.8°), whereas stiff SPM hips showed a similar CA_mid but a narrower CA_range (14.5° vs. 19.4°). Significant correlations were observed between spinopelvic alignment and CA parameters. Conclusions: Spinopelvic alignment influences both the optimal cup orientation and its tolerance range. Incorporating these factors may allow patient-specific optimization of acetabular component positioning. Level of Evidence: Level III, retrospective cohort study. See Instructions for Authors for a complete description of levels of evidence. Clinical Relevance: Personalized component positioning based on dynamic spinopelvic alignment may help reduce dislocation risk and improve implant stability in high-risk THA patients.