Dynamic Radiography Analysis of Functional Safe Zone During Sit-to-Stand Motion After Total Hip Arthroplasty and Its Association with Preoperative Factors

Background: Functional safe zones for implant positioning in total hip arthroplasty (THA) have primarily been evaluated in static postures. However, no studies have examined sit-to-stand movements, which carry a high risk of posterior dislocation. In this study, we recorded sit-to-stand motion post-THA using a dynamic x-ray system to analyze pelvic tilt and hip flexion angles during sitting and standing, including sex differences and the influence of sitting posture, and identify preoperative factors associated with standing alignment. Methods: We retrospectively evaluated 118 patients 6 months post-THA. Sequential lateral radiographs were used to capture maximal anterior pelvic tilt during standing. The sacral slope (SS) and pelvic–femoral angle (PFA) were measured; changes from the sitting position were calculated. Sex differences and correlations with sitting posture were assessed, and regression analyses identified predictors of standing SS and PFA. Results: The mean ΔSS was 29.6° ± 8.7°, and the mean ΔPFA was 24.8° ± 8.7°. ΔSS showed no sex difference (p = 0.331), whereas ΔPFA was larger in women (25.8° ± 8.6°) than in men (20.5° ± 8.3°, p = 0.011). ΔSS correlated negatively with sitting SS (r = −0.550, p < 0.001), and ΔPFA correlated positively with sitting PFA (r = 0.478, p < 0.001). Age, preoperative hip flexion, and pelvic incidence–lumbar lordosis (PI-LL) moderately predicted standing PFA. Conclusion: During sit-to-stand motions post-THA, the pelvis tilted anteriorly by approximately 30°, and the hip flexed by approximately 25°, influenced by sex and sitting posture. Standing hip flexion was affected by age, preoperative hip flexion angle, and PI-LL. Level of Evidence: Diagnostic Level III . See Instructions for Authors for a complete description of levels of evidence.