Although total hip arthroplasty (THA) is highly successful, standard cup orientation does not prevent all impingement-related complications. Functional planning integrates spinopelvic motion, but flexed-seated definitions of hip hypermobility remain unclear. We aimed to define clinically relevant flexed-seated femur–pelvis motion thresholds and test their association with simulated impingement and anteversion safe-zone constraints.

The existence of a pathological hip mobility threshold, independent of other risk factors for impingement with standard cup positioning, would provide additional evidence supporting the need to define a personalized cup orientation when planning total hip arthroplasty.

This retrospective consecutive cohort included 728 primary THA candidates with preoperative standing and flexed-seated lateral radiographs. Measured variables were spinopelvic tilt (SPT), pelvic incidence (PI), lumbar lordosis (LL), lumbar flexion, standing pelvic–femoral angle (PFAstanding), flexed-seated pelvic–femoral angle (PFAflexed-seated), and ΔPFA. The primary endpoint was simulated impingement at 40/20°. Secondary endpoints were impingement at 40/15° and patient-specific anteversion corridor metrics (minimum anteversion without anterior impingement, maximum anteversion without posterior impingement, safe-zone width = max–min, and corridor collapse when width ≤0). ROC analysis evaluated ΔPFA discrimination; logistic regression identified independent predictors. Within ΔPFA ≥110°, three mechanistic phenotypes were defined: high standing PFA only (≥200°), low flexed-seated PFA only (≤90°), and both.

ΔPFA discriminated impingement at 40/20° (AUC 0.79). At ΔPFA 110°, specificity was 94.4%, PPV 69.1%, sensitivity 34.7%, and NPV 80.0%, identifying a small high-risk subgroup. Compared with ΔPFA <110°, ΔPFA ≥110° was associated with higher adverse spinopelvic mobility (ΔSPT ≥20°: 76.9% vs 9.0%; OR 33.63, p < 0.001), higher impingement at 40/15° (78.8% vs 25.0%; OR 9.41, p < 0.001) and 40/20° (71.9% vs 19.9%; OR 10.02, p < 0.001), and smaller safe-zone width (median 4° vs 28°, p < 0.001). In multivariable analysis, ΔSPT ≥20° (OR 10.72, p < 0.001) and ΔPFA ≥110° (OR 2.56, p = 0.0032) independently predicted 40/20° impingement. Within ΔPFA ≥110°, the low flexed-seated phenotype had the highest corridor collapse (56.5%) and impingement rates (40/20°: 78.3%; 40/15°: 80.4%). Very low PFAflexed-seated (especially <80–85°) identified extreme outliers with frequent absence of an impingement-free corridor.

In flexed-seated analysis, ΔPFA ≥110° is a practical preoperative marker of elevated impingement risk, while low absolute PFAflexed-seated (≤85°, especially <80°) identifies the highest-risk hyperflexion outliers with corridor collapse. Combining ΔPFA and flexed-seated PFA phenotyping may improve patient-specific cup orientation planning before THA.

IV; retrospective study.