Influence of Pelvic Tilt and Bone Morphology on Extra-Articular Bony Impingement in Total Hip Arthroplasty.

Abstract

Extra-articular bony impingement may contribute to posture-specific groin pain after total hip arthroplasty (THA), but the relative roles of bony morphology and functional pelvic tilt remain unclear. CT reconstructions from 100 cadavers (200 hips) were used to build three-dimensional bone-on-bone collision detection models to quantify impingement-free range of motion (RoM). Femora were rotated in internal/external rotation and abduction/adduction from 30° hyperextension to 120° flexion in 5° flexion increments. Simulations were repeated with pelvic tilt ranging ±30° from neutral as extreme boundaries. The contribution of bony anatomy to early impingement was evaluated using Pearson correlations and two-stage regression, with nested cross-validation used for supplementary internal validation. Mean external rotation to extra-articular bony impingement in extension was 46.6° ± 14.8°, and mean internal rotation to impingement at 90° flexion was 34.4° ± 14.1°. In standing, 30° posterior pelvic tilt reduced external rotation clearance by 13.2° ± 10.2° (p < 0.001); while sitting, 30° anterior pelvic tilt reduced internal rotation clearance by 33.0° ± 12.1° (p < 0.001). Standing impingement was most frequently between the ischial tuberosity and posterior intertrochanteric crest or lesser trochanter, whereas seated impingement predominantly involved the anterior inferior iliac spine (AIIS) and intertrochanteric crest. Multivariable models showed strong in-sample fit and retained good held-out performance on nested cross-validation. High-risk morphologies combined with adverse pelvic tilt produced markedly earlier impingement, with the greatest reduction observed in the seated high-risk subgroup under extreme anterior tilt. These findings quantify posture-dependent extra-articular bony constraints and identify morphology-based phenotypes associated with reduced clearance, providing a biomechanical framework for future patient-specific and implant-based studies after THA.