Pelvic Tilt Increases the Risk of Impingement and Alters Impingement Type in Total Hip Arthroplasty: A Patient-Specific Simulation Study.

Impingement is a significant complication following total hip arthroplasty (THA), resulting in restricted range of motion (ROM). Pelvic tilt (PT) and its variation could alter both post-THA ROM and impingement types, which remains relatively unexplored in the literature. Therefore, this study aims to investigate the impact of PT changes on post-THA ROM and impingement types. Subject-specific ROM was simulated using 3D-CT and clinical data for 56 THA patients. Subsequently, the effect of no-PT, standing preoperative (StPT) and postoperative PT at 6 (StPT) and 12 months (StPT) on maximum ROM (flexion, extension, abduction, adduction, external rotation (ER) and internal rotation at 90° Flexion (IR@90°Flex)) and impingement types (implant-to-implant impingement (ITII), implant-to-bone impingement (ITBI), and bone-to-bone impingement (BTBI)) were investigated. Stong correlations existed between PT and flexion (R ≥ 0.686), extension (R ≥ 0.527), and IR@90°Flex (R ≥ 0.547). Anterior PT exceeding 8.1° and 11.8° were linked to decreased flexion below 110° and IR@90°Flex below 30°, respectively. Each 10° increase in anterior PT resulted in a 10° reduction in flexion and a 10.7° reduction in IR@90°Flex. Impingement types due to PT remained unchanged for flexion/extension, with increased ITII for abduction (8.9%), adduction (23.2%), and IR@90°Flex (16.1%), and increased BTBI (16.1%) for ER. In total, 12.5% and 19.6% of patients experienced clinically relevant ROM change for flexion and IR@90°Flex, respectively for StPT-StPT. However, it affected below 5.4% cases when comparing StPT and StPT. Minor changes in impingement type (< 6% of cases) were observed due to changes in PT before and after THA, as well as temporal changes in PT post-THA. However, PT had a substantial impact on impingement types when comparing ROM without considering PT to ROM with PT included. Specifically, anterior PT was associated with reduced flexion and IR@90°Flex, indicating a higher risk of impingement. PT changes over time may lead to clinically relevant alterations in ROM but not impingement types. Trial Registration: German Clinical Trials Register; Main ID: DRKS00000739.