Spinopelvic mobility patterns in patients with dislocation after THA: Direct anterior versus posterior approach

BACKGROUND: Dislocation remains a common reason for revision after total hip arthroplasty (THA), and adverse spinopelvic mobility is increasingly recognized as a major contributor to instability. Whether its prevalence differs between surgical approaches is unclear. This study compared adverse spinopelvic mobility and associated risk factors in patients who sustained a dislocation after a posterior approach (PA) or a direct anterior approach (DAA), and evaluated whether implant positioning differed between groups. METHODS: A retrospective analysis was performed on 133 patients with post-operative dislocation and available functional lateral radiographs and low-dose CT scans, including 85 PA and 48 DAA dislocations. Spinopelvic parameters, dynamic pelvic motion between standing, sitting, and supine postures, and established spinopelvic risk factors were assessed. Cup orientation, femoral version, combined anteversion, limb length, and global offset were compared. RESULTS: Spinopelvic morphology was similar, with no significant differences in standing spinopelvic tilt, pelvic incidence, PI-LL mismatch, lumbar lordosis, or lumbar flexion. However, dynamic motion differed markedly. A change in spinopelvic tilt (SPT) of ≥ 20° from standing to seated occurred in 41.3% of DAA dislocations versus 21.1% after PA (p = 0.029). A change in SPT ≤ - 13° from supine to standing occurred in 17.8% after DAA compared with 4.7% after PA (p = 0.048). Other spinopelvic risk factors showed no significant differences. Implant positioning was largely comparable; femoral and combined anteversion, cup inclination, and limb length were similar. Although femoral head size and global offset differed between groups, the association between surgical approach and adverse spinopelvic mobility persisted after accounting for these implant-related factors. CONCLUSION: Patients dislocating after DAA demonstrated a substantially higher prevalence of adverse spinopelvic mobility despite similar implant orientation and hip restoration. These findings suggest that dynamic pelvic behavior may contribute to anterior instability patterns and highlight the potential relevance of hip-spine assessment in patients undergoing anterior-approach THA. TRIAL REGISTRATION: Retrospectively registered, CNIL MR004 2,225,508.