Impact of spinopelvic mobility and functional safe zone conformity on functional outcomes after total hip arthroplasty

Abstract

Objective: This study aimed to evaluate the association between spinopelvic mobility, functional safe zone (FSZ) conformity, and patient-reported outcomes following total hip arthroplasty (THA). Methods: This retrospective observational study included 60 patients (22 males, 38 females; mean age 62.5 +/- 9.7 years) who underwent primary THA between October 2021 and October 2023 and completed at least 1 year of follow-up. Mean time since surgery was 22.18 +/- 6.31 months. Spinopelvic mobility was assessed using standing and relaxed-seated lateral radiographs and categorized as stiff ((sacral slope) Delta SS <= 10 degrees), normal (Delta SS 11 degrees-29 degrees), or hypermobile (Delta SS >= 30 degrees). Acetabular component orientation was evaluated according to FSZ conformity using the combined sagittal index. Clinical outcomes were assessed using the Harris Hip Score (HHS), Oxford Hip Score (OHS), Short Form-12 (SF-12), and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Group comparisons were performed using one-way analysis of variance (ANOVA) or Welch's ANOVA with Bonferroni or Games-Howell post-hoc analyses, as appropriate. Results: Spinopelvic mobility was classified as stiff in 18 patients (30%), normal in 34 patients (56.6%), and hypermobile in 8 patients (13.3%). Patients with normal spinopelvic mobility demonstrated significantly better postoperative HHS, OHS, WOMAC, and SF-12 scores compared with the stiff and hypermobile groups (all P < .001). Acetabular components were classified as FSZ-conforming in 38 hips and FSZ-non-conforming in 22 hips. The FSZ-conforming group demonstrated significantly superior HHS (P = .003), OHS (P = .025), WOMAC (P = .001), and SF-12 scores (P = .028) compared with the non-conforming group. Conclusion: Normal spinopelvic mobility and FSZ-conforming acetabular component alignment were associated with superior postoperative functional outcomes and quality of life after THA. These findings highlight the clinical relevance of dynamic spinopelvic assessment and patient-specific cup positioning during preoperative planning for THA.