Total hip arthroplasty improves global sagittal and coronal spinal imbalance and spinopelvic mobility : a prospective cohort study.

AIMS: The impact of total hip arthroplasty (THA) on global spinal alignment is under-reported. This study prospectively evaluates the effect of THA on coronal and sagittal global spinal balance and spinopelvic kinematics using dynamic whole-body EOS radiographs.

METHODS: We prospectively studied 163 patients (62 male, 101 female; mean age 68.4 years (SD 9.1)) undergoing primary unilateral THA for end-stage osteoarthritis. Full-body EOS images were acquired preoperatively and at a six-month follow-up in standing, relaxed-seated, and deep-seated postures. The primary outcome was change in sagittal vertical axis (SVA). Secondary outcomes included changes in pelvic tilt (PT), sacral slope (SS), proximal femoral angle (PFA), and coronal parameters (C7-central sacral vertical line (C7-CSVL), limb length discrepancy (LLD), and pelvic obliquity (PO)). Preoperative SVA > 50 mm defined sagittal imbalance; C7-CSVL > 20 mm defined coronal imbalance. Statistical analysis employed paired non-parametric tests and multivariate regression.

RESULTS: In the 58 patients with preoperative sagittal imbalance (SVA > 50 mm), mean SVA improved significantly from 77.9 mm (SD 24.4) to 59.9 mm (SD 36.1; p < 0.001), with 27.6% (16 out of 58) moving to the sagittal balance group postoperatively. Preoperative SVA was the sole independent predictor of SVA correction (R² 0.25; p = 0.009). In the 41 coronally imbalanced patients, LLD was corrected from 11.4 mm (SD 20.9) to 1.0 mm (SD 14.2; p < 0.001), with 31.7% (17 out of 41) becoming balanced after THA. From standing to deep-seated, there was an increase in PFA change from 48.8° (SD 18.5°) to 54.6° (SD 20.3°; p < 0.001) and reduced anterior pelvic tilt (ΔPT) from 15.8° (SD 14.0°) to 10.3° (SD 14.5°; p < 0.001).

CONCLUSION: THA improves global sagittal and coronal spinal imbalance, restoring spinopelvic mobility by increasing hip flexion and reducing compensatory pelvic tilt. These dynamic postoperative changes challenge the static assumptions underpinning current hip-spine classifications. Preoperative and postoperative analysis of global dynamic spinopelvic balance and mobility are crucial for managing hip-spine syndrome.