Does spinopelvic motion change after total hip arthroplasty?

Purpose: Spinopelvic motion plays an important role in functional acetabular cup position after total hip arthroplasty (THA). Sacral slope (SS) has been a useful surrogate for spinopelvic motion. The present study aimed to investigate statistical characteristics of spinopelvic motion before and after THA using changes in SS in supine, standing, and sitting positions.

Methods: A total of 76 patients (88 hips) were assessed. To classify spinopelvic mobility, defined as a change in SS from standing to sitting position (ΔSS_stand/sit), 10° ≤ ΔSS_stand/sit ≤ 30°, ΔSS_stand/sit < 10°, and ΔSS_stand/sit > 30° were considered normal, stiff, and hypermobile, respectively.

Results: Over ± 7° changes in SS between before and one year after THA were observed in 39 (44.3%) hips in the sitting position, 19 (21.6%) hips in the supine position, seven (7.9%) in the standing position. Percentages of hips with stiff spinopelvic mobility (11.4% vs. 22.7%) and hypermobile spinopelvic mobility (23.9% vs. 12.5%) between before THA and one year after THA were significantly different (p = 0.034 and p = 0.016, McNemar's test). At one year after THA, 40.0% (4/10) of hips with stiff spinopelvic mobility and 57.1% (12/21) of hips with hypermobile spinopelvic mobility shifted to normal spinopelvic mobility.

Conclusions: Change in SS between before THA and one year after THA had a high inter-subject variability especially in the sitting position. In addition, there was a distinct shift to normal spinopelvic mobility postoperatively in hips with stiff and hypermobile spinopelvic mobility pre-operatively.