Preoperative spinopelvic hypermobility resolves following total hip arthroplasty

Aims Spinopelvic mobility plays an important role in functional acetabular component position following total hip arthroplasty (THA). The primary aim of this study was to determine if spinopelvic hypermobility persists or resolves following THA. Our second aim was to identify patient demographic or radiological factors associated with hypermobility and resolution of hypermobility after THA. Methods This study investigated patients with preoperative posterior hypermobility, defined as a change in sacral slope (SS) from standing to sitting (Delta SSstand-sit) >= 30 degrees. Radiological spinopelvic parameters, including SS, pelvic incidence (PI), lumbar lordosis (LL), PI LL mismatch, anterior pelvic plane tilt (APPt), and spinopelvic tilt (SPT), were measured on preoperative imaging, and at six weeks and a minimum of one year postoperatively. The severity of bilateral hip osteoarthritis (OA) was graded using Kellgren-Lawrence criteria. Results A total of 136 patients were identified as having preoperative spinopelvic hypermobility. At one year after THA, 95% (129/136) of patients were no longer categorized as hypermobile on standing and sitting radiographs (Delta SSstand-sit < 30 degrees). Mean Delta SSstand-sit decreased from 36.4 degrees (SD 5.1 degrees) at baseline to 21.4 degrees (SD 6.6 degrees) at one year (p < 0.001). Mean SSseated increased from baseline (11.4 degrees (SD 8.8 degrees)) to one year after THA by 11.5 degrees (SD 7.4 degrees) (p < 0.001), which correlates to an 8.5 degrees (SD 5.5 degrees) mean decrease in seated functional cup anteversion. Contralateral hip OA was the only radiological predictor of hypermobility persisting at one year after surgery. The overall reoperation rate was 1.5%. Conclusion Spinopelvic hypermobility was found to resolve in the majority (95%) of patients one year after THA. The increase in SSseated was clinically significant, suggesting that current target recommendations for the hypermobile patient (decreased anteversion and inclination) should be revisited.