Total Hip Replacement influences Spinopelvic Mobility: A Prospective Observational Study.

BACKGROUND Abnormal spinopelvic mobility is identified as contributing element of total hip arthroplasty (THA) instability. Preoperative identification of THA patients at risk is still a remaining challenge. We therefore conducted this study 1)to evaluate if pre- and postoperative spinopelvic mobility differ, 2)to determine the interactions between the elements of the spinopelvic complex and 3)to identify preoperative parameter for predicting spinopelvic mobility. METHODS A prospective observational study assessing 197 THA patients was conducted with biplanar stereoradiography in standing and relaxed sitting position preoperatively and postoperatively. Two independent investigators determined spinopelvic mobility based on two different classifications (Δsacral slope(SS) and Δpelvic tilt(PT); Δ from standing to sitting;Δ 30° hypermobile). Multiple regression analysis and receiver operating characteristic (ROC) analysis were used to identify predictors for postoperative spinopelvic mobility. RESULTS Spinopelvic mobility significantly increased after THA based on ΔPT (Pre/Post 18.5°/22.8°;p<0.000) and ΔSS (Pre/Post 17.9°/22.4°;p<0.000). A distinct shift in the ratio from stiff (Pre/Post: 24%/9.7%) to hypermobile (Pre/Post: (10.2%/22.1%) mobility postoperatively was observed. ROC analysis predicted postoperative stiffness using preoperative PTStanding≥13.0° with a sensitivity of 90% and a specificity of 51% and hypermobility with preoperative SSStanding≥35.2° with a sensitivity of 81% and a specificity of 34%. Age at surgery, preoperative PTStanding and pelvic incidence were independent predictors of spinopelvic mobility (R2=0.24). CONCLUSIONS Definition of preoperative stiffness should be interpreted with caution by arthroplasty surgeons as mobility itself is influenced by THA. For the first time thresholds for standing preoperative parameters for predicting postoperative spinopelvic mobility could be provided. Preoperative standing only lateral assessment could serve as a screening tool for spinopelvic mobility.