How does corrective fusion surgery for adult spinal deformities affect pelvic inclination in the supine position as the reference plane for THA

PURPOSE To determine the changes in pelvic inclination in the supine and standing positions after spinal corrective surgery, and to identify the most predictive factor for changes in pelvic inclination with the supine position as the reference plane for total hip arthroplasty. METHODS We retrospectively analysed the data of 124 patients who underwent spinal corrective fusion surgery for adult spinal deformity between 2012 and 2016 at our institution. Spinal parameters were assessed preoperatively and postoperatively using whole spine radiographs in the standing position. The sacral slope (SS) was measured using spine and pelvis computed tomography. Differences between the preoperative and postoperative SS values in each position were calculated as Δsupine SSpre post and Δstanding SSpre post, respectively. We statistically analysed the correlations between Δsupine SS pre post and preoperative spinal parameters to determine the most useful predictor of Δ supine SSpre post. RESULTS The mean Δsupine SSpre post of 10.5°(-13°-50°) was significantly smaller than the mean Δstanding SSpre post of 13.2° (-19°-44°) (p = 0.02). Moreover, 21 patients (17%) had Δsupine SSpre post > 20°. The Δsupine SS pre post was correlated with preoperative LL (r = -0.34 p < 0.01), PT (r = 0.42 p < 0.01), and SVA (r = 0.37 p < 0.01). Preoperative supine SS (r = -0.54, p < 0.01) had the highest correlation with Δsupine SSpre post, whereas preoperative standing SS showed no correlation (r = -0.14 p = 0.12). CONCLUSION Preoperative supine SS is the most useful predictive factor for changes in supine pelvic inclination, and low preoperative values should be noted. This information should be considered for the management of patients with hip-spine syndrome.