Analysis of Spinopelvic Parameters with L5 as the New Sacrum after Fusion in High-Grade Spondylolisthesis: A Possible Explanation for Satisfactory Results with In-Situ Fusion

Study Design Retrospective case series. Purpose To correlate functional outcomes with spinopelvic parameters in patients with high-grade spondylolisthesis (HGS) treated with instrumented in-situ surgery or reduction and fusion. Overview of Literature Satisfactory functional outcomes are reported with reduction and in-situ fusion strategies in HGS. However, reasons for this are unclear. We hypothesize that following lumbosacral fusion, the L5 becomes part of the sacrum, which improves spinopelvic parameters, resulting in equivalent functional outcomes in both surgical methods. Methods Twenty-six patients undergoing HGS (reduction group A, 13; in-situ group B, 13) were clinically evaluated using the Oswestry Disability Index (ODI), short form-12 (SF-12), and Visual Analogue Scale (VAS) scores. Spinopelvic parameters, including pelvic incidence, pelvic tilt (PT), sacral slope (SS), lumbar lordosis (LL), lumbosacral kyphosis (LSK) angle, and sacrofemoral distance (SFD) were measured preoperatively from S1 and postoperatively from L5 as the new sacrum at 1 year follow-up. Sagittal alignment was assessed using the sagittal vertical axis. Results Both groups were comparable in terms of age, sex, severity of slip, and preoperative spinopelvic parameters (p>0.05). Postoperative VAS, SF-12, and ODI scores significantly improved in both groups (p<0.05). Compared with preoperative values, the mean postoperative PT, SFD, and LSK significantly changed in both groups. In reduction group, PT changed from 26.98° to 10.78°, SFD from 61.24 to 33.56 mm, and LSK from 74.76° to 109.61° (p<0.05). In in-situ fusion group PT changed from 26.78° to 11.08°, SFD from 62.9 to 36.99 mm, and LSK from 67.23° to 113.38° (p<0.05 for all). In both groups, SS and LL did not change significantly (p>0.05). Conclusions After fusion, the L5 becomes the new sacrum and influences spinopelvic parameters to change favorably. This possibly explains why reduction and in-situ fusion achieve equivalent functional outcomes in HGS.