The role of lumbosacral paraspinal muscle degeneration and low vertebral bone mineral density on distal instrumentation-related problems following long-instrumented spinal fusion for degenerative lumbar scoliosis: a retrospective cohort study

BACKGROUND: This study aimed to confirm the role of paraspinal muscle degeneration and low vertebral bone mineral density (vBMD) of the lumbosacral region in the development of distal instrumentation-related problems (DIPs) in degenerative lumbar scoliosis (DLS) patients undergoing long-instrumented spinal fusion. METHODS: From 2013 to 2019, 125 DLS patients with 24-month follow-up after long-instrumented spinal fusion in Beijing Chao-Yang Hospital were retrospectively recruited and divided into DIP and non-DIP groups. Demographic characteristics, surgical data, and radiographic parameters were statistically compared between the groups. Degeneration of the paraspinal muscle was evaluated using the relative gross cross-sectional area (rGCSA), relative functional cross-sectional area (rFCSA), ratio of the rFCSA to rGCSA, gross muscle-fat index, and functional muscle-fat index of the multifidus (MF), erector spinae (ES), paraspinal extensor muscle (PSE), and psoas major determined by preoperative magnetic resonance imaging (MRI). The vBMD of the lumbosacral region and lower instrumented vertebra (LIV) was assessed using Hounsfield unit (HU) values determined by computed tomography (CT) scans. The DeLong test was performed to select MRI and CT scan variables. Multivariable logistic regression analysis was applied to determine the independent predictive factors of DIPs. RESULTS: The incidence of DIPs was 16.0% (20/105). There were no significant differences in demographic characteristics or surgical data between the groups. The rFCSAs of the MF (65.74±21.51 vs. 92.37±21.68; P<0.001), ES (82.67±21.44 vs. 111.48±24.21; P<0.001) and PSE (144.31±36.12 vs. 208.48±41.57; P<0.001) and the HU values of the lumbosacral region (103.80±22.64 vs.. 132.19±19.17; P<0.001) and LIV (111.70±23.23 vs. 128.69±20.70; P=0.005) were significantly lower in the DIP group. Significantly less preoperative pelvic tilt and greater postoperative lumbosacral lordosis and sagittal vertical axis (SVA) values were observed in the DIP group. The rFCSA of the PSE, the HU value of the lumbosacral region, and the postoperative SVA value were detected as independent predictive factors of DIPs. CONCLUSIONS: Lower muscularity of the PSE, a lower vBMD of the lumbosacral region, and postoperative sagittal malalignment were independent predictive factors of DIPs. Surgeons should emphasize the preoperative evaluation of paraspinal muscle and bone mass in DLS patients.