Diagonal Paraspinal Sarcopenia and Osteoporosis contribute to the Rotatory Subluxation in patients with Degenerative Lumbar Scoliosis

Purpose This study aims to investigate the potential association between paraspinal sarcopenia, osteoporosis, and vertebral rotational subluxation (VRS) in patients with degenerative lumbar scoliosis (DLS). Methods This retrospective study analyzed standing anteroposterior radiographs to assess coronal (Cobb angle, CA; coronal balance distance, CBD; lateral translation, LT) and sagittal parameters (thoracic kyphosis, TK; lumbar lordosis, LL; sagittal vertical axis, SVA). Patients were categorized into Rotatory Subluxation (RS, LT ≥ 5mm) and Non-RS groups, with the RS group further subdivided into single and double level subgroups based on the frequency of RS occurrence. Bilateral paraspinal muscle (PSM) cross-sectional area (CSA) and fat infiltration rate (FI%) at L1-S1 were evaluated via lumbar MRI. Vertebral rotation angles and Hounsfield units (HU) were quantified using reconstructed axial CT images, with osteoporosis defined as L1 HU ≤ 110. Ratios of convex-to-concave measurements (Ro FI%, Ro CSA, Ro HU) were calculated. Spearman correlation and logistic regression analyses explored associations among paraspinal sarcopenia, osteoporosis, and VRS. Intraoperative multifidus muscle (MF) samples from RS-1 (upper RS level) and RS + 1 (below RS level) in DLS patients underwent histological analysis to assess regional fat infiltration and muscle atrophy. Results 166 patients were included in this study, 90 (54.2%) with VRS and 76 without. The apex vertebrae were predominantly at the RS-1 level (67%). Both single level and double level RS groups showed significantly higher FI% of PSM (erector spine, ES and MF) compared to the Non-RS group at various levels (P < 0.05). Patients with VRS generally exhibited osteoporosis. The HU value for both single level and double level RS patients were significantly lower than those in the Non-RS group at multiple levels (P < 0.05). Notably, the FI% of bilateral MF and HU value in RS group showed more severe asymmetry at multiple levels compared to the Non-RS group (P < 0.05). Both Ro HU and Ro FI% of MF muscle were generally < 1 above the RS level, while > 1 below the RS level, suggesting that asymmetric paraspinal sarcopenia and osteoporosis above and below the RS level was reversed. Logistic regression analysis showed that VRS was significantly associated with the Ro FI% above RS (odds ratio, 0.021; 95% confidence interval, 0.001–0.147; P < 0.001) and Ro FI% below RS (odds ratio, 1.956; 95% confidence interval, 0.930–4.114; P = 0.007). Cobb angle and osteoporosis were additional independent factors associated with VRS. Conclusion VRS in DLS is characterized by diagonal paraspinal sarcopenia patterns and vertebral osteoporosis. The reversed sarcopenia-osteoporosis gradient across subluxation levels suggests a biomechanical coupling mechanism driving curve progression. Preoperative quantification of these parameters may stratify progression risk and guide targeted rehabilitation.