Radiographic Muscle Sarcopenia Affects Regional Sagittal Alignment

Study Design. Retrospective cohort study. Objective. To describe differences in sagittal alignment and compensatory mechanisms in patients with and without sarcopenia. Summary of Background Data. Sarcopenia refers to the degenerative loss of muscle quantity and quality and impacts the sagittal balance of the spine. Skeletal muscle is essential to maintaining upright posture, and its progressive loss may compromise the compensatory mechanisms that preserve sagittal alignment. Sarcopenia can be diagnosed using thigh muscle thickness measured on a low-dose full-body radiograph. Methods. Full-body standing radiographs from 375 adult patients without prior spine surgery, hip, or knee arthroplasty were analyzed. Spinopelvic parameters, including pelvic tilt, pelvic incidence, lumbar lordosis (L1-S1), PI-LL, L4-S1 lordosis, sagittal vertebral axis (SVA), T1 pelvic angle, cervical SVA, T1 slope, C2-C7 lordosis, thoracic kyphosis (T4-T12), sacral slope, leg length discrepancy, and knee flexion, were generated through a machine-learning-based algorithm. Sarcopenia was defined as anteroposterior thigh muscle thickness <12.47 cm (males) and <10.68 cm (females), and lateral quadriceps thickness <3.23 cm (males) and <2.20 cm (females). Subgroup analyses examined patients with spinopelvic mismatch (108 [28.8%], PI-LL ≥10°) and sagittal imbalance (105 [28.0%], SVA ≥5 cm). Results. Sarcoopenia was prevalent in 12.8% of the study cohort, 13.0% in patients with spinopelvic mismatch, and 15.2% in patients with sagittal imbalance. The sarcopenic group had a higher cervical lordosis (−8.37 vs. −13.29°; P =0.021), T1 slope (33.11 vs. 36.90°; P =0.018), and thoracic kyphosis (39.57 vs. 44.19°; P =0.044). In subgroup analysis of patients with PI-LL mismatch or sagittal imbalance, those with sarcopenia had a higher L4-S1 lordosis. Conclusion. Sarcopenia is associated with altered regional alignment, manifesting as thoracic hyperkyphosis and cervical hyperlordosis. In the presence of sagittal deformity, compensatory mechanisms may differ in the distal and proximal segments of the lumbar spine.