BMI as an Independent Determinant of Spinal Alignment in Degenerative Scoliosis

Background/Objectives: Sagittal spinal alignment is a key determinant of function and quality of life in degenerative spinal disorders. While age is an established determinant of sagittal balance, the independent contribution of BMI remains insufficiently characterized, particularly in degenerative scoliosis cases. This study aimed to investigate the association between BMI and sagittal alignment, quantify its effect relative to age, and explore potential sex-specific differences. Methods: This retrospective single-center study included 293 adults with degenerative scoliosis who underwent standing full-spine EOS imaging. Multivariable linear regression models assessed the association between BMI and sagittal vertical axis (SVA), adjusting for age, sex, diabetes mellitus, and osteoporosis. Secondary analyses examined associations with coronal deformity (Cobb angle) and lumbar degeneration (Wilke score). Sex-stratified analyses and Receiver Operating Characteristic (ROC) analyses for sagittal decompensation were also performed. Results: The cohort had a median BMI of 26.2 kg/m² (56% female; median age 71 years). Median SVA measured 56.4 mm (interquartile range 30.6-87.1), and 58% of patients exhibited sagittal decompensation. In sex-stratified analyses, higher BMI was independently associated with greater SVA in women (p = 0.015), whereas no significant association was observed in men (p = 0.061). ROC analysis demonstrated limited overall discrimination; however, a BMI ≥ 35 kg/m² showed high specificity and positive predictive values exceeding 80% for sagittal decompensation. Conclusions: BMI is independently associated with sagittal alignment in degenerative scoliosis and contributes to sagittal decompensation to a degree comparable to age. These findings suggest that body weight represents a clinically relevant biomechanical factor that should be considered alongside age and spinopelvic parameters when interpreting sagittal alignment, although BMI's limited discriminatory performance for sagittal decompensation precludes its use as a standalone screening tool.