Abstract
Study Design: Systematic review with the proposal of a conceptual sarcopenia-integrated Global Alignment and Proportion modification (GAP-M) for future validation.Purpose: This study aims to synthesize evidence on sarcopenia and paraspinal muscle degeneration in adults with spinal deformity (ASD) and evaluate whether current evidence supports incorporating muscle-related metrics into the Global Alignment and Proportion (GAP) score.Overview of Literature: Alignment-based parameters alone cannot account for all mechanical complications following ASD correction. Regional paraspinal muscle degeneration, particularly near the upper instrumented vertebra (UIV), may weaken posterior dynamic support, whereas systemic sarcopenia reflects overall frailty and perioperative risk.Methods: A systematic search of PubMed/MEDLINE, Embase, Web of Science, Cochrane Library, and Google Scholar was conducted through May 23, 2026. Eligible studies included adults with ASD, used quantifiable muscle metrics, and reported alignment, mechanical, functional, or perioperative outcomes. Because definitions, measurement levels, and effect estimates varied widely across studies, a quantitative meta-analysis was not performed, and instead a narrative evidence-mapping approach was employed.Results: Of the 507 identified records, 229 were screened, and 50 full-text articles were examined for eligibility; 35 were excluded for documented reasons, leaving 15 primary studies (approximately 1,482 patients). No study directly evaluated whether adding sarcopenia metrics improves GAP score discrimination or reclassification. UIV-adjacent paraspinal muscle quality emerged as the most consistent predictor of proximal junctional kyphosis/failure (five of six studies), while psoas and systemic sarcopenia metrics demonstrated more mixed associations.Conclusions: Paraspinal muscle degeneration appears to be a clinically relevant risk marker in ASD surgery, although current evidence does not yet validate GAP-M for routine clinical use. We therefore propose a provisional 0 to +2-point Muscle-Quality Modifier as a testable framework that requires prospective multicenter validation with standardized UIV-adjacent imaging metrics and direct comparisons of GAP and GAP-M before clinical adoption.