Emerging pathogenetic mechanisms in adolescent idiopathic scoliosis: the role of inflammation and gut microbiota

PURPOSE: This narrative review was conducted to analyse current evidence on inflammation-related mechanisms contributing to the pathogenesis and progression of adolescent idiopathic scoliosis (AIS). METHODS: A comprehensive narrative synthesis of studies investigating inflammatory biomarkers, immune cell phenotypes, cytokine pathways, paraspinal muscle immunobiology, gut microbiota composition, and their mechanistic links to bone and muscle remodeling in AIS was performed. Evidence was integrated from clinical, genetic, histological, microbiological, and experimental models. RESULTS: Evidence indicates that AIS is associated with chronic low-grade inflammation affecting systemic immunity, bone metabolism, and paraspinal muscle structure. Altered cytokine activity (IL-6, IL-1β, TNF-α, IL-17) promotes osteoclastogenesis, extracellular matrix degradation, and reduced bone mineral density. Paraspinal muscles on the concave side exhibit fibrosis, macrophage imbalance, and impaired regeneration, consistent with persistent inflammatory signalling. Additionally, gut microbiota dysbiosis-characterized by reduced bacteria producing short chain fatty acids (SCFA) and increased pro-inflammatory taxa-may contribute to endotoxemia, immune activation, and disruption of the gut-bone-muscle axis. Inflammatory markers such as the neutrophil-to-lymphocyte ratio correlate with curve severity, and genetic and Mendelian randomization analyses suggest that specific microbial taxa may modulate AIS risk. CONCLUSIONS: Current evidence supports a multifactorial biological model of AIS in which chronic low-grade inflammation acts as a central integrator of systemic and local pathogenic processes. Altered cytokine signaling and immune cell imbalance promote dysregulated bone remodeling via the RANKL/RANK pathway, while persistent inflammatory activation within paraspinal muscles contributes to fibrosis, impaired regeneration, and biomechanical asymmetry. In parallel, gut microbiota dysbiosis may further amplify inflammatory signalling through intestinal barrier dysfunction, reduced production of anti-inflammatory microbial metabolites, and activation of the gut-bone-muscle axis. Although causal relationships remain to be fully established, these interconnected mechanisms provide a coherent framework linking immune dysregulation, musculoskeletal remodeling, and curve progression in AIS, highlighting opportunities for biomarker discovery and the development of targeted preventive and adjunctive therapeutic strategies.