Abstract
OBJECTIVES: To investigate the age-associated prevalence patterns of sacroiliac joint (SIJ) variations, evaluate their association with structural damage in axial spondyloarthritis (axSpA) patients, and compare their prevalence and morphological spectrum with European data. METHODS: This retrospective study analyzed high-resolution CT scans from 806 adults. Six predefined SIJ morphotypes were evaluated. Age-associated prevalence was modeled using generalized additive models (GAM) and Bayesian additive regression trees (BART) to capture non-linear dependencies. The association between SIJ variations and structural damage severity (Innsbruck CT grade) in axSpA patients was assessed with multivariable cumulative-link mixed models. European data were derived from a random-effects meta-analysis. RESULTS: SIJ variation prevalence showed a strong, monotonic increase with age (OR = 1.26/year, 95% CI 1.15–1.38), accelerating after 60 years and being more common in women (73.5% vs. 25.2%, P < 0.001). Critically, the presence of any SIJ variant was significantly associated with higher odds of severe structural damage in axSpA patients (OR = 2.23, 95% CI: 1.56–3.19, P < 0.001). BART modeling provided superior net benefit for risk prediction versus GAM, facilitating exploratory risk stratification. While overall prevalence was similar to European data (44.4% vs. 41.0%, P = 0.236), morphological distributions differed significantly: semicircular defects (11.2% vs. 4.8%) and crescent-shaped plates (12.4% vs. 3.6%) were more prevalent in our cohort. CONCLUSIONS: SIJ variations are strongly associated with age, suggesting a degenerative component, and constitute a relevant risk marker for severe structural damage in axSpA. The BART model effectively supports exploratory risk assessment. Our cohort shares a similar prevalence but demonstrates a distinct morphological spectrum compared to European populations. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12891-026-09629-9.