Age-related prevalence of sacroiliac joint variations and their association with structural damage in axial spondyloarthritis

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. 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. 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. 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.