Objective Sacroiliac (SI) joint and spinal inflammation are characteristic of ankylosing spondylitis (AS), but some AS patients have been identified who have discordant radiographic disease. We studied an AS subgroup with long-standing disease and fused SI joints. We identified factors associated with discrepant degrees of radiographic damage between the SI joints and spine. Methods From the PSOAS cohort, patients were included with disease duration ≥20 years and fused SI joints in a nested case-control design. Patients with and without syndesmophytes were used as cases and controls for analysis. We used CART analysis to determine risk factors for syndesmophytes presence and re-examined the risk factors' validity using univariable logistic regression models. Results There were 354 patients in the subgroup, 23 of whom lacked syndesmophytes. CART analysis showed females were less likely to have syndesmophytes. The next important predictor was age of symptom onset in males, with onset ≤16 years being less likely to have syndesmophytes. Univariable analysis confirmed females were less likely to have syndesmophytes (OR=0.17, 95% CI: 0.07-0.41). Syndesmophyte presence was associated with HLA-B27 positivity (p-value 0.03) and age of symptom onset >16 years old (OR 2.72, 95% CI: 1.15-6.45). All 23 patients who lacked syndesmophytes were HLA-B27 positive. Conclusion Using CART analysis and univariable modeling, women were less likely to have syndesmophytes despite advanced disease duration and SI joint disease. Patients with younger age of symptom onset were less likely to have syndesmophytes. All patients without syndesmophytes were HLAB27 positive, indicating HLA-B27 positivity may be more associated with sacroiliac disease than spinal disease.
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