Spontaneous ankylosis of the sacroiliac joint: prevalence and risk factors.

PURPOSE: Evidence on spontaneous sacroiliac joint (SIJ) ankylosis is lacking. The aim of this analysis was to assess the prevalence of spontaneous SIJ ankylosis and examined different ankylosis patterns and risk factors for spontaneous SIJ ankylosis. METHODS: Pelvic computed tomography (CT) data of 102 consecutive patients with spinal pathologies were compared to CT of a control group consisting of 102 consecutive patients without spinal pathologies. SIJ ankylosis patterns and risk factors for SIJ ankylosis, such as age, sex, and previous spinal fusion surgery were examined. RESULTS: Overall, 117 men and 86 women were examined between 2019 and 2020. Non-spinal patients were significantly older (mean age 70.5 years, standard deviation [SD] 11.4) than those in the spinal group (mean age 65.3 years, SD 14.3; p = 0.005). The prevalence of SIJ ankylosis was 24.5% in the non-spinal group and 23.5% in the spinal group. The anterior ankylosis type prevalence was 91.7% in the spinal group, compared to 48.0% in the non-spinal group. Factors associated with SIJ ankylosis were older age (odds ratio [OR] 1.04, 95% confidence interval [CI] 1.01-1.07, p = 0.004) and male sex (OR 5.14, 95% CI 2.29-11.55, p < 0.001). CONCLUSION: Spontaneous ankylosis of the SIJ was a frequent phenomenon in patients with and without spinal pathologies and more likely with older age and male sex. Anterior type SIJ ankylosis was substantially more frequent in patients with spinal pathologies. This may be due to strain exerted on the anterior SIJ aspects in patients with compromised posture due to spine degeneration.