Background Structural damage of the spine in ankylosing spondylitis (AS) is associated with worse physical function and impaired spinal mobility. Knowledge about predictors of new syndesmophyte formation is limited.

Objectives To assess the development of new syndesmophytes at the level of individual vertebral bodies and to assess predictors for this development.

Methods Clinical and radiological data from 132 patients from the Outcome in Ankylosing Spondylitis International Study for whom complete sets of radiographs were available at baseline and at 2- and 4-year follow-up were used. Univariable and multivariable logistic regression analyses were performed to identify predictors associated with development of new syndesmophytes within 4 years.

Results At baseline, 81 (61%) patients had syndesmophytes. New syndesmophytes developed in 44 (33%) patients within 2 years and in 63 (48%) patients within 4 years. The RR of developing new syndesmophytes was 5.0 (95% CI 2.5 to 10.2) at 4 years in patients with existing syndesmophytes as compared with patients without. In the univariable analysis, older age, worse functional status, male gender, erythrocyte sedimentation rate and existing syndesmophytes were associated with development of new syndesmophytes at 4 years. In the multivariable logistic regression analysis, only the presence of existing syndesmophytes was a significant predictor (OR 18.72, 95% CI 6.44 to 54.42). When existing syndesmophytes were taken out from the model, age (OR 1.07, 95% CI 1.03 to 1.11) and male gender (OR 3.98, 95% CI 1.47 to 10.77) were statistically significant contributors.

Conclusion In AS, patients with existing syndesmophytes are prone to develop new syndesmophytes over time.