Objectives To describe the evolution of radiographic abnormalities of the spine in patients with ankylosing spondylitis (AS).

Methods Patients with AS were followed prospectively with 2 yearly radiographs for 12 years. The modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) was scored by two readers (R1 and R2). New syndesmophytes at uninvolved vertebral corners were computed. Radiographic progression was investigated using generalised estimating equations.

Results 809 radiographs (presenting 520 at 2 yearly intervals) from 186 patients (70% men, mean age 43 (SD 12) years, mean 20 (SD 12) years since symptom onset and 83% HLA-B27 positive) were included. Mean mSASSS at baseline was 11.6 (16.2). While the course of progression in individual patients was highly variable, and still occurred in patients with decades of symptom duration, mean 2 year progression was 2.0 (3.5) mSASSS units. Over the entire follow-up, at least one new syndesmophyte was found in 55% (R1) and 63% (R2) of patients (38% (R1) and 39% (R2) of all intervals). In 24% of patients (39% of intervals), there was no progression. A progression ≥5 mSASSS units occurred in 22% of patients (or in 12% of intervals). At the group level, a linear time course model fitted the data best, with a constant rate over the entire 12 year interval of 0.98 mSASSS units/year. Radiographic progression occurred significantly faster in men, in HLA-B27 positive patients and in patients with a baseline mSASSS≥10.

Conclusions Long term radiographic progression in AS is highly variable in the individual patient, more severe in HLA-B27 positive men and still occurs after decades of disease. At the group level, however, progression in AS follows an approximately linear course.