A comparison of new bone formation in patients with ankylosing spondylitis and patients with diffuse idiopathic skeletal hyperostosis: a retrospective cohort study over six years.

OBJECTIVE Ankylosing spondylitis (AS) and diffuse idiopathic skeletal hyperostosis (DISH) are both characterized by new bone formation in the spine but presumably have a different pathogenesis. This study was undertaken to compare the natural course of new bone formation in AS and DISH. METHODS Lateral radiographs of the cervical and lumbar spine from AS and DISH patients obtained at ≥2 time points within 6 years were analyzed to quantify osteophyte development. Radiographs were scored in a blinded manner by 2 readers using the modified Stoke AS Spine Score (mSASSS). Bone spurs were categorized as having an angle of <45° or >45°. RESULTS AS patients (n = 146) were younger than DISH patients (n = 141) (mean ± SD 54.2 ± 12.3 years versus 60.3 ± 7.7 years). Symptom duration (mean ± SD) was 23.6 ± 11.2 years in AS patients and 21.6 ± 12.4 years in DISH patients. The mSASSS at baseline was lower in DISH patients (mean ± SD 14.3 ± 6.7) than in AS patients (20.5 ± 14.5) but had increased by a similar amount at followup (3.3 ± 4.2 versus 4.1 ± 9.5). The mean mSASSS progression rate per year (1.3 units) was also comparable. The mean ± SD number of syndesmophytes per patient was higher in AS (5.7 ± 5.5) than DISH (2.7 ± 2.8) patients (P < 0.001), while degenerative bone spurs (mean ± SD) were more frequent in DISH (1.4 ± 1.8) than AS (1.0 ± 1.4) patients. AS patients developed more new bone spurs with an angle of <45° than >45° per patient (mean ± SD 2.1 ± 2.7 versus 0.6 ± 0.9) (P < 0.001), while similar amounts of both types of bone formation were seen in DISH patients. CONCLUSION Our findings indicate that the rates of new bone formation in AS and DISH are largely similar. Both groups show osteophyte development, but as expected, syndesmophytes are more frequent in AS patients while DISH patients have more degenerative bone spurs. The nature of the different mechanisms of bone formation needs further study.