Objective To evaluate the ability of ultrasound (US) to detect the presence and change of Achilles erosions in spondyloarthritis (SpA).

Methods A blind prospective two-dimensional (2D) and three-dimensional (3D) US study of Achilles enthesis erosions in early SpA was undertaken. US examinations were performed at baseline and at 6 and 12 months of follow-up. Clinical outcomes measures were collected.

Results Bilateral Achilles entheses of 68 patients (35 women) were investigated. The mean Bath Ankylosing Spondylitis Disease Activity Index and C reactive protein (CRP) levels were 4.58±2.05 and 5.97±9.91 mg/l, respectively. The κ values for intrareader agreement for 2D and 3D images were 0.84 and 0.85 for two readers. 2D US visualised 10 erosions (7.4%) and 3D US visualised 13 erosions (9.6%) in 10 patients (14.7%). At 6 and 12 months of follow-up, 25% and 50% of basal erosions had disappeared, respectively and, of the new erosions that appeared at 6 months, 40% had disappeared 6 months later. A statistically significant association between erosion and CRP levels, entheseal Doppler signals and the number of tender and swollen joints was found.

Conclusions US examination of Achilles erosions is reliable and sensitive to change. An association was found between Achilles erosions and objective activity-based measurements of SpA outcomes.