Objective The objectives were: (1) to determine if ultrasound (US) can detect more erosions in erosive osteoarthritis (EOA) of the interphalangeal (IP) joints than conventional radiography (CR); and (2) to explore the frequency of structural and inflammatory findings in EOA and non-EOA.

Methods Structural changes and the anatomical phase were scored on CR in IP joints of 31 patients with EOA and 7 patients with non-EOA. Structural and inflammatory changes were scored by US. The frequency of sonographic findings was compared between the anatomical phases and between EOA and non-EOA by generalised estimation equation (GEE) modelling.

Results US detected 68 of 72 (94.4%) erosions seen on CR. US detected 45 additional erosive joints in EOA. The frequency of joint effusion and power Doppler signal was similar in EOA compared to non-EOA (p=0.91 and p=0.68, respectively). Statistically significantly more synovitis was present in full erosive phase compared to non-erosive phases in EOA (p=0.04). No differences in inflammatory findings were found between non-erosive phases in EOA and non-EOA.

Conclusion US is capable of detecting erosions in radiographic non-erosive phases. The highest frequency of synovitis is present in erosive joints but inflammatory findings are common in all anatomical phases of EOA and non-EOA.