Objective To investigate the potential of ultrasound (US) in the differential diagnosis between rheumatoid arthritis (RA) and psoriatic arthritis (PsA) at metacarpophalangeal (MCP) joints level.

Methods 18 RA patients and 20 PsA patients with clinical involvement of MCP joints were included. All US examinations were performed by two rheumatologists investigating: presence of joint cavity widening (JCW), synovial fluid and/or synovial hypertrophy, peritenon extensor tendon inflammation (PTI) and intra-articular or peri-tendinous power Doppler (PD) signal.

Results A total of 83 MCP joints in 18 RA patients were assessed. In all of these the authors found different degrees of JCW. 15 of 83 (18%) MCP joints showed synovial fluid, whereas 68 of 83 (82%) MCP joints showed synovial hypertrophy. In 72 of 83 (86.7%) MCP joints intra-articular PD was detected. No PTI pattern was found in these patients.

In PsA patients, a total of 82 MCP joints in 20 patients were assessed. 54 of 82 (65.8%) MCP joints showed PTI pattern (p = 0.001). In 50 of these 54 (92.5%) MCP joints extra-articular PD signal was detected (p = 0.001). 28 of 82 (34.1%) MCP joints showed different degrees of JCW. 6 of 28 (21.4%) MCP joints presented synovial fluid, whereas 22 of 28 (78.5%) MCP joints showed synovial hypertrophy. In 8 of 82 (9.7%) MCP joints the JCW and PTI patterns were found contemporaneously.

Conclusions Preliminary results demonstrate that PTI pattern is a higher characteristic of PsA, which suggests a potential role of US in the differential diagnosis between RA and PsA at MCP joints level.