It has been proposed that the defining difference between rheumatoid arthritis and spondyloarthropathy (including psoriatic arthritis) is the initial pathological lesion where the emphasis in psoriatic arthritis is on the enthesis and in rheumatoid arthritis on the synovium. Classical radiological descriptions of seronegative spondyloarthropathy include enthesopathy at major entheseal insertions characterised by erosions and exuberant new bone formation. In this study, the plain radiographic features of spondyloarthropathy are compared between psoriatic arthritis, other spondyloarthropathies and rheumatoid arthritis.
The CASPAR study collected clinical, radiological and laboratory data on 588 patients with physician diagnosed psoriatic arthritis and 525 controls with other inflammatory arthritis, 70% of which had rheumatoid arthritis. Plain radiographs of the pelvis and heels were part of the study protocol, although radiographs of other potential entheseal sites such as the knee, elbow and shoulder, were interpreted if available. All radiographs were read blind by two observers working in tandem.
Significant differences in entheseal erosion and entheseal new bone formation were found between psoriatic arthritis, ankylosing spondylitis, undifferentiated spondyloarthropathy, rheumatoid arthritis and other diagnoses (entheseal erosion, chi-squared 20.8, p=0.008; entheseal new bone formation, chi-squared 24.5, p=0.001). These differences were mainly due to a higher proportion of these features in ankylosing spondylitis. No differences in the plain radiographic features of enthesopathy were found between psoriatic arthritis and rheumatoid arthritis except in the case of entheseal new bone formation at sites of attachment of inguinal ligament, sartorius and rectus femoris muscles to the ilium (OR 3.01, 95% CI 1.13-8.02). Very few subjects with symptomatic heel involvement had radiographic changes and minimal differences were found between those with and without symptoms in terms of new bone formation and erosion at either calcaneal site.
New bone formation and erosion at major entheseal sites is most commonly seen in ankylosing spondylitis. Plain radiographic features of major enthesopathy are poor discriminators between psoriatic arthritis and rheumatoid arthritis.