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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.

Psoriatic arthritis (PsA) is a chronic inflammatory arthritis associated with psoriasis and, despite the larger estimated heritability for PsA, the majority of genetic susceptibility loci identified to date are shared with psoriasis. Here, we present results from a case–control association study on 1,962 PsA patients and 8,923 controls using the Immunochip genotyping array. We identify eight loci passing genome-wide significance, secondary independent effects at three loci and a distinct PsA-specific variant at the IL23R locus. We report two novel loci and evidence of a novel PsA-specific association at chromosome 5q31. Imputation of classical HLA alleles, amino acids and SNPs across the MHC region highlights three independent associations to class I genes. Finally, we find an enrichment of associated variants to markers of open chromatin in CD8+ memory primary T cells. This study identifies key insights into the genetics of PsA that could begin to explain fundamental differences between psoriasis and PsA., Psoriatic arthritis (PsA) is a chronic inflammatory arthritis with a significant genetic component. Here, the authors analyse immune-related genetic markers in 1,962 PsA patients and 8,923 controls to identify novel PsA risk loci and highlight distinct genetic differences between psoriasis and PsA.