We aimed to 1) identify clinical and genetic associations of sacroiliitis (SI) in patients with psoriatic arthritis (PsA), and 2) describe the different radiographic patterns of SI in PsA and their clinical and genetic associations.

283 PsA patients, fulfilling CASPAR criteria, underwent detailed skin and rheumatologic assessments. In addition, HLA-B*27 and B*080101 status was recorded, which have been shown as the key genetic markers of radiographic SI in PsA. Grade 2 Unilateral or bilateral radiographic changes of SI were required for inclusion and involvement was further defined as asymmetrical or symmetrical.

70 patients (25%) had radiographic SI; all either with a present or past history of backache. Regression analysis demonstrated a significant association of SI with peripheral joint erosions (p=0.043), PASI maximum (p=0.041), younger age of PsA onset (p=<0.001), presence of HLA-B*0801 (p=0.002) and only marginal significance with HLA-B*2705 (p=0.059). Asymmetrical SI was noted in 51 patients (73%). In striking contrast to those patients with symmetrical SI, patients with asymmetrical SI were more likely to be female (p=0.04), have a trend towards more severe nail disease (p=0.08) and peripheral joint erosions (p=0.08), more osteolysis (p=0.01), more HLA-B*0801 positivity (p=0.001) and much less HLA-B*270502 positivity (p=<0.001).

PsA developing at a younger age, severe skin disease, peripheral joint erosions, and HLA-B*0801 are significantly associated with SI, and there was only a marginal trend towards significance for HLA-B*2705. HLA-B*27 positive Axial-PsA patients resemble AS, while HLA-B*0801 positive Axial-PsA patients have asymmetrical and/or unilateral SI, which are typical of PsA.