Two patterns of sacroiliac joint bone marrow oedema are apparent in AxSpA determined by HLA-B27 status, body mass...

The sacroiliac joint (SIJ) plays a key role in load transmission, and in axial spondyloarthritis (axSpA), HLA-B27-positive disease is typically associated with sub-fibrocartilaginous bone marrow oedema (BMO). However, the relevance of other BMO patterns remains unclear. This study aimed to determine whether psoriasis and obesity in axSpA are associated with a distinct upper-quadrant SIJ BMO pattern, compared with the classical inferior pattern.

Semi-coronal SIJ MRIs from ASAS-classified axSpA patients were assessed using the Leeds axSpA MRI scoring system. Patients were categorized according to BMO distribution as predominantly upper, predominantly lower, or symmetrical. Associations between BMO localisation and demographic, clinical, and disease-related features were analysed.

Of 233 patients, 203 were evaluable (mean age 40.4 years [SD 13.3]; 60% male), and BMO was present in 164 (80.8%). Patients with predominant upper BMO (n=50) were older, had higher body mass index, longer disease duration, and a higher prevalence of psoriasis than those with predominant lower BMO (n=88) (all P<0.05). In this group, biologic DMARD therapy was associated with significant reductions in both upper and total SIJ BMO scores on follow-up MRI. In contrast, predominant lower BMO was associated with radiographic sacroiliitis, HLA-B27 positivity, and male sex, and was inversely related to age and body mass index.

Two predominant yet overlapping SIJ BMO patterns were identified: an upper, BMI- and psoriasis-associated peri-capsular pattern, and a lower, HLA-B27-associated sub-fibrocartilaginous pattern. Recognition of these regional signatures may aid phenotypic stratification and improve understanding of the biomechanical-immunological interface in axSpA.