Background: The sacroiliac joint (SIJ), often described as “bean-shaped,” is a critical anatomical structure that experiences diverse mechanical stresses from both the trunk and lower limbs. In axial spondyloarthritis (axSpA), particularly among HLA-B27-positive individuals, sub-fibrocartilaginous and sub-hyaline bone marrow oedema (BMO) is a hallmark finding on MRI. However, BMO patterns in capsular and peri-entheseal regions, though observed, remain less well-characterized. These differential patterns of BMO suggest a potential interplay between genetic predispositions, such as HLA-B27, and environmental or anatomical factors, including body mass index (BMI), age, and comorbid conditions like psoriasis-related arthritis (PsA). It is posited that while sub-fibrocartilaginous SIJ BMO predominates in HLA-B27-positive individuals, the superior SIJ region near the pericapsular tissue may exhibit greater susceptibility to BMO in older patients or those with higher BMI due to variations in joint loading. Additionally, the interaction of BMI with PsA could further amplify BMO patterns in the superior SIJ.

Objectives: This study aims to investigate whether regional SIJ BMO patterns may occur in axSpA patients according to HLA-B27 status, BMI, psoriasis, and other factors.

Methods: Cross-sectional evaluation of existing MRI scans from 203 patients with axSpA fulfilling the ASAS classification criteria. Semi-coronal SIJ MRI BMO patterns were scored using the Leeds MRI Scoring System, whereby the SIJ is divided into four quadrants: right upper, left upper, right lower, and left lower. The upper/lower distinction was made according to the vertical line passing through the midpoint of the first and second sacral foramina that is an approximate surrogate for the division of the two parts of the SIJ for upper body/torso and lower limb loading. Lesions were graded semi-quantitatively based on regional bone involvement, using a combination of lesion size and intensity, from 0 (no BMO) to 3 (severe BMO), giving a theoretical range of 0-24 [1]. The images were independently evaluated by two rheumatology experts (KA and ADM), with an interobserver correlation coefficient (ICC) exceeding 0.9. The evaluation was performed blinded to the clinical data. Presence or absence of BMO in each specific localization and corresponding BMO scores were analysed in relation to factors influencing disease phenotype, including HLA-B27 status, sex and psoriasis. The patients were categorized into groups based on the predominant SIJ BMO patterns. Patients with total combined sacral and iliac oedema scores in the lower regions exceeding those in the upper regions were classified as having predominant-lower BMO. Conversely, those with total combined sacral and iliac oedema scores in the upper regions surpassing those in the lower regions were categorized as having predominant-upper BMO. Patients with equal oedema scores were classified as diffuse-symmetrically distributed BMO.

Results: In the 203 patients, the mean age was 40.4 years (SD ± 13.3), and 60% were male. BMO was identified in 164 patients (81%). Patients with predominant upper BMO (n=26) demonstrated significantly higher BMI (p=0.002), older age at MRI (p=0.019), longer disease duration (p=0.04), and a greater prevalence of psoriasis (p=0.003) compared to those with predominant lower BMO (n=112). HLA-B27 positivity was significantly more frequent in the predominant lower BMO group (p=0.004) (Table 1). Excluding the effect of BMI (Mantel-Haenszel Chi-square test), the prevalence of psoriasis remained significantly higher in the predominant upper BMO group (p=0.023). Radiographic sacroiliitis was associated with a significantly higher total BMO score (p < 0.001). The total upper BMO score (iliac and sacral regions) was higher in patients without a history of uveitis (p<0.001). Conversely, the lower BMO score, encompassing the lower iliac and sacral regions, exhibited statistically significant negative correlations with age at MRI (p<0.001), disease duration (p=0.015), age at diagnosis (p=0.006), and BMI (p=0.03). However, a significant positive correlation was observed between lower BMO scores and CRP levels at MRI (p<0.001), and lower BMO scores were more common in males (p<0.001), HLA-B27-positive patients (p=0.01), non-psoriatic individuals (p=0.01).

Conclusion: These results suggest that two distinct clinico-pathological SIJ BMO patterns may be found in axSpA: an upper SIJ pattern associated with higher BMI and psoriasis, and a lower SIJ pattern linked to HLA-B27 positivity and systemic inflammation. These findings may have implications for the diagnosis and prognosis of axSpA.

REFERENCES: [1] Marzo-Ortega H, et al Arthritis and Rheumatism 2001; 44(9): 2112-7.

Acknowledgements: NIL.

Disclosure of Interests: Kerem Abacar: None declared, Andrea Di Matteo Janssen, Gabriele De Marco Janssen, Novartis, Kulveer Mankia Abbvie, ALLin Bio, Astra Zeneca, UCB, Lilly, Galapagos, Serac Healthcare, Zura Bio, Deepcure, Gilead, Lilly, Serac Healthcare, Astra Zeneca, Deepcure, Jake Weddell: None declared, Stephanie R Harrison Janssen and Novartis, Helena Marzo-Ortega AbbVie, Biogen, Eli-Lilly, Janssen, Moonlake, Novartis, Pfizer, Takeda and UCB, Janssen, Novartis and UCB, Dennis McGonagle AbbVie, Eli-Lilly, Janssen, Novartis, Pfizer, UCB, AbbVie, Eli-Lilly, Janssen, Novartis, Pfizer, UCB.

© The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.