OP0312 PATTERNS OF INFLAMMATORY AND STRUCTURAL SACROILIAC JOINT MRI LESIONS IN 1,946 EUROPEAN PATIENTS WITH AXIAL SPONDYLOARTHRITIS FROM ROUTINE CARE

Background: In 2009, the Assessment of SpondyloArthritis international Society (ASAS) definition of active sacroiliitis as detected by MRI emphasized the presence of sacroiliac joint (SIJ) subchondral bone marrow edema (BME). Later, concerns were raised that an overreliance on the presence of BME could lead to overdiagnosis of axial spondyloarthritis (axSpA) in routine care, since BME is not specific for this condition [1, 2]. Indeed, when an SIJ MRI is reported and used for diagnostic purposes, structural lesions and differential diagnoses must be considered in addition to BME [3, 4]. Objectives: To describe the occurrence and patterns of distribution of SIJ MRI features in patients with axSpA treated in routine care across Europe. Methods: Within the European Spondyloarthritis Research Collaboration Network (EuroSpA), patients with a clinical diagnosis of axSpA from 5 European registries, ATTRA (Czechia), Biorx.si (Slovenia), DANBIO (Denmark), ICEBIO (Iceland) and SCQM (Switzerland), and an available SIJ MRI were included. It was required that MRIs included both a T1-weighted sequence to allow for assessment of structural lesions and a short tau inversion recovery (STIR) or T2-weighted fat-saturated sequence to allow for assessment of active inflammatory lesions. MRIs were collected locally and evaluated centrally, using a web-based questionnaire, by two readers who were aware of patient age and sex but blinded to other clinical data and imaging. Each MRI was evaluated by an experienced musculoskeletal (MSK) radiologist and an experienced reader with more than 5 years of experience, or by two experienced MSK radiologists. The readers registered global assessments, including whether the MRI (1) was overall indicative of SpA, (2) fulfilled the ASAS definition of active sacroiliitis, and (3) indicated other conditions. In addition, the presence or absence of a list of inflammatory and structural SpA features, as defined by the ASAS MRI Group, was registered. MRIs were adjudicated by one of three expert MSK radiologists (all members of the ASAS MRI Group), using the same questionnaire, if the two primary readers disagreed on whether the SIJ MRI (1) was overall indicative of SpA, (2) fulfilled the ASAS definition of active sacroiliitis, and/or (3) indicated infection, neoplasia or fracture, according to one of the primary readers. Results: Overall, 1,946 patients with a clinical diagnosis of axSpA were included. Centrally read SIJ MRIs were indicative of SpA in 1,286 patients (66%), while not indicative of SpA in 660 patients (34%) (Table 1). In both groups, a minority of patients had received their first biological or targeted synthetic disease-modifying anti-rheumatic drug (b/tsDMARD) prior to the MRI (162 [13%] vs 116 [18%]), while others initiated b/tsDMARD after the MRI (738 [57%] vs 328 [50%]), and the remaining did not receive b/tsDMARD at any time point (386 [30%] vs 216 [33%]). The most prevalent lesions in the ‘MRI indicative of SpA' group were BME (75% [bilateral: 45%, unilateral: 30%]), erosions (75% [52%, 24%]), and fat lesions (56% [42%, 14%]). Advanced structural lesions (confluent erosions, backfill and/or ankylosis) were present in 779 of 1,286 patients (61%), with increasing frequency associated with increasing age, longer symptom duration and HLA-B27 positivity. When stratified by sex, the proportion of patients with inflammatory and structural lesions within the ‘MRI indicative of SpA' group were comparable in males and females, although females had slightly less backfill and ankylosis, and slightly more confluent erosions and BME than males (Figure 1). Figure 1Percentage of patients with unilateral/bilateral lesions, stratified by sex and whether the SIJ MRI is indicative of SpA (A and B) or not (C and D). Patients in the ‘MRI not indicative of SpA' group were more often female and more frequently had peripheral arthritis (378/517 [73%] vs 499/926 [54%], p<0.001). BME and sclerosis were the most frequent lesions in the ‘MRI not indicative of SpA' group. The most prevalent other conditions in the ‘MRI not indicative of SpA' group were osteoarthritis (142 [22%]), strain-related BME (135 [20%]) and osteitis condensans ilii (105 [16%]). Conclusion: Among routine care patients with a clinical diagnosis of axSpA, two-thirds of SIJ MRIs were interpreted as indicative of SpA at central reading. This large and comprehensive study provides firm knowledge of the MRI findings in real-world axSpA patients. REFERENCES: [1] Lambert et al. Ann Rheum Dis 2016;75:1958-63. [2] Diekhoff et al. Skeletal Radiol 2022;51:1721-30. [3] Diekhoff et al. Radiology 2024;311:e231786. [4] Mandl et al. Ann Rheum Dis 2015;74:1327-39. Table 1Demographics and MRI findingsMRI indicative of SpA(n = 1286)MRI not indicative of SpA(n = 660)P-valueAge37 ± 1242 ± 12<0.001Sex, male858 (67%)212 (32%)<0.001Symptom duration9.3 ± 109.6 ± 100.9SIJ MRI global assessmentsInflammatory lesions indicative of SpA897 (70%)14 (2%)<0.001Structural lesions indicative of SpA1202 (94%)13 (2%)<0.001ASAS-positive MRI (‘active sacroiliitis')853 (66%)9 (1%)<0.001SIJ MRI SpA lesionsBME75%48%<0.001BME depth >1 cm35%2%<0.001Inflammation in an erosion cavity33%1%<0.001Enthesitis13%3%<0.001Capsulitis12%1%<0.001Joint space fluid16%4%<0.001Sclerosis28%25%0.2Erosion75%8%<0.001Confluent erosion26%0.2%<0.001Fat lesion56%15%<0.001Fat lesion depth >1 cm38%5%<0.001Backfill29%0.2%<0.001Ankylosis23%0.5%<0.001Data are complete for all patients, except for symptom duration (1036 and 536 with available data). Values are n (percentage), mean ± SD, or percentage as appropriate. P-values are by Wilcoxon rank sum test, Chi-squared test, or Fisher's exact test as appropriate. Abbreviations: SpA, spondyloarthritis; SIJ, sacroiliac joint; MRI, magnetic resonance imaging; BME, bone marrow edema. Acknowledgements: The EuroSpA collaboration has been supported by Novartis Pharma AG since 2017 and UCB Biopharma SRL since 2022. This EuroSpA study was financially supported by Novartis. No financial sponsors had any influence on the data collection, statistical analyses, abstract preparation, or decision to submit. Disclosure of Interests: Simon Krabbe Novartis, AbbVie, MSD, Anna Enevold Fløistrup Hadsbjerg Novartis, Nora Vladimirova MSD, Novartis, Torsten Diekhoff Canon MS, Lilly, MSD, Novartis, Pfizer, UCB, UCB, Lilly, Canon MS, Robert G Lambert AbbVie, Iris Eshed: None declared, Adrian Ciurea: None declared, Kristýna Bubová: None declared, Monika Gregová: None declared, Michael Nissen Abbvie, Amgen, Eli-Lilly, Janssen, Novartis, Pfizer, UCB, Abbvie, Amgen, Eli-Lilly, Janssen, Novartis, Pfizer, UCB, Novartis, Burkhard Moeller Abbvie, Novartis, UCB, Janssen, Amgen, Raphael Micheroli Abbvie, Janssen, UCB, Susanne Juhl Pedersen MSD, Pfizer, AbbVie, UCB, Novartis, AbbVie, UCB, Novartis, AbbVie, MSD, Novartis, Danish Research Foundation, Jakub Závada Abbvie, Akord, Astra Zeneca, Celltrion, Eli-Lilly, Glaxo, Novartis, Pfizer, Sobi, Ziga Snoj: None declared, Karlo Pintaric: None declared, Bjorn Gudbjornsson: None declared, Ziga Rotar Abbvie, Amgen, Novartis, MSD, Medis, Biogen, Eli Lilly, Pfizer, Sanofi, Lek, Janssen, Abbvie, Novartis, Eli Lilly, Pfizer, Janssen, Sobi, Swixx BioPharma, AstraZeneca, Iwona Sudoł-Szopińska: None declared, Kasper K Gosvig: None declared, Manouk de Hooge: None declared, Maurice Donzallaz: None declared, Alexander Bernatschek: None declared, Karel Gorican: None declared, Marie Wetterslev: None declared, Stephanie Wichuk: None declared, Merete Lund Hetland Pfizer, Medac, Sandoz, Novartis, Abbvie, Former Chair of the Danish Rheumatology Quality Registry (DANBIO) which receives public funding and funding from pharmaceutical companies, Abbvie, Biogen, BMS, Celltrion, Eli Lilly, Janssen Biologics B.V, Lundbeck Fonden, MSD, Medac, Pfizer, Roche, Samsung Biopies, Sandoz, Novartis, Nordforsk, Lykke Midtbøll Ørnbjerg Novartis, UCB, Mikkel Østergaard Abbvie, BMS, Boehringer Ingelheim, Celgene, Eli Lilly, Galapagos, Gilead, Hospira, Janssen, MEDAC, Merck, Novartis, Novo, Orion, Pfizer, Regeneron, Roche, Sandoz, Sanofi, UCB, Abbvie, BMS, Boehringer Ingelheim, Celgene, Eli Lilly, Galapagos, Gilead, Hospira, Janssen, MEDAC, Merck, Novartis, Novo, Orion, Pfizer, Regeneron, Roche, Sandoz, Sanofi, UCB, AbbVie, Amgen, BMS, Merck, Celgene, Eli Lilly, Novartis, 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.

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