OP0255 THE ROLE OF PELVIC MORPHOLOGY IN AXIAL SPONDYLOARTHRITIS

Background: Axial Spondyloarthritis (axSpA) is a chronic inflammatory disease affecting the axial skeleton. It includes non-radiographic axSpA and radiographic axSpA [Ankylosing Spondylitis (AS)]. Male axSpA patients often have greater damage, while women report a higher disease burden. The role of pelvic morphology in the axSpA phenotype has not been explored. There is anatomic sexual dimorphism between the male and female pelvis. Given the phenotypic gender differences in axSpA, the role of pelvic morphometry is of interest. Objectives: The purpose of this study is to determine whether an association exists between pelvic dimensions and radiographic damage in patients with axSpA, as well as to compare these measurements in axSpA patients and healthy controls. Methods: This was a cross-sectional analysis comparing axSpA cases from a prospective cohort and non-axSpA controls from the UCSF radiology databank. Informed consent was obtained from axSpA cohort patients and this study was approved by the institutional IRB. To be included in the analysis, we limited inclusion to age ≤ 50 with an Anterior Posterior (AP) pelvis radiograph in the system. We excluded non-nulliparity, pelvic fracture history, BMI ≥ 30kg/m2, any prosthetic history and avascular necrosis. We measured the pelvic inlet, pelvic outlet, and subpubic angle (based on validated scoring methods) (Figure 1) and assessed its relation to sacroiliac joint (SIJ) damage (average SIJ score, New York criteria) and modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) in cases. AxSpA patients were also compared to age/gender matched controls. Pelvic measurements were performed by 2 blinded independent-trained readers in randomized, blinded image order. Inter-rater reliability was assessed. When examining the relationship between pelvic measurements and damage, linear regression was used to stratify by gender and adjust for potential confounders. Results: The axSpA cohort included 481 patients, of which 210 men and 89 women were included in this analysis and gender/age matched controls. Rater inter-class correlation was above 0.70 for pelvic outlet and above 0.80 for other measures. Cases and controls were similar (Table 1). The regression analysis showed a significant relationship between the sub-pubic angle and damage in the spine (coeff=-0.342, p=0.003) in men with axSpA. A sensitivity analysis, excluding mSASSS outliers (mSASSS ≥ 16) upheld the relationship (coeff=-1.40, p=0.002). Conclusion: In men with axSpA, there appears to be a relationship between sub-pubic angle and spinal radiographic damage. This is consistent with our finding that women have larger sub-pubic angles and lower spinal radiographic damage than men. A greater sub-pubic angle may protect against spinal involvement or associate with other protective factors. Further work should be performed to understand the contribution of pelvic anatomy to damage in axSpA. Disclosure of Interests: Ethan Zaccagnino: None declared, Rina Patel: None declared, Lianne S. Gensler Consultant of: AbbVie, Eli Lilly, Gilead, Novartis, Pfizer and UCB., Grant/research support from: Pfizer and UCB.