FRI0191 Similarities and differences between osteitis condensans ilii and axial spondyloarthritis patients presenting with chronic back pain in a rheumatology setting

Background Osteitis condensans ilii (OCI) is regarded as a non-inflammatory disorder, which is considered to be induced by mechanical stress (e.g., by pregnancy and delivery). The diagnosis is normally based on wide spread sclerosis of the sacroiliac joint without erosions or ankyloses on imaging. More recently, paraarticular bone marrow oedema has been described on MRI, which can occur early but also intermittently later in the course of the disease. The clinical characteristics of OCI patienst have not been well described. To date, there are no published systematic data on the characteristics of OCI as compared to axial spondyloarthritis (axSpA). Objectives The objective of this matched case-control study was to investigate demographic, clinical, and lab characteristics of OCI as compared to axSpA. Methods Using medical database search we have identified n=103 patients aged ≥18 years who were diagnosed with OCI upon presentation with chronic back pain in the Early Spondyloarthritis Clinic of the rheumatology department in the Charite University Hospital between January 2010 and May 2015. These patients were contacted in order to obtain an informed consent and to complete a survey on the disease-related history. n=60 OCI patients who provided an informed consent and completed the survey were included in the final analysis. These patients were matched with a 1:2 ratio according to the back pain duration to patients with definite axSpA diagnosed in the same setting in order to compare demographic, clinical and lab characteristics. Results The main characteristics of the two groups are presented in the table. Most importantly, all but 2 patients with OCI were females and had a significantly lower prevalence of inflammatory back pain, lower level of CRP stressing a rather non-inflammatory nature of this condition. All patients were referred because of possible axial SpA, therefore SpA features, although being lower than in axSpA patients (table 1), were higher than can normally expected in OCI patients. This is probably the reason why a statistical significance in comparison to axSpA was observed for uveitis only. There was no difference in age of back pain onset (but age Conclusions OCI manifesting with chronic back pain starting prior to 45 years of age represents an important differential diagnosis for axSpA. A constellation of a female sex (with a history of pregnancies), negative HLA-B27 and negative CRP seems to be of differential diagnostic value as compared to axSpA. Disclosure of Interest None declared