Background Idiopathic inflammatory myopathies are chronic, heterogeneous systemic autoimmune diseases with symmetrical proximal muscle weakness. During the disease course, osteoporosis and bone fractures are more common compared to the healthy population, which can be explained by the chronic inflammation, immobilization, spontaneous falls and steroid treatment, and affect crucially the patients' quality of life. Recently, a WHO fracture risk calculation tool, FRAX score is available, to measure the 10-year probability of osteoporotic fractures. It takes into account relevant clinical risk factors, such as rheumatoid arthritis, however myositis does not exist among the risk factors.

Objectives Estimation the effect of myositis and myositis related bone mineral densities on bone fracture risk calculated by FRAX tool.

Methods FRAX score was determined in 71 patients with idiopathic inflammatory myopathies and results were compared with the data from 50 age, sex and BMI matched patients with rheumatoid arthritis. Moreover, osteoporosis related biomarkers, disease related fractures and bone mineral densities were determined using DXA examinations. Statistical analysis was performed with IBM SPSS 20.0 software.

Results There were no significant differences between the demographical data, biomarkers (Ca, Vitamin D, parathormone level) of the two groups. Disease duration and cumulative steroid dose were higher in the myositis group. Results of the FRAX score without BMD were significantly lower in the patients with myositis, in both fracture risk: major osteoporotic (8,61±6,36%, vs. 15,59±12,66%; p: 0,002) and femur neck (2,66±3,24%, vs. 6,34±9,018; p: 0.003). T score results of the DXA examination were not significantly different between the two populations (Lumbar1–4: -0,9±1,43 vs.-0,829±1,38; p:0,829; Femoral neck: -1,4±1,08 vs. -1,02±1,08; p: 0,93), but the presence of osteopenia (60% vs. 39,5%) and osteoporosis (13,5% vs. 7%) were more frequent in the myositis group (p: 0,045). Disease related fracture was associated with disease duration in the myositis group and with antibody (RF, ACPA) presence in the RA group. FRAX score with BMD results showed no significant differences between the two populations (Major osteoporotic: 9,44±6,72 vs. 13,25±9,43; p: 0,053; Hip: 2,77±3,01 vs. 3,57±5,08; p: 0,811).

Conclusions As far as we know, this is the first study which examine the fracture risk using FRAX score in patients with idiopathic inflammatory myopathies. According to our data, we can conclude that existence of myositis might indicate similar, independent risk factor in fracture probability, like rheumatoid arthritis. Evaluation of fracture risk should be done with DXA result in patients with IIM, otherwise risk could be underestimated. An exact value of the “myositis related risk” could be determined by a 10-year prospective study.

Disclosure of Interest None declared