Background and Aims Our group introduced a quantitative marker of antiphospholipid antibodies (aPL) ‘‘antiphospholipid score (aPL-S)’’, which well reflected the risk of developing thrombosis (Otomo K, et al. Arthritis Rheum 2012). Idiopathic osteonecrosis (ION) has been shown to occur as a result of ischemia, however, the involvement of aPL in its pathophysiology remains to be elucidated. In this study, we aimed to identify the impact of aPL on the development of ION using aPL-S.

Methods A single centre retrospective study comprising 75 consecutive patients with systemic lupus erythematosus who underwent magnetic resonance imaging of hip joints from January 2000 to March 2016. aPL-S, as well as classical risk factors for ION, were evaluated in all the enrolled patients.

Results ION of the femoral head was observed in 33 out of 75 patients(44%). High aPL-S (p=0.009), aPL positivity (p=0.009), male (p=0.007), malar rash (p=0.010)and high dose (>0.8 mg/kg/day) glucocorticoids(p<0.001) were identified as risk factors for ION at univariate analysis. Multivariate analysis confirmed high aPL-S, male, malar rash and high dose glucocorticoids as independent variables. Six out of 8 patients(75%) with very high aPL-S (>30), developed ION. Conversely, systemic lupus erythematosus disease activity index and pulse methylprednisolone therapy were not identified as risk factors for ION.

Conclusions We newly identified aPL-S as a risk factor for ION. ION should be considered as one of the antiphosholipid antibody-associated-disease.