Background Magnetic resonance imaging (MRI) has become the imaging modality of choice in early SpA. Not only has this imaging technique proven useful in the identification of inflammatory lesions on T2 FS/STIR sequences, also structural lesions such as erosions, sclerosis, fat infiltration and ankylosis can be seen on T1 sequences. There is still ongoing debate concerning the possible inclusion of any structural lesion in the definition of a positive MRI in order to improve specificity. However, erosions and fat infiltration have also been found in respectively more than 10% and 17% of non-SpA patients on MRI of the sacroiliac joints. 
Objectives To evaluate the presence of structural lesions on MRI of the sacroiliac joints (MRI-SIJ) in young, asymptomatic subjects.
Methods Twenty-five military recruits volunteered to perform a MRI-SIJ before and after 6 week of intense physical training, of which 22 recruits underwent imaging at both time points. The MRIs were scored for structural lesions by 3 trained readers MdH, GV and TR, blinded for time sequence and clinical findings. Regarding the number of lesions a consensus was made by agreement of 2 out of 3 readers.
Results At baseline, structural lesions were present in 36.4% (8/22) of subjects of which 5 subjects presented with at least 1 erosion, one subject with sclerosis and 3 subjects presented fatty lesions. This increased to 50% subjects (11/22) with structural lesions on MRI-SIJ after 6 weeks of mechanical stress, of which 8 subjects with at least 1 erosion, one subject with sclerosis and 3 subjects with fatty lesions (P=0.453). The change scores for sclerosis, erosions and fatty lesions were respectively 0.0 (±0.0), 0.2 (±0.1) and 0.3 (±0.2). None of the subjects displayed ankylosis. The lesion distribution is visualized in Table1.
Conclusions Although not prevalent, structural lesions such as erosions, sclerosis and fatty lesions can be found in young asymptomatic subjects. However, these lesions do not seem to increase after 6 week of intense physical training.
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Acknowledgements ASAS research grant 2017.
Disclosure of Interest None declared