Blurring and irregularity of the subchondral cortex in pediatric sacroiliac joints on T1 images: incidence of normal findings that can mimic erosions.

OBJECTIVE To determine prevalence of variations of subchondral bone appearance that may mimic erosions on T1-weighted MRI of pediatric SIJ according to age and sex. METHODS With ethics committee approval and informed consent, SIJ MRI of 251 children (132 girls), mean age 12.4 years (range 6.1-18.0) were obtained, in two cohorts: 127 children imaged for non-rheumatologic reasons, and 124 children with low back pain but no features of sacroiliitis at initial clinical MRI review. MRI were reviewed by three experienced radiologists, blinded from each other, for three features of the cortical black line representing the subchondral bone plate on T1-weighted MRI: visibility, blurring and irregularity. RESULTS Based on agreement of 2 or more readers, the cortical black line was partially absent in 88.4% of the children, blurred in 34.7%, and irregular in 41.4%. All these features were most common on the iliac side of SIJ and at S1 level. Clearly visualized, sharply delineated SIJ with none of these features were seen in only 8.0% of children, or in 35.1% if we conservatively required agreement of all 3 readers to consider a feature present. There was no significant difference between sexes or cohorts; findings were similar across pediatric age groups. CONCLUSION Understanding the normal MRI appearance of the developing sacroiliac joint (SIJ) is necessary to distinguish physiologic findings from disease. At least two-thirds (65%) of normal pediatric SIJ showed at least one feature that is a component of the adult definition of SIJ erosions, risking over-diagnosis of sacroiliitis.