Evolution of osteitis condensans ilii-like sclerosis in first-time mothers by serial sacroiliac joint MRI during pregnancy and in the postpartum period

This study aims to evaluate the evolution of subchondral sacroiliac joint (SIJ) sclerosis from pregnancy to 12 months postpartum, and to explore preceding and concomitant magnetic resonance imaging (MRI) features, potentially indicating osteitis condensans ilii (OCI). One hundred three first-time mothers were recruited for serial SIJ MRIs. MRI scans were performed at pregnancy weeks 20 and 32, and at 3, 6, and 12 months postpartum. Three readers independently evaluated subchondral sclerosis, bone marrow oedema (BME), fat lesions, and erosions. Sclerosis evolution and associations with BME, fat lesions, erosions, and pain were analysed. The prevalence of iliac subchondral sclerosis increased gradually from 20% at pregnancy week 20% to 46% at 12 months postpartum, predominantly located in the upper-middle and anterior joint portions with depth expanding from a mean of 6.5 to 7.5 mm. BME and fat lesions were frequent with BME peaking at 3 months postpartum and fat lesions later in the postpartum period. Expanding depth of sclerosis at 12 months postpartum was associated with increasing prevalence of BME and fat lesions, but not with pain. At 12 months postpartum, BME and fat lesions meeting Assessment of SpondyloArthritis International Society thresholds were significantly more frequent with sclerosis ≥8 mm (50/40%) compared to sclerosis <8 mm (15/7%). Increasing depth of subchondral SIJ sclerosis during and after pregnancy was associated with BME peaking in the early and fat lesion in the late postpartum period suggesting that BME may precede the development of sclerosis and fat lesions. Sclerosis depth ≥8 mm, frequently accompanied by high-level BME and fat lesion, may serve as an MRI surrogate biomarker of radiographic OCI.