Fat infiltration on magnetic resonance imaging of the sacroiliac joints has limited diagnostic utility in nonradiographic axial spondyloarthritis.

OBJECTIVE: To explore whether morphological features of fat infiltration (FI) on sacroiliac joint (SIJ) magnetic resonance imaging (MRI) contribute to diagnostic utility in 2 inception cohorts of patients with nonradiographic axial spondyloarthritis (nr-axSpA).

METHODS: Four blinded readers assessed SIJ MRI in 2 cohorts (A/B) of 157 consecutive patients with back pain who were ≤ 50 years old, and in 20 healthy controls. Patients were classified according to clinical examination and pelvic radiography as having nr-axSpA (n = 51), ankylosing spondylitis (n = 34), or nonspecific back pain (n = 72). Readers recorded FI, bone marrow edema (BME), and erosion, predefined morphological features of FI (distinct border, homogeneity, subchondral location), and anatomical distribution of SIJ FI. The proportion of SIJ quadrants affected by FI and frequencies of various SIJ FI features were analyzed descriptively. We calculated positive/negative likelihood ratios (LR) to estimate the diagnostic utility of various features of FI, with and without associated BME, and erosion.

RESULTS: Of the patients with nr-axSpA in cohorts A/B, 45.0%/48.4% had FI in ≥ 2 SIJ quadrants. Of those, 25.0%/22.6% and 20.0%/25.8% showed FI with distinct border or homogeneous pattern, respectively, and 50% to 100% of those patients displayed concomitant BME or erosion. FI per se in ≥ 2 SIJ quadrants had no diagnostic utility (LR+ 1.62/1.91). FI with distinct border (LR+ 8.29/2.13) or homogeneity (LR+ 6.24/3.78) demonstrated small to moderate diagnostic utility.

CONCLUSION: SIJ FI per se was not of clinical utility in recognition of nr-axSpA. Distinct border or homogeneity of FI on SIJ MRI showed small to moderate diagnostic utility in nr-axSpA, but were strongly associated with concomitant BME or erosion, highlighting the contextual interpretation of SIJ MRI.