Imaging characteristics of osteitis condensans ilii. A follow-up study

Objective To evaluate the diagnostic ability of combined clinical and MRI findings in a cohort of patients initially diagnosed with axial spondyloarthritis (axSpA) and to detect potential patients with osteitis condensans ilii (OCI) for further analyses. Materials and methods We conducted a retrospective follow-up study of 129 consecutive patients aged ≥ 18 years (49 women/80 men) diagnosed with axSpA during the period 2012–2023. Uniform baseline MRI of the sacroiliac joints (SIJ) and spine with concomitant radiography and/or CT were reassessed by an experienced radiologist using fulfillment of thresholds for SIJ MRI changes consistent with axSpA and a global assessment of all imaging material . Clinical and biochemical findings at baseline and during follow-up were reevaluated and in combination with imaging findings used as a reference standard for axSpA and OCI changes, respectively. Imaging findings consistent with OCI were confirmed by a second experienced radiologist and OCI imaging characteristics were systematically assessed. Results axSpA was confirmed in 95 patients. Seven women had OCI and 27 (15 women/12 men) had non-specific SIJ bone marrow edema (BME). None of the OCI women fulfilled radiographic criteria for axSpA, but all fulfilled fat deposition thresholds on MRI consistent with axSpA and five also fulfilled thresholds for BME. The detected OCI MRI characteristics were as follows: sclerosis predominating at the anterior-middle iliac joint areas with BME and fat deposition at the periphery, and subchondral BME and fat deposition predominantly in the sacrum. Conclusion The location of sclerosis, BME, and fat deposition can be used to diagnose OCI changes on MRI.