Background/Purpose: Osteitis condensans ilii (OCI) is regarded to as a non-inflammatory disorder induced by mechanical stress and mechanical instability of the sacroiliac joints (SIJ). OCI is being increasingly recognized as an important differential diagnosis for axial spondyloarthritis (axSpA), due to onset at young age, possible inflammatory character of back pain and recently described presence of subchondral bone marrow edema on magnetic resonance imaging (MRI) of the SIJ. The objective of the study was to compare active and chronic inflammatory lesions of the SIJ as detected by MRI in patients with OCI and axSpA.

Methods: Using medical database search we identified n=103 patients aged ≥18 years who were diagnosed with OCI upon presentation with chronic back pain and suspicion of axSpA. A total of 27 patients had evaluable MRIs of the SIJ in STIR and T1-weighted sequences that were used for the current study. These patients were matched according to the back pain duration to 27 patients with definite axSpA. MRIs were scored in a blinded fashion according to the Berlin scoring system for osteitis, fatty metaplasia, erosions, sclerosis and ankylosis independently by 3 trained and calibrated readers. In addition, the preferential localization of lesions (ventral, mid, or dorsal part of the SIJ) was recorded.   

Results: There were no differences either in the osteitis score or in the proportion of patients with presence of osteitis on MRI of the SIJ between OCI and axSpA patients (table). The fatty metaplasia score was significantly lower in OCI as compared to axSpA, although the difference in the prevalence of the fatty lesions did not reach the level of statistical significance. There was a non-significant trend towards a higher sclerosis score in OCI patients. Importantly, there was a highly significant difference in the erosion score and in the prevalence of erosions: only 2 (7.4%) OCI vs. 18 (66.7%) axSpA patients had at least one erosion (table 1). Importantly, none of the OCI patients had high-grade (>5 erosions) erosive changes.

There were substantial differences concerning localization of the lesions: in OCI, ventral localization was recorded in 96% of the cases for osteitis, in 100% for fatty metaplasia, and in 96% for sclerosis, while in axSpA, osteitis was preferentially localized in the ventral part only in 29% of the cases, fatty metaplasia in 25%, sclerosis in 29% (p<0.001 for all comparisons vs. OCI). Ankylosis and erosions were localized in the mid part in almost all cases.

Conclusion: MRI of sacroiliac joints in OCI is characterized by preferential ventral localization of lesions (osteitis, fatty metaplasia, and sclerosis), absence of ankylosis and absence of extended erosive changes. Such a findings constellation should be taken into account as suggestive of OCI for the differential diagnosis of axSpA in clinical practice.