Osteitis condensans ilii: prevalence and characteristics of a neglected mimic of sacroiliitis

Objectives: Osteitis condensans ilii (OCI) is a benign condition characterised by triangular sclerosis of the iliac bone which may mimic radiographic sacroiliitis. Prevalence is estimated between 0.9 and 2.5%, with female predominance, but the most recent article reporting original epidemiological data in the general population was published in 1971. The aim of our study is to contribute updated figures about prevalence of OCI in Italy. Method: A retrospective review of pelvic radiographs was conducted. Consecutive patients visiting the emergency department of our Institution between 1st January and 31st December 2020 were enrolled. Individuals with a past diagnosis of axial spondyloarthritis were excluded. Presence of OCI was evaluated by two musculoskeletal radiologists. Clinical and radiologic features such as osteoarthritis and insertional enthesopathy were also assessed. Results: We included 1047 individuals (61% female) with a median age of 74 years. OCI was present in 10 cases, accounting for a prevalence in the general population of 1.0% (95% CI 0.5–1.7). All patients with OCI were women and, in the female sample, prevalence was 1.6% (95% CI 0.7–2.8). Clinical characteristics and associated radiographic features were not different between patients with OCI and women without OCI. Conclusions: The prevalence of OCI observed in our study is consistent with previous literature, and we confirm that it is more frequently retrieved in women. Longitudinal research is warranted to elucidate the evolution, while knowledge about the disorder is needed to raise the awareness of rheumatologists and radiologists and to properly identify and report the condition.Key Points• OCI may mimic sacroiliitis and is a major differential diagnosis of radiographic axial spondyloarthritis.• Prevalence of OCI in our sample is 1.0%, in line with previous literature.• OCI predominantly affects women, and our study suggests that the disorder can be incidentally identified even after childbearing age.• Increased awareness of the characteristics of OCI can facilitate identification and reporting of the disorder.