Osteitis condensans ilii masquerading as sacroiliitis: a case report of an underdiagnosed condition.

Introduction and Importance: Osteitis condensans ilii (OCI) is a benign, non-inflammatory condition characterized by triangular sclerosis of the iliac bone adjacent to the sacroiliac joint, typically with preserved joint space and normal inflammatory markers. It can mimic inflammatory sacroiliac disorders, leading to misdiagnosis. Case Presentation: We report the case of a 38-year-old multiparous woman with chronic low back pain that began during pregnancy and persisted for 2.5 years. The pain was mild, intermittent, and aggravated by activity. She had initially been treated for spondyloarthritis without improvement. Examination revealed normal spinal mobility and localized tenderness over the sacroiliac region. Laboratory tests, including inflammatory and autoimmune markers, were normal. The radiographs demonstrated bilateral triangular iliac sclerosis without erosions. MRI confirmed sclerosis with preserved sacroiliac joint space, consistent with OCI. Clinical Discussion: OCI is often underrecognized and may be mistaken for axial spondyloarthritis. Mechanical stress, particularly during pregnancy, is implicated; however, cases in men and nulliparous women suggest a multifactorial etiology. Imaging is crucial for diagnosis, as plain radiographs may be insufficient. Conclusion: OCI should be considered in patients with chronic low back pain to prevent unnecessary immunosuppressive therapy. Conservative management with physiotherapy, non-steroidal anti-inflammatory drugs (NSAIDs), and short-term muscle relaxants can provide significant symptomatic improvement, though complete resolution may require longer follow-up and adherence to rehabilitation measures. The overall prognosis is favorable.