Osteitis condensans ilii in a patient with ulcerative colitis: a mimic of ankylosing spondylitis or non-radiographic axial spondyloarthritis.

Osteitis condensans ilii (OCI) is known as one of the sacroiliitis diseases and the symptoms or radiographic findings of OCI sometimes resembled those of ankylosing spondylitis (AS) of non-radiographic axial spondyloarthritis (nr-axSpA). Bone marrow edoema (BME) around sacroiliac (SI) joints on magnetic resonance imaging (MRI) is one of the key findings in the classification criteria of axial SpA, but BME on MRI was also found in patients with OCI. The usage of SI joint MRI is increasing due to the need of accurate or early diagnosis of axial SpA in accordance with development of new biological treatments. Here was the case of a 38-year-old female patient with ulcerative colitis (UC) complicated with OCI, which was a mimic of AS or nr-axSpA. She had a persistent low back pain with BME around SI joints on MRI. She could meet the ASAS criteria of axial SpA and might be diagnosed with inflammatory bowel disease (IBD) related axial SpA. OCI is not a rare disease as a differential diagnosis for axial SpA especially in young women. We report this case with a literature review of OCI and would like the clinicians to be aware of the disease when diagnosing AS or nr-axSpA with chronic low back pain.