What causes brim sign in the pelvis on plain radiograph?
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The brim sign occurs due to Paget’s disease of bone involving the pelvis on plain radiograph. It is caused by abnormal bone remodeling characterized by increased osteoclastic activity leading to bone resorption in the early phase, followed by a compensatory excessive osteoblastic bone formation. This results in thickened, sclerotic bone along the pelvic brim (iliopectineal line). The sign is a radiological marker of pelvic involvement in Paget’s disease, a chronic disorder that causes abnormal, disorganized bone turnover.
Why is it called so?
It is named the “brim sign” or “pelvic brim sign” because the radiographic finding localizes to the pelvic brim, specifically along the iliopectineal line, which forms the anteromedial margin of the pelvic brim. The sign describes the characteristic appearance of cortical thickening and sclerosis outlining this pelvic margin on a plain radiograph.
Pathophysiology
The sign develops due to the pathological process of Paget’s disease of bone where increased osteoclastic resorption initially thins and weakens bone, prompting an exaggerated osteoblastic response that leads to excessive, disorganized new bone formation. Because the pelvic brim is a weight-bearing region, these remodeling changes are pronounced here, resulting in cortical thickening and sclerosis visible as a dense, sclerotic line on the radiograph tracing the pelvic brim anatomy.
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