Codman triangle is a radiologic sign seen most commonly on musculoskeletal plain films. It is the name given to a periosteal reaction that occurs when bone lesions grow so aggressively they lift the periosteum off the bone and do not allow the periosteum to lay down new bone. It is a pattern of interrupted periosteal reaction where the periosteum gets lifted at either end of the aggressive lesion, and the central aspect of the lesion does not have any overlying ossification. This state gives the appearance on plain films of a raised triangle of periosteum along the edges of the lesion. In this reaction, the periosteum may become lifted by the leading edge of the tumor, pus, or hemorrhage.
This radiologic sign classically presents with aggressive primary bone tumors, including osteosarcoma (most common), Ewing sarcoma, chondrosarcoma, metastasis, malignant fibrous histiocytoma, and malignant giant cell tumor. It may rarely appear with non-malignant lesions such as osteomyelitis, active aneurysmal bone cysts, trauma, and hematoma.
The reaction derives its name Ernest Amory Codman, a surgeon who first described the finding in an article in 1926 when discussing malignant bone sarcomas.