Codman triangle

Codman triangle (previously referred to as Codman's triangle) is a term used to describe the triangular area of new subperiosteal bone that is created when a lesion, often a tumour, raises the periosteum away from the bone. A Codman triangle is not actually a full triangle. Instead it is often a pseudotriangle on radiographic findings with ossification on the original bone and one additional side of the triangle which forms a two sided triangle with one open side. This two sided appearance in generated due to a tumor (or growth) that is growing at a rate which is faster than the periosteum can grow or expand, so instead of dimpling, the periosteum tears away and provides ossification on the second edge of the triangle.

The main causes for this sign are osteosarcoma, Ewing's sarcoma, eumycetoma, and a subperiosteal abscess.