Codman triangle

The Codman triangle (previously referred to as Codman's triangle) is the triangular area of new subperiosteal bone that is created when a lesion, often a tumor, raises the periosteum away from the bone.

This two sided appearance is 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.

[2] The advancing tumour displaces the periosteum away from the bone medulla.

The displaced and now lateral periosteum attempts to regenerate underlying bone.

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