Codman's triangle is the radiographic appearance of the rim of new subperiosteal bone which forms when a lesion such as a tumour lifts the periosteum away from the bone.
The small triangle of bone is seen at the advancing margin of the lesion.
The three main causes for a Codman's triangle are:
- Ewing's sarcoma
- subperiosteal abscess
Layering of the new bone may result in an "onion skin" appearance.