Describe Periosteal Reactions & Codman’s Triangle in detail.

Periosteal Reaction

A periosteal reaction is new bone laid down by the periosteum in response to an underlying process. Its pattern reflects the aggressiveness of the lesion: a solid, continuous reaction suggests a slow, benign process, whereas an interrupted (discontinuous) reaction suggests a rapidly growing, aggressive or malignant lesion that outpaces the periosteum.

Aggressive periosteal reactions: sunburst spiculation, laminated onion-skin, and Codman’s triangle.

Key Patterns

• Sunburst (‘sunray’) spiculation — spicules of new bone radiating perpendicular to the cortex, classic of osteosarcoma.
• Onion-skin (lamellated) — concentric layers, characteristic of Ewing’s sarcoma.
• Codman’s triangle — a cuff of new bone formed where an aggressive tumour lifts the periosteum off the cortex
• the periosteum ossifies only at the margin, leaving a triangular spur. It indicates an aggressive lesion (osteosarcoma, Ewing’s, sometimes infection).

• Periosteal reaction pattern reflects lesion aggressiveness.
• Solid/continuous = benign; interrupted = aggressive/malignant.
• Sunburst → osteosarcoma; onion-skin → Ewing’s.
• Codman’s triangle = periosteum lifted by an aggressive lesion (not tumour-specific).