It is a piece of dead bone in chronic osteomyelitis, surrounded by infected granulation tissue, trying to eat away the bone. The outer surface of this bone is smooth due to the erosion by granulation tissue, and the inner surface is rough. Image

Ref: Ortho Maheshwari – p161

1. Involucrum
2. Tension band principle
3. Anatomy – MCQ 62 – Growth at epiphyseal plate
4. AIIMS May 2011 – MCQ 154