- Involucrum is new bone formation surrounding dead bone (sequestrum).
- Commonly seen in chronic osteomyelitis.
- Represents body’s attempt to wall off infection.
- Important radiological and surgical concept.
What is Involucrum?
Involucrum refers to the formation of new reactive bone surrounding an area of devitalized bone (sequestrum) in chronic osteomyelitis. It represents a biological response where periosteal new bone encases infected necrotic bone in an attempt to contain infection and maintain structural stability.
Pathophysiology of Involucrum Formation
During osteomyelitis, infection leads to vascular compromise and bone necrosis. The necrotic segment separates as a sequestrum. In response, periosteal elevation stimulates osteogenesis resulting in involucrum formation. This new bone envelope may contain openings called cloacae that allow drainage of pus.
- Infection → vascular compromise
- Bone necrosis → sequestrum formation
- Periosteal reaction → involucrum
- Drainage tract → cloaca
Clinical Significance
- Indicator of chronicity in osteomyelitis.
- Suggests long-standing infection.
- Important in surgical planning.
- May maintain mechanical stability despite necrosis.
Radiological Features
Plain radiographs typically show dense periosteal new bone surrounding radiolucent areas representing sequestrum. CT imaging provides better visualization of cloacae and cortical destruction.
- Thickened cortical bone
- Irregular new bone formation
- Central sequestrum
Differential Concepts
- Sequestrum: Dead bone fragment.
- Involucrum: New reactive bone surrounding sequestrum.
- Cloaca: Opening for pus drainage.
Treatment Implications
Presence of involucrum often indicates chronic infection requiring surgical management. Treatment may include:
- Sequestrectomy
- Debridement
- Antibiotic therapy
- Reconstruction depending on defect size
Exam and MCQ Relevance
Involucrum is a classic orthopaedic exam concept frequently tested in postgraduate entrance exams and orthopaedic viva discussions. To test your understanding, practice infection and arthroscopy-related orthopaedic MCQs at:
OrthoMCQ.in – Orthopaedic MCQ Practice Platform
Frequently Asked Questions
Is involucrum always present in osteomyelitis?
No. It typically indicates chronic stages rather than acute infection.
Can involucrum be seen on X-ray?
Yes, especially in long-standing cases with significant periosteal reaction.
Does involucrum require surgery?
Often yes, particularly when sequestrum persists or infection fails to resolve.
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Written by: Dr Subhajit Maji, Orthopaedic Surgeon (AIIMS)
Last Updated: February 2026
This educational content is intended for informational purposes and should not replace professional medical consultation.