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Involucrum refers to the layer of new bone formed by the periosteum around a sequestrum (dead bone) in the setting of chronic osteomyelitis.

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It represents the body’s attempt to wall off the infection and stabilize the area by forming reactive bone around the infected focus.

Pathogenesis

Often perforated by a cloaca, allowing drainage of pus or formation of sinus tracts.

Process Step	Description
Infection	Bacterial osteomyelitis causes inflammation and marrow necrosis
Sequestrum formation	Devitalized necrotic bone becomes isolated from vascular supply
Periosteal reaction	The periosteum lays down new bone in response to cortical destruction
Involucrum	This new bone sheath surrounds and encases the sequestrum

Clinical Context

Chronic osteomyelitis (most common association)
Seen in:
- Post-traumatic infections
- Post-surgical infections (e.g., open fractures)
- Tuberculous osteomyelitis (less common)
- Diabetic foot infections

Radiology

Modality	Imaging features
XR	• Involucrum: Thick, irregular periosteal new bone encasing the original cortex
• Sequestrum: Inside Dense fragment within a lucent cavity
• Cloaca: Linear channel from inside to outside (if sinus present)
CT	• Bony detail: Clearly shows cortical disruption, involucrum, and sequestrum
• Sinus tract mapping: Excellent for evaluating cloaca and surgical planning
MR	• Involucrum: Seen as low signal intensity rim on all sequences
• Sequestrum inside: Non-enhancing T1/T2 dark fragment
• Adjacent findings: Marrow edema, soft tissue abscess, sinus tracts