Involucrum

An involucrum (plural: involucra) is a complication of osteomyelitis and represents a thick sheath of periosteal new bone surrounding a sequestrum.

Terminology

Involucrum should be used in the context of chronic osteomyelitis to describe an area of thickened viable bone that forms around an area of necrotic infected bone ¹.

Pathology

The formation of an involucrum can be analogised to the process of soft tissue walling off an abscess elsewhere in the body. The involucrum involves a layer of granulation tissue, surrounded by irregular expansile cortical or cancellous bone ¹. The inner layer serves to contain the infection, but further separates it from the body's immune system, and biofilm formation can occur, prolonging and complicating infection ². The outer layer, continuous with the healthy bone away from the site of infection, provides strength to the infected bone and allows continuity of function while the body combats the infection ².

Radiogaphic features

Plain radiograph

An involucrum will appear on plain radiographs as an irregular thickening of the cortex of an infected bone. There may be a cloaca or sequestrum visible, though these are more difficult to detect on plain radiographs than an involucrum. The trabecular pattern is likely to be more irregular and disordered than that of normal bone ³. 

CT

CT will also identify irregular cortical thickening, but is much more sensitive than plain radiographs to the other signs of chronic osteomyelitis like cloaca or sequestrum. Periosteal reaction and new bone formation may be identified in the outer layer.

MRI

The established parts thickened outer layer of an involucrum is new healthy bone and will follow cortical signal. The inner granulation layer may appear as a high signal rim on T1 weighted imaging, and periosteal new bone formation may also appear as high T1 weighted signal ^1,3.

Treatment and prognosis

Involucrum formation typically indicates chronic osteomyelitis that the body is walling off and isolating from the immune system. This means that surgical debridement, particularly of a sequestrum if present, is typically required for cure, though the thickened outer layer of involucrum may not itself need debrided ¹. If the infection is successfully treated, the irregular cortex may gradually remodel over time.