Sequestrum

A sequestrum (plural: sequestra) is a piece of dead bone^[1] that has become separated during necrosis from normal or sound bone. It is a complication (sequela) of osteomyelitis.

The pathological process is as follows:

• infection in the bone leads to an increase in intramedullary pressure due to inflammatory exudates
• the periosteum becomes stripped from the ostium, leading to vascular thrombosis
• lack of blood supply causes bone necrosis
• sequestra are formed

The sequestra are surrounded by sclerotic bone, which is relatively avascular (without a blood supply). So, antibiotics which travel to sites of infection via the bloodstream poorly penetrate these tissues, hence the difficulty in treating chronic osteomyelitis.^{[citation needed]}

Within the bone itself, the haversian canals become blocked with scar tissue, and the bone becomes surrounded by thickened periosteum.^{[citation needed]} Meantime, new bone (involucrum) forms. Openings in this involucrum allow debris and exudates (including pus) to pass from the sequestrum via sinus tracts to the skin.^{[citation needed]}

Rarely, a sequestrum may turn out to be an osteoid osteoma, a type of bone tumor.