Sequestrum

[Image:Bony sequestrum in a child femur.jpg|thumb|right|X-ray of a child's femur, showing a bony sequestrum indicated by the blue arrow.]
A sequestrum is a piece of dead bone that has become separated during the process of necrosis from normal or sound bone.
It is a complication 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
bone necrosis follows due to lack of blood supply
sequestra are formed

The sequestra are surrounded by sclerotic bone which is relatively avascular. Within the bone itself, the haversian canals become blocked with scar tissue, and the bone becomes surrounded by thickened periosteum.
Due to the avascular nature of this bone, antibiotics which travel to sites of infection via the bloodstream poorly penetrate these tissues, hence the difficulty in treating chronic osteomyelitis.
At the same time as this, new bone is forming. Openings in this involucrum allow debris and exudates to pass from the sequestrum via sinus tracts to the skin.
Rarely, a sequestrum may turn out to be an osteoid osteoma, a type of bone tumor.