A piece of dead bone which has become separated from the rest of the bone to which it initially belonged. It is a typical sign of bone necrosis or osteomyelitis (an infection of the bone). The inflammation results in a localised loss of blood supply, which leads to cell death. De-vitalised areas of bone are removed by osteoclasts, and a sequestrum may be formed. Eventually, most sequestra will also be removed by the osteoclasts, sometimes leading to abscesses and draining tracts or fistulas.