A sequestrum (plural: sequestra) is a piece of dead

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

that has become separated during the process of

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dig ...

from normal or sound bone. It is a complication (sequela) of

osteomyelitis Osteomyelitis (OM) is an infection of bone. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The long bones of the arms and legs are most commonly involved in children e.g. the femur and humerus, while the ...

. The pathological process is as follows: * infection in the bone leads to an increase in intramedullary pressure due to inflammatory exudates * the

periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones. Struc ...

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 (without a blood supply). Within the bone itself, the

haversian canals Haversian canals (sometimes canals of Havers) are a series of microscopic tubes in the outermost region of bone called cortical bone. They allow blood vessels and nerves to travel through them to supply the osteocytes. Structure Each Haversia ...

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 (known as involucrum). Openings in this involucrum allow debris and exudates (including pus) to pass from the sequestrum via sinus tracts to the skin. Rarely, a sequestrum may turn out to be an

osteoid osteoma An osteoid osteoma is a benign (non-cancerous) bone tumor that arises from osteoblasts and some components of osteoclasts. It was originally thought to be a smaller version of an osteoblastoma. Osteoid osteomas tend to be less than 1.5 cm ...

, a rare tumor of the bone.

References

{{Osteochondropathy Bone fractures Gross pathology