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, ...

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. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...

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

osteomyelitis Osteomyelitis (OM) is the infectious inflammation of bone marrow. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The feet, spine, and hips are the most commonly involved bones in adults. The cause is ...

. 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. (At the joints of long bones the bone's outer surface is lined with "articular cartila ...

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, osteonic canals or central canals) 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 ost ...

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 An involucrum (plural involucra) is a layer of new bone growth outside existing bone. There are two main contexts: * In pyogenic osteomyelitis where it is a layer of living bone that has formed around dead bone. It can be identified by radiogr ...

). 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