A sequestrum (plural: sequestra) is a piece of dead
bone
A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
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 dige ...
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
The medullary cavity (''medulla'', innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity.
Located in the ma ...
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 (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 Haversian ...
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 about dead bone. It can be identified by radiograph ...
). 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, a rare tumor of the bone.
References
{{Osteochondropathy
Bone fractures
Gross pathology