Bone Sequestrum Formation
The Anatomy of a SEQUESTRUM: (A), Normal (healthy) bone; (B), New bone (callus); (C), granulations lining involucrum; (D), cloaca or diptheritic tract; (E), sequestrum.
A sequestrum is defined as "a piece of dead bone that has become separated from sound bone during the process of necrosis". Bone sequestrae are fairly common occurrences in horses, especially in areas where the bone is not heavily protected by other tissues (such as muscle) and is particularly susceptible to external injury (such as a kick). With regard to the horse's limbs, the front of the cannon bones and the inside of the radius (front limb) and tibia (hind limb) are frequent sequestrum sites since these areas are covered only by skin and subcutaneous tissue. Bone overlaying the sinus cavities of the skull are also predisposed to sequestration following injury.
In order for a sequestrum to develop, the following must occur:
The piece of bone must separate from its parent bone (i.e. sequester itself).
The piece of bone must lose its blood supply (i.e. be dead).
The piece of bone must be infected (i.e. necrotic).
A piece of bone usually becomes separated from the parent bone in one of two ways:
The piece of bone is broken-off (fractured) due to direct trauma or
The periosteum overlying the piece of bone has become compromised, eventually resulting in lack of blood supply and underlying bone death.
Where does bone get its blood supply?
Compact or cortical bone generally receives blood supply from two sources:
The internal layer of the cortex receives blood from the marrow or medullary cavity existing at the bone's center.
The external layer (or outer shell) of the cortex receives blood from the periosteum.
The periosteum is a thin fibrous layer that covers most osseous (bone) tissue. Among other things, the periosteum is responsible for supplying the most superficial layer of the bone with blood and nutrients. Compromise of the periosteum, therefore, can result in disruption of bloodflow to its underlying osseous counterpart.
Bone will not sequester in the absence of contamination/ infection. In the case of a closed or sterile injury, for example, separated bone can reestablish blood supply, after which one of two scenarios can develop:
In cases of relatively small bone fragments, the piece may be resorbed into the bloodstream and the remaining parent defect heals via the formation of new bone.
In cases of larger fragments, the piece may reattach to its parent bone and heal via typical callus stabilization.
For this reason, closed injuries (i.e. those that do not penetrate the skin) are not likely to produce sequestrae.
Open injuries (those that do allow contamination of the wound), however, are more likely to result in sequestrum formation if bone is involved.
It is important to realize that fracture of bone overlying a skull sinus is always considered to be open and contaminated. Even though the overlying skin may not be broken, bacteria existing within the sinus cavity will have direct access to the fracture site from underneath. It is for this reason that presumably "closed" skull fractures often result in sequestrum formation.
Once blood supply has been lost, bacteria associated with the sequestrum prevent it from reestablishing vascularity.
An infected piece of dead bone is considered to be foreign to the body which cannot distinguish it from any other foreign object (such as wood, metal, or stone).
Since the piece of bone has no blood supply, the body has no way to resolve associated infection, which relies on the delivery of white blood cells by the bloodstream. As long as the sequestrum exists within the body, therefore, it will be a continuous source of infection. The Atlanta Equine Clinic has documented several cases of sequestration lasting for up to 18 years in horses.
Treatment involves surgical removal of the foreign object (sequestrum). Depending on its location, most sequestrae can be removed with the patient standing, sedated, and locally anesthetized. Once the sequestrum has been successfully removed, the underlying (normal) bone is debrided and cleaned. The wound can then heal via normal granulation, epithelialization and contraction.