Your horse sustained a small wound on the outside of the left fore cannon bone about two weeks ago. Despite cold hosing, bandaging and box rest, the wound has failed to heal. After a course of antibiotics prescribed by your vet, the discharge stops and the wound heals, only to swell up and start discharging again between five and seven days later. Frustration grows as you call for another visit. This time, the vet suggests a radiograph (X-ray) to see if a sequestrum has formed.
A sequestrum is a piece of dead bone that has lost its blood supply. The horse’s body treats this as a foreign object, rather like a splinter, and the immune system tries to reject it. That is the reason for the heat, pain, swelling, discharge and failure to heal. The immune system works hard to rid itself of this apparent foreign body, but is usually unsuccessful due to the size of bone involved.
How and why does it form?
Sequestra usually form in an area with minimal soft tissue covering the bone. Typically, this is the cannon bone region and over the splint bones, where the bone lies close to the skin; but any area with minimal soft tissue cover is at risk of a sequestrum following trauma. They include:
- the inside of the forearm or gaskin
- the bony prominence at the end of the stifle
- the top of the head
- the point of the hip
- the lower jaw
The original wound associated with a sequestrum can vary from a small nick to a large laceration. Usually, the wound causes sufficient trauma to damage blood supply
to the underlying bone and may even cause a small, superficial fracture.
The size of the sequestrum will vary with the original trauma and can range from a few millimetres to a few centimetres. The most common site of sequestrum formation is one of the splint bones. The second most common site is the front of the hind limb cannon bone, following a laceration.
The damage to the bone may not be evident immediately, even on a radiograph (X-ray). It takes three to four weeks for the body to start the rejection process in earnest. Only after some of the bone surrounding the sequestrum has been reabsorbed can a definitive diagnosis be made through radiographs or ultrasound.
Is it a serious problem?
In most cases, a sequestrum can be treated successfully with surgical removal of the damaged bone. The bone is usually small and not essential. Rarely, a sequestrum poses a more serious problem. This occurs when the damaged bone is within a joint or involves a significant portion of the thick outer portion of the bone (the cortex). In the latter case, the bone may be at risk of fracture, particularly if a general anaesthetic is performed and the horse has to stand up as it recovers.
Although the sequestrum may appear easy to remove, there is often significant scar tissue around the bone that makes this task quite challenging. Radiographs taken prior to surgery can help to identify the exact location of the dead bone. As long as the delay in surgical removal of a sequestrum will not compromise the horse further, surgery can be delayed until the vet is reasonably certain that the affected bone can withstand the stress of recovery.
Occasionally, a sequestrum may be very small and can be reabsorbed by the body without the need for removal. In most cases, sequestrum formation is an inconvenience rather than a serious cause of persistent lameness. Many horses with sequestra are in fact not lame at all, and removal of the sequestrum usually results in a full return to normal function.
Following removal of the dead bone, healing is usually rapid, since the cause of the infection has been removed. In most cases, there is minimal or no lameness before surgery, which means that most cases can return to full work reasonably quickly. The speed of return to work (within a few weeks or a month) would be determined by the bone involved, the amount of bone involved, the location of the sequestrum on the bone and the healing of the original injury.