Bone Cysts in the Horse

Lameness in the horse is one of the most common conditions that vets have to treat.  Most joint disease in the horse is as a result of wear and tear but there are a few conditions that have a definite cause such as chip fractures and osteochondrosis.  Another relatively common cause of joint disease occurs following the development of a bone cyst (or, correctly speaking, an osseous cyst-like lesion).  Bone cysts are found in a variety of joints, but the most common is the stifle, followed by the coffin, pastern and elbow joints.

Bone cysts are not true cysts, i.e. they are not full of fluid.  In fact, the word cyst is a misnomer as they are actually full of weakly formed bone and cartilage that is not as dense as the surrounding bone.  This damaged area therefore appears black on an X-ray and, as the area of bone loss is usually circular in outline, the term bone cyst has become commonly used.  Despite considerable research the exact cause as to why a bone cyst forms in a joint is still unclear.  However, it is generally thought that the underlying inciting cause is trauma to the bone directly beneath the smooth cartilage that covers the joint surface.  Horses that have weak bone underneath the cartilage are more susceptible to the formation of bone cysts.  Typically, these horses that are also predisposed to osteochondrosis (OCD), usually fast growing large breeds of horses.  Bone cysts typically develop in the maximal weight bearing areas of joints wheras chip fragments (OCD) occur in the gliding areas.

Lameness associated with a bone cyst usually occurs in one of three different situations.  The first and probably most common situation is in yearlings when the bone cyst is thought to develop.  The animal typically may be brought in from the field with a moderate to severe lameness and there is usually associated swelling within the affected joint.  In many cases the lameness settles down relatively quickly over the next few weeks but some of these foals and yearlings remain persistently lame.  The second situation when a bone cyst may cause lameness is in an adult when the horse may present with a sudden severe lameness on the affected limb.  In this situation it is thought that the bone cyst is already present and a small fissure or fracture occurs in the neck of the cyst and/or some of the contents of the cyst leaks into the joint.  In this situation the lameness can be very severe.  The third situation is when bone cysts are associated with lameness is in the older animal, where as a result of the cyst the horse may develop osteoarthritis.  The bone cyst in this situation is probably the underlying cause for the arthritis, however the arthritis is now the limiting factor rather than the cyst itself.

There are a variety of treatment options for lame horses with bone cysts and this is dependent on the joint affected, the degree of pain and the age of the horse.  There are three main treatments options.  The first is a prolonged period of rest, which is successful in many cases, particularly in yearlings when the cyst first forms.  Rest has only limited success in the older horse with a bone cyst and concurrent osteoarthritis.  The second treatment option, which until recently has been the more favoured approach, is the surgical removal of the contents of the cyst.  In youngsters this carries a better prognosis than long-term rest.  The surgery is undertaken either by keyhole surgery or via an open incision into the joint.  The success of this surgery is dependent on the affected joint; the stifle for example carries a reasonably favourable response, when the shoulder does not.  The surgery is also more successful in the younger group of horses.  Recently a third treatment option has become available and the aim of this has been to try to support and heal the damaged bone that is present within the cyst rather than remove the cyst contents.  Treatment has been undertaken by injecting into the cyst a variety of drugs, including steroids and bone substitutes.  Considerable development work is currently being undertaken into other drugs to inject into the area of weak bone to allow it to heal to the same strength and original state as the surrounding uninjured areas. The results so far in the stifle joint have been encouraging.