OTTB Health Series: Osselets

Thoroughbreds are often broken to saddle and put into racing training as two year olds, long before they reach physical maturity. Race training is physically demanding and horses may suffer muscle and bone injuries, including development of “osselets”.

Prospective owners may note that a horse of interest in CANTER’s listing has “old osselets”…or “green osselets”. They wonder what an osselet is, whether the osselets will limit the horse, what short-term treatment may be required and what long-term management strategies are recommended.

The Merck Veterinary Manual offers the following definition: Osselets refer to an inflammation, usually bilateral, of the periosteum on the dorsal distal epiphyseal surface of the third metacarpal bone and the associated capsule of the fetlock joint. The proximal end of the first phalanx may also be involved. Hence, osselets constitutes a form of periostitis and serous arthritis that may progress to degenerative joint disease. The exciting cause is the strain and repeated trauma of hard training in young horses and is recognized as an occupational hazard of the young Thoroughbred.

“…osselets often brew in both front ankles at once, the lameness might manifest itself as a short, choppy gait, with no one leg visibly more sore than the other. If only one fetlock is involved, the horse will “point” the affected leg and try to avoid bearing weight on it. At that stage, the condition is sometimes called “green osselets.” — The Blood Horse

Note the ankle enlargement. The left ankle is significantly more enlarged, which is common in OTTBs as they race and train "going left" much of the time.

Questions to ask if you are considering a horse with osselets:

Are the osselets “set” or “green”? When the injury is acute–that is, when the injury first flares up, before there is bony overgrowth in the joint–an osselet is referred to as “green”.

Are both fetlocks involved, or just one?

Was the stress on the fetlock caused by a back-end weakness or injury, initially? (Often we see that injuries are “diagonal” — that is, an injury in the left front may, in fact, be the result of over-compensation for soreness in the right hind.)

Are there current x-rays available?

Is the horse currently lame?

Is the “interstitial” or joint space compromised; simply, has the joint injury damaged the horse’s cartilage ?

Do the x-rays show ulceration of the articular surfaces of the joint?

• The horse may have a short, choppy or uneven gait. If the osselet is “green” the joint may be warm, swollen and painful on palpitation;
• X-rays and external examination may reveal the presence of new bone formation. Fragments of bone may break off and interfere with joint flexion;
• The ligaments may be involved and irritation at the site of attachment may be seen.

Osselets may not significantly hamper the horse, or the joint damage may leave the horse totally unrideable. In severe cases–where the horse has continued to train and race even after the initial inflammation appeared–the excess bony development may result in a “locked” ankle, with no articulation, or movement. At this stage degenerative joint disease may significantly affect soundness and usability.

It’s worth noting that it is not always good to rely on visual examination alone. “Ugly” ankles may not actually hamper the horse as much as ankles with a less abnormal appearance, but with significant damage to articular surfaces, loss of cartilage, or lots of “joint mice” (bony fragments that have broken off from the osseous overgrowth).

Understand whether or not the osselets are changing, or “set”. If the ankle is stable, the horse is not lame, the joint space is intact and the animal’s condition hasn’t changed significantly in its recent history, then the vet may actually be able to give you a “thumbs up”, depending upon what job you have in mind for the horse.  If the horse is young, the ankles are hot, there are chips in the joint and the joint spacing is compromised, the vet may suggest that you pass on the horse.

How Are Osselets Treated?

• Rest is very important and if the trauma is stopped in early cases long-term joint damage may be avoided.
• Relieving inflammation is important; cold hosing or ice wraps may be applied. Systemic anti-inflammatory drugs such as phenylbutazone (“Bute”) are often used to relieve swelling and pain.
• Corticosteroids may also be injected into the joint. But if a joint is injected, bute is given, and the horse is not taken out of training, this can lead to destruction of the joint surfaces and formation of excess bone.
• Injection of the joint with sodium hyaluronate may help to reestablish normal synovial viscosity. Synovial, or “joint fluid” is part of the the body’s normal shock absorbing system.

What is the Long-Term Prognosis for a Horse with Osselets?

Your veterinarian may recommend the following managing:

• If the osselet is “green” and the horse acutely lame, you may have to cold hose the horse 2-3 times per day, administer anti-inflammatory medication and keep the horse on stall rest or minimal turnout until the immediate joint trauma has passed.
• The horse with osselets requires good farrier care; a balanced and properly angled hoof is important to avoid stressing the joint.
• If the horse’s joint space is compromised–if they are missing cartilage–then you may want to discuss the use of medications to improve production of synovial fluid, which can cushion the joint. Medications like Adequan can be quite useful in the long-term management of joint conditions.

Other Resources to Learn About Osselets:

TheHorse.com

Knack Leg and Hoff Care for Horses

Equisearch