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By Barrie Grant, DVM, Dipl. ACVS

I was talking to one of my e-mail friends last night, and she said her horse had a "popped" knee. I hated to seem ignorant, but I have no idea what that means. Can you help?

"Popped knees" is a layman's term for enlargement in the front of the carpal joints of a horse. The knee looks like it has "popped out." Sometimes this is accompanied by marked lameness, sometimes not. The enlargement at the front of the knee usually is from the enlargement of the synovial structures--the three tendons that go over the front of the knees (flexor carpi radialis, flexor carpi ulnaris, and extensor carpi radialis), two joints, and one bursa between the tendon and the joints. Inflammation in any of those structures will cause an enlargement to the front of the knee and therefore give the appearance that the knee has "popped."

Causes vary. Most commonly, popped knees occur from a chip in the joint in racehorses or horses which perform at high speed. Infection, developmental abnormalities in young foals which rupture their tendons in front of the knee, and direct trauma to the knee are other causes.

It's important to identify which structures are inflamed, as treatment can vary. Sometimes all three structures can be involved, sometimes just one.

It's also important to get an early, accurate diagnosis. In fact, it's the most important thing you can do for treatment. The sooner a veterinarian can make an accurate diagnosis, the more successful the treatment.

Older horsemen might suggest do-it-yourself remedies, but that's a disservice to the horse. Just giving anti-inflammatory drugs or putting a bandage on the knee is not addressing the problem. If the horse owner puts off getting a proper veterinary diagnosis and tries to treat the horse himself for two or three months with old-fashioned liniments and bandages for a horse with chips in the joint, the owner is going to reduce the chance of a successful repair of that knee. Untreated, the injury can become worse and develop into a severe arthritic condition.

To diagnose a popped knee, the veterinarian combines a physical exam, radiographs, and ultrasound to determine which of the structures are involved. The physical exam consists of palpation of all the structures, observation, and results of flexion tests; infection will produce clinical signs of fever, lameness, and synovial fluid containing excessive amounts of white cells. X rays and ultrasound more clearly identify the structures involved. The response to local anesthetics placed into the joint also is important in arriving at a definitive diagnosis.

Treatment depends on the cause. Bone chips usually create inflammation in the joints, and sometimes inflammation of the tendon sheath. Bone chips usually are treated by removing them arthroscopically. This type of surgery often can be done in less than an hour with the horse under general anesthesia. Barring complications, the horse usually can return to work in 60 to 90 days.

For direct trauma to the soft tissues in the tendon sheath, veterinarians usually administer anti-inflammatory medications such as Bute or banamine, ice the injury, bandage or sweat the leg, and give the horse time off. That often takes care of soft tissue swelling, with full recovery taking a couple of weeks.

Infection is treated by draining and flushing the area and putting the horse on appropriate antibiotics. Horses which are quite lame need to be rested during the recovery period, which can range from three or four weeks, to three or four months.

For developmental abnormalities that cause tendon enlargement in foals, a veterinarian usually will put the affected leg in a half-cast for a couple of weeks. When the cast is removed, the leg is put in support bandages, and the foal is kept in a stall for a couple of months. The results usually are good.

Prognosis depends on what's wrong. With bone chips, it depends a little on the location of the bone chips; there are a number of classic locations for bone chips in racehorses' knees, and each one of them has a different prognosis. With removal of the bone chips, some horses can perform very successfully. Spend a Buck won the Kentucky Derby after surgery to remove a bone chip. However, if the knee has "popped" because there are severe slab fractures that require surgical fixation with screws, the prognosis is greatly reduced.

With direct trauma, successful recovery depends on which structures are damaged. Prognosis ranges from limited to full recovery.

The prognosis for popped knees due to infection depends on the invading organisms, how long they've been there, and the cause of injury. The horse might make a complete recovery, or might need to be euthanized.

With developmental abnormalities in foals, the prognosis is guarded to favorable. Some horses, after they reach maturity, never have problems.

The risk of your horse getting popped knees can be reduced. First, make sure your horse is not being over-trained. Over-training causes hardening of the carpal bones, then the bones are more likely to fracture (chip). Also, try to reduce situations that are traumatizing to your horse's knees.

Remember, if your horse does get popped knees, early veterinary diagnosis and treatment offers the best chance for recovery.

Barrie Grant, DVM, Dipl. ACVS, is a partner in the San Louis Rey Equine Hospital in Bonsall, Calif. He has served a term on the  AAEP Board of Directors.

Reviewed by original author in 2016.