OC a growing pain for young horses

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A radiograph showing an OC lesion in a horse’s hock (tibiotarsal joint).

There are several types of orthopedic diseases that can occur in foals and young horses.

Among the most recognized are juvenile osteoarthritis, which causes joints to degenerate; angular limb deformities, which make a foal look knock-kneed or bow-legged; and a condition called osteochondrosis (OC).

OC is the most common equine juvenile orthopedic disease, and is characterized by abnormal cartilage and bone formation in a horse’s joint as it grows. Horses with this problem can experience stiffness, lameness, and joint effusion, which can make the joints looks puffy, including the fetlock, hock, and stifle. These horses may be at more risk for developing osteoarthritis as they age if the osteochondrosis lesions are not treated.

Horses’ legs normally grow when the soft cartilage at the end of their long bones, known as growth plates, hardens into bone and gradually increases the length of the leg. For horses with OC, the cartilage at the top of certain growth plates does not undergo the normal hardening process and may break down. Extra fluid can form in the affected joints due to the inflammation caused by the abnormal cartilage. Osteochondrosis can also present as cracks in the cartilage and cysts in the underlying subchondral bone. Sometimes, when abnormal ossification or hardening of the cartilage occurs, bone and cartilage fragments may break off from the main bone and float free in the joint fluid, referred to as “joint mice.”

More often, these fragments remained loosely attached to the main bone and can resemble “pebbles” at the articular surface of the bone. When fragments or cartilage flaps develop within the joint, the lesion is then called osteochondrosis dessicans or OCD.

“Lameness and ‘puffy’ joints (joint effusion) are the two most common presenting complaints with horses that have OC,” said Dr Sarah Sampson, a board-certified equine surgeon who specializes in orthopedic disease and orthopedic magnetic resonance imaging (MRI). Dr Sampson, a former member of the WSU equine faculty, recently accepted a faculty position at Mississippi State University.

While any horse can get OC, it is most frequently seen in horses that grow rapidly and have a mature height of over 15 hands. Thoroughbreds, Arabians, Quarter horses, and Warmbloods are some of the most commonly affected breeds. While genetics may play a role in horses that get OC, foals born to mares that were over-fed during pregnancy, and foals that are over-fed after birth, may also have a higher risk of developing the condition.

OC is not apparent in all horses that have it. Symptoms are most frequently seen in competition horses about the time or soon after they begin training; however, some horses do not show signs at all or do not show signs until later in life.

“Sometimes OC is not found in a horse until a veterinarian performs a pre-purchase exam,” Dr. Sampson said.

“Radiographs (x-rays) can definitively diagnose OC if a bone fragment can be seen in the joint or if a defect in the subcondral bone is apparent. A horse with this joint problem may or may not be lame or have joint effusion. Whether a horse is lame can depend on the location of the cartilage abnormality and the level of work the horse is doing. Furthermore, if a horse has symptoms of joint effusion and lameness, but no obvious bone defects are found on radiographs, there can still be cartilage defects that the radiographs do not pick up, as cartilage is not visible on radiographs.

“If cartilage abnormalities are suspected, horses with these clinical signs may undergo arthroscopy to identify the lesion(s) within the joint,” she said. “Arthroscopy involves the placement of small incisions into the affected joint to enable the insertion of a small video camera that projects onto a TV monitor. Cartilage lesions can then be identified and treated during the surgical procedure.”

A radiograph of a bone cyst, one type of OC lesion found in a horse's stifle (femorotibial joint).
A radiograph of a bone cyst, one type of OC lesion found in a horse’s stifle (femorotibial joint).

The procedure is minimally invasive and gives surgeons a view of hard-to-see places like joints without having to resort to other more invasive forms of exploratory surgery, known as arthrotomy, which takes a horse longer to recover from. Tiny surgical instruments can be passed through the incisions to correct problems within the joint.

“An important part of treating OC is to accurately diagnose the problem so owners can pick the most appropriate treatment option for their horse,” Dr. Sampson said.

“In general, a four to six month rehabilitation plan is required after surgery, depending on what kind of lesion is present. For horses with many types of OC lesions, arthroscopic surgery is the treatment of choice to debride lesions and/or remove fragments in the joint. There are certain types of lesions that may require a more specific treatment and each horse must be evaluated individually. The lesion type, limb(s) affected, age of the horse, use of the horse, and history must also be taken into account.

“At this point in time, there are no proven ways to prevent OC. If a horse has no clinical signs of pain or inflammation, it is possible that the lesions will remain non-clinical for the remainder of the horse’s life, but there is no way to know if that will actually happen,” she said.

“We do know that over time, if inflammation is present in an affected joint, the horse will be at higher risk for osteoarthritis in the future. If OC lesions are treated appropriately, before the joint is osteoarthritic, these horses have a very good (~80%) prognosis for return to performance in many cases. The prognosis, however, is dependent on many factors so this can vary between horses. Horses that have lesions that are successfully treated can have a long and successful performance career.”

 

Courtesy Washington State University College of Veterinary Medicine

First published on Horsetalk in December, 2010

 

 

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