Written by Ty Wallis, DVM, MS, DACVS
QUESTION: When you find an OCD on a radiograph, how do you determine which are “safe” and which have to be removed?
Thank you for a great question that is commonly discussed during pre-purchase examinations. Osteochondrosis is a developmental orthopedic disease involving a small defect in the normal maturation process of bones. Bones are formed from a cartilage model, and typically this model calcifies/ossifies and turns to bone before, or shortly after, birth. When there is an alteration in this process, a small area of cartilage can fail to ossify. This weakened area then sustains trauma with exercise, and an osteochondrosis lesion forms. The area can either collapse inward, allowing joint fluid to flow into the bone forming a cyst (called a bone cyst or subchondral cystic lesion), or a flap, which is only partially attached, called an osteochondritis dissecans (or OCD lesion) can form. In the former instance, the lining that forms inside the cyst is responsible for enlargement of the cyst. In the latter instance, the OCD flap, once free from the parent bone, becomes mobile and inflamed and then ossifies, and it is this osteochondral fragment that is seen on radiographs.
Subchondral cystic lesions have been described in almost every bone in the horse, but are most frequently found in the medial femoral condyle. In Quarter Horses and Thoroughbreds, these are most often clinical, meaning they cause observable symptoms, but in Arabians and Half-Arabians they are commonly subclinical, meaning they don’t cause any symptoms such as joint effusion, lameness, or arthritis. When they are found during a lameness exam, and the lameness has been isolated to the joint involving the cyst, treatment is recommended. When they are found as an incidental finding on a pre-purchase exam, there are many factors to consider, such as age, appearance of the remainder of the joint on radiographs, presence of a good shelf of subchondral bone between the cyst and the joint cavity, the horse’s short-term and long-term goals and schedules, and the desires of the owner. Many times no treatment is performed. However, treatments which have been described include debridement (removal) of the cyst, with or without regenerative techniques to replace what is removed; injection of corticosteroid behind the lining of the cyst via arthroscopic, radiographic, or ultrasonographic guidance; and placement of a lag screw across the cyst. The lag screw technique, which is the most newly described, is currently showing the most promise in early clinical reports and the technique we currently perform in our practice.
OCD lesions are also described in almost every joint in the horse, but are most frequently seen in the hocks and fetlocks. Like cystic lesions, when OCD fragments are found in a joint which has been isolated as a source of pain during a lameness examination, surgical removal via arthroscopy is recommended. When they are found incidentally on a pre-purchase examination in the absence of lameness or a positive response to a flexion test, arthritic changes in the joint, or effusion of the joint, then a similar decision tree is followed as for incidental cysts described above. These are usually found in three- and four-year-olds, and they typically haven’t caused a clinical problem. Since these are most often in an area that is non-weight bearing within the joint, they don’t actually get caught between the bones making up the joint. The trouble they could potentially cause in the future is related to them being slightly mobile in the joint, causing the lining of the joint to become inflamed. The inflamed lining produces low quality joint fluid, and over time the cartilage, which relies heavily on high viscosity fluid for protection from wear and tear, becomes damaged by the fragment and leads to osteoarthritis. Therefore, removal is often recommended at a convenient time in the horse’s competition and training schedule to avoid damage in the future and to preserve value on resale, but it is not an emergency. In various locations in some joints, they may not cause a problem and may be left alone. Finally, in some instances, typically when identified in older horses, they may be ignored and the joint treated medically with corticosteroids and hyaluronic acid or IRAP II as needed.
Editor’s Note: Original article written for the Arabian Horse Life magazine.