Osteochondritis Dissecans of the Equine Hock and Stifle

Osteochondritis dissecans (OCD) is a type of skeletal maturation problem that affects joint cartilage and also often involves the subchondral bone just beneath the cartilage surface.

The cause of OCD has generally been considered a defect in bone mineralization at the joint surface. The paradigm has been that for some reason endochondral ossification does not occur correctly at a site, leaving an area of thickened retained cartilage which then is secondarily displaced as a flap or fragment. This end result is certainly attained, but not all cases follow this simple pattern. For instance, in some OCD lesions there is no thickening of the cartilage and no defects in the bone. Also, lesions have developed after endochondral ossification was complete and the surface appeared normal. Though there is still a lot to learn about this complicated disease, the important fact is that when these lesions do occur in the joint and result in flaps or fragments, they lead to an arthritis situation.

The stifle joint is one of the principal joints affected with OCD. Stifle OCD can be diagnosed in almost any breed, but it is more common in Thoroughbreds. The condition is found most often in young horses, but it can also show up clinically even after horses have won stakes races. The difference is when the retained cartilage fragment or flap becomes displaced.

As a general rule, when animals show up at a younger age with OCD, they tend to have more severe lesions. Clinically the animals present with joint swelling due to synovial effusion and varying degrees of lameness. Severe cases can be very lame and may be confused with wobblers because of the difficulty they have flexing the stifles in getting up and down. In older animals, an increase in the level of exercise may be part of the history. Horses will often have a bunny-hop gait behind and this also could be confused with a neurologic problem. Some horses will be very subtle in their lameness.

Joint distention (when the disease is clinically significant) is a regular feature. Careful palpation of the joint may identify free bodies or the surface irregularity associated with the damage within the joint. Bilateral involvement is common in the stifle, so careful examination of both stifles should be completed. In one major study of cases, 57% of animals had bilateral involvement. Lateral to medial radiographs provide the best means of diagnosis regarding specific location of the lesion and its size. The most common location is on the lateral trochlear ridge of the femur and shows up as an area of flattening, irregularity, or concavity. The area of the trochlear ridge adjacent to the bottom portion of the patella is most commonly involved. Various degrees of mineralization may be present within the flap tissue, affecting the radiographic signs, and free bodies may also be identified. OCD can also primarily affect the patella or the medial trochlear ridge of the femur. Generally the extent of damage to the joint identified at surgery is more extensive than would be predicted from radiographs. Although other joints can be involved concurrently, this is uncommon. In one study of 161 horses with stifle OCD, 5 also had OCD affecting the rear fetlocks, 4 had hock OCD, and one had OCD of a shoulder joint.

In general, arthroscopic surgery is recommended for the treatment of most of these cases. However, it has been recently identified that in low grade lesions detected very early, stall confinement could allow healing, presumably with reattachment of any separated cartilage. When there is a significant concave defect or flaps or fragments identified, arthroscopic surgery is recommended. The joint is thoroughly explored and this usually gives a better assessment of all damage. Suspicious lesions are probed and loose or detached tissue is elevated and removed. Loose bodies are also removed. The defect site is then debrided down to healthy tissue. Animals are usually stall rested for two weeks after surgery at which time hand walking is started. Restricted exercise is continued for two to three months after surgery, when training is started or the horse is turned out.  The total period of convalescence depends on the amount of damage.

In a recent study of 252 stifle joints in 161 horses, follow-up information was available for 134 horses. Of these 134 horses, 64% returned to their previous use or anticipated use (racing), 7% were in training, 16% were unsuccessful, and 13% were unsuccessful due to reasons unrelated to the stifle. The success rate was higher in horses having smaller lesions.

OCD occurs in a number of locations in the hock, including the intermediate ridge of the tibia (most common), the lateral trochlear ridge of the talus, the medial malleolus of the tibia, and the medial trochlear ridge of the talus. It is a very common disease in Standardbreds but is also quite common in Quarter Horses and Arabians. The most common clinical sign of hock OCD is joint distention of the tarsocrural joint. This manifests clinically as a bog spavin, which simply refers to the prominent swelling seen along the medial or inside aspect of the joint. Lameness can also be seen but is not common and is rarely prominent. Horses of all ages can be affected. Often in racehorses the disease does not show up until the horses are in training. In non-racehorses, the cases that are going to be clinical commonly show up in yearlings prior to going into training. The disease is confirmed with radiographs.

When clinical signs are present in association with OCD lesions in the hock, surgery is the recommended treatment. It is to be noted that the disease is often diagnosed as an incidental finding on radiographs of horses prior to sale. If there is no joint effusion or lameness, surgery is not normally recommended. In some instances in racehorses, lameness may be the only problem seen and it is only seen at racing speed or upper levels of performance. Certainly resolution of the joint effusion can only be expected with removal of abnormal tissue. The horses are treated arthroscopically with removal of fragments and debridement of any damage. The postoperative management is similar to OCD of the stifle but the convalescence time may be faster, as many of the lesions are localized and do not involve a major articulating surface. In a study involving 183 horses, 76% raced successfully or performed at their intended use after surgery. If secondary osteoarthritic change is identified at surgery in the cartilage, the prognosis is less favorable.

Resolution of the joint distention so there is a normal hock with no fluid is a critical criterion of success for non-racehorse owners. It was found in this study that the resolution of effusion was inferior for lesions involving the lateral trochlear ridge of the talus compared to the intermediate ridge of the tibia, so this should be taken this into account when giving a realistic prognosis to owners.