Had it not been for Amy, our friend in Oklahoma, I would not know about this disease, but it is more common than one would like.
Here is something I got online:
Overview
Osteochondritis dissecans (OCD) is a relatively common developmental disease that affects horses of all types. Although the exact cause of OCD and the steps leading up to it are not definitively known, there is a reasonable consensus about the general cause.
Causes and Risk Factors
Several causes of OCD are known, although the disease is generally considered to be multifactorial. As a result, the disease is not usually caused by any one factor, but rather a combination of several factors acting together. These known factors include:
Rapid growth and large body size: An unusually rapid phase of growth and/or growth to a large size can be associated with OCD formation.
Nutrition: Diets that are very high in energy or have an imbalance in trace minerals, low-copper diets in particular, can lead to OCD formation.
Genetics: Risk of OCD may also be partially inherited, although the mode of inheritance is not well defined and other factors are often required before an OCD fragment forms. Genetics can also be responsible for the horse's response to the other factors presented here, as well as rate of body growth.
Hormonal imbalances: Imbalance in certain hormones during development, including insulin and thyroid hormones, can encourage OCD formation.
Trauma and exercise: Trauma to a joint, including routine exercise, is often involved in formation and loosening of the OCD flap or fragment.
Incidence and Prevalence
Since all these factors are involved in a complex series of interactions it is not possible to predict which horses will develop OCD, and is therefore difficult to prevent the formation of OCD in individual animals. Clinical prevalence of OCD is usually between 5 and 25% in a given horse population, but radiographic signs of abnormal development can be as high as 60% in certain groups (Wittwer et al. J Vet Med A Physiol Pathol Clin Med 2006).
Signs and Symptoms
OCD may be detected as early as 5 months of age, or as late as skeletal maturity (approximately 3 years of age). However, in young horses, it is recommended that the final diagnosis should be made when the horse is about 1 year of age. The most common sign of OCD is effusion (swelling) in the joint of a young horse (Figure 2). Often, the joint swelling is first noticed shortly after the horse begins a formal training and exercise program. Depending on the location and severity of the OCD, the horse may be noticeably lame on the leg, may only be lame during high-speed work, or may not have detectable lameness at all. OCD can occur in virtually all joints, however there are several joints which are affected much more commonly. The most commonly involved joints are the hock, the stifle, and the fetlock. The shoulder can also be involved, although this is seen with less frequency in horses.
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:x :? :shock: :wink: 
:lol2: :lol2: :lol2: :wave:
