Osteochondritis Dissecans

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. In order to understand how OCD occurs, one must first understand how joints form. Differentiation of bones and joints occurs through a process known as endochondral ossification.

Endochondral ossification is responsible for the formation of normal bones as well as normal articular cartilage, the smooth covering at the ends of bones which allows joints to glide smoothly. Normal pain-free joint function depends on a smooth cartilage surface of consistent thickness with a strong supporting plate of bone underneath, known as the subchondral bone plate. Failure of the developmental process leads to a disturbance in the formation of joint cartilage and the bone supporting it (Figures 1a-d). The resultant cartilage and subchondral bone is irregular in thickness and weaker than in normally developing areas. There may be a loss of blood supply and necrosis of the abnormal tissue. Forces placed on these abnormal areas when the horse ambulates may cause further damage to the abnormal tissue. This can lead to several problems which are recognized as OCD, including local detachment and fracture of cartilage and bone. These detached areas may remain partially attached to the surrounding tissue, which then exist as loose flaps of tissue, or may become completely detached, resulting in free-floating islands which can travel throughout the joint. These loose flaps and islands of abnormal cartilage and bone can cause inflammation in the joint, and over time may cause damage to the normal cartilage.
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