However, there are problems associated with fast growth that owners should understand. Developmental orthopedic disease, or DOD, has many possible causes, but the major factors are rapid growth rate, a high-calorie diet, nutritional imbalances, genetics, hormones, and trauma to growing bones. OCD is characterized by fragments of cartilage within the joint, and it most commonly affects the hock, fetlock, and stifle. The causes of typical DOD lesions are often related to how animals' bones grow. Since the typical foal will need to grow to the size of its mother in a short span of time, horses' bones are specially engineered to grow quickly.
Bones grow by a process in which the cartilage plates at each end calcify, or harden into bone.
Little by little, this process adds length to the bones. In joints, cartilage in the growth plate forms bone that is eventually covered with a thin lining of cartilage. For unknown reasons, sometimes growth plate cartilage at the joint surface remains as an island of cartilage instead of converting to bone. If that occurs, OCD often results. The abnormally retained cartilage is weak and prone to mechanical damage, leading to defects such as cracks and cysts.