"Developmental Orthopaedic Disease (DOD) of young, growing horses includes any disturbance in changing the cartilaginous precursor of the skeleton to functional bone" (Anderson, 1996). One of the diseases included under this general term is Osteochondritis Dessicans (OCD) where a flap develops from the articular cartilage due to failure of cartilage to be properly converted into bone. It usually only affects young horses in any weight bearing joint of the body. OCD was first described in the horse in the femoropatellar joint (stifle joint), which is one of the most common sites of the disease although it may occur in several different joints at the same time.
In young growing horses, skeletal bones initially develop from cartilage, starting in the foetus and continuing after birth through to maturity. During development, two growth plates develop at each end of the long bones.
The epiphysis faces the joint, and the metaphysis faces the shaft of the bone and together are called the physis (Anderson, 1996). As the horse grows, there are several stages of normal cartilage and bone formation. Normal endochondral ossification (the conversion of cartilage to bone near the joint) involves the following steps: chondrocyte (cartilage cell) proliferation, maturation and growth, followed by chondrocyte degeneration, death and calcification, vascular invasion, and finally, bone formation and remodelling (Briggs, 1997). As these steps occur, nutrients are provided by two sources, diffusion through the cartilage from the synovial fluid (fluid in the joint) and from the capillaries that are located in the existing bone (Anderson, 1996).
In Osteochondrosis (a precursor to OCD), the cartilage is not replaced by bone in the normal manner. In affected areas, the cartilage might instead thicken and project into the bone of the epiphysis or the diaphysis. The thickening of...