Canada's pet information center - Dogs cats pets are welcome - Pets.ca

There are many reasons for a dog to have a sore leg. For example, it could have stepped on something sharp. If your dog looks like it is in pain, it is always a good idea to take it to the veterinarian to make sure there is nothing seriously wrong. One important cause of a sore leg is called “osteochondritis dissecans”. Osteochondritis dissecans (OCD) is a fairly common problem found in some large dogs (anything as big as or bigger than a border collie). The problem starts in the growing puppy, so the dog usually becomes sore when it is less than one year old. However, sometimes the signs are not severe at first and do not get noticed until the dog gets older.
           
OCD is a disease of joints. Joints are the fluid-filled spaces where the ends of bones meet (like your elbow, knee, and wrist). The bone ends are lined by cartilage cells (“articular cartilage”), which allow smooth movement of the joint. As a puppy with OCD gets older, for an unknown reason an area of articular cartilage does not mature properly. Normal articular cartilage is very thick in a puppy, and will gradually be replaced by bone as the dog matures until there is only a thin lining of articular cartilage cells at the ends of the bones. In OCD, there is an area of cartilage in the joint that is not replaced by bone and remains very thick.
           
This doesn’t cause a problem right away. But as the dog matures, this thick area of articular cartilage does not get enough blood supply. Without blood supply, it does not get nutrition and starts to die. The area begins to separate from the bone and form a flap of articular cartilage. In some cases the flap entirely separates from the bone and floats around in the joint fluid.
           
It may sound complicated, but the problem is clear; loose cartilage causes rubbing in the joint. This is painful! However, OCD is not painful until the cartilage flap forms. The dog then appears to be in pain especially after exercising. This is because exercising involves a lot of joint movement, allowing the flap to rub on the bone ends. Over months, the dog will gradually be in more pain. The flap begins to damage the normal articular cartilage and bone ends.
(Article continues here)