It's a scenario familiar to many pet owners - your dog is running across the yard chasing a ball or Frisbee, when in mid-stride he yelps in pain and yanks a hind leg off the ground. Moments later he's off again, but sporting a limp which goes away after 10 to 20 minutes.
What your dog likely experienced here is a luxated patella, or in layman's terms, a dislocated kneecap. In a normal knee, the patella fits into a groove at the end of the femur (thigh bone), and slides up and down as the knee joint bends back and forth. It also acts as a protective cover for the knee joint. The joint's movement follows a limited track, guided by the grooves in the femur.
Injuries or a congenital malformation can result in these grooves being too shallow. In these cases, when the dog is active the patella can luxate (or jump) sideways, usually toward the inside of the leg, causing the limb to lock up. Once the kneecap has become dislocated it cannot return to a normal position until the dog's quadriceps muscle relaxes, which takes several minutes. During the moment of dislocation the dog experiences a sharp pain, but once the kneecap is out of place, the dog is not in pain. It simply will not be able to put the leg down again until the muscles relax and the kneecap slides back into position.
Dogs who experience this problem display a variety of symptoms. They often hold the leg up for several days and show signs of pain and discomfort. If the problem affects both hind legs, the dog will likely change its entire posture, dropping its hindquarters and holding its rear legs out from the body as it walks. In the most severe cases, dogs will avoid using their back legs entirely, instead balancing on their front legs in order to walk. A luxated patella normally occurs in dogs that are middle-aged, and many of them will have a history of on-again, off-again lameness in one or both hind legs.
Alert owners who suspect their dog has a problem need to have it diagnosed by a veterinarian. Left uncorrected, the ridges along the dog's kneecap will wear away, the grooves in the femur will become even shallower, and the dog's lameness will become more pronounced. Also, the wear and tear will lead to arthritis, causing the dog's leg to become permanently swollen and painful. In severe cases, the arthritis resulting from this joint damage can become crippling.
Surgery is required to treat most, but not all, cases. Often surgery is performed to deepen the grooves in the femur and/or "tie down" the kneecap to prevent it from moving improperly.