The patella is the bone we know more familiarly as the knee cap. In a normal knee, the patella glides up and down as the knee joint is bent back and forth. The knee cap protects the knee joint and guides the action of muscles in the lower leg. Two bony ridges on the thigh bone guide the patella as it slides up and down. The entire knee system runs smoothly thanks to joint fluid.
A patella may become luxated — jumps out of the guiding grooves on the thigh bone — for different reasons. The ridges may not be prominent enough to hold the knee cap in place. A trauma or malformation can affect the patellar groove and lead to luxation.
When the patella leaves the guiding grooves, it is often stuck outside its normal place until the muscles in the lower leg relax and stretch out. An affected dog may hold his leg up or out after the initial incident — the joint is stuck in the flexed position. The initial jump of the knee cap can be painful and the dog may yelp, but after that initial hop, the dog feels no pain and can resume activity. After a few minutes, your dog is back to his usual self.
Most of the dogs who suffer from a luxating patella are middle-aged. They may have a history of occasional lameness in the affected leg or legs. Mid-run, the dog may stop and cry out, then be unable to flex the affected leg for a few minutes. Smaller breeds, like toy breed dogs, and dogs with extremely short legs are more commonly affected by luxating patella. Miniature and toy poodles, basset hounds, and dachshunds have higher incidences of patella luxation, but not all dogs of these breeds will have problems.
Some dogs do experience more severe problems because of a luxating patella. The joint may stay out of place for several days, instead of several minutes. The dog may show discomfort, or be reluctant to walk. He may change his posture, holding his rear legs farther out from the body to walk. Dogs with a luxating patella on both hind legs may try to hold their hindquarters off the ground entirely, or balance on their front legs.
If the cause of the luxation is not corrected (often with surgery), the ridges will wear away more and more. The dog will experience more frequent lameness, and arthritis can develop, further reducing mobility. Surgery can deepen the grooves at the base of the thigh bone to better contain the knee cap, or can tie down the patella to keep it from jumping out of its track. In most cases, the dog is fully recovered within a month of surgery.