What is a luxating patella? To luxate means to move out (actually pop out) of position. The patella should be located in the center of the knee joint and thus a luxated patella is a kneecap that has moved out of its central position. A patella resides in a groove that allows it to slide up and down as the leg is used. There are several conditions that allow the patella to move away from this groove. Small dogs especially seem to suffer from this syndrome. A study of 542 affected individuals revealed that dogs classified as small (adult weight 9g (20 lbs) or less) were twelve times as likely to be affected as medium, large or giant breed dogs. In addition, females were 1.5 times as likely to be affected. Some researchers have suggested a recessive method of inheritance, and the higher incidence in females could possibly be related to X-linked factors (the condition is passed through the mother) or hormonal influences.
The muscles of the thigh attach directly or indirectly to the top of the kneecap. There is a ligament,called the patellar ligament, which runs from the bottom of the kneecap to a point on the tibia(shin bone) just below the knee joint. When the thigh muscles contract, the force is transmitted through the patella and through the patellar ligament and results is a straightening of the knee joint. The patella stays in the center of the leg because the point of attachment of the patellar ligament is on the midline and because the patella slides in the groove on the lower end of the femur (thighbone). If the bones of the leg are misaligned as in “knocked knees” or “bowed legs” the pull exerted by the ligaments attached to the patella may over time or under stress cause the inner side of the groove to wear down and the patella is free to move out of the groove or dislocate. When this occurs, the dog has difficulty bearing weight on the leg. It may learn how to kick the leg and snap the patella back into its normal location. This behaviour is one of the first warning signs of potential problems.
Some dogs can tolerate this problem for many years, some for all of their lives. Since the joint surfaces are very slick and bathed in a slippery joint fluid, there is usually little or no discomfort early in the process. However, this abnormality predisposes the knee to other injuries, especially torn cruciate ligaments. Also, with advancing age, the joint may become arthritic and painful. The bones may actually curve in response to the abnormal location of the kneecap. What do you do if you are looking for a pet and you know that you want to do a dog sport such as agility? Your vet can test the patella for some conditions that may exist even in a young puppy. Some others, however, may not show up for many years. The best insurance is to ask your breeder whether they certify the patella of their breeding stock according to OFA standards. However, in very mild cases you may have one vet call the knee normal while another may issue a caution.
How will you know if your dog develops patella problems? In mild cases it may simply be a matter of the dog stopping from time to time and stretching its back leg straight out behind in order to allow the patella to slip back into place. Sometimes the dog may cry in pain when it does this which probably means that the problem is a bit more severe. Or the dog may need assistance to help slip the kneecap back into place. There are many avenues of help however. Chiropractic and message may relieve discomfort just as it does in humans especially in the milder cases. In very severe cases where the pain is intense and debilitating, surgery may be needed. There are orthopedic vets who have developed highly successful surgical techniques for major luxations and while the sport of agility may no longer be an option for the dog, many dogs return to normal activity after surgery.