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Medially luxating patellas: no fun having a trick knee

Do the terms "floating kneecap" or "trick knee" make you cringe? When I mention this to owners it usually does put a weird look on their face. These layman's terms refer to a condition called a medially luxating patella or luxating patella.

The patella, or kneecap, is an integral part of the knee joint. The patella not only protects the knee if the knee was to hit the ground, but also plays a role in stabilizing the joint. The kneecap is connected to the tibia via the straight patellar ligament and, in addition to the four major ligaments associated with the knee joint (anterior cruciate ligament, posterior cruciate ligament, and the medial and lateral collateral ligaments), aids in preventing the tibia (shin bone) from moving independently of the femur (thigh bone).

Unlike humans, patellar luxation in pets is a congenital (genetic) condition and is most common in dogs. At-risk breeds are most of the smaller breeds including toy poodles, Maltese, Jack Russell terriers, Yorkshire terriers, Pomeranians, Pekingeses, Chihuahuas, Cavalier King Charles spaniels, Papillons and Boston terriers. Also remember that many of the new "designer breeds" will also be at risk as they are combinations of the above mentioned breeds. Patella luxation can also happen in cats but it is much less common. Predisposed cat breeds include the Devon Rex and the Abyssynian.

Degree of severity

Patellar luxation is graded on degree of severity. How it is treated depends on that severity. There are four grades from least severe to most severe.

• Grade I — the least severe. The patella can be pushed out of the joint but immediately "pops" back into place and for the most part will stay in the appropriate location.

• Grade II. The patella can be pushed out of joint and stays out unless pulled back into normal position or the leg is extended.

• Grade III. The patella is usually out of joint (without actually manually pushing it) but will pop in when the leg is extended or it is pulled into place.

• Grade IV — the most severe. The kneecap is out all the time and cannot be reduced back into normal position.

Treatment

Treatment for a patellar luxation can be anything from no treatment, or monitoring to surgical correction. What we, as veterinarians, try to gauge is how is this defect is affecting the patient. A dog that has a less severe grade of luxation may never have pain or limping, or may have occasional limping that resolves on its own (the patella popped out and back in again on its own) and can live their whole lives without intervention.

However, a dog with a more severe luxation may hold its leg up more than the leg touches the ground — this would be an indication to intervene surgically. There was a recent study that proved that dogs with a Grade IV luxation (the most severe) were more at risk for rupturing their anterior cruciate ligament. For these cases many veterinarians are recommending surgery regardless of whether the dog is limping or not.

Surgery usually includes three procedures (don't worry, they are all done at the same time). First involves deepening of the trochlear groove. The trochlear groove is a groove on the femur (thighbone) that the patella sits in. Dogs with a luxating patella have a very shallow trochlear groove. By deepening the groove the patella will sit lower and be less likely to slip out.

Second, a tibial tuberosity transposition is performed. This refers to making a cut through the tibial tuberosity (the portion of the shin bone where the straight patellar ligament attaches) and pinning it slightly sideways from its original position. This lateral tension on the straight patellar ligament is designed to counteract the medial luxation. Lastly, sutures (or stitches) are placed in a way that also puts lateral tension on the patella to aid, or reinforce the tibial tuberosity transposition. Severe cases of patellar luxation involve cutting the femur, or thigh bone, and realigning it with plates, but luckily these cases are rare.

I hope this article sheds some light on a somewhat common orthopedic condition that is treated with a straightforward surgical procedure and in some cases does not need to be treated at all.

Dr. Kearns has been in practice for 13 years and is pictured with his son Matthew, as well as the newest member of the family, Jasmine, a Labrador retriever.

Author: 
Matthew Kearns, DVM