The patella (i.e., the kneecap) is a small, oval-shaped bone located near the femur and tibia of the hind leg in dogs and cats. The bone is situated underneath the patellar ligament in a deep groove, and can slide back and forth during knee extension and flexion. The patella plays a vital role in knee function, connecting the musculature between the two long bones, and protecting this highly mobile area. Luxation occurs when a joint becomes dislocated or forced out of its normal position. The patella can luxate (i.e., dislocate) or subluxate (i.e., partially dislocate) when an injury or genetic predisposition causes the bone to go awry. 

What causes patellar luxation?

Patellar luxation occurs when the small bone—and the ligament attached—moves out of the patellar groove, to the left or right of the knee. The majority of affected pets will have medial patellar luxation (MPL), in which the kneecap dislocates toward, not away from, the body. This can happen when the patellar ligament is not attached to a central location on the shin bone, or if other orthopedic abnormalities in the knee joint, such as a shallow patellar groove, cause the ligament and patella to dislocate. Many small-breed dogs are predisposed to MPL, including the Chihuahua, Yorkshire terrier, and Maltese, although any pet can develop the condition. While less common, cases in larger dog breeds, who typically develop lateral patellar luxation (LPL) as opposed to MPL, are on the rise. 

Does my pet have patellar luxation?

You should monitor your pet for telltale patellar luxation signs. If you notice your pet frequently walking on three legs (i.e., both forelimbs and one hindlimb), they may have patellar luxation. Most affected pets will have a temporary “skip” in their gait, where they raise the affected limb while walking or running until the patella moves back in place, and then resume their normal gait. In the vast majority of cases, owners do not report signs of pain in their pet, but simply an intermittent change in their walk. Sometimes, patellar luxation is found incidentally on veterinary examination. Each pet’s individual signs depend on luxation severity.

Veterinarians will often assign a grade to the dislocation, based on physical examination or X-ray. A grade one luxation occurs when the patella occasionally pops out of place, whereas with a grade four luxation, the patella is dislocated most of the time. Not surprisingly, the higher the grade, the more drastic the signs. A 50 percent chance exists that both kneecaps are affected in pets diagnosed with patellar luxation. 

Does patellar luxation lead to long-term problems?

For some affected pets, patellar luxation poses no pain, discomfort, or secondary problems. For others, the repetitive dislocation may wear down surrounding bone, causing luxation to worsen, and the patella to remain permanently out of place. The dislocation doesn’t necessarily cause discomfort, but the constant wear on surrounding joint structures can eventually lead to painful osteoarthritis. The likelihood of patellar luxation causing long-term problems depends on the dislocation severity, if both knees are affected, and hereditary factors. Patellar luxation may also put pets at risk for cranial cruciate ligament rupture—a painful condition that often requires surgery. Severe patellar luxation in puppies can lead to serious limb deformities, if left untreated. 

How is patellar luxation treated?

Treatment depends highly on luxation severity, and the affected pet’s signs. For small-breed dogs with grade one MPL, medical management is often a viable option. Orthopedic surgery to correct the luxation is typically recommended for pets with grade two MPL or higher, or those with associated orthopedic problems. Surgery is often the treatment of choice for puppies who may develop severe limb abnormalities without correction.

Patellar luxation surgery usually involves replacing the ligament attachment, deepening the patellar groove, and tightening the joint capsule. Occasionally, an implant is placed under the patella to keep the patella in place. According to the American College of Veterinary Surgeons, 90 percent of pet owners who choose surgical patellar luxation correction for their pet are satisfied with the outcome. 

What else do I need to know about patellar luxation?

Since patellar luxation development likely has a genetic component, pets with this condition should not be used for breeding purposes, regardless of their disease severity.

At Groves Veterinary Clinic, we take patellar luxation seriously. If your pet develops this condition, we will tailor our professional recommendations for your individual pet. Whether you pursue surgery or medical management, we will be with you every step of the way. For more information about patellar luxation or to set up a consultation, contact us.