Luxating Patella (Knee Cap) in Dogs

Patellar luxation is a dislocation of the knee cap either toward the inside (medial) or outside (lateral) of the leg. The patella and its ligament normally rides in a trough or trochlear groove in the centre of the leg (femur). It is one of the most common knee joint abnormalities in dogs.

Luxation may result from traumatic injury or congenital (present at birth) deformities. If the groove that the patellar ligament (knee cap) rides in is too shallow or if the distal attachment of the patellar ligament is medial instead of central, the patella will dislocate medially when the knee is bent. When this occurs, the dog (or cat) has difficulty bearing weight on the leg until the ligament snaps back into place. Sometimes only one knee is involved, but the disease can become bilateral (involving both legs) in 50% of cases.

Patellar luxation is most common in toy and miniature dog breeds, especially Poodles, Yorkshire Terriers, Pomeranians, Pekingese, Chihuahuas, Miniature Pinschers and Boston Terriers. It is uncommon in cats, but may be more common than suspected because most affected cats are not lame.

Clinical signs of patellar luxation vary depending upon severity and chronicity of the disease and may develop soon after birth or generally after four months of age. With mild forms of luxation, animals may occasionally pick up the affected leg when they run. As the disease progresses, animals may present with increased lameness with a decreased ability to jump. Pain is usually not associated with this condition unless it is the result of trauma or until degenerative arthritis has occurred due to chronicity of disease.

Some pets can tolerate this disease for many years, some for all of their lives, with minimal consequences. However, this weakness of the knee joint can predispose the knee and even the hip joint to other injuries such as degenerative arthritis and torn knee ligaments, both painful conditions.

Grade I — The patella can be manually luxated but easily comes back into normal position. Patient may or may not occasionally carry the affected leg.

Grade II — Patella luxates on flexion of the knee joint and remains out of place until manually replaced or patient extends and rotates joint. Patient intermittently carries the affected limb with the knee joint flexed.

Grade III — Patella remains luxated most of the time but can be manually reduced (placed back into position). Flexion and extension of the knee joint reluxates the patella. Patient transfers most of the body weight to the front legs, bunny hops or carries the affected legs, and appears bowlegged or knock-kneed.

Grade IV — Patella is permanently luxated and cannot be manually repositioned. The quadriceps muscle group starts to shorten, making it difficult to extend the leg fully. Patient transfers most of the body weight to the front legs, bunny hops or carries the affected legs, and appears bowlegged or knock-kneed.

Patellar luxation is diagnosed based upon history, physical exam findings, and radiographs.

Keep in mind that patellar luxation is a heritable disease and we do not recommend breeding affected dogs. It is also very common for the disease to affect both legs.

Treatment is based upon severity of signs and your pet’s age, breed and weight. Conservative therapy is often chosen in the early stages of the condition. However, if the patellar luxation is a grade III, your pet has persistent lameness, or other knee injuries occur secondary to the luxation, then surgery is the best option for your pet. Even with conservative medical therapy, your pet is at increased risk for torn ligaments in the knee and the condition can worsen over time, leading to degenerative joint disease which is a permanent and painful condition.
Surgical Therapy: The goal of surgery is to stabilize the knee and allow a normal return to function. A lateral incision is made over the knee joint of the affected leg. The joint is then examined for health of the cartilage, ligament stability, and presence of infection or osteophytes (bone spurs). Trochlear wedge resection involves deepening the groove that the patellar ligament rides in. The joint capsule is imbricated (some of the joint capsule is removed to tighten it) on the side opposite of the luxation to help pull the patella over. If the distal end of the patellar ligament is attached medially and not centrally, then the tibial tubercle (the point of attachment) is transpositioned so that it is centred on the tibia and then stabilized with pins and a tension band wire. After surgery a support bandage is placed around the leg to help with post-operative swelling and support.

If the surgery is performed before arthritis occurs, the prognosis is excellent and your pet should retain full use of his leg. However, if arthritis has already occurred, the joint will still be somewhat painful. Some arthritis can occur even post surgery. However, the amount of arthritis will be lessened with surgery. Recovery time post-surgery is varied and depends upon weight and age of the pet, pet’s physical activity, and degree of involvement of the surgical procedure. Most pets will begin toe-touching in 2 to 4 weeks. I would recommend ice packing for 15-30 minutes every 8 hours for 3 to 5 days after the surgery, and also recommend passive physical therapy (range of motion exercises) of the knee joint as soon as the pet will tolerate it.