Luxating Patella in Cats

What is a luxating patella?

A patella is a kneecap, and “luxating” means out of place or dislocated. So, a luxating patella is a kneecap that moves out of its normal location. The knee joint connects the femur (thighbone) and the tibia (shinbone). The patella is normally located in a groove (the trochlear groove) at the end of the femur, but with a luxating patella, it moves out of this position.

What causes patellar luxation?

The kneecap sits underneath a ligament called the patellar ligament. This ligament attaches the large thigh muscles to a point on the center front of the shinbone. When the thigh muscles contract, the force is transmitted through the patellar ligament, pulling on the shinbone and causing the knee to extend or straighten. The patella slides up and down in its groove and helps keep the patellar ligament in place during this movement.

The patella will luxate, or slip out of the groove, during extension of the leg if the trochlear groove is too shallow, if the cat is bowlegged or cow-hocked (i.e., the leg bones are excessively curved), or if the point of attachment on the tibia is off center. Most of the time, in cats, the patella luxates because the trochlear groove is too shallow and the direction of luxation is towards the inside of the leg, which is called medial luxation.

Although any breed of cat can be afflicted with patellar luxation, there may be a genetic predisposition to this problem in Devon rex, Siamese, and Abyssinian cats. In some cases, patellar luxation may occur because of trauma to the bones or muscles of the hind leg.

How severe is this condition?

There are four grades of patellar luxation, and a higher grade means the condition is more severe:

• Grade I patellar luxation means the patella can be luxated out of the groove by putting pressure on it, but as soon as the pressure is released, the patella goes back in place.
• Grade II patellar luxation means the patella intermittently pops out on its own and remains out until the leg is hyperextended and rotated so the patella returns to the groove.
• With a Grade III patellar luxation, the patella is out of the groove most of the time, but it can be manually pushed back into the groove.
• With a Grade IV patellar luxation, the patella is permanently out of the groove and cannot be manually repositioned.

In some affected cats, both kneecaps may luxate to a different degree.

What are the clinical signs of patellar luxation?

The signs of patellar luxation include intermittent lameness, an unusual “skipping” on the affected leg when the cat walks or runs, or difficulty jumping. With a medial luxation, the lower part of the leg will usually be held out to the side as long as the kneecap is out of place. When it returns to its normal position, the cat can resume normal walking.

Many cats with this condition learn how to kick the leg to the side, which causes the knee to hyperextend and “snaps” the patella back into place. Over time, the soft tissues around the joint become damaged and/or stretched and it becomes easier for the kneecap to pop out of place.

How is patellar luxation diagnosed?

Your veterinarian will diagnose a luxating patella by feeling the displaced kneecap during palpation of the leg. In some cases, particularly with young cats that have been born with this problem, the luxating patella will be detected during a routine physical examination. In other cases, a medical history of intermittent lameness may suggest this problem and your veterinarian will examine the leg to confirm patellar luxation.

Radiographs (X-rays) are usually performed to determine the extent of the problem and to detect whether there are any other changes or abnormalities in the joint, particularly if the luxation is caused by trauma.

Can a luxating patella cause long-term problems?

A luxating patella can cause long-term problems for your cat, but it depends on the grade of luxation and whether both legs are affected to the same degree. The higher the grade, the more likely your cat will develop long-term problems. Some cats, especially with Grade I patellar luxation, can tolerate this condition for many years, even their entire lives; however, as the cat ages, arthritis develops and results in decreased mobility and joint pain. Once arthritis develops, it cannot be reversed. In addition, patellar luxation predisposes the knee to other injuries, especially torn cruciate ligaments.

Can a luxating patella be corrected?

A luxating patella can be surgically corrected, especially if the patella luxates frequently. With mild Grade I luxation, surgery may not be necessary. For most cats with patellar luxation, surgery is recommended sooner rather than later to minimize the likelihood that arthritis will develop. The higher the grade of luxation, the sooner the surgery should be performed.

 Surgical repair may involve several techniques, depending on the reason that the patella is moving out of place, including:

1. Tibial tuberosity transposition: Moving the patellar ligament’s point of attachment on the tibia (shinbone) to its proper location.
2. Trochleoplasty: Deepening the groove in the femur where the patella glides so it is harder for it to move out of place.
3. Altering the inappropriate tension on the patella by releasing or cutting tight connective tissue that is pulling the patella out of place, and tightening the stretched tissues that are trying to pull it into the correct position (imbrication). This type of surgery is often used in addition to one of the above interventions to help prevent the patella from luxating again.
4. Placing an implant on the inside of the knee, making the patella unable to slip over it.

After surgery, recovery is usually rapid, especially with appropriate pain management.

What is the prognosis?

If your veterinarian performs surgery before arthritis or another knee injury occurs, the prognosis is excellent. Your cat should regain full use of the leg. However, if arthritis has already developed in the knee joint, your cat may experience intermittent pain in the leg and it may progress.

Prescription anti-inflammatories, antibody therapy (Solensia®), joint supplements, and therapeutic mobility diets may slow the progression of arthritis and help control any discomfort. Weight reduction is also recommended for overweight cats. Post-operative physical therapy (rehabilitation) may be recommended to help improve recovery. Your veterinarian can help you determine the best post-operative plan for your cat.