What is a luxating patella?
The knee joint connects the femur, (thighbone) and the tibia (shinbone). The patella (kneecap) is normally located in a groove called the trochlear groove, found at the end of the femur. The term luxating means out of place or dislocated. Therefore, a luxating patella is a kneecap that 'pops out' or moves out of its normal location.
Pet owners may notice a skip in their dog’s step or see their dog run on three legs; then, suddenly, she will be back on all four legs as if nothing happened.
"...a luxating patella is a kneecap that 'pops out' or moves out of its normal location."
What causes a 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 shin bone (tibia). When the thigh muscles contract, the force is transmitted through the patellar ligament, pulling on the shin bone. This results in extension or straightening of the knee. The patella slides up and down in its groove (trochlear groove) and helps keep the patellar ligament in place during this movement.
In some dogs, especially ones that are bowlegged, the patella may luxate because the point of attachment of the patellar ligament is not in the center of the shinbone. In these cases, it is almost always located too far toward the middle of the body or the inside of the leg. As the thigh muscles contract, the force applied to the patella pulls it to the inside of the knee. After several months or years of this abnormal movement, the inner side of the groove in the femur may wear down. Once this happens, the patella is then free to dislocate or slide toward the inside of the knee.
Many toy and small breed dogs, including Maltese, Chihuahua, French Poodle, and Bichon Frise, have a genetic predisposition for a luxating patella.
How severe is this condition?
There are four grades of patellar luxation, and a higher grade means that the condition is more severe. Grade I patellar luxation means that 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 that the patella will intermittently pop out on its own and will remain out until the leg is hyperextended and rotated so that the patella returns to the groove. With a Grade III patellar luxation, the patella is out of the groove the majority 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 affected dogs, one or both kneecaps may luxate, sometimes to a different degree. Approximately 50% of affected dogs have both knees involved.
"Approximately 50% of affected dogs have both knees involved."
Can a luxating patella cause long-term problems?
This depends upon the grade of the luxation and whether both legs are affected to the same degree. The higher the grade, the more likely your dog will develop long-term problems. Some dogs, especially with Grade I patellar luxation, can tolerate this condition for many years, even for their entire lives; however, as the dog ages, arthritis develops and results not only in decreased mobility but joint pain as well. Once arthritis develops, it cannot be reversed.
In addition, patellar luxation predisposes the knee to other injuries, especially torn cruciate ligaments (see handout "Cruciate Ligament Rupture in Dogs" for more information).
Can a luxating patella be corrected?
Surgery should be performed if your dog has recurrent or persistent lameness or if other knee injuries occur secondary to the luxating patella. Grade I patellar luxations are not usually surgically repaired, but Grades II-IV are.
Surgical repair generally involves three steps:
- The point of attachment of the patellar ligament to the shin bone is moved to its proper location.
- The groove in the femur is deepened so the patella can stay in place.
- The capsule around the joint is tightened. When the patella luxates, the joint capsule stretches; tightening it helps to prevent the patella from luxating again. An implant may also be placed on the inside of the knee, making the patella unable to slip over it.
Post-operatively, the recovery period is usually very 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."
If your veterinarian performs surgery before arthritis or another knee injury occurs, the prognosis is excellent. Your dog should regain full use of her leg. However, if arthritis has already developed in the knee joint, your dog may experience intermittent pain in the leg and it may progress. The higher the grade of patellar luxation, the higher the likelihood of reoccurrence postoperatively.
Prescription anti-inflammatories, joint supplements, and/or therapeutic mobility diets may slow the progression of arthritis and help control any discomfort. Weight reduction is also recommended for overweight dogs. Post-operative physiotherapy may be recommended. Your veterinarian can help you determine the best post-operative plan for your dog.