One of the problems our doctors at the Animal Clinic at Thorndale frequently hear about from owners is that their dog can no longer jump up on the sofa or bed. There can be many causes for this, usually involving the rear limbs or back. One of the most common causes in toy breeds for not being able to jump up is a luxating patella. Yet our doctors have even seen this problem in dogs as large as a Newfoundland, so size is not everything. Luxating patella dogs tend to limp intermittently. Observant owners may even notice their dog stretching the rear leg out behind them to “pop” the knee cap back in place.
(Dislocated Kneecap or “Trick knee”)
The patella, or kneecap, should be located in the center of the knee joint. The term “luxating” means out of place or dislocated. Therefore, a luxating patella is a kneecap that moves out of its normal location.
Contributing Factors
The muscles of the thigh attach, directly or indirectly, to the top of the kneecap. There is a ligament, the patellar ligament, running from the bottom of the kneecap to a point on the tibia just below the knee joint. When the thigh muscles contract, the force is transmitted through the patella and through the patellar ligament to the point on the top of the tibia. This results in extension (straightening) of the knee. The patella stays in the center of the leg because the point of attachment of the patellar ligament is on the midline and because the patella slides in a groove on the lower end of the femur between two boney rollers like structures called condyles. The femur is the long bone which fits between the knee and the hip.
Prevalence
Patellar luxation is most common in small toy breeds of dogs.
Causes/Transmission
The patella luxates when the point of attachment of the patellar ligament is not on the midline of the tibia. It is almost always located too far medial (toward the midline of the body). As the thigh muscles contract, the force is pulled medial. After several months or years of this abnormal movement, the inner side of the groove in the femur wears down. Once the side of the groove wears down, the patella is then free to dislocate. When this occurs, the dog has difficulty bearing weight on the leg. The dog may learn to kick the leg out behind it and snap the patella back into its normal location. However, because the side of the groove is gone, it dislocates again easily.
Clinical Signs
Some dogs can tolerate this problem for many years, some for all of their lives. However, this weakness in the knee predisposes the knee to other injuries, especially torn cruciate ligaments. Also, arthritic changes may take place in the joint and make it painful. Overweight dogs can be more severely affected. Some dogs may be lame intermittently. Some owners can feel the “pop” of the knee cap as it slides in and out of the groove while flexing and extending the knee joint.
Diagnosis
Luxating patellae can be detected with a routine orthopedic examination of the knee joint by your regular veterinarian.
Therapy
A luxating patella can be repaired surgically by relocating the point of attachment of the patellar ligament and by deepening the groove in the femur. This should be done if your dog has a persistent lameness or if other knee injuries occur secondary to the luxating patella.
Prognosis
Surgical repair is generally very successful. The prognosis is more favorable when the luxation is not severe or if repair occurs before arthritis develops.