Feb 22 2013

Luxating Patella or Kneecap in Dogs


Do you have a small or toy breed dog that as it walks or runs has an occasional skip in its gait? Your dog may have a knee cap that is popping in and out of its associated groove known as a medial luxating patella.

The patella, or “kneecap,” is normally located in a groove on the end of the femur, or thighbone.

The term luxating means “out of place” or “dislocated”. Therefore, a luxating patella is a kneecap that moves out of its normal location. It generally resumes its normal anatomical orientation after only a brief period of luxation in most dogs, thus the skip!

What causes a patellar luxation?

The large muscles of the thigh (quadriceps) attach to the top of the kneecap. A ligament, known as the patellar ligament, attaches the quadriceps muscle to a point on the center front of tibia (the bone in the lower leg) just below the knee joint. The kneecap sits on the undersurface of this ligament. When the thigh muscles contract, the force is transmitted through the patellar ligament, pulling on the tibia. This results in extension or straitening of the knee. The patella slides up and down in its groove (the trochlear groove) and helps keep the patellar ligament in place during this movement.

Many toy or small breed dogs, including Maltese, Yorkshire Terriers,Chihuahuas, Poodles and the like, have a genetic predisposition for a luxating patella due to a congenitally shallow trochlear groove. In some dogs, (especially ones that are bowlegged) the patella may luxate because the point of attachment of the patellar ligament is not on the midline of the tibia. In these cases, is almost always located too far medially. As the thigh muscles contract, the force applied onto the patella pulls it medially or to the inside of the knee. After several months or years of this abnormal movement, the inner side of the trochlear groove in the femur may wear down. Once the side of the groove wears down, the patella is then free to dislocate or slide toward the inside of the knee.

When the patella luxates, the dog has difficulty bearing weight on the leg, although there are rarely any signs of pain. It may learn how to kick the leg to the side, which will hyperextend the knee and snap the patella back into its normal location. Medically, there are several degrees or grades of patella luxation, ranging from a mild form in which the kneecap only luxates when force is applied directly to it, to moderate luxation in which the kneecap pops out frequently, all the way to a severe luxation, in which the kneecap may be dislocated the majority of the time. In affected dogs, one or both kneecap may luxate, sometimes to a different degree.

Does a luxating patella cause any long-term problems for my dog?

This depends upon the grade of the luxation and whether both legs are affected to the same degree. Some dogs can tolerate this condition for many years, even for their entire life.

As this condition is a congenital (meaning born with) and hereditary (meaning passed to offspring), a dog with medial luxating of the knees should not be bred and should be neutered or spayed. Your veterinarian will feel your small breed dogs knees at every physical examination to see if your dog is showing symptoms of knee cap looseness. Typically, if a dog less than 1-2 years is showing symptoms it will normally become a surgical candidate.

Patellar luxation predisposes the knee to other injuries, especially torn cruciate ligaments. With advancing age, arthritic changes often take place in the joint due to the abnormal movements and stress, resulting in pain or decreased mobility.

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. Surgical repair generally involved three to four steps.

  1. The point of attachment of the patellar ligament to the tibia maybe transplanted to its proper location.
  2. The groove in the femur is deepened so the patella will stay in place.
  3. A non absorbable suture is placed around the knee cap and tied anchored on the lateral side of the knee.
  4. 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. 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 other knee injury occurs, the prognosis is excellent. Your dog should regain full use of its leg. However, it arthritis has already developed in the knee joint, your dog may experience intermittent pain in the leg and the arthritis may progress. Long-term anti-inflammatory medications and joint supplements such as glucosamine are used to ensure your pet remains pain-free. Being overweight is also a predisposing factor.

As always, call your family veterinarian with questions and concerns.


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