Medial luxating patella:

Medial luxating patella is a genetic disease of small and toy breed dogs. The breeds most prone to this condition are: Affenpinsher, Boston Terrier, Cairn Terrier, Chihuahua, Cocker Spaniel, Papillon, Pekinese, Pomeranian, Toy and Miniature Poodle, Silky Terrier, Yorkshire Terrier, Pug, Dachshund, Basset hound, and Corgi. Although present from birth, the looseness generally cannot be found on examination until 7 months of age. There are three conditions of the knee that cause the luxation to happen.

• One is the fascia (tissue over the muscles of the thigh) being too lose, allowing too much movement
• The groove at the end of the femur is too shallow
• The patellar ligament’s attachment on the tibia (shinbone) is to high up.

With the excessive laxity (looseness) of the joint, the patella moves out of the joint when the knee bends. This laxity can be mild to severe. With time, all of the abnormal motion causes wearing down of the cartilage covering the bones in the joint. When this happens for long enough, eventually bone on bone contact occurs. This is very painful. The condition at this point is called osteroarthritis or degenerative arthritis. The laxity of a dog’s kneecaps is put into stages (or grades) depending on findings during an exam.

Grade 1: Mild luxation, generally noted only on examination of the knee. The patella can be moved out of joint but immediately returns to its proper place.

Grade 2: Moderate luxation. The patella slips out of the groove causing lameness. Eventually it returns to its normal position, usually by a stretching of the leg. With time, clinical osteoarthritis may occur.

Grade 3: Severe luxation. Lameness is more pronounced. It can be occasional to complete non-weight bearing. The patella is mostly out of joint, but can be manipulated back into place. Left untreated, severe osteoarthritis will occur.

Grade 4: Very severe luxation. Lameness is constant. The patella is out of joint and cannot be replaced into its normal position. Osteoarthritis occurs earlier and is more severe.

Symptoms depend on severity of the luxation. The symptoms generally don’t set in until the dog is 5-8 years old. For mild (grade 1) cases, no problems may occur. Sometimes an owner will notice a slight “hitch” or stretch in the leg while running. For more serious cases, the lameness can be more pronounced. It can even eventually lead to bony changes in the joint and a tearing of the cranial cruciate ligament. As the dog ages, the ligament can stretch more, causing increasing laxity. A dog that was Grade1 at age three can become a grade 2 by age five for example.

How is a luxating patella diagnosed? How can a veterinarian tell this is the problem? A veterinarian can physically move a luxating patella by bending the knee and trying (gently) to force the patella out of joint. At this point a grade can be placed to the degree of laxity. Radiographs are also recommended. These allow the veterinarian to assess laxity as well as arthritic changes that would affect treatment and prognosis. Rarely other diagnostic tools such as ultrasound, MRI, Nuclear scans, and arthroscopy are used to assess the knee.

Treatment for a luxating patella depends on severity of the laxity and presence of clinical signs.

• For grade 1 luxations, conservative management is recommended. This includes reduction in jumping and dietary supplements to minimize any arthritic changes down the road.
• For grade 2 luxations, sometimes conservative management is used if symptoms are very mild. For more symptomatic dogs or those with radiographic changes, surgery is recommended before arthritis occurs. Early surgical intervention minimizes arthritis, which allows more maximal return to full function.
• For grade 3 and 4 luxations, early surgical intervention is always recommended.

If surgery is recommended, what does that mean? There are three ways that a luxated patella is repaired. General anesthesia is required. The knee is opened up surgically. At this point the surgeon can evaluate the knee. Other ligaments can be checked visually and arthritic changes remodeled by scraping away abnormal bone formation. An approach to correct the problem is then made.

• The fascia can be tightened down. This is done by removing a portion of the fascia, which is then surgically closed. Think of it as a type of hemming, which keeps the kneecap in place. This is done for mild cases and in combination with other procedures for more advanced cases.
• The patellar groove on the end of the femur is deepened. Scraping a deeper canal or using a saw to cut out a piece in a pie shape, removing the sharp end and replacing it can do this. This does damage the cartilage.
• The end of the ligament, where it attaches to the tibia (shin) can be removed and reinserted further down on the tibia, tightening the ligament. A pin must be placed for this to heal.

Recovery from the surgery generally takes about 6 weeks. Anti-inflamatories, dietary supplements, and sometimes narcotics are used to reduce pain post-operatively. An injection of Adequan (a chemical called a proteoaminoglycan) given every 4 days for 8 injections helps to re-establish the cartilage and clean up inflammation in the joint after surgery. Rest is absolutely required. Sometimes a soft splint is used to immobilize the joint after surgery. Surgery is generally very successful. The earlier performed the better the results.

Because the problem is genetic, sometimes both knees will require surgery.

Because the problem cannot be detected at initial exams, it is best to purchase or adopt a puppy from parents certified free of the condition. There is no means to identify parents via a blood test for the genes, as the genes responsible have not been identified. It is best to use a breeder who uses a breed registry such as the OFA (Orthopedic Foundation for Animals) or the Genetic Disease Control in Animals (GDC) to check the lineage for history of defects. If your dog is diagnosed, the breeder should be notified, as the parents may not have the disease, but are carrying the genes.

There is also a much rarer condition called lateral patella luxation, where the knee moves towards the outward side of the joint. It is seen in large/giant breed dogs such as the Great Dane, Irish Wolfhound, Rottweiler, and St. Bernard. Symptoms of lameness occur by 5-6 months of age. Treatment is very similar to medial luxating patella.

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