Ron Hines DVM PhD 4/29/06

Trick knee cap or patellar luxation due to misaligned knees are quite common in toy breeds of dogs. In my practice, I see them only rarely in standard and large breeds. One leg may be worse than the other but both sides are always affects to some extent. The kneecap or patella and its ligaments connect the anterior muscle mass of the thigh to the tibia or shinbone of the lower leg (see diagram). Two ligaments, the medial (inner) and lateral (outer) patellar ligaments normally keep the knee cap riding smoothly in a groove in the femur called the trochlear groove. Two conditions allow for the kneecap to jump out of its groove. The first is a weak patellar ligament; the second is a patella groove that is too shallow. In over ninety percent of the cases I see, the lateral patellar ligament is the culprit allowing the kneecap to displace to the inside (medial) portion of the leg. Because this problem begins early in life while the bones are still growing, severe cases cause bone shape to become distorted and muscle groups to shift their positions.

Symptoms of the Problem:
Toy and miniature dogs of all ages are presented to me with this problem. I can identify these dogs as early as eight weeks of age. The problem is a genetic defect so parents with the problem should not be bred. In most of these cases, gentle thumb pressure on the kneecap while it is in motion is sufficient to cause it to jump out of the trochlear groove and move to an abnormal, position on the inner side of the knee. On the rare occasions when I see this condition in larger breeds, the kneecap is as likely to move to the outside (lateral) side of the legs as to the inside. Occasionally, the kneecap will luxate both directions. Mature and adolescent dogs usually come to me with a history of intermittent weight-bearing lameness and a skipping gait that comes and goes without much pain.

Treatment:
When the problem occurs only due to my manipulation or only occasionally on its own, no surgery is necessary and no medicines need to be administered. When the knee locks up frequently or the dog exhibits pain it its knee surgery is required. There are a number of surgical techniques that attempt to fix this problem. Some veterinarians relocate the patellar ligament and a small portion of bone (the tibial tuberosity) where it attaches to the tibia or shinbone. Others remove a portion of the medial patellar ligament and reinforce the lateral patellar ligament with suture. Often, the groove in which the patella rides is deepened. I usually deepen the trochlear groove and reinforce and tighten the lateral patella ligament with polyester suture material. Since the medial ligament has usually shrunk and contracted, I either stretch it back to its normal length or partially sever it. If I find on X-ray that the femur is so distorted and bowed that this combination of surgery will not be sufficient, I refer the patient to an orthopedic specialist who actually cuts and realigns the femur (femoral osteotomy). The younger a dog is when this surgery is performed, the less the bones will have distorted and the more the likelihood that the surgery will be a success. Older dogs already have some degree of arthritis because of misaligned bones and this arthritis cannot be cured through surgery.

Postoperative Care:
Because it is impossible to cleans and maintain an animal as aseptically as a human being, I begin all orthopedic cases on a broad-spectrum antibiotic several days prior to surgery. I continue this medicine for a week following the surgery. The affected leg or legs are bandaged for three days following the surgery and the pet is limited to short leashed walks for an additional two weeks. Because the surgery is relatively straight forward, few post surgical complications occur. I try to do both legs at the same time even if the current problem is confined to a single leg. This is because left unattended, problems with the lesser-affected knee or sub clinical problems will lead to arthritis of that knee.

Although I wrote this article to pertain to dogs, I have also performed this surgery on several persian cats. Surgery and aftercare are esentially the same. The surgery in cats is more challenging because all the elements that make up the knee are more delicate.

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