Medial Patellar Luxation Starting at $2000
The medial luxating patella, commonly called a “trick knee,” is an extremely common problem in toy breed dogs. An owner typically notices a little skip in the dog’s step. The dog may even run on three legs, holding one hind leg up, and then miraculously be back on four legs as if nothing has happened. In fact, something has happened: the kneecap, or patella, has slipped out of the smooth groove in which it normally rides up and down. It has slipped medially, toward the opposite leg, as opposed to laterally, which would be away from the dog entirely. With the patella dislocated or luxated medially, the knee cannot extend properly and stays bent. With luck, the patient will be able to slip the kneecap back where it belongs and be back to normal in only a few steps. For some dogs, resetting the kneecap and regaining normal extension of the rear leg is only attainable with surgical correction known as medial patellar luxation surgery. Approximately 50 percent of affected dogs have both knees involved while the other 50 percent has only one knee involved.
Medial patellar luxations are graded to assess severity.
Grade I: The kneecap can be moved out of place manually but will fall back into its natural position once the manipulator lets go.
Grade 2: The kneecap occasionally slips out of its groove, spontaneously creating an intermittent lameness. The kneecap will go back in place on its own. Whether it needs surgery depends on how often lameness is a problem and how long lameness lasts when it is a problem. Another reason to proceed with surgical intervention is for prevention. If the ridge of bone on the inside of the knee wears down, the Grade 2 will progress to Grade 3. Surgery can prevent this progression.
Grade 3: The patella is out of place all the time but can be manipulated back into its normal position manually, though it will not stay there. At this point, the dog may seem to crouch or slant down in the rear. Lameness or rear weakness may be persistent. The dog may have a bow-legged appearance.
Grade 4: The patella is out of place all the time and no amount of manipulation can return it to its proper place. Such a dog has extreme difficulty extending its knees and walks with its knees bent virtually all the time in a somewhat crouched position. In this situation, the rotation in the long bones that accompanies the luxation will interfere with surgical correction and total resolution of the lameness may not be possible.
A dog’s kneecap should not be out of place. This alters the entire weight-bearing stress of the rear leg, which, in time, leads to changes in the hips, long bones, and ultimately arthritis. How severe the changes are depends on how severe the luxation is and how long that degree of luxation has been going on. In time, the legs will turn outward with their muscles turning inward, making the dog bow-legged. The luxation is not considered a painful condition but after enough time and conformational change, arthritis sets in, which is indeed painful. Dogs with Grade I luxations do not require surgical repair. Often they can be managed with weight control and periodic anti-inflammatory medication. Dogs with Grade 3 or 4 disease definitely should have surgery.
Also called lateral reinforcement, this procedure alone may be adequate for a mild case of canine medial patellar luxation but is often used as an adjunctive procedure to supplement one of the other surgeries. When the patella slips out of its groove, the joint capsule surrounding stretches to allow this motion. Imbrication simply involves taking a tuck in the joint capsule. The tightened joint capsule does not allow for the slipping of the kneecap and the kneecap is confined to its proper groove.
The patella rides in a groove at the bottom of the femur (thigh bone). In toy breed dogs, this groove is shallow, which allows the patella to slip. If the groove is deepened, the patella stays where it belongs. The normal groove in the femur is lined by slippery lubricated cartilage, called hyaline cartilage. Surgeons peel or cut away this cartilage, they slice the bone underneath to deepen the groove, and they replace the cartilage. Techniques that do not preserve the original cartilage are no longer recommended.
This is also called tibial tuberosity transposition. If the knock-kneed conformation has already started to set in, the tibias (or leg bones) will have rotated. In particular, the crest on the tibia where the thigh muscle (the quadriceps femoris) attaches may have migrated inward. If this is the case, the crest will have to be removed and pinned back where it belongs to straighten out the leg. Severe rotation of the tibias may involve cutting through the entire bone and rotating it back into place.
Some surgeons prefer to stage MPL surgery for dogs. By doing one leg at a time, at least eight weeks apart, the patient will have one good rear leg upon which to walk. If the patient is less than one year old, it may be a good idea to do both legs at the same time to prevent conformational problems in the leg that is not operated on first. Every surgeon will have their own philosophy on whether to do both legs at once. Consult your surgeon about pros and cons and decide.
Post-operative care is necessary after medial patellar luxation surgery. If imbrication was the only procedure, expect three to four weeks of confinement to a large crate or pen, during which the pet will need to be carried outside for bathroom functions. If you chose another procedure, expect six to eight weeks of confinement depending on the surgeon’s preference. During this time easy walking (no running or jumping) is helpful. The dog should be using the leg by two weeks post-operatively though some dogs must be retrained to use the leg after surgery. Physical therapy is in order if the dog is not using the leg after one month.
Wendy Brooks, DVM, DABVP
Date Published: 12/11/2006
Date Reviewed/Revised: 08/29/2019