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The medial luxating patella is an extremely common problem in toy and small 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. With the patella dislocated (or “luxated”) medially, the knee cannot extend properly and stays bent. Hopefully, the patient will be able to slip the kneecap back where it belongs and be back to normal in only a few steps.
When Is Surgery Indicated?
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:  Same as Grade 1 except that the kneecap does not move back to its normal position when the manipulator lets go. These dogs are likely to progress to arthritis development and should be considered for surgery to prevent conformational damage.
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 remain there.
Grade 4: The patella is not only out of place all the time but cannot even be manipulated back into place by hand. Such a dog has extreme difficulty extending his knees and walks with his knees bent virtually all the time.
When the patella is out of place the entire weight-bearing stress of the rear leg is altered which, in time, leads to changes in the hips, long bones, and ultimately arthritis. How severe the changes are depend on how severe the luxation is (i.e. the grade as described above) and how long that degree of luxation has been going on. In time, the legs will actually turn outward with its 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.
What Surgical Procedures Are Available?
Lateral Imbrication
This procedure alone may be adequate for a mild case but is often used as an adjuctive procedure to supplement one of the other surgeries. When the patella slips out of its groove, the joint capsule surrounding it is stretched 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.
Trochlear Modification
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”. This cartilage is peeled or cut away, the bone underneath is sliced out to form a deeper groove, and the cartilage is replaced. Techniques that do not preserve the original cartilage are no longer recommended.
Tibial Crest 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 actually cutting through the entire bone and de-rotating it back into place.
Should Both Knees Be Repaired At Once?
Some surgeons feel that doing one leg at a time, 8 weeks or more apart, is beneficial as the patient will have one good rear leg upon which to walk. If the patient is very young (under age one year) it may be a good idea to do both legs at the same time so as to prevent conformational problems in the leg not operated first. All options should be discussed between you and your veterinarian to establish the best plan of action for repairing the knee.