Medially Luxating Patella

Medially luxating patella a condition where the patella (knee cap) pops in and out of place in response to extension, flexion, or weight bearing of the knee joint. The disease is congenital in origin, resulting from inherited shallow patellar grooves (the groove in which the patella normally sits in). The disease is sometimes complicated by an inherited abnormally medially located attachment point for the patellar tendon. The luxations almost always occur in the medial direction (toward the center of the body). For example, in the left limb, the tendency is for the patella to luxate to the right. The opposite is true for the right limb. The majority of luxating patella cases are bilateral.

Luxations cause inflammation and pain in the knee joint. Also, because the chief purpose of the patella is to redistrubute tendon forces to best handle weight bearing and stress, patellar luxations tend to throw off the physics of the entire leg, causing excessive wear on the hips and ankles. This predisposes to degenerative joint disease in these other joints. What’s more, over time, as the luxations occur, the medial ridge of the patellar groove becomes worn, increasing the frequency of luxations. Eventually, if left untreated, the disease can progress to the point where the patella spends all of its time out of place. Patellar luxations are graded on the following scale:

Grade I: Patella luxates occasionally. Pain, discomfort, and lameness intermittent.

Grade II: Patella luxates regularly. Pain, discomfort, and lameness more frequently observed than normal gait.

Grade III: Patella spends more time luxated than in place. Pain, discomfort, and lamness almost always apparent.

Grade IV: Patella always luxated. Pain and discomfort severe to the point of severe to non-weight bearing lameness.

Medially luxating patella most commonly occurs in toy to small sized dogs. However, the disease is occasionally reported in larger dogs and in cats. Treatment is surgical and geared toward deepening the patellar groove and anchoring the patella laterally. The most accepted surgical approach in small dogs is known as the trochleoplasty, tibial tuberosity transposition.

Trochleoplasty

Deepening of the trochlear groove, or trochleoplasty, can be accomplished with a variety of techniques. A chondroplasty technique involves cutting out a taco-shaped wedge of cartilage, removing a small portion of bone beneath it, and then replacing the cartilage. The result is a deeper groove. This procedure can only be performed on very young dogs, because their cartilage is thicker.

Trochlear recession involves cutting out the cartilage and bone in such a way as to create a deeper trough. This trough will then fill in with scar tissue over time. Because this scar tissue is not as good as cartilage for joint function, this technique has given way to others that attempt to preserve normal cartilage. It can, however, be useful in carefully selected cases.

Wedge recession creates a taco-shaped piece of cartilage and underlying bone. Then, the bone below the wedge is removed and the wedge is replaced, forming a deeper groove. Block recession is identical in principle to wedge recession, except that a rectangular piece of cartilage and bone, rather than a wedge, is removed.

Tibial Tuberosity Transposition

The kneecap attaches to the lower leg via its patellar tendon at a bony site called the tibial tuberosity. Many times this site forms abnormally on the inside, as with MPL, or on the outside, as with LPL. In this procedure, the surgeon moves the tibial tuberosity back into proper alignment and secures it in place with a pin or wire. Realigning the joint, kneecap, and tendon prevents dislocation from reccurring.

Osteotomy
In severe cases, with malformation of the tibia or femur, corrective bone cuts known as osteotomies may be required.

Prevention:
Early detection and correction is the best way to prevent severe lameness and dysfunction. Breeding affected animals should be discouraged; however, the disease is so prevalent in some breeds that this may not be practical.

By: Roger L. Welton, DVM
Founder, Web-DVM
President, Maybeck Animal Hospital
Author Canine and Feline 101