When patellar luxations are present early in life, the major muscle groups of the thigh pull toward the inside of the leg, putting abnormal pressure on the knee joint cartilage. The result is a bowlegged stance and an abnormal pull on the patella (see illustrations below). Thus, a number of anatomic pelvic limb deformities can result from the structural manifestation of medial patellar luxation. These include bowed legs, coxofemoral (hip) joint abnormalities, and outward rotation of the limb.

When the patella is in its normal position, its cartilage surface glides smoothly and painlessly along the cartilage surface of the trochlear groove with little or no discomfort. As the patella pops out of its groove, these cartilage surfaces rub each other. The animal may cry and try to straighten the leg to pop the patella back into position or may hold the limb up until muscle relaxation allows the kneecap to reposition itself. This resembles an intermittent lameness. There is little or no discomfort until the cartilage is eroded to a point where bone touches bone. From this point on, each time the patella pops out into its abnormal, luxated position, it will cause pain. This explains why many dogs have no clinical lameness until they reach adulthood when progressive cartilage wear creates an acutely painful condition.

Because there is great individual variation in the pathologic deformities seen, a graded classification of medial patellar luxation (Putnam 1968) has been formulated as a basis for recommending which type of surgical repair is most appropriate for each individual. In the following description each classification is addressed:

GRADE I
The anatomic alignment of the stifle is normal with the patella luxating only when pushed out of the socket.

GRADE II
The patella luxates upon flexion of the joint and remains luxated until returned by manual pressure.

GRADE III
The patella is permanently dislocated but can be reduced manually with the limb extended.

GRADE IV
The patella is permanently dislocated and cannot be manually reduced.

Treatment

The procedures for repair of medial patellar luxation deal with repositioning and stabilizing the kneecap in the patellar groove of the femur. Depending on the severity of the deformities, the technique may be as simple as soft tissue reconstruction or as complicated as multiple corrective osteotomies (straightening the bone).

The most commonly accepted surgical procedures include:

Deepening the trochlear groove.


Tightening the tissues around the joint.


De-rotating the femur or tibia.


Repositioning the patellar ligament attachment to the tibia.

Postoperative Care