Medial patellar luxation has been attributed to many factors. Medial displacement of the quadriceps, a shallow femoral trochlear groove, and medial displacement of the tibial tuberosity all play a role in the pathogenesis of medial patellar luxation. With medial luxation of the patella there is often lateral torsion of the distal femur. The origin of this deforming torsional force has not been clearly established. A large potential for axial and torsional growth exists in the cartilage columns of the growth plates. Growth plates yield to forces rapidly by either increasing or decreasing their rate of growth, whereas mature bone responds to changes in forces through bone deposition or resorption. Therefore, remodeling of mature bone is much slower. This is not present in all dogs with medial patellar luxation.

Deformities of the distal femur and proximal tibia are commonly associated with medial patellar luxation. These deformities displace the trochlea from the line of contraction of the quadriceps muscle group and predispose to luxation. Contraction of the quadriceps muscle group, acting in a straight line with the tibial tuberosity, tends to displace the patella medially. The malalignment of the extensor mechanism contributes to the development of further skeletal deformities following the Wolf's Law of bone remodeling. Axial compression on a physis retards normal growth, while tensile forces on a physis accelerate growth. Mild forces originating from postural abnormalities, gravitational forces, or muscle forces are sufficient to affect the physis. The medial malalignment of the quadriceps muscles seen with medial patellar luxation produces enough pressure on the medial aspect of the distal femoral physis to retard growth medially. Concurrently, there is less pressure on the lateral aspect of the physis, which accelerates growth laterally. Decreased length of the medial cortex relative to the increased length of the lateral cortex results in lateral bowing of the distal third of the femur. Medial bowing of the proximal tibia is produced by compression of the lateral aspect of the proximal tibial physis (retarding growth laterally) and distractive forces on the medial aspect of the physis (accelerating growth medially).

Femoropatellar instability is a common cause of lameness in dogs. The condition varies from mild instability without associated clinical signs to complete, irreducible luxation of the patella and severe lameness. The luxation may be intermittent, lateral or medial, traumatic or developmental. The most common diagnosis is congenital or developmental medial patellar luxation in small dogs. Medial patellar luxation is diagnosed relatively frequently in large dogs. Lateral patellar luxation in small dogs is rare and is usually congenital. Lateral patellar luxation in large or giant breed dogs is often a distinct syndrome associated with severe limb deformities and carries a much more guarded prognosis. Cats also develop patellar luxation - less commonly than dogs, and usually medial.

Careful physical examination is necessary to characterize the femoropatellar instability and to rule out cranial cruciate ligament rupture. The animal is observed to evaluate overall conformation and gait. Examine the stifle joint with the animal standing, for comparison with the contralateral stifle and to judge the influence of quadriceps muscle contraction. One hand is placed over the patella, and the other hand used to pick up the tibia and place the joint through a range of motion. Patellar tracking is evaluated to determine crepitance or pain. Spontaneous luxation is easily detected as a snapping or popping sensation. With the joint in extension, the patella is isolated between the thumb and index finger and pushed medially and laterally. In a normal joint, the patella may subluxate slightly but does not luxate. Tracing the tendon from the tibial tuberosity to the patella is an easy way to find the patella in puppies and toy breeds. Radiographs can be used to diagnose patellar luxation, and to determine the extent of bony deformity and degenerative joint disease. Patella position is often difficult to assess on radiographs. Skyline views of the distal femur are useful both pre- and postoperatively to evaluate the depth and contour of the femoral trochlea.

Grade I medial patellar luxation without clinical signs can be managed conservatively; if lameness develops the dog may be reevaluated. Grade IV medial patellar luxation is corrected surgically early in life to prevent severe bony deformity and lameness. The gray zones involve grades II and Ill medial patellar luxation in small dogs with intermittent lameness. These dogs generally have mild degenerative joint disease, and it does not markedly progress. In this situation, routine surgical correction cannot be recommended. If, however, lameness is frequent and of concern to the owner, if there is severe internal rotation to the tibia, and if the cranial cruciate ligament is torn, surgical correction should be undertaken.

Surgical correction of patellar luxation involves realignment of the extensor mechanism and stabilization of the patella in the femoral trochlea. The goal of surgical procedures associated with the femoral trochlea is to deepen the trochlear groove so that at least 50% of the patella is seated in the trochlear groove. The simplest method for accomplishing this is removing the articular surface and some subchondral until you have achieved the necessary depth. This defect will fill in with fibrocartilage and has produced acceptable clinical results in dogs. Preservation of the majority of the articular cartilage of the trochlear groove can be accomplished by recession tracheloplasty. A V-shaped wedge or block of the trochlear groove is removed with a small saw. Additional subchondral bone is removed with a saw, curette or rongeurs, and the articular wedge is replaced into the defect. Pressure from the patella and friction generated between the wedge or block and the subchondral bone negate the need for internal fixation. In young animals (<6 months old), the cartilage can be separated from the subchondral bone by gentle elevation with a periosteal elevator. The flap is replaced after sufficient subchondral bone is removed.

The tibial tuberosity is elevated and reattached to the proximal tibial in a position which is aligned with the femoral trochlea and extensor mechanism. Pin and tension band fixation is used to stabilize the tuberosity in its new position. This procedure can be used for both medial and lateral luxations. The quadriceps muscles can be released from the cranial portion of the femur to better align the extensor mechanism. An additional procedure has been described advocating transposition of the rectus femoris muscle from its origin on the pelvis to the trochanteric crest of the femur. It is believed that this helps to better align the quadriceps muscle group as all other muscles in the group originate on the femur; however, it does require a separate incision and the dog must be positioned in lateral recumbency on the surgery table.

Correction of deformities of the tibia and femur are an important component of the surgical treatment of medial patellar luxation. These corrections should be done in young animals with remodeling potential, if possible. In older animals, the entire limb has developed abnormally, with permanent bony and ligamentous abnormalities. Simply rotating the tibia medially or laterally does not correct these problems. Osteotomies are complex surgical procedures, but if performed by surgeon with experience can lead to excellent outcomes. Another strategy for correction of the tibial rotation is to use a heavy non-absorbable suture material from the fabella to the tibial tuberosity or distal patella. This technique is especially advantageous in dogs with combined medial patellar luxation and cranial cruciate ligament rupture, as it also reduces craniocaudal laxity. Osteotomies such as TPLO and TTA can also be used for treating medial patellar luxation and cranial cruciate ligament rupture. These techniques provide options for larger dogs with the combination of these two problems. Prognosis is highly dependent on the age of the animal at the time of treatment, the severity of bony changes that have developed, the severity of articular cartilage lesions and the severity of degenerative joint disease that has developed.