Patella luxation: Diagnosis and surgical decision making (Proceedings)

Patella luxation is a common orthopedic condition in dogs. It may cause varying degrees of lameness, pain, and progression of osteoarthritis. It is most commonly diagnosed in small breed dogs, however large breed dogs are also affected. Both large and small breed dogs more commonly exhibit medial luxation (MPL) than lateral. Lateral patella luxation (LPL) is rare in small breed dogs, but is seen more frequently in large breed dogs.

Etiology and Pathophysiology

Patella luxation in dogs is rarely the result of trauma. Most commonly it is due to congenital conformational abnormalities, resulting in malalignment of the stifle extensor mechanism. The extensor mechanism consists of the quadriceps muscle group, the patella, and the patellar ligament. The muscles of the quadriceps originate from the proximal femur and the caudal ilium, just cranial to the acetabulum. Extension of the stifle begins with quadriceps contraction. This force is transmitted to the patella, which articulates with the distal femur, and transmits the force to the patellar ligament. The patellar ligament inserts on the tibial tuberosity. During quadriceps contranction, the extensor mechanism must be aligned with the trochlear groove of the distal femur, or patella luxation results.

Malalignment of the extensor mechanism and the trochlear groove may be the result of a variety of skeletal abnormalities, from the hip to the proximal tibia. These include coxa vara/valga, femoral varus/valgus, tibial torsion, and medialization of the tibial tuberosity. The most clinically relevant abnormalities are femoral varus/valgus, and medialization of the tibial tuberosity.

In addition to extensor mechanism malalignment, dogs with patella luxation may exhibit a shallow trochlear groove, erosion of the medial trochlear ridge, varying degrees of degenerative joint disease and periarticular fibrous tissue proliferation. Most of these changes are probably secondary to extensor malalignment.

Presentation and Physical Exam

Patella luxation is most commonly diagnosed in young to middle aged dogs. They may present for lameness evaluation or luxation may be an incidental finding on physical examination. Physical examination is best initiated with the patient standing. Both left and right stifles can be palpated simultaneously with the dog in a more relaxed position than in lateral recumbency. The patella can be palpated between the thumb and index finger, and its location relative to the femoral condyles and trochlear groove evaluated. If the patella cannot be palpated immediately, it can be located by following the patellar ligament proximally from its distal attachment on the tibial tuberosity.

Patella Luxation is Graded on a scale of I-IV

Grade I: The patella can be luxated with manual pressure, but immediately reduces when pressure is released.

Grade II: The patella can be luxated with manual pressure, and spontaneously luxates during ambulation. It easily reduces by extending the stifle or by manual pressure, and resides in the trochlear groove a majority of the time.

Grade III: The patella resides outside of the trochlear groove a majority of the time. It can be reduced by manual pressure.

Grade IV: The patella resides outside of the trochlear groove continually, and cannot be reduced.

If the patella is found to be luxated, attempt to reduce it by extending the stifle and moving the patella medially or laterally with the thumb or index finger. If the patella is found to be reduced, its stability is best evaluated by extending the stifle and attempting to force the patella medially or laterally. With the patella in its most mediolaterally displaced position, the stifle can now be flexed. The normal patella will track back into the trochlear groove, while a luxating patella will remain deviated. In addition, internally or externally rotating the tibia can aid in luxating or reducing the patella. For example, internal rotation of the tibia facilitates medial patella luxation, while external rotation facilitates reduction of a medial luxation.

The degree of patella luxation is associated with differing degrees of lameness. Dogs with a grade I luxation most often are asymptomatic. Dogs with grade II luxation may be asymptomatic, or may present with a complaint of an intermittent "skipping" lameness. Owners often report that these episodes resolve after the dog "stretches his leg out behind him". Dogs with grade III and IV luxation more typically present with chronic lameness. It may be worse after exercise or inactivity, but unlike grade II dogs, the lameness does not resolve. These dogs often appear bowlegged and may seem to walk in a crouched position, due to their inability to extend the stifle completely.

A complete orthopedic examination is essential in these patients, as concurrent disease may be present. In particular, young small breed dogs with MPL and hindlimb lameness may also be affected by Legg-Perthes disease- aseptic necrosis of the femoral head. Older dogs with acute onset of lameness and patella luxation should be closely examined for ruptured cranial cruciate ligament.

Radiographic Findings

Standard lateral and cranial-caudal radiographs of the stifle should be taken, to rule out concurrent orthopedic conditions and evaluate conformation. A luxated patella may be visible on radiographs, however grade I and II luxations are intermittent and the patella may be reduced at the time of radiography. Varying degrees of joint effusion and degenerative joint disease may also be seen. A V-D pelvic radiograph, including the stifles and proximal tibiae can help to evaluate femoral conformation. With a straight film, femoral varus/valgus can be assessed. However, findings on this view can be misleading, especially if the hindlimbs are internally/externally rotated or adducted/abducted. Caution should be used when diagnosing varus/valgus or torsional abnormalities, and multiple radiographs may be necessary.

Surgical Treatment

The decision to surgically correct patella luxation is based primarily on clinical signs. There is no evidence that operating asymptomatic dogs to prevent progression of osteoarthritis results in a superior outcome over waiting to operate dogs once they exhibit clinical signs. If a dog shows persistent (greater than a few weeks) or recurrent lameness (greater than a few episodes), even if intermittent, surgery should be considered.

In cases with significant varus/valgus deformities of the femur, closing wedge osteotomies may be necessary to correct malalignment of the extensor mechanism. However, the majority of dogs can be corrected by deepening the trochlear groove, transposing the tibial tuberosity, and resection/release of medial/lateral fascia. A standard craniolateral approach to the stifle joint is made. The joint is evaluated first for other abnormalities, such as cruciate ligament rupture, meniscal injury, or OCD lesion.

Next, the depth of the trochlear groove is evaluated. A common rule of thumb is that the groove depth should equal about ½ the thickness of the patella. This can be difficult to gauge in surgery. The groove should be wide enough to accommodate the entire patella width. The trochlear groove can be deepened by a variety of techniques- trochlear wedge recession, abrasion trochleoplasty, trochlear block recession, and trochlear chondroplasty. The easiest technique that preserves some hyaline cartilage in the groove is trochlear wedge recession. Using a hobby saw, a "V" shaped wedge of articular cartilage and subchondral bone is removed form the trochlear groove. An additional cut is made parallel to either the medial or lateral cuts, removing a thin piece of bone and cartilage. The wedge is replaced in the groove, and is now recessed due to removal of this piece. The point of the wedge may need to be modified to allow the wedge to sit firmly in the groove. The most common error in performing this technique is failure to deepen the proximal extent of the groove, which allows the patella to luxate when the stifle is in extension.

After deepening the trochlear groove, alignment of the stifle extensor mechanism is assessed. Correcting alignment abnormalities is integral to the success of the surgery. While standing at the foot of the dog, with the dog in dorsal recumbency, flex the stifle and hock. Palpate the quadriceps muscle group, the patella, tibial tuberosity, and the hock. Essentially, these structures should be in a straight line.

To correct tibial tuberosity position, a tibial tuberosity transposition is performed. After elevating the cranioproximal portion of the cranial tibial muscle, the proximal aspect of the tibial tuberosity is cut using an osteotome or bone cutter. It is important to cut a large enough piece of the tuberosity to allow secure purchase of implants after transposition. A small piece can be difficult to secure, and is prone to splitting. The periosteum and soft tissues should be left intact at the distal aspect of the osteotomy. This will add stability to the fragement. If the periosteum is cut, be sure that the fragment does not move proximally (creating a patella alta) before stabilizing it. The fragment is transposed laterally to correct for an MPL. A releasing incision in the medial joint capsule and fascia will be necessary to allow transposition of the fragment and associated patella. A flat recipient site is created lateral to the donor site by using a rasp or scalpel blade in small dogs. In addition a shelf or 'dovetail' can be made to accept the tuberosity fragment.

Once the ideal position of the tuberosity is determined, the fragment is secured in place using pin and tension band technique. For very small dogs, 0.045" K-wires can be used without tension band. For dogs 20-40# in weight, 0.062" K-wires can be used with a tension band. For larger dogs, small IM pins + tension band is indicated. When placing the K-wires, be sure to avoid the joint and measure K-wires for appropriate length. Orthopedic wire (18-20ga) is used for the tension band. When placing the tension band, drill the hole for the orthopedic wire distal to the distal end of the osteotomy.

Once the wedge recession and tuberosity transposition have been performed, recheck alignment and evaluated stability of the patella. Because soft tissues have not been closed, the patella can still be luxated with direct pressure. However, the patella should not luxate with internal/external rotation of the tibia and flexion/extension of the stifle. In the case of MPL, redundant joint capsule and fascia on the lateral aspect of the joint is removed and the joint is closed. Once the lateral joint capsule is closed, the patella should no longer be able to luxate medially with direct pressure (however it can luxate laterally, due to the open medial joint capsule). Typically, the medial joint capsule is not closed entirely, as this will tend to pull the patella medially. A few interrupted sutures can be placed in the proximal aspect of the joint capsule, working distally until the patella can no longer be luxated.

Prognosis for dogs following surgical correction of MPL is very good to excellent. Multiple studies have graded outcomes of excellent or good in >90% of operated cases. The best outcomes and fewest complications seem to occur when techniques to address trochlear groove depth, tibial tuberosity position, and soft tissue redundancy all are performed together. The most common complications are implant failure and persistent luxation. However, only a minority of dogs with persistent luxation exhibit lameness on follow-up examination.