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Patella Luxation

Patella Luxation Update
Michael Weh, DVM, DACVS-SA

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

Etiology and Pathophysiologypatella1

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 contraction, 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.

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.

Radiographic Findings

Standard lateral and cranial-caudal radiographs of the stifle should be taken to rule out concurrent orthopedic conditions and evaluate conformation. A patella2luxated patella may be visible on
radiographs; however, grade I and II luxations are intermittent, and the patella may appear 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. For more complex deformities, computed tomography can be useful in diagnosing the underlying malformation.

Surgical Treatmentpatella3

The decision to surgically correct patella luxation is based primarily on 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.

The majority of dogs with patella luxation can be corrected by deepening the trochlear groove, transposing the tibial tuberosity, and resection/release of medial/lateral fascia. For cases with significant varus/valgus deformities of the femur, or rotational deformities of the femur or tibia, corrective osteotomies may be necessary to align the extensor mechanism.

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.

What’s New

Several studies in recent years have focused specifically on the role of excessive femoral varus in contributing to patella luxation. In certain patients, especially large breed dogs, evaluating distal femoral varus and correcting this when it is excessive helps improve outcome and decrease likelihood of persistent or recurrent luxation following surgery. In cases with complex angular limb deformities related to patella luxation, computed tomography can be particularly useful. The CT scan allows reconstruction of imaging data in multiple plans for ease of evaluating limb alignment as well as three dimensional reconstruction and modeling for pre-surgical planning. With the CT scanner in our Seattle hospital, we can take advantage of this technology to facilitate planning and improve outcomes.