Definition:

Medial patella luxation is a displacement of the patella (knee cap) from the trochlear groove.

Cause:

Medial patella luxation is a common cause of lameness in small breed dogs, and is also seen in large dogs and cats. In most cases the cause is congenital (present at birth) but it can be acquired secondary to trauma.

The congenital form is most common in toy and miniature breeds of dogs such as the Miniature and Toy Poodle, Pomeranian, Chihuahua, Yorkshire Terrier and Pekingese and may occur simultaneously with other pelvic limb deformities.

Pathophysiology:

When patellar luxation is present early in life, the major muscle groups of the thigh pull toward the inside of the leg, putting abnormal pressure on the stifle (knee joint) cartilage. The result is a bowlegged stance and an abnormal pull on the patella. 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, and if not corrected

Clinical Signs (symptoms):

Your pet 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 causes an intermittent lameness or non-weightbearing. 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.

Grades:

There are four assigned grades based on the severity of the luxation. Grade 1 – “in-in” (in all the time, can be pushed out, but immediately returns to the trochlear groove) Grade 2 – “in-out” (in most of the time, dog can self-correct when it comes out, when pushed out the patella generally will remain out for a period of time, even with stifle joint flexion/extension) Grade 3 – “out-in” – the patella is luxated either all the time or most of the time, but can be pushed back into the trochlear groove manually Grade 4 – “out-out” – the patella is permanently out and cannot be pushed back in. These dogs will often have an obvious limb deformity) The prognosis for grade 1 to 3 is good, and is more guarded in grade 4.

Medical or Surgical management?

Dogs with grade 1 and asymptomatic Grade 2 patella luxations may just be monitored. They may, however, acutely worsen with no or mild trauma, causing persistent lameness or increased frequency of clinical signs.

Dogs with Symptomatic Grade 2 – if typical clinical signs are present (intermittent non-weightbearing or a persistent lameness), surgical correction is recommended

Dogs with Grade 3 and Grade 4 – early surgery is recommended to try to minimize the development of cartilage erosions, arthritis (degenerative joint disease) and cranial cruciate ligament rupture. Even if these dogs have open growth plates still, early surgical correction is essential, especially grade 4.

Patella luxation in cats: cats can be very debilitated even with grade 2 patella luxations and early surgical stabilization is recommended.

 

Surgical correction

consists of a stabilizing and realigning the patella. Stabilization is achieved by performing a trochleoplasty (deepening the groove that the patella lies in while preserving the articular cartilage), and joint capsule and fascial tightening. Realignment is achieved with a tibial tuberosity transposition (moving a portion of the attachment of the patella to realign the mechanical forces of the quadriceps muscle and reattaching the bone with small pins).

It is important that your pet have strict rest after surgery to allow proper healing. Usually full function is returned in 2 months. Potential complications from surgery include re-luxation and pin migration.