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Patellar Luxation

Understanding Your Pet's Medical Diagnosis

Patellar luxation is a dislocation of the kneecap (patella). The kneecap may dislocate toward the inside (medial) or outside (lateral) of the leg, or may move in both directions. It may result from injury or congenital (present at birth) deformities. Both legs may be affected.

What is a patellar luxation?

The normal position of the patella (kneecap) is in a deep groove located in the front lower aspect of the thigh bone (femur). A luxated patella is displaced out of its normal position. It may luxate (come out of the "joint") to the inside (medial) or outside (lateral) of the leg. Some pets are prone to a patellar luxation because they have a shallow groove, or no groove at all, where the kneecap normally sits. This variation in anatomy allows the kneecap to "slip out" with movement. The knee also may have a tendency to twist, causing the patella to luxate, due to abnormal positioning of the muscles supporting the knee joint. Patellar luxations are graded from mild (grade I) to severe (grade IV). A pet with a grade I patellar luxation has a kneecap that is loose but tends to stay in its normal position. A pet with a grade IV luxation has a kneecap that is permanently out of its normal position. Patellar luxations are found in both dogs and cats.

What causes a patellar luxation ?

Animals may be born with a loose kneecap, or develop the problem as their bones grow. Both of these scenarios are considered hereditary because the genes that cause patella luxation are passed from the affected animal's parents. Occasionally, trauma to the knee area can cause a patellar luxation.

What are Signs of patellar luxation ?

Clinical signs related to patellar luxation depend on the grade (severity) of luxation, amount of arthritis present, how long the disease has been left untreated, and the presence of other knee joint abnormalities (e.g., cruciate ligament rupture). Bilateral (both sides) knee involvement is found in about half of dogs with this disorder.

Common signs to look for include:

The affected leg is intermittently carried with the knee joint flexed
Persistent, abnormal hind leg carriage and function in neonates and puppies
Occasional "skipping" hind leg lameness
Sudden signs of lameness following minor trauma
Crouching, bowlegged or knock-kneed stance
There may be no signs, and the diagnosis may occur during routine physical examination by you veterinarian
Worsening over time

How is a patellar luxation diagnosed ?

The diagnosis of patellar luxation is based on your observations at home, your veterinarian's physical examination, and radiographs (X-rays). Most dogs with patellar luxation have a "medial luxtation" (a kneecap that is displaced to the inside).

The crippling effects of patellar luxation are related to the severity and duration of the luxation. The milder forms, especially in small breeds, show little or no signs, and only minimal treatment is required. Severe cases cause more intense pain, with limping.

How is patellar luxation treated ?

Dogs with mild luxation (grade I) are often treated medically. Surgical stabilization of the kneecap is the treatment of choice for dogs with grades II to IV luxation. There are many procedures that your veterinarian can choose from. Multiple procedures may be required to correct the abnormalities. (They can usually be done during the same surgery.)

Two common procedures are:

Trochleoplasty: the groove in which the kneecap rides is deepened.
Tibial tubercle transposition: A small piece of bone is detached from the lower leg bone (tibia) and moved to help straighten the leg muscles around the knee joint

Forms of Surgical Correction of Medial Patellar Luxation

The type of surgery done depends on the type of problem present. Often the appropriate surgery can only be determined during the surgical proceedure. The simplist proceedure involves placing a suture of a synthetic material around the febella (a small bone behind the knee joint) that passes around the patella. This can prevent the knee cap from being able to luxate. In addition the tissues adjacent to the knee joint are tightened in a way that increases the strength of the tissues holding the knee cap in place.However, if allignment is not good there will still be stresses applied to the knee cap that may lead to the development of arthritis later in life, even if the knee cap does not slip out of place. Also, with time the synthetic suture may break.

To prevent these stresses from occuring we try to deepen the "groove" that the patella slides in without damaging the smooth cartilage that the patella slides on. My usual proceedure is called a recession sulcoplasty. In this proceedure a wedge of bone including the cartilage is removed, another small piece of bone is removed to deepen the site that the bone was removed from, and the wedge of bone is returned to the site. Since the site has been deepened, the bone fits deeper into the site and the edges of the site act to prevent luxation of the patella.

If the allignment between the muscles of the thigh, the knee cap and the attachment of the tendon of the knee cap are not in a straight line there will still be stresses applied to the patella and the "groove" that lead to arthritis. In these cases we try to line things up by moving the attachment of the ligament of the knee cap as it attaches to the lower leg. This is called a transposition of the tibial insertion. A bone chisel is used to cut the bone free as it "inserts"onto the tibia. The soft tissues associated with this insertion are left intact. The bone is moved into allignment and then it is tack back in place with a small bone pin. When this is done you can usually feel the bone pin present under the skin of the shin just below the knee.

Usually there is a combination to these proceedures done. We will let you know which were needed to complete the repair of you pet when you pick it up after surgery. In general the fewest numbers of repairs necessary to provide good alignment and stability are done. Since there is not a lot of stress placed on the corrections it is not necessary to restrict exercise much unless there was also damage present to the other ligaments of the knee (the cruciate ligaments and the collateral ligaments).

What is the prognosis for animals with patellar luxation ?

More than 90% of the animals treated surgically will have improved joint function and leg use. Recurrence is possible following surgical correction, but it is generally less severe than the initial episode. Arthritis often develops in knee joints that have had patellar luxation for some time, and if arthritis is advanced, the prognosis will be less favorable.