PATELLAR LUXATION

What is Patellar Luxation?

The patella, or kneecap, is part of the stifle joint (knee). In patellar luxation, the kneecap luxates, or pops out of place, either in a medial or lateral position.
Bilateral involvement is most common, but unilateral is not uncommon. Animals can be affected by the time they are 8 weeks of age. The most notable finding is a knock-knee (genu valgum) stance. The patella is usually reducible, and laxity of the medial collateral ligament may be evident. The medial retinacular tissues of the stifle joint are often thickened, and the foot can be seen to twist laterally as weight is placed on the limb.

Patellar Luxation Categories

Patellar luxations fall into several categories:
1. Medial luxation; toy, miniature, and large breeds
2. Lateral luxation; toy and miniature breeds
3. Lateral luxation; large and giant breeds.
4. Luxation resulting from trauma; various breeds, of no importance to the certification process.
Numbers 1, 2 and 3 are either known to be heritable or strongly suspected.

Medial Luxation in Toy, Miniature Breeds

Although the luxation may not be present at birth, the anatomical deformities that cause these luxations are present at that time and are responsible for subsequent recurrent patellar luxation. Patellar luxation should be considered an inherited disease.

Clinical Signs

Three classes of patients are identifiable:
1. Neonates and older puppies often show clinical signs of abnormal hind-leg carriage and function from the time they start walking; these present grades 3 and 4 generally.
2. Young to mature animals with grade 2 to 3 luxations usually have exhibited abnormal or intermittently abnormal gaits all their lives but are presented when the problem symptomatically worsens.
3. Older animals with grade 1 and 2 luxations may exhibit sudden signs of lameness because of further breakdown of soft tissues as result of minor trauma or because of worsening of degenerative joint disease pain.

Signs vary dramatically with the degree of luxation. In grades 1 and 2, lameness is evident only when the patella is in the luxated position. The leg is carried with the stifle joint flexed but may be touched to the ground every third or fourth step at fast gaits. Grade 3 and 4 animals exhibit a crouching, bowlegged stance (genu varum) with the feet turned inward and with most of the weight transferred to the front legs.

Permanent luxation renders the quadriceps ineffective in extending the stifle. Extension of the stifle will allow reduction of the luxation in grades 1 and 2. Pain is present in some cases, especially when chondromalacia of the patella and femoral condyle is present. Most animals; however, seem to show little irritation upon palpation.

Lateral Luxation in Toy and Miniature Breeds

Lateral luxation in small breeds is most often seen late in the animal’s life, from 5 to 8 years of age. The heritability is unknown. Skeletal abnormalities are relatively minor in this syndrome, which seems to represent a breakdown in soft tissue in response to, as yet, obscure skeletal derangement. Thus, most lateral luxations are grades 1 and 2, and the bony changes are similar, but opposite, to those described for medial luxation. The dog has more functional disability with lateral luxation than with medial luxation.

Clinical Signs

In mature animals, signs may develop rapidly and may be associated with minor trauma or strenuous activity. A knock-knee or genu valgum stance, sometimes described as seal-like, is characteristic.
Sudden bilateral luxation may render the animal unable to stand and so simulate neurological disease. Physical examination is as described for medial luxation.

Diagnosing Patellar Luxation

Examination and Certification

The dog is examined awake (chemical restraint is not recommended) and classified by the attending veterinarian according to the application and general information instructions. The veterinarian then completes the application form indicating the the results of the dog’s patella evaluation.

The application and fee can then be mailed to OFA. The attending veterinarian and owner is encouraged to submit all evaluations, whether normal or abnormal, for the purpose of completeness of data. There is no OFA fee for entering an abnormal evaluation of the patella in the data bank.

A breed database number will be issued to all dogs found to be normal at 12 months of age or older. The breed database number will contain the age at evaluation and it is recommended that dogs be periodically reexamined as some luxations will not be evident until later in life.

Preliminary Evaluations

Evaluation of dogs under 12 months of age is encouraged if the owner desires to breed at this age. The most opportune time to gather breeding data is at 6-8 weeks of age prior to the puppy’s release to the new owner.

Grades of Patellar Luxation

The Patellar Luxation Database is for dogs 12 months and over. Examinations performed on dogs less than 12 months will be treated as Consultations and no OFA breed numbers will be assigned.
A method of classifying the degree of luxation and bony deformity is useful for diagnosis, and can be applied to either medial or lateral luxations by reversing the medial-lateral directional references. The position of the patella can easily be palpated starting at the tibial tubercle and working proximal along the patellar ligament to the patella.

Grade 1

Manually the patella easily luxates at full extension of the stifle joint, but returns to the trochlea when released. No crepitation is apparent. The medial, or very occasionally, lateral deviation of the tibial crest (with lateral luxation of the patella) is only minimal, and there is very slight rotation of the tibia. Flexion and extension of the stifle is in a straight line with no abduction of the hock.

Grade 2

There is frequent patellar luxation, which, in some cases, becomes more or less permanent. The limb is sometimes carried, although weight bearing routinely occurs with the stifle remaining slightly flexed. Especially under anesthesia it is often possible to reduce the luxation by manually turning the tibia laterally, but the patella reluxates with ease when manual tension of the joint is released. As much as 30 degrees of medial tibial torsion and a slight medial deviation of the tibial crest may exist. When the patella is resting medially the hock is slightly abducted. If the condition is bilateral, more weight is thrown onto the forelimbs.

Many dogs with this grade live with the condition reasonably well for many years, but the constant luxation of the patella over the medial trochlear ridge of the trochlea causes erosion of the articulating surface of the patella and also the proximal area of the medial lip. This results in crepitation becoming apparent when the patella is luxated manually.

Grade 3

The patella is permanently luxated with torsion of the tibia and deviation of the tibial crest of between 30 degrees and 50 degrees from the cranial/caudal plane. Although the luxation is not intermittent, many animals use the limb with the stifle held in a semi flexed position. The trochlea is very shallow or even flattened.

Grade 4

The tibia is medially twisted and the tibial crest may show further deviation medially with the result that it lies 50 degrees to 90 degrees from the cranial/caudal plane. The patella is permanently luxated. The patella lies just above the medial condyle and a space can be palpated between the patellar ligament and the distal end of the femur. The trochlea is absent or even convex. The limb is carried, or the animal moves in a crouched position, with the limb flexed.

Medial and lateral patellar luxation

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It is much more common for the kneecap to ride on the inside than on the outside surface of the knee. This condition typically affects small and miniature breeds such as the poodle, Pomeranian, Chihuahua, Schipperke, Bichon Frise, and pug. It is also seen in the cocker spaniel, golden retriever, Labrador retriever, and mixed breeds.

1. Patella
2. Femur
3. Patellar ligament
4. Tibial Tuberosity
5. Medial Luxation of Patella
6. Lateral Luxation of Patella

Outside dislocation of the kneecap, or lateral patellar luxation, occurs more often in large breeds such as the Great Dane, Saint Bernard, and Bernese mountain dog. Lateral luxation is often accompanied by malformation of the femur, the large bone in the thigh. In these cases the prognosis is more uncertain, as major limb reconstructive surgery may be needed.
Generally, the prognosis is better when corrective surgery is performed early in the course of the disease; many of the malformations in the bones and joint occur over time and can be prevented with early correction.

Clinical Signs

Lameness that is often intermittent, and may be unilateral or bilateral; thick, swollen stifles; pain on range-of-motion; crepitus; palpable luxation; inability to jump or walk normally; medial displacement of quadriceps muscle group; lateral bowing of the distal third of the femur.

Symptoms

Intermittent or consistent lameness; bowlegged stance; reluctance to walk or jump; occasionally holding a rear leg out to the side when walking.

Description

Medial patellar luxation, or MPL, is a very common disease of small and miniature breeds in which the kneecap occasionally rides on the inside of its normal groove. Primarily congenital, although occasionally acquired through trauma, MPL causes lameness in one or both rear limbs. The degree of lameness is determined by the severity and duration of the disease, as well as the extent of existing arthritis.
Patellar luxation is graded on a scale from I to IV, with IV being the most severe. The disease can progress from the less severe to more severe grades over time. The more severe forms are often accompanied by malformation of the femur and tibia, as well as varying amounts of arthritis.
Dogs are frequently presented to a veterinarian for intermittent lameness, often because it is becoming more frequent or severe. When the patella, or kneecap, pops out of its normal trochlear groove, the dog feels pain, and owners may report a hitch in the gait. The dog will frequently extend the knee out from the body in order to get the patella to pop back in to the trochlear groove. As MPL progresses, the structures that hold the patella in place become looser, and thus the problem becomes more frequent. This dislocation causes pain, and as the frequency increases, so does the lameness.
Unfortunately, many of the severe Grade III or IV cases go unnoticed for months or years because the affected animals are usually miniature breeds that are often carried much of the time by their owners. Their inability to jump or straighten out their hind legs may go unnoticed because of their small size and sedentary lifestyle.
Lateral patellar luxation, or LPL, is less common than MPL and occurs when the kneecap occasionally rides on the outside of its normal groove. It, too, can be congenital or acquired, with the congenital form again being more common. While it can occur in any dog, it is more common in large and giant breeds. LPL is frequently accompanied by malformation of the femur and/or tibia. The disease can produce marked lameness and progress to crippling arthritis. Because of the accompanying bony malformations, extensive surgery may be required to correct this problem.

Diagnosis

The examining veterinarian will often make a diagnosis from a physical examination and history. However, x-rays are needed to determine the degree of arthritis, and evaluate for any malformation of the femur and tibia, the two major bones in the leg, which are joined together at the knee.

Prognosis

The prognosis for a Grade I patellar luxation is very good. These dogs may not need surgery. However close observation for signs of worsening is important. If surgery is indicated and performed early on, most animals regain normal functionality.
The prognosis for Grades II and III depends on how much arthritis and malformation have occurred. If caught and treated early, both have a good to excellent prognosis. If there is significant bony malformation or arthritis, the prognosis is guarded to fair.
The prognosis for Grade IV patellar luxation is guarded. Most of these animals have moderate to severe bony malformations and significant arthritis. If correction is performed, it is important to initiate early physical therapy to help restore function.

Transmission or Cause

The congenital condition is probably genetic in nature, and as such, affected animals should not be bred. Trauma or injury can also cause patella luxation.

Treatment

Treatment involves replacing the kneecap into the groove, and preventing it from popping in and out. The following procedures can be used alone or in combination as necessary to maintain the proper function of the knee.

Imbrication

Tightening the joint capsule, known as imbrication, is done on the opposite side of the luxation to prevent the kneecap from having enough slack to pop out of the trochlear groove. Thus a medial patellar luxation is treated with a lateral imbrication, and vice-versa. Additionally, the joint capsule can be loosened on the side of the luxation; this is called a release incision. This procedure relieves the tension that the joint capsule is placing on the patella, thus allowing it to ride in the trochlea.
In severe cases a synthetic suture is sometimes necessary to keep the kneecap in place. This suture is placed on the side opposite the luxation, and goes from behind the femur to the patellar tendon. It also prevents the kneecap from popping over to the other side.

Trochleoplasty

Deepening of the trochlear groove, or trochleoplasty, can be accomplished with a variety of techniques. A chondroplasty technique involves cutting out a taco-shaped wedge of cartilage, removing a small portion of bone beneath it, and then replacing the cartilage. The result is a deeper groove. This procedure can only be performed on very young dogs, because their cartilage is thicker.
Trochlear recession involves cutting out the cartilage and bone in such a way as to create a deeper trough. This trough will then fill in with scar tissue over time. Because this scar tissue is not as good as cartilage for joint function, this technique has given way to others that attempt to preserve normal cartilage. It can, however, be useful in carefully selected cases.
Wedge recession creates a taco-shaped piece of cartilage and underlying bone. Then, the bone below the wedge is removed and the wedge is replaced, forming a deeper groove. Block recession is identical in principle to wedge recession, except that a rectangular piece of cartilage and bone, rather than a wedge, is removed.

Tibial Tuberosity Transposition

The kneecap attaches to the lower leg via its patellar tendon at a bony site called the tibial tuberosity. Many times this site forms abnormally on the inside, as with MPL, or on the outside, as with LPL. In this procedure, the surgeon moves the tibial tuberosity back into proper alignment and secures it in place with a pin or wire. Realigning the joint, kneecap, and tendon prevents dislocation from reoccurring.

Osteotomy

In severe cases, with malformation of the tibia or femur, corrective bone cuts known as osteotomies may be required.

Prevention:

Early detection and correction is the best way to prevent severe lameness and dysfunction. Breeding affected animals should be discouraged; however, the disease is so prevalent in some breeds that this may not be practical.

Links:

http://www.offa.org/patluxgeninfo.html

http://www.workingdogs.com/vcpatlux.htm

http://www.dpklubben.se/PDF/patella.pdf

http://jalostus.kennelliitto.fi/frmTerveys.aspx?R=185

http://www.kisallatortopedia.hu/diszplazia-szures/terdizulet-patellaficam