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Dog: Patellar Luxation

General information

Other common/scientific names: kneecap dislocation, stifle dislocation

The stifle or knee in a dog is made up of two bones: the femur which is the thigh bone and the tibia which is the shin bone. The end of the femur has an indentation called the trochlear groove. The patella or kneecap sits in this groove. The quadriceps muscles of the thigh attach to the top of the patella which is then attached to the tibia by the patellar ligament. These structures, along with ligaments and menisci in the joint, help to support and stabilize the knee.

Patella luxation occurs when the patella slips out of the groove. These can range in severity to a luxation that only occurs when the knee is in extension and then readily snaps back in place. Or, the luxation can be permanent where the kneecap is never in its proper position.

The groove and the underneath side of the patella are lined with smooth cartilage. When the patella is in its normal position, its glides smoothly and painlessly along the surface of the trochlear groove. When the patella pops out of the groove, the cartilage on the patella rubs against bone causing the cartilage to deteriorate. Over time, this results in pain from bone on bone friction and arthritis. Chronic patellar luxation in young, growing dogs can also result in twisting of the femur and tibia causing further deformities in the hind limb conformation.

Most patellar luxations are toward the medial or inside of the leg. These occur most frequently in small or toy breed dogs including Yorkshire Terriers, Poodles, Boston Terriers and Chihuahuas. Lateral (to the outside) patellar luxation can occur but is more common in large breed dogs such as Great Danes, St. Bernards and Irish Wolfhounds.

Abb. GGDE4H7T
Abb. GGDE4H7T: This is an illustration of the knee or stifle of the dog.
The arrows on the patella indicate the direction of a luxation.

Causes

Patellar luxations are either inherited or caused by trauma. The inherited form can be seen at birth (congenital) or develop during the first year of life. The inherited form may result from an anatomical abnormality at the level of the hip resulting in misalignment of the thigh muscles attached to the patella. This pulls the patella out of the groove. Additionally, affected dogs may have too shallow of a trochlear groove which allows the patella to slip out of place.

Traumatic patellar luxation can occur at any age and is usually secondary to being hit by a car. Cranial cruciate ligament rupture commonly accompanies this type of injury.

Cardinal symptom

Lameness

Symptoms

Clinical signs of patellar luxation include intermittent lameness, skipping gait and unwillingness to jump. When the patella is luxated, the knee stays bent and cannot extend. Many times, the patella slides back into place and the leg is back to normal in a couple of steps. As arthritis develops, the knee joint stiffens and the lameness become constant. Congenital patellar luxation most commonly occurs in both legs.

Diagnosis

Diagnosis of patellar luxation is made by physical examination. Radiographs are not needed for diagnosis but are recommended to assess the conformation and the degree of arthritis. Patellar luxations are graded to assess severity:

  • Normal: No evidence of patella luxation.
  • Grade I: The anatomic position of the knee is normal. The patella can be manually moved out of place but returns back to position.
  • Grade II: The patella luxates when the leg is flexed but remains out of place until manually replaced into groove.
  • Grade III: The patella is permanently dislocated but can be manually replaced into groove.
  • Grade IV: The patella is permanently dislocated and cannot be manually replaced into groove.

The Orthopedic Foundation for Animals (OFA) was originally founded to standardize radiographic findings of dogs in regards to hip dyplasia and provide information to dog owners to assist them with the selection of good breeding animals. The OFA also provides evaluation and certification for other orthopedic and genetic diseases including patellar luxations. Dogs are examined at 12 months of age or older and graded according to the physical examination findings. Dogs classified as normal are given a certification number from the OFA and recognized by the American Kennel Club (AKC).

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Abb. GGDEDTXG: Medial Patellar Luxation.
This is a radiograph showing both patellae (arrows) dislocated to the inside of the knee. (A) femur (B) tibia (C) fibula.

Treatment

Mild cases of patellar luxation can be treated with exercise restriction and NSAIDs(non-steroidal anti-inflammatory). However, most cases should be treated with surgery to prevent arthritis. There are several surgical procedures available depending on the degree of luxation. A combination of these techniques is commonly used:

  • Deepening the trochlear groove.
  • Tightening the joint capsule along the patella.
  • Repositioning the tibial crest (a bony projection at the knee joint) to straighten the leg. Bone pins are used for this realignment.
  • Straightening the femur by removing bone. The cut bone is held together with bone plates and screws. This is more commonly used in large breed dogs.

Abb. GGDEGROI
Abb. GGDEGROI: Post Surgery.
This is a radiograph of a dog which has had the left patella (arrow) surgically replaced its groove. Note the right patella is still dislocated to the inside of the thigh.

Prognosis

If surgery is performed before arthritic changes occur, a good prognosis is likely. Severe patellar luxations with arthritis have a poorer prognosis.

Prevention

Prevention is aimed at breeding individuals that have OFA normal certification. Dogs with even mild (grade I) patellar luxation should not be used for breeding. Breed organizations and breed clubs should recognize patellar luxation and encourage OFA certification. Prospective buyers should check pedigrees. If no documentation is available, the dog should not be used for breeding.

Tips

For dogs undergoing surgery, it is very important that the dog does not lick or pull at the sutures. A special collar (E-collar or Elizabethan collar) is worn by the dog to prevent this. Incisions should be observed daily for swelling, discharge or redness.

CAUTION

If you notice your pup or dog is lame or carrying a leg for more than 3 days, contact your veterinarian. Early diagnosis of orthopedic problems is the key to successful treatment.

Update version: 4/24/2014, © Copyright by www.enpevet.de
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