Patella Luxation in the Dog

Patella luxation is a condition affecting the dog that I am seeing more and more frequently due to the current popularity of certain small breed dogs. 

Overview

The Patella (otherwise known as the kneecap) usually runs in a groove surrounded by 2 ridges at the end of the femur (thigh bone).  It is attached to the quadriceps muscle at the top end; and the patella ligament at the bottom end - which attaches onto a bony prominence (tibial tuberosity) at the top of the tibia (shin bone).  When the quadriceps muscle contracts it pulls the patella up the groove and consequently pulls on the tibia (via the patella ligament) to straighten (extend the lower leg).  When the quadriceps muscle relaxes the patella then slides back down the groove and the lower leg can flex.  This system is lubricated with joint fluid and lined with cartilage so that the patella can glide up and down with ease. In addition 2 flat ligaments either side of the patella help to keep the patella stable within the groove.

Cause

Patella luxation basically means that the patella has moved out of the groove in the femur (see diagram).  It most commonly luxates to the inside (medial) aspect of the joint but can luxate to the outside (lateral).  Patella luxation can occur after a traumatic incident which damages the ligaments either side of the patella; however it is more commonly seen as a genetic birth defect in certain breeds.  Certain toy and miniature breed dogs such as Chihuahuas, miniature poodles, Pomeranians and small cross breeds (Bichon/Shih tzu etc) are more commonly affected with medial patella luxation.  It is much less common in large breed dogs however it can occur, especially in Labradors.    As the dog grows 3 problems can increase the chance of the patella jumping to the inside (medial). 

1. The groove in the femur is abnormally flattened so that the patella doesn’t sit deeply.
2. The lateral (outside) ligament becomes stretched allowing the patella to slide to the inside.
3. The tuberosity that the patella ligament attaches to doesn’t sit in the centre of the tibia but rather to the inside, which again pulls the patella medially.          

Clinical signs

In the beginning this is usually an intermittent problem.  Most owners may notice a yelp and their dog will hold the leg in a flexed position for a few strides then they seem to recover rapidly and runaround as if nothing has happened.  Sometimes all owners will notice will be an intermittent skip. What actually happens is that as the patella glides over to the inside it can cause brief acute pain.  Then when the patella is sitting on the inside, your dog is unable to straighten his leg until the quadriceps muscle relaxes and the patella can pop back into its correct place.  As the condition progresses the knee joint may become painful and arthritic as the cartilage covering the joint erodes away and the inside ridge flattens completely.  Usually both legs are affected to varying degrees.

Diagnosis

This is not a difficult disease to diagnose by your veterinary surgeon, as it is fairly easy to palpate the patella popping in and out of the groove when the joint is extended (straightened).  If however the patella is sitting permanently on the inside, then x-rays may be needed to determine the position of the patella. Patella luxation is graded into 4 categories:

• Grade 1 - Usually not painful.  The patella pops out of place intermittently and can be easily massaged back into place.
• Grade  2 - The patella can still be massaged easily into place but pops in and out more frequently often just when the joint is flexed. 
• Grade  3 - The patella is less stable and frequently pops out again when replaced. These dogs often have evidence of arthritis and can have persistent pain    
• Grade  4 - The patella is usually located on the inside of the joint and the dos has advanced arthritis in the joint.  If both knees are affected then the dog has a crouching stance and will have difficulty walking.

Treatment

For dogs with grade 1 disease showing no signs of discomfort, no treatment may be necessary.  Weight loss if overweight, will help to reduce weight on the knees and controlled exercise helps to strengthen the quadriceps and help to stabilise the patella.  Grade 2 dogs need to be judged on an individual basis depending on how the dogs are coping and if it is progressing. Grade 3 and 4 dogs definitely will benefit from surgically correction.

Surgical options

Four techniques are commonly performed to stabilise the patella.  These are:

Trochleoplasty

This basically involves removing a v shaped wedge of cartilage from the groove, removing under lying bone and then replacing the cartilage wedge to make the groove deeper so that the patella sits deeper.

Tibial Crest Transposition

This involves moving the patella ligament where it attaches to the top of the tibia to a more central location (rather than attaching more to the inside).

Lateral patella ligament imbrication

This involves tightening the ligament on the outside of the patella to pull the patella over.

Medial ligament releasing incision

This involves incising the medial patella ligament if it has tightened and is pulling the patella over to the inside.

Your vet may perform one or all of these techniques to help to correct the luxating patella.  Aftercare involves strict rest initially followed by controlled exercise.

Prevention

As this is a genetic disease then any affected dogs should not be bred from and the breeder should be informed.