By: Julie Mayer DVM
September/October 2011 Issue
A luxating patella occurs when the knee cap moves out of its natural position. The patella (knee cap) lies in a cartilaginous groove at the end of the femur at the stifle. The patella in dogs is shaped like an almond and its purpose is to assist in knee extension. The patella resides in the tendon of the quadriceps muscle group which attaches to the bone below the femur, the tibia. When this muscle group contracts, it pulls on the tendon and the knee cap, thereby extending the stifle. If the patella is pulled out of its normal groove with knee extension, this is called a luxating patella.
The causes of this condition can be congenital, genetic and/or traumatic. Breeds with a predisposition for luxating patellae are Miniature and Toy Poodles, Maltese, Jack Russell Terriers, Yorkshire Terriers, Pomeranians, Pekingese, Chihuahuas, Cavalier King Charles Spaniels, Papillons and Boston Terriers. Large breed dogs prone to this condition include Labrador Retrievers, Golden Retrievers, Akitas, Malamutes, Boxers, Huskies and St. Bernards. Apart from breed predilection, if a dog has poor conformation, such as no angulation in the hock, then this can also cause luxating patellae.
This condition is usually diagnosed early on. The initial symptoms include occasional limping, an intermittent skip in the gait, sudden loss of support on the limb, abnormal sitting posture with the knee placed outward; all of which are usually intermittent. Sometimes, chronic cases can lead to erosion of the cartilage on the femur from the constant friction, and eventually, to osteoarthritis. In this case, pain is usually involved and lameness is more constant and severe.
Occasionally, a luxating patella can lead to a ruptured cranial cruciate ligament. The literature states that at least 15% to 20% of dogs with patellar luxation will eventually rupture their cranial cruciate ligament. Two main reasons why this scenario may follow are:
- a luxating patella will change the biomechanics of the knee and subject the cranial cruciate ligament to more stress and strain, and
- if the luxating patella is chronic with arthritic changes, the inflamed environment inside the joint will cause a breakdown of the ligaments (especially cruciate ligaments).
A luxating patella is usually diagnosed by feel and is assigned a grade based on the severity of the condition. Grade 1 is the least severe and the knee cap easily slips back into place on its own whereas Grade 4 means the knee cap is actually stuck and fixed outside its normal resting position in the groove of the femur. A radiograph of the stifles can be performed to see if there is osteoarthritis present or any sign of cranial cruciate ligament damage.
Surgery is not always necessary for this condition. Many small dogs live their entire life with luxating patellae and it never results in arthritis or pain, nor does it interfere with the dog’s life. Grade 3 or 4 luxations normally require surgery as greater pain or discomfort will be involved, along with reduced function of the leg or associated damage such as a cranial cruciate ligament rupture. Every situation is different.
The surgical procedure usually involves carving out a deeper groove in the end of the femur so the patella will remain in the groove with move- ment. If a ruptured cranial cruciate ligament is present, it can be cor- rected at the same time.
If your dog suffers from this condition, you can’t change his DNA but you can help him with supportive nutrients and exercise.