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Luxating Patella
The patella is the bone we know as the knee cap. A groove in the end of the femur allows the patella to glide up and down when the knee joint is bent back and forth. In so doing, the patella guides the action of the quadriceps muscle in the lower leg. The patella also protects the knee joint.
Looking at the lower front portion of the femur (the thigh bone) in a normal dog, you will notice two bony ridges that form a fairly deep groove in which the patella is supposed to slide up and down. These structures limit the patella’s movement to one restricted place, and in so doing, control the activity of the quadriceps muscle.
The entire system is constantly lubricated by joint fluid. It works so that there is total freedom of motion between the structures.
In some dogs, because of malformation or trauma, the ridges forming the patellar groove are not prominent, and a too-shallow groove is created. In a dog with shallow grooves, the patella will luxate (jump out of the groove) sideways, especially toward the inside. This causes the leg to 'lock up' with the foot held off the ground.
When the patella luxates from the groove of the femur, it usually cannot return to its normal position until the quadriceps muscle relaxes and increases in length. This explains why the affected dog may be forced to hold his leg up for a few minutes or so after the initial incident. While the muscles are contracted and the patella is luxated from its correct position, the joint is held in the flexed or bent position. The dogs “yelp” is from the pain caused by the knee cap sliding across the bony ridges of the femur. Once out of position, the animal feels no discomfort and continues his activity.
Most dogs are middle-aged, with a history of intermittent (on-again-off-again) lameness in the affected rear leg(s). An affected dog commonly stops and cries out in pain as he is running. The affected leg will be extended rearward, and for a while the dog is unable to flex it back into the normal position.
Uncorrected, the patellar ridges will wear, the groove will become even shallower, and the dog will become progressively more lame. Arthritis will prematurely affect the joint, causing a permanently swollen knee with poor mobility. Therefore, a good evaluation needs to be done by your veterinarian early in the condition to prevent long-term arthritic crippling.
As would be expected, medical therapy has little corrective ability in this disorder and surgery is therefore required and is the treatment of choice. A surgical treatment is not necessary in every individual with this condition.
Surgery can alter both the affected structures and the movement of the patella. The groove at the base of the femur may be surgically deepened to better contain the knee cap. This procedure is known as a trochlear modification. The knee cap itself may be "tied down" laterally (on the outside) to prevent it from deviating medially (toward the inside). This is called a lateral imbrication. The bony protuberance at the site of the attachment of the quadriceps tendon on the tibia may be cut off and then re-attached in a more lateral position. This is termed a tibial crest transposition. All of these procedures work well and the type performed depends on the individual case and the clinician. The animal should respond quickly after surgery and is usually completely recovered within 30-60 days, depending upon the procedure. This time scale can be reduced by swimming regularly and physiotherapy.
Surgery has approximately a 90% success rate.
Success is defined as the return of good function of the limb Unfortunately surgery will not remove the arthritis that may already be present in the knee. As a result, your pet may have some stiffness of the limb in the mornings or after laying down for a nap. In addition, your pet may have some lameness after heavy exercise By having the surgery done earlier, the chance of developing significant arthritis is decreased Dogs that have a higher grade of patellar luxation may have increased risk for reluxation of the patella.