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Luxating Patella
A Maltese runs across the yard chasing a tossed ball. In mid-stride, it yelps in pain and pulls its left hind leg off of the ground.
After a second, he continues limping on in three-legged fashion. After ten minutes, the rear leg drops back down to the ground
and he uses it normally. This episode occurs maybe once a week. It never really seems to bother him that much  a yip of pain,
a short period of lameness, and in a few minutes he is back to his old self. Typically, he is a small or toy breed such as a
Maltese.

A Luxating Patella may affect some animals much more severely, They may hold the leg up for several days and show
considerable discomfort. Dogs who have a luxating patella on both hind legs may change their entire posture, dropping their
hindquarters and holding the rear legs farther out from the body as they walk. Those most severely affected may not even use
their rear legs, walking by balancing themselves on their front legs like a circus act, holding their hindquarters completely off
the ground.

Normal knee anatomy

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.


What occurs when the patella is luxated?

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 its 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 yip 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 its activity.



What are the symptoms?

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.



What are the risks?

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.



Treatment for Luxating Patella

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. The knee cap itself may be "tied down" laterally (on the outside) to prevent
it from deviating medially (toward the inside). 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. 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 thirty days, using its legs in normal fashion.