A Luxating patella may affect some dogs much more severely than others, some may hold the affected leg up for several minutes others 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.

What is a luxating patella?

The problem outlined above is the luxation of the Patella (or knee cap).

This bone is encased within the tendon of the

Quadriceps muscle and slides over the distal end of  the femur. As this muscle group contracts, it causes the lower portion of the leg below the knee to move forward or straighten in relationship to the upper leg (Figure 1).

This is an integral part of walking or running – bringing the lower leg forward for the next step. The knee cap slides back and forth on the lower femur, thereby maintaining the relative position of the quadriceps in relationship to the tibia (the major long bone immediately below the knee joint).

The tendon of the quadriceps attaches to this bone at the uppermost part of its length.

Looking at the lower front portion of the femur in a normal dog, you will notice two bony ridges that form a fairly deep groove in which the patella is suppose to slide up and down (Figure 2).

These structures limit the patella’s movement to one restricted place and, in so doing, control the activity of the quadriceps upon the tibia.

In older dogs these may, along with the other structures within this joint, develop roughened arthritic spurs. However, in younger or non-arthritic animals, these ridges and the bony surfaces between them are perfectly smooth. The backside of the knee cap that articulates with these joint areas has a surface that is just as smooth.

The entire system is constantly lubricated by joint fluid.
This enables total freedom of motion between the structures. In the dogs described above, lameness is caused by the patella luxating (moving out of position) during the normal flexing and straightening of the knee joint. When this occurs, the kneecap moves to the medial side (or inner surface) of the joint. It rides over the ridge on the medial side of the femur so it is no longer within its groove. Whenever the patella passes over one of these ridges it is said to be luxated (dislocated or out of joint). The rotational force that is exerted by the quadriceps during contraction brings about a natural tendency for this to occur but the deviation is normally controlled by the bony structures involved.

When the patella luxates from the groove of the femur, it usually cannot return to its normal position until the quadriceps muscles relax 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. pain is caused by the kneecap sliding across the bony ridges of the femur. Once out of position, the animal feels no discomfort and continues its activity.

This condition is usually gradedaccording to its severity.

There are other less common causes of patella luxation but these two examples account for over ninety percent of all cases. Surprisingly, trauma or other acquired lesions are rarely a problem.

The repeated sliding of the kneecap over the femoral ridges can possibly lead to development of a painful arthritis in the area. In fact, arthritis that is secondary to the initial problem is, in most cases, the most serious consequence of the disorder. Therefore, a good evaluation needs to be done early in the condition to prevent long-term arthritic crippling.

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 kneecap. The kneecap itself may be "tied down" laterally to prevent it from deviating medially. 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.

The animal should respond quickly after surgery and is usually completely recovered within thirty days, using its legs in normal fashion.

Breeding Considerations

Because of the strong genetic relationships, we really feel that animals with this disorder should not be used for breeding. They can still be excellent pets - and those that do require surgery will usually lead perfectly normal lives without any restrictions on activity.