This is an inherited defect, usually of small breed dogs, in which the patella or kneecap does not remain in its normal position.
How does this happen?
Let’s start by looking at the normal structure of the knee in the picture above.
At the end of the femur or thighbone, there are two ridges with a deep groove between them. The kneecap is a piece of unattached bone that rides in this groove. The muscles from the front of the thigh funnel into one large tendon that runs over the top of the kneecap and attaches to the front of the tibia, or shin. As the knee flexes and extends, the kneecap keeps the tendon itself from rubbing. Cartilage and lubricating fluids keep the kneecap moving smoothly. This deep groove limits the movement of the patella and therefore controls the action of the thigh muscles.
What causes the kneecap to luxate?
In some dogs, again, most commonly as an inherited trait, the ridges on the femur are not prominent, and the groove is too shallow. In a dog with shallow grooves, the patella will luxate, or jump out of the groove, sideways, especially toward the inside or medial side of the knee.
The symptoms of a luxating patella vary with the severity. Some dogs will only luxate the patella occasionally, in others it is out of place all the time. Owners are often unaware their dog has a problem until their veterinarian finds it.
For young dogs, there might be no symptoms or, at most, an occasional limp. With time, the abnormal movement of the kneecap will create excessive wear. As the joint becomes arthritic, we can see progressive pain, lameness, and swelling of the joint.
Severe cases may require surgery to keep the kneecap in place. Minor cases can do well without surgery, but may need treatment for arthritis later in life. Because this is an inherited trait, any animal diagnosed with a medially luxating patella should not be used for breeding.