The patella, or kneecap, is part of the stifle (knee) joint. The patella is normally located in a groove on the end of the femur (thigh bone) just above the stifle (knee). The term luxating means ‘out of place’ or ‘dislocated’. In patellar luxation, the kneecap, which normally sits on the groove of the femur, moves or pops out of place, either in a medial (inward) or lateral (outward) position.
A luxating patella can occur from a traumatic injury but more commonly is associated with joint or limb structure abnormalities, such as the groove of the femur where the kneecap sits being too shallow, or the area where the kneecap attaches to the shinbone (tibia) is off-center. These limb and joint changes result in an alteration of forces placed on the knee and, in turn, luxation of the patella.
For many toy or small breed dogs such as Yorkshire terriers, Chihuahuas, Miniature Poodles, and Pomeranians, there is a genetic predisposition for developing a luxating patella. In most small breed dogs, this movement occurs medially or toward the inside of the limb. A luxating patella can happen laterally or toward the outside of the limb, but this is less common and usually only occurs in larger breeds.