A luxating patella, or trick knee in layman's terms, is a condition in which the patella, or kneecap, dislocates or moves out of its normal location.
Patella luxation is a common condition in dogs, particularly small and miniature breeds. The condition usually becomes evident between the ages of 4 and 6 months. It can occur in cats, as well, especially domestic short-haired cats.
It also occurs in humans, where it can be associated with damage to the anterior cruciate ligament.