A luxating patella, sometimes called a trick knee, is a condition in which the patella, or kneecap, dislocates or moves out of its normal location.
Patellar 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.
There have been several reports of patella luxation in other species such as miniature pigs, alpacas, llamas, cattle and goats.