The patella, otherwise known as the kneecap sits in a groove at the end of the femur. This groove allows the patella to slide up and down when the knee joint is bent forwards and backwards and controls the movement of the quadriceps muscle in the lower leg.  A luxating patella occurs when the patella becomes dislocated from its normal position jumping out of the groove and moving sideways, which causes the leg to lock and the dog will hold its leg up. The dog can only put its leg back down once the quadriceps muscles in the hind legs relax and lengthen.

This condition can occur as part of a trauma to the knee but is often a genetic cause and a consequence of bad breeding. Signs of this condition can start from as young as 4 months old when the dog is in the growth phase and malformation of the groove and patella occurs.