The patella is a small bone sliding in a groove in the stifle joint - equivalent to the kneecap in the human knee. If the groove is too shallow the patella may slip sideways out of the groove (patella luxation), causing the dog to limp, until it slips back - often seen as a limp, a skip and a hop followed by normal movement. The patella may slip out of its groove easily or only occasionally. In a badly affected dog the joint is painful and becomes arthritic. The mode of inheritance is polygenic (controlled by several or many genes) and can only be combated by breeding from unaffected parents. A veterinary surgeon can test the joint by feeling how firmly seated the patella is in its groove.
An operation is available to deepen a shallow groove so that the patella is held in place properly. Post-operative care is rather intensive. The dog needs to be comforted because its leg is unfortable even with modern painkillers, and its leg must be rested for days before carefully being used again.