The stifle joint corresponds to the human knee joint. It is the largest joint in the horse but also the weakest joint. It is a joint that has the ability to flex, extend and act as a locking mechanism. A horse is unable to flex the hock without flexing the stifle and hip joints.
In the case of a slipped stifle, also referred to as an uncontrollable locking of the stifle, the hind limb is locked in extention and the hock can not be flexed. This can start intermittently during forward motion. This can also progress to the point of the joint being stuck and a horse hopping on 3 legs. In some cases backing or a jump forward can release the sticking of the joint. The legs can move somewhat uncoordinated and jerky in their movement. One or both stifles can be affected.
Causes are commonly related to poor structure but can be due to over straining the joint. Veterinary advice is essential and depending on the diagnosis it may just be a matter of rest and then some slow type strengthening exercises. In some cases there are a variety of injection possibilities as options, and in extreme cases a vet may advise surgery.