The knee joint is a hinge joint with strong ligaments between the thigh and shin bones. Smooth cartilage covers the end of bones to reduce friction. There are also two extra pieces of thick cartilage called menisci which act as shock absorbers.
The knee ligaments and capsule (the bag that surrounds the joint), prevent the bones moving in the wrong direction or dislocating. The thigh muscles (quadriceps) also help to hold the knee joint in place and produce movement.
Problems with knee joints are common. See below for information on conditions / self help.