The knee is a complex joint that flexes, extends, and twists from side to side and remains stable throughout.
It consists of essentially three joints formed the lower end of the femur (Thigh bone), upper end of the tibia (Shin bone) and the patella (Knee cap)
The ends are covered with hyaline cartilage that allows the joint to move smoothly. The knee is stabilised (Held together) by the muscles, ligaments, tendons and the menisci (cartilages) of the knee joint. Each and every one of these structures can be injured and torn giving rise to symptoms and disability. Patients present with a history of knee pain, swelling, giving way, restriction of the range of movement and locking of the knee.
More information: http://orthoinfo.aaos.org/menus/leg.cfm