The knee is the largest weight-bearing joint in the body. The knee is made up of the lower end of the thigh bone (femur), which moves on the upper end of the shin bone (tibia), and the knee cap (patella), which slides in a groove on the end of the femur. Large ligaments attach to the femur and tibia to provide stability. The long thigh muscles give the knee strength for movement. The joint surfaces where these three bones move against each other are covered with articular cartilage, a smooth tissue that cushions the bones and enables them to move easily. All remaining surfaces of the knee are covered by a thin, smooth tissue liner called the synovial membrane. In a healthy knee, this membrane releases a special fluid that lubricates the knee and almost eliminates the friction. Normally, all of these components work in harmony. But disease or injury can disrupt this harmony, resulting in pain, muscle weakness, and less function.