- Thigh bone (Femur)
- Leg bone (Tibia)
- Knee cap (Patella)

For a knee to function normally, the quality of smoothness where each bone moves upon the other becomes important in the function of the knee joint. The surfaces of all three bones coming into contact with each other are normally covered with a smooth gliding surface known as articular cartilage, a smooth substance that cushions the bones and enables them to move easily. Normally, all of these components work in harmony. But disease or injury can disrupt this harmony, resulting in pain, muscle weakness, and decreases functions of the knee. All remaining surfaces of the knee are covered by a thin, smooth tissue liner called the synovial membrane. This membrane releases a special fluid that lubricates the knee which reduces friction to nearly zero in a healthy knee.