The knee is the largest joint in the body and is often the site of pain and injury. The joint is made up of the lower end of the femur (thigh bone) and the upper end of the tibia (shin bone). The patella (kneecap) slides along a groove on the femur, and covers the front of the joint. The meniscus and cartilage cushion the spaces between these bones, and act as shock absorbers during movement.

The quadriceps muscle groups on the front of the upper leg help straighten the leg from a bent position, while the hamstring muscle group, on the back of the upper leg, help to bend the knee.

Several large fibrous bands of tissue, called ligaments, support the knee on both sides. They provide strength and stability to the joint. The four ligaments that connect the femur and tibia are:

    * The medial collateral ligament (MCL) provides stability to the inside of the knee.
    * The lateral collateral ligament (LCL) provides stability to the outside of the knee.
    * The anterior cruciate ligament (ACL), in the center of the knee, limits rotation and the forward movement of the tibia.
    * The posterior cruciate ligament (PCL), also in the center of the knee, limits backward movement of the tibia.

Tendons are tough cords of tissue that connect muscle to bone. In the knee, the quadriceps tendon connects the quadriceps muscle to the patella and helps provide power to extend the leg. The patellar tendon connects the patella to the tibia.