The knee is the largest joint in the body. A healthy knee moves easily, allowing you to walk, turn, and do many other activities without pain. A complex network of bones, cartilage, ligaments, muscles, and tendons work together to make a knee flexible.
There are three bones in your knee joint. Your thighbone (femur) sits on top of your shinbone (tibia). When you bend or straighten your knee, the rounded end of your thighbone rolls and glides across the relatively flat upper surface of your shinbone. The third bone is often called the kneecap (patella), which is attached to the muscles, allowing you to straighten your knee. Your kneecap provides leverage and reduces strain on these muscles.
In a healthy knee joint, the surfaces of these bones are very smooth and covered with a tough protective tissue called articular or hyaline cartilage. Normally, cartilage is a tough tissue that absorbs shock and weight-bearing joint movements, especially during sports activities.
Ligaments (another type of soft tissue) lie along the sides and back of the knee, holding the bones of the knee joint in place. These ligaments work with the muscles that control the bones and the tendons that connect the muscles to the bones so you can bend and straighten your knee. Fluid-filled sacs (bursae) cushion the area where skin or tendons glide across bone. The knee also has a lining (synovium) that secretes a clear liquid called synovial fluid. This fluid lubricates the joint, further reducing friction and making movement easier