The knee is the largest, hardest working joint in the body and is also the most easily injured. A healthy knee moves easily, allowing you to do many activities without pain. A complex network of bones, cartilage, ligaments, muscles and tendons work together to make the knee flexible.
There are three bones in your knee joint, the Femur, Tibia and Patella. Your thighbone (femur) sits on top of your shinbone (tibia). When you bend or straighten your knee, the rounded end of your thighbone 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 cartilage which allows the bones to slide against one another with minimal friction. Once cartilage is damaged, it cannot heal itself and the progressive deterioration leads to a loss of cartilage and exposed bone.
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, bones and tendons 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.
As you might expect, there are many different reasons why you might have knee pain including injury, infection, and arthritis.