The knee joint is a junction of three bones. The femur and the tibia meet to form a hinge joint. In front of them is the patella (kneecap). The patella sits over the other bones and slides when the leg moves.
The ends of the three bones are covered with articular cartilage. This is a tough elastic material that basically cushions the joint. Also helping to cushion the knee are two C-shaped pads of cartilage called menisci. They lie between the tibia and femur. There is a lateral mensicus and a medial meniscus.
Ligaments help to stabilize the knee. These are stron elastic bands of tissue that connect one bone to another. The four main stabilizing ligaments of the knee are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL).
There are two basic groups of muscles at the knee. In the front of the knee are the quadricep muscles that work to straighten the leg out. In the back of the knee are the hamstring muscles which help to flex the knee.