While there are four bones that come together at the knee, only the femur (thigh bone) and the tibia (shin bone) form the joint itself. The head of the fibula (strut bone on the outside of the leg) provides some stability, and the patella (kneecap) helps with joint and muscle function. Movement and weight-bearing occur where the ends of the femur called the femoral condyles match up with the top flat surfaces of the tibia (tibial plateaus).
There are two major muscle groups that are balanced and allow movement of the knee joint. When the quadriceps muscles on the front of the thigh contract, the knee extends or straightens. The hamstring muscles on the back of the thigh flex or bend the knee when they contract. The muscles cross the knee joint and are attached to the tibia by tendons. The quadriceps tendon is a little special, in that it contains the patella within it. The patella allows the quadriceps muscle/tendon unit to work more efficiently. This tendon is renamed the patellar tendon in the area below the kneecap to its attachment to the tibia.