The Knee Joint
The knee joint is formed by the end of the femur, or thigh bone, and the top of the tibial, or leg bone, which meet to form a hinged-type joint. The knee joint also includes the patella (knee cap), which is a bone that gives additional power to the muscles, which allow the knee to straighten (extend). The knee joint has a surface named 'articular cartilage' that covers the surfaces of the tibial and femur bones that meet. The underside of the knee cap is also covered with articular cartilage, which makes a joint as it moves across the end of the femur bone. Between the articular cartilage surfaces of the femur and tibia are structures which are called meniscus and help to cushion the load that goes through the knee joint when you walk. There is a meniscus on the outside half of the joint called the lateral meniscus, and one on the inside of the joint, called the medial meniscus. These are made from tissue named fibrous cartilage. Theses menisci essentially act as shock absorbers between the bones.
Ligaments are structures that help to stabilize the knee by acting like tethers to movement. Ligaments along the inside and outside of the knee are named collateral ligaments (medial on the side and lateral on the outside). These ligaments limit both the sideways and rotatory motion of the knee. The ligaments in the center of the knee are named cruciate ligaments because they cross each other. The anterior cruciate ligament (or ACL) connects the tibia to the femur with the function of helping to limit rotation and forward slipping of the tibia. The posterior cruciate ligament (or PCL, posterior to the ACL) again help in limiting rotatory motion and prevents backward sliding of the femur on the tibia. These ligaments of your knee, along with the muscles of your leg, work together to absorb the stress your knee receives as you walk, run, and jump.
When arthritis affects your knee joint, the articular cartilage on the bones are worn away, and there will be symptoms of grinding in the knee, swelling, and pain with activity. For most patients, arthritis of the knee is painful with weight bearing activities and not so painful at rest. This is because at rest, the ligaments relax and the joint surfaces are not in such close contact as they are during weight bearing.