The knee joint is comprised of the bottom of the femur articulating (meeting) with the top of the tibia. There is a thick layer of a smooth, firm coating called articular cartilage between them to cushion the impact of the two bones during movement. In healthy joints, this cartilage is one centimeter thick (about half an inch). The patella (knee cap) is coated in the same articular cartilage on the side facing the femur to allow it to glide over the joint and create efficient bending of the knee. Ligaments in and around the knee stabilize the joint, and a capsule full of a smooth, vicous liquid called synovial fluid envelopes the joint to provide extra lubrication.