Joints are the junctions between two or more bones. Some joints do not normally move, such as those located between the plates of the skull. Other joints allow a large and complex range of motion. The configuration of a joint determines the degree and direction of possible motion. For example, the shoulder joints, which have a ball-and-socket design, allow inward and outward rotation as well as forward, backward, and sideways motion of the arms. Hinge joints of the knees, fingers, and toes allow only bending (flexion) and straightening (extension).
The components of joints provide stability and reduce the risk of damage from constant use. In a joint, the ends of the bones are covered with cartilage. Cartilage is a smooth, tough, resilient, and protective tissue composed of collagen, water, and proteoglycans that reduces friction as joints move. (Collagen is a tough fibrous tissue, and proteoglycans are substances that help provide the cartilage's resilience.) Joints also have a lining (synovial tissue) that encloses them to form the joint capsule. Cells in the synovial tissue produce a small amount of clear fluid (synovial fluid), which provides nourishment to the cartilage and further reduces friction while facilitating movement.
Inside the Knee
The knee is designed for its own protection. It is completely surrounded by a joint capsule that is flexible enough to allow movement but strong enough to hold the joint together. The capsule is lined with synovial tissue, which secretes synovial fluid to lubricate the joint. Wear-resistant cartilage covering the ends of the thighbone (femur) and shinbone (tibia) helps reduce friction during movement. Pads of cartilage (menisci) act as cushions between the two bones and help distribute body weight in the joint. Fluid-filled sacs (bursae) provide cushioning between structures such as the tibia and the tendon attached to the kneecap (patellar tendon). Five ligaments along the sides and the back of the knee reinforce the joint capsule, adding stability. The kneecap (patella) protects the front of the joint.