Joints

Bones come together to form joints. The configuration of a joint determines the degree and direction of possible motion. Some joints do not move, except in very young children (during and for a short time after birth). Examples of such joints are those located between the plates of the skull. Other joints allow a large and complex range of 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 in the elbows, 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--a smooth, tough, resilient protective tissue composed of collagen, water, and proteoglycans that reduces friction as joints move. (Collagen is a tough fibrous tissue; proteoglycans are substances that 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.

See the figure Inside the Knee.

The components of a joint work together to facilitate movement that is balanced and causes no damage to any component of the musculoskeletal system. For example, when the knee joint is bent to take a step, the hamstring muscles on the back of the thigh contract and shorten, pulling the lower leg in and bending the knee. At the same time, the quadriceps muscles on the front of the thigh relax, so that the knee is able to bend. Within the knee joint, the cartilage and synovial fluid minimize friction. Also within the knee joint lie two pads of cartilage, called the medial meniscus and the lateral meniscus; these pads act as cushions between the bones at the knee and give the knee joint increased stability. Five ligaments around the knee joint help to keep the bones properly aligned. Bursas provide cushioning between structures such as the shinbone (tibia) and the tendon attached to the kneecap (patellar tendon).