Joints: where bones meet

The shoulder and the hip are the only ball-and-socket joints in the body.

Three kinds by how much they move

A joint, or articulation, is any place where two bones come together. Some joints do not move at all, some move a little and some move freely. Immovable joints hold bones in very close contact, separated only by a thin layer of fibrous tissue; the sutures between the skull bones are examples. Slightly movable joints are joined by cartilage. The ribs connected to the sternum by costal cartilage, the joint at the front of the pelvis (the pubic symphysis) and the joints between vertebrae with their discs are all of this kind.

Freely movable joints

Most joints in the adult body move freely. In these joints the ends of the bones are covered with smooth articular cartilage and separated by a space called the joint cavity. A tough capsule surrounds the joint. Its outer layer includes the ligaments that hold the bones together, and its inner layer, the synovial membrane, makes synovial fluid that keeps the joint slippery. Because they all have this membrane, freely movable joints are also called synovial joints.

Ball-and-socket, hinge and pivot

Synovial joints are sorted by the shape of the bone ends, and the shape decides how the joint can move. In a ball-and-socket joint, the rounded head of one bone sits in a cup on the other. It allows the widest range of motion: forward and back, out to the side, rotation and full circles. The hip and shoulder are the only ball-and-socket joints in the body. The hip socket is deep and held by strong ligaments, which gives stability for standing and walking, while the shoulder is shallower and moves more freely. Choose Ball-and-socket in the demo list and watch the arms trace circles.

A hinge joint works like a door hinge: it only bends and straightens. The elbow, the knee, the ankle and the joints between the finger and toe bones are hinge joints. Choose Hinge in the list and the elbows bend while the upper arms stay still. A pivot joint lets one bone rotate around another. Between the first two neck vertebrae, the atlas turns around a peg that points up from the axis, and that is how you turn your head to say no. A second pivot joint near the elbow lets the radius rotate when you turn your palm up and down. Choose Pivot to see the head turn on the atlas.

Three more shapes

The list has three more types. Condyloid joints, such as the wrist and the knuckles at the base of the fingers, let a rounded end move in an oval cup: bend, straighten and move side to side. The saddle joint at the base of the thumb fits two curved surfaces together like a rider on a saddle, which gives the thumb its wide range. Plane joints between small flat surfaces, such as those between the tarsal bones of the foot or between the small processes of neighbouring vertebrae, allow only slight gliding.

Try a simple test on yourself. Keep your upper arm still and bend your elbow: the forearm moves in only one plane, which is what a hinge does. Now swing your whole arm in a circle from the shoulder: a ball-and-socket joint makes that possible. Finally, keep your shoulders still and turn your head left and right: most of that turning happens at the pivot between the atlas and the axis.