A joint is a junction between two bones. There are three basic types of joints. A
synarthrosis or syndesmosis is a joint in which the bones are held together by fibrous
tissue that permits almost no movement. The joints between the bones of the skull are of
this type. The second type is an amphiarthrosis, in which the bones are connected by
cartilage. An example of this type of joint is found in the spine, where pads of cartilage
(the intervertebral discs) are interposed between the vertebral bodies. Amphiarthroses
allow a moderate degree of movement.
The third type of joint (the type that has the greatest freedom of movement) is called
a diarthrosis or synovial joint. All diarthroses have certain features in common. The
opposing surfaces of the two bones (the articular surfaces) are covered with articular
cartilage. This provides a smooth surface which minimizes friction. The articular surfaces
are enclosed by a joint capsule, a sleeve of fibrous connective tissue that fits over the
ends of the two bones. The inside of the joint capsule is lined with a membrane called
synovium that secretes a lubricating fluid (synovial fluid). The fluid-filled space inside
the synovial membrane is called the joint cavity.
The joint capsule is firmly attached to the bones, holding them together; yet it is
flexible, allowing a free range-of-motion. The capsule is reinforced by tough bands of
fibrous connective tissue called ligaments. Most ligaments are located outside the
capsule; however, some joints have ligaments that pass through the joint cavity (e.g., the
cruciate ligaments of the knee).
Diarthroses come in a variety of shapes, allowing different types of movement. For
example, ball-and-socket joints (such as the hip and shoulder) can move in all directions,
and have the greatest range-of-motion. Hinge joints (such as the elbow) can move in only
two directions, flexion (bending) and extension (straightening). The knee is mostly a
hinge; however, a slight degree of rotation occurs as the knee flexes and extends. The
atlanto-axial joint (between the first two cervical vertebrae) is a pivot, permitting the
head to rotate on the neck. Some of the intercarpal joints (between the small bones of the
wrist) are plane joints, allowing the bones to glide over each other. The first
carpometacarpal (CMC) joint (the joint at the base of the thumb) is saddle-shaped; it can
move in all directions, but with less range than a ball-and-socket. This is what makes the
thumb opposable, so that it is possible to grasp objects between the thumb and fingers.
Movement at a joint is produced by the contraction of muscles. A tendon is a bundle of
fibrous connective tissue that attaches a muscle to a bone. Another structure associated
with some joints is a bursa, a sac, lined with synovium, but separate from the joint
cavity. Bursae may be located between bones and tendons, between ligaments and tendons,
etc. They act as cushions, reducing friction between the moving parts around a joint.
Further information about specific joints is presented in separate sections for the