Joints are the areas where two or more bones meet. Most joints are
mobile, allowing the bones to move. Joints consist of the following:
cartilage - at the joint, the bones are covered with cartilage
(a connective tissue), which is made up of cells and fibers and
is wear-resistant. Cartilage helps reduce the friction of
synovial membrane - a tissue called the synovial membrane lines
the joint and seals it into a joint capsule. The synovial
membrane secretes synovial fluid (a clear, sticky fluid) around
the joint to lubricate it.
ligaments - strong ligaments (tough, elastic bands of connective
tissue) surround the joint to give support and limit the joint's
tendons - tendons (another type of tough connective tissue) on
each side of a joint attach to muscles that control movement of
bursas - fluid-filled sacs, called bursas, between bones,
ligaments, or other adjacent structures help cushion the
friction in a joint.
synovial fluid - a clear, sticky fluid secreted by the synovial
meniscus - a curved part of cartilage in the knees and other
There are many types of joints, including joints that do not move in
adults, such as the suture joints in the skull. Joints that do not move
are called "fixed." Other joints may move a little, such as the
vertebrae. Examples of mobile joints include the following:
Ball-and-socket joints, such as the shoulder and hip joints,
allow backward, forward, sideways, and rotating movements.
Hinge joints, such as in the fingers, knees, elbows, and toes,
allow only bending and straightening movements.
Pivot joints, such as the neck joints, allow limited rotating
Ellipsoidal joints, such as the wrist joint, allows all types of
movement except pivotal movements