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JOINT - A joint, or articulation, is
a junction between bones of the skeleton. The construction of a joint can be
viewed on a stability versus mobility continuum. Stability is increased when
the bones fit snugly together and the ligaments and tendons are tight,
restricting movement. Movement is increased with a looser construction though
stability is decreased. There are three types of joints: fibrous,
cartilaginous, and synovial.
Fibrous joints are joints in which two
bones are connected by a fibrous connective tissue; the joint space is
completely filled with fibrous tissue, allowing no movement. These joints can
be found in between the plates of the skull, in the bones of the forearm and
other areas.
Cartilaginous joints are bones joined by
cartilage. The cartilage fills the joint and very little movement is allowed;
they are found between the ribs and sternum, and between vertebrae discs of the
back.
Synovial joints are the most common joints
in the body. The ends of the bones of these joints are capped with smooth
cartilage allowing them to slide easily over each other. In some of these
joints, an example of which is the knee, accessory cartilaginous structure,
which provide extra cushioning and improve the stability of the joint, are
found. In a synovial joint, the ends of the bones are enclosed in a fibrous
joint capsule. Tendons and ligaments intermix at some point in the capsule. The
tautness of the capsule determines the stability and range movement of the
joint. Under the capsule is a synovial membrane which secretes a fluid called
synovial fluid. This fluid lubricates and cushions the joint, and provides
nutrients to the cartilage. There are six types of synovial joints: ball and
socket (e.g., hip joint)_ hinge (e.g., elbow), saddle (e.g. thumb), condyloid
(e.g., knee), pivot (e.g. neck), and plane (e.g., wrist). Each type of joint
differs in range of movement and stability.
Double-jointedness is caused by a joint
constructed to permit an extreme range of motion. This can be due to the lack
of ligaments or a difference in bony configuration. |
