TYPES OF JOINT
There are several types of joint. Some are fixed (such as the
skull joints), some allow only a little movement (the vertebral
joints, for example), and some are mobile. Ofthe mobile joints,
ball-and-socket have the widest range ofmovement. Pivot
joints allow rotation only. Ellipsoidal joints allow movement
in most directions. Hinge joints mainly allow bending.
In an ellipsoidal joint, the oval-shaped end of
one bone fits into an elliptical cavityin another,
allowing all types of movement except pivoting.
An example is the wrist joint.
This is the simplest joint; the cylindrical end of one
bone fits into a groove in another to allow bending and
straightening, as in the fingers. The knee and elbow are
modified hinge joints that allow some rotation as well.
STRUCTURE OF A FIXED JOINT
Fixed joints are firmly secured by
fibrous tissue. The joints between
the bones of the skull (known as
sutures) are an example.
STRUCTURE OF A MOBILE JOINT
The surfaces of the bone ends are
coated with very smooth cartilage to
reduce friction as they move against each
other. The ends are sealed within a tough
fibrous capsule; this is lined with synovial
membrane, which produces a sticky
lubricating fluid. The joint is surrounded
by strong ligaments that support it
and prevent excessive movement. Its
movement is controlled by muscles that
are attached to bone by tendons on either
side of the joint. Most mobile joints have
at least one bursa (fluid-filled sac) nearby,
which cushions a pressure point.