A joint, or articulation,
is the location where two bones come together. Joints keep the bones
in position, while allowing them to move next to each other. The ends of
the bones are covered with cartilage, which provides a cushion between
the two bones. Further cushioning is provided by synovial fluid that
fills a capsule surrounding the ends of the bones.
Connective tissue forms a
capsule or cavity around the ends of the bones where they join together,
this is called the synovial capsule. Lining the inside of this capsule
is a membrane which secretes thick synovial fluid. The fluid helps
to lubricate the joint and reduce friction as the bones move next to each
other. Ligaments and tendons also surround the joint providing further
support.
There are several types of
joints in your body. Joints are classified depending on the type of movement
the joint allows.
Hinge joint - The hinge joint
allows flexion and extension of the joint. The joint acts like a
hinge on a door, allowing the joint to “open” and “close.” The elbow and
knee joints are hinge joints, as well as the joints in the fingers and
toes.
Ball and socket - The ball and
socket joints are made up of one bone that is shaped like a round ball
sitting inside another bone with a socket. This type of bone placement
allows for rotation in a circular motion. Examples are the shoulder
and hip.
Pivot - A pivot
joint allows bones to pivot or rotate against each other. The atlas
(first cervical vertebrae) and the axis (second cervical vertebrae) pivot
against each other. The top of the radius also pivots against the bottom
of the humerus in the elbow.
Condyloid - Similar to the ball
and socket joint, the condyloid joint allows circular motion. In
the condyloid joint, the ball component rests against the end of a bone
rather than inside a socket. The carpals of the wrist rest against
the end of the radius bone of the forearm, making a condyloid joint.
At the knee, there is a condyloid joint between the femur and the top of
the tibia.
Saddle - The saddle joint allows
movement in various directions. For example, where the thumb
meets the wrist, the bones fit up against each other like a saddle fits
over the back of a horse.
