The Joints

6 min read

Joints, in anatomy, a structure that separates two or more adjacent elements of the skeletal system.

Depending on the type of joint, such separated elements may or may not move on one another.
This article discusses the joints of the human body particularly their structure but also their ligaments, nerve and blood supply, and nutrition.

In order to describe the main types of joint structures, it is helpful first to summarize the motions made possible by joints.

These motions include spinning, swinging, gliding, rolling, and approximation.

Spin is a movement of a bone around its own long axis, it is denoted by the anatomical term rotation.
An important example of spin is provided by the radius (outer bone of the forearm) this bone can spin upon the lower end of the humerus (upper arm) in all positions of the elbow.
When an individual presses the back of the hand against the mouth, the forearm is pronated, or twisted;

When the palm of the hand is pressed against the mouth, the forearm is supinated, or untwisted.

Swing, or angular movement, brings about a change in the angle between the long axis of the moving bone and some reference line in the fixed bone.

Flexion (bending) and extension (straightening) of the elbow are examples of swing.

A swing (to the right or left) of one bone away from another is called abduction, the reverse, adduction.

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
A type of tissue that covers the surface of a bone at a joint;

Cartilage helps reduce the friction of movement within a joint;

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 movement;

Tendons
Tendons (another type of tough connective tissue) on each side of a joint attach to muscles that control the movement of the joint;

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 membrane;

Femur
This is the thighbone;

Tibia
This is the shin bone;

Patella
This is the kneecap;

Meniscus
This is a curved part of cartilage in the knees and other joints.

What are the different types of joints?
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:

1. Ball and socket joints;
Such as the shoulder and hip joints, allow backward, forward, sideways, and rotating movements.

2. Hinge joints;
Such as in the fingers, knees, elbows, and toes, allow only bending and straightening movements.

3. Pivot joints;
Such as the neck joints, allow limited rotating movements.

4. Ellipsoidal joints;
Such as the wrist joint, allow all types of movement except pivotal movements.

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