The human skeleton is formed from over 200 bones. With the exception of one bone, the hyoid bone located in the front portion of the neck, which is the only bone in the human skeleton that does not articulate or attach to another bone, according to Live Science, all other bones are connected to one another to form joints.
Not all joints are the same, nor are their functions. Some help to make the body move, while others help in twisting or turning motions. Still others help us walk, run and jump. Although many joints fall into each category, there are three basic types of joints in the human body.
Synarthrosis Joints
The term, synarthrosis, refers to the classification of joints that are essentially immovable. Joints which are synarthrotic are very close together and are separated only by a thin film of fibrous tissue. These joints are also referred to as fibrous joints.
The most common example of a synarthrosis type of joint are the lines between the bones of the human skull. These lines are called sutures. From birth and during the early developmental years, these skull bones, and their respective synarthrosis joints, while not movable per se, have some level of flexibility to allow for growth and enlargement of the brain. Over time, these joints cease to be flexible and prevent any further movement of the skull.
Amphiarthrosis Joints
The amphiarthrosis classification of joints refers to those joints that allow only slight movement. The connection between the bones of an amphiarthrotic joint are a material called hyaline cartilage. These joints offer a level of movement together with a level of sturdiness.
Examples of amphiarthrotic joints can be found in the rib cage where the ribs connect to the sternum, and throughout the entire spinal column. In the case of the rib cage, the bones and joints offer reasonable protection for the lungs and heart, while at the same time being just flexible enough to allow for expansion and contraction during inhaling and exhaling of the lungs.
Regarding the spinal column, amphiarthrotic joints are found between each vertebra from the neck to the sacrum. These are also known as inter-vertebral discs. Because of their attachment to each vertebra, as well as their limited mobility and flexibility, they allow each segment of the spinal column to move forward and back. Because there are many of these vertebrae and amphiarthrotic joints, the spine can display an overall high level of flexibility.
Diarthrosis Joints
Diarthrotic joints, also known as synovial joints, is a classification of joints that can exhibit essentially free movement. The major elements of movement include bending, straightening, also known as flexion and extension, and rotational movement.
The joints are formed from the attachment between two bones by a surrounding joint capsule. The capsule encloses the joint cavity, which is lined by a membrane called the synovium. This synovium produces lubricating fluid that helps with movement and nourishment of the joints' cartilage surfaces.
Examples of diarthrotic joints can include the hips, shoulders, knees, finger joints, wrist and elbows which, together, display virtually all of the different ranges of motion.
