There are over 200 bones in the human skeleton and all but one, the hyoid bone, found in the front portion of the neck and protects the larynx and voice box, connect to another to form a movable segment called a joint. Joints help us perform all the necessary activities for function and living, such as movement, walking, sitting, bending, turning and grasping. Some joints are basic and are responsible for minimal activities, while others are considerably more complex and contribute to more complicated activities.
Synarthroses
Synarthrotic joints possess essentially no real movement capabilities. They typically consist of two bones separated by a thin layer of dense, fibrous tissue that binds them to one another.
The most common example of synarthrotic joints are those found in the skull, where the plates of the skull are connected by the dense fibrous tissue. These joints or attachments are called sutures and are immovable in adult skulls.
Amphiarthroses
These joint types offer limited movement. Bones involving this joint type are held together by fibers of collagen of interosseous (between bones) ligaments. Two major types of amphiarthrotic joints are symphyses and syndesmoses.
Examples of a symphysis joint can be found in the front portion of the pelvis where the two halves come together below the belly button. This is called the symphyses pubis. What little motion exists is frequently found in pregnant women when preparing for giving birth. This symphysis softens and allows the pelvis to expand to accommodate the baby during delivery.
An example of a syndesmosis can be found in the forearm and the lower leg. In the forearm, the radius and ulna bones run parallel to each other and are only connected at their ends, which allows for rotational movements at the wrist. A similar example is found in the lower leg where the tibia, or shin bone, and the smaller fibula run parallel and connect at their ends, only here the fibula is an attachment bone for ligaments and tendons, and is a non-weight-bearing bone and leg rotation is limited.
Diarthroses
Diarthrotic joints consist of two bones connected by a joint capsule that contains a membrane called the synovium, which produces synovial fluid for lubrication of the joint cartilage surfaces. This joint type offers free movement in several planes of motion.
Examples of diarthrotic joints are the ball-and-socket hip joint; the hinge joint, as seen in the elbow; the saddle joint at the base of the thumb; the pivot joints, located at the junction between the radius and ulna of the forearm closest to the elbow, which allows for bending and straightening as well as rotation of the hand and wrist; gliding joints as seen where the small carpal bones of the wrist glide against each other, and where the clavicle, or collar bone connects to the acromion, or the front part of the roof of the shoulder. The last joint is the condyloid joint, where the small carpal bones move against the end of the radius bone at the wrist, including bending, straightening and side-to-side glide.
