Functional Classification of Joints
The functional classification of joints is determined by the amount of mobility between the adjacent bones. Joints are functionally classified as a synarthrosis or immobile joint, an amphiarthrosis or slightly moveable joint, or as a diarthrosis, a freely moveable joint. Fibrous and cartilaginous joints can be functionally classified as either synarthroses or amphiarthroses, whereas all synovial joints are classified as diarthroses.
Synarthrosis
An immobile or nearly immobile joint is a synarthrosis. The immobile nature of these joints provide for a strong union between the articulating bones. This is important at locations where the bones protect internal organs. Examples include sutures, the fibrous joints between the bones of the skull that surround and protect the brain, and the manubriosternal joint, the cartilaginous joint that unites the manubrium and body of the sternum to protect the heart.
Amphiarthrosis
An amphiarthrosis is a joint that has limited mobility. An example of this type of joint is the cartilaginous joint that unites the bodies of adjacent vertebrae. A thick pad of fibrocartilage called an intervertebral disc fills the gap between the vertebrae. Each intervertebral disc strongly unites the vertebrae but still allows for limited movement between them. However, the small movements available between adjacent vertebrae can sum together along the length of the vertebral column to provide for large ranges of body movements.
Another example of an amphiarthrosis is the pubic symphysis of the pelvis. This is a cartilaginous joint in which the pubic regions of the right and left hip bones are strongly anchored to each other by fibrocartilage. This joint normally has very little mobility. The strength of the pubic symphysis is important in conferring weight-bearing stability to the pelvis.
Diarthrosis
A freely mobile joint is classified as a diarthrosis. These types of joints include all synovial joints of the body, which provide the majority of body movements. Most diarthrotic joints are found in the appendicular skeleton and give the limbs a wide range of motion. These joints are divided into three categories based on the number of axes of motion provided by each. An axis in anatomy is described as the movements in reference to the three anatomical planes: the transverse plane, the frontal plane, and the sagittal plane. Diarthroses are classified as uniaxial (for movement in one plane) such as the elbow joint, biaxial (for movement in two planes) like Metacarpophalangeal joint, or multiaxial joints (for movement in all three anatomical planes), like the shoulder joint.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 9.1: Functional Classification of Joints.
