Joints are the junctions where two or more bones or bone and cartilage meet. These connections are fundamental to the skeletal system, providing both stability to support weight and flexibility necessary for movement. The human body contains hundreds of these connections, which are broadly classified according to the amount of motion they permit. This classification organizes the body’s articulations based on their functional role in locomotion and structural support.
Understanding Functional Joint Classification
Joints are functionally grouped into three major classes based on their mobility. The most stable joints, which permit little to no movement, are termed synarthroses. At the opposite end of the spectrum are diarthroses, which are joints designed for a wide range of motion. The intermediate classification is the amphiarthrosis, which refers to a joint that allows for only slight or limited mobility.
This slightly movable status provides a balance between stability and flexibility, often serving a load-bearing or shock-absorbing function. While movement at a single amphiarthrotic joint is small, when multiple joints are linked together, such as in the vertebral column, their combined minor movements produce a substantial range of motion for the entire structure. The classification of a joint as an amphiarthrosis is determined by this functional attribute of limited movement.
Structural Categories of Amphiarthrotic Joints
While the functional classification describes joint mobility, the structural classification describes the material connecting the bones. Amphiarthrotic joints are held together by either dense fibrous connective tissue or cartilage. The two primary structural types that align with the amphiarthrotic classification are the syndesmosis and the symphysis.
A syndesmosis is a fibrous joint where two parallel bones are united by a band or sheet of dense fibrous connective tissue. This tissue may be an interosseous membrane, which is a broad, flat sheet, or discrete ligaments. Movement is restricted, allowing only a small degree of give or rotation. The length of the connecting fibers determines the amount of movement allowed.
A symphysis is a cartilaginous joint where the articulating surfaces are separated by a thick pad of fibrocartilage. Fibrocartilage is a tough, resilient material that withstands significant compression and tension forces. This structure strongly unites the bones but permits a small amount of movement due to the pad’s compressibility.
Key Examples of Moderately Movable Joints
A primary example of an amphiarthrotic joint that is a symphysis is the pubic symphysis. This joint unites the right and left hip bones at the anterior pelvis via a fibrocartilage pad. Its limited mobility maintains the weight-bearing stability of the pelvic girdle. The joint can exhibit increased mobility during pregnancy, as the hormone relaxin softens the fibrocartilage to allow for expansion during childbirth.
Another example of a symphysis is the connection between adjacent vertebrae formed by the intervertebral discs. Each disc is a thick pad of fibrocartilage that binds the vertebral bodies together. This fibrocartilage allows for slight compression and bending between individual vertebrae, which collectively enables the extensive flexibility of the vertebral column. The disc also absorbs shock generated by walking or running.
An example of an amphiarthrosis that is a syndesmosis is the distal tibiofibular joint, located just above the ankle. The tibia and fibula are anchored together by ligaments and fibrous connective tissue. This firm connection stabilizes the ankle joint and prevents separation of the lower leg bones. Similarly, the interosseous membrane between the shafts of the radius and ulna in the forearm is a syndesmosis that permits only slight movement between the two bones.