Joints, or articulations, are the points where two or more bones meet. They are classified based on the amount of movement they permit, ranging from completely immobile to freely movable. This article focuses on the amphiarthrotic joint, a category that balances rigidity and mobility.
How Amphiarthrotic Joints are Defined
The term amphiarthrosis refers to a joint that possesses limited mobility, placing it between the completely immobile and the freely movable joints in the functional classification system. This slight movement is a defining characteristic, differentiating it from an immobile joint, which provides a strong, fixed union between bones. Amphiarthroses are designed primarily for stability while permitting minor shifts.
The minimal movement allowed serves a specific mechanical purpose in the body. The slight flexibility enables the joint to function as a shock absorber, cushioning the impact of forces like walking or jumping. Although the movement at any single joint is small, the cumulative effect of several amphiarthroses results in a larger range of body motion, which is important in the axial skeleton and pelvic region.
The Structural Classifications of Amphiarthroses
Amphiarthroses are categorized structurally based on the type of connective tissue holding the bones together. The two main structural types are certain cartilaginous joints and certain fibrous joints. These tissues lack the fluid-filled joint cavity characteristic of freely movable joints.
One structural type is the cartilaginous joint known as a symphysis, where bones are united by fibrocartilage. Fibrocartilage is a robust tissue containing thick collagen fibers, giving it a high capacity to resist pulling and compressive forces. This strong material allows the adjacent bones to be strongly united while still permitting limited movement.
The other structural type includes fibrous joints classified as a syndesmosis, where bones are joined by a ligament or an interosseous membrane. A syndesmosis permits slightly more movement than other fibrous joint types. This arrangement allows for the slight movement necessary for its amphiarthrotic function.
Primary Anatomical Examples
Pubic Symphysis
The pubic symphysis is a secondary cartilaginous joint where the right and left hip bones meet at the front of the pelvis. This symphysis consists of a fibrocartilaginous disc designed to withstand tension and compressive forces. The joint normally allows a very small amount of gliding and separation, typically around two millimeters of movement in adults. This limited mobility is important for stabilizing the pelvis and absorbing shock during movement.
Intervertebral Joints
Another prominent example of a symphysis is the intervertebral joints found along the spinal column. The bodies of adjacent vertebrae are joined by thick pads of fibrocartilage called intervertebral discs. These discs strongly unite the vertebrae while allowing for small individual movements between them. The cumulative effect of these minor movements provides the larger range of motion needed for bending and twisting the trunk.
Distal Tibiofibular Joint
An example of a syndesmosis is the distal tibiofibular joint, located at the ankle. The distal ends of the tibia and fibula are held together by strong ligamentous structures, including the anterior and posterior inferior tibiofibular ligaments. This fibrous articulation is crucial for maintaining the integrity of the ankle structure, preventing excessive displacement of the fibula. The slight movement at this syndesmosis accommodates the subtle shifting of the bones during weight-bearing actions.