A joint, or articulation, is the point where two or more bones meet. Joints are categorized functionally based on the degree of movement they permit. The three main functional classes are synarthrosis (immovable joints), diarthrosis (freely movable joints), and amphiarthrosis. Amphiarthrosis joints allow only limited motion between the articulating bones.
Structural Basis for Slight Movement
The limited motion characteristic of amphiarthrosis joints is directly determined by the material that connects the adjacent bones. Unlike freely movable joints that feature a fluid-filled cavity, amphiarthroses are joined by dense connective tissue or cartilage, which physically restricts movement. This structural arrangement provides a high degree of strength and stability while permitting minor shifts and adjustments.
One structural category resulting in amphiarthrosis is the cartilaginous joint, specifically the symphysis. In a symphysis, articulating bone surfaces are covered by hyaline cartilage, but they are ultimately united by a broad, flattened disc of fibrocartilage. Fibrocartilage is a tough, resilient material that withstands significant compression, allowing for slight deformation under stress and thus limited movement.
The other structural type that functions as an amphiarthrosis is a fibrous joint known as a syndesmosis. Here, the bones are connected by a strong, cord-like ligament or a broad sheet of dense fibrous connective tissue called an interosseous membrane. The length of the connecting fibers in a syndesmosis dictates the small amount of give or movement available. This dense, unyielding tissue ensures the bones remain strongly anchored together, even as it facilitates minimal movement.
Key Locations and Examples in the Human Body
Amphiarthrosis joints are strategically placed in the body where strength and weight-bearing stability must be coupled with the ability to absorb shock and allow minor flexibility. The pubic symphysis, located at the anterior part of the pelvis, is a prime example of a cartilaginous symphysis. This joint connects the two hip bones with a pad of fibrocartilage, providing the pelvic girdle with the necessary stability to support upper body weight while permitting minute movements, which are particularly relevant during childbirth or walking.
The intervertebral discs, which sit between the bodies of adjacent vertebrae, also represent cartilaginous symphyses. Each disc is a thick pad of fibrocartilage that strongly unites the vertebrae, limiting movement between any two segments. However, the small movements available at each disc sum up along the entire length of the column to produce the overall large range of motion required for bending and twisting the trunk.
A clear example of a fibrous syndesmosis functioning as an amphiarthrosis is the distal tibiofibular joint. This joint, found near the ankle, connects the distal ends of the tibia and fibula with a strong ligamentous connection. The dense connective tissue here prevents the two bones from separating, which is essential for maintaining the integrity of the ankle joint during weight-bearing. This robust structure allows for only a subtle torsional movement, or twisting, which is necessary for proper ankle function.
Essential Function of Amphiarthrosis Joints
Amphiarthrosis joints balance structural strength and necessary flexibility in areas subjected to significant mechanical stress. These joints are uniquely designed to manage compressive forces by acting as efficient shock absorbers. The fibrocartilage in joints like the intervertebral discs compresses and rebounds, distributing the forces generated by activities such as running or jumping, protecting the bones and nervous tissue from impact damage.
The limited mobility they offer is directly tied to their function in weight distribution and stability. The slight yielding in the pubic symphysis and the intervertebral joints allows the body to shift its center of gravity slightly without compromising the structural integrity of the pelvis or the spine. This controlled flexibility ensures that the weight-bearing skeleton remains robust and capable of supporting the body’s mass against gravity.
By being only slightly movable, amphiarthrosis joints provide greater stability compared to the widely mobile synovial joints. The strong fibrous and cartilaginous attachments minimize the risk of dislocation while still accommodating the minor positional adjustments necessary for postural changes and movement. This combination of strength and subtle movement makes them indispensable for the core structures of the axial skeleton.