A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together (articulate with each other) to form a connection. Joints are classified both structurally and functionally. Structural classifications of joints take into account whether the adjacent bones are strongly anchored to each other by fibrous connective tissue or cartilage, or whether the adjacent bones articulate with each other within a fluid-filled space called a joint cavity. Functional classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints. The amount of movement available at a particular joint of the body is related to the functional requirements for that joint. Thus immobile or slightly moveable joints serve to protect internal organs, give stability to the body, and allow for limited body movement. In contrast, freely moveable joints allow for much more extensive movements of the body and limbs.
Structural Classification of Joints
The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces contact each other within a fluid- filled joint cavity. These differences serve to divide the joints of the body into three structural classifications.
- A fibrous joint is where the adjacent bones are united by fibrous connective tissue.
- At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage.
- At a synovial joint, the articulating surfaces of the bones are not directly connected, but instead come into contact with each other within a joint cavity that is filled with a lubricating fluid. Synovial joints allow for free movement between the bones and are the most common joints of the body.
Functional Classification of Joints
The functional classification of joints is determined by the amount of mobility found between the adjacent bones. Joints are thus functionally classified as:
- Synarthrosis or immobile joint,
- Amphiarthrosis or slightly moveable joint,
- Diarthrosis, which is a freely moveable joint (arthroun = “to fasten by a joint”).
Depending on their location, fibrous joints may be functionally classified as a synarthrosis (immobile joint) or an amphiarthrosis (slightly mobile joint). Cartilaginous joints are also functionally classified as either a synarthrosis or an amphiarthrosis joint. All synovial joints are functionally classified as a diarthrosis joint.