The structural classification is based on the material that binds the bones together and whether or not a joint cavity is present.
There are three primary structural classifications:
1) Fibrous joints:
These joints are connected by dense connective tissue, primarily composed of collagen.
They do not have a joint cavity.
Types:
Sutures: Found between the bones of the skull. These joints are immovable (synarthrosis).
Syndesmoses: Bones are connected by a ligament or an interosseous membrane, allowing for very limited movement. Example: the distal connection between the tibia and fibula.
Gomphoses: A type of joint where a peg fits into a socket, such as the connection between a tooth and its socket in the jawbone (alveolar socket).
2) Cartilaginous joints:
In these joints, bones are connected entirely by cartilage (either hyaline or fibrocartilage).
They also lack a joint cavity.
Types:
Synchondroses: Bones are united by hyaline cartilage. Most are temporary joints that eventually ossify. Example: the epiphyseal plates in long bones during growth.
Symphyses: Bones are united by fibrocartilage. These joints allow for slight movement (amphiarthrosis). Example: the pubic symphysis and intervertebral discs.
3) Synovial joints:
These joints have a fluid-filled joint cavity, surrounded by a joint capsule, and allow for a wide range of movements.
Components
Articular Cartilage: Covers the ends of bones, reducing friction and absorbing shock.
Joint (Synovial) Cavity: The space between the bones, filled with synovial fluid.
Articular Capsule: A two-layered structure that encloses the joint. The outer layer is fibrous, while the inner (synovial membrane) secretes synovial fluid.
Synovial Fluid: Lubricates the joint, nourishes the articular cartilage, and acts as a shock absorber.
Reinforcing Ligaments: Strengthen and support the joint.
Nerves and Blood Vessels: Provide the joint with sensory input and nutrients.