Joints

A joint, or articulation, is any place where two bones meet. Joints are what allow the rigid skeleton to move, while also holding it together. Anatomists classify joints in two complementary ways: by the tissue connecting the bones (structural) and by how much movement they permit (functional).

Structural Classification

By structure, there are three joint types:

• Fibrous joints: bones are joined directly by tough fibrous connective tissue, with no joint cavity. Examples include the sutures of the skull and the joint between the tibia and fibula.
• Cartilaginous joints: bones are united by cartilage (hyaline cartilage or fibrocartilage). Examples include the intervertebral discs and the pubic symphysis.
• Synovial joints: the bone surfaces are separated by a fluid-filled joint cavity. These are the most movable joints in the body, such as the knee, hip, shoulder, and elbow.

Functional Classification

By the amount of motion allowed, joints are:

• Synarthroses: immovable joints (generally fibrous), such as skull sutures.
• Amphiarthroses: slightly movable joints (generally cartilaginous), such as the discs between vertebrae.
• Diarthroses: freely movable joints (synovial), such as the knee or shoulder.

In general, fibrous joints are synarthroses, cartilaginous joints are amphiarthroses, and synovial joints are diarthroses.

Synovial Joint Anatomy and Clinical Relevance

Synovial joints have a fibrous capsule lined by a synovial membrane that secretes lubricating synovial fluid. The ends of the bones are capped with smooth articular (hyaline) cartilage that reduces friction, and ligaments add stability. Synovial joints come in several shapes—hinge (elbow), ball-and-socket (hip), pivot, saddle, plane, and condyloid—each permitting characteristic movements.

Because joints bear load and move constantly, they are prone to wear and disease. Osteoarthritis is the breakdown of articular cartilage with aging or overuse, while rheumatoid arthritis is an autoimmune inflammation of the synovial membrane. Sprains injure the supporting ligaments. This page is educational and not medical advice; persistent joint pain should be evaluated by a clinician.