Summary
Highlights
Joints, also known as articulations, are points of contact between two bones. They serve several functions including holding body weight, absorbing shock, and enabling movement. The structure of a joint dictates its movement capabilities and strength; strong joints offer little movement, while movable joints tend to be weaker.
Joints are classified structurally based on the tissues holding them together. Fibrous joints connect bones with fibrous connective tissue and lack a cavity, permitting little to no movement. Cartilaginous joints connect bones with cartilage, also lacking a cavity and offering limited movement. Synovial joints are unique, featuring a synovial cavity between bones, allowing for significant movement.
Functionally, joints are classified by their degree of movement. Synarthrosis joints are immovable (e.g., sutures), amphiarthrosis joints are slightly movable (e.g., syndesmoses), and diarthrosis joints are freely movable (e.g., synovial joints).
Fibrous joints lack a synovial cavity and have bones held together by fibrous connective tissue. Examples include sutures (immovable, synarthrosis) in the skull, syndesmoses (slightly movable, amphiarthrosis) found between the tibia and fibula, and gomphoses (immovable, synarthrosis) which secure teeth in their sockets.
Cartilaginous joints have bones united by cartilage with no cavity. Synchondroses involve hyaline cartilage (e.g., epiphyseal plates, sternum-rib joints) and are immovable (synarthrosis). Symphyses involve fibrocartilage (e.g., intervertebral discs, pubic symphysis) and are slightly movable (amphiarthrosis), with specific hormonal effects like relaxin during childbirth.
Synovial joints feature a distinct synovial cavity between articulating bones, filled with viscous synovial fluid for lubrication. They also possess articular cartilage (hyaline cartilage) on bone ends to absorb shock, an articular capsule with a fibrous layer for strength and a synovial membrane for fluid production, reinforcing ligaments, nerves, and blood vessels. All synovial joints are freely movable (diarthroses).
Synovial joints are classified by the axes of movement they permit: monoaxial (uniaxial) for movement around one axis, biaxial for movement around two axes, and multiaxial for movement around several axes. Examples include plane joints (gliding, often non-axial or monoaxial), saddle joints (biaxial, like the thumb), hinge joints (monoaxial, like elbows and knees), pivot joints (monoaxial, like the radioulnar joint), ball and socket joints (multiaxial, like the shoulder), and ellipsoid/condylar joints (biaxial, modified ball and socket).
Precise anatomical terms describe various joint movements. Gliding movements occur in plane joints. Angular movements increase or decrease angles between bones, including flexion (decreasing angle), extension (increasing angle), abduction (away from midline), adduction (towards midline), and circumduction (circular motion combining the others). Rotational movements include pronation (palm posterior) and supination (palm anterior). Special movements encompass inversion/eversion (foot sole inward/outward), dorsiflexion/plantar flexion (foot up/down), opposition/reposition (thumb to/from other digits), protraction/retraction (body part forward/backward), and elevation/depression (body part upward/downward).