Joints

Joints, or articulations, are junctions where two or more bones meet. They enable movement and provide stability, transmit loads, and accommodate growth and remodeling. Joints are classified structurally as fibrous, cartilaginous, or synovial, and functionally as synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).

Fibrous joints are bound by dense connective tissue and include sutures in the skull, syndesmoses between long bones, and gomphoses at teeth. Cartilaginous joints use cartilage to link bones, as in synchondroses between epiphyses and diaphyses in the developing skeleton and symphyses between vertebrae or the pubic symphysis. Synovial joints are the most mobile and feature a joint cavity enclosed by a fibrous joint capsule, articular cartilage covering the bone ends, and a synovial membrane that produces lubricating fluid. Accessory structures such as ligaments, fibrocartilaginous discs or menisci, and bursae may add stability and shock absorption.

Most synovial joints are classified by shape and movement: ball-and-socket (shoulder, hip) permit multi-axial rotation; hinge (elbow, knee) allow angular motion in one plane; pivot (atlas–axis; radius–ulna) enable rotation; condylar (wrist) permit biaxial motion; saddle (thumb) provides complex, yet still biaxial movement; plane joints (acromioclavicular, intercarpal) permit gliding movements. The joints also distribute forces across articular surfaces and function in proprioception and reflexes.

Common disorders include osteoarthritis, rheumatoid arthritis, sprains, and dislocations, reflecting injury or degeneration of cartilage, ligaments, or synovial tissue. Joints are central to daily movement, posture, and overall biomechanics.