Synarthroses

Synarthroses are immovable joints in which adjacent bones are held together by connective tissue or cartilage, producing little or no movement under normal conditions. They serve to stabilize the skeleton and protect soft tissues, and they are contrasted with amphiarthroses (slightly movable) and diarthroses (freely movable). The functional immobility can persist throughout life or be present only during growth.

Fibrous synarthroses include sutures and gomphoses. Sutures occur between the bones of the skull and are bound by dense fibrous tissue that interlocks to provide stability while accommodating brain growth in infancy. Over time, many sutures ossify and fuse, forming synostoses. Gomphoses are the immobile joints between a tooth and its alveolar socket, stabilized by the periodontal ligament; these joints permit negligible movement.

Cartilaginous synarthroses are joined by hyaline cartilage or fibrocartilage. The classic example is a synchondrosis, where bones are united by a thin plate of hyaline cartilage. Synchondroses are common during growth and typically become fused (synostosis) as the skeleton matures, reducing or eliminating movement at the joint. Other cartilaginous immovable joints may occur transiently in development or in specific anatomical regions.

Development and aging influence synarthroses. In early life, some immobile joints accommodate growth and brain development (notably skull sutures). With aging, many sutures and synchondroses gradually ossify, converting into synostoses and further reducing any potential motion.

In summary, synarthroses are immobile joints formed by fibrous or cartilaginous connections, including sutures, gomphoses, and synchondroses, with ossification to synostoses common as individuals age.