Articulations: Synarthrosis and Amphiarthrosis

It's common to think of the skeletal system as being made up of only bones, and performing only the function of supporting the body. However, the skeletal system also contains other structures, and performs a variety of functions for the body.

While the bones of the skeletal system are fascinating, it is our ability to move segments of the skeleton in relation to one another that allows us to move around. Each connection of bones is called an articulationor a joint. Articulations are classified based on material at the joint and the movement allowed at the joint.

Synarthrosis Articulations Immovable articulations are synarthrosis articulations ("syn" means together and "arthrosis" means joint); immovable articulations sounds like a contradiction, but all regions where bones come together are called articulations, so there are articulations that don't move, including in the skull, where bones have fused, and where your teeth meet your jaw. These synarthroses are joined with fibrous connective tissue. Some synarthroses are formed by hyaline cartilage, such as the articulation between the first rib and the sternum (via costal cartilage). This immoveable joint helps stabilize the shoulder girdle and the cartilage can ossify in adults with age. The epiphyseal plate or “growth plate” at the end of long bones is also a synarthrosis until hyaline cartilage ossification is completed around the time of puberty.

Amphiarthrosis Articulations

There are some articulations which have limited motion called amphiarthrosis articulations. They are held in place with fibrocartilage or fibrous connective tissue. The anterior pelvic girdle joint between pubic bones (pubic symphysis) and the intervertebral joints of the spinal column (discs) are examples of cartilaginous amphiarthroses. The two parallel bones in the arms and legs are considered amphiarthrosis articulations with a sheet-like interosseous membrane. The lateral distal articulation between tibia and fibula at ankle is also an amphiarthrosis with a fibrous ligament.