The human body contains 206 bones that provide structural support. At joints, bones contact other bones, cartilage or teeth. Arthrologists, or people who study joints, classify these points of contact by either structure or function. There are three types of joint structures: fibrous, cartilaginous and synovial. Likewise, there are three types of joint function: synarthrotic (immovable), amphiarthrotic (semi-movable) and diarthrotic (movable).
Joints that are commonly referred to as fixed joints are synarthrotic joints. Structurally, all of these joints are fibrous because fibrous connective tissue joins the bones. Fibrous joints occur when bones are in close contact and when there is no joint cavity for one bone to fit into. Fibrous joints fall into one of three categories: sutures, gomphoses and syndesmoses.
Sutures are found exclusively in the skull. A dense connective tissue holds the skull bones together. In children, this tissue is less rigid, allowing for the bones to grow. By adulthood, these sutures completely ossify to make the joint completely immovable.
Found exclusively with teeth, gomphoses look like pegs that fit into sockets. In the case of teeth, the roots form the pegs, and the alveolar processes of the jaw bones form the sockets. Periodontal ligaments hold the teeth in place so that they do not move.
A third type of fibrous joint, syndesmoses, feature ligaments or sheets of connective tissue that attach two bones. Some arthrologists classify syndesmoses as amphiarthrotic (semi-movable) joints while others classify them as synarthrotic (immovable). Two examples of this type of joint are the tibiofibular joint in the lower leg and the radioulnar joint of the lower arm.