A joint occurs where two bones come together and can be classified by the material that binds them together and by their range of motion. With the former method, the three common types of joints are fibrous, cartilaginous, and synovial. When classification is based on the range of motion, there are three forms: synarthrosis (immovable joint), amphiarthrosis (slightly movable joint), and diarthrosis (freely movable joint).
A fibrous joint (under which most joints are classified as synarthroses forms because they are immovable) occur where bones are attached by the collagen fibers of fibrous connective tissue. There are three types of fibrous joints: sutures, syndesmoses, and gomphoses.
- A suture is a fibrous joint found connecting most bones in the skull. Sutures form strong connections that allow little movement between bones to help protect the brain. The spaces between bones in the skull are relatively wide in babies to allow flexibility during childbirth and to allow the skull and brain to enlarge with growth. These spaces become increasingly narrow into adulthood. Synostosis is a bony junction that forms when connective tissue between two bones is replaced with bone tissue. These joints are immovable and, once fully fused, considered a single bone. This can occur in the sutures later in adulthood and occurs during normal development of the coxal bone (hip bone), frontal bone (front of cranium), and mandibular bone (lower jawbone).
- A syndesmosis joint is a fibrous joint between bones that are held together by a ligament. This type of fibrous joint is found between the long parallel bones in the lower legs (tibia and fibula) and in the forearms (ulna and radius). In the leg, this syndesmosis joint allows for little movement, providing needed stability. In the arm, this joint is more flexible to allow rotation during forearm movements.
- A gomphosis is a narrow fibrous peg-and-socket joint found only where the roots of teeth insert into the sockets of the jaw bones.
A cartilaginous joint occur where bones are separated by cartilage. Compared to fibrous joints, the cartilage allows slight movements between the bones. There are two types of cartilaginous joints: synchondroses and symphyses.
- A synchondrosis joint is a cartilaginous joint where bones are joined together by hyaline cartilage. Hyaline cartilage is a firm, translucent cartilage. Synchondroses can be temporary or permanent. An example of a temporary synchondrosis is the epiphyseal plate, or growth plate, where cartilage is replaced by bone before adulthood. Permanent synchondroses are located in the thoracic cage, where the costal (relating to ribs) cartilage of the first rib meets the broad upper part (manubrium) of the sternum. Synchondrosis joints are primarily classified as synarthrosis joints because they are immovable.
- A symphysis joint is a slightly movable and permanent cartilaginous joint where the bones are joined by fibrocartilage. Fibrocartilage is a strong, flexible cartilage that contains thick, fibrous collagen fibers. One example is the pubic symphysis, where the pubis regions of the coxal bones attach via a narrow stretch of fibrocartilage. Intervertebral joints are symphyses in which adjacent vertebral bodies articulate via the intervertebral discs. This provides cushioning between vertebrae. Symphysis joints are classified as amphiarthrosis joints because they are slightly movable.