Joint

From ArticleWorld

A joint is the junction between two bones, generally for the purpose of movement of the bones in relationship to one another. Joints have the dual purposes of giving adequate movement along with an effective stability at the point of bony connection.

Structural Classifications

There are three structural classifications of joints. “Fibrous” joints connect bones by means of dense connective tissue. Such joints are actually rather inflexible except in children where the joints are not completely formed. The cranial sutures are examples of fibrous joints. In adults, they can appear to be completely ossified.

“Cartilaginous” joints have only cartilage to connect the bones. Slight movement or flexibility is allowed by such joints. The connection between the ribs and the sternum, the pubic symphysis and the articular surfaces in the spinal column are all considered cartilaginous joints.

“Synovial” joints are what one thinks of when thinking of a joint. They are also sometimes called “diarthrosis joints” and generally permit more movement than the others. Synovial joints have some sort of space (containing synovial fluid) between the articular surfaces of the bones.

Functional Classifications

Joints can be classified as to their function. “Syndesmoses” involve the union of two bones by ligaments. “Synchondroses” involve two bones joined and stabilized by cartilage. “Synostoses” involve the bony union of two joints to the point where they become a single bone. The cranial sutures are this type of joint. None of the above joint types are moveable and are simply joining two stable bones together.

The term “amphiarthroses” refers to joints that allow a slight degree of movement. Most of these are cartilaginous joints such as the rib articulation with the vertebra.

“Diarthrosis” joints permit a great degree of movement. Each joint is stabilized by a number of ligaments that form the articular capsule. Inside the joint, the bony articular surfaces are protected by smooth cartilage. If the joints do not fit tightly together, a cartilaginous meniscus surrounds the borders of the joint allowing for a snug fit for the bony surfaces.

All around the inside of the joint is a synovial membrane that secretes a lubricating synovial fluid that facilitates movement.

Joint Shapes

There are several different joint shapes. The ball-and-socket joint is like the hip joint. The saddle joint is like that of the connection between the metacarpal and the carpal bones. A condyloid joint is like the knee joint where two odd surfaces come together with little rotation allowed. A pivot joint is like that of the radius and ulna that slide across one another. A hinge joint is like the elbow joint that allows for flexion and extension about the joint. Gliding joints are multifaceted and are like those involving the small bones of the wrist which move in many different ways.