In the human body, joints are points of articulation—locations where bones meet.
Types of Joints
There are three main types. The first—fibrous joints—are stable and allow for little to no mobility. For instance, the sutures of the skull are held together by fibrous connective tissue and do not allow movement between adjacent bones.
The second kind is cartilaginous joints—where fibrocartilage connects bones, which makes them strong and flexible to allow for bending motions. Examples are the intervertebral discs between the vertebrae in vertebrates.
The third and most common group, synovial joints, allow for the greatest mobility. In these joints, the bony surfaces are not directly connected. Rather, they are covered by a thin layer of cartilage and surrounded by an articular capsule that is filled with synovial fluid to provide lubrication. One example is the glenohumeral joint of the shoulder, also known as a ball and socket joint. The rounded part—the head of the humerus—is located within the glenoid cavity, the socket. The surrounding capsule is then supported by ligaments and rotator cuff muscles to yield a wide range of motion, in all directions, to enable reaching an object.