Many different types of joints exist in the human body, but the kind which allows the greatest range of motion is the ball and socket joint. These joints are present where one bone ends with a spherical knob that lies in a circular depression in the other bone. This arrangement theoretically allows for 360 degrees of rotation — in other words, a full circle. Each shoulder has a ball and socket joint where the upper arm meets the shoulder blade. The hips also have this type of joint on either side, where the femur meets the pelvic bone.
Both the shoulder and hip, in addition to having a ball and socket joint, are known as synovial joints. These are the most common type of joints in the human body. In a synovial joint, the two bones are not connected in the same way as other joints, but have additional tissue around the moving bones to provide lubrication and nourishment. On the end of each bone, where it touches the other, is a layer of cartilage which allows the bones to move past each other with a minimum of friction. Surrounding the cartilage is a fluid-like substance called synovial fluid. This aids in lubricating the joint, as well as helping the cells of the cartilage to operate efficiently.
The cartilage and synovial fluid are contained within the synovial membrane, which in turn is contained in a fibrous structure called the joint capsule. Beyond the joint capsule are the ligaments which hold the bones in place, and the muscles and tendons that move the bones. This is the basic structure of every ball and socket joint.
There are slight variations in some cases. The shoulder joint, for instance, contains a small sac called the bursa, filled with synovial fluid, which helps the many tendons, bones, and muscles in the shoulder to glide past each other without friction. It also acts as a cushion between the joint itself and nearby bones.
Due to the complex nature of ball and socket joints, they are usually the ones that are most subject to disease and wear. Surgical replacement of the hips and shoulders is not uncommon if the joints become worn enough that they cause severe pain when used. Other diseases and problems characterized by inflammation and/or degeneration, such as arthritis, can exact a particularly heavy toll these joints, because of how much we depend on them for movement.
