Anatomy of the shoulder
The shoulder joint complex is actually made up of three smaller joints; the gleno-humeral joint, the acromio-clavicular joint and the sterno-clavicular joint.
The gleno-humeral joint
This joint is a ball and socket joint made up of the humeral component (the ball on the end of the upper arm), and the glenoid (the socket which is actually part of the shoulder blade or scapula). The socket is very shallow and the ball sits in this much like a golf ball sits in a tee. In order to increase the stability of the joint the glenoid has a cartilage ring around it so that the socket is effectively made deeper.
There is a surrounding ligamentous capsule which is quite loose and allows a high degree of movement. Four short muscles and their tendons surround the joint and add to the control and stability of the joint. These are known as the rotator cuff and are often requiring rehabilitation in the event of shoulder injuries. Over the top of the gleno-humeral joint is the acromial process of the scapula. In this space are tendons of the rotator cuff and a bursa, which is a fluid filled sac.
The acromio-clavicular joint
This is the joint which attaches the shoulder blade (acromion) to the collar bone (clavicle). It is situated on the top of the shoulder and is supported by strong ligaments.
The sterno-clavicular joint
This joint joins the clavicle to the sternum at the front of the chest. It also relies on strong ligaments for primary support.
The shoulder joint has a large range of movement which is allowed by the combination of movement at all of these joints as well as the movement of the shoulder blade on the back of the chest wall. There are twenty different muscles involved in the movement of the shoulder including those which control the movement of the arm, the clavicle and the scapula. Injuries to the shoulder are common and include fractures, dislocations, ligament and tendon injuries, muscle tears and inflammatory disorders.