The shoulder is a remarkable joint with one of the highest degrees of motion of any joint in the human body. Like most joints in the human body, it consists of multiple structures which work together in a complex manner to produce a strong, stable joint with an extraordinary degree of motion.
At the centre of the shoulder lies the shoulder blade (scapula) which has a socket (glenoid) towards its outer aspect. The upper arm bone (humerus) forms a ball shape (humeral head) at the end, which forms a ball-andsocket joint with the glenoid called the gleno-humeral joint which is what most people think of as the shoulder joint.
The collar bone (clavicle) helps suspend your upper limb from your chest. At its outer edge, it is attached to an area of your scapula called the (acromion). The joint where the clavicle and the acromion of the scapula meet is called the acromioclavicular joint sometimes called the AC joint. The AC joint is upported by ligaments between the acromion and clavicle, and some ligaments from another portion of the scapular called the coronoid process to the under surface of the clavicle. The AC joint can be damaged during injuries or may become inflamed or worn by disease processes, with both being frequent causes of shoulder problems. The other end of the clavicle is attached to the breastbone (sternum) and forms the joint called the sternoclavicular joint (SC