The anatomy of your shoulder allows it to have the greatest flexibility and range of motion of any joint in your body. Unfortunately, that anatomy can also allow your shoulder to become unstable or vulnerable to injury because the ball of the upper arm is larger than the shoulder socket that holds it. Muscles, tendons and ligaments serve to hold your shoulder in a stable position by supporting the bones.
Your shoulder has three major joints. The glenohumeral, or GH joint, the acromioclavicular, or AC joint, and the scapulothoracic, or ST joint. Each of these joints can be injured and cause discomfort or pain.
When people talk about the shoulder joint, they're usually referring to the glenohumeral joint, which is the ball-and-socket that links the upper arm to the body. The large, rounded end of the upper arm bone, called the humerus, moves within the shallow, scooped-out glenoid of the scapula or shoulder blade.
The acromioclavicular joint, or the AC joint, joins the scapula, or shoulder blade, to the clavicle. The AC joint can become separated - - often during contact sports, for example when a football player falls on the tip of his shoulder.
The third shoulder joint is the scapulothoracic or ST joint, which is formed by the scapula as it lies over the back of the rib cage, which is called the thorax. The ST joint is attached to the thorax by muscles and tendons. Your posture is closely linked with the healthy functioning of your ST joint, which in turn affects the shoulder structure.
The rotator cuff in the shoulder is formed by a group of four muscles and tendons that encircle and stabilize the shoulder joint. The rotator cuff can be vulnerable to tears and weakening due to a number of causes, including trauma, strain and overuse.
Your shoulder also contains structures called bursa sacs, which are fluid-filled membranes within and around the shoulder. Bursa sacs cushion the joints and help minimize friction between the bones and muscles.