With the greatest range of motion than any joint in the body, the shoulder is key to enjoying full use of the hands and upper body. The shoulder is made up of three bones – the scapula or shoulder blade, the humerus or upper arm bone, and the clavicle or collarbone. The two joints that facilitate shoulder movement include the acromioclavicular (AC) joint, located between the acromion (a bony projection sitting at the top of the shoulder blade or scapula), and the clavicle. The acromion serves as the point of origin for the deltoid muscle, while joining the clavicle to form the AC joint.
The bones of the shoulder are secured by muscles, tendons and ligaments. Tendons are tough cords of tissue that attach the shoulder muscles to the bone and assist the muscles in moving the shoulder. Ligaments attach the shoulder bones to each other and provide stability. The deltoid muscle is among the strongest muscle in the muscle joint and attaches to the humerus midway down the bone, facilitating the movement of the arm lifting up and away from the side of the body. This type of motion is called abduction.
Beneath the deltoid muscle is a group of four muscles and supporting tendons – called the Rotator Cuff – which helps control the broad range of shoulder joint motion. The muscles of the rotator cuff consist of the infraspinatus and the teres minor (posterior, or behind) and the supraspinatus (superior, or top) – which are responsible for outward rotation. And the subscapularis (anterior, or front) is responsible for inward rotation.
The subscapularis muscle is attached to the surface of the scapula and runs down across the front of the humeral head – settling into a bony ball on the humerous known as the lesser tuberosity. It plays an important role in preventing the shoulder from slipping out of the front of the joint.
The rotator cuff muscles function to help keep the shoulder “in socket” and properly “located.” The labrum is located around the socket and also helps keep the shoulder in place. Because the shoulder is stabilized by muscles and not just ligaments as most other joints in the body, the components of the rotator cuff can adjust length and tension accordingly to suit the required range of motion.
These muscles are attached to the humeral head by tendons and help keep the humeral head, or ball, centered within the glenoid, or socket, when the deltoid and other large shoulder muscles are utilized and when the arm is lifted overhead. The rotator cuff is surrounded by a fluid-filled sac called the bursa, which permits the smooth gliding between tendon, muscle and bone. The bursa also provides a cushion between the rotator cuff and the acromion.
Because of the large range of motion the shoulder joint can achieve and correspondingly support, it is often the site of sports-related and other injuries. In fact, according to the American Academy of Orthopaedic Surgeons, approximately four million people in the United States annually seek medical care for shoulder sprain, dislocation or other instability problem. Rotator cuff tears and shoulder instability conditions are among the most common shoulder injuries.