Shoulder Anatomy and Physiology

The omni-directional shoulder is the most mobile and the most versatile of the human joints. The shoulder, in conjunction with the elbow joint and the ulnar and radial bones of the forearm, may be operated to create a powerful lever to perform lifting or prying movements. The ability of the shoulder to rotate is essential to all throwing, catching, or shooting actions in a multitude of sports. The construction of the shoulder permits the arm to be rotated 360° from every position relative to its connection to the shoulder.

The inherent flexibility and the range of motion present in the shoulder is due to the unique structure of the joint. Virtually every joint in the body brings two or more bones into close, one-on-one contact. The musculoskeletal framework of the shoulder mechanism relies to a much greater degree on the connective tissues of the shoulder, as opposed to its bones, to provide the joint with its strength as well as its ability to absorb significant force. The shoulder has the greatest risk of soft tissue injury of any of the joints that propel human limbs.

The skeletal components of the shoulder include humerus, the long bone of the upper arm that is a part of the elbow and the shoulder joints; the scapula, or the shoulder blade, positioned on the posterior (rear) of the shoulder on the upper back; the clavicle, or the collarbone, aligned between the neck and the outer limit of the shoulder; and the sternum, or breastbone, which is not a primary bone in the formation or function of the shoulder joint. The sternum is a bone that supports the opposing end of the clavicle from the shoulder.

Each of the shoulder bones has articular cartilage covering the head of the bone that comes into contact with another bone, reducing the degree of friction created by two bones moving against one another. Articular cartilage also provides a measure of cushioning between the bones. Osteoarthritis is a common form of disease in the shoulder region among athletes who, through the wear and tear of repetitive motions such as throwing, sustain a gradual thinning or wearing away of the articular cartilage.

Shoulder movement and any corresponding injuries are commonly described with reference to the shoulder joint, as if the structure had a single means of flexion, extension, and rotation. The shoulder comprises four separate joints, each an integrated device that is capable of a degree of independent movement.

The glenohumoral joint is the largest and the most prominent of the joints that contribute to shoulder movement. It is this part of the shoulder that is most commonly described as the shoulder joint. The glenohumoral structure is created by the meeting of the head of the humerus and the portion of the scapula known as the glenoid. These two bones create a "ball and socket" mechanism, which permits the shoulder to rotate freely. The flexibility of the glenohumoral joint is the result of its own intricate ligament structure as well as the position of the rotator cuff relative to the glenohumoral structure. The joint is powered in part through its connection to the biceps tendon, which provides the linkage to the biceps muscle. The remainder of the joint's movement is directed through the muscles and tendons of the rotator cuff. The entire glenohumoral joint is enclosed in a loose membrane capsule that contains a small quantity of synovial fluid that also assists in the movement of the joint.

The acromioclavicular (AC) joint is formed between the clavicle and the region of the scapula known as the acromion. It is the AC joint that is subject to injury when a shoulder is said to become "separated," which refers to the clavicle becoming detached within the AC joint.

The sternoclavicular joint is created at the junction between the sternum and the clavicle, at the base of the neck.

The scapulothoracic articulation is not a joint in the technical sense of two or more bones in conjunction; this structure is a muscle and tendon mechanism that permits the scapula to slide without obstruction along the upper back as the arm is raised or extended during the movements of the shoulder.

The rotator cuff is the most important soft tissue structure in the shoulder. The rotator cuff is essential to any action in relation to the throwing or catching of objects in sport. The rotator cuff, positioned on the top of the glenohumoral joint, provides the muscle power to assist the shoulder in movement, permitting circular motion. The rotator cuff also limits the joint from being overextended when the arm is extended upward. Of the four muscles and tendons that form the rotator cuff, the supraspinatus muscle and its tendon, which extends from the rotator cuff to the top of the humerus, are the most frequently injured of the rotator cuff tissues. Baseball pitchers, athletes who often sustain an injury described generally as a rotator cuff tear, most often sustain a specific injury to the supraspinatus tendon.

The next most important of the soft tissue features of the shoulder are the two bursa, located at the highest part of the shoulder. The bursa, which are

X-ray of a shoulder.
small, fluid-filled sacs constructed of fibrous material, provide cushioning to the joint; the bursa work particularly to protect the rotator cuff from excessive contact with the acromion, a bony extension of the scapula. Overuse of the shoulder, most often through repetitive throwing, will often irritate the bursa fibers, creating an often chronic condition known as bursitis.

SEE ALSO Back anatomy and physiology; Bone, ligaments, tendons; Musculoskeletal injuries; Shoulder injuries.