The shoulder is composed of three bones, three joints, and the muscles, tendons and ligaments that connect them. The three bones are the humerus (arm bone), the scapula (shoulder blade) and the clavicle (collarbone). The three joints are the glenohumeral, acromioclavicular, and sternoclavicular.
The largest joint is the glenohumeral joint which is a ball and socket joint made up by the shoulder blade and the arm bone. The socket, or glenoid, is part of the shoulder blade and is very shallow, resembling a golf tee in appearance. The humerus, or arm bone, has a ball on the upper end that rolls in the socket to allow arm motion. Because the socket is not deep, the ball can move freely without the shaft of the arm bone hitting the edge of the socket, which would limit motion. Instead, the shoulder relies on ligaments and a group of tendons called the rotator cuff to hold the ball in the socket. Because the shoulder relies primarily on soft tissue for stability, its range of motion is substantially greater than other joints. However, this also makes the shoulder prone to injury.
The acromioclavicular joint is made up of the collarbone and a part of the shoulder blade called the acromion. It is located almost directly over the glenohumeral joint. In contrast to the glenohumeral joint, there is very little motion at this joint. It is held together by the acromioclavicular and coracoclavicular ligaments. Injuries to these ligaments are known as a ‘separated shoulder’.
The sternoclavicular joint connects the collarbone to the sternum, or breastbone, and helps support the shoulder. It is rarely injured, except in high-velocity trauma, but arthritis may develop in the joint leading to the appearance of a tender bump where the bones connect.
The deltoid muscle is a large powerful muscle, which attaches the shoulder blade and collarbone to the arm bone. Its function is to lift, flex and extend the shoulder joint. It provides most of the power in the shoulder.
The rotator cuff is a group of four muscles that surround the humeral head. They are the supraspinatus, infraspinatus, teres minor and subscapularis. Their job is to stabilize the humeral head in the socket and to assist in elevation and rotation of the shoulder. The portion of a muscle that attaches to bone is referred to as a tendon, and the rotator cuff has four tendons, which are located in a small space between the humeral head and the acromion. If the rotator cuff becomes injured and swells, this small space may lead to further injury and possible tearing, or detachment, of the rotator cuff.
The biceps is a muscle that flexes the elbow and turns the forearm up. It has two attachments to the shoulder blade. One to the coracoid process in the front, which is seldom injured, and one inside the shoulder joint to the top of the socket, which is frequently injured.
Other muscles that attach near the shoulder include the latissimus dorsii in the back and the pectoralis major in the chest. These large muscles act to internally rotate the humerus.
Inside the shoulder joint, the labrum is a cartilage ring that surrounds the socket and makes it deeper. The labrum is soft and flexible, so it can provide an extra layer of stability without interfering with motion. Several ligaments, or fibrous connective tissue that hold bones in position, assist the rotator cuff by helping to hold the ball in the socket.
Mechanism of Injury
The shoulder can be injured as a result of trauma, repetitive use, or degeneration that occurs with age. Trauma is a leading cause of injury in the young. Falls on to an outstretched hand, or falls that lead to a sudden twisting injury to the shoulder can lead to fractures or dislocations. Repetitive use is a common cause of shoulder injury. People whose occupations require a large amount of overhead work, such as plumbers, electricians, carpenters and mechanics are particularly susceptible. Athletes who play sports that require overhead use of the shoulder, such as baseball, tennis, volleyball and swimming, can develop shoulder pain. Finally, as the body ages, it loses flexibility. The same is true for the rotator cuff and ligaments that hold the shoulder in to the socket. They become stiffer and more easily torn. Normal wear and tear on the joint can accumulate over a lifetime, and can lead to tears of the rotator cuff without any specific injury.