Anatomy 101: The Shoulder

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While just about everyone knows what a shoulder is and can readily point to their shoulder, a precise definition appears to have eluded anatomists. There is a shoulder blade (scapula bone), a shoulder joint (site of connection between the arm and shoulder glenohumoral joint) and a tip of the shoulder (the outward/lateral projecting point of the shoulder acromion) but it is quite difficult to arrive at a specific definition of the shoulder in many anatomy books.

What can be considered to form the skeletal structure of the shoulder includes the scapula, clavicle (collar bone) and head (rounded upper portion) of the humerus bone of the arm. The rounded head of the humerus projects into a space in the scapula called the glenoid cavity to form the glenohumeral joint. The clavicle protrudes across the upper region of the chest, just beneath the skin, from the midline breast bone (sternum) to the tip of the scapula (acromion). In this way, the clavicle serves as a strut providing structural support for the glenohumral joint and maintains this joint at a distance from the chest to enable free movement of the arm.

As the head of the humerus is rounded, it creates a ball-and socket glenohumoral joint which permits a considerable range of arm movement. A number of ligaments and four notable muscles, collectively called the rotator cuff, hold the arm within this joint and, in combination with the deltoid muscle (covering the upper potion of the shoulder) and latissimus dorsi (located on the back) are responsible for producing the extensive array of arm movements. Unfortunately, these permissive arm movements come at the expense of increased potential for injury.

A shoulder dislocation, which actually is a dislocation of the arm, results when the head of the humerus becomes dislodged from within the glenoid cavity. The glenoid cavity that houses the head of the humerus is relatively shallow and therefore does not completely encompass the head of the humerus. Moreover, the ligaments and rotator cuff muscles that secure the head of the humerus are not all that strong, in part, to permit a wide range of motion. As a result, this head of the humerus has the potential to slide out from this glenohumoral cavity when an excessive force is applied onto the outstretched arm to produce a shoulder dislocation.

A shoulder separation occurs when the clavicle detaches from its attachment to the tip of the shoulder - acromion. These can result from a strong force applied to the acromion, which then slides the acromion beneath the clavicle, tearing the ligaments that connect these two structures. Such injuries are commonly observed in football as a result of a severe fall on the side of the shoulder. This injury is relatively easy to diagnose as the free/detached end of the clavicle will be protruding and require external support to prevent the shoulder from drooping.

Generalized pain in the shoulder can result from many conditions. Tears of rotator cuff muscles and/or degeneration of bursa that normally function to reduce the frictional motions of tendons during movement can produce shoulder pain. Some conditions producing shoulder pain may have nothing to do with the shoulder. Some spinal diseases, pressure from additional ribs and/or irritations of the chest and abdomen can produce referred pain that is experienced in the shoulder.

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Anatomy 101: The Shoulder