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Shoulder Injuries and Disorders

The shoulder is a complex joint formed by the articulation of several bones surrounded by muscular attachments and ligaments to form what is the most versatile joint in the human skeleton. The complexity provides for outstanding range of motion but predisposes to injury. Mechanical stresses pose a risk for dislocation or fracture that may result in debilitating injury. Given the musculoskeletal and supporting structural relations a broad range of disorders are possible. In general, the most common disorders plaguing the shoulder joint can be classified and further differentiated based on the rotator cuff origin of the disorder, bony fracture involvement, and/or the arthritic components of the ailment.

The shoulder joint is frequently injured in a wide variety of activities ranging from sport related to repetitive overuse in the work environment. The shoulder offers tremendous range of motion and functions as the most mobile joint in the human body partly as a result of the unique bony articulation and supporting soft tissue architecture unique to the shoulder joint. The drawback to this increased mobility is the lack of structural restriction on undesirable and potentially hazardous movements.

The bony portion of the shoulder joint consists primarily of three structures, the collar bone (clavicle), the shoulder blade (scapula), and the upper arm (humerus). The term shoulder actually encompasses several less prominent articulations. The clavicle meets with the acromion process of the scapula to form the acromioclavicular joint (AC joint). A ball and socket joint is formed when the glenoid labrum (soft tissue lip around the periphery of the glenoid fossa of the scapula) contacts the head of the humerus (glenohumeral joint). The labrum is a key structural component that widens and deepens the articulation between the head of the humerus and the scapular head to create a unique dynamic between of stability and range of motion.

The muscular support of the shoulder is equally complex with three major groups of muscles providing support to the bony structures. Four muscles comprise the rotator cuff that aid in mobility as well as stability. The largest muscles of the chest and back also contribute to shoulder stability and both heads of the biceps originate from the scapula.

Dislocation

The shoulder joint is the most frequently dislocated major joint in the body. Dislocation results when the shoulder is forced beyond the limit of its normal range and the communication between the humerus (ball) and glenoid of scapula (socket) are displaced. Major complications of dislocation include nerve damage that may result in motor and sensory impairment. Following a shoulder dislocation, an athlete for example, will try to hold the injured extremity to the side, gripping the injured arm at the forearm with the opposite hand. Should the injury be significant enough to damage the major nerve supply to the region, denervation of the deltoid muscle will result as well as loss of sensation of the proximal lateral aspect of the arm.

Rotator Cuff

The rotator cuff represents the upper half of the joint capsule of the shoulder joint reinforced by the tendons of insertion of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles. Injury to the rotator cuff, a frequent cause of shoulder pain and disability, has a high incidence during athletic activities. Injury to the rotator cuff may result in a diminished range of motion at the shoulder joint, pain, weakness, and limitation of normal movements. The pain of a rotator cuff injury is often exacerbated by activities that will necessitate overhead positioning of the arm. Pain occurring at night is also common in such injuries as those affected will often roll onto the injured shoulder and are awoken as a result of the discomfort experienced. Weakness and a decreased range of motion are often a result of rotator cuff injury but pain alone results from bursitis and rotator cuff tendonitis. Each of these entities can be the result of what is known as the impingement syndrome.

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