Shoulder Impingement

The shoulder is the most complex joint in the body. It is capable of moving in more than 16,000 positions. Many of its ailments, including the most common ones, involve biomechanical mechanisms that are unique to the shoulder. The most common shoulder problem for which professional help is sought out for is shoulder impingement (Haig 1996). Shoulder impingement is primarily an overuse injury that involves a mechanical compression of the supraspinatus tendon, subacromial bursa, and the long head of the biceps tendon, all of which are located under the coracoacromial arch (Prentice 2001). Impingement has been described as a continuum during which repetitive compression eventually leads to irritation and inflammation that progresses to fibrosis and eventually to rupture of the rotator cuff. Because impingement involves a spectrum of lesions of tissue in the shoulder, a working knowledge of its structural relationships will facilitate an understanding of the factors that result in abnormalities. This paper will provide knowledge of the anatomy, biomechanics, and correct rehabilitation involved with shoulder impingement.

Impingement syndrome was originally described by Dr. Charles Neer as mechanical impingement of the supraspinatus muscle and the long head of the biceps tendon underneath the acromial arch. Neer classified three stages of impingement. Stage I is characterized by edema and hemorrhage of the rotator cuff and suprahumeral tissue. Stage II is characterized by fibrosis of the glenohumeral capsule and subacromial bursa and tendonitis of the involved tendons. Patients usually demonstrate a loss of active and passive range of motion because of capsular fibrosis. Stage III is the most difficult to treat and is characterized by disruption of the rotator cuff tendons. This includes rotator cuff tears, biceps rupture, and bone changes. Since this is a continuous disease process, there is often overlap of signs and symptoms (Hawkins and Abrams 1987).

For descriptive purposes, factors related to shoulder impingement can be divided into intrinsic and extrinsic categories. Intrinsic factors directly involve the subacromial space and include changes in vascularity of the rotator cuff, degeneration, and anatomy or bony anomalies. Extrinsic factors include muscle imbalances and motor control problems of the rotator cuff and parascapular muscles, functional arc of movement, postural changes, training errors, and occupational or environmental hazards. More likely, the cause of impingement has multiple factors. However, all factors may be important and the key factor in any case depends on individual circumstances.

According to Neer, the anterioinferior one third of the acromion is thought to be the causative factor in mechanical wear of the rotator cuff through a process called impingement (Donatelli 2004). Neer believes that the supraspinatus and long head of the biceps are subjected to repeated compression when the arm is raised in forward flexion. The result of repeated forward flexion is that the suprahumeral tissue is effectively driven directly under the anterioinferior one third of the acromion.

A force couple is defined as two forces of equal magnitude, but in opposite direction that produce rotation on a body. Two primary force couples are used in the shoulder to control the scapula and humerus. The scapular force couple is formed by the upper fibers of the trapezius, levator scapulae, and the upper fibers of the serratus anterior. The lower portion of the force couple is formed by the lower fibers of the trapezius and lower fibers of the serratus anterior. Simultaneous contraction of these muscles produces a smooth rhythmic motion to rotate and protract the scapula along the posterior thorax during elevation of the arm (Donatelli 2004). The scapular muscles function to rhythmically position the glenoid relative to the humeral head, therefore maintaining a normal length-tension relationship with the rotator cuff (Prentice 2001).

The internal rotators must be capable of producing humeral rotation on the order of 7,000 degrees per second. Thus the subscapularis tends to be stronger than the infraspinatus and teres minor, creating a strength imbalance. The opposite can also occur due to a poorly conditioned or fatigued subscapularis. The subscapularis is unable to control the excessive external rotation and extension of the humeral head. Over-hand throwing athletes are the most susceptible to develop rotator cuff muscle imbalance.

The primary intrinsic factors can be divided into vascular, degenerative, and anatomic categories. Rotator cuff vascularity is mostly affected in an area known as the critical zone. This zone is located approximately one centimeter medial to the insertion of the supraspinatus tendon. It is in this are where the supraspinatus is most likely to rupture. Rathbun and Macnab noted that the critical zone of the rotator cuff had an adequate blood supply when the vessels were injected with the arm in the abducted position, but this area was hypovascular when the injection was given with the arm in the adducted position. The authors propose a hypothesis of transient hypovascularity in the critical zone as a result of the vessels being "wrung out" when the arm was in the adducted position (Donatelli 2004). The authors indicated that most degenerative rotator cuff tears occur with in this zone, suggesting that hypovascularity of the supraspinatus tendon may play a role in the pathogenesis of rotator cuff tears. Although there is not yet any definitive scientific evidence of a direct cause and effect relationship, the finding seems to indicate a vascular predisposition to the pathogenesis of rotator cuff disease and impingement.

Evidence indicates that there is a natural age-related degeneration of the rotator cuff tendons. Rotator cuff tendon ruptures in older patients normally occur bilaterally and in the presence of preexisting tendon degeneration. The result of rotator cuff tendon degeneration puts the elderly population at a greater risk for developing type III shoulder impingement.

An abnormally shaped acromion will also cause impingement on the cuff tendons. Three types of acromions have been identified. They are: type I (flat), type II (curved), and type III (hooked). In a study performed by Morrison and Bigliani, 70% of rotator cuff tears were associated with type II and III acromions. None had type I. Although the causal relationship between the shape of the acromion and rotator cuff tears or impingement can be concluded, the clinical findings support Neer's theory of impingement occurring primarily along the anterioinferior acromion (Donatelli 2004). Haig (1996) describes shoulder impingement as producing an atrophic, "worn away" appearance of the cuff tendons, which are frequently retracted.

As compared to many joints in the body, rehabilitation