PATHOMECHANICS OF SHOULDER INJURIES

Extreme external rotation during throwing makes the shoulder prone to injury. As the shoulder externally rotates, the humeral head translates anteriorly. At maximum external rotation, the posterior rotator cuff may become impinged between the glenoid labrum and the humeral head. This "over-rotation" injury can cause degeneration of both the superior labrum and the rotator cuff.

Many rotator cuff tears in throwers are located from the midsupraspinatus posterior to the midinfraspinatus area. These injuries are usually due to tensile failure, as the rotator cuff muscles tried to resist distraction, horizontal adduction, and internal rotation at the shoulder during arm deceleration. Biomechanical research supports this explanation, as the posterior shoulder muscles (i.e., teres minor, infraspinatus, and posterior deltoid) are very active in producing compressive force, posterior force, and horizontal adduction torque during this phase (Figure 3).

Translation, subluxation, and compression of the humeral head can cause forceful entrapment of the labrum between the humeral head and the glenoid rim, resulting in labral tearing. The rapid rotations and large forces and torques produced at the shoulder during throwing make prevention of humeral translation quite challenging. Capsular laxity, as well as muscle weakness or fatigue, makes maintaining joint stability even more difficult, further increasing the chance of injury.

Degeneration of the labrum can also be caused by the "shoulder grinding factor." This factor results when the humeral head translates and subluxes during arm acceleration and arm deceleration. Humeral displacement combined with the compressive force and internal rotation during these phases causes the humerus to grind on the labrum.

During the arm deceleration phase, a large inferior force must be produced. The inability to generate sufficient inferior force can lead to superior translation of the humerus. Because the arm is abducted, horizontally adducted, and internally rotated, superior translation of the humerus can cause impingement of the greater tuberosity, rotator cuff muscles, or biceps against the inferior surface of the acromion or coracoacromial ligament. Supraspinatus, infraspinatus, and bicipital tendinitis, or even abrasion wear may occur.

Another type of shoulder injury is the SLAP lesion tear of the labrum. Although Snyder et al. described the most common injury mechanism for a SLAP lesion as a fall on an abducted and flexed arm, they did observe incidents of SLAP lesions from throwing. Andrews et al. observed a similar pathology, a tear to the anterosuperior labrum, in a series of 73 baseball pitchers and other throwing athletes. A traumatic episode was not present for most of these patients, and injury was believed to result from repetitive overuse throwing. In throwing, a SLAP lesion can result from the force of the long head of the biceps brachii pulling the labrum away from the glenoid. Biceps force is particularly strong during the arm deceleration phase, as the biceps contracts to both decelerate elbow extension and resist glenohumeral distraction. Laxity in the shoulder joint may result in increased shoulder compressive force needed, further increasing the demand on the biceps tendon labrum complex. This is supported by the findings of Glousman et al., who showed that the EMG activity level in the biceps was larger in shoulders with chronic anterior instability. Biceps force may be especially high in throwers with improper mechanics; in an EMG comparison between professional and amateur pitchers, biceps activity was greater for the amateurs. Because total force generated by the biceps is a combination of its contribution to elbow flexion torque and shoulder compressive force, evaluation of both of these loads may help explain the relevance of the EMG findings. With proper baseball pitching mechanics, maximum elbow flexion torque occurs before maximum shoulder compressive force. With improper mechanics these two loads may occur closer together in time, requiring a greater maximum force by the biceps.