Walch et al6 first described internal impingement as intra-articular contact between the posterosuperior rotator cuff (infraspinatus) and posterior glenoid labrum at maximal shoulder abduction and external rotation in tennis players. Anatomic lesions associated with internal impingement include injury to the articular surface of the rotator cuff (especially the infra-spinatus tendon in throwers and the supraspinatus tendon in tennis players), posterior and superior labrum, and anterior capsular structures. Posterior capsular thickening and contracture have also been reported as common findings11 (Box 23-2).
Injury to the posterosuperior rotator cuff (infraspinatus) due to direct abutment against the glenoid and labrum with the shoulder in abduction and external rotation
Labral fraying or detachment SLAP (superior labrum anterior to posterior) kissing lesions
Mild excessive anteroinferior capsular laxity is often present Bony and soft-tissue alterations or adaptations that allow excessive external rotation
Posterior capsular contracture with loss of external rotation
The biomechanical etiology for injury to these structures is controversial. Two possible causes have been reported, although neither is universally accepted. The most prevalent theory has been termed rotational instability, which describes the ability of the throwing shoulder to overrotate into a position of hyperex-ternal rotation during the late cocking and acceleration phases of the throwing motion. At maximal external rotation, the undersurface of the rotator cuff becomes entrapped between the humeral head and posterior superior glenoid labrum. This extreme position is resisted actively by the subscapularis and passively by the anterior band of the inferior glenohumeral ligament. Most shoulders, even those without symptoms, can achieve this position, and it is not considered pathologic.2,22 However, this physiologic posture gradually creates pathology in overhead throwers because throwing imparts progressive microtrauma to the anterior capsular structures. Fatigue of the sub-scapularis or overload of the inferior glenohumeral ligament may lead to subtle anterior instability that can worsen the impingement by facilitating this rotational instability. If these restraints fail to keep the humeral head centered in the glenoid, then the amount of external rotation will be accentuated.23 This, in turn, imparts even greater stress to the rotator cuff and labrum. Repetitive posterior impingement, particularly the aggressive act of throwing a baseball, may lead to fraying of either the labrum or rotator cuff and eventually pain. This cascade can result in altered mechanics, which will further irritate and fatigue the rotator cuff and scapular musculature, increase pain, and decrease effectiveness.
Another hypothesis to explain the injury cascade of internal impingement has focused on the presence of posterior capsular tightness and contracture as the predominant cause of injury.11 Burkhart and Morgan theorized that repetitive distraction microtrauma in tension that occurs in the follow-through phase of the throwing motion in the posterior shoulder leads to scarring and capsular contracture posteroinferiorly. This has been demonstrated to cause obligate posterior and superior migration of the humeral head on the glenoid, allowing relaxation of the anterior capsule, and excessive external rotation, which may lead to injury of the posterior labrum and rotator cuff.12,24 In addition to excessive external rotation, there is a disproportionate loss of internal rotation that they termed glenohumeral internal rotation deficit (GIRD). However, the authors of this chapter believe that overhand athletes, particularly baseball players, do not commonly present with clinical tightness of the posterior capsule.
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Since World War II, there has been a tremendous change in the makeup and direction of kid baseball, as it is called. Adults, showing an unprecedented interest in the activity, have initiated and developed programs in thousands of towns across the United States programs that providebr wholesome recreation for millions of youngsters and are often a source of pride and joy to the community in which they exist.