In our evaluation of 193 pitchers examined during their preseason physical examinations, we identified a statistically significant association between decreased total arc of motion, decreased shoulder strength, and GIRD.
The pitching motion is composed of a carefully orchestrated kinetic chain that transfers energy from the legs, up through the pelvis and trunk, into the upper extremity, and finally to the baseball. As energy is funneled from the trunk into the shoulder and elbow, these progressively smaller joints undergo tremendous stresses.24 During delivery of a pitch, as the point of maximum ER is reached, the shoulder experiences up to a 111 Nm IR torque and up to 1,090 N in compressive force across the glenohumeral joint, immediately after the ball is released.24,25 As pitchers repeat this sequence numerous times over the course of a contest, season, and career, anatomic changes occur in the shoulder to accommodate the chronic stress.24 In addition to the kinetic chain-of-energy transfer, the evaluation of the scapula is necessary to exclude diagnosis of scapular dyskinesis. Kinsella et al26 noted that protracted or excessive IR of the scapula leads to rotator cuff weakness, which causes a poor length-tension relationship of the rotator cuff complex. This is often seen because the origin of the rotator cuff complex is from the scapula.
Further evidence suggests that shoulder adaptive changes occur before skeletal maturity in throwing athletes.16,27,28 Meister et al16 measured shoulder IR and ER in 294 baseball players between the ages of 8 and 16 years and found significant differences in ER and IR between dominant and non-dominant arms. They noted that the total arc of motion decreased as age increased, with the most dramatic decrease occurring in 13- and 14-year-olds. They postulated that this loss of motion was a result of bone and soft-tissue adaptation, particularly in response to stress at the proximal humeral physis during throwing.16 Between the ages of 13 and 16 years, this physis undergoes rapid growth and may be particularly susceptible to the stress of throwing. Levine et al27 made similar observations in a study of pitchers across a wider age range (8 to 28 years old). Crockett et al11 reported that increased humeral head retroversion at least partially accounts for the highly consistent findings of increased ER and decreased IR in pitchers. The authors speculated that this osseous adaptation plays a central role in the development of GIRD and may confer a competitive as well as potentially protective advantage.11 Kinsella et al26 noted increased humeral retroversion changes of the thrower's arc of motion, with a gain in ER motion with a loss of IR motion. This increase allows the thrower’s shoulder to increase ER before the greater tuberosity engagement with the posterior superior labrum in the abduction ER position.26 The change in the arc of motion can lead to posterior cuff and labral pathology in throwers. Thomas et al29 also found increased retroversion of the humerus to lead to posterior capsular thickness or GIRD, which creates negative effects on the shoulder kinematics. The humeral retroversion leads to a loss of IR and a symmetric gain of the ER motion, in most cases with an arc loss of motion of 10° to 17°.29
While the studies described earlier support the theory of osseous adaptive changes to the shoulder in response to the demands of pitching in younger age groups, other researchers attribute changes in shoulder rotation to soft-tissue microtrauma and the gradual onset of a pathologic process, even after skeletal maturity. Burkhart et al6 postulated that the observed increase in ER and decrease in IR is primarily a result of repetitive microtrauma to and eventual stretching of the anterior capsule of the glenohumeral joint during the cocking phase of throwing. The subsequent acquired loss of IR arises from posteroinferior capsular contracture, which functions as the “essential lesion” in the development of GIRD. Tehranzadeh et al28 performed a retrospective review of magnetic resonance arthrograms in six professional pitchers who had presented with pain and reduced velocity and had been diagnosed with GIRD. They noted that posterior capsular thickening was a clear and consistent finding in all six patients. Associated lesions seen on imaging were partial undersurface rotator cuff tears and type 1 and 2 SLAP (superior labrum anterior-to-posterior) tears. Burkhart et al23 noted that professional throwers who were examined in the preseason with an IR rotation deficit >25° had a higher rate of shoulder difficulties during the upcoming season compared with the contralateral shoulder. Osseous as well as soft-tissue adaptations are likely both involved in the development of GIRD; however, the relative importance of each is unknown. Clabbers et al30 placed cadaver specimens in the late-cocking position after imbricating the posterior capsule to assess the relationship between posterior capsular tightness and changes in the glenohumeral relationship. The imbricated posterior capsule leads to relative posterior and superior migration of the humeral head. Huffman et al31 and Grossman et al32 found translation of the humeral head with posterior capsule tightness in addition to the anterior capsular laxity of the arm in the abduction ER position. The posterior inferior capsular contracture leads to an inferior tether that does not allow the dominant arm to achieve a full ER of the humerus. The subsequent results of the new center of rotation leads to an increased contact along the posterior cuff and labrum, leading to eccentric fiber failure and articular surface tearing. In contrast, Bailey et al33 found posterior rotator cuff stiffness to be the primary cause of ROM deficits with the baseball cohort. Within the two cohorts, baseball players with instrument-assisted soft-tissue mobilization with self-stretching noted a decrease in the posterior rotator cuff stiffness and an increase in ROM compared with the healthy baseball players. Harshbarger et al34 found a relationship between rotator cuff stiffness and strength. Similarly, this study found a loss of motion within the total arc led to a decrease in shoulder strength.
The small number of professional baseball pitchers within one professional organization led to statistical challenges of the power of this study. Recognizing the limitations of this study, we feel that further studies accounting for playing time missed may provide clearer answers to the proposed effects of shoulder strength, GIRD, TRM and shoulder injuries. It may provide an insight for the medical staff to identify players at a higher risk of injury during the spring training examination and to create a threshold of shoulder strength to avoid risk of shoulder issues in the ensuing season.
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