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Lower degrees of dominant humeral torsion results in severe upper extremity injuries for pitchers
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Major League Baseball pitchers with lower degrees of dominant humeral torsion had more severe upper extremity injuries, and pitchers with lower side-to-side differences in torsion experienced more severe dominant upper extremity injuries, according to results of this recently published study.
In the study, 25 professional pitchers from a single Major League Baseball organization underwent CT on dominant and nondominant humeri. Image data were processed with a 3-D volume-rendering post-processing program. Researchers then modified the software program to model a simplified throwing motion to measure potential internal impingement distances in a small subset of players. The researchers followed the players for 2 years and recorded the number of days missed from pitching as a measure of injury severity and incidence.
Overall, the mean dominant humeral torsion was 38.5°, while the mean nondominant humeral torsion was 27.6°. Overall, 45% of pitchers were injured during follow-up. Five players had shoulder injuries, seven players had elbow injuries and two players had finger injuries.
According to the study results, dominant humeral torsion was a statistically significant predictor of severe injuries, but not of milder injuries. Researchers found a strong correlation between a high number of days missed because of injury and lower degrees of dominant humeral torsion and smaller differences between dominant and nondominant humeral torsions. Study results showed no significant association between the incidence of shoulder injury and minimum glenoid-tuberosity distance in the dominant or nondominant shoulder or degree of dominant glenoid version.
“As these results show, the interplay of dominant humeral torsion, torsion difference and shoulder injury is complex,” the researchers wrote.
Disclosure: This study was supported by grant funding from Major League Baseball.
Perspective
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Samer S. Hasan, MD, PhD
Several studies into professional baseball pitchers, including the study by Polster and colleagues, have observed a bilateral difference in humeral torsion, with the dominant side demonstrating increased torsion. This occurs during skeletal maturation and can be explained as an adaptation to repetitive stress. Specifically, the proximal humeral physis undergoes asymmetrical bone remodeling in response to high torque shoulder external rotations during pitching. The resultant increased humeral torsion serves to maximize the external rotation available during the late-cocking phase of throwing and, in turn, the arc of rotation available for internal rotation acceleration.
Increased humeral torsion may also protect the throwing arm from injury, but this effect remains incompletely understood. Polster and colleagues found that lower degrees of dominant humeral torsion correlated with more serious upper extremity injuries in their small sample study. Increased humeral head torsion may protect the arm from injury by limiting the twisting of the long head of the biceps and glenohumeral ligaments during external rotation, but this has not been verified clinically or experimentally.
Professional pitchers are an ideal population to study because they are uniformly skilled and conditioned, and are followed closely and systematically. However, studies into professional athletes are often limited by small sample size and abbreviated follow-up. Additionally, observations in professional pitchers may not generalize to the far larger and more heterogeneous population of young overhead throwers. For example, Whiteley and colleagues studied a small group of adolescent baseball players and found that lower degrees of nondominant, rather than dominant, humeral torsion predicted a higher risk of injury. Large-scale longitudinal studies of overhead throwers are needed to determine if increased torsion confers protection from specific injuries such as labrum, rotator cuff, or ulnar collateral ligament tears. Such studies may require alternative strategies to image the proximal humerus, such as with ultrasonography, in order to avoid the radiation exposure during computed tomography.
Ultimately, it is hoped that an enhanced understanding of glenohumeral joint adaptations will refine existing youth baseball injury prevention strategies centered around proper conditioning, adequate rest intervals, controlled introduction of breaking pitches, and strict pitch count enforcement.
Reference:
Whiteley RJ. J Sci Med Sport. 2010;doi:10.1016/j.jsams.2009.06.001.
Perspective
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Kevin E. Wilk, PT, DPT, FAPTA
In this article, the investigators reported a strong relationship between lower degrees of dominant humeral torsion and more severe injuries to the upper extremity, as well as a trend relating side to side differences in torsion with more severe upper extremity injuries.
These conclusions are in agreement with the findings published by Pieper who reported a lower injury rate in team handball players with exhibited greater humeral torsion in their throwing arm. These results suggest that the greater degree of glenohumeral joint external rotation may be protective for the overhead thrower. Higher degrees of humeral torsion results in greater glenohumeral joint external rotation and lesser degrees of internal rotation. Wilk and colleagues reported a lower shoulder injury rate in professional baseball pitchers who exhibited higher degrees of glenohumeral joint external rotation. Sweitzer and colleagues reported a lower injury rate in baseball pitchers with greater glenoid retroversion.
The humeral torsion values reported by Polster and colleagues are similar to previously reported humeral torsion values of others. A surprising finding in this study was that three pitchers of the 25 pitchers studied exhibited greater humeral torsion on their nondominant arm. Based on the conclusions of this study, these three pitchers would be at high risk for a severe upper extremity injury.
Lastly, we all realize that shoulder and elbow injuries in the overhead athlete are multifactorial. There are numerous causes of these injuries: improper mechanics, arm fatigue, tissue overload, glenohumeral internal rotation deficit (GIRD), total range of motion, external factors and others. But to this point, if increased humeral torsion is protective, then greater glenohumeral joint external rotation and lesser internal rotation will occur. Thus, GIRD will be present in that situation. GIRD may not be the most significant factor regarding shoulder/elbow injuries in the overhead athlete. Several recent studies have found that GIRD did not exhibit a relationship to injuries but rather the side to side difference in total rotational motion.
Further research is necessary to determine the role of humeral torsion, GIRD, total range of motion and other factors and their relationship to shoulder/elbow injuries in the overhead thrower.
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