Relationship Between Humeral Torsion and Injury in Professional Baseball Pitchers

Background: High levels of humeral torsion allow baseball pitchers to achieve maximum external rotation in the late cocking phase of pitching with lower twisting and shear forces on the long head of the biceps tendon and rotator cuff tendons. Hypothesis: Humeral torsion is inversely related to the incidence and severity of shoulder injuries and other upper extremity injuries in professional baseball pitchers. Study Design: Case-control study; Level of evidence, 3. Methods: A total of 25 professional pitchers from a single Major League Baseball organization were prospectively recruited into this study. Computed tomography (CT) was performed on dominant and nondominant humeri, and image data were processed with a 3-dimensional volume-rendering postprocessing program. The software program was then modified to model a simplified throwing motion and to measure potential internal impingement distances in a small subset of players. Players were followed for 2 years after CT, and the number of days missed from pitching activities was recorded as a measure of injury severity and incidence. Results: The mean dominant humeral torsion was 38.5 degrees 8.9 degrees; the mean nondominant humeral torsion was 27.6 degrees +/- 8.0 degrees. The difference between dominant and nondominant torsions was significant (P < .0001). Among the 11 pitchers (44%) injured during follow-up, 5 players had shoulder injuries, 7 had elbow injuries, and 2 had finger injuries. Dominant humeral torsion was a statistically significant predictor of severe injuries (30 days; P = .048) but not of milder injuries. Among injured players, higher numbers of days missed because of injury were strongly correlated with lower degrees of dominant humeral torsion (r = -0.78; P = .005) and smaller differences between dominant and nondominant humeral torsions (r = -0.59; P = .055). There was 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. Conclusion: A strong relationship was found between lower degrees of dominant humeral torsion and more severe upper extremity injuries as well as a trend relating lower side-to-side differences in torsion with more severe dominant upper extremity injuries. In addition, there was a higher incidence of severe injuries in players with lower degrees of dominant torsion. If future studies confirm these results, humeral torsion measurements could play a role in risk assessment in pitchers.