The Development of Humeral Retrotorsion and Its Relationship to Throwing Sports

Abstract

Context: Several investigations have noted that throwing athletes exhibit a more posteriorly oriented humeral head (humeral retrotorsion) in the dominant arm. This asymmetry is believed to represent an adaptive response to the stress of throwing that occurs during childhood. The significance of this alteration and factors that affect its development are currently not clear.

Evidence acquisition: Basic science, research studies, and review articles were searched through PubMed with search terms including humeral torsion, humeral retrotorsion, and with 1 of the following: pediatric, adult, baseball, pitching, shoulder, and range of motion. The references from each article were reviewed for further inclusion. This review included articles through March 2015.

Study design: Clinical review.

Level of evidence: Level 4.

Results: The throwing motion creates stressors that result in bony adaptations that occur while skeletally immature. These osseous changes likely contribute to the observed shift in the arc of rotational range of motion noted in throwing athletes and may play a protective role against injury. However, too much or too little retrotorsion may predispose the shoulder to injury. The degree of "optimal" humeral retrotorsion and factors that influence its development are not fully understood.

Conclusion: Evidence supports the assertion that the throwing motion creates stressors that alter bony anatomy while young. It is important to determine what specific factors affect this adaptation and its relationship to injury.

Keywords: humeral retrotorsion; humeral torsion; pediatric; shoulder range of motion; throwing athlete.

Figure 1.

Schematic drawing of the left humerus, viewed from above. The dashed lines represent a line perpendicular to the proximal humeral articular surface (representing proximal humerus orientation) and the transepicondylar axis (representing distal humerus orientation). The difference between these angles is the degree of humeral retrotorsion present.

Figure 2.

Anterior view of the proximal left humerus. Dashed line, location of the proximal humeral growth plate; red arrows, insertion points of medial rotator muscles (pectoralis major, lattisimus dorsi, teres major) inferior to the growth plate; blue arrow, insertion of lateral rotators superior to growth plate.

Figure 3.

Posterior view of the proximal humerus. Dashed line, location of the proximal humeral growth plate; blue arrows, insertion of supraspinatus, infraspinatus, and teres minor superior to growth plate.

Figure 4.

Anterior view of the proximal left humerus indicating the opposing rotational forces exerted by muscles above and below the physis (dashed line). These forces influence proximal humeral positioning during normal development. Assuming the humeral head begins in a greater degree of retrotorsion (more posteromedially oriented position), these forces would facilitate derotation, resulting in a more anteromedial humeral head orientation.

Figure 5.

(a) While pitching, the weight of the forearm and ball produce an external rotation torsional force about the long axis of the humerus distally. (b) Body momentum, joint capsule, and muscular forces create an internal rotation torque at the proximal humerus. These opposing forces facilitate a more posteriorly oriented humeral head consistent with increased humeral retrotorsion.

Figure 6.

(a and b) Demonstration of how an increase in humeral retrotorsion would alter shoulder range of motion. The humeral head remains in the same orientation; however, if there is an increase in humeral retrotorsion (b), there is an apparent gain in external rotation range of motion.