Age-related Humeral Retrotorsion Asymmetry in Junior Tennis Players: A Cross-sectional Ultrasound Study.

BACKGROUND: Humeral retrotorsion normally decreases during growth through physiologic derotation. In overhead athletes, however, repetitive loading of the dominant arm may restrict this process and lead to side-to-side asymmetry in humeral retrotorsion. Although tennis involves repeated overhead loading, it remains unclear whether junior tennis players develop such asymmetry and when it first becomes evident. Therefore, the purpose of this study was to quantify bilateral humeral retrotorsion angle (HRA) using an ultrasound-assisted technique, determine the magnitude of side-to-side difference, and identify the age at which dominant-to-nondominant differences first become statistically evident using single-year age strata.

METHODS: In this cross-sectional, age-stratified study, 213 competitive junior tennis players (200 boys and 13 girls; mean age, 13.0 ± 2.5 years; range, 9-17 years) underwent bilateral ultrasound-assisted assessment of HRA during tournament medical check-ups in Hokkaido, Japan. HRA was measured in both shoulders using a standardized ultrasound-guided positioning technique, and side-to-side difference (ΔHRA) was calculated as dominant minus nondominant HRA. Dominant and nondominant HRA were compared overall and within each single-year age stratum using paired t tests with Holm adjustment. Pearson correlation coefficients were used to assess associations between age and HRA.

RESULTS: Overall, HRA was significantly greater in the dominant arm than in the nondominant arm (78° ± 12° vs. 70° ± 12°, P < .001), with a mean ΔHRA of 8°. In age-stratified analyses, dominant-to-nondominant differences were not significant at age 9 years but were significant from age 10 years onward, and mean ΔHRA remained approximately 10° from age 13 years onward. HRA decreased with age in both shoulders, but the decline was smaller on the dominant side (dominant r = -0.377; nondominant r = -0.577; P < .001 for both).

CONCLUSION: Junior tennis players demonstrated greater dominant-arm HRA than nondominant-arm HRA, with side-to-side asymmetry becoming evident from age 10 years onward and remaining approximately 10° from age 13 years onward. These findings indicate that humeral retrotorsion asymmetry emerges during late childhood in tennis players and likely reflects altered physiologic derotation of the dominant humerus during growth. This osseous adaptation may help clinicians interpret shoulder rotational findings in young overhead athletes.