Age-Related Medial Elbow Laxity in Youth Baseball Players: Ultrasonographic Evaluation.

Background: Medial elbow joint instability is a critical concern among young baseball players and is associated with throwing-related injuries. Greater medial elbow joint space (MEJS) gapping has been proposed as a key indicator of valgus instability. However, age-specific differences remain unclear, particularly in elementary school-age pitchers. Hypothesis/Purpose: This study aimed to clarify age-related differences in MEJS gapping by comparing joint space changes between resting and gravity stress conditions among young baseball players. It was hypothesized that 12-year-old players would show greater changes in MEJS compared with younger players. Study Design: Cross-sectional study; Level of evidence, 3. Methods: This study included 190 young baseball players (mean age, 11 ± 0.6 years) from 22 teams. Ultrasonographic assessment of the MEJS was performed during a resting and a gravity-induced valgus stress test. The change in MEJS was defined as the distance between the medial humeral epicondyle and the sublime tubercle of the ulna measured under each condition. The Kruskal-Wallis test was used to compare changes in MEJS among 3 age groups (10, 11, and 12 years), and the Wilcoxon rank-sum test was used to assess differences between the throwing and nonthrowing sides. Multiple regression analysis was performed to identify factors associated with the change in MEJS on the throwing side. Results: Significant age-related differences in MEJS changes were observed ( Conclusion: Our study demonstrated that 10-year-old players showed greater MEJS changes than 12-year-old players, highlighting the need for age-specific interpretation. These changes may reflect physiological joint laxity related to growth rather than pathological instability, with implications for ultrasonographic interpretation and injury risk.