Influence of medial epicondyle morphology on the contribution of flexor–pronator muscle contractions to dynamic elbow stability

Background: Elbow injuries are common among baseball players, often involving the medial epicondyle of the humerus in childhood and the ulnar collateral ligament in adulthood. During skeletal immaturity, excessive valgus stress may lead to fragmentation or hypertrophy of the medial epicondyle. If such morphological abnormalities persist into adulthood, they may compromise elbow stability. The forearm flexor-pronator muscles contribute to dynamic elbow stability under valgus stress during throwing. However, the influence of medial epicondyle morphology on this stabilization remains unclear. This study aimed to clarify the influence of medial epicondyle morphological variations on the dynamic stability of the elbow joint, range of motion, and clinical signs of valgus instability by comparing participants classified according to ultrasonography. Methods: The medial epicondyle morphology in 48 male collegiate baseball players (19.3 ± 0.9 years; years of athletic experience, 11.6 ± 0.6 years) was classified into 3 groups based on ultrasonographic evaluation: normal, hypertrophic, and fragmented. Dynamic elbow stability was evaluated by measuring the medial joint space distances in both horizontal and vertical directions using ultrasonography, under 3 conditions: rest (gravity eliminated), gravity stress, and contraction of the flexor-pronator muscles. Elbow range of motion and clinical signs of valgus instability were evaluated using the Moving Valgus Stress Test and Elbow Valgus Stress Test. Joint space distances were analyzed using a two-way mixed-design analysis of variance, with morphology and condition as factors. Group differences in range of motion and test outcomes were assessed using one-way analysis of variance or chi-squared tests, as appropriate. Results: Among the 48 players, 29 were classified as normal (60.4%), 7 as hypertrophic (14.6%), and 12 as fragmented (25.0%). The fragmented group exhibited significantly smaller vertical joint space distances during muscle contraction compared with the normal group, indicating a reduced ability of the forearm muscles to resist valgus stress by medially displacing the ulna. This group also showed significantly limited elbow extension and a higher positive rate on the Moving Valgus Stress Test. Conclusion: Fragmentation of the medial epicondyle may contribute to compromised medial elbow dynamic stability in adult baseball players. These findings highlight the potential long-term consequences of unresolved fragmentation and support the importance of early detection.