Characteristics of ulnar neuropathy in baseball players: Focusing on the entrapment point of the ulnar nerve and valgus instability.

Background: The region where the ulnar nerve (UN) is swollen in baseball players with ulnar neuropathy is not apparent. This study investigated the UN's cross-sectional area (CSA) at each entrapment point in baseball players. We also aimed to clarify the relationship between valgus instability and the CSA of the UN. Methods: Forty baseball players were separated into healthy and ulnar neuropathy groups. The CSA and valgus instability were measured using ultrasonography (US). Relative to the medial epicondyle (MEC), the arcade of Struthers (SA) was 5 cm proximal, the cubital tunnel was the posterior part, and Osborne's ligament was defined as 3 cm distal. The ulnohumeral joint space was imaged as a low-echo space between the distal-medial corner of the trochlea and the proximal edge of the sublime tubercle. Results: The UN in the ulnar neuropathy group had significant swelling in the cubital tunnel and Osborne's ligament. We found a weak positive correlation between the CSA and ulnohumeral joint space, and the ulnohumeral joint space at rest and valgus stress. Conclusion: Evaluation and treatment of UN, especially cubital tunnel and Osborne's ligament, are necessary for the rehabilitation of baseball players presenting with ulnar neuropathy and valgus instability.