The role of the flexor pronator muscles as dynamic stabilizers against elbow valgus stress in patients with medial ulnar collateral ligament insufficiency.

Abstract Background Flexor pronator muscles (FPMs) have been reported to act as a dynamic stabilizer against valgus forces in overhead throwing athletes. Several studies have demonstrated the anatomical, biomechanical and clinical effects of the FPMs. However, no studies have investigated the in vivo kinematics of FPMs against the valgus forces on the elbow. The current study aimed to clarify the clinical contribution of the FPMs as a dynamic stabilizer in medial ulnar collateral ligament (MUCL) insufficiency. Methods Eighteen baseball players who suffered from MUCL injury participated in this study. The elbow was flexed to 90 degrees and the forearm was placed in the supinated position. Manual valgus stress was applied to the elbow joint until maximal shoulder external rotation was achieved. The widths of the ulnohumeral joint space were measured using ultrasonography and any changes in the medial elbow pain was recorded before and after isometric forearm pronation. Results All the subjects had MUCL tenderness and felt medial elbow pain when elbow valgus stress was applied. The width of the medial joint space was significantly larger on the injured side compared with the healthy side (5.2±1.0mm vs. 3.5±0.7mm) in elbow valgus stress. During isometric forearm pronation, the width of the medial joint space was significantly decreased (3.4±0.9mm, vs. 2.8±0.9mm) and medial elbow pain had completely diminished. Conclusion Isometric forearm pronation reduces valgus stress induced widening of the medial joint space and medial elbow pain in patients with MUCL insufficiency.