Ultrasonographic assessment of the flexor pronator muscles as a dynamic stabilizer of the elbow against valgus force.

Flexor pronator muscles (FPMs) play a key role in stabilizing the elbow joint against valgus forces. However, no studies have investigated the in vivo kinematics of FPMs against these forces on the elbow. This study aimed to clarify the in vivo contribution of each FPM as a dynamic stabilizer in a clinical situation.Twelve healthy volunteers participated in this study. Verbal informed consent was obtained from all subjects. The elbow was flexed to 90 degrees, and the forearm was placed in the neutral position. Manual valgus stress was applied to the elbow joint until maximal shoulder external rotation was achieved. The width of the ulnohumeral joint space and the ulnar shift of the sublime tubercle were measured before and after isometric contraction of FPMs using ultrasonography.The horizontal distances were decreased 1.1±0.6 mm after forearm pronation, 0.6±0.5 mm after wrist palmar flexion, 0.1±0.4 mm after wrist ulnar flexion, and 0.2±0.5 mm after finger flexion. Significant changes were observed during forearm pronation, wrist palmar flexion, and finger flexion but not during wrist ulnar flexion (p<0.05). The sublime tubercle was significantly shifted 0.5±0.1 mm medially after forearm pronation, 0.2±0.1 mm medially after wrist palmar flexion, and 0.1±0.1 mm laterally after wrist ulnar flexion and finger flexion (p<0.05). The FPMs, especially the pronator teres and the flexor carpi radialis, function as dynamic stabilizers against elbow valgus stress. The results of this study may be useful in developing injury prevention and rehabilitation strategies for throwing injuries of the elbow.