Anterior Monteggia fracture-dislocations are frequently complicated by persistent radial head instability leading to suboptimal outcomes. In this biomechanical investigation using a cadaveric elbow motion simulator, we examined the effects of ulnar extension angulation, soft tissue disruption, biceps loading and elbow motion on radial head translation. Our results showed significant anterior radial head translation with progressive ulnar extension angulation, with greater soft tissue injuries and increased biceps loading (P=.000). There was no significant difference in radial head translation between simulated active and passive elbow flexion (P=.251). These findings support the importance of an anatomic reduction of the ulnar fracture. However, in cases with significant soft tissue disruption, even an anatomic reduction of the ulna may not be sufficient to restore radial head alignment. Moreover, postoperative immobilization with the elbow in a flexed position to avoid elbow motion and to relax the biceps should be considered in patients with unstable Monteggia injuries.