A New Fracture Model for "Terrible Triad" Injuries of the Elbow: Influence of Forearm Rotation on Injury Patterns

Objective: The purpose of this study was to evaluate the influence of forearm rotation on failure patterns of the elbow under axial loads. Methods: Fourteen upper extremities were resected mid-humerus and mounted on a custom apparatus, which allowed rotation of the ulna, radius, and humerus about a fixed wrist while loading in axial compression. Seven specimens were loaded to failure with the forearm in pronation and 7 in supination. Results: Six of the 7 elbows axially loaded in pronation resulted in fractures of the radial head and coronoid with posterior dislocation (terrible triad). Six of the 7 elbows loaded in supination dislocated without fracture. One of the 7 elbows tested in supination had a terrible triad-type elbow injury. Five of the 6 specimens with ulna external rotation had damage to the lateral ligaments; all 8 specimens with internal rotation had damage to the medial ligaments. There were no significant differences in biomechanical parameters between pronation and supination. Conclusions: The forearm position during axial load was the primary determinant of fracture-dislocation pattern. When the forearm was pronated, a terrible triad injury pattern most often occurred. When the forearm was supinated, a dislocation without fracture most often occurred. In both forearm rotation positions, when the ulna internally rotated during failure, the medial structures were the first to be disrupted. When the ulna externally rotated, the lateral structures were the first to be disrupted. Understanding the pathomechanics of elbow dislocation may improve diagnosis and treatment of these injuries.