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Abstract

Introduction: The elbow is the second most commonly dislocated joint in the body. Forty-nine percent of these are complex dislocations (ie, associated with a fracture). There are classic patterns for elbow injuries. Previous experiments have attempted to recreate these injury patterns using external forces. There is a common belief that elbow dislocations are caused by an axial force accompanied by flexion and an additional varus, valgus, or rotation force that leads to posterior dislocation with injury to either the medial or lateral structures. The elbow must flex to at least 20 degrees to unlock the olecranon before a dislocation can occur. The hypothesis of this study was that elbow injury patterns are influenced both by forearm orientation and the compressive interaction of joint surfaces in the fully extended elbow. Therefore, the objective of this study was to investigate the joint kinematics and injury pattern of the elbow that resulted from an axial load to failure with the forearm in either pronation or supination. Methods: Fourteen upper extremities were resected through the distal 1/3 humerus. Soft tissues were removed, leaving only the joint capsule, ligaments, interosseous membrane, and muscular insertions of the brachialis, supinator, and triceps. The valgus angle of the specimens was determined using markers and digital photos in supination, pronation, and neutral. A custom apparatus was used that allowed free rotation of the ulna, radius, and humerus about a fixed wrist. A compressive load was applied with a material tester (Instron 4411, Canton, MA) at a rate of two millimeters per second until failure occurred. Force and displacement data were collected. An optoelectronic, three-dimensional camera system (Optotrak 3020; Northern Digital Inc, Canada) was used to measure kinematic data for the distal humerus, proximal radius, and proximal ulna. Following failure, the elbow was carefully inspected and photographed to document all fractures and disrupted tissues. Results: Of the fourteen specimens, seven elbows failed in pronation and seven elbows failed in supination. In pronation, 6/7 elbows had a terrible triad type elbow injury with fracture of the radial head and coronoid with posterior dislocation. In supination, 6/7 elbows dislocated without coronoid or radial head fractures. Four out of fourteen elbows had damage to the lateral structures and ten out of fourteen had injury to the medial structures. In each case, damage to either the medial or lateral structures correlated with internal or external rotation of the ulna as compared to the humerus. Load to failure of the extended elbow in pronation most commonly led to a terrible triad (fracture of radial head and coronoid with posterior dislocation). In supination, independent rotation of the ulna and humerus enabled the elbow to dislocate without a fracture of the radial head or coronoid. No additional forces were necessary to allow the ulna to rotate out of the olecranon fossa. Post failure dissection revealed injury to the medial or lateral structures. The documented structural injuries correlated with the calculated rotations of the humerus and ulna relative to each other. When the ulna rotated externally relative to the humerus, the lateral structures were damaged. When the ulna rotated internally relative to the humerus, there was damage to the medial structures. The post failure dissection demonstrated rotation laxity at the elbow after reduction. Conclusions: During axial compression of the extended elbow, the ulna hinges either medially or laterally and allows the elbow to dislocate. The ligaments on the contralateral side are either torn or attenuated. In our study, the medial structures were more commonly disrupted. The terrible triad injury pattern can be produced with the elbow pronated in full extension. If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use). If noted, the author indicates something of value received. The codes are identified as follows: a, research or institutional support; b, miscellaneous funding; c, royalties; d, stock options; e, consultant or employee. *The Food and Drug Administration has not cleared the drug and/or medical device for the use described in this presentation (ie, the drug or medical device is being discussed for an “off-label” use).