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Two theories of the pathogenesis of the anterior Monteggia fracture are the direct blow theory and the hyperpronation theory. There are objections to both. An alternate theory is proposed which has implications concerning treatment: that this fracture is a hyperextension injury in which the radial head is dislocated anteriorly by strong contraction of the biceps muscle, the ulna then fracturing largely as the result of a longitudinal compressive force. One reason for failure of closed treatment, besides the interposition of soft tissues preventing reduction of the radial head, is failure of the surgeon to consider the appropriate pronation-supination position of immobilization. It is recommended that the arm be immobilized in a cast in 100 to 110 degrees of elbow flexion, with the cast molded over the lateral side of the ulna to prevent radialward bowing of the ulnar fracture. As long as sufficient relaxation of the biceps is achieved, the reduction of the radial head is easily maintained.