A 7-year-old boy falls from playground equipment and sustains a displaced supracondylar fracture of the humerus. His hand is warm, capillary refill is 1.5 seconds and a Doppler signal is present at the wrist, although the radial pulse cannot be felt; he can flex and extend his fingers. The elbow is markedly swollen, but there is no pain with passive finger movement. Which one of the following is the most appropriate next step?