The pattern of 3-dimensional deformity of the forearm in missed Monteggia fracture and Monteggia-like lesions is very complicated. Recently, computer-assisted preoperative planning using 3-dimensional computed tomography (CT) modeling has been used for the treatment of malunited fractures. We developed a 4-dimensional simulation system by adding the anatomic motion axis to the 3-dimensional CT scan data of the affected arm in 1 position. Our system is based on the motion axis of the forearm passing through the center axis of the radial head and capitellum to the ulnar fovea. We simulate the osteotomy angle and amount of correction to make the radius move around the rotational axis of the forearm without interference between bones. The advantage of our method is the determination of a precise and flexible osteotomy site preoperatively using only the affected side CT image based on the rotational motion of the forearm. A 4-dimensional preoperative simulation system would be useful for chronic Monteggia fracture and Monteggia-like lesions because it would improve preoperative planning.