Background: This study aimed to investigate whether the distance between the radial nerve and rotational center of the elbow joint when observing from the lateral surface of the humerus changes according to passive elbow joint flexion for safe external fixation with a hinged fixator of the elbow joint. Methods: Twenty fresh-frozen cadaveric arms were dissected. The points where the radial nerve crosses over the posterior aspect of the humerus, crosses through the lateral center, and crosses over the anterior aspect of the humerus were defined in the lateral view of the elbow joint, using fluoroscopy, as R1, R2, and R3, respectively. The distances between the rotational center and each point on the radial nerve were measured when the flexion angle of the elbow joint was 10 degrees, 50 degrees, 90 degrees, and 130 degrees. Results: The distances between the rotational center and R1, R2, and R3 were 118 mm, 94 mm, and 65 mm, respectively, when the flexion angle was 10 degrees; 112 mm, 93 mm, and 74 mm, respectively, for 50 degrees; 108 mm, 93 mm, and 77 mm, respectively, for 90 degrees; and 103 mm, 94 mm, and 83 mm, respectively, for 130 degrees. The distance between the rotational center and R2 was constant regardless of the flexion angle. With elbow joint extension, the distances between R1 and R3 increased; the safe zone, a region where the radial nerve would not be located on the humerus, was the smallest in extension. When the elbow joint was flexed, the distances between R1 and R3 decreased; the safe zone was the largest in flexion. Conclusions: This study showed that the radial nerve location on the humerus varied based on the flexion angle of the elbow joint; the safe zone may change. A half-pin can be likely inserted safely, avoiding the elbow joint extension position.
