A line is drawn from the olecranon to the metaphysis of the distal end of the ulna. The maximum vertical distance from the straight line to the dorsal edge of the ulna is the maximum arcuate distance of the ulna and is recorded as MUB (H), A is the distance from the olecranon to the epiphysis of the distal ulna, and B is the distance from the position of the maximum arcuate sign to the epiphysis of the distal ulna. The maximum bow ratio is recorded as R-MUB (H/A), and the position of the maximum bow distance is recorded as P-MUB (B/A).