Preoperative planning (a–c) and intraoperative protractor method (d). (a) Two metallic markers are placed on the skin, one at the tibial tuberosity and the other at the anterior tibial crest. (b) The markers are 20 cm apart. Anteroposterior radiographs of the leg are obtained. (c) We defined the line going along the two markers as the tuberosity line (red line). The osteotomy line (blue line) is perpendicular to the anatomical axis of tibia (yellow line). We calculated the angle between the tuberosity line and the anatomical axis of tibia. Finally, we calculated the angle of the osteotomy line with the tuberosity line and called it the osteotomy angle. In this case, the angle of the osteotomy line with the tuberosity line is 88°. (d) The 20-cm bar is in accord with the tuberosity line, and the 10-cm bar is in synchrony with the osteotomy line. We set the osteotomy angle of the protractor based on a preoperative measurement. In this case, the protractor is set at 88°. We cut the bone parallel to the osteotomy line (blue line) determined by the protractor.