*2.2. Smart Phone Application*

The SP application (SP app) used in this study was the Smart Protractor application (Smart Tools Co., Dae-gu, South Korea). Measurement with the SP app is obtained by positioning a virtual protractor, visible on the SP screen, on photography obtained using the SP camera. The assistant takes a photo of the Schanz pins, saves it, measures the angles, and observes the value. The picture should be taken with the camera positioned along the imaginary line between two pins. After the photo has been taken and saved, two red lines appear on the screen. The lines can be dragged across the screen to place the virtual goniometer on the axis of the Schanz pins, finely adjusting them until the goniometer is positioned correctly. Pictures judged subjectively wrong by the surgeon because of a perspective error can be deleted. The angle-SP measurement was repeated three times, and the average value was determined.

**Figure 7.** (**A**) A 15-year-old girl with a tibia shaft fracture; (**B**) minimally invasive plate osteosynthesis was carried out; (**C**) postoperative CT scan showing a moderate difference (over 16 degrees) in rotation compared with the noninjured side.

**Figure 8.** (**A**,**B**) Parallel Schanz pins placed in the proximal and distal tibia; (**C**) screws were removed at the proximal segment; (**D**) external rotation of the distal segment and the smartphone application measured about 14◦ correction; (**E**) screws were fixed at the proximal segment; (**F**) postoperative radiographs showing the improved alignment after revision procedure.
