*2.1. Surgical Technique*

Preoperative evaluation included calculation of the required angulation of rotational correction and identification of any additional deformities. The surgical plan included derotational osteotomy at the previously fractured area and use of either an IM nail or plate for fixation, selected based on the anatomical location of the pre-existing implant.

During surgery, the patient was placed in a supine or lateral position on a radiolucent table, and their whole lower extremity was draped. Prior to commencement of the osteotomy, two parallel 5 mm Schanz pins were carefully placed above and below the previous fracture such that they did not interfere with the pre-existing or new implants (nail or plate) to allow accurate measurement of the correction (Figure 2).

**Figure 1.** (**A**) Preoperative anteroposterior and lateral radiographs showing nonunion after retrograde nailing; (**B**) CT scan showing an evident difference in the femoral torsional angle between the affected and unaffected sides; (**C**) externally rotated foot on the right side indicating retroversion of the femur.

**Figure 2.** (**A**,**B**) Two parallel Schanz pins placed above and below the nonunion without interfering with the nail; (**C**) internal rotation of the distal segment after removal of the previous nail; (**D**) the smartphone application measured a 22◦ correction.

Thereafter, the pre-existing implant placed during the earlier osteosynthesis was removed, and in patients with nonunion or malunion, percutaneous osteotomy was performed at the planned site. Minimally invasive osteotomy was performed using a 1–2 cm incision and multiaxial drilling with C-arm control, and the final procedure was completed by connecting the multiple drill holes using a half-inch osteotome (Figure 3). In patients with postoperative malalignment, the proximal or distal fixation was disassembled without removing the full implant (Figures 4–9).

After reaming the medullary canal, a new IM nail was inserted, and derotation was carried out by manually rotating the distal part of the limb. The extent of rotation was estimated by measuring the angle between the two Schanz pins. The intraoperative angle of correction was measured by an assistant standing at the end of the table using a free protractor SP app (angle-SP). The corrective osteotomy aimed to achieve an angle of correction equivalent to the rotational alignment of the contralateral side, determined using a preoperative CT scan. A maximum difference of 5◦ between the measured and target values was considered acceptable. Distal fixation was then performed while maintaining the rotational correction, and the Schanz pins were removed. Clinical examination was

performed after removal of the drapes to confirm rotational correction, and the patient was then sent to the recovery room.

**Figure 3.** (**A**) Postoperative radiographs showing antegrade nailing with bone graft at the nonunion site; (**B**) CT scan showing similar angles of anteversion; (**C**) similar rotation in foot position; (**D**) complete healing observed 1 year postoperatively.

**Figure 4.** (**A**) An 11-year-old male patient diagnosed with a femoral-shaft fracture; (**B**) IM nailing was carried out; (**C**) postoperative CT scan showing the decreased angle of anteversion compared with the noninjured side.

**Figure 5.** (**A**,**B**) Parallel Schanz pins placed in the proximal and distal femur; (**C**,**D**) internal rotation of the distal segment after removal of the distal interlocking screws; (**E**) the smartphone application measured a 20◦ correction.

**Figure 6.** (**A**) Postoperative radiographs showing revised fixation of distal interlocking; (**B**) CT scan showing similar angles of anteversion; (**C**) complete healing observed 6 months postoperatively.
