**3. Results**

Between March 2013 and February 2021, of the 31 identified patients who underwent derotational osteotomy in the femur or tibia, 19 patients treated with minimally invasive osteotomy with the SP app were included. Among 19 cases (17 cases of the femur, 2 cases of the tibia), 17 cases were treated using IM nailing, and 2 cases using MIPO. All patients had a minimum follow-up of 1 year. The mean age of the patients at the time of surgery was 37.9 years (range: 11–77 years).

Four patients were diagnosed with acute postoperative malalignment, 15 patients with late manifestation (nonunion *n* = 12; malunion, *n* = 3), 11 patients with externally rotated deformities, and 8 patients with internally rotated deformities. Preoperative CT scans showed a mean difference of 22.3◦ (range: 11.2◦–38.3◦) in rotational angle. Among those exhibiting nonunion, four patients presented with additional angular malalignments that were simultaneously corrected. Fixation was carried out using IM and MIPO in 17 and 2 patients, respectively, and additional bone grafting was carried out in 8 patients. Sixteen patients were operated on in the supine position, while three were in the lateral position.

The mean intraoperative angle-SP was 21.6◦ (range: 10.2◦–36.1◦), while the postoperative angle-CT was 21.3◦ (range: 13.9◦–39.2◦). The mean difference in angle of rotation was 2.3◦, and the variations in value were within the accepted range of 5◦ (range: −4.2◦ to +4.8◦). A statistically significant positive correlation between angle-SP and angle-CT was observed (Pearson's correlation coefficient r = 0.972; *p*-value = <0.001).

The mean duration of healing was 17.7 weeks (range: 12–24 weeks) in 18 out of 19 patients, and all of them exhibited acceptable improvement in gait with symmetric angles of foot progression. One patient exhibited nonunion requiring a secondary bone graft, while three patients presented with minor complications related to the Schanz pins (bent pin *n* = 2; broken pin, *n* = 1), which likely occurred during manual derotational correction. None of the patients developed any infections (Table 1).



#### **4. Discussion**

Rotational osteotomy is a commonly used surgical treatment measure for the correction of malalignment caused by congenital, developmental, or posttraumatic factors. Techniques involving intraoperative fluoroscopic imaging have been used to ensure accurate correction, particularly along the coronal and sagittal planes. However, no reliable methods of assessing the intraoperative rotational alignment of the lower limbs have been reported to date. SP technology has been used previously in various orthopedic surgeries [18,19], with experimental studies examining its accuracy in the measurement of rotational deformities [14,15]. The findings were largely similar to those obtained using CT scanning with markers placed over artificial bones without any soft tissue coverage. However, angle measurement may be simpler in saw bone models as the markers are clearly visible, and this is in contrast to actual bony tissues in the extremities, which are typically enveloped by muscle and soft tissues that can interfere with visualization of the markers and prevent accurate measurement of angles. The findings of this study showed a strong correlation between angle-SP and postoperative angle-CT and, to the best of our knowledge, this is the first clinical series to examine derotational osteotomy and report accurate intraoperative measurement of correction using an SP app with an integrated gyroscope. Therefore, SP-assisted derotational osteotomy may be considered a suitable alternative for traditional osteotomies using open measuring.

Several fixation techniques for acute fractures also use fluoroscopic imaging to accurately measure rotation intraoperatively. They typically use anatomical landmarks (such as the lesser trochanter profile, patellar and fibular position, and native femoral torsion of the hip compared to the posterior femoral condylar plane) on the contralateral unaffected extremity as a template [20–22]. While these methods are also applicable in corrective osteotomy, variations in local anatomy may raise uncertainty regarding the accuracy of techniques based on visual estimation [23]. High rates of malrotation have been reported in patients with highly comminuted fractures, pre-existing anatomical deformities, or bilateral injuries [3,5]. Another limitation of techniques using fluoroscopy is the additional operative time required. Intraoperative use of mobile CT scanning [24,25] or computer navigation [26] is considered an ideal method for assessment of rotation, although their use is limited because of high associated costs, increased radiation exposure, and logistical issues in the operating room. In comparison, the proposed technique using an SP app measures the angle of rotation intuitively in real-time, is more economically viable, minimizes radiation exposure, and is also logistically convenient.

A recently introduced innovative surgical technique that corrects femoral malrotation using 3D printing technology [27] has the advantage of using customized cutting guides that allow accurate estimation of the angle of correction of malrotation. Additionally, it also requires shorter operating time and less radiation exposure. However, this technique requires a considerably invasive surgical approach as the 3D printed guides must be fixed proximal and distal to the osteotomy site, thus increasing the risk of blood loss, infection, and disturbed bone healing. Therefore, this technique may be unsuitable for the correction of postsurgical malalignment where the exposure of the previous fracture is unnecessary. In comparison, the proposed technique using SP app-assisted measurement requires minimal surgical exposure for percutaneous osteotomy and can even be carried out without opening the fracture site. Therefore, it allows for a less invasive approach, consequently reducing surgical morbidity and risk of infection. Additionally, it can also promote bone healing, as it allows preservation of the periosteal blood supply and the surrounding soft tissue. In the current study, 18 out of 19 patients exhibited primary bone healing without any infectious complications. Therefore, we believe that the proposed technique for derotational osteotomy sufficiently meets the requirement of minimal invasiveness.

Precise intraoperative measurement of the angle of derotation is important as it contributes to the functional outcomes after corrective osteotomy. Care should be taken to place markers during the procedure in order to allow accurate measurement of this parameter. After osteotomy, intraoperative measurement of the angle between the two markers

is essential. A previous study examining version abnormalities of the femur used flat triangular osteotomy templates to estimate the angle between the markers visually [9]. However, its measurement may be rough and inaccurate, since the angles of triangles cannot match the different amount of the rotational malalignment. Although a sterilized goniometer may also be considered a suitable alternative, direct measurement may not be easy based on the distance between the markers. In comparison, the SP app can act as a customized alternative of the virtual goniometer that can determine the magnitude of rotational deformity precisely. This clinical trial showed a maximum difference of <5◦ between the affected and unaffected sides, suggesting that the use of a contemporary SP app is likely to achieve accuracy of measurement during derotational osteotomy.

Various operative techniques for correcting rotational deformities have been described in the literature. The conventional method is open exposure at the deformity level [28,29]. The correction angle is planned on preoperative CT scans and intraoperatively marked on the bone with Kirschner wires or Schanz screws. Subsequently, a transverse osteotomy and derotation are performed [28]. In our study, minimally invasive derotational osteotomy was carried out. This technique allows closed osteotomy without stripping the surrounding soft tissues, reducing surgical morbidity and risk of infection. The periosteum is left intact, which improves callus formation and bone healing. However, it is difficult to perform accurate angle correction, and there are disadvantages in that radiation exposure and intraoperative time increase. The proposed SP app technique can intuitively measure the rotation angle in real time and precisely determine the rotational deformity's magnitude. So, it can minimize radiation exposure and also decrease intraoperative time. However, the minimally invasive technique may be technically challenging surgery and requires a steep learning curve.

This study had several limitations, including a small sample size and no comparator groups. Future studies with a larger number of patients and appropriate comparison groups treated using other methods are necessary to confirm the efficacy of using the SP app for this purpose. This is the first study to carry out a large series of derotational osteotomies for the treatment of posttraumatic malalignments only. However, there are several factors that may affect the accuracy of measurement when using this technique. Firstly, errors may occur if the height and direction of the SP are not parallel to the two markers, emphasizing the need to ensure that the camera is placed on an imaginary line that is parallel to the markers when measuring. Secondly, errors may also occur if the markers are bent, with three patients in the current study exhibiting Schanz pins that were bent or broken during manual derotation. In such cases, reinsertion of the markers is essential if the fault is detected prior to measurement, and rotation of the distal limb should be carried out without holding the markers. Thirdly, loss of derotation during distal fixation of the implant can lead to over or undercorrection. An innovative technique using electromagnetic tracking (EMT) to monitor the angle of derotation continuously during surgery has been proposed previously [30]. However, this method is technically difficult and requires sterilization of specific parts (sensors or pointers) prior to use in the surgical setting. In comparison, the SP app does not require any complex equipment, and also offers minimal discrepancies between intraoperative derotation and postoperative results.
