**1. Introduction**

Intramedullary (IM) nailing and minimally invasive plate osteosynthesis (MIPO) are the most commonly used treatment measures for diaphyseal and metaphyseal fractures of long bones, respectively [1,2]. These closed-reduction surgical techniques provide better treatment outcomes (e.g., excellent fracture healing and rapid patient recovery) compared to open reduction with internal fixation, as they require smaller incisions and minimal soft tissue dissection. Malrotation, a common complication of IM nailing or MIPO, is often overlooked in comparison to angular deformities of the coronal and sagittal planes. Postoperative malrotations exceeding 10◦ have been observed in 50% of patients with femoral and tibial fractures [3–6], and restoration of the alignment is essential as abnormal loading may lead to pain, instability, and early degeneration [7].

Although challenging, a variety of osteotomy procedures have been used previously for the correction of rotational malalignment. During open osteotomy, the surgeon marks

**Citation:** Oh, C.-W.; Park, K.-H.; Kim, J.-W.; Kim, D.-H.; Seo, I.; Lee, J.-H.; Kim, J.-W.; Yoon, S.-H. Minimally Invasive Derotational Osteotomy of Long Bones: Smartphone Application Used to Improve the Accuracy of Correction. *J. Clin. Med.* **2023**, *12*, 1335. https://doi.org/10.3390/ jcm12041335

Academic Editors: Umile Giuseppe Longo and Pengde Kang

Received: 29 December 2022 Revised: 22 January 2023 Accepted: 6 February 2023 Published: 7 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

reference lines over the bone and/or uses angle templates to visually determine the desired correction intraoperatively [8]. This traditional technique can disturb bone healing and increase blood loss through extensile exposure, and this risk can be decreased by using minimally invasive corrective osteotomy techniques with fluoroscopic guidance [9,10]. Although intraoperative C-arm images have been used for objective measurement of the lesser trochanter profile, cortical step sign, and diameter difference sign [11,12], visual fluoroscopic estimation may prove to be imprecise when determining the angle of rotation [13].

Although the gyroscopic function of smartphone (SP) technology has previously been used to measure angles in bone models simulating rotational deformities [14,15], there is limited evidence of its efficacy in measuring the rotational angle during surgical corrective osteotomy. Therefore, in the current study, a consecutive series of derotational osteotomies were performed using an SP application (SP app) intraoperatively, and a comparison of the desired angle, measured using the SP app, and the corrected angle, measured using a postoperative CT scan, was carried out. The hypothesis being tested was that the proposed osteotomy technique using an SP app would achieve accurate correction of rotational deformity in a reproducible manner.
