**5. Conclusions**

In conclusion, the SP app allows precise assessment of intraoperative angles during derotation osteotomy. SP app may help the newly introduced minimally invasive derotational osteotomy. It is a reliable and reproducible procedure to predict accurate angle measurement and produce excellent bone healing and function.

**Author Contributions:** Conceptualization: K.-H.P. and C.-W.O.; methodology: J.-W.K. (Joon-Woo Kim) and D.-H.K.; validation: S.-H.Y.; formal analysis: K.-H.P. and J.-H.L.; investigation: I.S. and C.-W.O.; data curation: I.S. and D.-H.K.; original draft preparation: K.-H.P. and C.-W.O.; draft review and editing: C.-W.O. and J.-W.K. (Joon-Woo Kim); visualization: K.-H.P. and S.-H.Y.; supervision: C.-W.O. and J.-W.K. (Ji-Wan Kim). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a Biomedical Research Institute grant, Kyungpook National University Hospital (2020).

**Institutional Review Board Statement:** The design and protocol of this study were approved by the institutional review board of Kyungpook National University Hospital (IRB no: KNUH 2020-11-009, approved on 4 December 2020).

**Informed Consent Statement:** Informed consent was obtained from all individual participants included in this study.

**Data Availability Statement:** Source data may be shared by the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
