Background: Femoral derotational osteotomy surgery is a sur- gical intervention that involves the derotation of the femur at different sites to achieve the desired femoral anteversion angle. However, current intraoperative techniques to perform and mea- sure the degree of correction are controversial, this can result in undesired degree of femoral version angle. In addition, until now there is no ideal method has been described to do so. Our idea is to reach the desired anteversion angle using arc and chord method preoperatively.

Methods: CT images of 20 femurs with excessive anteversion or retroversion deformities were retrospectively selected and three-di- mensional (3D) geometric models were obtained. 15° of femoral anteversion were chosen as the target angle. Geometric arc and chord formulas were separately applied on axial CT images at the mid-point of the femurs to calculate the degree of correction. Then, virtual mid-shaft femoral derotational osteotomy was performed, then post-op femoral anteversion angle was measured.

Results: There was no statistical difference between the postop- erative measured anteversion angles and the targeted angle in all examples. The arc and the chord lengths were not statistically different which indicates both formulas can be used to estimate the degree of correction.

Conclusion: Surgeons can practically estimate the degree of cor- rection pre-operatively by applying either arc or chord formula on axial CT images which can help determinate the the degree of correction, without having the intraoperative measurements nor using any special equipment or unnecessary radiation dose.