A Simple and Practical Method to Determine Amount of Rotation in Femoral Midshaft Derotational Osteotomy: Geometric Arc and Chord Formulas

Objective: Femoral derotational osteotomy is a surgical intervention that involves the derotation of the femur at certain sites to achieve the desired femoral anteversion angle. However, current intraoperative techniques measuring the degree of correction and adequately performing it is still insufficient to achieve the desired femoral anteversion angle. The aim of this study is to investigate whether the geometric arc and chord formulas are applicable to achieve the target femoral anteversion angle in mid-shaft femoral derotational osteotomy. Methods: CT images of 20 femurs with excessive anteversion or retroversion deformities were retrospectively selected, and three-dimensional (3D) geometric models were obtained. 15⁰ 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 postoperative 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: The arc and chord formulas provided an accurate target femoral anteversion angle. Using one of these formulas preoperatively can facilitate to achieve desired femoral anteversion angle without having the intraoperative measurements nor using any special equipment.