Optimal additional support screw position for prevention of hinge fracture in biplanar closed wedge distal femoral osteotomy.

Background: The purpose of this study was to examine the biomechanical significance of supplemental fixation using a positional screw in prevention of the hinge fracture in lateral closed-wedge distal femoral osteotomy (LCW-DFO) by means of a three-dimensional finite element analysis. Methods: The three-dimensional numerical knee models with LCW-DFO were developed. To assess the mechanical efficacy of the positional screw and determine its optimal position and orientation, in total, 13 screwing methods were analyzed. In the first four methods, the screw was supported by the cortical bone only on the medial surface (mono-cortical). In the other 9 models, the screw was supported by both medial and lateral cortical bones (bi-cortical). Under 1000 N of vertical force and 5 Nm of rotational torques, the highest shear stress value around the medial hinge area was adopted as an analytical parameter. Results: In mono-cortical methods, with the cancellous bone support, all methods were able to reduce the highest stress value compared to the value without the screw, while the efficacy was rather inferior when the screw was in horizontal direction. Without the cancellous bone support, however, all methods were not able to reduce the stress value. In bi-cortical methods, with the cancellous bone support, almost all screw augmentation methods were able to reduce the stress value. When screwing from the medial to the lateral, it only gets worse when going extremely posterior. Without the cancellous bone support, all methods were able to reduce the stress value. Conclusion: The mechanical efficacy of the bi-cortical method was proven regardless of the quality of the local cancellous bone.