Femur Deformity Correction Planning in Sagittal Plane Using Mechanical Axis

Recently, deformity correction planning using reference lines and angles is a standard procedure. At the same time, both anatomical and mechanical axes are used for the frontal plane. However, until nowadays, there was no algorithm for planning femoral deformity correction in the sagittal plane based on the mechanical axes of the bone fragments. The aim of this study was to develop a method of femur deformity correction planning in the sagittal plane based on mechanical axes. Materials and Methods: On the basis of computer tomography data of 23 adults with nondeformed femurs, we measured the angular relationships between the anatomical axis of the proximal femur in sagittal plane and mechanical axis (angle maj) and the angular relationships between the femoral neck axis and mechanical axis in sagittal plane (angle laj). Results: It was found that angle maj was 16.0 degrees +/- 7.6 degrees, and angle laj was 10.2 degrees +/- 2.4 degrees. Based on the data obtained, we developed a method for planning the correction of femoral deformities in the sagittal plane. According to this method, the mechanical axis of the proximal bone fragment can be determined using any of three options: (1) Joint line based. In this method, the proximal joint line of the femur is drawn, then from the center of the femur head with the angle 85 degrees (mean value of mPPFA) to this line is drawn a line that is mechanical axis; (2) Femoral neck based. The neck axis is drawn first, then from the center of the femur head with the angle 16 degrees (mean value angle maj) to this line is drawn a line that is mechanical axis; and (3) Anatomical axis based. First, the anatomical axis of the proximal femur is drawn, then a line parallel to the anatomical axis from the center of the femoral head is drawn, then a line at an angle of 10 degrees (mean value angle laj) to it is drawn, that is the mechanical axis. Determination of the mechanical axis of the distal fragment in sagittal plane is made by the following: the distal joint line is drawn and divided into five equal segments. Then, the point located on the border of the front 2/5 and the rear 3/5 of the segment is found. From that point, at an angle of 81 degrees (mean mPDFA), a mechanical axis of the distal femur fragment is drawn. The intersection of the mechanical axes of the proximal and distal fragments defines the apex of the deformity. Conclusion: The proposed method for planning deformity correction based on mechanical axes for the sagittal plane complements the existing planning methods for the frontal plane and improves the quality, namely the accuracy of preoperative planning.

Automated summary

The study proposed a method for planning femur deformity correction in the sagittal plane based on mechanical axes, improving the quality of correction by measuring angular relationships and developing correction methods.