Direction of screw insertion for internal fixation plate in distal femoral osteotomy: Evaluation using axial computer tomography imaging

Purpose: In distal femoral osteotomy (DFO), using longer distal screws in fixation plates may improve stability. This study examined the insertion direction of three distal screws at the horizontal cross-section to determine if posterior angulation enables deeper placement. Methods: Forty-seven varus knees that underwent DFO were included (medial closed-wedge DFO [MCWDFO], 30 knees; lateral closed-wedge DFO [LCWDFO], 17 knees). Postoperative plain CT images were obtained from a plane parallel to the three distal screws, with the most distal screw designated as A, the anterior of the second distal row as B, and the posterior of the second distal row as C. For each case, a curve passing through the center of the bony cortex on the cross-section parallel to each screw and over its entire length was drawn, and the curve and the lower edge of the screw were projected onto a graph. The maximum angle at which the lower edge of each screw touches the intercondylar region without interfering with the intercondylar region was designated as (AnA), (AnB), and (AnC) for A-, B-, and C-screws, respectively. The angle between the line connecting the insertion points of the B- and C-screws on the plate and the tangent line to the medial and lateral bony cortex was designated as (AnP). Results: In the MCWDFO group, the mean values for each parameter were AnA, 10.9 ± 5.4; AnB, 27.0 ± 4.2; AnC, 9.2 ± 3.4; and AnP, −2.6 ± 6.9. In the LCWDFO group, the mean values for each parameter were AnA, 18.2 ± 6.9; AnB, 30.4 ± 7.1; AnC, 16.1 ± 7.2; AnP, −0.2 ± 6.1°. Conclusions: The medial surface is inclined compared to the epicondylar axis and posterior condyle, usually resulting in plate positioning that is parallel to the placement surface. The optimal screw insertion from the anterior to posterior was generally achieved; however, there was still room for posterior angulation margins of 9–11° for A- and C-screws and approximately 27° for the B-screw. In contrast, the lateral surface is flatter with less inclination, causing anterior plate placement and wider posterior angulation – approximately 16–18° for A- and C-screws and 30° for the B-screw – allowing a greater range of posterior swing than the medial side.