Three-Dimensional Analysis of the Morphological Features in the Femur of Atypical Fracture and Practical Implications of Intramedullary Nailing

Purpose: This study analyzed the morphological features of the contralateral femur without an atypical fracture by constructing a three-dimensional model with an actual size medullary canal. Materials and Methods: Lateral and anterior bowing of the shaft were measured for 21 models, and the shape of the medullary canal was analyzed. To eliminate the projection error, the anteroposterior (AP) femur was rotated internally to the extent that the centerline of the head and neck, which is the ideal position of cephalomedullary nail screw, was neutral, and the lateral femur matched the medial and lateral condyle exactly. Results: The lateral bowing and anterior bowing was an average of 5.5 ° (range, 2.8 ° -10.7 ° ; standard deviation [SD], 2.4 ° ) and 13.1 ° (range, 6.2 ° -21.4 ° ; SD, 3.2 ° ), respectively. In the area where lateral bowing increased, the lateral cortex became thicker, and the medullary canal was straightened. On the lateral femur, the anterior angle was increased significantly, and the diameter of curvature averaged 1,370.2 mm (range, 896-1,996 mm; SD, 249.5 mm). Conclusion: Even if the anterolateral bowing increases in the atypical femur, the medullary canal tends to be straightened in the AP direction. So, it might be considered as a reference to the modification of an intramedullary nail to increase the conformity.