Changes in the alignment of the leg due to flexion and rotation A 3D simulation

Background: This three-dimensional (3D) simulation study was designed to analyze the impact of rotation and flexion of the leg on commonly used radiological alignment measurements. Methods: Segmented computed tomography (CT) and the 3D bone models of legs (n = 60) were projected onto the coronal plane to mimic the long leg radiograph. Rotation by up to 15 degrees inwards or outwards and flexion along the femoral intercondylar axis by up to 30 degrees were performed. In this way, 28 combinations of rotation and flexion were obtained for each leg. The results were statistically analyzed on a descriptive level and using regression models. Results: The mean deviation of the hip-knee-ankle angle (HKA) was small with the knee extended across all rotation levels. With rotation alone, the medial proximal tibial angle (MPTA) and mechanical lateral distal femoral angle (mLDFA) changed in opposite directions; however, for all parameters the effects increased significantly (p < 0.05) when flexion and rotation were present. Thus, the HKA changed by up to 8.4?degrees and -8.5 degrees in the varus/valgus direction. Mechanical axis deviation (MAD) showed a medial deviation up to -25.4 mm in internal rotation and a lateral deviation up to 25.2 mm in external rotation. Conclusion: The simultaneous presence of rotation and flexion of the leg in the 2D long leg radiograph has a large and clinically relevant influence on radiological measurements. If only one of the factors is present the effect is relatively small. Nevertheless, this must be considered when analyzing alignment of the lower limb. Therefore, 3D planning of surgery (e.g., using CT imaging) should be generously considered.