Dynamic Positional Changes in the Popliteal Artery and Vastus Medialis and Lateralis Muscles During Knee Flexion and Extension: An Open MRI-Based Anatomical Study

Background/Objectives: In periarticular knee surgery, such as osteotomies, ligament reconstruction, and fracture fixation, surgeons face a dilemma: ensuring the safety of the popliteal artery (PA) while securing adequate surgical access to the bone. While macroscopic anatomical studies suggest knee flexion protects the PA, they often fail to account for physiological muscle tension in living knees. This study aimed to quantitatively evaluate the dynamic positional changes in the PA and the vastus medialis and lateralis muscles (VM and VL, respectively) using Open Magnetic Resonance Imaging (MRI) to determine the optimal limb position for each surgical step. Methods: Twenty-three living knees were evaluated using Open MRI. The shortest perpendicular distances from the posterior aspect of the femur and tibia to the PA, and from the femoral cortex to the posterior border of the VM and VL, were measured at 10° knee-flexed position (representing the extended position) and 90° knee-flexed position. Results: The PA shifted significantly away from the bone in 90° knee-flexed position compared to extension at the distal femur (0 and 1 cm proximal to the intercondylar line (Blumensaat’s line)) and the proximal tibia (0, 1, and 2 cm distal to the joint line) (Q < 0.05). Conversely, both the VM and VL moved significantly closer to the femur in flexion at all measured levels (0–4 cm) (Q < 0.05), often causing the muscles to compress tightly against the bone. Conclusions: The vascular safety margin is maximized in flexion, whereas surgical exposure for the distal femur is optimized in extension due to vastus muscle relaxation. We suggest performing superficial exposure and femoral plate insertion in extension, and surgical maneuvers involving the posterior cortex in flexion to minimize neurovascular and soft tissue complications.