In vivo three-dimensional kinematic comparison of normal knees between flexion and extension activities

Background/Objective: Normal knee kinematics during flexion and extension activities over the whole range of motion remains unknown. This study aimed to clarify in vivo kinematics during knee flexion and extension activities of normal knees by comparing continuous flexion and extension activities up to a high flexion angle. Methods: Twenty knees of 10 Japanese volunteers were enrolled in this study. Each volunteer performed a continuous squatting motion under fluoroscopy, and a two- or three-dimensional registration technique was used. Rotation and anteroposterior translation of the medial and lateral sides of the femur relative to the tibia at each flexion angle were evaluated. Results: Femoral external rotation was significantly smaller from 10 degrees to 40 degrees flexion during extension activities than during flexion activities. However, the femoral external rotation was larger from 120 degrees to 130 degrees flexion during extension activities than during flexion activities. From 10 degrees to 60 degrees of flexion, the medial side was significantly more posteriorly located during extension activities than during flexion activities. Furthermore, the lateral side was significantly more posteriorly located at 130 degrees of flexion during extension activities than during flexion activities. Conclusion: In vivo kinematics of normal knees during extension activities differ from those during flexion activities in early and high flexion.