A new method for the evaluation of the end-to-end distance of the knee ligaments and popliteal complex during passive knee flexion

Background: Accurate knowledge about the length variation of the knee ligaments (ACL, PCL, MCL and LCL) and the popliteal complex during knee flexion/extension is essential for modelling and clinical applications. The aimof the present study is to provide this information by using an original technique able to faithfully reproduce the continuous passive knee flexion–extension kinematics and to reliably identify each ligament/tendon attachment site. Methods: Twelve lower limbs (femur, tibia, fibula, patella)were tested and set inmotion (0–120°) using an ad hoc rig. Tibio-femoral kinematics was obtained using an optoelectronic system. A 3D digital model of each bone was obtained using low-dosage stereoradiography. Knee specimens were dissected and the insertion of each ligament and popliteal complex were marked with radio opaque paint. ACL, PCL and MCL were separated into two bundles. Bone epiphyses were CT-scanned to obtain a digital model of each ligament insertion. Bones and attachment site models were registered and the end-to-end distance variation of each ligament/tendon was computed over knee flexion. Results: A tibial internal rotation of 18°±4°with respect to the femur was observed. The different bundles of the ACL, MCL and LCL shortened, whereas all bundles of the PCL lengthened. The popliteal complex was found to shorten until 30° of knee flexion and then to lengthen. Conclusion: The end-to-end distance variation of the knee ligaments and popliteal complex can be estimated during knee flexion using a robust and reliable method to mark the ligaments/tendon insertions with radiopaque paint.