Preliminary design of a knee external fixator based on planar geometric synthesis

The paper presents the validation of a methodology for the optimal synthesis of a planar mechanical system able to reproduce knee kinematics in a limited range of motion. Such a mechanism could be effectively used in the design and placement of a moving external fixator for the knee joint articulation in either post-traumatic and pathological treatment or rehabilitation. The purpose is to help patient's recovery through a limited and occasional movement of the articulation without loading the injured area. In order to validate the actual method, knee kinematics is estimated by means of plane radiographs of the articular joint of an healthy volunteer at different flexion angles. Each of the lateral radiographs shows the relative tibia-femur motion in different configurations. The whole movement is reconstructed by interpolation, assuming a relative gliding law. Geometric synthesis, based on Burmester's theory, is thus used to identify a set of compatible four-bar-linkages (f b.l.), which reproduce the estimated rigid body motion.