Mechanism for human knee joint prosthesis

Abstract

The human knee can be considered a simple hinge joint but a more detailed analysis will show that real knee joints have significantly complex kinematics. The complex movement of knee joints is determined by unique shape of bones, especially the surfaces shape of the femur end and the beginning of the tibia. Analysis has shown that the knee is in fact, rolling“ joint where the tibia rolls on the femur end, and that motion is supported by Ligaments. Ligament arrangement around bones and shapes of contact surfaces defines movement, which can be represented by a four-bar mechanism. Modern above-knee prostheses consist of a four-bar mechanism created by suitable metal bars (linkages) connected with hinges, which will require some maintenance and lubrication, which can be solved using compliant mechanisms. Compliant mechanisms can offer several advantages over classical mechanisms, such are, no requirements for lubrication and maintenance of hinges can provide preload necessary for proper prosthesis stability, etc... These are good advantages but a synthesis of compliant mechanisms can be quite challenging, especially in a case where new geometry and topology must be designed to achieve required movement. An interesting case is simply the conversion of a classical mechanism into a compliant one, where hinges will be replaced with deformable (compliant) hinges. With this approach, the simplicity of the classical planar mechanism can be retained with the advantages of compliant mechanisms.