Conceptual Design for a Surface-Guided Total Knee Replacement With Normal Kinematics
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The objective of this thesis was to develop a concept and methodologies for designing a total knee replacement (TKR) with normal kinematics and a high range of motion. The design philosophy was that a TKR can function similar to the normal knee, provided that after TKR the inherent passive characteristics of the joint are restored to normal with minimum disruption in the functions of the remaining structures of the joint. As the first step prior to design, cadaver experiments were conducted and biomechanical models of the passive knee were developed to study the mechanics of the normal knee. The guiding roles of the tibial articular surface including the menisci, the combined effects of the cruciates and contact forces, and the elongation patterns of the cruciates were investigated. Based on the results obtained from these studies and the relevant information in the literature, design requirements for a TKR with normal kinematics were identified, and an innovative design concept was introduced. On the medial compartment of this design the shape of the articular surfaces resembled a ball-and-socket joint, and on the lateral side a pair of guiding bearing surfaces mimicked the guiding roles of the cruciate ligaments. The novelty in the design concept lies in the design of the shape of the lateral articular surfaces. The progressive variations of the curvature of the medial and lateral aspects of the lateral condyle generate the desired guiding effect for the full cycle of extension and flexion. The bearing spacing defined as the distance between the medial and lateral contact points was kept constant throughout the motion, as this was proved to be necessary to ensure compatibility between the geometry of the bearing surfaces and the desired pattern of motion. Appropriate methodologies were developed to generate the complete shapes of the bearing surfaces and to build the prototypes based on the constraints of the bone geometries and kinematics of a sample cadaver knee. The kinematic test of the prototype proved the viability of the design concept and methodologies. The novel design philosophy, concept and methodologies developed in this thesis provide a foundation for a new generation of TKR with normal kinematics.