Challenges in the Design of Biomimetic Orthopaedic Implants Next Generation Knee Arthroplasty

Abstract

Designing orthopaedic implants to reconstruct native anatomy and function continues to be a major challenge in the development of new products. This thesis provides insights and suggestions for designing next generation knee arthroplasty with a focus on native joint kinematics. Like in many other specialties, biomimetics is the ultimate objective in joint reconstruction. Knee arthroplasty is the most successful treatment option for severe osteoarthritis, providing patients with a reliable solution to relief pain and regain mobility. However, native joint function is not fully restored and for an aging population with growing interest in demanding activities, several limitations are present. Particularly, existing total knee arthroplasty (TKA) delivers kinematic deficits, associated with abnormal feeling of the knee, following joint surgery. These limitations are related to sacrificed ligaments, and none-anatomic geometries of current implant designs. Mostly, the anterior cruciate ligament (ACL) is sacrificed and its function is lost. Therefore, a biomimetic process was implemented to reverse engineer articular surfaces, by directly incorporating native knee kinematics. For the first aim, we designed a biomimetic implant that would preserve all major ligaments of the native knee joint. Therefore, an anatomically designed femoral component was virtually moved through native knee kinematics, to carve out a tibial articular surface, through the biomimetic process. This anatomical surface was incorporated into a novel implant, specifically designed to preserve the native ACL. For the second aim, we established a dynamic simulation platform to reliably evaluate implant design variations and the effect of ligaments. Knee kinematics, driven by ligament function and contacting implant geometry, allowed for comparison of different knee arthroplasty systems, against in vivo and simulated native knee motion. The first computational study revealed that a biomimetic, bi-cruciate retaining (BCR) implant provides activity dependent kinematics, similar to healthy knees in vivo, during deep knee bend, chair sit and level walking. This was in contrast to symmetric BCR systems, which nonetheless showed kinematic improvements over ACL-sacrificing implants. This indicates that restoring anatomic knee geometry together with ACL preservation is required for optimal implant function. The second computational study showed that a novel ACL-substituting mechanism could improve kinematic deficits of contemporary PCL-retaining (ACL-sacrificing) implants. ACL-substituting and ACL-retaining implants both provided similar improvements over the ACL-sacrificing implant in comparison to a native knee model. This was shown during walking, stair-ascent, chair-sit and deep knee bend simulations, indicating that ACL-substitution may be a valuable treatment option when ACL preservation is not feasible. For the third aim, we evaluated an extended treatment indication for unicompartmental knee arthroplasty (UKA), via in vivo kinematic analysis, using a moving fluoroscope. UKA is known for better functionality over TKA due to ACL retention and partial preservation of the native articulation. An ACL-deficient population, undergoing an altered surgical technique, with appropriate adaptation of implant placement, was compared against conventional UKA patients, with an intact ACL. The first in vivo study showed kinematic similarities between ACL-deficient and conventional UKA patients, in contrast to TKA. A posterior femoral shift was observed with ACL-deficient UKA, while kinematic trends and range of motion were similar during deep knee bend, downhill walking, level walking and stair descent. Based on kinematics, this confirms that the indication for UKA can be extended to include selected ACL-deficient patients. The second kinematic study revealed no differences in ground reaction forces between ACL-deficient and conventional UKA patients. Adequate kinematic and kinetic symmetries among implanted and contralateral leg indicate that UKA may not always be a contraindication due to ACL deficiency. In summary, this thesis outlines the importance of biomimetic implants for the objective of “forgotten knees”, following joint arthroplasty. Native knee function may be restored with anatomic geometries and ligament retention. Preservation or substitution of either the articular geometry or the ACL, results in kinematic improvements over conventional TKA. Long-term clinical outcome studies will be required for the ultimate proof of this concept.