Design and Initial Verification of a Novel Total Knee Replacement that Incorporates Synthetic Ligaments to Influence Knee Stability

In the United States, the number of patients under the age of 65 who are receiving total knee replacements (TKRs) is rising due to increasing demand for and access to this life-changing orthopaedic procedure. Although this younger population tends to have a higher life expectancy, they have also been shown to have a lower implant survival rate than patients over the age of 65, possibly due to their more active lifestyles. Thus, there will be a rising demand for implants that have both a higher functionality and survivorship to meet performance demands of younger patient\u27s lifestyles. The purpose of this study was to design and initially verify a novel TKR design that incorporates artificial ligaments into a knee replacement whose stability and eventual kinematic performance will be driven by both geometry and ligamentous structure. A computational model was first developed that incorporated synthetic ligaments into an existing knee replacement within an anatomical knee model using the AnyBody modeling software system. Simulated A/P drawer tests at different flexion angles were analyzed for over 2,916 possible anterior and posterior cruciate ligament location and length combinations to determine the effects of ligament length and location on the A/P stability of the TKR. A complete physical model was then designed and constructed, and the computational model was verified by performing mechanical testing on an Instron system. A/P drawer tests were performed under 710 N of simulated body weight. Tibial A/P displacement was tracked for the TKR system with and without cruciate ligaments to determine the effect of ligament placement on resulting TKR stability. Ligament length and location were found to significantly influence knee laxity and knee flexion. Knee flexion was determined to be more sensitive to the ACL attachment location on the femur than on the tibia. As ACL insertion location moved posteriorly on the femur, it was found to decrease ACL ligament strain enabling a higher range of flexion. In general, as ACL and PCL length increased, the A/P laxity of the TKR system increased linearly. Interestingly, range of motion was found to be more dependent on ligament attachment location than ligament lengths. Knee replacement stability is clearly affected by synthetic ligament length and location within a TKR system. A knee replacement that incorporates synthetic ligaments with calibrated location and lengths should be able to significantly influence kinematic performance of the TKR system, possibly influencing long-term functional outcomes