Evaluation of Rapid Manufacturing Solutions for Improved Knee Implants Using Finite Element Analysis

Knee joint is an important part of human body; failure of knee joint may occur mainly because of surface to surface contact of femoral and tibial surfaces owing to the dry out of bursae fluid. The damaged surfaces must be replaced by artificial implants made of metals, ceramics, or composite materials. This process is known as total knee replacement. Three-dimensional physical model of the knee implant is modeled in NX Unigraphics 7.5 software. This model is imported to ANSYS-WORKBENCH 13, solver used is MECHANICAL-APDL, which is based on Finite Element Method (FEM). The metallic implant is made of porous inside and dense outside. This reduces the overall weight of the implant. The failure of bearing component in knee implant is due to high stresses at the contact regions of femoral component and polyethylene insert causes wear and decreases the life of the implant. New improved knee joint implant is made, which reduces the stresses at the contact regions and increases the life of knee implant. The stiffness of artificial implants is around 110 GPa to 210 GPa, while that of the human bone is around 17 GPa. A metallic implant made of titanium-β alloy having stiffness of around 40 GPa to 60 GPa is used. This reduces the effect of stress shielding between the bone and implant

Design of customised total knee implants with musculoskeletal dynamic simulations

Effects of a customised total knee implant (CTKI) on the contact forces and relative motions of the tibiofemoral and patellofemoral joints have been investigated with computer simulations by applying the patient-specific muscle forces on the lower limb and the joint reaction forces at the ankle and hip joints. Firstly, a method was proposed and realized to create a CTKI based on the geometry of a patient’s knee joint using ANSYS Mechanical APDL. Secondly, a patient-specific musculoskeletal model was built to calculate the muscle forces and joint reaction forces during a squat motion. Finally, a dynamic finite element (FE) model was created in ANSYS incorporating the aforementioned forces and the CTKI to calculate the contact forces and relative motions of the tibiofemoral and patellofemoral joints. In addition, an off-the-shelf symmetric total knee implant (STKI) with cruciate ligaments (CLs) retained was simulated for comparison analysis. Knee joint collateral ligaments with nonlinear properties and pretensions were created in the dynamic FE model. A series of dynamic simulations of a squat motion with different initial laxities of the collateral ligaments were performed on the CTKI model under three treatment scenarios of CLs: both CLs retained, anterior cruciate ligament (ACL) removed and both CLs removed. Results showed that only the CTKI model with both CLs retained resulted in similar femoral external rotation and posterior translation with those of the healthy knees. There were not big differences in the tibiofemoral compressive forces among the three scenarios. All the three tibiofemoral compressive forces showed good agreement with other research results from either in-vivo measurements or simulations. The CTKI has better mobility than the traditional STKI designs. The curvatures of the tibial bearing surfaces have been varied in the transverse and longitudinal directions. Compared with the STKI, the CTKIs could restore patient’s knee function to normal, though the tibiofemoral compressive force observed in CTKIs was larger than that of the STKI in the late 25° of simulated knee flexion angles, which was caused by the large passive knee ligament forces and the larger knee motion ranges. The patella has also been studied and compared between the unresurfaced and resurfaced patellar components. The laxity of patellofemoral ligament was firstly tested on the unresurfaced patellar component. Then, the same dynamic boundary conditions were applied on three different patellar button components. Differences were found in the patellar internal rotation and medial tilt motions between the unresurfaced and resurfaced patellar components. The original patellar button component showed contact between the patellar bone and the femoral component apart from contact between the patellar component and the femoral component. The scaled-up button was able to avoid the contact between the patellar component and the femoral component and reduce the patellar medial translation. However, it resulted in larger patellofemoral force than that of the original and flat patellar components. The patellofemoral forces on the scaled-up patellar component were more fluctuating due to less conformity of the contact surfaces. The scaled-up patellar components were found to have two contact areas on the patellofemoral joint, while the original one had only one contact area