Numerical Study of Biopolymer Implants for Distal Femoral CondylesÐ Finite Element Simulations
Creators: name: Olsen, Luke 
Publisher: name: University of Nevada, Las Vegas
Year: 2018

description: Approximately 12% of the adult population in the United States is affected by
         Osteoarthritis (OA) [1, 2]. Because of this, OA is the considered the most chronic degenerative joint disease, and is subject to continuous research into treatment. OA mainly manifests itself by degrading the articular cartilage in joints, such as the knee, and can eventually lead to complete loss of cartilage and potentially bone damage, leading to pain and discomfort for the patient [3]. For severe OA, the most common treatment is total knee arthroplasty (TKA) [4]. This procedure includes removing portions of the femur and tibia, and replacing the articulating surfaces with metal implants, commonly known as total knee replacement (TKR) implants. This can be a costly, and potentially painful, procedure and recovery for the patients [5-12]. To avoid these consequences, research is being conducted to develop alternative implants. One such implant is designed to replace only the damaged cartilage, and not the bone [13, 14, 43]. This paper focused on the creation and testing of a finite element model framework of this 2mm thick biopolymer implant, which could be used to determine the implant's feasibility and to serve as a baseline approximation into stress and deformation values for future testing.
         The simulations in this paper considered an in-vivo loading case for the tibiofemoral joint and a possible experimental loading case for the implant, and used the standard student version of COMSOL Multiphysics. The implant material tested was Bionate 80A, and used the material properties of cartilage to serve as a model benchmark, with all materials assumed to be linearly elastic. The first loading case was a 90-degree squat where the joint started at full extension, or standing, with a full body weight load (BW), and ended at 90-degree flexion at 300% BW load [21, 22, 28]. This loading case was considered to be a potential experimental loading case. The second loading case was a heel-strike to toe-off walking gait, which experiences peak load of 261% BW [21, 28]. With these loading cases, two generations of simulations were created. These simulations were tested by performing an initial feasibility study of the presented implant as well as testing natural cartilage. The first-generation model, known as the Simple Contact Simulation (SC-SIM), assumed that the contact area is perfectly circular, and the load is evenly distributed along this surface [43]. This model was created by using experimental data to create an expression for contact area size as a function of angle of flexion and load. This expression was used to drive the size of two cylinders, which were used to partition the surface of the implant, then the load was applied to this surface as a force per area. The second-generation model, known as the Function Driven Contact Simulation (FDC-SIM), is a modified version of the SC-SIM. Instead of using partition cylinders and uniform loads, the FDC-SIM uses a parametric equation to drive the contact shape to mimic the general shape found in existing publications, and applies a compact-supported load using a modified Gauss curve. The FDC-SIM itself had two variants, where one was full time-dependent and the other was a parametric sweep of time.
         The results of the simulation showed that the uniform load over the partition surfaces of the SC-SIM causes large stress concentrations near the outer edge of the contact. For the squat at 300% BW load, these stresses exceeded the yield of Bionate 80A, and was above the determined range of in-vivo loading stress and deformation [32, 35, 39, 52, 56]. On the other hand, performing the squat at 150% BW produced stress values within the cartilage stress range, and produced factors of safety for the Bionate 80A implant that were comparable to healthy cartilage [43]. Furthermore, this 150% BW squat had loads that were more comparable to a few of the
         experiments used to validate the results. The SC-SIM walking gait still produced higher than expected stresses due to the large stress concentrations, but like the 150%BW squat, the results were closer to those found under similar loads. The more accurate contact load and distribution of the FDC-SIM yielded stress and deformation values that were within the range of the reference data for both the gait and the squat. The time-dependent variant resulted in slightly higher stresses and deformations than the parametric sweep variant. Additionally, the time-dependent case estimated a wear depth of 2.4x10-3mm for the squat and 1.68x10-4mm for the gait for 2 million cycles.
         From these results, the high stress concentrations and long computation time makes the SC-SIM not the optimal choice for simulating the implant, despite the reasonable results from the walking gait. Since the FDC-SIM results were verifiable for the cartilage loading case, this shows that this model can properly predict the stress and deformation under in-vivo loading cases, even with the assumption of linearly elastic material. With the FDC-SIM verified, the results of the Bionate 80A indicate an initial feasibility of the implant for in-vivo loading, and the results can serve as possible theoretical values to serve as a baseline for future UNLV experimentation.