Finite Element Analysis to Investigate Lipid Absorption in UHMWPE Under Mechanical Loading

Ultra-High-Molecular-Weight Polyethylene (UHMWPE) has long been considered the gold standard for tibial inserts in total knee arthroplasty (TKA) due to its mechanical strength, low friction, and biocompatibility. Despite advancements in crosslinking and post-irradiation stabilization, oxidative degradation remains a limitation, contributing to long-term implant failure. Recent studies have shown that in vivo oxidative degradation still occurs, suggesting alternative degradation pathways. One involves the diffusion of lipid species from synovial fluid into the UHMWPE matrix. Lipids diffuse particularly into amorphous regions and, by inter- acting with them, alter the chemical and mechanical structure over time. The combination of cyclic mechanical stress and thermal effects from daily movements may increase polymer chain mobility, accelerating oxidative reactions and loss of integrity. This thesis presents two computational model to simulate the time-dependent diffusion of lipid species into crosslinked UHMWPE under mechanical loading. Two approaches were implemented to couple stress and diffusion finite element. The first approach involves two sequential finite element analyses in Abaqus: one to simulate the mechanical stress field, and the other to model mass diffusion using a stress-dependent diffusivity. A custom MATLAB script is used to iteratively couple the two simulations by updating diffusion parameters based on the local stress state, thus reducing computational cost. This method allows more flexibility in post-processing but requires iterative data exchange between Abaqus and MATLAB. In contrast, the second approach offers a fully integrated simulation within Abaqus. This approach takes advantage of the mathematical similarity between mass diffusion and heat transfer equations, since the two formulations differ only for the variables involved. Abaqus fully coupled temperature–displacement simulation is used to model mass diffusion by treating temperature as the equivalent for concentration. The model is applied to a simplified 2D geometry representing one-quarter of a symmetric experimental setup. Material properties, including diffusion coefficient (D), solubility (S) and stress-diffusion factor (kp) were derived from literature and experiments and calibrated for virgin and remelted UHMWPE. Simulations predict palmitic acid accumulation and mass flux, identifying regions vulnerable to early softening or failure under high stress. These findings highlight the role of mechanical loading in lipid transport and provide a predictive framework for long-term degradation under clinically relevant conditions.

Author dc:creator

• Isabella Paris (23292070)

• Biomedical Engineering
• Computational Modeling
• Lipid Diffusion

Statement dc:rights

• In Copyright
• Open Access after 2028-01-01

DOI dc:identifier

https://doi.org/10.25417/uic.31451770.v1

OAI identifier oai:identifier

oai:figshare.com:article/31451770