• total ankle replacement
• Ansys
• PyMAPDL
• wear model
• finite element analysis

The ankle is one of the joints most affected by osteoarthritis, with a progressive degeneration of the cartilage that can lead, in severe cases, to a complete anatomical and functional compromise of the joint itself. Among the currently available solutions, Total Ankle Replacement (TAR) has gained relevance in the last two decades as an alternative to arthrodesis. While the latter involves surgical fusion of the ankle, limiting mobility and presenting other possible complications, TAR allows for a close approximation of physiological movement and pain reduction. However, TARs still exhibit lower performance compared to more common hip and knee replacements, and polyethylene insert wear, along with aseptic loosening and instability, is considered one of the main causes of early revision surgery.

This thesis focuses on the use of computer modelling and simulation to study and predict wear in TARs. The results obtained In Silico can be used to generate information supporting the safety and effectiveness of new devices, reducing time and costs of traditional experimental tests. In particular, this work aims to optimize a finite element model developed to predict wear in the polyethylene insert of the commercial FAR ankle prosthesis produced by the company Adler Ortho and to automate the procedure used to perform simulations.

The first phase of the study focuses on optimizing, within the Ansys Mechanical APDL environment, the finite element model built based on the implant geometry provided directly by the company and the boundary conditions established by ISO 22622-2019, which defines the loading and kinematic conditions to be considered for the experimental wear tests. Quasi-static analyses are performed to simulate 10 gait cycles with the implementation of a double contact model to evaluate three variables of interest on both insert surfaces: maximum contact pressure, total wear volume, and wear depth.

The second part of the study involves the automation of the entire simulation pipeline through PyAnsys, specifically PyMAPDL, a new programming language devised by Ansys. This language allows the use of both APDL and Python within a single programming interface, in this specific case, Jupyter Lab. The significant advantage of PyAnsys lies in its ability to automate the analyses, allowing a series of simulations to be run without any intervention from the user, except for the writing of the initial code, including the "post-processing" phase and result saving. This innovative approach also facilitated a detailed examination of additional wear-related parameters, such as contact surfaces and worn areas. Subsequently, using the power of the new tool, a mesh convergence analysis was conducted, along with a sensitivity analysis to some key model parameters.

The results obtained are consistent with the data reported in other studies that have considered wear of the ankle prosthesis insert, focusing particularly on the talar side, subject to higher contact pressures, and consequently greater wear, as observed in this study. The contact pressures documented for the side of the insert in contact with the talar surface range from 5.7 to 25 MPa, in line with the results of this study, which report values ranging from 3 to 28 MPa.

A good agreement is also observed in the estimated wear volume. In particular, the literature reports that the total wear volume related to the talar side of the insert falls within a range between 16 mm3 and 66 mm3 when the prosthesis undergoes 5 million cycles. These values are in line with what emerged in our study. Assuming a linear trend for the wear volume and extrapolating the results obtained for 5 million cycles, it is found that the volume loss for the same contact surface is approximately 55 mm3.