Towards robust prosthetic socket design through simulated pressure casting

Abstract

Every year a considerable number of people undergo a lower leg amputation and start using prostheses. After a period of testing and fitting of various prostheses, gradually more suitable for the patient and sized on his/her needs, the definitive prosthesis is created. However, the use of prostheses, even if definitive, especially if prolonged over time, can lead to problems for the patient. In fact, despite the final fitting, problems may still arise due to the routine use of the prosthesis or, in specific cases such as athletes, to achieve high-level performance. Because of this use, pathological remodelling phenomena or even lesions and tissue damage often occur. As a result, a major line of scientific research focuses improving prosthetic socket design. Techniques are moving gradually from hands-on plaster casting and modification to hands-off plaster methods like pcasting, and digital methods known as CAD/CAM. Bioengineering contributions involve modelling and characterizing the amputated limb, to provide predictive tools to prosthetists. Modelling has been applied to analyse conventional hands-on plaster sockets, and CAD/CAM, but simulations of the pcast procedure have not been reported in the literature. The aim of this work was to provide a basic model and a biomechanical FEM simulation routine for the lower leg stump. Specifically, the p-cast procedure useful to form the socket mold has been simulated, consolidated, and tested through biomechanically accurate and usable FEM simulations. The final future aim will be to optimize the socket design. Specifically, for future work, the mechanical behaviour of the leg in contact with the socket could be simulated and optimized to increase its robustness. Also identifying biomechanical factors (input design parameters) to improve the mechanical stump/socket interaction. Finite element simulations of the p-cast process were performed, on the meshed model of a lower leg stump derived from medical imaging data. Different variations of the biomechanical and general simulation properties have been implemented (patient weight, different material properties of the stump, constitutive models of the liner, etc...). From the FEM simulations several contour plots of the different mechanical quantities of interest (displacements, strains, first and third principal strains and max shear strains) have been produced, showing their distribution for the different built models. Furthermore, key values such as maximum, minimum, and average values of the main mechanical quantities were also extracted from the models and showed accordingly. The obtained results showed the differences and the similarities of the models, in terms of quantitative comparison and different distribution of the strain, displacements, principal strains. The results showed that even slightly changing the material properties such as Young's modulus and Poisson's ratio to simulate old patients and in particular sportive patients has substantial effects on the distribution and above all on the intensity of displacements and strains. It can be said that these changes are significant but do not affect the robustness of the system, as the intensities do not vary by many orders of magnitude with respect to the reference model. Furthermore, biomechanical considerations on the cases examined have been illustrated in this work. Different liners produce different states of displacements and strains, also a different shape deformation of the liner and of the stump themselves. Furthermore, it has been shown that heavier patients generate higher pressures in the p-cast process as well as displacements and strains, while the sportive model has all these values very low. On the one hand, it might be interesting to explore the effects of these changes also considering other factors, but above all the natural continuation of the work would be to include the socket in the FEM simulations, to fully evaluate the performance and robustness of the prosthesis/stump bio-mechanical interactions.

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ORCID for Alex Dickinson: orcid.org/0000-0002-9647-1944

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