Finite element analysis in colorectal surgery: non-linear effects induced by material model and geometry

The use of continuum mechanics, especially Finite Element Analysis (FEA) has gained an extensive application in the medical field, in order to simulate soft tissues. In particular, colorectal simulations can be used to understand the interaction between colon and the surrounding tissues, and also, between colon and surgical instruments. Although several works have been introduced considering small displacements, FEA applied to colorectal surgical scenarios with large displacements is still a challenge. This work aims to investigate how FEA can describe non-linear effects induced by material properties and different approximating geometries for colon. More in detail, it shows a comparison between simulations that are performed using well-known hyperelastic models (principally Mooney-Rivlin and, in one case, Yeoh) and the linear one. These different mechanical behaviours are applied on different geometrical models (planar, cylindrical and a 3D-shape from digital acquisitions) with the aim of evaluating also the effects of geometric non-linearity. Increasing the displacements imposed by the surgical instruments, the adoption of a hyperelastic model shows lower stresses than the linear elastic one that seems to overestimate the averaged stress. Moreover, the details of the geometrical models affect the results in terms of stress-strain distribution, since it provides a better localisation of the effects related to the hypothesis of large strains.