Experimental and numerical investigation of waterjet interaction with liver in connection with surgical technique.

Hepatectomy, or liver resection, is a process by which through surgery part or all of the liver is removed. In this operation, less bleeding, negligible damage and fast removal are the most important requirements. Surgery through waterjet is one of the most efficient techniques which is widely used in hepatectomy. Some clinical studies are conducted to investigate waterjet method in liver resection. In the present study interaction of waterjet with liver during the process of the surgery is investigated in terms of mechanical engineering. For this purpose, a system of waterjet is designed to consider the interaction of waterjet with liver at different nozzle diameter and velocities. For validation, SPH-FEM model is used to analyze waterjet interaction with hyperelastic liver. In this model, liver cutting is simulated using element deletion defined by a subroutine code based on maximum principal strain criterion. Depth of cut along with degraded volume are measured experimentally and compared with simulated method. Results show that good agreement exists between experimental and simulation finding. By comparing depth of cut in the experimental and simulation results, it can be seen that liver behavior changes from brittle to ductile by increasing waterjet velocity during the experimental tests. For the simulation, maximum principal strain threshold is set to be between 0.1 and 0.4. However, the best agreement between experimental and simulation results exists at maximum principal strain threshold equal to 0.2. The findings can help surgeons to find the best working range of waterjet device and the most efficient operation.

Subject specific finite element modelling of periprosthetic femoral fractures in different load cases.

Periprosthetic femoral fractures (PFF) around total hip replacements are one of the biggest challenges for orthopaedic surgeons. To understand the risk factors and formation of these fractures, the development of a reliable finite element (FE) model incorporating bone failure is essential. Due to the anisotropic and complex hierarchical structure of bone, the mechanical behaviour under large strains is difficult to predict. In this study, a state-of-the-art subject specific FE modelling technique for bone is utilised to generate and investigate PFF. A bilinear constitutive law is applied to bone tissue in subject specific FE models of five human femurs which are virtually implanted with a straight hip stem to numerically analyse PFF. The material parameters of the models are expressed as a function of bone ash density and mapped node wise to the FE mesh. In this way the subject specific, heterogeneous structure of bone is mimicked. For material mapping of the parameters, computed tomography (CT) images of the original fresh-frozen femurs are used. Periprosthetic fractures are generated by deleting elements on the basis of a critical plastic strain failure criterion. The models are analysed under physiological and clinically relevant conditions in two different load cases re-enacting stumbling and a sideways fall on the hip. The results of the analyses are quantified with experimental data from previous work. With regard to fracture pattern, stiffness and failure load the simulations of the load case stumbling delivered the most stable and accurate results. In general, mapping of material properties was found to be an appropriate way to reproduce PFF with finite element models.

An explicit micro-FE approach to investigate the post-yield behaviour of trabecular bone under large deformations.

Homogenised finite element (FE) analyses are able to predict osteoporosis-related bone fractures and become useful for clinical applications. The predictions of FE analyses depend on the apparent, heterogeneous, anisotropic, elastic, and yield material properties, which are typically determined by implicit micro-FE (µFE) analyses of trabecular bone. The objective of this study is to explore an explicit µFE approach to determine the apparent post-yield behaviour of trabecular bone, beyond the elastic and yield properties. The material behaviour of bone tissue was described by elasto-plasticity with a von Mises yield criterion closed by a planar cap for positive hydrostatic stresses to distinguish the post-yield behaviour in tension and compression. Two ultimate strains for tension and compression were calibrated to trigger element deletion and reproduce damage of trabecular bone. A convergence analysis was undertaken to assess the role of the mesh. Thirteen load cases using periodicity-compatible mixed uniform boundary conditions were applied to three human trabecular bone samples of increasing volume fractions. The effect of densification in large strains was explored. The convergence study revealed a strong dependence of the apparent ultimate stresses and strains on element size. An apparent quadric strength surface for trabecular bone was successfully fitted in a normalised stress space. The effect of densification was reproduced and correlated well with former experimental results. This study demonstrates the potential of the explicit FE formulation and the element deletion technique to reproduce damage in trabecular bone using µFE analyses. The proper account of the mesh sensitivity remains challenging for practical computing times.