Characterizing the Nonlinear Mechanical Response of Liver to Surgical Manipulation

Computer-aided medical technologies such as simulators for surgical training and planning require accurate representation of soft tissue behavior under large deformations. Limited datasets, and unrealistic models for soft tissues currently hinder the advancement of surgical simulation. This work identifies the nonlinear mechanical response of liver through the development of a physically-based constitutive model. The effects of perfusion on the viscoelastic response of liver are identified, and a perfusion apparatus is created that approximates the in vivo condition. Indentation tests measuring the response of whole, perfused, porcine livers under finite deformations (~30% nominal strain) are conducted. Results indicate a time dependent, nonlinear, viscoelastic, force-displacement behavior. A constitutive model describes the time varying response through the combined contributions of three subsystems: collagenous capsule, parenchyma, and fluid filled vessels. Solving the inverse problem through iterative finite element modeling identifies the seven independent material parameters. The model is capable of capturing the salient features of the data. Modifications to the model and methods for ensuring the uniqueness of the parameters are presented. Additionally, a technique is described for assessing the tradeoff between accuracy and computation time necessary for real-time implementation. This work presents data and modeling efforts to realize a constitutive model for liver whose parameters have a physical basis. The model can predict the behavior of liver independent of loading modality. Implementing such physically-based constitutive models into simulation systems provide realistic behavior, and ensure that errors made are not from the virtual environment.