Characterization of soft tissue cutting for haptic display: experiments and computational models

Real-time medical simulation for robotic surgery planning and surgery training requires realistic yet computationally fast models of the mechanical behavior of soft tissue. This work presents a study to develop such a model to enable fast haptics display in simulation of softtissue cutting. An apparatus was developed and experiments were conducted to generate force-displacement data for cutting of soft tissue such as pig liver. The force-displacement curve of cutting pig liver revealed a characteristic pattern: the overall curve is formed by repeating units consisting of a local deformation segment followed by a local crack-growth segment. The modeling effort reported here focused on characterizing the tissue in the local deformation segment for fast haptic display. The deformation resistance of the tissue was quantified in terms of the local effective modulus (LEM) consistent with experimental forcedisplacement data. An algorithm was developed to determine LEM by solving an inverse problem with iterative finite element models. To enable faster simulation of cutting of a threedimensional (3D) liver specimen of naturally varying thickness, three levels of model order reduction were studied. Additionally, the variation of the LEM with cutting speed was determined. The values of LEM decreased as the cutting speed increased. This thesis also includes the characteristic response of soft tissue to the growth of a cut (cracking) with a scalpel blade. The experimentally measured cut-force versus cut-length data was used to determine the soft tissue’s resistance to fracture (resistance to crack extension) in scalpel cutting. The resistance to fracture of the soft tissue is defined as the amount of mechanical work needed to cause a cut (crack) to extend for a unit length in a soft-tissue sample of unit thickness. The equipment, method, and model are applicable for all soft tissue.Finally, the method of determining the property of the pig liver tissue during cutting was verified. Dual C-arm fluoroscopes were used to obtain the motion of the beads embedded inside the specimen during cutting. The experimentally measured displacement field was compared to the displacement field obtained through finite element model based on the LEM values at each localized area.Ph.D., Mechanical Engineering and Mechanics -- Drexel University, 200