In situ Measurement of Cadaveric Soft Tissue Mechanical Properties and Fulcrum Forces for Use in Physics-Based Surgical Simulation Yi-Je Lim ∗ Suvranu De † Daniel B. Jones ‡ Tejinder P. Singh ° Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute Department of Surgery, Beth Israel Deaconess Medical Center Department of Surgery, Albany Medical College ABSTRACT Development of a realistic surgery simulator that delivers high fidelity visual and haptic feedback, based on the physics of deformable objects, requires the use of empirical data on the behavior of soft tissues when subjected to external loads. Measurements on live human patients present significant risks, thus making the use of cadavers a logical alternative. Cadavers are widely used in present day surgical training, are relatively easy to procure through excellent donor programs and have the right anatomy, which makes them better candidates for training than the porcine model. To investigate the static and dynamic properties of soft tissue, we have developed a high precision tactile stimulator by modifying an exisitng haptic interface device, the Phantom, and used it to record the force-displacement behavior of intra-abdominal organs of fresh human cadavers at the US Surgical in Connecticut and Albany Medical College. Another paraemeter of great practical significace, but mostly overlooked in the literature, is the fulcrum force at the point of entry of the trocar into the abdomen. The length of the surgical tools, coupled with a thick abdominal wall of overweight or obese patients, tend to produce torques at the wrist of the surgeon which are often large enough to mask any haptic sensation from the organs being operated on. To ascertain the true role of haptics in endoscopic surgery one needs to measure these fulcrum forces. In this paper we discuss techniques and some preliminary results in this direction. Keywords: soft tissue, biomechanical properties, human cadaver, indentation experiment, surgical simulation * limy@rpi.edu † des@rpi.edu ‡ djones1@bidmc.harvard.edu ° SinghT@mail.amc.edu I. INTRODUCTION To provide realistic visual and haptic feedback to the user, physics-based computational techniques, such as the finite element method [1-5] or meshfree methods [6] have been advocated. These techniques are based on numerically solving partial differential equations that govern the physical behavior of soft tissues, subject to boundary and/or initial conditions. While the differential equations represent conservation of momentum, they contain a few coefficients, known as the constitutive tensor, which must be determined experimentally. For instance, in linear elasticity, the stress ( ij σ ) and strain ( kl ε ) tensors are related by the rank four elasticity tensor ( ijkl C ) through the Hookean relationship {, , , } {1, 2, 3} ij ijkl kl C ijkl σ ε = ∈ (1) For a homogeneous, isotropic material the elasticity tensor has just two independent constants (Young’s modulus, E, and Poison’s ratio, ν) [7], which must be empirically determined. Notice that only a few well-conceived experiments are necessary to determine these constants, in contrast to, for example, spring-mass models, where hundreds of thousands of stiffness values have to be assigned using elaborate curve fitting techniques [8] sometimes using neural networks. Measurement of soft tissue properties is a well- established research area [9,10]. However, the major challenge in this field is that soft tissues exhibit complicated mechanical behavior including nonlinear, inhomogeneous, anisotropic, and rate dependent response. For surgical simulation, ideally it is necessary to measure, and then model the in vivo mechanical response of the soft tissues operated on. However, the current efforts are either aimed at obtaining ex vivo properties [9,11], which are grossly different from in vivo conditions [12,13], or utilizing animal models such as the porcine model [12,13] which have fundamental differences in anatomy and tissue consistency compared to humans. The use of fresh human cadavers is a risk-free alternative to live human experiments, and is pursued in this paper. Cadavers 259 Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems 2006 March 25 - 26, Alexandria, Virginia, USA 1-4244-0226-3/06/$20.00 ©2006 IEEE