In vivo mechanical behavior of intra-abdominal organs.

For realistic surgical simulation in a virtual environment, in vivo material properties of biological tissues are required for simulating the deformations and the reaction forces from the tool-tissue interactions. In this paper, the in vivo static and dynamic mechanical behavior of the liver and lower esophagus of pigs were presented both in linear and nonlinear regions under compressive and shear indentations. A robotic device was programmed to function as a mechanical stimulator with a 2-mm flat-tipped cylindrical probe attached to its tip. A series of ramp and hold stimuli, as well as sinusoidal indentation stimuli, were delivered to the organs and reaction forces were measured. The conditions for these indentation stimuli were designed such that they were similar to conditions in an operating room. Experiments were also carried out on the organs for ex vivo and in vitro conditions. Results show that the breathing and pulse rate significantly affect the measured force responses of the organs. From the obtained force-displacement relationships, steady-state impedances as well as dynamic impedances of both organs were calculated. The results also show that in vivo steady-state impedance of the lower esophagus is significantly higher than that of the liver. The in vivo steady-state response of the liver, however, exhibits a greater degree of nonlinearity than that of the lower esophagus. The in vivo steady-state response of the lower esophagus in the three orthogonal directions also indicates that the lower esophagus is not significantly anisotropic. The impedance of both organs under sinusoidal indentations (0-5 Hz) are fairly similar each other. Magnitudes of the impedance over the stimulus frequencies are fairly constant. The impedance phase angles decrease over the range of stimulus frequencies applied. Comparison of the measurements obtained from the in vivo, ex vivo, and in vitro experiments shows that the mechanical properties of the biological tissues change significantly after the death of the animal. The tissues generally become stiffer and exhibit greater nonlinearity. The degree of change in their mechanical properties is dependent on the amount of time after the death of the animal. These data can be further utilized in the computing of the material parameters of tissue models for laparoscopic surgery simulators as well as open surgery simulators.