Measurements and modelling of the compliance of human and porcine organs

Stress-strain data obtained from animal and human tissue have several appli cations including medical diagnosis, assisting in surgical instrument desig n and the production of realistic computer-based simulators for training in minimal access surgery. Such data may also be useful for corroborating mat hematical models of tissue response. This paper presents data obtained from ex-vivo and in-vivo tissue indentation tests using a small indentor that i s similar to instruments used in minimal access surgery. In addition, unifo rm stress tests provide basic material property data, via an exponential st ress-strain law, to allow a finite element method to be used to predict the response for the non-uniform stresses produced by the small indentor. Data are obtained from harvested pig liver and spleen using a static compliance probe. Data for human liver are obtained from volunteer patients, undergoi ng minor open surgery, using a sterile hand-held compliance probe. All the results demonstrate highly non-linear stress-strain behaviour. Pig spleen i s shown to be much more compliant than pig liver with mean elastic moduli o f 0.11 and 4.0 MPa respectively. The right lobe of human liver had a mean e lastic modulus of about 0.27 MPa. However, a single case of a diseased live r had a mean modulus of 0.74 MPa - nearly three times the stiffness. It was found that an exponential stress-strain law could accurately fit uniform s tress test data and that subsequent finite element modelling for non-unifor m stress around a small indentor matched measured force characteristics. (C ) 2001 Elsevier Science B.V. All rights reserved.