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Finite element analysis of soft tissues is a well-developed method thatallows estimation of mechanical quantities (e.g. stresses, strains). Aconstitutive law has to be used to characterise the individual tissues. This iscomplex as biological tissues are generally visco-hyperelastic, anisotropic,and heterogenous. A specific characteristic, their nearly incompressibility,was well reported in the literature, but very little effort has been made tocompare volume variations computed by the simulations with in vivomeasurements. In the present study, volume changes of the fat pad duringcontrolled indentations of the human heel region were estimated from segmentedmedical images using digital volume correlation. Indentations were repeatedwith high and mild intensity normal and shear loads. The experiment wasreproduced using finite element modelling with several values of Poisson'sratio for the fat pad, extracted from literature values (from 0.4500 to0.4999). Estimated fat pad volume changes were compared to the measured ones toassess the best value of Poisson's ratio in each indentation case. The impactof the Poisson's ratio on the Jacobian of the deformation gradient and thevolumetric strains was also computed. A single value of Poisson's ratio couldnot fit all the indentation cases. Estimated volume changes were between 0.9 %- 11.7 % with a Poisson's ratio from 0.4500 to 0.4999. The best fit wasobtained with a 0.4900 Poisson's ratio except for the high normal load where avalue of 0.4999 resulted in less error. In conclusion, special care should betaken when setting the Poisson's ratio as the resulting estimated deformationsmay become unrealistic when the value is far from incompressible materials.

computer methods in biomechanics and biomedical engineering

Current poisson's ratio values of finite element models are too low to  consider soft tissues nearly-incompressible: illustration on the human heel  region