NONLINEAR, 3-DIMENSIONAL FINITE-ELEMENT MODEL OF SKIN BIOMECHANICS

Objective: The aim of this study was to expand the rigour and scope of soft-tissue finite-element modelling through the introduction of nonl inear biomechanics. The capability to simulate ''tissue'' movement in three-dimensional space was a priority. Method: A computer-based finit e-element technique was used to approximate the exact solution to the governing differential equations. Common fusiform defects were ''close d'' in two- and three-dimensional space. Strains of approximately 17% were introduced. Skin was modelled as a nonlinear elastic anisotropic material in a laminated-composite structure undergoing large deformati ons and large strain. The finite-element software package for nonlinea r biomechanical analysis was run on a university-based, multi-user wor kstation. Repeated simulations were performed. The key independent var iables were the magnitude of the subcutaneous adhesion and the degree of undermining. The two dependent variables were the ''closure'' force and the distortion field. Results: The absolute values computed for ' 'closure'' force, ranging from 4.4 N to 5.2 N, were consistent with pr evious animal studies. The periphery of the distortion field varied fr om 4.7 to 5.9 cm from the defect midline. The force of subcutaneous ad hesion was varied from 2 to 20 kN/m (+900%) and led to a -19% and +16% change in the distortion field width and ''closure'' force, respectiv ely. Undermining was progressively increased from 1 to 5 cm (+400%) an d produced a +5% and -12% change in the width of the distortion field and the ''closure'' force, respectively. Conclusions: The application of nonlinear biomechanics to soft-tissue finite-element modelling has been rewarding. The results correlate well with surgical experience. W ith specific regard to undermining, additional insight has been gained . Undermining broad-based soft-tissue naps has progressively limited b enefits. The computational demonstration of this result, consistent wi th prior animal studies, has not been previously published. Future app lication of this technology may permit the development of more complex flaps. Animal experimentation may be reduced and/or deferred until po stulated flap designs have been simulated and refined.