Nonlinear, three-dimensional finite-element model of skin biomechanics

The aim of this study was to expand the rigour and scope of soft-tissue finite-element modelling through the introduction of nonlinear biomechanics. The capability to simulate "tissue" movement in three-dimensional space was a priority.

A computer-based finite-element technique was used to approximate the exact solution to the governing differential equations. Common fusiform defects were "closed" 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 deformations and large strain. The finite-element software package for nonlinear biomechanical analysis was run on a university-based, multi-user workstation. Repeated simulations were performed. The key independent variables were the magnitude of the subcutaneous adhesion and the degree of undermining. The two dependent variables were the "closure" force and the distortion field.

The absolute values computed for "closure" force, ranging from 4.4 N to 5.2 N, were consistent with previous animal studies. The periphery of the distortion field varied from 4.7 to 5.9 cm from the defect midline. The force of subcutaneous adhesion 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, respectively. Undermining was progressively increased from 1 to 5 cm (+400%) and produced a +5% and -12% change in the width of the distortion field and the "closure" force, respectively.

The application of nonlinear biomechanics to soft-tissue finite-element modelling has been rewarding. The results correlate well with surgical experience. With specific regard to undermining, additional insight has been gained. Undermining broad-based soft-tissue flaps has progressively limited benefits. The computational demonstration of this result, consistent with prior animal studies, has not been previously published. Future application of this technology may permit the development of more complex flaps. Animal experimentation may be reduced and/or deferred until postulated flap designs have been simulated and refined.