Toward a biomechanical tool to evaluate rupture potential of abdominal aortic aneurysm: identification of a finite strain constitutive model and evaluation of its applicability

Knowledge of the wall stresses in an abdominal aortic aneurysm (AAA) may be helpful in evaluating the need for surgical intervention to avoid rupture. This must be preceded by the development of a more suitable finite strain constitutive model for AAA, as none currently exists. Additionally, reliabl e stress analysis of in vivo AAA for the purposes of clinical diagnostics r equires patient-specific values of the material parameters, which are diffi cult to determine noninvasively. The purpose of this work, therefore, was t hree-fold: (1) to develop a finite strain constitutive model for AAA; (2) t o estimate the variation of model parameters within a sample population; an d (3) to evaluate the sensitivity of computed stress distribution in AAA du e to this biologic variation. We propose here a two parameter, hyperelastic , isotropic, incompressible material model and utilize experimental data fr om 69 freshly excised AAA specimens to both develop the functional form of the model and estimate its material parameters. Parametric analyses were pe rformed via repeated finite element computations to determine the effect of varying each of the two model parameters on the stress distribution in a t hree-dimensional AAA model. The agreement between experimental data and the proposed functional form of the constitutive law was very good (R-2 > 0.9) , Our finite element simulations showed that the computed AAA wall stresses changed by only 4% or less when both the parameters were varied within the 95% confidence intervals for the patient population studied, This observat ion indicates that in lieu of the patient-specific material parameters, whi ch are difficult to determine the use of population mean values is sufficie ntly accurate for the model to be reasonably employed in a clinical setting . We believe that this is an important advancement toward the development o f a computational tool for the estimation of rupture potential for individu al AAA, for which there is great clinical need, (C) 2000 Elsevier Science L td. All rights reserved.