MECHANICAL PROPERTIES CHARACTERIZATION OF ABDOMINAL AORTIC ANEURYSM TISSUE USING BIAXIAL TESTING

摘要

An abdominal aortic aneurysm (AAA) is an abnormal, localized enlargement of the aorta. If untreated, a AAA will continue to enlarge in size and eventually rupture. Currently, AAA diameter is used as the principal indicator of impending rupture. However, this method it is not totally reliable. In an effort to improve the estimation of rupture risk, some researchers are currently studying the mechanical wall stresses of AAAs using patient-specific medical imaging techniques and finite element modeling. The accuracy of these models depends significantly on the constitutive law used to describe the mechanical properties of the AAA tissue. To date, only isotropic constitutive laws have been used in these models. The mechanical properties of the AAA wall have been presented in several studies. In all known cases the experimental testing of AAA tissue has been performed only uniaxially. These studies have consistently shown that the aneurysmal wall is considerably stiffer than the nonaneurysmal wall. In 1994 Chang and Roach [3] presented testing results of seven AAA wall specimens obtained from seven patients. Their specimens were taken only from the anterior wall of the aneurysm and only in the longitudinal direction. A one-dimensional power-law model was presented to relate the resulting true stress to the engineering strain. In 2001 Thubrikar et al. presented results of forty-seven AAA wall specimens obtained from five patients. Their specimens were taken from various locations on the aneurysm, and in both longitudinal and circumferential directions. Again, a one-dimensional power-law model was used to relate the resulting true stress to the engineering strain. The results of this work show that the AAA wall mechanical properties vary with both location on the aneurysm (stiffer on anterior side) and direction (stiffer in circumferential direction). Raghavan et al. presented results of seventy-one wall specimens taken from a total of sixty-one patients. Specimens were taken in both longitudinal and circumferential directions, and a two-parameter isotropic nonlinear elastic constitutive model was developed to relate the Cauchy stress to the deformation. While uniaxial testing is very useful in characterizing mechanical properties, it alone is not sufficient to characterize the properties of nonlinearly elastic materials. Additional multiple-loading experiments must be performed to fully characterize the elastic properties of these materials. Such experiments are typically performed by subjecting material test specimens to either biaxial loading or to simultaneous tension and torsion. For the case of the AAA wall, given that the testing specimens are planar in geometry, biaxial loading experiments are most appropriate. Biaxial loading experiments have been used frequently as a means to characterize the non-linear elastic behavior of many biological tissues. These include muscle, skin, and artery.