Wall stress as a possible mechanism for the development of transverse intimal tears in aortic dissections.

In aortic dissection intimal tear develops in a transverse direction. Since dissection is associated with the aneurysm of the aorta, its mechanism was investigated by analysing the pressure induced wall stress as a function of 'growth' of the aneurysm. The stresses were determined using a finite element analysis where the aorta was modelled as an isotropic, nonlinear, hyperelastic material. Growth of the aneurysm was simulated by dilating an aortic segment in increments of 10% of the initial diameter. At each dilation luminal pressure of 120 mm Hg was applied and stress determined. In the aneurysm bulb, longitudinal stress increased significantly as the bulb became larger, while circumferential stress changed only a little. In the undilated segment both the longitudinal and circumferential stresses remained relatively unchanged. The increase in the longitudinal stress in the bulb occurred primarily due to change in shape of the aorta from cylindrical to ellipsoidal to spherical. Hence, as the aneurysm 'grows', the longitudinal stress in the bulb increases rapidly and could be responsible for the transverse tear in the aortic dissection.