Evolving anisotropy and degree of elastolytic insult in abdominal aortic aneurysms: Potential clinical relevance?

Abstract

Accurately estimating patient-specific rupture risk remains a primary challenge in timing interventions for abdominal aortic aneurysms (AAAs). By re-analyzing published biaxial mechanical testing data from surgically repaired human AAAs, material anisotropy emerged as a potentially important determinant of patient-specific lesion progression. That is, based on a new classification scheme, we discovered that anisotropic aneurysmal specimens correlated with increased patient age at surgery when compared with more isotropic specimens (79.7 vs. 70.9 years, p<0.002), despite no significant difference in maximum diameter. Furthermore, using an idealized axisymmetric, finite-element growth and remodeling model of AAA progression, we found that both the initial axial extent of elastin loss and ongoing damage to elastin in the shoulder region of the AAA directly affected the degree of anisotropy as the lesion evolved, with more extensive insults increasing the anisotropy. This effect appeared to be mediated by alterations in axial loading and subsequent differences in orientation of deposited collagen fibers. While the observed increased age before surgical intervention may suggest a potential benefit of anisotropic remodeling, future biaxial tests coupled with pre-surgical data on expansion rates and detailed theoretical analyses of the biostability of a lesion as a function of anisotropy will be required to verify its clinical relevance to patient-specific rupture risk.

Correlation of Wall Microstructure and Heterogeneous Distributions of Strain in Evolving Murine Abdominal Aortic Aneurysms

“…Since the infrarenal aorta is axially constrained at the renal arteries proximally and the iliac bifurcation distally, a remodeling lesion that extends axially may unload the aorta axially in the shoulder and neck regions proximal and distal to the AAA. Such an axial unloading (and the resulting anisotropic remodeling) has been predicted computationally [22], and an expected increase in tortuosity that may occur with axial unloading has been observed [25]. Interestingly, 8 of the 10 shoulder samples in this data set had axial strains greater than all other apical and control samples (all with strain >0.01) (Figure 10).…”

“…Increasing evidence suggests that mechanical changes in the axial (not just circumferential) direction may play important roles in determining overall growth and remodeling of these complex lesions [22,23]. Similarly, volumetric expansion may be an independent factor from maximum diameter for AAA rupture risk [24], suggesting that axial expansion may be clinically important.…”

Inverse problems in the mechanical characterization of elastic arteries

“…The current perspective is however elsewhere, as the proper calibration of growth and remodeling models represent nowadays a new challenge. This is a new class of inverse problems related to the mechanobiological characterization of arteries 78,79 instead of their purely mechanical characterization.…”

Patient-specific stress analyses in the ascending thoracic aorta using a finite-element implementation of the constrained mixture theory

“…A sixth advantage of the present model is its mechanobiological foundation. By spliting the contribution of elastin, collagen and SMCs to the wall stress and by taking into account their respective deposition stretches, this permits to: 1-model the transfer of stress from collagen to elastin when elastin gets damaged which is usually the onset of aneurysm growth [59]; 2-vary collagen mass fractions by taking into account the turnover of collagen which can be mechanobiologically driven. In that case the objective is to preserve a constant (homeostatic) stress in the collagen fibers (maximum stress must return to a given target, σ 0 , which is homeostatic) [54]; 3-Adapt the deposition stretch of collagen to the configuration in which it is deposited during the growth.…”