Regional distribution of wall thickness and failure properties of human abdominal aortic aneurysm

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Abstract

The regional distribution of wall thickness and failure properties in human abdominal aortic aneurysm (AAA) was explored. Three unruptured and one ruptured AAA were harvested as a whole during necropsy. Thickness was measured at about every 1.5 cm2 wall surface area for an average of 100 measurement sites per AAA. Multiple longitudinally oriented rectangular specimen strips were cut at various locations from each AAA for a total of 48 strips. The strips were subjected to uniaxial extension until failure. Wall thickness varied regionally and between AAA from as low as 0.23 mm at a rupture site to 4.26 mm at a calcified site (median=1.48 mm). Wall thickness was slightly lower in the posterior and right regions. The failure tension (ultimate) of specimen strips varied regionally and between AAA from 5.5 N/cm close to a blister site in the ruptured AAA to 42.3 N/cm at the undilated neck of a 4 cm diameter unruptured AAA (median=14.8 N/cm). Failure stress (ultimate) varied from 33.6 to 235.1 N/cm2 (median=126.6 N/cm2). There was no perceptible pattern in failure properties along the circumference. Failure tension of specimen strips at or close to blisters was mostly low. The rupture site in the ruptured aneurysm had the lowest recorded wall thickness of 0.23 mm with only slightly higher readings within a 1 cm radius. The failure tension of the specimen strip close to the rupture site was low (11.1 N/cm) compared to its neighborhood in the ruptured aneurysm.

Introduction

Knowledge of the regional distribution of wall thickness and failure properties in an abdominal aortic aneurysm (AAA) can help in understanding its natural history, developing methods to predict rupture risk and in designing vascular prostheses. Earlier studies have reported on wall thickness and failure strength by testing specimens taken from the anterior midsection of an AAA (He and Roach, 1994; Raghavan et al., 1996; Vorp et al., 2001). However, they did not provide information on regional variation in these characteristics. A recent report (Thubrikar et al., 2001) discussed efforts at studying regional variation in properties of five AAA. Despite useful insights from these studies, our understanding of the regional variations in wall thickness and failure properties remains poor. Further, we know little about how these properties are affected by localized wall characteristics. A key difficulty in studying such issues is the sparse availability of wholly harvested AAA. In the largest study of its nature in recent times, University of Sao Paulo investigators, da Silva et al. (2000) reported on a 5-year study where 38 ruptured and 40 unruptured AAA were harvested as a whole during autopsies for investigation of morphological characteristics. In this study, we seek to explore regional variations in failure properties within AAA that were harvested whole under the ongoing Sao Paulo study. Our study objective is exploratory. It compliments earlier work by Thubrikar et al. (2001), but additionally provides a first look into issues hitherto unaddressed in the literature.

Section snippets

Methods

Three unruptured and one ruptured AAA were excised from cadavers submitted for necropsy for the elucidation of the cause of death at the Service for Confirmation of Deaths at São Paulo University School of Medicine (SP, Brazil). Following death, the corpses were transported immediately to the Service where they were frozen until necropsy. All AAA in this study were harvested at the Service within 24 h of death. No chemical preparation was done to the cadavers or specimens. The study was approved

Results

Of the four specimens, three were stored in a frozen state after harvest until they were tested, while one specimen was studied immediately following harvest (see Table 1 for storage time, patient demographics, morphology and number of specimen strips). Wall thickness was measured at a total of 394 sites from the four AAA. It varied from 0.23 mm at a rupture site to 4.26 mm at a calcified site with the median, 5th and 95th percentile measurements of 1.48, 0.80 and 2.83 mm, respectively. The wall

Discussion

Wall thickness and failure properties of an AAA represent the biomechanical manifestation of tissue degeneration and/or regeneration. If experimental testing of a sufficiently large number of AAA tissues ex vivo reveals some underlying consistency, then a stochastic model can be built for the prediction of thickness and failure property distribution in a future patient's intact AAA (Wang et al., 2001). In a remarkable first study of its nature, Thubrikar et al. (2001) cut longitudinal and

Acknowledgments

This study was supported by travel grants from NSF International Division: Americas Program (to Raghavan) and FAPESP, Brazil (to da Silva) and a grant from the American Heart Association Heartland Affiliate. The authors thank Elizabeth Rikkers, Lisandra Colón (University of Lowa) and Marcos lkeda (University of São Paulo) for assistance with digitization, study of freezing effects and mechanical testing respectively.

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