Variations of dissection properties and mass fractions with thrombus age in human abdominal aortic aneurysms J. Tong a , T. Cohnert b , P. Regitnig c , J. Kohlbacher c,d , R. Birner-Gruenberger c,d,e , A.J. Schriefl a , G. Sommer a , G.A. Holzapfel a,f,n a Graz University of Technology, Institute of Biomechanics, Graz, Austria b Medical University of Graz, Clinical Department of Vascular Surgery, Graz, Austria c Medical University of Graz, Institute of Pathology, Graz, Austria d Medical University of Graz, Center of Medical Research, Graz, Austria e ACIB GmbH, Graz, Austria f Royal Institute of Technology (KTH), Department of Solid Mechanics, Stockholm, Sweden article info Article history: Accepted 13 October 2013 Keywords: Intraluminal thrombus Thrombus-covered wall Dissection Thrombus age Mass fraction abstract Introduction: Thrombus ages, defined as four relative age phases, are related to different compositions of the intraluminal thrombus (ILT) in the abdominal aortic aneurysm (AAA) (Tong et al., 2011b). Experimental studies indicate a correlation between the relative thrombus age and the strength of the thrombus-covered wall. Methods: On 32 AAA samples we performed peeling tests with the aim to dissect the material (i) through the ILT thickness, (ii) within the individual ILT layers and (iii) within the aneurysm wall underneath the thrombus by using two extension rates (1 mm/min, 1 mm/s). Histological investigations and mass fraction analysis were performed to characterize the dissected morphology, to determine the relative thrombus age, and to quantify dry weight percentages of elastin and collagen in the AAA wall. Results: A remarkably lower dissection energy was needed to dissect within the individual ILT layers and through the thicknesses of old thrombi. With increasing ILT age the dissection energy of the underlying intima–media composite continuously decreased and the anisotropic dissection properties for that composite vanished. The quantified dissection properties were rate dependent for both tissue types (ILT and wall). Histology showed that single fibrin fibers or smaller protein clots within the ILT generate smooth dissected surfaces during the peeling. There was a notable decrease in mass fraction of elastin within the thrombus-covered intima–media composite with ILT age, whereas no significant change was found for that of collagen. Conclusions: These findings suggest that intraluminal thrombus aging leads to a higher propensity of dissection for the ILT and the intima–media composite of the aneurysmal wall. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction An abdominal aortic aneurysm (AAA) is a vascular pathology associated with permanent and irreversible localized dilations. Rupture of an AAA is a mechanical failure of the aneurysm wall, which occurs when the peak wall stress exceeds the local strength of the aortic tissue. In order to seek a more reliable criterion for the AAA rupture assessment, the majority of recent studies have focused on the development of non-invasive methods to predict the rupture risk of patient-specific AAA models on a computational basis. Detailed information of patient-specific geometries of intraluminal thrombi (ILT) and walls, advanced anisotropic constitutive models for AAA walls and the interaction between the fluid and the structure have improved the reliability of finite element simulations to a great extent (Wang et al., 2002; Vande Geest et al., 2006b; Rissland et al., 2009; Maier et al., 2010; Xenos et al., 2010). Aneurysmal degeneration, from a pathohisto- logical point of view, is mainly attributed to loss of elastin and collagen remodeling within the aortic wall (Dobrin et al., 1984; Carmo et al., 2002; Schriefl et al., 2012a). As a key issue, rupture locations of AAA have been identified by several studies (Golledge et al., 1999; Doyle et al., 2009, 2010) using experimental techni- ques and computational validations. For example, Doyle et al. (2009) reported the rupture site of a silicon rubber AAA model, i.e. at the inflection point, which is in agreement with the peak stress regions as numerically predicted. In addition to that, they continuously measured the internal rupture pressures of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jbiomech.2013.10.027 n Corresponding author at: Graz University of Technology, Institute of Biomechanics, Center of Biomedical Engineering, Kronesgasse 5-I, 8010 Graz, Austria. Tel.: þ43 316 873 1625. E-mail address: holzapfel@tugraz.at (G.A. Holzapfel). Journal of Biomechanics 47 (2014) 14–23