Experimental Assessment of Stress Patterns in Abdominal Aortic Aneurysms using the Photoelastic Method

Abstract

Abdominal aortic aneurysm (AAA) sac size is a major clinical problem. The maximum sac diameter is the major determining factor for treatment. There is a direct correlation between the wall stress and rupture sites for AAA with maximum wall stress not being dependent on the maximum diameter of the vessel but on the morphology of vessel. The reflective photoelastic method was developed to experimentally evaluate the stresses and strains in a model AAA. The epoxy resin material was used as the self‐supporting structure and had similar mechanical properties to the aneurysmal aorta. For both static and dynamic fluid testing the high‐stressed regions were located proximally and distally to the aneurysm. This correlates to reported in vivo rupture sites. There was a low‐stress region at the location of greatest diameter at the centre of the aneurysm. The direction of the maximum principal stress was found to be in the circumferential direction. The maximum stress for a high blood pressure of 22.7 kPa (170 mmHg) was 35% greater than that for a normal blood pressure of 16.0 kPa (120 mmHg). The photoelastic method is a powerful and innovative method of analysing stresses in AAA models, producing results, which are visual and easy to interpret.

Use of the photoelastic method and finite element analysis in the assessment of wall strain in abdominal aortic aneurysm models

“…There were certain limitations to the earlier work by our group (Morris et al, 2004), in particular, normal incidence was not considered on the curved surfaces on the model. The null-balance compensator must be directly normal to the surface of investigation in order to accurately quantify the fringe order (N).…”

“…PL-3 epoxy resin and PC-11 reflective coating (Vishay Precision Group Inc., Micro-Measurements, NC, USA) were selected for this study as before (Callanan et al, 2011;Morris et al, 2004). According to the manufacturer, the epoxy resin is linearly elastic with a Young's modulus (E) of 1.4 MPa and Poisson's ratio (v) of 0.42.…”

“…PL-3 has a strain optical coefficient (K) of 0.002. However, it is known from earlier work (Callanan et al, 2011;Morris et al, 2004) that the material is nonlinear and that the thickness of the PC-11 reflective coating alters the mechanical behaviour of the PL-3 resin.…”

“…Liquid coating was then brushed onto the inner surface and the model was rotated at a slow speed using a custom-made rotating device to ensure uniform application. PC-11 was then used to re-seal each model (Callanan et al, 2011;Morris et al, 2004).…”

“…The method has been used in the biomedical industry for over 70 years, in particular for bone analysis (Meyer et al, 2002;Milch, 1940), dental implants (Ochiai et al, 2003) and in the investigation of ligaments (Kawada et al, 1999;Yamamoto et al, 1998). Morris et al (2004) applied the photoelastic method to an idealised AAA model and reported the wall stresses and strains. In a follow-up study using the same idealised AAA model, Callanan et al (2011) compared the photoelastic method to FEA showing that the two techniques are in good agreement when utilising non-linear elasticity.…”