Morphometric model of abdominal aortic aneurysms and the significance of the structural changes in the aortic wall for rupture risk assessment

Abstract

Objectives: The aims of this study were to evaluate the morphometric and structural characteristics of abdominal aortic aneurysms (AAA), and to form a morphometric model of the AAA that could be applicable in the development of mathematical and computation models for rupture risk assessment. Material and Methods: The following morphometric parameters significant for biomechanical stability and compliance of the aortic wall were analyzed: the thickness of the wall, the thickness of the media and the thickness of the adventitia. Morphometry was performed with the Olympus BX 41 microscope and the Olympus C - 5060 wide zoom digital camera with an application of the Olympus DP-soft Image Analyzer program. The media-to-wall and adventitia-to-wall ratios were calculated. Parameters were correlated with the diameters of the aneurysms (established by MSCT angiography), the patients' age and gender, the presence of a thrombus and the grade of inflammation. Results and Discussion: Our results showed that an increase in the AAA diameter affected the structure of the aortic wall in the following ways: 1. the thickness of the aortic wall significantly increased, with the greatest increase for aneurysms with diameters between 41 and 60 mm (ANOVA F=268.561; p<0.001); 2. the thickness of the adventitia and its proportion in the wall thickness significantly increased, in the same group (ANOVA F=376.727, p<0.001); 3. the thickness of the media and its proportion in the wall thickness significantly decreased, with the greatest increase for aneurysms with diameters >60mm (ANOVA F=265.865; p<0,001). The supposed influence of the latter two factors reduced the adaptability of the vascular wall and augmented the rupture risk since the aortic wall media is responsible for the elastic properties of the blood vessels, while increased and fibrotic adventitia did not provide sufficient compliance. We confirmed, by means of the Univariate Analysis of Variance, that the increase of the adventitia and destruction of the media were even greater in aneurysms with inflammation and in patients over 65 years old. Female patients with small aneurysms (d<40mm) are at a special rupture risk. They have a significantly thinner wall (F=35.164; p<0.001), with a significantly thinner media (F=35.473; p<0.001) and a significantly thicker adventitia (F=21.146; p<0.001) than male patients. Small-diameter aneurysms with a thrombus are also under special rupture risk. Destruction of the media was advanced in this group, with an exceptionally small medial thickness compared to larger aneurysms with a thrombus (F=237.770; p<0.001). Conclusions and Clinical Relevance: A correction of AAA computation models with histomorphometric data is necessary for an accurate prediction of rupture risk. Parameters used in computation models are based on CT scans, but not all parameters are easily assessed with a CT scan (i.e., the wall thickness). Most of the computation models operate under the assumption that aneurysms are homogenous structures, which is not the case. On the contrary, aneurysms are heterogeneous, with extreme variations of structure among patients and in different parts of the same aneurysm. Some data are not uniquely defined and recognized, (i.e., the influence of a thrombus on the wall structure). Hence, the goal of morphometric models is to provide sufficient data for the construction of an improved and adjustable model for rupture risk prediction that will combine many different factors and enable a tailored decision making process for each patient. © 2009 Nova Science Publishers, Inc.