It is generally acknowledged that rupture of an abdominal aortic aneurysm (AAA) occurs when the stress acting on the wall over the cardiac cycle exceeds the strength of the wall. Peak wall stress computations appear to give a more accurate rupture risk assessment than AAA diameter, which is currently used for a diagnosis. Despite the numerous studies utilizing patient-specific wall stress modeling of AAAs, none investigated the effect of wall calcifications on wall stress. The objective of this study was to evaluate the influence of calcifications on patient-specific finite element stress computations. In addition, we assessed whether the effect of calcifications could be predicted directly from the CT-scans by relating the effect to the amount of calcification present in the AAA wall. For 6 AAAs, the location and extent of calcification was identified from CT-scans. A finite element model was created for each AAA and the areas of calcification were defined node-wise in the mesh of the model. Comparisons are made between maximum principal stress distributions, computed without calcifications and with calcifications with varying material properties. Peak stresses are determined from the stress results and related to a calcification index (CI), a quantification of the amount of calcification in the AAA wall. At calcification sites, local stresses increased, leading to a peak stress increase of 22% in the most severe case. Our results displayed a weak correlation between the CI and the increase in peak stress. Additionally, the results showed a marked influence of the calcification elastic modulus on computed stresses. Inclusion of calcifications in finite element analysis of AAAs resulted in a marked alteration of the stress distributions and should therefore be included in rupture risk assessment. The results also suggest that the location and shape of the calcified regions--not only the relative amount--are considerations that influence the effect on AAA wall stress. The dependency of the effect of the wall stress on the calcification elastic modulus points out the importance of determination of the material properties of calcified AAA wall.
"FEA models are used to estimate AAA's wall stress based on medical images. While some of these models assume a nonlinear constitutive model of vascular wall to evaluate its stress distribution at a fixed point in time     , other efforts have been done to not only calculate AAA wall stress distribution during its growth, but also account for the evolution of vascular wall material properties  . Some latter FEA studies have been done regarding the ability of arterial wall to adapt with its mechanical environment   . "
[Show abstract][Hide abstract] ABSTRACT: Objectives: Abdominal aortic aneurysms (AAA) that rupture have a high mortality rate. Rupture occurs when local mechanical stress exceeds the local mechanical strength of an AAA, so stress profiles such as those from finite element analysis (FEA) are useful. The role and effect of surrounding tissues, like the vertebral column, which have not been extensively studied, are examined in this paper.
Methods: Longitudinal CT scans from ten patients with AAAs were studied to see the effect of surrounding tissues on AAAs. Segmentation was performed to distinguish the AAA from other tissues and we studied how these surrounding tissues affected the shape and curvature of the AAA. Previously established methods by Veldenz et al. were used to split the AAA into 8 sections and examine the specific effects of surrounding tissues on these sections . Three-dimensional models were created to better examine these effects over time. Registration was done in order to compare AAAs longitudinally.
Results: The vertebral column and osteophytes were observed to have been affecting the shape and the curvature of the AAA. Interaction with the spine caused focal flattening in certain areas of the AAA. In 16 of the 41 CT scans, the right posterior dorsal section (section 5), had the highest radius of curvature, which was by far the section that had the maximum radius for a specified CT scan. Evolution of the growing AAA showed increased flattening in this section when comparing the last CT scan to the first scan.
Conclusion: Surrounding tissues have a clear influence on the geometry of an AAA, which may in turn affect the stress profile of AAA. Incorporating these structures in FEA and G&R models will provide a better estimate of stress.
Clinical Relevance: Currently, size is the only variable considered when deciding whether to undergo elective surgery to repair AAA since it is an easy enough measure for clinicians to utilize. However, this may not be the best indicator of rupture risk because small aneurysms also contribute to a high mortality rate. AAA’s wall stress is a superior indicator and may be better predicted with the inclusion of these surrounding tissues, which then could be used by clinicians in their decision-making process on whether to operate on an AAA.
"It is generally acknowledged that AAA rupture occurs when the stress acting on the wall during the cardiac cycle exceeds the strength of the wall. Speelman et al(18) suggested that there was a weak correlation between aortic calcification and increased peak stress on the wall. However, Li et al(19) suggested that aortic calcification increased AAA peak wall stress and decreased the biomechanical stability of the AAA (19). "
[Show abstract][Hide abstract] ABSTRACT: Vascular calcification is a prominent feature of atherosclerosis. The mineral composition and quantity within calcified arterial plaques remains unelucidated; therefore, the aim of this study was to analyze the mineral composition of such plaques. Calcified arterial plaques were obtained from patients with abdominal aortic aneurysms (AAAs) and carotid artery stenoses. Calcified aneurysmal plaques were obtained during the routine open repair of AAAs, while calcified carotid plaques were collected from patients who underwent carotid endarterectomy (CEA). Following the appropriate preparation of each sample, inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to analyze the calcium and phosphate levels, while flame atomic absorption spectrometry (FAAS) was used to analyze the levels of iron and zinc. The levels of these mineral components were evaluated. In the aortic and carotid plaques, the mean calcium concentration was 9.83 and 11.94 wt.%, respectively, and the mean phosphate concentration was 4.31 and 6.08 wt.%, respectively. It was not possible to analyze the absolute concentration of iron in the carotid plaques due to the concentration being below the measurement limit. The zinc concentration was variable between samples. In conclusion, the main components of aortic and carotid plaques are calcium and phosphate. The mineral concentrations of the plaques in the present study may be used as reference values for further studies on vascular calcification. More studies are required to elucidate the correlation between the mineral components and vascular calcification.
Experimental and therapeutic medicine 01/2014; 7(1):23-26. DOI:10.3892/etm.2013.1385 · 1.27 Impact Factor
"Its rupture and the risk associated with its treatment pose an enormous social and economic burden in our aging society and there has been continuous demand for better risk assessment and patient management based on advances in medical imaging and computational biomechanics. Classical finite element (FE) analysis using a nonlinear constitutive model of vascular wall and a medical image-based geometric model provides a better estimation of wall stress as a more reliable biomarker of rupture (Dorfmann et al., 2010; Fillinger et al., 2002; Raghavan and Vorp, 2000; Rissland et al., 2009; Speelman et al., 2007). These previous studies, however, estimate stress distribution of an AAA at a fixed time and do not account for the continuous evolution in material properties, and thus, strength of the wall. "
[Show abstract][Hide abstract] ABSTRACT: Advances in theoretical modeling of biological tissue growth and remodeling (G&R) and computational
biomechanics have been helpfulto capture salientfeatures of vascular remodeling during the progression
of vascular diseases. Nevertheless, application of such advances to individualized diagnosis and clinical
treatment of diseases such as abdominal aortic aneurysm (AAA) remains challenging. As a step toward
that goal, in this paper, we present a computational framework necessary towards patient-speciﬁc modeling of AAA growth. Prior to AAA simulations, using an inverse optimization method, initial material
parameters are identiﬁed for a healthy aorta such that a homeostatic condition is satisﬁed for the given
medical image-based geometrical model under physiological conditions. Various shapes of AAAs are
then computationally created by inducing elastin degradation with different spatio-temporal distributions. The simulation results emphasize the role of extent of elastin damage, geometric complexity of an
enlarged AAA, and sensitivity of stress-mediated collagen turnover on the wall stress distribution and
the rate of expansion. The results also show that the distributions of stress and local expansion initially
correspond to the extent of elastin damage, but change via stress-mediated tissue G&R depending on
the aneurysm shape. Finally, we suggest that the current framework can be utilized along with medical
images from an individual patient to predict the AAA shape and mechanical properties in the near future
via an inverse scheme.
Mechanics Research Communications 06/2012; 42:107-117. DOI:10.1016/j.mechrescom.2012.01.008 · 1.50 Impact Factor
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