Diameter-related variations of geometrical, mechanical, and mass fraction data in the anterior portion of abdominal aortic aneurysms.

Maximum aortic diameter is an important measure in rupture prediction of abdominal aortic aneurysms (AAAs). Analyzing the variations of geometrical, material, and biochemical properties with increased AAA diameters advances understanding of the effect of lesion enlargement on patient specific vascular properties. 96 AAA samples were harvested during open surgical aneurysm repair. Geometrical factors such as the maximum intraluminal thrombus (ILT) thickness, wall thickness, and AAA expansion rate were measured. Biaxial extension and peeling tests were performed to characterize the biaxial mechanical responses and to quantify the dissection properties of aneurysmal tissue. Mass fraction analysis quantified the dry weight percentages of elastin and collagen within the AAA wall. Linear regression models were used to correlate geometrical, mechanical, and mass fraction data with maximum AAA diameter. Both ILT thickness and AAA expansion rate increased and were positively correlated with maximum AAA diameter, while there was a slight increase in wall thickness for AAAs with a larger maximum diameter. For the biaxial mechanical responses, mean peak stretches and maximum tangential moduli in the circumferential and longitudinal axes did not correlate with maximum AAA diameters. However, the quantified energy to propagate tissue dissections within intima-media composites showed a significant inverse correlation with maximum AAA diameter. Elastin content decreased significantly with increasing AAA diameter. Larger AAA diameters are associated with thicker ILTs, higher AAA expansion rates, and pronounced elastin loss, and may also lead to a higher propensity for tissue dissection and aneurysm rupture.

Tong J ，Cohnert T ，Holzapfel GA 《-》

On the influence of wall calcification and intraluminal thrombus on prediction of abdominal aortic aneurysm rupture.

Parameters other than maximum diameter that predict rupture of abdominal aortic aneurysms (AAAs) may be helpful for risk-benefit analysis in individual patients. The aim of this study was to characterize the biomechanical-structural characteristics associated with AAA walls to better identify the related mechanistic variables required for an accurate prediction of rupture risk. Anterior AAA wall (n = 40) and intraluminal thrombus (ILT; n = 114) samples were acquired from 18 patients undergoing open surgical repair. Biomechanical characterization was performed using controlled circumferential stretching tests combined with a speckle-strain tracking technique to quantify the spatial heterogeneity in deformation and localized strains in the AAA walls containing calcification. After mechanical testing, the accompanying microstructural characteristics of the AAA wall and ILT types were examined using electron microscopy. No significant correlation was found between the AAA diameter and the wall mechanical properties in terms of Cauchy stress (rs = -0.139; P = .596) or stiffness (rs = -0.451; P = .069). Quantification of significant localized peak strains, which were concentrated in the tissue regions surrounding calcification, reveals that peak strains increased by a mean of 174% as a result of calcification and corresponding peak stresses by 18.2%. Four ILT types characteristic of diverse stages in the evolving tissue microstructure were directly associated with distinct mechanical stiffness properties of the ILT and underlying AAA wall. ILT types were independent of geometric factors, including ILT volume and AAA diameter measures (ILT stiffness and AAA diameter [rs = -0.511; P = .074]; ILT stiffness and ILT volume [rs = -0.245; P = .467]). AAA wall stiffness properties are controlled by the load-bearing capacity of the noncalcified tissue portion, and low stiffness properties represent a highly degraded vulnerable wall. The presence of calcification that is contiguous with the inner wall causes severe tissue overstretching in surrounding tissue areas. The results highlight the use of additional biomechanical measures, detailing the biomechanical-structural characteristics of AAA tissue, that may be a helpful adjunct to improve the accuracy of rupture prediction.

Barrett HE ，Cunnane EM ，Hidayat H ，O'Brien JM ，Moloney MA ，Kavanagh EG ，Walsh MT ... -  《-》

Gender differences in biomechanical properties, thrombus age, mass fraction and clinical factors of abdominal aortic aneurysms.

The main purpose of the present study is the investigation of gender differences in the biomechanical properties, thrombus age, mass fraction and key clinical factors of abdominal aortic aneurysms (AAAs). A total of 90 AAA samples (78 males and 12 females) were harvested from open surgical aneurysm repairs. Biaxial extension and peeling tests were performed to characterise the biaxial mechanical responses and to determine dissection properties of both the intraluminal thrombi (ILTs) and the thrombus-covered walls. Relative thrombus age was determined by characterising the ILT histological microstructure. Mass fraction analyses quantified dry weight percentages of elastin and collagen within the AAA walls. Moreover, we statistically compared clinical factors between male and female. The luminal layers of the female thrombi and the female AAA walls showed a significantly lower tissue stiffness (modulus) in the longitudinal direction when compared to males. Gender differences were also shown in the dissection properties of the intima-media composite within the AAA walls, in which a statistically significantly lower energy to propagate a dissection was quantified for females than for males. Moreover, 82% of female thrombi were relatively older (ILT age phases III and IV), twice that of male thrombi (43%). A pronounced lower elastin content was identified for the intima-media composites of male AAA walls, whereas female AAA walls had significantly lower dry weight percentages of collagen. Regarding clinical factors, nicotine pack years, serum creatinine and AAA expansion rate were found to be much higher for male patients. These findings may help to explain higher risks for AAA growth in males and the ruptures of smaller-sized AAAs in females.

Tong J ，Schriefl AJ ，Cohnert T ，Holzapfel GA ... -  《-》

Interaction of biomechanics with extracellular matrix components in abdominal aortic aneurysm wall.

Little is known about the interactions between extracellular matrix (ECM) proteins and locally acting mechanical conditions and material macroscopic properties in abdominal aortic aneurysm (AAA). In this study, ECM components were investigated with correlation to corresponding biomechanical properties and loads in aneurysmal arterial wall tissue. Fifty-four tissue samples from 31 AAA patients (30♂; max. diameter Dmax 5.98 ± 1.42 cm) were excised from the aneurysm sac. Samples were divided for corresponding immunohistological and mechanical analysis. Collagen I and III, total collagen, elastin, and proteoglycans were quantified by computational image analysis of histological staining. Pre-surgical CT data were used for 3D segmentation of the AAA and calculation of mechanical conditions by advanced finite element analysis. AAA wall stiffness and strength were assessed by repeated cyclical, sinusoidal and destructive tensile testing. Amounts of collagen I, III, and total collagen were increased with higher local wall stress (p = .002, .017, .030, respectively) and strain (p = .002, .012, .020, respectively). AAA wall failure tension exhibited a positive correlation with collagen I, total collagen, and proteoglycans (p = .037, .038, .022, respectively). α-Stiffness correlated with collagen I, III, and total collagen (p = .011, .038, and .008), while β-stiffness correlated only with proteoglycans (p = .028). In contrast, increased thrombus thickness was associated with decreased collagen I, III, and total collagen (p = .003, .020, .015, respectively), and AAA diameter was negatively associated with elastin (p = .006). The present results indicate that in AAA, increased locally acting biomechanical conditions (stress and strain) involve increased synthesis of collagen and proteoglycans with increased failure tension. These findings confirm the presence of adaptive biological processes to maintain the mechanical stability of AAA wall.

Tanios F ，Gee MW ，Pelisek J ，Kehl S ，Biehler J ，Grabher-Meier V ，Wall WA ，Eckstein HH ，Reeps C ... -  《-》

Prediction of Rupture Sites in Abdominal Aortic Aneurysms After Finite Element Analysis.

To associate regions of highest local rupture risk from finite element analysis (FEA) to subsequent rupture sites in abdominal aortic aneurysms (AAA). This retrospective multicenter study analyzed computed tomography angiography (CTA) data from 13 asymptomatic AAA patients (mean age 76 years; 8 men) experiencing rupture at a later point in time between 2005 and 2011. All patients had CTA scans before and during the rupture event. FEA was performed to calculate peak wall stress (PWS), peak wall rupture risk (PWRR), rupture risk equivalent diameters (RRED), and the intraluminal thrombus volume (ILTV). PWS and PWRR locations in the prerupture state were compared with subsequent CTA rupture findings. Visible contrast extravasation was considered a definite (n=5) rupture sign, while a periaortic hematoma was an indefinite (n=8) sign. A statistical comparison was performed between the 13-patient asymptomatic AAA group before and during rupture and a 23-patient diameter-matched asymptomatic AAA control group that underwent elective surgery. The asymptomatic AAAs before rupture showed significantly higher PWRR and RRED values compared to the matched asymptomatic AAA control group (median values 0.74 vs 0.52 and 77 vs 59 mm, respectively; p<0.0001 for both). No statistical differences could be found for PWS and ILTV. Ruptured AAAs showed the highest maximum diameters, PWRR, and RRED values. In 7 of the ruptured AAAs (2 definite and 5 indefinite rupture signs), CTA rupture sites correlated with prerupture PWRR locations. The location of the PWRR in unruptured AAAs predicted future rupture sites in several cases. Asymptomatic AAA patients with high PWRR and RRED values have an increased rupture risk.

Erhart P ，Roy J ，de Vries JP ，Liljeqvist ML ，Grond-Ginsbach C ，Hyhlik-Dürr A ，Böckler D ... -  《-》