Effect of diabetes mellitus on the dissection properties of the rabbit descending thoracic aortas

Region-specific delamination strength of ascending thoracic aortic aneurysm of elderly hypertensive patients with bicuspid and tricuspid aortic valves

Both ageing and hypertension are clinical factors that may lead to a higher propensity for dissection or rupture of ascending thoracic aortic aneurysms (ATAAs). This study sought to investigate effect of valve morphology on regional delamination strength of ATAAs in the elderly hypertensive patients. Whole fresh ATAA samples were harvested from 23 hypertensive patients (age, 71 ± 8 years) who underwent elective aortic surgery. Peeling tests were performed to measure region-specific delamination strengths of the ATAAs, which were compared between patients with bicuspid aortic valve (BAV) and tricuspid aortic valve (TAV). The regional delamination strengths of the ATAAs were further correlated with patient ages and aortic diameters for BAV and TAV groups. In the anterior and right lateral regions, the longitudinal delamination strengths of the ATAAs were statistically significantly higher for BAV patients than TAV patients (33 ± 7 vs. 23 ± 8 mN/mm, p = 0.01; 30 ± 7 vs. 19 ± 9 mN/mm, p = 0.02). For both BAV and TAV patients, the left lateral region exhibited significantly higher delamination strengths in both directions than the right lateral region. Histology revealed that disruption of elastic fibers in the right lateral region of the ATAAs was more severe for the TAV patients than the BAV patients. A strong inverse correlation between longitudinal delamination strength and age was identified in the right lateral region of the ATAAs of the TAV patients. Results suggest that TAV-ATAAs are more vulnerable to aortic dissection than BAV-ATAAs for the elderly hypertensive patients. Regardless of valve morphotypes, the right lateral region may be a special quadrant which is more likely to initiate dissection when compared with other regions.

Effect of hypertension on the delamination and tensile strength of ascending thoracic aortic aneurysm with a focus on right lateral region

Hypertension is a major predisposing factor to initiate thoracic aortopathy. The objective of this study is to investigate effect of hypertension on delamination and tensile strength of ascending thoracic aortic aneurysms (ATAAs). A total of 35 fresh ATAA samples were harvested from 19 hypertensive and 16 non-hypertensive patients during elective aortic surgery. Peeling tests with two extension rates were performed to determine delamination strength, while uniaxial tensile (UT) tests were employed to measure failure stresses. The delamination strength and failure stresses of the ATAAs were further correlated with patient ages for hypertensive and non-hypertensive groups. The delamination strength to peel apart the ATAA tissue along the longitudinal direction was statistically significantly lower for the hypertensive patients than that of the non-hypertensive patients (35 ± 11 vs. 49 ± 9 mN/mm, p = 0.02). A higher delamination strength was measured if peeling was performed with a higher extension rate. The circumferential failure stresses were significantly lower for the hypertensive ATAAs than those of the non-hypertensive ATAAs (1.03 ± 0.27 vs. 1.43 ± 0.38 MPa, p = 0.02). Histology showed that laminar structures of elastic fibers were mainly disrupted in the hypertensive ATAAs. The longitudinal delamination strength of the ATAAs was significantly decreased and strongly correlated with ages for the hypertensive patients. Strong inverse correlations were also identified between the circumferential and longitudinal failure stresses of the ATAAs and ages for the hypertensive patients. Results suggest that the ATAAs of the elderly hypertensive patients may have a higher propensity for dissection or rupture. The dissection properties of the ATAA tissue are rate dependent.

Failure Properties of Healthy and Diabetic Rabbit Thoracic Aortas and Their Potential Correlation with Mass Fractions of Collagen

PURPOSE

Diabetes Mellitus (DM) plays an important role in aortic remodeling and alters the wall mechanics. The purpose of this study is to investigate and compare multi-directional failure properties of healthy and diabetic thoracic aortas.

METHODS

Thirty adult rabbits (1.6-2.2 kg) were collected and type 1 diabetic rabbit model was induced by injection of alloxan. A total of 10 control and 20 diabetic (with different time exposure to diabetic condition) rabbit descending thoracic aortas were harvested. Uniaxial tensile (UT) and radial tension (RT) tests were performed to determine circumferential, axial and radial failure stresses of the control and diabetic aortas, which were further correlated with mass fractions (MFs) of collagen.

RESULTS

Throughout the UT test, there was a clear indication of anisotropic mechanical responses for some diabetic aorta specimens in the high loading domain. There was a trend towards an increase in the mean circumferential and axial failure stresses for the diabetic aortas when compared to the control aortas. However, differences were not statistically significant. The quantified failure stresses in the circumferential direction were, in general, higher than the stress values in the axial direction for both control and diabetic groups. For the RT test, the radial failure stresses of the diabetic aortas (in 8 weeks) were significantly higher than those of the control aortas (95 ± 17 vs. 63 ± 15 kPa, p = 0.01). Strong correlations were identified between the circumferential failure stresses and the MFs of collagen for both control and diabetic aortas. Nevertheless, this correlation was not present in the axial and radial directions.

CONCLUSION

The results suggest that there is a lower propensity of radial tear occurrence within the diabetic aortic wall. More importantly, time exposure to diabetic condition is not a factor that may change failure properties of the rabbit descending thoracic aortas in the circumferential and axial directions.

Effect of Glycation on Interlamellar Bonding of Arterial Elastin

BACKGROUND

Interlamellar bonding in the arterial wall is often compromised by cardiovascular diseases. However, several recent nationwide and hospital-based studies have uniformly reported reduced risk of thoracic aortic dissection in patients with diabetes. As one of the primary structural constituents in the arterial wall, elastin plays an important role in providing its interlamellar structural integrity.

OBJECTIVE

The purpose of this study is to examine the effects of glycation on the interlamellar bonding properties of arterial elastin.

METHODS

Purified elastin network was isolated from porcine descending thoracic aorta and incubated in 2 M glucose solution for 7, 14 or 21 days at 37 °C. Peeling and direct tension tests were performed to provide complimentary information on understanding the interlamellar layer separation properties of elastin network with glycation effect. Peeling tests were simulated using a cohesive zone model (CZM). Multiphoton imaging was used to visualize the interlamellar elastin fibers in samples subjected to peeling and direct tension.

RESULTS

Peeling and direct tension tests show that interlamellar energy release rate and strength both increases with the duration of glucose treatment. The traction at damage initiation estimated for the CZM agrees well with the interlamellar strength measurements from direct tension tests. Glycation was also found to increase the interlamellar failure strain of arterial elastin. Multiphoton imaging confirmed the contribution of radially running elastin fibers to resisting dissection.

CONCLUSIONS

Nonenzymatic glycation reduces the propensity of arterial elastin to dissection. This study also suggests that the CZM effectively describes the interlamellar bonding properties of arterial elastin.