Fatigue exhaustion of the mitral valve tissue.

Sudden failure and rupture of the tissue is a rare but serious short-term complication after the mitral valve surgical repair. Excessive cyclic loading on the suture line of the repair can progressively damage the surrounding tissue and finally cause tissue rupture. Moreover, mechanical over-tension, which occurs in a diseased mitral valve, gradually leads to tissue floppiness, mitral annular dilation, and leaflet rupture. In this work, the rupture mechanics of mitral valve is studied by characterizing the fracture toughness exhaustion of healthy tissue. Results of this study show that fracture toughness of the posterior mitral valve is lower than its anterior counterpart, indicating that posterior tissue is more prone to failure. Moreover, the decrease in fracture toughness by increasing the number of fatigue cycles shows that excessive mechanical loading leads to progressive failure and rupture of mitral valve tissue within a damage accumulative process.

Obtaining the biomechanical behavior of ascending aortic aneurysm via the use of novel speckle tracking echocardiography.

INTRODUCTION: Ex vivo measurement of ascending aortic biomechanical properties may help understand the risk for rupture or dissection of dilated ascending aortas. A validated in vivo method that can predict aortic biomechanics does not exist. Speckle tracking transesophageal echocardiography (TEE) has been used to measure ventricular stiffness; we sought to determine whether speckle TEE could be adapted to estimate aortic stiffness in vivo and compare these findings with those obtained by ex vivo tissue measurements. METHODS: A total of 17 patients undergoing ascending aortic resection were recruited to with a mean aortic diameter was 56.16 ± 15 mm. Intraoperative speckle TEE tracking analysis was used to calculate aortic stiffness index using the following equation: ß2=ln(SBP/DBP)/AoS, where ß2 is the stiffness index; SBP is systolic blood pressure; DBP is diastolic blood pressure; and AoS is the circumferential strain. Ex vivo stiffness was obtained by mechanical tissue testing according to previously described methods. The aortic ring at the pulmonary trunk was divided into 4 equal quadrants. RESULTS: The in vivo stiffness index for the inner curvature, anterior wall, outer curvature, and posterior wall were 0.0544 ± 0.0490, 0.0295 ± 0.0199, 0.0411 ± 0.0328, and 0.0502 ± 0.0320, respectively. The mean ex vivo 25% apparent stiffness for inner curvature, anterior wall, outer curvature, and posterior wall were 0.0616 ± 0.0758 MPa, 0.0352 ± 0.00992 MPa, 0.0405 ± 0.0199 MPa, and 0.0327 ± 0.0106 MPa, respectively. The patient-matched ex vivo 25% apparent stiffness and in vivo stiffness index were not significantly different (P = .8617, 2-way analysis of variance with repeated measures). CONCLUSIONS: The use of speckle TEE appears to be a promising technique to estimate ex vivo mechanical properties of the ascending aortic tissue.

Evaluating ascending aortic aneurysm tissue toughness: Dependence on collagen and elastin contents.

Ascending thoracic aortic aneurysms (ATAAs) can lead to a dissection or rupture of the aorta, causing death or disability of the patients. Surgical interventions used to treat this disease are associated with risks of mortality and morbidity. Several studies have investigated the rupture mechanisms of ATAAs; however, underlying reasons behind aortic rupture (failure) have not been fully elucidated and further investigations are necessary. The rupture of pathological aortic tissue is a local phenomenon resulting from defects or tears in the vessel wall. In this work, the toughness-based rupture properties of ATAAs have been examined. The toughness, biaxial tensile properties, and histological properties of aneurysmal and control human ascending thoracic aortas (ATAs) were characterized from four quadrants of surgically excised aortic rings. The aneurysmal tissue population included aortas from patients with bicuspid aortic valves (BAV) and tricuspid aortic valves (TAV). The toughness, incremental modulus, and thickness properties of the aortas were determined and compared regionally. Additionally, to further explore the rupture propensity of ATAAs, the inter-correlation of the toughness properties with histological characteristics have been explored. We found no correlation between toughness and incremental modulus. However, toughness decreased significantly with the amount of collagen. In the outer curvature, there was an increase in incremental modulus with collagen+elastin content, but a decrease in toughness. These results suggest tissue remodeling could affect toughness and stiffness differently in ascending aortic aneurysms.

Investigation on the Regional Loss Factor and Its Anisotropy for Aortic Aneurysms.

An aortic aneurysm is a lethal arterial disease that mainly occurs in the thoracic and abdominal regions of the aorta. Thoracic aortic aneurysms are prevalent in the root/ascending parts of the aorta and can lead to aortic rupture resulting in the sudden death of patients. Understanding the biomechanical and histopathological changes associated with ascending thoracic aortic aneurysms (ATAAs), this study investigates the mechanical properties of the aorta during strip-biaxial tensile cycles. The loss factor-defined as the ratio of dissipated energy to the energy absorbed during a tensile cycle-the incremental modulus, and their anisotropy indexes were compared with the media fiber compositions for aneurysmal (n = 26) and control (n = 4) human ascending aortas. The aneurysmal aortas were categorized into the aortas with bicuspid aortic valves (BAV) and tricuspid aortic valves (TAV). The strip-biaxial loss factor correlates well with the diameter of the aortas with BAV and TAV (for the axial direction, respectively, R² = 0.71, p = 0.0022 and R² = 0.54, p = 0.0096). The loss factor increases significantly with patients' age in the BAV group (for the axial direction: R² = 0.45, p = 0.0164). The loss factor is isotropic for all TAV quadrants, whereas it is on average only isotropic in the anterior and outer curvature regions of the BAV group. The results suggest that loss factor may be a useful surrogate measure to describe the histopathology of aneurysmal tissue and to demonstrate the differences between ATAAs with the BAV and TAV.

Characterization of the toughness and elastic properties of fresh and cryopreserved arteries.

Surgical interventions are used to manage severe complications of heart valve diseases and to prevent the eventual rupture of an aortic aneurysm. Soft-tissue allografts, xenografts, and prosthetic grafts are used in these interventions; however, there are pre-surgical difficulties and post-surgical complications in using these grafts. One of these is the rupture potential of cryopreserved allografts at the time of transplantation and/or after the thawing process for the cryopreserved tissue. Moreover, a number of clinical observations report the patency of prosthetic grafts and aneurysm of cryopreserved allografts after the transplantation. This work aims to study the effect of cryopreservation on the resistance of arterial tissue to crack growth and propagation; we examined the biomechanical parameters which could be used in designing more efficient prosthetic grafts. Investigation of the toughness properties can also be helpful to understand the failure mechanisms of pathological arterial tissues. The toughness and biaxial tensile properties of the post-cryopreserved and fresh arteries have been examined.