Arterial occlusive disease remains a major health issue in the developed world and a rapidly growing problem in the developing world. Although a growing number of patients are now being effectively treated with minimally invasive techniques, there remains a tremendous pressure on the vascular community to develop a synthetic small-diameter vascular graft with improved long-term patency rates. The field of tissue engineering offers an exciting alternative in the search for living organ replacement structures. Several methodologies have emerged for constructing blood vessel replacements with biological functionality. Common strategies include cell-seeded biodegradable synthetic scaffolds, cell self-assembly, cell-seeded gels and xenogeneic acellular materials. A wide range of materials are being investigated as potential scaffolds for vascular tissue engineering applications. Some are commercialised and others are still in development. Recently, researchers have studied the role of fibrin gel as a three-dimensional scaffold in vascular tissue engineering. This overview describes the properties of fibrin gel in vascular tissue engineering and highlights some recent progress and difficulties encountered in the development of cell fibrin scaffold technology.
Objective
Abdominal aortic aneurysm rupture (AAA) is believed to occur when the local mechanical stress exceeds the local mechanical strength of the wall tissue. Based on this hypothesis, the knowledge of the stress acting on the wall of an unruptured aneurysm could be useful in determining the risk of rupture. The role of asymmetry has previously been identified in idealised AAA models, and is now studied using realistic AAAs in the current work.
Methods
Fifteen patient-specific AAAs were studied to estimate the relationship between wall stress and geometrical parameters. 3D AAA models were reconstructed from CT scan data. The stress distribution on the AAA wall was evaluated by the finite element method, and peak wall stress was compared to both diameter and centreline asymmetry. A simple method of determining asymmetry was adapted and developed. Statistical analyses were also performed to determine potential significance of results.
Results
Mean von Mises peak wall stress ± standard deviation was shown to be 0.4505 ± 0.14 MPa, with a range of 0.3157 – 0.9048 MPa. Posterior wall stress increases with anterior centreline asymmetry. Peak stress increased by 48% and posterior wall stress increased by 38% when asymmetry was introduced into a realistic AAA model.
Conclusion
The relationship between posterior wall stress and AAA asymmetry showed that excessive bulging of one surface results in elevated wall stress on the opposite surface. Assessing the degree of bulging and asymmetry that is experienced in an individual AAA may be of benefit to surgeons in the decision making process, and may provide a useful adjunct to diameter as a surgical intervention guide.
The in vivo healing process of vascular grafts involves the interaction of many contributing factors. The ability of vascular grafts to provide an environment which allows successful accomplishment of this process is extremely difficult. Poor endothelisation, inflammation, infection, occlusion, thrombosis, hyperplasia and pseudoaneurysms are common issues with synthetic grafts in vivo. Advanced materials composed of decellularised extracellular matrices (ECM) have been shown to promote the healing process via modulation of the host immune response, resistance to bacterial infections, allowing re-innervation and reestablishing homeostasis in the healing region. The physiological balance within the newly developed vascular tissue is maintained via the recreation of correct biorheology and mechanotransduction factors including host immune response, infection control, homing and the attraction of progenitor cells and infiltration by host tissue. Here, we review the progress in this tissue engineering approach, the enhancement potential of ECM materials and future prospects to reach the clinical environment.
A Spiral Computerized Tomography (CT) scan of the aorta were obtained from a single subject and three model variations were examined. Computational fluid dynamics modeling of all three models showed variations in the velocity contours along the aortic arch with differences in the boundary layer growth and recirculation regions. Further down-stream, all three models showed very similar velocity profiles during maximum velocity with differences occurring in the decelerating part of the pulse. Flow patterns obtained from transient 3-D computational fluid dynamics are influenced by different reconstruction methods and the pulsatility of the flow. Caution is required when analyzing models based on CT scans.
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