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Abstract
The success of endoluminal stent-graft treatment for abdominal aortic aneurysm relies on maintenance of an effective seal when the stent expands into the healthy artery. Clinical observation of aortic neck dilation following endoluminal grafting has led to the hypothesis that excessive stent expansion forces may cause remodelling and dilation of the artery to accommodate the strong forces. This may lead to failure of the seal, hence so-called endoleak. In this research, we analysed the force field generated by aortic stent-grafts and investigated in vitro approaches for studying the effects of these forces on cells within the vascular wall.
The pressure-deformation behaviour of ovine arteries was examined experimentally and was found to vary with artery type. A finite element model of abdominal aorta (AA) characterised by Mooney-Rivlin hyperelastic material properties was validated. The property inputs were derived from the polynomial form of the strain energy density function proposed by Patel and Vaishnav.
Stent-artery contact simulations revealed stresses 1.2-19 times higher than within a normal vessel at 120 mmHg when contacted by a zig-zag, square cross-section stent that expanded the AA by 3-16%. Streses 1.3-23 times normal were predicted for circular cross-section stents at the same range of expansions. The stress distribution was determined to be concentrated at the contacting surface and within the inner region of the aortic wall. These results confirmed that the forces within the vessel wall are likely to place unnatural physiological demands on the cells within. We then developed an in vitro system for studying the impact of this mechanical stress on cells within a three dimensional (3D) structure.
A 20 wt% poly(vinyl alcohol) (PVA) - 5 wt% collagen tubular construct was developed to support cells, and was shown to sustain physiological blood pressures. Two cell-seeding techniques were examined, direct cell encapsulation and surface cell-seeding. Both demonstrated the capability of entrapping viable cells within the construct that remained viable for up to 4 days.
In conclusion, stent contact does create abnormal stress concentrations within the vessel wall with a magnitude severely higher than physiological levels. A feasible tubular construct and an in vitro system were developed, enabling further assessments on the effects of these abnormality on the cells.