Movement and Dislocation of Modular Stent-Grafts Due to Pulsatile Flow and the Pressure Difference between the Stent-Graft and the Aneurysm Sac

Abstract

Purpose:

To investigate the stability and movement of modular aortic stent-grafts subjected to oscillating forces from pulsatile blood flow, with particular reference to the thoracic aorta.

Methods:

Analytical mathematical modeling was used to understand the forces on modular grafts. In a benchtop experiment, a transparent acrylic box was filled with water to mimic an aneurysm. Two stent-grafts were placed inside the box in a nested, arched configuration where one component was partly inside the other. A pump produced a pulsatile !5-L/min flow of water through the stent-grafts at a mean inlet pressure of !100 mmHg (∼13,330 Pa), with systolic and diastolic pressures of ∼130 and ∼80 mmHg, respectively (pulse pressure 50 mmHg). The movement of the 2 modular stent-grafts was observed.

Results:

The curved stent-graft system oscillated transversely when there was zero mean pressure difference between the stent-graft and the aneurysm. As the mean pressure difference was increased, this transverse graft movement was damped and then disappeared. A relatively large pressure difference caused the stent-graft to inflate and become sturdier. In terms of stability, the analytical mathematical model for a 30-mm-diameter Zenith modular stent-graft curved through 90° (with the ends of the graft fixed in place) showed that the modular components will separate at a pressure difference of 0 mmHg for 1 stent segment overlap (20 mm) and at an average 59 mmHg pressure difference for 2 stent overlaps, but the device would not separate at a pressure difference of 90 mmHg for 3 stent overlaps.

Conclusion:

Transverse cyclic movement of the curved stent-graft system with pulsation indicates a pressurized sac. When the pressure difference is large and there is a blood-tight seal between the aneurysm and the stent-graft, then the transverse movement of the stent-graft is minimal, but the risk for modular separation is highest. Curved thoracic endografts are subject to forces that may cause migration or separation, the latter being more likely if the seal between the graft and the sac is blood tight, if the blood pressure is high, and if the diameter of the graft is small and the sac large. Operators should plan for maximum overlap of modular components when treating large or long thoracic aneurysms.