Experimental Analysis of Endovascular Treatment of AAA and Predictors of Long Term Outcomes

Abstract

This chapter describes experimental investigations of parameters which are likely to reduce the ability of an implanted stent-graft for the treatment of Abdominal Aortic Aneurysm (AAA) to resist migration. Idealised AAA analogues were manufactured with realistic wall properties. Both proximal stents and complete stent-graft devices were deployed inside these models and the force required to cause migration during physiological flow was investigated. The effect of stent-graft morphology on the columnar rigidity generated by a stent-graft and on the migration force transmitted to the proximal end of the device was also investigated. Lower wall compliance and pulsatile wall motions due to physiological flow were seen to reduce the fixation of an implanted proximal stent from 8.4 ± 0.32 to 3.7 ± 0.06 N. The results also show that high systolic pressure or low proximal fixation length reduce the force required to migrate a graft from 4.62 ± 0.25 to 2.57 ± 0.11 N in a flexible stent-graft with little longitudinal rigidity. In a fully stented device these correlations were less clear due to the complex compressive behaviour of the device and the increase in iliac fixation when the proximal fixation length was reduced. Longitudinal rigidity was measured in terms of the amount of force to cause 5 mm compression of the graft and was found to provide up to 11.53 N of resistance to migration in a fully stented device which is greater than the resistance afforded by passive proximal stents alone. Even the flexible stent-graft was shown to require up to 5.88 N of compressive force to cause 5 mm of device compression due to internal pressure assisting the device in holding its shape. Increasing iliac bifurcation angle or placing the devices in a tortuous configuration was found to reduce the longitudinal rigidity of both devices. The results also showed that the drag force acting on a stent-graft may be somewhat attenuated by compressive forces set up in a non rigid stent-graft model. Both an increase in iliac bifurcation angle and tortuosity was found to increase the migration force on the proximal end of the device from 2.56 to 4.92 N. Tortuosity and higher iliac leg angle were both found to have the double disadvantage of increasing the migration force and decreasing device longitudinal rigidity, while longitudinal rigidity was shown to be crucial to the success of passively fixated stent-grafts. The test methods described in this chapter could be useful in the future preclinical evaluation of stent-grafts and could be useful in the design phase of next generation EVAR devices.