Fluid Pressure Derived Force is the Main Contributor to Iliac Limb Displacement Forces – Shear Force and Redirection of Flow are Negligible

Artikel i vetenskaplig tidskrift, 2019

Introduction - Complications after EVAR, including aortic rupture, continue to be a problem also in long term follow-up1. There is still a need for life-long surveillance and late re-interventions raises the question about long term EVAR-durability. Additional iliac stent grafts due to distal endoleak type I are among the more frequent among these late re-interventions2,3,4. There are indications that graft migration at iliac landing zones and graft interconnections are among the most common causes and that wide iliac arteries predispose for these complications5. Flow induced displacement forces have, in an experimental model, been shown to be significant also at distal landing zones in EVAR stent grafts6 and particularly high in grafts with large distal diameters7. These findings indicate that late EVAR failures are linked to flow induced displacement forces and subsequent graft migration. We have therefore conducted a study of fluid structure interaction with the aim to differentiate the magnitude of the different force components and numerically evaluate the forces acting on iliac limb stent grafts in different graft configurations.

Methods - The displacement forces in iliac limb stent grafts were numerically evaluated using a finite volume approach for fluid-structure interaction (FSI) with the open source tool FOAM-extend-3.1. The grafts were modelled with homogeneous properties in three configurations; tapered (16 mm proximal diameter, 12 mm distal diameter), non tapered (16-16 mm) and bellbottom (16-27 mm), all at 90° angulation (see 16-27 in figure). Experimentally determined pulsatile conditions were applied for different pressures; 145/80, 170/90, 195/100 mmHg.

Results - Stent graft displacement forces increased with higher fluid pressure in all graft configurations. Maximum forces in bellbottom grafts (proximal end 2.3 N, distal end 8.1 N) were particularly high compared to tapered (proximal end 2.5 N, distal end 1.2 N) and non tapered grafts (proximal end 2.5 N, distal end 2.5 N). The effects of shear stress and redirection of flow were together less than 2.8 % of the total forces in all graft configurations, whereas pressure derived forces were the main contributor to the forces acting on the stent graft. The flexibility of the stent graft absorbed up to 15 % of the forces.

Conclusion - The forces acting on stent grafts during pulsatile flow are significant and particularly high in the distal end of grafts with large distal diameter. The main contributor to these displacement forces is pressure while the contribution of shear and redirection of flow are negligible. The flexibility of the stent graft tends to absorb a significant part of the forces.