Objectives: Significant stent graft remodeling commonly occurs after endovascular repair of thoracic aortic dissections because of continuing expansion of the true lumen. A suboptimal proximal landing zone, minimal oversizing, and lack of a healthy distal attachment site are unique factors affecting long-term stent graft stability. We used computational fluid dynamic techniques to analyze the biomechanical factors associated with stent graft remodeling in these patients.
Patients and methods: A series of computational fluid dynamic models were constructed to investigate the biomechanical factors affecting the drag force on a thoracic stent graft. The resultant drag force as a net change of fluid momentum was calculated on the basis of varying three-dimensional geometry and deployment positions. A series of 12 patients with type B aortic dissections treated by thoracic stent graft and followed up for more than 12 months were then studied. Computed tomography transaxial images of each patient shortly after stent graft deployment and on subsequent follow-up were used to generate three-dimensional geometric models that were then fitted with a surface mesh. Computational fluid dynamic simulations were then performed on each stent graft model according to its geometric parameters to determine the actual change in drag force experienced by the stent graft as it remodels over time.
Results: The drag force on the stent graft model increases linearly with its internal diameter and becomes highest when the deployment position is closer to the proximal arch. Aortic curvature is not a significant factor. Serial computed tomography scans of patients showed an increase in mean inlet area from 1030 mm(2) to 1140 mm(2), and mean outlet area from 586 mm(2) to 884 mm(2) (increase of 11% and 58%, respectively; P = .05, .01). These increases are associated with a change in resultant drag force on the stent graft from 21.0 N to 24.8 N (mean increase, 19.5%; range, 0%-63.2%; P = .002). There is a positive relationship between increase in drag force and increase in stent-graft area.
Conclusion: The drag force on thoracic stent grafts is high. A significant change in stent-graft diameter occurs after endovascular repair for type B dissections, which is associated with an increase in hemodynamic drag force. These stent grafts may be subjected to a higher risk of distal migration, and continuing surveillance is mandatory.