Computational fluid dynamic analysis of the effect of morphologic features on distraction forces in fenestrated stent grafts.

OBJECTIVE: Secure fixation of endovascular stent grafts is essential for successful endovascular aneurysm repair. Hemodynamic distraction forces are generated by blood pressure and blood flow and act against fixation force to encourage migration that may eventually lead to late stent graft failure. The aim of this in silico study was to determine which morphologic features were associated with greater distraction force. METHODS: Computer models of 54 in situ fenestrated stent grafts were constructed from postoperative computed tomography scans by use of image processing software. Computational fluid dynamic analysis was then performed by use of a commercial finite volume solver with boundary conditions representative of peak systole. Distraction force results were obtained for each component of the stent graft. Distraction force was correlated with lumen cross-sectional area (XSA) at the inlet and outlet of components and was compared between groups of components, depending on the magnitude of four predefined angles within the aortoiliac territory that we describe in detail. RESULTS: Median total resultant distraction force (RDF) acting on the fenestrated proximal bodies was 4.8 N (1.3-15.7 N); bifurcated distal bodies, 5.6N (1.0-8.0 N); and limb extensions, 1.7 N (0.6-8.4N). Inlet XSA exhibited strong, positive correlation with total RDF in proximal body and distal body components (Spearman correlation coefficient ρ, 0.883 and 0.802, respectively). Outlet XSA exhibited a similarly strong, positive correlation with total RDF in limb extension components (ρ, 0.822). Outlet angulation ≥ 45 degrees was associated with greater total RDF in the limb extension components only (P = .004). CONCLUSIONS: For a given blood pressure, XSA was the most important morphologic determinant of total RDF. Angulation within the aorta was not large enough to influence this, whereas iliac angulation affecting outlet angulation of limb extension components was associated with significantly greater total RDF.

Closure technique after carotid endarterectomy influences local hemodynamics.

BACKGROUND: Meta-analysis supports patch angioplasty after carotid endarterectomy (CEA); however, studies indicate considerable variation in practice. The hemodynamic effect of a patch is unclear and this study attempted to elucidate this and guide patch width selection. METHODS: Four groups were selected: healthy volunteers and patients undergoing CEA with primary closure, trimmed patch (5 mm), or 8-mm patch angioplasty. Computer-generated three-dimensional models of carotid bifurcations were produced from transverse ultrasound images recorded at 1-mm intervals. Rapid prototyping generated models for flow visualization studies. Computational fluid dynamic studies were performed for each model and validated by flow visualization. Mean wall shear stress (WSS) and oscillatory shear index (OSI) maps were created for each model using pulsatile inflow at 300 mL/min. WSS of <0.4 Pa and OSI >0.3 were considered pathological, predisposing to accretion of intimal hyperplasia. The resultant WSS and OSI maps were compared. RESULTS: The four groups comprised 8 normal carotid arteries, 6 primary closures, 6 trimmed patches, and seven 8-mm patches. Flow visualization identified flow separation and recirculation at the bifurcation increased with a patch and was related to the patch width. Computational fluid dynamic identified that primary closure had the fewest areas of low WSS or elevated OSI but did have mild common carotid artery stenoses at the proximal arteriotomy that caused turbulence. Trimmed patches had more regions of abnormal WSS and OSI at the bifurcation, but 8-mm patches had the largest areas of deleteriously low WSS and high OSI. Qualitative comparison among the four groups confirmed that incorporation of a patch increased areas of low WSS and high OSI at the bifurcation and that this was related to patch width. CONCLUSIONS: Closure technique after CEA influences the hemodynamic profile. Patching does not appear to generate favorable flow dynamics. However, a trimmed 5-mm patch may offer hemodynamic benefits over an 8-mm patch and may be the preferred option.

Fractured superior mesenteric artery stents after fenestrated endovascular aneurysm repair.

Stent fracture after fenestrated endovascular aneurysm repair is a recognized complication. In this report, we record the occurrence of superior mesenteric artery stent fractures in our series and describe the management of embolized stent fragments during secondary intervention.

The accuracy of computed tomography central luminal line measurements in quantifying stent graft migration.

PURPOSE: This study evaluated the accuracy of central luminal line (CLL) measurements in quantifying stent graft migration. The bias of the CLL technique together with observer variability were assessed. METHODS: Stent grafts were deployed in plastic aortic phantoms at fixed locations from two side branches. Each phantom was filled with iodinated contrast, and a 2-mm multislice computed tomography (CT) scan was performed. The stent graft was then displaced caudally, its new location determined, and again, a CT scan performed. This created a series of 15 cases with known stent graft migration. CLLs were used to measure stent graft position on the CT scans and calculate migration (3 observers). In vivo stent graft migration was then evaluated in a similar manner using a series of follow-up CT scans from nine patients (2 observers). All CLL measurements were performed independently and were repeated on a separate occasion. RESULTS: The mean difference in CLL migration between the actual and observed measurements (bias) in the aortic phantoms was <1 mm. The 95% confidence intervals for the bias were within the interval (-1 and 1 mm), and the 95% limits of agreement were within -3 mm and +3 mm. The 95% limits of agreement for measurements within and between observers were -4 to 2 mm and -2 to 2 mm, respectively. The phantom study generated a coefficient of repeatability (RC) of 1 mm for within-observer measurements. Clinically, CLLs generated 95% limits of agreement within and between observers of -3 to 4 mm (RC, 2 mm) and -3 to +3 mm, respectively. CONCLUSIONS: Bias from CLL-determined migration is small and insignificant from a practical point of view. A small amount of measurement variability within and between observers does exist; it should be feasible to detect changes in stent graft position that are ≥4 mm.

Factors and forces influencing stent-graft migration after endovascular aortic aneurysm repair.

PURPOSE: To assess and validate the clinical features predisposing to stent-graft migration and to calculate the distal displacement forces exerted at the proximal fixation site following endovascular aortic aneurysm repair (EVAR). METHODS: Demographic, anatomical, and graft-related features from 2862 patients were analyzed in a regression model to identify variables associated with stent-graft migration, which was defined as device movement >5 mm or considered significant by the investigator. Using the principles of continuity and momentum, a mathematical model of blood flow was created. The pulse pressure, proximal aortic and distal iliac diameters, and the degree of iliac angulation were varied in the calculations, and the distal displacement force exerted at the proximal fixation site was calculated. RESULTS: Ninety-nine patients developed stent-graft migration, which was clinically relevant in 85 (3.0%). Hypertension (p=0.015), smoking (p=0.009), maximal aortic diameter (p=0.004), and distal transverse aortic diameter (p=0.03) correlated with migration in the univariate analysis, but iliac angulation did not quite achieve significance (p=0.06). On multivariate analysis, current smoking, hypertension, distal transverse aortic diameter, maximum common iliac diameter, and increasing proximal graft size were significantly associated with stent-graft migration. The mathematical model calculated the distal displacement force exerted on the proximal fixation site of the stent-graft and validated the clinical findings. The ratio of graft-diameter change from proximal aorta to distal iliac influenced the greatest increase in the displacement force. CONCLUSIONS: The mathematical model validated hypertension, aneurysm morphology, and endograft size as clinical factors significantly associated with stent-graft migration. These findings may have important implications for the choice and design of future stent-grafts.