The shear stess profile of the pivotal fenestrated endograft at the level of the renal branches: A computational study for complex aortic aneurysms

Fate of target visceral vessels in fenestrated and branched complex endovascular aortic repair

OBJECTIVE

To assess branch vessel outcomes after endovascular repair of complex aortic aneurysms analyzing possible factors influencing early and long-term results.

METHODS

The Italian Multicentre Fenestrated and Branched registry enrolled 596 consecutive patients treated with fenestrated and branched endografts for complex aortic disease from January 2008 to December 2019 by four Italian academic centers. The primary end points of the study were technical success (defined as target visceral vessel [TVV] patency and absence of bridging device-related endoleak at final intraoperative control), and freedom from TVV instability (defined as the combined results of type IC/IIIC endoleaks and patency loss) during follow-up. Secondary end points were overall survival and TVV-related reinterventions.

RESULTS

We excluded 591 patients (3 patients with a surgical debranching and 2 patients who died before completion from the study cohort) were treated for a total of 1991 visceral vessels targeted by either a directional branch or a fenestration. The overall technical success rate was 98.4%. Failure was related to the use of an off-the-shelf (OTS) device (custom-made device vs OTS, HR, 0.220; P = .007) and a preoperative TVV stenosis of >50% (HR, 12.460; P < .001). The mean follow-up time was 25.1 months (interquartile range, 3-39 months). The overall estimated survival rates were 87%, 77.4%, and 67.8% at 1, 3, and 5 years, respectively (standard error [SE], 0.015, 0.022, and 0.032). During follow-up, TVV branch instability was observed in 91 vessels (5%): 48 type IC/IIIC endoleaks (2.6%) and 43 stenoses-thromboses (2.4%). The extent of aneurysm disease (thoracoabdominal aortic aneurysm [TAAA] types I-III vs TAAA type IV/juxtarenal aortic aneurysm/pararenal aortic aneurysm) was the only independent predictor for developing a TVV-related type IC/IIIC endoleak (HR, 3.899; 95% confidence interval [CI]:, 1.924-7.900; P < .001). Risk of patency loss was independently associated with branch configuration (HR, 8.883; P < .001; 95% CI, 3.750-21.043) and renal arteries (HR, 2.848; P = .030; 95% CI, 1.108-7.319). Estimated rates at 1, 3, and 5 years of freedom from TVV instability and freedom from TVV-related reintervention were 96.6%, 93.8%, and 90% (SE, 0.005, 0.007, and 0.014) and 97.4%, 95.0%, and 91.6% (SE, 0.004, 0.007, and 0.013), respectively.

CONCLUSIONS

Intraoperative failure to bridge a TVV was associated with a preoperative TVV stenosis of >50% and the use of OTS devices. Midterm outcomes were satisfying, with an estimated 5-year freedom from TVV instability and reintervention of 90.0% and 91.6%, respectively. During follow-up, the larger extent of aneurysm disease was associated with an increased risk of TVV-related endoleaks, whereas a branch configuration and renal arteries were more prone to patency loss.

Systematic Review of the Current In Vitro Experience of the Endovascular Treatment of Juxtarenal Abdominal Aortic Aneurysms by Fenestrated and Parallel Endografting

OBJECTIVE

To identify and analyze the published in vitro benchtop experiments for the assessment of endovascular techniques used for the treatment of juxtarenal abdominal aortic aneurysms (jAAAs).

DATA SOURCES

Scopus, PubMed, and Web of Science.

REVIEW METHODS

A systematic literature search was carried out throughout March 2021 following PRISMA guidelines. Two investigators independently performed title and abstract screening to reveal all benchtop testing evaluating the endovascular treatment of jAAA.

RESULTS

A total of 19 studies were included, 8 evaluating fenestrated (FEVAR) and 11 parallel grafts (PGs). FEVAR studies used different custom testing apparatus (n=7) or 3D-printed models (n=1) to analyze dislodgement and migration resistance, misalignment consequences and causation, and bridging stents' radial force, flareability, fatigue, and fracture resistance. All PG studies used silicone-based models to analyze optimal oversizing, sealing length, gutter behavior, and possible reduction. Test evaluation in FEVAR in vitro testing was based on pullout force analysis (N=5), photo evaluation (n=1), fluoroscopy (n=1), X-rays (n=4), CT analysis (n=3), macro- and microscopic evaluation (n=4), water permeability (n=1), and fatigue simulator testing (n=1), while it was based on CT analysis in all PG studies adding ECG-gate in one study. The most frequently tested devices were Zenit (Cook) (n=7), Endurant (Medtronic) (n=5), and Excluder (Gore) (n=5) as main grafts, and Advanta V12 (n=14) as the bridging device.

CONCLUSIONS

This systematic review presents a broad analysis of the current in vitro methods evaluating the endovascular treatment of jAAA. Fundamental issues have been benchtop tested in both FEVAR and PGs. The analysis of the included studies allowed to recommend an optimal testing design. In vitro testing is a potential tool to further elucidate points of attention hard to investigate in vivo to finally enhance the endovascular treatment outcomes. Future in vitro studies are needed to evaluate the in vitro performance of all indistinctively used devices in the clinical practice.

Impact of target visceral vessel anatomical configuration on early complications following endovascular repair of thoracoabdominal aortic aneurysms

Impact of target visceral vessel anatomical configuration on early complicatins following endovascular repair of thoracoabdominal aortic aneurysms Objectives: Fenestrated and branched endovascular aortic repair (fEVAR-bEVAR) is a viable treatment option for thoracoabdominal aortic aneurysms but target visceral stent (TVS) endoleak and thrombosis remain a limiting factor. This study aims to evaluate TVS anatomy impact on one-year risk of thrombosis and endoleak.

METHODS

Patients treated with fEVAR-bEVAR for thoracoabdominal aneurysms between 2008-2020 in our centre were enrolled. We recorded comorbidities, operative details, one-month postoperative CT scan (anatomical reference), and TVS behaviour: thrombosis and endoleak at one-year follow-up. For each TVS, different points were identified using a centre-lumen-line: (A) TVS origin, (B) end of branch/fenestration, (C) visceral vessel entry, (D) end of TVS, (E) 1-cm distally. We analyzed TVS tortuosity ((centre-lumen-line/straight distance)-1, in %), image vector analysis of each segment in 2D (antero-posterior, left-right) and 3D (craneo-caudal displacement), and centre-lumen-line analysis (bending in ABC and CDE). Three independent observers performed a blind analysis, and anatomical differences between bEVAR/fEVAR, and cases with/without 1-year thrombosis and TVS endoleak, were compared using Kaplan-Meier curves (Log-Rank test), and T-Test/Wilcoxon signed-ranks test respectively.

RESULTS

54 patients (72±713 years mean age; 182 TVS: 50 branches, 132 fenestrations) met the inclusion criteria. bEVAR cases had longer stents, with more caudal 3D angulation and greater ABC angulated segment. After excluding bEVAR cases (low case number), 97 fEVAR TVS were analyzed. Five thrombosis and seven endoleaks were observed. While anatomical configuration showed no association to thrombosis, it was related to endoleak: these cases presented more tortuous stents (5.97%±0.10, 21.40%±0,22, P=.011), with more angulated centre-lumen-line at ABC segment (5.69°±15.77°, 7.18°±7.77°, P=.012), and more upward-pointing stents in the origin of the stent (AB: 89.07°±24.46°, 109.09°±16.56°, P=.012; BC: 87.86°±21.10°, 113.11°±22.23°, P=.026).

CONCLUSIONS

Anatomical configuration of the TVS is associated with stent type I-III endoleak, but not thrombosis, at one-year following fEVAR. Cases with endoleak presented more tortuous stents, with a more angulated exit from the endograft and upward-pointing of the origin of the stent.

Geometric and hemodynamic analysis of fenestrated and multibranched aortic endografts

OBJECTIVE

The objective of this study was to determine the influence of hemodynamic force on the development of type III endoleak and branch thrombosis after complex endovascular thoracoabdominal aortic aneurysm repair.

METHODS

Patients with thoracoabdominal aortic aneurysm, within surgical range, treated with a fenestrated or branched endovascular aneurysm repair from 2014 to 2018 and with 3-month control computed tomography angiography were selected. Demographic variables, aneurysm anatomy, and endograft conformation were analyzed retrospectively from a prospective registry. The hemodynamic force was calculated using the mass and momentum conservation equations.

RESULTS

Twenty-eight patients were included; the mean follow-up period was 24.7 ± 19.3 months. There were 102 abdominal vessels successfully catheterized (19 celiac arteries, 29 superior mesenteric arteries, 27 right renal arteries, 26 left renal arteries, and 1 polar renal artery). The rate of type III endoleak was 11.5% (n = 12); six cases were associated with branches that received two stents (P < .001). A higher rate of endoleak was observed with wider stents (8.50 ± 1.0 mm vs 7.17 ± 1.3 mm; P = .001) but not with longer stents (P = .530). All cases of type III endoleak affected visceral arteries (eight celiac arteries and four superior mesenteric arteries). The freedom from type III endoleak at 24 months was 86%. The rate of thrombosis was 5.9% (n = 6). A higher rate of thrombosis was observed in smaller vessels (5.00 ± 1.3 mm vs 7.16 ± 1.8 mm; P = .001), with higher stent oversizing (36.87% ± 23.6% vs 5.52% ± 15.0%; P < .001), and with a higher angle of curvature (124.33 ± 86.1 degrees vs 57.71 ± 27.9 degrees; P < .001). All cases of thrombosis were related to renal arteries (two left renal arteries, two right renal arteries, and two polar renal arteries). The freedom from thrombosis at 24 months was 92%. The area under the curve for the angle of curvature was 0.802 (95% confidence interval, 0.661-0.943; P = .013), and the cutoff point was established at 59.5 degrees (sensitivity, 100%; specificity, 60.4%). The receiver operating characteristic curve for the stent oversize showed an area under the curve of 0.903 (95% confidence interval, 0.821-0.984; P = .001), and the cutoff point was 14.5% (sensitivity, 100%; specificity, 77.1%). A higher hemodynamic force was associated with thrombosis (23.35 × 10^-3 N ± 18.7 × 10^-3 N vs 12.31 × 10^-3 N ± 6.8 × 10^-3 N; P = .001) but not with endoleak (P = .796). The freedom from endoleak and thrombosis at 24 months was 86% and 90%, respectively.

CONCLUSIONS

Longer stents should be preferred to avoid type III endoleak. A higher angle of curvature leads to a higher hemodynamic force that results in a higher rate of thrombosis. Accordingly, we recommend maintaining the angle of curvature under 59.9 degrees. Small vessels and excessive stent oversizing entail a higher risk of thrombosis; as such, we advise a maximum stent oversize of 14.5%. Renal arteries are more susceptible to thrombosis, whereas visceral arteries are more prone to endoleak.

Performance of BeGraft and BeGraft+ Stent-Grafts as Bridging Devices for Fenestrated Endovascular Aneurysm Repair: An In Vitro Study

Purpose: To investigate 2 generations of balloon-expandable covered stents as potential bridging devices using an in vitro model of stent-graft fenestrations. Materials and Methods: Twenty BeGraft and 20 BeGraft+ cobalt-chromium stents covered in expanded polytetrafluoroethylene (ePTFE) in 6- and 8-mm diameters were tested in sheets mimicking stent-graft fenestrations. Microscopy and radiography were employed to evaluate stent morphology after flaring. In vitro bench tests measured maximum pullout (perpendicular displacement) and the shear stress (axial displacement) forces needed to dislocate the stents. Results: No alteration of ePTFE coverage was detected in the flared stents. Digital radiography and computed tomography showed marked alteration of the stent geometry, which was more pronounced in the BeGraft group. No fractures were detected. Median (minimum-maximum) pullout forces for the 6-mm stent-grafts were 17.1 N (15.8-19.6) for the BeGraft device and 30.4 N (20.2-31.9) for the BeGraft+ device (p=0.006). Median (minimum-maximum) pullout forces for the 8-mm stent-grafts were 11.3 N (11-12.1) for the BeGraft device and 21.8 N (18.2-25.5) for the BeGraft+ device (p<0.001). The shear stress test showed median forces of 10.5 vs 15.28 N at 150% of the stent diameter for the 6-mm BeGraft and BeGraft+ stent-grafts, respectively, and 15.23 vs 20.72 N at 150% stent diameter for the 8-mm models (p=0.016 and 0.017, respectively). Conclusion: Flaring changed the stent geometry but did not provoke stent fractures. The BeGraft+ is superior to the BeGraft in terms of pullout and shear stress forces, demonstrating greater resilience.

Effect of a Flared Renal Stent on the Performance of Fenestrated Stent-Grafts at Rest and Exercise Conditions

PURPOSE

To quantify the hemodynamic impact of a flared renal stent on the performance of fenestrated stent-grafts (FSGs) by analyzing flow patterns and wall shear stress-derived parameters in flared and nonflared FSGs in different physiologic scenarios.

METHODS

Hypothetical models of FSGs were created with and without flaring of the proximal portion of the renal stent. Flared FSGs with different dilation angles and protrusion lengths were examined, as well as a nonplanar flared FSG to account for lumbar curvature. Laminar and pulsatile blood flow was simulated by numerically solving Navier-Stokes equations. A physiologically realistic flow rate waveform was prescribed at the inlet, while downstream vasculature was modeled using a lumped parameter 3-element windkessel model. No slip boundary conditions were imposed at the FSG walls, which were assumed to be rigid. While resting simulations were performed on all the FSGs, exercise simulations were also performed on a flared FSG to quantify the effect of flaring in different physiologic scenarios.

RESULTS

For cycle-averaged inflow of 2.94 L/min (rest) and 4.63 L/min (exercise), 27% of blood flow was channeled into each renal branch at rest and 21% under exercise for all the flared FSGs examined. Although the renal flow waveform was not affected by flaring, flow within the flared FSGs was disturbed. This flow disturbance led to high endothelial cell activation potential (ECAP) values at the renal ostia for all the flared geometries. Reducing the dilation angle or protrusion length and exercise lowered the ECAP values for flared FSGs.

CONCLUSION

Flaring of renal stents has a negligible effect on the time dependence of renal flow rate waveforms and can maintain sufficient renal perfusion at rest and exercise. Local flow patterns are, however, strongly dependent on renal flaring, which creates a local flow disturbance and may increase the thrombogenicity at the renal ostia. Smaller dilation angles, shorter protrusion lengths, and moderate lower limb exercise are likely to reduce the risk of thrombosis in flared geometries.

Hemodynamic Functions of Fenestrated Stent Graft under Resting, Hypertension, and Exercise Conditions

The aim of this study was to assess the hemodynamic performance of a patient-specific fenestrated stent graft (FSG) under different physiological conditions, including normal resting, hypertension, and hypertension with moderate lower limb exercise. A patient-specific FSG model was constructed from computed tomography images and was discretized into a fine unstructured mesh comprising tetrahedral and prism elements. Blood flow was simulated using Navier-Stokes equations, and physiologically realistic boundary conditions were utilized to yield clinically relevant results. For a given cycle-averaged inflow of 2.08 L/min at normal resting and hypertension conditions, approximately 25% of flow was channeled into each renal artery. When hypertension was combined with exercise, the cycle-averaged inflow increased to 6.39 L/min but only 6.29% of this was channeled into each renal artery, which led to a 438.46% increase in the iliac flow. For all the simulated scenarios and throughout the cardiac cycle, the instantaneous flow streamlines in the FSG were well organized without any notable flow recirculation. This well-organized flow led to low values of endothelial cell activation potential, which is a hemodynamic metric used to identify regions at risk of thrombosis. The displacement forces acting on the FSG varied with the physiological conditions, and the cycle-averaged displacement force at normal rest, hypertension, and hypertension with exercise was 6.46, 8.77, and 8.99 N, respectively. The numerical results from this study suggest that the analyzed FSG can maintain sufficient blood perfusion to the end organs at all the simulated conditions. Even though the FSG was found to have a low risk of thrombosis at rest and hypertension, this risk can be reduced even further with moderate lower limb exercise.