Fenestrated Stent Graft Repair of Abdominal Aortic Aneurysm: Hemodynamic Analysis of the Effect of Fenestrated Stents on the Renal Arteries

Objective: We wanted to investigate the hemodynamic effect of fenestrated stents on the renal arteries with using a fluid structure interaction method. Materials and Methods: Two representative patients who each had abdominal aortic aneurysm that was treated with fenestrated stent grafts were selected for the study. 3D realistic aorta models for the main artery branches and aneurysm were generated based on the multislice CT scans from two patients with different aortic geometries. The simulated fenestrated stents were designed and modelled based on the 3D intraluminal appearance, and these were placed inside the renal artery with an intra-aortic protrusion of 5.0-7.0 mm to reflect the actual patients' treatment. The stent wire thickness was simulated with a diameter of 0.4 mm and hemodynamic analysis was performed at different cardiac cycles. Results: Our results showed that the effect of the fenestrated stent wires on the renal blood flow was minimal because the flow velocity was not significantly affected when compared to that calculated at pre-stent graft implantation, and this was despite the presence of recirculation patterns at the proximal part of the renal arteries. The wall pressure was found to be significantly decreased after fenestration, yet no significant change of the wall shear stress was noticed at post-fenestration, although the wall shear stress was shown to decrease slightly at the proximal aneurysm necks. Conclusion: Our analysis demonstrates that the hemodynamic effect of fenestrated renal stents on the renal arteries is insignificant. Further studies are needed to investigate the effect of different lengths of stent protrusion with variable stent thicknesses on the renal blood flow, and this is valuable for understanding the long-term outcomes of fenestrated repair

Endograft Sizing for Abdominal Aortic Aneurysms

While a tight seal and fixation of aortic stent-grafts to the vessel wall are vital for positive outcomes in treating abdominal aortic aneurysms (AAAs), optimal aortic stent-graft sizing for endovascular aneurysm repair (EVAR) remains debatable. We performed a holistic review of the data surrounding the sizing of endografts using instructions for use (IFU) guidelines, as well as experimental, computational, and clinical studies. Most clinical studies that have investigated the role of sizing and outcomes are limited by the strict selection criteria, or the inability to account for the multitude of confounders associated with sizing. Currently, oversizing of endografts between 10 and 20% remains safe and favored, but sizing outside the IFU guidelines frequently occurs. Oversizing up to 25% appears to be associated with decreased rates of proximal endoleak and aneurysm sac enlargement, while excessive oversizing (>30%) has been linked to graft infolding, collapse, and aortic dilatation. It is unclear, however, whether there is an association between oversizing associated with neck dilatation and graft migration. During sizing, surgeons should take an individual approach and consider several factors including device type, calcification and/or thrombus of apposition site, hemodynamics, and aortoiliac morphology

Computational analysis of the hemodynamic performance of novel endovascular and surgical procedures for complex aortic diseases

Novel branched stent-grafts (BSG) have been developed for endovascular repair of complex thoracic aortic aneurysms (TAA) involving the aortic arch or thoracoabdominal aorta, but their haemodynamic performance has not been adequately studied. In addition, surgical replacement of the ascending aorta with a Dacron graft remains the gold standard for type A aortic dissection (TAAD), although 12% of patients are at risk of aortic rupture due to further dilatation of the residual dissected aorta. The underlying mechanisms for progressive aortic dilatation following TAAD repair are poorly understood, but haemodynamic and biomechanical factors are believed to play an important role. Therefore, the present study aims to provide more insights into the haemodynamics in novel BSGs developed for treating complex aortic diseases, and a comprehensive evaluation of flow and biomechanical conditions in post-surgery TAADs by means of state-of-the-art computational methods. The first part of this thesis focuses on evaluating the haemodynamic performance of novel BSG designs, including thoracoabdominal branch endoprosthesis (TAMBE) and dual-branched thoracic endograft. Haemodynamics in idealised and patient-specific BSG models has been analysed by examining side branch outflow waveforms, wall shear stress related indices, and displacement forces, in order to assess their long-term durability. The numerical results show that all the stent-graft models examined in this study are capable of providing normal blood perfusion to side vessels, and are at low risk of in-stent thrombosis and device migration. Furthermore, it has been shown that geometric variations in TAMBE do not affect the key haemodynamic results, indicating its potential suitability for a variety of visceral artery anatomies. Comparisons of dual-branched thoracic endograft models with different inner tunnel diameters suggest that BSGs with larger inner tunnel diameters than the respective vessels would be preferred. Comparisons between the pre- and post-intervention models show that insertion of a dual-branched stent-graft significantly alters the flow pattern in the aortic arch, some of which may have a detrimental effect in the long term, thus requiring follow-up studies. The second part of the thesis provides a comprehensive analysis of the haemodynamic and biomechanical conditions in surgically repaired TAAD. Geometric and haemodynamic parameters have been analyzed and compared between the group of patients with stable aortic diameter and another group with progressive aortic dilatation. The number of re-entry tears (6±5 vs 2±1;P= 0.02) and luminal pressure difference (1.3 ±1 vs 11.7 ±14.6 mmHg;P= 0.001) have been identified as potential predictors of progressive aortic dilatation in TAAD patients following surgical repair. This is an important finding and can potentially assist clinicians to make the most appropriate choice or surgical plan for individual patients. Based on the finite element analysis of four patient-specific cases, there are no clear differences in biomechanical parameters between the stable and unstable groups. Furthermore, a preliminary fluid-solid interaction (FSI) simulation performed on a single TAAD model has demonstrated the important influence of wall compliance on pressures in the true and false lumen. Compared to a rigid wall model, the FSI simulation results show a reduction in systolic pressure by up to 10 mmHg and a slight increase in diastolic pressure. However, pressures in the true and false lumen are affected in the same way, so that the luminal pressure difference remains the same between the rigid and FSI models.Open Acces

Efficiently simulating an endograft deployment : a methodology for detailed CFD analyses

Numerical models of endografts for the simulation of endovascular aneurysm repair are increasingly important in the improvement of device designs and patient outcomes. Nevertheless, current finite element analysis (FEA) models of complete endograft devices come at a high computational cost, requiring days of runtime, therefore restricting their applicability. In the current study, an efficient FEA model of the Anaconda™ endograft (Terumo Aortic, UK) was developed, able to yield results in just over 4 h, an order of magnitude less than similar models found in the literature. The model was used to replicate a physical device that was deployed in a 3D printed aorta and comparison of the two shapes illustrated a less than 5 mm placement error of the model in the regions of interest, consistent with other more computationally intensive models in the literature. Furthermore, the final goal of the study was to utilize the deployed fabric model in a hemodynamic analysis that would incorporate realistic fabric folds, a feature that is almost always omitted in similar simulations. By successfully exporting the deployed graft geometry into a flow analysis, it was illustrated that the inclusion of fabric wrinkles enabled clinically significant flow patterns such as flow stagnation and recirculation to be detected, paving the way for this modelling methodology to be used in future for stent design optimisation

A computational fluid dynamics analysis of the distraction forces experienced by stent-grafts following fenestrated Endovascular Aneurysm Repair

Introduction One option for repair of abdominal aortic aneurysms with inadequate length of infrarenal neck is fenestrated Endovascular Aneurysm Repair. Significant complications may be caused by stent-graft migration and component distraction which are both resisted by fixation force and provoked by haemodynamic distraction force. The hypotheses tested in this thesis are that larger angulation of vessels is associated with greater distraction force and that greater distraction force is associated with higher incidence of migration and component distraction. Method Interobserver variation of a new method of angle measurement was compared with the standard method currently in use in our unit. Computer models of complete fenestrated stent-grafts and their individual components (proximal body, distal body and limb extensions) were then constructed based on the postoperative computed tomography scans of 54 patients. Computational Fluid Dynamic analysis in steady state was used to quantify the distraction force acting on each device. Blood pressure was kept constant at 160mmHg and the impact of morphological features upon distraction force was assessed. To test the second hypothesis, patient-specific blood pressures were used to obtain in situ distraction forces that were then related to the incidence of migration and component distraction. Results There were no significant differences between the old and new methods of angle measurement (p=.723, WSR). Inlet cross-sectional area (XSA) exhibited a strong, positive correlation with total RDF in complete stent-grafts, proximal body and distal body components. Outlet angulation ≥45° was significantly associated with greater total RDF in complete stent-grafts and limb extension components (Median total RDF in complete stent-grafts with angle <45° = 2.6N vs 6.2N in those ≥45°, p<.001. Limb extensions: 1.4N vs 2.1N, p=.004, MWU). There was no significant difference between total RDF acting on the proximal or distal bodies that underwent migration or component distraction versus those that did not. Limb extensions that were observed to migrate were exposed to significantly greater total RDF compared to those that did not migrate (Median total RDF 2.9N, range 2.7-6.3N versus 1.6N, range 0.4-3.8N, p=.003, MWU). Conclusions For a given blood pressure XSA was the most important morphological determinant of total RDF. Outlet angulation of complete stent-grafts and limb extensions was associated with significantly greater total RDF. In limb extensions, greater distraction force was significantly associated with migration. The results suggest caution when planning distal seal in ectatic iliac vessels

Aneurysm sac filling. A new approach in the treatment of abdominal aortic aneurysm

Σκοπός: Η διενέργεια ολοκληρωμένης ανασκόπησης των πρόσφατων στοιχείων της βιβλιογραφίας που αφορούν σε προσεγγίσεις πλήρωσης του ανευρυσματικού σάκου ως τρόπο θεραπείας των ανευρυσμάτων της κοιλιακής αορτής (ΑΚΑ). Μεθοδολογία: Η βιβλιογραφική αναζήτηση διενεργήθει από τις 15 Δεκεμβρίου 2014 έως και 29 Δεκεμβρίου 2014 χρησιμοποιώντας τη βάση δεδομένων του Pubmed. Οι λέξεις κλειδιά που χρησιμοποιήθηκαν για την αναζήτηση, περιλάμβαναν τα ακόλουθα: ‘Aneurysm sac filling’, ‘Abdominal aneurysm sac filling’, ‘Nellix’ και ‘Endovascular aneurysm sealing’. Τα αποτελέσματα των άρθρων που εντοπίστηκαν ταξινομήθηκαν βάσει των πληροφοριών που δίνονται στον τίτλο και την περίληψη. Στην ανασκόπηση επιλέχθηκαν οι μελέτες που ήταν στην Αγγλική γλώσσα και ήταν σχετικές με το βασικό αντικείμενο αυτής της μελέτης. Αποτελέσματα: Στα αποτελέσματα του Pubmed εντοπίστηκαν συνολικά 350 άρθρα. Πέντε μελέτες αφορούσαν στον άμεσο τρόπο πλήρωσης του ανευρυσματικού σάκου με τη χρήση πολυμερών υλικών (2 in vitro, 2 in vivo και 1 ex vivo, εργαστηριακές μελέτες) και 12 μελέτες που αφορούσαν στον έμμεσο τρόπο πλήρωσης του σάκου μέσω της ενδαγγειακής συσκευής Nellix. (2 πολυκεντρικές κλινικές μελέτες, 2 μη τυχαιοποιημένες προοπτικές μελέτες, 1 σχόλιο, 1 in vitro ερευνητική μελέτη, 4 αναφορές κλινικών περιστατικών, 1 περίληψη 4 κέντρων με μεμονωμένες αναφορές και 1 ανασκόπηση) Συμπεράσματα: Τα σύγχρονα στοιχεία των προτεινόμενων προσεγγίσεων για πλήρωση του ανευρυσματικού σάκου ως τρόπο θεραπείας του ΑΚΑ είναι περιορισμένα σε αριθμό και βασίζονται μόνο σε λίγες ερευνητικές μελέτες και στα πρώτα κλινικά αποτελέσματα μικρού αριθμού ασθενών, μετά από χρήση της ενδαγγειακής συσκευής Nellix. Παρόλα αυτά, τα αποτελέσματα των πρώτων διαθέσιμων μελετών είναι ενθαρρυντικά και παραθέτουν πως η πλήρωση του σάκου, λόγω των ιδιοτήτων της ασφαλούς στερέωσης και στεγανοποίησης σε ποικίλες ανατομίες, δύναται να αποτελέσει μια νέα πρόκληση στην ενδαγγειακή αντιμετώπιση των ΑΚΑ. Τα μακροπρόθεσμα αποτελέσματα των ερευνητικών και κλινικών μελετών που είναι σε εξέλιξη καθώς και η συνέχιση της διεπιστημονικής συνεργασίας της ιατρικής, βιοχημείας, βιομηχανικής και επιστήμης υπολογιστών, δύνανται στο άμεσο μέλλον να δώσουν απαντήσεις για την ασφάλεια και διάρκεια στο χρόνο, της νέας προσέγγισης στην ενδαγγειακή αποκατάσταση του ανευρύσματος της κοιλιακής αορτής μέσω πλήρωσης του αορτικού σάκου, δεδομένου του απειλητικού για τη ζωή ποσοστού και τη μακρόχρονη παρακολούθηση μετά από ενδαγγειακή αποκατάσταση ανευρύσματος κοιλιακής αορτής.Objective: To conduct a comprehensive review of the recent evidence on suggested approaches for the aneurysm sac filling as a treatment of abdominal aortic aneurysm (AAA). Methods: A bibliographic search was performed between December 15th 2014 and December 29th 2014 using the Pubmed database. The search keywords included: ‘aneurysm sac filling’, ‘abdominal aorta aneurysm sac filling’, ‘Nellix’ and ‘endovascular aneurysm sealing’. The detected articles were sorted based on the information provided in the title and the abstract. Studies written in English and relevant to the main objective of this study were considered eligible and included in the review. Results: Pubmed results detected 350 articles in total; from those, 17 independent studies were eligible. Five studies referred to direct sac filling using elastomer or polymer materials (2 in vitro, 2 in vivo and 1 ex vivo, experimental studies) and 12 studies referred to indirect sac filling using the endovascular Nellix device (2 multicenter clinical trials, 2 non randomized prospective cohort studies, 1 commentary, 1 in vitro experimental study, 4 case reports, 1 abstract of 4-centers single reports and 1 review). Conclusions: The current evidence on the recently suggested approaches for aneurysm sac filling for the treatment of AAA is very limited and relies only in few experimental studies and some early results in a small number of patients that have used the Nellix endovascular device in clinical practice. However, the findings from the first available studies are encouraging and suggest that sac filling, due to the secure fixation and sealing properties in various anatomies could be a new challenge in EVAR. The long term on progress results of the experimental and clinical studies along with the continuous multidisciplinary collaboration of medicine, biochemistry, bioengineering and computer science, guarantee the ability to give answers for the effectiveness of aneurysm sac filling as a safe, durable and cost-efficient method for EVAR, regarding the life threatening rate and the long term surveillance after EVAR

The role of contrast enhanced ultrasonography in post-operative surveillance of endovascular aortic aneurysm stent graft repair

MD (Res)Abdominal aortic aneurysms are common and responsible for many deaths. They are treated increasingly by EndoVascular Aneurysm Repair (EVAR) rather than conventional surgery. Approximately 25% of EVAR patients require re-intervention to prevent aneurysm enlargement which can rupture despite previous repair. All EVAR patients undergo life-long surveillance for complications such as stent-graft migration or endoleak. Computed Tomography (CT) has been the ‘gold-standard’ for surveillance accounting for 65% of EVAR costs, and exposes patients to cumulative radiation and nephrotoxic contrast. Duplex Ultrasound Scanning (DUS) has been proposed as an alternative for surveillance with lesser cost and patient risk. However, clinical studies have reported varying results. The addition of microbubble contrast significantly improves endoleak detection rates, making it comparable with CT. The physical properties that affect endoleak detection with DUS have not been determined. It is also unknown specifically which endoleaks’ detection are improved by Contrast Enhanced Aortic Duplex UltraSound Scanning (CEADUSS). To investigate the physical properties of endoleaks, I constructed an EVAR phantom model with a simulated endoleak of variable velocity (fast/slow), position (near/far) and plane (anterior/lateral/posterior). Preliminary studies investigated the behavior of microbubble contrast in the phantom system, and then laboratory experiments tested subjects over 36 variable endoleaks using DUS and CEADUSS. These laboratory experiments were translated clinically with a pilot study of CEADUSS in 10 patients with endoleaks on CT not detected by DUS, undefined endoleak type or origin, or a sac enlargement with no endoleak present. My experiments reveal an insight into factors influencing ultrasound endoleak detection. With this knowledge, the use of these modalities for surveillance protocols can be increased, reducing current CT burden, radiation and nephrotoxic contrast exposure, and overall EVAR cost. Clinical assessment of an endoleak, specifically noting physical characteristics (plane, position and velocity) will improve detection and surveillance

Dynamics in endovascular surgery

Rauwerda, J.A. [Promotor]Wisselink, W. [Promotor]Marcus, J.T. [Copromotor