Matthew G. Doyle

Increasing angulation decreases measured aortic stent graft pullout forces

OBJECTIVE Experimentally measured pullout forces for stent grafts (SGs) are used in clinical discussions and as reference values in bench studies and computer simulations. Previous values of these forces are available from studies in which the SG was pulled out in the straight caudal direction. However, clinical and numerical studies have suggested that displacement forces acting on SGs are directed more anteriorly. The objective of this study was to measure pullout forces as a function of angulation and to test the hypothesis that pullout forces decrease with increasing angulation. METHODS Six different SGs (Bolton Treovance, Cook Zenith Flex, Cook Zenith LP, Medtronic Endurant, Medtronic Talent, and Vascutek Anaconda) were deployed in fresh bovine aortas, then pulled out by an electronic motor at 1 mm/s, while tension force was measured continuously with a digital load cell. The SG off-axis angulation was changed from 0 to 90 degrees in increments of 10 degrees. The test system was submerged in a custom-built temperature-controlled saline bath at 37°C. At least three tests were performed for each device at each angle (with the exception of the Cook Zenith Flex, which experienced plastic deformation of its barbs after a single test per device). Each aortic specimen was used only once and then discarded. Hand-sutured graft anastomoses were also tested at 0 degrees to provide a reference value. RESULTS A total of 374 pullout tests were performed for the SGs and anastomoses. Sixty-four tests were excluded because of failure of the aorta or apparatus before device pullout. The remaining 310 tests showed pullout forces that demonstrated a decrease in the average pullout force for all six devices from 0 to 90 degrees (Bolton Treovance from 39.3 N to 23.9 N; Cook Zenith Flex from 59.8 N to 48.9 N; Cook Zenith LP from 50.3 N to 41.8 N; Medtronic Endurant from 29.9 N to 25.8 N; Medtronic Talent from 6.0 N to 5.5 N; and Vascutek Anaconda from 37.0 N to 30.3 N). For reference, the mean pullout force for the hand-sutured anastomoses was 63 N. CONCLUSIONS This study reports for the first time the change in pullout force with angulation, showing a general pullout force decrease with increasing angle. With a larger number of samples than in previous studies, our results provide updated benchmark data that can be used for clinical discussions, computational and experimental studies, and future device design.

Numerical Simulations of Blood Flow in Artificial and Natural Hearts With Fluid–Structure Interaction

This article describes two ongoing numerical studies of fluid-structure interaction in the cardiovascular system: an idealized pulsatile ventricular assist device (VAD), consisting of two fluid chambers separated by a flexible diaphragm; and blood flow and heart wall motion during passive filling of a canine heart. Simulations have been performed for the VAD and compared with the results of a previous study and to our own preliminary experimental results. Detailed measurements of the flow field in the VAD model and additional simulations are in progress. Preliminary simulations using both an idealized model of the natural heart as well as a realistic model have identified the limitations of the current numerical methods in dealing with large three-dimensional deformations. Ongoing research aims at extending the range of simulations to include large deformations and to incorporate an anisotropic material model for the heart wall to account for the muscle fibers.

Application of parallel processing to the simulation of heart mechanics

Simulations of the mechanics of the left ventricle of the heart with fluid-structure interaction benefit greatly from the parallel processing power of a high performance computing cluster, such as HPCVL. The objective of this paper is to describe the computational requirements for our simulations. Results of parallelization studies show that, as expected, increasing the number of threads per job reduces the total wall clock time for the simulations. Further, the speed-up factor increases with increasing problem size. Comparative simulations with different computational meshes and time steps show that our numerical solutions are nearly independent of the mesh density in the solid wall (myocardium) and the time step duration. The results of these tests allow our simulations to continue with the confidence that we are optimizing our computational resources while minimizing errors due to choices in spatial or temporal resolution.

Analysis of Iliac Artery Geometric Properties in Fenestrated Aortic Stent Graft Rotation

Introduction: A complication of fenestrated endovascular aneurysm repair is the potential for stent graft rotation during deployment causing fenestration misalignment and branch artery occlusion. The objective of this study is to demonstrate that this rotation is caused by a buildup of rotational energy as the device is delivered through the iliac arteries and to quantify iliac artery geometric properties associated with device rotation. Methods: A retrospective clinical study was undertaken in which iliac artery geometric properties were assessed from preoperative imaging for 42 cases divided into 2 groups: 27 in the nonrotation group and 15 in the rotation group. Preoperative computed tomography scans were segmented, and the iliac artery centerlines were determined. Iliac artery tortuosity, curvature, torsion, and diameter were calculated from the centerline and the segmented vessel geometry. Results: The total iliac artery net torsion was found to be higher in the rotation group compared to the nonrotation group (23.5 ± 14.7 vs 14.6 ± 12.8 mm−1; P = .05). No statistically significant differences were found for the mean values of tortuosity, curvature, torsion, or diameter between the 2 groups. Conclusion: Stent graft rotation occurred in 36% of the cases considered in this study. Cases with high iliac artery total net torsion were found to be more likely to have stent graft rotation upon deployment. This retrospective study provides a framework for prospectively studying the influence of iliac artery geometric properties on fenestrated stent graft rotation.

Prediction of advanced endovascular stent graft rotation and its associated morbidity and mortality

Objective: Advanced endovascular aneurysm repair (EVAR) with fenestrated and branched stent grafts is increasingly being used to repair complex aortic aneurysms; however, these devices can rotate unpredictably during deployment, leading to device misalignment. The objectives of this study were to quantify the short‐term clinical outcomes in patients with intraoperative stent graft rotation and to identify quantitative anatomic markers of the arterial geometry that can predict stent graft rotation preoperatively. Methods: A prospective study evaluating all patients undergoing advanced EVAR was conducted at two university‐affiliated hospitals between November 2015 and December 2016. Stent graft rotation (defined as ≥10 degrees) was measured on intraoperative fluoroscopic video of the deployment sequence. Standard preoperative computed tomography angiography imaging was used to calculate the geometric properties of the arterial anatomy. Any in‐hospital and 30‐day complications were prospectively documented, and a composite outcome of any end‐organ ischemia or death was used as the primary end point. Results: Thirty‐nine patients undergoing advanced EVAR were enrolled in the study with a mean age of 75 years (interquartile range [IQR], 71‐80 years) and a mean aneurysm diameter of 64 mm (IQR, 59‐65 mm). The incidence of stent graft rotation was 37% (n = 14), with a mean rotation of 25 degrees (IQR, 21‐28 degrees). A nominal logistic regression model identified iliac artery torsion, volume of iliac artery calcification, and stent graft length as the primary predictive factors. The total net torsion and the total volume of calcific plaque were higher in patients with stent graft rotation, 8.9 ± 0.8 mm−1 vs 4.1 ± 0.5 mm−1 (P < .0001) and 1054 ± 144 mm3 vs 525 ± 83 mm3 (P < .01), respectively. The length of the implanted stent grafts was also higher in patients with intraoperative rotation, 172 ± 9 mm vs 156 ± 8 mm (P < .01). The composite outcome of any end‐organ ischemia or death was also substantially higher in patients with stent graft rotation (36% vs 0%; P = .004). In addition, patients with stent graft rotation had significantly higher combined rates of type Ib and type III endoleaks (43% vs 8%; P = .03). Conclusions: Patients with intraoperative stent graft rotation have a significantly higher rate of severe postoperative complications, and this is strongly associated with higher levels of iliac artery torsion, calcification, and stent graft length. These findings suggest that preoperative quantitative analysis of iliac artery torsion and calcification may improve risk stratification of patients before advanced EVAR.

The Influence of Surgical Technique on Device Rotation and Fenestration Alignment in Advanced Endovascular Aneurysm Repair

ICH, Intracranial hemorrhage; MI, myocardial infarction; N/A, not applicable. ICH and MI were significantly increased among patients who underwent CEA within 2 days of symptom onset (Fisher exact test, P 1⁄4 .05). Stroke rate was nonsignificantly increased in patients who received CEA within 2 days of symptoms (Fisher exact test, P 1⁄4 .32). Values are reported as number (%). Sean A. Crawford, MD, Matthew G. Doyle, PhD, Cristina H. Amon, ScD, MS, P Eng, Thomas L. Forbes, MD, FRCSC. Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada; Division of Vascular Surgery, University Health Network, Peter Munk Cardiac Centre, University of Toronto, Toronto, Ontario, Canada; Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada

Predicting rotation in fenestrated endovascular aneurysm repair using finite element analysis

Fenestrated endovascular aneurysm repair (FEVAR) is a minimally invasive method of abdominal aortic aneurysm (AAA) repair utilized in patients with complex vessel anatomies. Stent grafts (SG) used in this process contain fenestrations within the device that need to be aligned with the visceral arteries upon successful SG deployment. Proper alignment is crucial to maintain blood flow to these arteries and avoid surgical complications. During fenestrated SG deployment, rotation of the SG can occur during the unsheathing process. This leads to misalignment of the vessels, and the fenestrations and is associated with poor clinical outcomes. The aim of this study was to develop a computational model of the FEVAR process to predict SG rotation. Six patient-specific cases are presented and compared with surgical case data. Realistic material properties, frictional effects, deployment methods, and boundary conditions are included in the model. A mean simulation error of 2 deg (range 1-4 deg) was observed. This model was then used to conduct a parameter study of frictional properties to see if rotation could be minimized. This study showed that increasing or decreasing the coefficients of friction (COF) between the sheath and the vessel walls would decrease the amount of rotation observed. Our model accurately predicts the amount of SG rotation observed during FEVAR and can be used as a preoperative planning tool within the surgical workflow.

Computational Fluid Dynamics Simulations of Flow in the Renal Arteries after Stent Graft Implantation

The objective of this work is to report a computational fluid dynamics study assessing the hemodynamic effects of fenestration misalignment, towards understanding post-surgical complications of fenestrated endovascular aneurysm repair for abdominal aortic aneurysms. Idealized models were constructed based on geometries from a patient with an infrarenal aortic aneurysm. Fenestrated stent grafts were simulated in the models, with combinations of different fenestration misalignments and takeoff angles. Computational fluid dynamics simulations were performed by solving the governing equations for blood flow under physiologically realistic boundary conditions. Hemodynamic results of renal artery flow rate and time-averaged wall shear stresses were analyzed to build connections between the degree of fenestration misalignment, the takeoff angle, and changes in flow dynamics. Keywords-abdominal aortic aneurysms; computational fluid dynamics; fenestrated endovascular aneurysm repair; hemodynamics; time-averaged wall shear stress

Microdevice arrays with strain sensors for 3D mechanical stimulation and monitoring of engineered tissues

Native and engineered tissue development are regulated by the integrative effects of multiple microenvironmental stimuli. Microfabricated bioreactor array platforms can efficiently dissect cue-response networks, and have recently integrated critical 2D and 3D mechanical stimulation for greater physiological relevance. However, a limitation of these approaches is that assessment of tissue functional properties is typically limited to end-point analyses. Here we report a new deformable membrane platform with integrated strain sensors that enables mechanical stretching or compression of 3D cell-hydrogel arrays and simultaneous measurement of hydrogel construct stiffness in situ. We tested the ability of the integrated strain sensors to measure the evolution of the stiffness of cell-hydrogel constructs for two cases. First, we demonstrated in situ stiffness monitoring of degradable poly (ethylene glycol)-norbornene (PEG-NB) hydrogels embedded with mesenchymal stromal cells (MSCs) and cultured with or without cyclic tensile stimulation for up to 15 days. Whereas statically-cultured hydrogels degraded and softened throughout the culture period, mechanically-stimulated gels initially softened and then recovered their stiffness corresponding to extensive cell network and collagen production. Second, we demonstrated in situ measurement of compressive stiffening of MSC-seeded PEG-NB gels cultured statically under osteogenic conditions, corresponding to increased mineralization and cellularization. This measurement technique can be generalized to other relevant bioreactor and organ-on-a-chip platforms to facilitate online, non-invasive, and high-throughput functional analysis, and to provide insights into the dynamics of engineered tissue development that are otherwise not available.

PC040 Analysis of Fenestrated Endovascular Aneurysm Repair Complication Frequency With Respect to Stent Graft Misalignment

Objectives: The objective of the current study is to evaluate the incidence of and clinical outcomes following fenestrated stent graft misalignment during complex endovascular aortic repairs. Methods: A retrospective record review was performed for all fenestrated endovascular aneurysm repairs (FEVAR) performed at our center between January 2008 and April 2015. Data were gathered from physician notes, radiology records, laboratory reports, and intraoperative imaging. Vertical stent graft misalignment was defined as