Ilija D. Šutalo

Type II endoleaks: when is intervention indicated and what is the index of suspicion for types I or III?

One of the principal reasons for failure of endovascular aneurysm repair (EVAR) is the occurrence of endoleaks, which regardless of size or type can transmit systemic pressure to the aneurysm sac. There is little debate that type I endoleaks (poor proximal or distal sealing) are associated with continued risk of aneurysm rupture and require treatment. Similarly, with type III endoleak, there is agreement that the defect in the device needs to be addressed; however, what to do with type II endoleaks and their effect on long-term outcome are not so clear. Aneurysm sac change is a primary parameter for determining the presence of an endoleak and assessing its impact. While diameter measurement has been the most commonly used method for determining sac changes, volume measurement has now been proven superior for monitoring structural changes in the 3-dimensional sac. Determining the source of an endoleak and the direction of flow are necessary for proper classification; however, while computed tomographic angiography has high sensitivity and specificity for detecting endoleaks, it is limited in its ability to show the direction of flow. Contrast-enhanced duplex ultrasound, on the other hand, is better able to quantify flow and characterize endoleaks. Flow is evidence of pressure, and increasing intrasac pressure increases wall tension, thus inducing progressive aneurysm expansion until rupture. Hence, determining intrasac pressure is becoming a vital component of endoleak assessment. All endoleaks can create systemic pressure inside the aneurysm sac, and there are a variety of intrasac pressure transducers being evaluated to assess this effect. A clinical pathway for patients with suspected type II endoleaks is based on a combination of imaging and pressure measurements. Imaging alone requires at least two interval examinations to determine the trend, while pressure measurements give immediate reassurance or an indication to intervene. Although still under development, pressure measurement is destined for general use and will provide a scientific basis for the management of type II endoleaks.

Movement and Dislocation of Modular Stent-Grafts Due to Pulsatile Flow and the Pressure Difference Between the Stent-Graft and the Aneurysm Sac

Purpose: To investigate the stability and movement of modular aortic stent-grafts subjected to oscillating forces from pulsatile blood flow, with particular reference to the thoracic aorta. Methods: Analytical mathematical modeling was used to understand the forces on modular grafts. In a benchtop experiment, a transparent acrylic box was filled with water to mimic an aneurysm. Two stent-grafts were placed inside the box in a nested, arched configuration where one component was partly inside the other. A pump produced a pulsatile !5-L/min flow of water through the stent-grafts at a mean inlet pressure of !100 mmHg (∼13,330 Pa), with systolic and diastolic pressures of ∼130 and ∼80 mmHg, respectively (pulse pressure 50 mmHg). The movement of the 2 modular stent-grafts was observed. Results: The curved stent-graft system oscillated transversely when there was zero mean pressure difference between the stent-graft and the aneurysm. As the mean pressure difference was increased, this transverse graft movement was damped and then disappeared. A relatively large pressure difference caused the stent-graft to inflate and become sturdier. In terms of stability, the analytical mathematical model for a 30-mm-diameter Zenith modular stent-graft curved through 90° (with the ends of the graft fixed in place) showed that the modular components will separate at a pressure difference of 0 mmHg for 1 stent segment overlap (20 mm) and at an average 59 mmHg pressure difference for 2 stent overlaps, but the device would not separate at a pressure difference of 90 mmHg for 3 stent overlaps. Conclusion: Transverse cyclic movement of the curved stent-graft system with pulsation indicates a pressurized sac. When the pressure difference is large and there is a blood-tight seal between the aneurysm and the stent-graft, then the transverse movement of the stent-graft is minimal, but the risk for modular separation is highest. Curved thoracic endografts are subject to forces that may cause migration or separation, the latter being more likely if the seal between the graft and the sac is blood tight, if the blood pressure is high, and if the diameter of the graft is small and the sac large. Operators should plan for maximum overlap of modular components when treating large or long thoracic aneurysms.

Dynamics of pulsatile flow in fractal models of vascular branching networks.

Efficient regulation of blood flow is critically important to the normal function of many organs, especially the brain. To investigate the circulation of blood in complex, multi-branching vascular networks, a computer model consisting of a virtual fractal model of the vasculature and a mathematical model describing the transport of blood has been developed. Although limited by some constraints, in particular, the use of simplistic, uniformly distributed model for cerebral vasculature and the omission of anastomosis, the proposed computer model was found to provide insights into blood circulation in the cerebral vascular branching network plus the physiological and pathological factors which may affect its functionality. The numerical study conducted on a model of the middle cerebral artery region signified the important effects of vessel compliance, blood viscosity variation as a function of the blood hematocrit, and flow velocity profile on the distributions of flow and pressure in the vascular network.

Mode of failure of rib fixation with absorbable plates: a clinical and numerical modeling study.

BACKGROUND Rib fractures as a result of trauma are a relatively common injury. There is an increasing interest in the operative stabilization of these injuries. Absorbable fixation plates are an option for improved rib fracture treatment. The aim of this study was to review our plating failures and to create a numerical model of muscle forces on fractured ribs to identify the mechanism by which these rib fixations have failed. METHODS Thirteen patients who had 58 ribs fixed with absorbable prostheses were reviewed. Finite element modeling was used to simulate the fixation of a lateral rib fracture using an absorbable plate and screw system. Internal pressure, intercostal forces, and appropriate displacement and rotational constraints were enforced at the rib ends. RESULTS Ten rib fixation failures were noted in the clinical series. The modeling results showed that stresses on the plate differ during inspiration and expiration. Failure to use the two central screws resulted in higher stresses on the plating system. During inspiration simulations, the screws on both rib parts are active in keeping the rib and plate surfaces unseparated. However, during expiration, there is a greater stress on the screws on the posterior part of the broken rib, and separation of the plate from the rib seems to be more likely to occur at this site. CONCLUSIONS This study indicates that the likely mode of failure of this absorbable plating system occurs on the posterior part of the rib, which correlates with the clinical failures seen.

Structural integrity of intramedullary rib fixation using a single bioresorbable screw

BACKGROUND Operative management of flail chest injury is receiving increasing interest. However, we have noticed in our own practice the difficulty in achieving reliable results with posterior rib fracture fixation. In this article, we analyze and model the physiologic forces acting on posterior rib fractures and assess the suitability of an intramedullary screw fixation technique in this site. METHODS Computerized finite element analysis (FEA) was used to model a typical sixth rib and analyze the physiologic forces that act on the rib in vivo. A fracture in the posterior aspect of the rib was incorporated into the model, and an intramedullary screw fixation concept was assessed, using both a bioabsorbable polymer screw and a stainless steel screw. The records of 120 consecutive patients with flail chest were reviewed, and 26 patients were identified as having multiple posterior rib fractures with displacement. These patients formed a clinical correlation group by which to assess the FEA model. RESULTS FEA modeling of the posterior rib fracture showed likely posterior displacement in response to physiologic forces. Review of the 26 patients with flail chest and displaced posterior fractures confirmed the direction of displacement. Modeling of an intramedullary screw fixation showed significant stresses in the bone/screw contact areas (stainless steel solution) and the prosthesis itself (bioabsorbable polymer solution) CONCLUSION This FEA model demonstrates that physiologic forces cause posterior displacement at posterior rib fracture sites. Fixation solutions to counteract these forces need to overcome significant stresses at both the bone/prosthesis contact regions and within the prosthetic material itself. LEVEL OF EVIDENCE Epidemiologic/therapeutic study, level V.

Flow visualisation and computational prediction in thickener rake models

Abstract Although thickener rakes are essential in the transport of bed material to the underflow, few details of the flow resulting from rake action in thickeners have been published. In this investigation, flow visualisation and velocity measurements in a small-scale thickener rake model were undertaken along with computational modelling in order to better understand the flow patterns around rake components and global flow patterns in thickener beds. Experiments were carried out for a range of rake parameters in a small-scale thickener rake model and an optically clear polymeric fluid, Carbopol 980 solution, was used to simulate the sediment bed. It was found that rake blades suck material behind them as well as pushing material in front of them toward the underflow. For inward raking thickeners the overall transport pattern of material from the periphery of the thickener to the underflow was spiral motion. A computational fluid dynamics model was used to compute the flow around the rakes and showed the same transport patterns as in the experiments. Velocity measurements were undertaken in the model using particle image velocimetry. The computational predictions were found to be in good agreement with these measurements, indicating the validity of the computational model in scaling up to full-scale units.

Numerical Investigation of Hemodynamics of Lateral Cerebral Aneurysm Following Coil Embolization

Abstract Cerebral aneurysms can be treated by coil embolization within the aneurysm sac to alter the local hemodynamics and lower the wall shear stress (WSS) by making the aneurysmal flow inactive. This study investigates the hemodynamics of a lateral wall cerebral aneurysm with coils incorporating fluid-structure interaction (FSI) where the effect of apparent viscosity on thrombus formation is analysed considering the non-Newtonian behaviour of the blood. Three-dimensional transient incompressible laminar flow fields were predicted inside the aneurysm with coils at the proximal and distal neck ends with straight and curved parent vessels. The predictions showed the WSS and the effective stress were highest at the neck region, but the maximum wall displacement occurred at the dome. The coils at the distal neck performed better compared to the coils at the proximal neck in terms of reduced flow rate and higher apparent viscosity. The cerebral aneurysm with coils and curved parent vessel was subjected to higher inflow, displacement and WSS but lower apparent viscosity compared to the one with a straight parent vessel, and therefore has a greater risk of aneurysm wall damage. Hypertension increased the effective stress and displacement on the aneurysm. In patients with hypertension, more emphasis should be placed on ensuring that coils are densely packed at the distal end, especially for curved parent vessels.

Modeling of Antegrade and Retrograde Flow Into a Branch Artery of the Aorta: Implications for Endovascular Stent-Grafting and Extra-Anatomical Visceral Bypass

Purpose: To compare antegrade and retrograde flow characteristics in a branch of a conduit under typical pulsatile pressure and flows, seeking an answer to the question: “Does it matter whether inflow to a branch vessel is antegrade or retrograde?” Methods: A model was built to simulate an abdominal aorta with a branch designed to approximate a typical renal artery. Experiments were conducted to measure the flow rates from 40- and 200-mm-long inflow conduit tubes simulating a branch with antegrade and retrograde inflow configurations. For the base case with a flush origin of the branch, the pressure difference between the main conduit and branch vessel was adjusted so that the average branch flow rate was 1.22 L/min, representing average renal artery flow. A pump produced a pulsatile 5-L/min flow of a glycerol/water solution through a tube to mimic blood flow through the aorta at a mean inlet pressure of 97 mmHg, with systolic and diastolic pressures of 121 and 78 mmHg, respectively. Computational fluid dynamics (CFD) simulations were performed for the flush, antegrade inflow, and retrograde inflow cases. The CFD-predicted flow rates at the branch vessel outlet for all 3 geometries were compared with the experiments. Results: From the experiments, the mean time-average branch vessel outflow rate through a 40-mm conduit for the antegrade case was 1.22±0.01 L/min, which was the same as the retrograde case (1.21±0.01 L/min; within the experimental error). However, the branch vessel outflow flow rate through a 200-mm conduit for the retrograde case was 0.07 L/min lower than the antegrade. The results from the CFD model were in good agreement with the experiments. Conclusion: The experiments and CFD results suggest that there is negligible difference in the outflow rates to a branch vessel in antegrade and retrograde directions for 40-mm-long conduits. However, for a 200-mm conduit, the flow to a branch vessel through the retrograde path is lower than for the antegrade direction, which has implications for the insertion of branches to stent-grafts and extra-anatomical surgical bypass for visceral revascularization.

Modelling of flow through the circle of Willis and cerebral vasculature

The blood flow through the circle of Willis was modelled by coupling a Computational Fluid Dynamics (CFD) model of the circle of Willis with a branching tree model of the cerebral vasculature. The cerebral small vascular networks, which often cannot be accurately obtained by medical imaging, were modelled using a branching tree fractal model that accurately simulated the cerebral vasculature geometries and flow. This provided realistic mass flow boundary conditions for the outlet arteries of the circle of Willis. CFD was used to model the three-dimensional transient flow through a simplified and a patient-specific circle of Willis. The patient specific geometry was obtained directly from Computed Tomography (CT) images. A pipe network model was also used to predict the flow through the simplified circle of Willis and the predictions for the flow rates were within 4% of the CFD predictions. The coupled CFD and branching tree model provided useful insight into the variation of the flow through the circle of Willis.

Density segregation of granular material in a rotating cylindrical tumbler

Many mining operations use large quantities of water to separate valuable minerals from less valuable gangue. This dependence on liquid separation has an environmental impact in terms of energy and water use and also implies a cap on production due to the availability of water. To address these problems, the CSIRO has developed the CSIRO Rotational Classifier, which - by using the phenomena of rotational segregation - can quickly separate dry granular material in terms of size and/or density without the use of any liquids. The purpose of this paper is to obtain a deeper understanding of how rotational segregation can separate particles of different densities in a rotating cylinder, free from any interstitial fluids. This was accomplished by analyzing a cross section at the 20% fill level in a 50% full classifier, which contained a 50-50 ratio of glass and lead beads. The granular bed was sampled at different time intervals over a 60 second period with a classifier rotation rate of 2 rpm. These experiments resulted in a high segregation level of 0.9 in 20 seconds and 0.95 by 60 seconds (where a level of 1 implies full segregation). The results then underwent image analysis and were subsequently compared to results from a discrete element method (DEM) model where similar segregation ratios, albeit at longer timescales, were obtained. This study gave a further insight into the segregation process particularly in terms of axial formation of the segregated core which may one day be used in the separation of minerals.