Abstract
Background/Purpose
Computational fluid dynamics (CFD) have been recently adopted in many fields of cardiovascular medicine and in interventional cardiology. Using CFD analysis we compared the use of different PCI procedures, with and without the utilization of a proximal optimization technique (POT), on a complex coronary artery bifurcation.
Methods/Materials
For the analysis, we considered a hypothetic model of a left anterior descending artery-diagonal Medina 1,1,1 bifurcation type with a diameter of the proximal main branch (MB) and the side branch (SB) set at 3.5 mm and 2.5 mm, respectively. The bifurcation angle has been set to 50°. For the stent simulation, we reconstructed a third-generation, ultra-thin strut everolimus-eluting stent (ORSIRO stent, Biotronik IC, Bulack, Switzerland).
Results
The Nano-crush and the modified T techniques seem able to restore the most physiologic fluid dynamic profile. Conversely, the DK-crush and the culotte demonstrated an intermediate and worst effect, respectively. The addition of a final POT resulted favorably for both Nano-crush and reverse modified T techniques, whereas a neutral and lack of significant effects have been observed for the DK-crush and culotte technique, respectively.
Conclusion
Different double-stenting techniques (DST) have a different impact on coronary flow physiology. Both Nano-crush and modified T techniques achieved the most physiologic profile. The addition of a final POT appears to be a favourable step for both Nano-crush and modified T.
Highlights
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The optimal treatment of complex bifurcations remains in a matter of speculation. Indeed, clinical trials, designed to assess stenting techniques for coronary bifurcated lesions, have failed to prove the benefit of 2-stenting techniques.
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Over the last years, computational fluid analysis (CF) has been largely used in many field of interventional cardiology.
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Different double-stenting techniques (DST) have a different impact of the coronary flow physiology. Both the Nano-crush and the modified T achieved the most physiologic profile. The addition of a final POT appears to be a favourable step for both Nano-crush and modified T.
1
Introduction
The treatment of coronary artery bifurcations often remains a challenge in modern interventional cardiology. Several technical and clinical problems could be encountered during percutaneous coronary intervention (PCI) involving coronary artery bifurcations. Indeed, the anatomy, the type of lesion, and the technique adopted influence the final PCI result. From an interventional point of view, stent deformation or overlap, incomplete lesion coverage, stent thrombosis or restenosis, wire trapping and possible plaque shift could cause severe coronary flow problems . The evaluation of these issues is often difficult from a clinical and/or interventional point of view. Conversely, a bio-physics evaluation could be able to describe better the entity of the problem and suggest possible solutions. Over the last 15 years, computational fluid dynamic (CFD) analysis has been widely used in the field of cardiovascular medicine . An integrated mathematical, physics and clinical approach have improved understandings and applications of interventional cardiology from stent design to implantation. The analysis of the fluids dynamic and deformation forces which act on coronary artery lesions or stented vessels could improve the interventional treatment, by restoring and achieving a more physiologic hemodynamic profile. Indeed, abnormal wall shear stress (WSS) or altered coronary vascular geometry and related blood hemodynamic disturbances could promote restenosis. While simple bifurcation lesion could be treated with a single-stent technique , the optimal treatment of complex bifurcations still remains a matter of speculation . Coronary bifurcations are involved in about 20% of all PCI but the optimal management of these types of lesions remains an issue of considerable debate. The latest European Bifurcation Club (EBC) recommendations for PCI of bifurcation lesions remain to use main branch (MB) stenting with a proximal optimisation technique (POT) and provisional side branch (SB) stenting as a preferred approach. However, the document suggests that more scientific works on the optimal endovascular approach for the treatment of non-left main (LM) and LM bifurcation lesions are needed to improve the outcome of these patients .
Obviously, the aim of PCI remains to restore, as much as possible, the original blood flow physiology. In this setting, the evaluation of different PCI techniques, in terms of safety and efficacy, but also considering their success in the long-term outcome, appears mandatory. For these reasons, considering that a randomized trial, which should evaluate different double-stent techniques, in terms of hemodynamic physiology and stents stresses, appears difficult and expensive to realize, we used CFD analysis with the aim to establish, from a mathematical point of view, the best double stenting strategies for the treatment of a complex coronary artery bifurcation. In the present manuscript, we compared the impact on coronary bifurcation physiology of different PCI techniques, such as the Nano-crush , the modified T, the DK-crush and the culotte technique. We also evaluated the benefit of the proximal optimization technique (POT) in each above interventional technique.
2
Material and methods
2.1
Bifurcation model
For the computational domain analysis, we considered a hypothetic model of a left anterior descending artery-diagonal bifurcation. A schematic representation of the geometry is given in Fig. 1 . The diameter of the proximal main branch (PMB) and side (SB) were 3.5 mm and 2.5 mm, respectively . The bifurcation angle, defined as the angle between the axis of the side branch at its origin and the axis of the main branch have been set to 50° . The vessel sizes and angles adopted in the analysis have been selected considering a retrospective evaluation of our institutional database of 1453 patients (mean age 70.7 ± 12.4, 886 females) who underwent PCI for the treatment of bifurcations lesions in the last 5 years from 1° January 2011 to 1° January 2016. The bifurcation angle was measured after the diagnostic angiography using an electronic goniometer. Angles were represented as followed: 870 patients (59.8%) presented an angle approximating 50° (45°-55°), 364 patients (25.0%) an angle approximating 30° (<30°-45°) and 219 (15.0%) an angle approximating 60° (56°->70°).The model has been created resembling a Medina 1,1,1 bifurcation type lesion with a stenosis of 80% at the proximal MB, 80% at the SB, and a stenosis of 80% at the distal MB ( Fig. 2 ). Moreover, the diameter of the distal main branch (DMB) has been calculated from the diameters of the PMB and the SB by the scaling law of Finnet:
The vessel’s size of patients with a bifurcation angle of 50° were in accordance with the Finnet law in 64.0% of cases (n = 557). Coronary arteries with different diameters or bifurcation angles could influence the velocity, wall shear stress and pressure, but these variations could be considered irrelevant from a fluid dynamic point of view, so in the analysis has been not accounted.
The model was constructed using Rhinoceros v. 4.0 Evaluation (McNeel& Associates, Indianapolis, IN).
2.2
Stent simulation
For the stent simulation, we reconstructed the strut design and linkage pattern of a third-generation, everolimus-eluting stent (ORSIRO stent, Biotronik IC, Bulack, Switzerland), used in our institution. The strut thickness of this stent is characterized by a very ultrathin strut (60 μm up to 3.0 mm diameter stent and 80 μm up to 4.0 mm stent) . Computer Aided Design (CAD) software was used to reproduce the stented geometry as accurately as possible (SolidWorks 2009, Solidworks Corp, Concorn, MA). In a first step, we created the solid model of the coronary artery bifurcation and then the expanded stent geometry. For this purpose, a hollow tube with outer diameter equal with both the nominal expanded diameter and thickness of the stent was created. Then, a 2-dimensional sketch with the stent’s strut was propagated and wrapped around the tube. Through a cut out, the obtained ring of the stent was propagated axially to create the full-length, expanded model.
2.3
Virtual implantation
After placing the stent model in the correct position, the stenting procedure was performed following all the proper procedural steps: detailed data filled into the model included balloon size and dilation pressure, stent size and implantation pressure, balloon size and pressure for the kissing balloon and POT. Using Boolean operation, the modified solid model was subtracted from the bifurcation model to obtain the final geometry. We assumed that after stent deployment and implantation, both in the main and side branch, there was no residual stenosis.
2.4
CFD analysis
Over the last few years, CFD analysis has been widely used in the field of cardiovascular medicine. Altered flow conditions, such as flow reversal, shear stress, and flow separation, are well-recognized risk factors in the development of arterial diseases. In this context, both simulation and finite element (FE) analysis have been adopted as novel, non-invasive strategies, to describe and also predict hemodynamic alterations and long-term adaptation of the cardiovascular system, before and after surgical or interventional procedures. This means that CFD could be a very useful tool also for interventional cardiologists for planning and also modifying their therapeutic strategies . In the present study, to analyze the spatially resolved velocity, pressure and wall shear stress we used CFD simulations. We modeled blood as a non-Newtonian, viscous and incompressible fluid. Density was defined as 1060 kg/m3, according to the standard values cited in the literature. Blood was represented by the Navier–Stokes equation
and continuity equation
where v is the 3D velocity vector, P pressure, r density and τ the shear stress term. Instead, the Carreau model was adopted for viscosity
Given that coronary artery perfusion is primarily diastolic, at the inlet we considered a steady blood flow velocity (0.18 m/s) and pressure (10,665 Pa equivalent to 80 mmHg) . The hemodynamic parameters ( Table 1 ) that were assessed at stented coronary artery bifurcation were static pressure (Pa), Reynolds number, vorticity magnitude (1/s), stream function (Kg/s), strain rate (1/s), wall shear stress (WSS) (Pa) and skin friction coefficient. Pre- and post-stenting visual analysis was obtained also for the dynamic pressure (Pa), radial velocity (m/s), velocity magnitude (m/s), vorticity magnitude (m/s) and strain rate (1/s). The numeric grid was created from geometry using ANSYS Meshing 14.0 (Ansys, Inc., Canonsburg, PA) while the simulations were conducted using the commercial software ANSYS FLUENT 14.0 (Ansys, Inc., Canonsburg, PA).
| Physiologic model | Pathologic model | |
|---|---|---|
| Static pressure (Pa) [mmHg] | 10,670.42 [80.025] | 10,816.22 [81.121] |
| Reynolds number | 0.264 | 0.658 |
| Vorticity magnitude (1/s) | 63.920 | 6190.47 |
| Stream Function (Kg/s) | 0.089 | 0.148 |
| Strain Rate (1/s) | 64.732 | 7060.36 |
| Wall shear stress (Pa) [mmHg] | 2.185 | 5.568 |
| Skin friction coefficient | 0.165 | 0.457 |
2.5
Fluid parameters
Static and dynamic pressure in the vessel has been evaluated in Pascal. Practically, low static pressure is generally related to increased vessel wall thickness. Dynamic pressure has been defined as
Cells Reynolds Number, indicated in text as Reynolds number (Re) represents the value of the Reynolds number in a cell. As known, Re is a dimensionless parameter that is the ratio of inertia forces to viscous forces. Re has been defined as
where ρ is density, u is velocity magnitude, μ is the effective viscosity (laminar plus turbulent), and d is Cell Volume 1/2 for 2D and Cell Volume 1/3 in 3D or axisymmetric cases . Vorticity Magnitude represents the magnitude of the vorticity vector while vorticity is a measure of the rotation of a fluid element as it moves in the domain, and it has been defined as the curl of the velocity vector:
Stream function has been considered as the relationship between the streamlines and the statement of conservation of mass. Streamline could be defined as a line which is tangent to the velocity vector of the flowing fluid. For a 2D planar flow, as in or case, the stream function, ψ has been defined as
where ψ is constant along a streamline and the difference between constant values of stream function defining two streamlines is the mass rate of flow between the streamlines.
Strain rate, also known as shear rate, has been evaluated to correlate shear stress to the viscosity. WSS (Pa) has been defined as the force which is tangentially acting to the surface due to friction. As well known, low wall shear stress is related to the development of greater plaques and necrotic core progression with a constrictive remodeling whereas high wall shear stress segment develops greater necrotic core and calcium progression with expansive remodeling .
Finally, the skin friction coefficient has been considered as a non-dimensional parameter which has been defined as the ratio of the wall shear stress and the reference dynamic pressure.
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