Changes of flow characteristics by stenting in aneurysm models: influence of aneurysm geometry and stent porosity

An endovascular technique using a stent has been developed and successfully applied in the treatment of wide neck aneurysms. A stent can facilitate thrombosis in the aneurysm pouch while maintaining biocompatible passage of the parent artery. Insertion of the stent changes the flow characteristics inside the aneurysm pouch, which can affect the intra-aneurysmal embolization process. The purpose of this study is to clarify the velocity and wall shear stress changes that are caused by stenting in fusiform and lateral aneurysm models. We used a flow visualization technique that incorporated a photochromic dye in order to observe the flow fields and measure the wall shear rates. The intra-aneurysmal flow motion was significantly reduced in the stented aneurysm models. Coherent inflow along the distal wall of the aneurysm was diminished and inflow was distributed along the pores of the stent wall in the stented models. Also, sluggish intra-aneurysmal vortex motion was well maintained in the stented aneurysm models during the deceleration phase. A less porous stent generally reduced the intraneurysmal fluid motion further, but the porosity effect was not significant. The magnitude and pulsatility of the wall shear rate were reduced by stenting, and the reductions were more significant in the lateral aneurysm models compared to the fusiform aneurysm models. The hemodynamic changes that were observed in our study can help explain the efficacy of in vivo thrombus formation caused by stenting. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8780-y, 8719Xx     

The Stress–Strain Behavior of Coronary Stent Struts is Size Dependent

Coronary stents are used to re-establish the vascular lumen and flow conditions within the coronary arteries; the typical thickness of a stent strut is 100 m, and average grain sizes of approximately 25 m exist in stainless steel stents. The purpose of this study is to investigate the effect of strut size on the stress strain behavior of 316 L stainless steel. Other materials have shown a size dependence at the micron size scale; however, at present there are no studies that show a material property size dependence in coronary stents. Electropolished stainless steel stent struts within the size range of 60–500 m were tensile tested. The results showed that within the size range of coronary stent struts a size dependent stress–strain relationship is required to describe the material. Finite element models of the final phase of fracture, i.e., void growth models, explained partially the reason for this size effect. This study demonstrated that a size based stress–strain relationship must be used to describe the tensile behavior material of 316 L stainless steel at the size scale of coronary stent struts. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr, 8780Rb, 8719Uv     

Enhancement of Stent-Induced Thromboembolism by Residual Stenoses: Contribution of Hemodynamics

In vitro stent-induced thromboembolism was altered by the presence of residual stenoses placed upstream or placed upstream and downstream of the stent. Heparinized (3 /ml) bovine blood was gravity fed through a conduit with a deployed coronary stent. Embolism was continuously monitored using a light-scattering microemboli detector, and the thrombus accumulated on the stent at the conclusion of the experiment was assessed gravimetrically. Gaussian stenoses (75% reduction in the cross-sectional area) were placed upstream or upstream and downstream of the stent to alter flow characteristics in the stent region. The presence of stenoses enhanced embolization from the stent in all cases, while end-point thrombus accumulation on the stent decreased with only an upstream stenosis present, and increased when upstream and downstream stenoses were present. Computational fluid dynamics with and without hypothetical model thrombi were used to ascertain the alterations in the flow environment caused by the stenoses and thrombi. Combining the computed hemodynamic parameters with experimental results indicated that increased radial transport of blood components and low wall shear stress provided by the stenoses and thrombi may explain the enhancement of end-point thrombus accumulation. Analysis further showed that thrombi growing at the stenosis-induced reattachment and separation points will be subjected to high shear forces which may explain the increased embolism when stenoses are present. © 2000 Biomedical Engineering Society. PAC00: 8719Uv, 8719Xx, 8780-y     

Fluid and solid mechanical implications of vascular stenting

Vascular stents have emerged as an effective treatment for occlusive vascular disease. Despite their success and widespread use, outcomes for patients receiving stents are still hampered by thrombosis and restensosis. As arteries attempt to adapt to the mechanical changes created by stents, they may in fact create a new flow-limiting situation similar to that which they were intended to correct. In vitro fluid mechanics and solid mechanics studies of stented vessels have revealed important information about how stents alter the mechanical environment in the arteries into which they are placed. Adverse nonlaminar flow patterns have been demonstrated as well as remarkably high stress concentrations in the vessel wall. In vivo studies of stented vessels have also shown a strong relationship between stent design and their dynamic performance within arteries. Alterations in pressure and flow pulses distal to the stent have been observed, as well as regional changes in vascular compliance. Considering the influence of flow and stress on the vascular response and the suboptimal clinical outcomes associated with stenting, knowledge gained from stent/artery mechanics studies should play an increasingly important role in improving the long-term patency of these devices. © 2002 Biomedical Engineering Society. PAC2002: 8719Rr, 8780-y, 8719Uv     

A rapid and computationally inexpensive method to virtually implant current and next-generation stents into subject-specific computational fluid dynamics models

Computational modeling is often used to quantify hemodynamic alterations induced by stenting, but frequently uses simplified device or vascular representations. Based on a series of Boolean operations, we developed an efficient and robust method for assessing the influence of current and next-generation stents on local hemodynamics and vascular biomechanics quantified by computational fluid dynamics. Stent designs were parameterized to allow easy control over design features including the number, width and circumferential or longitudinal spacing of struts, as well as the implantation diameter and overall length. The approach allowed stents to be automatically regenerated for rapid analysis of the contribution of design features to resulting hemodynamic alterations. The applicability of the method was demonstrated with patient-specific models of a stented coronary artery bifurcation and basilar trunk aneurysm constructed from medical imaging data. In the coronary bifurcation, we analyzed the hemodynamic difference between closed-cell and open-cell stent geometries. We investigated the impact of decreased strut size in stents with a constant porosity for increasing flow stasis within the stented basilar aneurysm model. These examples demonstrate the current method can be used to investigate differences in stent performance in complex vascular beds for a variety of stenting procedures and clinical scenarios.     

Alteration of hemodynamics in aneurysm models by stenting: Influence of stent porosity

Recent developments in minimally invasive approach to cerebrovascular diseases include the placement of stents in arteries for treatment of aneurysms. Preliminary clinical observations and experimental studies have shown that intravascular stents traversing the orifice may lead to thrombosis and subsequent occlusion of the aneurysm. The alterations in vessel local hemodynamics due to the introduction of a stent are not yet well understood. We investigated changes in local hemodynamics resulting from stent implantation. Pulsatile flow patterns in an experimental flow appraratus were visualized using laser-induced fluorescence of rhodamine dye. The test cells were constructed in a rectangular shape to facilitate an undisturbed longitudinal view of flow patterns in parent vessel and aneurysm models with and without porous stents. Woven nitinol stents of various porosities (76%, 80%, 82%, and 85%) were investigated. The selected fluid dynamic similarity parameters (Reynolds and Womersley numbers) represented conditions usually found in high-flow, larger arteries in humans (such as the carotid artery) and low-flow, smaller arteries (such as the vertebral artery). The mean Reynolds number for the larger arteries was 180, with maximum/minimum values of 490/−30 and the Womersley number was 5.3. The mean Reynolds number for the smaller arteries was 90, with maximum/minimum values of 230/2, and the Womersley number was 2.7. For the larger arteries modeled, placement of a stent of the lowest porosity across the aneurysm orifice resulted in reduction of aneurysmal vortex speed and decreased interaction with parent vessel flow. For smaller arteries, a stent of the same porosity led to a substantial reduction of parent vessel/aneurysmal flow interaction and the appearance of a nonrecirculating crescent of fluid rich in rhodamine dye in the aneurysm dome. Our results can help explainin vivo thrombus formation within an aneurysm after placement of a stent that is compatible with local hemodynamics.     

Construction of 3 animal experimental models in the development of honeycomb microporous covered stents for the treatment of large wide-necked cerebral aneurysms

The treatment of large or wide-necked cerebral aneurysms is extremely difficult, and carries a high risk of rupture, even when surgical or endovascular methods are available. We are developing novel honeycomb microporous covered stents for treating such aneurysms. In this study, 3 experimental animal models were designed and evaluated quantitatively before preclinical study. The stents were prepared using specially designed balloon-expandable stents (diameter 3.5–5.0 mm, length 16–28 mm) by dip-coating to completely cover their struts with polyurethane film (thickness 20 µm) and microprocessing to form the honeycomb pattern after expansion. (1) In an internal carotid artery canine model (n = 4), all stents mounted on the delivery catheter passed smoothly through the tortuous vessel with minimal arterial damage. (2) In an the large, wide-necked, outer-sidewall aneurysm canine model, almost all parts of the aneurysms had embolized immediately after stenting (n = 4), and histological examination at 2 months revealed neointimal formation with complete endothelialization at all stented segments and entirely organized aneurysms. (3) In a perforating artery rabbit model, all lumbar arteries remained patent (n = 3), with minimal change in the vascular flow pattern for over 1 year, even after placement of a second, overlapping stent (n = 3). At 2 months after stenting, the luminal surface was covered with complete thin neointimal formation. Excellent embolization performance of the honeycomb microporous covered stents without disturbing branching flow was confirmed at the aneurysms in this proof-of-concept study.     

Intramyocardial Influences on Blood Flow Distributions in the Myocardial Wall

Flow velocity wave forms of coronary arterial inflow and venous outflow of myocardium are influenced by cardiac contraction and relaxation: arterial flow is exclusively diastolic; venous outflow is systolic. We first discuss the intramyocardial microvascular flow dynamics, then present some results of visualization of transmural microvessels by our needle-probe charge coupled device (CCD) microscope, along with an interpretation of the arteriolar and venular hemodynamics through a cardiac cycle. After describing a hierarchical system of coronary microvessels (small artery, arteriole, and capillary), we emphasize the importance of spatial heterogeneity of blood supply to myocardium with reference to a minimal vascular control unit (400 m). An understanding of mechanoenergetic interaction is fundamentally important to an understanding of intramyocardial coronary circulation, and the Physiome Project will provide powerful tools for understanding the integrated role of the intramyocardial microcirculation system. © 2000 Biomedical Engineering Society. PAC00: 8719Hh, 8719Ff, 8719Tt     

Filament lengths in frog semitendinosus and tibialis anterior muscle fibres

Summary In frog semitendinosus muscle the descending limb of the length-tension curve is shifted rightward relative to that of tibialis anterior. Both the plateau right corner and the zero-force intercept are equally shifted. To investigate the reason for this shift, we compared filament lengths in the two muscles. Single fibres were mechanically skinned, stretched to reveal filaments clearly, incubated in a solution containing one of several antibodies to enhance filament visualization, and examined by electron microscopy. We found no differences of filament length. Thick filament lengths were 1.62 and 1.61 m, respectively. I-segment lengths were measured by two methods. With the first, filament length was the same for both muscles, 1.95 or 1.98 m, depending on the value taken for the troponin repeat; with the second it was 1.92 and 1.94 m, respectively, for the two muscles. These differences are insignificant. Thus, the reported differences of shape of the length-tension curve are not explainable in terms of differences of filament length.     

Directional Wall Strength in Saccular Brain Aneurysms from Polarized Light Microscopy

The aneurysm wall, which must withstand arterial blood pressure, is composed of layered collagen. Wall strength is related to both collagen fiber strength and orientation. When the aneurysm enlarges, the amount and organization of the collagen fibers change, potentially increasing the risk of rupture. We studied the directional organization and molecular strength of the collagen fibers layer by layer across the walls of four aneurysms in order to measure their mechanical integrity. The technique incorporates the birefringent properties of collagen, enabling us to use linearly polarized light for measuring the orientation of the fibers, and the Sénarmont compensator to measure the birefringence and thus mechanical strength. Intact aneurysms were obtained at autopsy, fixed at physiological pressure, sectioned at 4 m, and stained with 0.05% picrosirius red. By combining birefringence and orientation data we estimated tensile strength as a function of direction on the aneurysmal wall. The average breaking strength of the wall ranged from 0.73 to 1.9 MPa. Comparing the weakest to the strongest direction, the breaking strength varied by a factor of up to 2×, implying a significant degree of mechanical anisotropy. © 2000 Biomedical Engineering Society. PAC00: 8719Rr, 8719Uv, 8764Rr