Stent Geometry Research Articles

Influence of translational disorder on the mechanical properties of hexachiral honeycomb systems

Chiral honeycombs are one of the most important and oft studied classes of auxetic systems due to their vast number of potential applications which range from stent geometries to composites, sensors and satellite components. Despite numerous works on these systems, however, relatively few studies have investigated the effect of structural disorder on these structures. In view of this, in this study, the effect of translational disorder on hexachiral honeycombs were investigated through a Finite Element approach. It was found that this type of disorder has minimal effect on the Poisson's ratios of these systems provided that the ligament length to thickness ratio remains sufficiently large and the overall length to width ratio of the disordered system does not differ considerably from that of its ordered counterpart. This makes it ideal for use in various applications and products such as sandwich composites with an auxetic core.

Early aortic transcatheter heart valve thrombosis: diagnostic value of contrast-enhanced multidetector computed tomography.

Early stent valve thrombosis after transcatheter aortic valve implantation (TAVI) is a rare complication, which is diagnosed based on the appearance of clinical symptoms of heart failure and echocardiographic findings. After TAVI, transthoracic echocardiography is performed to assess transcatheter heart valve (THV) function. However, preliminary reports indicate the potential additive clinical value of multidetector computed tomography (MDCT) for the diagnosis of THV thrombosis. We sought to determine the value of MDCT for the diagnosis of THV thrombosis and the frequency of this complication after balloon-expandable TAVI. MDCT was performed in 140 patients within 1 to 3 months after TAVI with the Edwards Sapien XT THV to assess the presence of THV thrombosis and THV stent geometry. Post-TAVI MDCT identified THV thrombosis in 5 patients (4%). Of note, 4 of these patients were asymptomatic and had a normal transthoracic echocardiographic examination without signs of thrombus formation or flow obstruction. In patients with THV thrombosis, a left ventricular ejection fraction of <35% was present in 3 (60%), whereas 2 (40%) did not receive standard post-TAVI dual-antiplatelet therapy. Neither THV underexpansion nor THV noncircularity was detected in patients with THV thrombosis. Post-TAVI MDCT is a valuable tool for the diagnosis of THV thrombosis, and this complication seems to be more common than previously anticipated. Larger studies are required to identify specific risk factors of THV thrombosis.

Mechanical properties of different airway stents

Airway stents improve pulmonary function and quality of life in patients suffering from airway obstruction. The aim of this study was to compare main types of stents (silicone, balloon-dilated metal, self-expanding metal, and covered self-expanding metal) in terms of their mechanical properties and the radial forces they exert on the trachea. Mechanical measurements were carried out using a force gauge and specially designed adaptors fabricated in our lab. Numerical simulations were performed for eight different stent geometries, inserted into trachea models. The results show a clear correlation between stent diameter (oversizing) and the levels of stress it exerts on the trachea. Compared with uncovered metal stents, metal stents that are covered with less stiff material exert significantly less stress on the trachea while still maintaining strong contact with it. The use of such stents may reduce formation of mucosa necrosis and fistulas while still preventing stent migration. Silicone stents produce the lowest levels of stress, which may be due to weak contact between the stent and the trachea and can explain their propensity for migration. Unexpectedly, stents made of the same materials exerted different stresses due to differences in their structure. Stenosis significantly increases stress levels in all stents.

Stent platforms anno 2015: is there still a place for bare metal stents at the front line?

Coronary stenting is the preferred treatment for symptomatic stenotic coronary artery disease since the mid-1990s. Early limitations of bare metal stents (BMS), namely (sub)acute stent thrombosis, were relatively quickly mastered by recognising the importance of optimal stent expansion and the introduction of dual antiplatelet therapy. The main limitation affecting the long-term outcome of BMS, namely excessive neointima formation causing stent restenosis, was way more challenging. The quest for solving this problem triggered the proposal of a variety of strategies including intravascular ultrasound-guided stent optimisation, rota-stenting, brachytherapy and local and systemic pharmacotherapy. Finally, drug-eluting stents (DES) were successfully developed to overcome this problem at the beginning of the millennium. Restenosis rates have dropped dramatically since and DES are superior to BMS in almost every situation [1]. This had established DES as standard of care. While DES do not induce the fast neointimal growth that leads to restenosis in BMS typically within the first 6 months after implantation, we recognised a new enemy: (very) late stent failure beyond 12 months, caused by development of early neoatherosclerosis within the DES on the one hand [2] and impaired healing on the other hand. Against this background, the quest for improving stent technology is ongoing. The concept of BMS, where both neoatherosclerosis and impaired healing play a less apparent clinical role, is again of interest. Especially as recent advances in material and production methods have created new possibilities in stent design and allow for the production of very thin metal stents struts. This is of note, as the pivotal role of stent design and geometry on neointima formation and stent thrombosis is well recognised. More specifically, experimental and clinical studies have confirmed again and again the importance of reducing stent strut thickness, with thinner struts, eliciting less restenosis than thicker struts in both BMS and DES [3]. Thicker struts increase blood flow separation, stagnation and reattachment which can ultimately lead to an increased rate of stent thrombosis [4]. In the current issue of the Netherlands Heart Journal, Suttorp et al. present a result of today’s quest for optimised stent design and deserve to be congratulated on their work. The SOLSTICE registry [5] included a series of patients treated in four Benelux centres with the SolarFlex BMS (DISA Vascular, Cape Town, South Africa, CE approval February 2014), a balloon-expandable Cobalt Cromium stent with remarkable thin struts of 65 µm. The clinical outcome up to 6 months is encouraging with a major adverse cardiac event (MACE) rate of 5.8 %. SOLSTICE is a registry, with all its advantages and disadvantages. The registry focuses on practical aspects such as device success. The patient population is more likely to reflect a real-world cathlab population with a considerable amount of patients with acute coronary syndromes (47.1 %) and diabetes (29 %) being included. However, relatively simple lesions were selected for treatment as reflected by the use of single stent (as per protocol), stent diameter (mean 3.1 mm), stent length (mean 17.4 mm) and low grade lesion angulation and tortuosity. Follow-up is clinical and extends up to 6 months, which is known to be the period when the peak of neointimal growth after BMS implantation has been reached [6]. Importantly, no follow-up angiography has been performed, which is in line with the contemporary focus on clinical outcome (as opposed to the focus on angiographic outcome in the early days of BMS) and the notion that follow-up angiography in itself can trigger revascularisation events. This ‘oculo-stenotic reflex’ has been shown to potentially double the revascularisation rate. When comparing MACE and target-vessel revascularisation rates of contemporary studies with clinical follow-up of BMS studies from the past with mandatory angiographic follow-up, this has to be taken into account. The SOLSTICE registry reports a target-vessel failure rate of 5 % which is remarkably low for BMS. Having said that, it would have been particularly interesting to gain insights into the pathophysiological mechanism of the stent failure cases which could further increase the knowledge about the role of strut thickness in vascular response and how to avoid unfavourable outcome. What can we expect in 2015—will we be back to the future with the renaissance of sophisticated BMS? Certainly, current developments in stent design make for an interesting time in the catheterisation laboratory. There is innovation, allowing the realisation of more challenging stent designs, both in stent geometry as well as material characteristics. We will be able to offer our patients ‘stents to stay’ and bioresorbable ‘stents to disappear’, a concept that has only just begun. Carefully designed mechanistic studies and clinical observations will be needed to determine the role and most appropriate indications for their use.

Geometry and respiratory-induced deformation of abdominal branch vessels and stents after complex endovascular aneurysm repair

This study quantified the geometry and respiration-induced deformation of abdominal branch vessels and stents after fenestrated (F-) and snorkel (Sn-) endovascular aneurysm repair (EVAR). Twenty patients (80% male; mean age, 75.2 ± 7.4 years; mean aneurysm diameter, 6.2 ± 1.8 cm) underwent computed tomography angiography during inspiratory and expiratory breath hold protocols after F-EVAR (n = 11) or Sn-EVAR (n = 9). Centerlines for the aorta and visceral vessels were extracted from three-dimensional models. Branch angles were computed relative to the orthogonal plane at the branch ostia, and end-stent angles of the left renal artery (LRA) and right renal artery (RRA) were computed relative to the distal stent orientation. The radius of peak curvature was defined by the circumscribed circle at the highest curvature. Sn-renal branches were more downward-angled than F-renal branches (P < .04). At the distal ends of the RRA stents, Sn-RRAs were angled greater than F-RRAs (P < .03) and had a smaller radius of peak curvature (P < .03). With expiration, the end-stent angle of Sn-LRAs increased by 4° ± 4° (P < .02) and exhibited a significant reduction of radius of curvature (P < .04). The unstented celiac arteries were more downward-angled (P < .02, inspiration), with a smaller radius of curvature (P < .00001), than the unstented superior mesenteric arteries. With expiration, the celiac arteries angled upwards by 9° ± 9° (P < .0005), which was greater than the superior mesenteric arteries (P < .03). At a median postoperative follow-up of 12.6 months (range, 1.0-37.1 months), branch vessel patency was 100%, serum creatinine levels remained stable, and one reintervention was required for a type III endoleak at the main body-LRA stent interface. Sn-renals were angled more inferiorly at the branch and more angulated at the stent end than F-renals due to stent placement strategies. Sn-LRAs exhibited a significant change in end-stent angle and curvature during respiration, a finding that may compromise long-term durability for parallel stent graft configurations. Further investigation is warranted to better optimize anatomic, patient, and branch vessel stent selection between fenestrated and snorkel strategies and their relationship to long-term patency.

S0220301 脳動脈瘤治療用ステントにおける自己組織化マップを用いた最適形状解析

As evaluation of hemodynamics in an aneurysm, a lot of parameters have been introduced. However, the relationship between stent geometry and each hemodynamic parameter is not clear enough. We applied Self-organizing Maps (SOM) to search the detailed relationship between stent strut and hemodynamic parameters. Two thousands of stent design candidates evaluated with five hemodynamic parameters and 25 stent geometry parameters were stored during the optimization process. By using SOM software, 2,000 design candidates were visualized into 30 2D-maps while preserving their own features in the original 30-dimensional design space. Each map was divided into 20 clusters to characterize stent design and flow reduction. By comparing each cluster, critical design for reduction effect of each hemodynamic parameter is suggested. Strut placement on the inflow area is important to reduce maximum shear rate. In addition to the inflow area, strut on the outflow area is required to reduce velocity, vorticity, and average shear rate.

Feasibility of a priori numerical assessment of plaque scaffolding after carotid artery stenting in clinical routine: proof of concept.

Carotid artery stenting (CAS) is an alternative procedure for the treatment of severely stenosed carotid artery lesions in high-risk patients. Appropriate patient selection and stent design are paramount to achieve a low stroke and death rate in these complex high-risk procedures. This study introduces and evaluates a novel virtual, patient-specific, pre-operative environment to quantify scaffolding parameters based on routine imaging techniques. Two patients who underwent CAS with two different sizes of the Acculink stent (Abbott Vascular, Santa Clara, CA, USA) were studied. Pre-operative data were used to build the numerical models for the virtual procedure. Numerical results were validated with post-operative angiography. Using novel virtual geometrical tools, incomplete stent apposition, free cell area and largest fitting sphere in the stent cell were evaluated in situ as quantitative measures of successful stent placement and to assess potential risk factors for CAS complications. A quantitative validation of the numerical outcome with post-operative images noted differences in lumen diameter of 5.31 ± 8.05% and 4.12 ± 9.84%, demonstrating the reliability of the proposed methodology. The quantitative measurements of the scaffolding parameters on the virtually deployed stent geometry highlight the variability of the device behavior in relation to the target lesion. The free cell area depends on the target diameter and oversizing, while the largest fitting spheres and apposition values are influenced by the local concavity and convexity of the vessel. The proposed virtual environment may be an additional tool for endovascular specialists especially in complex anatomical cases where stent design and positioning may have a higher impact on procedural success and outcome.

Computational analysis of the radial mechanical performance of PLLA coronary artery stents

Stents have been an effective tool to restore and maintain the patency of narrowed blood vessels, but they must have sufficient radial strength. Biodegradable stent materials have substantially lower mechanical properties than permanent stents. The stent geometry and material properties must be considered simultaneously when assessing stent performance. Material tests were performed to determine the mechanical characteristics of high-molecular-weight poly-l-lactic acid (PLLA). The results were used to calibrate an anisotropic elastic-plastic material model. Three distinct geometries were analysed with a range of material stiffness values in a finite element analysis to investigate their comparative effect on the radial strength, recoil, and radial stiffness. The performance of the different geometries varies substantially, with one particular geometry, with the highest material stiffness of 9GPa, exceeding the desired radial strength of 300mmHg.

On the RF heating of coronary stents at 7.0 Tesla MRI.

Examine radiofrequency (RF) induced heating of coronary stents at 7.0 Tesla (T) to derive an analytical approach which supports RF heating assessment of arbitrary stent geometries and RF coils. Simulations are performed to detail electromagnetic fields (EMF), local specific absorption rates (SAR) and temperature changes. For validation E-field measurements and RF heating experiments are conducted. To progress to clinical setups RF coils tailored for cardiac MRI at 7.0T and coronary stents are incorporated into EMF simulations using a human voxel model. Our simulations of coronary stents at 297 MHz were confirmed by E-field and temperature measurements. An analytical solution which describes SAR(1g tissue voxel) induced by an arbitrary coronary stent interfering with E-fields generated by an arbitrary RF coil was derived. The analytical approach yielded a conservative estimation of induced SAR(1g tissue voxel) maxima without the need for integrating the stent into EMF simulations of the human voxel model. The proposed analytical approach can be applied for any patient, coronary stent type, RF coil configuration and RF transmission regime. The generalized approach is of value for RF heating assessment of other passive electrically conductive implants and provides a novel design criterion for RF coils.

TCT-442 Comparison of contemporary DES of different stent geometry and Absorb in a Swine Carotid – Jugular Thrombogenicity Shunt Model

Polymer coatings of drug eluting stents (DES) have been shown to offer a protective barrier against acute thrombus formation compared with bare metal stents (BMS). While this anti-thrombotic function has been assigned to contemporary DES utilizing permanent polymeric coatings, the impact of

Accuracy and reproducibility of patient-specific hemodynamic models of stented intracranial aneurysms: report on the Virtual Intracranial Stenting Challenge 2011.

Validation studies are prerequisites for computational fluid dynamics (CFD) simulations to be accepted as part of clinical decision-making. This paper reports on the 2011 edition of the Virtual Intracranial Stenting Challenge. The challenge aimed to assess the reproducibility with which research groups can simulate the velocity field in an intracranial aneurysm, both untreated and treated with five different configurations of high-porosity stents. Particle imaging velocimetry (PIV) measurements were obtained to validate the untreated velocity field. Six participants, totaling three CFD solvers, were provided with surface meshes of the vascular geometry and the deployed stent geometries, and flow rate boundary conditions for all inlets and outlets. As output, they were invited to submit an abstract to the 8th International Interdisciplinary Cerebrovascular Symposium 2011 (ICS'11), outlining their methods and giving their interpretation of the performance of each stent configuration. After the challenge, all CFD solutions were collected and analyzed. To quantitatively analyze the data, we calculated the root-mean-square error (RMSE) over uniformly distributed nodes on a plane slicing the main flow jet along its axis and normalized it with the maximum velocity on the slice of the untreated case (NRMSE). Good agreement was found between CFD and PIV with a NRMSE of 7.28%. Excellent agreement was found between CFD solutions, both untreated and treated. The maximum difference between any two groups (along a line perpendicular to the main flow jet) was 4.0 mm/s, i.e. 4.1% of the maximum velocity of the untreated case, and the average NRMSE was 0.47% (range 0.28-1.03%). In conclusion, given geometry and flow rates, research groups can accurately simulate the velocity field inside an intracranial aneurysm-as assessed by comparison with in vitro measurements-and find excellent agreement on the hemodynamic effect of different stent configurations.

NUMERICAL STUDY ON THE EFFECTS OF THE NUMBER AND GEOMETRIES OF DRUG-ELUTING STENT LINKS ON THE DRUG CONCENTRATION

The paper investigated the effects of the link numbers and geometries of drug-eluting stents (DESs) on the hemodynamics and the distribution of drug concentration, and the final results could be used as a guide for the optimization of DES design. Four 3D virtual stents with different numbers and geometries of links were modeled and numerically studied with respect to the distribution of wall shear stress (WSS) and drug concentration. Results have showed (1) The geometries of links have big impact on the distribution of WSS regions and the uniform drug concentration distribution on the vascular wall, but little effect on the mean drug concentration on the vascular wall; the S-shaped link stents had less low-WSS regions and the reason is that the flow direction was considered in the design of this type of stents. (2) The drug concentration distribution on the vascular wall had remarkable difference from that in blood. (3) The 6S link stent had the least low-WSS regions and with high and uniformly distributed drug concentration, so this type of stent was concluded to be the best design in four stents. Overall, it is not reliable to use the drug concentration in blood to deduce the drug concentration on the vascular wall. In DES design, the configuration of links should be, as much as possible, in line with the blood flow direction, so it can decrease the low-WSS regions. Meanwhile, in order to get more uniformly distributed drug on the vascular wall, the distribution of drug on the stent surface should be taken into account. Additionally, because of the increment of contact area between the stents and vascular wall, the drug concentration on the vascular wall increased with the number of links.

Development of biodegradable magnesium alloy stents with coating

Biodegradable stents are attracting the attention of many researchers in biomedical and materials research fields since they can absolve their specific function for the expected period of time and then gradually disappear. This feature allows avoiding the risk of long-term complications such as restenosis or mechanical instability of the device when the vessel grows in size in pediatric patients. Up to now biodegradable stents made of polymers or magnesium alloys have been proposed. However, both the solutions have limitations. The polymers have low mechanical properties, which lead to devices that cannot withstand the natural contraction of the blood vessel: the restenosis appears just after the implant, and can be ascribed to the compliance of the stent. The magnesium alloys have much higher mechanical properties, but they dissolve too fast in the human body. In this work we present some results of an ongoing study aiming to the development of biodegradable stents made of a magnesium alloy that is coated with a polymer having a high corrosion resistance. The mechanical action on the blood vessel is given by the magnesium stent for the desired period, being the stent protected against fast corrosion by the coating. The coating will dissolve in a longer term, thus delaying the exposition of the magnesium stent to the corrosive environment. We dealt with the problem exploiting the potentialities of a combined approach of experimental and computational methods (both standard and ad-hoc developed) for designing magnesium alloy, coating and scaffold geometry from different points of views. Our study required the following steps: i) selection of a Mg alloy suitable for stent production, having sufficient strength and elongation capability; ii) computational optimization of the stent geometry to minimize stress and strain after stent deployment, improve scaffolding ability and corrosion resistance; iii) development of a numerical model for studying stent degradation to support the selection of the best geometry; iv) optimization of the alloy microstructure and production of Mg alloy tubes for stent manufacturing; v) set up, in terms of laser cut and surface finishing, of the procedure to manufacture magnesium stents; vi) selection of a coating able to assure enough corrosion resistance and computational evaluation of the coating adhesion. In the paper the multi-disciplinary approach used to go through the steps above is summarized. The obtained results suggest that developed methodology is effective at designing innovative biomedical devices.

Simulation of transcatheter aortic valve implantation through patient-specific finite element analysis: Two clinical cases

Transcatheter aortic valve implantation (TAVI) is a minimally invasive procedure introduced to treat aortic valve stenosis in elder patients. Its clinical outcomes are strictly related to patient selection, operator skills, and dedicated pre-procedural planning based on accurate medical imaging analysis. The goal of this work is to define a finite element framework to realistically reproduce TAVI and evaluate the impact of aortic root anatomy on procedure outcomes starting from two real patient datasets. Patient-specific aortic root models including native leaflets, calcific plaques extracted from medical images, and an accurate stent geometry based on micro-tomography reconstruction are key aspects included in the present study. Through the proposed simulation strategy we observe that, in both patients, stent apposition significantly induces anatomical configuration changes, while it leads to different stress distributions on the aortic wall. Moreover, for one patient, a possible risk of paravalvular leakage has been found while an asymmetric coaptation occurs in both investigated cases. Post-operative clinical data, that have been analyzed to prove reliability of the performed simulations, show a good agreement with analysis results. The proposed work thus represents a further step towards the use of realistic computer-based simulations of TAVI procedures, aiming at improving the efficacy of the operation technique and supporting device optimization.

Reproducibility of the Carpet View system: a novel technical solution for display and off line analysis of OCT images.

The optical coherence tomography (OCT) evaluation of the stent anatomy requires the inspection of sequential cross section (CS). However stent coils cannot be appreciated in the conventional format as the OCT CS simply display stent struts, that are poorly representative of the stent architecture. The aim of the present study was to validate a new software (Carpet View), which unfolds the stented segment, reconstructing it as an open structure and displaying the stent meshwork. 21 patients were studied with frequency domain OCT after the deployment of different stents: seven bio-absorbable scaffolds (Dream), seven bare metal stent (Vision/Multilink8), seven drug eluting stent (Cre8). Conventional CS reconstructions were post-processed with the Carpet View software and analyzed by the same reader twice (intra-observer variability) and by two different readers (inter-observer variability). A small average difference in the number of all struts was obtained with the two methods (conventional vs carpet view reconstruction). Using the carpet view, high intra-observer and inter-observer correlations were found for the number of struts obtained in each coil. The Pearson correlation values were 0.98 (p = 0.0001) and 0.96 (p = 0.0001) respectively. The same number of coils was found when analyses were repeated by the same reader or by a different reader whilst mild differences in the count of stent junctions were reported. The Carpet View can be used to address the stent geometry with high reproducibility. This approach enables the matching of the same stent portion during serial time points and promises to improve the stent assessment.

FEA study on the stress distributions in the polymer coatings of cardiovascular drug-eluting stent medical devices.

Cardiovascular drug-eluting stents (DES) are widely applied medical products to treat diseased narrowed arteries. Despite their wide application, there still are many clinical adverse effects associated with DES implantation. One of the major issues is that the coatings comprised of drug and polymer phases are often delaminated during the deployment of the stent, which can lead to more serious clinical complications. In the present work, we conducted a 3D finite-element analysis (FEA) computational study to quantitatively estimate the stress distributions in the coating components of DES devices. To adequately represent the skeleton design of modern DES products, we adopted the strut geometry of a SYNERGY stent along with a full coating of poly(lactic-co-glycolic) acid. The FEA computation results clearly indicate that the curved regions (i.e., kink) are subject to much higher stress accumulation in the coating. In addition, it was found that the local shear and normal stress distribution profiles in the polymer coatings are different from those based on von-Mises stresses near the kink area.

Uncertainty assessment of imaging techniques for the 3D reconstruction of stent geometry

This paper presents a quantitative assessment of uncertainty for the 3D reconstruction of stents. This study investigates a CP stent (Numed, USA) used in congenital heart disease applications with a focus on the variance in measurements of stent geometry. The stent was mounted on a model of patient implantation site geometry, reconstructed from magnetic resonance images, and imaged using micro-computed tomography (CT), conventional CT, biplane fluoroscopy and optical stereo-photogrammetry. Image data were post-processed to retrieve the 3D stent geometry. Stent strut length, separation angle and cell asymmetry were derived and repeatability was assessed for each technique along with variation in relation to μCT data, assumed to represent the gold standard. The results demonstrate the performance of biplanar reconstruction methods is comparable with volumetric CT scans in evaluating 3D stent geometry. Uncertainty on the evaluation of strut length, separation angle and cell asymmetry using biplanar fluoroscopy is of the order ±0.2mm, 3° and 0.03, respectively. These results support the use of biplanar fluoroscopy for in vivo measurement of 3D stent geometry and provide quantitative assessment of uncertainty in the measurement of geometric parameters.

In vitro encrustation of a semi-permanent polymer-covered nitinol ureter stent: an artificial urine model

To measure and compare the percentage of surface and luminal thickness of encrustation in Allium and conventional double J ureteric stents after exposure for 6 weeks to an accelerated encrustation model. An artificial urine solution was prepared and three stents were immersed into each of six containers allocated to each stent type, representing each week of encrustation. Slight agitation was accomplished by placing a magnetic stirrer at the bottom of each container. Images were obtained by examination under a stereomicroscope and analyzed with the aid of specialized image analysis software (Image J). By week 2, nearly 100 % of the stent surface was covered by a thin layer of encrustation, gradually increasing in thickness through weeks 3-6. On completion of 6 weeks of encrustation, the 10 mm length double J stent specimens did not show visible encrustation, while the 60 mm long Allium stents showed 100 % surface coverage. This was most evident in the mid-section of the stents compared to the ends, suggesting a correlation between stent length and encrustation formation. There was also no blockage of the lumen of either stents between weeks 1-6. The designed accelerated encrustation model was successful and showed 80 % surface coverage after 6 weeks. In our study, there appears to be a slightly reduced level of surface encrustation to that of earlier reports. A correlation between stent length and geometry was suggested. This model may be used to compare encrustation for a variety of polymeric stent materials.

Contribution of Mechanical and Fluid Stresses to the Magnitude of In-stent Restenosis at the Level of Individual Stent Struts

Structural and fluid stresses acting on the artery wall are proposed as mechanical mediators of in-stent restenosis (ISR). This study reports an investigation of the correlation between stresses obtained from computational simulations with the magnitude of ISR at the level of individual stent struts observed in an in vivo model of restenosis. Structural and fluid dynamic analyses were undertaken in a model based on volumetric micro-CT data from an in vivo stent deployment in a porcine right coronary artery. Structural and fluid mechanics were compared with histological data from the same stented vessel sample. Interpretation of the combined data at the level of individual stent struts was possible by identifying the location of each 2-D histological section within the 3-D micro-CT volume. Linear correlation between structural and fluid stimuli and neointimal thickness at the level of individual struts is less clear when individual stimuli are considered [compressive force (CF), R 2 = 0.19, wall shear stress (WSS), R 2 = 0.25, oscillatory shear index (OSI), R 2 = 0.28]. Closer correlation is observed if combined structural and fluid stimuli are assumed to stimulate ISR (CF/WSS, R 2 = 0.64). The use of micro-CT to characterise stent geometry after deployment enhances the clinical relevance of computational simulations, allowing direct comparison with histology. The results support the combined role of both structural and fluid mechanics to determine the magnitude of ISR at the level of individual struts. This finding is consistent with other studies which consider these stimuli averaged over a transverse section of the vessel.

On the suitability of hexagonal honeycombs as stent geometries

It is a well-known fact that the mechanical properties of coronary stents are defined mainly by two components, the constituting material and the design pattern of the stent itself. The latter especially has attracted the interest of entrepreneurs and scientists alike with a plethora of patents being issued for numerous stent designs. Despite this widespread interest, the suitability of said designs are seldom studied. Accordingly, in this work we have investigated the properties of stent designs based on the hexagonal honeycomb geometry. Stent patterns based upon re-entrant, non re-entrant, and hybrid honeycomb geometries were studied with respect to their behaviour at extremely high strains using Finite Element Analysis. The data collected indicates that although the non re-entrant and hybrid geometries may be more suited to stent designs than the re-entrant geometry in terms of tolerance to high strains, none of these systems convey all the ideal properties desired in a stent, even if the former two have the potential of exhibiting some of them.