Mis-sizing of stent promotes intimal hyperplasia: impact of endothelial shear and intramural stress

Design and Simulation of the Biomechanics of Multi-Layered Composite Poly(Vinyl Alcohol) Coronary Artery Grafts

Coronary artery disease is among the primary causes of death worldwide. While synthetic grafts allow replacement of diseased tissue, mismatched mechanical properties between graft and native tissue remains a major cause of graft failure. Multi-layered grafts could overcome these mechanical incompatibilities by mimicking the structural heterogeneity of the artery wall. However, the layer-specific biomechanics of synthetic grafts under physiological conditions and their impact on endothelial function is often overlooked and/or poorly understood. In this study, the transmural biomechanics of four synthetic graft designs were simulated under physiological pressure, relative to the coronary artery wall, using finite element analysis. Using poly(vinyl alcohol) (PVA)/gelatin cryogel as the representative biomaterial, the following conclusions are drawn: (I) the maximum circumferential stress occurs at the luminal surface of both the grafts and the artery; (II) circumferential stress varies discontinuously across the media and adventitia, and is influenced by the stiffness of the adventitia; (III) unlike native tissue, PVA/gelatin does not exhibit strain stiffening below diastolic pressure; and (IV) for both PVA/gelatin and native tissue, the magnitude of stress and strain distribution is heavily dependent on the constitutive models used to model material hyperelasticity. While these results build on the current literature surrounding PVA-based arterial grafts, the proposed method has exciting potential toward the wider design of multi-layer scaffolds. Such finite element analyses could help guide the future validation of multi-layered grafts for the treatment of coronary artery disease.

Comparison of coronary vessel sizing using coronary angiography versus intravascular ultrasound in Egyptian patients

Background: Coronary artery disease (CAD) is a leading causeof death worldwide. Intravascular imaging is an important toolin the arsenal of each interventional cardiologist. While angiographyprovides a two-dimensional image of a three-dimensionalstructure, intravascular imaging enhances understanding by providingdetailed cross-sectional images. This study aimed to investigatethe discrepancies in coronary vessel sizing between quantitativecoronary angiography (QCA) and intravascular ultrasound.Methods: This cohort study was conducted on 69 patients whowere referred for elective coronary angiography. Patients weresubjected to history taking, examination, blood samples, electrocardiogram(E.C.G.), and echocardiography. Then, a comparisonof each vessel’s luminal diameter by QCA and IVUS was done.Results: The study included 69 patients; The mean age was 54.7± 9.7. There was a statistically significant difference between thestudied vessels regarding the discrepancy between luminal diametersmeasured by IVUS and QCA. IVUS luminal diameter waslarger than QCA luminal diameter (the median difference in measuresof QCA and IVUS in the left main artery, LAD, LCX, and RCAwere -0.8, -0.55, -0.4, and -0.5 respectively). Furthermore, thereis a statistically significant difference between the studied vesselsregarding the presence of a difference >0.75 mm between theluminal diameters measured by IVUS and QCA (Difference >0.75mm in the left main artery, LAD, left circumflex and RCA were55.8%, 21.7%, 30.8%, and 15.4% respectively). Conclusion: Coronarylesions were underestimated by QCA in comparison to IVUSregarding luminal diameter, especially the left main (LM).

Three-Year Clinical Outcomes Following Implantation of LifeStent Self-Expanding Nitinol Stents in Patients With Femoropopliteal Artery Lesions

The aim of this study was to evaluate midterm clinical outcomes after implantation of LifeStent self-expanding nitinol stents for the treatment of femoropopliteal lesions. This retrospective, multicenter, non-randomized study examined 260 femoropopliteal lesions in 250 consecutive patients with peripheral artery disease implanted with LifeStents from April 2016 to April 2017. The prevalence of chronic total occlusion (CTO), lesion length ≥25 cm, and distal reference vessel diameter (RVD) <5 mm was 58%, 35%, and 50%, respectively. The 3-year restenosis rate in the overall population was estimated to be 72.9% and a major adverse limb event was observed in 36.9%. Multivariate analysis revealed that chronic limb-threatening ischemia (CLTI) (odds ratio [OR]: 8.04; 95% confidence interval [CI]: 1.86-34.7), CTO (OR: 4.87; 95% CI: 1.43-16.6), lesion length ≥25 cm (OR: 5.95; 95% CI: 1.11-32.0), and distal RVD <5 mm (OR: 4.43; 95% CI: 1.34-14.6) were independent risk factors for 3-year restenosis. The present study demonstrated the midterm clinical outcomes and risk factors for restenosis after implantation of the LifeStent in femoropopliteal artery lesions. CLTI, CTO, lesion length ≥25 cm, and distal RVD <5 mm predicted decreased patency after a 3-year follow-up.

The Role of Elevated Wall Shear Stress in Progression of Pulmonary Vein Stenosis: Evidence from Two Case Studies

Pulmonary vein stenosis is a serious condition characterized by restriction or blockage due to fibrotic tissue ingrowth that develops in the pulmonary veins of infants or children. It is often progressive and can lead to severe pulmonary hypertension and death. Efforts to halt or reverse disease progression include surgery and catheter-based balloon dilation and stent implantation. Its cause and mechanism of progression are unknown. In this pilot study, we propose and explore the hypothesis that elevated wall shear stress at discrete pulmonary venous sites triggers stenosis. To assess this theory, we retrospectively analyzed cardiac catheterization, lung scan, and X-ray computed tomography data to estimate wall shear stress in the pulmonary veins at multiple time points during disease progression in two patients. Results are consistent with the existence of a level of elevated wall shear stress above which the disease is progressive and below which progression is halted. The analysis also suggests the possibility of predicting the target lumen size necessary in a given vein to reduce wall shear stress to normal levels and remove the trigger for stenosis progression.

Computational Simulations of Provisional Stenting of a Diseased Coronary Artery Bifurcation Model

Although stenting of non-branched arterial segments has acceptable clinical outcomes, in-stent restenosis (ISR) and stent thrombosis remain clinically significant issues for vascular bifurcations (15-28% restenosis). Local fluid and solid stresses appear to play an important role in restenosis and thrombosis. The combined role of wall shear stress (WSS) and circumferential wall stresses (CWS) is unclear in the case of stenting at vascular bifurcations. Using numerical simulations, we computed the fluid shear, solid stresses and the stress ratio at the the bifurcation region. Stenting of main vessel increased the maximum CWS in the the side branch (SB), resulting in a nearly two-fold increase of stress ratio in the SB compared to the MB (5.1 × 10⁵ vs. 9.2 × 10⁵). The existence of plaque decreased WSS and increased CWS near the carina, increasing the stress ratio at the SB. The changes of stress ratio were highly consistent with clinical data on bifurcation stenting. Fluid dynamics and solids mechanics should be considered in planning of stenting for a specific bifurcation, as their combined biomechanical effect may play an important role in stent restenosis and thrombosis.

Promising results of stent graft placement for cephalic arch stenosis after repeated failure of angioplasty in patients on hemodialysis

Objective

Cephalic arch stenosis (CAS) causes repeated dysfunction and failure of arteriovenous access. Percutaneous transluminal angioplasty is the standard initial treatment for CAS, but its outcome is unsatisfactory. This study aimed to evaluate the outcome of stent graft placement for CAS in patients on hemodialysis.

Methods

A retrospective chart review from a tertiary medical center was performed in patients receiving stent graft placement for CAS between January 2012 and 2016. Patency was analyzed using the Kaplan–Meier method.

Results

Twenty-one patients received stent graft placement for CAS. Technical and clinical success rates were 100%. Primary target lesion patency was 95% (95% confidence interval [CI], 86%–100%), 76% (95% CI, 58%–94%), and 43% (95% CI, 22%–64%) at 3, 6, and 12 months, respectively. No significant difference in patency was observed between the arteriovenous fistula and arteriovenous graft groups. Assisted primary patency was 95% (95% CI, 86%–100%), 71% (95% CI, 52%–91%), and 57% (95% CI, 36%–78%) at 3, 6, and 12 months, respectively. Secondary patency was 100% at 3, 6, and 12 months.

Conclusions

After repeated failed angioplasty for cephalic arch stenosis, patients on hemodialysis who receive stent graft placement have effective and durable outcomes.

Increase of wall shear stress caused by arteriovenous fistula reduces neointimal hyperplasia after stent implantation in healthy arteries

BACKGROUND AND OBJECTIVES

Wall shear stress plays a critical role in neointimal hyperplasia after stent implantation. It has been found that there is an inverse relation between wall shear stress and neointimal hyperplasia. This study hypothesized that the increase of arterial wall shear stress caused by arteriovenous fistula could reduce neointimal hyperplasia after stents implantation.

METHODS AND RESULTS

Thirty-six male rabbits were randomly divided into three groups: STENT, rabbits received stent implantation into right common carotid artery; STENT/arteriovenous fistula, rabbits received stent implantation into right common carotid artery and carotid-jugular arteriovenous fistula; Control, rabbits received no treatment. After 21 days, stented common carotid artery specimens were harvested for histological staining and protein expression analysis. In STENT group, wall shear stress maintained at a low level from 43.2 to 48.9% of baseline. In STENT/arteriovenous fistula group, wall shear stress gradually increased to 86% over baseline. There was a more significant neointimal hyperplasia in group STENT compared with the STENT/arteriovenous fistula group (neointima area: 0.87 mm² versus 0.19 mm²; neointima-to-media area ratio: 1.13 versus 0.18). Western blot analysis demonstrated that the protein level of endothelial nitric oxide synthase in STENT group was significantly lower than that in STENT/arteriovenous fistula group, but the protein levels of proliferating cell nuclear antigen, vascular cell adhesion molecule 1, phospho-p38 mitogen-activated protein kinase (Pp38), and phospho-c-Jun N-terminal kinase in STENT group were significantly higher than that in the STENT group.

CONCLUSION

High wall shear stress caused by arteriovenous fistula as associated with the induction in neointimal hyperplasia after stent implantation. The underlying mechanisms may be related to modulating the expression and activation of endothelial nitric oxide synthase, vascular cell adhesion molecule 1, p38, and c-Jun N-terminal kinase.

Role of Vessel Microstructure in the Longevity of End-to-Side Grafts

Compliance mismatch between the graft and the host artery of an end-to-side (ETS) arterial bypass graft anastomosis increases the intramural stress in the ETS graft-artery junction, and thus may compromise its long-term patency. The present study takes into account the effects of collagen fibers to demonstrate how their orientations alter the stresses. The stresses in an ETS bypass graft anastomosis, as a man-made bifurcation, are compared to those of its natural counterpart with different fiber orientations. Both of the ETS bypass graft anastomosis and its natural counterpart have identical geometric and material models and only their collagen fiber orientations are different. The results indicate that the fiber orientation mismatch between the graft and the host artery may increase the stresses at both the heel and toe regions of the ETS anastomosis (the maximum principal stress at the heel and toe regions increased by 72% and 12%, respectively). Our observations, thus, propose that the mismatch between the collagen fiber orientations of the graft and the host artery, independent of the effect of the suture line, may induce aberrant stresses to the anastomosis of the bypass graft.

An in silico study of the influence of vessel wall deformation on neointimal hyperplasia progression in peripheral bypass grafts

Neointimal hyperplasia (NIH) is a major obstacle to graft patency in the peripheral arteries. A complex interaction of biomechanical factors contribute to NIH development and progression, and although haemodynamic markers such as wall shear stress have been linked to the disease, these have so far been insufficient to fully capture its behaviour. Using a computational model linking computational fluid dynamics (CFD) simulations of blood flow with a biochemical model representing NIH growth mechanisms, we analyse the effect of compliance mismatch, due to the presence of surgical stitches and/or to the change in distensibility between artery and vein graft, on the haemodynamics in the lumen and, subsequently, on NIH progression. The model enabled to simulate NIH at proximal and distal anastomoses of three patient-specific end-to-side saphenous vein grafts under two compliance-mismatch configurations, and a rigid wall case for comparison, obtaining values of stenosis similar to those observed in the computed tomography (CT) scans. The maximum difference in time-averaged wall shear stress between the rigid and compliant models was 3.4 Pa, and differences in estimation of NIH progression were only observed in one patient. The impact of compliance on the haemodynamic-driven development of NIH was small in the patient-specific cases considered.

Search for an Optimal Design of a Bioprosthetic Venous Valve: In silico and in vitro Studies

OBJECTIVE/BACKGROUND

Valve incompetence is a progressive disease of the venous system that may eventually lead to venous hypertension, pain, and ulcers. There is a need for a venous valve prosthesis to replace incompetent valves. Computational and experimental investigations on venous valve design and associated haemodynamics will undoubtedly advance prosthesis design and treatments. Here, the objective is to investigate the effect of venous valve on the fluid and solid mechanics. The hypothesis is that there exists a valve geometry that maximises leaflet shear stress (LSS) but minimises leaflet intramural stress (LIS; i.e., minimise stress ratio = LIS/LSS).

METHODS

To address the hypothesis, fully dynamic fluid-structure interaction (FSI) models were developed. The entire cycle of valve opening and closure was simulated. The flow validation experiments were conducted using a stented venous valve prosthesis and a pulse duplicator flow loop.

RESULTS

Agreement between the output of FSI simulations and output of pulse duplicator was confirmed. The maximum flow rates were within 6% difference, and the total flow during the cycle was within 10% difference. The simulated high stress ratio region at the leaflet base (five times the leaflet average) predicted the disease location of the vast majority of explanted venous valves reported in clinical literature. The study found that the reduced valve height and leaflet dome shape resulted in optimal performance to provide the lowest stress ratio.

CONCLUSION

This study proposes an effective design of venous prostheses and elaborates on the correlations of venous valve with clinical observations.

Electrical Conductance Device for Stent Sizing

The minimum stent area (MSA) has been clinically established as a significant predictor of restenosis, thrombosis, and ischemia using intra-vascular ultrasound (IVUS). Unfortunately, IVUS measurements are far from routine because of significant cost of IVUS, the training required, the subjectivity of image interpretation and the time added to the procedure. The objective of this study is to verify the accuracy of a conductance catheter for stent sizing. Here, we introduce an easy and entirely objective device and method for real time determination of MSA. A 10 kHz, 35 μA rms current is passed through the external electrodes of an intravascular catheter while the conductance is measured across a separate set of electrodes. Both phantom and ex vivo validations of metal stent sizing in five porcine carotid arteries were confirmed. The accuracy of the measurements were found to be excellent in phantoms (root mean square, rms, of 3.4% of actual value) and in ex-vivo vessels (rms = 3.2% of measured value). An offset of conductance occurs when a conductive metal stent (e.g., bare metal stent) is deployed in the vessel, while the slope remains the same. This offset is absent in the case of drug eluting stent where the metal is coated (i.e., insulated) or non-metal bioresorbable stent. The present device makes easy, accurate and reproducible measurements of the size of stented blood vessels within 3.2% rms error. This device provides an alternative method to sizing of stent (i.e., MSA) in real-time without subjective interpretation and with less cost than IVUS.

Numerical study of incomplete stent apposition caused by deploying undersized stent in arteries with elliptical cross-sections

Incomplete stent apposition (ISA) is one of the causes leading to post-stent complications, which can be found when an undersized or under-expanded stent is deployed at lesions. Previous research efforts have focused on ISA in idealized coronary arterial geometry with circular cross-sections. However, arterial cross-section eccentricity plays an important role in both location and severity of ISA. Computational fluid dynamics (CFD) simulations are carried out to systematically study the effects of ISA in arteries with elliptical cross-sections, as such stents are partially embedded on the minor axis sides of the ellipse and malapposed elsewhere. Overall, ISA leads to high time-averaged WSS (TAWSS) at the proximal end of the stent and low TAWSS at the ISA transition region and the distal end. Shear rate depends on both malapposition distance and blood stream locations, which is found to be significantly higher at the inner stent surface than the outer surface. The proximal high shear rate signifies increasing possibility in platelet activation, when coupled with low TAWSS at the transition and distal region which may indicate a nidus for in-stent thrombosis.

Hemodynamics and pathology of an enlarging abdominal aortic aneurysm model in rabbits

Hemodynamics may play an essential role in the initiation and progression of abdominal aortic aneurysm (AAA). We aimed to study the mechanism of self-healing process by the changes of hemodynamics and pathology in an enlarging AAA in rabbits. Seventy-two rabbits were randomly divided into three groups. Rabbits underwent extrinsic coarctation and received a 10-minute elastase incubation in Group A and Group B. Absorbable suture used in Group A was terminated by balloon dilation at week 4. Diameter was measured after 1, 3, 5, and 15 weeks, computational fluid dynamics analysis was performed at week 3 and week 15. Rabbits were sacrificed after 1, 5, and 15 weeks for pathological and quantitative studies. The higher velocity magnitude, intensified bulk flow and obvious vortex formation were observed in Group A at week 3 instead of week 15. Both low wall shear stress and high relative residence time increased in Group B, however, high oscillatory shear index had relatively less increase compared with Group A. Aortic diameter reached a plateau at 5 weeks in Group A, which was significantly lower than in week 15 in Group B. Intimal hyperplasia, intima-media thickness increased significantly in Group A at week 5, significantly higher than in week 15 in Group B. Marked destruction of elastin fibers and smooth muscle cells occurred at week 1, and increased significantly at week 15 in Group A. Aneurysm exhibited strong expression of matrix metalloproteinase 9 and mouse anti-rabbit macrophage 11 at week 1, and showed a tendency to decrease. Matrix metalloproteinase 2 expression decreased significantly in Group B at week 15 compared with week 5 and Group A. In conclusion, the self-healing of rabbit AAA may attributed to the regeneration of smooth muscle cells. The turbulence flow caused by coarctation is associated with continuous growth of rabbit AAA and prevents the self-healing phenomenon.

Human Femoral Vein Diameter and Topography of Valves and Tributaries: A Post Mortem Analysis

The femoral vein (FV) is a clinically important vessel. Failure of its valves can lead to chronic venous insufficiency (CVI) with severe manifestations such as painful ulcers. Although they are crucial for identifying suitable implant sites for therapeutic valves, studies on the topography of FV tributaries and valves are rare. Moreover, the femoral vein diameter (FVD) must be known to assess the morphometric requirements for valve implants. To reassess the anatomical requirements for valve implants, 155 FVs from 82 human corpses were examined. FVDs and tributary and valve topographies were assessed using a laboratory straightedge. The FVD increased from 6 mm in the distal femoropopliteal vein to 11 mm in the iliofemoral vein proximal to the saphenofemoral junction (SFJ). Diameters were significantly bigger in males than females. Height correlated positively with FVD. Distal to the SFJ, within a distance of 38 cm, one to eight valves were present. Up to two valves were present within 10 cm proximal to the SFJ. Individual tributary and valve topography must be considered to ensure appropriate design and successful implantation of a venous valve for CVI therapy in the FV. A suitable implant site would be proximal to the SFJ via an infrainguinal transfemoral access. Clin. Anat. 31:1065-1076, 2018. © 2018 Wiley Periodicals, Inc.

LumenRECON Guidewire: Pilot Study of a Novel, Nonimaging Technology for Accurate Vessel Sizing and Delivery of Therapy in Femoropopliteal Disease

BACKGROUND

Proper vessel sizing during endovascular interventions is crucial to avoid adverse procedural and clinical outcomes. LumenRECON (LR) is a novel, nonimaging, 0.035-inch wire-based technology that uses the physics-based principle of Ohm's law to provide a simple, real-time luminal size while also providing a platform for therapy delivery. This study evaluated the accuracy, reliability, and safety of the LR system in patients presenting for a femoropopliteal artery intervention.

METHODS AND RESULTS

This multicenter, prospective pilot study of 24 patients presenting for peripheral intervention compared LR measurements of femoropopliteal artery size to angiographic visual estimation, duplex ultrasound, quantitative angiography, and intravascular ultrasound. The primary effectiveness and safety end point was comparison against core laboratory adjudicated intravascular ultrasound values and major adverse events, respectively. Additional preclinical studies were also performed in vitro and in vivo in swine to determine the accuracy of the LR guidewire system. No intra- or postprocedure device-related adverse events occurred. A balloon or stent was successfully delivered in 12 patients (50%) over the LR wire. Differences in repeatability between successive LR measurements was 2.5±0.40% (R²=0.96) with no significant bias. Differences in measurements of LR to other modalities were 0.5±1.7%, 5.0±1.8%, -1.5±2.0%, and 6.8±3.4% for intravascular ultrasound core laboratory, quantitative angiography, angiographic, and duplex ultrasound, respectively.

CONCLUSIONS

This study demonstrates that through a physics-based principle, LR provides a real-time, safe, reproducible, and accurate vessel size of the femoropopliteal artery during intervention and can additionally serve as a conduit for therapy delivery over its wire-based platform.

Patient-Specific Modeling of Stented Coronary Arteries Reconstructed from Optical Coherence Tomography: Towards a Widespread Clinical Use of Fluid Dynamics Analyses

The recent widespread application of optical coherence tomography (OCT) in interventional cardiology has improved patient-specific modeling of stented coronary arteries for the investigation of local hemodynamics. In this review, the workflow for the creation of fluid dynamics models of stented coronary arteries from OCT images is presented. The algorithms for lumen contours and stent strut detection from OCT as well as the reconstruction methods of stented geometries are discussed. Furthermore, the state of the art of studies that investigate the hemodynamics of OCT-based stented coronary artery geometries is reported. Although those studies analyzed few patient-specific cases, the application of the current reconstruction methods of stented geometries to large populations is possible. However, the improvement of these methods and the reduction of the time needed for the entire modeling process are crucial for a widespread clinical use of the OCT-based models and future in silico clinical trials.

Biomechanical impact of provisional stenting and balloon dilatation on coronary bifurcation: clinical implications

In-stent restenosis (ISR) and stent thrombosis remain clinically significant problems for bifurcations. Although the role of wall shear stress (WSS) has been well investigated, the role of circumferential wall stresses (CWS) has not been well studied in provisional stenting with and without final kissing balloon (FKB). We hypothesized that the perturbation of CWS at the SB in provisional stenting and balloon dilatation is an important factor in addition to WSS, and, hence, may affect restenosis rates (i.e., higher CWS correlates with higher restenosis). To test this hypothesis, we developed computational models of stent, FKB at bifurcation, and finite element simulations that considered both fluid and solid mechanics of the vessel wall. We computed the stress ratio (CWS/WSS) to show potential correlation with restenosis in clinical studies (i.e., higher stress ratio correlates with higher restenosis). Our simulation results show that stenting in the main branch (MB) increases the maximum CWS in the side branch (SB) and, hence, yields a higher stress ratio in the SB, as compared with the MB. FKB dilatation decreases the CWS and increases WSS, which collectively lowers the stress ratio in the SB. The changes of stress ratio were correlated positively with clinical data in provisional stenting and FKB. Both fluid and solid mechanics need to be evaluated when considering various stenting techniques at bifurcations, as solid stresses also play an important role in clinical outcome. An integrative index of bifurcation mechanics is the stress ratio that considers both CWS and WSS.NEW & NOTEWORTHY Although the role of wall shear stress (WSS) has been well investigated, the role of circumferential wall stresses (CWS) has not been well studied in provisional stenting with and without final kissing balloon. Both fluid and solid mechanics need to be evaluated when considering various stenting techniques at bifurcations. An integrative index of bifurcation mechanics is the stress ratio that considers both CWS and WSS.

Long-term Results of Tapered Stents in Endovascular Treatment of Carotid Stenosis

BACKGROUND

Recent technological developments allowed significant improvements in interventional approach to carotid artery stenosis. Tapered stents were specifically designed for extracranial carotid artery stenting (CAS) to deal with vessels mismatch with the objective of decreasing the risk of thrombosis. Nevertheless, whether the stent geometry may affect the outcomes of CAS is uncertain. This study aims to investigate the impact of stent configuration on perioperative and long-term results of this procedure.

METHODS

All CAS procedures performed between 2005 and 2012 at a single high-volume center were reviewed. Primary end points of the study were 30-day mortality and any ipsilateral neurological event. Secondary end points were any late (>30 days) neurological event and restenosis >50%.

RESULTS

About 1,368 procedures were performed in the period 2005-2012. 9.1% of the patients were symptomatic for recent focal neurological event. 883 patients were treated with a cylindrical stent while the others underwent a tapered design device implantation. Technical success was achieved in 96% of the procedures. No perioperative mortality was recorded; perioperative stroke occurred in 1.2% vs. 1.6% (P = not significant) in tapered and straight stent group respectively. Stent design did not predict perioperative ipsilateral neurological events neither at bivariate nor at multivariate analysis. At a mean follow-up of 30 months, late neurological events occurred in 26 cases (1.9%); conic stents resulted protective at Kaplan-Meier analysis (P = 0.027).

CONCLUSIONS

The use of conic stents appears to be associated with similar perioperative results when compared with straight stents. Late outcomes suggest a lower risk of restenosis and late neurological events in patients with conical shape stents.

Microstructure-based biomechanics of coronary arteries in health and disease

Coronary atherosclerosis is the major cause of mortality and disability in developed nations. A deeper understanding of mechanical properties of coronary arteries and hence their mechanical response to stress is significant for clinical prevention and treatment. Microstructure-based models of blood vessels can provide predictions of arterial mechanical response at the macro- and micro-mechanical level for each constituent structure. Such models must be based on quantitative data of structural parameters (constituent content, orientation angle and dimension) and mechanical properties of individual adventitia and media layers of normal arteries as well as change of structural and mechanical properties of atherosclerotic arteries. The microstructural constitutive models of healthy coronary arteries consist of three major mechanical components: collagen, elastin, and smooth muscle cells, while the models of atherosclerotic arteries should account for additional constituents including intima, fibrous plaque, lipid, calcification, etc. This review surveys the literature on morphology, mechanical properties, and microstructural constitutive models of normal and atherosclerotic coronary arteries. It also provides an overview of current gaps in knowledge that must be filed in order to advance this important area of research for understanding initiation, progression and clinical treatment of vascular disease. Patient-specific structural models are highlighted to provide diagnosis, virtual planning of therapy and prognosis when realistic patient-specific geometries and material properties of diseased vessels can be acquired by advanced imaging techniques.

Effect of pulmonary conduit oversizing on hemodynamics in children

PURPOSE

Implanting the largest valved conduit possible - oversizing - to reconstruct an absent connection from the right ventricle to the pulmonary artery in certain types of congenital heart defects has been suggested as a compensating measure for somatic outgrowth of the patient. However, one effect that has not been investigated yet is the hemodynamic consequence. For this purpose, virtual implantation and flow simulations were conducted in this study.

METHODS

To isolate the effects of conduit oversizing on the hemodynamics observed after conduit implantation and outgrowth, calculated wall shear stresses (WSS) of image-based computational fluid dynamic (CFD) simulations were used as indicators. Three different sizes of valved conduits (20 mm, 22 mm, and 24 mm), including the largest possible conduit size, virtually implanted in a child-sized healthy pulmonary artery and the corresponding adult-sized model were investigated.

RESULTS

The child and adult models show a decrease of the mean WSS (approx. 26%) in the whole domain with an increase of the conduit size. When looking at the mean WSS at the anastomosis, for the child model the WSS is significantly increased (approx. 40%) when oversizing (Z-score +3.21). In contrast, the stresses are decreased for the adult model (34%) when using the largest conduit (Z-score +0.25).

CONCLUSIONS

Based on the results of this study, it must be considered that choosing a prosthesis size that will lead to high WSS and an associated intimal reaction, possibly leading to stenosis, can defeat the benefit of having a nominally larger orifice area directly after implantation.

Impact of bifurcation dual stenting on endothelial shear stress

Despite advances in percutaneous coronary interventions and the introduction of drug eluding stents, in-stent restenosis and stent thrombosis remain a clinically significant problem for bifurcations. The aim of this study is to determine the effect of dual bifurcation stenting on hemodynamic parameters known to influence restenosis and thrombosis. We hypothesized that double stenting, especially with a longer side branch (SB) stent, likely has a negative effect on wall shear stress (WSS), WSS gradient (WSSG), and oscillatory shear index (OSI). To test this hypothesis, we developed computational models of dual stents at bifurcations and non-Newtonian blood simulations. The models were then interfaced, meshed, and solved in a validated finite-element package. Longer and shorter stents at the SB and provisional stenting were compared. It was found that stents placed in the SB at a bifurcation lowered WSS, but elevated WSSG and OSI. Dual stenting with longer SB stent had the most adverse impact on SB endothelial WSS, WSSG, and OSI, with low WSS region up to 50% more than the case with shorter SB stent. The simulations also demonstrated flow disturbances resulting from SB stent struts protruding into the main flow field near the carina, which may have implications on stent thrombosis. The simulations predict a negative hemodynamic role for SB stenting, which is exaggerated with a longer stent, consistent with clinical trial findings that dual-stenting is comparable or inferior to provisional stenting.

Nitinol Stent Oversizing in Patients Undergoing Popliteal Artery Revascularization: A Finite Element Study

Nitinol stent oversizing is frequently performed in peripheral arteries to ensure a desirable lumen gain. However, the clinical effect of mis-sizing remains controversial. The goal of this study was to provide a better understanding of the structural and hemodynamic effects of Nitinol stent oversizing. Five patient-specific numerical models of non-calcified popliteal arteries were developed to simulate the deployment of Nitinol stents with oversizing ratios ranging from 1.1 to 1.8. In addition to arterial biomechanics, computational fluid dynamics methods were adopted to simulate the physiological blood flow inside the stented arteries. Results showed that stent oversizing led to a limited increase in the acute lumen gain, albeit at the cost of a significant increase in arterial wall stresses. Furthermore, localized areas affected by low Wall Shear Stress increased with higher oversizing ratios. Stents were also negatively impacted by the procedure as their fatigue safety factors gradually decreased with oversizing. These adverse effects to both the artery walls and stents may create circumstances for restenosis. Although the ideal oversizing ratio is stent-specific, this study showed that Nitinol stent oversizing has a very small impact on the immediate lumen gain, which contradicts the clinical motivations of the procedure.

Studying the Interaction of Stent-Grafts and Treated Abdominal Aortic Aneurysms

Since the advent of endovascular repair of aortic aneurysms (EVAR), clinical focus has been on preventing loss of sealing at the level of the infrarenal neck, which leads to type I endoleak and repressurization of the aneurysm sac. Enhanced mechanisms for central fixation and seal have consequently lowered the incidence of migration and endoleaks. However, endograft limb thrombosis and its causal mechanisms have not been addressed adequately in the literature. This article reviews the pathophysiological mechanisms associated with limb thrombosis in order to facilitate better clinical judgment to prevent iliac adverse effects.

Temporal correlation between wall shear stress and in-stent stenosis after Wingspan stent in swine model

BACKGROUND AND PURPOSE

A recent randomized clinical trial on intracranial atherosclerosis was discontinued because of the higher frequency of stroke and death in the angioplasty and stent placement group than in the medical treatment group. An in-depth understanding of the relationship between biologic responses and flow dynamics is still required to identify the current limitations of intracranial stent placement.

MATERIALS AND METHODS

Five Wingspan stents were deployed in tapered swine ascending pharyngeal arteries. Temporal wall shear stress distributions and in-stent stenosis were evaluated at days 0, 7, 14, and 28 after stent placement. The physiologic role of wall shear stress was analyzed regarding its correlation with in-stent stenosis.

RESULTS

In-stent stenosis reached a peak of nearly 40% at day 14 and decreased mainly at the distal stent segment until day 28. The wall shear stress demonstrated a characteristic pattern with time on the basis of the in-stent stenosis change. The wall shear stress gradient increased from the proximal to distal segment until day 14. At day 28, the trend was reversed dramatically, decreasing from the proximal to the distal segment. A significant correlation between the in-stent stenosis growth until day 14 and low wall shear stress values just after stent placement was detected. In-stent stenosis regression between days 14 and 28 was also associated with the high wall shear stress values at day 14.

CONCLUSIONS

These data suggest that the physiologic wall shear stress can control the biphasic in-stent stenosis change in tapered arteries.

From Histology and Imaging Data to Models for In-Stent Restenosis

The implantation of stents has been used to treat coronary artery stenosis for several decades. Although stenting is successful in restoring the vessel lumen and is a minimally invasive approach, the long-term outcomes are often compromised by in-stent restenosis (ISR). Animal models have provided insights into the pathophysiology of ISR and are widely used to evaluate candidate drug inhibitors of ISR. Such biological models allow the response of the vessel to stent implantation to be studied without the variation of lesion characteristics encountered in patient studies.

This paper describes the development of complementary in silico models employed to improve the understanding of the biological response to stenting using a porcine model of restenosis. This includes experimental quantification using microCT imaging and histology and the use of this data to establish numerical models of restenosis. Comparison of in silico results with histology is used to examine the relationship between spatial localization of fluid and solid mechanics stimuli immediately post-stenting. Multi-scale simulation methods are employed to study the evolution of neointimal growth over time and the variation in the extent of neointimal hyperplasia within the stented region. Interpretation of model results through direct comparison with the biological response contributes to more detailed understanding of the pathophysiology of ISR, and suggests the focus for follow-up studies.

In conclusion we outline the challenges which remain to both complete our understanding of the mechanisms responsible for restenosis and translate these models to applications in stent design and treatment planning at both population-based and patient-specific levels.

Structural analysis for Wingspan stent in a perforator model

Perforator infarction represents a critical problem after intracranial Wingspan stent. To explore the mechanism of perforator infarction, we simulated the stent-artery interaction at an atheromatous plaque with perforator. Structural deformation and biomechanical stress distribution after stenting were analyzed. High radial stress values were located along the stent struts, which surrounded the area with high circumferential stress. Stretched perforator orifice in a circumferential direction after stenting was simulated. These results show that structural deformation could play a role in the mechanism of perforator occlusion after Wingspan stenting.

Simulation of a pulsatile non-Newtonian flow past a stenosed 2D artery with atherosclerosis

Atherosclerotic plaque can cause severe stenosis in the artery lumen. Blood flow through a substantially narrowed artery may have different flow characteristics and produce different forces acting on the plaque surface and artery wall. The disturbed flow and force fields in the lumen may have serious implications on vascular endothelial cells, smooth muscle cells, and circulating blood cells. In this work a simplified model is used to simulate a pulsatile non-Newtonian blood flow past a stenosed artery caused by atherosclerotic plaques of different severity. The focus is on a systematic parameter study of the effects of plaque size/geometry, flow Reynolds number, shear-rate dependent viscosity and flow pulsatility on the fluid wall shear stress and its gradient, fluid wall normal stress, and flow shear rate. The computational results obtained from this idealized model may shed light on the flow and force characteristics of more realistic blood flow through an atherosclerotic vessel.

Impact of stent mis-sizing and mis-positioning on coronary fluid wall shear and intramural stress

Stent deployments with geographical miss (GM) are associated with increased risk of target-vessel revascularization and periprocedural myocardial infarction. The aim of the current study was to investigate the underlying biomechanical mechanisms for adverse events with GM. The hypothesis is that stent deployment with GM [longitudinal GM, or LGM (i.e., stent not centered on the lesion); or radial GM, RGM (i.e., stent oversizing)] results in unfavorable fluid wall shear stress (WSS), WSS gradient (WSSG), oscillatory shear index (OSI), and intramural circumferential wall stress (CWS). Three-dimensional computational models of stents and plaque were created using a computer-assisted design package. The models were then solved with validated finite element and computational fluid dynamic packages. The dynamic process of large deformation stent deployment was modeled to expand the stent to the desired vessel size. Stent deployed with GM resulted in a 45% increase in vessel CWS compared with stents that were centered and fully covered the lesion. A 20% oversized stent resulted in 72% higher CWS than a correct sized stent. The linkages between the struts had much higher stress than the main struts (i.e., 180 MPa vs. 80 MPa). Additionally, LGM and RGM reduced endothelial WSS and increased WSSG and OSI. The simulations suggest that both LGM and RGM adversely reduce WSS but increase WSSG, OSI, and CWS. These findings highlight the potential mechanical mechanism of the higher adverse events and underscore the importance of stent positioning and sizing for improved clinical outcome.

Simulated biomechanical responses at a curved arterial segment after Wingspan Stent deployment in swine

OBJECTIVES

Endovascular treatment with the Wingspan Stent is frequently associated with in-stent restenosis at the curved portion, leading to late-phase stroke. To explore the cause of stroke complications after treatment with the Wingspan Stent, we simulated the biomechanical responses at a curved arterial segment using the finite element method.

METHODS

A Wingspan stent was deployed at a slightly curved ascending pharyngeal artery (APA) in swine. Several stress distributions modeling solid mechanics were analyzed with structural deformation. Histopathological analysis of the selected APA was assessed at 28 days after stenting.

RESULTS

Arterial straightening was simulated in this study. Both radial stress (RS) and circumferential stress (CS) concentrations increased at both stent ends. Marked lower axial stress (AS) concentration was observed at the outer wall of an arterial curvature. The proximal stent segment, ending in the curved portion, significantly impacted the solid mechanical environment. Eccentric neointimal hyperplasia was observed at the curved segment.

DISCUSSION

These results show that the Wingspan stent exaggerated the non-uniform stress distributions in a curved artery. The understanding of stent-arterial wall interactions is of value to identify the current limitations of intracranial stenting, and will help to improve this treatment methodology and future devices.

Which diameter and angle rule provides optimal flow patterns in a coronary bifurcation?

The branching angle and diameter ratio in epicardial coronary artery bifurcations are two important determinants of atherogenesis. Murray's cubed diameter law and bifurcation angle have been assumed to yield optimal flows through a bifurcation. In contrast, we have recently shown a 7/3 diameter law (HK diameter model), based on minimum energy hypothesis in an entire tree structure. Here, we derive a bifurcation angle rule corresponding to the HK diameter model and critically evaluate the streamline flow through HK and Murray-type bifurcations. The bifurcations from coronary casts were found to obey the HK diameter model and angle rule much more than Murray's model. A finite element model was used to investigate flow patterns for coronary artery bifurcations of various types. The inlet velocity and pressure boundary conditions were measured by ComboWire. Y-bifurcation of Murray type decreased wall shear stress-WSS (10%-40%) and created an increased oscillatory shear index-OSI in atherosclerosis-prone regions as compared with HK-type bifurcations. The HK-type bifurcations were found to have more optimal flow patterns (i.e., higher WSS and lower OSI) than Murray-type bifurcations which have been traditionally believed to be optimized. This study has implications for changes in bifurcation angles and diameters in percutaneous coronary intervention.