Intramural Stress Increases Exponentially with Stent Diameter: A Stress Threshold for Neointimal Hyperplasia

Preoperative Sizing to Lower In-Stent Restenosis in Peripheral Arterial Occlusive Disease

BACKGROUND

The primary treatment for lower-extremity peripheral arterial occlusive disease (PAOD) is angioplasty stenting. Its main complication is in-stent restenosis. Poor selection of stent dimensions has been identified as a factor contributing to early in-stent restenosis. The aim of this study is to determine whether the implantation of stents, selected based on arterial morphological reconstruction using a sizing software program, reduces the occurrence of in-stent restenosis. The study also aims to evaluate the potential benefits of routine preoperative sizing.

METHODS

Between January 2016 and December 2020, all patients treated for PAOD through scheduled angioplasty stenting in our department were included in the study. Using systematic preoperative computed tomography angiography, precise reconstruction and sizing were performed to select the ideal length and diameter of stents, resulting in the selection of a so-called IDEAL stent. During the procedure, the surgeon implanted either the IDEAL stent or a different one, named the ACTUAL stent, based on intraoperative data and/or availability. We compared the in-stent restenosis rate between IDEAL and ACTUAL stents.

RESULTS

There were no significant differences in the overall characteristics between the IDEAL and ACTUAL stent groups. The in-stent restenosis rate at 1 year was 13% (N = 28/212, P = 0.994) in the IDEAL group and 17% (N = 25/149, P = 0.994) in the ACTUAL group. Among the ACTUAL stents, a total of 19.6% of stents with a diameter mismatch when chosen based on arteriography showed a significantly higher restenosis rate during the first year of follow-up (P = 0.02).

CONCLUSIONS

Our study did not demonstrate a significant difference in 1-year restenosis rate between the IDEAL and the ACTUAL stent groups. It specifically revealed the significant impact of diameter selection on the intrastent restenosis rate during the first year of follow-up. Stents chosen based on arteriographic criteria, which exhibited diameter discordance, compared to the IDEAL stents group selected using sizing reconstructions, could be either oversized or undersized. This led to a significantly higher restenosis rate at 1 year postoperatively.

Treating post-angioplasty dissection in the femoropopliteal arteries using the tack endovascular system: Tack optimized balloon angioplasty II 24-month results

OBJECTIVES

The TOBA (Tack Optimized Balloon Angioplasty) II trial is a prospective, single-arm, multicenter study that investigated Tack treatment for patients with dissection after angioplasty in the superficial femoral artery and/or proximal popliteal artery. The Tack device is a nitinol-based, short (6 mm), stent-like implant with low outward force that can be deployed in a targeted fashion to treat vascular dissection. TOBA II primary results through 12 months have been published previously. This report provides follow-up safety and efficacy results through 24 months (RC).

METHODS

The TOBA II trial enrolled 213 patients with Rutherford classification 2 to 4 and a de novo or non-stented restenotic lesion in the superficial femoral artery and/or proximal popliteal artery who developed a dissection of any grade after treatment with plain balloon or drug-coated balloon (DCB) angioplasty. Participants were followed for 30 days, 6 months, 12 months, 24 months, and 36 months following the procedure. Evaluations included clinically driven target lesion revascularization (CD-TLR), ankle-brachial index, Rutherford classification, peripheral artery questionnaire, quality of life assessed by the EQ-5D-3L, and the Walking Impairment Questionnaire.

RESULTS

At enrollment, mean age was 68.2 ± 9.1 years, 70.9% were male, and 95.8% of patients were categorized as RC 2 or 3. The distribution of balloon types in the study were standard balloons: 42.3%; and drug-coated balloons: 57.7%. At 24-month follow-up, 167 patients (78.4%) had available data. The overall survival rate at 24 months was 95.4% and there were no major amputations during this time. After 24 months of follow-up, the Kaplan-Meier freedom from CD-TLR was 77.7%. Rutherford classification, ankle-brachial index, and quality of life were significantly improved compared with baseline through 24 months.

CONCLUSIONS

The TOBA II 24-month data demonstrate durable intermediate-term outcomes with the use of the Tack Endovascular System. Tack deployment was a safe and effective therapeutic option for dissection repair following angioplasty.

FEA of Drug-Eluting Stents and Sensitivity Analysis of a Continuum Damage Model for the Degradation of PLGA Coating

Drug-Eluting Stents (DES) are commonly used in coronary angioplasty operations as a solution against artery stenosis and restenosis. Computational Bioengineering allows for the in-silico analysis of their performance. The scope of this work is to develop a DES Digital Twin, focusing on the mechanical integrity of its biodegradable coating throughout the operational lifecycle. The implementation leverages the Finite Element Method (FEM) to compute the developed mechanical stress field on the DES during the inflation/deflation stage, followed by the degradation of the polymer-based coating. The simulation of the degradation process is based on a Continuum Damage Mechanics (CDM) model that considers bulk degradation. The CDM algorithm is implemented on the NX Nastran solver through a user-defined material (UMAT) subroutine. For benchmarking purposes and to compare with the baseline design of the BioCoStent project, this conceptual study implements an alternative stent design, to study the effect of the geometry on the developed stresses. Additionally, the effect of the degradation rate on the polymer-based coating's lifecycle is studied via sensitivity analysis.

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.

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.

Restenosis After Tack Implantation is Associated with Less Complex Patterns of Restenosis Compared to Stent Implantation

In-stent restenosis is complex, difficult to treat and has led to a ‘leave less metal behind’ approach to femoropopliteal intervention. Postangioplasty dissection often requires scaffolding to maintain patency. The Tack Endovascular System provides minimal-metal dissection repair that preserves future treatment options. Tack implants are designed to minimise the inflammation and neointimal hyperplasia that lead to in-stent restenosis. An independent angiographic core laboratory evaluated the restenosis patterns in clinically driven target lesion revascularisation (CD-TLR) during the 12 months following the index procedure in the Tack Optimized Balloon Angioplasty (TOBA) II study and compared these results to those published for nitinol stent implantation. Of the 213 patients in TOBA II, 31 (14.6%) required a CD-TLR. Of these, 28 had angiograms that were evaluated by the core laboratory, and 45.2%, 16.1%, and 29% were graded as Tosaka class I, II and III, respectively. There were no significant differences (p>0.05) in lesion length, degree of calcification or dissection class between the three groups. Tack restenotic lesion classification and analysis show a prevalence of both class I and shorter lesions relative to in-stent restenosis, which may be beneficial to long-term patient outcomes.

In Silico analysis of stent deployment- effect of stent design

Coronary artery disease (CAD) remains the leading cause of death in Europe and worldwide. One of the most common pathologic processes involved in CAD is atherosclerosis. Coronary stents are expandable scaffolds that are used to widen the occluded arteries and enable the blood flow restoration. To achieve an adequate delivery and placement of coronary stents different parameters play a significant role. Due to the strain that the stents are exposed to and the forces they should withstand, the stent design is dominant. This study focuses on investigating the effect of the stent design in two finite element models using two stents with difference in the strut thickness. The in silico deployment is performed in a reconstructed patient specific arterial segment. The results are analyzed in terms of stress in the stent and the arterial wall and demonstrate how stent expansion is extensively affected by the scaffold's design.

In silico assessment of the effects of material on stent deployment

Coronary stents are expandable scaffolds that are used to widen occluded diseased arteries and restore blood flow. Because of the strain they are exposed to and forces they must resist as well as the importance of surface interactions, material properties are dominant. Indeed, a common differentiating factors amongst commercially available stents is their material. Several performance requirements relate to stent materials including radial strength for adequate arterial support post-deployment. This study investigated the effect of the stent material in three finite element models using different stents made of: (i) Cobalt-Chromium (CoCr), (ii) Stainless Steel (SS316L), and (iii) Platinum Chromium (PtCr). Deployment was investigated in a patient specific arterial geometry, created based on a fusion of angiographic data and intravascular ultrasound images. In silico results show that: (i) the maximum von Mises stress occurs for the CoCr, however the curved areas of the stent links present higher stresses compared to the straight stent segments for all stents, (ii) more areas of high inner arterial stress exist in the case of the CoCr stent deployment, (iii) there is no significant difference in the percentage of arterial stress volume distribution among all models.

Midterm Results of Low-Profile Stents to Treat Atherosclerotic Iliac Artery Disease

PURPOSE

To evaluate the safety and effectiveness of low-profile 4-F stents for the treatment of atherosclerotic iliac artery lesions.

METHODS

Between January 2009 and December 2015, 63 patients (mean age 69.3 years; 42 men) received low-profile stents (Astron Pulsar or Pulsar-18) at the discretion of the operator to treat iliac artery occlusive disease. The majority of patients (40, 63.5%) had critical limb ischemia; 36 of 82 lesions were total occlusions. All procedures were performed with 4-F equipment. Outcomes evaluated included mortality, patency (primary, assisted primary, and secondary), absence of target lesion revascularization (TLR), and limb salvage. Associations of patient and procedure variables with patency and TLR outcomes were sought with univariate and multivariate analysis.

RESULTS

Via a brachial (n=46/63) or femoral (n=17/63) access, 82 stents were successfully deployed to treat the 82 lesions, with <30% residual stenosis. The overall access-related complication rate was 1.6% (brachial artery occlusion). Mean duration of follow-up was 24.1±22.3 months (range 1-72), during which 3 patients died and 1 major amputation occurred at 10 months. The 4-year Kaplan-Meier estimate of primary patency was 76.9% (95% CI 70.2% to 83.6%); the assisted primary and secondary patency estimates were 79.3% (95% CI 73% to 85.6%) and 91% (95% CI 84.5% to 97.5%). The 4-year freedom from TLR estimate was 73.8% (95% CI 67.4% to 80.2%). On multivariate analysis, the only associations confirmed involved Rutherford category 5/6 with primary patency (hazard ratio [HR] 5.7, 95% CI 4.4 to 7, p=0.02) and assisted primary patency (HR 6.1, 95% CI 4.9 to 7.3, p=0.01).

CONCLUSION

Use of a low-profile 4-F stent in atherosclerotic iliac lesions was safe and effective. At 4 years, the overall patency and the absence of TLR were good. Midterm outcomes were poor in patients with Rutherford category 5/6 ischemia. Finally, the use of stents with a ≥6-mm diameter and postdeployment balloon dilation are recommended in all cases.

Preoperative CT-scan-based sizing and in-stent restenosis in peripheral endovascular revascularizations

Objectives This study evaluates the effect of stent sizing with CT-scan on the incidence of restenosis in peripheral arterial disease. Methods This retrospective study included 59 patients with 66 arterial lesions who underwent a endovascular procedure for peripheral arterial disease between April 2013 and October 2013. All patients had de novo iliac or femoral lesions, were candidates for an endovascular procedure alone and underwent CTA preoperatively. The stent actually implanted, whose dimensions were chosen on the basis of the operator's experience on an intraoperative 2D angiography, was compared to the "ideal" stent chosen retrospectively on the basis of precise lesion sizing by the preoperative CTA. Planning was considered "discordant" if there was a difference in length of more than 20 mm and/or a difference in diameter of more than 1 mm between the ideal stent and the actual stent. Results For iliac lesions, discordance essentially concerned stent diameter (36.1%), whereas stent length was the main reason for discordance for femoral lesions (36.7%). The median length of follow-up was 18 months (range 6-24). For iliac lesions, freedom from restenosis at 24 months was higher for patients with concordant planning (90% vs. 62.5%, p = 0.045). Most restenoses occurred in the external iliac artery, where there was a tendency towards oversizing of the implanted stent. For femoral lesions, the restenosis-free rate at 24 months was higher for patients with concordant planning (77.8% vs. 50%, p = 0.057). A multivariate analysis was conducted on the prediction of restenosis. Among factors, only discordant planning was found to be a significant predictor of restenosis with an odds ratio of 0.115 (95% confidence interval, 0.02-0.674; p = 0.016). Conclusion The absence of sizing for peripheral lesions engenders a tendency to choose the wrong stent, in particular in terms of diameter in iliac arteries and length in femoral arteries.

Use of covered stents in the central vein: a feasibility study in a canine model

PURPOSE

To evaluate the feasibility of using covered stents vs. bare stents in a model of central vein stenosis with an arteriovenous graft created to mimic the conditions in hemodialysis patients.

METHODS

In 7 mongrel dogs, an expanded polytetrafluoroethylene-covered nitinol stent was placed in one common iliac vein and a bare stent was placed in the contralateral vein. Arteriovenous grafts were created bilaterally between the common femoral artery and vein to induce endothelial damage. Neointima formation in the covered stents was compared to the bare stents at 12 weeks using microscopy and histochemical staining.

RESULTS

Two dogs were excluded due to thrombosis and infection of the arteriovenous grafts, but all stents in the remaining 5 dogs were patent. Smooth, complete neointimal coverage was observed on the inner surface of all the covered stents without intraluminal thrombus. In contrast, incomplete neointimal coverage was seen in all bare stents, with small focal thrombi adhering to the neointima on 3 bare stents. Focal nodular neointimal hyperplasia with denudation of the endothelium was observed in only 2 bare stents. Mean neointimal thickening was significantly greater in the covered stents. Eccentric neointimal thickening was observed at the inflow and outflow segments of both types of stents.

CONCLUSION

Covered stents are technically feasible for the treatment of central vein stenosis, and they demonstrate complete, smooth neointimal coverage in normal central veins, but they also display greater neointimal thickening than bare stents.

The consequences of the mechanical environment of peripheral arteries for nitinol stenting

The use of stents in peripheral arteries has not been as successful as in coronary arteries, with high rates of restenosis and stent fracture common. Normal joint flexion induces a range of forces on the arteries, which has an unknown effect on the outcomes of stenting. The objective of this study is to determine how physiological levels of vessel bending and compression following stent implantation will influence the magnitude of stent stresses and hence the risks of fatigue fracture. A further objective is to compare how this mechanical environment will influence arterial stresses following implantation of either stainless steel or nitinol stents. To this end, models of both nitinol and stainless steel stents deployed in peripheral arteries were created, with appropriate loading conditions applied. At high levels of bending and compression, the strain amplitude threshold value for fatigue failure is exceeded for nitinol stents. Bending was predicted to induce high stresses in the artery following stenting, with higher arterial stresses predicted following implantation of a stainless steel stent compared to a nitinol stent. Both bending and compression may contribute to stent fracture by increasing the strain amplitude within the stent, with the dominant factor dependant on location within the arterial tree. For the specific stent types investigated in this study, the model predictions suggest that compression is the dominant mechanical factor in terms of stent fatigue in the femoral arteries, whereas bending is the most significant factor in the popliteal artery. To increase fatigue life and reduce arterial injury, location specific stent designs are required for peripheral arteries.

Self-expanding nitinol renal artery stents: comparison of safety and efficacy of bare versus Polyzene-F nanocoated stents in a porcine model

OBJECTIVE

To investigate the safety and efficacy of a Polyzene-F nanocoat on new low-profile self-expandable nitinol stents in minipig renal arteries.

MATERIALS AND METHODS

Ten bare nitinol stents (BNS) and 10 stents coated with a 50 nm-thin Polyzene-F coating were randomly implanted into renal arteries of 10 minipigs (4- and 12-week follow-up, 5 animals/group). Thrombogenicity, on-stent surface endothelialization, vessel wall injury, late in-stent stenosis, and peristrut vessel wall inflammation were determined by quantitative angiography and postmortem histomorphometry.

RESULTS

In 6 of 10 BNS, >50% stenosis was found, but no stenosis was found in stents with a nanothin Polyzene-F coating. Histomorphometry showed a statistically significant (p < 0.05) different average maximum luminal loss of 55.16% ± 8.43% at 12 weeks in BNS versus 39.77% ± 7.41% in stents with a nanothin Polyzene-F coating. Stents with a nanothin Polyzene-F coating had a significantly (p < 0.05) lower inflammation score after 12 weeks, 1.31 ± 1.17 versus 2.17 ± 0.85 in BNS. The results for vessel wall injury (0.6 ± 0.58 for Polyzene-F-coated stents; 0.72 ± 0.98 for BNS) and re-endothelialization, (1.16 ± 0.43 and 1.23 ± 0.54, respectively) were not statistically significant at 12-week follow-up. No thrombus deposition was observed on the stents at either follow-up time point.

CONCLUSION

Nitinol stents with a nanothin Polyzene-F coating successfully decreased in-stent stenosis and vessel wall inflammation compared with BNS. Endothelialization and vessel wall injury were found to be equal. These studies warrant long-term pig studies (≥120 days) because 12 weeks may not be sufficient time for complete healing; thereafter, human studies may be warranted.

The role of vessel geometry and material properties on the mechanics of stenting in the coronary and peripheral arteries

There have been notably higher rates of restenosis with stents used to restore blood flow to many stenosed peripheral arteries compared with their coronary counterparts. The mechanical environment of arteries such as the femoral and popliteal (and the stent fracture that this can cause) has previously been identified as a contributing factor to the relatively low success rates for this procedure. The aim of this study was to investigate how other factors, namely the differences in geometries and mechanical properties of the arteries and the stents used in them, might influence the outcome in these different arteries. Finite element models of the stents and arteries were created, and the results compared in terms of stresses induced in the arteries, the lumen gain, and the deformation of the stent due to pulsatile loading. It was found that deploying a Nitinol stent in a peripheral artery induced lower stresses in the vessel wall than expanding a stainless steel stent in a coronary artery, although the lumen gain was also lower. The predicted strain amplitude induced in Nitinol stents by the cardiac cycle was below the value required to cause fatigue failure. This study does not provide any evidence to suggest that differences in the geometry and material properties between peripheral and coronary arteries, or the types of stent used to restore vessel patency, are the dominate factors responsible for the higher rates of restenosis observed in peripheral arteries.

Assessment of tissue prolapse after balloon-expandable stenting: influence of stent cell geometry

Restenosis is a re-narrowing or blockage of an artery at the same site where treatment, such as a balloon angioplasty or stent procedure, has already taken place. Several clinical trials have shown a significant reduction in the restenosis rates with endovascular stenting. The purpose of stenting is to maintain the arterial lumen open by a scaffolding action that provides radial support. However, stenting can cause a vascular injury during the deployment. Indeed, in-stent restenosis remains a major problem in percutaneous coronary intervention, requiring patients to undergo repeated procedures and surgery. The loading imposed by the deployment of the stent on the artery is involved in the restenosis process. Furthermore, it is well known that the stent design plays a role in the outcome of the stenting interventional procedure. This study compares the mechanical effects of the expansion of five different designs of balloon-expandable stents in a coronary artery by means of numerical models based on the finite element method. An index for the evaluation of the tissue prolapse based on the expanded configuration reached by the stent cells is proposed. The effects of the balloon inflation and deflation are included in the present study. Wall stresses and tissue prolapse of the vessel wall within the stent cells are evaluated and compared among the different stent designs. Results show that the printed area does not predict prolapse, and that the proposed index (PI) does correlate with tissue prolapse.

Simulation of stent deployment in a realistic human coronary artery

Background

The process of restenosis after a stenting procedure is related to local biomechanical environment. Arterial wall stresses caused by the interaction of the stent with the vascular wall and possibly stress induced stent strut fracture are two important parameters. The knowledge of these parameters after stent deployment in a patient derived 3D reconstruction of a diseased coronary artery might give insights in the understanding of the process of restenosis.

Methods

3D reconstruction of a mildly stenosed coronary artery was carried out based on a combination of biplane angiography and intravascular ultrasound. Finite element method computations were performed to simulate the deployment of a stent inside the reconstructed coronary artery model at inflation pressure of 1.0 MPa. Strut thickness of the stent was varied to investigate stresses in the stent and the vessel wall.

Results

Deformed configurations, pressure-lumen area relationship and stress distribution in the arterial wall and stent struts were studied. The simulations show how the stent pushes the arterial wall towards the outside allowing the expansion of the occluded artery. Higher stresses in the arterial wall are present behind the stent struts and in regions where the arterial wall was thin. Values of 200 MPa for the peak stresses in the stent strut were detected near the connecting parts between the stent struts, and they were only just below the fatigue stress. Decreasing strut thickness might reduce arterial damage without increasing stresses in the struts significantly.

Conclusion

The method presented in this paper can be used to predict stresses in the stent struts and the vessel wall, and thus evaluate whether a specific stent design is optimal for a specific patient.