Carotid Geometry as a Predictor of In-Stent Neointimal Hyperplasia - A Computational Fluid Dynamics Study

Comparison of blood viscosity models in different degrees of carotid artery stenosis

Background

Carotid atherosclerotic vascular disease significantly contributes to strokes, presenting a heightened risk of early recurrent ischemia. Computational fluid dynamics (CFD) has shown potential in predicting subsequent stroke recurrence in patients with carotid stenosis.

Objective

This study aims to investigate the differences in computational time and accuracy of four key hemodynamic indices-wall shear stress (WSS), time-averaged wall shear stress (TAWSS), Oscillatory Shear Index (OSI), and relative residence time (RRT)-across different viscosity models, thereby providing optimal model selection for clinical cases and offering guidance for clinicians' decision-making.

Methods

A three-dimensional vessel model was established using computed tomography angiography (CTA), followed by CFD simulations to calculate WSS, TAWSS, OSI, and RRT. The accuracy of the simulations was validated by comparing the results with those from Razavi et al. (10.1016/j.jbiomech.2011.04.023). Numerical errors in different parameters under varying stenosis levels and viscosity models were analyzed.

Results

In the transient state, when degree of stenosis is 38%, 72%-84%, the performance difference between the two is less than 6%. For TAWSS, the difference is 0% when degree of stenosis is 12%, 18%, 26%, 54%, and 76%. For OSI, the difference is 0% when stenosis is 16%, 18%, 26%. For RRT, the difference between the two is 0% when degree of stenosis is 18% and 84%. WSS exhibited an increasing trend with higher degrees of stenosis. TAWSS demonstrated significant variation in moderate to severe stenosis, while OSI increased markedly in cases of moderate to severe stenosis. High RRT values in severely stenotic regions indicated a propensity for atherosclerotic lesion development.

Conclusion

This study systematically quantified the discrepancies between Newtonian and non-Newtonian blood viscosity models in predicting hemodynamic parameters across different degrees of carotid artery stenosis. Statistical analyses revealed significant differences between the two models in WSS, TAWSS, OSI, and RRT (p < 0.001 for all parameters). Newtonian models, while computationally efficient, overestimated TAWSS, OSI, and RRT while underestimating WSS, particularly in moderate to severe stenosis. In contrast, non-Newtonian models provided more physiologically accurate predictions, especially in regions with high shear stress variations. The results emphasize the importance of selecting appropriate viscosity models for CFD-based patient-specific risk assessment, particularly in stroke prediction, plaque evaluation, and surgical planning. Non-Newtonian models should be prioritized in high-risk patients where flow disturbances are more pronounced, whereas Newtonian models remain suitable for early screening and rapid assessments.

Correlation of computed fractional flow and in-stent restenosis in patients with intracranial atherosclerotic stenosis

OBJECTIVE

Fractional flow (FF) reserve has been developed as a gold standard for coronary intervention. Intracranial FF is also a valuable hemodynamic index to assess the severity of narrowing in intracranial atherosclerotic stenosis (ICAS). This study aims to investigate the predictive value of FF in assessing restenosis following endovascular treatment in patients with symptomatic ICAS.

METHODS

This retrospective study recruited 67 patients with symptomatic ICAS who received intracranial stenting between March 2019 and January 2024. FF was measured by dedicated software (AccuICAD) before and after stenting. During follow-up, patients were categorized into two groups based on the occurrence of in-stent restenosis (ISR): ISR group and non-ISR group. Multivariate regression analysis and Kaplan-Meier survival analysis were performed to identify the predictive factors for ISR.

RESULTS

Post-FF was significantly different between the ISR and non-ISR groups (0.84±0.09 vs 0.92±0.06, respectively, P<0.01). Univariate and multivariate Cox regression analyses identified post-FF (HR 0.0, 95% CI 0.0 to 0.08, P=0.005) and smoking (HR 3.06, 95% CI 1.02 to 9.19, P=0.047) as the two predictors of ISR. Receiver operating characteristic curve analysis confirmed the predictive value of post-FF for ISR (AUC=0.783, 95% CI 0.645 to 0.920, P=0.003), with a cut-off value of 0.94. Kaplan-Meier survival analysis further demonstrated that patients with a post-FF value >0.94 had a significantly lower incidence of ISR (P=0.001).

CONCLUSION

In this study, post-FF effectively predicted ISR, providing an intraoperative evaluation value for stenting in ICAS.

Local artery geometry characteristics associated with middle cerebral atherosclerotic stenosis

OBJECTIVES

The impact of artery geometry on intracranial atherosclerosis (ICAS) is unclear. To investigate the association between local artery geometry characteristics and middle cerebral atherosclerotic stenosis.

METHODS

This was a case-control study. ICAS patients with more than 50% stenosis in one M1 pre-bifurcation segment but no significant stenosis on the other side were included. The stenosis degree, diameter, length, angle, and morphology (straight-shape, U-shape, and S-shape) of the M1 pre-bifurcation segments were measured by using digital subtraction angiography (DSA) combined with computed tomography angiography (CTA) and 3D reconstruction imaging. The geometry characteristics on both sides were compared. The conditional multivariate Logistics regression was used to investigate the association between stenotic sides and the geometry characteristics.

RESULT

A total of 133 patients, 266 M1 pre-bifurcation segments, were included. Compared with non-stenotic sides, stenotic sides showed smaller diameters (2.873 ± 0.456 mm vs. 2.993 ± 0.360 mm, P = 0.002) and larger tortuosity index (7.61% vs. 6.45%, P = 0.048). The S-shape M1 pre-bifurcation segment was independently associated with the stenotic sides (OR = 3.509, 95% CI 1.176-10.468, P = 0.024). The stenotic side was worse fitted to Murray's law (mean square error 0.326 vs. 0.313).

CONCLUSIONS

The morphology of the M1 pre-bifurcation segment was associated with middle cerebral atherosclerotic stenosis.

Effects of stent shape on focal hemodynamics in intracranial atherosclerotic stenosis: A simulation study with computational fluid dynamics modeling

Background and aims

The shape of a stent could influence focal hemodynamics and subsequently plaque growth or in-stent restenosis in intracranial atherosclerotic stenosis (ICAS). In this preliminary study, we aim to investigate the associations between stent shapes and focal hemodynamics in ICAS, using computational fluid dynamics (CFD) simulations with manually manipulated stents of different shapes.

Methods

We built an idealized artery model, and reconstructed four patient-specific models of ICAS. In each model, three variations of stent geometry (i.e., enlarged, inner-narrowed, and outer-narrowed) were developed. We performed static CFD simulation on the idealized model and three patient-specific models, and transient CFD simulation of three cardiac cycles on one patient-specific model. Pressure, wall shear stress (WSS), and low-density lipoprotein (LDL) filtration rate were quantified in the CFD models, and compared between models with an inner- or outer-narrowed stent vs. an enlarged stent. The absolute difference in each hemodynamic parameter was obtained by subtracting values from two models; a normalized difference (ND) was calculated as the ratio of the absolute difference and the value in the enlarged stent model, both area-averaged throughout the arterial wall.

Results

The differences in focal pressure in models with different stent geometry were negligible (ND<1% for all cases). However, there were significant differences in the WSS and LDL filtration rate with different stent geometry, with ND >20% in a static model. Observable differences in WSS and LDL filtration rate mainly appeared in area adjacent to and immediately distal to the stent. In the transient simulation, the LDL filtration rate had milder temporal fluctuations than WSS.

Conclusions

The stent geometry might influence the focal WSS and LDL filtration rate in ICAS, with negligible effect on pressure. Future studies are warranted to verify the relevance of the changes in these hemodynamic parameters in governing plaque growth and possibly in-stent restenosis in ICAS.

The Need to Shift from Morphological to Structural Assessment for Carotid Plaque Vulnerability

Degree of luminal stenosis is generally considered to be an important indicator for judging the risk of atherosclerosis burden. However, patients with the same or similar degree of stenosis may have significant differences in plaque morphology and biomechanical factors. This study investigated three patients with carotid atherosclerosis within a similar range of stenosis. Using our developed fluid-structure interaction (FSI) modelling method, this study analyzed and compared the morphological and biomechanical parameters of the three patients. Although their degrees of carotid stenosis were similar, the plaque components showed a significant difference. The distribution range of time-averaged wall shear stress (TAWSS) of patient 2 was wider than that of patient 1 and patient 3. Patient 2 also had a much smaller plaque stress compared to the other two patients. There were significant differences in TAWSS and plaque stresses among three patients. This study suggests that plaque vulnerability is not determined by a single morphological factor, but rather by the combined structure. It is necessary to transform the morphological assessment into a structural assessment of the risk of plaque rupture.

Hemodynamic and Geometric Risk Factors for In-Stent Restenosis in Patients with Intracranial Atherosclerotic Stenosis

Methods

Severe ICAS patients managed with percutaneous transluminal angioplasty and stenting (PTAS) were included in the retrospective cohort study and were divided into two groups according to whether ISR occurred at follow-up (ISR group and no-ISR group). Computational fluid dynamics models were built based on digital subtraction angiography before and after PTAS to simulate blood flow and quantify hemodynamic parameters. The associations between vessel geometry, hemodynamics, and ISR in ICAS patients were investigated.

Results

Among 39 patients, ISR occurred in seven patients (17.95%) after a mean follow-up period of 6.69 ± 3.24 months. Stenting decreased vessel angulation (51.11° [40.07°-67.27°] vs. 15.97° [0.00°-36.16°], P = 0.000) and vessel tortuosity (0.09 [0.06-0.13] vs. 0.01 [0.00-0.03], P = 0.000). Meanwhile, the translational pressure ratio (PR) dramatically increased (0.07 [0.00-0.31] vs. 0.62 [0.41-0.82], P = 0.000) with the wall shear stress ratio decreased (13.93 [8.37-40.30] vs. 2.90 [1.69-4.48], P = 0.000). In the multivariate analysis, smaller Δ tortuosity (P = 0.038) was independently associated with the occurrence of ISR, and smaller post-PTAS translesional PR was also a predictive factor of marginal significance (P = 0.059).

Conclusion

PTAS decreased vessel angulation, vessel tortuosity, and translesional wall shear stress ratio while it increased translesional pressure ratio (PR) dramatically in ICAS patients. Smaller Δ tortuosity was found to be a risk factor for ISR, and smaller post-PTAS translesional PR was also a predictive factor of marginal significance, indicating that both geometric and hemodynamic parameters played important roles in the occurrence of ISR after PTAS.

Influence of Stent Flexibility on Artery Wall Stress and Wall Shear Stress in Bifurcation Lesions

Purpose

Stent flexibility can influence clinical outcome, especially in bifurcation lesions. For instance, an overly rigid stent can impose mechanical stress on the artery at the stent edges and alter both arterial geometry and blood flow dynamics in bifurcations. This study investigated the influence of stent flexibility on vessel geometry, histology, wall stress, and blood flow dynamics in arterial bifurcations.

Materials and Methods

We compared arterial angulation, stenosis, histopathology, simulated wall shear stress (WSS), and simulated blood flow velocity distribution in swine coronary artery bifurcations following placement of the less flexible Multi-link 8 or more flexible Kaname stent (4.1 ± 0.5 vs 1.5 ± 0.1 mN, p < 0.05, t-test). Stents were implanted into six coronary artery bifurcations each using the single-stent crossover technique without side branch strut dilatation. Outcomes were examined after 28 days.

Results

Implantation of both stents significantly increased site angulation (Multi-link 8: 148° ± 8° to 172° ± 2°, p < 0.05, paired t-test; Kaname: 152° ± 5° to 164° ± 4°, p < 0.05, paired t-test), but the change tended to be greater after Multi-link 8 stent implantation (24° ± 15° vs 11° ± 7°, p = 0.1, t-test), suggesting greater straightening of the bifurcation. The Multi-link 8 stent induced greater neointimal thickness than the Kaname stent (0.53 ± 0.3 mm vs 0.26 ± 0.1 mm, p < 0.05, t-test). The distribution of neointimal hyperplasia following stent implantation as revealed by longitudinal histopathology matched the distribution of WSS simulated using computational fluid dynamics (CFD). The endothelium at low WSS areas exhibited aberrant cell morphology and leukocyte adhesion. A CFD model of a curved bifurcation suggested that the region of low WSS is expanded by artery straightening.

Conclusion

In bifurcated lesions, stent flexibility influences not only mechanical stress on the artery but also WSS, which may induce local neointimal hyperplasia.

Blood flow simulations in patient-specific geometries of the carotid artery: A systematic review

Computational Fluid Dynamics (CFD) and Fluid-Structure Interaction (FSI) are currently widely applied in the study of blood flow parameters and their alterations under pathological conditions, which are important indicators for diagnosis of atherosclerosis. In this manuscript, a systematic review of the published literature was conducted, according to the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses, on the simulation studies of blood flow in patient-specific geometries of the carotid artery bifurcation. Scopus, PubMed and ScienceDirect databases were used in the literature search, which was completed on the 3rd of August 2020. Forty-nine articles were included after the selection process and were organized in two distinct categories: the CFD studies (36/49 articles), which comprise only the fluid analysis and the FSI studies (13/49 articles), which includes both fluid and Fluid-Structure domain in the analysis. The data of the research works was structured in different categories (Geometry, Viscosity models, Type of Flow, Boundary Conditions, Flow Parameters, Type of Solver and Validation). The aim of this systematic review is to demonstrate the methodology in the modelling, simulation and analysis of carotid blood flow and also identify potential gaps and challenges in this research field.