Elsevier

Journal of Biomechanics

Volume 49, Issue 9, 14 June 2016, Pages 1570-1582
Journal of Biomechanics

Impact of bifurcation angle and other anatomical characteristics on blood flow – A computational study of non-stented and stented coronary arteries

https://doi.org/10.1016/j.jbiomech.2016.03.038Get rights and content

Abstract

The hemodynamic influence of vessel shape such as bifurcation angle is not fully understood with clinical and quantitative observations being equivocal. The aim of this study is to use computational modeling to study the hemodynamic effect of shape characteristics, in particular bifurcation angle (BA), for non-stented and stented coronary arteries.

Nine bifurcations with angles of 40°, 60° and 80°, representative of ±1 SD of 101 asymptomatic computed tomography angiogram cases (average age 54±8 years; 57 females), were generated for (1) a non-stented idealized, (2) stented idealized, and (3) non-stented patient-specific geometry. Only the bifurcation angle was changed while the geometries were constant to eliminate flow effects induced by other vessel shape characteristics. The commercially available Biomatrix stent was used as a template and virtually inserted into each branch, simulating the T-stenting technique. Three patient-specific geometries with additional shape variation and ±2 SD BA variation (33°, 42° and 117°) were also computed. Computational fluid dynamics (CFD) analysis was performed for all 12 geometries to simulate physiological conditions, enabling the quantification of the hemodynamic stress distributions, including a threshold analysis of adversely low and high wall shear stress (WSS), low time-averaged WSS (TAWSS), high spatial WSS gradient (WSSG) and high Oscillatory Shear Index (OSI) area.

The bifurcation angle had a minor impact on the areas of adverse hemodynamics in the idealized non-stented geometries, which fully disappeared once stented and was not apparent for patient geometries. High WSS regions were located close to the carina around peak-flow, and WSSG increased significantly after stenting for the idealized bifurcations. Additional shape variations affected the hemodynamic profiles, suggesting that BA alone has little effect on a patient׳s hemodynamic profile. Incoming flow angle, diameter and tortuosity appear to have stronger effects.

This suggests that other bifurcation shape characteristics and stent placement/strategy may be more important than bifurcation angle in atherosclerotic disease development, progression, and stent outcome.

Introduction

Coronary arterial bifurcations are prone to atherosclerosis development, and 20–30% of all percutaneous coronary interventions (PCI) involve bifurcations. They are also vulnerable to stent thrombosis and restenosis, with a 3–10 fold higher risk compared to non-bifurcation lesions (Lakovou et al., 2005).

The bifurcation angle (BA), the angle between the daughter vessels after branching (angle B using European Bifurcation Club nomenclature), has been suggested as risk predictor in clinical settings (Dzavik et al., 2006, Sun and Cao, 2011, Watanabe et al., 2014).

Sun and Cao (2011) determined the relationship between angle and plaque formation by analyzing computed tomography angiograms of 22 patients with left main disease, finding a direct correlation between a wide BA and dimensional changes downstream of the lesion. Others found a correlation between wide BA and presence of lipid arc using optical coherence tomography (OCT) in 47 lesions with various Medina classifications (location of plaque burden at the bifurcation) (Watanabe et al., 2014).

Dzavik et al. (2006) studied the outcome of 133 patients who underwent PCI with “crush” stenting and identified a correlation between major cardiac events (MACE), including myocardial infarction, repeated target lesion, bypass graft surgery, repeated PCI or death, and wide BAs with 23% and 7% events in those above and below the median BA respectively. It is not clear, however, how these observations were influenced by the crush stenting technique. In contrast, another clinical trial of 266 patients found no relationship between BA and adverse clinical events (Girasis et al., 2010).

Computational studies have been used to understand the hemodynamic mechanisms underlying clinical observations (Morlacchi and Migliavacca, 2013), but few have focused on BA in the coronaries (Chaichana et al., 2011, Malvè et al., 2015). Chaichana et al. (2011) simulated the flow in eight idealized, and four patient-specific left main bifurcations. Direct correlations between BA and disturbed flow patterns were observed in the wide angle models. Other patient-specific computational studies have shown that BA had no impact on the bifurcation hemodynamics, and found tortuosity to be a more prominent factor (Lee et al., 2008, Malvè et al., 2015).

Coronary BA studies (Chaichana et al., 2011) have found consistent observations between idealized and patient-specific geometries, and concluded that idealized geometries are representative of patient-specific geometries when comparing the impact of vessel shape on hemodynamics.

Many studies have considered the hemodynamic effects of stents (Gijsen et al., 2008, LaDisa et al., 2003, Lewis, 2008). However, to our knowledge no computational study has considered if the presence of stents alters vessel shape-induced flow. In short, does the presence of a stent mitigate or aggravate the effect of BA or other shape characteristics?

The aim of this study was therefore to investigate the relationship between hemodynamic measures and shape characteristics, in particular the BA. Idealized non-stented and stented geometries with a common ‘Y’- bifurcation shape were chosen, and only the BA varied to eliminate other shape-induced flow effects. A second step, a non-stented patient-specific geometry was modified only in its BA. Finally, patient-specific geometries which varied in other shape characteristics (inflow angle, diameter, and tortuosity) were studied to elucidate how other coronary shape characteristic compare with BA.

Section snippets

Idealized non-stented coronary arteries

An average simplified geometry of the left main bifurcation dividing into the left anterior descending (LAD) and circumflex (LCX) arteries, was derived from CT angiograms from 101 asymptomatic patients (average age 54±8 years; 57 females) (Medrano-Gracia et al., 2014). This yielded an average LAD diameter of approximately 4 mm and 3.5 mm for the LCX, and the BA was 62°±22° (mean ± 1 SD) between the two. Based on these findings, three representative idealized Y-shaped bifurcation geometries were

Results

Endothelial cells respond to shear stress and ideally cover the stent surface within a few days after PCI (Jiménez and Davies, 2009, Malek et al., 1999). For these reasons, all reported quantities were normalized with to area being considered such as the vessel and stent surface (where applicable), plus 1 mm upstream of the stent in the main vessel, and 3 mm downstream of the stent in both daughter vessels (Fig. 1b). Table 2 summarizes the area-averaged statistics of the TAWSS distribution.

TAWSS

Discussion

This study was performed to explore the relationship between shape characteristics and hemodynamic stress distribution before and after stenting of idealized Y-shaped, non-stented BA modified and non-modified patient-specific bifurcations. The results provide four primary findings.

First, the BA alone had minor impact on the stress distribution, and is consistent with a statistical shape-stress analysis of patient-specific bifurcations (Malvè et al., 2015), where it was shown that BA is a weak

Conclusion

In conclusion, the hemodynamic impact of Y-shaped BA between 40°, 60° and 80° for non-stented and stented bifurcations showed minor changes of adverse hemodynamic thresholds during the cardiac cycle. This suggests that other vessel shape characteristics (such as tortuosity, diameter or incoming angle), and stent placement may introduce flow patterns relevant for atherosclerotic disease development and progression.

Disclosure

J.O. provides consulting advice to the Advisory Boards of Abbott Vascular.

Acknowledgments

S.B. was funded by the Auckland Heart Group Charitable Trust. P.M.G. was supported by the Green Lane Research and Educational Fund. The authors wish to acknowledge NeSI high performance computing facilities (https://www.nesi.org.nz) for their support of this research. New Zealand׳s national facilities are provided by the New Zealand eScience Infrastructure and funded jointly by NeSI׳s collaborator institutions through the Ministry of Business, Innovation & Employment Research Infrastructure.

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