Coronary artery stent design for challenging disease: insights into patient specific modelling

Abstract

In the last two decades, numerical methods have been a widely recognised tool for investigating stenting procedures. Initially, computer models of stenting were restricted to ideal vessels and in some cases by two dimensional analysis due to limited computational capabilities and resources. Nevertheless, nowadays, the increased computational power along with the development of solid imaging processing techniques, have launched a new category in computational stenting, that of imaged-based computational modelling.

Recent clinical evidence has shown that new generation stents are better in terms of in-stent restenosis and stent thrombosis. However, improving stent performance regarding one factor can impair others and, as a result, a compromised approach is likely to be necessary. This fact seems to be more evident in challenging anatomies where a long and flexible stent has to be implanted. Challenging anatomies can be characterised by long and tortuous geometry, comprising non-focal and highly calcified plaque. Common complications of percutaneous coronary intervention in such anatomies include stent malapposition and stent longitudinal deformation.

The aims of this doctoral work were (i) to reconstruct diseased patient-specific coronary artery segments, (ii) simulate the deployment of state of the art stents into these segments following model validation and verification, (iii) assess the degree of stent malapposition and stent longitudinal deformation, (iv) design stent systems to mitigate the risk of stent malapposition and longitudinal deformation in these segments and (v) analyse optimum stent deployments according to a patient-specific vessel.

Patient-specific cases were reconstructed by combining coronary angiography and ultrasonography to an acceptable accuracy level for the computational purposes of this project. Then, after generating contemporary virtual stent/balloon models, they were validated/calibrated against experimental data. In addition, novel varying diameter balloon models and a modified stent were generated to mitigate the risk of stent malapposition and longitudinal deformation, respectively. After developing an inexpensive numerical methodology for image-based stenting simulations, numerous patient-specific structural simulations were carried out to investigate the effect of i) different stent design in stent malapposition and longitudinal deformation and ii) different dilation system design in stent malapposition. Finally, a multi-objective optimisation framework was presented to investigate the optimum dilation protocol in a patient-specific segment via structural and surrogate modelling.

Results indicate that stent malapposition, for the simulated patient specific cases, is dependent on the so-called “reference diameter”. Remarkably, the proposed balloon models demonstrated superior results of performance especially as far as stent malapposition is concerned. In particular, they led to an approximately 40% reduction in malapposed struts when compared with the baseline models, whilst maintaining a relatively low stressed mechanical environment. As for stent longitudinal deformation, the outcomes indicated that (i) it is significantly different between the stent platforms in a manner consistent with physical testing in a laboratory environment, (ii) there was a smaller range of variation for simulations of in vivo performance relative to models of in vitro experiments, and (iii) the modified stent design demonstrated considerably higher longitudinal integrity. Interestingly, it was shown that stent longitudinal stability may differ significantly after a localised in vivo force compared to a distributed in vitro force. Lastly, the multi-objective optimisation study demonstrated that given a patient-specific vessel, different optimum dilation strategies could be extracted according to the interventional cardiologist’s preference.

Significantly, this work computationally investigates stent longitudinal deformation and stent malapposition of patient-specific reconstructed vessels. Such numerical models can provide three dimensional qualitative and quantitative information in the investigated clinical problems. Moreover, they may represent a potentially valuable tool for predicting stent malapposition, avoiding stent deformations and, consequently, optimising the interventional protocol according to any patient-specific case.

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