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Simulation: surgery tailored to you

Pete Wilton

Bioengineering is an exciting and diverse field covering everything from carbon nanotubes to microrobotics and from gene therapy to virtual drug testing.

Oxford’s IBME recently hosted Bioengineering '09, a conference bringing together the latest research in this area.

I caught up with conference organiser Yiannis Ventikos of Oxford University’s Department of Engineering Science to ask him about his group’s research into modelling stents – just one example of what can happen when engineering science meets clinical practice:

OxSciBlog: How does modelling help inform surgical interventions?
Yiannis Ventikos: Uniquely, computational modelling, or ‘simulation’, has the capability to predict the outcome of interventions, in a personalised and patient-specific fashion. Appropriate computer models can answer questions like: ‘how will this disease progress?’, ‘should we intervene aggressively, say surgery, or should we follow a more conservative approach, for example pharmaceutical?’ or ‘if we have decided on intervention, which of the available scenarios is the preferred one?’

The unique power of simulation is that it answers these questions in a personalised fashion. Statistics, demographics and epidemiology of the type commonly used in clinical decision-making are used to validate the models, but once the models are validated, they answer these questions, sometimes from first principles, for the specific genetics, family history and morphology/physiology of each individual patient.

So, the answers obtained are not based on some vague average but on the particular characteristics of the person in need of treatment for the specific disease.

OSB: Why is understanding how different stents work so important?
YV: Stents are used to treat many types of vascular pathologies. A relatively new approach used in interventional neuroradiology is to implant open stents at the neck of cerebral aneurysms in order to reduce blood flow into them. This procedure is a minimally invasive one, since the stents can be delivered using a catheter, through the artery, entering the body via a tiny incision, usually at the thigh.

Oxford and the Department of Neuroradiology are amongst the world’s leading centres where such aneurysms in the brain are treated and therefore making sure that this procedure is done optimally is really important. The open stents used in this procedure are effectively cylindrical-shaped wire meshes that allow blood to pass towards side vessels, but reduce flow adequately towards the aneurysm sack so as to promote stagnation and the formation of a stable clot in the aneurysm.

OSB: How can modelling help to evaluate stents?
YV: Open stents for cerebral aneurysms sound like an interventionist’s dream treatment, a minimally invasive approach that actually addresses the problem for a very wide range of aneurysm morphologies. There is a problem however: not all stents successfully occlude (close) all aneurysms.

The problem here is one of fluid dynamics: putting what is effectively a grid in front of the aneurysm opening must lead to a reduction of inflow that is adequate to induce stagnation and clotting. There is no intuitive way to estimate that however. The reduction of inflow depends on the stent weaving, but also on the local haemodynamics: the speed and direction of blood flow, the morphology and orientation of the opening, and above all, on the interplay of all of the above. Second-guessing whether a stent will work or not is no good – if a wrong decision is made, further interventions are then needed that burden the patient.

Simulation answers this question brilliantly: stents from different manufacturers (or new designs) are virtually placed into position, in a similarly virtual anatomically accurate representation of the vasculature, and their performance is evaluated preoperatively.

Not only can we choose the best device for a particular aneurysm, we can also decide on the best orientation and positioning of the implant. It’s all virtual, done on the computer a day or two before the intervention and it is 100 per cent safe and repeatable. It gives us confidence that when the time for the actual intervention comes, the clinician’s skills will be supplemented with robust knowledge of which device will do the best job for a particular patient, for a particular aneurysm actually, and how this device should be used for best results.

OSB: What are the next steps in your research?
YV: The conceptual framework described above (predictive, simulation-based interventional planning) can be applied to a whole host of conditions. Similarly, the techniques and tools we develop for one disease are often applicable, with little modification, to other diseases too.

We are expanding this technique now to account for a different type of implant, a stentgraft, as used for the treatment of acute aortic disease (like for example, aortic dissections). There, closed conduits (made of reinforced synthetic material) are used to line and so strengthen weak, diseased aortas. Again, models that are as representative of the real situation as possible are being devised; models that incorporate the blood flow dynamics, the vascular wall adaptation and its interaction with the implant, biochemical processes like thrombosis etc.