Carotid plaque response to arterial stenting : an experimental and numerical investigation

Each year, approximately 8,500 people suffer a stroke in Ireland, of which 2,500 are fatal. Cardiovascular disease (CVD) is the leading cause of death in the European Union attributing to over two million deaths each year. CVD across all member states has an annual cost to the EU economy of €192 billion. Of the cardiovascular diseases, stroke is second most common cause of death, after heart disease (European Heart Network, 2008). Carotid artery disease is a blockage of the carotid arteries due to a build-up of atherosclerosis. Blood is supplied to the brain by the carotid arteries, which are located on either side of the neck and serve as the main conduit for blood flow to the brain. The brain requires oxygen and nutrients (eg. glucose) which are supplied through the circulation. The most common type of stroke is an ischemic stroke which is caused by a thrombus (blood clot) travelling in the blood stream and lodging in the carotid artery, thus depriving the brain of the necessary nutrients. If the blood supply is not restored, these brain cells will die. The current gold standard for treating diseased carotid arteries is endarterectomy (CEA), in which plaque is removed through an invasive surgical procedure requiring general anaesthesia. Carotid artery stenting (CAS) is an alternative to the more invasive CEA as this method employs a stent and/or balloon to dilate the occluded artery in order to restore blood flow to the brain (Laroche et al, 2006). The main advantages of CAS are that it is minimally-invasive, involves reduced scarring and risk of infection, and both the procedural time and associated healthcare costs are significantly reduced. The main concern with the CAS surgical procedure is that the impact of stent deployment on arterial plaque is not predictable and the behaviour of the plaque is equally unknown. This work addressed the primary issue of plaque dislodgement associated with CAS, which can cause minor/major stroke due to the size of dislodged plaque trapped distally in the carotid artery. A portable tensile test rig was designed and developed to effectively clamp the biological tissue and determine the radial properties of the plaque. The ability of the plaque to withstand the deployed stent during the CAS surgery without fracture was then accessed. A number of fresh patient carotid plaques were tested on this rig post-CEA and a predictive model of plaque response to stent deployment was developed. The experimental data determined at the hospital site post-endarterectomy was used as a predictive model to determine pre-operatively whether a type of plaque, as identified during ultrasound screening prior to surgery, would rupture during a CAS procedure. The current trend of moving from traditional open surgery to minimally invasive techniques will require that the mechanical properties of the carotid plaque are understood when treating diseased carotid arteries. From work undertaken, and published during this study, the Stress values determined did agree with those reported in current literature (Holzapfel et al, 2004). Also the Yeoh Strain Energy Function did fit modes of deformation using the data obtained from uniaxial tests (Chen & Wu, 1997). It is expected that the material coefficients identified in the work can be used for the design and optimisation of future interventional medical devices when treating diseased carotid arteries.