• Highly important disease: causes failure of arterial stents, bypass grafts, vascular access grafts, balloon angioplasty, AV fistulae.
• Pathology: accumulation of smooth muscle cells and extracellular matrix in intima.
• Cause: possibly vessel damage but, similar to atherosclerosis, IH is influenced by flow-favours low wall shear/stagnation regions and haemodynamic forces.
Each year over a million Arterial Bypass Grafts (ABGs) including Peripheral Vascular Disease (PVD) and Coronary Artery Disease (CAD) are performed, utilising either autologous vessels or prosthetic grafts commonly manufactured from Dacron (Polyethylene Terephthalate, PET) and expanded Polytetrafluoroethylene (ePTFE). Graft failure is currently a major concern for medical practitioners in treating Peripheral Vascular Disease (PVD) and Coronary Artery Disease (CAD). For instance, almost 35,000 Coronary Artery Bypass Graft (CABG) procedures are performed each year in the UK according to the British Heart Foundation; however, over 50% of CABGs fail within 10 years. In 1999, an estimated 688,000 bypass surgeries were performed in the United States but up to 10% of these procedures failed within 30 days of surgery. Similarly, stenosis at the graft-vein junction caused by Intimal Hyperplasia (IH) is the major cause of failure of arterio-venous access grafts used for haemodialysis. Early graft failure (within 30 days) is attributable to surgical technical errors and resulting thrombosis, while late graft failures are mainly caused by progression of atherosclerosis and IH.
It is now widely accepted that haemodynamic factors play an important role in the formation and development of IH and acute thrombosis, which are the main causes of PVD and CAD failures. Consequently, much research has been conducted in the past few decades to design grafts with longer patency, ideally longer than the life-span of the patient. Given the extent of graft failure, the motivation behind this multidisciplinary project starting in 2015 was to improve the patency of the current bypass grafts by developing a novel and optimised blood flow augmentation technique.
One of the most significant contributions to the improvement of haemodynamics in grafts was based on a research which showed that the ‘spiral flow’ is a natural phenomenon in the whole arterial system and is induced by the twisting of the left ventricle during contraction and then accentuated upon entering the aortic arch. The benefit of this flow pattern lies in removing unfavourable haemodynamic environment such as turbulence, stagnation and oscillatory shear stress, which are believed to be the main causes of intimal hyperplasia at anastomotic configurations.
In order to address the challenges posed by bypass graft failure rates mentioned above, the present project has brought together nuclear thermal-hydraulic experts, computer modelling specialists, biomedical engineers and clinicians to find a solution to a haemodynamics-based medical problem. This multi-disciplinary engineering venture has resulted in a unique product which makes use of both non-planar helicity and an optimised internal ridge within the graft to achieve a significantly improved haemodynamic condition within the anastomosis (an anastomosis is a surgical connection between autologous/prosthetic grafts and veins/arteries inside the human body).
Once the initial stages of the design were completed, a ‘multi-objective optimisation’ technique was used to achieve the optimum configuration to achieve the most favourable haemodynamic conditions around the anastomosis. At this stage, advanced Computational Fluid Dynamics (CFD) technique combined with high performance computing facilities were used to test several 100 combinations of design parameters to find the final geometrical configuration which consisted of non-planar (i.e. out-of-plane) helicity and an internal ridge.