HELICAL STE NTS C.G.Caro Department of Bioengineering Imperial College

ARTERIAL STENTING Arterial stenting for atherosclerosis increasing Challenges of stenting: Intimal hyperplasia (IH), stent kinking and fracture Drug-eluting stents retard IH development, but can delay endothelial repair and risk of in-stent thrombosis

INTIMAL HYPERPLASIA Highly important disease : causes failure of arterial stents, bypass grafts, vascular access grafts, balloon angioplasty, a-v fistulae Pathology: accumulation of smooth muscle cells and extracellular matrix in intima Causation: possibly vessel damage but, like atherosclerosis, IH influenced by flow - favours low wall shear/stagnation regions

LARGER ARTERY FLOW AND GEOMETRY Reynolds number >> 1 (inertially dominated flow) Three-dimensional (non-planar) geometry

FEATURES OF INERTIALLY DOMINATED NON-PLANAR FLOWS Swirling Cross-mixing Inhibition of flow stagnation, separation and instability Relatively uniform distribution of wall shear Enhanced blood-wall mass transport (including oxygen)

Planar Non-planar Toe 1-D 3-D MRI of flow in model bypass graft

Computational study of swirling and mixing in a helical tube Reynolds number 200, amplitude ratio 0.5, pitch 5 diameters

from “Studies of Planar & Non-Plaar flow”July 1998 Re = 1000 Files: bend side.avi, bend top.avi approx 0’20”

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Helical Stent

Angiograms post stent implantation in pig common carotid arteries Helical stent: helical deformation of artery and swirling of blood flow immediately after implantation of stent Straight stent: no swirling of flow Rt Straight Lt Helical Rt Straight Lt Helical 3547

Transverse sections of pig common carotid arteries one month post stent implantation

MECHANICAL PROPERTIES OF HELICAL STENTS Cadaver superficial femoral artery: helical stents kink less than conventional stents on flexing knee Computational studies: lower local stresses on deforming helical than conventional stents Bench tests: helical stents far less likely to fracture than conventional stents

CONCLUSIONS Less intimal hyperplasia with helical than conventional (straight) stents Improvement due to swirling and mixing, improving wall shear and blood-wall mass transport, including of oxygen? Intimal hyperplasia caused by vessel injury and compromised wall shear and mass transport? First-in-man studies planned

ACKNOWLEDGEMENTS To many colleagues over many years, including at Veryan Medical Limited

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Bed wall Toe wall Bed Toe x y MRI CFD