Post on 06-Jan-2016
description
HELICAL STENTS
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”
U-bend
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
RtStraight
LtHelical
RtStraight
LtHelical
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
U-bend movie
Bed wall
Toe wall
Bed
Toe
x
y
MRI
CFD