helical ste nts

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