Stent Fracture

OVERVIEW
SRI International, Stanford University, and a consortium of stent manufacturers, are teaming in an effort to improve the durability of endovascular implants. The objectives, approach, and research plans were presented and discussed at a kickoff meeting at SRI April 28, 2004.

The program was motivated by the great success of stenting in the coronary arteries, the subsequent strong interest in applying the technology to the peripheral vasculature, and the observed thrombosis and restenosis associated with stent fracture.

The program focuses on the superficial femoral artery (SFA), a vessel subjected to large and repetitive multimode deformation and prone to atherosclerotic lesions. Clinical trials show that stents in the SFA fracture at a significant rate. To design stents that resist fracture, the duty cycle the stents experience must be known. The team intends to define stent duty cycles in terms of displacements and loads from measurements of in-vivo displacements of the SFA via MRA imaging and SFA stents via CT scanning, with the help of finite element analyses. The team further intends to determine how stents deteriorate and fail in the body environment by acquiring and examining explanted stents.

The research program is being performed in two phases. The aims of the initial phase are to develop imaging methods for arteries and implanted stents, to establish a repository and database for explanted stents, and to begin to define the boundary conditions imposed on stents by the body environment. These aims are listed below in brief detail. The second phase will apply and build on the first phase results to generate data necessary for designing more robust stents.

SPECIFIC AIMS
AIM 1: Explore and identify methods to acquire 3D anatomy of the superficial femoral artery in healthy subjects using contrast-enhanced magnetic resonance angiography (MRA).

• Investigate capabilities of 0.5T GE interventional open-magnet to acquire 3D contrast-enhanced MRAs of the SFA during seated and squatting postures.
• Determine what appropriate body postures are possible in the 1.5T GE conventional magnet for contrast-enhanced MRAs of the SFA.
• Image 3 healthy adults using the identified MRI protocol.
• Use custom software to construct three dimensional geometric models of the SFA in order to quantify vessel deformations.

AIM 2: Explore and identify methods to acquire 3D images of implanted stents using a new rotational computed tomography (CT) machine.

• Develop and optimize new acquisition protocols for high-resolution measurement of stent geometry in vitro and in vivo with rotational CT.
• Evaluate the new imaging protocol using simple vascular phantoms, and in a single ambulatory patient volunteer.
• Identify boundaries of stents from rotational CT image data and quantify stent deformation.

AIM 3: Identify deterioration and failure modes of SFA stents exposed to the in vivo environment.

• Acquire stents that have been explanted from the SFAs of peripheral vascular disease patients recovered from autopsies and amputations.
• Examine these stents with optical and scanning electron microscopy to seek evidence of corrosion, erosion, wear and fracture.
• Use the type and extent of microdamage, as well as knowledge about macroscale deformation from Aim 2, to determine failure modes of these stents.

AIM 4: Define boundary conditions for finite element analyses of stent response that are representative of the clinical environment in the SFA and practical to perform with current FE codes.

• Collect information on current FEA practices for stent design and analysis.
• Perform FE analyses of a generic Nitinol stent using imaging data generated in Aims 1 and 2.
• Investigate stent/artery interaction and effects of different definitions of boundary conditons on the response of the stent/artery system.

Contact Us
Don Shockey
Director, Center for Fracture Physics
Phone: 650-859-2587
Email: donald.shockey@sri.com