Stent Design and Engineer
Coating Over Flow Removal Tool
Team #3 (Vimage)
Vincent Hoang
10/29/04
Executive Summary
In this paper the different design and engineers of stent are discuss and compared.
Some of the important parameter of a coronary stent are the structure, geometry and
dimensions. These different parameters play an important role in deliverability, visibility
scaffolding performance and procedural success.
Some of the parameter can also
influences the occurrence of restenosis and other cardial vascular disease.
parameters examine here are the type of stent such as self-expansion
The
vs. balloon
expansion, physical design such as coil vs. tube, dimensional effect such as length and
width and the geometry such as number of struts and strut thickness.
Introduction
At the turn of the 20th century, one of the scientific breakthroughs is the
introduction of the drug-eluting stent, also known as drug-coated stent.
With the
technological advancement in engineering and design, drug-coated stent has become the
ultimate device in treating cardial vascular disease. As a result, others stent parameters
are taken lightly and considered less important by many specialists. The under
appreciation of the stent design, that has evolved for the pass 18 years, will affect the
overall performance of the stent. The structure, geometry and dimension of a stent play a
crucial role in the delivery process, performance of the stent and the procedural success.
The stent was introduced to replace the tradition invasive surgery and angioplasty
balloon that was use in treating cardial vascular disease. In early stage of angioplasty,
cardiologists use balloon to expand the artery that is clogged. However, in some cases the
wall of the coronary artery became weakened after the balloon is dilated, resulting an
acute or sub acute collapse of the artery. The stent was created in the 1980s in response
to the danger posed by the angioplasty balloon. The stent was able diminish the danger
of collapsing artery wall, but it gave birth to the problem of restenosis. After years of
engineering and design, the stent is equipped with biocompatible and radio visible
coating with reduce restenosis.
The coating on stent is not the only factor that is
responsible for today stent performance and success in procedures, but many others stent
parameters.
Report Body
When designing and engineering a stent the geometry, structure and dimensions
of the stent need to be taken into consideration. The parameters need to be chosen
appropriately to maximize the stent function and performances. Some of the parameter
issues are: self-expanding vs. balloon-expandable, coil vs. tube, slotted tube vs. modular,
long vs. short, wide vs. narrow, percent metal coverage, more struts vs. less, thick vs. thin
struts, square vs. round strut cross-section, rough vs. smooth and type of alloy use. All
the parameters of a stent are study in clinical trial, and the result provided valuable
information in designing an optimal stent.
There are two possible designs for a stent, a self-expanding or a balloon
expandable stent. The ealier balloon expandable stent is a stent that is made of a coil,
mesh or zigzag design and is constraint with a sheath. When the sheath is removed, the
stent elastically spring open to a predetermine size that reflect the coronary artery
diameter. Today, the stent is pre-mounted on a balloon and the inflation of the balloon
plastically expands the stent with respect to the balloon diameter. In self-expanding
stent, such as the Medtronic nitinol self-expanding coil stent, the smart material auto
expands to a calculated size. However, in some cases over time these self-expanding
stent continue to expand. As a result, self- expanding stent is not recommended and is
rarely used these days in design for they have concerning safety issues.
Ballon expandable stent can be divided into two groups, one is the tube design
and the other is the coil design. Figure 1.1 illustrates the difference between coil and a
tube design. Coil design incorporates a continuous wound wire or a series of flat sheet
coils. The strut width is greater; there are gap between struts, and no connections
between struts which give it more flexibility. However, the design lack radial strength,
and the wide gap allow tissues to dangle. As a result, coil design has become obsolete
and replace by the more superior in radial strength, the tube design.
Coil
Tube
Figure 1.1 Coil vs. Tube
Now, the design is narrow down to balloon expandable, and tubular, there are two
type of specification, a slotted tube and modular tube. In clinical study, it is found that
slotted tube stent resisted restenosis more when compare with modular tube. Beside this,
the two are relatively similar in that they are both laser cut from a tube of metal,
connecting elements and flexibility.
The dimension of the stent, such as length and width can influence the
deliverability, visibility and flexibility of the stent. “Work form Thoraxcentre has clearly
demonstrated that restenosis rates are predictable from stent and vessel dimensions.” In
their clinical study, the data clearly showed that short stent has lower cases of restenosis
than long stent. Similar study in width of the stent suggested that wide diameter stent is
more favorable than the narrow one. Figure 1.2 shows a visual of a wide diameter
compare to narrower diameter. The drug-eluting stent does not solve this problem,
therefore stent size and length is important parameter when engineering and designing
stent.
Wide
Narrow
Figure 1.2 Wide vs. Narrow
Stent geometry is another important engineer aspect. In the study in rabbit iliac
arteries, the result indicated that less struts induce less chance of restenosis compare to
more struts. Figure 1.3 illustrates the difference in less strut and more strut. The strut
thickness also attribute to this affects. Thinners struts have can slice into tissue more
easily and therefore more deeply than thicker strut. But, unlike thicker strut, they cause
less angulations and stretching of the arterial wall. The strut thickness is directly related
to the percent of metal coverage, in that the amount distribute should correspond to
amount require for a thin strut. Another note worthy stent geometry is the cross-section
of the strut. Round strut cross-section area is ideal for smoothness design. On the other
hand, square strut cross-section area in not recommend because it interferes with blood
flow due to their sharp edge which can slice blood cells. With today advance in material
science, material such as Co-Cr (Cobalt-Chromium) allows stent to be design with lesser
strut and be thinner. Figure 1.3, and 1.4 show a visual representation of these parameters.
More struts
Less struts
Figure 1.3 More struts vs. less struts
Square Thick
Square Thin
Round
Figure 1.4 Thick vs. Thin struts (A, B) & Square vs. round strut cross-section(B,C)
Smoothness of a stent can affect the performance and biocompatible of the stent.
Early study shows that a smooth surface can reduce thrombus adhesion and neointimal
growth. In today cardial vascular medical product, a stent quality is judge by how smooth
it is. “Quality of the surface of all stents may be improved by the simple expedient of
pressure-rising to remove surface contaminants.” But, to obtain smoothness, the stent
need to be treated with acid-pickling and then electrochemical polishing. The process
removes slag which includes depositions and burrs, formed on the surface of stents due to
the laser cutting production process.
Lastly, the materials that stent are manufacture from remain one of the top priority
in design and engineer.
For 15 years, the standard material from which balloon
expandable stents were made was 316L stainless steel which has good radial strength,
deformability, resisted to corrosion. They can also be coated with another alloy thru
galvanization process such as argon ion bombardment, but research showed that extra
coating lead to porosity and surface cracks. Currently, new promising material such as
cobalt, chromium and nickel are competing to be the primary material use in stent. In
addition, the 1st generation of polymer such as polyester stent also joined the competition.
References
-[1] Morton Allison C, Crossman. D, Gunn. J, The influence of physical stent parameters
upon restenosis, Elsevier SAS, 2004.
-[2] Lau. KW, Johan A, Sigwart U, Hung JS, A stent is not just a stent: stent construction
and design do matter in its clinical performance, Singapore Medical Journal, 2004.
-[3] Zhao Hui, Sohier. J, Scheerder. I, Electrochamical polishing of 316L stainless steel
slotted tube coronary stents, Kluwer Academic Publishers 2002.
-[4] Rogers, Campbell D.K, MD, Optimal Stent Design for Drug Delivery, MedReviews,
LLC, 2004.
-[5] McClean Dougal R, MD, Eigler N, MD, Stent Design: Implications for Restenosis,
MedReviews, LLC 2002