Acta Polytechnica CTU Proceedings


doi:10.14311/APP.2016.3.0015
Acta Polytechnica CTU Proceedings 3:15–18, 2016 © Czech Technical University in Prague, 2016

available online at http://ojs.cvut.cz/ojs/index.php/app

FLEXIBILITY TEST OF THE TOOLS OF ANGIOPLASTY

István Hajdu∗, Dóra Károly, Liza Pelyhe

Budapest University of Technology and Economics, Faculty of Mechanical Engineering, Department of
Materials Science and Engineering, Bertalan Lajos u. 7, 1111, Budapest, Hungary

∗ corresponding author: hajdu999@gmail.com

Abstract. Coronary angioplasty is a procedure used to treat the narrowed coronary arteries.
Physicians operate with many different tools during the intervention, the main devices are the following:
guidewires, guiding catheters, balloon catheters and stents.

One of the most important properties of the tools of angioplasty is flexibility. This article introduces
a flexibility measuring device and a testing method. With the help of this the flexibility of the tools of
angioplasty can be compared easily.

Keywords: angioplasty, coronary stent, balloon catheter, flexibility.

1. Introduction
Coronary artery disease (CAD) is a leading cause
of morbidity and mortality in both developing and
developed countries [1]. During life fat and cholesterol
deposits (called plaques) emerge inevitably in the
coronary vessel walls. They gradually increase over
time, first they cause coronary stenosis, then complete
blockage of the coronary vessels.
Percutaneous transluminal coronary angioplasty

(PTCA) is an invasive procedure performed to re-
duce blockages in coronary arteries [2]. During the
process a balloon catheter is guided into the narrowed
section of the coronary artery. Physicians expand
this at the place of the stenosis, thereby the balloon
compresses the plaque and ensures continuous flow of
blood.

Usually, a metal mesh is also expanded with the bal-
loon, called stent (Fig. 1). Stents are commonly made
of CoCr, PtCr or stainless steel. The most important
requirements for stent materials are biocompatibil-
ity and hemocompatibility. The stent sholuld not
produce a toxic, injurious or immunologic response
in living tissue. It is also important for physicians
to see the stent during the intervention, therefore it
should be visible under X-rays. MRI compatibility
is another main property of stents, that is why these
devices are made from non-magnetic metals (for ex-
ample austenitic stainless steel). The stent should
be flexible to follow the curves of the vessel during
the pulsation of the heart, but it should be rigid to
compress the plaque [2, 3].
Sometimes arteries will still become clogged and

narrowed again even with a stent. This phenomenon is
called restenosis [4]. Low endothelial shear stress can
affect the emergence of atherosclerosis and restenosis.
Stent design, strut thickness and so flexibility have a
main role in this field [5].
Stent and stent-system properties are mechanical

parameters which are important during the deploy-
ment, dilation and long term use of the devices. We

Figure 1. Balloon catheter, stent-system and stent.

can say that flexibility is one of the most important
properties of the tools of angioplasty. The catheter
and with it the stent have to follow the curves of the
vascular network without damaging or losing their
function during deployment, and the stent has to fol-
low the curves of the vessel after dilation at the place
of the stenosis.

There are many methods to measure the flexibility
of the tools of angioplasty, for example the company
Certiga Engineering Solutions developed a special
equipment. They push the tested tool into a spiral
tube as long as it breaks. Then they measure the
radius at the place of the breaking; they characterize
the flexibility with this value [6].

In the study of Mori and Saito the four-points bend-
ing test was utilized. Acryl resin bars were attached
to both ends of the stent prior to bending measure-
ments, thereby allowing for application of a constant
moment to the specimen. Through contact with the
punch on the acryl resin bars, stent bending with a
constant moment and without radial deformation can
be achieved [7].

Another method is based on the mechanical model
of a loaded beam. Dr. Péter Szabadíts characterize
the flexibility with the flexural strength of the beam.

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http://dx.doi.org/10.14311/APP.2016.3.0015
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I. Hajdu, D. Károly, L. Pelyhe Acta Polytechnica CTU Proceedings

Figure 2. The used mechanical model.

We also used this model, but by us to the charac-
terization of flexibility the reciprocal of the flexural
strength was used [8].

The European Standard (EN) does not contain any
method for the test of the flexibility. The U.S. Food
and Drug Administration recommends to wrap the
catheter around a series of mandrels with successively
smaller radius until the catheter kinks or the lumen
collapses. But there is no standard for the stents
flexibility measurement [9].
Our aim was to design and manufacture a device

which is appropriate for the measurement of the flex-
ibility. The method should be fast and easy and
adaptable to all kind of tools of angioplasty.

2. Materials and Methods
For the flexibility tests a mechanical model of a loaded
beam was used (one end of the cantilever beam was
clamped and the other end was loaded; Fig. 2).
The following equation was used to describe the

model:

EI =
F L3

3f
(1)

With the help of this equation, the flexibility can
be easily calculated:

F lexibility =
1

EI
(2)

For the tests a measuring device was needed (Fig.
3), which can be impacted into the testing machine
(Instron 5965). After designing the device it was
fabricated with PolyJet and FFF rapid prototyping
technologies. This device consist of a bottom and a
top holder, 33 guide bushes, 2 bolts and 5 spacers. All
of the guide bushes had a mortise in their centers with
different diameters from 0,4 to 4,7 millimeters. The
spacers have the following length: 1, 2, 4, 8 and 16
millimeters; with these between 1 and 31 millimeters
all discrete quantities can be assembled. Several stents,
balloon catheters and stent-systems were investigated
as a loaded beam in the model.
The investigated devices were the following: 3

balloon catheters with different diameters; 10 stent-
systems, from these 6 were the same type, they had
difference just in the size of the diameter; and 5 stents

Figure 3. Flexibility measuring device.

Figure 4. The investigated stents.

with different diameters and strut widths (Fig. 4).
The investigated stents were made from platinum
chromium (PtCr) or cobalt chromium (CoCr) alloys.

The investigations were performed by stereo micro-
scope and testing machine. First, the diameter of the
tools was measured with the help of the stereo micro-
scope. Then it was impacted into the right size guide
bush, which was embed to the top holder. The guide
bushes had a bigger outside diameter as the diameter
of the mortise on the top holder, so they were fitting
tight. After that, the required length was adjusted
with the spacers and the whole measuring device with
the tested tool was impacted into the testing machine.
The tested tools were loaded with a perpendicular
force at the end.
The load force, the length and deflection of the

beam was measured. After the tests the flexibility
was calculated with the help of equation (2).

Every device was loaded five times, between the
measurements each was rotated about its axis with
the same angles.

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vol. 3/2016 Flexibility Test of the Tools of Angioplasty

Table 1. The results.

Figure 5. Correlation between flexibility and diame-
ter in the case of stent-systems.

3. Results
Table 1. contains basic information and flexibility
of the investigated devices. Diameter and length are
the nominal sizes. The maximum deflection was 20%
of the clamping length, but it was maximized in 3
millimeters. Strut width was measured with the help
of stereo microscope.
Three main factors influences the flexibility of the

tools of angioplasty, these are the followings: diameter,
material and stent pattern. Primarily the relationship
between the flexibility and diameter was investigated.
The results show that between the flexibility and the
diameter there is a power function dependence (Fig.
5).

In the case of stents the relationship between flexi-
bility and strut width was also investigated; between

Figure 6. Correlation between flexibility and strut
width in the case of stents.

the flexibility and the strut width there is a power
function dependence (Fig. 6).
Based on the measurement the flexibility of stents

and balloon catheters are about with one order of
magnitude greater than the flexibility of stent-systems
(Fig. 7).

4. Conclusions
Based on the experiments the measuring device is
adequate for the investigation of flexibility. In every
case the relative standard deviation of the data is
lower than 10% (in most cases it is lower than 1%),
so the data is homogeneous, this means that it isn’t
needed to measure more times in different positions.
The flexibility depends on the diameter, raw material,

17



I. Hajdu, D. Károly, L. Pelyhe Acta Polytechnica CTU Proceedings

Figure 7. Correlation between flexibility and diame-
ter of the investigated devices.

strut width and stent pattern.
The manufacturers don’t give any concrete, numer-

ical data about flexibility of the tools of angioplasty,
that’s why these investigations are important. With
the help of these results, physicians can easily com-
pare stents or stent-systems according to flexibility,
and they can purposefully choose the most suitable
tools for the intervention.

List of symbols
E Young’s modulus [MPa]
I Moment of inertia of beam [mm4]
F Force [N]
L Length of beam [mm]
f Deflection of beam [mm]

References
[1] K. Iwasaki, et al. Prevalence of subclinical coronary
artery disease in ischemic stroke patients. Journal of
Cardiology 65:71–75, 2015.
doi:10.1016/j.jjcc.2014.04.004.

[2] G. Mani, M. D. Feldman, D. Patel, C. M. Agrawal.
Coronary stents: A materials perspective. Biomaterials
28:1689–1710, 2007.

[3] D. János. Az értágítóbetétek anyagainak fejlödése.
Kohászat 5-6:44–48, 2013.

[4] L. Petrini, F. Migliavacca, F. Auricchio, G. Dubini.
Numerical investigation of the intravascular coronary
stent flexibility. Journal of Biomechanics 37:495–501,
2004.

[5] K. C. Koskinas, et al. Role of endothelial shear stress
in stent restenosis and thrombosis : Pathophysiologic
mechanisms and implications for clinical translation.
Journal of the American College of Cardiology
59:1337–1349, 2012. doi:10.1016/j.jacc.2011.10.903.

[6] Stm-3015 - stent system testing ”flexibility” according
to din en 14299:2004 7.3.3.
http://www.certiga.com/en/stm_3015.en.htm.

[7] K. Mori, T. Saito. Effects of stent structure on stent
flexibility measurement. Annals of Biomedical
Engineering 33:733–742, 2005.

[8] P. Szabadíts, Z. Puskás, J. Dobránszky. Flexibility
and trackability of laser cut coronary stent systems.
Acta Bioeng Biomech 11:8–11, 2009.

[9] Class ii special controls guidance document for certain
percutaneous transluminal coronary angioplasty (ptca)
catheters, 2010 september. http://www.fda.gov/
medicaldevices/deviceregulationandguidance/
guidancedocuments/ucm225145.htm.

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http://dx.doi.org/10.1016/j.jjcc.2014.04.004
http://dx.doi.org/10.1016/j.jacc.2011.10.903
http://www.certiga.com/en/stm_3015.en.htm
http://www.fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm225145.htm
http://www.fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm225145.htm
http://www.fda.gov/medicaldevices/deviceregulationandguidance/guidancedocuments/ucm225145.htm

	Acta Polytechnica CTU Proceedings 3:15–18, 2016
	1 Introduction
	2 Materials and Methods
	3 Results
	4 Conclusions
	List of symbols
	References