Development of an in vitro measurement method for improved

DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203114

Paula Rosam*, Michael Stiehm, Finja Borowski, Jonas Keiler, Andreas Wree, Alper Öner,

Klaus-Peter Schmitz and Wolfram Schmidt

Development of an in vitro measurement

method for improved assessment of the side

branch expansion capacity

Abstract: The expansion capacity and accessibility of the

side branch is essential for the stenting of complex

bifurcations. Since previous measurement methods only

provide limited information based on geometrical data of

stent cells, a new measurement approach was developed

which considers the mechanical deformation capacity of the

stent design. This approach provides essential information on

the stent with regard to the application of bifurcation

stenting. Four different commercially available coronary

stents (nominal diameter 3.0 mm) were dilated and a central

strut cell was over-expanded by means balloon catheters of

increasing nominal diameter (2.0 to 5.0 mm). After balloon

inflation, the remaining cell size was investigated for

maximum cell diameter and strut fractures. Large expansion

capacity without cell damage is taken as a measure of the

accessibility of the side branch. In none of the expansion

experiments the desired target size could be achieved, which

is due to the elastic recoil of the stent cells. Deviations from

the target diameter between 14-38% were determined.

However, larger diameters also showed a constriction of the

balloon, so that in some cases the target diameter could not

be achieved at all. No strut fractures occurred even at

maximum balloon diameter and pressure (5.0 mm non-

compliant balloons). As a result the side branch accessibility

differs depending on the individual stent designs. No

particular risk for the stent was found by extensive over-

dilatation.

Keywords: stents, side branch accessibility, expansion

capacity, strut deformation, bifurcation

https://doi.org/10.1515/cdbme-2020-3114

1 Introduction

The term over-expansion capacity of stent designs is used to

evaluate how the stent geometry changes when over-

dilatation is performed [1,2]. This parameter is also

suggested in the context of bifurcation stenting for the

selection of suitable stents [3], including a description of how

side branch accessibility changes when over-dilatation occurs

(cell opening).

This is a common approach in various kinds of

bifurcation stenting, in which the side branch has to be

stented additionally. High grade stenoses as seen in figure 1

obstruct the blood flow extensively along the bifurcation and

must be treated.

Figure 1: MicroCT scan of an anatomical bifurcation preparation

from the Department of Anatomy (Rostock). Arrows are indicating

flow direction. LAD: left anterior descending artery, D1: (diagonal)

side branch.

This side-branch accessibility was tested by an approach

similar to above-mentioned, namely without extra

mechanical stress by a penetrating balloon / stent system

(catheter) for the “side-branch-struts”.

_____

*Corresponding author: Paula Rosam: Institute for

ImplantatTechnology and Biomaterials e.V., Friedrich-Barnewitz-

Str. 4, 18119 Rostock-Warnemünde, Germany,

mail: paula.rosam@uni-rostock.de

Finja Borowski, Michael Stiehm, Klaus Peter Schmitz: Institute

for ImplantatTechnology and Biomaterials e.V., 18119 Rostock-

Warnemünde, Germany

Jonas Keiler, Andreas Wree: Rostock University Medical Center,

Department of Anatomy, Rostock, Germany

Alper Öner: Rostock University Medical Center, Department for

Cardiology, Rostock, Germany

Wolfram Schmidt: Institute for Biomedical Engineering, Rostock

University Medical Center, 18119 Rostock-Warnemünde,

Germany

Paula Rosam et al., Development of an in vitro measurement method for improved assessment of the side branch expansion capacity — 2

However, the result is only one criterion for success. If the

passage with wire and catheter is successful, the dilatation of

the side branch also causes a considerable deformation of the

cell. The question arises how far a stent cell can be dilated

without breaking and thus not becoming a risk to balloon and

vessel. Balloon ruptures at sharp-edged strut fractures

prevent the expansion, but also the safe evacuation and

withdrawal of the balloon. This aspect has already been

investigated for bioresorbable scaffolds (BVS Absorb,

Abbott Vascular) and has been evaluated as critical [4].

There is a lack of systematic studies for permanent stents.

2 Material and method

For the following coronary permanent drug eluting stents

DES (each n = 1)

▪ Abbott Xience Sierra 3.0/15

▪ Boston Scientific Promus PREMIER Select 3.0/16

▪ BIOTRONIK Orsiro 3.0/15

▪ Medtronic Resolute Integrity 3.0/15

the resulting cell sizes were determined after probing one

mesh each with a guide wire (ChoiCE

PT 0.014" x 182 cm,

REF H74912160011, Boston Scientific). This mesh was then

dilated using ascending diameter balloon catheters (see Table

1). The balloon pressure corresponds to the nominal pressure

of the balloon (NP) and the expected maximum balloon

diameter corresponds to the nominal diameter of the balloon.

The used non-compliant balloons for larger test diameters

allow at the same time maximum pressure and represent a

highly challenging case. The procedure is shown in Figure 2.

Table 1: Balloon catheters used for over-expansion of the stent

cells; balloon parameters are: dimension, balloon pressure and

compliance

Ballon catheter

Dimension

[mm]

Ballon pressure

[atm]

comment

Biotronik Pantera

2.0/20

semi-

compliant

Biotronik Pantera

3.0/20

semi-

compliant

Biotronik Pantera

3.5/25

semi-

compliant

Biotronik Pantera

LEO

4.0/30

non-

compliant

Biotronik Pantera

LEO

5.0/30

non-

compliant

Figure 2: Xience Sierra 3.0/15 with guide wire ChoiCE PT and

balloon Pantera 2.0/20 in a Y-silicone model

After balloon expansion and retraction, the cell size was

determined optically (light microscope SZX16, magnification

1.25x, Olympus). The maximum inscribed circle was

measured with image analysis software (Stream Start, V1.7,

Olympus). The diameter of this circle was used as a measure

of the achieved cell size (see Figure 3).

Figure 3: Xience Sierra 3.0/15 - Mesh expanded with balloon

Pantera Leo 5.0/30. Diameter measurement of the inscribed circle

was used as a measure for the mesh size

Due to the three-dimensional deformation of the stent, a strict

vertical top view was not possible. Horizontal distortions

may occur due to the position of the stents under the

microscope. As long as the inscribed circle is vertically

limited by the stent structure, the diameter of the circle is still

correct, as no distortions occur in this direction.

3 Results

During diameter increase of the dilating balloon, an

increasing constriction of the balloon through the mesh was

detected. This effect was strongest at the maximum diameter

(d = 5.0 mm)

Paula Rosam et al., Development of an in vitro measurement method for improved assessment of the side branch expansion capacity — 3

The following Figure 4 shows an example of the increasing

deformation of a stent mesh when stretched from 2.0 to

5.0 mm. As a result of the severe deformation of the cell, in

none of the investigated stents were torn. However, at

maximum expansion an almost circular cell without visible

reserve for further diameter increase was seen. A strong

deformation of adjacent cells also occurred.

Figure 4: Side branch expansion of the Resolute Integrity 3.0/15.

Increasing deformation of a stent mesh when stretched with

balloons from 2.0 to 5.0 mm.

The cell diameters reached during the over-expansion process

of all stents are shown in Figure 5.

Figure 5: Stent cell diameter in relation to the diameter of the

expanding balloon.

4 Discussion

The achieved cell diameters were mostly about 0.5 mm

smaller than the targeted diameters. On the one hand this is

due to the constriction of the balloons which was especially

seen with large balloon diameters. This is remarkable

because especially for the diameters 4.0 and 5.0 mm non-

compliant balloons were used, which were dilated with high

balloon pressure (14 atm). The result indicates considerable

mechanical stress on the cell. On the other hand, every plastic

deformation is accompanied by an elastic component, which

leads to a restoring movement after mechanical loading.

Thus, the cell size also shows elastic recoil.

At the target diameter of 2.0 mm, the difference to the

achieved mesh size was maximal (28 - 38%). The mesh

structure was only slightly deformed and the proportion of

elastic deformation dominates. As the target diameter

increases, the difference initially becomes smaller (at

3.5 mm: 14 - 17%) since the proportion of plastic

deformation increases. At the largest target diameter of

5.0 mm, there is a clear obstruction of the balloon expansion

(constriction), so that the stent mesh was not expanded to this

target diameter.

Figure 6 provides an overview of the measured

deviations from the target diameter.

Over-expansion of stent cells larger than the diameter of

the main branch is of little relevance to clinical practice.

However, since the stents investigated were examined at

nominal diameter, they have a capacity for over-dilatation up

to 3.5 - 4.75 mm according to the manufacturer's

specifications [5–8]

Paula Rosam et al., Development of an in vitro measurement method for improved assessment of the side branch expansion capacity — 4

Figure 6: Difference between target diameter and achieved mesh

size as a function of the diameter of the expanding balloon

On the other hand since it is assumed that secondary

branches are smaller than the main vessel, dilatation up to

5.0 mm is a very extreme case. The absence of strut failure is

a sign of high stent quality and material strength. It facilitates

low-risk use in bifurcation stenting from mechanical point of

view. The deformation of adjacent cells including the kinking

of the stent would probably have been less severe in a closed

vascular model, thus representing a worst-case scenario. In

such a model, however, the measurement could not have

been performed with the same accuracy.

The novel investigation shows a new relevance

compared to previous measurements of side branch

accessibility. In a common benchmark of different stents, as

it is also performed in our test laboratory, only the size of the

expanded mesh is measured after conventional stent

dilatation to nominal pressure. The corresponding

measurement results are shown in Table 2.

Table 2: Tested stents and their side branch accessibility

Tested Stents

side branch accessibility [mm]

Abbott Xience Sierra

3.0/15

0.967

Boston Scientific Promus

PREMIER Select 3.0/16

0.891

BIOTRONIK Orsiro

3.0/15

0.871

Medtronic Resolute Integrity

3.0/15

0.640

These values show no correlation with the new in vitro

measurements. They are much lower than needed for probing

the side branch.

However, since an additional widening and deformation

of the meshes occurs, especially with regard to the stenting of

bifurcations, the side branch expansion capacity should also

be measured in the future.

Author Statement

Research funding: Financial support by the European

Regional Development Fund (ERDF) and the European

Social Fund (ESF) within the collaborative research between

economy and science of the state Mecklenburg-Vorpommern

within the project “TheraMagna” is gratefully acknowledged.

Conflict of interest: Authors state no conflict of interest.

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2 3 3.5 4 5

Deviation of the opened meshes from

the target diameter in %

Target diameter of the balloon [mm]

Orsiro Resolute Integrity Xience Sierra Promus Premier Select