stent system.pdf

Journal of Materials Processing Technology 162–163 (2005) 196–202

Biomechanical analysis of stent–oesophagus system

W. Kajzer ∗ , M. Kaczmarek, J. Marciniak

Institute of Engineering Materials and Biomaterials, Silesian University of Technology, ul. Konarskiego 18a, 44-100 Gliwice, Poland

Abstract

The paper presents the biomechanical analysis of a stent–oesophagus system. In particular, stresses and strains for the oesophageal stent

(Z-type) reﬂecting its implantation technique were calculated. The obtained results are the basis for the optimization of the geometrical features

of the stent and mechanical properties of the metallic biomaterial.

Keywords: Biomaterials; Oesophageal stent; Finite element method

1. Introduction

repositioned. The second disadvantage is the tumor ingrowth

causing restenosis. In the new generation of oesophageal

stents, that problem was eliminated by silicon-covered stents

[3] . However, the effectiveness of self-expanding stents was

conﬁrmed in long-term clinical examinations [4–11] .

A basic indication for oesophagus stenting is non-

resectable oesophagus tumor or benign obstruction. A

stenting technique is relatively simple and ensures a direct

and lasting therapeutic effect. This method of treatment

must be taken into consideration in 60% of the patients with

an oesophagus cancer recognition. Nowadays, two types of

stents are widely used. The ﬁrst group are conventional stents

(plastic)—a characteristic feature of this type of stents is a ne-

cessity of a dilation of oesophagus lumen because the diame-

ter of a stent is constant. The main disadvantage of this type of

stent is that these bulky devices require general anesthesia for

the placement and have a high complication rate. The second

group are metallic stents made of the Cr Ni Mo stainless

steel or shape memory alloys. The advantage of these stents

in comparison to plastic ones is the smaller diameter. Thanks

to this, a large dilation of the stricture is not required what

reduces a risk of the oesophagus wall injury. Furthermore, the

small diameter makes the stent “trackable”. Clinical data have

shown that the percentage of serious complications connected

with the conventional stents is 47% whereas the use of the

self-expanding stents causes only 16% of the complications.

The effectiveness in patency reaches 90% and almost 100% in

relation to oesophageal ﬁstulas [1,2] . However, there are two

basic problems related to the application of self-expanding

stents. When the stent is expanded, it cannot be removed or

2. The aim of the work

The aim of the work was the biomechanical analysis of

the given geometrical form of the oesophageal stent used in

the digestive system treatment. A three-dimensional model of

the stent implanted into the oesophagus and the mechanical

parameters of the stent were used to evaluate the relationship

between the stent and the oesophagus tissue [12–14] . The

performed analysis concerned the stresses and strains distri-

bution in the elements of the modeled system. The research

connected with the experimental veriﬁcation enables to work

out the physical model, taking into consideration the physi-

ological conditions of a human. The obtained results are the

basis for the optimization of the geometrical and material

features of the stents and at the same time, they allow to set

the conditions useful for the implantation technique.

3. Methods

3.1. Geometrical model of the stent–oesophagus system

∗ Corresponding author.

E-mail address: wojciech.kajzer@polsl.pl (W. Kajzer).

In order to carry out the biomechanical analysis with the

use of the ﬁnite element method, a geometrical model of

doi:10.1016/j.jmatprotec.2005.02.209

W. Kajzer et al. / Journal of Materials Processing Technology 162–163 (2005) 196–202

197

model of the oesophagus was in the form of the thin-walled

pipe of the following parameters [16] : the inner diameter

d wn = 24 mm, the thickness of the wall g sp = 3 mm, the length

of the oesophagus l p =40mm.

Fig. 1. The model of the oesophageal stent used in the FEM analysis.

Fig. 2. The model of the oesophagus.

the oesophageal stent was worked out. The following pa-

rameters were set [15] : the length of the stent l = 120 mm,

the length of the reﬂux valve l z = 80 mm, the diameter of

the stent wire d d = 0.37 mm, the thickness of the plastic

layer g = 0.3 mm, the diameter of the stent in the distal

part d 1 = 25 mm, the diameter of the stent in the middle

part d 2 = 18 mm. These stents are made of the austenitic

stainless steel. The stent was covered with the plastic layer

( Fig. 1 ).

In order to analyze the stent–oesophagus system, a geo-

metrical model of the oesophagus was also worked out. The

Fig. 3. The SOLID95 ﬁnite element.

Fig. 4. Discrete model of: (a) the oesophagus; (b–d) the oesophageal stent;

and (e) the stent–oesophagus system.

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W. Kajzer et al. / Journal of Materials Processing Technology 162–163 (2005) 196–202

The length of the oesophagus model was equal to the single

segment of the stent increased in both the directions of 10 mm

( Fig. 2 ).

3.2. Discrete model of the stent–oesophagus system

In order to perform the ﬁnite element analysis, a mesh-

ing of the geometrical model of the implant was done. The

SOLID95 ﬁnite element was used ( Fig. 3 ). This element al-

lows to take into consideration the physical non-linearities

and large displacements.

The discrete model of the stent and the oesophagus is

presented in Fig. 4 .

Due to the repeatability of the structure, the calculations

were carried out for the single segment of the stent (distal

part). In the analysis, the two stages were applied. The ﬁrst

stage was a clamping of the stent up to the diameter which

allows to insert the stent into the oesophagus. The second

stage was a slow expansion of the stent up to the initial di-

ameter, a contact between the stent and the oesophagus and

then the elastic work of the stent with the oesophagus. The

Fig. 5. Stresses distribution in the oesophageal stent (MPa), the radial dis-

placement, 7.5 mm: (a) the stent and (b) the plastic layer.

100%), the radial

displacement, 7.5 mm: (a) the plastic layer and (b) the stent wire.

Fig. 7. Radial displacements (mm) during the work of the system: (a) the

stent–oesophagus system; (b) the stent; and (c) the oesophagus.

Fig. 6. Strains distribution in the oesophageal stent (

W. Kajzer et al. / Journal of Materials Processing Technology 162–163 (2005) 196–202

199

simulation of the clamping and the expansion was performed

in the displacemental manner.

4. Results

4.1. The analysis of the stent–oesophagus system

4.1.1. The ﬁrst stage—clamping of the stent up to the

diameter which enables to insert the stent into the

oesophagus

The stresses and strains in the elements of the oesophageal

stent for the given radial displacement equal to 7.5 mm are

presented in Figs. 5 and 6 . The aim of the idea of the ra-

dial displacement was the diameter reduction of the distal

part of the stent of 15 mm. It allowed to implant the stent

into the oesophagus. The obtained stresses were in the range

0–1213 MPa ( Fig. 5 ) and reached the maximum value in the

bent regions of the stent. The stresses distribution for the

plastic layer are presented in Fig. 5 b. The stresses were in

the range 0–8 MPa. The analysis of the model has shown that

100%) during the work of the system: (a) the

stent–oesophagus system; (b) the stent; and (c) the oesophagus.

the strains were in the range 0–201% ( Fig. 6 a). The high-

est strains were localized in the plastic layer bordered on the

metallic segment in the bent region. The strains distribution

for the metallic segment is presented in Fig. 6 b. The maxi-

mum values were equal to 1%.

Fig. 8. Stresses distribution (MPa) during the work of the system: (a) the

stent–oesophagus system; (b) the stent; and (c) the oesophagus.

Fig. 10. Stresses distribution of the oesophageal stent (MPa), the radial dis-

placement, 7.5 mm.

Fig. 9. Strains distribution (

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W. Kajzer et al. / Journal of Materials Processing Technology 162–163 (2005) 196–202

Fig. 11. Strains distribution (

100%) in the single segment of the oe-

sophageal stent.

4.1.2. The second stage—slow expansion of the stent up

to the initial diameter, the contact between the stent and

the oesophagus and the elastic work of the stent with the

oesophagus

In the next step, the geometrical features of the stent during

the expansion in the oesophagus were analyzed. The expan-

sion was realized with the use of the radial displacement up

Fig. 13. Stresses distribution (MPa) during the work of the system: (a) the

stent–oesophagus system; (b) the stent; and (c) the oesophagus.

to the total expansion of the stent. The stresses, the strains

and the displacements in the stent–oesophagus system are

presented in Figs. 7–9 . The analysis has shown that the ra-

dial displacements, after the total expansion of the stent, were

in the range 0–0.69 mm ( Fig. 7 a). The maximum radial dis-

placements in the stent were equal to 0.2 mm ( Fig. 7 b) and in

the oesophagus were 0.69 mm ( Fig. 7 c). The stresses were in

the range 0–247 MPa ( Fig. 8 a and b) and the maximum val-

ues were localized in the bent region of the metallic segment.

The stresses distribution in the oesophagus was in the range

0–2.6 MPa ( Fig. 8 c). The strains were in the range 0–27%

( Fig. 9 a and b). The highest strains for the layer were local-

ized at the ends of the analyzed segment. Fig. 9 c shows the

strain distribution in the oesophagus.

4.2. The analysis of the stent wire–oesophagus system

Fig. 12. Radial displacements (mm) during the work of the system: (a) the

stent–oesophagus system; (b) the stent; and (c) the oesophagus.

4.2.1. The ﬁrst stage—clamping of the stent up to the

diameter which enables to insert the stent into the

oesophagus

The boundary conditions were the same as for the ﬁrst

variant and the radial displacement of 7.5 mm were set. The

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