Mechanical Characteristics of Self-expandable Metallic Stents: In Vitro Study with Three of Stress

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J Korean Radiol Soc 1998; 39 : 49? - 502

Mechanical Characteristics of Self-expandable Metallic Stents . In Vitro Study with Three Types ofStress

¹

Byung Hee Lee, M.D., Kie Hwan Kim, M.D., Soo Yil Chin, M.D.

Purpose : To obtain objective and comparable data for mechanical characteristics of self-expandable metallic stents widely used in the treatment of biliary obstruction.

Materials and Methods : The stents tested were the 6 and 8 mm-band Hanaro spiral stent, Gianturco-Rösch Z stent, Wallstent, Ultraflex stent, and Memotherm stent. Each was subjected to three types of load : point, area, and circular. We analyzed their mechanical characteristics (resistance force, expansile force, and elasticity) according to these three types of stress.

Results : With regard to point loads, the Memotherm stent showed the highest resistance force and expansile force. The 8 mm-band Hanaro stent showed the lowest resistance force and the Gianturco-Rösch Z stent and Ultraflex stent showed lower expansile force. With regard to area loads, the Ultraflex stent showed the highest resistance force. The 6 mm-band Hanaro stent, Gianturco-Rösch Z stent, and Ultraflex stent showed higher expansile force. The 8 mm-band Hanaro stent showed the lowest value in both resistance force and expansile force. For circular loads, the Memotherm stent showed the highest resistance force and the Ultraflex stent and Wallstent showed lower value. Under all types of stress, the Hanaro stent and Memotherm stent were completely elastic, and the Ultraflex stent and Wallstent showed a wide gap between resistance force and expansile force.

Conclusion : In clinical practice, awareness of the mechanical characteristics of each stent might help in choosing the one which is most suitable, according to type of biliary 0 bstruction.

Index words : Stents and prostheses

Metallic stents have found a wide spectrum of applications in modern interventional radiology. To date, various types of metallic stents made of different materials have been designed to enhance clinical effi- cacy and are now widely used in the vascular, biliary, and gastrointestinal system (1- 8).

The primary technical success of stent placement depends directl y on the mechanical characteristics of the stent. These include resistance force, expansile force, and longitudinal flexibility. In a paticular type of stent, these characteristics depend on the stent’s

'Department of Radiology, Korea Cancer Center Hospital Received April27, 1998; Accepted June 2, 1998

Address reprint requests to : Byung Hee Lee, M.D., Department of Radiology, Korea Cancer Center Hospital "215-4, Gongneung-dong, Nowon-gu, Seoul 139-706, Korea Tel. 82-2-970-1253 Fax, 82-2-972-3093

construction and the nature ofthe materials used.

It is crucial to choose the most suitable stent, according to different types of obstruction, and is thus important to evaluate the mechanical characteristics of the metallic stent widely used. Only a few reports have described the mechanical characteristics of metallic stents (9 - 11). Thus, this stud y aimed to obtain objective and comparable data relating to the mechanical characteristics of widely used self-expandable metallic stents and to evaluate their suitability for the treatment ofthe different types of obstructions seen in clinical practice.

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Byung Hee Lee. et al : Mechanical Characteristics of Self-expandable Metallic Stents

Materials and Methods

To evaluate their Ìnechanical characteristics, six types of selεexpandable stents were tested in vitro.

These were the 6 and 8 mm-band Hanaro spiral stent (SooHo Medi-tech, Seoul, Korea), the Gianturco-Rösch Z stent (Cook, Bloomington, U. S. A.), the Wallstent (Schneider, Bulach, Switzerland), the Ultraf1ex stent (nitinol Strecker stent; Medi-tech/Boston Scientific, Natick, U. S. A.), and the Memotherm stent (Ang- iomed, Karlsruhe, Germany). To obtain objective data, stents designed for the biliary system were selected and the study was carried out in a standardized fashion. The stents used were 65 - 70 mm in length and 10 mm in diameter. For each type of stress, one stent of each design was tested.

For the measuremen t of strength, a method similar to that described by Flueckiger et al was used (9 - 11).

Tests were performed at 35 - 37⁰C and three types of stress - point load, area load, and circular load - were applied to each stent (Fig. 1). Clinically these implied focal eccentric, diffuse eccentric, and concentric stenosis, respectively. Forces were measured with a micropro-cessor force gauge (EFG 250, Mecmesin, England).

Point load was applied using a rod with a round tip, 5 mm in diameter, and area load using a round plate 30 mm in diameter. We measured resistance and expansile forces under both these load condition. Resistance forces were measured as stress was applied until the stents were compressed to 3 mm oftheir diameter, and expansile forces were recorded during subsequent relaxation. If the stent was distorted before the compressed distance reached 7 mm, no more stress was applied and relaxation was begun. The values of measured forces were plotted as applied force versus compressed distance. Circular loads were applied using a 30 mm-wide loop of nonelastic photographic film, with the loop wrapped around the center of the stent. One end of this was connected to a gauge which monitored force by applying increasing tension to the other end of the loop. Because it was technically impossible to

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monitor expansile force, only resistance force was measured under circular load. The values of measured forces were plotted as applied force versus the length of photographic film pulled. In all stents, forces were measured at the middle of their length. Additionally, in the case of the Gianturco-Rösch Z stent, point and area loads were applied at the connecting portion between stent bodies as well as at their center.

Results

In all stents, diameter ofthe stents decreased as force increased.

According to the results 0 btained for point and area load, all stents showed different values between resistance forces and expansile forces at the same diameter of compressed stent, and the values of resistance forces were higher than those of expansile forces. The ranges of force gap were different in each stent. We defined

‘

elasticity’ of a stent as the gap between resistance force and expansile force (Fig. 2).

Point Loads

Resistance Force

At an indentation value of 3 mm, all stents showed similar resistance forces. Over this indentation, the Memotherm stent showed the highest resistance force, followed by the 6 mm-band Hanaro stent, the Walls- tent, the Ultraf1ex stent, and the Gianturco-Rösch Z stent. The 8 mm-band Hanaro stent showed the lowest resistance force (Fig. 3A).

Expansile Force

During re-expansion to 7 mm of stent diameter, the Memotherm stent showed the highest expansile force, followed by the 6 mm-band Hanaro stent, the Wall-

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Fig. 2. Force (newtons) versus compressed distance plot of the 6 mm-band Hanaro stent and the Wallstent subjected to area load. Elasticity is different in two stents. H6: 6 mm-band Hanaro stent. W: Wallstent.

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J Korean Radiol Soc 1998; 39 : 497 - 502

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Fig. 3. Force (newtons) versus compressed distance plot of six stents subjected to point loads. A. Resistance forces.

8. Expansile forces. M: Memotherm stent. H6: 6 mm-band Hanaro stent. W : Wallstent. U: Ultraf1ex stent. G: Gianturco- Rösch Z stent. H8 : 8 mm-band Hanaro stent

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Fig. 4. Force (newtons) versus compressed distance plot of the Gianturco-Rösch Z stent subjected to point loads. A:

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stent, the 8 mm-band Hanaro stent, the Ultraflex stent, and the Gianturco-Rösch Z stent. During expansion over 7 mm of diameter, however, the 6 mm-band Hanaro stent and theWallstent showed a higher expansile force than the Memotherm stent (Fig. 3B).

The Hanaro stent and Memotherm stent were completely elastic and reattained their original diameter after removal ofthe load. The Wallstent showed inter mediate elasticity. The Gianturco-Rö5Ch Z stent and Ultraflex stent, however, showed lower elasticity, indicating a wide gap between resistance and expansile forces.

In all except the Gianturco-Rösch Z stent, both re sistance force and expansile force were uniform throughout their length. In the Gianturco-Rösch Z stent, however, both resistance and expansile force were markedly lower at the connections between stent bodies than at the middle ofthe body (Fig. 4).

Area Loads

Resistance Force

The Ultraflex stent, 6 mm- band Hanaro stent, and Gianturco-Rösch Z stent showed higher resistance forces than the Memotherm stent and 8 mm-band Hanaro stent (Fig. 5A). In the case ofthe Wallstent, the area load curve was interesting ; at an indentation of 2 mm, its value was higher, but above that value, the curve reached a plateau. The Gianturco-Rösch Z stent showed no force difference between the middle of its body and the connecting portion under the area load.

Expansile force

The 6 mm-band Hanaro stent, Gianturco-Rösch Z stent, and Ultraflex stent demonstrated a higher expansile force than the Wallstent and 8 mm-band Hanaro stent (Fig. 5B).

The Hanaro stent, Memotherm stent, and Gianturco- Rösch Z stent showed good elasticity, but in the Ultraflex stent and Wallstent there was a wide gap between resistance force and expansile force (Fig. 2).

Circular Loads

Resistance force

At an indentation of over 2 mm, the Memotherm stent showed the highest resistance force, followed by the 6 mm-band Hanaro stent, Gianturco-Rösch Z stent, 8 mm-band Hanaro stent, Wallstent, and Ultraflex stent(Fig.6).

Discussion

Metallic stents are now widely used to treat or palli-

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Fig. S. Force (newtons) versus compressed distance plot of six stents subjected to area loads

A. Resistance forces. B. Expansile forces. U: Ultraf1ex stent. H6: 6 mm-band Hanaro stent. G : Gianturco-Rösch Z stent. M: Memotherm stent. W: Wallstent. H8: 8 mm-band Hanaro stent.

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Fig. 6. Resistance force (newtons) versus the length of film pulled plot of six stents subjected to circular loads. M Memotherm stent. H6: 6 mm-band Hanaro stent. H8: 8 mm-band Hanaro stent. G: Gianturco-Rösch Z stent. 、^I:

Wallstent. U: Ultraf1ex stent

ate obstructions. To date, various types of meta11ic stents, made of different materials, have been designed (2- 6). They are categorized into two groups, self-expandable and ba11oon-expandable. In biliary inter- vention, balloon-expandable metallic stents have now been largely abandoned; they need ba1100ning prior to stent placement and compared with self-expandable stents, have no benefit in long term patency (4, 8). In this study we therefore dealt with six self-expandable stents widely used in the biliary system.

The ideal stent would combine the virtues of sim- plicity of use, low introduction profile, radiopacity, proper mechanical characteristics, and high patency

rate. The primary technical success of stent placement

in many types of obstructive lesion depends direct1y on the mechanical characteristics of the stent. When choosing the type most appropriate to a particular clinical setting, awareness of the mechanical characteristics of each stent is thus important.

The mechanical characteristics of stents include ex-

pansile force, resistance force, elasticity, longitudinal f1exibility, biocompatibility, and shortening. For each type of stent, these depend on the mode of construction, amount of mechanically effective mass of metaL and the mechanical characteristics of the metal such as plasticity, elasticity, and temperature dependence (9) During the early stage of making a meta11ic stent, stainless steel was used. The Wa11stent, Gianturco-Rösch Z stent, and Hanaro stent are made of biocompatible stainless steel wires. To enhance expandability and elasticity, nitinol - a material with thermoelastic charac-teristics - has recentl y been used. It is used for the Memotherm stent and Ultraflex stent, for example.

The mechanical characteristics tested in vitro in- cluded expansile and resistance forces. Three types of load were applied to each stent, namely point, area, and circular, which under clinical conditions imply, respectively, focal eccentric, diffuse eccentric, and concentnc stenoSlS.

The resistance force of a meta11ic stent is the strength to sustain its fu11y expanded state against an extrinsic growing mass. We experienced one hundred cases with metallic stent placement and in none of these was the stent compressed by a tumor (8). Furthermore, because a11 stents are designed to be inserted in their compressed state, resistance forces in the clinical situation are less important than expansile forces. Our results showed, fortunately, that the order of stents according to resistance force was similar to that according to expansile force.

Expansile force, on the other hand, is closely related to the primary effectiveness of stent placement; this is because it is closely related to the time that the stent takes to achieve effective luminal diameter against a surrounding tumor. According to our results, the 6 mm-band Hanaro stent showed higher resistance and

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J Korean Radiol Soc 1 998 ; 39 : 497 - 502

expandability under all types of stress. The Memo therm stent showed the highest expandability under point and circular load, though this was lower under area load. Conversely, the Ultraflex stent showed the highest expandable force under area load but a lower value under other loads. The Wallstent and Gianturco- Rösch Z stent showed somewhat lower expandability under all stresses. These results coincide with those of a study by Flueckiger et al (9). With regard to expandability itself, the Memotherm stent has an ad- vantage in focal eccentric and concentric stenosis, the Ultraflex stent in diffuse eccentric stenosis, and the 6 mm-band Hanaro stent in all types ofstenosis.

The Gianturco-Rösch Z stent showed uneven expandability along its length. As it has lower expansile force in areas of connections between individual stent bodies, in short-segment stenoses the exact placement ofthe Gianturco-Rösch Z stent is more difficult to control. This uneven expandability, however, has not been a problem in clinical use ofthe Gianturco-Rösch Z stent in longer stenoses.

In our study, the elasticity of six stents differed. The Hanaro stent and Memotherm stent were completely elastic, but the Ultraf1ex stent and Wallstent showed a wide gap between resistance force and expansile force.

It is uncertain why the Ultraf1ex stent and Wallstent a- re less elastic, but their mode of construction appears to be related. Although the Ultraflex stent is made of nitinol wire, which has thermoelastic characteristics, it is knitted into a cylindrically shaped f1exible mesh, which is not interconnected. We found that the con- figuration of the Ultraflex stent was easily distorted as stress increased, especially when a circular load was applied. When it expands from the compressed state, the vector of radial force is thus dispersed. Because of its design an eccentric load on the Wallstent causes concentric constriction over a considerable length. Ac- cordingly, if an eccentric load is applied to this stent, it is unable to offer adequate mechanical resistance at precisely this segment (9)

Ideally, the diameter of a stent on insertion and the surface area when expanded should both be as small as possible. These two factors limit the mechanically effective mass of a metal, which could be expected to be oflimited strength.

In this study we tested the mechanical characteristics of metallic stents in vitro, and the results ob

diameter in the curved lumen. Though we did not test this featurein the present study, flexibility in general decreases as expandability increases; the former depends on the mode of stent’s construction. In other words, the inter-connection of mesh structures and construction of the stent inf1uence the stent' s longitudinal flexi bili ty.

Although the Ultraf1ex stent showed lower strength under point and circular loads, it has superior f1exi- bility. It is constructed of mesh struts that are not fixed to one another, and this permits good longitudinal f1exibility (5). The Wallstent is composed of 18 biocompatible stainless steel monofilaments, free to move over each other because the cross points between them are not soldered together. It is pliable and flexible in the longitudinal axis (9). In the case of the Gianturco- Rösch Z stent and Memotherm stent, however, each connecting portion is fixed, and this decreases their longitudinal flexibility

Shortening of a stent in clinical application is ano ther mechanical characteristic to be considered. The Hanaro stent, Memotherm stent, and Gianturco-Rösch Z stent shorten only minimally, and if, during deploy- ment, this is the case - or there is no shortening- exact placement should be easy. In the case of the Wallstent and Ultraf1ex stent, however, shortening after placement is reported to be approximately 35 - 40 % ofthe length ofthe unexpanded stent (5, 12) This could make proper positioning at the desired seg ment difficult, especially in cases involving hilar ob structlOn

When choosing a stent, another important aspect is its design of construction. The Wallstent has sharp wires on both ends. Though such complications are rare, these may cause CBD ulceration, duodenal ulcerations when the stent is placed across the papilla for low CBD obstructions, or bleeding from duodenal mucosa. The Ultraf1ex stent, however, has soft looped ends which should not cause injury to the ductal or bowel mucosa (5)

In conclusion, the Memotherm stent showed the highest expandable force under both point and circular load. The Ultraf1ex stent showed the highest strength under area load. The 6 mm-band Hanaro stent showed favorable expansile force and superior elasticity under all types of stress. Under these conditions, the 8 mm-band Hanaro stent showed the lowest value.

In clinical practice, these results may help in choosing the most suitable stent, according to

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the forces generated in stents during our experiments.

References

1. Irving JD, Adam A, Dick R, Dondelinger RF, Lunderquist A, Roche A. Gianturco expandable metallic biliary stents: results of a European clinical trial. Radi%gy 1989; 172: 321-326

2. Adam A. Metallic biliary endoprostheses. Cardiovasc Intervent Radio/1994; 17: 127-132

3‘ Ferro C, Perona F, Ambrogi C. Barile A, Cianni R. Malignant biliary obstruction treated by Wallstents and Strecker tantalum stents: a retrospective review. Cardiovasc Intervent Radio/1995; 18: 25-29 4. Rossi P, Bezzi M, Rossi M, et al. Metallic stents in malignant biliary

obstruction : results of a multicenter European study of 240 patients. JVasc Intervent Radio/1994; 5 : 279-285

5. Bezzi M, Orsi F, Salvatori FM, Maccioni F, Possi P. Self-expandable nitinol stent for the management of biliary obstruction ‘ long-term clinical results. JVasc Intervent Radio/1994; 5 : 287-293

6. Cragg AH, De Jong SC, Barnhart WH, Landas SK, Smith TP. Nitinol intravascular stent: results of preclinical evaluation. Radi%.밍

1993; 189: 775-778

7. Lee BH, DO YS, Lee JH, Kim KH, Chin SY. New selεexpandable

spiral metallic stent: preliminary clinical evaluation in malignant biliary obstruction. JVasc Intervent Radio/1995; 6 : 635-640 8. Lee BH, Choe DH, Lee JH, Kim KH. Chin SY. Metallic stents in ma-

lignant biliary obstruction: prospective long-term clinical results AJR 1997; 168: 741-745

9. Flueckiger F, Sternthal H. Klein GE, Aschauer M, Szolar D, Kleinhappl G. Strength, elasticity, and plasticity of expandable metal stents: in vitro studies with three types of stress. J Vasc Intervent Radio/1994; 5 : 745-750

10. Fallone BG, Wallace S, Gianturco C. Elastic characteristics of the self-expanding metallic stents. Invest Radio/1988; 23 : 370-376 11. Maeda M, Timmermans HA, Uchida BT, Keller FS, Rosch J. In vitro

comparison of the spiral Z stent and the Gianturco Z stent. J Vasc Intervent Radio/1992; 3 : 565-569

12. Gillams A, Dick R, Dooley JS, Wallsten H, EI-Din A‘ Self-expandable stainless steel braided endoprosthesis for biliary strictures.

Radi%gy 1990; 174: 137-140

대한빙사선의학회지 1998; 39 :497-502

자가팽창성 금속스텐트의 물리적 성잘 :

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l 원자력병원진단방사선과 이병희·검기환·진수일

목 적 : 담도폐쇄의 치료에 널리 사용되는자가팽창성 금속스텐트의 물리적 성질을객관적으로비교한자료를얻 는데목적이있다.

대상 및 방법 : 실험 에 사용된 스텐트는 6 mm-band와 8 mm-band 하나로 스텐트， Gianturco-Rösch Z stent, W alls ten t, Ultr aflex 스텐트， Memotherm 스벤트였다. 각 스텐트에 point load, area load, circular load 등 3가지 부 하를가하면서 각부하에 따른스텐트의 물리적 성질 (저항력，팽창력，탄성)을측정하여 비교하였다.

결 과 : Point load에서는 Memotherm 스텐트가가장높은저항력과팽창력을보였고， 8mm-band 하나로스텐 트가 가장 낮은 저 항력 을， Gianturco-Rösch Z stent 와 Ultraflex 스텐트가 낮은 팽 창력 을 나타내 었다. Areaload에 서 는 Ultraflex 스텐트가 가장 높은 저 항력 을， 6mm-band 하나로 스텐트， Gianturco-Rsch Z stent, Ultraflex 스텐트 가 높은 팽 창력을 보였으며， 8mm-band 하나로 스텐트는 가장 낮은 저 항력 과 팽 창력 을 나타내 였다. Circular loads 에서는 Memotherm 스텐트가 가장 높은 저 항력 을， Ultraflex 스텐트와 Wallstent가 낮은 저 항력 을 보였다. 모든 종 류의 부하실험 에서 하나로 스텐트와 Memotherm 스텐트는 높은 탄성 을 보인 반면， Ultraflex 스텐트와 Wallstent 는저항력과팽창력사이에 큰차이를보였다.

결 론 각금속스텐트의 물리적 성질을알고있으면 임상에서 여러가지 종류의 담도협착에 따른적절한스텐트를 선택하는데 도움이 되리라생각된다.

Mechanical Characteristics of Self-expandable Metallic Stents: In Vitro Study with Three of Stress

Mechanical Characteristics of Self-expandable Metallic Stents . In Vitro Study with Three Types ofStress

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