bestent—the serpentine balloon expandable stent: review of mechanical properties and clinical...

BeStent—The Serpentine Balloon Expandable Stent:Review of Mechanical Properties and Clinical Experience

Ariel Roguin and Rafael Beyar

Division of Invasive Cardiology, Rambam Medical Center, Heart System Research Center, Technion-Israel Institute ofTechnology, Haifa, Israel

Abstract: The objective of this study was to present theengineering and clinical aspects of a new balloon expand-able coronary stent. A new tubular, serpentine designstainless steel balloon-expandable stent, the beStent, wasdesigned based on clinical requirements for stents and hasbeen clinically evaluated in multiple sites. The stent is fea-tured by terminal gold markers and rotational junctionsthat assure no shortening upon expansion and lead to or-thogonal locking, maximizing radial strength. In terms ofmethods and results, the stent was clinically evaluated inthe framework of a pilot evaluation in a variety of lesiontypes. The short- and long-term results evaluated duringthe course of the beStent multicenter pilot evaluation andin our single center study are reported. A variety of pa-

tients were included, including patients with long complexlesions, restenosis lesions, and total occlusions. Short-termclinical success with stenting was achieved in more than97% of the cases. Subacute thrombosis was low in 1% ofthe cases. Clinical restenosis rates were acceptable with anoverall 85% 6 month event free survival. In conclusion, themechanical features of the stent providing its flexibility,scaffolding properties, radial strength, and absence ofshortening were tested in a clinical study, showing that it issafe and effective for treating simple as well as long andcomplex lesions associated with coronary disease with arelatively low rate of complications. Key Words: Angio-plasty—Coronary arteries—Stents.

Percutaneous transluminal coronary angioplasty(PTCA) is an effective treatment for selected pa-tients with symptomatic coronary artery disease.Yet, despite great improvement in angioplasty tech-nology and operational techniques, certain limita-tions remain. Primarily, abrupt closure complica-tions between 2% and 8% have been the majorcause of in hospital morbidity and mortality (1,2).Second, long-term success is compromised by the oc-currence of restenosis in approximately 30% of pa-tients within the first 6 months (1–7).

Stents have been clearly shown to have benefit inthe prevention of restenosis in short lesions and inthe treatment of suboptimal results and acute andthreatened closure conditions (8–16). The effect ofstent treatment is mediated primarily by mechanicalfactors, preventing the arterial elastic recoil and

compressing plaque material against the wall, thuscreating a bigger initial lumen. The variation in stentmaterial and design may have substantial effect onthe angiographic and clinical outcome. The majorityof stents used today are balloon expandable. Thesestents are generally limited due to the trade-off be-tween flexibility, scaffolding properties, visibility,and versatile sizes optimizing metal surface cover-age. A new serpentine design stent, the beStent, wasdesigned to optimize flexibility, scaffolding proper-ties, radial strength, and visibility and to eliminateshortening upon expansion. We report here the ma-jor design characteristics of the stent. In addition, wereview the initial experience with this stent in thefirst 100 consecutive patients in 2 centers in Israeland an international multicenter pilot evaluation ofthe stent treating both simple and complex diseasemorphology.

METHODS

Stent designThe stent before and after expansion is shown in

Fig. 1. The technical specifications of the stent and

Received November 1997.Presented in part at the XIth World Congress of the Interna-

tional Society for Artificial Organs, held June 29–July 1, 1997, inProvidence, Rhode Island, U.S.A.

Address correspondence and reprint requests to Dr. ArielRoguin, Cardiology Department, Rambam Medical Center, BatGalim, Haifa, 31-096, POB 9601, Israel.

Artificial Organs22(3):243–249, Blackwell Science, Inc.© 1998 International Society for Artificial Organs

243

its mechanisms of expansion are described in Table1. The stent is made of a 316L stainless steel tube cutinto a unique serpentine design. Upon expansion,the stent utilizes the principles of rotational non-stress junctions and orthogonal locking. The featuresof no shortening upon expansion and 2 radiopaqueend markers for adequate visibility allow precise po-sitioning. The rotational junctions (Fig. 2) dissipatestress concentration upon expansion and lead to or-thogonal locking, maximizing radial strength. Thestent lengths used in this study were 15, 25, and 35mm, available in both small diameter (BES series,2.5–3.0 mm) and large diameter ranges (BEL series,3.0–5.5 mm).

Stent mechanical propertiesStents are designed to provide adequate support

to the arterial wall, thus preventing both the elasticrecoil of the artery and prolapse of atheromatousmaterial between the stent struts. Therefore, themesh density and the area coverage are importantfor adequate scaffolding, which assures no interstrut

plaque prolapse. The metal content of the stent isbetween 14–18% and together with the serpentinedesign provides adequate scaffolding properties forthe most irregular arterial surfaces.

The radial force of a stent is important to preventelastic recoil, i.e., the elastic properties of the arterialwall that force the artery to decrease to a smallerdiameter. The radial force of a stent can be mea-sured by a special compression chamber system inwhich the stent within a rubber tube is compressedby negative transmural pressure and its diameter iscontinuously monitored with sonocrystals. A graphshowing the resistance to collapse of the variousstent designs listed in Table 2 is shown in Fig. 3. Notethat balloon expandable slotted stents are character-ized by an initially linear relationship between thecompression radial strain and transmural pressure;however, as higher pressures are reached, the stentsuddenly collapses, associated with a highly nonlin-ear strain-pressure relationship. There is variability

Fig. 1. Shown are the unmounted, unexpanded (A), and ex-panded stent (B). Of note is the serpentine design of the stentand the gold markers delineating the stent ends.

TABLE 1. Stent technical specification, stent expansion,and delivery data

Material composition Stainless steel 316LStent design Tubular, cut into a serpentine

mesh shapeDegree of radiopacity Stent body is minimally radiopaque;

gold markers delineate stent endsMetallic surface area 12–18%, depending on stent

expansionStrut dimensions 110 × 75 mProfile on the balloon Less than 1 mm on a low profile

balloonFlexibility High longitudinal flexibilityShortening upon

expansionNone

Sizes (Bes-small; Model Bes: 2.5–3.0 mm vesselsBel-large) Model Bel: 3.0–5.5 mm vessels

Lengths 8, 15, 25, 35 mmMechanism of

expanision duringballoon dilatation

Rotation of the cross junctionsminimizes stress concentrations,leads to orthogonal locking ofthe stent, maximizing radialstrength, and results in noshortening

Minimal internaldiameter ofguiding catheter

Guiding catheter compatibility isdetermined by the balloon type.With most of the low profileballoons a 6 F-guiding cathetercan be used

Position of radiopaquemarkers

Gold markers delineate the stentends, allowing precisepositioning

Recommendenddeployment pressure

6–8 atm. If a high pressureballoon is used for deployment,it can be used for the pressuredilatation

High pressure balloondilatation

Recommended at 14–16 atm,keeping the distal end of theballoon at the distal end of thestent

Recrossability ofexpanded stent

Can be crossed with balloon,stent, intravascular ultrasound, etc.

A. ROGUIN AND R. BEYAR244

Artif Organs, Vol. 22, No. 3, 1998

between different stents as shown in Fig. 3 attributedto design aspects presented in Table 2. However, itmay be seen that the common slotted tubular stents(beStent, Crown, Nir, Palmaz-Schatz) share verysimilar mechanical properties of resistance to col-lapse.

Clinical experience

The Israeli stent registry

Patients. The Israeli Registry of the first 100 pa-tients has been recently reported (8). BetweenMarch and November 1996, 148 stents were de-ployed in 100 consecutive patients undergoing elec-tive angioplasty at 2 hospitals in Israel. All patientssigned informed consent for the procedure obtainedunder Helsinki approval by the local institutional re-

view board and the Israel Ministry of Health. Theinclusion criteria included 3–5 mm native vessels, denovo and restenotic lesions, and no myocardial in-farction (MI) 8 days prior to implantation. The indi-cations for stenting in this cohort of patients weretreatment with suboptimal results, bailout condi-tions, and prevention of restenosis. The patients’ages ranged between 32 and 78 years (mean, 58 ±12). The majority of the patients were males (78%)with the common risk factors for coronary arterydisease (active or former smoking, 88; active smoker,19; hypercholesterolemia, 45; family history, 16; hy-pertension, 34; and diabetes, 30). A substantial por-tion of the population had had previous MI (52%).A history of previous bypass surgery was present in12 patients and previous PTCA in 26 patients. Mostpatients were in angina CCS classes II and III (49and 34, respectively).

All patients were clinically monitored with regularvisits at 1, 3, and 6 months after implantation at ourpostangioplasty outpatient clinic. All main adversecoronary and cerebral events (MACCEs) i.e., death,MI, coronary artery bypass graft (CABG), re-PTCA,and cardiovascular accident (CVA) were recorded.All patients received aspirin before stent deploy-ment, heparin after sheath insertion, and ticlopidinefor 1 month.

Patient, angiographic, and procedural characteris-tics. There were 85 de novo and 18 restenotic lesionsand total occlusion in 23 cases. The majority of thelesions were complex, class B2 or C (62%). Only 31(30%) of the lesions were short with a substantial

Fig. 2. The mechanism of stent expansion is shown. As the stentexpands, the cross junctions rotate, and the serpentine wiresstrengthen both in the longitudinal and radial directions. The ro-tating junctions (magnified) are the major mechanism for rela-tively even stress distribution in the stent and the absence ofshortening. The concentration of force in the radial direction uponexpansion leads to the orthogonal locking principle.

Fig. 3. Shown is the resistance to collapse of various stent de-signs. Of note is that the balloon expandable slotted stents arecharacterized by an initially linear relationship between strain andpressure; however, as higher pressures are reached, the stentsuddenly collapses, associated with a highly nonlinear relation-ship. The different stents’ metal compositions and strut thick-nesses appearing in the diagram are reported in Table 2.

TABLE 2. Design parameters of the stents included inFig. 3

Stent Metal Design

Strutthickness

(mm)

AVE Micro-II Stainless steel316L

Wavy pattern 200

BeStent Stainless steel316L

Serpentine tubularmesh

75

Cardiocoil Nitinol Self expanding coil 120Crown Stainless steel

316LWavy slotted tube 68

Nir Stainless steel316L

Multicellular slottedtube

100

Palmaz-Schatz Stainless steel316L

Articulated slottedtube

68

Spiral Art. Ps Stainless steel316L

Slotted tubewith spiralarticulation

97

Wiktor Tantalum Wavy coil design 127

BESTENT TREATMENT 245


number with tubular (n 4 43, 42%) and diffuse dis-ease (n 4 29, 28%).

Stent implantation procedure. The stent wasmounted on the appropriate balloon for delivery. Ifpossible, a semicompliant or a noncompliant balloonwas generally used so that the same balloon could beused for high (14 atm or higher) pressure inflation,thus optimizing balloon usage. The stent was sleddedon the balloon using the special tube and stylet sys-tem for mounting. It was then gently crimped on theballoon, which was mounted over the angioplastywire and then firmly crimped on the balloon, verify-ing a smooth stent surface with no protruding strutsand no stent slippage over the balloon. The stent waspositioned using its gold end markers. Initial deploy-ment was done at a pressure of 8 atm. The secondballoon inflation was made to 14 atm or higher afterballoon repositioning so that it did not protrude dis-tal to the stent. The stent reached the lesions and wassuccessfully implanted in all but 2 cases in which 35mm stents had to be withdrawn and exchanged for25 and 15 mm stents, which were successfully deliv-ered. In 1 of them, the stent was successfully with-drawn and in 1 case the stent embolized.

In hospital clinical success and complications. Theacute and in hospital results are detailed in Table 3.There were no procedural complications or mortal-ity associated with stent implantation. Two stent

thromboses on Days 1 and 2 were successfullytreated with repeat PTCA without MI. Retroperito-neal bleeding complicated by sepsis and dissemi-nated coagulopathy led to death due to septic shockon Day 14 in another patient. There were no othermajor complications or adverse clinical events in thisfirst complex cohort of patients during hospitaliza-tion.

Thirty day and 6 month follow-up. All the patientswere under close clinical follow-up and were seenroutinely at 1, 3, and 6 months after the procedure,or as needed. During the 30 day follow-up, there was1 death (21 days after the procedure) that occurredin a patient with severe arterial hypertension second-ary to a polycystic kidney who had a 25 mm stentimplanted in a distal right coronary artery (RCA).The death was caused by hypertensive crisis withoutevidence of a coronary event. There were no cardiacdeaths, repeat revascularizations, or MIs during this30 day period. During the 30 day to 6 month followup period, events occurred in 11 patients, 1 inferiorwall MI in a patient stented in the left anterior de-scending coronary artery (LAD) (sent later to elec-tive CABG), 3 elective CABGs, and 8 repeat angio-plasties. There were no deaths nor CVAs (Table 2).The 6 month event free survival was 85%.

The multicenter pilot registry (Phase 1A)The short-term clinical results of a multicenter

registry during the initial evaluation of the beStentwere reported recently (24). Two hundred eighty-four stents were used in a total of 217 patients (age,57.9 ± 3.10 years; M, 178; F, 39) in 8 centers world-wide for variable indications. Stents of 15, 25, and 35mm length were used. The arteries treated wereLAD (n 4 112, 42%), circumflex (n 4 54, 20.2%),RCA (n 4 95, 35.5%) left main (n 4 1, 0.4%), andvein graft (n 4 5, 1.9%). Lesion types were A in 42patients (16.5%), B1 in 53 patients (20.7%), B2 in 81patients (31.8%), and C in 79 patients (31%). Onehundred fifty-nine patients required 1 stent, 40 pa-tients required 2 stents, and 18 patients required 3 ormore stents. Anticoagulation protocol included pro-cedural heparin with aspirin with or without ticlopi-dine. Smooth angiographic results were obtained inall cases with no plaque herniation. Acute angio-graphic success was obtained in 97% of the patientsand acute clinical success in 95% of the patients.Complications within 30 days were 3 (1.4%) deaths(2 noncardiac), 2 (0.9%) myocardial infarctions, and2 (0.9%), stent thromboses.

The long-term results of these patients have beenreported to be favorable compared to those of pa-tients with other available stents (25). The 6 month

TABLE 3. Procedural, in hospital, 30 days, and 6 monthcomplications, of the first 100 patients treated in Israel

Complications

Acute(<24 h of

procedure)In

hospital30

days6

months Total

MajorCombined 1 2 1 11 15

All death 0 1a 1b 0 2Cardiac death 0 0 0 0 0MI 1 0 0 1c 1CABG 0 0 0 3 3Stent

thrombosis 1 1 0 0 2Repeat

angioplasty 1 1 0 8 10CVA/TIA/

Stroke 0 0 0 0 0Minor

Surgicalvascularrepair 0 1 0 0 1

Bloodtransfusion 0 1 0 0 1

a Death due to retroperitoneal bleeding, leading to shock andsepsis (14 days after procedure).

b Death due to hypertensive crisis (21 days after implantation).c MI in a nonrevascularized lesion territory.TIA, transient ischemic attack.



event free survival was 83.8% for all lesions. The 6month event free survival was 90% for lesionsshorter than 15 mm. Therefore, it was concluded thatbased on the short- and mid-term results, the beStentis safe for treatment of complex lesions of variablelength and complexity, providing excellent resultswith a low complication rate despite unfavorable ba-sic clinical and angiographic characteristics.

DISCUSSION

General principles of coronary stentsStents are divided according to the principles of

deployment into self expanding and balloon expand-able stents. In addition to these classifications, stentscan be also classified according to their design intotubular mesh and coil designs. The majority of stentsused today are balloon expandable stents. While theaction of these stents is similar and directed towardsprevention of elastic recoil preserving the artificiallumen, there is marked variability between stents inlongitudinal flexibility, scaffolding properties, visibil-ity, shortening upon expansion, and radial force.While the characteristics of profile, longitudinal flex-ibility, and smoothness of the stent surface deter-mine the acute success of delivery of the stent to thelesion, it is not clear whether these differences mayhave an effect on the long-term response to stenting.

There are other properties that may have an effecton the short- and long-term results of stenting. Thesurface characteristics and the degree of metal pol-ishing, the charge of the metal surface, and type ofcoating that may interfere with the biocompatibilityof stenting all may have potential effects. Yet today,while metal uncovered stents have acceptable bio-compatibility, the additional effect of stent coatingremains unproved at large.

Unique features of the stentThe current study with the beStent deals with a

mesh design metal stent that has optimized the metalproperties of the stent. In terms of the engineeringdesign and principles, the serpentine design allowshomogenous distribution of stress along the struts.This unique feature is due to the rotation of thejunctions, which minimizes stress concentrations at asingle point and dissipates it along the struts. Theorthogonal locking mechanism is attributed to theconcentration of the strength in the radial directionupon expansion. This mechanism, maximizing radialstrength, is reached when the stent is dilated almostto its maximum dimension. In addition to limitingstress concentration and directing the forces in theradial direction, the rotational junctions provide theengineering basis for the true nonshortening of thistubular stent, which together with the radiopaque

markers, facilitates precise positioning. In terms ofthe clinical performance of the stent, it can be easilycrimped on any balloon, including low profile bal-loons of the last generations. Therefore, this stentcan be used to access difficult and tortuous locations.The combination of radiopaque markers with noshortening allows precise positioning of the stentrelative to ostial lesions, side branches, and otherstents. Its flat, homogeneous mesh design upon ex-pansion provides adequate scaffolding for irregularlesions with typical smooth angiographic results atthe end of the procedure. The orthogonal lockingmechanism provides the stent with its optimal radialstrength.

Clinical results in perspective to other studiesThe current results with the stent are typical of the

current practice of stenting today and better that theolder Stress and Benestent studies (1,2). The acutesuccess with stenting today is high and typicallyhigher than 97%. The long-term results are also fa-vorable and depend on the length of the lesion cov-ered with the stent rather than its complexity. It wasshown in our series that short lesions covered with 1stent have long-term event rates of only 10% if clini-cal endpoints are considered. It is acceptable that ifangiography is used routinely for all patients, as iscustomary for an angiographic based study and insome clinical centers, the degree of revascularizationincreases with an overall long-term event rate atabout 20%. For long lesions, the long-term prognosisis worse. The event rate with long lesions may be upto 30% (26), this being primarily due to aggressiveproliferative responses in some cases that lead tostent restenosis. Stent restenosis is yet an unresolvedclinical problem with potential solution either by ra-diation or by local drug therapy (14).

Present and future studies with the beStentThe beStent is currently under clinical evaluation

in a variety of studies. The Registry for OptimalStent Evaluation (ROSE) study conducted in Eu-rope is an angiographic study aimed at studying theangiographic restenosis rates and quantitative angio-graphic parameters regarding late loss after stenting.The beStent Trial (BEST) study conducted in theU.S.A. compares the beStent to the Palmaz-Schatzstent in a controlled randomized study for treatmentof de novo and restenosis lesions. Different armswithin this study test the performance of the stent fortreatment of vein grafts and bail-out conditions. TheSUCCESS study currently in planning will comparestenting to balloon angioplasty in long lesions forwhich the benefit of stenting over balloon angio-plasty has not been established. Other studies will



examine the performance of the stent or a mini ver-sion of the stent for treatment of small vessels, anarea for which the benefit of stents has not beendemonstrated.

Limitations of the stentSimilar to other balloon expandable stents, the

metal surface of the crimped stent is not as smoothas the surface of the balloon. Therefore, care shouldbe taken in excessive tortuous lesions, particularlywith a long stent, to have sufficient predilatation toreach the lesion. Tight crimping on the balloon ismandatory before implantation, and extra timeshould be allowed to complete the crimping toachieve the smoothest stent surface possible. The fu-ture direction in stenting is to provide the physicianwith premounted stents and to avoid the need formanual crimping of the stent over the balloon.

CONCLUSIONS

Based on the initial experience with the serpentinedesign stent, it can be concluded that it provides thecardiologist with a versatile tool that optimizes metalstent features and allows for treatment of coronarynarrowings of variable size, length, and complexity.The acute and 6 month results in the initial group ofpatients are favorable despite the complex lesionmorphology present in a large portion of this study.The initial experience with the beStent demonstratesits safety and efficacy for treating simple as well aslong and complex lesions associated with coronarydisease with a relatively low rate of complications.

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