JOURNAL OF ENDOUROLOGYVolume 7, Number 2, 1993Mary Ann Liebert, Inc., Publishers

Comparative Evaluation of Materials Used for InternalUreteral Stents

HAL K. MARDIS, MD, R. MICHAEL KROEGER, MD, JON J. MORTON, MD, andJOHN M. DONOVAN, MD

ABSTRACT

The materials used in the fabrication of self-retained internal ureteral stents should provide strength, flexibil-ity, low surface friction, radiopacity, biodurability, biocompatibiiity, and reasonable unit cost. Polymericbiomaterials currently used for stent construction include polyurethane, silicone, Silitek,™ C-Flex,™ andPercuflex. ™ Comparative evaluation of these materials in the context of the requirements for stent structureand function suggests advantages and disadvantages for all of them. We believe that the most importantattributes for an internal ureteral stent are ease of insertion, effective restoration and maintenance of flow,resistance to migration, significant biodurability, and biocompatibiiity. Based on our physical testing of stentsfabricated from these materials, as well as clinical and laboratory experience, we believe that C-Flex andPercuflex are the most suitable materials for stent construction.

INTRODUCTION

INTUBATION OF THE LOWER URINARY SYSTEM forrelief of obstruction has been recognized throughout the re-

corded history of medicine, and intubation of the ureters hasbeen described throughout the past century. The goal of a self-retained internal ureteral catheter for relief of upper tract ob-struction has evolved over the past 25 years. It is apparent thatthe success of the current self-retained internal ureteral stent isrelated to the versatile polymeric biomaterials now available forthis application.

We have evaluated these materials by means of end-applica-tion testing (e.g., physical testing of the stent in its manufac-tured and sterilized form) and have reviewed the publishedclinical and experimental experience in an attempt to determinewhether one material may offer an advantage over the others. Agroup of definitions relevant to these materials is given in Table1. The test procedures used to evaluate the physical propertiesof these materials are traceable to The American Society for

Testing and Materials (ASTM).* A description of the varioustypes of polymers utilized in the construction of these stentstested is given in Table 2

MATERIAL STRENGTH

The "high" polymers (as the biochemist calls them) are giantmolecules composed of thousands of chemical monomers thatlink by polymerization into chains of immense tensile strength.A comparison of the tensile strength of various 6F ureteral stentmaterials is given in Table 3. The advantage of high tensilestrength in a polymer is the ability to extrude the material with a

relatively large internal diameter (ID) relative to the outsidediameter (OD). Comparative ID/OD measurement of a group of7F stents is given in Table 4. It seems logical that flow restora-tion and maintenance will be enhanced if the internal diameteris as large as possible. Another advantage to a material withhigh tensile strength is the ability to create relatively large

Department of Surgery (Urology), University of Nebraska Medical Center, Nebraska Methodist Hospital and Childrens Memorial Hospital,Omaha, NE

*1916RoseSt., Philadelphia, PA 19103-1087.

105

MARDIS ET AL.

Table 1. Definitions

BiomaterialA substance, natural or synthetic, which, at some stage in treatment, interfaces with tissue

PolymerA substance consisting of molecules characterized by the repetition of one or more typesof chemical units (monomers)

CopolymerA substance formed of repeated sequences of polymers with differing chemical structure("blocks")

DegradationA deleterious change in the chemical structure, physical properties, or surfacecharacteristics of a polymer

ElastomerA macromolecular polymer that can re-form to an original configuration after substantialdeformation

ThermoplasticA polymer that can be softened by heating and hardened by cooling and that, in a softstate, can be shaped by molding or extrusion

ThermosetA polymer that can be cured by heat to the extent that it is substantially nonmeltable

sideholes along the shaft of the stent. We have shown that stents condition of diuresis.2 Under high-flow conditions,3 it is obvi-with sideholes have a remarkable drainage advantage over ous that drainage occurs around as well as through these stentsstents without. ' We believe that a stent with a large ID and large (Fig. 2), yet we see no reason to limit the restoration of flow tosideholes can be presumed to be better than one in which the any one parameter alone.strength of the material limits these parameters (Fig. 1). The The concept of reinforcing the strength of the polymericfunction of the sideholes is most important in the "low-flow" material by incorporating a wire spiral within the extrusion has

Table 2. Stent Materials

PolyurethaneA generic class of condensation polymers, the repeating units of the polymeric chain beingderived from polyisocyanate and a polyol. Includes elastomers that may differ in theirproperties

SiliconeA generic class of condensation polymers with chains of alternating silicon and oxygenatoms. Includes elastomers that may differ in their properties

SilitekA proprietary silicone-based block copolymer from Medical Engineering Corporation*

C-FlexA proprietary silicone-modifled styrene/ethylene/butylene block copolymer from ConceptPolymer Technologiest

PercuflexA proprietary olefinic block copolymer from Boston Scientific Corporation^ (Medi-Tech,Microvasive)*Racine, WI 53404.tClearwater, FL 33546.ÏWatertown, MA 12172.

COMPARATIVE EVALUATION OF STENT MATERIALS 107

Table 3. Tensile Strength of Various 6F Internal Ureteral Stent Materials

Manufacturer Material Designation PSI at Break

High StrengthCook*CookSurgitekBardtSurgitekMicrovasivei

Medium StrengthCookCookMicrovasiveMicrovasiveMicrovasiveMicrovasive

PolyurethanePolyurethaneSilitekPolyurethaneSilitekPercuflex

C-FlexC-FlexC-FlexC-FlexC-FlexC-Flex

Sof-Flex pigtailDouble pigtailMulti-floDouble pigtailUropassDouble pigtail

Passive spiralDouble pigtailMacaluso J-MaxxMardis firm stentHydro-plus stentMardis Soft-Stent

250024402440242021401795

1215120512001200900900

Low StrengthCookCookCookBardSurgitekBard

SiliconeSiliconeSiliconeSiliconeSiliconeSilicone

Filiform double pigtailMulti-lengthDouble pigtailDouble-4Finney double-JFluoro-4

615613612612526503

*Cook Urological, Spencer, IN.tC. R. Bard, Inc., Covington, GA.¿Microvasive Urology Division, Watertown, MA.

This test considers surface area in cross-section, derived from measurement of internal andexternal tubing and sidehole diameters. Three samples of each stent design were tested at 10in/min crosshead speed, 100 lb. load cell, and 1 in jaw separation. The stents invariably breakacross the sidehole. Results indicate the load (psi) of cross-sectional stent area required to breakthe stent. (ASTM Method D-412; tests performed by Concept Polymer Technologies.)

historical interest because the Gibbons silicone stent in its initialform had this type of wire reinforcement.4 This feature im-proved its flow characteristics because the ID and sideholescould be made larger in this relatively weak material comparedwith the double-J silicone rubber stents without reinforcement.5

Finally, with a closed-end stent, inserted with a stylet in thetraditional retrograde ureteral catheterization technique, theclosed end limits flow and the ability to exchange the stent byinserting a wire through it during its removal. A dissolvablematerial has evolved that can be bonded as the tip of the stent to

Table 4. Comparison of Inside (ID) and Outside (OD) Diameters of Various 7F StentMaterials in Manufactured Form*

Manufacturer MaterialID(in)

OD(in)

IDIODRatio

SurgitekBardCook filiformMicrovasiveCookMicrovasive

SiliconeSiliconeSiliconeC-Flex

PolyurethanePercuflex

0.0390.0420.0440.0460.0490.059

0.0980.0940.0900.0880.0900.089

0.3980.4470.4890.5230.5440.663

*The ID/OD ratio is a measure of the potential internal flow capacity of these stents. These datado not reflect the potential for drainage outside of the stent, which may be important in high-flowconditions of diuresis.

108 MARDIS ET AL.

FIG. 1. Comparison of Percuflex™ 8.3F pigtail stent coil(left) and silicone 8.5F J stent hook (right). Tensile strengthadvantage of Percuflex allows larger sideholes and inside diam-eters, which markedly improves internal drainage capacity inthese stents compared with those made of silicone.

a nondissolvable shaft material, thus providing a closed tip forinsertion should this be desired. Rapid dissolution of the tipleaves an open end for drainage and subsequent guidewire-stentexchange if needed (Fig. 3).

COIL RETENTION

Most elastomeric polymers have "memory": the ability toresume an original configuration after distortion. Memory is a

direct consequence of cross-linking: the physical entanglementor chemical bonding between the polymeric chains. This allowsthe stent to be formed with a retention coil, which can easily bestraightened out over a guidewire, yet will re-form when theguidewire is removed, thus facilitating endourologic place-ment.6,7

Our retention strength test results of various materials andcoil configurations are given in Table 5. The configuration ofthe coil does introduce a variable that is independent of thematerial used. The simple hook or J configuration is weakbecause there is less polymer length to which memory has beenafforded compared with the pigtail or spiral configurations.Hence, because of the relative weakness of the silicone materialand the inadequacy of the J configuration, these stents are proneto spontaneous migration. Conversely, pigtail configurations,when formed of a relatively strong material, may fail to uncoilas the stent is withdrawn. A cross-coil or spiral configurationseems to lend itself more easily to uncoiling. Experience sug-gests that coil strength in the range of 20 g or more is sufficientto minimize the problem of migration. Another strategy is touse a relatively firm spiral retention coil in the renal pelvis and a

soft cross-coil design for the bladder end of the stent (Fig. 4).

FIG. 2. Drainage outside and inside stent. A. Retromax™ 14F/7F stent 3 days after percutaneous retrograde endopyelotomy.This stent has sideholes only in the coils and lower third of shaft. B. Antegrade nephrostogram via Foley catheter. C. Lower half ofureter. D. Bladder coil. There is obvious drainage of contrast around as well as through this Percuflex stent.

COMPARATIVE EVALUATION OF STENT MATERIALS 109

FIG. 3. Fader-tip™ 6F Percuflex stent (Microvasive Urology Division). A. Upper coil tip has 0.025-inch opening, but0.038-inch wire can be used as stylet for stent insertion. B. Appearance 30 minutes after contact with artificial urine. C.Appearance 60 minutes after placement; tip has completely dissolved, converting closed end to open end.

SURFACE FRICTION

Surface friction or "drag" is a property of all elastomers. Themost obvious example is the natural polymer polyisoprene, thesource of latex rubber.

Standard Stent Materials

We have measured the surface coefficient of friction of vari-ous stent materials (Table 6). The degree of polymeric cross-

linking also affects this physical property; thus these data showa wide variation in surface friction of stents even though thebasic material is the same. In general, the softer (and more

rubbery) the elastomer, the higher will be the coefficient offriction. This fact has important implications with respect to theproblems that a soft stent material offers because of the inter-face friction between the stent and the apparatus (guidewire,etc.) used for its insertion, as well as that between the surface ofthe stent and the wall of the ureter. We believe that surfacefriction also has implications with respect to biocompatibiiityand resistance to encrustation.8-1 '

Table 5. Proximal (Renal Pelvis) Coil Retention Strength of Various 6F Internal Ureteral Stent Materials

Manufacturer Material Designation Coil Type Fk (g)*High StrengthMicrovasiveCookMicrovasiveCookCookMicrovasive

Hydrogel on firm C-FlexPolyurethaneC-FlexC-FlexC-FlexPercuflex

Macaluso J-MaxxDouble pigtailMardis firm stentPassive spiralDouble pigtailDouble pigtail

SpiralPigtailCross-coilSpiralPigtailPigtail

908987655239

Medium Strength

CookCookCookMicrovasiveMicrovasiveMicrovasive

C-FlexPolyurethaneSiliconeC-FlexC-FlexHydrogel on soft C-Flex

Towers peripheral PigtailSof-Flex pigtail PigtailFiliform double-pigtail PigtailMardis Soft-Stent Cross-coilMulti-coil SpiralHydro-plus Cross-coil

272524242222

Low Strength

BardBardBardBardSurgitekSurgitekSurgitekSurgitekSurgitek

PolyurethaneSiliconeSiliconeSiliconeHydrogel on soft polyurethaneSilitekSilitekSiliconeSilicone

Soft pigtailFluoro-4Double-4Coil stentLubri-flexMulti-flowUropassTractfinderFinney double-J

Cross-coilCross-coilCross-coilSpiralCross-coilCross-coilCross-coilHookHook

19181614109865

*Fk reflects the mean load cell force required to pull the coil through a 5 x 30-mm length of hydrophilic tubing at 12inches/minute using a pull test (tests performed in a 37° C waterbath and stents soaked for 20 minutes prior to testing). This testevaluates the resistance of the proximal (renal pelvis) stent coil as it is drawn through a simulated ureteropelvic junction and is amodification of ASTM Method D-1894.

FIG. 4. C-Flex stent with Hydro-plus™ graft on its surface. A. Renal pelvis coil is spiral with high retention strength and will notmigrate. B. Bladder coil is soft polymer fused to firmer shaft. Collar at beginning of shaft is for stent pusher, which slips over softcoil during insertion (J-Maxx™ by Microvasive).

COMPARATIVE EVALUATION OF STENT MATERIALS 111

Table 6. Kinetic Surface Coefficient of Friction among Various 6F UreteralStent Materials

Manufacturer Material Designation Fk(g) Uk*

High Friction

SurgitekSurgitekCookBardSurgitekMicrovasiveCook

Medium Friction

SilitekSiliconeSiliconeSiliconeSilitekC-FlexC-Flex

Uropass 274 1.98Finney double-J 262 1.89Multi-length 261 1.88Double-4 250 1.80Multi-flo 179 1.28Mardis Soft-Stent 161 1.16Towers peripheral 137 0.99

MicrovasiveBardCookCookMicrovasiveCookCookBard

Low Friction

C-FlexPolyurethanePolyurethaneC-FlexPercuflexC-FlexSiliconeSilicone

Mardis firm stent 104 0.75Soft pigtail 104 0.75Sof-Flex 96 0.69Double pigtail 86 0.62Double pigtail 84 0.60Passive spiral 81 0.58Filiform double pigtail 81 0.58Fluoro-4 69 0.50

SurgitekMicrovasiveMicrovasive

Hydrogel on polyurethaneHydrogel on soft C-FlexHydrogel on firm C-Flex

Lubri-flex 29 0.21Mardis Hydro-plus 15 0.11Macaluso J-Maxx 14 0.10

*The coefficient Uk is a measure of the relative difficulty with which the surface of onematerial will slide over that of another, standard reference, material. A high surface friction mayproduce problems during stent insertion. This test is a modification of ASTM Method D-1894.

HydrogelsA hydrogel is a polymeric biomaterial that will imbibe water

and retain a significant fraction of it within its structure yet willnot dissolve in water.I2 Hydrogels are composed of polymericchains. The mechanisms by which water is drawn into thepolymer are variable. The mechanism of importance to theendourologist is water structured along the surface of the gel,which produces a very low coefficient of friction.5 It is ofinterest that the urothelium has on its surface a layer of gly-cosaminoglycans (GAGs), which are hydrophilic and appear toprotect the surface cells against a number of disease pro-cesses.13-15

A limitation of hydrogels for endourologic applications istheir obvious lack of mechanical strength.16 Fortunately, tech-niques have evolved for grafting these gels on the surface ofstronger polymers, and this represents their current applicationas a stent material. ' '

•

'7

RADIOPACITY

None of the polymers used for stent fabrication is sufficientlyradiopaque to permit fluoroscopic visualization during en-

dourologic placement. Thus, metallic salts must be added to thepolymer mix prior to extrusion. Barium, bismuth, tantalum,and tungsten are most commonly used. The type of metal and

the percentage of it added to the overall mix can create a widevariation in the actual radiopacity of otherwise-similar stents

(Fig. 5). In addition to radiographie densitometry, we haveused a CT scanner to evaluate the radiopacity of a group ofstents (Table 7). We believe that stent radiopacity should ap-

FIG. 5. Radiograph of two 6F silicone ureteral stents (46kVp, 1/120 sec, 100 Mas). Left: Filiform double-pigtail stent(Cook Urological) with metallic filler as radiopacifier. Right:Double-J (Surgitek) with barium sulfate as radiopacifier. Supe-rior radiographie image of metallic filler is obvious.

112 MARDIS ET AL.

Table 7. Radiopacity of Various 6F Ureteral Stent Materials

Manufacturer Material DesignationHounsfield

Units*

High RadiopacityCookMicrovasiveBardSurgitekMicrovasiveMicrovasive

Medium Radiopacity

SurgitekSurgitekCookMicrovasiveBardSurgitekLow RadiopacityCookBardBardCookSurgitek

SiliconePercuflexSiliconeSiliconeC-FlexC-Flex

SilitekPolyurethanePolyurethaneC-FlexPolyurethanePolyurethane

C-FlexPolyurethaneSiliconeSiliconeSilicone

Filiform double pigtailDouble pigtailFluoro-4TractfinderMacaluso J-MaxxMardis firm stent

UropassDual-flexSof-FlexMardis Soft-StentSoft pigtailLubri-flex

Towers peripheralDouble pigtailDouble-4Coil stentFinney double-J

272627171551146510771076

863828810431420420

360277177131125

*Each image was magnified, and a CT number (Hounsfield units) was calculated based on arelative linear attenuation coefficient for each stent normalized to a reference material (water).Water has a CT number of zero; bone is +500 and air —500 ± 5. Equipment was a SiemensDR-2 CT scanner with 10-mm section, 120 kVp, 350 Mas.

proach that of bone (500 Hounsfield units) in order to ensure

adequate fluoroscopic imaging.

BIODURABILITY

From the moment a stent material is placed within the urinarysystem, there occurs surface absorption of biomolecules, usu-

ally proteins. This is followed within minutes by cellular inter-actions and a tendency for crystalloid encrustation on the pro-teinaceous platform. Chemicals within the urine also begin to

penetrate porous areas on the surface of the polymer. Resis-tance of the material to these attacks provides the basis for thedurability of the stent within the urinary system. Degradation ofthe polymer can ultimately result in the loss of critical charac-teristics such as strength, elasticity, and flexibility. There havebeen reports of spontaneous breakage of internal ureteral stentsfabricated of polyethylene and polyurethane after many monthsof time indwelling.17'8 In contrast, clinical reports and deviceretrieval studies have shown that C-Flex and Percuflex will not

degrade significantly for as long as 2 years.5'9,20 Siliconerubber, probably because of its thermoset (heat-stable) cross-

linkage, has been found to retain flexibility and elasticity even

after 10 years within a ureter, although encrustation had clearlyproduced device failure (HK Mardis, unpublished data).

Although encrustation remains the primary limiting factor forprolonged stent function, it seems prudent to limit long-term(more than 6 months) stent usage to those made of silicone,C-Flex, or Percuflex.'9,20 All stents must be monitored forposition and continued efficacy of function at least at 3-monthintervals. Patients with a history of stone formation should havethe stents evaluated and possibly changed more frequently.

BIOCOMPATIBILITY

Absolute biocompatibiiity (indicating that a biomaterial can

be present within a physiologic environment without the mate-rial adversely affecting the environment or the environmentadversely affecting the material) is a Utopian state of affairs thatcannot be expected.21 However, stent materials generally havesufficient relative biocompatibiiity so that significant adverseeffects are not anticipated until the device has been indwellingfor several weeks. Biocompatibiiity at the interface between thestent and the ureter depends on characteristics that discourageexcess cell proliferation and maintain normal cell differentia-tion within the immediate surroundings.8 With the relativelynonporous polymers used for stents, biocompatibiiity is influ-enced in large part by the surface properties of the material.8,11

COMPARATIVE EVALUATION OF STENT MATERIALS 113

FIG. 6. Scanning electron microscopy of three stent material surfaces (X 800, 30° slant). A. Silicone. B. C-Flex with hydrogelgraft. C. Polyurethane. Irregular surface of polyurethane may contribute to its poor biocompatibiiity compared with silicone andC-Flex. (From Mardis HK, Kroeger RM: Ureteral stents: materials. Urol Clin North Am 1988; 15:471, with permission.)

In an attempt to evaluate the biocompatibiiity of stent materi-als objectively, Marx et al, in a controlled study, left stents in31 ureters of 19 mongrel dogs for 6 weeks.22 Silicone, C-Flex,and polyurethane caused similar mild ureteral edema, but ure-

ters stented with Silitek demonstrated fairly marked edema.Epithelial ulcération and erosion occurred with all polyurethanestents and rarely with the other materials. A statistically signif-icant advantage was demonstrated for C-Flex compared withpolyurethane. Those investigators concluded that silicone andC-Flex are more suitable for ureteral stenting than polyurethaneor Silitek. Percuflex was not studied in this experiment.

We have evaluated various stent material surfaces by means

of electron microscopy (Fig. 6). Ratner believes that the bio-compatibiiity of nonporous polymeric materials is enhanced bya smooth surface.8

One wonders if the generally poor early results of the innova-tive Davis "intubated ureterotomy," described nearly 50 years

ago,23 were attributable to the use of latex rubber, subsequentlyproved to be a poor biomaterial,24-26 for the stent. When thebiocompatibiiity of silicone rubber was recognized nearly twodecades later,27 alternative open surgical pyeloureteroplastyprocedures had become the standard of care for ureteropelvicjunction obstruction, and the Davis procedure was rarely used.We believe that the recent success of percutaneous and uretero-

scopic endopyelotomy23,29,30 can be attributed in some part tothe biocompatibiiity of the stent materials now used in theseprocedures (Fig. 7).

DISCUSSION

The decision to use a stent of one material rather than anotherhas many ramifications. Clearly, the issues presented herein are

less important for short-term (a few days or weeks) stents than

114 MARDIS ET AL.

FIG. 7. Percuflex Retromax endopyelotomy stents. Left: 14Ftapering to 7F for adults. Right: 10F tapering to 5F for children.We believe biocompatibiiity of stent is an important factor forendopyelotomy. These pédiatrie stents have enabled us to per-form endopyelotomy on children as young as 8 months, and thestents are well tolerated by these youngsters.

for those needed for months or years. Cost is also an issue, andthe simpler biomaterials such as polyurethane are obviously lessexpensive than complex copolymers or silicones. The cost ofthe stent itself, however, is not significant compared with thecost of the operating, anesthesia, and recovery services requiredfor stent placement, even when done as an outpatient proce-dure. Hence, we do not believe that the unit cost of the stent can

be the sole criterion for its selection.

SiliconeOur experience demonstrates fundamental limitations of sili-

cone for stent fabrication. Low tensile strength limits the ID andthe aperture of the sideholes, which clearly reduces the efficacyof the stent in restoration and maintenance of flow. Moreover,weak coil strength produces a significant risk of stent migra-tion, and the high surface coefficient of friction creates difficul-ties during stent insertion. In addition to these deficiencies,silicone is costly compared with the other materials, primarilybecause of the fabrication requirements and historical pricingpolicies. The excellent biodurability and biocompatibiiity ofthis material notwithstanding, silicone rubber would not appearto be the best choice of material for ureteral stents.

PolyurethaneBecause it is a highly versatile and inexpensive biomaterial,

polyurethane will continue to be utilized for stent fabrication.We believe, however, that its questionable biodurability and

biocompatibiiity suggests that stents of this type are appropriateonly for short-term utilization.

Silitek

A silicone-based copolymer, Silitek has excellent tensilestrength, yet provides this at the expense of a low ID/OD ratioand small sidehole apertures and thus a relatively low flowcapacity. Also, it shares the relatively weak coil retentionstrength and high unit cost of silicone rubber stents. Althoughwe have done no device retrieval studies with this material, we

anticipate that it will prove to be biodurable. Its surface issimilar to that of silicone rubber.

C-Flex

The silicone-modified thermoplastic elastomer called C-Flexwas designed specifically as a biomaterial. It is a relativelyinexpensive block copolymer yielding good surface and pro-cessing properties. It does not have the strength of polyure-thane, Silitek, or Percuflex but is sufficiently strong to providegood flow rates and coil retention strength, particularly in diam-eters of 7F and 8F. It has proved biodurability and biocompati-biiity.5'9 Furthermore, this material has the longest record ofhydrogel grafting on its surface, and with this modification, ithas the lowest surface coefficient of friction that we have mea-

sured.

PercuflexPercuflex is another thermoplastic block copolymer. It has

impressive tensile and coil strength and thus can be extrudedwith the best ID/OD ratio and sidehole efficiency that we havemeasured. Both clinical and device retrieval studies have con-

firmed its long-term biodurability and biocompatibiiity.5,20 Itssurface characteristics are good, and it has a relatively lowcoefficient of friction even without a hydrogel graft. Our stud-ies suggest that Percuflex may represent one of the most bal-anced stent materials.

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Address reprint requests to:Hal K. Mardis, MD

HIS. 90th St.Omaha, NE 68114

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