Durability of Resin-Dentin BondsYasuo ShonoV Masamichi TerashitaV Jun ShimadaV Yoshio Kozono^ /

Ricardo M. CarvalhoV Carl M. Russell^/ David H. Pashiey

Purpose: The purpose of this study was to determine if the durabiiity of resin-dentin bonds couid be evalu-ated more quickly if the bond specimen was divided into 1 x 1 x S mm beams incubated at 37C for a 90-day period.

Materials and Methods: Extracted human third molars were prepared for bonding by removing the oc-ciusal surface near the dento-enamel junction (superficial dentin group) or near the pulp (deep dentingroup). The teeth were bonded either with MaoBond, One Step or Ciearfil Liner Bond 2, and then builtup toform a flat resin composite crown. After 24 hours in water, eaoh buildup was verticaiiy divided into slabs 1mm thick, the top half of whioh was resin, with the bottom haif as dentin. Each siab was then verticallysectioned at 1-mm increments to create 1 X 1 X 8-mm beams of resin-bonded dentin. They were incubatedfor 1 day or 90 days at 37"C, foliowed by measurement of the tensiie bond strengths. The results were an-alyzed by the Least-Squares Means method at the 95% confidence level.

Results: iVlacBond gave the highest p < 0,05] 1-day bond strengths to superficial dentin, but signifioantlylower bond strengths were measured in deep dentin. There were no significant differences in the bondstrengths of either One Step or Ciearfii Liner Bond 2 to superficial vs deep dentin at 1 day, but at 90 daystheir bond strengths to deep dentin had fallen significantiy (p < 0,05), Prepoiymerized cylinders of resincomposite bonded together with One Step showed littie variation in bond strength over the 90-day experi-ment. SEM examination of the failed bonds showed increased porosrty in intertubuiar dentin overtime.Conclusion: The results indicate that division of large specimens into many smaii beams acceierated thedeterioration of bond strength in deep dentin in aii three bonding systems and in both superficial anddeep dentin in the iVlacBond treated specimens. This method seems promising for studying the durabiiityof resin-dentin bonds.

J Adhesive Dent 1999:1:211-218. Submitted for pijblic3tion:12.03.99: accepted for publication:25.05.99.

Shear and tensile tests have been widely used tomeasure resin-dentin strength in dentai re-search for many years. Bonding is usualiy done inthe center of flattened coronai dentin, using cylin-ders of resin composite 3 to 5 mm in diameter.S'i2This approach worked very well with the eariy com-mercially availabie adhesive resins, whioh yielded

= Department of Operative Dentistry, Kyushu Dentai College, Ki-takyushu, Japan.

" Department of Dental Materials Sciences, Kyushu Dentai College,Kitaifyushu, Japan.

^ Oepa'lriient of Restorative Dentistry, University of Sao Pauio,Bauru, SP, Brazil.

" Department of Oral Biology and Maniliofacial Pathoiogy, Schooi ofDentistry, Medicai Coiiege of Georgia, Augusta, Georgia, USA.

Reprint requests: Dr. David H. Pasbtey, Department of Oral Biology &Msxitiofaciai Pathology, Schooi of Dentistry, Medica/ College of Geor-gia, Augusta, Georgia 30912-1129

bond Strengths of 10 MPa or less, and exhibited a^ d-hesive failures. However, newer, improved productshave been deveioped which routineiy provide adhe-sive strength of 15 MPa or more, and almost aiwaysproduce cohesive faiiure in the dentin or the resinduring testing,!^ In the case of large surface areas,non-uniform stress i ng^^.ig seems to initiate crackformation in the resin-bonded substrate, resultingin catastrophic failure. Cohesive faiiures of dentinpreclude evaluation of the true interfaciai bondstrength when using large surface areas. In 1994,Sano developed the so-cailed microtensile bondstrength test,^2,i3 using bonded areas as small as0,5 mm2. One of the advantages of this microten-siie bond strength test is that when cross-sectionaibond areas were iess than 1 mm^, aimost all of thefaiiures were adhesive in nature, even though thebond strengths v^ fere 25 MPa or more. There are

211

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many advantages to that technique, but a greatdeal of tooth structure is sacrificed to create anhour-glass configuration.i'' This limits the numberof specimens that can be prepared from a singietooth. We recently deveioped a new version of themicrotensiie test technique to evaluate regionalvariations in resin-dentin bond strength.^^ In es-sence, a resin composite buildup of a tooth speci-men is vertically, seriaiiy sectioned into slabs. Eachslab is sectioned into 1-mm-wide beams, each witha cross-sectional area of 1 mm^. The resulting 20to 25 individuai values can be compared individu-ally as a function of position within the sametooth,16 or an overall mean and standard deviationcan be calculated, as weii as the frequency distribu-tion or spatial distribution of the bond strengths.

The durability of resin-dentin bonds is of vitai im-portance if resin composites are to be successfullyused as esthetic restorations over long time peri-ods. During prolonged storage, it is assumed thatwater siowly diffuses from the externai surface intothe bonded interface. In a 3 to 4-mm-diametercylindricai resin bond, water must diffuse 1.5 to 2mm to reach the center of the specimen from theperiphery. Some investi gato rs^ reported that it maytake as iong as 3 to 5-years for the adhesive toslowly hydrolyze. However, within that 3 to 5-yearperiod, the tested adhesive products are usually su-perceded by a new generation of products. Thus,when long-term observations on the durabiiity ofproducts are pubiished, the products have usuallybeen replaced with an improved version. It wouldbe desirable to develop an accelerated aging modelthat could chalienge the durability of resin-dentinbonds in a relatively short period of time. One ap-proach to this is to divide bonded specimens into 1X 1 X 8-mm beams as described above,

Uniike conventionai bonding methods, the centerof the 1 x 1 mm2 resin-bonded dentin beams usedin the microtensiie test is only 0.5 mm from any ex-ternai surface. We speculated that such a speci-men might provide a useful method of acceleratingwhatever hydrolysis of resin-dentin bonds mightoccur over time.i^ This has recently been confirm-ed by Dickens et al. ' ' They compared the bondstrengths of Scotchbond Multi-Purpose Plus and anexperimental bonding system tested 1 day (23C)after bonding, compared to after storage at 60Cfor 7 days. The specimens had cross-sectionalbonded areas of either 0.7 mm2 or 12 mm^. Therewas a much iarger faii in bond strengths in the 0.7mm2 vs the 12 mm^ groups over the 7-day storage

period indicating an acceierated aging effect. ThereIS some concern, however, that O-C temperaturesmay be too high for such studies.i

The purpose of this study was to test the hypoth-esis that small (ca, 1 x 1 x 8 mm) beams of resin-bonded dentin specimens will exhibit iower bondstrengths after incubation for 90 days, comparedto 1 day, by measuring their microtensiie bondstrengths.

MATERiALS AND iVIETHODS

reeth; Human third molars were used within 1month of extraction. They were stored in 4^0 wateruntil used. There were two preparation variables: Inthe superficial dentin group (SD), the bonded oc-clusal superficial dentin was prepared just beneaththe dentin-enamel junct ion. In the deep dentingroup (DD), the dentin surface was prepared within0.5 mm of the highest puip horn. These flat sur-faces were created with an Isomet saw (BuehierLtd., Lake Bluff, IL, USA) with water as a iubricant.Three bonding systems were compared: iVlacBond(MB) (Tokuso Corporation, Tokyo, Japan); OneStep(OS) (Bisco, inc., Schaumburg, IL, USA); and ClearfiiLiner Bond 2 (LB 2) (Kuraray Co., Ltd,, Osaka,Japan) were used according to the manufacturer'sinstructions (Table 1). Specimens bonded with OneStep were built up with ZIOO resin composite (3iVlDental Products, St, Paul, MN, USA). Those speci-mens bonded with iVIacBond were built up in Pai-fique Estelite resin composite (Tokuso Corp., Tokyo,Japan), and the specimens bonded with ClearfiiLiner Bond Ii were buiit up in AP-X resin composite(Kuraray Co., Ltd., Osaka, Japan). After 24 h of stor-age in water at 37C, the specimens were verticaiiyseriaiiy sectioned at 1-mm thickness intervais,using the Isomet iow speed diamond saw (Fig 1).Then a second section was made perpendicular tcthe bonded interface to make 1 x 1-mm beams. Theprocedure created a total of about 20 beams with across-sectional area of 1 mm^ (ie, 1 x 1 mm). Thetop half of each beam was resin composite and thebottom half was dentin. Controls consisted of pairsof prepoiymerized resin composite cylinders 6 mmin diameter that were bonded together with OneStep adhesive resin. These were sectioned intoslabs and then into 1 x 1 x 8-mm stioks as de-scribed above. In each bonding group, three teethwere used and each bonded tooth was preparedinto an array of about 20 individual beams 1 x 1 x 8

212 Journal tff Abgsive Dentistry

Shono et al

Table 1 Adhesive Systems

Adhesive Systems

MacBond (MB)(Tokuso Corp,Tokyo, Japan)

One Step (OS)(Biscc, Schaumburg,IL, USA

Clearfii LinerBond 2 (LB 2)(Kuraray, Osaka,Japan)

Etchant

Self-etchingmaleic acid,Mac 10alcohol

32% phosphoricacid (Uni-Etch)

Self-etchingPhenyi-PHEMA, water

Adhesives

Mac-10HEMA

BPDM, bis-GMAHEMA, acetone,photoinitiator

MDPHEMAbis-CMA

Procedures^

a(20s)b,e,g, (lOslf(lOs)bis-GMA

a(15 s)b,c,d,gf(10s

a(30 s),g,ef(20 s)

Abbreviations: MAC-10= ii-methaceyloiy.H-undecanB-dicarbaiiylic acid; MDP = lO-methacryloxy-decamethylere phosphoric acid: HEMA = hydronyetliyl-methacrylale, Pheryl.p ^ 2.methacry-loxyaihyl phenyl phosphoric aciO; bis-GMA = 2,2.Bis [4.2-hydroxy.3.methacryloyoxypropDxyphenylpropane.

sProcedjresi (a) acid-etching; (b) rinse: (c) blol-dry: (d] apply 2 coats of adhesive; |e) applied onelayer of adhesive; (f) light-cjre; g) blow dry.

Fig 1 Schematic illustration of how resincomposite buildup was verticaily sec-tioned into slabs that were, in turn, di-vided into beams that were individuallytested.

NEW VERSION OF MICROTENSILE BOND TEST METHOD

mm. Of the 60 individual beams in each group, 10were used for SEM studies and the remaining indi-vidual beams were mixed together in each group.

Incubation: The 60 beams from the threebonded teeth or composite cylinders were incu-

bated in separate vials containing steriie 0,85%NaCi solution containing penicillin (10,000 units/ml), streptomycin (10 pg/mi), and a few drops ofION NaOiH and phenol red as a pH indicator toachieve a pH of 7.0. Although ail glassware and

Vcl 1 , 213

Shono et al

Specimens were clean, they were not sterile. Oncesealed, the vials were placed in an inoubator-shakerfor the specified time interval at 37C,

Measuring bond s t r e n g t : At the appropriatetime, a vial was opened and each individual beamwas removed and bonded to steel "grips" of a Ben-cor Multi-T device (Danville Engineering, Danville,CA. USA) using Zapit-brand cyanoacryiate {DentalVentures of America, Ventura, CA, USA) (Fig 1) topermit measurement of the tensile bond strength ofeach specimen in an Instron machine operated at1 mm/min with a 49-N load cell. Ten specimenswere used for SEM observations (5 at day 1 and 5at 90 days). Selected specimens of the failed bondswere also examined by SEM to look for microcrackor other defects aiong the edges of the beams andto examine the quaiity ofthe dentin interface.

Ca/C(j/ations: Load at failure was divided by thecross-sectionai area to yield the stress at failure inMPa. The data were entered into an Excel spread-sheet to permit calculation of descriptive statistics(mean SD), The mode of failure was evaluatedusing a stereomicroscope at 18X magnification.

Statistics: The data were collected in a three-wayanalysis of variance (ANOVA) design, with bondingmateriai, dentin depth, and time of incubation thethree factors. In addition, the interaction of the fac-tors was tested. The response variable was tensiiebond strength. Group comparisons were done bythe Least-Squares Means method. Least-squaresmeans are the expected vaiue of a group or sub-group means that one expects for a balanced de-sign involving the group variable with all covariatesat their mean vaiue. Statistical significance was setin advance at the 0.05 level. Aii anaiyses were con-ducted with SAS software for the personal com-puter (SAS Institute, Cary, NC).

Scanning electron microscopy: Scanning elec-tron microscopy was done on the dentin sides of se-lected failed bonds to obtain information about thetypes of faiiures and the appearance of the bondedinterface after prolonged storage. The specimenswere rinsed with distilled water, allowed to air dryovernight, gold coated, and examined in a JEOLSEM.

image analysis: The SEM prints of the failedbonds were scanned to convert them from analogto digital form. They were then subjected to imageanalysis using NIH Image PC (Scion, Eredrick, MD,USA). Four areas were selected in each micrographthat avoided resin tags in dentinai tubules or areasof pure adhesive resin. Each of these regions of in-

ter tubular dentin had the same surface area(24 nm^). The porosity of the intertubular dentirwas integrated to yield a total surface area vaiue.The same gray scale window was used to evaluateall SEM prints. Then means and standard devia-tions were calculated for the intertubuiar porosityobserved at each time period (1 day vs 90 days).One-way ANOVA was performed to determine ifthere were any statistically significant differencesamong the surface porosities as a function of time.Statistical significance was considered as a a 0,05,

RESULTS

Table 2 shows the mean microtensi le bondstrengths (MPa) of the three bonding systems to su-perficial (SD) and deep dentin (DD) at 1 day vs 90days. The highest 24 h tensile bond strengths wereobtained with MacBond on superficial dentin. After90 days of incubat ion, the resin-dentin bondstrength of MacBond in superficiai dentin hadfallen to iess than half the initial value (p < 0.001),Bonds made to deep dentin were only haif as large(28 MPa) as those obtained on superficial dentin at1 day (p < 0.001) and they fell to iow values (8.3MPa] after 90 days. One Step gave iower 24 h bondstrengths to superficial dentin (ca 20 MPa), butthey did not change over 90 days (Table 2), One-Step bonds to deep dentin were similar to those ofMacBond in that the 24 h bond strengths were 26Mpa, which feli to low values (5.9) after 90 days.Liner Bond 2 gave moderate 24 h bond strengths tosuperficial dentin (ca 26 MPa) that did not changeover 90 days, while bonds made to deep dentin fellfrom 23 MPa at 1 day to 6 MPa at 90 days (Table2).

In contrast to the variable resuits obtained withresin-dentin bonds, the oontrol group of prepolymer-\ze cylinders of resin composite bonded to com-posite specimens gave similar bond strengths at 1day vs, 90 days, with little variation (note the smallstandard deviations).

Most bond failures of all of the bonding materi-als were adhesive in nature (Table 3), Only a fewbeams failed in dentin or in resin. There was no cor-relation between failure mode and bond strength.Figure 2a shows the fractographs of the One Stepsystem bonded to superficial dentin after 90 days.Figure 2b shows the dentin side of a failed bond todeep dentin after 1 day, and Fig 3 shows deepdentin after 90 days. After 90 days of immersion in

214_Thfi lniirnal-QlAdt)gsive Dentistry

Shono et al

Table 2 Tensile bond strengths (MPa) of superficial dentin and deepdentin at 1 vs 90 days.

Materiai/Location

MacBond/Superficial dentinMacBond/Deep dentinOneStep/Superficiai dentinOne Step/Deep dentinLiner Bond 2/Superficiai dentinLiner Bond 2/Deep dentinComposite/One Step/Composite

'P-: 0.001; groups identified by diffretisticallysignficantiy different. Numbersgroup.

lday

M0.613.1 (51)'i27.711.6 (53]i'19.920.1 (52)026.3114,6(501i'25.418,9 (51)"22.5*17,2 (53)36.11.9 (52)

t supe'scripls are significann psrenthese; indicate the

*

NS.

NS*

NS

Ilydiffenumbe

90 days

19.811.2(53)'^8.36-0{52)='20.2tl3.1(51)=5.96.4{53)='23.1tl2.4(53)

Shoho et al

Fig 2a Secondary electron image of the dentin side of aspecimen of superficial dentin that was bonded with OneStep and then tested for bond strength 90 days later, it ap-pears that the resin tags puiied outof the tubules but that theintertubuiar dentin remained infiitrated with resin.

Fig 2b Secondary eiectron image of the dentin side of aspecimen of deep dentin that was bonded with One Step andthen tested to faiiure after 1 day. Most ofthe resin tags brokecohesively at the top of the hybrid layer, although a few cameloose or puiied out. The intertubuiar dentin remained sealedwith resin.

Fig 3 Secondary electron image of the dentin side of a speci-men of deep dentin that was bonded with One Step, then in-cubated for 90 days prior to bond testing, iVlost of the resintags broke cohesively but they appeared to be surrounded byvoids. The intertubuiar regions were very porous and con-sisted of a ioose reticuiar network.

studies, although some investigators have gone asiong as 5 years.^

Burrows et ai^ bonded bovine dentin with 12 dif-ferent bonding systems and then measured tensilebond strength after 1 day, 1, 3, 6 months and 1year, Ciearfil Liner Bond 2 and Superbond D Linerwere unaffected by storage for 1 year, but most ofthe other bond strengths fell with time, Gwinnettand YuS found a significant ioss of initial resin-dentin bond strength foiiowing immersion in waterfor 6 months. Burrow et ai^ reported the effects of

3 years of water storage on the bond strength ofbonds made with or without primer (3% N-methacry-loyl-5-aminosaiicyiic acid) using Clearfii Photobondas the bonding agent. Bovine dentin bonded withthe primer gave stabie bonds (ca 10 MPa) for 1year but they feli to 5,5 MPa by 3 years.

In the current study, the bond strengths ofMacBond to superficial dentin were significantlyhigher (p < 0.001) than those made to deep dentinwhen tested at 1 day, but there were no significantdifferences between superficial or deep dentin

216ve Dentistry

Shono tal

bond strengths with the other 2 bonding systems at1 day (Table 2). Apparently, One Step and LinerBond 2 could form bonds equally well to either sub-strate. However, the bonds made to deep dentin byall of the test materiais deteriorated over the next90 days, resulting in significantly (p < 0.001) iowerbond strengths in all cases. Scanning eiectron mi-croscopy of the failed bonds reveaied extensiveporosity in the intertubular regions of deep dentinthat was not as evident in superficial dentin at 90days. The loss of intertubuiar mass appears to bedue to both a ioss of resin and coiiagen fibrils. Wespeculate that the higher water content of deepdentin is a result of the higher tubule density anddiameter,^ causing more rapid hydrolysis of thesetwo phases of the hybrid layer. The susceptibility ofthe resin to hydrolysis^ is probabiy due to its low de-gree of polymerization.^ The 'nearest neighbor"tubuie in deep dentin is only about 3 pm away (Fig2b), whiie in superficial dentin it is over 10 pm dis-tant (Fig 2a). It is clear that more information isneeded about the degree of conversion of mono-mers to polymers, their amount of cross iinking,their concentration, and how well they envelop col-lagen fibrils before improvements can be made inthe durabiiity of bonding to deep dentin. Resinbends made to superficial dentin may or may notdeteriorate over time (Table 2) depending on themateriai.

Although the storage solution consisted of un-buffered isotonic sodium chloride containing peni-cillin and streptomycin, the presence of these anti-biotics provided significant buffer capacity. Whenthe solution was adjusted to pH 7.0, it maintainedthat pH for the duration of the 90-day experiment{data not shown). Thus, the fall in bond strength ndeep dentin specimens cannot be expiained by lowpH. An alternative explanation is that calcium mayhave siowiy dissolved from the dentin into the cai-cium-free storage soiution. Since changes in me-dium calcium were not measured, the potentiai ofcalcium-free media to cause deterioration of resin-dentin bonds remains specuiative. The ioss of massfrom the bonded interface shown in Fig. 3 cannotbe due to a loss of calcium since that region hadbeen acid-etched prior to bonding.

The use of smail (ca, 1 mm^) cross-sectionalareas apparently accelerates the degradation ofdentin bonds," When incubated at 37''C for 90days, the method easily detected s igni f icantdecreases in resin bond strength to deep dentin(Table 2). One of the three adhesive systems

(iViacBond) tested on superficial dentin also de-tected a significant decrease in resin-dentin bondstrength over the 90-day test period. Whether theother two adhesives (One Step and Liner Bond 2)bonded to superficial dentin would have exhibitedlower bond strengths had we incubated themlonger, remains to be determined in future experi-ments.

The fact that specimens of the same size madefrom resin composite cylinders bonded to eachother with One Step showed stabie bond strengthover the 90 day experiment (Table 2) implicatesdentin as being the weakest link in resin bonds toteeth. However, the One Step bonds to prepoiymer-ized cylinders of resin composite were made in theabsence of water. That is, wet bonding had notbeen done. In the future, we will inciude anothergroup of bonded composite cyiinders that arebonded dry vs wet.

From this study, we can conclude that the newversion of the microtensiie test method may serveas a good model to test the durabiiity of resin-dentin bonds.

ACKNOWLEDGMENTS

The authors wish io thank Shirley Johnslon for secretarial support.Tiiis work was supported, in part, bji grants DE06427 from theNIDCRandby granl-in-aid C1O67I7% from the Ministry of Educa-tion of FAPESP 95/3S95-9. Japan.

REFERENCES

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3. Calais JG, Sderholm K-J. Influence of filler type and waterexposure on fiexural strength of exprimentai compositeresins. J Dent Res 1988;57:S36-840.

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Shono et ai ^ -

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10, Nakabayashi N. Pashley DH, Hybridization of Dental HarQ Tis-sues. Chicago:Quintessencepl99a:85,

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14, Shono Y, Terashita M, Pashley EL, Brewer PD, Pashley DH,Effects of cross-sectional area on resin-enamel tensile bondstrength. Dent Mater 1997;13:290-296,

15, Shono Y, Carvalho RM, Russell CM, Terashita M, Pashley DH,Durability of resin-dentin bonds, [Prooeedirgs of the Fifth In-ternational Conference on Composites Engineering, 1998,Las Vegas], Las Vegas, NV:825-826,

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