Dental Research

Bond strengths of dentinal bonding systems to enamel and dentinRichard S. McGuckin* / John M. Powers** / Lan Li

Contemporary, third-generation dentinal bonding products have become highlv specializedin producing high bond strengths to dentin. This investigation compared in vitro bondstrengths of six dentinal bonding systems and their matched composite resins to htunan en-amel and dentin. The effects of treatment by dentinal primers on enamel bond strengths as wellas the effects of phosphoric acid on the strengths of dentinal bonds were measured. The u.se ofdentinal primer on enamel improved the bond strengths of Prisma Universal Bond 3/PrismaA PH and XR Bond/Herctdite systems and had no effect on Denthesive/Charisma. Scotch-bond 2/Silux. and Tenure/Perfection, while the enamel bond strengths of Gluma/Pekaluxdeclined. Pretreatment of dentin with phosphoric acid improved the bond strengths ofDenthesive/Charisma. Prisma Universal Bond 3/Prisma APH and XR Bond/Hercidite, buthad no effect on Gluma/Pekalux, Scotchbond 2/Silux and Tenure/Perfection.(Quintessence Int 1994;25:791-796:)

Introduction

To attain strong resin bonds to dentin. dental investiga-tors have studied dentin as a substrate' ^ in great detail.Their findings have assisted manufacturers to developthree generations'*" of bonding resins with increasinglygreater affinity for dentin. Higher dentinal bondstrengths have been made possible in part by tailoringthe composition of bonding agents to the dentinal sur-face' and by attachment to the histochemical features'of dentin.

Pursuit of improved dentinal bonding may havecaused the profession to overlook the effect of special-ty dentinal bonding systems on enamel. Enamel and

•* Associate Professor and Interim Ctiair. Department of Pros-thodcntics. University of Texas. Houston, Health Science Cen-ter, Dental Brancti, PO Box 2006S, Houston. Texas 77225.

"* Professor and Seclion Head, Section of Orat Biomaterials, De-partment of Basic Sciences, Uuiversity of Texas, Houston.Health Science Center.

*** Visiting Research Fcltow, Department of Prosthodontics,Tianjin Medical College, People's Republic of China.

Address all correspondence to Dr Richard S. McGuckin.

dentin differ greatly from each other in mineral, pro-tein, and water contents. Clinically, it is very difficultand inconvenient to treat each substrate differently;therefore, investigation ofthe effect of specialty denti-nal bonding agents on enamel is a worthy endeavor.

Early investigators etched enamel and dentin alike.Later, etching dentin was discouraged because of sus-pected pulpal inflammation. Recently, phosphoric acidhas been questioned as a cause of pulpal inflamma-tion, Some clinicians'*'' have advocated acid pretreat-ment of all dentin, the "total-etch" technique, to re-move smear layers to improve bonding. These re-searchers have not specified the dentinal bondingsystem to be used for such pretreatment. Acid etchingof dentin may not be efficient or effective for everybonding system because of differences in their manage-ment of smear layers and their theorized bondingmechanisms.

In vitro investigation of dentina] bond strengths hasyielded inconsistent results,'" Contributing to the pTob-lem are variations in materials management and testingmethodology In an effort to bring a degree of stan-dardization into the field, Munksgaard and Asmus-sen" proposed that bonds formed with enamel be usedas controls and that dentinal bond strengths be ex-

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Dentai Research

Table 1

Code

D

G

P

S

T

X

Codes, batch numbers, and manufacturers of products tested

Bondingagentand composite resin

Denthesive

CharismaGluma

PekaluxPrisma Universal Bond 3Prisma APHScotchbond 2SiluxPlusTenure

Visar SealPerfectionXRBondHercuhte XRV

Batch Nos.

A—y3.12.3!B—93,12,131.046Conditioner— 30.06,94.2194,06.30.021Sealer— 4108S2Cleanser— 4108S2Primer— 008OF22044D209209009119038ALlACA—461031B—46204ÍÍConditioner—29103613912728404307269001138

Manufacturers

Kulzer

Miles Dental

Caulk/Dentsply

3M Dental

Den-Mat

Kerr/Sybron

pressed as a ratio of the enamel bond controls. Theirsuggestion remains largely unnoticed in the researchcommunity.

The purpose of this investigation was to determine invitro bond strengths of six dentinal bonding systemsand their composite resins to human enamel and den-tin. The effects of treatments by dentinal primers onbond strengths to enamel and the effect of phosphoricacid eiching on dentinal bond strengths were also in-vestigated.

Method and materials

Six third-generation dentinal bonding systems weretested (Table 1),

Forty-eight unrestored, caries-free, extracted humanmaxillary incisors and canines were selected. Previous-ly stored in formalin for an undetermined lime, thespecimens were transferred lo saline with 2% sodiumazide for 3 weeks.'^ Each specimen was embedded inresin (Sampl-Kwick; Buehler) with the facial surfaceexposed about 2 mm above the resin. The sites for bondformation were made by abrading the tooth on new

sections of 600-grit silicon carbide paper with copiouswater irrigation (Automet, Buehler). Bond sites wereapproximately 4 mm in diameter. Four bond sites werecreated sequentially—two sites in enamel and thentwo in dentin. Each tooth was used four times, with thedebonded surface abraded to form a new bond site.

Eight teeth were selected randomly for testing eachdentinal bonding system. A crossover experimental de-sign was employed to overcome the effects of bondingat different depths of enamel and dentin.' ' Specimens1 to 4 received the following pretreatments at eachlevel of bonding: first, enamel-dentin primer; second,enamel-no dentin primer; first, dentin-eteh; second,dentin-no etch. Specimens 5 to 8 received the followingpretreatment sequence: first, cnamcl-no dentinalprimer; second, enamel-dentinal primer: first, dentin-no etch; second, dentin-etch. All dentinal bond siteswere located in superficial dentin and were examinedunder magnification for the absence of enamel rem-nants within the bond site.

All enamel sites were etched by using agitation witheach system's supplied phosphoric acid for the timerecommended by the manufacturer. The surface was

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Dental Research

Table 2 Shear bond strengths (MPa) of dentinal bonding agents to human enamel treated with and without primerand to dentin treated with and without phosphoric acid

Etched enamelNo primerPrimer

DentinUnetchedEtched

DBS/EBS**

D

14.0(3,7)*12.3(1.8)

5.9(3,9)10.0(4,4)

4S%

G

9.2(3,2)6,7(4.0)

7.3(3.0)8.1(4,1)

110%

P

9.7(2.6)13.7(2.9)

8.2(3.S)12.3(3.7)

60%

S

8.7(4.0)8,5(4,2)

5.9(4.7)5,3(2.5)

69%

T

6,1(4,3)6,0(5,3)

8,0(3,2)9,9(3,7)

130%

X

14.0(3.0)16.1(3.8)

10.7{3.6)12.7(4.5)

66%

Means with standard deviations in parentheses from eigtit replications. Significant differences were observed by two-way ANOVA. Tukeyintervals al the .05 significance level among bonding agents and among enamel and deniin treaiments were 2.7 and 2.Û MPa, respectively.Vertical lines to (he right of (he ila(a indica(e that there were no significant Jiffcrences between tneans.Ratio of unetctied dentinat bond strength (o primed enamel bond strength.

rinsed for 10 seconds and then dried with oil-free com-pressed air for 10 seconds. Dentinal primer was appliedto the enamel before the resin adhesive was applied inone group of specimens. The remaining enamel speci-mens did not have dentinal primer applied.

One group of dentinal sites was prepared by etchingfor 30 seconds with the manufacturer's supplied phos-phoric acid etchant, followed by rinsing and drying for10 seconds eaeh. The remaining dentinal bond siteswere not pretreated with acid. Dentinal primer was ap-plied as instructed to all dentinal bond sites, followedby application of the bonding resin.

Each system's universal shade composite resin wasplaced in two increments into a nylon tube that was1.9mm in internal diameter and 3.0 mm long (RSN 4/2,Small Parts Inc). Each addition was light cured for 40seconds with a curing light (Elipar. ESPE-Premier)that had been tested for output intensity (model 100,Curing Radiometer, Demetron Research) prior to use.

The specimens were stored in 37 °C tap water for 24hours and loaded in shear to failure, with the nylontube in place, on a testing machine (model 8501, Ins-tron) in shear using a cable-loop test. Crosshead speedwas 0.05 cm/min. The bond area was 3 mm'. Bondstrengths were measured in megaPascals. A total of 192bonds were tested. No spontaneous failures occurred.

The means and standard deviations (n = 8) for thetested specimens were analyzed by a two-way analysis

of varianee (ANOVA)''' because analysis of variance ofthe crossover design showed no significant differencesbetween specimens 1 to 4 and 5 to 8. The means werecompared by Tukey confidence intervals calculated atthe .05 significance level." Differences between meansequal to or greater than the Tukey interval were consid-ered statistically significant.

Results

The means and standard deviations of the bondstrengths for the systems and treatments tested areshown in Table 2 and Figs 1 and 2.

Tlie two-way ANOVA (P = .05) disclosed that thebonding systems were a more important factor thantreatments, and that their interaction was statisticallysignificant, Tukey intervals at the .05 significance levelwere 2.7 MPa among dentinal bonding systems and2.0MPa among the enamel and dentinal treatments.The overall coefficient of variation for the investiga-tion was 39%, which compares favorably with that ofother investigations,"^'^

Application of dentinal bonding primer on enamelbefore the bonding resin was applied resulted in bondstrengths for Prisma Universal Bond 3 (P) and XRBond (X) systems that were 41% and 15% greater, re-spectively, than those to enamel without dentinal prim-er. Denthesive (D). Scotchbond 2 (S), and Tenure (T)

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Dental Research

Fig 1 Shear bond strengths of dentinai bonding systemsto enamel witti and without dentinal primer.

Fig 2 Shear bond strengths to dentin witin and withoutphosphoric acid etching.

showed no change when dentinal primer was applied toenamel. The enamel bond strengths of the Glumasystem (G) were lowered by 27% when tbe dentinalprimer (Gluma 3) was applied to enamel before the ad-hesive resin.

Phosphoric acid etching of dentin before apphcationof the dentinal primer and resin adhesive improveddentinal bond strengtbs for the Denthesive, PrismaUniversal Bond 3, and XR Bond systems by 69%, 50%,and 19%, respectively, over the strength of unetcheddentin. Gluma, Scotchbond 2, and Tenure demonstrat-ed no statistically significant change when dentin waspretreated with acid etching before primer and resinapphcation.

As suggested by Munksgaard and Asmussen,'' theratios of bond strengths of unetched dentin to primedenamel are presented in Table 2. These ratios rangedfrom 48% to 130% in this investigation.

The rank order of bond strengths of dentinal bond-ing systems to etcbed, unprimed enamel was X = D >p = G = S > T. When ranked, the bond strengths toetched and primed enamel were X > P = ^ D > S = G =T. The rank order of bond strengths to unetcbed,primed dentin was X = P = T and X > G = D = S.Bond strengths to etched and primed dentin wereranked X = P = D a n d X > T = G > S ,

In all subgroups, the XR Bond-Herculite systembad consistently higher bonds to dentin and enamel,while Tenu re-Perfection had the lowest enamel bondsstrengths and Scotchbond 2 with Silux Plus had thelowesf bond strengths to etched and unetched dentin.

Discussion

Histochemical and structural differences between en-amel and dentin have been responsible for the histori-cal course of adhesive development away from applica-tion of enamel bonding resins to dentin and toward theuse of specifically designed systems that utilize theunitjue properties of dentin for bond formation. Ironi-cally, dentinal bonding products may now be just asunsuitable for enamel as the initia! enamel bondingproducts were for dentin. This could be especiallymeaningful wben dentinal bonding systems designed tointeract with dentinal proteins (20 wt%) attempt tobond to enamel, which is composed of less than 1 wt%protein.''•"

Clinically, use of different treatments of enamel anddentin is difficult and impractical for restorative den-tists. Covering etched enamel with dentin primers isprescribed by some suppliers and is difficult to controlif contact is to be avoided. Knowing how a dentinalbonding system bonds lo enamel and dentin may be im-portant lo clinical longevity of composite resin restora-tions.

The present results supported the suggestion thatsome dentinal bonding systems may not effective forbonding to enamel. Bond strengths to enamel werelowered by 27% when Gluma 3 was applied to enamel,Althougb three systems were statistically unaffected(Denthesive, Scotchbond 2, and Tenure), two of thethree demonstrated a statistically nonsignificant trendtoward lower bond enamel strengtbs following applica-

794 Quintessence International Voiume 25, Number 11/1994

Dental Research

tiort of deniinal primer to enairtel, Cotiversely. twosystems (Prisma Universal Botid 3 and XR Bond) dem-onstrated impro\tíd enamel bond strengths when thedentinal primer was applied.

Table 2 presents the ratio of dentinal bond strengthsto primed enamel bond strengths. Percentages lessthan 100% suggest belter enamel bonding; conversely,ratios exceeding 100% suggest higher dentinal bondstrengths,

Munksgaard and Asmussen" reported dontinalbond strengths in the range of 24% to 74% of enamelbond strengths. In the present sttidy, the values rangedfrom 48% to 130%. Expressing dentinal bondstrengths only as a ratio could be misleading. The infer-ence could be made that ratios greater than 100%imply low enamel bond strengths. For clarity, the ratioalone is inadequate; the mean bond strength valuesshould be reported also.

Acid etching of dentin has its origins in the transferof methodology from enamel bonding. Buonocore^"first proposed acid etching of enamel to improve resinbonds, and his group was first to propose etching ofdentin.-' Later, acid etching was judged responsible foradverse pulpal réactions" and the practice was discour-aged. Recently, however, the idea that phosphoric acidis the cause of pulpal irritation has been qticstioned,-'and several clinicians '* bave advocated acid etching ofdentin. The objective of acid pretreatment of dentin isto remove weak dentinal smear layers. Exposed dentin(without a smear layer) theoretically allows bondingsystem components to make intimate contact withintertubular dentin for resin interpénétration."''-'forming a hybrid layer ^ or mechanical retention indentinal tubules or microtubules,-' -

Smear layers impart a protective function to dentin.Their particulate components are forced into dentinaltubules, formtng tubular plugs"'' that reduce outwardfluid flow to the cut surface and lower the rate of bacte-rial penetration into the pulp via the dentinal tubules.First- and second-generation bonding systems accept-ed the smear layer and bonded to tt. Their botidstrengths were comparatively low, however.

Third-generation agents manage the smear iayer bytotally removing it (usually with acid), dissolving andreprecipitating its constituents (Scotchhond 2), or de-positing a completely new. nonendogenous (aluminumoxalate) smear layer (Tenure). Acid pretreatment hasbeen shown to completely remove the adherent smearlayer,™ demineralizc peritubular dentin 30 ¡im deep,"increase permeability,'' facilitate wetting of the dentt-nal surface/- and denature dentinal collagen,^' Al-

though removal of smear layers has produced higherinitial dentinal bond strengths, should bond failure oc-cur, greater pulpal sequela may be possible because ofthe loss of the protective smear plugs and the presenceof open dcutinal tubules.

Acid pretieatment of dentin prior to application of aprimer that dissolves and reprecipitates smear layersmay remove the hypothetical "raw materials" for themechanistii of action, Sitnilarly. acid denaturing of den-tinal protein tnay render collagen less effective tosystems conceptually designed to bond to proteins.Hence, we question the practice of indiscriminate acidconditioning of all dentin without regard for the restor-ative bonding system being used. This concern receivessupport frotn the present findings. While some systems(Denthcsive, Pristna Universal Bond 3, and XR Bond)detnonstrated improved dentinal bond strengths. Glu-ma, Scotchbond 2. and Tenure realized no improve-ment in bond strengths from acid etching of dentin.Those systems that benefited most from acid precondi-tioning were the ones that feature acidic primersystems. Applying acidic solutions twice may pose agreater danger to pulpal tissue than if only one applica-tion is used,'"' If no improvement in dentinal bondstrength for a given bonding sysiem is achieved by acidpretreatment of dentin, the efficacy of etching dentin isquestionable in light of the risk-to-benefit ratio.

The relationship among bond strengths, marginaladaption, and microleakage is not fully understood,Tliis investigation supports the work of Airoldi et al,"who found that denttnal bonding agents have a deleter-ious effect on resin adaption to etched enamel cavitymargins. Reticf et al'"found thai etchtng dentin lowersthe shear bond strengths but is accompanied by re-duced quantitative microleakage.

Many questions regarding application of dentinalbonding agents to enamel beg for more investigation.Clearly more research is needed concerning the appro-priateness of acid etching of dentin and compatabilitywith any given dentinal bonding system. Additionally,long-term maintenance of bond strengths to and resis-tance to microleakage of etched and unetchcd dentin invivo is in need of further study.

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