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The effect of application sustained seating pressureon adhesive luting procedure

Nicoletta Chieffia,∗, Stefano Chersonia,b, Federica Papacchinia, Michele Vanoa,c,Cecilia Goraccia, Carel L. Davidsona, Franklin R. Taya,d, Marco Ferraria

a Department of Dental Materials, University of Siena, Siena, Italyb Department of Dental Sciences, University of Bologna, Bologna, Italyc Department of Oral Surgery, University of Pisa, Pisa, Italyd Department of Oral Biology & Maxillofacial Pathology, Medical College of Georgia, Augusta, GA, USA

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rticle history:

eceived 25 August 2005

ccepted 4 January 2006

eywords:

eating pressure

esin cement

entin adhesive system

ond strength

Objectives. To evaluate the effect of short versus long application seating pressure on the bond

strength of resin blocks, luted with a dual-cured resin cement (Panavia F) to pre-coated or

non pre-coated dentin with an hydrophobic light-cured adhesive (Clearfil Protect Bond).

Methods. Sixteen non-carious human third molars were randomly divided into six Groups

(four teeth each). Cylindrical composite blocks were luted with Panavia F (Group Ia) and

with Clearfil Protect Bond with Panavia F (Group IIa) and seating pressure was applied for

5 s. In Groups Ib and IIb, the two bonding procedures were respectively repeated, but the

resin cylinder was maintained under constant pressure during the entire 3 min polymeriza-

tion period for the resin cement. After storing in distilled water for 24 h, 0.9 mm × 0.9 mm

sticks were produced from these luted specimens for microtensile bond testing and SEM

examination.

Results. The use of Clearfil Protect Bond with Panavia F produced higher bond strengths

than the use of Panavia F (p < 0.05). Extending the time of pressure application up to 3 min

increased the bond strength (p < 0.001) and improved the integrity of the interfacial quality.

Significance. The application of sustained seating pressure during luting procedures and the

additional use of a hydrophobic light-cured adhesive both improve the final bond strength

of the resin cement.

© 2006 Elsevier Ltd and Academy of Dental Materials. All rights reserved.

. Introduction

t has been widely demonstrated that simplified self-etchdhesives can attract fluid from dentin after polymerization1–6]. This fluid absorption is due to relatively high concentra-ions of ionic or acidic monomers in the self-etching primersin the two-step systems) [1] or in adhesive (in the single-tep systems) [2–6] essential to enable these systems to dif-use through the smear layer and demineralize the underlyingentin [7].

∗ Corresponding author. Tel.: +39 0577 233131; fax: +39 0577 233117.E-mail addresses: nchieffi@libero.it (N. Chieffi), ferrarimar@unisi.it (M. Ferrari).

Recently, it was reported that incompatibility may bepresent when auto-cured or dual-cured resin cements wereused in conjunction with simplified self-etch dentin adhe-sives [8,9]. Adverse chemical interaction and adhesive per-meability were identified as the two main factors responsiblefor the reduction in bond strength, when slow setting resincements were coupled to bondings on hydrated dentin [10].Even when the interaction between acidic simplified dentinadhesives and the basic amine catalysts in dual-curable resincements may be eliminated by the use of sulphinate-type

109-5641/$ – see front matter © 2006 Elsevier Ltd and Academy of Dental Materials. All rights reserved.oi:10.1016/j.dental.2006.01.006

DENTAL-904; No. of Pages 6

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Table 1 – Batch number (#), composition and manufacturer of the materials employed in this study

Materials Components Manufacturer

Panavia F (#41170) Primer a: HEMA, 10-MDP, 5-NMSA, water, accelerator Kuraray Medical Inc., Tokyo, JapanPrimer b: 5-NMSA, accelerator, water, sodium benzene sulfinate

Paste A: 10-MDP, hydrophobic aromatic dimethacrylate, hydrophobicaliphatic dimethacrylate, hydrophilic dimethacrylate, silanated silica,photoinitiator, benzoyl peroxidePaste B: hydrophobic aromatic dimethacrylate, hydrophobic aliphaticdimethacrylate, hydrophilic dimethacrylate, sodium aromaticsulfinate, accelerator, sodium fluoride, silanated barium glass

Clearfil Protect Bond (#41112) Primer: HEMA, 10-MDP, 12-MDPBa, hydrophilic dimethacrylate, water Kuraray Medical Inc., Tokyo, Japan

Bond: 10-MDP, Bis-GMA, HEMA, hydrophobic dimethacrylate,di-Camphoroquinone; N,N-diethanol-p-toluidine, silanated colloidalsilica, surface treated sodium fluoride

Abbreviations: HEMA: 2-hydroxyethyl methacrylate; 10-MDP: 10-methacryoloyloxydecyl dihydrogen phosphate; 5-NMSA: N-methacryloxyl-5-aminosalicylic acid; 12-MDPB: 12-methacryloyloxydodecylpyridinium bromide; Bis-GMA: bis-phenol A diglycidylmethacrylate.a MDPB has been included as for its antibacterial properties.

ternary catalysts in the self-etching primer and the resincement [11], adhesive permeability remains a possible causeof bond strength reduction. This is because auto/dual-curedresin cements have longer setting times than light-cured resincements and, in time, water can diffuse from the underlyingbonded dentin across the polymerized hydrophilic adhesivelayer via an osmotic gradient [6]. A previous study concludedthat the additional application of a more hydrophobic resinlayer resulted in improved coupling of the dual-cured resincement to dentin [12]. Another recent investigation [13] indi-cated that pressure application during the entire course ofthe setting of a dual-cured resin cement improves the bondstrength and reduces fluid interference from the underlyingdentin, with bonding covered dentin.

The objective of this study was to evaluate the effect of thesustained seating pressure on the integrity of the adhesivebond in resin blocks, in order to simulate indirect restora-tions, luted with a dual-cured resin cement. The effect of seat-ing pressure was examined by comparing the use of PanaviaF as recommended by the manufacturer (i.e., resin cementapplied to dentin primed with the one-step self-etch adhe-sive included in the kit), versus the application of Panavia F todentin that had first undergone a bonding procedure with thetwo-step self-etch adhesive Clearfil Protect Bond.

The null hypotheses tested were that: (a) the applicationof sustained seating pressure during the curing of the resin

cement has no influence on the microtensile bond strength ofresin blocks to dentin; and (b) the additional application of thetwo-step self-etch adhesive has no influence on the microten-sile bond strength of the dual-cured resin cement.

2. Materials and methods

Sixteen non-carious human third molars that had been storedin a 0.5% chloramine T solution at 4 ◦C were used within 1month following extraction. All the teeth were submitted tobonding in their normal hydrated state, as they were retrievedfrom the storage medium. The composition of the materials isshown in Table 1. The operating procedures for the four exper-imental and control Groups are summarized in Table 2.

2.1. Microtensile examination

2.1.1. Specimen preparationThe occlusal enamel was removed using a slow-speed sawwith a diamond-impregnated disk (Isomet, Buehler Ltd., LakeBluff, IL, USA) under water cooling. The roots of all teeth werecut at the same level, in order to obtained tooth samples witha standardized height (5 mm). A 180-grit silicon carbide paperwas used under running water to create a clinically relevantsmear layer on the dentin surface.

ir m

Sea

nt (P >

Table 2 – Procedures employed for the four Groups and the

Group designation

I (a and b) Panavia FII (a and b) Clearfil Protect Bond + Panavia F

Groups with the same superscripts (A–C) are not significantly differe∗ Values are means (standard deviations) in MPa.

icrotensile bond strengths

Microtensile bond strength (MPa)*

A Bting pressure: for 5 s Seating pressure: for 3 min

9.78 (5.66)C 20.71 (7.31)B

13.03 (7.01)B,C 31.11 (11.85)A

0.05).

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Composite cylinders (Tetric-Ceram, Ivoclar-Vivadent,Schaan, Liechtenstein) 10 mm in diameter and 6 mm inheight were prepared using a split aluminum mold, in orderto limit and standardize the bonding area. Prior to cementingprocedures, the bonding surface of each resin cylinder wassandblasted, etched with phosphoric acid, rinsed with waterand dried.

2.1.2. Cementing proceduresThe specimens were randomly divided in four Groups (N = 4).In Group Ia, the resin cement Panavia F (Kuraray Medical Inc.,Tokyo, Japan) was used according to manufacturer’s instruc-tions. ED Primer (A + B) was mixed and applied with a brush onthe dentin surface and left in place for 30 s. After drying theetched surface with gentle air flow, Panavia F resin cement(A + B) was mixed and applied to each of the freshly preparedcomposite cylinders.

In Group IIa, the light-curing self-etch adhesive ClearfilProtect Bond (Kuraray Medical Inc., Tokyo, Japan) wasused according to manufacturer’s instructions. This mate-rial also contains 12-methacryloyloxydodecylpyridinium bro-mide (MDPB) monomer to include antibacterial propertieswhich may be useful in cases of vital teeth. The self-etching primer was applied to the dentin surface with abrush and left in place for 20 s. After drying the etched sur-face with mild air flow, the bonding was applied on theetched-primed dentin, gently air flowed and light-cured for1ad

tTtrf

weimmTattidmM

2Afwtoica

Fig. 1 – The metallic tool used to standardized the seatingpressure application. The tooth was positioned on the basalsurface of the tool and the composite cylinder wascemented, applying the weight on the upper surface of thetool.

DVA, Corona, CA, USA). The device was attached to a universaltesting machine (Triax digital 50, Controls, Milano, Italy) andloaded in tension at a crosshead speed of 0.5 mm/min untilfailure.

2.1.4. Statistical analysisThe population of bond strength values was normally dis-tributed according to the Kolmogorov–Smirnov test (p > 0.05).The two-way ANOVA was applied with bond strength as thedependent variable, material and pressure application time asfactors. Tukey’s test was used for post hoc comparisons.

2.2. Scanning electron microscopy (SEM)

2.2.1. Specimen preparationAfter microtensile bond testing, eight pairs (fractured com-posite and dentin sides) of specimens from each Group wererandomly selected for SEM examination. Each specimen wasmounted on metallic stubs, sputtered with gold/palladiumand observed with a SEM (JSM—6060LV, JEOL, Tokyo, Japan)operating at 10 or 15 kV.

Images of the two complementary detached interfaces foreach fractured specimen were taken at different standardizedmagnifications: 100–6000×.

0 s. Then Panavia F resin cement (A + B) was mixed andpplied on each of the freshly prepared composite cylin-ers.

In these two Groups, the composite block was cemented tohe dentin surface using a seating pressure of 1.25 MPa for 5 s.o ensure optimal polymerization of the resin cement alonghe exposed margins, a layer of Oxyguard was applied afteremoval of the excess cement with a probe and left in placeor at least 3 min.

In Group Ib and in Group IIb, the bonding proceduresere the same as described for Group Ia and Group IIa,

xcept for the application of the cementation pressure. Dur-ng the entire 3 min period necessary for complete poly-

erization of the resin cement, the resin cylinder wasaintained under a constant seating pressure of 1.25 MPa.

he excess cement was removed and the Oxiguard waspplied while the specimen was under pressure. In ordero standardize the applied pressure, a metallic tool (Fig. 1)hat delivered 10 kg was used. This resulted in a seat-ng force of 98.1 N. The pressure [N/m2] was calculated,ividing this force [N] by the surface area [m2] of theetal weight. The value obtained was finally converted intoPa.

.1.3. Bond strength evaluationfter cementation, all the specimens were stored in water

or 24 h at 37 ◦C. Each tooth was then sectioned verticallyith the Isomet saw into a series of slabs. The slabs were

hen sectioned vertically into 0.9 mm × 0.9 mm sticks, basedn the “non-trimming” version of the microtensile bond test-

ng technique [14]. Each stick was measured using a digitalaliper to determine the cross-sectional area. The sticks werettached to a testing device with cyanoacrylate glue (Zapit,

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Fig. 2 – SEM imagine of fracture composite side of aspecimen from Group Ia, where the seating pressure wasapplied for 5 s. The micrograph (4000×) shows thestructural design of the defects observed.

3. Results

3.1. Microtensile bond strengths

Bond strength results of the four Groups are shown in Table 2.The selection of the luting material was a significant fac-

tor with regard to the bond strength obtained; with ClearfilProtect Bond + Panavia F measuring significantly higher bondstrengths than Panavia F (p < 0.05).

Pressure application time was also a significant factor.Extending the time of pressure application up to 3 min sig-nificantly increased the bond strength (p < 0.001).

The interaction between cement and time of pressureapplication was also significant to obtain the highest bondstrength. According to the Tukey’s test applied for post hoccomparisons, Group IIb achieved the highest bond strengthand the difference was statistically significant (p < 0.05). GroupIb and Group IIa were comparable (p > 0.05). The latter wascomparable to Group Ia, which had the lowest bond strength.

3.2. SEM observations

SEM analysis of the specimens from those Groups where theseating pressure was sustained for a few seconds, revealed thepresence of structural defects. These defects consisted of com-partmentalized structural entities that were nearly identical

Fig. 3 – SEM image of fractured composite sides of aspecimen from Group IIa, where the seating pressure wasapplied for five seconds. (a) A medium magnification view(1000×) of composite side, showing structural defects inmost of the adhesive layer, almost completely detachedfrom the resin cement (b) a high magnification view (4000×)of an area in photo a, showing resin globules.

of Group Ib where the seating pressure was sustained for3 min.

In almost all specimens examined in Group IIb the fracturewas located between the resin cement and the composite resinblock and a strong adhesion between the adhesive layer andthe resin cement was present (Fig. 4).

4. Discussion

As the microtensile bond strength of the resin cement was sig-nificantly higher when the seating pressure was maintainedthroughout the entire setting period of the cement, the firstnull hypothesis tested in this study has to be rejected.

If the SEM evaluation had not been performed, the lowerbond strength of the control Group could have been attributedto voids caused by air introduced during hand mixing of thetwo component pastes of the dual cured cement. However,

in almost all of the fractured composite interfaces examinedfrom Groups Ia (Fig. 2).

The most significant result in Group IIa (pre-coated light-cured adhesive dentin, with a few seconds of pressure appli-cation time), was that separation was between the adhesiveand the resin cement (Fig. 3a). Moreover, resin globules thatrepresented the separation phase of resin components werepresent between the detached adhesive and the resin cement(Fig. 3b).

In contrast to this, such features (structural defects andresin globules) were not found at the fractured interfaces

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Fig. 4 – SEM image of dentin side of a specimen from GroupIIb, where the seating pressure was sustained for 3 min.The micrograph (200×) demonstrates that the fracture wasbetween the resin cement and the composite block.

when examining the micrographs another explanation maybe found.

As a result of the incompatibility of the absorbed water withthe hydrophobic components in the resin cement, materialdiscontinuity (presence of globules) [15] and irregular features[12,16] along the adhesive–composite interface were previ-ously reported. The osmotic permeability of the simplifiedadhesive systems has been described in experiments per-formed with pulpal pressure simulations [1,5], and also inthose experiments performed without a perfusion system[3,4,17,18]. In these latter studies, as well as in the presentinvestigation, it is reasonable that fluid interferences couldhave occurred as the samples were retrieved from the stor-age medium and bonded in their normal hydrated state. Still,transmission of dentin fluid through simplified dentin adhe-sives was demonstrated to occur in vivo, in endodonticallytreated dentin [19].

It is clear that such structural defects (Fig. 2) observed inthose Groups where the seating pressure was only applied fora few seconds, were therefore caused by water that had per-meated through the etched and primed dentin. These defectswere even observed in those samples that had been pre-treated with the hydrophobic light-cured adhesive (Fig. 3aand b). Thus, it is feasible that permeability of the light-curedself-etch adhesive may have compromised the ultimate bondstrength result of the resin cement.

Apparently the application of a sustained seating pressuresratcsi

lbt

the additional application of a hydrophobic light-cured adhe-sive improves the final bond strength of the resin cement.This latter result confirms those studies [20,21] showingthat the application of an additional hydrophilic resin layerimproves resin cement bond strength. Moreover, as the exten-sive and prolonged antibacterial activity of the adhesive testedin the present investigation has been previously demon-strated [22,23], clinical protection against oral bacteria can beexpected from this product.

Also the selection of cement in combination with pres-sure application time was found to be significant to obtainthe highest bond strength. In particular, Group IIb (pre-coatedlight-cured adhesive dentin, with 3 min pressure applicationtime) achieved the highest bond strength among all the otherGroups. Thus, it can be speculated that, since the inherentself-etch adhesive system permeability was eliminated by thesustained seating pressure, the application of the hydropho-bic light-cured adhesive was more efficient in improving thebond strength of the resin cement, because the incompatibil-ity factors had been reduced. This finding confirms a previ-ous investigation [3], showing that when simplified adhesiveswere bonded to dehydrated dentin, no compromised bondstrength resulted.

An unexpected outcome of this study was the difference inthe kind of fracture observed in Group IIa, located betweenthe adhesive and the resin cement (Fig. 3a and b) and inGroup IIb, between the resin cement and the composite block

r

uppresses the absorption of water and globule formation,esulting in a better quality of adhesive interface. These resultsre in agreement with those obtained in a previous investiga-ion [13]. As a consequence, reduction of water infiltration,oming from the underlying dentin into the primer-cementystem may account for the higher bond strength as observedn Groups Ib and IIb.

Since Clearfil Protect Bond together with Panavia F, regard-ess of pressure application time, had significantly higherond strengths than Panavia F, the second null hypothesisested in this study has also to be rejected. This means that

(Fig. 4). From SEM analysis, it should be stressed that whenthe short seating pressure was applied (Group IIa), the waterthat had permeated through the adhesive interfered with thehydrophobic resin cement and resulted in a weaker adhesion.Thus, failure in the adhesive/resin-cement bonding inter-face resulted. On the other hand, with 3 min seating pres-sure, a stronger adhesion between adhesive and resin cementoccurred due to the reduction of the adhesive permeability.Thus, the weakest bonding interface was between the resincement and the composite restoration. In order to improvethe adhesion of the latter bonding interface, the compos-ite block surface could be treated with hydrofluoric acid andbonding.

The results of this study demonstrated that: to improvecoupling of the resin cement to dentin, both the additionalself-etch adhesive containing the light-cured hydrophobicbonding layer and the sustained seating pressure duringcementation are effective. The latter reduces the incompati-bility between simplified-step adhesives and dual-cured resincements, since it reduces the adhesive permeability associ-ated with simplified self-etch adhesives. However, since pulpalpressure was not simulated in this study, the results of this invitro study still cannot fully predict the clinical behavior ofthese materials applied on vital teeth.

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