`No-bottle' vs `multi-bottle' dentin adhesivesÐa microtensile bondstrength and morphological study

R. Frankenbergera, J. PerdigaÄob,*, B.T. Rosac,1, M. Lopesd

aPoliclinic for Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Glueckstrasse 11, D-91054 Erlangen, GermanybUniversity of Minnesota, Division of Operative Dentistry and Minnesota Dental Research Center for Biomaterials and Biomechanics, 8-450 Moos Tower,

515 Delaware St. SE, Minneapolis, MN 55455, USAcFormer Graduate Student, University of North Carolina at Chapel Hill, Department of Operative Dentistry, CB no. 7450 Brauer Hall, Chapel Hill, NC 27599-

7450, USAd Minnesota Dental Research Center for Biomaterials and Biomechanics, 16-212 Moos Tower, 515 Delaware St. SE, Minneapolis, MN 55455, USA

Received 11 April 2000; revised 18 August 2000; accepted 30 August 2000

Abstract

Purpose: To compare the adhesive capability of the new adhesive Prompt L-Pop (ESPE) with that of two total-etch adhesive systemsÐ

EBS Multi (ESPE) and Prime&Bond NT (Dentsply).

Methods: Extracted human molars were bonded and prepared for microtensile dentin bond strength (mTBS) testing using Prompt L-Pop,

EBS Multi, and Prime&Bond NT combined with Pertac II (composite) or Hytac Aplitip (compomer). Prompt L-Pop was applied using ®ve

different protocols: (1) as an `all-in-one' self-conditioning adhesive, as per manufacturer's instructions (LP); (2) as a self-etching primer

combined with a separate bonding resin (LP/self-etch 2-step); (3) as a classical primer used upon etching dentin with phosphoric acid,

followed by a bonding resin (LP/total-etch 3-step); (4) as a multi-application `all-in-one` self-conditioning adhesive (LP/multi-coat) to leave

a visibly glossy dentin surface; and (5) as a ®lled adhesive, upon adding quartz ®llers to its composition (LP/®lled). After 24 h of storage in

water at 378C the mTBS were measured in an Instron machine. Corresponding interfaces of the same specimens were micromorphologically

analyzed using SEM and TEM.

Results: When used with a composite resin, LP/®lled and LP/multi-coat resulted in signi®cantly higher mTBS than LP. The addition of an

extra adhesive bonding resin (LP/self-etch 2-step) had no effect on bond strength. The use of Prompt L-Pop as a primer of a fourth-generation

adhesive (LP/total-etch 3-step) replacing the EBS Multi primer, resulted in lower bond strengths than those of the original EBS Multi. LP/

multi-coat showed similar mTBS to Prime&Bond NT (P&BNT). When used with a compomer, LP exhibited higher bond strengths than when

used with the resin composite and was as effective as the experimental groups LP/®lled and LP/multi-coat and the control group P&BNT. The

SEM evaluation showed an inconsistent hybrid layer for the LP specimens, whereas in both the LP/®lled and LP/multi-coat specimens a

hybrid layer was clearly evident. Under the TEM all groups displayed dentin hybridization with dissolved smear plugs in the specimens that

had been conditioned with Prompt L-Pop without a separate etching step. In many tubules of specimens conditioned with LP (as per

manufacturer's instructions), ®llers of the resin composite were present within the dentinal tubules.

Conclusions: When combined with a resin composite, Prompt L-Pop resulted in statistically lower bond strengths when applied in one

layer than when applied in multiple layers. Prompt L-Pop also resulted in higher bond strengths when used with a polyacid-modi®ed

composite resin than with a composite resin. When applied in multiple coats, Prompt L-Pop results in bond strengths that are not statistically

different from those of Prime&Bond NT, a total-etch adhesive. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All

rights reserved.

1. Introduction

The increasing demand for esthetic restorations has

generated intensive research on adhesive materials with

special focus on amalgam alternatives [1±3]. Successful

adhesion to dental hard tissues is a fundamental requirement

prior to the insertion of tooth-colored materials such as

direct resin composites [4±7]. The polymerization shrink-

age of resin composites produces stress between bonded

restorations and tooth walls, potentially resulting in gap

formation [8±10]. Only a perfect adaptation is capable of

preventing microleakage and, consequently, recurrent caries

and pulpal irritation [5,10].

Bonding to enamel has been known as being clinically

Dental Materials 17 (2001) 373±380

dentalmaterials

0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.

PII: S0109-5641(00)00084-1

www.elsevier.com/locate/dental

* Corresponding author. Tel.: 11-612-625-8486; fax: 11-612-625-7440.

E-mail address: perdi001@tc.umn.edu (J. PerdigaÄo).1 Currently in Private Practice in Londrina, ParanaÂ, Brazil.

very reliable. Since its introduction in 1955, the acid-etch-

technique has provided an ideal surface morphology as a

result of the use of 30±40% phosphoric acid [2,11]. The

resulting etch pattern is characterized by abundant micro-

porosities which allow the penetration of polymerizable

monomers into those porosities to form resin tags that

provide micromechanical retention [2].

Successful attempts of bonding to dentin in a similar

fashion have been reported more recently [12±15]. Due to

the speci®c properties of human dentin, such as the tubular

structure and its intrinsic wetness, bonding to dentin is more

dif®cult to achieve than bonding to enamel [16±19]. The

bonding mechanism of recent dentin bonding agents is

based on the penetration of ambiphilic molecules into

acid-etched dentin [20±22]. Water-chasing solvents, such

as ethanol or acetone, are commonly utilized to facilitate

penetration of the monomers and to obtain a direct contact

of resin with the collagen ®bers, which results in a mixed

zone of polymerized resin and entangled collagen ®brils, the

hybrid layer [12,13,20]. Dentin bonding agents that rely on

dentin hybridization have been reported to result in higher

bond strengths especially when the dentin is left moist rather

than being desiccated with air [12,13].

A different philosophy is advocated by `self-etching

primers', which have been evaluated in several studies

[24±26]. Self-etching primers condition and prime enamel

and dentin simultaneously without rinsing. The mild acidity

of these materials is responsible for their inability to remove

the smear plugs upon conditioning. Nevertheless, self-etch-

ing primers hybridize dentin for up to 2 mm and have been

reported to withstand stresses from polymerization shrink-

age clinically [27]. These materials have the ability to

dissolve hydroxyapatite partially, both within the smear

layer and the dentin surface, resulting in a resin-in®ltrated

zone with entrapped minerals [10,17]. These attempts at

creating adhesion to dentin without a separate etching

step may be a noteworthy alternative to the total etch

technique.

The simultaneous treatment of enamel and dentin with one

material in one application is a major step towards simpli®ca-

tion of the clinical procedure with the ®nal goal of decreasing

the possibility of error during the application of current adhe-

sive materials. This ultimate simpli®cation is a new bonding

concept known as `all-in-one'. A new `all-in-one' adhesive,

(Prompt L-Pop, ESPE, Seefeld, Germany), has been reported

to result in enamel bond strengths comparable to those

obtained with total-etch multi-step adhesives [26,28]. Prompt

L-Pop is now recommended to be used with composites and

compomers, which represents a modi®cation of the ®rst

version recommended for compomers exclusively, without a

separate light-curing step.

The null hypothesis tested in this project is two-fold. (1)

The use of Prompt L-Pop as a multi-step adhesive system

does not result in higher bond strengths than when used as

per manufacturer's directions. (2) The bond strengths

obtained with Prompt L-Pop are not signi®cantly lower

than those obtained with either a water- or an acetone-

based total-etch adhesive.

2. Materials and methods

2.1. Specimen preparation and microtensile testing

A total of 32 caries-free extracted human third molars

were used in this investigation. The teeth were stored in a

0.1% thymol solution at room temperature for up to 4 weeks

after extraction and were consecutively debrided and exam-

ined to ensure that they were free of defects. The occlusal

enamel was removed and 700±900 mm thick enamel-dentin

discs were cut from the mid-coronal level of the tooth,

perpendicular to the tooth axis by slow-speed diamond-

saw sectioning (Isomet 1000, Buehler Ltd, Lake Bluff, IL,

USA) under water. A standardized smear layer was created

on both surfaces by wet sanding with 600-grit silicon

carbide abrasive paper (Buehler Ltd) for 60 s. Both surfaces

were treated with the dentin adhesive systems (n� 2)

displayed in Table 1. The crowns of the cut teeth were

reconstructed with four consecutive 1 mm-layers of a mini-

®lled hybrid resin composite or a compomer (Pertac II or

Hytac Aplitip, ESPE). Each layer was light-cured for 40 s

with a photocuring unit (Elipar Highlight, ESPE). The inten-

sity of the light was monitored periodically with a radio-

meter (Demetron/Kerr, Danbury, CT, USA) to ensure that

400 mW/cm2 were exceeded. The disks were then cut in

half and stored in distilled water to fabricate the SEM and

the TEM specimens.

The mTBS specimens were stored in distilled water for

24 h at 378C and then sectioned in 800 mm-disks parallel to

the tooth axis. The disks were modi®ed to obtain an hour-

glass shape using a diamond bur mounted in a high-speed

handpiece, under continuous water spray. The area of

the tested interface was adjusted to 1.00±1.25 mm2. The

hourglass shaped specimens were mounted in a special jig

(Bencor Multi-T, Danville Engineering, San Ramon, CA,

USA) with cyanoacrylate cement (Zapit, Dental Ventures

of America, Corona, CA, USA) and debonded using a

universal testing machine (Model 4411, Instron Co., Canton,

MA, USA) with a 50 N load cell traveling at a crosshead

speed of 1 mm/min. Microtensile bond strengths were deter-

mined by computing the ratio of maximum load (N) by the

adhesion area. After the mTBS test, the dentin sides of the

fractured interfaces were analyzed under a stereo light

microscope at 40 £ magni®cation to determine the fracture

mode.

2.2. Statistical analysis

The statistical analysis was performed using a software

package (SPSS 8.0 for Windows, SPSS Inc., Chicago, IL,

USA). The values of bond strength were non-normally

distributed (proved by Kolmogorov±Smirnov test), there-

fore non-parametric tests, the Wilcoxon matched-pairs

R. Frankenberger et al. / Dental Materials 17 (2001) 373±380374

signed-ranks test for dependent pairwise comparisons (same

adhesive), and the Mann±Whitney U-test (different adhe-

sives) for multiple comparisons (correction method accord-

ing to Bonferroni±Holm) at the 0.05 level of signi®cance

(a ) were performed. To assess the in¯uence of different

adhesives and restoratives, the levels of signi®cance were

adjusted to a*� 1 2 (1 2 a)1/k (k� number of performed

pairwise tests).

2.3. Scanning Electron Microscopy (SEM) analysis

The hemisected disks assigned to SEM investigation

(n� 2) were ®xed in 2% paraformaldehyde, dehydrated in

ascending concentrations of ethanol (50±70±95±100%) for

1 h, and ®nally embedded in epoxy resin (Epo-thin, Buehler

Ltd) exposing the adhesive interfaces of the middle of the

tooth. The exposed interfaces were polished and treated

with 6 N hydrochloric acid for 30 s followed by a 10 min

immersion in 2.5% sodium hypochlorite to enable the visua-

lization of a three-dimensional interface relief. The speci-

mens were sputter-coated for 90 s at 20 mA (Polaron E

5100, Polaron Instruments, Watford, UK) and mounted on

aluminum specimen holders to be observed at £ 4000

magni®cation at 15 kV, using a scanning electron micro-

scope (JSM 6300V, JEOL USA, Peabody, MA, USA).

2.4. Transmission Electron Microscopy (TEM) analysis

For TEM evaluation, the resin composite layers of the

corresponding hemisected disks were thinned and six

small sticks with a cross-section area of 1.0±1.5 mm2

were cut from the central area using a slow-speed diamond

saw, under continuous water-cooling exposing the opposite

sides of the interfaces viewed by SEM.

Three of the resulting six sticks in each group were kept

in distilled water, while the remaining three sticks were

demineralized in 10% buffered EDTA for 72 h. All the

disks were consecutively ®xed using 2.5% glutaraldehyde/

2% paraformaldehyde in 0.1 M sodium cacodylate buffer at

pH 7.4 for 12 h at 48C. After the ®xation process, the disks

were rinsed with 0.1 M sodium cacodylate buffer at pH 7.4

for 2 h at 48C. Post-®xation was performed with 2% osmium

tetroxide in 0.1 M sodium cacodylate buffer for 1 h

followed by washing in 0.1 M sodium cacodylate buffer at

pH 7.4 for 1 h. After rinsing in deionized water for 20 min,

both the demineralized and the non-demineralized sticks

were dehydrated in ascending grades of ethanol in analogy

to the SEM specimens. The specimens were immersed in

propylene oxide for 20 min and embedded in a 50/50 mix of

propylene oxide and epoxy resin (Ted Pella, Redding, CA,

USA) in a rotator for 6 h at 6 rpm. Finally, the specimens

were immersed in 100% epoxy resin and set under vacuum

for 12 h for resin in®ltration into the sticks. After position-

ing of the specimens in rubber molds, the sticks were

embedded in fresh resin, exposing the central area of the

interface. The resin-embedded sticks were then dried in an

oven for 12 h at 658C. The blocks were sectioned in an

R. Frankenberger et al. / Dental Materials 17 (2001) 373±380 375T

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ultra-microtome (MT 2-B Ultramicrotome, Ivan Sorvall

Inc., Norwalk, CT, USA), and the demineralized sections

were additionally stained with 2% uranyl acetate for 10 min.

and 3% lead citrate for 5 min.

After drying at room temperature, the sections were

analyzed using a Transmission Electron Microscope

(Philips CM-12, Philips Electronic Instruments Inc.,

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3. Results

3.1. Microtensile bond strength

When Prompt L-Pop was dried with air to evaporate the

water used as solvent, dull areas were often apparent on the

cut and smeared tooth surface, which, under magni®cation

glasses looked like dry spots. An overview of the results is

shown in Table 2. When used with composite, LP resulted in

lower mean bond strengths than when used with a compomer.

The use of an additional bonding resin (LP/self-etch 2-step)

revealed no positive effect on bond strength (P . 0.05),

whereas phosphoric acid etching prior to the application of

Prompt L-Pop as conventional primer (LP/total-etch 3-step)

followed by a bonding resin, resulted in an increase in bond

strengths (P , 0.05). Both LP/®lled and LP/multi-coat groups

resulted in signi®cantly higher mTBS than Prompt L-Pop

applied as per manufacturer's instructions (LP) at P , 0.05.

Prime & Bond NT (P&BNT) resulted in similar mean bond

strengths to LP/multi (P . 0.05). EBS Multi resulted in statis-

tically higher dentin bond strengths than any other experimen-

tal group.

For the groups restored with the compomer, Prompt L-

Pop behaved somewhat differently as compared to the

groups restored with composite. LP (material used as per

manufacturer's instructions) achieved statistically similar

mTBS to LP/multi-coat and LP/®lled at P . 0.05. L-Pop

resulted in higher mTBS than LP/self-etch 2-step (group

with additional application of the EBS Multi adhesive)

and LP with the resin composite at P , 0.05. LP/total-

etch 3-step with the compomer showed lower m-TBS than

with the resin composite. Experimental applications of

Prompt L-Pop alone (LP/multi-coat, LP/®lled) revealed

similar bonding behavior compared with P&BNT at

P . 0.05. EBS Multi resulted in signi®cantly higher bond

strengths than the specimens bonded with LP at P , 0.05.

All specimens exhibited adhesive interface failure.

3.2. Electron microscopy

The SEM evaluation showed no microscopical presence of

an acid resistant hybrid layer for the LP specimens (Fig. 1).

However, the specimens treated with Prompt L-Pop in multi-

ple layers (LP/multi-coat) exhibited a distinct hybrid zone

approximately 1 mm thick (Fig. 2). The ®lled experimental

version of the Prompt L-Pop adhesive (LP/®lled) resulted in

a thicker and more consistent hybrid layer of 1±2 mm thick-

ness (Fig. 3) and a visible layer of resin between the hybrid

zone and the restorative material (Fig. 3). The application of

one and multiple coats of Prompt L-Pop resulted in a barely

visible adhesive layer in the micrographs (Figs. 1 and 2).

P&BNT and EBS Multi exhibited similar morphology under

the SEM, showing hybrid layers approximately 3±4 mm thick

and the characteristic triangular peritubular hybridization

(Figs. 4 and 5).

The TEM investigation exhibited the formation of a

hybrid layer for all groups. The interfaces produced by the

use of Prompt L-Pop, in accordance with the manufacturer's

protocol, showed characteristic features. In some tubule

ori®ces the dissolved smear plugs appear as voids within

the resin-®lled tubule (Fig. 6). Intertubular and triangular

peritubular in®ltration of demineralized dentin appears thin-

ner compared with PBNT (not shown) (Fig. 6). An adhesive

layer between hybrid zone and onlying resin composite was

not detectable, therefore most of the funnel-shaped tubular

ori®ces were ®lled with ®llers of the restorative material to a

depth of up to 3 mm. The magni®cation of a hybridized

region at a tubule ori®ce of an LP specimen shows a super-

®cial area of lower electron density and a deeper zone with

more pronounced electron density (Fig. 7). The analysis of

the LP/multi-coat specimens revealed a superior saturation

of the upper half of the hybrid layer combined with visible

penetration of lateral branches (Fig. 8), and less penetration

of ®llers from the restoratives into the tubules. The control

groups showed thicker hybrid layers in analogy to the SEM

observations.

R. Frankenberger et al. / Dental Materials 17 (2001) 373±380376

Table 2

Results of microtensile bond strength test for different restorative materials [MPa] (SD). Asterisks show signi®cant differences between groups with the same

adhesive for bonding of resin composite and compomer (Wilcoxon test, P , 0.05). Groups with same superscript letters among one restorative material are not

statistically different (Mann±Whitney U-test, P . 0.05)

Pertac II Signi®cance (P , 0.05) Hytac ApliTip

LP 5.2 (6.1)D * 13.5 (6.3)c

LP/total-etch 2-step 11.3 (9.6)CD n.s. 6.9 (5.1)d

LP/total-etch 3-step 18.6 (6.9)B * 12.1 (3.3)c

LP/multi-coat 16.4 (5.0)BC n.s. 18.1 (6.3)bc

LP/®lled 12.8 (5.7)C n.s. 16.9 (6.9)bc

P&BNT 19.0 (3.9)B n.s. 19.4 (7.6)b

EBS 28.9 (8.7)A n.s. 23.8 (6.3)ab

4. Discussion

This in vitro study was design to clarify the effect of

alternative application protocols and ®ller content of a

new dentin-bonding agent on dentin bond strength and

micromorphological appearance of the resin±dentin inter-

face. Prompt L-Pop is a recent attempt to the simpli®cation

of the rather complicated methods for bonding to dentin.

This may be advantageous with respect to the technique

sensitivity claimed for modern dentin bonding agents

[29,30]. Almost all results presently available concerning

in vitro performance of dentin bonding systems were carried

out in strict accordance with the manufacturers' recommen-

dations. However, Ciucchi demonstrated that different

general dental practitioners achieved rather inconsistent

dentin shear bond strengths even when using identical

adhesive systems [30]. Therefore it is an obvious assump-

tion that incorrect application procedures may affect dentin

bonding behaviour, as reported by Peschke for the particular

adhesive system OptiBond FL (Kerr Co., Orange, CA) [31].

On the other hand, packing all required qualities of an

effective dentin adhesive such as wettability, acidity,

capability of penetration, and ®nally cohesive strength and

loading capacity into one chemical composition may be

R. Frankenberger et al. / Dental Materials 17 (2001) 373±380 377

Fig. 1. Scanning electron micrograph of interface bonded with Prompt

L-Pop in one layer (®nal magni®cation: £ 4000). Resin tags (T) are present

with lateral branches (asterisks). No acid-resistant hybrid layer is detect-

able. The resin composite (RC) lies directly on the dentin without an inter-

mediate adhesive layer (circle).

Fig. 2. Scanning electron micrograph of a specimen bonded with Prompt L-

Pop with multiple coats (®nal magni®cation: £ 4000). Note the thin 1mm

thick hybrid layer (H, arrowheads). Resin tags (T) are present revealing

lateral branches (asterisk).

Fig. 3. Scanning electron micrograph of interface bonded with Prompt L-

Pop in the experimentally ®lled version (®nal magni®cation: £ 4000). Note

the visible thickness of the adhesive layer (A, arrowheads) beneath the resin

composite (RC). The hybrid layer (H, arrowheads) is 2 mm thick. The

tubular resin tags (T) show lateral branches (asterisks).

Fig. 4. SEM resin±dentin interface formed by Prime&Bond NT (®nal

magni®cation: £ 4000) The hybrid layer (H, arrowheads) is approximately

4 mm thick. The resin tags show a triangular shape at the entrances (T) and

lateral branches (asterisks).

hard to accomplish. As reported for total-etch one-bottle

adhesives, the compromise of wettablity and stability

seems to result in an overall poorer adhesive performance

compared with multi-step systems. The latter have different

chemical compositions for priming and bonding the acid-

conditioned dentin [17,23,32]. Nevertheless, clinicians

prefer materials with an easy application technique when

performing adhesive dentistry in daily practice; therefore

all of these simpli®ed products are usually accepted with

enthusiasm by clinicians [33,34].

First results for Prompt L-Pop, initially marketed as a

simpli®ed bonding agent for compomers only, were very

promising [22,26,28,34,35]. All the reported high bond

strengths to enamel have been recognized as encouraging,

due to the fact that the resulting etch pattern in enamel

showed a nearly identical performance to that caused by

etching with phosphoric acid [34]. The version of Prompt

L-Pop used in the present project is now marketed in an

improved formula for bonding compomers and resin

composites as well, therefore, both restorative materials

were tested. The evaluation methods carried out in the

present study are commonly accepted. Microtensile bond

R. Frankenberger et al. / Dental Materials 17 (2001) 373±380378

Fig. 5. SEM interface produced by EBS Multi (®nal magni®cation: £4000).

The interface shows a pronounced appearance with a hybrid layer (H), resin

tags (T) and short lateral branches (asterisks).

Fig. 6. Transmission electron micrograph of interface bonded with Prompt

L-Pop in one layer (non-decalci®ed specimen, ®nal magni®cation:

£ 6300). The presence of ®llers within the tubule ori®ce is evident. The

resin-in®ltrated (H) zone with a thickness ,1 mm extends from the inter-

tubular to the peritubular area where the penetration is characteristically

triangular. D� dentin.

Fig. 7. Higher magni®cation of a hybridized zone at a tubule ori®ce produced

by Prompt L-Pop according to the manufacturer' recommendations

(decalci®ed specimen, £ 17,000). The upper half of the hybrid layer (H)

appears to be less saturated with adhesive compared to the bottom half of the

penetrated demineralized dentin with higher electron density. D� dentin.

Fig. 8. Transmission electron micrograph of interface bonded with Prompt

L-Pop in multiple coats (®nal magni®cation: £ 28,000). The hybrid layer

(H) is more saturated than in Fig. 7. Note the fully hybridized lateral (H)

branch below the bottom of the hybrid layer (asterik). D� dentin.

strength testing enables the investigation of interfacial

bonds with reduced probability of pulling out dentin from

the ¯at surface similar to that often recorded when testing in

shear or conventional tensile mode [36±38]. The analysis of

the debonded dentin samples clearly demonstrated that

particular advantage compared to other bond strength test-

ing methodologies. The interfacial analysis by use of SEM

and TEM represents an ideal complement to the mTBS

method allowing us to look into the clues of micro- and

ultra-morphological appearances of the different attempts

for successful bonding to human dentin [21,35]. To have a

valid control for the demineralized TEM samples, non-

decalci®ed specimens were also evaluated (Fig. 6) [39].

The behavior of Prompt L-Pop exhibited a non-uniform

characteristic. During the application of Prompt L-Pop on

¯at dentin surfaces a major problem became clearly evident.

After applying the material for 15 s, LP has to be dried with

air to evaporate the water used as solvent. If this procedure

is carried out carefully, dull areas are often apparent on the

cut and smeared tooth surface, representing dry spots where

the material is probably too thin for being successfully

photopolymerized. The dry spots may have been responsi-

ble for the areas without the interfacial characteristics of

hybridization under the SEM (Fig. 1). TEM investigation

disclosed lower resin saturation in the upper half of the

hybrid layer, which may be attributed to oxygen inhibition

of the thin resin layer after evaporating the water, prevent-

ing the adhesive from polymerizing. There have been

reports in the literature showing that additional coats of

the adhesive system may solve this particular problem

[32,33]. The observation of dry spots was indeed completely

prevented by applying multiple coats of the bonding mate-

rial after drying the solvent until the whole dentin surface

became thoroughly glossy. As a result of multiple coats and

immediate light curing, an adequate and apparently poly-

merizable resin layer is created, resulting in signi®canlty

higher mTBS for the LP/multi-coat groups, which also stres-

ses the need for light curing prior to inserting the restorative.

The same effect was evident when the adhesive was modi-

®ed with quartz ®llers. The noticeably higher viscosity of

the experimentally ®lled Prompt L-Pop may have prevented

the formation of dry spots on the surface and may have

formed an adhesive layer above the hybrid layer. The

SEM disclosed a hybrid layer thicker than that in the other

groups treated with Prompt L-Pop without ®llers. Addition-

ally, the TEM analysis of the interfaces bonded with Prompt

L-Pop applied in one layer (LP) demonstrated an extremely

thin appearance exhibiting ®ller particles of the resin compo-

site plugged into the dentinal tubules during the application of

the material onto the ¯at dentin surface (Fig. 6).

The application of the more hydrophilic polyacid-modi®ed

composite Hytac Aplitip [28] to dentin showed that LP bonds

better to compomers than to the more hydrophobic resins, such

as composites. This is demonstrated by the results of Prompt

L-Pop used according to the manufacturer's protocol (LP).

Although the same problems with dry spots occurred during

the application procedure, the effect seems to be compensated

by a possible greater chemical af®nity of Prompt L-Pop to

Hytac than to the corresponding composite [28]. This might

explain the fact that the experimentally modi®ed groups show

no increase in bond strength.

The use of the EBS un®lled resin resulted in detrimental

effects for Prompt L-Pop regardless of additional phospho-

ric acid etching. When used with EBS alone, the compomer

also exhibited lower bond strengths than when bonded to the

resin composite Pertac II. This might be attributed to some

kind of chemical incompatibility between the EBS bonding

agent and the polyacid-modi®ed material Hytac Aplitip.

This study clari®es the fact that total-etch adhesive

systems, in which a separate phosphoric acid etching step

is used prior to priming or combined priming and bonding,

still represent the dominant method to achieve high bond

strengths to dentin. The bene®cial effect of light-curing the

bonding for the adhesive performance of resin composites

was previously reported [23]. Prompt L-Pop is now used

with composites, which represents a modi®cation of the

®rst version recommended for compomers exclusively with-

out a separate light-curing step.

Altogether, the present mTBS and micromorphological

results point out low bond strengths and narrow ®lm thick-

ness of the all-in-one self-conditioning adhesive when

applied in one layer. However, either the addition of quartz

®llers or the additional application of layers onto the dentin

surface demonstrated to be capable of compensating the

tendency towards the formation of dry spots and consecu-

tively incomplete penetration of the dentin surface. The

ultramorphological analysis con®ms the assumptions gener-

ated by the results of the mTBS investigation. Among the

modi®ed application protocols carried out in the course of

this study, only multiple coating or ®ller addition showed

the desired effect on more saturation within the resin±dentin

interface (higher electron-density, as observed in Fig. 8) as

well as higher mTBS.

The null hypotheses were rejected. (1) The use of Prompt

L-Pop as a multi-step adhesive system resulted in higher

bond strengths than when used as per manufacturer's direc-

tions. (2) When applied in multiple coats, Prompt L-Pop

resulted in bond strengths that were not statistically different

from those of P&BNT, a total-etch adhesive. However, the

bond strengths obtained with EBS Multi, a water-one-based

total-etch adhesive, were signi®cantly higher than those

obtained with Prompt L-Pop.

Acknowledgements

This research project was supported by a grant sponsored

by ESPE, Seefeld, Germany, as the ESPE Dental Talent

Award 1998 `Young Lecturer of the Year'. We are grateful

to Wallace Ambrose at the Electron Microscopy Labora-

tory, Dental Research Center, University of North Carolina

at Chapel Hill for his helpful comments.

R. Frankenberger et al. / Dental Materials 17 (2001) 373±380 379

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