The influence of the dentin smear layer on adhesion: a self-etching

primer vs. a total-etch system

Sofia S.A. Oliveiraa, Megan K. Pugacha, Joan F. Hiltonb, Larry G. Watanabea,Sally J. Marshalla, Grayson W. Marshall Jr.a,*

aDepartment of Preventive and Restorative Dental Sciences, University of California, 707 Parnassus Avenue D2246, San Francisco, CA 94143 0758, USAbDepartment of Epidemiology and Biostatistics, University of California, San Francisco, CA, USA

Abstract

Objective. To determine the effect of dentin smear layers created by various abrasives on the adhesion of a self-etching primer (SE) and

total-etch (SB) bonding systems.

Methods. Polished human dentin disks were further abraded with 0.05 mm alumina slurry, 240-, 320- or 600-grit abrasive papers, # 245

carbide, # 250.9 F diamond or # 250.9 C diamond burs. Shear bond strength (SBS) was evaluated by single-plane lap shear, after bonding

with SE or SB and with a restorative composite. Smear layers were characterized by thickness, using SEM; surface roughness using AFM;

and reaction to the conditioners, based on the percentage of open tubules, using SEM.

Results. Overall, SBS was lower when SB was used than when SE was used. SBS decreased with increasing coarseness of the abrasive in

the SE group. Among burs, the carbide group had the highest SBS, and 320- and 240-grit papers had SBS close to the carbide group. Surface

roughness and smear layer thickness varied strongly with coarseness. After conditioning with SE primer, the tubule openness of specimens

abraded by carbide bur did not differ from 240- or 320-grit paper, but did differ from the 600-grit.

Significance. Even though affected by different surface preparation methods, SE yielded higher SBS than SB. The higher SBS and thin

smear layer of the carbide bur group, suggests its use when self-etching materials are used in vivo. Overall, the 320-grit abrasive paper

surface finish yielded results closer to that of the carbide bur and its use is recommended in vitro as a clinical simulator when using the SE

material.

2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Keywords: Self-etching primers; Smear layer; Burs; Abrasive papers; Dentin adhesion

1. Introduction

The smear layer has been defined as a layer of debris on

the surface of dental tissues created by cutting a tooth [1]. It

varies in thickness, roughness, density and degree of

attachment to the underlying tooth structure according to

the surface preparation [2–8].

As part of restorative procedures required by adhesive

dentistry, the smear layer must be removed, modified or

impregnated by the resin to allow for bonding between the

tooth and the restorative material [9–11]. The poor

performance of early dentin adhesive systems was thought

to occur because the smear layer was not removed, resulting

in bonding of the adhesive to the surface of the smeared

debris [12], and not to the underlying dentin [1]. As a result,

an acidic conditioner was introduced to dissolve and remove

this layer, allowing the direct contact of the resin with

partially demineralized dentin.

Some studies [7,13,14], show no difference in bond

strength of total-etch adhesive systems to different dentin

smear layers, probably because these systems completely

remove smeared debris from the surface. Nevertheless, the

topography of the dentin surface after removal of the smear

layer would reflect the coarseness of the abrasive and

coarser abrasives would have increased surface area. It

would be reasonable to assume that this roughness

influences the bond strength of the adhesive agents [4,15].

The introduction of self-etching materials in which the

acid conditioning step is eliminated by use of a primer

containing an acidic monomer raises new questions. It has

been stated that the self-etching primers, most likely due to

their intrinsic acidity, have the ability to permeate dentin

smears and impregnate the underlying dentin [16]. The

smear layer components are probably incorporated within

the bonding layers [17], since the dissolved matter is not

Dental Materials 19 (2003) 758–767

www.elsevier.com/locate/dental

0109-5641/$ - see front matter 2003 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/S0109-5641(03)00023-X

* Corresponding author. Tel.: þ1-415-476-9119; fax: þ1-415-476-0858.

E-mail address: graymar@itsa.ucsf.edu (G.W. Marshall Jr.).

rinsed away. Koibuchi et al. [18] demonstrated that this

hybridized smear layer has an effect on the bond strength of

the self-etching materials and Ogata et al. [19] showed that

different smear layers had different effects on the bond

strength of a self-etching primer to dentin.

Several studies have evaluated the importance of surface

preparation method on bond strength. [13,20] and have

attempted to define the most clinically relevant smear layer

preparation for use in in vitro tests. The preparation of the

sample’s surface with a bur in vitro is complex and time

consuming and may be difficult to standardize. [3,5–7]

Most in vitro bond strength studies prepare dentin surfaces

with a 600-grit abrasive paper, even though they do not

indicate the clinical relevance of that surfacing procedure.

[21–26] Others treat tooth surfaces with 400-grit, [27,28]

320-grit, [29,30] or even 60-grit, [6] abrasive paper in vitro.

In order to perform clinically relevant research in dentin

adhesive systems in vitro, an increased understanding of the

smear layer is important so that relevant standards for

various approaches to dentin bonding can be developed.

The first objective of this study was to establish a

standard in vitro method to create a smear layer that most

closely mimics those produced by clinical burs. Secondly,

we tested the hypotheses that, when a total-etch adhesive

system is used, shear bond strength does not depend on the

surface preparation method (specifically, the type or

coarseness of the abrasive used to create a smear layer),

but when a self-etching primer is used shear bond strength

does depend on these factors. Finally, we hope to better

understand how the self-etching materials interact with the

smear layer.

2. Materials and methods

The specimens used in this study were prepared from

randomly selected human non-carious third molars. All the

teeth were recently extracted (less than three months) from

patients needing extractions as part of their dental treatment

and as approved by the UCSF Committee on Human

Research. They were gamma irradiated and refrigerated in

Hank’s balanced salt solution prior to use. For shear bond

strength tests, teeth were sagittally sectioned into 2

segments with a low-speed diamond saw (Buehler, Lake

Bluff IL, USA) and proximal surfaces were abraded to

remove the enamel and expose the superficial dentin

surface. For characterization of smear layers, dentin disks

were sectioned from approximately the same depth

(occlusal surface ,2 mm above the pulp horns) and a

smear layer on the occlusal side of each disk was prepared.

Smear layers were prepared with seven different surface

preparation methods: # 250.9 F fine diamond bur and #250.9

C coarse diamond bur (Premier Dental Products Co,

Canada- batch numbers 166 and 176, respectively), #245

carbide plain fissure bur (Midwest-Des Plaines, IL- batch

number 0086) all operated in a highspeed handpiece,

0.05 mm alumina powder slurry (Buehler Micropolish,

Buehler, Lake Bluff, IL), 600-, 320- and 240-grit SiO2

abrasive papers (Carbimet Buehler-met, Buehler, Lake

Bluff, IL). These preparation methods were classified by

type and coarseness (0.05 mm alumina powder slurry -

lowest coarseness; burs: carbide ,fine diamond ,coarse

diamond; SiO2 papers: 600-grit , 320-grit , 240-grit).

Before surface preparation, all samples were polished

through 0.05 mm alumina powder slurry (Buehler Micro-

polish, Buehler, Lake Bluff, IL) to create a baseline surface

finish. All the surface preparations were performed with

water flow (25 ml/min).

The smear layers created by the burs were prepared in a

device developed in our laboratory that firmly holds the bur

in a high-speed hand piece while moving the sample with a

constant load of 150 g. This load was found to be the mean

pressure exerted by most clinicians at the tip of the bur. [31]

Each bur was used under constant water spray, to prepare a

maximum of 5 surfaces. Each sample prepared with paper

was abraded with one of the different grits, for 5 s with a

weight ,150 g, measured on a digital scale (Mettler PC-

2000- Mettler Instruments Corp, NJ). Since it was difficult

with the rougher abrasive papers to maintain constant

weight an effort was made to keep it between 100 and 300 g.

The adhesive and restorative materials used in this study

are listed in Table 1 along with the manufacturers

compositions, batch numbers and codes. All were used

following manufacturers directions.

Four outcome variables were analyzed in two phases. We

examined the distribution of each outcome prior to analysis;

all were analyzed on the natural scale. In phase A, we

analyzed the shear bond strength associated with different

surface preparation methods when bonded with Clearfil SE

bond (SE; Kuraray America, Inc. New York, NY) or Single

Bond Adhesive system (SB; 3M ESPE, St Paul, MN). We

then identified the best clinical abrasive (bur) when the SE is

used (i.e. that associated with the highest shear bond

strength), and compared the paper abrasives with this bur to

identify the best in vitro abrasive (paper). In phase B, we

characterized the surface preparation methods according to

the smear layers’ roughness, thickness, and reaction to

conditioners (Fig. 1). We theorized that the paper abra-

sive(s) with the shear bond strength closest to that of the best

bur also would be closest with respect to characteristics of

the smear layer, indirectly explaining the variation in shear

bond strength. Specifically, we hypothesized that shear bond

strength for the SE should be enhanced by a rough dentin

surface, a thin smear layer, and increased tubule openness

after application of the acidic primer.

2.1. Bond strength

Following smear layer preparation, each specimen was

mounted in a single-plane lap shear device as described by

Watanabe et al, 1996; 1999. [30,32] A Mylar mask with a 3-

mm diameter hole was placed on each prepared surface to

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767 759

standardize the bonded area. One group, consisting of seven

different surface treatments (n ¼ 12 per treatment), was

tested with SE and an analogous group with SB. The

adhesive systems were applied following manufacturers’

instructions and the composite was applied in 1 mm

increments. The intensity of the curing light was monitored

periodically with a curing radiometer (acceptable range of

500–600 mw/cm2) (Model 100, Demetron Research Cor-

poration, Danbury, CT, USA). Samples were stored for 24 h

at 37 8C and 100% humidity before testing. Shear bond

strength was evaluated at a crosshead speed of 5 mm/min,

using a universal testing machine (Instron model 1122,

Instron Corp., Canton, MA, USA).

For each conditioner, we obtained the mean (standard

deviation) shear bond strength by coarseness level (high,

medium, low, extra low) and abrasive type (bur or paper,

slurry), and used ANOVA and Tukey’s studentized range

test to compare the means as a function of these two factors.

The findings based on Tukey’s statistic were described via

letters identifying mean differences that were statistically

significant based on two-sided 0.05- level tests.

Failed samples were examined in the SEM at 25 £

magnification on the bonded surface, followed by 1000–

2000 £ on the cross-section of the failed bonded area to

determine mode of failure.

2.2. Smear layer characteristics

After surface preparation, each dentin disk was cross-

fractured into 3 segments by applying a shearing force into

pre-cut grooves on the pulp side of the disk (Fig. 1).

Separate segments were evaluated for roughness (9

segments per treatment), thickness (6 segments per treat-

ment), and reaction to the conditioners (for each condi-

tioner, 9 segments per treatment), as described below.

Prior to analyzing the smear layer characteristics, we

used mixed-effects models to estimate the correlation

among segments within teeth (and among tubules within

segments). If the correlation was less than 0.10, we used

segments as the units of analysis; otherwise we conducted

the analysis at the disk level. We then used these models to

estimate and compare the mean roughness of the smear

layers formed by the different abrasives and to determine the

dependence of roughness on abrasive type and coarseness

level. Smear layer thickness and reaction to conditioners

were analyzed in the same manner as smear layer roughness.

(1) Smear layer roughness (Root Mean Square rough-

ness, Rq) was measured using an atomic force microscope

(AFM—Digital Instruments—Nanoscope III, Santa Bar-

bara, CA, USA). All measurements were made in water to

prevent sample dehydration. One 50 mm £ 50 mm image

was taken of each segment and the whole image was

measured for surface roughness, using the roughness

analysis option from the AFM software (Nanoscope III,

version 5.12r2. Digital Instruments. Santa Barbara, CA,

USA).

(2) Smear layer thickness was evaluated using a scanning

electron microscope (SEM; ISI ABT SX-40A wet SEM,

Topcon Instruments, Pleasanton, CA, USA). Immediately

after surface preparation and prior to segmentation, disks

Table 1

Restorative and adhesive materials

Material Code Composition Batch # Function

Single Bond Adhesive

system—3M St Paul, MN

SB 35% phosphoric acid Bisphenol A diglycidyl ether dimethacrylate,

HEMA, dimethacrylate, solvent, water.

OEU Adhesive system

Clearfil SE Bond—Kuraray

America, USA

SE Primer- 10-MDP, HEMA, hydrophilic dimethacrylate,

dl-Camphorquinone, N, N-diethanol-p-toluidine, water

Primer-00101A Adhesive system

Resin-10-MDP, Bis-GMA, HEMA, hydrophilic dimethacrylate, dl-

Camphorquinone, N, N-diethanol-p-toluidine, Silanated colloidal silica.

Adhesive-00103A

Z-100—3M St Paul, MN Z100 Bisphenol A diglycidyl ether dimethacrylate; Silanated zirconium silica

synthetic mineral.

OKA Restorative material

35% phosphoric

acid—Fisher Scientific, USA

PA 35% phosphoric acid diluted from 85% phosphoric acid 933812 Acid conditioner

Fig. 1. Sample preparation for smear layer classification. Different samples

were used for each classification method. The occlusal surface of each

dentin disk was prepared with the abrasive, and the samples were fractured

by applying a shearing force in pre-cut grooves on the pulp side. Smear

layer roughness was measured using software from the AFM in images

from the surface of the dentin segments. Smear layer thickness was

measured on the cross section of the dentin segments. Smear layer removal

was evaluated by SEM imaging of the sample surface following smear layer

preparation by each abrasive and conditioning with either the primer of

Clearfil SE Bond or a 35% aqueous phosphoric acid.

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767760

were treated for SEM analysis. They were fixed in 2.5%

glutaraldehyde in a 0.1 M sodium cacodylate buffer

(pH ¼ 7.4) for 12 h at 4 8C, rinsed with 0.2 M sodium

cacodylate for one hour in three different baths, and rinsed

for one minute with deionized water. The disks were then

dehydrated in ascending grades of ethanol to 100%,

transferred to HMDS and allowed to air-dry for 10 min.

[33] Finally the disks were segmented and sputter-coated

with 200 nm gold/palladium in a sputtering system

(Hummer VII, Anatech Ltd, Alexandria, VA). One

SEM image at 5000 £ was taken of the cross-section of

each segment, and the smear layer thickness was measured

at ten equally spaced points along the smear layer surface,

using image analysis software (Ultrascan 2.1.1, Soft

Imaging Software, Kevex Sigma, Noran Instruments, Inc.,

Madison, WI). Each segment was tilted 208 in each direction

to ensure that the smear layer width was measured

accurately.

(3) Smear layers’ reactions to the conditioners were

analyzed on the SEM micrographs of the segment

surfaces. After surface abrasion and prior to segmenta-

tion, each conditioner was applied to 3 disk surfaces

(Fig. 1). The primer from SE was applied for 20 s and

then air-dried with a gentle stream of air, following the

manufacturer’s instructions. These disks were immedi-

ately placed in 100% ethanol for 5 min to dissolve the

monomer of the SE primer, and then soaked in de-

ionized water for 5 min to reverse any dehydration from

the ethanol. A prior study showed that this procedure

removed the monomer and reversed the effect of the

ethanol dehydration. [34] Phosphoric acid liquid was

applied to 3 other disks for 15 s with a brush and rinsed

with deionized water for 10 s. The 35% phosphoric acid

used for this procedure was prepared by diluting 85%

phosphoric acid (Fisher Scientific, USA) with de-ionized

water, since the conditioner from the SB contains a silica

thickener, which leaves a precipitate that interfered with

the surface analysis. The same ethanol and de-ionized

water treatments were applied to these disks as were

used after the SE treatment. All 6 disks were then treated

for SEM evaluation (as described above) and segmented

before sputtering.

One 5000 £ SEM micrograph was taken per segment.

The numbers of tubules that were open, partially open,

plugged with smear layer or closed, were determined by

visual evaluation of the SEM micrographs for each surface

treatment. Tubules were considered closed when the

structure of the peritubular dentin was not visible, whereas

plugged tubules were clogged below the surface, with the

tubule structure and peritubular dentin visible. The extent of

openness was coded as follows: 100% ¼ completely open,

66% ¼ partially open, 33% ¼ plugged, 0% ¼ fully closed.

In order to perform a segment-level analysis, the mean

openness per segment was calculated (on average, there

were 9 tubules per segment) and analyzed using mixed-

effects models, as described above.

3. Results

3.1. Bond strength test

Overall, shear bond strength was greater when SE was

used (35.5 ^ 8.8 MPa) than when SB was used

(21.8 ^ 7.3 MPa; P , 0:001). In the SB group, when

specimens abraded with 0.05 mm alumina slurry were

included in the model, shear bond strength varied by both

coarseness level ðP ¼ 0:039Þ and abrasive type ðP ¼

0:029Þ; however, when these specimens were excluded,

shear bond strength did not vary with either factor (model,

P ¼ 0:53).

In the SE group, the mean shear bond strength (standard

deviation) was 35.5 (8.8) MPa. When specimens abraded

with 0.05 mm alumina slurry were included in the model,

shear bond strength varied by both coarseness level ðP ¼

0:053Þ and abrasive type ðP ¼ 0:043Þ: When these speci-

mens were excluded, the statistical significance of both

factors increased (P ¼ 0:005 and P ¼ 0:003; respectively).

Regardless of abrasive type, shear bond strength decreased

with increasing coarseness of the abrasive (Table 2) and was

significantly lower when the coarsest abrasives were used

(coarse diamond bur or 240-grit paper), according to

Tukey’s test. Among the bur abrasives, the carbide bur

yielded higher bond strength than either the fine diamond

bur [by 2.1 (95% CI, 22.9–7.1) MPa] or the coarse

diamond bur [by 7.3 MPa (95% CI, 2.2–12.4) MPa]. Thus it

was selected as the standard against which the paper

abrasives were then compared. Although no paper abrasive

differed significantly from the carbide bur ðP ¼ 0:15Þ; the

320-grit paper and the 240-grit paper yielded shear bond

strengths within 0.5 MPa of the shear bond strength of the

carbide bur (Table 2). Thus they appeared to offer better

simulations of the clinical bur than the 600-grit paper.

Examination of the surfaces fractured during the shear

bond strength test showed a common cohesive failure

through the adhesive layer for all abrasives except for the

coarsest when used in conjunction with SE. In the latter

cases (coarse diamond bur and 240-grit paper), residual

smear layer appeared to remain on the de-bonded surfaces

Table 2

Abrasive and adhesive influence on shear bond strength (MPa)

Abrasive Mean shear bond strength

(std. dev.)a

Type Coarseness Clearfil SE Single bond

Alumina slurry 0.05 mm Extra low 35.1 (13.8)A,B 17.0 (6.6)A

Abrasive paper 600-grit Low 42.0 (7.5)A 25.4 (6.8)A

Abrasive paper 320-grit Medium 36.6 (7.6)A,B 21.7 (6.6)A

Abrasive paper 240-grit High 35.7 (8.7)A,B 22.4 (9.0)A

Bur Carbide Low 36.2 (5.3)A,B 22.0 (6.8)A

Bur Fine diamond Medium 34.1 (5.8)A,B 20.4 (7.9)A

Bur Coarse diamond High 28.9 (7.0)B 23.6 (6.2)A

a The outcomes are significantly different if the superscript letters differ.

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767 761

(Fig. 2). Although demarcation between the bonded and

unbonded areas was clear, it was apparent that the striations

and smear layer are continuous across the bonded and

unbonded areas.

3.2. Smear layer characteristics

We compared the smear layers created by three paper

abrasives with the carbide bur with respect to three

characteristics of the smear layer: roughness of the dentin

surface, thickness of the smear layer, and reaction to the

conditioner (by means of tubule openness).

Surface roughness. AFM measurements made of the

surface roughness (Rq) for disks abraded with a coarse

diamond bur exceeded 1000 nm, which was too large to

measure; hence no data were available for analysis. In disks

abraded with 0.05 mm alumina slurry, surface roughness

ranged from 16.8–28.6 nm, whereas for other abrasives it

was 220 nm or higher (Table 3). Because the effect of

0.05 mm alumina slurry on surface roughness was very

different from the effects of other abrasives and this skewed

the overall distribution; these samples were excluded from

further analysis. The correlation among segments within

disks was low (0.05), enabling us to use segments as the

units of analysis of surface roughness (n ¼ 45 segments; 9

per abrasive).

Surface roughness varied strongly by coarseness level

(median (range): Low, 347 (220–618) nm; Medium, 769

(479 – 1232) nm; and High, 726 (541 – 1389) nm;

P , 0:001) but not by abrasive type (P ¼ 0:20). Of the

SiO2 papers, the 600-grit paper produced a surface rough-

ness most similar to that of the carbide bur (difference,

158 ^ 91 nm; the 320-grit paper differed by

280 ^ 113 nm).

Thickness of the smear layer. Smear layer thickness

measurements for each segment were made by SEM

analysis of the fractured segments (Fig. 1). Specimens

treated with 0.05 mm alumina slurry were not analyzed

because smear layers were too thin to measure. Since the

correlation among segments within a given tooth was low

(0.09) but the correlation among measurements per segment

was high (0.58), we averaged over the measurements within

each segment and used segments as the units of analysis of

smear layer thickness (n ¼ 36 segments; 9 per abrasive).

Fig. 2. Fractured surfaces from shear bond strength samples from the Clearfil SE bond group. (a) 25 £ magnification SEM micrograph of the 600-grit abrasive

paper subgroup sample shows almost all the surface covered with adhesive (x), which suggests a mainly cohesive failure within the adhesive layer. (b) SEM

micrographs (25 £ magnification) of the coarse diamond bur subgroup sample show some areas (z) where there is no adhesive and failure appears to have

occurred below it. From the similarity with the surface around the bonded area, we can see that striations are the same in the area that was conditioned and the

dentin surface around it. These areas (z) appear not to have been conditioned and look just like the smear layer around it. Therefore we would consider this

failure to be cohesive within the smear layer.

Table 3

Smear layer characteristics produced by different abrasives

Abrasive Smear layer characteristics [mean (std. dev.)]a

Type Coarseness Roughness (nm)a Thickness (mm) Smear layer reaction to the SE

primer.b (Tubule Openness; %)

Alumina slurry 0.05 mm Extra low 21.7 (3.3) N/A 66.0 (14.4)A

Abrasive paper 600-grit Low 267.7 (27.0)A 1.4 (0.2)A 48.8 (11.8)A,B

Abrasive paper 320-grit Medium 757.5 (80.3)B 2.0 (0.4)A,B 33.4 (17.2)C,B

Abrasive paper 240-grit High 821.2 (225.6)B 3.0 (0.7)C 16.4 (13.9)C

Bur Carbide Low 425.9 (94.6)A 1.8 (0.1)A,B 22.2 (17.8)C

Bur Fine diamond Medium 909.7 (92.2)B 2.0 (0.2)A,B 18.5 (13.0)C

Bur Coarse diamond High .1000 2.4 (1.1)C,B 15.5 (16.1)C

a The outcomes are significantly different if the superscript letters differ. Abrasives without letters were excluded from the analysis.b The SB system had tubule openness of 100% for all abrasives.

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767762

As was true for surface roughness, the thickness of the

smear layer increased significantly with the coarseness of

the abrasive [median (range): Low, 1.6 (1.2–1.9) mm;

Medium, 2.0 (1.5–2.6) mm; and High, 2.8 (1.6–4.5) mm.

P , 0:001)] but did not differ significantly by abrasive type

(burs vs. papers; P ¼ 0:68). The thickness produced by the

carbide bur (1.76 ^ 0.13 mm; Table 3) was slightly more

than that of the 600-grit paper, by 0.36 ^ 0.17 mm, and

slightly less that of the 320-grit SiO3 paper, by

0.22 ^ 0.32 mm.

Tubule openness. SEM micrographs of all smear layers

treated with 35% phosphoric acid showed the tubules 100%

opened so that further analysis with this conditioner was not

needed. Typical SEM micrographs for the effect of the SE

primer on smear layers created with various abrasives are

shown in Fig. 3. When tubule openness (varying from 0% to

100% open) was analyzed, low correlation among measure-

ments within segments (0.0006) would have enabled us to

use tubules as the units of analysis. However, we averaged

the data within segments to make these the units of analysis

(n ¼ 63 segments; 9 per abrasive), to approximate this

analysis with the others in sample size and power. Overall,

the mean tubule openness for SE was 33 ^ 36%; 10% were

completely open, 21% were partially open, 30% were

mostly closed, and 39% were fully closed. After reducing

the data to segment-specific means, the overall mean tubule

openness was 32 ^ 23%.

Tubule openness decreased with increasing coarseness

level and was lower among burs than among paper abrasives

(both, P , 0:001; Table 2 and Fig. 4). The tubule openness

of specimens abraded with a carbide bur (22 ^ 18%) did not

differ from those abraded with 240-grit or 320-grit paper,

but did differ from the level of 600-grit specimens (Table 3).

4. Discussion

The self-etching materials were introduced to the dental

market at a time when dentists desired easier and less

technique-sensitive adhesive materials. Although these

qualities can be very appealing to clinicians, care should

be taken to evaluate how these new materials interact with

the dentin surface. Since the current self-etching materials

have higher pH values than the acids used with total-etch

adhesive systems, and the self-etching materials are not

rinsed away, the smear layer or its components are

incorporated into the bonded layers.

With the methods used in this study we found that the

total-etch system (SB) completely removed the smear layer,

regardless of the abrasive used and that shear bond strength

Fig. 3. SEM micrographs of the reaction of the smear layer to the Clearfil SE primer treatment. (a) 600-grit smear layer; (b) 320-grit smear layer; (c) carbide bur

smear layer; (d) coarse diamond bur smear layer. Open tubules (O), partially opened tubules (PO), plugged tubules (P) and closed tubules (C), are indicated in

the images.

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767 763

for the SB system was not sensitive to the method used to

create the smear layer. This was not true for the SE system

and the results confirmed the hypothesis that the method

used to produce the smear layer affects the bond strength of

this system.

The evaluation of the smear layer modification by the

primer showed a significant inverse association between

coarseness level and the tubule openness (Figs. 3 and 4).

Thicker smear layers resulted in increased number of closed

tubules after SE treatment.

To determine if the rinsing step influenced the retention

of the smear layer on the dentin surface, an additional

experiment was performed (Fig. 5) where dentin samples

with a smear layer created by a 240-grit abrasive paper

(which gave the thickest smear layer; Table 3) were acid

etched with SE primer for 20 s (Fig. 5–a) and air dried for

5 s. These were compared with those etched with different

concentrations of phosphoric acid [0.13%, pH ¼ 2 (same as

SE), Fig. 5b; 20%, pH ¼ 0.21, Fig. 5c; and 35%,

pH ¼ 20.28,Fig. 5d] for 15 s and rinsed with de-ionized

water for 10 s. The smear layer was successfully rinsed

away after being etched with the acid only when 35 %

phosphoric acid was used (Fig. 5d). This suggests that more

diluted acids were not strong enough to etch through the

whole thickness of the 240-grit smear layers and that even

after rinsing part of the smear layer was still attached to the

dentin surface. Pashley and Carvalho. [11] suggested that

the dentin smear layer interferes with the self-etching

primer adhesion. Our results support this suggestion.

Additionally, the decreased bond strength and increased

smear layer thickness with the coarseness of the abrasive

when the SE was used (Tables 2 and 3), and the presence of

smear layer on the de-bonded samples from the coarser

abrasives in the SE group (Fig. 2), further support this

hypothesis. Nonetheless, the shear bond strength results of

SE were, overall, significantly higher than those bonded

with SB. This was true despite the fact that SE was less

effective in completely removing the smear layer. This

challenges the general consensus that it is necessary to

remove the smear layer in order to achieve high bond

strengths. [9] Preliminary work in our lab evaluated the

hybrid layer for the self-etching primer and total etch system

studied in this work. We found that the hybrid layer had a

substantially higher modulus for the self-etching system and

this may offer a possible explanation for the higher bond

strengths found in this investigation. We hope a more

complete evaluation will confirm these findings in the near

future. On the other hand, it was reported [35] that the

phosphoric acid used in these concentrations causes the

denaturation of the top layer of collagen, which could

explain its lower bond strength.

When SE was used, the carbide bur gave the thinnest

smear layer of all the burs (Table 3), and it produced the

highest bond strength among the bur groups (Table 2).

This finding emphasizes the importance of using, clinically, a

bur that creates a thin smear layer when applying the current

SE materials as an adhesive system for bonded restorations.

In order to generate relevant data, laboratory testing

should be performed on surfaces that closely resemble those

created under clinical conditions. According to our results

on the smear layer characteristics, the carbide bur group

yielded smear layer thickness between those of 320- and

600-grit papers, and its roughness was closer to the 600-grit

paper. But when the adhesive system was used (shear bond

strength and reaction to the conditioner), the smear layer

created by the carbide bur gave results that were similar to

the 320-grit smear layer or were in between those of the

320- and 240-grit papers.

Fig. 4. Reaction of the smear layers, by surface preparation method, to the primer of Clearfil SE Bond. Tubule openness decreased with increased coarseness

level and was lower among burs than paper abrasives (both, P , 0:001). The tubule openness was similar for specimens abraded with carbide bur, 240-grit or

320-grit paper.

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767764

An explanation for this difference could rely on the

difference between the two abrasive methods. Using a bur

may produce a denser smear layer than that produced by

abrasive papers, which may affect the primer’s ability to

etch through the smear layer. We found that, on average, the

thickness of smear layer did not vary by abrasive type, but

burs tended to have more closed tubules than the papers.

Thus openness may be more closely related to denseness of

smear layer. The suggestion that burs may leave a thinner

but denser smear layer than the sandpaper was discussed by

Tao et al. [14] If the 320-grit abrasive paper creates a less

compact smear layer than the carbide bur, this might

compensate for its slightly higher thickness, so that both

react similarly to the self-etching primer, as seen in the

results from the shear bond strength and reaction to the

conditioners. Regardless, within the limitations of our study,

the 320-grit paper yielded reactions to the conditioners

(shear bond strength and openness of tubules) that were

similar to those produced by the carbide bur. Thus the 320-

grit abrasive paper would be the most clinically relevant

SiO2 paper of the three we studied.

Even though in a previous study there was a similar result,

[36] it is still perplexing that in the shear bond strength tests

the lowest values were found for the 0.05 mm alumina group

with either SB or SE, even though it presented the thinnest

smear layer with the most open tubules. This might be

explained by our observation that although the 35%

phosphoric acid removes the whole smear layer, the striated

topography created by the dentin surface preparation

remained intact (Fig. 6). It is reasonable to assume that by

Fig. 5. 240-grit abrasive paper smear layer samples etched with (a)—Clearfil SE primer (SE), and different concentrations of phosphoric acid: (b)—0.13%

(pH ¼ 2; similar to SE primer), (c)—20% (pH ¼ 0.21), and (d)—35% (pH ¼ 20.28). Only in (d) the smear layer was successfully rinsed away after being etched

with the acid. These pictures represent the inability of an acid with higher pH to etch through the thick smear layer created by the 240-grit abrasive paper.

Fig. 6. Fine diamond bur smear layer treated with 35% aqueous phosphoric

acid. Although the surface is clean of smear, the striations due to the

preparation were still evident.

S.S.A. Oliveira et al. / Dental Materials 19 (2003) 758–767 765

increasing the surface area, the bond strength would be

higher, since it would increase the true area of the surface

bonded by the resin. [37] Thus the lower surface area

resulting from the 0.05 mm alumina slurry would probably

give lower bond strengths in the SB group. For the SE group

however, the smear layer thickness seems to play a major role

in the bond strength, as discussed above, so a balance

between roughness and thickness should be achieved in order

to produce higher bond strengths. Another, possible

explanation for the low bond strength of the 0.05 mm

alumina group is that by highly polishing with an alumina

powder slurry, we modify the surface in some way that

interferes with bonding. Finally it should be noted that the

polished surfaces are of theoretical interest but are not

relevant to clinical settings.

The finding that the self-etching primer did not totally

remove the smear layer or open all the tubules for the bur-

abraded samples may be important from the clinical

standpoint. As noted by Pashley, [38] the combination of

the smear layer and smear plugs reduce dentin permeability.

Removing this barrier would increase dentin permeability,

[12] producing an outward dentinal fluid movement from the

pulp, which could interfere with dentin adhesion [39] and

dilute the adhesive agents. [11] Also, since some resin

components are hypertonic, they can osmotically increase

dentinal fluid flow toward the dentin surface, resulting in a

displacement of the odontoblasts. [38] and post-operative

pain. Since the self-etching materials do not completely

remove the smear plugs, they may have the potential to

promote less post-operative sensitivity and be less disturbed

by moisture changes of the dentin substrate, [40] without

sacrificing shear bond strength.

Another concern is that the bacteria present in the smear

layer might be retained with it and affect the pulp. However,

if the restoration is well sealed, this may not be a concern

because it would prevent the bacteria from subsisting. Even

though high bond strength and low microleakage may not

always be correlated, high bond strengths are necessary to

overcome curing stresses and prevent the formation of gaps

between the restoration and tooth substrate. However, it is

possible that retaining part of the smear layer or its

components could affect the long-term bond strength by

degrading over time. In this study, the smear layer has been

shown to be important in the adhesion of self-etching

primers. Research on long-term effects is needed to more

fully understand its implications.

In conclusion, shear bond strength of the SB system was

not sensitive to the abrasive used except for the very smooth

surfaces produced by the 0.05 mm alumina slurry. In general

thick smear layers seemed to interfere with the adhesion

capabilities of the self-etching primer studied, although this

system still showed higher bond strengths than the etch-and-

rinse adhesive system. This suggests that self-etching

primers should be used in vivo with a surface preparation

method that creates a thin smear layer. In this study we

found that, compared with those produced by the two

diamond burs, the carbide bur yielded the highest shear

bond strength and the thinnest smear layer. Therefore,

surface preparation for in vitro tests of this SE system

should consider the use of an abrasive paper that creates a

smear layer with similar characteristics to the carbide bur,

which we believe to be the 320-grit SiO2 paper.

Acknowledgements

Sofia Oliveira is supported by a PhD fellowship from the

Portuguese Ministry for Science and Technology, Praxis

XXI program. This study was supported by NIH/NIDCR

Grant P0.1 DE09859.

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