Shear Bond Strength of Composite Resin to Moist Enamel

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					    Shear Bond Strength of Composite
         Resin to Moist Enamel
                                Dr. Nasrien Ateyah

Assistant Professor
Division of Operative Dentistry
Department of Restorative Dental Sciences
College of Dentistry
P.O. Box 5967, Riyadh 11432

Objective: To compare the effect of shear bond strength of three composite resins to

etched moist enamel.

Materials & Methods: Sixty composite resin specimens were prepared, they were

divided into three main groups, twenty specimens for each material to be tested for shear

bond to etched enamel and compared in dry and moist conditions.

Results: The bond strength of Prodigy and Z-100 was not statistically affected by the

moist etched enamel surface. Spectrum TPH showed a significant increase in bond

strength (50%) under moist condition.

Conclusion: Moist etched enamel was not significantly affected the shear bond strength

of the studied composite resin materials. However, spectrum TPH had the highest bond

strength when compared with the dry samples.

              Shear Bond Strength of Composite Resin to Moist Enamel


       The reliability of bond composite resin to enamel forms the basis of much of

adhesive dentistry. The technique was pioneered by Bunocore in 1955, and was made a

practical reality with the development of modern composite resins using Bis GMA or

other long-chain dimethacrylate resins as their basis.      The technique comprises the

pretreatment of enamel surface with acid to render it microporous with the subsequent

infiltration of the porous surfaces with a low viscosity resin (Gwinett and Matsui, 1967;

Soetopo et al., 1978 and Gilpatrick et al., 1991).

       This produces a micromechanical bond of considerable strength. The bonding

resin for early composite materials comprised simply of the binding dimethacrylate resin

itself that may also have contained some diluents to improve flow. It was established at a

relatively early stage in the evaluation of this technique that contamination of the dried

etched enamel surface with water, saliva or gingival crevicular fluid resulted in failure of

resin penetration and dramatic reduction in bond strength (Baier, 1992; Barghi et al.,

1991; Hormati et al., 1980 and Silverstone et al., 1985).

       The developments of bonding resins from simple dimethacrylate resins to a

complex and chemically active resin systems that has the ability to bond to dentin, rather

than to enamel, improving their bonding efficiency. Moreover, these resins are better

when bonded to dry etched enamel (Swift and Triolo, 1992, and Hadavi et al., 1993).

       Recent concepts in dentine bonding showed that the etched dentin surface should

be fully hydrated to permit maximal penetration of the resin into the exposed collagen

network, thus forming the ‘hybrid layer’. In a clinical situation, it is not possible to

produce damp dentin with dry etched enamel around it (Walls et al., 2001).

       There are a few data in the literature showing the effects of bonding adhesive

resin systems to damp etched enamel and compared with dry etched enamel (Walls et al.,

2001). Some authors reported reduction in bond strengths of composite materials when

moist etched enamel was compared with dry etched enamel (Airoldi et al., 1992 and

Hadavi et al., 1993), others showed that it has no effect (Woronko et al., 1996).

However, some investigators revealed improved bonding composite to moist etched

enamel surface (Swift and Triolo, 1992; Xie et al., 1993 and Powers et al., 1995).

       The aim of this study was to compare the shear bond strength of three composite

resins and their bonding systems to etched moist enamel.

Materials and Methods:

        The composite resins and their bonding system used are listed in Table 1. Sixty

defect-free bovine incisor teeth were collected, debrided, and stored in 0.2% thymol in

distilled water and kept in refrigerator until being used. The facial surface of each tooth

was grounded on a rotary grinding machine (Jean Wirtz Automat polishing machine

Dusseldort, West Germany) using 240-grit silicon carbide paper followed by 400-grit

paper under water cooling until a flat enamel surface with a minimum diameter of 5mm

was produced. The flattened surfaces of each tooth were centered horizontally in Teflon

mold and mounted with cold cure acrylic resin. Before bonding, teeth were refined

slightly using 600-grit silicon carbide paper to remove any acrylic flashes from the

enamel surfaces and to assure clean and fresh surfaces for bonding.

        Ten specimens were randomly assigned to each of six groups with various

treatment are presented in Table 1.

        The enamel surface was etched, according to the manufacturer’s recommended

time, with the acidic conditioner of different concentration appropriate for each system,

then washed and dried with oil-free air until the typical chalk-like appearance of etched

enamel was obtained.

        For moist specimen surfaces, the etched enamel was dried as described before

then moistened thoroughly with water soaked in a small sponge applicator until the

surface become saturated with fluid. Excess water was then removed by blotting the

surface with a cotton pledget to leave a damp etched surface similar to simulate a clinical


       Each group was then subjected to bonding procedure as described by the

manufacturer’s directions.

       Split Teflon mold (5mm in diameter, 3mm thickness) were used as matrices for

the production of columns of composite bonded to the enamel surface, which was firmly

attached to the tooth with a metal ring.

       The composites were cured according to the manufacturer’s instruction using a

light-curing unit (ESPE Dental Elipar Highlight, Seefeld, Germany).

       All specimens were stored in distilled water at 37C and 100% relative humidity

for 24-hours between preparation and bond strength testing.

       The shear bond strengths were then determined using an Instron universal testing

machine (Instron Cor. 8500 Canton, Massachusetts, USA) with cross head speed of

0.5mm per minute. Knife-edge shear probe was attached to the cross head until fracture

occurred. Shear forces were recorded in the units of megapascal (MPa), and obtained

directly from the computer output.

Statistical Analysis

       Student t-test was used to determine the effects of bonding composite resin

materials to dry or moist enamel surfaces for each material.       One-way analysis of

variance (ANOVA) was used at a significant level of p0.05.           Tukey’s multiple

comparison test was used to determine the nature of differences.

Table 1:       Restorative materials manufacturers and their adhesive bonding

  Composite Restorative                       Surface
                                  N                           Adhesive Bonding System
       Material                              Condition
                                 10          Dry             Etchant:
       Spectrum TPH
                                                                     34% Phosphoric acid
      (Dentsply Caulk
 International Inc. Milford,
                                 10             Moist
                                                              Bonding Agent:
   DE 19963-0359, USA)
                                                                     Prime & Bond NT
                                 10             Dry          Etchant:
          Prodigy                                                    37.5% Phosphoric
  (Kerr, USA 1717 West           10             Moist               acid
Collins Avenue Orange, CA
 92867 (800) KERR-123)                                        Bonding Agent:
                                                                     OptiBond Solo Plus
                                 10             Dry          Etchant:
           Z-100                                                     35% Phosphoric acid
 (3M Dental Products, St.        10             Moist
 Paul, Illinois MN55144,                                      Bonding Agent:
            1000                                                    ScotchBond

Table 2:       Mean bond strength and standard deviation (SD) of composite resins
               to etched enamel (MPa)

        Composite                          Dry                             Moist

       Spectrum TPH                    10.25 (1.5)*                    19.95 (5.5)

           Prodigy                      16.35 (4.8)                    20.08 (3.7)

           Z-100                        17.45 (4.7)                    15.55 (4.5)

   -   Vertical lines connect means that are not significantly different
   -   *Significantly different from the other surface condition group


       The data obtained for bond strength are given in Table 2. Prodigy and Z-100

composite resins showed no significant differences between moist and dry surfaces.

Spectrum TPH composite resin demonstrated greater bond strength to moist enamel

(19.95). These values were statistically significant when compared with that to dry

enamel (10.25).

       Between the materials, comparison was made for different surface conditions. It

was evident that there was no significant difference between Spectrum TPH (19.95),

Prodigy (20.08) and Z-100 (15.55) on moist enamel. For dry enamel surface, Spectrum

TPH (10.25) showed significantly lower bond strength when compared with the rest of

materials used [Fig. 1 and 2].

          25                                              Dry


( MPa )



               Spectrum TPH      Prodigy          Z-100

Figure 1. Mean bond strength of moist and dry samples for all test groups.

                                                           Moist       Dry


( MPa )



               Spectrum TPH             Prodigy                    Z-100

   Figure 2. Mean bond strength of moist and dry samples of three test materials.


       Traditionally, the etched enamel-composite resin bond has been the most reliable

adhesive bond known to dentistry (Pain and Stewart, 2000).

       With the advent of universal enamel/dentin bonding systems, dentin primers have

been used with enamel bonding system. However, the application of bonding materials

to both enamel and dentin might create an important variable, since sufficient dryness of

enamel and/or dentin cannot be completely attained before composite application.

Therefore, it is very important that these variables be studied carefully in order to ensure

that it will not interfere with the enamel – resin bond.

       To obtain good bond strengths of composite resin to enamel, stringent isolation

procedures need to be followed. The way in which composite resin adapts to tooth

substance depends more on the nature of the tooth surface than on the properties of the

composite resin (Barnes, 1977). Contamination of etched enamel with moisture, debris,

dentin primers, and other contaminants, such as saliva, plasma, cements, and handpiece

lubricants has shown to interfere with the enamel-resin bond (Silverstone & Dogon,

1975; Soetopo et al., 1978; Hormati et al., 1980; Powers et al., 1995; Xie et al., 1993).

       Most of the available enamel/dentin bonding systems are comprised of an acid

etchant, dentin primer, and low-viscosity adhesive resin supplied separately in one bottle.

Other brands combined the primer and adhesive in one bottle. Most of the researchers

recommend etching the enamel and dentin surfaces and leaving the dentin surface moist

after rinsing the etchant and before application of the hydrophilic primer on the

hydrophilic primer/adhesive combination following the manufacturer direction. There is

no evidence to clarify whether the enamel in the same tooth preparation should be left

moist or dry before application of primer. However, it is quite impractical to leave the

dentin moist clinically after rinsing the etchant off but thoroughly dry enamel adjacent to

it. Therefore, in all probability, in most clinical situation involving both enamel and

dentin, the enamel surface is occasionally left moist along with the dentin. This problem

has been realized by few of the previous researchers (Pain and Stewart, 2000).

       Kanca (1992) studied the effect of surface wetness of enamel on composite resin

bond strengths. His results revealed that the bond strengths of composite resins to etched

and moist enamel were equal or higher than those tested for dry enamel. These results

coincide the results of the present investigation. Wakefield et al. (1996) revealed that

surface wetness had no significant effect on shear bond strength of seven different dentin

bonding agents to enamel. Iwami et al. (1998) found that humidity of oral ambient air

did not significantly influence shear bond strengths of enamel to composite resin.

However, some clinical studies showed significant lowering of bond strengths to enamel

when moisture contaminates etched enamel surfaces (Barghi et al., 1991). Although,

Knight et al. (1993) showed that moisture contamination of etched enamel surfaces does

adversely affect composite resin adaptation to enamel.

       Davidson et al., 1984 stated that, bond strength of resin composite to etched

enamel serves two purposes, to help to retain the restoration in place and to help to resist

the forces of polymerization shrinkage and hence micro-leakage. They concluded that

shear bond strength of the order of 17 to 20 MPa is required to achieve their objective.

Their results were also in agreement with Fielzer et al., 1993. The results of the present

investigation coincides to Davidson et al., 1984, in spite of the used composite materials

and their etching and bonding system were differs. Perhaps the improvement of the

bonding materials used with Prodigy and Z-100 in the present investigation is responsible

for the higher bonding values.    In addition, the concentration of the etchants may also

have a significant effect on the higher value of the bond strength of most composite

materials used. Since the amount of mechanical locks developed with the use of etchant

depends on the increased concentration of the etchant. Thus, increasing the wettability

of bonding material (monomer) in moist conditions. This was in agreement with Walls et

al., 2001. Nakabayashi et al., 1991 who proposed that water triggered the formation of a

hybrid layer, lead to adhesion. They pointed out that certain monomers could infiltrate

enamel prisms and combine with the hydroxyapatite to form this tooth-resin layer.

Clinical Implication

       Within the limits of this study, it would appear that bonding to moist enamel

should not be a problem clinically with modern moist dentin bonding agents.

       Consequently, we would still recommend the routine isolation of the operative

field during etching enamel and the occasional contaminant of moist during the

application of dentin bonding agent could be considered safe if it was difficult to control



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