Braz Dent J (1998) 9(1): 11-18 ISSN 0103-6440
| Introduction | Material and Methods | Results | Discussion | Conclusion | References |
The tensile bond strength of one third-generation (XR Bond) and one fourth-generation (Optibond) adhesive system to dentin was compared. The influence of acid etching of dentin on the bond strength in the XR Bond system was also evaluated. In addition, two kinds of curing (photo and dual) of the Optibond system were evaluated. The Optibond system showed a statistically significant higher tensile bond strength to dentin than the XR Bond system. Etching of the dentin before application to XR Bond did not significantly improve the bond strength to dentin. Both photo and dual curing showed similar bond strengths two hours and seven days after sample preparation.
Key Words: adhesion, composite resin, dentin adhesives, tensile strength, dentin.
Acid etching of enamel is a safe procedure for obtaining adhesion of
composite resins. However, the adhesion between composite resin and dentin
is more difficult to obtain, mainly due to structure and composition of
the substrate. While enamel is almost completely mineralized tissue, dentin
is a living tissue that consists of inorganic compounds (45%), organic
compounds (33%) and water (Holtan et al., 1994). When dentin is mechanically
instrumented a layer of debris is deposited on the surface called smear
layer (Marshall Jr., 1993).
The differences among the dentin bonding system generations are based on the treatment of smear layer. In the first and second generations, the objective was to obtain a chemical adhesion between the adhesive and the smear layer, which might be kept for protecting the dental pulp. Unfortunately, the smear layer is superficially attached to the intact dentin surface, thus very poor results were obtained with these adhesives (Burke and McCaughey, 1995).
In the attempt to improve the bond strength of dentin adhesives, primer solutions were included that can alter the smear layer in the third generation adhesive systems. In the fourth generation systems, the smear layer is totally removed by etching with organic acids, which is followed by application of primer and adhesive forming the hybrid layer (Nakabayashi et al., 1992, 1995), thus a high bond strength could be achieved.
On the other hand, if we use a third generation dentin bonding agent with a fourth generation proceeding, e.g., acid etching of dentin, the results are controversial (Davis et al., 1991; McHenry et al., 1991; Barkmeier and Jefferies, 1992; McGuckin and Powers, 1992).
After a few years of usage, manufacturers replace their dentin adhesive systems stating that their new generation obtains significantly higher bond strengths than the previous generation.
Recently, it has been shown that the degree of polymerization of the dentin bonding agents is directly correlated to bond strength (Yanagawa and Finger, 1994). Thus, if dual curing is used with a dentin adhesive, a better degree of polymerization can be expected and a higher bond strength can be achieved compared to a dentin adhesive that is only photo cured.
The aim of this study was to compare the tensile bond strength of a third-generation (XR Bond) and a fourth-generation (Optibond) dentin adhesive system, both from the same manufacturer. The influence of acid etching of dentin in the XR Bond system was also investigated. The performance of two kinds of curing (dual and photo) with a fourth-generation dentin bonding agent (Optibond) was compared, both after two hours and after seven days.
A third-generation (XR Bond) and a fourth-generation (Optibond) dentin
adhesive system were tested (Sybron/Kerr; Roumlus, MI, USA).
Sixty recently extracted non-carious human third molars were used. The teeth were stored in distilled water until testing. The enamel was removed with diamond burs and the dentin was grounded with 220 grit sandpaper. The teeth were embedded in acrylic resin, parallel to their long axis and were randomly divided into six groups (N = 10 in each group).
Group 1 (XR Bond using manufacturer’s instructions). Only in the area of dentin adhesion, XR Primer was scrubbed for 30 seconds, then air-thinned and photo cured for 20 seconds, with a Optilux II Visible Light Cure Unit (Gnatus; Ribeirão Preto, SP, Brazil). The adhesive XR Bond was applied and photo cured for 20 seconds, and a cone shaped standard polyvinyl siloxane matrix was then used for sample preparation. The minor basis of this matrix, in contact with the tooth, measured 2 mm. The composite resin, Herculite (Sybron/Kerr), was applied on the matrix in three steps, which were photo cured for 40 seconds each.
Group 2 (XR Bond with acid etching). Dentin was acid etched with 37% phosphoric acid for 20 seconds; the surface was washed with air/water spray keeping the dentin slightly moist. The application of adhesive and composite resin was carried out as for group 1.
Group 3 (Optibond, photo cure/2 hours). Acid etching of dentin was done with 37% phosphoric acid for 20 seconds, and the surface was then washed and dentin was kept slightly moist. Optibond Prime was scrubbed for 30 seconds on the etched dentin and photo cured for 20 seconds. Optibond Photo Cure Adhesive (Bottle 2) was then applied and photo cured for 20 seconds. Sample preparation was done as for groups 1 and 2.
Group 4 (Optibond, dual cure/2 hours). After the etching and washing of the dentin, Optibond Prime was scrubbed on the surface and photo cured. One drop of both bottles 3A and 3B of Optibond dual cure were mixed and applied to the surface and photo cured for 20 seconds. The composite resin was applied as for group 3.
The samples of these 4 groups were stored in distilled water for two hours at 37oC. They were then mounted in a jig and the tensile bond strength was measured in a Wolpert machine, at a crosshead speed of 0.5 mm/min (Bianchi, 1994) (see Figure 1).
Figure 1 - Graphic presentation of methodology used. A, Region of enamel and dentin removal. B, Sample prepared after embedding tooth in acrylic resin. C, Measurement of tensile strength..
Group 5 (Optibond, photo cure/7 days). This group was treated in the
same manner as Group 3. However, the samples were stored in distilled water
for 24 hours, they were then submitted to thermal cycling (700 cycles of
one min, between 5oC and 55oC) and stored in distilled water until the
period of seven days was completed, at which time the tensile bond strength
Group 6 (Optibond, dual cure/7 days). The same procedures as Group 4 were carried out for this group, but the storage of samples was the same that of Group 5. The samples were measured for tensile bond strength after seven days.
The values obtained at the moment of sample failure were transformed in MPa in accordance to their adhesion area. Statistical analysis was carried out using the ANOVA and Tukey’s tests.
Statistical analysis was divided into two steps. In the first step Groups
1, 2, 3, and 4 were compared. When the one-way ANOVA test was used, statistically
significant differences were found among groups (P<0.01). These differences
were further evaluated using the Tukey’s test, which showed that Groups
3 and 4 had similar means that were statistically superior to Groups 1
and 2. Groups 1 and 2 also had similar means (Table
In the second step, Groups 3, 4, 5, and 6 were compared. When the one-way ANOVA was used, statistically significant differences were not found among the groups (P>0.05) which showed that these groups had similar performance (Table 2).
Development of new dentin bonding agent generations has improved the
bond strength to dentin. The results of this study have showed a improvement
in adhesion (± 65%), when we compare a third-generation dentin bonding
agent (XR Bond) with a fourth-generation dentin bonding agent (Optibond),
both from the same manufacturer. The tensile bond strengths are in agreement
with those found in recent literature for both systems and the values found
for Optibond (Groups 3 and 4) are similar to the optimal values reported
by Retief et al. (1994) for preventing microleakage in composite restorations.
The difference in bond strength between XR Bond and Optibond is related to their mechanisms of adhesion to dentin. XR Primer modifies and removes, in part, the smear layer. XR Bond has to diffuse through the smear layer to reach the dentin. Likewise, a micromechanical adhesion occurs (Barkmeier and Cooley, 1992; Holtan et al., 1994).
On the other hand, Optibond requires the removal of the smear layer with acid etching. This etching also demineralizes the dentin exposing a collagen network. When the hydrophilic primer (Optibond Prime) is applied, it diffuses into peritubular and intertubular demineralized dentin. When the adhesive (Optibond) is applied, its wetting ability is improved and can encapsulate the exposed collagen fiber, forming a film referred to as a hybrid layer. Thus, high bond strength can be achieved.
Although McGuckin and Powers (1992) verified that acid etching of dentin before the application of XR Primer/XR Bond significantly improved the bond strength, in this study, when we used the XR Bond system like a fourth-generation dentin bonding agent, e.g., with acid etching of dentin, there was no increase in bond strength. Similar findings are also reported in several studies when third-generation dentin bonding agents were applied with fourth generation procedures (Barkmeier and Jeeferies, 1992; Davis et al., 1991). McHenry et al. (1991) observed a decrease in the bond strength when acid etching of dentin was performed before the application of XR Primer/XR Bond.
The similar values for the XR Bond system with and without acid etching may be related to the adhesion mechanism of the third-generation dentin bonding agent. In these systems smear layer is modified, replaced or removed in part but it is still present on the dentin surface when the adhesive is applied. With acid etching, the smear layer is totally removed and the dentin is demineralized, which allows the dentin adhesive to have a better diffusion into demineralized dentin. However, the improvement in wetting ability could be compensated for by the smear layer removal. Smear layer takes place in adhesion of the XR Bond system, thus there is no difference in adhesion between the two proceedings (Barkmeier and Cooley, 1992).
Even though different kinds of polymerization have been linked to different properties of composite resins (Fusayama, 1993; El-Badrawy and El-Mowafy, 1995), in the present study there were no differences between the two kinds of curing of the Optibond system, at both periods of two hours and seven days. In some procedures light access is difficult for curing the composite resin and thus, the composite resin polymerization may not be complete. The degree of polymerization has also been related to bond strength between resin and dentin (Yanagawa and Finger, 1994). If dual curing is available, a better conversion of free radicals takes place and better polymerization would be expected. Thus, higher bond strength could occur in groups with dual curing than in groups that are only photo cured. However, in our work this did not occur.
When the results of the different groups before and after thermal cycling and storage were analyzed a decreasing trend in the bond strength was observed, but this was not statistically significant. The decrease observed in the tensile bond strength is a usual finding after thermal cycling (Bianchi, 1994), because thermal cycling causes stress at the dentin/resin interface. New dentin adhesive systems provide stable bond strength even after long storage. If more time for the test had been allowed, this trend could have become statistically significant (Nakabayashi et al., 1995).
1. Optibond, both photo and dual cured, had higher bonding strength
to dentin than XR Bond.
2. The bond strength of XR Bond was the same with and without dentin acid etching.
3. Optibond photo cure and Optibond dual cure had similar bond strengths at both periods of time.
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Correspondence: Profa. Míriam Lacalle Turbino, Departamento de Dentística, Faculdade de Odontologia, USP, Av. Prof. Lineu Prestes, 2227, Cidade Universitária, 05508-900 São Paulo, SP, Brasil. E-mail: email@example.com
Accepted August 15, 1997
Electronic publication: October, 1998