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RESEARCH REPORT |
1 Paediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, 34 Hospital Road, Hong Kong SAR, China;
2 Bisco Inc., Schaumburg, IL, USA;
3 Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, SP, Brazil; and
4 Department of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA, USA;
* corresponding author, kfctay{at}netvigator.com
| ABSTRACT |
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KEY WORDS: oxalate desensitizer acid-etch single-bottle adhesives pH fluoride.
| INTRODUCTION |
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nerve fibers (Närhi et al., 1994), which results in post-operative sensitivity. One way of relieving post-operative sensitivity clinically is the adjunctive use of oxalate desensitizers on acid-etched dentin prior to adhesive application (Pashley et al., 2001; Tay et al., 2003b). Depletion of calcium ions from the surface dentin forces the oxalate ions to diffuse further down into the dentinal tubule, until calcium ions are encountered for reaction. The calcium oxalate crystals that are formed result in subsurface tubular occlusion and reduction in the hydraulic conductance of dentin. However, the solubility of calcium oxalate is affected by pH, since the anion is the conjugate base of a weak acid (Kotz and Treichel, 1999). Preliminary screening of the compatibility of oxalate desensitizers with total-etch adhesive systems revealed complete compatibility with One-Step (Bisco) and Single Bond (3M ESPE) and poor compatibility with Prime&Bond NT (Dentsply) and OptiBond Solo Plus (Kerr). This led to a more detailed investigation seeking the reasons for compatibility vs. incompatibility. The purpose of this study was to determine if the acidity of total-etch adhesives may influence their bonding to oxalate-desensitizer-treated acid-etched dentin.
Thus, the objectives of this study were: (1) to compare the microtensile bond strengths of 4 single-bottle total-etch adhesives of different acidities to oxalate-desensitizer-treated acid-etched dentin; and (2) to examine, with the use of scanning and transmission electron microscopy, the ultrastructure of the bonded interface. The null hypothesis tested was that there is no difference in the bonding of single-bottle total-etch adhesives of different acidities to oxalate-desensitizer-treated acid-etched dentin.
| MATERIALS & METHODS |
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Experimental Design
Four single-bottle total-etch adhesives and 2 oxalate desensitizers were investigated. The adhesives were: One-Step (OS; Bisco Inc., Schaumburg, IL, USA), Single Bond (SB; 3M ESPE, St. Paul, MN, USA), OptiBond Solo Plus (OB; Kerr Co., Orange, CA, USA), and Prime&Bond NT (PB; Dentsply De Trey, Konstanz, Germany). The oxalate desensitizers were: BisBlock (BB; Bisco Inc.) and Super Seal (SS, Phoenix Dental Inc., Fenton, MI, USA). Each adhesive was divided into 3 experimental groups with 7 teeth each. Five restored teeth were used for microtensile bond strength evaluation and 2 teeth for ultrastructural examination.
Microtensile Bond Strength (µTBS)
Bonding was performed on the occlusal surfaces of deep, coronal dentin. The 3 experimental groups were:
Group 1: The surface to be bonded was etched with a 32% phosphoric acid gel (Uni-Etch, Bisco Inc.) for 15 sec and rinsed with water for 20 sec before bonding occurred.
Group 2: After the surface was acid-etched, BisBlock was applied with a rubbing motion for 30 sec and rinsed with water for 60 sec before bonding occurred.
Group 3: After the surface was acid-etched, Super Seal was applied with a rubbing motion for 30 sec and rinsed with water for 60 sec before bonding occurred.
The treated teeth were bonded according to the manufacturers instructions. Bonded surfaces were air-dried and light-cured for 10 sec. Composite build-ups were performed with the use of a light-cured composite (Z250, 3M ESPE) in 5 1-mm increments and individually light-cured for 40 sec. The teeth were stored in distilled water at 37°C for 24 hrs. Bonded teeth were then sectioned occluso-gingivally into 0.9 x 0.9-mm composite-dentin beams (Shono et al., 1999). Eight beams were retrieved from the 2 widest slabs of each tooth. Five teeth from each group yielded 40 beams for bond strength evaluation. Specimens were stressed to failure under tension in a Bencor Multi-T device (Danville Engineering, San Ramon, CA, USA) with the use of a universal testing machine, Model 4440 (Instron, Inc., Canton, MA, USA), at a crosshead speed of 1 mm/min. Beams with premature bond failure were assigned a null bond strength value and were included in the compilation of the mean bond strength. The data collected were analyzed with SigmaStat Version 2.03 (SPSS, Chicago, IL, USA). Since the µTBS data were not normally distributed (Kolmogorov-Smirnof test), the data were analyzed by Kruskal-Wallis one-way ANOVA on ranks and Dunns multiple-comparison tests, with statistical significance set at
= 0.05.
Scanning Electron Microscopy
Four representative fractured beams from each group with µTBS close to the mean bond strength of that group were selected for fractographic analysis by scanning electron microscopy (SEM). The dentin sides of the fractured specimens were air-dried, coated with gold/palladium, and examined with a SEM (Cambridge Stereoscan 440, Cambridge, UK), operating at 1020 kV.
Transmission Electron Microscopy
Two teeth from each group were acid-etched and similarly treated with the oxalate desensitizers in the manner as previously described. The teeth were sectioned into 0.9-mm slabs. The 2 widest slabs were coated with 2 layers of nail varnish applied 1 mm from the bonded interfaces. They were immersed in a 50 wt% ammoniacal silver nitrate solution for 24 hrs, according to the silver impregnation protocol reported by Tay et al.(2002). After reduction of the diamine silver ions, the silver-impregnated slabs were processed for transmission electron microscopy (TEM) without further demineralization, and were examined, unstained, by TEM (Philips EM208S, Philips, Eindhoven, The Netherlands) at 80 kV.
Measurement of pH and F Concentration of Adhesives
Water-free adhesives that are dissolved in polar solvents do not usually dissociate into ionic species. To circumvent this problem, we dispensed 2-mL specimens of each adhesive into clean glass vials containing 3 mL of 70% ethanol and 30% distilled water. The pH values of the 4 adhesive solutions were measured at ambient temperature (2225°C) by means of a digital pH meter (Model 501, Orion Research, Inc., Beverly, MA, USA). Three readings were taken for each adhesive, and the mean pH value was calculated for each adhesive.
Similarly to the pH measurement, a 2-mL specimen of each simplified-step adhesive was dispensed into a clean glass vial containing 3 mL of 70% ethanol and 30% distilled water. The solution was buffered with TISAB III (Orion Research Inc.). The fluoride concentrations were determined at ambient temperature by an ion-selective electrode (Cat. No. 96, Orion Research Inc.).
| RESULTS |
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TEM Results
Bonded interfaces from PB-SS specimens revealed extensive nanoleakage of silver within the hybrid layer, and electronlucent spherical globules with silver deposits were observed in the adhesive layer. Discontinuous electronlucent spaces could be identified along the adhesive-dentin interface (Fig. 2A
). Nanoleakage was also observed within the entire hybrid layer from OB-BB (Fig. 2B
). The dentinal tubules were filled with silver deposits, with no subsurface oxalate crystals identified. Discontinuous spaces with scalloped margins were found between the hybrid and adhesive layers in OB-BB. No major differences could be recognized between the 2 oxalate desensitizers after the application of OB and PB. In contrast to OB and PB, isolated silver deposits were found in only some areas of the hybrid layer of SB-SS (Fig. 2C
) and OS-BB (Fig. 2D
).
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| DISCUSSION |
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The bond strengths of OS and SB were not compromised with the adjunctive use of the oxalate desensitizers. Optimal resin infiltration and hybrid layer formation occurred in the presence of bi-pyramidal calcium oxalate dihydrate crystals in the dentinal tubules. These crystals can be seen more clearly in cryofractured specimens (Fig. 3
). In contrast, significantly lower bond strengths were observed in OB and PB when applied to oxalate-desensitizer-treated dentin, that could be attributed to the presence of spherical globules along the adhesive-dentin interface. These spherical globules closely resemble the loosely bound spherical calcium fluoride (CaF2)-like material formed on enamel (Dijkman et al., 1983; Nelson et al., 1983, 1984) and demineralized dentin surfaces (Itota et al., 2002) following topical fluoride treatment. These spherical globules were removed by the KOH extraction method (Caslavska et al., 1975), confirming that the globules were KOH-soluble fluoride (Appendix). Regardless of the composition of the spherical globules found on the surfaces of oxalate-treated specimens bonded with the 2 incompatible adhesive systems (OB and PB), their presence at the bonded interface and in the adhesive layer could serve as stress-raisers that would create debonding at lower stresses than would occur in their absence. TEM examination of the bonded interface also revealed discontinuous electronlucent spaces with distinct "scalloped" margins. These were not true gaps along the adhesive-dentin interfaces, since they were not infiltrated by silver deposits. The "scalloped" margins correlated nicely with the shapes of spherical globules.
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Although the presence of fluoride ion is necessary for formation of the spherical globules in OB- and PB-bonded specimens, the availability of calcium ions on the dentin surface is also critical. Since the dentin surface was completely deprived of calcium phosphate following phosphoric-acid-etching (Van Meerbeek et al., 2003), the calcium ions could have been derived from the dissolution of calcium oxalate crystals in the dentinal tubules. The solubility of calcium oxalate (CaC2O4) is affected by pH, since the anion is the conjugate base of a weak acid. The low pH values of OB and PB may increase the solubility of calcium oxalates in the dentinal tubules. According to Le Châteliers principle (Silberberg, 2003), as calcium oxalate crystals are exposed to more H3O+, more calcium oxalate dissolves into calcium and oxalate ions, to compensate for the depletion of oxalate ions and maintain the equilibrium constant. This is supported by the SEM observation of considerably fewer, and smaller, calcium oxalate crystals in the dentinal tubules following the application of OB and PB to oxalate-desensitizer-treated dentin.
In conclusion, the results of the present in vitro study indicated that the bond strength of simplified total-etch adhesives to oxalate-desensitizer-treated acid-etched dentin may be compromised by the acidity and the availability of fluorides from some total-etch adhesives. In vivo studies are needed to confirm these in vitro observations. Clinicians should be aware of the potential drop in bond strength with the use of PB and OB on oxalate-desensitizer-treated acid-etched dentin.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Received August 27, 2004; Last revision May 12, 2005; Accepted May 13, 2005
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