Effect of pre-heated dual-cured resin cements on the bond strength of indirect restorations to dentin

Abstract

This study evaluated the effects of resin luting agents (LA) polymerized using increased temperature on the in vitro microtensile bond strength (mTBS) of indirect restorations to dentin. The occlusal dentin surfaces of 40 human third molars were exposed and flattened. The teeth were assigned to 8 groups (n = 5) according to the LA temperature (25°C o r 50°C), curing mode (dual- or self-curing mode), and product (Excite DSC/Variolink II [VII] and XP Bond/Calibra [Cal]). The bonding agents were applied to the dentin surfaces according to manufacturers' instructions. For preheated groups, the LAs were heated to 50°C, subsequently mixed on a heated stirrer surface, and applied to the previously heated pre-polymerized resin discs (2 mm thickness, TPH-Spectrum). The discs were bonded to the dentin surfaces, and the LAs were either exposed to a curing light according to manufacturers' instructions or allowed to self-cure. Specimens were stored in relative humidity at 37°C for 7 days. Specimens were mesio-distally and bucco-lingually sectioned to obtain multiple bonded beams with a 1-mm² cross-sectional area for mTBS testing. Data (MPa) were analyzed by 2-way ANOVA and Tukey's post hoc test (a = 5%) for each product. Specimen failure patterns were analyzed using a scanning electron microscope. VII groups showed higher mTBS at 50°C than at 25°C regardless of curing mode (p = 0.05). Cal groups showed similar mTBS at 25°C and 50°C in all activation modes. The use of some dual-polymerizing LAs at 50°C may improve the mTBS of indirect restorations to dentin.

Resin Cements; Temperature; Dental Bonding

RESTORATIVE DENTISTRY

Effect of pre-heated dual-cured resin cements on the bond strength of indirect restorations to dentin

Alexandre Morais^I; Alline Rachid Abreu dos Santos^I; Marcelo Giannini^II; Andre Figueiredo Reis^I; José Augusto Rodrigues^I; César Augusto Galvão Arrais^I

^IDepartment of Restorative Dentistry, Dental Research Division, Guarulhos University – UnG, Guarulhos, SP, Brazil

^IIDepartment of Operative Dentistry, University of Campinas, Piracicaba School of Dentistry, Piracicaba, SP, Brazil

^{Corresponding author}

ABSTRACT

This study evaluated the effects of resin luting agents (LA) polymerized using increased temperature on the in vitro microtensile bond strength (mTBS) of indirect restorations to dentin. The occlusal dentin surfaces of 40 human third molars were exposed and flattened. The teeth were assigned to 8 groups (n = 5) according to the LA temperature (25°C o r 50°C), curing mode (dual- or self-curing mode), and product (Excite DSC/Variolink II [VII] and XP Bond/Calibra [Cal]). The bonding agents were applied to the dentin surfaces according to manufacturers' instructions. For preheated groups, the LAs were heated to 50°C, subsequently mixed on a heated stirrer surface, and applied to the previously heated pre-polymerized resin discs (2 mm thickness, TPH-Spectrum). The discs were bonded to the dentin surfaces, and the LAs were either exposed to a curing light according to manufacturers' instructions or allowed to self-cure. Specimens were stored in relative humidity at 37°C for 7 days. Specimens were mesio-distally and bucco-lingually sectioned to obtain multiple bonded beams with a 1-mm² cross-sectional area for mTBS testing. Data (MPa) were analyzed by 2-way ANOVA and Tukey's post hoc test (a = 5%) for each product. Specimen failure patterns were analyzed using a scanning electron microscope. VII groups showed higher mTBS at 50°C than at 25°C regardless of curing mode (p = 0.05). Cal groups showed similar mTBS at 25°C and 50°C in all activation modes. The use of some dual-polymerizing LAs at 50°C may improve the mTBS of indirect restorations to dentin.

Descriptors: Resin Cements; Temperature; Dental Bonding.

Introduction

Dual-polymerizing resin luting agents (LAs) are widely used to bond esthetic indirect restorations because of their low solubility, low viscosity, clinically acceptable film thickness, better mechanical properties than conventional cements, ability to bond tooth to restorative material when used with bonding agents, and lower microleakage compared to other luting materials.^1-3 In addition to the presence of photoiniators in their composition, self-curing components are added to ensure an optimal polymerization even when activating light is severely attenuated due to the presence of an indirect restoration interposed between LA and light-curing unit tip.^4-8 However, these components are not as effective as the dual-curing mode to provide an optimal polymerization.^4,5,7,9,10 Such a low effectiveness is related to the slow activation and propagation rates of polymerization observed in the self-curing mechanism.¹¹

Composite resin preheating to 60°C prior to resin polymerization decreases the resin viscosity and allows for an increased free radical mobility.^12-15 Consequently, these materials achieve higher monomer conversion at high temperature than when they are achieved at room temperature.^12,13,16-18 When monomer conversion is closely related to the polymer mechanical properties,^19-21 improved bond strength of indirect restorations to dentin would be expected even when LA polymerization relies solely on self-curing components. Although concerns about the possible effects of temperature on pulp may arise, previous findings have demonstrated that any increase in composite resin temperature to 54°C and 60°C does not significantly change the intrapulpal temperatures.¹⁸ However, few studies have evaluated the effects of preheating resin LAs on the bond strength of indirect restorations to dentin.^22,23

The aim of this study was to evaluate the effects of preheating dual-polymerizing LAs on the bond strength of indirect restorations to dentin. The research hypothesis was that preheated LAs provide greater bond strength to dentin than cements used at room temperature regardless of activating mode.

Methodology

Indirect restorative procedures

Forty freshly extracted, erupted human third molars were used. The research protocol was approved by the Human Assurance Committee of Guarulhos University (90/09). Teeth were stored in saturated thymol (Symrise GmbH & Co, Holzminden, Germany) at 5°C for no longer than 3 months and were sectioned perpendicular to the long axis using double-face diamond discs (7020; KG Sorensen, Cotia, Brazil) to expose middle-depth occlusal dentin surface. The dentin surfaces were wet ground (Arotec, Cotia, Brazil) with 600-grit SiC papers (Carborundum; Saint-Gobain Abrasivos, Guarulhos, Brazil) to create a flat surface with a standard smear layer.

Forty pre-cured resin composite discs (2-mm thickness and 10-mm diameter; A2 shade, TPH Spectrum; Dentsply Caulk, Milford, DE, USA) were created to simulate overlying laboratory-processed composite resin restorations that had one surface airborne-particle abraded using 50-mm aluminum oxide particles (Asfer Industria Quimica Ltd., Sao Caetano do Sul, Brazil) for 10 s (distance from the tip: 1.5 cm; Bio-Art, São Carlos, Brazil). The dentin surfaces were acid etched with 35% phosphoric acid (Brazil Dentsply, Rio de Janeiro, Brazil) for 15 s and thoroughly water-rinsed. The excess water was removed using an absorbent paper. Two dual-polymerizing cementing systems (bonding agents / LAs) were used (Table 1):

• Excite DSC/Variolink II (Ivoclar Vivadent, Schann, Liechtenstein) and

• XP Bond/self-cure activator/Calibra (Dentsply Caulk, Milford, USA).

The adhesive systems were applied and light activated (Optilux 501; power density: 650 mW/cm², Demetron Kerr, Danbury, CT, USA) according to the manufacturers' instructions. Curing light intensity was continuously measured using a radiometer (Cure Rite, Dentsply Caulk). The LAs were applied at 25°C or were heated to 50°C prior to their use and were exposed to light from the same light-curing unit used to polymerize the bonding agents (dual-curing mode), or were allowed to self-cure (self-curing mode), resulting in 8 experimental groups (n = 5). For experimental groups using LAs heated to 50°C, base and catalyst pastes were equally dispensed on a glass plate laying on a heating stirrer surface (Cientec, Piracicaba, Brazil) that was previously set to 50°C. Cement and glass plate temperatures were continuously measured using a K-type thermocouple (Novus, Porto Alegre, Brazil) to ensure that both pastes reached 50°C. Based on the temperature control as measured using the thermocouple, it took approximately 30 s to 40 s for the LAs to reach 50°C. Only a small portion of the LA to be used for the cementing procedure was heated immediately before use; therefore, only one LA was heated at a time.

The resin composite discs were also placed on the heated plate for approximately 1 min; therefore, both LA and indirect restoration had the same temperature during cementation. The LAs were mixed on the heated glass plate and were applied to abraded resin disc surfaces. For the room temperature groups (25°C), the same procedures were performed without heat. For the dual-cured groups, the disc was positioned and fixed to the adhesive-coated dentin surfaces under a 500-g load for 5 s, and a light-polymerizing unit tip was placed against it. The LA layer was then exposed to a curing light for 40 s. For the self-cured groups, the disc laying on the adhesive-coated dentin surface was subjected to the same load for 5 min. A 3-mm layer of self-curing composite resin (Alpha Plast, DFL Industria e Comercio S.A., Rio de Janeiro, Brazil) was placed on an untreated, pre-cured resin disc surface to facilitate specimen gripping during the testing process.

Microtensile bond strength test (mTBS)

The restored teeth were dark-stored in relative humidity at 37°C for 7 days. The teeth were sectioned vertically and serially into several 1.0-mm slabs using a diamond saw (Buehler Ltd, Lake Bluff, USA). Each slab was further sectioned to produce beams with a cross-sectional area of approximately 1.0 mm² at the bonded interface. Each bonded stick was attached to the testing jig grips with cyanoacrylate glue (Loctite Super Bonder Gel; Henkel, Düsseldorf, Germany) for mTBS testing using a universal testing machine (EZ Test; Shimadzu Co., Kyoto, Japan) at a crosshead speed of 1 mm/minute until failure. After testing, the specimens were carefully removed from the fixtures with a scalpel blade, and the cross-sectional area at the site of the fracture was measured to the nearest 0.01 mm using a digital micrometer (Mitutoyo America Corp., Aurora, IL, USA). Specimen cross-sectional area was calculated to produce mTBS data in units of stress (MPa). Five beams were tested per tooth, and mTBS average obtained from these beams represented the value for each restored tooth. Therefore, the experimental unit was the restored tooth.

Statistical analysis

Two-way analysis of variance (ANOVA) (temperature and polymerizing mode factors) was performed for each product. The statistical tests indicating significant differences were followed by Tukey HSD post hoc test at a preset alpha of 5%. The statistical analyses were performed using software (SAS for Windows V8; SAS Institute, Cary, NC, USA). Post hoc power analysis was performed for the statistical analysis of the mTBS data using statistical software (IBM SPSS 19, SPSS Inc., IBM Company, Armonk, NY, USA).

Failure mode analysis

All fractured surfaces of the tested specimens were allowed to air-dry overnight at 37°C, sputter coated (MED 010-Balzers, Balzers, Liechtenstein) with a thin gold layer, and examined using a scanning electron microscope (SEM - LEO 435 VP; LEO Electron Microscopy Ltd, Cambridge, UK). The failure patterns were classified as follows:

• adhesive along the dentin surface,

• cohesive within the dentin,

• cohesive within the LA and

• mixed, when simultaneously exhibiting the remnants of both the hybrid layer and the LA.

Results

Microtensile bond strength

Post hoc power analysis demonstrated statistical power greater than 95% at a pre-set alpha of 0.05. For Calibra, no significant differences in any factors were detected by ANOVA. In contrast, for Variolink II, ANOVA detected significant differences for the temperature (p = 0.05) and activation mode (p < 0.0001) factors. No significant difference in mTBS values was noted between teeth that were restored using Calibra at 50°C and 25°C, regardless of the activation mode (Table 2). Additionally, for all temperatures, no significant mTBS difference was observed between the dual- and the self-curing groups. In contrast, Variolink II at 50°C promoted higher mTBS values than at 25°C in both activation modes (p = 0.05), whereas the dual-curing mode promoted higher mTBS values than the self-curing mode regardless of temperature (p < 0.0001).

Failure pattern analysis

The distribution of failure modes among groups is shown in Figure 1. Figures 2A and 2B show SEM images of a mixed failure that was predominantly observed in the experimental groups. Only dual-polymerizing Calibra groups polymerized at 25°C exhibited failures predominantly located within the dentin. A decrease in cohesive failures within dentin was observed when Calibra was heated to 50°C. For Variolink II, the cohesive failures within dentin in both the dual-polymerizing and auto-polymerized groups at 25°C were eliminated when both groups were polymerized at 50°C.

Discussion

In this study, the temperature of 50°C was chosen based on previously reported data.^12,13,18 However, it must be emphasized that a temperature decline is expected, similar to the temperature decline observed in the preheated composite resins that are placed in tooth cavities.¹⁶ For this reason, the indirect restoration was also heated to delay any reduction in the LA temperature. Therefore, the present results should not be extrapolated to clinical situations in which the LA is heated and the indirect restoration is maintained at room temperature. Moreover, any comparison between products was neglected because the evaluated resin cements use different bonding agents. For this reason, if product comparisons were made, the possible differences in the mTBS values could be related to the differences in bonding agent compositions rather than to increased LA temperature.

Variolink II at 50°C promoted higher mTBS than at 25°C in both curing modes. Therefore, the research hypothesis was accepted for this product regardless of the activation mode. This finding is in agreement with those of studies that evaluated preheated LAs,^22,23 and demonstrates that the previously shown²⁴ increased LA monomer conversion and improved mechanical properties do increase bond strength to dentin. Possibly, decreased resin viscosity and high radical mobility were not the only factors contributing to the greater bond strength. The self-curing component benzoyl peroxide (BPO) from Variolink II is unstable and can be activated by heat.^25,26 Thus, BPO decomposes faster at higher temperatures and creates more radicals more rapidly than it does at room temperature.²⁶ As a consequence, more radical formation may have contributed to the increased mTBS values. Moreover, the use of this LA at a high temperature also reduced the percentage of the cohesive failures within the dentin. Therefore, a high temperature could have influenced the mechanical properties at the bonding interface below the LA layer. However, further studies evaluating such effects are needed to confirm this observation.

Regardless of temperature, lower mTBS was observed when Variolink II relied solely on the self-polymerizing components rather than when the LA was exposed to curing light. Therefore, the self-curing mode was less effective than the dual-curing mode, even at a high temperature. This finding might be attributed to the low BPO content in Variolink II (information from the manufacturer's material safety data sheet), which explains why this product is not indicated in clinical situations in which the activating light is entirely blocked by an indirect restoration.¹⁰

Conversely, Calibra was not influenced by heat because no significant difference in the mTBS values was noted between the groups polymerized at 25°C and 50°C. Therefore, the research hypothesis was rejected for this LA. Curiously, at 25°C, no significant difference in the mTBS was noted between the dual- and self-curing groups, indicating that the self-curing mode in Calibra was as effective as the dual-curing mode. This finding might be attributed to the higher BPO content (approximately 2%) in Calibra compared to Variolink II (approximately 1%), according to the manufacturers' material safety data sheets. Because an increased temperature decomposes BPO more quickly, LAs with higher BPO content at a high temperature may set faster than those with low BPO content. This was observed during specimen preparation when Calibra started setting immediately after the indirect restoration was cemented onto the tooth. Therefore, the polymer network may have been rigid during the light exposure. Thus, any possible benefit from a high temperature on the bond strength was masked by the tension created at the bonding interfaces caused by the premature LA setting. This observation confirmed previous findings.^22,23 For this reason, the use of Calibra at 50°C is concerning because its working time was apparently not sufficient to allow for a proper seating of the indirect restoration on the prepared tooth. For this reason, manufacturers should provide more detailed information regarding the content of the self-curing components to allow clinicians to choose when to heat LAs prior to their application.

Conclusion

An increased temperature prior to polymerization can promote higher mTBS to dentin in indirect restorations. However, the effectiveness of LA preheating on mTBS was product-dependent because these effects were observed in the LAs with a low content of self-curing components.

Received for publication on Oct 27, 2011

Accepted for publication on Jan 04, 2012

Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.

• 1.  Hill EE, Lott J. A clinically focused discussion of luting materials. Aust Dent J. 2011 Jun;56 Suppl 1:67-76.
• 2.  Fonseca RG, Santos JG, Adabo GL. Influence of activation modes on diametral tensile strength of dual-curing resin cements. Braz Oral Res. 2005 Oct-Dec;19(4):267-71.
• 3.  Leinfelder KF. Indirect posterior composite resins. Compend Contin Educ Dent. 2005 Jul;26(7):495-503.
• 4.  Arrais CA, Giannini M, Rueggeberg FA. Kinetic analysis of monomer conversion in auto- and dual-polymerizing modes of commercial resin luting cements. J Prosthet Dent. 2009 Feb;101(2):128-36.
• 5.  Braga RR, Cesar PF, Gonzaga CC. Mechanical properties of resin cements with different activation modes. J Oral Rehabil. 2002 Mar;29(3):257-62.
• 6.  El-Mowafy OM, Rubo MH. Influence of composite inlay/onlay thickness on hardening of dual-cured resin cements. J Can Dent Assoc. 2000 Mar;66(3):147.
• 7.  Arrais CA, Rueggeberg FA, Waller JL, de Goes MF, Giannini M. Effect of curing mode on the polymerization characteristics of dual-cured resin cement systems. J Dent. 2008 Jun;36(6):418-26.
• 8.  el-Mowafy OM, Rubo MH, el-Badrawy WA. Hardening of new resin cements cured through a ceramic inlay. Oper Dent. 1999 Jan-Feb;24(1):38-44.
• 9.  Pereira SG, Fulgencio R, Nunes TG, Toledano M, Osorio R, Carvalho RM. Effect of curing protocol on the polymerization of dual-cured resin cements. Dent Mater. 2010 Jul;26(7):710-8.
• 10. Pegoraro TA, da Silva NR, Carvalho RM. Cements for use in esthetic dentistry. Dent Clin North Am. 2007 Apr;51(2):453-71, x.
• 11. Warnock RD, Rueggeberg FA. Curing kinetics of a photo-polymerized dental sealant. Am J Dent. 2004 Dec;17(6):457-61.
• 12. Daronch M, Rueggeberg FA, De Goes MF. Monomer conversion of pre-heated composite. J Dent Res. 2005 Jul;84(7):663-7.
• 13. Daronch M, Rueggeberg FA, De Goes MF, Giudici R. Polymerization kinetics of pre-heated composite. J Dent Res. 2006 Jan;85(1):38-43.
• 14. Deb S, Di Silvio L, Mackler HE, Millar BJ. Pre-warming of dental composites. Dent Mater. 2011 Apr;27(4):e51-9.
• 15. Froes-Salgado NR, Silva LM, Kawano Y, Francci C, Reis A, Loguercio AD. Composite pre-heating: effects on marginal adaptation, degree of conversion and mechanical properties. Dent Mater. 2010 Sep;26(9):908-14.
• 16. Daronch M, Rueggeberg FA, Moss L, de Goes MF. Clinically relevant issues related to preheating composites. J Esthet Restor Dent. 2006 18(6):340-50; discussion 51.
• 17. Blalock JS, Holmes RG, Rueggeberg FA. Effect of temperature on unpolymerized composite resin film thickness. J Prosthet Dent. 2006 Dec;96(6):424-32.
• 18. Daronch M, Rueggeberg FA, Hall G, De Goes MF. Effect of composite temperature on in vitro intrapulpal temperature rise. Dent Mater. 2007 Oct;23(10):1283-8.
• 19. Ferracane JL. Correlation between hardness and degree of conversion during the setting reaction of unfilled dental restorative resins. Dent Mater. 1985 Feb;1(1):11-4.
• 20. Ferracane JL, Berge HX, Condon JR. In vitro aging of dental composites in water--effect of degree of conversion, filler volume, and filler/matrix coupling. J Biomed Mater Res. 1998 Dec 5;42(3):465-72.
• 21. Peutzfeldt A. Dual-cure resin cements: in vitro wear and effect of quantity of remaining double bonds, filler volume, and light curing. Acta Odontol Scand. 1995 Feb;53(1):29-34.
• 22. Cantoro A, Goracci C, Carvalho CA, Coniglio I, Ferrari M. Bonding potential of self-adhesive luting agents used at different temperatures to lute composite onlays. J Dent. 2009 Jun;37(6):454-61.
• 23. Cantoro A, Goracci C, Papacchini F, Mazzitelli C, Fadda GM, Ferrari M. Effect of pre-cure temperature on the bonding potential of self-etch and self-adhesive resin cements. Dent Mater. 2008 May;24(5):577-83.
• 24. Franca FA, Oliveira M, Rodrigues JA, Arrais CA. Pre-heated dual-cured resin cements: analysis of the degree of conversion and ultimate tensile strength. Braz Oral Res. 2010 Apr;25(2):174-9.
• 25. Smith DC, Bains ME. The acrylic denture base - some effects of residual monomer and peroxide. British Dental Journal. 1959;106:331-6.
• 26. Zaman F, Beezer AE, Mitchell JC, Clarkson Q, Elliot J, Davis AF, et al. The stability of benzoyl peroxide by isothermal microcalorimetry. Int J Pharm. 2001 Oct 4;227(1-2):133-7.

Publication Dates

• Publication in this collection
  10 Apr 2012
• Date of issue
  Apr 2012

History

• Received
  27 Oct 2011
• Accepted
  04 Jan 2012