Influence of preheating the bonding agent of a conventional three-step adhesive system and the light activated resin cement on dentin bond strength.

AIMS: to evaluate the influence of preheating the bonding agent (Scotchbond Multipurpose Adhesive/3M ESPE) and the light-activated resin cement (RelyX Venner/3M ESPE) on dentin microtensile bond strength.
MATERIALS AND METHODS: The exposed flat dentin surface of 40 human third molars were randomly distributed into four groups for cementation (SR Adoro/Ivoclar Vivadent) (n = 10): G1-bond and resin cement, both at room temperature (22°C), G2-bond preheated to 58°C and cement at room temperature (22°C), G3-bond at room temperature (22°C) and the cement preheated to 58°C, G4-bond preheated to 58°C and cement preheated to 58°C. Sticks of dentin/block set measuring approximately 1 mm(2) were obtained and used for the microtensile bond strength test. All sticks had their failure mode classified. STATISTICAL ANALYSIS USED: Factorial analysis of variance was applied, 2 × 2 (bond × cement) (P < 0.05).
RESULTS: Preheating the bonding agent (P = 0.8411) or the cement (P = 0.7155), yielded no significant difference. The interaction bond × cement was not significant (P = 0.9389).
CONCLUSIONS: Preheating the bond and/or the light-activated resin cement did not influence dentin bond strength or fracture failure mode.

INTRODUCTION

Resin materials do not undergo complete polymerization, especially when used at room temperature[1] and when they receive insufficient energy by light activation.[2] Incomplete monomer conversion may result in residual monomers and nonreactive photoinitiators, which can leak out to the saliva causing allergic reactions and bacterial build-up around restorations.[34] Monomer conversion, therefore, plays a pivotal role in clinical performance and, consequently, in the physical and mechanical properties of the polymers used in dentistry.

A higher degree of monomer conversion in resin materials can be obtained using an effective polymerization method described by Friedman in 2003, who heated the composite resin to 54-60°C prior to light activation.[5] This technique, named “preheating dental materials”, reduces the viscosity of the composite resin without changing its original composition and increases the rate of monomer conversion, producing a polymer with improved physical and mechanical properties.[16]

Preheating composite resins immediately before light-curing leads to an increased monomer conversion, which produces some beneficial effects on the resin material.[78910] Some studies have also evaluated the effects of preheating dual resin cements,[168] self-etching adhesive systems,[11] and single-bottle adhesive systems.[12] The use of preheated composite resin for cementation was also demonstrated, since it reduces viscosity and facilitates restoration fitting.[13141516] However, none of these studies used a conventional three-step adhesive system associated with a light-curing resin cement, which is the recommended method for veneer fitting. Therefore, the aim of this study was to evaluate the influence of the preheating technique on dentin bond strength, using the preheated bonding agent (bond) of a three-step system and preheated light-curing resin cement.

MATERIALS AND METHODS

Teeth preparation

This study was approved by the Ethics Committee in Research (number 2010/0373). The occlusal enamel of 40 sound human third molars were removed using a flexible diamond disk (Diamond Wafering Blades, Buehler, USA) mounted in a precision electric saw (Isomet 1000, Precision Saw, Buehler Lake Bluffilh, Hong Kong, China). The dentin surfaces were polished using grade 600 aluminum oxide sandpaper for 10 s on a metallographic polisher (Politriz Aropol2V, Arotec S. A. Ind. Com., Brazil). The apical two-thirds of the teeth were removed to expose the pulp chamber, so that it could be filled with microhybrid composite resin (Filtek Z250, shade A1, 3M ESPE, batch N108579, Saint Paul, MN, USA) following adhesive (Adper Single Bond 2, 3M ESPE, batch 8RF, Irvine, CA, USA). The teeth were randomly divided into four experimental groups [Table 1].

Table 1

Study groups, materials, and temperatures used

Indirect composite resin block preparation

An addition silicone mold (5 × 5 × 4 mm) was used (Elite Transparent, Zhermack, Rovigo, Italy) to obtain ceromer blocks. Indirect composite (SR Adoro, shade B2 Dentin, batch G26588, Ivoclar Vivadent, Shaan, Liechtenstein) was applied to the molds in 2 mm increments and light-cured for 20 s each layer (Targis Quick, Ivoclar Vivadent, Shaan, Liechtenstein, Germany). An additional polymerization was achieved using a curing oven (Targis Power, Ivoclar Vivadent, Shaan, Liechtenstein, Germany) for 25 min. The blocks were sandblasted (Microjato Bioart, Sao Carlos, SP, Brazil) with extra fine aluminum oxide, etched with 35% phosphoric acid (Ultra-Etch, batch B4CTW, Ultradent Products Inc., South Jordan, Utah, USA) and washed under running water. They were then air dried to receive silane (Dentsply, batch 207897B, Petropolis, RJ, Brazil) and bond (Scotchbond Multi-Purpose Adhesive, batch 9RK, 3M ESPE, Saint Paul, MN, USA).

Cementation

The occlusal dentin surface was etched with 35% phosphoric acid (Ultra-Etch, batch B4CTW, Ultradent Products Inc., South Jordan, Utah, USA) for 15 s and rinsed with water spray also for 15 s. The excess of water was removed using absorbent paper for up to 5 s. Primer was applied (Scotchbond Multi-Purpose Primer, batch N154838, 3M ESPE, Saint Paul, MN, USA). For the warm bond group, the bonding agent (Scotchbond Multi-Purpose Bond, batch 9RK, 3M ESPE, Saint Paul, MN, USA) was heated in its carrier compule (Accudose, Posterior HV, Centrix System/DFL, Rio de Janeiro, RJ, Brazil), which was placed in a Calset Tri Tray device (AdDent Inc., Danbury, CT, USA) in order to reach 58°C.

Bond was applied onto the prepared tooth directly from its compule, using a disposable applicator that did not require removal from the heating equipment.

The resin cement RelyX Veneer (3M ESPE, shade Opaque White, batch N145321, Saint Paul, MN, USA) was applied directly onto the composite block, which was then cemented onto the dentin. For the warm cement group, RelyX Veneer (3M ESPE, shade Opaque White, batch N145321, Saint Paul, MN, USA) was heated in a compule, which was placed in a Calset Tri Tray device until 58°C. A universal CoMax syringe (AdDent Inc., Danbury, CT, USA) was used to secure the compule for cement application. The syringe was also heated in the same Calset Tri Tray equipment to minimize heat loss during the procedure.

The blocks were positioned and light-cured for 40 s on each side (buccal, lingual, palatal, mesial, and occlusal), using a halogen curing light (Optilight Plus, Gnatus, Ribeirao Preto, SP, Brazil) set for 450 mW/cm2. The tooth/block set was placed in an incubator (ECB 1.3 digital, Odontobrás Ind. e Com. Med. Odont. Ltda, Ribeirao Preto, SP, Brazil) at 37 ± 2°C for 24 h.

Microtensile bond strength test

The tooth/block set was sectioned vertically in an apical direction using a flexible diamond disk (Diamond Wafering Blades, Buehler, USA) mounted on an electric precision saw (Isomet 1000 Precision Saw, Buehler Lake Bluffilh, Hong Kong, China), producing four parallel slices mesiodistally, four parallel slices buccal-lingual/palatally, which yielded sticks with an area of 1 mm2, approximately. The central sticks were selected for testing, whilst the peripheral ones were excluded from the sample.

Microtensile bond strength was measured on a universal testing machine (EMIC, model DL2000, São Jose dos Pinhais, PR, Brazil) using a loading cell of 20 kgf at 0.5 mm/min until failure. The values were obtained in kgf and converted into MPa according to the bonding area, for statistical analysis.

Failure mode evaluation

The area of bonding failure was visually assessed using a stereoscopic magnifier (EK3ST, Eikonal Equipamentos Ópticos e Analíticos, Sao Paulo, SP, Brazil) at 40× magnification, and it was classified into ceromer cohesive, dentin cohesive, adhesive, or mixed failure.

Statistical analysis

For each tooth, the microtensile bond strength values of all beams were averaged for statistical purposes, each tooth serving as a statistical unit. Factorial analysis of variance (ANOVA) was applied, 2 × 2 (bond × cement), with P < 0.05 (SAS Institute Inc., Cary, NC, USA, Release 9.2, 2008). The distribution of the failure modes were described according to the groups.

RESULTS

Analysis of variance revealed no significant difference between the heated and nonheated bond (P = 0.8411). No significance was detected between the heated and non-heated resin cement either (P = 0.7155). The bond vs cement interaction was also not significant (P = 0.9389) [Table 2].

Table 2

Mean bond strength values (standard deviation) for the groups, in MPa

Table 3 shows the frequency of failure mode per group. For the nonheated bond and cement group, the most frequent failure mode was dentin cohesive (39.5%) and the least frequent was mixed (2.3%). In the heated bond group, the commonest failure was adhesive (53.8%) and the least frequent was mixed (7.7%). In the heated cement group, dentin cohesive failure was the most frequent (41.6%), whereas the least frequent was adhesive (8.3%). In the heated bond and cement group, adhesive (50%) and mixed (4.2%) were the most and least frequent failure mode, respectively.

Table 3

Distribution of the failure modes (%) according to the groups

DISCUSSION

Some studies have suggested the use of a heat source to raise the temperature of the composite prior to its application.[13591617181920212223] The energy generated by preheating the composite increases the rate of collisions between the nonreactive groups and the free radicals, resulting in a more complete polymerization reaction.[2316171824] Reis et al.,[12] observed an increase in bond strength at the tooth/resin interface when heated adhesives were used. This was attributed to the higher temperature, which together with the increase in reaction rate, caused an increase in molecule collision per unit of time. According to the adhesive system used, it was observed that at higher temperatures (37-50°C), the speed of penetration into the etched dentin was significantly increased. It is possible that the difference in boiling points for the solvents from each adhesive system, such as acetone (56.5°C PB (Prime and Bond)) and ethanol (78.3°C SB (Single Bond 2)), could be responsible for the difference in behavior at higher temperatures.

The results from this study, however, have shown no significant difference in dentin bond strength using techniques either with or without heating. These findings corroborate those by Wagner et al.,[19] and Fróes-Salgado et al.,[20] who found no significance in the effect of heat on composite monomer conversion, probably due to the rapid drop in temperature whilst handling the material.

Despite the temperature of the material being constant until its use, time was needed to execute its application to the substrate. Furthermore, such materials are dispensed at very low quantities, which can cause them to reach room temperature rapidly, as identified in other similar studies.[102125]

Preheating would play a fundamental role in converting and polymerizing the cement, since the ceromer block thickness would work as a light barrier to photoactivation. Preheating it could lead to an increase in polymerization depth[11] and greater molecular mobility,[231618] thus increasing the propagation of polymer chains, and ultimately, optimizing polymerization.[1825] It has been observed that when using preheated dual cure resin cements[8] or microhybrid composite resin[13] for cementation, there was a decrease in the negative influence of restoration thickness and also an increase in the conversion rate, with some variation between different commercial brands.[8] Special care must be taken with cements containing benzoyl peroxide, since an increase in temperature may lead to the formation of free radicals, thus causing premature cure.[6] In the present study, a light-curing resin cement was selected due to an increased bond strength generated by preheating it, since this cement contains a higher amount of monomers than a composite. However, such expectation was not met, as no difference was observed between the groups tested, despite a subtle higher material fluidity in the preheated groups.

Daronch et al.,[3] reported that a composite preheated to 60°C can lose 35-40% of its heat in 40 s after being removed from the heat source. After 2 and 5 min, the drop in temperature can reach 50 and 90%, respectively.[14] In clinical conditions, the temperature of a composite preheated to 60°C is maintained merely at 6-8°C above the intraoral temperature after application.[21] It is likely that, by the time the material is light-cured, the temperature will have reached a level at which an increase in molecular mobility becomes improbable and, consequently, no optimization of mechanical properties would be achieved.[61016202425] Therefore, the present study suggests that preheating the material is an unnecessary step.