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Brazilian Journal of Oral Sciences
Piracicaba Dental School - UNICAMP
EISSN: 1677-3225
Vol. 5, Num. 17, 2006, pp. 1028-1033

Brazilian Journal of Oral Sciences, Vol. 5, No. 17, Apr-June, 2006, pp. 1028-1033

Influence of time elapsed between application and adhesive polymerization on the bond strength to dentin

Carlos José Soares1 Gisele Rodrigues da Silva2 Rodrigo Borges Fonseca3 Paulo Sérgio Quagliatto1

1DDS, MS, PhD, Biomechanics group, Professor at Department of Operative Dentistry, Dental School, Federal University of Uberlândia, Minas Gerais, Brazil.
2DDS, Graduate Student, Dental School - Federal University of Uberlândia, Minas Gerais, Brazil.
3DDS, MS, PhD Student, Dental School–Piracicaba Dental School, State University of Campinas, Piracicaba, Brazil.
Correspondence to: Carlos José Soares Universidade Federal de Uberlândia Faculdade de Odontologia –Área de Dentística e Materiais Odontológicos Av. Pará 1720 - Bloco 2B - sala 2B24 -Campus Umuarama CEP: 38400-902 - Uberlândia/MG - Brasil Phone:+55(34)3218-2255 Fax:+55(34)3218-2279 E-mail: carlosjsoares@umuarama.ufu.br

Received for publication: June 20, 2005 Accepted: May 18, 2006

Code Number: os06019

Abstract

The aim of this study was to evaluate the influence of time elapsed between dentin adhesive application and its polymerization on the bond strength to dentin. Superficial bovine dentin received the following treatments: Single Bond (3M/ESPE Dental Products Division, St.Paul, MN, USA), Excite (Ivoclar/Vivadent, Schaan, Liechtenstein), Prime&Bond 2.1 (Dentsply/Caulk, Milford, DE, USA) and One-Up Bond F (Tokuyama Corp.,Tokyo, Japan) being polymerized immediately after its application, 10 and 20 seconds after application. After, indirect composite restoration was fixed with resinous cement RelyX Arc (3M/ESPE). The restoration/tooth interface was observed on Scanning Electron Microscopy (SEM). Bond strength was accessed through a shear bond test on a testing machine at 0.5 mm/min. Data were compared by ANOVA followed by Tukey test (α=0.05). The failure modes was observed by means of stereo-microscope (40X magnification), being classified as adhesive, dentin cohesive, resin cohesive and mixed failure. There was no significant difference on the bond strength between the 10s and 20s groups for the majority of the materials. However, both the failure modes and the SEM showed better interaction between dentin and adhesive systems after elapsed 20s its application. It was concluded that longer periods of adhesive system application resulted on better interaction between dentin and adhesive systems evaluated.

Key Words: dental materials, adhesives, dental bonding, dental restoration

Introduction

Bonding resin composites to acid-etched enamel surfaces is a clinically well-established treatment procedure in restorative dentistry. When compared to enamel, dentin is a complex hydrated biological substrate. Its morphological and functional characteristics are determinants of the quality of resin-dentin bond achieved with adhesive agents1. Sclerotic2, caries-affected and deep dentin3 have been considered unfavorable substrates for bonding. Other factors may interfere with the quality of bonding including the presence of smear layer1, the surface moisture degree4-6, the number of dentinal tubules per mm2, as well as the relative amount of intratubular and intertubular dentine3, the type of dentin, bovine or human dentin and the type of adhesive to be used4. When bonding of composite resins to dentin is considered, the sealing of the tubules, the bond strength that obtained and the post-operative sensitivity can be related to the vehicles that are used by different proprietary adhesive systems7-8. Total-etch adhesive systems usually employ one or more resin monomers associated with a solvent like water (boiling temperature 100°C), ethanol (boiling temperature 78.3°C), acetone (boiling temperature 56.5°C) or a mixture of more than one solvent. It is clear that bond strength of totaletch adhesive systems to dentin depends on a proper combination of surface moisture4-6 and solvent type9.

Some studies have shown that while water-based systems require a rather dry dentin surface, the systems where vapor pressure is high, such as the acetone-based system, require a rather wetter dentin surface for better bond strength9. When dentin is extensively air-dried, the water within the collagen matrix is removed and collagen fibrils are brought into close contact. They can now form a weak interpeptide bond that renders the matrix shrunk, stiff and practically not permeable to resin adhesives10. One way to revert such undesirable situation is to apply a solution that can break these interpeptide bonds and re-expand the matrix. Water has the highest capability of re-expanding demineralized dentin matrix11. However, water-containing adhesives can rewet the dentin structure that has been over-dried12 and so, overcome the difficulties that were mentioned for acetone-containing systems but with some risk of over-wetting13.

The self-etching materials were introduced to the dental market at a time when dentists desired easier and less technique-sensitive adhesive materials. These materials contain acidic adhesive monomers and water, so that it works as an etching as well as a primer, and thus offers simplification of the clinical procedure.

For a great number of adhesives systems manufactures recommends that light-curing should be done at least 10 to 20 seconds after application although some commercial brands suggests 30 seconds. Thus, the aim of this study was to evaluate the influence of time elapsed between dentin adhesive application and its polymerization on the bond strength to bovine dentin. The hypothesis to be tested here is that different elapsed time intervals have an influence on the effective adhesion establishment to different adhesive systems.

Material and Methods

Four commercial adhesive systems were employed on this study (Table 1), three two-step bonding systems, Single Bond (3M/ESPE Dental Products Division, St. Paul, MN, USA), Excite (Ivoclar/Vivadent, Schaan, Liechtenstein), Prime&bond 2.1 (Dentsply/Caulk, Milford, DE, USA) and one single-application self-etching system, One-Up Bond F (Tokuyama Corp.,Tokyo, Japan). One hundred and eighty freshly mandibular incisors extracted from two-to-three year old cattle were stored in thymol 0.2% at 4oC for up to two weeks and randomly assigned to twelve groups (N=15), as substitute for human teeth.

The roots of the teeth were cut-off with a double faced diamond disk (KG Sorensen, Barueri, SP, Brazil) and any tissue remnant and debris were removed. The crowns were then mounted in a cylinder with self-curing polystyrene resin (Hutchinson of Brazil, Tabuão da Serra, SP, Brazil) and their labial surface was ground with wet 180-, 320- and 600-grit SiC abrasive paper to obtain plain superficial dentin surfaces and to produce a standardized smear layer.

An adhesive tape with hole with 3mm in diameter was positioned on the flat dentin surface to demarcate the bonding region. This methodology ensured that the area where the bonding agent is applied will be standardized and it is also measurable, facilitating the obtainment of accurate bond strength values12. A condensable silicone impression was taken of a 3mm high and 3mm diameter metallic cylinder, producing a mold to build indirect composite restorations. A composite resin (TPH Spectrum, Caulk/Dentsply, Milford, DE, USA) was inserted in three increments and each one was light-polymerized for 40 seconds with a conventional halogen light curing unit XL 2500 -600mW/cm2 (3M/ESPE, Sedbauer AG, Grafenau 94475, Germany), then samples were post-polymerized in a autoclave at 110ºC for 15 minutes14. For total-etch adhesive systems, Single Bond, Prime&bond 2.1 and Excite, the specimens were etched with a phosphoric acid (37%; MagicAcid, Vigodent, Brazil) for 15 seconds and after that it was copiously washed for 30 seconds and dried with absorbent paper. Dentin surface was coated with adhesive system. For self-etching adhesive system, One-Up Bond F, the liquid of bottles A and B were mixed and applied over dentin surface. Since the manufactures of employed adhesive systems recommends light-curing after 10, 15 or 20 seconds, the polymerization was standardized after following periods, which would enable better comparison between groups: immediately after its application (negative control), 10 and 20 seconds after application, for 10 seconds with a conventional halogen light curing unit XL 2500.

The cylinders of composite resin were sandblasted with 50µm aluminum oxide for 10 seconds (Microjet, Bioart, São Carlos, SP, Brazil) and coated with a silane-coupling agent (3M/ESPE Dental Products Division, St. Paul, MN, USA)15. Dual-curing resin cement Rely X Arc (3M/ESPE Dental Products Division, St. Paul, MN, USA) was mixed and applied to the treated surface of the restorations. The restoration was set in place under a load of 500 grams for 5 minutes, the cement excesses removed with a brush and light-polymerized for 40 seconds in five different points of the bonded resin cylinder. The specimens were then stored in 37°C distilled water for 24 hours and tested in shear mode with a blade in an EMIC 2000 DL (São José dos Pinhais, SP, Brazil) testing machine at a crosshead speed of 0.5mm/min16. Shear bond strength values in MPa were calculated at the peak load at failure divided by the specimen surface area. Data were analyzed by two-way ANOVA and Tukey test (α=0.05).

The fractured specimens were examined using a stereo-microscope (Leica, Hanau, Germany) at 40X magnification. The failure modes were classified into types; cohesive failures within the resin cement or within dentin, failure across the bonding interface (adhesive failure) or mixed failure and presented as percentages.

Scanning Electron Microscopy observation (SEM)

On each group, the restoration/tooth interface was observed by Scanning Electron Microscopy (DSM 940A, Zeiss, Germany). One sample of each group was embedded in polystyrene resin, and then longitudinally sectioned with a double-faced diamond disk. The sectioned surfaces were polished in sequence with wet 600-, 1200- and 1500-grit SiC paper followed by diamond pastes (6µm, 3µm, 1µm, 0.5m and 0.25µm). The sample’s surface was etched with 37% phosphoric acid for 15 seconds, rinsed with water spray and dehydrated in ascending concentrations of ethanol (50%, 70% and 95% for 10 minutes each; 100% for 30 minutes), critical-point dried (Balzers CPD 030, Balzers Union, Liechtenstein). The samples were fixed on metallic stubs with colloidal silver, stored in silica-gel for 12 hours and sputtercoated with a thin film of gold-palladium on a sputtering system (MED 010, Balzers Union, Liechtenstein) and examined under SEM. Representative areas of the interface were photographed at 1000X and 2000X magnification.

Results

The mean shear bond strengths to bovine dentin are summarized in table 2. Data presented normal and homogeneous test distribution enabling a parametric analysis. Two-way ANOVA (α=0.05) (4x3) revealed significance in respect to the adhesive system and its application period, and also for the interaction between these factors. Tukey test (α=0.05) showed that elapsed 20 seconds adhesive application before its polymerization was not significantly greater bond strength than 10 seconds, except for Prime&bond 2.1 system, but it resulted on higher values when compared with immediate polymerization.

When considering the adhesive system alone, for the period of 20 seconds, Single Bond (18.9±3.2)a showed higher bond strength than both Prime&bond 2.1 (11.7±2.2)c and One-Up Bond F (14.7±3)bc, and similar values to Excite (17.1±5.7)ab.

When compared, SEM photomicrographs of groups of 0, 10 seconds (Figure 1) and 20 seconds after adhesive application, the latter showed better interaction between dentin and adhesive system (Figure 2). The adhesive failure mode took place for almost all samples that were submitted to light polymerization right after adhesive application. Similarly, for almost all PB 2.1 samples light polymerization after 10 or 20 seconds resulted on adhesive failures, but for OB, SB and EX, both cohesive and mixed failures could be noted (Figure 3).

Discussion

The employment of human teeth on laboratory research in order to evaluate restorative procedures has been a routine. However, the increase in dental health has meant that obtaining extracted human teeth has been difficult. As a substitute for human teeth, bovine permanent incisors have been employed on dental research17. Bovine teeth, besides being easily obtained, enable the standardization of teeth age and reduce the risks of contamination. Previous studies showed radiodensity18, number and diameter of dentinal tubules19 and bond strength20 similar between human and bovine teeth. The classical principles of Operative Dentistry have been challenged during the last two decades due to the development of adhesive technologies. However, the dentin adhesion mechanism is still evoluing. Adhesive materials may react with dentin mechanically and chemically or through both manners. Demineralization of intertubular dentin and maintenance of interfibrillar porosity is required for resin monomer infiltration into dentin and hybrid layer formation21.

The application of acetone-based or alcohol-based hydrophilic primers in single or two-bottle adhesive systems, prepares the dentinal substrate facilitating the penetration of resin monomers. The type of solvent depends both on the primer solubility and on the adhesive composition5. HEMA containing materials (2-hydroyxylethyl methacrylate) enables the employment of different types of solvents, such as water and ethanol (Single Bond), ethanol (Excite) or water (One-Up Bond F). As the volatility of the solvent increases, the adhesive system application becomes more techniquesensitive in relation to possibility of excessive dentin drying and as a consequence disintegration and collapsement of the collagen matrix can occur12. On the other hand, water, in excess, remains a micromorphological contaminant to dentin hybridization, being referred to as the overwet phenomenon13. Bond strength on dentin was affected by shorter periods, 0 and 10 seconds elapsed between Prime&bond 2.1 application and its polymerization. When acetone, which presents vapor pressure-ca-200mmHg22, tries to displace water by resin monomers the collagen matrix may get chemically dehydrated similar to a physical dehydration caused by air-drying. This fact suggests that its higher volatile solvent might had difficulty the resin monomer interlocking into the collagen matrix, especially in shorter periods of time. Since that vapor pressure of ethanol, ca-44mmHg22, and water, ca-24.0mmHg22 are shorter than its acetone, adhesives systems contained this solvents (Excite, Single Bond, One-Up Bond F) are less techniquesensitive to react with dentin in shorter periods of application. Irrespective of the adhesive system composition it was observed that the increase on the duration of application resulted on higher percentage of dentin cohesive failures and mixed failures and simultaneously, microscopically observations showed more homogeneous hybrid layers, suggesting better interaction between dentin and adhesive systems. El-Din et al.23, studied the duration of application of one- and two-bottle adhesive systems, concluding that higher periods of application produces better interlocking of resin monomers and consequently higher bond strength. Single Bond and Excite showed higher bond strength than others adhesive systems analyzed. The first contains water and ethanol; the presence of water reduces the possibility of failures related to excessive dentine drying. This product also has a polyalkenoic acid, which forms a complex with the calcium of the dentin surface and with the nozzle of the dentine tubules stabilizing the bonded interface. Similarly, Excite contains phosphate groups that interact with calcium of enamel and dentin, while methacrylate adheres to the other polymerizable components of the adhesive system.

The intensive search for an adhesive system that promotes stable and efficient bond together with better biocompatibility resulted in new adhesive technologies. Self-etching bonding systems reduced clinical steps through the elimination of rinsing and drying and, as a consequence, reduced the technique-sensitiveness. These adhesive bonding systems are characterized by the addition of polymerizable organic acids or acidic monomers, they are effective on moist dentine because water seems to be essential for the ionization of the acidic monomers and subsequent demineralization of dental hard tissues, although stated that these adhesives can be used in dried dentin because generally they have water in their composition24.

One-Up Bond F contains 11-methacryloyloxy-1, 1undecanedicarboxylic acid (MAC-10), an unsaturated tricarboxylated methacrylate monomer. It is a bi-functional acidic monomer that penetrates beyond the smear layer into intact dentin, incorporating it into the hybrid layer, thus, the possibility of incomplete infiltration into dentin is reduced. Despite the presence of short resin tags, a good seal is achieved as the smear plugs are left intact, preventing the movement of fluid, which could result in post-operative sensitivity25. In spite of the fact that lower periods of application negatively affect its bond strength, this system suffered the least effect of immediate polymerization. In addition the presence of an acidic surface over the dental substrate treated by One-Up bond F and immediately polymerized is an important aspect since tertiary amines, which are responsible for the chemical polymerization process of the resinous cement, become inhibited leading to lower bond strength values. In spite of this fact, the quality of the resin-infiltrated layer, rather than its absolute thickness is a more important criterion to produce higher bond strength because there is no correlation between hybrid layer thickness and dentin bond strength3, thus, the hybrid layer and the presence of resin tags may not be the only mechanisms influencing dentin strengths26.

It has been a challenge for Adhesive Dentistry to produce a single unit between dental structure and restorative materials. Thus, its is important to know the chemical composition and adhesion mechanism of different adhesive systems in order to establish the best technique protocol, enabling longevity of indirect restorative procedures.

Acknowledgements

The authors are indebted to Dr. Elliot W. Kitajima (NAP/ MEPA-ESALQ/USP) for SEM equipment support. We also wish to thank 3M/ESPE Dental Products, Ivoclar-Vivadent, Caulk/Dentsply and Tokuyama for donating their products, and the financial support granted by FAPEMIG/UFU(D009/ 2003).

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Copyright 2006 - Piracicaba Dental School - UNICAMP São Paulo - Brazil


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