INTRODUCTION

Most current dental composite resins are methacrylate-based monomers, such as bisphenol-A glycidyl methacrylate (Bis-GMA), urethane dimethacrylate (UDMA), and triethylene glycol dimethacrylate (TEGDMA), with well-known high volumetric shrinkage¹ ranging from 1.9 to 3.5 vol%.² Light-cured restorative materials exhibit a significant percentage of unreacted methacrylate groups due to incomplete conversion of the C=C bonds.³ However, a higher monomer conversion rate results in greater shrinkage strain.⁴ Polymerization stress can result in many adverse effects, such as cuspal deflection,⁵ debonding at the adhesive interface, postoperative sensitivity,^6,7 microleakage,⁶ secondary caries, marginal discoloration, and restoration and dental fractures.⁷

A low-shrinkage monomer, silorane, was developed from the reaction of oxirane and siloxane molecules.^5,8,9 Methacrylate polymerizes by the free-radical cure, while silorane chemistry presents a cationic ring-opening polymerization mechanism,^5,8 and more light-curing time is needed to initiate the polymerization reaction by cation formation.^1,5,10 Silorane exhibits low polymerization shrinkage,^5,8,11 less than 1 vol%,² and the mechanical properties are comparable to those of conventional methacrylate dental composites.⁹ Silorane composite is used with a dedicated two-step self-etch adhesive system; both primer and bond agents are light-cured, creating distinct layers.¹

During restorative procedures, the distance between the guide tip of the light-curing unit and the resinous material surface in deep cavities reduces the irradiance that reaches the restorative material and decreases the monomer conversion and physical properties.¹² The increase in curing time improves monomer conversion into polymers, thereby improving the physical properties of the material.^4,13,14

Nanoindentation is a reliable technique to obtain site specific nanohardness (H) and reduced elastic modulus (Er) of the resin-dentin interface components.¹⁵ This technique would allow users to determine how extended light-curing time affects the properties of individual components of the resin-dentin bonded interface of different resin-based dental restorative materials.

The objective of this study was to evaluate the influence of different monomers and curing time on the H and Er of resin-dentin bonded interface components after 24 hours and six months of water storage. The null hypotheses tested were that (1) there would be no difference between the two distinct resin-based materials, (2) there would be no difference in how the light-curing times affect H and E_r, and (3) the aging would not affect the mechanical properties tested.

METHODS AND MATERIALS

The local Institutional Review Board approved the study protocol (#031/2010) for use of extracted human molars. Twelve freshly noncarious, unrestored human molars were collected and stored in 0.1% thymol solution at 4°C. The teeth were cleaned, stored in distilled water at 4°C, and used within three months after extraction.

The roots were embedded in polystyrene resin (Piraglass, Piracicaba, Brazil), and the occlusal surfaces were ground with 320-grit silicon carbide (SiC) grinding paper (CarbiMet 2 Abrasive Discs, Buehler Inc, Lake Bluff, IL, USA) under running water (APL-4, Arotec, São Paulo, SP, Brazil) until the distance between the occlusal surface and the cementum-enamel junction was 5 mm. Standardized Class II slot preparations with cervical margins in root dentin/cementum were performed in one of the interproximal surfaces of each molar using a high-speed diamond bur (No. 3100, KG Sorensen, Barueri, SP, Brazil) under water spray to final dimensions of 4 mm width, 6 mm height (1 mm below the cementum-enamel junction), and 2 mm axial depth. A custom-made preparation device allowed the standardization of the cavity dimensions. Each bur was used to cut three preparations.

The composition of the restorative materials is shown in Table 1. Two composite resins (Filtek Z250 and Filtek LS) and two self-etching adhesive systems (Clearfil SE Bond and Filtek LS Adhesive) were used to restore the cavities.

Table 1:  Materials Used

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The preparations were randomized into four treatments, and nanoindentation measurements were performed after 24 hours and six months of storage on the same specimen, for a total of eight experimental groups (n=3) (Table 2). Restoration procedures were carried out using the following protocols. For the methacrylate groups, Clearfil SE Bond primer (bottle A) was first vigorously scrubbed with applicator brushes for 20 seconds; a mild air stream was applied for solvent volatilization; then a coat of adhesive resin (bottle B) was applied, followed by a gentle air stream, and light-curing for 10 seconds (G1 and G5) or 20 seconds (G2 and G6). For the silorane groups, Filtek LS Adhesive primer (bottle 1) was actively applied for 15 seconds, a mild air stream was applied, and then it was cured for 10 seconds (G3 and G7) or 20 seconds (G4 and G8). Afterward, adhesive resin (bottle 2) was applied, followed by a gentle air stream, and light-curing for 10 seconds or 20 seconds. The composite resins were incrementally placed using three horizontal layers (each approximately 2 mm) and light-cured for 20 seconds or 40 seconds, according to experimental groups (Table 2).

Table 2:  Experimental Groups

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Resin-based materials were light-cured from the occlusal surface using a second-generation light-emitting diode (LED) unit (Bluephase 16i, Vivadent, Bürs, Austria) at 1390 mW/cm² of irradiance (at 0 mm). The output light power (mW) was measured with a power meter (Ophir Optronics, Har-Hotzvim, Jerusalem, Israel). The tip diameter was measured with a digital caliper (Mitutoyo Sul Americana, Suzano, SP, Brazil) and recorded as 7 mm; the tip area was determined in centimeters squared. Irradiance (mW/cm²) was calculated by dividing the output light power by the tip area. Irradiance was also calculated by positioning a spacer device (with heights of 4 mm and 6 mm) between the light guide tip of the curing unit and the surface of the power meter as well as beneath the resin disks for both composites (2 mm thick, simulating the first increment) and 4 mm from the top surface of the resin disk. The distance between the tip of the light-curing unit and the bottom of the cavity was 6 mm, resulting in 610 mW/cm² of irradiance. The increments of composite were approximately 2 mm thick, the total irradiance of the top surface of the first composite increment was 990 mW/cm² (4 mm distance between the tip of the light-curing device and the top surface of the first composite increment). The irradiance at the bottom surface at 6 mm (beneath the composite resin at 2 mm thickness) was 380 ± 5 mW/cm².

The restoration was cut in half, and one of the halves was embedded in epoxy resin (Buehler Inc) and polished manually with 800-, 1000-, and 1200-grit SiC grinding paper (CarbiMet 2 Abrasive Discs, Buehler Inc) under running water. A standardized metallographic polishing technique was used, the specimens were polished to a mirrorlike finish with polycrystalline diamond suspensions of grades 9, 6, 3, and 1 μm (MetaDi Supreme, Buehler Inc), and 0.05 μm alumina suspension polish (MasterPrep, Buehler Inc) on soft polishing pads (MicroCloth, Buehler Inc). Between each polishing stage, specimens were cleaned using an ultrasonic cleaner (CD-4800, Practical Systems Inc., Odessa, FL, USA) with distilled water for five minutes.

Nanoindentation measurements were performed in the restoration components with a Berkovich fluid tip attached to a TI 700 Ubi-1 nanoindenter (Hysitron Inc, Minneapolis, MN, USA). The bottom of the block of epoxy resin was fixed on a metal disc and stabilized on the equipment platform with magnets. Wax was placed around the boundaries of the block and filled with Hanks' balanced salt solution (BioWhittaker, Lonza Walkersville Inc, Walkersville, MD, USA) to keep the specimen hydrated throughout the testing procedure.

Three regions were selected visually using an optical microscope coupled to the equipment. A three-axis piezo scanner (TriboScan, Hysitron Inc) was attached to the microscope to control the tip positioning and the in situ scanning probe imaging; the load-displacement transducer with a probe attached was used to indent the specimen while collecting the load-displacement data. Figure 1 shows the four aggregated images, two topography forward images of the silorane and two gradient forward imaging modes of the methacrylate restorative system. The specimens were stored in Hank's solution for six months at 37°C, changed weekly,¹⁶ and remeasured.

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An elongated pyramidal Berkovich fluid diamond tip (curvature radius ≈ 100 nm, Hysitron Inc) was used for imaging and testing on the hydrated samples. At each of the three regions previously selected with the optical microscope, three indentations were made with maximum load values of 100 μN for the hybrid layer and 1200 μN for the other components of the restoration (dentin, layer of the adhesive, and composite) under a standard trapezoidal load function of 10-40-10 seconds. Nine indentations were made in each component per specimen (n=3). The indentation load-displacement data collected were used to calculate the H and E_r by the TriboScope software (version 8.2.0.14, Hysitron Inc), using the Oliver-Pharr method.¹⁷ The H and E_r averages of the nine measurements were used to determine the individual properties of the dentin, hybrid layer, adhesive, and composite resin of each specimen.

Nanomechanical properties readings after 24 hours and six months were performed in the same specimen, and proc-mixed analysis of variance (ANOVA) was used for repeated measures. The H and E_r data for dentin, hybrid layer, adhesive, and composite were analyzed by three-way ANOVA (factors were material, curing time, and aging) and Tukey's test (alpha=0.05). The H and E_r of the bond layer of the Filtek LS System Adhesive were subjected to two-way ANOVA (factors were curing time and aging) and Tukey's test at a preset alpha of 0.05; because this layer was only for the silorane adhesive system, silorane- and methacrylate-based materials were not compared.

RESULTS

ANOVA showed no interaction for any factor studied (p>0.05). The H and Er data are exhibited in Tables 3 and 4, respectively.

Table 3:  Nanohardness (H [MPa]) of the Restoration Components According to Light-curing Time, Restorative System, and Aging

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Table 4:  Reduced Elastic Modulus (E_r [GPa]) of the Restoration Components According to Light-curing Time, Restorative System, and Aging

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The H and E_r values of the methacrylate resin systems were higher than those for the silorane-based materials (p≤0.05), except for the Er of the adhesive at 24 hours for both curing times (p>0.05). Mechanical properties of the underlying intertubular dentin were not influenced by the material or curing time (p>0.05), and it decreased over time (p≤0.05).

The greater light-curing time did not improve the H and E_r of the materials (p>0.05). The six months of storage aging decreased the H and E_r values of all dentin-resin interface components (p≤0.05), except for the E_r of the adhesive (p>0.05). The H and E_r of the resin composites were not affected by the aging factor (p>0.05).

DISCUSSION

Longevity of adhesive restorations is dependent on adequate and stable adhesion of the restorative materials to dental hard tissues over time. The first null hypothesis tested was rejected; the methacrylate restorative system showed higher nanomechanical properties than the silorane system, except for the E_r of the adhesive at 24 hours. Self-etch adhesives are less technique sensitive because there are no rinsing and drying steps; this maintains the ideal dentin humidity and reduces the risk of inaccuracies during application.¹⁸ Two-step self-etch adhesive contains an acid primer that demineralizes and penetrates monomers into the dentin subsurface simultaneously; this is followed by application of a solvent-free hydrophobic resin to improve the mechanical properties.¹⁹ One-step self-etch adhesives contain a mixture of acid, organic solvents, water, and hydrophilic and hydrophobic monomers in a single bottle.¹⁹

Clearfil SE Bond consists of a weakly acidic and hydrophilic self-etch primer and a solvent-free viscous hydrophobic resin coating layer that can increase the mechanical properties due to highly cross-linking monomers.¹⁹ A specific two-step self-etch adhesive (Filtek LS System Adhesive) was formulated for silorane low-shrinkage composite. The hydrophilic and solvated self-etch LS primer is applied and light-cured, creating the hybrid layer (Fig. 1-II).^1,20 Then, LS bond is applied as a low-viscosity bifunctional hydrophobic monomer (phosphorylated methacrylate) and also light-cured, creating distinct layers (primer and bond, Fig. 1-I). It reacts with methacrylate by the acrylate groups and with oxirane by the phosphate groups.¹¹ So LS primer represents a one-step self-etch adhesive.^1,20 The solvated adhesives, such as silorane adhesive primer, have been related to poor mechanical properties, despite the improvement in degree of conversion (DC) from the increased mobility of the molecules over solvent-free adhesives.²¹ Thus, the resin bond component of the methacrylate adhesive applied after the primer may exhibit better mechanical behavior, as demonstrated by the higher H and E_r of the hybrid layer and adhesive, compared with the silorane bonding system.

Sufficient cations are necessary to initiate the cationic ring-opening polymerization of the silorane composite; the onset of this reaction is slow, and additional light-curing is required compared with the free-radical cure of the methacrylate monomers into a polymer network.^1,5,10 Higher DC, Knoop microhardness, and depth of cure were found for methacrylate compared with the silorane composite.²² Thus, the superior physical properties of the methacrylate-based composite probably resulted in higher H and E_r values than silorane.

The second null hypothesis was accepted as the light-curing time did not influence the H and E_r values of the restorative systems. Improvements in the physical properties of resin-based materials have been related to increased curing times because of the higher DC.^4,13,14,18 Light intensity is reduced approximately 10% by interposing 1 mm of air between the guide tip of the light-curing unit and the material surface irradiated.²³ Special care should be taken in deep cavities and when the curing unit is at low light power during the polymerization of resinous materials. A light-curing time of 40 seconds is recommended for silorane composite resin using quartz-tungsten-halogen with irradiance between 500 and 1400 mW/cm² and for LEDs with output between 500 and 1000 mW/cm². For LEDs with irradiance between 1000 and 1500 mW/cm², a light-curing time of 20 seconds is recommended. An irradiation of 10 seconds is recommended to cure the primer and bond of silorane system adhesive, without a specific recommendation of minimum irradiance. In this study, a high light power LED of 1390 mW/cm² was used, indicating 20 seconds and 10 seconds of light polymerization for composite and adhesive, respectively. However, the irradiance achieved on the surface of the first composite increment was 990 mW/cm² at 4 mm from the guide tip and 610 mW/cm² at 6 mm for the adhesive system.

The second-generation LED unit used exhibits a narrow spectrum (between 410 and 530 nm, with a peak on the curve at 454 nm), which includes the maximum energy absorption peak of camphorquinone at 468 nm, which absorbs wavelengths from 360 to 510 nm.²⁴ However, the extended curing time available for light polymerization did not improve the H and E_r of the materials, perhaps because the high light power was sufficient to form more cross-linked polymers, which are less susceptible to degradation than linear polymers,³ but results in the deceleration of the polymerization reaction and in limits on the conversion rate.²⁵

The third null hypothesis was rejected because the long-term storage decreased the mechanical properties of most resin-dentin interface components. Bonding interface components can be degraded by hydrolysis; the water sorption results in polymer plasticization by swelling and reducing the frictional forces between the polymer chains, thereby decreasing the mechanical properties.¹⁸

The 10-methacryloyloxydecyl dihydrogen phosphate (MDP) monomer contained in the methacrylate adhesive tested in contact with the dental tissues form the MDP-calcium salt hardly dissolved in water; therefore the bond between MDP and hydroxyapatite should be stable.²⁶ Thus, the chemical interaction improves the resistance to hydrolytic breakdown and debonding stress, keeping the restoration margins sealed for a longer period.²⁷ Moreover, application of the hydrophobic bonding agent after the hydrophilic primer improves the mechanical properties by the presence of the cross-linking monomers,¹⁹ which may contribute to reducing the adhesive interface degradation over time.

All-in-one adhesives, such as the one-step self-etch silorane adhesive primer, act as permeable membranes and can be more susceptible to aging.²⁸ These adhesives are strongly influenced by the light intensity of the curing unit.¹⁹ The bond of the silorane adhesive is also solvent-free and contains more monomers with more cross-linking ability.^19,21 The nanomechanical properties of the components of the adhesive interface (hybrid layer, adhesive, and bond layer of the silorane adhesive) were reduced after storage, except for the E_r of the adhesive. Although the solvated silorane adhesive primer showed similar E_r as the methacrylate adhesive at 24 hours, after aging it exhibited significant lower properties, likely because of greater susceptibility to plasticization by the greater amount of hydrophilic monomers²⁸ and possible residual solvent.

Siloxane species present in the silorane composite exhibit high hydrophobicity,^8,9 and the H and E_r of this material was not affected after six months of storage, probably because of the hydrophobic nature of the siloxane species.^8,9 The higher conversion rate of the methacrylate composite compared with silorane²² may have compensated for its lower hydrophobicity and increased the plasticization resistance of the Filtek Z250, for which properties also did not decrease with aging. The mechanical properties of the underlying intertubular dentin decreased over time regardless of the restorative material. Dentin tissue contains collagenolytic enzymes, such as matrix metalloproteinases and cysteine cathepsins, which are responsible for the hydrolytic degradation of the collagen matrix.²⁹

The conversion of the monomers into structured polymers is related to the increase of the physical properties of the resinous material; this polymerization reaction is dependent on various factors, such as design and size of the tip guide, distance of the light guide tip from the material surface, power density, exposure duration, shade and opacity of the composite, increment thickness, and composition of the materials, resulting in clinical performance improvement of light-cured materials and more durability of the adhesive restorations.¹²

CONCLUSIONS

Overall, methacrylate restorative systems exhibited higher H and E_r than silorane; increased light-curing time did not influence the nanomechanical properties, which were significantly reduced after long-term storage.