Vol 14, No X, 2012 1

Effect of Photoactivation Mode on the Hardness and Bond Strength of Methacrylate- and Silorane Monomer-based CompositesWilliam C. Brandta / Renata F. S. Lacerdab / Eduardo J. C. Souza-Juniorc / Mario A. C. Sinhoretid

Purpose: To evaluate the Knoop hardness (KH) and the bond strength (BS) at the tooth/restoration interface of conventional methacrylate- (Filtek Supreme) and silorane-based (Filtek P90) composites photoactivated by differ-ent methods using an LED Freelight 2.

Materials and Methods: Bond strength was tested in a universal testing machine by the “push-out” test in re-stored cavities measuring 2 × 1.5 × 2 mm with a C-factor of 2.2, prepared in 60 bovine teeth. To restore the cavities, the respective adhesive system of each composite was used (Single Bond 2 and P90 system adhe-sives). The composites were photoactivated by 3 different methods: continuous light: 40 s at 1000 mW/cm2; soft-start: 10 s at 150 mW/cm2 + 38 s at 1000 mW/cm2; pulse delay: 5 s at 150 mW/cm2, followed by a 3-min wait (without photoactivation) and 39 s at 1000 mW/cm2. Before the push-out test was performed, the KH was analyzed at the top and bottom of the restorations. Data were statistically anaylzed using ANOVA and Tukey’s test.

Results: The photoactivation methods produced no differences in BS or KH in the same composite, while Filtek P90 (28.0 MPa) showed higher BS values than Filtek Supreme (22.3 MPa) and a lower KH.

Conclusion: The composite Filtek P90 was capable of increasing bond strength, but presented lower Knoop hardness.

Keywords: resin composite, photoactivation, bond strength, silorane, methacrylate.

J Adhes Dent 2012;14:7pages Submitted for publication: 17.05.11; accepted for publication: 28.12.11 XXX

a Professor, Department of Prosthodontics, Dentistry School, University of Tau-baté, Taubaté, SP, Brazil. Idea, experimental design, hypothesis, performed push-out test, wrote manuscript.

b MS Student, Department of Prosthodontics, Dentistry School, University of Taubaté, Taubaté, SP, Brazil. Cleaned and preparated bovine teeth, per-formed hardness test.

c PhD Student, Department of Restorative Dentistry, Dental Materials Area, Piracicaba School of Dentistry, State University of Campinas, SP, Brazil. Con-tributed substantially to discussion and review.

d Professor, Department of Restorative Dentistry, Dental Materials Area, Pi-racicaba School of Dentistry, State University of Campinas, SP, Brazil. Idea, hypothesis, statistical analysis.

Correspondence: William Cunha Brandt, Department of Prosthodontics, Den-tistry School, University of Taubaté, UNITAU, Rua Expedicionário Ernesto Pereira, 110, 12020-330, Taubaté, SP, Brazil. Tel: +55-12-3625-4149, Fax: +55-12-3632-4968. e-mail: williamcbrandt@yahoo.com.br

Light-cured resin composites are commonly used in daily clinical practice to restore anterior and pos-

terior teeth because of their many advantages: good esthetics, bonding to tooth structure, and mechanical properties. However, these materials undergo significant volumetric shrinkage when polymerized.9 In vitro mea-surements of polymerization shrinkage of resin compos-ites range from 0.9% to 2.8% by volume.18

As part of bonded preparations, the contraction of these composites induces the development of mechan-ical stress inside the material.9 The stress is transmitted via bonded interfaces to tooth structures. In light-curing composites, fast conversion induces a fast increase in composite stiffness, causing high shrinkage stress at the interface.4 Such stress may disrupt the bond be-tween the composite and the cavity walls or may even cause cohesive failure of the restorative material or the surrounding tooth tissue, in addition to postoperative sensitivity.28

The rate of monomer conversion depends upon many factors, such as photoinitiator chemistry, filler morph-ology, pigments, and irradiance (mW/cm2). The role played by the irradiance applied to the composite is funda-mental, because it is a factor that can be controlled by the operator through modulated photoactivation methods, as opposed to the other factors just mentioned. The higher the irradiance is, the faster the monomer conversion and the higher the stress generation. Photoactivation using low irradiance could reduce the stress, because it would allow flow during the earlier stages of polymerization and enable a certain degree of polymer chain relaxation before reaching the rubbery stage.4,11,27

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Studies on alternative photoactivation methods have shown the beneficial effects of modulated polymeriza-tion, including the decrease of shrinkage stress. Many photoactivation methods, such pulse-delay and soft-start modes, have been examined for their ability to reduce the shrinkage stress of dental composites.1,4

Modulation of the light energy, as in the soft-start and pulse-delay methods, has been shown to be effective in decreasing the shrinkage stress of dental composite polymerization, but its clinical use is difficult, because it increases the clinical time and is dependent on the irradi-ance of the light-curing unit, which the dentist does not usually know. Although manufacturers have incorporated soft-start mode in the light-curing units (LCUs), they have not increased the total curing time, wich can cause an incomplete polymerization of composite resin. Moreover, these methods can reduce the stress, but they do not reduce the final shrinkage of the material.1,4,9,27

Therefore, with the objective of decreasing polymeriza-tion shrinkage and, consequently, the stress generated at the tooth/restoration interface, new monomers have been studied and introduced into the composition of den-tal composites. The monomers bis-GMA, bis-EMA, UDMA, and TEG-DMA can be substituted by alternative monomers that have low polymerization shrinkage.10,21,29

Recently, a silorane-based composite (Filtek P90), a synthesized monomer starting from oxirane and siloxane, was introduced on the market. Silorane-based compos-ites differ from the methacrylate-based composites due to the polymerization process that occurs via a cationic ring-opening reaction, which decreases the volumetric contraction of the composite when compared with other methacrylate-based composites, in which polymeriza-tion proceeds by addition.29 Another difference between silorane-based composites and methacrylate-based composites is related to the adhesive system used. The adhesives currently available on the market have been developed for traditional methacrylate materials and will, therefore, lead to insufficient results in combination with Filtek P90 restorative.29

When methacrylate monomers are replaced by silorane, not only can the polymerization shrinkage be reduced, but also the stress caused by it.12,29 Thus, many problems related to composite restorations, such as microleakage, marginal staining, secondary caries, and postoperative sensitivity, can be overcome.5

However, few studies have verified the effectiveness of silorane-based composites regarding their properties and benefits in terms of bond strength when different photoactivation methods were used. Therefore, the aim of this study was to evaluate the Knoop hardness and bond strength between the tooth and restoration of con-ventional methacrylate- and silorane-based composites photoactivated by different methods. The bond strength was evaluated with the push-out test, which is very useful for verifying the effect of polymerization shrinkage on com-posite restorations and its influence on bond strength.15 Knoop hardness was performed to indirectly assess the degree of conversion of composite restorations.24 There-fore, because of the difference in composition between

the composite resins analyzed, they may have different behaviors. Thus,the hypotheses tested were:1. The photoactivation methods can influence the tooth/

restoration bond strength for restorations made with a methacrylate-based composite (Filtek Supreme), but not for restorations made with a silorane-based composite (Filtek P90);

2. The silorane-based composite can produce higher tooth/restoration bond strength than the methacry-late-based composite, regardless of the photoactiva-tion method used;

3. The methacrylate-based composites will obtain higher Knoop hardness values than silorane-based compos-ites, regardless of the photoactivation method used.

MATERIALS AND METHODS

Restorative ProceduresSixty bovine incisors were obtained, cleaned, and stored in 0.5% chloramine-T solution at 4°C for a week. After removing the root portions, the buccal aspects were wet ground with 400-, 600- and 1200-grit SiC abrasive papers to obtain flat surfaces in dentin. Standardized conical cavities (approximately 2 mm top diameter × 1.5 mm bottom diameter × 2 mm height) were then prepared, using #3131 diamond burs (KG Sorensen; Ba-rueri, SP, Brazil) at high speed under air-water cooling. A custom made preparation device allowed the cavity di-mensions to be standardized. A digital caliper (Mitutoyo; Kawasaki, Japan) was used to check the dimensions of the cavities. The burs were replaced after every five preparations. In order to expose the bottom surface of the cavities, the lingual aspects were ground following the same procedure described for flattening the buccal aspects. By following these procedures, a cavity with a C-factor of 2.2 was obtained, according to equation 1. The adhesive systems Single Bond 2 (3M ESPE; St Paul, MN, USA, lot 8RW) and P90 System Adhesive (3M ESPE, lots 7AF and 7AL) were then applied to the cavi-ties, according to the manufacturer’s instructions. The specimens were placed on a glass slab and the restor-ative procedures were carried out using the resin com-posites Filtek Supreme (3M ESPE, shade A3, lot 7KY) and Filtek P90 (3M ESPE, shade A3, lot 8BL), which were bulk inserted into each cavity from its wider side. Table 1 shows the composition of the materials used. Different photoactivation procedures, as described in Table 2, were tested. For each method, 10 specimens were prepared. Prior to the polymerization procedures, the output power of the LED FreeLight 2 (3M ESPE) was measured with a calibrated power meter (Ophir Optron-ics; Danvers, MA, USA), and the diameter of the light-guide tip was checked with a digital caliper (Mitutoyo). Light irradiance (mW/cm2) was computed as the ratio of the output power to the area of the tip. Different polymerization times were used in order to maintain a total radiant exposure of approximately 40 J/cm2 for all samples. Irradiance at high light intensity (1000 mW/cm2) was carried out with the light-guide tip positioned

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directly on the restoration, which had been previously covered with a polyester strip. To produce an output of 150 mW/cm2, a standard black acrylic cylinder separa-tor was used to allow the light-guide tip to be positioned 1.3 cm away from the restoration surface, and the irradi-ance was confirmed with the power meter. The different times of photoactivation were controlled with a digital watch. Additionally, the light spectrum profile emitted by the curing unit was analyzed with a computer-controlled spectrometer (USB 2000, Ocean Optics; Dunedin, FL, USA). Equation 1:

C-factor = bonded area = (π/2) × h × (D+d) unbonded area π(D/2)2 + π(d/2)2

,

where: h is the height of the cavity, D is the diameter of the top and d is the diameter of the bottom surface.

Hardness MeasurementsAfter light-curing procedures, the specimens were stored in distilled water at 37°C for 24 h. Thereafter, both the top and bottom surfaces were wet-polished with 1200-grit SiC paper to obtain a flat surface. Knoop hardness measurements were taken on both surfaces using an indenter (HMV-2, Shimadzu; Tokyo, Japan) under a 0.49 N load (equivalent to 50 gf) for 15 s. Five readings were performed for each surface. The Knoop hardness num-ber (KHN, Kgf/mm2) for each surface was recorded as the mean of the five indentations. Data were submitted to three-way ANOVA (resin composite vs photoactivation method vs surface) followed by Tukey’s test (α = 0.05).

Push-out TestThe push-out test (Fig 1) was performed in a univer-sal testing machine (model 4411, Instron; Canton, MA, USA). An acrylic device with a central orifice was adapted to the base of the machine. Each specimen was placed in the device with the top of its cavity against the acrylic surface. The bottom surface of the

Fig 1 Schematic representation of the push-out test. A: tooth crown; B: preparation made using a specialized device on the buccal face; C: lateral view of the cavity; D: lateral view of the cavity with the lingual face ground; E: lateral view of the restored specimen (2.0 mm in height, buccal diameter 2.0 mm, lingual diameter 1.5 mm); F: lateral view of the specimen showing the direction of specimen push-out.

AB

C

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Table 2 Description of the photoactivation methods

Photoactivation method

Exposure protocol

Continuous light 1000 mW/cm2 for 40 s

Soft-start 150 mW/cm2 for 10 s + 1000 mW/cm2 for 38 s

Pulse-delay 150 mW/cm2 for 5 s (3 min without photoactivation) + 1000 mW/cm2 for 39 s

Table 1 Composition of the composites and adhesive systems employed (manufacturer information)

Product Composition Photoinitiator

Filtek P90 Silorane resin and 76% by weight (mean: 0.47 μm) quartz and yttrium fluoride Camphorquinone, iodonium salt and electron donor

P90 self-etching primer HEMA, bis-GMA, water, ethanol, phosphoric acid-methacryloxyhexylesters, silane-treated silica, 1,6-hexanediol dimethacrylate, copolymer of acrylic and itaconic acid, (dimethylamino)ethyl methacrylate

Camphorquinone, phos-phine oxide

P90 Bond agent Substituted dimethacrylate, silane-treated silica, TEG-DMA, phosphoric acid methacryloxy-hexylesters, 1,6- hexanediol dimethacrylate

Camphorquinone

Filtek Supreme Bis-GMA, bis-EMA, UDMA, TEG-DMA, 72.5% by weight (mean: 75 nm) silica nanofiller and 0.6/1.4 μm clusters of silica

Camphorquinone and elec-tron donor

Single Bond 2 Ethyl alcohol, silane-treated silica (nanofiller), bis-GMA, 2-hydroxyethyl methacrylate, glycerol 1,3-dimethacrylate, copolymer of acrylic and itaconic acids, water, diurethane dimethacrylate

Camphorquinone

Bis-EMA: ethoxylated bisphenol A dimethacrylate; bis-GMA: bisphenol A diglycidyl ether dimethacrylate; TEG-DMA: triethylene glycol dimethacrylate; HEMA: 2-hydroxyethyl methacrylate; UDMA: urethane dimethacrylate.

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restoration was then loaded with a 1-mm-diameter cy-lindrical plunger at a crosshead speed of 0.5 mm/min until failure of the tooth/composite bond in the lateral walls of the cavity. The plunger tip was positioned so that it touched only the filling material, without stressing the surrounding walls. The load required for failure was recorded by the testing machine and transformed into MPa taking the area of each cavity into account. Data were submitted to two-way ANOVA (resin composite vs photoactivation method) and Tukey’s test (α = 0.05). After testing, the fractured specimens were examined using a stereomicroscope (Carl Zeiss; Manaus, AM, Bra-zil) at a magnification of 40X. Their failure modes were classified as follows: adhesive failure, cohesive failure within the composite or mixed failure involving adhe-sive, dentin and composite. Additionally, representative fractured specimens were sputter coated with gold and examined by SEM (JSM 5600LV, JEOL; Peabody, MA, USA).

RESULTS

The Knoop hardness assessment means are summa-rized in Table 3. For top and bottom hardness, irrespec-tive of the light-curing method, no significant differences were detected. On the other hand, significant differ-ences were detected between Filtek Supreme and Filtek P90 for both top and bottom surfaces (p < 0.05). Filtek Supreme showed higher Knoop hardness means than Filtek P90.

The push-out test values are shown in Table 4. Irrespec-tive of the light-curing method, no significant differences were detected in the bond strength. On the other hand, significant differences were detected between the bond strengths of Filtek Supreme and Filtek P90 (p < 0.05). Filtek P90 showed higher bond strength values than Filtek Supreme.

Figures 2 and 3 depict the percentage of failure modes in the push-out test for the resin composites Filtek Su-preme and Filtek P90, respectively. For both Filtek Su-preme and Filtek P90 photoactivated with continuous light, more adhesive failures occurred. For soft-start and pulse-delay photoactivation, adhesive failure was also the most frequently observed mode, but with an increase in the percentage of mixed and cohesive failure compared to continuous-light photoactivation.

DISCUSSION

The push-out test is generally used to evaluate the bond strength of endodontic cements in the radicular dentin.17,22 However, in the present study, the push-out test was adapted to evaluate the bond strength of restorative composites in a simulated Class I cav-ity.4,14,15,19

Other bond strength tests, eg, shear bond strength, tensile bond strength, microshear bond strength, and microtensile bond strength, are usually carried out to evaluate the bond strength of resin composites. However, these tests are generally performed on flat surfaces. In this situation, the C-factor is very low and the development of shrinkage stress is not directed to the bonding inter-face. The advantage of using the push-out test was that the bond strength could be evaluated in a high C-factor cavity (2.2), with high stress generation directed to the bonding area.13 The entire bonding area was submitted to the compressive force at the same time, allowing the push-out bond strength to be evaluated in a cavity. In ad-dition, the reliability of the push-out test was confirmed by low variability of the data, since the results showed low standard deviations.

The polymerization shrinkage of dental composites is still the main cause of flaws in restorations. The shrink-age of the material can cause postoperative sensitivity

Table 3 Means (standard deviations) for top and bottom hardness (KHN, Kgf/mm2) of the resin composites Filtek Supreme and Filtek P90

Resin composite Region Continuous light Soft-start Pulse delay

Filtek Supreme Top 58.4 (3.6) A,a 61.4 (6.3) A,a 62.7 (3.0) A,a

Bottom 61.2 (3.4) A,a 59.7 (4.8) A,a 63.9 (2.6) A,a

Filtek P90 Top 42.8 (6.2) A,b 41.5 (4.5) A,b 38.6 (3.7) A,b

Bottom 40.0 (3.0) A,b 40.6 (3.4) A,b 40.7 (2.7) A,b

Means followed by different superscript capital letters in the same line and small letters in the same column are significantly different (p < 0.05).

Table 4 Means (standard deviations) for push-out test (MPa)

Compositon Continuous light

Soft-start Pulse delay

Filtek Supreme

22.7 (9.4) A,b 23.0 (7.9) A,b 21.1 (7.6) A,b

Filtek P90 29.4 (9.0) A,a 26.9 (7.3) A,a 27.3 (4.8) A,a

Means followed by different superscript capital letters in the same line and small letters in the same column are significantly different (p < 0.05).

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and/or debonding, and consequently, marginal staining, microleakage, and secondary caries.5 Thus, numerous researchers have endeavored to reduce the shrinkage stress with the objective of reducing the problems caused by polymerization shrinkage, which is inherent to the ma-terial.1,4,6,27

One way to reduce the shrinkage stress is through modulation of the light energy. Photoactivation methods such as soft-start and pulse delay employ lower initial ir-radiation, thus decreasing the initial polymerization rate of the composite and prolonging the viscous-elastic stage of polymerization. This extends the viscous-elastic stage, that is, more time is allowed for the composite to flow be-fore reaching the rubbery stage. However, there are other factors that influence stress generation. In addition to

decreasing the light energy, and consequently, decreasing the rate of polymerization, the C-factor and volume of the material are very important factors.3

In the present study, the different photoactivation meth-ods did not produce differences in bond strength. Conse-quently, the first hypothesis was rejected. One of the rea-sons for this could be the small volume of material used. Although the cavity had a high C-factor (2.2), it was equiva-lent to the use of a single increment (with a maximum thick-ness of 2 mm), which was probably not enough to create differences in the bond strength.3 In the methacrylate-based resin composite (Filtek Supreme), the generation of radical species is achieved by a two-component system consisting of camphorquinone (CQ), which is the actual photoinitiator, and a tertiary amine responsible for the

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Fig 2 Percentage of failure modes using the push-out test for the resin composite Filtek Supreme.

Fig 3 Percentage of failure modes using the push-out test for the resin composite Filtek P90.

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hydrogen transfer reaction. In contrast, photoactivation in the silorane-based composite (Filtek P90) is achieved with a three-component initiating system consisting of cam-phorquinone, an iodonium salt, and an electron donor. In spite of the inclusion of the iodonium salt to increase the rate of polymerization, Filtek P90 still possesses a reduced initial speed of polymerization reaction when compared with traditional composites.29 Thus, it could be that the modification of curing mode neither effectively interfered in the polymerization of composites Filtek P90 and Filtek Supreme nor increased the bond strength values.

Another fact is related to LCU used. The LED curing unit FreeLight 2 emits light in the region of greater absorption of the photoinitiator CQ. This good correlation between the spectrum of emission of the LCU and the spectrum of absorption of CQ may have provided a sufficient quantity of protons to impair the decrease of the polymerization rate of the composites used, the same as using a low ini-tial irradiance. Because of this, the rate of polymerization may not be sufficiently reduced, thus not prolonging the viscous-elastic stage of polymerization, and not allowing more time for the composite to flow before reaching the rubbery stage. This might explain the absence of differ-ences in the bond strength values.

Many studies demonstrated that the modulation of the light energy could increase the bond strength values in composite resin restorations, mainly when a halogen LCU was used.4,9,11 When an LED light-curing unit is used instead of a halogen LCU, those benefits are decreased, or even lost, due to better correspondence between the light-emission spectrum of LED LCUs and the light absorp-tion spectrum of CQ, the most common photoinitiator.7,27

Differences in the degree of conversion may also in-fluence the bond strength, because if a photoactivation method produces a low degree of conversion, low polymeri-zation shrinkage results, which improves the bond strength values. However, the Knoop hardness at the top and bottom of the samples showed no differences among the different photoactivation methods within the same composite. In this study, Knoop hardness was an indirect measure of the cur-ing extent or degree of conversion.24 Lower values of Knoop hardness or degree of conversion can influence not only the properties of the material, but also the bond strength val-ues, because a composite restoration with a lower degree of conversion possesses low contraction and consequently, lower shrinkage stress which can improve the bond strength values. The different irradiances in the modulated groups were compensated by the long light-exposure time (mW/cm2 × time in seconds) to maintain the total radiant expo-sure of approximately 40 J/cm2 for all samples. This result is in agreement with those of previous studies.4,8,28

Although the bond strength values showed no differ-ences among the different photoactivation methods, there was a decrease in the prevalence of adhesive fail-ures when soft-start and pulse-delay modes were used instead of continuous light. The decrease in adhesive fail-ures could be an indication of a better adaptation of the composite to the cavity walls, consequently increasing the prevalence of mixed and even cohesive failures, in spite of not being sufficient to increase the bond strength values.

Although the Knoop hardness values did not differ between the photoactivation methods, another explana-tion for the higher prevalence of failure within the com-posite may be due to lower mechanical properties for composites photoactivated with soft-start and pulse-delay modes. Some studies have shown that these photoactiva-tion methods produce polymers with lower cross-linking density, which consequently affects their mechanical properties.4 Composite Filtek P90 showed higher bond strength values than Filtek Supreme. Thus, the second null hypothesis was accepted.

The silorane network is generated by the cationic ring-opening polymerization of the cycloaliphatic oxirane moieties, which are known for their low shrinkage and low polymerization stress. The low polymerization shrink-age and shrinkage stress can lead to an increase in the bond strength.4 The failure mode also showed differences between Filtek Supreme and Filtek P90 (Figs 2 and 3). Composite Filtek P90 yielded a larger number of cohesive failures. This might have occurred due to the lower stress caused by Filtek P90, better adaptation between the com-posite and the cavity walls, and therefore a better bond between the tooth/restoration was obtained, producing a bond that was stronger than the cohesive strength of the material. The Knoop hardness results support this explana-tion: Filtek P90 presented lower mean Knoop hardness val-ues than did Filtek Supreme, which could suggest reduced mechanical properties and therefore an increase in cohe-sive failures. Thus, the third hypothesis was accepted.

This higher bond strength values of Filtek P90 in rela-tion to Filtek Supreme could also be a result of better monomer cross-linking in the case of Filtek Supreme, as indicated by the higher Knoop hardness. Better cross-linking is known to result in a higher modulus of elasticity, which in turn increases shrinkage stresses, thus interfer-ing with the quality of the bond.29 An increased rigidity may also directly influence the bond strength test itself, in that it promotes stress formation along the adhesive interface during the debonding test.16

Different adhesive systems were used. Filtek P90 has its own adhesive system, because it possesses a differ-ent composition than the methacrylate-based composites such as Filtek Supreme. The P90 system adhesive is a self-etching adhesive, which differs from Single Bond 2 – an etch-and-rinse adhesive – used with Filtek Supreme. The use of different adhesive systems might have contrib-uted to the differences found in the bond strength values. Many studies show differences in the hybrid layer formed by self-etching adhesives and etch-and-rinse adhesives. In general, self-etching adhesives form a less pronounced hybrid layer than do etch-and-rinse adhesives.25 However, bond strength tests show similar results between them.2

As mentioned earlier, the use of different adhesive systems influenced the results, which should be consid-ered when comparing the results of the two restorative systems investigated. However, as the P90 System Adhe-sive was developed for use with Filtek P90, it is difficult to compare it with Filtek Supreme.29 The silorane-based composite Filtek P90 presented lower Knoop hardness that the methacrylate-based composite Filtek Supreme,

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which could suggest reduced mechanical properties. Fur-ther studies related to the properties exhibited by the composite, such as wear and ultimate tensile strength, should be conducted.

Finally, is important to point out some limitations of this study. The use of bovine teeth requires caution in the in-terpretation of the results. The objective of this study was to evaluate the effect of different photoactivation methods on composite behavior under confinement conditions (eg, in a prepared cavity). Nevertheless, the use of bovine inci-sors is supported by numerous authors.20,23,26

CONCLUSION

Under the limitations of this study, the photoactivation methods produced no differences in bond strength or Knoop hardness in the same composite when a small volume of composite resin was used. However, the higher prevalence of cohesive and mixed failures in com-posite cured with the soft-start and pulse-delay modes may indicate lower mechanical properties of the com-posite resins used here.

The silorane-based composite (Filtek P90) used in combination with its proprietary bonding system produced higher push-out bond strengths than the methacrylate-based resin composite (Filtek Supreme) in combination with a universal bonding system (Single Bond 2). The poten-tial association between this difference in bond strength and the lower mechanical properties of the silorane-based composite compared with traditional methacrylate-based resin composites as indicated by the hardness measure-ments should be the objective of further studies.

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Clinical relevance: The silorane-based composite Filtek P90 in combination with its respective bond-ing system seems to achieve better bond strengths to dentin when compared to a methacrylate-based restorative system. However, the potential effects of the lower mechanical properties of the silorane-based  on the longevity of posterior restorations remains to be determined.

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