A comparison of microhardness of indirect composite restorative materials.

The purpose of this study was to compare the microhardness of four indirect composite resins. Forty cylindrical samples were prepared according to the manufacturers recommendations using a Teflon mold. Ten specimens were produced from each tested material, constituting four groups (n=10) as follows: G1 - Artglass; G2 - Sinfony; G3 - Solidex; G4 - Targis. Microhardness was determined by the Vickers indentation technique with a load of 300g for 10 seconds. Four indentations were made on each sample, determining the mean microhardness values for each specimen. Descriptive statistics data for the experimental conditions were: G1 - Artglass (mean ±standard deviation: 55.26 ± 1.15HVN; median: 52.6); G2 - Sinfony (31.22 ± 0.65HVN; 31.30); G3 - Solidex (52.25 ± 1.55HVN; 52.60); G4 - Targis (72.14 ± 2.82HVN; 73.30). An exploratory data analysis was performed to determine the most appropriate statistical test through: (I) Levene's for homogeneity of variances; (II) ANOVA on ranks (Kruskal-Wallis); (III) Dunn's multiple comparison test (0.05). Targis presented the highest microhardness values while Sinfony presented the lowest. Artglass and Solidex were found as intermediate materials. These results indicate that distinct mechanical properties may be observed at specific materials. The composition of each material as well as variations on polymerization methods are possibly responsibles for the difference found in microhardness. Therefore, indirect composite resin materials that guarantee both good esthetics and adequate mechanical properties may be considered as substitutes of natural teeth.


INTRODUCTION
Esthetic Dentistry has been widely advocated, since there has been an increasing demand for materials that resemble the natural tooth.Therefore, dental treatments that provide a harmonious smile have been widely requested in dental offices.
Since Buonocore 2 introduced the acid-etching concept followed by Bowen's research 1 on composite resin, new perspectives were born for Restorative Dentistry.Initially, composite resins presented low wear resistance due to the weak bonding of the filler content to the organic matrix 11 .When later improvements were achieved regarding the material's composition, better chemical and mechanical properties were accomplished, thus improving its clinical performance and extending its utilization to posterior teeth 4 .The incorporation of smaller particles associated to higher filler content has guaranteed higher mechanical properties, better marginal sealing and longer color stability 9,10 .
Indirect composite resins have been introduced with high expectations to overcome direct composite resins drawbacks.Among the proposed advantages are the potential for achieving positive interproximal contacts, less polymerization shrinkage and better marginal sealing because of the polymerization process that takes place in a laboratory setting 7 .Such enhanced properties are the result of a higher degree of conversion obtained from the utilization of different polymerization procedures that involve heat, pressure, light, vacuum, or nitrogen atmosphere 16 .The degree of conversion increases when multifunctional monomers are present, offering extra reactive sites that enlarge the polymer chains.Better mechanical properties may also be ensured through reinforcements of glass and polyethylene fibers added to indirect composite resin materials 6 .
Therefore, indirect composite restorations have become a popular alternative to all-ceramic restorations for the esthetic treatment of posterior teeth.Since there has been a rapid introduction of new dental restorative composite resins, the selection of the appropriate material becomes rather difficult.As mechanical properties are one of the most important characteristics when deciding for a suitable material, scientific validation on the efficacy of these new technologies is necessary 14 .
Microhardness tests are considered an efficient method to investigate the physical strength of a material and therefore may be one an appropriate indicative method to guide indirect composite application.The hardness of a material is a relative measure of its resistance to indentation when a specific, constant load is applied.Thus, hardness may be described as a measure of the ability of a material to resist indentation or scratching 13 .
Aiming to clear the confusion with respect to clinical decision-making when selecting an indirect composite resin, the purpose of this study was to investigate the microhardness of four indirect resins by using the indentation technique.
Ten cylindrical specimens of each of the test composites were prepared by placing the materials into a Teflon matrix (5mm deep and 5mm in diameter) for polymerization according to the manufacturer's recommendations.All samples were inserted into a polyester resin (Arotec T208 -Valglass Comércio e Indústria Ltda., São José dos Campos -Brazil) in order to ease sample handling.Special care was taken to leave the tested surface uncovered by the polyester resin.Standardized surfaces were obtained though a sequential sandpaper finishing to ensure that both upper and lower surfaces were parallel to each other.After 24 hours, all the specimens were submitted to the proposed testing, constituting four groups (n=10) as follows: G1 -Artglass; G2 -Sinfony; G3 -Solidex; G4 -Targis.
Hardness was determined by the indentation technique performed on a microhardness tester (Digital Microhardness Tester FM -Future-Tech Corporation, Kawasaki -Japan) with a load of 300g for 10 seconds.Four indentations were made on each sample using a   Regarding data variability, it may be verified that:

Microhardness (HVN) descriptive data obtained are demonstrated in
(I) there was not overlapping of data correspondent to the interquartil range (which is the most stable and important data of the distribution) among the tested materials (Figure 1); (II) Targis resulted into higher standard deviation (Figure 2) when compared to the other tested materials (Levene's Test: F 3;36 = 4.544; p=0.008).Statistical significance was observed between the median values of the tested materials when data were submitted to Kruskal-Wallis test (kw=17.67;df=3; p=0.001).
The results obtained from Dunn's multiple comparison test (0.05) are displayed on Table 2

DISCUSSION
This study performed microhardness tests in order to evaluate some of the indirect composite resin systems commercially available.Theoretically, all indirect resin materials should present similar mechanical properties since the composition of the filler content of such materials is almost identical, basically constituted of oxygen, aluminum, silicon, and barium 13 .However, the results obtained in this study demonstrate that the different tested materials present intrinsic characteristics, which resulted into specific behaviors.
The overall properties of a composite are influenced by the type, size, and volume fraction of the filler particles and the degree to which the filler is bonded to the resin matrix.Therefore, the type of  matrix and the degree to which conversion occurs during polymerization might also influence the mechanical properties, especially when aging occurs in the oral environment 3 .
There may be yet a positive correlation between the method of polymerization and the microhardness property.Tanoue, et al. 15 pointed out that achieving the best mechanical and physical properties is directly related to a combination of composite material and curing unit from the same manufacturer.It could be observed that Targis, which was polymerized trough light (450-500nm) integrated to tempering heat (95ºC), presented the highest microhardness numbers.Yamaga et al. 18 reported that heat might facilitate monomer conversion by breaking the double bonds on the polymer network into single bonds, thus optimizing the polymerization of the residual monomers.The indirect composites that suffered polymerization under light-activation only (400-550nm) were found to hold intermediate mean microhardness values (Artglass and Solidex).On the other hand, Sinfony presented the lowest mechanical property tested, even when its polymerization method associated light (400-500nm) and vacuum.This suggests that the composition of the material influences the degree of conversion during polymerization resulting into lower resistance to indentation.
The presence of filler particles increases the compressive strength and hardness of the resin matrix 17 .Initially, it was thought that increasing the level of filler content in composites could optimize properties such as wear resistance, compressive strength, hardness, water sorption, and elastic modulus 12 .Later researches have reported that there is no correlation between filler content and mechanical properties 5,13,15 .This study verified that Targis resulted into the highest microhardness mean values when compared to the other tested materials (Artglass, Solidex, and Sinfony) although it did not present the highest filler content.Interestingly, the indirect composite with the lowest filler content (Sinfony) presented the lowest mean microhardness data.Although divergence exists when considering a possible correlation between filler particle content and composite mechanical properties, it must be pointed out that perhaps the manufacturer's information about the filler particle size and filler content is not as closely monitored as they advertise 8 .Therefore, further research is necessary to determine indirect composite behavior in order to assist clinicians in a better understanding of their clinical indications.

CONCLUSION
Within the limits of this study, different indirect composite resins presented distinct microhardness mean values through the indentation technique under constant load of 300gf.Such differences may be related to the intrinsic composition of each material as well as the variation of their polymerization methods.

CLINICAL IMPLICATIONS
As proper substitutes of natural teeth, indirect composite resins should gather both adequate mechanical properties and good esthetic in order to produce successful results.

FIGURE 2 -
FIGURE 2-Mean microhardness values (HVN) and standard deviation for each tested material

FIGURE 1 -
FIGURE 1-Box and Whisker Plot representation of microhardness values (HVN) for each indirect composite resin

TABLE 1 -
Product informationVickers diamond point in order to determine the mean microhardness values for each specimen.