Study the effect of different shades and thickness on surface hardness of light cure composite restoration ( A Comparative In Vitro Study )

Objectives: Study performed to determined the effect of cavity depth & shade of composite on surface microhardness of the restoration. Materials and Methods: Eighty cylindrical specimens were prepared, (Group R) 40 specimens of Nano filled composanceram, divided into four subgroups(R1,2.5 mm,A1shade / R2, 2.5mm, C2 shade / R3,4.5mm, A1shade / R4, 4.5mm, C2shade). And (Group E) 40specimens of Micro hybrid, 3G SDI, divided into four subgroups as the same as first group. After curing with visible light cure 230v/50Hz, output 470 nm, measuring the Vicker Hardness number of top and bottom surface of each specimen, then analyzed by Student ttest. Results: Showed highly significant in difference in relation between top and bottom surfaces for all subgroups,and also there was highly significant indifference in relation to different types of composite, while there was non-significant indifference in relation to difference in shade in case of thin thickness, while it was significant in difference in case of thick thickness of specimens. Conclusions: In case of deep cavity it wise to use thinner composite increments to improve polymerization and should avoid thicker increment especially in a dark shade of composite.

Since the manufacturing of composite as restorative materials, many attempts have been made to increase the longevity of the dental restoration.Although development have been made on mechanical properties, such as wear fracture resistance have been improved, the challenged by shrinkage caused by polymerization contraction still need more scientific resolve, In addition adequate light activation pf polymers is important to ensure optimum polymerization, as incomplete curing of the deeper portion increase the risk of restoration fracture or marginal leakage (2,3) .
The depth of curing is of important not only in order to reach optimum mechanical properties including hardness but also to ensure that the clinical problem do not occur due to incompletely polymerized material in the deepest part of the cavity.Hybrid or Nanocomposite contain a graded of small and colloidal silica filler particles to achieve an optimal balance among the mechanical properties of strength, polymerization shrinkage, wear resistance and polishability (3)(4)(5) .
Dental composite restorations have a major controversy: the degree to which the composite cure that is proportional to the amount of photoactivation to which they are exposed.So, the composite will polymerize to a depth which proportion with the penetration of a light beam in the bulk material.This depth of cure has a significant influence on both physical and biological properties of restorations.The depth of cure is the thickness of the material to which the light can harden (6)(7)(8) .
Although there exists no universal agreement definition concerning depth of cure and how to determine it quantity, there is some kind of acceptance that depth of cure is limited to that distance from the top surface of sample where no more resin material can be scratched off.Because dental fillings need a maximum polymerization level, many investigators have studied the effect of different factors on the depth of cure (9) .The depth of cure was affected by incorporation and composition of a composite rather than the irradiance from light units The hardness of composites decreased with increasing the depth.At the composite surface, filler type, exposure duration and resin shade predominated as the most influential factors, respectively (10,11) .
The cure of outer surfaces of photoactivated restoration was not greatly affected by light intensities but curing of the inner parts of these materials was affected by light intensities (12) .
The factors in close relation effect to the surface microhardness values of restorative materials include the filler volume, composition resin type, and polymerization degree.A reduced polymerization is associated with a higher affinity to intrinsic discoloration due to colorants under clinical condition (13) .
However, hardness can be defined as the ability of the material to resist permanent surface indentation, penetration, and abrasion.Hardness is indicative of the ease of finishing of a structure and its resistance to scratching (13,14) .
Besides, the indentation produced on the surface of a material from an applied force of a sharp point or an abrasive particle results from the interaction of numerous properties.Among the properties that are related to the hardness of the material, there is strength, proportional limit and ductility.The hardness of material such as composite resin can be determined by Vickers, and Knoop tests, which are classified as microhardness tests in (15,16) .
The purpose of this study was to evaluate the influence of shade, types and thickness of filling on the hardness of composite resins.

Materials and Methods:
A clear cylindrical specimens (16 mm in diameter, a package strip of drug tablets) used as a mold to prepare the specimens (figure 1).
To resemble the shade of dentine and enamel, the outer surface of the mold(boundary, bottom of the base) painted by the white color nail painting (figure 2).
The molds filled avoiding leaving bubbles by putting the mass in the center of the mold and moving it toward the boundaries so that all volume will be filled homogeneously.
Stripe of a cellulose acetate placing on top, A glass slab pressed on the top of this to express the excess material, Then the specimens were cured from one end of the mould(superficial part) for 30seconds (figure3).The specimen light cured followed the manufacturer's instructions using a light system (Polil, Italy).The power output density used is 230v/50Hz, output 470 nm/, where the tip of light unite 1mm distance from the top surface.

Surface Hardness test
Vickers Hardness test is among the oldest methods used to test material used in dentistry , The method depend on indenting the test material with a diamond indenter in the form of a pyramid with a square base and an angle of 136 degrees.
The full load 100gm is applied for 10 seconds, then measured the diagonal of each indentation that appear on the screen (figure6).
The Vicker Hardness measured by using digital microhardness tester (figure 7).
The surface hardness will be measured twice the time for each sample.2-#HV of the bottom surface (inferior layer).

Results:
Table Minimum value appears in a subgroups (R4#), While the maximum value found in a subgroup (R3*), and the comparison were highly significant in the difference between the top and the bottom surface among all subgroups.Minimum value appears in a subgroup (E1#), While the maximum value found in a subgroup (E3*),and the comparison were highly significant in the difference between the top and the bottom surface among all subgroups.
Table (3) representing the descriptive statistic (Mean, minimum value, the maximum value and standard deviation) and inferential statistic through Student t-test to a comparison between different composite materials of the same thickness and shade.
Minimum value appears in a subgroup (ME1), While the maximum value found in a subgroup (MR3), and the comparison were highly significant indifference among all subgroups.Minimum value appear in subgroup (ME1), While the maximum value found in subgroup(MR3), and the comparison were non-significant in difference between subgroup(MR1 &MR2) and(ME1&ME2),While it were significant in difference between subgroup (MR3&MR4) and (ME3&ME4).

Discussion:
This study evaluated the influence of the resin composite types, shades , and the thickness (depth) of composite filling on the microhardness of the top and the bottom resin composite surfaces, the results of this study showed that these three factors were capable of affecting polymerization microhardness.
Regarding the depth of the cavity, the results indicate that the sample thickness had no effect on the top surface hardness for both two types of resin composites, as expected.The top surface hardness of composites were less independent of light intensity than the bottom surface.The top surface is actually receiving the maximum energy from the curing light (3,12,17) .
In relation to the bottom surface, when comparing the microhardness with top surface there were statistically highly significant in differences for all subgroups.These results come with the suggestion stated by Moor et al; 2008 who stated that the composite resin increments should be thinner than 2 mm are more suitable for achieving adequate polymerization in bottom surface for both Nano filled and micro hybrid composite resin., in which the lightcuring tip is 1mm distant from the composite resin, as in deep restorations, was confirmed.The composite resin is known to be capable of dispersing the light; thus, when light passes through the bulk of the composite, the irradiance is reduced due to light scattering caused by filler particles and resin matrix (4,7,8) .
Regarding types of the composite resins, the results were generally dependent on the properties of the material evaluated, especially with regard to filler  features( Nano filled and micro hybrid).The Nano filled composite (Group R) for instance, presented the highest top and bottom VK values which were highly significant in differences from Micro hybrid composite (Group E), probably because of its large particles and the highest filler content (5) , these particles when packed together result in a precisely engineered mixture that allows light transmission for the deepest layers (17) .These results come with an agreement with Rueggeberg et al; 1993 who concluded that the filler type is one of the most influential factors in the depth of cure.
Regarding shade of composite was also consider as factor affect on microhardness, Apparently, the darker shade C2 in subgroups (MR2, MR4, ME2 & ME4) influenced the passage of the light through the composite, and the light power density produce by light cure unit was not enough to reach a minimum degree of conversion especially in deep cavity due to the effects of colorants substances added to the darker shade of composite (19) .
In case of subgroup (MR4 & ME4), the hardness results observed in the superficial areas of the composite might be explained by the fact the light easily excites the surface of light cured composite.However with the increase of composite depth part of this light is spread, absorbed ,or its passage become more difficult because of the increase of density of polymer formed which reduces the activation of camphor quinone molecules (20,21) .
Results showed that resin shade is a factor that can alter polymerization efficacy.In this study, A1 composite (Nano filled and micro hybrid) showed highest hardness means and was statistically nonsignificant in difference from C2 in thin thickness like subgroups(MR1 &MR2),(ME1 &ME2), while it were statistically significant in difference in case of deep cavity(thick thickness) like subgroups (MR3 &MR4), (ME3 &ME4).This may be due to Light transmission through the dark shades is diminished because of opacity (19) .
Opaque shades decrease the capacity of the light to penetrate into the bulk of the resin composite (21) , the light intensity is greatly reduced due to light scattering, thus decreasing the effectiveness of cure at the bottom surface (22) .These results confirm the observation made by Aguiar et al;2006 (23) , who reported higher hardness values for the composite shade A1 in comparison with C2.

Conclusions:
Within the limits of this study, it can be concluded that: 1. Resin composite has the capacity of reducing light penetration and, consequently, polymerization effectiveness of the bottom surface of the sample.

2.
In deep cavities, it is wise to use thinner composite resin increments to improve polymerization at the bottom surface.
3. The light curing method including variations in the depth of cure, and the composite shade influence the composite microhardness.
4. Clinicians should avoid thicker increments when working with composite restorations.Extended light-curing time might be indicated depending on the composite shade and on the light-curing device.

Figure 1 :Figure 2 :
Figure 1: Package strip of drug tablet

FigureFigure 7 :
Figure 3: Using light cure for polymerization of each specimen separately

Table ( 2
) representing the descriptive statistic (Mean, minimum value, maximum value and standard deviation) and inferential statistic through Student t-test to a comparison between each subgroup in relation to top and bottom surface of micro hybrid composite.

Table ( 4
) representing the descriptive statistic (Mean, minimum value, maximum value, standard