Introduction
The development of new technologies have improved composite resin restoration survival to more than 17 years1 and has decreased the annual failure rate to 2.2%, which is similar to dental amalgam.2 The degree of conversion (DC) of the composite is an important aspect related to the durability of the restorations because it is directly related to the physical and mechanical properties of the material.3 The DC depends upon factors such as monomer structure, amount and type of filler particles, composite shade,4 light curing time, irradiance, and depth.5
The polymerization process generates a polymer network through the substitution of the carbon double links (C=C) by simple covalent links (C-C). The DC, which is represented by the reduction of the C=C rate, has been shown to maintain a direct relationship with the composite resin microhardness3,5 so a hardness test can be used to indirectly evaluate it.3 The polymerization starts with the excitation of the camphoroquinone molecules by the blue light corresponding to a spectrum range of 400-500 nm. The absorption peak of the camphoroquinone is around 470 nm. Thus, the narrower the light spectrum is around this peak, the more effective the polymerization will be.
The quartz-tungsten-halogen (QTH) light-curing units (LCUs) still are the most employed curing devices in clinical practice. Their light is produced through the incandescence of a light filament and it presents a wide range of wavelengths requiring a filter to select only the spectrum corresponding to the blue light. The use of these LCUs along time initiates a degradation process of the device due to the generation of heat and damage to the bulb, the reflector, and the filter reducing the life-span of these units considerably.
The light emitting diodes (LEDs) have been considered as an alternative technology to the QTH devices since they present some advantages over the conventional halogen lamps. LEDs produce blue light eliminating the need for a selective filter. In addition, the blue light is produced in a narrower spectrum very close to the 470 nm of the camphoroquinone absorption peak. Furthermore, as no filtering process is present, the heat generated by the first generation LEDs is very low and the life span of these LCUs is considerably higher (about 10,000 hours) than QTH devices.6 On the other hand, these first generation LEDs only present a low light power density resulting in concern about the degree of conversion of the composites and the longevity of the restorations cured with them.7 Increase in light-curing time could overcome the low light intensity, thus, improving the composite properties.8,9
Concerns remain about the effectiveness of this new technology when used with dark shade composites. The effect of the shade is negligible.11
The basic composite insertion and polymerization protocol usually recommends the use of increments not thicker than 2 mm to guarantee an effective polymerization. Further, the light guide should be as close as possible to the composite surface to guarantee the light will not be dissipated. However, some clinical situations present a real challenge to the utilization of these recommended polymerization techniques, such as accessing the floor of Class II proximal boxes where the distance between the light guide and the material surface is generally greater.12 For such situations, the increase of the light-curing time has been strongly recommended.9
Thus, the aim of this study was to evaluate the influence of the light curing method, the composite shade, and the polymerization depth on composite microhardness.
