## 4 Discussion  
In this study, the effect of the resin type and placement technique on the polymerization efficiency of RBCs polymerized by an LED LCU was evaluated. The DC results obtained showed that there were statistically significant differences in terms of RBC type and placement technique. Therefore, the null hypothesis of the study that the DC would be affected by the resin type and placement technique was accepted. Several factors influencing the DC can be classified into material (i.e., composite)-related, LCU-related, and placement technique-related. Composite-related factors include resin composition, filler content, filler particle size, load and distribution, color, and thickness of the RBC [3; 19; 33; 34]. Light source-related factors include the type of light source, wavelength, light intensity, light-tip size, the distance of the light tip from the surface, type and concentration of photoinitiator, curing mode and time of the material, and post-irradiation time [2; 7; 9; 16; 27; 35]. The placement technique-related factors include the type, viscosity, and properties of the RBC material, size and depth of restoration, and application technique, which minimize the effects of polymerization shrinkage and shrinkage stress when curing RBC materials. The Bis-GMA-based RBCs used in the present study were placed with three placement techniques and polymerized with an LED LCU at a continuous constant intensity using recommended curing times. In general, conventional RBCs are placed and light-polymerized in 2 mm increments [25]. RBC samples were placed with the conventional standard technique into 4 mm diameter and 2 mm depth models for Group 1 of this in vitro study. The DCs of 2 mm thick RBCs placed with the standard technique ranged between 62.8 and 32.3%. Different DC values were obtained for each RBC, but each RBC exhibited the same DC at the top and bottom surfaces. This result is compatible with the concept accepted by many researchers and instructions from manufacturers that the material should be at least 2 mm in thickness for sufficient polymerization. Similarly, [PERSON] et al. (1997) stated that different materials reacted differently to the same light intensity and the same period of exposure, even when the same shades were used [23]. On the other hand, it was declared that the distance from the cusp tip to the gingival floor of Class II molar cavities is at least 8 mm in 15% of maxillary molars [36]. As this cavity depth simulates the clinical conditions, RBC samples were placed into 8 mm depth molds with the bulk and incremental placement techniques for Groups 2 and 3 in this in vitro study. While the DCs of RBCs placed with the bulk technique ranged between 59.0 and 32.1%, the DC of RBCs placed with the incremental technique ranged between 53.5 and 29.2%. Different DC values were obtained for each RBC placed with bulk and incremental techniques at the top, middle, and bottom surfaces. DC values diminished gradually from the top to the bottom surfaces. This result is in agreement with the results of [PERSON] et al. (2003) and [PERSON] et al. (1997) [23; 37]. It has been thought that these differences result from the bulk or thickness of RBCs and the distance between the tip of the LED LCU and the bottom surface of the RBC tested. Although the minimum DC required for a clinically successful restoration has not yet been precisely determined, it is recommended that the DC value for the occlusal surface of the restorations should be at least 55% [19; 22; 29]. Differences in DC values may be related to changes in the resin matrix composition, filler content, and the size and distribution of filler particles of the tested RBCs [14; 29]. The final DCs of the different monomer system increased in the following order: Bis-GMA < Bis-EMA < UDMA < TEGDMA [29; 38; 39]. Packable Filtek P60 and micro-hybrid Filtek Z250 both contain Bis-GMA, UDMA, Bis-EMA, and TEGDMA, and the basic component is Bis-GMA.