KHN and DC of composites using 3rdgen LED LCU 400 exposure received by the composite was not suf- ﬁcient to cause this behavior. According to the sci- entiﬁc literature, a bottom-to-top surface hardness ratio >0.8 is deﬁned as sufﬁciently cured14,18 For this study, all experimental groups were within this pa- rameter. Related to the DC, there were no statistically differences among the groups for either the top or bottom surfaces, the exception to this trend involved Filtek Supreme XT light-cured by 2nd- generation LED LCU (Table 4). This result may be because this particular composite and shade may contain photoinitiators in addition to CQ. It may thus not have been completely excited by the nar- row emission spectrum of Radii Cal. CQ is yellow,9 which makes it difﬁcult to produce translucent or light shades and co-initiators are used to avoid this concern. Further, associations between CQ and PPD have been shown to improve the DC1; this re- sults in lower quantities of CQ being used in some composites formulations.2 The 3rd-generation LED LCU presented a broader emission spectrum, and these additional lower wavelengths had an effect on the polymerization of the Filtek Supreme XT composite.9 Thus, in agreement with several stud- ies, we found that the effectiveness of curing asso- ciated with LED LCUs was material-dependent.2,9,19 For the “LCU” factor, two experimental condi- tions showed signiﬁcant differences in the DC. The ﬁrst experimental condition was the top surface of Filtek Supreme XT, which showed a lower DC when light-cured by 2nd-generation LED LCUs than when light-cured by 3rd-generation LED LCUs. This dif- ference can also be explained by the overlap of the emission spectrum of LCU and emission absorp- tion of the composite resin, as mentioned before. For the bottom surface of Supreme XT, this effect was not observed because resin composites ﬁlter shorter light wavelengths more than longer ones. Therefore, the shorter wavelengths delivered by 3rd-generation LED LCUs may not reach the bottom of 2mm thick specimens.9,15 The second experi- mental condition in which a difference was noted was top surface of Z350 lightcured by Ultralume 5, which showed a signiﬁcant lower DC than when this surface was light-cured by Radii Cal. This compos- ite, selected in the A1 shade, probably has a higher content of CQ and thus favors excitement by wave- lengths above 440nm (these wavelengths are more prevalent in 2nd-generation LED LCUs) (Table 2). European Journal of Dentistry For the “surface” factor, the top surface of all samples presented an equal or higher DC than the bottom surface (Table 4). However, top surface is not an adequate clinical indicator of an adequately polymerized composite restoration, since even a poor light source may produce a well-cured surface that conceals inadequate or unpolymerized resin in deeper parts of the cavity.20 The conversion ratio (DC of bottom surface/DC of top surface) should be greater than 0.8 and thus the polymerization gradient should thus not exceed 0.1 to 0.2.21 The conversion ratio obtained in this study was found to be greater than 0.9 for all experimental conditions (Table 4). CONCLUSIONS The results of the present study require rejec- tion of the ﬁrst and second hypothesis, since 2nd and 3rd-generation LED LCUs provide different levels of microhardness and degrees of conversion. The ad- ditional UV LED does not improve the DC or KHN at bottom surface of the studied composites. REFERENCES 1. Jimenez-Planas A, Martin J, Abalos C, Llamas R. De- velopments in polymerization lamps. Quintessence Int 2008;39:e74-84. 2. Polydorou O, Manolakis A, Hellwig E, Hahn P. Evaluation of the curing depth of two translucent composite materials us- ing a halogen and two LED curing units. Clin Oral Investig 2008;12:45-51. 3. Cavalcante LM, Valentino TA, Carlini B, Jr., Silikas N, Pi- menta LA. Inﬂuence of different exposure time required to stabilize hardness values of composite resin restorations. J Contemp Dent Pract 2009;10:42-50. 4. Ruttermann S, Tomruk S, Raab WH, Janda R. Effect of En- ergy Density on the Physical Properties of Resin-Based Re- storative Materials when Polymerized with Quartz-Tungsten Halogen or LED-Light. Eur J Dent 2010;4:183-191. 5. Ceballos L, Fuentes MV, Tafalla H, Martinez A, Flores J, Ro- driguez J. Curing effectiveness of resin composites at differ- ent exposure times using LED and halogen units. Med Oral Patol Oral Cir Bucal 2009;14:E51-56. 6. Beriat NC, Ertan AA, Canay S, Gurpinar A, Onur MA. Effect of different polymerization methods on the cytotoxicity of den- tal composites. Eur J Dent 2010;4:287-292. 7. Kramer N, Lohbauer U, Garcia-Godoy F, Frankenberger R. Light curing of resin-based composites in the LED era. Am J Dent 2008;21:135-142.