Background and Aim: The use of light emitting diode (LED) light curing units has recently increased due to optimal properties such as longer durability, no need for filter and less heat generation compared to quartz tungsten halogen (QTH) devices. The aim of this study was to assess the shear bond strength of composite to dentin following lightcuring with QTH and LED light curing units for different time periods. Materials and Methods: In this experimental study, 60 sound extracted human molar and premolar teeth with no decay or restorations were collected. The buccal surface of the teeth was ground to expose adequate amount of dentin. The teeth were randomly divided into six groups. After acid etching and bonding, composite was packed in plastic cylindrical molds and placed on dentin surface. Groups one, two and three were cured by a LED unit (Wood Pecker) with ramp method for 20, 30 and 40 seconds, respectively. Groups four, five and six were cured with a QTH unit (Optilux 501) with a light intensity of 500 mW/cm2 for 20, 30 and 40 seconds, respectively. After keeping the samples for two weeks in distilled water at room temperature, shear bond strength was measured by a universal testing machine at a crosshead speed of 1mm/minute. The data were analyzed by Two-way ANOVA, one-way ANOVA and Tukey’ s HSD test. Results: One-way ANOVA showed a significant difference among the groups and the mean shear bond strength was the highest following light curing by LED device for 40 seconds (18. 63 MPa). Pairwise comparisons by Tukey’ s test showed significant differences in shear bond strength of groups cured with LED unit (P= 0. 059 for the difference between 20 seconds and 40 seconds and P=0. 004 for the difference between 30 seconds and 40 seconds). Conclusion: Use of LED units (ramp method) yielded superior results in terms of shear bond strength compared to QTH. Also, 40 seconds of curing is recommended in use of LED devices.

Background: Despite improvements in the optical properties of composite resins, their color stability is still a matter of discussion. This study sought to assess the effect of light intensity and curing time by a light-emitting diode (LED) light-curing unit on color stability of a methacrylate-based composite resin. Methods: In this in vitro study, 60 discs (8 × 2 mm) were fabricated of A2 shade of Z250 composite. Specimens were polished and divided into 4 groups (n = 15) for curing for 20 or 40 seconds with a light intensity of 600 or 1200 mW/cm2. After immersion in distilled water at 37° C for 24 hours, colorimetry was performed using a spectroradiometer. The specimens were then immersed in tea solution for 7 days (3 times a day, each time for one hour) and were subjected to colorimetry again. Color change (Δ E) was calculated and analyzed using two-way ANOVA. Results: Significant color change was noted following an increase in curing time (P < 0. 05). No improvement in color stability was noted after increasing the light intensity (P > 0. 05). The interaction effect of light intensity and curing time on color change was significant (P < 0. 05). Conclusions: Curing time is an important factor affecting the color stability of composite resin polymerized with LED light curing unit. On the other hand, increasing the light intensity over the standard threshold showed no significant effect.

Introduction: Characteristics of light-curing units (LCU), including irradiation uniformity and adequate light intensity, emission spectrum, exposure duration, correct light-curing technique, and regular monitoring of the LCU, are important factors in the success of composite resin restorations. In addition, infection control methods for LCUs have drawn great attention in recent years to control infection.Search Strategy: An electronic search was carried out in PubMed and Science Direct databases from 1985 to 2016 using these key word Light-curing unit, dental restoration failures, composite resin restorations, and dental restoration repair.Conclusion: This article tries to draw attention to the importance of LCUs and discusses the relevant issues. Additionally, the consequences of delivering too little or too much light energy, the concerns over leakage from undercured resins, and the ocular hazards are discussed. Practical recommendations are provided to help clinicians improve their use of the LCU so that their patients can receive safe and potentially longer lasting resin restorations.

Background and Aim: Excessive heat produced while curing of light-activated dental restorations may cause irreversible damage to dental pulp. The Aims of this study wereto measure the temperature rise(TR) induced by two different light curing units and to evaluate the relathionship between the measured TR in dentin discs of different thicknesses and different distances of light guide tips from dentin disc.Materials and Methods: In this experimental study, Quartz-Tungsten-Halogen (QTH) (400 mw/cm2) and Light-Emitting Diode (LED) systems (1500 mw/cm2) were used as the curing units. Dentin discs of 0.5, 1.0 and 1.5 mm thicknesses were prepared. Distances of 1, 2 and 3 mm from the tip of the light guide and dentin discs were set. Temperatures were recorded using a digital laser thermometer. Data were statistically analyzed using two-way ANOVA, Duncan and T-test.Results: Results showed that both dentin thickness and distance between tip of curing units and dentin disks had significant effects on temperature rise in both curing units (P<0.001 ).Conclusion: LED produced the highest and the QTH the lowest TR for all tested conditions. Reduced thermal insults were related to increased dentin thickness, increased distance between the light guide tip and dentin, and decreased energy emitted from the light-curing unit.

Introduction: The degree of polymerization depends on the type of light-curing unit. The aim of this study was to compare the hardness of composite resin cured by LED and Halogen light curing units.Methods: In this experimental study, 20 cylindrical samples of Tetric Ceram composite were prepared. Half of them were cured with Ultralume 2 LED and the other half with Astralis 7 Halogen light curing unit. In the depths of 0, 1, 2 and 3 mm from surface, one point in peripheral and one point in central portion were marked, then the hardness of these points was measured by Vickers test. The data was analyzed by a pvalue less than 0.05 considered as significant.Results: The mean hardness of samples cured by LED was more than halogen group in different depths and this difference was statistically significant in peripheral points (p=.048) but this was not significant in central points (p=0.644). The mean hardness in both groups had a decreasing trend from surface to the deep parts in central and peripheral parts and this was more in the central parts.Conclusions: Composites cured by LED light curing unit showed more hardness in similar depths, besides the hardness of composites in central parts is more than the peripheral ones in both groups.

Background: Success rate in composite restorations is associated with the degree of polymerization and light intensity. Objectives: The aim of this study was to evaluate the light intensity of curing units in private offices in Qazvin 2003.Methods: In this analytical study the light intensity of 62 light curing units were investigated before and after cleaning their light emitting tips using a radiometer. The results were classified in 3 groups as follows: A: sufficient intensity> 300 mw/cm2, B: borderline intensity (200-299 mw/cm2), and C: insufficient intensity<200 mw/cm2. Data were analyzed using chi-square and Pearson correlation tests.Findings: Mean intensity of curing units was 350.96 mw/cm2 which increased significantly to 371.6 mw/cm2 (p=0.01). Before cleaning, 67.8% of curing tips were in group A, 14.5% in B and 17.7% in C. After cleaning, 72.6% were in sufficient intensity group, 11.3% in borderline intensity, and 14.1% in insufficient intensity. Removing curing tips' contamination caused a significant increase in light intensity (p=0.01).Conclusion: Periodic assessment of light intensity and quality control of light curing units are mandatory.

Background and Aim: The factors such as adhesive compositions and light curing cement could be affected on shear bond strength between enamel and brackets and could increase orthodontic techniques, so that, the aim of this study was to evaluate the shear bond strength of orthodontic brackets using different light curing cements. Methods & Materials: Thirty human extracted intact permanent premolars were used in this study. After cleaning with ultrasnic scalar, teeth were disinfected with 1% hypoclorite for ten minutes and stored in tab water at room temperature and were divided into three groups, and the roots were mounted in acrylic resin. Brackets was bonded on buccal surface of teeth with composite resin according to their manufacturer's instructions. The specimens were thermocycled (5C˚to 55C˚for 500 cycles), shear bond strength was determined using universal testing machine at a cross head speed of Imm/min.The data was analysed by SPSS software, ANOVA and LSD tests. Result: The mean value of shear bond strength for Ideal makoo, 3M and Heliomolar was 19.52±4.36 Mpa, 19.84±5.19 Mpa and 17.04±3.53 Mpa respectively.Conclusion: There was not a statistically significant difference in shear bond strength between the three light curing cements, therefore Ideal Makoo cement is recommended for orthodontic bonding groups.

Background and Objectives: Incomplete polymerization of resin cement leads to a decrease in its physical properties increases its water sorption, solubility and pulpal irritation.Measurement of surface hardness is one of the methods for assessment the degree of polymerization. The aim in this study was to evaluate the effect of two photo-curing units with two curing times on the microhardness of two resin cements.Materials and Methods: In this labrotory study total numbers of 80 porcelain disks were made. After etching with HF (hydrofluoric acid) and using silane and bonding agent, samples were divided randomly into 8 groups (n=10). Dual-cured and light-cured cements were used in groups 1-4 and 5-8, respectively. In prepared samples from each type of cement, 20 samples were irradiated using QTH (Quartz Tungsten Halogen), (10 samples were cured for 30 seconds; groups 1 and 5 and 10 samples were cured for 60 seconds; groups 2 and 6). The remaining 20 samples were cured using LED (light-emitting diode) in the same manner (groups 3 and 7) and (groups 4 and 8), respectively. Microhardness of the cements was measured. In order to compare the microhardness in study groups, three-factor ANOVA was used.Results: There were statistically significant differences in microhardness mean values depending on the time, the type of cement and the curing unit (P<0.0005).Conclusion: Microhardness of the cements increased with an increase in curing time with two light-curing units. The use of LED resulted in an increased microhardness in two cements.Regardless of the time and the type of the light-curing unit, microhardness of the dual-cured cement was higher than that of the light-cured cement.