Year : 2009 | Volume
: 20 | Issue : 3 | Page : 308--312
Effect of different light curing units on Knoop hardness and temperature of resin composite
Ricardo Danil Guiraldo1, Simonides Consani2, Rafael Leonardo Xediek Consani3, Wilson Batista Mendes4, Thais Lympius5, Mario Alexandre Coelho Sinhoreti2,
1 Pythagoras College, Belo Horizonte MG, Brazil
2 Department of Restorative Dentistry, Piracicaba Dental School, State University of Campinas, Piracicaba SP, Brazil
3 Department of Prosthodontics and Periodontics, Piracicaba Dental School, State University of Campinas, Piracicaba SP, Brazil
4 Coordinator of the Prosthodontics Post-graduate Program in Pythagoras College, Belo Horizonte, MG, Brazil
5 Piracicaba Dental School, State University of Campinas, Piracicaba SP, Brazil
Ricardo Danil Guiraldo
Pythagoras College, Belo Horizonte MG
Aim: To evaluate the influence of quartz tungsten halogen and plasma arc curing (PAC) lights on Knoop hardness and change in polymerization temperature of resin composite.
Materials and Methods: Filtek Z250 and Esthet X composites were used in the shade A3. The temperature increase was registered with Type-k thermocouple connected to a digital thermometer (Iopetherm 46). A self-cured polymerized acrylic resin base was built in order to guide the thermocouple and to support the dentin disk of 1.0 mm thickness obtained from bovine tooth. On the acrylic resin base, elastomer mold of 2.0 mm was adapted. The temperature increase was measured after composite light curing. After 24 h, the specimens were submitted to Knoop hardness test (HMV-2000, Shimadzu, Tokyo, Japan). Data were submitted to ANOVA and Tukey«SQ»s test (a = 0.05).
Results: For both composites, there were no significant differences (P > 0.05) in the top surface hardness; however, PAC promoted statistically lower (P < 0.05) Knoop hardness number values in the bottom. The mean temperature increase showed no significant statistical differences (P > 0.05).
Conclusion: The standardized radiant exposure showed no influence on the temperature increase of the composite, however, showed significant effect on hardness values.
|How to cite this article:|
Guiraldo RD, Consani S, Xediek Consani RL, Mendes WB, Lympius T, Coelho Sinhoreti MA. Effect of different light curing units on Knoop hardness and temperature of resin composite.Indian J Dent Res 2009;20:308-312
|How to cite this URL:|
Guiraldo RD, Consani S, Xediek Consani RL, Mendes WB, Lympius T, Coelho Sinhoreti MA. Effect of different light curing units on Knoop hardness and temperature of resin composite. Indian J Dent Res [serial online] 2009 [cited 2019 Sep 18 ];20:308-312
Available from: http://www.ijdr.in/text.asp?2009/20/3/308/57370
A dental composite can be defined as a three-dimensional combination of at least two chemically different materials, with an interface separating the components.  Basically, they are composed of an organic matrix, fillers (glass, quartz, and/or melted silica), and a bonding agent, usually an organic silane, with a dual characteristic enabling chemical bonding with the filler particle and copolymerization with monomers of the organic matrix. 
Light curing is performed with visible light in the blue area range of the electromagnetic spectrum to excite camphorquinone (the most commonly used photoinitiator in composite resins) that possesses an absorption spectrum in the interval between 400 and 500 nm. The most efficient wavelength for polymerization would be 468- 470 nm.  According to previous study,  the association of camphorquinone (CQ) and amine is used as a photoinitiator/co-initiator system since the introduction of visible light activated resin composites. The absorption of light by CQ excites the molecule, a state known as 'triplet', with very short half-life. When the excited CQ interacts with an amine molecule, it results in an excited state complex called 'exciplex'. In this state, CQ can abstract a hydrogen atom from the tertiary amine resulting in the formation of a free radical.
Among the photoactivation units available in the market, the most traditional ones are those that use conventional quartz tungsten halogen (QTH) light as light curing unit (LCU). However, the main irradiation produced by those lamps is in the infrared spectrum that is absorbed by the composite and results in high molecular vibration and heat generation.  Thus, light sources that use halogen lamps need absorbent filters that reduce the passage of infrared energy to the tooth.  The efficiency of these filters varies according to the manufacturer and as a result the energy not absorbed can produce heat. Aiming clinical time reduction, high power units, such as xenon plasma arc (plasma arc curing, PAC) curing units, were introduced in the market. According to Burgess et al.,  PAC units produce significant heat as the energy passes through the electrodes. For this reason, these units also require filters to reduce tooth heating.
Studies have suggested that visible light photoactivation can also contribute to temperature increase inside the pulp chamber, ,, probably, causing damages to the pulp. , However, when the bulk increment technique was used, there was no difference in temperature increase among the photoactivation methods (continuous, intermittent light, or soft-start),  as well as for different colors from a same composite.  Pulp damage can be generated when a source of external heat is applied on the dental structure, because the increase of the temperature inside the pulp chamber can result in irreversible inflammatory lesions to the pulp tissue. ,
Most studies that have compared temperature increase during light curing have used different radiant exposures. ,,, It is claimed that photo-polymerization process of dental composite seems to be radiant exposure dependent.  Thus, to verify the temperature increase produced by different light curing methods with the same curing effectiveness, it is necessary to standardize the light radiant exposure.
When the total amount of exposure represented by the product of the light intensity and the irradiation time was constant, the depth of cure and distributions of degree of conversion were the same regardless of the light intensity or irradiation time.  Increase in degree of conversion can play important roles in the mechanical properties of composites, such as decrease of solubility, improvement of dimensional stability, lesser color alteration, and greater biocompatibility.  It is likely that composites with low properties will have short clinical life.
The objective of this study was to standardize the light radiant exposures during light curing by QTH and PAC units to evaluate its influence on the hardness and generation of heat. It has been hypothesized that the LCUs with similar light radiant exposures do not influence on hardness and temperature increase of the composites.
Materials and Methods
Two restorative resin composites of shade A3 were used in this study [Table 1]: Filtek Z250 (3M-ESPE, St. Paul, MN, USA) and Esthet X (Dentsply-Caulk, Milford, DE, USA).
The two LCUs used included a conventional halogen light curing (XL 2500, 3M-ESPE) and a PAC unit (Apollo 95E, DMD, Westlake Village, CA, USA). The characteristics of the LCUs are shown in [Table 2].
The irradiance was measured by a radiometer Model 100 Curing Radiometer (Demetron Research Corporation, Danbury, CT, USA). Then the radiant exposure was calculated by the calculation: Radiant exposure (J/cm 2 ) is the intensity (mW/cm 2 ) multiplied by time (seconds) and divided by 1,000 [Table 2]. The spectral distributions were obtained using a spectrometer (USB 2000, Ocean Optics, Dunedin, FL, USA)-[Figure 1].
Temperature change was recorded by a thermocouple K connected to a digital thermometer (Iopetherm 46, IOPE, Sao Paulo, Brazil), with 0.1°C of accuracy. A base of chemically polymerized acrylic resin (JET, Artigos Odontolσgicos Clαssico, Sao Paulo, SP, Brazil) was prepared to serve as guide for the thermocouple and as support for the dentin discs of 1.0 mm thickness. Circulars molds made of elastomer (2 mm height and 3 mm diameter) were adapted on the acrylic resin base to standardize the composite thickness. A similar apparatus for temperature measurement developed by Schneider et al.  was used [Figure 2].
The composite was inserted into the circular elastomer mold, covered with a polyester strip and photoactivated by LCUs. For photoactivation, the curing tips were positioned close to the elastomer mold/restorative composite set. For QTH, photoactivation was performed for 20 s according to the manufacturer's recommendation. For PAC, photoactivation was performed for 10 s based on previous literature  and in this current study this time of exposure was sufficient to standardize the radiant exposure.
All the measurements were performed in a temperature/ humidity-controlled room, with a constant temperature of 21°C and 30% relative humidity. For temperature measurements, the initial temperature was recorded following temperature stabilization (21°C); the composite was then light cured and temperature peak was registered. The initial temperature was deducted from the final temperature, and the temperature increase was obtained.
The temperature increase data were submitted to two-away ANOVA and the means compared by Tukey's test, at 5% of significance level (α = 0.05).
Knoop hardness test
After the photoactivation procedure, the specimens were then placed in Ύ inch diameter PVC rings filled with self- curing acrylic resin (JET, Artigos Odontolσgicos Clαssico, Sao Paulo, SP, Brazil) to keep them fixed. After 24 h, each group was flatted with SiC sandpapers with #200, 400, and 600 grit (Saint-Gobain, Recife, Pernambuco, Brazil) to obtain polished and flattened surfaces.
Indentations and Knoop hardness number (KHN) measurements were performed sequentially, in a hardness tester machine HMV-2000 (Shimadzu, Tokyo, Japan). Three indentations were performed in different polymerization depths (surface and bottom), with load of 50 g during 15 s.
The results of the KHN test were submitted to three-away ANOVA and the means compared by Tukey's test, at 5% of significance level (α = 0.05).
As shown in [Table 3], to the composite factor, independent of other factors, the KHN of Filtek Z250 was higher than Esthet X (P 0.05).
The hypothesis that the LCUs with similar light radiant exposures do not influence on hardness and temperature increase of the composites was partially accepted.
The KHT was employed in the current study due to the fact that the shape of the Knoop indenter (rhombohedral pyramid) causes elastic recovery of the projected impression to occur primarily along the shorter diagonal. The stresses are therefore distributed in such a manner that only the dimensions of the minor axis are subject to change by relaxation.  Thus, KHT provides a good parameter to estimate monomer conversion in a light-curing composite. 
The hardness of composites is influenced by several factors, such as organic matrix composition,  type and amount of filer particles,  and degree of conversion.  In this current study, the KHN of Filtek Z250 was higher than Esthet X [Table 3]. The higher KHN may be explained by differences in filler type and organic matrix composition between both materials [Table 1]. These results are in accordance to Correr et al.  Craig  suggested that composites with harder filler particles exhibit higher surface hardness. However, the bond of filler particles to the polymeric matrix also affects their hardness.
The PAC units have higher intensities of light emitted over a narrow range of wavelength and as a result offer shorter curing time in comparison to QTH.  However, the high irradiance delivered by PAC for a few seconds is not enough to produce optimum properties in resin composites and has a great influence on the degree of polymerization.  In this current study, their time of exposure was increased from 3 s (manufacturer's recommendation) to 10 s  to standardize the radiant exposure. Even so, PAC produced lower values of hardness when compared to QTH, at the bottom for Filtek Z250 [Table 4] and Esthet X [Table 5]. Thus, it can be suggested that the lower exposure time for PAC unit led to inefficient curing even with similar radiant exposure. This fact did not occur in hardness on the top surface, where the light did not pass through composites.
The KHT is used to establish a correlation between the hardness and the degree of monomer conversion (DC).  The DC directly influences mechanical properties of dental resin composite.  After photoactivation, it is desirable that the restorative material be sufficiently polymerized to convert more monomer into polymer molecules, in order to attain the best mechanical properties. Insufficiently polymerized composite resin may present quite a large number of problems, such as poor color stability, greater stain uptake, and risk of pulp aggression by nonpolymerized monomers and portions of the material with different values of Young's modulus. It has been reported that loading well- polymerized composite layers that are placed on poorly polymerized layers can lead to the composite restoration to bending inward and becoming displaced, causing marginal fracture, open margins, and cusp deflection. 
The increase of the temperature caused by photoactivation would result from the radiant exposure emitted by the LCUs. , There was no statistically significant difference for temperature increase when different methods of photoactivation were considered [Table 6]. Probably for this reason, in this study, the radiant exposure was similar (14 J/ cm 2 ) between QTH and PAC. Moreover, the polymerization process produces exothermal heat; however, it is a secondary factor in the temperature increase. , Thus, there was no difference in temperature increase between the composites Filtek Z250 and Esthet X.
The results of the present study seem to indicate that the standardization of the radiant exposure showed no statistical influence on the temperature increase of the composite, however, showed significant effect on the hardness.
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