ORIGINAL ARTICLE The effect of light curing units, curing time, and veneering materials on resin cement microhardness y Nurcan Ozakar Ilday a *, Yusuf Ziya Bayindir a , Funda Bayindir b , Aysel Gurpinar a a Department of Restorative Dentistry, Atatu¨rk University, Erzurum, Turkey b Department of Prosthodontics, Atatu¨rk University, Erzurum, Turkey Received 1 February 2012; Final revision received 26 April 2012 Available online 31 October 2012 KEYWORDS curing lights; microhardness; resin cements; veneering materials Abstract Background/purpose: Several factors may affects microhardness of resin cement under veneering materials. The aim of this study was to evaluate the effect of different veneering materials, light-curing units and curing times (20/3, 40/6) on the microhardness of dual-cured resin cement. Materials and methods: We pressed dual-cured resin cement specimens (Clearfil SA cement, 5 mm diameter, 1 mm thick) between two microscopic glass slides covered with transparent polystyrene matrix strips to remove excess material, then irradiated them through a ceramic disc and a composite disc (A2 Esthet X HD, Dentsply, Caulk) with three types of high-power light-curing units as follows: conventional halogen light (quartz tungsten halogen) for 20/40 s, light-emitting diodes for 20/40 s and xenon plasma arc for 3/6 s. The control group specimens were cured under two layer transparent polyester matrix strips (n Z 5). After dry storage in the dark (24 h/37 C), we re- corded specimens’ Vickers microhardness numbers (50 gF load/15 s) and made three indentations on the bottom surface of each one. Data were analyzed using analysis of variance and post-hoc comparisons using Duncan’s test and the Student t test with a significance level of 5%. Results: Analysis of variance revealed significant differences in microhardness resulting from the different curing units, veneering materials and polymerization times (P < 0.05). The light-emitting diode curing unit produced higher microhardness values compared to the conventional halogen light and plasma arc light sources (P < 0.05). Both veneering materials, ceramic, and composite resin, exhibited significantly lower microhardness values than those of the control group (P < 0.05). Extended polymerization time increased mean surface microhardness values of the resin cement specimens (P < 0.05). * Corresponding author. Department of Restorative Dentistry, Atatu ¨rk University, Erzurum, Turkey. E-mail addresses: [email protected](N.O. Ilday), [email protected](Y.Z. Bayindir), [email protected](F. Bayindir), [email protected](A. Gurpinar). y This study was first presented at the BaSS Congress in Bucharest, Romania, April 28 th eMay 1 st , 2011. 1991-7902/$36 Copyright ª 2012, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved. http://dx.doi.org/10.1016/j.jds.2012.09.014 Available online at www.sciencedirect.com journal homepage: www.e-jds.com Journal of Dental Sciences (2013) 8, 141e146
Transcript
Journal of Dental Sciences (2013) 8, 141e146
Available online at www.sciencedirect.com
journal homepage: www.e- jds.com
ORIGINAL ARTICLE
The effect of light curing units, curing time,and veneering materials on resin cementmicrohardnessy
Nurcan Ozakar Ilday a*, Yusuf Ziya Bayindir a, Funda Bayindir b,Aysel Gurpinar a
aDepartment of Restorative Dentistry, Ataturk University, Erzurum, TurkeybDepartment of Prosthodontics, Ataturk University, Erzurum, Turkey
Received 1 February 2012; Final revision received 26 April 2012Available online 31 October 2012
Abstract Background/purpose: Several factorsmayaffectsmicrohardness of resin cement underveneeringmaterials.Theaimof this studywastoevaluatetheeffectofdifferentveneeringmaterials,light-curing units and curing times (20/3, 40/6) on the microhardness of dual-cured resin cement.Materials andmethods: Wepressed dual-cured resin cement specimens (Clearfil SA cement, 5mmdiameter, 1mmthick) between twomicroscopic glass slides coveredwith transparent polystyrenematrix strips to remove excess material, then irradiated them through a ceramic disc andacompositedisc (A2Esthet XHD,Dentsply,Caulk)with three typesofhigh-power light-curingunitsas follows: conventional halogen light (quartz tungsten halogen) for 20/40 s, light-emitting diodesfor 20/40 s and xenon plasma arc for 3/6 s. The control group specimens were cured under twolayer transparent polyester matrix strips (nZ 5). After dry storage in the dark (24 h/37�C), we re-corded specimens’ Vickersmicrohardness numbers (50 gF load/15 s) andmade three indentationson the bottom surface of each one. Data were analyzed using analysis of variance and post-hoccomparisons using Duncan’s test and the Student t test with a significance level of 5%.Results: Analysis of variance revealed significant differences in microhardness resulting from thedifferentcuringunits, veneeringmaterials andpolymerization times (P<0.05).The light-emittingdiode curing unit produced higher microhardness values compared to the conventional halogenlight and plasma arc light sources (P < 0.05). Both veneering materials, ceramic, and compositeresin, exhibited significantly lower microhardness values than those of the control group(P < 0.05). Extended polymerization time increased mean surface microhardness values of theresin cement specimens (P < 0.05).
of Restorative Dentistry, Ataturk University, Erzurum, Turkey.ail.com (N.O. Ilday), [email protected] (Y.Z. Bayindir), [email protected] (F. Bayindir),
he BaSS Congress in Bucharest, Romania, April 28theMay 1st, 2011.
iation for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.9.014
Conclusion: Light-curing units, curing time, and veneering materials are important factors forachieving adequate dual cure resin composite microhardness. High-intensity light and longercuring times resulted in the highest microhardness values.Copyrightª 2012, Association for Dental Sciences of the Republic of China. Published by ElsevierTaiwan LLC. All rights reserved.
Introduction
Recent high demand for esthetic restorations has increasedthe use of composites and ceramics and diminished the useof amalgam. Inlays, onlays, laminated veneers, andceramo-ceramic crowns are commonly cemented with dual-cured resin cements because light transmission throughindirect restorative materials is critical and the chemicalreaction theoretically guarantees a satisfactory polymeri-zation.1,2 These cements provide adhesion to substratesdue to their compatibility with silane agents and adhesivesystems, and offer low solubility, easy handling properties,acceptable working time and favorable esthetic results.3,4
In addition, resin cements are used to improve thecompressive strength of all ceramic crowns compared toglass ionomer and zinc phosphate cement, allowing moreeffective stress transfer from the restoration to the sup-porting tooth.5,6
The hardness of a material is the resistance of a solid tolocal deformation. Hardness is also dependent on thedegree of polymerization of the resin matrix.7 Further-more, mechanical properties and the biocompatibility ofresin cements are directly related to the degree of mono-mer conversion.4,6
During adhesive cementation procedures, different lightsources and veneering materials may affect the polymeri-zation of resin luting agents.1,6 Polymerization of thesematerials can be accomplished with different light sources,including quartz tungsten halogen (QTH), light-emittingdiodes (LEDs) and xenon plasma arc (PAC). QTH has theadvantage of low cost. Its drawbacks include highertemperatures and irradiance decline over time due to bulband filter aging.8,9 LED units offer certain advantages overconventional light-curing units: many LEDs are wireless andthey have an estimated lifetime of about 10,000 h (incontrast, QTH bulbs have a lifetime of 50e100 h).4,10 PACcuring units emit at higher intensities and were primarilydesigned to save irradiation time.11 When using PAC units,the manufacturers recommend 3 s of exposure time topolymerize composite resins with camphorquinone asa photoinitiator system.12
Doubts about the effectiveness of light activation of resincements with different light-curing units (LCUs) and beneathdifferent veneering materials still persist. Peutzfeldt13 re-ported that when dual-cure cements are adequately light-activated there is an increase in the degree of conversioncompared to dual-cured resin cements subjected to chem-ical activation alone. This confirms the importance of lightexposure to increase the degree of conversion of dual-cureresin cements. One difficulty with indirect adhesive resto-rations is achieving an adequate degree of polymerization ofthe resin luting or base material beneath the restoration,
especially if using light- or dual-cured resin material.14 Thisstudy evaluated the effect of different veneering materials,different LCUs and two different curing times (20/3 and 40/6) on the microhardness of dual-cured resin cement (ClearfilSA cement, Kuraray, Japan). The null hypothesis was thatdifferent values of resin cement hardness would be obtainedwith different veneering materials, LCUs, and curing times.
Materials and methods
Dual-cure resin cement (Clearfil SA Cement), shade A2, wasfirmly compressed under a 5-kg load for 3 minutes ina silicon cylindrical mold (1 mm thick and 5 mm in diam-eter), placed between two glass slides covered by a poly-styrene strip to produce a uniform thickness, and thenplaced against a black background. This background actedas a supporting surface and reduced the reflectivity of theunderlying surface toward each specimen. Resin cementswere polymerized on veneering materials (5 mm in diam-eter and 1 mm thick) prefabricated from ceramic material(feldspathic ceramic material) (Duceram Plus; DuceraDental GmbH & Co. KG, Rosbach, Germany; VITA shade A2)and composite resin (Esthet X HD A2, Dentsply, Milford,USA) used for simulated veneering material.
We prepared the veneering materials 1 week before theexperiment with exposure times sufficient to obtaina maximum initial degree of polymerization. Excess resinwas removed before polymerization. Two-layer transparentpolyester double strips were used as a control group. ThreeLCUs were used for polymerization of resin cementsamples:
� conventional QTH (Hilux curing light, Benlioglu Inc.,Ankara, Turkey) LCU for 20/40 s at 450 mW/cm2;
� LEDs (Elipar S10, 3M ESPE, Germany) for 20/40 s at1200 mW/cm2; and
� PAC (Valo Curing Light, Ultradent Products Inc., USA)for 3/6 s at 4500 mW/cm2.
The characteristics of each LCU are shown in Table 1. Wemeasured the power (mW) of the three light sources usinga power meter (Hilux Curing Light Meter, Benlioglu DentalInc., Ankara, Turkey). The light tips were in close contactwith either the glass slide or ceramic/composite disc (tipdiameter 8 mm) (Fig. 1). After light curing, we stored thespecimens dry in light-proof containers in a darkenedincubator at 37�C for 24 hours. In order to obtain a smoothplanar surface for hardness testing, we polished the bottomsurfaces using 400, 600 and 1200 grit SiC papers.
We created 18 groups (n Z 5) on the basis of differentcombinations of veneering materials (double strip, ceramicand composite disc), LCUs (QTH, LED and PAC) and
Figure 1 Schematic illustration of specimen preparation.
Effect of different parameters on resin cement microhardness 143
polymerization times (20/20/3 s and 40/40/6 s). We ob-tained microhardness measurements using a microhardnesstester (Buehler OmniMet MHT1600-4980T, Buehler, IL, USA),taking three readings with a 50-g load over 10 s on eachbottom surface, and converting the average into a Vickershardness number (VHN) (ISO 6507-1).
Data were analyzed statistically using SPSS 16.0 forWindows (SPSS Inc., Chicago, IL, USA) statistical software.Two-way analysis of variance (ANOVA) was used to comparethe variables: LCU, veneering material and curing time. Weused on-way ANOVA for intragroup comparisons. Post hoctests were calculated using Duncan’s test and the Student ttest. A confidence interval of 95% was set for all tests(a Z 0.05).
Results
Mean microhardness values and standard deviations for thetop surface dual-cure resins polymerized under veneeringmaterials are given in Table 2.
Xenon plasma arc Valo Curing Light, Ultradent ProducBatch number: V02640
Two-way ANOVA revealed a statistically significantdifference (P < 0.05) among the factors investigated(veneering materials, LCUs, and curing time) and alsoamong their interactions (P < 0.05). The LED curing unitsource produced a higher microhardness value compared tothe QTH and PAC light sources (P < 0.05). Both veneeringmaterials (ceramic and composite resin), exhibited signifi-cantly lower microhardness values than those of the controlgroup (P < 0.05). The control groups exhibited significantlyhigher VHN means than those groups in which the cementwas light cured through ceramic and composite restoration.
Microhardness values of resin cement specimens wereincreased efficiently by extending the polymerization time(P < 0.05). The specimens polymerized with LED/40 sbeneath double strip produced a significantly highermicrohardness than any others The specimens polymerizedwith LED/40 s beneath double strip specimens produceda significantly higher microhardness than any other speci-mens (lower VHN, 77.28 � 2.98, P < 0.05). Those poly-merized beneath ceramic specimens with PAC at a 3-s
Intensity(mW/cm2)
Time (s) Total energy(mWs/cm2)
nkara, Turkey 450 20 900040 18,000
1200 20 24,00040 48,000
ts Inc., USA 4500 3 13,5006 27,000
Table 2 Mean microhardness (Vickers hardness number) values for dual-cure resin composite under each experimentalcondition of the test groups.
Curing lights Veneering materials Vickers hardness number (�standard deviation)
*For each LCU, means followed by different uppercase letters in the columns differed statistically by Duncan’s test and lowercase lettersin the rows differed statistically by the Student t test at the 5% level.
144 N.O. Ilday et al
curing time produced a significantly lower microhardnessthan any others examined (VHN, 23.22 � 3.36, P < 0.05, seeTable 2). The presence of veneering material resulted inlower VHNs, while a longer curing time resulted in higherVHNs (P < 0.001).
Discussion
This study evaluated the VHN of dual-cure resin cementlight-cured through a 1-mm-thick composite or ceramic discusing three different LCUs for two different curing times.The effectiveness of material curing may be assesseddirectly or indirectly: direct methods, such as infraredspectroscopy, are complex, expensive and time-consuming;indirect methods include visual, scraping, and hardnesstesting.15 Many studies have used surface hardness testingbecause of its relative simplicity and good correlation withthe degree of conversion using infrared spectroscopy.14,16
Moreover, the hardness test is more sensitive thaninfrared spectroscopy in detecting small changes in thedegree of conversion after the network is cross-linked.17
Our study used microhardness measurements to estimatethe quality of resin curing under veneering materials, sincethe mechanical properties of resin-based materials can bedirectly related to the extent of the conversion of thepolymer network.15,18
The microhardness of dual-cure resin cements isaffected by energy density of the LCUs.19 Energy density isobtained from the emitted light intensity and curing time.Our results (Table 2) show that lower hardness values wereobtained when the resin cement Clearfil SA was light-curedwith PAC compared to QTH and LED. Iriyama et al. observedsimilar results for resin cement Rely X light cured with QTH,LED, and PAC. 20 Light curing with PAC for 3 and 6 s, despitebeing very fast, does not provide sufficient polymerizationof composites, and imperfect polymerization will lead toimperfect properties. Experiments with PAC curingdemonstrated that 3 � 3 s light curing at constant highenergy densities is sufficient for the polymerization ofhybrid resin composites.21 Lower light energy may affectpolymer development, primarily by decreasing the double-
bond conversion, since the polymerization process isdependent on radiant exposure delivered to materials.High-intensity lights may favor the formation of moredensely cross-linked networks by generating a multitude ofpolymer growth centers. More densely cross-linked poly-mers will therefore provide higher hardness outcomes.6 Sohand Yap have stated that light curing at high intensity wouldlead to a highly cross-linked polymer chain, and thus togreater hardness.22 In regions exposed to low energydensity, the polymer chain is more linear with highermobility and lower hardness values.
Adequate polymerization of resin-based luting cement iscritical for stability, optimal mechanical features, and theclinical performance of indirect restorations.23,24 In addi-tion, maximum bond strength of dual-cured cements is onlyachieved when light activation is done properly.25 Thedegree of conversion in a polymerization reaction isdependent on the energy delivered during light curing,characterized as the product of light intensity and exposuretime.1 Longer light exposure times result in greatercomposite resin cure depth, conversion degree and hard-ness.3 Some have argued that the low irradiation output ofLCUs may be compensated for by increasing the irradiationtime, without affecting the conversion degree forcomposites, in such a way that different LCUs can have thesame energy density ([mW/cm2] � T).25 The degree ofpolymerization with PAC LCU can be compensated for withlonger exposure times. PAC techniques require a significantincrease in irradiation time when applied to indirect poly-merization.25,26 Our results show that the increased expo-sure time of the high-power PAC light (6 s) can compensatefor the attenuation of light through the 1-mm-thickveneering material and the dual-cure resin cement,resulting in hardness values close to those of other groups.
In this study, the LED LCU generally produced highermicrohardness values, especially at 40 s polymerization.This probably due to the higher energy density used inthese groups. This result is in accordance with a study bySantos et al,4 who reported higher VHN values with LED LCUcompared to QTH LCU. It has previously been reported thatLED units are the most efficient ones because they arecapable of converting electrical current into the correct
Effect of different parameters on resin cement microhardness 145
wavelength, similar to the absorption wavelength of cam-phorquinone. Ozyes‚il et al, however, observed similardegrees of conversion for resin cement Variolink II lightcuring with conventional QTH and PAC.27
When light is transmitted through a ceramic orcomposite, it is absorbed and reflected, losing intensity.Hasegava et al observed a reduction in light when trans-mitted through laminated veneers during resin cementpolymerization. Our study confirmed that the presence ofa 1-mm-thick veneering material interposed during curingreduced VHNs.28 This finding is also corroborated by otherauthors.14,29 These low hardness figures may be ascribed tolight attenuation by the veneering material or the resincement itself.3 They may also be attributed to the differentrefraction indexes and opacity of the veneering materials,because of their distinct nature (composite and ceramic).1
In our study, there were no significant differences betweenthe veneering materials (composite and ceramic; exceptfor double strips), which involve distinct optical charac-teristics and compositions. Whereas Esthet X HD is anindirect composite resin, Duceram Plus is a feldspathicporcelain.
The light cure polymerization of resin cements is affectedby chemical composition, filler particle size, shade, and thethickness of overlying restorations, as well as light intensityand time of exposure.30e32 Favorable polymerization of resincement is crucial in order to achieve optimal cement prop-erties to prolong the clinical life of the overlying indirectrestoration. Additional studies should be conducted tofurther evaluate light curing using other types of veneeringmaterials, resin cements with different filler loads andmonomer and photoinitiatorecatalyst compositions.
Within the limits of an in vitro investigation, weconcluded that the LED curing unit was associated with thehighest hardness values for surface hardness of the dual-cure resin cement (Clearfil SA cement) under veneeringmaterials. LED LCUs may be considered more effective thanQTH and PAC LCUs for polymerization of the dual-cure resincement material (Clearfil SA cement). The presence ofsimulated veneering restoration material inhibits the poly-merization of the underlying dual-cure resin cement mate-rial. Increasing the polymerization time had a positive effecton dual-cure resin cement microhardness for all LCUs. Inconclusion, prolonged exposure time is necessary in thepresence of veneering materials, especially PAC LCUs.
Acknowledgments
The authors are indebted to the Department of MechanicalEngineering at Ataturk University for use of theirequipment.
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