Minimum Radiant Exposure and Irradiance for Triggering Adequate Polymerization of a Photo-Polymerized Resin Cement

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Article Minimum Radiant Exposure and Irradiance for Triggering Adequate Polymerization of a Photo-Polymerized Resin Cement

Qi Li 1,2,†, Hong-Lei Lin 1,†, Ming Zheng 1,*, Mutlu Ozcan 3 and Hao Yu 1,*

1 Fujian Key Laboratory of Oral Diseases, Fujian Provincial Engineering Research Center of Oral Biomaterial, Stomatological Key Laboratory of Fujian College and University, School and Hospital of Stomatology, Fujian Medical University, Fuzhou 350000, China; [email protected] (Q.L.); [email protected] (H.-L.L.) 2 Fujian Provincial Governmental Hospital, Fuzhou 350000, China 3 Division of Dental Biomaterials, Clinic for Reconstructive Dentistry, Center for Dental Medicine, University of Zurich, 8006 Zurich, Switzerland; [email protected] * Correspondence: [email protected] (M.Z.); [email protected] or [email protected] (H.Y.) † Qi Li and Hong-Lei Lin contributed equally to this work.

Abstract: This study aimed to establish the minimum radiant exposure and irradiance to trigger an adequate polymerization of a photo-polymerized resin cement. In total, 220 disc-shaped specimens (diameter of 10 mm and thickness of 0.1 mm) were fabricated using a photo-polymerized resin cement (Variolink N-transparent, Ivoclar Vivadent). To investigate the minimum radiant exposure, the specimens were polymerized with radiant exposures of 1, 2, 3, 4, 5, 6, and 18 J/cm2 (n = 20). During polymerization, the irradiance was maintained at 200 mW/cm2. To investigate the minimum 2 irradiance, the specimens were polymerized with irradiances of 50, 100, 150, and 200 mW/cm (n = 20). During polymerization, the radiant exposure was maintained at the previously determined Citation: Li, Q.; Lin, H.-L.; Zheng, minimum radiant exposure. The Vickers microhardness (HV) and degree of conversion (DC) of the M.; Ozcan, M.; Yu, H. Minimum carbon double bond of the specimens were measured to determine the degree of polymerization Radiant Exposure and Irradiance for of the specimens. The results were analyzed using one-way analysis of variance (ANOVA) and Triggering Adequate Polymerization Tukey’s test (p < 0.05). In the investigation of the minimum radiant exposure, the HV and DC of of a Photo-Polymerized Resin the specimens polymerized with a radiant exposure from 1 to 5 J/cm2 were significantly lower than Cement. Materials 2021, 14, 2341. 2 p https://doi.org/10.3390/ma14092341 those with 18 J/cm (all < 0.05). However, no significant difference in HV and DC was found between the specimens polymerized with 6 J/cm2 and 18 J/cm2 (p > 0.05). In the investigation of the 2 Academic Editors: Francesco Baino minimum irradiance, the specimens polymerized with an irradiance of 50 mW/cm had significantly 2 and Gianrico Spagnuolo lower HV and DC than the specimens polymerized with an irradiance of 200 mW/cm (p < 0.05). However, no significant difference in the HV and DC was found among the specimens cured with Received: 9 April 2021 irradiances of 100, 150, and 200 mW/cm2 (p > 0.05). In conclusion, the minimum radiant exposure Accepted: 26 April 2021 and irradiance to trigger an adequate polymerization of the light-cured resin cement were 6 J/cm2 Published: 30 April 2021 and 100 mW/cm2, respectively.

Publisher’s Note: MDPI stays neutral Keywords: radiant exposure; irradiance; polymerization; photo-polymerized resin cement with regard to jurisdictional claims in published maps and institutional afﬁl- iations. 1. Introduction All-ceramic restorations are increasingly preferred by dentists and patients because of their natural appearance, biocompatibility, durability, and high compressive resistance [1]. Copyright: © 2021 by the authors. A high bond strength for the adhesion complex formed between the ceramic, resin cement, Licensee MDPI, Basel, Switzerland. and dental hard tissues is vital for the success of a ceramic restoration [2]. Inadequate This article is an open access article polymerization of resin cements leads to microleakage, poor retention of the restoration, distributed under the terms and poor marginal adaptation, easy fracture, unstable color, and cytotoxicity [3–6]. conditions of the Creative Commons Resin cements are classiﬁed into three types according to their activation modes: Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ chemical-cured (CC), photo-polymerized/light-cured (LC), and dual-cured (DC) resin 4.0/). cements [7]. LC resin cements are most used in aesthetic rehabilitation due to their better

Materials 2021, 14, 2341. https://doi.org/10.3390/ma14092341 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 8

Materials 2021, 14, 2341 Resin cements are classified into three types according to their activation modes:2 of 7 chemical-cured (CC), photo-polymerized/light-cured (LC), and dual-cured (DC) resin cements [7]. LC resin cements are most used in aesthetic rehabilitation due to their better color stability than that of the other cements [7]. The polymerization of LC resin materials color stability than that of the other cements [7]. The polymerization of LC resin materials is dependent on the total energy of the irradiation, e.g., the radiant exposure (RE), which is dependent on the total energy of the irradiation, e.g., the radiant exposure (RE), which is the product of the irradiance (mW/cm2) (also known as light intensity) and irradiation is the product of the irradiance (mW/cm2) (also known as light intensity) and irradiation time [8]. Different combinations of irradiance and light curing time provide similar mate- time [8]. Different combinations of irradiance and light curing time provide similar material rial properties at constant REs, which is known as the exposure reciprocity law (ERL) [9]. properties at constant REs, which is known as the exposure reciprocity law (ERL) [9]. In dental practice, LC resin composites are used to restore dental defects, and LC In dental practice, LC resin composites are used to restore dental defects, and LC resin cements are used to bond the dental restoration. The REs required to adequately resin cements are used to bond the dental restoration. The REs required to adequately polymerize 1.5 and 2 mm thick LC resin composites were reported to be 18 and 24 J/cm2, polymerize 1.5 and 2 mm thick LC resin composites were reported to be 18 and 24 J/cm2, re- respectively [9–11]. Moreover, a rating scale developed by the manufacturer (De- spectively [9–11]. Moreover, a rating scale developed by the manufacturer (Demetron/Kerr, metron/Kerr, CA, USA) showed that the polymerization from 2 to 3 mm thick LC resin CA, USA) showed that the polymerization from 2 to 3 mm thick LC resin composites would composites would be inadequate when the irradiance is below 200 mW/cm2 [12]. Alt- be inadequate when the irradiance is below 200 mW/cm2 [12]. Although resin cements havehough a similarresin cements composition have a assimilar resin composition composites as [13 resin], the composites thickness of[13], resin the cementsthickness in of clinicalresin cements applications in clinical is much applications thinner (~0.1 is much mm) [ 14thinner,15]. Therefore, (~0.1 mm) it [14,15]. is conceivable Therefore, that theit is REconceivable and irradiance that the requirements RE and irradiance would be requir differentements for LCwould resin be cements. different However, for LC resin limited ce- informationments. However, is available limited in information the literature. is available in the literature. AlthoughAlthough the the irradiance irradiance of contemporaryof contemporary light-emitting light-emitting diode diode (LED) (LED) light curinglight curing units 2 (LCUs)units (LCUs) is commonly is commonly far higher far than higher the than abovementioned the abovementioned threshold threshold (200 mW/cm (2002 )[mW/cm16], the) actual[16], the irradiance actual irradiance reaching reaching the LC resin the LC cement resin is cement reduced is reduced drastically drastically duringlight during curing light throughcuring through a ceramic a restoration.ceramic restoration. A previous A previo study reportedus study thatreported the irradiance that the irradiance decreased byde- >80%,creased 95%, by and>80%, 99% 95%, through and 99% 1.5, through 3.0, and 6.01.5, mm 3.0, thickand 6.0 glass mm ceramic thick glass discs, ceramic respectively, discs, comparedrespectively, to the compared irradiance to the with irradiance no ceramic with disc no at ceramic 0 mm distancedisc at 0 [mm16]. distance Considering [16]. suchCon- massivesidering attenuationsuch massive of theattenuation irradiance, of the it is irradi necessaryance, forit is dentists necessary to knowfor dentists the minimum to know requiredthe minimum irradiance required and requiredirradiance RE and to adequatelyrequired RE polymerize to adequately an LC polymerize resin cement, an LC which resin werecement, the purposeswhich were of thisthe purposes study. of this study.

2.2. MaterialsMaterials andand MethodsMethods TheThe experimentalexperimental protocol protocol is is exhibited exhibited in in Figure Figure1 .1.

FigureFigure 1. 1.Flow Flow diagram diagram for for this this study. study.

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2.1. Specimen Preparation 2.1.An Specimen LED LCU Preparation (Bluephase N, Ivoclar Vivadent, Liechtenstein) was used in the present study. TheAn actual LED LCU irradiance (Bluephase of the N, LED Ivoclar LCU Vivadent, (700 mW/cm Liechtenstein)2) was determined was used by in an the optical present powerstudy. meter The (Sensor actual S1350C, irradiance Console of the PM100D, LED LCU Thorlabs, (700 mW/cm Newton,2) wasNJ, USA). determined Four glass by an ceramicoptical plates power (Empress meter (Sensor HT, Ivoclar S1350C, Vivadent, Console Schaan, PM100D, Liechtenstein) Thorlabs, Newton, with different NJ, USA). thick- Four nessesglass (1, ceramic 1.5, 2, platesand 3 (Empressmm; light HT,attenuation Ivoclar Vivadent, rate: 71.5, Schaan, 78.5, 86, Liechtenstein) and 93%) were with used different to attenuatethicknesses the irradiance (1, 1.5, 2, and of the 3 mm; light light from attenuation the unit at rate: 200, 71.5,150, 100, 78.5, and 86, and50 mW/cm 93%) were2. used to attenuateThe specimens the irradiance were fabricated of the light with from a custom the unitized at 200, stainless-steel 150, 100, and mold 50mW/cm (thickness2. of 0.1 mm Theand specimensinternal diameter were fabricated of 10 mm) with using a customized a LC resin cement stainless-steel (Variolink mold N-transpar- (thickness of ent,0.1 Ivoclar mm and Vivadent, internal Liechten diameterstein). of 10 An mm) appropriate using a LC amount resin cement of the (Variolink resin cement N-transparent, was ap- pliedIvoclar to the Vivadent, stainless-steel Liechtenstein). mold. Light An curing appropriate was performed amount ofby the placing resin the cement tip of was the appliedLCU in contactto the stainless-steel with the ceramic mold. plate. Light A curingblack countertop was performed was used by placing to support the tip the of stainless- the LCU in steelcontact mold withwith thethe ceramicspecimen plate. and Adecrease black countertop the light reflectivity was used to[17]. support A polyester the stainless-steel film was placedmold on with the thetop specimenand bottom and of decrease the specimen the light to isolate reﬂectivity it from [17 the]. A glass polyester ceramic ﬁlm plate was and placed blackon countertop the top and and bottom prevent of the oxygen-inhibition specimen to isolate during it from polymerization the glass ceramic [7,17] plate (Figure and 2). black countertop and prevent oxygen-inhibition during polymerization [7,17] (Figure2).

Figure 2. Polymerization apparatus for the LC resin cement. Figure 2. Polymerization apparatus for the LC resin cement. 2.2. Determination of the Minimum RE 2.2. DeterminationThe software of the G*power Minimum (version RE 3.1, Dusseldorf, Germany) was used to calculate the numberThe software of specimens. G*power The (version power 3.1, for theDussel primarydorf, Germany) outcome (surface was used microhardness) to calculate the was numbercalculated of specimens. based on aThe two-sided power fort-test the with primary a significance outcome level (surface of 5% microhardness) and a statistical was power calculatedof 80%. Thebased results on a indicatedtwo-sided that t-test a minimum with a significance sample size level of 10 of was 5% required and a statistical per group. power ofThe 80%. experiment The results was indicated performed that ina minimum a darkroom sample environment. size of 10 Based was required on a previous per 2 group.study [12], 200 mW/cm was considered adequate to trigger polymerization of the 0.1 mm thickThe LC experiment resin cement was specimens. performed Six in experimentala darkroom environment. groups (1 J, 2 J,Based 3 J, 4 J,on 5 J,a andprevious 6 J) and study1 positive [12], 200 control mW/cm group2 was were considered adopted adequate according to to tri thegger RE polymerization (n = 20). According of the to 0.1 previous mm 2 thickstudies LC resin [9,11 cement], 18 J/cm specimens.was selected Six experimental in the positive groups control (1 J, group.2 J, 3 J, 4 The J, 5 transmittedJ, and 6 J) and light 2 1 positivefrom 200 control mW/cm groupwas were irradiated adopted according onto the specimens to the RE (n from = 20). groups According 1 J, 2 to J, previous 3 J, 4 J, 5 J, studies6 J,and [9,11], the 18 control J/cm2 was for5, selected 10, 15, in 20, the 25, positive 30, and control 90 s, providing group. The REs transmitted of 1, 2, 3, light 4, 5, 6, 2 fromand 200 18 mW/cm J/cm , respectively.2 was irradiated A digital onto the microhardness specimens from tester groups (HXD-1000TM/LCD, 1 J, 2 J, 3 J, 4 J, 5 Baoling,J, 6 J, ≥ andChina) the control was used for 5, to 10, produce 15, 20, 325, microindentations 30, and 90 s, providing (1 at the REs center of 1, and2, 3, others4, 5, 6, atand1 18 mm J/cmfrom2, respectively. the center) A on digital the upper microhardness surface of te eachster specimen.(HXD-1000TM/LCD, The microindentations Baoling, China) were n wasthen used measured to produce to determine3 microindentations the Vickers (1 microhardnessat the center and (HV) others of the at specimens≥1 mm from ( the= 10). Measurements were performed under a load of 50 gf for 15 s [17–19]. A Fourier transform center) on the upper surface of each specimen. The microindentations were then measured infrared spectrometer (Nicolet iS50, Thermo Scientific, Waltham, WJ, USA) was used to to determine the Vickers microhardness (HV) of the specimens (n = 10). Measurements test the degree of conversion (DC) of carbon double bond of the specimens (n = 10). The were performed under a load of 50 gf for 15 s [17–19]. A Fourier transform infrared spec- absorbance spectra of uncured resin materials and cured resin specimens were acquired trometer (Nicolet iS50, Thermo Scientific, Waltham, WJ, USA) was used to test the degree by scanning the specimens 32 times over a 4000–400 cm−1 range with a resolution of of conversion (DC) of carbon double bond of the specimens (n = 10). The absorbance spec- 4 cm−1 (OMNIC spectra Software, version 6.2, Thermo Scientific, Madison, WI, USA). The tra of uncured resin materials and cured resin specimens were acquired by scanning the DC was calculated from the absorbance intensity of aliphatic carbon double bond peaks specimens 32 times over a 4000–400 cm−1 range with a resolution of 4 cm−1 (OMNIC spectra (1638 cm−1) and the ratio of the absorbance intensity of aromatic carbon double bond peaks Software, version 6.2, Thermo Scientific, Madison, WI, USA). The DC was calculated from (1608 cm−1) of cured (C) and uncured (U) resin. DC was calculated as follows: DC (%) = −1 the absorbance intensity−1 of −aliphati1 c carbon double bond −peaks1 (1638− cm1 ) and the ratio [1 − (1638 cm /1608 cm )Peak height after curing/(1638 cm /1608 cm )Peak height before curing] of the absorbance intensity of aromatic carbon double bond peaks (1608 cm−1) of cured (C) × 100 [16,20]. All data were collected at 25 ◦C ± 1 ◦C and 60 ± 5% humidity conditions and

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−1 Materials 2021, 14, 2341 and uncured (U) resin. DC was calculated as follows: DC (%) = [1 − (1638 cm /16084 of 7 cm−1)Peak height after curing/(1638 cm−1/1608 cm−1)Peak height before curing] × 100 [16,20]. All data were collected at 25 °C ± 1 °C and 60 ± 5% humidity conditions and shielded from ambient and room light. Before the measurements, all specimens were placed in the dark for 15 min [21].shielded According from ambient to the statistical and room analysis, light. Before when the the measurements, mean value of all HV specimens and DC were of certain placed experimentalin the dark forgroups 15 min did [not21]. ha Accordingve significantly to the different statistical mean analysis, values when of HV the and mean DC from value thatof HVof the and control DC of group, certain the experimental minimum groupsRE of the did experimental not have significantly groups was different considered mean thevalues minimum of HV RE and to DC trigger from an that adequate of the control polymerization group, the of minimum the resin REcements. of the experimental groups was considered the minimum RE to trigger an adequate polymerization of the 2.3.resin Determination cements. of the Minimum Irradiance 2.3.The Determination experiment of thewas Minimum performed Irradiance in a darkroom environment. Three experimental groups (50 mW, 100 mW, and 150 mW) and 1 positive control group were adopted ac- The experiment was performed in a darkroom environment. Three experimental cording to the irradiance (n = 20). The transmitted light from 50, 100, 150, and 200 mW/cm2 groups (50 mW, 100 mW, and 150 mW) and 1 positive control group were adopted according was irradiated to the specimens from the groups 50 mW, 100 mW, 150 mW, and a positive to the irradiance (n = 20). The transmitted light from 50, 100, 150, and 200 mW/cm2 was control for a certain period of time (200 mW/cm2 for 30 s) to provide the previously deter- irradiated to the specimens from the groups 50 mW, 100 mW, 150 mW, and a positive minedcontrol minimum for a certain RE. periodThe HV of and time DC (200 of mW/cmthe specimens2 for 30 were s) to measured provide theas previouslydescribed above.determined According minimum to the RE. statistical The HV analysis, and DC ofwhen the specimenscertain experimental were measured groups as describeddid not haveabove. significantly According different to the statistical mean HV analysis, and DC when from certain the control experimental group, groupsthe minimum did not irra- have diancesignificantly of the experimental different mean groups HV and was DC consid fromered the controlthe minimum group, theirradiance minimum to trigger irradiance an adequateof the experimental polymerization groups of the was resin considered cements. the minimum irradiance to trigger an adequate polymerization of the resin cements. 2.4. Statistical Analysis 2.4.Statistical Statistical Analysisanalysis was performed using the SPSS 25.0 software package (Armonk, NY, USA).Statistical Shapiro-Wilk analysis was test performed was used usingto test the the SPSS data 25.0 normality. software packageOne-way (Armonk, analysis NY,of varianceUSA). Shapiro-Wilk (ANOVA) and test Tukey’s was used test to testwere the performed data normality. on the One-way HV and analysis DC. p < of 0.05 variance was considered(ANOVA) to and be Tukey’s statistically test weresignificant. performed on the HV and DC. p < 0.05 was considered to be statistically significant. 3. Results 3. Results In the minimum RE experiment, all mean HV and DC values of the specimens in groupsIn 1 theJ, 2 minimumJ, 3 J, 4 J, and RE 5 experiment, J were significantly all mean different HV and from DC valuesthose in of the the control specimens group in (allgroups p < 0.05). 1 J, 2However, J, 3 J, 4 J, andno significant 5 J were significantly difference in different mean HV from and those DC was in the found control between group group(all p <6J 0.05).and the However, control nogroup significant (p > 0.05) difference (Figure 3). in Therefore, mean HV and6 J/cm DC2 was was determined found between as 2 thegroup minimum 6J and RE the to control trigger group an adequate (p > 0.05) polymerization (Figure3). Therefore, of the resin 6 J/cm cements.was determined as the minimum RE to trigger an adequate polymerization of the resin cements.

group 1J 25.5 * group 1J 20.1 * ns group 2J 35.5 * group 2J 23.5 *

group 3J 43.0 * group 3J 32.9 *

group 4J 53.9 * group 4J 44.6 *

group 5J 61.4 * group 5J 61.3 * group 6J 75.3 group 6J 76.5 ns ns control group 75.0 control group 77.0

0 20406080100 0 20406080100 Vickers microhardness(HV) Degree of conversion(DC)(%)

Figure 3. HV and DC of the LC resin cements triggered by different REs in the minimum RE Figureexperiment 3. HV and (mean DC with of the SD, LCn resin= 10). cements * indicates triggere thatd the by HVdifferent or DC REs of thein the group minimum was significantly RE ex- perimentdifferent (mean from with that ofSD, the n = control 10). * indicates group ( pthat< 0.05).the HV “ns” or DC indicates of the group that there was significantly was no significant dif- ferentdifference from inthat the of HV the orcontrol DC between group (p two < 0.05). groups “ns” (p indicates> 0.05). that there was no significant difference in the HV or DC between two groups (p > 0.05). In the minimum irradiance experiment, the specimens in the 50 mW group had significantly different mean values of HV and DC from those in the control group (p < 0.05). The specimens in the 150 mW and 100 mW groups did not have significantly different mean values of HV and DC from those in the control group (all p > 0.05) (Figure4). Therefore, 100 mW/cm2 was determined as the minimum irradiance to trigger adequate

polymerization of the resin cements. Materials 2021, 14, x FOR PEER REVIEW 5 of 8

In the minimum irradiance experiment, the specimens in the 50 mW group had significantly different mean values of HV and DC from those in the control group (p < 0.05). The specimens in the 150 mW and 100 mW groups did not have significantly different mean values of HV and DC from those in the control group (all p > 0.05) (Figure 4). There- Materials 2021, 14, 2341 fore, 100 mW/cm2 was determined as the minimum irradiance to trigger adequate5 of 7 polymerization of the resin cements.

group 150mW 75.7 group 150mW 74.0

group 100mW 75.0 group 100mW 72.0 ns ns

group 50mW 66.5 * ns group 50mW 52.4 * ns

control group 75.1 control group 78.4

0 20406080100 0 20406080100 Vickers microhardness(HV) Degree of conversion(DC)(%)

Figure 4. HV and DC of the LC resin cements triggered by different irradiances in the minimum Figureirradiance 4. HV experiment and DC of (meanthe LC withresin SD,cementsn = 10).triggere * indicatesd by different that the irradiances HV or DC in ofthe the minimum group was irradiancesignificantly experiment different (mean from thatwith of SD, the n control= 10). * indicates group (p that< 0.05). the HV “ns” or indicatesDC of the that group there was was sig- no nificantlysignificant different difference from in thethat HV of the or DCcontrol between group two (p < groups 0.05). “ns” (p > 0.05).indicates that there was no significant difference in the HV or DC between two groups (p > 0.05). 4. Discussion 4. DiscussionWhen an LCU irradiates a resin cement through a ceramic restoration, the actual irradianceWhen reachingan LCU irradiates the cement a isresin reduced cement to athrough certain extenta ceramic due torestoration, the attenuation the actual effect ir- of radiancethe ceramic reaching on the the light, cement and is the reduced polymerization to a certain of theextent cement due to may the be attenuation compromised effect [16 of]. theAlthough ceramic previous on the light, studies and have the beenpolymerization performed of regarding the cement the adequatemay be compromised polymerization [16]. of AlthoughLC resin compositesprevious studies for restorative have been purposes performed (filling) regarding [10–12 the], informationadequate polymerization regarding the adequate polymerization of LC resin cements for bonding is limited. The present study, of LC resin composites for restorative purposes (filling) [10–12], information regarding for the first time, investigated the minimum irradiance and RE required to adequately the adequate polymerization of LC resin cements for bonding is limited. The present polymerize an LC resin cement. study, for the first time, investigated the minimum irradiance and RE required to ade- The extent of polymerization of resin cements can be measured by several methods. quately polymerize an LC resin cement. Fourier transform infrared spectroscopy (FTIR) can directly measure the extent of polymer- The extent of polymerization of resin cements can be measured by several methods. ization of a resin by assessing the DC of the carbon double bond, which evaluate the change Fourier transform infrared spectroscopy (FTIR) can directly measure the extent of in ratio of characteristic absorbance peaks of the cured and uncured resin. Commonly, polymerization of a resin by assessing the DC of the carbon double bond, which evaluate the aliphatic carbon double bond peak height or area at 1638 cm−1 is compared to the the change in ratio of characteristic absorbance peaks of the cured and uncured resin. aromatic carbon double bond peak height or area at 1608 cm−1 [9,16,20]. Furthermore, Commonly, the aliphatic carbon double bond peak height or area at 1638 cm−1 is compared the microhardness has been shown to be a simple and reliable indicator of the carbon to the aromatic carbon double bond peak height or area at 1608 cm−1 [9,16,20]. Further- double bond conversion, and it has been used to indirectly measure the extent of poly- more,merization the microhardness of resins [22– 24has]. been Therefore, shown the to DCbe a and simple microhardness and reliable testindicator were usedof the in car- the bonpresent double study. bond Previous conversion, studies and on theit has internal been fitused and to marginal indirectly adaptation measure ofthe restorations extent of polymerizationshowed that a of gap resins between [22–24]. a restoration Therefore, andthe DC a tooth and rangedmicrohardness was tens test to were hundreds used in of themicrons present [14 study.,15,25, 26Previous]. Therefore, studies the on cement the internal thickness fit and of the marginal polymerization adaptation model of restora- was set tionsas 0.1 showed mm in thisthat study.a gap between a restoration and a tooth ranged was tens to hundreds of micronsIn the [14,15,25,26]. present study, Therefore, the minimum the ceme REnt and thickness irradiance of the to polymerization trigger an adequate model poly-was setmerization as 0.1 mm of in the this LC study. resin cement were 6 J/cm2 and 100 mW/cm2, respectively, which is obviouslyIn the present different study, from the the minimum previous RE results and irradiance for an LC to resin trigger composite an adequate (18 J/cm polymer-2 and ization200 mW/cm of the 2LCfor resin a 1.5 cement mm thick were LC 6 resinJ/cm2 composite)and 100 mW/cm [9–112]., respectively, This difference which may is be obvi- due ouslyto the different different from thicknesses the previous of the resu resinlts cement for an andLC composite.resin composite Moreover, (18 J/cm the2 scrapingand 200 mW/cmtest was2 for used a 1.5 in amm previous thick LC study resin regarding composite) LC [9–11]. resin compositesThis difference [11]. may However, be due forto the LC differentresin cements thicknesses with aof thickness the resin ofcement 0.1 mm, and it composite. is difficult Moreover, to perform the the scraping scraping test test. was In accordance with previous studies, the DC and microhardness test were adopted. According to a previous study [16], when light curing the LC resin cement through 1.5, 3.0, and 6.0 mm ceramic restorations, the light attenuation rates were >80%, 95%, and 99%, respectively. Theoretically, to achieve the minimum irradiance of 100 mW/cm2

and RE of 6 J/cm2 established in the present study, the irradiance of the LCU should be at least 500, 2000, and 10,000 mW/cm2 when light curing the LC resin cement through 1.5, 3, and 6 mm ceramic restorations, respectively, and the irradiation time should be more than 60 s. However, the irradiance of most commercially available LCUs is below Materials 2021, 14, 2341 6 of 7

2000 mW/cm2 [16,17,27,28]. Therefore, LC resin cement should not be used when the ceramic restorations exceed 3.0 mm in thickness. The results of this study also tested the effectiveness of the ERL. In the present study, the specimens were inadequately polymerized when the irradiance was 50 mW/cm2, regardless of the irradiation time of 120 s. This apparent violation of the ERL is similar to what occurred in previous studies. Although the RE plays an important role, the irradiance and irradiation time independently affect the polymerization of resin [29–33]. Therefore, both time and irradiance should be considered critical in the polymerization process. The minimum irradiance and RE values should be 50–100 mW/cm2, and 5–6 J/cm2, respectively. However, the samples were not tested at these experimental settings due to technical limitation, which was a limitation of the present study. Furthermore, the cement thickness and resin cements may vary in clinical conditions [34]. Only 1 LC resin cement with a thickness of 0.1 mm was tested in the present study, which may be considered another limitation. Therefore, the present ﬁndings should be interpreted with caution. Future studies are required to evaluate different types of resin cements.

5. Conclusions From this study, it can be concluded that the minimum RE and irradiance to trigger an adequate polymerization of a 0.1 mm thick photo-polymerized resin cement are 6 J/cm2 and 100 mW/cm2, respectively.

Author Contributions: All authors discussed and agreed upon the idea, and made scientific contributions. Conceptualization, M.Z., M.O. and H.Y.; methodology and investigation, Q.L. and H.-L.L.; writing—original draft preparation, Q.L.; writing—review and editing, M.O. and H.Y.; supervision, M.Z. and H.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Startup Fund for Scientific Research, Fujian Medical University, grant number 2017XQ1112 and the Joint Fund for Scientific and Technological Innovation of Fujian Province, grant number 2019Y9030. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Further data may be requested by contacting the corresponding author. We declare that any data regarding the study will willingly provided. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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