SCIENTIFIC SECTION

Journal of Orthodontics, Vol. 41, 2014, 292–298

Degree of conversion of resin-based orthodontic bonding materials cured with single-wave or dual-wave LED light-curing units Ario Santini1, Niall McGuinness2 and Noor Azreen Md Nor3 1

Director Biomaterial Research, Edinburgh Dental Institute, Lauriston Buildings, Lauriston Place, Edinburgh EH3 9HA Orthodontic Consultant , Edinburgh Dental Institute, Lauriston Buildings, Lauriston Place, Edinburgh EH3 9HA 3 Postgraduate Student, Department of Orthodontics, Edinburgh Dental Institute, Lauriston Buildings, Lauriston Place, Edinburgh EH3 9HA 2

Aim: To evaluate the degree of conversion (DC) of orthodontic adhesives (RBOAs) cured with dual peak or single peak light-emitting diode (LED) light-curing units (LCUs). Materials and methods: Standardized samples of RBOAs, APCPlus, OpalH BondH and LightBondTM were prepared (n53) and cured with one of two dual peak LCUs (bluephaseH G2-Ivoclar-Vivadent or Valo-Ultradent) or a single peak control (bluephaseH Ivoclar-Vivadent). The DC was determined using micro-Raman spectroscopy. The presence or absence of initiators other than camphorquinone was confirmed by high-performance liquid chromatography and nuclear magnetic resonance spectroscopy. Data were analysed using general linear model in Minitab 15 (Minitab Inc., State College, PA, USA). Results: There was no significant difference in DC between APCPlus, and OpalH Bond (confidence interval: 23.89– to 2.48); significant difference between APCPlus and LightBondTM (218.55 to 212.18) and OpalH Bond and LightbondTM (217.85 to 211.48); no significant difference between bluephase (single peak) and dual peak LCUs, bluephase G2 (24.896 to 1.476) and Valo (23.935 to 2.437) and between bluephase G2 and Valo (22.225 to 4.147). APCPlus and OpalH Bond showed higher DC values than LightBondTM (P,0.05). LucirinH TPO was found only in Vit-l-escence. Conclusion: LucirinH TPO was not identified in the three orthodontic adhesives. All three LCUs performed similarly with the orthodontic adhesives: orthodontic adhesive make had a greater effect on DC than the LCUs. It is strongly suggested that manufacturers of resin-based orthodontic materials test report whether or not dual peak LCUs should be used with their materials. Dual peak LED LCUs, though suitable in the majority of cases, may not be recommended for certain non LucirinH TPO-containing materials. Key words: Orthodontic adhesive, degree of conversion, Raman, LucirinH TPO Received 13 March 2014; accepted 2 May 2014

Introduction Following the development of bisphenol A diglycidyl ether dimethacrylates (bis-GMA) (Bowen, 1979) and bonding to enamel (Buonocore, 1955), there was an increased use of dental resin-based composite materials (RBCs), fissure sealants and adhesives, including orthodontic adhesives (Newman, 1965). The advantage of an orthodontic light cured adhesive system is that it allows ample time to accurately position the bracket on the tooth before curing (Dunn and Taloumis, 2002). To achieve optimum mechanical properties a high monomer to polymer conversion is required (Bishara et al., 2003; Witzel et al., 2005; Cassoni et al., 2008; Lopez-Suevos and Dickens, 2008; Turssi et al., 2005; Yan et al., 2010). Insufficient conversion has been associated with elution of substances from RBCs with potentially toxic effects, including hormonal disruption (Eliades Address for correspondence: Ario Santini, BDS, DDS, PhD, FDS, FFGDP (UK), DipFMed. FADM. Director Biomaterial Research, Edinburgh Dental Institute, Lauriston Buildings, Lauriston Place, Edinburgh EH3 9HA Tel: 0131 536 4970, FAX 0131 536 4971 Email: [email protected] # 2014 British Orthodontic Society

et al., 1995; Geurtsen et al., 1998; Miletic et al., 2009; Manojlovich et al., 2011; Polydorou et al., 2011) and these have also been identified in association with orthodontic bracket adhesives (Kloukos et al., 2013). An important factor for adequate polymerization efficiency is that the emission spectrum of the light source must match the absorption spectrum of the initiators (Santini, 2010). The concentration of the photoinitiators is also known to influence radical formation in camphorquinone-amine (CQ-amine) systems and this is known to vary among commercial brands (Shintani et al., 1985; Alvim et al., 2007). There is evidence that higher concentrations of photoinitiators improve the degree of conversion (DC) and mechanical properties of the formed polymer (Yoshida and Greener, 1994; Rueggeberg et al., 1997; Kallyanna and Yamuna, 1998; Moin Jan et al., 2001; Schroeder and Vallo, 2007); though above a certain

DOI 10.1179/1465313314Y.0000000101

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Degree of conversion of resin-based orthodontic bonding materials

threshold, no benefits are observed (Jakubiak et al., 2001). Moreover, aesthetics may be compromised due to CQamine’s yellow colour (Taira et al., 1988; Park et al., 1999) producing a non-acceptable aesthetic result (Janda et al., 2004; Janda et al., 2007). To counteract this effect CQ-amine substitutes have been explored involving adiketone and acylphosphine oxide derivatives and current evidence suggests that some of these alternative initiators result in more acceptable aesthetic properties (Uhl et al., 2003; Arikawa et al., 2009; Cadenaro et al., 2010; Ikemura and Endo, 2010; Ikemura et al., 2010; Brandt et al., 2011). However, the light absorbance of these materials is below 420 nm which falls out of the emission range of most single peak light-emitting diode, light curing units (LED LCU) (Neumann et al., 2005; Arikawa et al., 2009; Brandt et al., 2011; Ikemura and Endo, 2011). This emission– absorption mismatch may result in insufficient polymerization and DC, which may adversely affect the mechanical properties and biocompatibility of the cured materials. Dual peak LCUs have been developed to increase polymerization efficiency of materials containing initiators other than or in addition to a CQ-amine system. These LCUs have a ‘primary’ peak at about 468 nm to initiate CQ and a ‘secondary’ peak at about 400 nm. It has been shown that such an LCU is efficient in photo activation of dental adhesives containing 2,4,6-trimethylbenzoyldiphenylphosphine oxide (Ilie and Hickel, 2008). In a recent study (Miletic and Santini, 2012), unfilled resin materials containing both Lucirin (ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate) (TPO) and CQ-amine initiators were Table 1

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effectively cured using a dual peak LCU and resin mixture with the same weight percentage of initiators were better cured when TPO was the only initiator, compared to CQamine only or combined TPO and CQ-amine system. Manufacturers tend not to indicate the initiator system used in their products and this includes resin based, light activated orthodontic adhesive materials. Aesthetics play an important role in orthodontics, especially with the increasing use of ceramic brackets. This calls for an investigation of the curing efficiency of dual wave LED LCUs with respect to orthodontic adhesives, irrespective of their initiating system. The aim of this study was to measure the DC of orthodontic adhesives under orthodontic brackets cured with dual peak or single peak LCUs. The null hypothesis was that there is no significant difference in the DC of different material cured with either a dual peak or a single peak LCU. Materials and methods Table 1 lists the materials used in the present study. For the identification of TPO in resin-based materials: five hundred milligrams of each RBC material were weighed on an analytical balance with an accuracy of 0.1 mg and suspended in 10 ml of dichloromethane. The samples were ultrasonicated for 5 min in a heated ultrasonic bath, and then centrifuged at 10,000 rev/min for 3 min. The supernatant was transferred to a 50 ml round bottomed flask and a further 10 ml of dichloromethane was added to the centrifuged solid. The sonication and centrifugation steps

Materials and light-curing units used in the present study.

Material

Manufacturer

Composition*

Vit-l-escence** A1 shade Herculite XRV Ultra** Extra light shade Tetric EvoCeram A1** LOT M35813

Ultradent Products Inc., South Jordan, UT, USA Kerr Corporation, Orange, CA, USA

OpalH Bond***

OpalH BondHBondHBondH Orthodontics, 505W 10200 South Jordan, UT 84095, USA 3M Orthodontic Products 3M Center, St Paul, MN 55144-1000, USA

Bis-GMA, barium borosilicate and other fillers, camphorquinone, an amine co-initiator and a proprietary initiator Uncured methacrylate ester monomers, titanium dioxide and pigments, 4 methoxyphenol, benzoyl peroxide, trimethylolpropane triacrylate, initiators Bis-GMA, urethane dimethacrylate, ethoxylated bis-EMA (16.8 wt-%); barium glass filler, ytterbiumtrifluoride, mixed oxide (48.5 wt-%); prepolymers (34 wt-%); additives, catalysts, stabilizers and pigments (,1 wt-%) Bisphenol A diglycidyl ether dimethacrylates (bis-GMA) Ethyl 4-dimethylaminobenzoate Silane-treated glass Silane-treated quartz Polyethylene glycol dimethacrylate Citric acid dimethacrylate oligomer Bisphenol A diglycidyl ether dimethacrylates (Bis-GMA) Dimethyl siloxane, reaction product with silica Diphenyliodonium hexafluorophosphate DL-camphorquinone 2,6-di-tert-butyl-p-cresol (BHT) Bisphenol A diglycidyl ether dimethacrylates (Bis-GMA) Triethylene glycol dimethacrylate

APC2 Plus***

Light Bond2***

Ivoclar-Vivadent, Schaan, Liechtenstein

Reliance Orthodontic Products, Inc., PO Box 678, Itasca, IL 60143, USA

*According to manufacturers’ technical data. **Used in preliminary study. ***Used in main study.

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Table 2 Curing parameters of the three LED light-curing units obtained using MARC2 RC. BluephaseH G2

Valo

BluephaseH

Serial no. 215502 (dual peak)

Serial no. V07839 (dual peak)

(single peak)

Ivoclar-Vivadent, Schaan, Liechtenstein y1200 mW/cm2, ¡10%

Ultradent Products Inc., South Jordan, UT, USA y1400 mW/cm2, ¡10%

Ivoclar-Vivadent, Schaan, Liechtenstein y1100 mW/cm2, ¡10%

1310 mW/cm2 460.72 409.75 25.7 2.3 23.4

1233 mW/cm2 459.47 410.39 24.8 3.9 20.9

1777 mW/cm2 455.72

LCU

Manufacturer Tip irradiance according to manufacturers’ technical data Max tip irradiance by MARC2 Peak wavelength(s) (nm) Total energy (J/cm2) Energy (J/cm2) at 380–420 nm Energy (J/cm2) at 420–540 nm

were repeated and the dichloromethane was once again decanted into the round bottomed flask. The solvent was evaporated under vacuum to provide a viscous waxy residue of approximately 200 mg, which was then used for analysis. Samples of the materials were prepared for highperformance liquid chromatography (HPLC) analysis by dissolving 20 mg in 1 ml of HPLC grade methanol and for nuclear magnetic resonance (NMR) spectroscopy by dissolving 20 mg in 1 ml of deuterated chloroform or deuterated dimethylsulfoxide. Reference samples of bis-GMA, urethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGDMA) and CQ (Esstech Inc., USA), 2,2-bis(4-(2-methacryloxyethoxy)phenylpropane (bis-EMA) and 2,4,6-trimethylbenzoyldiphenylphosphine oxide (LucirinH TPO) (Ivoclar-Vivadent, Liechtenstein) were prepared to enable accurate comparison of HPLC retention times and NMR spectra. Samples were prepared as follows: For HPLC, a 10 mM solution of each sample was prepared in 1 ml of HPLC grade methanol and serial dilutions to prepare 1 mM and 0.1 mM solutions were also carried out. For NMR analysis, 10 mg of each material were dissolved in deuterated chloroform or deuterated dimethylsulfoxide. HPLC analysis was carried out on an Agilent 1100 Series HPLC system (Agilent Technologies, UK) equipped with a G1311A QuatPump, a G1313A Standard Autosampler and a G1365B UV-Vis Detector. The column used was a reverse-phase Agilent Poroshell 120 SB C18 2.7 mM (4.6650 mm) (Agilent Technologies). The mobile phase was a mixture of water (HPLC Grade, containing 0.1% formic acid) and methanol (HPLC Grade, containing 0.1% formic acid) and a gradient was applied over 15 min according to previously determined methanol/water ratios. The flow rate was 1 ml/min and the injection volume was 10 ml. Identification of TPO in RBCs was confirmed by comparison of the HPLC elution profiles of the RBC material with samples spiked with a known concentration

23.4 0.2 23.2

of TPO. 1H NMR spectroscopy was carried out on all materials using a Bruker AVANCE III spectrometer (Bruker, UK) fitted with a cryo probe, at 500 MHz. 31 P NMR spectroscopy was carried out on a Bruker AVANCE III spectrometer fitted with a heteronuclear probe, at 161.9 MHz. Chemical shifts were calculated in ppm relative to internal standards and were compared to reference spectra of the individual pure compounds. The presence of TPO was only confirmed after positive identification by both HPLC and NMR analysis. Samples of the orthodontic adhesives (n53) were prepared using orthodontic metal brackets (VictorySeriesTM; 3M Unitek, USA). Nine metal brackets were obtained with APCPlus already applied to the flat surface of each bracket by the manufacturer. Three of them were randomly distributed to Group 1 (APCPlus) and used as supplied by the manufacturer. Six brackets had APCPlus removed by dissolving in methanol, ultrasonicating and drying in an oven before being randomly allocated to either to Group 2 (OpalH Bond) or Group 3 (LightBondTM). Subsequently, each bracket was turned upside down and pressed on a Mylar strip which was then placed on a buccal surface of an extracted upper central incisor embedded horizontally in gypsum. Excess material from the periphery of the bracket was removed using a dental probe. Light curing was done at two mm tip-to-surface distance with the light tip at 45u. A custom-built set-up was used to allow fixed and reproducible angulations and distances. Each sample was cured for 5 s from the mesial and then 5 s from the distal aspect of the bracket. After light curing, the brackets were kept dry in labeled, light-proof containers in a water bath at 37uC for 24 h before micro-Raman spectroscopic analysis. Curing parameters of the LCUs, light irradiance, total energy and peak wavelengths as well as their spectral range were monitored using MARCTM (BlueLight Analytics Inc., Canada), which incorporates a spectroradiometer

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(USB 4000; Ocean Optics). The experiment was carried at 22¡2uC and 51¡5% relative humidity which were continuously monitored by a USB-502 logger (Measurement Computing Corp., USA). Micro-Raman spectroscopic analysis Micro-Raman analysis was done 24 h post-curing with the samples being retained in the moulds and on metal brackets. A micro-Raman spectrometer (LabRam 300; Horiba JobinYvon, UK) was calibrated at the beginning of each session internally for zero and using a silicon sample for coefficient values. Standard micro-Raman parameters used in this study were: 20 mW HeNe laser with 632.817 nm wavelength, spatial resolution y1.5 mm, spectral resolution y2.5 cm21, 6100 magnification. The DC was calculated according to the following formula: DC~(1{Rcured =Runcured )|100 where R is the ratio of peak heights at 1639 and 1609/cm in cured and uncured materials which served as a reference. Three point spectra were taken from the top and bottom surface of each sample of the RBCs and fissure sealants. For orthodontic adhesives, the brackets were placed with the adhesive upwards under the microscope objective. Three point spectra per sample were taken for orthodontic adhesives. Statistical analysis Statistical analysis was performed using Minitab 15 (Minitab Inc., State College, PA, USA). Tukey’s posttest was used to evaluate differences between groups. Results TPO was identified in Tetric EvoCeram and Vit-l-escence, but not in Herculite XRV Ultra by HPLC and NMR analyses. Table 3 gives DC values (mean¡SD) of orthodontic adhesives cured with the three different LCUs. Figure 1 shows the emission spectra of the three tested

Table 3

Figure 1 Emission spectra of the three tested lightcuring units

light-curing units. The interaction of the material and the LCU factors was not significant (P50.848), therefore the effects of the individual factors were considered separately. Material factor: The effect of material on DC was statistically significant (P,0.001). LCU factor: The effect of LCUs on DC was not statistically significant (P50.441). Tukey’s method compares all possible pairs of level means for the specified factors. For the DC analysis, all pairwise comparisons were tested for the material and LCU factors. The confidence level chosen for the intervals was 95%, corresponding to a family error rate of 0.05 (5%). With respect to the materials, the confidence intervals for the comparisons showed that there was no significant difference in DC between APC Plus, and OpalH Bond (23.89 to 2.48), but there was a significant difference between APC Plus and LightBondTM (218.55 to 212.18) and OpalH Bond and LightBondTM (217.85 to 211.48). With respect to LCU factor, the confidence intervals for the comparisons showed that there was no significant difference between bluephase (single peak) and the two dual peak LCUs, bluephase G2 (24.896 to 1.476) and Valo (23.935 to 2.437).

DC values (mean¡SD) of orthodontic adhesives cured with different LCUs Bluephase G2

Bluephase

Valo dual peak

Dual peak

Single peak

APCz

Mean (SD)

61.0 (6.5)1,a

59.8 (4.2)1,b

63.3 (5.9)1,c

OpalH Bond

Mean (SD)

60.9 (5.3)2,a

59.6 (5.2)2,b

61.5 (5.3)2,c

LightBond2

Mean (SD)

46.3 (5.1)3

45.9 (3.2)3

45.8 (2.7)3

The same letter vertical superscripts indicate no significant difference with respect to material. Same number horizontal superscripts indicate no significant difference with respect to LCU.

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There was a significant difference between the two dual peak LCUs, bluephase G2 and Valo (22.225 to 4.147). These results can also be compared with the adjusted P-values, which indicate the same results. Discussion A preliminary study was considered necessary to confirm the ability to identify CQ and LucirinH TPO in resin composite materials, and the methods used are detailed so that future study comparisons can be made. It is logical to assume that manufacturers of dual peak LCUs do so because some of their own brand RBCs contains initiators other than CQ-amine, and for this reason Vit-l-escence and Tetric EvoCeram were chosen in the preliminary study, as it was expected that lighter shades of this material would contain LucirinH TPO. Herculite XRV Ultra, a RBC by a manufacturer who does not produce a dual peak LED LCUs was chosen in anticipation that it would not contain LucirinH TPO. HPLC and NMR analyses confirmed these initial expectations for both materials and the ability to identify the materials, and were then used against orthodontic adhesives. In the main study, orthodontic adhesives were chosen to cover a range of materials frequently used in clinical practice. No traces of LucirinH TPO were identified using either HPLC or NMR in any of the three studied orthodontic adhesives. The orthodontic adhesives were cured against tooth enamel to allow comparison with previous studies (Eliades et al., 1995; Niepraschk et al., 2007). This study differs significantly from others in that the use of the MARCTM RC allowed more accurate measurement of parameters such as LCU tip irradiance, and spectral range. No study could be found after an orthodontic literature review, which detailed such data with respect to light curing orthodontic adhesive. In the main, previous studies have used manufacturers’ tip irradiance data or tip irradiance measured by clinical radiometers, the later which are documented as being inaccurate Kemeyama et al., 2013), and though perhaps suitable to gauge clinical LCU power reduction with time, are not suitable as a data source for experimental calculation of applied total energy. This is demonstrated by the data derived from MARCTM RC, in the current study, which shows there is frequently a variance in manufacturers’ tip irradiance values and the measured value. Vit-l-escence showed higher DC values when cured with dual wave LED LCUs, Valo and bluephaseH G2, compared to the single peak control, bluephaseH. This could be associated with the presence of LucirinH TPO in Vit-l-escence which was more efficiently initiated by both dual wave LCUs. It has been previously reported

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that the single peak bluephaseH LCU resulted in DC values in excess of 70% in unfilled experimental resins containing 1 wt-% LucirinH TPO (Miletic and Santini, 2012). However, this may not be sufficient for commercial RBCs due to lower concentrations of LucirinH TPO and the presence of fillers and pigments, which are known to decrease the DC of RBCs (Hadis et al, 2011; Leprince et al., 2011). The light, opaque white shade of Vit-l-escence used in a previous study (Santini et al., 2012) showed a higher DC than the A1 shade used in the present study. A higher amount of LucirinH TPO in the whiter shade could probably be associated with increased monomer to polymer conversion. However, it was not possible to quantify the amount of LucirinH TPO in the two shades of Vit-l-escence so the validity of the above statement contentious. No LucirinH TPO was identified in Herculite XRV Ultra and this may be one of the reasons why the single peak LED LCU showed higher DC values than the dual peak LCUs. Additionally, higher total energy in the absorption region of CQ for the single peak compared to the two dual peak LCUs (Table 2) may have increased photochemical reaction efficiency in the CQ-amine system. Each of the three LCUs showed similar results in so far as APCPlus and OpalH Bond had DCs of approximately 60% and LightBondTM a diminished DC of approximately 45%. Moreover, the results indicate that the material had a greater effect on DC than the LCU type, and the difference in the total energy delivered by the LCUs did not influence the final DC in orthodontic adhesives. As all materials were shown to have CQ as an initiator and LucirinH TPO was absent in all three, constituents or composition of the materials, other than photoinitiators, would seem to be implicated in this outcome. Brackets, as a physical barrier, significantly attenuated the amount of energy absorbed by orthodontic adhesives making the LCU tip energy less relevant. Of the three tested orthodontic adhesives, LightBondTM showed the lowest DC values. No data in the literature could be found on the DC of the three orthodontic adhesives used in the present study. For other types of light-cured orthodontic adhesives, the DC varied in the range of 40– 60% when cured directly using a halogen or a plasma arc LCU (Bang, 2004). The DC of an orthodontic adhesive cured through metal brackets was reported to be higher for halogen and LED LCUs than a plasma arc LCU and increased with longer curing times (Niepraschk et al., 2007). In another study, the DC was in the range of 60– 70% when directly cured without brackets and 45–60% when cured through the brackets, which is in agreement with the present results (Eliades et al., 1995).

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The two dual peak LCUs showed slightly different distribution of energy delivered in the 380–420 and 420– 540 nm regions but similar total energy as well as the wavelengths of their two emission peaks, and may identified with the similar results obtained with Valo and bluephaseH G2. It is becoming increasingly more evident that manufactures of RBMs should give more information of constituents, especially photoinitiators contained in their materials, and manufacturers of LCUs give information, not only of the tip irradiance but spectral range of their LCUs. Conclusions N The two dual peak LCUs performed slightly better than the dual peak LCU with the LucirinH TPOcontaining RBC. The tested RBC with no LucirinH TPO showed a N higher DC when cured with the dual peak LCU. N LucirinH TPO was not identified in any of the three orthodontic adhesives. N All three LCUs performed similar with the orthodontic adhesives. N The make of the orthodontic adhesive had a greater effect on DC than the LCUs. In the material factor, the confidence intervals for the comparisons show: 1. The difference between the means for APCz, and OpalH Bond (23.89 to 2.48); not significant. 2. The differences between the means for APCz LightbondTM (218.55 to 212.18); significant. 3. The differences between the means for OpalHBond and LightbondTM (217.85 to 211.48); significant. In the LCU factor, the confidence intervals for the comparisons show: 1. The differences between the means for bluephase and bluephase G2 (24.896 to 1.476) and Valo (23.935 to 2.437); not significant. 2. The differences between means for bluephase G2 and Valo (22.225 to 4.147); not significant. These results can also be compared with the adjusted pvalues which indicate the same results as above. Disclaimer statements Contributors All authors were engaged in study design, data collection and writing of the paper. Guarantor: Ario Santini. Funding None. Conflicts of interest There are no conflicts of interest.

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