ORIGINAL ARTICLE
Effect of intraoral aging on the setting status of resin composite and glass ionomer orthodontic adhesives Anna Iliadi,a Stefan Baumgartner,b Athanasios E. Athanasiou,c Theodore Eliades,d and George Eliadese Athens and Thessaloniki, Greece, Zurich, Switzerland, and Dubai, United Arab Emirates Introduction: The aim of this study was to assess the effect of intraoral aging on the setting status of a resin composite and a glass ionomer adhesive, relative to control specimens stored in water. Methods: Metallic brackets were bonded with resin composite orthodontic adhesive (Transbond XT; 3M Unitek, Monrovia, Calif) or a glass ionomer cement (Fuji I; GC, Tokyo, Japan) to recently extracted premolars and kept in water for 6 months. The same materials were also bonded to the premolars of orthodontic patients. After 6 months, the teeth were carefully extracted, with the brackets intact on their buccal surfaces. All teeth were embedded in epoxy resin and sectioned buccolingually. Fourier transform infrared microscopy and Raman microscopy were used for the estimation of the degree of cure in the composite and the salt yield in the glass ionomer adhesives. Results: The control samples of the composite showed significantly lower degrees of cure than did the retrieved specimens (52.40% 6 3.21% vs 57.62% 6 1.32% by Fourier transform infrared microscopy, and 61.40% 6 2.61% vs 67.40% 6 3.44% by Raman microscopy). Raman microscopy significantly overestimated the degree of cure and failed to provide reliable information for the salt yield in the glass ionomer cement. Fourier transform infrared microscopy showed increased, but no statistically significant difference in, aluminum-carboxylate salts in the retrieved specimens. Conclusions: Enhanced oxidation of residual carbon-carbon bonds in the composite and slightly increased dissolution of the weaker calcium-salt phase in the glass ionomer cement were the main differences in the intraorally aged specimens in comparison with the specimens stored in water. (Am J Orthod Dentofacial Orthop 2014;145:425-33)
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espite many studies from orthodontic materials research, the laboratory configurations and setups used for exposing the materials to water and various electrolytes fail to simulate the actual oral environment, which includes saliva, pH fluctuation, microbial attack, complex cyclic loading, and enzymatic degradation.1 Since the intraoral environment is a
a Postgraduate student, Department of Orthodontics, School of Dentistry, University of Athens, Athens, Greece. b Attending dental physician, Department of Orthodontics and Paediatric Dentistry, Center of Dental Medicine, University of Zurich, Zurich, Switzerland. c Professor and director of orthodontics, Dubai School of Dental Medicine, Dubai, United Arab Emirates; professor, Department of Orthodontics, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece. d Professor and director, Department of Orthodontics and Paediatric Dentistry, Center of Dental Medicine, University of Zurich, Zurich, Switzerland. e Professor and head, Department of Dental Biomaterials, School of Dentistry, University of Athens, Athens, Greece. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Theodore Eliades, Department of Orthodontics and Paediatric Dentistry, Center of Dental Medicine, University of Zurich, Plattenstarsse 11, CH-8032, Zurich, Switzerland; e-mail, [email protected]. Submitted, July 2013; revised and accepted, November 2013. 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.11.020
complex milieu capable of affecting the physical and mechanical properties of an adhesive, laboratory data should be carefully interpreted. Thus, there is a notable lack of evidence regarding the intraoral aging pattern of orthodontic materials and the associated phenomena, such as surface alterations, structural changes, and variations of mechanical properties. The intraoral exposure might affect the longevity of the adhesive (eg, aging of polymeric adhesives) by modification of structural qualities. The traditional orthodontic adhesives for bracket bonding are resin composite materials, introduced in the mid-1970s. These orthodontic adhesives have been used successfully on acid-etched enamel over the last 40 years. The development of visible light-cured materials offered additional advantages in the field—ie, high early bond strength,2 minimal oxygen inhibition,3 and prolonged working time for optimal bracket placement—which have brought innovation and practicality to modern orthodontic treatment. Disadvantages of the resin composite adhesives include loss of enamel during acid conditioning,4 loss of the superficial fluoride-rich enamel zone during debonding,5 and demineralization 425
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of enamel around the brackets because of poor patient compliance with respect to oral hygiene.6,7 The importance of the curing efficiency on the performance of resin composites has long been documented. It has been shown that the degree of cure (%DC, the amount of carbon-carbon bonds reacted after setting) of resin composites modulates the composite's physical and mechanical properties,8,9 color stability,10 solubility and degradation,11 and biocompatibility.12 Glass ionomer cements possess unique benefits that make them useful in clinical orthodontics. They chemically adhere to both enamel and metal,13,14 release fluoride with its known caries-inhibiting properties,15,16 and can absorb fluoride from sources such as fluoride solutions, thus recharging their fluoride reservoir.17 Furthermore, acid conditioning is not required, so that both bonding and debonding procedures can reduce iatrogenic damage to enamel, and the bonding field does not have to be dry, thus increasing the speed and ease of bracket placement.18 But it is a concern that bond strengths for glass ionomer cements are not of the same magnitude as those for resin composite adhesives; as a result, the failure rate of brackets attached to teeth is higher.18,19 To improve bracket cementation, resinmodified glass ionomers were introduced.18 Because the phenomena accompanying intraoral aging of the material might not be reflected in shortterm or long-term performance of the adhesives under laboratory conditions, intraoral aging might modify the critical properties of orthodontic adhesives. The purpose of this study was to assess the effect of 6 months of intraoral aging on the setting efficiency of resin composite and conventional glass ionomer orthodontic adhesives relative to control specimens stored in water. The null hypothesis tested was that there is no difference in the setting status between laboratory and intraorally aged resin composite and glass ionomer orthodontic adhesives. MATERIAL AND METHODS
The control sample consisted of 14 sound premolars, recently extracted for orthodontic reasons and kept for 1 week in distilled water with the addition of 0.5% sodium azide at 8 C. The teeth were randomly classified into 2 groups of 7 specimens each, cleaned and polished with a nonfluoride paste (Clean Polish; Hawe-Neos Dental, Bioggio, Switzerland). Metallic brackets (Mini Sprint; Forestadent, Pforzheim, Germany) were bonded to the middle buccal enamel surfaces with a conventional glass ionomer cement (Fuji I; GC, Tokyo, Japan) or a composite resin adhesive (Transbond XT; 3M Unitek, Monrovia, Calif). Glass ionomer cement was placed directly on blot-dried enamel. The resin composite adhesive, used
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without primer or bonding resin, was applied on air-dried, acid-etched (for 30 seconds) enamel with a 35% phosphoric acid gel (Transbond XT etching gel, 3M Unitek) and photopolymerized from the incisal and cervical bracket edges (10 seconds each) with a halogen light-curing unit (850 mW/cm2 standard light intensity, Trilight; 3M ESPE, Seefeld, Germany). The tip of the unit was placed in contact with the bracket edges at approximately a 45 angle. All teeth were stored in 200 mL of distilled water at 37 C for 6 months. The water was replaced once a month to prevent saturation from leachable components. The test sample consisted of specimens obtained from patients recruited in the Graduate Orthodontic Clinic of the Aristotle University of Thessaloniki, Thessaloniki, Greece, with an age range of 13 to 36 years (mean, 22 years); 55% were male, and 45% were female. Briefly, the patients had been selected using the following selection criteria: need for extractions for orthodontic purposes of 2 or 4 premolars, intact premolar buccal surfaces, and no previous glass ionomer restorative fillings in the oral cavity. The original protocol had been approved by the ethical committee of the university (14/18.07.2005); after detailed explanations of the purpose of this research and the clinical procedures, consent had been obtained. Furthermore, no orthodontic treatment had been started before the extractions and 6 months after bonding with the adhesives to be tested. The patients and the buccal premolar surfaces had been randomly classified (numbers 1-7 for patients; letters C [composite] or G [glass ionomer] for tooth surfaces) in 7 contralateral pairs (total of 14 teeth) to receive sequentially alternating resin composite and glass ionomer adhesives. The tooth surfaces had been cleaned and polished with the nonfluoride paste. The same brand of metallic brackets had been bonded with either the conventional glass ionomer cement (Fuji I) or the composite resin (Transbond XT) as reported before. After 6 months, the teeth were carefully extracted, with the brackets intact on their buccal surfaces, and thoroughly rinsed with distilled water. The roots of all teeth were removed by a horizontal section 1 mm below the cervical enamel margins, and the crowns were embedded in epoxy resin (CaldoFix; Struers A/S, Ballerup, Denmark) with the proximal surfaces parallel to the horizontal plane. The specimens were sectioned buccolingually using a microtome under continuous coolant (Isomet; Buehler, Lake Bluff, Ill). The sections were then polished with silicon carbide papers (600-1200 grit sizes) under water coolant and ultrasonicated in distilled water for 2 minutes to remove the surface smears (Fig 1). The specimens were stored in a desiccator for 5 days to remove loosely bound water
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Fig 1. Reflected light microscopic images of representative specimens used for the analysis of the orthodontic adhesives: A, bracket bonded with the light-cured resin composite adhesive; B, bracket bonded with the conventional glass ionomer cement. Note the porosity in the glass ionomer layer mainly attributed to hand mixing (magnification 12 times; bar, 1 mm).
and then studied by Fourier transform infrared microscopy (FTIRM) and Raman microscopy (RM). FTIRM was performed with a microscope (AutoImage; Perkin-Elmer, Bacon, United Kingdom) equipped with a germanium micro-attenuated total reflectance objective and attached to a Fourier transform infrared spectrometer (Spectrum GX; Perkin Elmer). Spectra were acquired at 4 points per specimen (2 in the bulk adhesive, 2 at the margins) under the following conditions: mercury cadmium telluride detector cooled by liquid nitrogen, 4000 to 750 cm 1 wave number range, 4 cm 1 resolution, 50-mm-diameter sampling window (attenuated total reflectance objective), 128 spectra coaddition, and 1mm depth of analysis at 1000 cm 1. All spectra were subjected to baseline and attenuated total reflectance corrections. Spectra of the resin composite paste and the glass ionomer mix before setting were used as references of the unset materials. RM was performed by a Raman spectrometer (EZRaman I; Enwave Optronics, Irvine, Calif) attached to a microscope (MBE; Leica Microsystems, Wetzlar, Germany)
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under the following conditions: 785 nm laser excitation, 40-times magnification, 100 to 200 mW nominal power, 1.5-mm working distance, 2000 to 200 cm 1 wave number range, and 5 cm 1 resolution. Four regions were analyzed per specimen as above. For the resin composite adhesive, the percentage degree of cure (%DC) after storage was assessed on a relative basis by the 2-band method and the tangent baseline technique.20 The aliphatic (C 5 C) bond stretching vibrations at 1638 cm 1 were chosen as the analytical band, whereas the aromatic (C.C) bond-stretching vibrations at 1605 cm 1, which are unaffected by the polymerization reaction, were chosen as the reference band. The %DC was calculated according to the equation: %DC 5 100 3 [1 (A/B 3 C/D)], where A is the net peak absorbance height of the polymerized material at 1638 cm 1, B is the absorbance height of the unpolymerized material at 1638 cm 1, C is the absorbance height of the unpolymerized material at 1605 cm 1, and D is the absorbance height of the polymerized material at 1605 cm 1. The %DC values for each specimen were the averages of the 4 measurements. For the glass ionomer cement, the following peaks were used to monitor the acid-base reaction: the carboxyl peaks of polyacrylic acid (1740 cm 1) and tartaric acid (1730 cm 1), and the peaks of calcium polyacrylate (1540 cm 1), aluminum polyacrylate (1610 cm 1), calcium tartrate (1595 cm 1), and aluminum tartrate (1670 cm 1).21,22 Because the carboxyl groups are completely neutralized during the early setting stages of conventional glass ionomers, the salt yield was calculated by measuring the ratio of the peak areas of aluminum salts (1700-1600 cm 1) to calcium salts (1600-1500 cm 1) using the tangent baseline technique and the same analysis mode as above (4 regions per specimen). The results of the %DC were statistically analyzed with 2-way analysis of variance (ANOVA) (a 5 0.05) with analytical method and storage conditions as independent parameters. Statistical analysis of the salt ratio values was assessed by a t test. RESULTS
Representative FTIRM spectra of the resin composite adhesive are shown in Figure 2. All the set specimens demonstrated a reduction in the peak high at the 1638 cm 1 band (C 5 C stretching vibrations) because of the conversion (C 5 C / C C) after light curing. The spectra profiles were similar, except for a slight reduction of the C 5 C peak in intraorally exposed specimens. The RM spectra of the resin composite adhesive clearly demonstrated the characteristic bands (C 5 C stretching vibrations 1638 cm 1 and aromatic C.C
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Fig 2. FTIRM spectra of Transbond XT orthodontic adhesive at unset state and at set state after 6 months of storage in water and 6 months of intraoral exposure (absorbance scale, 2000-1300 cm 1 region).
stretching vibrations 1605 cm 1) used for the calculation of the %DC as well (Fig 3). The results of the %DC after the FTIRM and RM analyses are presented in Table I. Statistical analysis of the %DC results by 2-way ANOVA showed significant differences in analytical method and storage conditions (P \0.001), with no interaction between them (P 5 0.757). RM showed significantly higher %DC values for both storage conditions than did FTIRM (P \0.001). Within the same analytical technique, the retrieved specimens showed significantly higher %DC than did those stored in water (P 5 0.009 for FTIRM; P 5 0.004 for RM). In 2 of the 7 retrieved specimens, FTIRM analysis of the adhesive at the outer 50-mm regions showed the maximum %DC values. FTIRM spectra of the glass ionomer cement are illustrated in Figure 4. The characteristic broad carboxylic peak of the unset material (1730 cm 1) disappeared after setting, and new peaks appeared assigned to calcium polyacrylate (1550-1520 cm 1 and 1410-1390 cm 1), aluminum polyacrylate (1620-1580 cm 1 and 1470-1430 cm 1), calcium tartrate (1630-1600 cm 1), and aluminum tartrate (1670-1650 cm 1). The contribution of water (1642 cm 1) was greater in some water-stored specimens in comparison with the retrieved ones. Because of the shape complexity and partial overlapping of the peak regions, the peak area ratios (aluminum salts/calcium salts) instead of the sum of the absolute area values were used.
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A representative RM spectrum of a retrieved glass ionomer specimen, with the peaks of calcium and aluminum salts, is presented in Figure 5. The strong peak at 1457 cm 1 is attributed to methylene group (CH2) deformation and the peaks at 1553 cm 1 and 1614 cm 1 to calcium and aluminum salts, respectively. These peaks, though, are poorly resolved in RM because of the strong CH2 peak at the region.21 Therefore, peak measurements were limited only to FTIRM. The results are presented in Table II. Statistical analysis of the salt ratio values by t test showed no statistically significant difference between the mean values (P 5 0.181), although the ratios in the retrieved specimens had higher mean values. DISCUSSION
Notwithstanding numerous in-vitro studies, little is known about adhesives behaving at true clinical conditions. In this study, the bonded premolars remained in the subjects' mouths for 6 months, thus experiencing the in-vivo environment, before being extracted and tested. The split-mouth design, by the symmetric bonding of the 2 orthodontic adhesives at each patient's premolars, was an important prerequisite enabling both materials to be tested on teeth with the same morphology under the same intraoral conditions. Moreover, the water-stored specimens were used as the controls, since in many experiments water storage has been used as an aging medium.
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Fig 3. RM spectra of an unset (top) and a retrieved Transbond XT sample (bottom), after 6 months of intraoral storage (absorbance scale, 1750-1550 cm 1 region). Table I. Results of the degree of cure (%DC) of the
light-cured adhesive tested (n 5 7) Stored in water (6 mo, 37 C) Retrieved
%DC, FTIRM 52.4 (3.2)1,a 42.0 (min)-57.1 (max) 57.6 (1.3)2,c 56.2 (min)-59.4 (max)
%DC, RM 61.4 (2.6)3,b 59.9 (min)-64.1 (max) 67.4 (3.4)4,d 64.0 (min)-72.3 (max)
Values are mean (SD) or minimum and maximum. Same superscripts show means with no statistically significant differences (P .0.05) between storage conditions per method (numbers) and methods per storage condition (letters).
The light-cured orthodontic adhesive tested is a commonly used resin composite for bracket bonding. The glass ionomer cement used is indicated for bonding
orthodontic appliances and bands. We used the same product for bracket bonding instead of the proposed resin-modified glass ionomer analog (Fuji Ortho LC, GC) to clearly identify the effect of the immersion environment on individual reactions (free radical in composite, acid-base in glass ionomer), without the complex spectral interferences produced from dual set materials (free radical and acid-base, as in Fuji Ortho LC).23 Nevertheless, this should not be considered as a contraindication, since conventional glass ionomers are still used for bracket bonding.16 The glass ionomer was cemented on blot-dried, but not desiccated, enamel to facilitate acid ionization and ionic bonding to enamel. No protective varnishes were used, as in restorative applications, since the material area exposed to water
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Fig 4. FTIRM spectra of Fuji I at unset state and at set state after 6 months of storage in water and 6 months of intraoral exposure (absorbance scale, 2000-1300 cm 1 region). COOH, carboxyl groups; H2O, water; COOAl, aluminum carboxylate salt; COOCa, calcium carboxylate salt.
Fig 5. An RM spectrum of a retrieved Fuji I samples after 6 months of intraoral storage (absorbance scale, 2200-200 cm 1 region). The calcium (COOCa, 1553 cm 1) and aluminum (COOAl, 1614 cm 1) carboxylate salts are poorly resolved.
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Table II. Results of aluminum to calcium salts ratio
(n 5 7)
Stored in water (6 mo, 37 C) Retrieved
Aluminum-calcium salts, FTIRM 0.6 (0.2)a 0.3 (min)-0.7 (max) 0.8 (0.3)a 0.4 (min)-1.1 (max)
Aluminum-calcium salts, RM — —
Values are mean (SD) or minimum and maximum. Same superscript letters show means with no statistically significant differences (P .0.05) between storage conditions per method.
and intraorally was limited to the cross-sectional thickness of the cement (approximately 250 mm as measured under the microscopes). For both types of orthodontic adhesives, the effect of the storage environment was assessed on the outcome of their setting reactions, since establishment of the main material matrices strongly depends on the C 5 C conversion (resin composite) and the polysalt hydrogel formation (glass ionomer). The Fourier transform infrared method has long been considered as an appropriate analytical technique in quantifying the %DC of resin composites24 and studying the setting reactions of various categories of glass ionomer cements such as traditional, reinforced, or resin reinforced.21-23 The use of a Fourier transform infrared microscope with a 50-mm tip attenuated total reflectance objective made possible selective probing within the cross-sectional thickness of the orthodontic adhesive materials between the bracket base and the enamel. RM, a complementary technique to FTIRM, offers improved spatial resolution, has no interferences from water, better resolves the polyacrylic acid peak from the various polycarboxylate salts formed,22 and has been frequently used for measurement of %DC in resin composites.25-27 However, the main problems of RM are the thermal effects from the focused laser beam and the fluorescence of organic targets, leading to high background scattering.28,29 For this reason, precise calibrations of the laser wavelength and power are required. In this study, the glass ionomer regions probed by the Raman laser showed cracking and pitting when examined under the optical microscope of the unit. This implies that the Raman analysis, as performed, induced severe dehydration in the glass ionomer specimens. The numbers of specimens used for the %DC and salt yield measurements were greater in our study (n 5 7) in comparison with in-vitro studies (n 5 3-5) to compensate for possible variances induced by the intraoral environment.30,31 The results of the FTIRM study showed that the intraorally aged resin composite adhesive showed higher
%DC values than did the water-stored controls. This might not be attributed to postcuring reactions but, rather, to interactions with the oral environment. Transbond XT is composed of bisphenol-A diglycidylether dimethacrylate and bisphenol-A bis (2-hydroxyethyl ether) dimethacrylate comonomers at a 2:1 weight ratio, 70% to 80% wt silanated quartz filler and 5% wt silanated silica filler.32 The set material is characterized by a crosslinked polymer network with a minor amount of residual monomer and a much greater amount of immobilized pendant C 5 C groups that can be subjected to hydrolytic, chemical, or enzymatic degradation, reducing thus the final amount of residual C 5 C unsaturation. Such a mechanism has been observed in some resin composites after accelerated exposure to topical acidulated phosphate fluoride agents in water, apparently due to fluoride addition to C 5 C bonds or acidic ester hydrolysis of the methacrylate ester groups.33 Our findings imply that the complex intraoral environment might cause breakdown of residual C 5 C bonds more effectively than isothermal water storage. The reduced C 5 C unsaturation found at the periphery of the bracket bases in 2 intraorally exposed specimens might be explained by their exposure to a more aggressive environment. Oxidation of residual C 5 C groups has been shown to result in network degradation and release of reactive compounds, such as formaldehyde,34 methacrylic acid, and others. Concerns have been expressed on the biologic performance of these compounds, since along with residual monomers, they can induce detrimental biologic effects in cell cultures.35 Moreover, the extent of %DC in adhesives with bisphenol-A components has been inversely associated with bisphenol-A release,36 a compound with known estrogenic activity.37 By increasing the %DC of the adhesive or modifying the chemistry of the adhesives, this concern might be diminished. The latter seems quite promising in orthodontic adhesives, by using aliphatic or single aromatic ring dimethacrylate compounds as substitutes for adhesives with bisphenol-A components.38 Conventional glass ionomer cements set by an acid-base reaction that leads to the formation of a polycarboxylate salt cement matrix. The product tested is a powder-liquid cement containing a powder of silicon dioxide, aluminum oxide, strontium carbonate, sodium fluoride, calcium carbonate, and calcium phosphate, and a liquid containing an aqueous solution of liquid polyacrylic and tartaric acids.39 Upon setting, salts are formed with divalent cations (Ca21, Sr21) that transform particle surfaces to a silica hydrogel. This structure is further stabilized by Al31 ions that coordinate to 6 atoms providing enhanced polyacrylate crosslinking
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and the final strength to the material.40 In this study, the intraorally exposed specimens demonstrated higher aluminum salt yields, although these were not statistically significant, possibly because of partial dissolution of the weaker divalent calcium salts. This might be implicated with the erosive dissolution of the material during low oral pH transient periods and the associated release of therapeutic compounds such as calcium, strontium, and fluoride.41 These results showed a different pattern of chemical changes in the resin composite orthodontic adhesive after intraoral exposure vs conventional water storage. This pattern was associated with an increased conversion from oxidation of residual C 5 C bonds in the retrieved specimens; this confirms the null hypothesis. However, in the traditional glass ionomer cement, the retrieved specimens demonstrated greater aluminum salt formation that did the specimens stored in water, but without a statistically significant difference, a finding that partially rejects the null hypothesis. These findings suggest that retrieval analysis of intraorally exposed specimens, which can be applied to most orthodontic treatment modalities, can provide a more reliable aging pattern than in-vitro water storage for complex material issues, such as their setting reactions. CONCLUSIONS
Intraoral aging affected the setting status of the resin composite adhesive to a statistically significant extent in comparison with water storage. The glass ionomer setting was affected as well, but not to a statistically significant extent. ACKNOWLEDGMENTS
Dr E. Chatzistavrou is acknowledged for her contributing to the study. REFERENCES 1. Matasa CG. Microbial attack of orthodontic adhesives. Am J Orthod Dentofacial Orthop 1995;108:132-41. 2. Eliades T, Viazis AD, Eliades G. Bonding of ceramic brackets to enamel: morphologic and structural considerations. Am J Orthod Dentofacial Orthop 1991;99:369-75. 3. Lekka MP, Papagiannoulis L, Eliades GC, Caputo AA. A comparative in vitro study of visible light-cured sealants. J Oral Rehabil 1989;16:287-99. € 4. Berghauser Pont H, Ozcan M, Bagis B, Ren Y. Loss of surface enamel after bracket debonding: an in-vivo and ex-vivo evaluation. Am J Orthod Dentofacial Orthop 2010;138:387.e1-9. 5. Diedrich P. Enamel alterations from bracket bonding and debonding: a study with the scanning electron microscope. Am J Orthod 1981;79:500-22. 6. Zachrisson BU, Zachrisson S. Caries incidence and oral hygiene during orthodontic treatment. Scand J Dent Res 1971;79:394-401.
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7. Ogaard B, R€ olla G, Arends J. Orthodontic appliances and enamel demineralization. Part 1. Lesion development. Am J Orthod Dentofacial Orthop 1988;94:68-73. 8. Asmussen E. Restorative resins: hardness and strength vs. quantity of remaining double bonds. Scand J Dent Res 1982;90:484-9. 9. Ferracane JL, Greener EH. The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J Biomed Mater Res 1986;20:121-31. 10. Ferracane JL, Moser JB, Greener EH. Ultraviolet light-induced yellowing of dental restorative resins. J Prosthet Dent 1985;54: 483-7. 11. Eliades T, Eliades G, Brantley WA, Johnston WM. Polymerization efficiency of chemically cured and visible light-cured orthodontic adhesives: degree of cure. Am J Orthod Dentofacial Orthop 1995;108:294-301. 12. Rathbun MA, Craig RG, Hanks CT, Filisko FE. Cytotoxicity of a BIS-GMA dental composite before and after leaching in organic solvents. J Biomed Mater Res 1991;25:443-57. 13. Hotz P, McLean JW, Sced I, Wilson AD. The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 1977; 142:41-7. 14. Nasu T. Polyacrylic acid-metal adhesive bond joint characterization by x-ray photoelectron spectroscopy. J Biomed Mater Res 1986;20:347-62. 15. Marcusson A, Norevall LI, Persson M. White spot reduction when using glass-ionomer cement for bonding in orthodontics: a longitudinal and comparative study. Eur J Orthod 1997;19: 233-42. 16. Shungin D, Olsson AI, Persson M. Orthodontic treatment-related white spot lesions: a 14-year prospective quantitative follow-up, including bonding material assessment. Am J Orthod Dentofacial Orthop 2010;138:136.e1-8. 17. Forsten L. Fluoride release and uptake by glass-ionomers. Scand J Dent Res 1991;99:241-5. 18. McComb D. Luting in orthodontic practice. In: Davidson C, Mjor IA, editors. Advances in glass-ionomer cements. Carol Stream, Ill: Quintessence; 1999. p. 171-82. 19. Fajen VB, Duncanson MG Jr, Nanda RS, Currier GF, Angolkar PV. An in vitro evaluation of bond strength of three glass ionomer cements. Am J Orthod Dentofacial Orthop 1990;97:316-22. 20. Rueggeberg FA, Hashinger DT, Fairhurst CW. Calibration of FTIR conversion analysis of contemporary dental resin composites. Dent Mater 1990;6:241-9. 21. Nicholson JW, Brookman PJ, Lacy OM, Wilson AD. Fourier transform infrared spectroscopic study of the role of tartaric acid in glass-ionomer dental cements. J Dent Res 1988;67: 1451-4. 22. Young AM, Sherpa A, Pearson G, Schottlander B, Waters DN. Use of Raman spectroscopy in the characterisation of the acid-base reaction in glass-ionomer cements. Biomaterials 2000;21: 1971-9. 23. Kakaboura A, Eliades G, Palaghias G. An FTIR study on the setting mechanism of resin-modified glass ionomer restoratives. Dent Mater 1996;12:173-8. 24. Yoon TH, Lee YK, Lim BS, Kim CW. Degree of polymerization of resin composites by different light sources. J Oral Rehabil 2002; 29:1165-73. 25. Miletic VJ, Santini A. Remaining unreacted methacrylate groups in resin-based composite with respect to sample preparation and storing conditions using micro-Raman spectroscopy. J Biomed Mater Res B Appl Biomater 2008;87:468-74.
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Iliadi et al
26. Acquaviva PA, Cerutti F, Adami G, Gagliani M, Ferrari M, Gherlone E, et al. Degree of conversion of three composite materials employed in the adhesive cementation of indirect restorations: a micro-Raman analysis. J Dent 2009;37:610-5. 27. Lohbauer U, Pelka M, Belli R, Schmitt J, Mocker E, Jandt KD, et al. Degree of conversion of luting resins around ceramic inlays in natural deep cavities: a micro-Raman spectroscopy analysis. Oper Dent 2010;35:579-86. 28. West YD. Research report on measuring the Raman spectra of temperature-sensitive solid samples. Spectrochim Acta A Mol Biomol Spectrosc 1997;53:35-42. 29. Bowie BT, Chase DB, Griffiths PR. Factors affecting the performance of bench-top Raman spectrometers. Appl Spectrosc 2000; 54:164A-73A. 30. Rueggeburg FA, Craig RG. Correlation of parameters used to estimate monomer conversion in a light-cured composite. J Dent Res 1988;67:932-7. 31. Mohamad D, Young RJ, Mann AB, Watts DC. Post-polymerisation of dental resin composite evaluated with nanoindentation and micro-Raman spectroscopy. Arch Orofac Sci 2007;2:26-31. 32. 3M Unitek. Material safety data sheet for 3M Unitek Transbond XT light cure adhesive (712-031, 712-036, 712-066). Available at: http://multimedia.3m.com/mws/mediawebserver?SSSSSuUn_ zu8l00x4YtB5YtUPv70k17zHvu9lxtD7SSSSSS. Accessed June 30, 2013. 33. Papagiannoulis L, Tzoutzas J, Eliades G. Effect of topical fluoride agents on the morphologic characteristics and composition of resin composite restorative materials. J Prosthet Dent 1997;77:405-13.
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34. Øysaed H, Ruyter IE, Sjøvik-Kleven U. Release of formaldehyde from dental composites. J Dent Res 1988;67:1289-94. 35. Matasa CG. Polymers in orthodontics: a danger present? In: Graber TM, Eliades T, Athanasiou AE, editors. Risk management in orthodontics: experts' guide to malpractice. Chicago: Quintessence; 2000. p. 113-31. 36. Sunitha C, Kailasam V, Padmanabhan S, Chitharanjan AB. Bisphenol A release from an orthodontic adhesive and its correlation with the degree of conversion on varying light-curing tip distances. Am J Orthod Dentofacial Orthop 2011;140: 239-44. 37. Gioka C, Eliades T, Zinelis S, Pratsinis H, Athanasiou AE, Eliades G, et al. Characterization and in vitro estrogenicity of orthodontic adhesive particulates produced by simulated debonding. Dent Mater 2009;25:376-82. 38. Papakonstantinou AE, Eliades T, Cellesi F, Watts DC, Silikas N. Evaluation of UDMA's potential as a substitute for Bis-GMA in orthodontic adhesives. Dent Mater 2013;29:898-905. 39. Saito S, Tosaki S, Hirota K. Characteristics of glass-ionomer cements. In: Davidson C, Mjor IA, editors. Advances in glass-ionomer cements. Carol Stream, Ill: Quintessence; 1999. p. 15-50. 40. Mitchell CA, Douglas WH, Cheng YS. Fracture toughness of conventional, resin-modified glass-ionomer and composite luting cements. Dent Mater 1999;15:7-13. 41. Wilson AD, Groffman DM, Kuhn AT. The release of fluoride and other chemical species from a glass-ionomer cement. Biomaterials 1985;6:431-3.
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Effect of intraoral aging on the setting status of resin composite and glass ionomer orthodontic adhesives Anna Iliadi,a Stefan Baumgartner,b Athanasios E. Athanasiou,c Theodore Eliades,d and George Eliadese Athens and Thessaloniki, Greece, Zurich, Switzerland, and Dubai, United Arab Emirates Introduction: The aim of this study was to assess the effect of intraoral aging on the setting status of a resin composite and a glass ionomer adhesive, relative to control specimens stored in water. Methods: Metallic brackets were bonded with resin composite orthodontic adhesive (Transbond XT; 3M Unitek, Monrovia, Calif) or a glass ionomer cement (Fuji I; GC, Tokyo, Japan) to recently extracted premolars and kept in water for 6 months. The same materials were also bonded to the premolars of orthodontic patients. After 6 months, the teeth were carefully extracted, with the brackets intact on their buccal surfaces. All teeth were embedded in epoxy resin and sectioned buccolingually. Fourier transform infrared microscopy and Raman microscopy were used for the estimation of the degree of cure in the composite and the salt yield in the glass ionomer adhesives. Results: The control samples of the composite showed significantly lower degrees of cure than did the retrieved specimens (52.40% 6 3.21% vs 57.62% 6 1.32% by Fourier transform infrared microscopy, and 61.40% 6 2.61% vs 67.40% 6 3.44% by Raman microscopy). Raman microscopy significantly overestimated the degree of cure and failed to provide reliable information for the salt yield in the glass ionomer cement. Fourier transform infrared microscopy showed increased, but no statistically significant difference in, aluminum-carboxylate salts in the retrieved specimens. Conclusions: Enhanced oxidation of residual carbon-carbon bonds in the composite and slightly increased dissolution of the weaker calcium-salt phase in the glass ionomer cement were the main differences in the intraorally aged specimens in comparison with the specimens stored in water. (Am J Orthod Dentofacial Orthop 2014;145:425-33)
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espite many studies from orthodontic materials research, the laboratory configurations and setups used for exposing the materials to water and various electrolytes fail to simulate the actual oral environment, which includes saliva, pH fluctuation, microbial attack, complex cyclic loading, and enzymatic degradation.1 Since the intraoral environment is a
a Postgraduate student, Department of Orthodontics, School of Dentistry, University of Athens, Athens, Greece. b Attending dental physician, Department of Orthodontics and Paediatric Dentistry, Center of Dental Medicine, University of Zurich, Zurich, Switzerland. c Professor and director of orthodontics, Dubai School of Dental Medicine, Dubai, United Arab Emirates; professor, Department of Orthodontics, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece. d Professor and director, Department of Orthodontics and Paediatric Dentistry, Center of Dental Medicine, University of Zurich, Zurich, Switzerland. e Professor and head, Department of Dental Biomaterials, School of Dentistry, University of Athens, Athens, Greece. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Theodore Eliades, Department of Orthodontics and Paediatric Dentistry, Center of Dental Medicine, University of Zurich, Plattenstarsse 11, CH-8032, Zurich, Switzerland; e-mail, [email protected]. Submitted, July 2013; revised and accepted, November 2013. 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.11.020
complex milieu capable of affecting the physical and mechanical properties of an adhesive, laboratory data should be carefully interpreted. Thus, there is a notable lack of evidence regarding the intraoral aging pattern of orthodontic materials and the associated phenomena, such as surface alterations, structural changes, and variations of mechanical properties. The intraoral exposure might affect the longevity of the adhesive (eg, aging of polymeric adhesives) by modification of structural qualities. The traditional orthodontic adhesives for bracket bonding are resin composite materials, introduced in the mid-1970s. These orthodontic adhesives have been used successfully on acid-etched enamel over the last 40 years. The development of visible light-cured materials offered additional advantages in the field—ie, high early bond strength,2 minimal oxygen inhibition,3 and prolonged working time for optimal bracket placement—which have brought innovation and practicality to modern orthodontic treatment. Disadvantages of the resin composite adhesives include loss of enamel during acid conditioning,4 loss of the superficial fluoride-rich enamel zone during debonding,5 and demineralization 425
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of enamel around the brackets because of poor patient compliance with respect to oral hygiene.6,7 The importance of the curing efficiency on the performance of resin composites has long been documented. It has been shown that the degree of cure (%DC, the amount of carbon-carbon bonds reacted after setting) of resin composites modulates the composite's physical and mechanical properties,8,9 color stability,10 solubility and degradation,11 and biocompatibility.12 Glass ionomer cements possess unique benefits that make them useful in clinical orthodontics. They chemically adhere to both enamel and metal,13,14 release fluoride with its known caries-inhibiting properties,15,16 and can absorb fluoride from sources such as fluoride solutions, thus recharging their fluoride reservoir.17 Furthermore, acid conditioning is not required, so that both bonding and debonding procedures can reduce iatrogenic damage to enamel, and the bonding field does not have to be dry, thus increasing the speed and ease of bracket placement.18 But it is a concern that bond strengths for glass ionomer cements are not of the same magnitude as those for resin composite adhesives; as a result, the failure rate of brackets attached to teeth is higher.18,19 To improve bracket cementation, resinmodified glass ionomers were introduced.18 Because the phenomena accompanying intraoral aging of the material might not be reflected in shortterm or long-term performance of the adhesives under laboratory conditions, intraoral aging might modify the critical properties of orthodontic adhesives. The purpose of this study was to assess the effect of 6 months of intraoral aging on the setting efficiency of resin composite and conventional glass ionomer orthodontic adhesives relative to control specimens stored in water. The null hypothesis tested was that there is no difference in the setting status between laboratory and intraorally aged resin composite and glass ionomer orthodontic adhesives. MATERIAL AND METHODS
The control sample consisted of 14 sound premolars, recently extracted for orthodontic reasons and kept for 1 week in distilled water with the addition of 0.5% sodium azide at 8 C. The teeth were randomly classified into 2 groups of 7 specimens each, cleaned and polished with a nonfluoride paste (Clean Polish; Hawe-Neos Dental, Bioggio, Switzerland). Metallic brackets (Mini Sprint; Forestadent, Pforzheim, Germany) were bonded to the middle buccal enamel surfaces with a conventional glass ionomer cement (Fuji I; GC, Tokyo, Japan) or a composite resin adhesive (Transbond XT; 3M Unitek, Monrovia, Calif). Glass ionomer cement was placed directly on blot-dried enamel. The resin composite adhesive, used
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without primer or bonding resin, was applied on air-dried, acid-etched (for 30 seconds) enamel with a 35% phosphoric acid gel (Transbond XT etching gel, 3M Unitek) and photopolymerized from the incisal and cervical bracket edges (10 seconds each) with a halogen light-curing unit (850 mW/cm2 standard light intensity, Trilight; 3M ESPE, Seefeld, Germany). The tip of the unit was placed in contact with the bracket edges at approximately a 45 angle. All teeth were stored in 200 mL of distilled water at 37 C for 6 months. The water was replaced once a month to prevent saturation from leachable components. The test sample consisted of specimens obtained from patients recruited in the Graduate Orthodontic Clinic of the Aristotle University of Thessaloniki, Thessaloniki, Greece, with an age range of 13 to 36 years (mean, 22 years); 55% were male, and 45% were female. Briefly, the patients had been selected using the following selection criteria: need for extractions for orthodontic purposes of 2 or 4 premolars, intact premolar buccal surfaces, and no previous glass ionomer restorative fillings in the oral cavity. The original protocol had been approved by the ethical committee of the university (14/18.07.2005); after detailed explanations of the purpose of this research and the clinical procedures, consent had been obtained. Furthermore, no orthodontic treatment had been started before the extractions and 6 months after bonding with the adhesives to be tested. The patients and the buccal premolar surfaces had been randomly classified (numbers 1-7 for patients; letters C [composite] or G [glass ionomer] for tooth surfaces) in 7 contralateral pairs (total of 14 teeth) to receive sequentially alternating resin composite and glass ionomer adhesives. The tooth surfaces had been cleaned and polished with the nonfluoride paste. The same brand of metallic brackets had been bonded with either the conventional glass ionomer cement (Fuji I) or the composite resin (Transbond XT) as reported before. After 6 months, the teeth were carefully extracted, with the brackets intact on their buccal surfaces, and thoroughly rinsed with distilled water. The roots of all teeth were removed by a horizontal section 1 mm below the cervical enamel margins, and the crowns were embedded in epoxy resin (CaldoFix; Struers A/S, Ballerup, Denmark) with the proximal surfaces parallel to the horizontal plane. The specimens were sectioned buccolingually using a microtome under continuous coolant (Isomet; Buehler, Lake Bluff, Ill). The sections were then polished with silicon carbide papers (600-1200 grit sizes) under water coolant and ultrasonicated in distilled water for 2 minutes to remove the surface smears (Fig 1). The specimens were stored in a desiccator for 5 days to remove loosely bound water
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Fig 1. Reflected light microscopic images of representative specimens used for the analysis of the orthodontic adhesives: A, bracket bonded with the light-cured resin composite adhesive; B, bracket bonded with the conventional glass ionomer cement. Note the porosity in the glass ionomer layer mainly attributed to hand mixing (magnification 12 times; bar, 1 mm).
and then studied by Fourier transform infrared microscopy (FTIRM) and Raman microscopy (RM). FTIRM was performed with a microscope (AutoImage; Perkin-Elmer, Bacon, United Kingdom) equipped with a germanium micro-attenuated total reflectance objective and attached to a Fourier transform infrared spectrometer (Spectrum GX; Perkin Elmer). Spectra were acquired at 4 points per specimen (2 in the bulk adhesive, 2 at the margins) under the following conditions: mercury cadmium telluride detector cooled by liquid nitrogen, 4000 to 750 cm 1 wave number range, 4 cm 1 resolution, 50-mm-diameter sampling window (attenuated total reflectance objective), 128 spectra coaddition, and 1mm depth of analysis at 1000 cm 1. All spectra were subjected to baseline and attenuated total reflectance corrections. Spectra of the resin composite paste and the glass ionomer mix before setting were used as references of the unset materials. RM was performed by a Raman spectrometer (EZRaman I; Enwave Optronics, Irvine, Calif) attached to a microscope (MBE; Leica Microsystems, Wetzlar, Germany)
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under the following conditions: 785 nm laser excitation, 40-times magnification, 100 to 200 mW nominal power, 1.5-mm working distance, 2000 to 200 cm 1 wave number range, and 5 cm 1 resolution. Four regions were analyzed per specimen as above. For the resin composite adhesive, the percentage degree of cure (%DC) after storage was assessed on a relative basis by the 2-band method and the tangent baseline technique.20 The aliphatic (C 5 C) bond stretching vibrations at 1638 cm 1 were chosen as the analytical band, whereas the aromatic (C.C) bond-stretching vibrations at 1605 cm 1, which are unaffected by the polymerization reaction, were chosen as the reference band. The %DC was calculated according to the equation: %DC 5 100 3 [1 (A/B 3 C/D)], where A is the net peak absorbance height of the polymerized material at 1638 cm 1, B is the absorbance height of the unpolymerized material at 1638 cm 1, C is the absorbance height of the unpolymerized material at 1605 cm 1, and D is the absorbance height of the polymerized material at 1605 cm 1. The %DC values for each specimen were the averages of the 4 measurements. For the glass ionomer cement, the following peaks were used to monitor the acid-base reaction: the carboxyl peaks of polyacrylic acid (1740 cm 1) and tartaric acid (1730 cm 1), and the peaks of calcium polyacrylate (1540 cm 1), aluminum polyacrylate (1610 cm 1), calcium tartrate (1595 cm 1), and aluminum tartrate (1670 cm 1).21,22 Because the carboxyl groups are completely neutralized during the early setting stages of conventional glass ionomers, the salt yield was calculated by measuring the ratio of the peak areas of aluminum salts (1700-1600 cm 1) to calcium salts (1600-1500 cm 1) using the tangent baseline technique and the same analysis mode as above (4 regions per specimen). The results of the %DC were statistically analyzed with 2-way analysis of variance (ANOVA) (a 5 0.05) with analytical method and storage conditions as independent parameters. Statistical analysis of the salt ratio values was assessed by a t test. RESULTS
Representative FTIRM spectra of the resin composite adhesive are shown in Figure 2. All the set specimens demonstrated a reduction in the peak high at the 1638 cm 1 band (C 5 C stretching vibrations) because of the conversion (C 5 C / C C) after light curing. The spectra profiles were similar, except for a slight reduction of the C 5 C peak in intraorally exposed specimens. The RM spectra of the resin composite adhesive clearly demonstrated the characteristic bands (C 5 C stretching vibrations 1638 cm 1 and aromatic C.C
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Fig 2. FTIRM spectra of Transbond XT orthodontic adhesive at unset state and at set state after 6 months of storage in water and 6 months of intraoral exposure (absorbance scale, 2000-1300 cm 1 region).
stretching vibrations 1605 cm 1) used for the calculation of the %DC as well (Fig 3). The results of the %DC after the FTIRM and RM analyses are presented in Table I. Statistical analysis of the %DC results by 2-way ANOVA showed significant differences in analytical method and storage conditions (P \0.001), with no interaction between them (P 5 0.757). RM showed significantly higher %DC values for both storage conditions than did FTIRM (P \0.001). Within the same analytical technique, the retrieved specimens showed significantly higher %DC than did those stored in water (P 5 0.009 for FTIRM; P 5 0.004 for RM). In 2 of the 7 retrieved specimens, FTIRM analysis of the adhesive at the outer 50-mm regions showed the maximum %DC values. FTIRM spectra of the glass ionomer cement are illustrated in Figure 4. The characteristic broad carboxylic peak of the unset material (1730 cm 1) disappeared after setting, and new peaks appeared assigned to calcium polyacrylate (1550-1520 cm 1 and 1410-1390 cm 1), aluminum polyacrylate (1620-1580 cm 1 and 1470-1430 cm 1), calcium tartrate (1630-1600 cm 1), and aluminum tartrate (1670-1650 cm 1). The contribution of water (1642 cm 1) was greater in some water-stored specimens in comparison with the retrieved ones. Because of the shape complexity and partial overlapping of the peak regions, the peak area ratios (aluminum salts/calcium salts) instead of the sum of the absolute area values were used.
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A representative RM spectrum of a retrieved glass ionomer specimen, with the peaks of calcium and aluminum salts, is presented in Figure 5. The strong peak at 1457 cm 1 is attributed to methylene group (CH2) deformation and the peaks at 1553 cm 1 and 1614 cm 1 to calcium and aluminum salts, respectively. These peaks, though, are poorly resolved in RM because of the strong CH2 peak at the region.21 Therefore, peak measurements were limited only to FTIRM. The results are presented in Table II. Statistical analysis of the salt ratio values by t test showed no statistically significant difference between the mean values (P 5 0.181), although the ratios in the retrieved specimens had higher mean values. DISCUSSION
Notwithstanding numerous in-vitro studies, little is known about adhesives behaving at true clinical conditions. In this study, the bonded premolars remained in the subjects' mouths for 6 months, thus experiencing the in-vivo environment, before being extracted and tested. The split-mouth design, by the symmetric bonding of the 2 orthodontic adhesives at each patient's premolars, was an important prerequisite enabling both materials to be tested on teeth with the same morphology under the same intraoral conditions. Moreover, the water-stored specimens were used as the controls, since in many experiments water storage has been used as an aging medium.
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Fig 3. RM spectra of an unset (top) and a retrieved Transbond XT sample (bottom), after 6 months of intraoral storage (absorbance scale, 1750-1550 cm 1 region). Table I. Results of the degree of cure (%DC) of the
light-cured adhesive tested (n 5 7) Stored in water (6 mo, 37 C) Retrieved
%DC, FTIRM 52.4 (3.2)1,a 42.0 (min)-57.1 (max) 57.6 (1.3)2,c 56.2 (min)-59.4 (max)
%DC, RM 61.4 (2.6)3,b 59.9 (min)-64.1 (max) 67.4 (3.4)4,d 64.0 (min)-72.3 (max)
Values are mean (SD) or minimum and maximum. Same superscripts show means with no statistically significant differences (P .0.05) between storage conditions per method (numbers) and methods per storage condition (letters).
The light-cured orthodontic adhesive tested is a commonly used resin composite for bracket bonding. The glass ionomer cement used is indicated for bonding
orthodontic appliances and bands. We used the same product for bracket bonding instead of the proposed resin-modified glass ionomer analog (Fuji Ortho LC, GC) to clearly identify the effect of the immersion environment on individual reactions (free radical in composite, acid-base in glass ionomer), without the complex spectral interferences produced from dual set materials (free radical and acid-base, as in Fuji Ortho LC).23 Nevertheless, this should not be considered as a contraindication, since conventional glass ionomers are still used for bracket bonding.16 The glass ionomer was cemented on blot-dried, but not desiccated, enamel to facilitate acid ionization and ionic bonding to enamel. No protective varnishes were used, as in restorative applications, since the material area exposed to water
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Fig 4. FTIRM spectra of Fuji I at unset state and at set state after 6 months of storage in water and 6 months of intraoral exposure (absorbance scale, 2000-1300 cm 1 region). COOH, carboxyl groups; H2O, water; COOAl, aluminum carboxylate salt; COOCa, calcium carboxylate salt.
Fig 5. An RM spectrum of a retrieved Fuji I samples after 6 months of intraoral storage (absorbance scale, 2200-200 cm 1 region). The calcium (COOCa, 1553 cm 1) and aluminum (COOAl, 1614 cm 1) carboxylate salts are poorly resolved.
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Table II. Results of aluminum to calcium salts ratio
(n 5 7)
Stored in water (6 mo, 37 C) Retrieved
Aluminum-calcium salts, FTIRM 0.6 (0.2)a 0.3 (min)-0.7 (max) 0.8 (0.3)a 0.4 (min)-1.1 (max)
Aluminum-calcium salts, RM — —
Values are mean (SD) or minimum and maximum. Same superscript letters show means with no statistically significant differences (P .0.05) between storage conditions per method.
and intraorally was limited to the cross-sectional thickness of the cement (approximately 250 mm as measured under the microscopes). For both types of orthodontic adhesives, the effect of the storage environment was assessed on the outcome of their setting reactions, since establishment of the main material matrices strongly depends on the C 5 C conversion (resin composite) and the polysalt hydrogel formation (glass ionomer). The Fourier transform infrared method has long been considered as an appropriate analytical technique in quantifying the %DC of resin composites24 and studying the setting reactions of various categories of glass ionomer cements such as traditional, reinforced, or resin reinforced.21-23 The use of a Fourier transform infrared microscope with a 50-mm tip attenuated total reflectance objective made possible selective probing within the cross-sectional thickness of the orthodontic adhesive materials between the bracket base and the enamel. RM, a complementary technique to FTIRM, offers improved spatial resolution, has no interferences from water, better resolves the polyacrylic acid peak from the various polycarboxylate salts formed,22 and has been frequently used for measurement of %DC in resin composites.25-27 However, the main problems of RM are the thermal effects from the focused laser beam and the fluorescence of organic targets, leading to high background scattering.28,29 For this reason, precise calibrations of the laser wavelength and power are required. In this study, the glass ionomer regions probed by the Raman laser showed cracking and pitting when examined under the optical microscope of the unit. This implies that the Raman analysis, as performed, induced severe dehydration in the glass ionomer specimens. The numbers of specimens used for the %DC and salt yield measurements were greater in our study (n 5 7) in comparison with in-vitro studies (n 5 3-5) to compensate for possible variances induced by the intraoral environment.30,31 The results of the FTIRM study showed that the intraorally aged resin composite adhesive showed higher
%DC values than did the water-stored controls. This might not be attributed to postcuring reactions but, rather, to interactions with the oral environment. Transbond XT is composed of bisphenol-A diglycidylether dimethacrylate and bisphenol-A bis (2-hydroxyethyl ether) dimethacrylate comonomers at a 2:1 weight ratio, 70% to 80% wt silanated quartz filler and 5% wt silanated silica filler.32 The set material is characterized by a crosslinked polymer network with a minor amount of residual monomer and a much greater amount of immobilized pendant C 5 C groups that can be subjected to hydrolytic, chemical, or enzymatic degradation, reducing thus the final amount of residual C 5 C unsaturation. Such a mechanism has been observed in some resin composites after accelerated exposure to topical acidulated phosphate fluoride agents in water, apparently due to fluoride addition to C 5 C bonds or acidic ester hydrolysis of the methacrylate ester groups.33 Our findings imply that the complex intraoral environment might cause breakdown of residual C 5 C bonds more effectively than isothermal water storage. The reduced C 5 C unsaturation found at the periphery of the bracket bases in 2 intraorally exposed specimens might be explained by their exposure to a more aggressive environment. Oxidation of residual C 5 C groups has been shown to result in network degradation and release of reactive compounds, such as formaldehyde,34 methacrylic acid, and others. Concerns have been expressed on the biologic performance of these compounds, since along with residual monomers, they can induce detrimental biologic effects in cell cultures.35 Moreover, the extent of %DC in adhesives with bisphenol-A components has been inversely associated with bisphenol-A release,36 a compound with known estrogenic activity.37 By increasing the %DC of the adhesive or modifying the chemistry of the adhesives, this concern might be diminished. The latter seems quite promising in orthodontic adhesives, by using aliphatic or single aromatic ring dimethacrylate compounds as substitutes for adhesives with bisphenol-A components.38 Conventional glass ionomer cements set by an acid-base reaction that leads to the formation of a polycarboxylate salt cement matrix. The product tested is a powder-liquid cement containing a powder of silicon dioxide, aluminum oxide, strontium carbonate, sodium fluoride, calcium carbonate, and calcium phosphate, and a liquid containing an aqueous solution of liquid polyacrylic and tartaric acids.39 Upon setting, salts are formed with divalent cations (Ca21, Sr21) that transform particle surfaces to a silica hydrogel. This structure is further stabilized by Al31 ions that coordinate to 6 atoms providing enhanced polyacrylate crosslinking
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and the final strength to the material.40 In this study, the intraorally exposed specimens demonstrated higher aluminum salt yields, although these were not statistically significant, possibly because of partial dissolution of the weaker divalent calcium salts. This might be implicated with the erosive dissolution of the material during low oral pH transient periods and the associated release of therapeutic compounds such as calcium, strontium, and fluoride.41 These results showed a different pattern of chemical changes in the resin composite orthodontic adhesive after intraoral exposure vs conventional water storage. This pattern was associated with an increased conversion from oxidation of residual C 5 C bonds in the retrieved specimens; this confirms the null hypothesis. However, in the traditional glass ionomer cement, the retrieved specimens demonstrated greater aluminum salt formation that did the specimens stored in water, but without a statistically significant difference, a finding that partially rejects the null hypothesis. These findings suggest that retrieval analysis of intraorally exposed specimens, which can be applied to most orthodontic treatment modalities, can provide a more reliable aging pattern than in-vitro water storage for complex material issues, such as their setting reactions. CONCLUSIONS
Intraoral aging affected the setting status of the resin composite adhesive to a statistically significant extent in comparison with water storage. The glass ionomer setting was affected as well, but not to a statistically significant extent. ACKNOWLEDGMENTS
Dr E. Chatzistavrou is acknowledged for her contributing to the study. REFERENCES 1. Matasa CG. Microbial attack of orthodontic adhesives. Am J Orthod Dentofacial Orthop 1995;108:132-41. 2. Eliades T, Viazis AD, Eliades G. Bonding of ceramic brackets to enamel: morphologic and structural considerations. Am J Orthod Dentofacial Orthop 1991;99:369-75. 3. Lekka MP, Papagiannoulis L, Eliades GC, Caputo AA. A comparative in vitro study of visible light-cured sealants. J Oral Rehabil 1989;16:287-99. € 4. Berghauser Pont H, Ozcan M, Bagis B, Ren Y. Loss of surface enamel after bracket debonding: an in-vivo and ex-vivo evaluation. Am J Orthod Dentofacial Orthop 2010;138:387.e1-9. 5. Diedrich P. Enamel alterations from bracket bonding and debonding: a study with the scanning electron microscope. Am J Orthod 1981;79:500-22. 6. Zachrisson BU, Zachrisson S. Caries incidence and oral hygiene during orthodontic treatment. Scand J Dent Res 1971;79:394-401.
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7. Ogaard B, R€ olla G, Arends J. Orthodontic appliances and enamel demineralization. Part 1. Lesion development. Am J Orthod Dentofacial Orthop 1988;94:68-73. 8. Asmussen E. Restorative resins: hardness and strength vs. quantity of remaining double bonds. Scand J Dent Res 1982;90:484-9. 9. Ferracane JL, Greener EH. The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J Biomed Mater Res 1986;20:121-31. 10. Ferracane JL, Moser JB, Greener EH. Ultraviolet light-induced yellowing of dental restorative resins. J Prosthet Dent 1985;54: 483-7. 11. Eliades T, Eliades G, Brantley WA, Johnston WM. Polymerization efficiency of chemically cured and visible light-cured orthodontic adhesives: degree of cure. Am J Orthod Dentofacial Orthop 1995;108:294-301. 12. Rathbun MA, Craig RG, Hanks CT, Filisko FE. Cytotoxicity of a BIS-GMA dental composite before and after leaching in organic solvents. J Biomed Mater Res 1991;25:443-57. 13. Hotz P, McLean JW, Sced I, Wilson AD. The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 1977; 142:41-7. 14. Nasu T. Polyacrylic acid-metal adhesive bond joint characterization by x-ray photoelectron spectroscopy. J Biomed Mater Res 1986;20:347-62. 15. Marcusson A, Norevall LI, Persson M. White spot reduction when using glass-ionomer cement for bonding in orthodontics: a longitudinal and comparative study. Eur J Orthod 1997;19: 233-42. 16. Shungin D, Olsson AI, Persson M. Orthodontic treatment-related white spot lesions: a 14-year prospective quantitative follow-up, including bonding material assessment. Am J Orthod Dentofacial Orthop 2010;138:136.e1-8. 17. Forsten L. Fluoride release and uptake by glass-ionomers. Scand J Dent Res 1991;99:241-5. 18. McComb D. Luting in orthodontic practice. In: Davidson C, Mjor IA, editors. Advances in glass-ionomer cements. Carol Stream, Ill: Quintessence; 1999. p. 171-82. 19. Fajen VB, Duncanson MG Jr, Nanda RS, Currier GF, Angolkar PV. An in vitro evaluation of bond strength of three glass ionomer cements. Am J Orthod Dentofacial Orthop 1990;97:316-22. 20. Rueggeberg FA, Hashinger DT, Fairhurst CW. Calibration of FTIR conversion analysis of contemporary dental resin composites. Dent Mater 1990;6:241-9. 21. Nicholson JW, Brookman PJ, Lacy OM, Wilson AD. Fourier transform infrared spectroscopic study of the role of tartaric acid in glass-ionomer dental cements. J Dent Res 1988;67: 1451-4. 22. Young AM, Sherpa A, Pearson G, Schottlander B, Waters DN. Use of Raman spectroscopy in the characterisation of the acid-base reaction in glass-ionomer cements. Biomaterials 2000;21: 1971-9. 23. Kakaboura A, Eliades G, Palaghias G. An FTIR study on the setting mechanism of resin-modified glass ionomer restoratives. Dent Mater 1996;12:173-8. 24. Yoon TH, Lee YK, Lim BS, Kim CW. Degree of polymerization of resin composites by different light sources. J Oral Rehabil 2002; 29:1165-73. 25. Miletic VJ, Santini A. Remaining unreacted methacrylate groups in resin-based composite with respect to sample preparation and storing conditions using micro-Raman spectroscopy. J Biomed Mater Res B Appl Biomater 2008;87:468-74.
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Iliadi et al
26. Acquaviva PA, Cerutti F, Adami G, Gagliani M, Ferrari M, Gherlone E, et al. Degree of conversion of three composite materials employed in the adhesive cementation of indirect restorations: a micro-Raman analysis. J Dent 2009;37:610-5. 27. Lohbauer U, Pelka M, Belli R, Schmitt J, Mocker E, Jandt KD, et al. Degree of conversion of luting resins around ceramic inlays in natural deep cavities: a micro-Raman spectroscopy analysis. Oper Dent 2010;35:579-86. 28. West YD. Research report on measuring the Raman spectra of temperature-sensitive solid samples. Spectrochim Acta A Mol Biomol Spectrosc 1997;53:35-42. 29. Bowie BT, Chase DB, Griffiths PR. Factors affecting the performance of bench-top Raman spectrometers. Appl Spectrosc 2000; 54:164A-73A. 30. Rueggeburg FA, Craig RG. Correlation of parameters used to estimate monomer conversion in a light-cured composite. J Dent Res 1988;67:932-7. 31. Mohamad D, Young RJ, Mann AB, Watts DC. Post-polymerisation of dental resin composite evaluated with nanoindentation and micro-Raman spectroscopy. Arch Orofac Sci 2007;2:26-31. 32. 3M Unitek. Material safety data sheet for 3M Unitek Transbond XT light cure adhesive (712-031, 712-036, 712-066). Available at: http://multimedia.3m.com/mws/mediawebserver?SSSSSuUn_ zu8l00x4YtB5YtUPv70k17zHvu9lxtD7SSSSSS. Accessed June 30, 2013. 33. Papagiannoulis L, Tzoutzas J, Eliades G. Effect of topical fluoride agents on the morphologic characteristics and composition of resin composite restorative materials. J Prosthet Dent 1997;77:405-13.
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34. Øysaed H, Ruyter IE, Sjøvik-Kleven U. Release of formaldehyde from dental composites. J Dent Res 1988;67:1289-94. 35. Matasa CG. Polymers in orthodontics: a danger present? In: Graber TM, Eliades T, Athanasiou AE, editors. Risk management in orthodontics: experts' guide to malpractice. Chicago: Quintessence; 2000. p. 113-31. 36. Sunitha C, Kailasam V, Padmanabhan S, Chitharanjan AB. Bisphenol A release from an orthodontic adhesive and its correlation with the degree of conversion on varying light-curing tip distances. Am J Orthod Dentofacial Orthop 2011;140: 239-44. 37. Gioka C, Eliades T, Zinelis S, Pratsinis H, Athanasiou AE, Eliades G, et al. Characterization and in vitro estrogenicity of orthodontic adhesive particulates produced by simulated debonding. Dent Mater 2009;25:376-82. 38. Papakonstantinou AE, Eliades T, Cellesi F, Watts DC, Silikas N. Evaluation of UDMA's potential as a substitute for Bis-GMA in orthodontic adhesives. Dent Mater 2013;29:898-905. 39. Saito S, Tosaki S, Hirota K. Characteristics of glass-ionomer cements. In: Davidson C, Mjor IA, editors. Advances in glass-ionomer cements. Carol Stream, Ill: Quintessence; 1999. p. 15-50. 40. Mitchell CA, Douglas WH, Cheng YS. Fracture toughness of conventional, resin-modified glass-ionomer and composite luting cements. Dent Mater 1999;15:7-13. 41. Wilson AD, Groffman DM, Kuhn AT. The release of fluoride and other chemical species from a glass-ionomer cement. Biomaterials 1985;6:431-3.
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