Sorption and Solubility Testing of Orthodontic Bonding Cements in Different Solutions

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1 Sorption and Solubility Testing of Orthodontic Bonding Cements in Different Solutions Manuel Toledano, 1 Raquel Osorio, 1 Estrella Osorio, 1 Fátima S. Aguilera, 1 Alejandro Romeo, 2 Blanca de la Higuera, 2 Franklin García-Godoy 3 1 Department of Dental Materials, University of Granada, Avda Fuerzas Armadas, n 1, 1 B, Granada, Spain 2 Department of Orthodontic, University of Barcelona, Barcelona, Spain 3 College of Dental Medicine, Nova Southeastern University, Fort Lauderdale, Florida Received 10 November 2004; revised 11 April 2005; accepted 13 April 2005 Published online 28 September 2005 in Wiley InterScience ( DOI: /jbm.b Abstract: To evaluate and compare the solubility and sorption of orthodontic bonding cements after immersion in different solutions, five different cements were used: a fluoridecontaining resin composite, a light-cured glass ionomer cement, a light-cured resin composite, a paste paste chemically cured resin composite, and a liquid paste chemically cured resin composite. Five different solutions were employed: distilled water, artificial saliva, an alcoholfree mouthrinse solution (Orthokin), a 5% alcohol mouthrinse solution (Perioaid), and a 75% ethanol/water solution. Five disc specimens (15 mm 0.85 mm) were used for each experimental condition. Materials were handled following manufacturers instructions and were ground wet with silicon carbide paper. Solubility and sorption of the materials were calculated by means of weighing the samples before and after immersion and desiccation. Data were analyzed by two-way ANOVA and Student Newman Keuls test (p < 0.05). The light-cured glass ionomer cement showed the lowest solubility and the highest sorption values. When using alcohol-containing solutions as storage media, solubility of the paste paste chemically cured resin composite increased, and sorption values for the tested chemically cured resin composites were also increased. The use of alcohol-free mouthrinses does not affect sorption and solubility of orthodontic cements. The chemically cured (paste paste) composite resin cement, requiring a mixing procedure, was the most affected by immersion in alcohol-containing solutions Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 76B: , 2006 Keywords: solubility; sorption; alcohol; orthodontic bonding cements INTRODUCTION Orthodontic brackets are predominantly bonded to teeth using composite resins. Efforts to minimize the formation of white spot lesions include the use of fluoride-releasing bonding agents, as resin modified glass ionomers have the potential to minimize demineralization around orthodontic brackets. 1 3 All these cements are exposed intermittently or continuously to chemical agents such as those found in saliva or food. This process could provide a path for their rapid degradation 4 which would produce a potential impact on both the structural stability of the materials and their mechanical properties, 5 7 Correspondence to: R. Osorio ( toledano@ugr.es) Contract grant sponsor: CICYT/FEDER MAT; contract grant number: C03-02 Contract grant sponsor: RED CYTED VIII.J Contract grant sponsor: Enterprise-University of Granada, Spain; contract grant number: Wiley Periodicals, Inc. leading to higher bracket debonding rate during the orthodontic treatment. When resin samples are stored in wet conditions, some of the components, such as unreacted monomers or fillers, dissolve and are leached out of the samples. This results in loss of weight and can be measured as solubility or leaching. 8 Sorption will also occur by the polymer matrix and could cause hydrolytic corrosion of the network structure and debonding of the silanated fillers. 7 It will affect the suitability of the clinical applications of these materials. 7 Several factors contribute to the process of elution from dental composites: unreacted monomers, size, and chemical composition of the elutable specimens and chemistry of the solvent. 5 Orthodontic bonding cements are either exposed intermittently or continuously to chemical agents such as those found in saliva, food, and mouthrinses. 9,10 Most of the mouthrinses include ethanol in their composition, and agents such as ethanol may accelerate and increase the expected water deg- 251

2 252 TOLEDANO ET AL. TABLE I. Names, Type of Product, Required Mixing Procedure, Setting Mechanism, and Manufacturers of the Different Tested Cements Name and Type of Product Setting Mechanism/Presentation Form Manufacturer Kurasper F Light-activated Kuraray, Osaka, Japan Fluoride-containing resin composite Liquid paste Fuji Ortho LC Light-activated GC America Inc., Alsip, IL, USA Resin-modified glass ionomer cement Liquid powder* Concise Chemically cured 3M-ESPE, St Paul, MN, USA Resin composite Paste paste* Light Bond Light-activated Reliance, Itasca, IL, USA Resin composite Liquid paste System-One No-mix, chemically cured ORMCO Corp., Glendora, CA, USA Resin composite Liquid paste * Requiring a mixing procedure. radation of composite resins. 11 Subsurface and surface degradation of composites has been reproduced in vitro by storing of composites in ethanol, which appears in FDA guidelines as a food-simulating liquid. 12,13 Moreover, mechanical properties of composite resins are also compromised by aging in alcohol-containing solutions. 12 The purpose of this study was to measure the solubility and sorption of five cements for orthodontic bracket bonding, in five different solutions: artificial saliva, distilled water, 75% ethanol/water, an alcohol-free mouthrinse (Orthokin), and a 5% alcohol-containing mouthrinse solution (Perioaid). The null hypothesis to be tested is that there are no differences in sorption and solubility of these cements after being immersed in these different solutions. MATERIAL AND METHODS Five commercial cements for bracket adhesion were tested: Kurasper-F, Fuji Ortho LC, Concise, Light Bond, and System-One (Table I). Sorption and solubility measurements were performed immersing the specimens in five different solutions: artificial saliva, distilled water, 75% ethanol/water, an alcohol-free mouthrinse (Orthokin), and a 5% alcoholcontaining mouthrinse solution (Perioaid) (Table II). For sorption and solubility measurements, five disk specimens were prepared for each material. Products were handled following the manufacturer s instructions. A mold for the preparation of a disc specimen mm in diameter and mm thickness was used, following the instructions of Söderholm. 7 For products that are presented as liquid/ paste, the liquid was painted on two glass microscope slides (Kurasper F was applied only in one slide, as manufacturers indicate only 1 layer application) and the specimens were allow to polymerize chemically or in the case of light-activated cements the specimens were light-cured, under the two glass microscope slides, with an activated light source (Translux CL, Kulzer, Wehrheim, Germany) polymerization unit. The light was tested for light output (600 mw/cm 2 )by means of a Demetron radiometer (Model 100, Demetron Research Corp., Danbury, CT). The samples were irradiated on both sides and in different positions for 40 s until the entire area was exposed. After removal from the mold, the specimens were ground wet with silicon carbide paper (500 to 1200 grits) on both sides to a thickness of mm, so that unpolymerized and poorly polymerized surface layers would be removed. Thickness measurements were made with a digital caliper Mitutoyo (Mitutoyo, Tokyo, Japan). Disks were conditioned by placing them in a desiccator, containing calcium sulfate, at 37 C until a constant weight had been achieved (a constant weight is considered when no weight change is obtained during three consecutive measurements at the accuracy of the scale: g) (m 0 ). The disks were placed in a glass vial containing 100 ml of distilled water, artificial saliva, Orthokin mouthrinse, Perioaid mouthrinse, and 75% ethanol/water solution (Table II). The vials were wrapped in aluminum foil to avoid light exposure and placed in an incubator at 37 C at intervals (1 h, 6 h, 24 h, and TABLE II. Composition of Different Tested Solutions Solution Main Ingredients Artificial saliva Potassium biphosphate 0.02 g, potassium chloride 0.12 g, potassium thiocyanate g, sodium biphosphate g, sodium chloride 0.07 g, sodium bicarbonate 0.15 g, urea 0.15 g, lactic acid (ph 6.7), excipient 100 ml Water Distilled water Ethanol/water 75% ethanol, 25% distilled water Orthokin Chlorhexidine digluconate 0.06 %, zinc acetate 0.34 %, excp. 100 ml Perioaid Chlorhexidine digluconate 0.12 %, ethanol 5%, excipient 100 ml

3 SORPTION AND SOLUBILITY OF ORTHODONTIC CEMENTS 253 TABLE III. Mean Solubility Values [ g/mm 3 (SD)] Obtained for Orthodontic Bonding Cements in Different Solutions Saliva Water Ethanol/Water Orthokin Perioaid Kurasper F 1.2 (0.5) b1 0.4 (0.5) b1 4.1 (2.5) a1 0.2 (0.2) b1 1.2 (1.0) a1 FujiOrtho LC 3.7 (2.7) a1 2.4 (3.7) a1 3.2 (1.5) a1 4.1 (2.8) a1 4.2 (5.1) b1 Concise 1.5 (0.6) c1 2.5 (1.2) b (9.9) c3 5.8 (4.6) b (9.4) c3 Light Bond 0.6 (0.4) bc1 1.6 (1.7) b1 3.1 (5.1) b1 1.3 (1.9) b1 1.9 (2.9) a1 System-One 0.5 (0.7) bc1 1.6 (0.9) b1 6.6 (9.4) b1 1.7 (1.4) b1 5.5 (4.3) a1 Letters show differences between materials in the same solution and numbers show differences between solutions in each material ( p 0.05). subsequently at 2-day intervals) removed, blot-dried, weighed, and then returned to water. This was continued until a constant weight was achieved (m 1 ). The discs were removed from the water and replaced in a desiccator containing calcium sulfate, at 37 C and were reweighed until a constant weight had been achieved. It was subsequently dried by placing it into a vacuum oven (25 inches of mercury) at 60 C for 24 h and then reweighed for the last time (m 2 ). These steps were carried out to evaluate sorption (A) and solubility (S) according to Oysaed and Ruyter 14 formula: A m 1 m 2 /V and S m 0 m 2 /V, where: m 0 is the sample weight before immersion, m 1 is the sample weight after immersion, and m 2 is the sample weight after immersion and desiccation, and V is the sample volume. For weight measurements, an analytic scale (A & D Instruments, Frankfurt, Germany) was used, with an accuracy of g; the complete device was mounted on an antivibration table. Means and standard deviations for solubility and sorption values ( g/mm 3 ) were calculated for each group. Two-way analysis of variance and Student Newman Keuls multiple comparisons tests were performed with p 0.05 as the level of significance. RESULTS Solubility mean values ( m/mm 3 ) and standard deviations are presented in Table III. Solubility was different for the tested materials (F 39.68; p ) and influenced by the tested solutions (F 6.5; p 0.003). Interactions between these variables were also significant (F 3.37; p 0.003). Power of the ANOVA test was 81%. Sorption mean values ( m/mm 3 ) and standard deviations are presented in Table IV. Sorption was different for the tested materials (F 218.5; p ) and affected by the tested solutions (F 2.89; p 0.04). Interactions between these variables were also significant (F 7.24; p ). Power of the ANOVA test was 93%. The light-cured glass ionomer (Fuji Ortho LC) showed the lowest solubility and the highest sorption, followed by the other materials. When ethanol/water solution or the alcoholcontaining mouthrinse (Perioaid) were used as storage solutions, the solubility values of the paste paste chemically cured resin composite (Concise) increased, showing higher solubility than the rest of the materials. Sorption also increased in the two chemically cured resin composites (Concise and System-One) when using alcohol-containing storage solutions, attaining similar values to those of the light-cured glass ionomer cement Fuji Ortho LC. DISCUSSION In the present study, composite resins attained similar solubility and sorption values when using distilled water, artificial saliva or the non-alcohol containing mouthrinse (Orthokin) as storage solutions. Range of solubility and sorption values for these composite resins are from 1.6 to 2.5 g/mm 3 and from 2.5 to 3.7 g/mm 3 respectively, and are similar to those previously published. 14,15 Solubility values are slightly lower, compared to those obtained by Oysaed and Ruyter 14 ( g/mm 3 ), probably due to the absence of radiopaque constituents, which sometimes appear in the restorative composite resins and result in higher solubility. 16 It is difficult to make direct comparisons between studies, as the results are often for different time periods, specimen size, expressed in different units. 17 Different size specimens will take different periods of time for water to completely infiltrate throughout the polymer matrix, and the materials that absorbed more water would also take longer to stabilize. 18 Furthermore, the constant handling of the specimens may TABLE IV. Mean Sorption Values [ g/mm 3 (SD)] Obtained for Orthodontic Bonding Cements in Different Solutions Saliva Water Ethanol/Water Orthokin Perioaid Kurasper F 4.0 (0.3) a1 4.8 (1.6) a1 5.1 (0.9) a1 4.2 (1.9) a1 5.1 (2.7) a1 FujiOrtho LC 16.1 (0.1) b1 9.3 (1.3) b (3.0) b (2.1) b (15) b1 Concise 3.6 (0.7) a1 3.5 (1.5) a (2.9) b2 3.9 (1.2) a (2.6) b1 Light Bond 2.5 (0.5) a1 2.7 (0.9) a1 3.2 (1.1) a1 2.7 (1.5) a1 3.2 (1.3) a1 System-One 3.8 (1.8) a1 3.1 (1.1) a (2.5) b2 3.5 (1.0) a (1.9) b2 Letters show differences between materials in the same solution and numbers show differences between solutions in each material ( p 0.05).

4 254 TOLEDANO ET AL. have also caused minute wear of the surface, leading to a weight reduction. 19 So, studies determining the solubility and sorption of resin-based materials are important mainly for their relative values, although numerical comparisons are not always possible. Other factors influencing solubility values include filler concentration and particle sizes, the coupling agents, the particles themselves, 8 and the type of solvent. 20 In the present study, when using alcohol-containing solutions as storage media, the solubility of the paste paste chemically cured composite resin (Concise) was increased. Two reasons may explain the variations in solubility: (1) elution of components is more complete in alcohol or organic solvents compared to water, solutions of ethyl alcohol in water have been shown to be the best solvents for dental composite networks, 5,12,13 and especially for uncured Bis-GMA resins 13,21 ; (2) the degree of conversion of the monomers varies somewhat among materials 22,23 ; the mixing method of this paste paste composite resin may generate air voids, 17 and air voids incorporated in the material lead to inhibition zones with unpolymerized materials 24 which may accelerate the solubility. 14,25 A low degree of conversion results in more leachable ingredients. 6,22,23,25 The higher sorption values were attained by the tested glass ionomer cement (Fuji Ortho LC). Networks with high cross-linking density (composite resins) have reduced solvent uptake, but the extent of water uptake has a minimal relationship to the degree of conversion of the polymer network 22 and it is highly dependent upon the chemistry of the monomers, the potential for hydrogen bonding and polar interactions, which exist in glass ionomer cements. When chemically cured composites were stored in alcohol-containing solutions the sorption increased and values were similar to those of glass ionomers. It may be due to the enhanced ability of ethanol to penetrate and swell the crosslinked polymer network in comparison to water. 13,26 A high level of porosity will also facilitate fluid transport into and out of the network leading to enhanced water uptake. 26,27 The uptake of water or solvent by composite resins may cause swelling, and a reduction of interchain interactions that may soften the resin. This effect would be greatest for surface properties and at a slower pace involve more of the matrix and be reflected in bulk mechanical properties. 26,28 The water destroys some of the filler matrix bonds and causes the surrounding matrix to swell and plasticize, which will facilitate filler pull-out, 29 but the hydrolytic stability of coupling agents also varied in the different fillers. 14 However, no conclusive evidence exists showing that hydrolytic degradation of the fillers is a key factor in the wear process of dental composites. 20 Ferracane and Marker 13 demonstrated that the fracture toughness of composites aged in ethanol was significantly lower than for specimens aged in water, probably due to polymer degradation as well as to the reduction in modulus brought on by softening of the resin matrix. A reduction of bracket-enamel shear bond strength has been shown to occur after storing the specimens in 75% ethanol/water solution for 1 week, attaining similar bond-strength values for the lightcured glass ionomer cement than those of some composite resins. 30 As this solution is recommended as a food simulator when testing dental materials, 12,13 these results may be considered clinically relevant; and they might explain the similar failure rate obtained when in vivo studies are conducted with both types of cements, 31 even when bond strength of glass ionomer is reported to be lower than that of composite resins in most of the in vitro studies. 30,32 The light-cured glass ionomer cement (Fuji Ortho LC) and the fluoride containing composite resin (Kurasper F) obtained no differences in sorption or solubility mean values between solutions, but they did obtained negative solubility values. When specimens are immersed in water (initial weight measurement: m 0 ), both weight increases by water uptake, and decreases by dissolution of components occurred at the same time, 15,27 and final weight is recorded as m 1. After the complete desiccation of the specimens (m 2 is obtained), water gained by composite resins, usually held within pores, between polymer chains or even at the resin filler interface, 26 is lost (in these cases m 0 m 2 and positive values for solubility are obtained). When glass ionomers are tested, in an initial phase, cements absorb water and disintegration of a surface layer is the main problem. Glass particles, ions, and some of the organic materials can be found in the solvent. 33 But the gained water participates in a continued acid-base setting reaction, and leads to the formation of a silica gel (supporting the hypothesis of a possible a self-healing process in glass ionomers). 3,15,34,35 This water will not be lost by desiccation (so, m 0 m 2 and negative values for solubility may be obtained), thus resulting in a great reduction of the encountered solubility values during the first hours and days 2,33 (Table III). Moreover, the particles participate in the setting reaction; and may lead to the formation of a silica gel at the particle peripheries that links the filler core to the matrix phase, 36 and reduces disintegration of surface layer. All these phenomena, occurring in glass ionomer-based materials, make difficult comparisons of their water solubility values to those of composite resins. Even when resin-modified glass ionomers are hybrid materials between composite resin and glass ionomers, water solubility values of these cements may be better determined by the old ADA specification test (N 8, American Dental Association, 1978) than by means of weight changes. The determination of true water sorption is difficult. Using artificial saliva as storage media did not affect the sorption and solubility values of tested materials. Other authors neither found changes in IR (infrared) spectra after storing composite resins in artificial saliva, in comparison with water. 11 Saito 37 stated that glass ionomers were less soluble in artificial saliva than they were in water. The presence of resin component in Fuji Ortho could have helped to obtain a more consistent and stable chemical link among all constituents of the material. Lee and colleagues 4 also stated that resin-based materials are not susceptible to chemical breakdown by artificial saliva. Nevertheless, an ion exchange mechanism has been previously shown to occur at the filler surface of cements, which implies that the original filler

5 SORPTION AND SOLUBILITY OF ORTHODONTIC CEMENTS 255 ions are substituted by ions from the solution. 20 As is the case with most in vitro studies, caution must be used when the results are extrapolated to the oral environment. The oral environment is also likely to cause more pronounced filler degradation than that revealed by storage in artificial saliva as it differs from real saliva, particularly in its lack of anhydrases, amylases, peroxidases, lysozimes, 20 and other esterases that may cause a reduction in wear resistance of resinbased materials. 38 Moreover, interactions between oral microbes and the polymer network also may be possible because bacteria can colonize on composite surface and may exert a degrading effect. 39 Curing of specimens in vitro represents an ideal processing procedure. 25 In the present study, the light activation of light-cured resin materials has been performed directly on the specimens surfaces, but when metal brackets are bonded to enamel during the clinical procedure, an incomplete polymerization may occur. 32,40 The light must travel through a considerable thickness of enamel and dentin and, although illumination of the tooth from different aspects is ideal, it is not always possible. Curing light-activated materials during orthodontic bonding, even with longer illumination times, does not result in the same degree of polymerization that is obtained by direct illumination. 32,40 So, higher solubility and sorption than reported values are expected in the oral environment. 20 Our results should not be extrapolated to other systems because of the effects and interactions of the filler composition and variation in chemistry. Additional investigations in these directions may yield information for better understanding of these cements. It may be concluded that the use of alcohol-free mouthrinses would be preferable when undergoing orthodontic treatment with fixed appliances bonded with a chemically cured resin composite, especially if it is a paste paste product requiring a mixing procedure. A decrease in mechanical properties of these cements is expected to occur during the orthodontic treatment. REFERENCES 1. Marcushamer M, García-Godoy F, Chan DC. 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