Improvement of enamel bond strengths for conventional and resin-modified glass ionomers: acid-etching vs. conditioning

Improvement of enamel bond strengths for conventional and resin-modified glass ionomers: acid-etching vs. conditioning first-author corresp ‡Corresponding Author ‡Corresponding Author †E-mail:fbp@zju.edu.cn †E-mail:fbp@zju.edu.cn fbp@zju.edu.cn Hospital of Stomatology Affiliated to Zhejiang University School of Medicine, Hangzhou 310006, China Hospital of Stomatology Affiliated to Zhejiang University School of Medicine, Hangzhou 310006, China Objective: 2 This study deals with the effect of phosphoric acid etching and conditioning on enamel micro-tensile bond strengths (μTBSs) of conventional and resin-modified glass ionomer cements (GICs/RMGICs). Methods: 2 Forty-eight bovine incisors were prepared into rectangular blocks. Highly-polished labial enamel surfaces were either acid-etched, conditioned with liquids of cements, or not further treated (control). Subsequently, two matching pre-treated enamel surfaces were cemented together with one of four cements [two GICs: Fuji I (GC), Ketac Cem Easymix (3M ESPE); two RMGICs: Fuji Plus (GC), RelyX Luting (3M ESPE)] in preparation for μTBS tests. Pre-treated enamel surfaces and cement-enamel interfaces were analyzed by scanning electron microscopy (SEM). Results: 2 Phosphoric acid etching significantly increased the enamel μTBS of GICs/RMGICs. Conditioning with the liquids of the cements produced significantly weaker or equivalent enamel μTBS compared to the control. Regardless of etching, RMGICs yielded stronger enamel μTBS than GICs. A visible hybrid layer was found at certain enamel-cement interfaces of the etched enamels. Conclusions: 2 Phosphoric acid etching significantly increased the enamel μTBSs of GICs/RMGICs. Phosphoric acid etching should be recommended to etch the enamel margins before the cementation of the prostheses such as inlays and onlays, using GICs/RMGICs to improve the bond strengths. RMGICs provided stronger enamel bond strength than GICs and conditioning did not increase enamel bond strength. Glass ionomer cements Surface treatments Bovine enamels Scanning electron microscopy (SEM) Micro-tensile bond strengths Introduction 1 Glass ionomer cements (GICs) are composed of calcium fluoroaluminosilicate glasses and an aqueous solution of polyelectrolyte (Wilson and Kent, 1972 1972 ; Sidhu, 2011 2011 ; Moshaverinia et al., 2012 2012 ). The latter is a homo-or co-polymer of unsaturated carboxylic acids (Wilson and Kent, 1972 1972 ; Moshaverinia et al., 2012 2012 ). Compared with other restorative materials, GICs exhibit several clinical advantages, including physicochemical bonding to tooth structures (Glasspoole et al., 2002 2002 ), fluoride release over a long period (Kent et al., 1979 1979 ; Khouw-Liu et al., 1999 1999 ), and good biocompatibility (Tanumiharja et al., 2000 2000 ; Xie et al., 2000 2000 ). Moreover, GICs are considered smart materials, self-adhering to tooth hard tissues without any surface pretreatment, due to the ionic bond formation between the hydroxyapatite (HAp) of tooth hard tissues and carboxyl groups of polyalkenoic acid (Wilson and Kent, 1972 1972 ; Yoshida et al., 2000 2000 ). However, GICs have some disadvantages, such as low early strength and moisture sensitivity (Fajen et al., 1990 1990 ; Wiltshire, 1994 1994 ; Khouw-Liu et al., 1999 1999 ; Itoh et al., 1999 1999 ; Xie et al., 2000 2000 ; Chitnis et al., 2006 2006 ). In order to improve the chemical and physical properties of GICs, resins were incorporated into GICs. As a result, resin-modified glass ionomer cements (RMGICs) were introduced to the dental profession in 1988, which contain fluoroaluminosilicate glasses, polyacrylic acid, as well as a methacrylate monomer, such as hydroxyethyl methacrylate (HEMA) (Antonucci et al., 1988 1988 ; Nicholson and Czarnecka, 2008 2008 ). Surface pretreatments are of great importance for dental adhesion and cementation. Enamel bond strengths depend on the etching agents, acid concentration, and etching time (Bishara et al., 2000b 2000b ; Movahhed et al., 2005 2005 ; Espinosa et al., 2010 2010 ). Phosphoric acid etching on enamel surfaces has been widely used as a standard bonding procedure using etch-and-rinse adhesives (Buonocore, 1955 1955 ; de Munck et al., 2005 2005 ; van Meerbeek et al., 2010 2010 ; Pashley et al., 2011 2011 ). More recently, selective enamel etching with phosphoric acid has been highly recommended to improve the performance of the mild self-etch adhesives (van Landuyt et al., 2006 2006 ; Erickson et al., 2009 2009 ; van Meerbeek et al., 2011 2011 ). However, the use of selective enamel etching or phosphoric acid etching for GICs/RMGICs has not been adequately explored, probably because these materials can adhere to the tooth hard tissue surfaces by chemical mechanisms (Wilson and Kent, 1972 1972 ; Yoshida et al., 2000 2000 ; 2001 2001 ; Fukuda et al., 2003 2003 ). A review of the dental literature reveals many controversial reports about the effect of acid-etching and conditioning on the enamel bond strengths of GICs/RMGICs (Powis et al., 1982 1982 ; Silverman et al., 1995 1995 ; Attin et al., 1996 1996 ; Cacciafesta et al., 1998 1998 ; 1999 1999 ; 2003 2003 ; Bishara et al., 2000b 2000b ; Valente et al., 2002 2002 ; Coutinho et al., 2006 2006 ). Several investigators demonstrated that GICs/RMGICs bonded to the enamel effectively without a need to etch or condition the teeth before bonding procedures (Silverman et al., 1995 1995 ; Cacciafesta et al., 1998 1998 ; 1999 1999 ). On the contrary, some laboratory studies revealed the improvements in the enamel bond strengths of GICs/RMGICs after enamel surface pretreatments with various solutions (Powis et al., 1982 1982 ; Cortes et al., 1993 1993 ; Attin et al., 1996 1996 ; Bishara et al., 2000a 2000a ; 2000b 2000b ; Glasspoole et al., 2002 2002 ; Valente et al., 2002 2002 ; Cacciafesta et al., 2003 2003 ; Coutinho et al., 2006 2006 ). In general, most of the previous studies focused on the surface treatments of RMGICs (Cortes et al., 1993 1993 ; Silverman et al., 1995 1995 ; Attin et al., 1996 1996 ; Cacciafesta et al., 1998 1998 ; 1999 1999 ; 2003 2003 ; Bishara et al., 2000a 2000a ; 2000b 2000b ; Valente et al., 2002 2002 ; Coutinho et al., 2006 2006 ), while few studies dealt with the pretreatments of the enamel surfaces for GICs (Powis et al., 1982 1982 ; Glasspoole et al., 2002 2002 ). Until recently, the effect of acid-etching or conditioning on the enamel bond strengths of GICs/RMGICs has not been investigated in detail. The null hypothesis tested in this study was that phosphoric acid etching and conditioning with the liquids of the cements would not significantly increase the enamel bond strengths of GICs/RMGICs. Therefore, the aim of this study was: (1) to investigate the effect of phosphoric acid etching and conditioning with the liquids of the cements on the enamel micro-tensile bond strengths (μTBS) of two GICs and two RMGICs, and (2) to analyze the micro-morphologies of the enamel-cement interfaces and the enamel surfaces by scanning electron microscopy (SEM) whether the polished enamel surfaces were etched, conditioned, or not further treated. Materials and methods 1 Specimen preparations 2 This research protocol was carried out in accordance with the international Ethical Guidelines and Declaration of Helsinki and approved by the Ethics Committee of Zhejiang University School of Stomatology. Fifty-four non-carious bovine mandibular incisors were stored in 0.1% (w/v) thymol solution at 4 °C, and used for this study within three months after extraction. They were prepared into rectangular blocks. After all labial enamel surfaces were serially wet-ground with 300-, 600-, 1 200-, and 2 500-grit SiC abrasive paper for 30 s, ending with 4 000-grit SiC paper for 1 min, 48 incisors were randomly assigned into four groups according to four cements [two GICs: Fuji I (GC, Japan), Ketac Cem Easymix (3M ESPE, Germany); two RMGICs: Fuji Plus (GC, Japan), RelyX Luting (3M ESPE, Germany)]. The polished enamel surfaces of all the specimens were either etched with 37% (w/v) phosphoric acid (ETCH-37, Bisco Inc., Schaumburg, IL, USA) for 15 s, conditioned with the liquids of the cements for 15 s, or not further treated (control). Subsequently, they were water-sprayed for 30 s, and gently air-dried, before two matching pretreated enamel surfaces were luted together with one of the four cements under finger pressure for 5 min. The specimens were covered by Vaseline at the enamel-cement interfaces and stored in tap water. The materials used in the study are summarized in Table 1 1 . P: powder; L: liquid; GIC: glass ionomer cement; RMGIC: resin-modified glass ionomer cement; HEMA: hydroxyethyl methacrylate Material P/L Nature Component Etch-37 37% phosphoric acid  Bisco Inc., Schaumburg, IL, USA  LOT: 0900005544 Ketac Cem Easymix 3.8/1 GICs Powder: glass powder, polycarboxylic acid, pigments;  3M ESPE AG, Seefeld, Germany Liquid: water, tartaric acid, conservation agents  LOT: 376688 (P), 371540 (L) Fuji I 1.8/1 GICs Powder: fluoroaluminosilicate glass, polyacrylic acid;  GC, Tokyo, Japan Liquid: polyacrylic acid, water, polycarboxylic acid  LOT: 0812021 (P), 0811261 (L) Fuji Plus 2/1 RMGICs Powder: fluoroaluminosilicate glass;  GC, Tokyo, Japan Liquid: copolymer of acrylic and maleic acids, HEMA, tartaric acid, water, chemical initiators  LOT: 0901211 (P), 0901221 (L) RelyX Luting 1.6/1 RMGICs Powder: fluoroaluminosilicate glass;  3M ESPE AG, Seefeld, Germany Liquid: polyalkenoic acid and HEMA, water, chemical initiators  LOT: N119359 (P), N119360 (L) μTBS measurement 2 After all the specimens had been stored in tap water at room temperature for 24 h and the 24 specimens resulting from 48 incisors had been luted together, every pair of incisors were perpendicularly sectioned through the enamel-cement interfaces using a low-speed saw (Isomet 1000, Buehler, Lake Bluff, IL, USA) under continuous water cooling. The specimens were prepared into multiple beams about 9 mm long with a rectangular cross-sectional area of approximately 1 mm². The μTBS tests were performed with a micro-tensile tester (Bisco Inc., USA) at a crosshead speed of 1 mm/min until fracture. The μTBS was calculated in MPa. Mode of failure 2 The failure mode of all the debonded specimens was determined under a light microscopy (Nikon Eclipse 80i, Tokyo, Japan) at 100-fold magnifications. The failure mode was categorized into three types as following: Type 1, no cements remained on the fractured surfaces, revealing a smooth enamel surface; Type 2, a few cements remained on the fractured surfaces; and Type 3, most of the fractured surfaces were covered with cement residues. SEM 2 Two 1-mm thick pieces from each subgroup were obtained during the specimen sectioning for μTBS tests. The sectioned surfaces were immersed in 0.1 mol/L HCl for 10–30 s to expose the enamel-cement interfaces for SEM observations. The enamel surfaces of another six bovine incisors were treated as per the above-mentioned for SEM observations. In addition, two debonded specimens from each subgroup were randomly selected after μTBS tests for SEM observations. After all the specimens were dehydrated in ascending concentrations of ethanol and gold sputter-coated, the cement-enamel interfaces, fractured surfaces, and the pre-treated enamel surfaces were analyzed by SEM (ZEISS ULTRA 55, Germany). Statistics 2 Statistical analysis was performed with statistical software (SPSS software, Version 17.0, SPSS Inc., Chicago, IL, USA). The data were analyzed with 3×4 factorial design analysis of variance (ANOVA). Post-hoc least significant difference (LSD) multiple comparisons were used to analyze the statistical differences in μTBS data among the four groups with different surface treatments. Results 1 μTBS 2 The μTBS data are summarized in Table 2 2 . Compared with the control, phosphoric acid etching significantly increased the enamel μTBSs of GICs/RMGICs ( P < 0.01). Conditioning with the liquids of the cements produced significantly weaker enamel μTBS (Ketac Cem Easymix and Fuji Plus) ( P < 0.05) than or equivalent enamel μTBS (Fuji I and RelyX Luting) ( P > 0.05) as the control. Regardless of etching, RMGICs (RelyX Luting, Fuji Plus) yielded stronger enamel μTBS than GICs (Fuji I, Ketac Cem Easymix) ( P < 0.01). In addition, Fuji Plus yielded the strongest enamel μTBS among the four GICs on highly-polished enamel surfaces (control) ( P < 0.01). Data are expressed as mean±SD ( n ), analyzed by 3×4 factorial design ANOVA followed by post-hoc LSD multiple comparisons; n is the number of the specimens for μTBS tests. The different superscript letters in each column or the different superscript symbols in each row indicate significant differences ( P < 0.05) 2 Surface treatment 4 μTBS (MPa) Fuji I Easymix Fuji Plus RelyX Luting Polishing 4.75±2.22 (11) **a 8.27±2.08 (12) ##a 22.32±3.65 (15) ♦♦b 15.45±5.70 (14) ▲▲g Etching 11.32±3.34 (13) ***c 12.09±4.79 (20) ###c 29.15±4.00 (10) ♦♦♦d 27.79±5.47 (10) ▲▲▲d Conditioning 6.40±2.81 (21) **e 5.29±3.52 (14) #e 17.02±7.00 (13) ♦f 17.52±4.67(13) ▲▲f Failure analysis 2 The failure analysis data are graphically presented in Fig. 1 1 . Type 1 failure revealed a smooth surface on the fractured surfaces, occasionally occurring in this study (Fig. 2a 2a ). Type 2 failure revealed some cement residues on fractured surfaces on small amounts of fractured specimens (Fig. 2b 2b ). Type 3 failure revealed that the fractured surface was mainly covered with cement residues on the majority of fractured specimens (Fig. 2c 2c ). Type 1 failure revealed a smooth surface on a fractured surface. Type 2 failure revealed some cement residues on some fractured surfaces, mainly occurring on the highly-polished enamel surfaces (control group) or on the enamel surface conditioned with liquids of GICs. Type 3 failure revealed cohesive failure of cements that fractured surfaces were mainly covered with cement residues on the majority of fractured specimens SEM fractography revealed smooth surface on the fractured surface of one control specimen luted with Fuji I (a), some cements residues on fractured surfaces of a control specimen luted with RelyX Luting (b), and amount of cement residues on the fractured surface of an etched specimen luted with RelyX Luting (c). G: resin-modified glass ionomer cements (RelyX Luting); E: enamel SEM observation 2 The polished enamel surface revealed a very smooth smear layer with several scratches (Fig. 3a 3a ). The enamel surfaces etched with phosphoric acid or conditioned with the liquids of cements such as Fuji I and Ketac Cem Easymix showed a typical honeycomb etching pattern (Figs. 3b‒3d 3b‒3d ). The enamel conditioned with the liquids of cements such as RelyX Luting and Fuji Plus exhibited the preferential dissolution of the interprismatic matrixes without the complete removal of the polishing scratches (Figs. 3e and 3f 3e and 3f ). (a) The highly-polished enamel surface presented a smooth smear layer with some scratches; (b) The acid-etched enamel surface revealed a typical etching pattern; (c, d) The enamel surfaces conditioned with the liquids of Fuji I (c) or Ketac Cem Easymix (d) showed the complete removal of the smear layer and preferential dissolution of interprismatic matrixes; (e) The enamel surfaces conditioned with the liquids of Fuji Plus looked like the polished enamel surface; (f) Conditioning with the liquid of RelyX Luting produced the shallow pittings scattered on the smear layer An obvious, continuous, and uniform hybrid layer was found at the enamel-cement interfaces of the etched enamel when Fuji I, RelyX Luting, and Fuji Plus were used (Figs. 4b 4b , 6b 6b , and 7b 7b ), but not detected when Ketac Cem Easymix was used (Fig. 5b 5b ). Furthermore, typical resin tags penetrating into the partially decalcified enamel substrate with network structures were found at the cement-enamel interfaces of the etched enamels, forming the thick hybrid layers between the cements and the etched enamels (Figs. 4b 4b , 6b 6b , and 7b 7b ). In contrary, the hybrid layers at the enamel-cement interfaces were thin or almost invisible when the enamel surfaces were merely highly polished (control) (Figs. 4a 4a , 5a 5a , 6a 6a and 7a 7a ) or they were further conditioned with the liquids of the cements (Figs. 4c 4c , 5c 5c , 6c 6c and 7c 7c ). (a) The continuous micro-gaps were found between the polished enamel and the cement, and the hybrid layer was not visible; (b) Thick tag-like extensions penetrating into the acid-etched enamel substrate produced the thick hybrid layer between the enamel and the cement; (c) Conditioning with the liquid of the cement (Fuji I) resulted in a very thin hybrid layer between the enamel and the cement. G: glass ionomer cements; E: enamel; HL: hybrid layer (a) The discontinuous micro-gaps were found between the polished enamel and the cement, and the hybrid layer was almost invisible; (b, c) The very thin hybrid layers were found between the enamel and the cement when the enamel surfaces were either acid-etched (b) or conditioned with the liquid of the cement (Ketac Cem Easymix) (c). G: glass ionomer cements; E: enamel (a) The discontinuous micro-gaps with an invisible hybrid layer were found at the enamel-cement interface; (b) Typical resin tags penetrated into the acid-etched enamel substrate with network structures at the cement-enamel interfaces; (c) Polyalkenoic acid conditioning with the liquid of Fuji Plus produced the very thin hybrid layer. G: resin-modified glass ionomer cements; E: enamel; HL: hybrid layer (a) A few micro-gaps with invisible hybrid layer were found at the enamel-cement interface; (b) Tag-like extensions penetrating into the acid-etched enamel substrate appeared as the visible hybrid layer at the enamel-cement interfaces; (c) The polyalkenoic acid conditioning with the liquid of RelyX Luting produced the very thin, continuous hybrid layer at the enamel-cement interfaces. G: resin-modified glass ionomer cements; E: enamel; HL: hybrid layer Discussion 1 In this study, bovine incisors were used as a substitute for human teeth to evaluate the enamel bond strengths of GICs/RMGICs because bovine enamel possesses similar chemical compositions, physical properties, and microstructure as human enamel (Feagin et al., 1969 1969 ; Putt et al., 1980 1980 ; Yassen et al., 2011 2011 ; Wang et al., 2012 2012 ). Furthermore, bovine and human enamel possess the same bonding/cementing behavior with dental adhesives and cements (Nakamichi et al., 1983 1983 ; Jiang et al., 2010 2010 ; Yassen et al., 2011 2011 ). Two matching pretreated enamel surfaces directly cemented together with GICs/RMGICs under finger pressure were chosen in this study, because dental prostheses are usually inserted onto or into prepared teeth under finger pressure and this method was also reported in a previous study (Altintas et al., 2008 2008 ). The thickness of the cement layer in the present study was about 10–30 μm. This finding demonstrated that finger pressure is feasible and reliable for this study. In addition, fractography of most debonded specimens revealed cohesion failure of cements (Type 3 failure) due to low mechanical strengths of GICs/RMGICs. This helps to explain the reason that bond strength of GICs/RMGICs was low (Fajen et al., 1990 1990 ; Wiltshire, 1994 1994 ; Itoh et al., 1999 1999 ). Etching or conditioning 2 In the previous studies, the application of 37% phosphoric acid for 15 s to etching the enamel surfaces was recommended, because 15-s etching produced reliable, clinically acceptable enamel bond strengths (Gardner and Hobson, 2001 2001 ; Glasspoole et al., 2002 2002 ; Coutinho et al., 2006 2006 ). Moreover, the extension of etching time would not increase the enamel bond strengths due to the excessive loss of the enamel substrate (Legler et al., 1990 1990 ). Thus, 15-s etching was applied in this study. GICs/RMGICs possess the twofold bonding mechanisms to tooth hard tissues via micromechanical interlocking and chemical bonding at the cement-enamel interfaces (Powis et al., 1982 1982 ; Yoshida et al., 2000 2000 ; Glasspoole et al., 2002 2002 ; Fukuda et al., 2003 2003 ; Coutinho et al., 2006 2006 ). In the present study, 37% phosphoric acid etching significantly improved enamel bond strengths of GICs/RMGICs. This is in complete agreement with the previous reports (Cortes et al., 1993 1993 ; Bishara et al., 2000a 2000a ; Glasspoole et al., 2002 2002 ; Cacciafesta et al., 2003 2003 ; Coutinho et al., 2006 2006 ). Cacciafesta et al. ( 2003 2003 ) found a substantial increase in bond strength to enamel with Fuji Ortho LC, when the enamel was etched with 37% phosphoric acid in comparison to the enamel conditioned with 10% (w/v) polyacrylic acid. Coutinho et al. ( 2006 2006 ) demonstrated that the pretreatments of the enamel surfaces with 37% phosphoric acid and 25% (w/v) polyalkenoic acid could significantly increase the enamel bond strengths for both Fuji BOND LC and Fuji LC, but acid-etching yielded much stronger enamel bond strengths than conditioning. Contrarily, Valente et al. ( 2002 2002 ) showed that 37% phosphoric acid etching for 30 s produced enamel bond strengths for Fuji Ortho LC no stronger than either 25% phosphoric acid etching or 10% polyacrylic acid conditioning. These controversial findings might be related to different surface treatments, different modes of measurement, different cements used, and different operators (Cortes et al., 1993 1993 ; Bishara et al., 2000a 2000a ; Glasspoole et al., 2002 2002 ; Valente et al., 2002 2002 ; Cacciafesta et al., 2003 2003 ; Coutinho et al., 2006 2006 ; Jiang et al., 2010 2010 ). The role of phosphoric acid etching involves the effective removal of the smear layer (Fig. 3b 3b ) and provides good wetting of the surface with GICs, which can chemically interact with the underlying enamel (Torii et al., 2002 2002 ; Brauchli et al., 2010 2010 ). In addition, phosphoric acid etching produces micro-porosities in the enamel substrate and increases the surface area (Fig. 3b 3b ) for chemical bonding and micromechanical interlocking (Figs. 4b 4b , 6b 6b , and 7b 7b ) (Glasspoole et al., 2002 2002 ; van Meerbeek et al., 2010 2010 ). The SEM findings in this study for the first time revealed that certain cements could infiltrate the partially demineralized enamel substrate to form the typical micromechanical interlocking (Figs. 4b 4b , 6b 6b , and 7b 7b ), while other cements could not (Fig. 5b 5b ). The discrepancies might result from the permeability and acidity of the cements themselves. Furthermore, the typical micromechanical interlocking was not directly associated with the enamel bond strengths (Figs. 4b 4b , 6b 6b , and 7b 7b ; Table 2 2 ) in this study. In other words, the visible hybrid layer at the enamel-cement interface did not possess a stronger enamel bond strength than the invisible hybrid layer. This finding might be attributed to the sub-micro- or nano-scale hybrid layer at the enamel-cement interfaces, namely, the nano-interaction zone (NIZ) (Koshiro et al., 2006 2006 ). Thus, micro- and nano-mechanical interlocking might play an important role in the presence of polymer tags penetrating into the micro-porosities of the enamel surfaces treated with etch-and-rinse adhesives or self-etch adhesives (Hannig et al., 2002 2002 ). This was the reason that the acid-etching could substantially increase the enamel bond strengths of GICs/RMGICs in the present study. In order to maximize the chemical bonding on the enamel surfaces, a diluted polyalkenoic acid conditioner was recommended to treat the enamel surfaces prior to bonding (Coutinho et al., 2006 2006 ). The liquids of GICs/RMGICs are polyalkenoic acid in the majority of common cements, while only a few liquids of common cements, such as Ketac Cem Easymix, contain tartaric acid (Table 1 1 ). However, conditioning with liquids of cements either did not increase the enamel μTBS, or even reduced the enamel μTBS in comparison with the control in the present study. Moreover, the SEM findings revealed that the hybrid layers were thin or even invisible when the polished enamel surfaces were conditioned with the liquids of the cements (Figs. 4c 4c , 5c 5c , 6c 6c and 7c 7c ). Compared with 37% phosphoric acid, polyalkenoic acid in the conditioner might not be at a sufficiently high concentration to facilitate cleaning and wetting the enamel (Valente et al., 2002 2002 ). The SEM findings in this study demonstrated that polyalkenoic acid conditioning, using liquids of Fuji Plus and RelyX Luting, could not effectively remove the polishing scratches on the smear layer (Figs. 3e and 3f 3e and 3f ). Oddly, conditioning with the liquids of Fuji I/Easymix could completely remove the smear layer with exposures of the prisms and the interprisms, but still did not increase the enamel bond strengths (Table 2 2 ; Figs. 4c and 4d 4c and 4d ). The tartaric acid used in the liquid of Ketac Cem Easymix (Table 1 1 ) could simultaneously decalcify and adhere to the enamel surface (Fu et al., 2005 2005 ), but it could not increase the enamel bond strengths of the cement Ketac Cem Easymix. Still, these findings could be explained by the fact that various minerals or salts resulting from the chemical interaction of polyalkenoic acid or carboxylic acid with the enamel HAp, possibly deposited onto the enamel surfaces, and interfered with the intimate contact between the cements and the enamel substrate. In other words, the HAp at the enamel surfaces was saturated with the ionic bond provided by the carboxyl groups of polyalkenoic acid or carboxylic acid in the conditioner. This might block the further formation of the ionic bond between the enamel HAp and the carboxyl groups of GICs/RMGICs themselves, suggesting unfavorable surfaces for the effective bonding (Kanca, 1993 1993 ; Valente et al., 2002 2002 ). Some researchers have advocated for there being no need to etch or condition tooth hard tissues before bonding procedures because GICs/RMGICs can self-adhere to tooth hard tissues (Wilson and Kent, 1972 1972 ; Yoshida et al., 2000 2000 ; 2001 2001 ; Fukuda et al., 2003 2003 ), and especially, RMGICs can bond to the enamel effectively (Silverman et al., 1995 1995 ; Cacciafesta et al., 1998 1998 ; 1999 1999 ). However, others have insisted that enamel surface pretreatments could improve the bond strengths (Powis et al., 1982 1982 ; Cortes et al., 1993 1993 ; Attin et al., 1996 1996 ; Bishara et al., 2000a 2000a ; 2000b 2000b ; Glasspoole et al., 2002 2002 ; Valente et al., 2002 2002 ; Cacciafesta et al., 2003 2003 ; Coutinho et al., 2006 2006 ). Interestingly, the findings in the present study revealed that the polished enamel surfaces produced much stronger, or at least, no weaker enamel μTBS than the conditioned enamel surfaces. This might be the reason why some researchers assert that it is needless to condition or etch the enamel surfaces before the bonding procedure. However, the acid-etched enamel surfaces resulted in significantly stronger enamel bond strengths than either the polished or the conditioned enamel surfaces in this study. According to our limited study, it is strongly recommended to use phosphoric acid to etch the enamel margins/surfaces before the cementation of dental fixed prostheses to improve the bond strengths. The integrity and bond durability of prostheses might be increased, with better bond performance (Erickson et al., 2009 2009 ). RMGIC vs. GIC 2 RMGICs were invented to overcome the disadvantages of GICs, such as low early mechanical strength and moisture sensitivity (Nicholson and Czarnecka, 2008 2008 ; Yelamanchili and Darvell, 2008 2008 ). As expected, the enamel bond strengths of RMGICs measured in this study were greater than those of GICs, regardless of etching or conditioning. RMGICs were able to bear more acidic challenges than GICs when they were briefly attacked by HCl solution (Figs. 6 6 and 7 7 ). The infiltration of the monomers of RMGICs into the enamel substrate and the monomers themselves, could play a role in the improvement of the bond strengths (Tanumiharja et al., 2000 2000 ; Chitnis et al., 2006 2006 ; van Landuyt et al., 2007 2007 ). Interestingly, the liquids of RMGICs in the present study possessed a weaker ability to condition enamel surfaces than those of GICs (Figs. 3e and 3f 3e and 3f ). This implies that the etching ability of polyalkenoic acid in RMGICs is much weaker than that of tartaric acid in Ketac Cem Easymix and polyacrylic acid/some polycarboxylic acid in Fuji I (Table 1 1 ). Whether the polished enamel surfaces were etched, conditioned or not, RMGICs yielded much stronger enamel bond strengths than GICs. Thus, the enamel bond strengths of the cements do not rely on the etching patterns. They depend on the mechanical strengths of the cements themselves. The variations in the bond strengths of the cements were probably related to their individual material compositions rather than their bonding mechanisms (Triana et al., 1994 1994 ; Glasspoole et al., 2002 2002 ; Coutinho et al., 2006 2006 ; Yelamanchili and Darvell, 2008 2008 ). Phosphoric acid-etching can increase the enamel μTBS of GICs/RMGICs, but conditioning with the liquids of the cements does not increase the enamel μTBS of GICs/RMGICs. Conclusions 1 Phosphoric acid etching significantly increases the enamel μTBS of GICs/RMGICs. Hence, it is recommended to etch enamel margins/surfaces before cementation of dental prostheses such as inlays or onlays using GICs/RMGICs. RMGICs provide stronger enamel bond strength than GICs, regardless of etching or not. Conditioning with the liquids of GICs/RMGICs does not increase enamel bond strength. References S Altintas , AU Eldeniz , A Usumez . Shear bond strength of four resin cements used to lute ceramic core material to human dentin . J Prosthodont , 2008 . 17 ( 8 ): 634 - 640 . 18761570 DOI: 10.1111/j.1532-849X.2008.00348.x http://doi.org/10.1111/j.1532-849X.2008.00348.x . Shear bond strength of four resin cements used to lute ceramic core material to human dentin JM Antonucci , JE McKinney , JW Stansbury . Resin-Modified Glass Ionomer Cement . US Patent Application , 7-160 856 . 1988 . Resin-Modified Glass Ionomer Cement T Attin , W Buchalla , E Hellwig . Influence of enamel conditioning on bond strength of resin-modified glass ionomer restorative materials and polyacid-modified composites . J Prosthet Dent , 1996 . 76 ( 1 ): 29 - 33 . 8814631 DOI: 10.1016/S0022-3913(96)90342-X http://doi.org/10.1016/S0022-3913(96)90342-X . Influence of enamel conditioning on bond strength of resin-modified glass ionomer restorative materials and polyacid-modified composites SE Bishara , L VonWald , JF Laffoon , JR Jakobsen . Effect of altering the type of enamel conditioner on the shear bond strength of a resin-reinforced glass ionomer adhesive . Am J Orthod Dentofacial Orthop , 2000 . 118 ( 3 ): 288 - 294 . 10982929 DOI: 10.1067/mod.2000.104903 http://doi.org/10.1067/mod.2000.104903 . Effect of altering the type of enamel conditioner on the shear bond strength of a resin-reinforced glass ionomer adhesive SE Bishara , L VonWald , JF Laffoon , JR Jakobsen . Effect of changing enamel conditioner concentration on the shear bond strength of a resin-modified glass ionomer adhesive . Am J Orthod Dentofacial Orthop , 2000 . 118 ( 3 ): 311 - 316 . 10982933 DOI: 10.1067/mod.2000.108682 http://doi.org/10.1067/mod.2000.108682 . Effect of changing enamel conditioner concentration on the shear bond strength of a resin-modified glass ionomer adhesive L Brauchli , T Muscillo , M Steineck , A Wichelhaus . Influence of enamel conditioning on the shear bond strength of different adhesives . J Orofac Orthop , 2010 . 71 ( 6 ): 411 - 420 . 21082304 DOI: 10.1007/s00056-010-1036-2 http://doi.org/10.1007/s00056-010-1036-2 . Influence of enamel conditioning on the shear bond strength of different adhesives MG Buonocore . A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces . J Dent Res , 1955 . 34 ( 6 ): 849 - 853 . 13271655 DOI: 10.1177/00220345550340060801 http://doi.org/10.1177/00220345550340060801 . A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces V Cacciafesta , C Bosch , B Melsen . Clinical comparison between a resin-reinforced self-cured glass ionomer cement and a composite resin for direct bonding of orthodontic brackets. Part 1: wetting with water . Clin Orthod Res , 1998 . 1 ( 1 ): 29 - 36 . 9918643 . Clinical comparison between a resin-reinforced self-cured glass ionomer cement and a composite resin for direct bonding of orthodontic brackets. Part 1: wetting with water V Cacciafesta , C Bosch , B Melsen . Clinical comparison between a resin-reinforced self-cured glass ionomer cement and a composite resin for direct bonding of orthodontic brackets. Part 2: bonding on dry enamel and on enamel soaked with saliva . Clin Orthod Res , 1999 . 2 ( 4 ): 186 - 193 . 10806942 . Clinical comparison between a resin-reinforced self-cured glass ionomer cement and a composite resin for direct bonding of orthodontic brackets. Part 2: bonding on dry enamel and on enamel soaked with saliva V Cacciafesta , MF Sfondrini , L Baluga , A Scribante , C Klersy . Use of a self-etching primer in combination with a resin-modified glass ionomer: effect of water and saliva contamination on shear bond strength . Am J Orthod Dentofacial Orthop , 2003 . 124 ( 4 ): 420 - 426 . 14560273 DOI: 10.1016/S0889-5406(03)00572-9 http://doi.org/10.1016/S0889-5406(03)00572-9 . Use of a self-etching primer in combination with a resin-modified glass ionomer: effect of water and saliva contamination on shear bond strength D Chitnis , WJ Dunn , DA Gonzalesc . Comparison of in-vitro bond strengths between resin-modified glass ionomer, polyacid-modified composite resin, and giomer adhesive systems . Am J Orthod Dentofacial Orthop , 2006 . 129 ( 3 ): 330.e11 - e16 . DOI: 10.1016/j.ajodo.2005.11.011 http://doi.org/10.1016/j.ajodo.2005.11.011 . Comparison of in-vitro bond strengths between resin-modified glass ionomer, polyacid-modified composite resin, and giomer adhesive systems O Cortes , F Garcia-Godoy , JR Boj . Bond strength of resin-reinforced glass ionomer cements after enamel etching . Am J Dent , 1993 . 6 ( 6 ): 299 - 301 . 7880480 . Bond strength of resin-reinforced glass ionomer cements after enamel etching E Coutinho , K van Landuyt , J de Munck , A Poitevin , Y Yoshida , S Inoue , M Peumans , K Suzuki , P Lambrechts , B van Meerbeek . Development of a self-etch adhesive for resin-modified glass-ionomers . J Dent Res , 2006 . 85 ( 4 ): 349 - 353 . 16567557 DOI: 10.1177/154405910608500413 http://doi.org/10.1177/154405910608500413 . Development of a self-etch adhesive for resin-modified glass-ionomers J de Munck , K van Landuyt , M Peumans , A Poitevin , P Lambrenchts , M Braem , B van Meerbeek . A critical review of the durability of adhesion to tooth tissue: methods and results . J Dent Res , 2005 . 84 ( 2 ): 118 - 132 . 15668328 DOI: 10.1177/154405910508400204 http://doi.org/10.1177/154405910508400204 . A critical review of the durability of adhesion to tooth tissue: methods and results RL Erickson , WW Barkmeier , NS Kimmes . Bond strength of self-etch adhesives to pre-etched enamel . Dent Mater , 2009 . 25 ( 10 ): 1187 - 1194 . 19473695 DOI: 10.1016/j.dental.2009.04.004 http://doi.org/10.1016/j.dental.2009.04.004 . Bond strength of self-etch adhesives to pre-etched enamel R Espinosa , R Valencia , M Uribe , I Ceja , J Cruz , M Saadia . Resin replica in enamel deproteinization and its effect on acid etching . J Clin Pediatr Dent , 2010 . 35 ( 1 ): 47 - 51 . 21189764 . Resin replica in enamel deproteinization and its effect on acid etching VB Fajen , Jr MG Duncanson , RS Nanda , GF Currier , PV Angolkar . An in vitro evaluation of bond strength of three glass ionomer cements . Am J Orthod Dentofacial Orthop , 1990 . 97 ( 4 ): 316 - 322 . 2181867 DOI: 10.1016/0889-5406(90)70104-K http://doi.org/10.1016/0889-5406(90)70104-K . An in vitro evaluation of bond strength of three glass ionomer cements F Feagin , T Koulourides , W Pigman . The characterization of enamel surface demineralization, remineralization, and associated hardness changes in human and bovine material . Arch Oral Biol , 1969 . 14 ( 12 ): 1407 - 1417 . 4903322 DOI: 10.1016/0003-9969(69)90258-1 http://doi.org/10.1016/0003-9969(69)90258-1 . The characterization of enamel surface demineralization, remineralization, and associated hardness changes in human and bovine material B Fu , Q Shen , W Qian , X Sun , M Hannig . Interfacial interaction of tartaric acid with hydroxyapatite and enamel . J Mater Sci Mater Med , 2005 . 16 ( 9 ): 827 - 831 . 16167111 DOI: 10.1007/s10856-005-3581-6 http://doi.org/10.1007/s10856-005-3581-6 . Interfacial interaction of tartaric acid with hydroxyapatite and enamel R Fukuda , Y Yoshida , Y Nakayama , M Okazaki , S Inoue , H Sano , K Suzuki , H Shintani , B van Meerbeek . Bonding efficacy of polyalkenoic acids to hydroxyapatite, enamel and dentin . Biomaterials , 2003 . 24 ( 11 ): 1861 - 1867 . 12615476 DOI: 10.1016/S0142-9612(02)00575-6 http://doi.org/10.1016/S0142-9612(02)00575-6 . Bonding efficacy of polyalkenoic acids to hydroxyapatite, enamel and dentin A Gardner , R Hobson . Variations in acid-etch patterns with different acids and etch times . Am J Orthod Dentofacial Orthop , 2001 . 120 ( 1 ): 64 - 67 . 11455380 DOI: 10.1067/mod.2001.114643 http://doi.org/10.1067/mod.2001.114643 . Variations in acid-etch patterns with different acids and etch times EA Glasspoole , RL Erickson , CL Davidson . Effect of surface treatments on the bond strength of glass ionomers to enamel . Dent Mater , 2002 . 18 ( 6 ): 454 - 462 . 12098574 DOI: 10.1016/S0109-5641(01)00068-9 http://doi.org/10.1016/S0109-5641(01)00068-9 . Effect of surface treatments on the bond strength of glass ionomers to enamel M Hannig , H Bock , B Bott , W Hoth-Hannig . Inter-crystallite nanoretention of self-etching adhesives at enamel imaged by transmission electron microscopy . Eur J Oral Sci , 2002 . 110 ( 6 ): 464 - 470 . 12507221 DOI: 10.1034/j.1600-0722.2002.21326.x http://doi.org/10.1034/j.1600-0722.2002.21326.x . Inter-crystallite nanoretention of self-etching adhesives at enamel imaged by transmission electron microscopy T Itoh , T Fukushima , Y Inoue , S Arita , K Miyazaki . Effect of water, saliva and blood contamination on bonding of metal brackets with a 4-META/MMA/TBB resin to etched enamel . Am J Dent , 1999 . 12 ( 6 ): 299 - 304 . 10850251 . Effect of water, saliva and blood contamination on bonding of metal brackets with a 4-META/MMA/TBB resin to etched enamel Q Jiang , H Pan , B Liang , B Fu , M Hannig . Effect of saliva contamination and decontamination on bovine enamel bond strength of four self-etching adhesives . Oper Dent , 2010 . 35 ( 2 ): 194 - 202 . 20420063 DOI: 10.2341/09-151-L http://doi.org/10.2341/09-151-L . Effect of saliva contamination and decontamination on bovine enamel bond strength of four self-etching adhesives J Kanca . Etchant composition and bond strength to dentin . Am J Dent , 1993 . 6 ( 6 ): 287 - 290 . 7880477 . Etchant composition and bond strength to dentin BE Kent , BG Lewis , AD Wilson . Glass ionomer cement formulations: I. The preparation of novel fluoroaluminosilicate glasses high in fluorine . J Dent Res , 1979 . 58 ( 6 ): 1607 - 1619 . 286706 DOI: 10.1177/00220345790580061001 http://doi.org/10.1177/00220345790580061001 . Glass ionomer cement formulations: I. The preparation of novel fluoroaluminosilicate glasses high in fluorine VHW Khouw-Liu , HM Anstice , GJ Pearson . An in vitro investigation of a poly(vinyl phosphonic acid) based cement with four conventional glass-ionomer cements. Part 1: flexural strength and fluoride release . J Dent , 1999 . 27 ( 5 ): 351 - 357 . 10377610 DOI: 10.1016/S0300-5712(98)00061-X http://doi.org/10.1016/S0300-5712(98)00061-X . An in vitro investigation of a poly(vinyl phosphonic acid) based cement with four conventional glass-ionomer cements. Part 1: flexural strength and fluoride release K Koshiro , SK Sidhu , S Inoue , T Ikeda , H Sano . New concept of resin-dentin interfacial adhesion: the nanointeraction zone . J Biomed Mater Res B Appl Biomater , 2006 . 77B ( 2 ): 401 - 408 . DOI: 10.1002/jbm.b.30450 http://doi.org/10.1002/jbm.b.30450 . New concept of resin-dentin interfacial adhesion: the nanointeraction zone LR Legler , DH Retief , EL Bradley . Effects of phosphoric acid concentration and etch duration on enamel depth of etch: an in vitro study . Am J Orthod Dentofacial Orthop , 1990 . 98 ( 2 ): 154 - 160 . 2198801 DOI: 10.1016/0889-5406(90)70009-2 http://doi.org/10.1016/0889-5406(90)70009-2 . Effects of phosphoric acid concentration and etch duration on enamel depth of etch: an in vitro study A Moshaverinia , N Roohpour , WWL Chee , SR Schricker . A review of polyelectrolyte modifications in conventional glass-ionomer dental cements . J Mater Chem , 2012 . 22 ( 7 ): 2824 - 2833 . DOI: 10.1039/c2jm14880c http://doi.org/10.1039/c2jm14880c . A review of polyelectrolyte modifications in conventional glass-ionomer dental cements HZ Movahhed , B Ogaard , M Syverud . An in vitro comparison of the shear bond strength of a resin-reinforced glass ionomer cement and a composite adhesive for bonding orthodontic brackets . Eur J Orthod , 2005 . 27 ( 5 ): 477 - 483 . 16043469 DOI: 10.1093/ejo/cji051 http://doi.org/10.1093/ejo/cji051 . An in vitro comparison of the shear bond strength of a resin-reinforced glass ionomer cement and a composite adhesive for bonding orthodontic brackets I Nakamichi , M Iwaku , T Fusayama . Bovine teeth as possible substitutes in the adhesion test . J Dent Res , 1983 . 62 ( 10 ): 1076 - 1081 . 6352757 DOI: 10.1177/00220345830620101501 http://doi.org/10.1177/00220345830620101501 . Bovine teeth as possible substitutes in the adhesion test JW Nicholson , B Czarnecka . The biocompatibility of resin-modified glass-ionomer cements for dentistry . Dent Mater , 2008 . 24 ( 12 ): 1702 - 1708 . 18539324 DOI: 10.1016/j.dental.2008.04.005 http://doi.org/10.1016/j.dental.2008.04.005 . The biocompatibility of resin-modified glass-ionomer cements for dentistry DH Pashley , FR Tay , L Breschi , L Tjäderhane , RM Carvalho , M Carrilho , A Tezvergil-Mutluay . State of the art etch-and-rinse adhesives . Dent Mater , 2011 . 27 ( 1 ): 1 - 16 . 21112620 DOI: 10.1016/j.dental.2010.10.016 http://doi.org/10.1016/j.dental.2010.10.016 . State of the art etch-and-rinse adhesives DR Powis , T Follerås , SA Merson , AD Wilson . Improved adhesion of a glass ionomer cement to dentin and enamel . J Dent Res , 1982 . 61 ( 12 ): 1416 - 1422 . 6757292 DOI: 10.1177/00220345820610120801 http://doi.org/10.1177/00220345820610120801 . Improved adhesion of a glass ionomer cement to dentin and enamel MS Putt , CJ Kleber , JC Muhler . A comparison of the polishing properties of human and bovine enamel . J Dent Res , 1980 . 59 ( 7 ): 1177 6929820 DOI: 10.1177/00220345800590072401 http://doi.org/10.1177/00220345800590072401 . A comparison of the polishing properties of human and bovine enamel SK Sidhu . Glass-ionomer cement restorative materials: a sticky subject? . Aust Dent J , 2011 . 56 ( s1 ): 23 - 30 . 21564113 DOI: 10.1111/j.1834-7819.2010.01293.x http://doi.org/10.1111/j.1834-7819.2010.01293.x . Glass-ionomer cement restorative materials: a sticky subject? E Silverman , M Cohen , RS Demke , M Silverman . A new light-cured glass ionomer cement that bonds brackets to teeth without etching in the presence of saliva . Am J Orthod Dentofacial Orthop , 1995 . 108 ( 3 ): 231 - 236 . 7661137 DOI: 10.1016/S0889-5406(95)70014-5 http://doi.org/10.1016/S0889-5406(95)70014-5 . A new light-cured glass ionomer cement that bonds brackets to teeth without etching in the presence of saliva M Tanumiharja , MF Burrow , MJ Tyas . Microtensile bond strengths of glass ionomer (polyalkenoate) cements to dentine using four conditioners . J Dent , 2000 . 28 ( 5 ): 361 - 366 . 10785303 DOI: 10.1016/S0300-5712(00)00009-9 http://doi.org/10.1016/S0300-5712(00)00009-9 . Microtensile bond strengths of glass ionomer (polyalkenoate) cements to dentine using four conditioners Y Torii , K Itou , R Hikasa , S Iwata , Y Nishitani . Enamel tensile bond strength and morphology of resin-enamel interface created by acid etching system with or without moisture and self-etching priming system . J Oral Rehabil , 2002 . 29 ( 6 ): 528 - 533 . 12071920 DOI: 10.1046/j.1365-2842.2002.00855.x http://doi.org/10.1046/j.1365-2842.2002.00855.x . Enamel tensile bond strength and morphology of resin-enamel interface created by acid etching system with or without moisture and self-etching priming system R Triana , C Prado , J Garro , F García-Godoy . Dentin bond strength of fluoride-releasing materials . Am J Dent , 1994 . 7 ( 5 ): 252 - 254 . 7986447 . Dentin bond strength of fluoride-releasing materials RM Valente , WG de Rijk , JL Drummond , CA Evans . Etching conditions for resin-modified glass ionomer cement for orthodontic brackets . Am J Orthod Dentofacial Orthop , 2002 . 121 ( 5 ): 516 - 520 . 12045770 DOI: 10.1067/mod.2002.122165 http://doi.org/10.1067/mod.2002.122165 . Etching conditions for resin-modified glass ionomer cement for orthodontic brackets KL van Landuyt , P Kanumilli , J de Munck , M Peumans , P Lambrechts , B van Meerbeek . Bond strength of a mild self-etch adhesive with and without prior acid-etching . J Dent , 2006 . 34 ( 1 ): 77 - 85 . 15979226 DOI: 10.1016/j.jdent.2005.04.001 http://doi.org/10.1016/j.jdent.2005.04.001 . Bond strength of a mild self-etch adhesive with and without prior acid-etching KL van Landuyt , J Snauwaert , J de Munck , M Peumans , Y Yoshida , A Poitevin , E Coutinho , K Suzuki , P Lambrechts , B van Meerbeek . Systematic review of the chemical composition of contemporary dental adhesives . Biomaterials , 2007 . 28 ( 26 ): 3757 - 3785 . 17543382 DOI: 10.1016/j.biomaterials.2007.04.044 http://doi.org/10.1016/j.biomaterials.2007.04.044 . Systematic review of the chemical composition of contemporary dental adhesives B van Meerbeek , M Peumans , A Poitevin , A Mine , A van Ende , A Neves , J de Munck . Relationship between bond-strength tests and clinical outcomes . Dent Mater , 2010 . 26 ( 2 ): e100 - e121 . 20006379 DOI: 10.1016/j.dental.2009.11.148 http://doi.org/10.1016/j.dental.2009.11.148 . Relationship between bond-strength tests and clinical outcomes B van Meerbeek , K Yoshihara , Y Yoshida , A Mine , J de Munck , KL van Landuyt . State of the art of self-etch adhesives . Dent Mater , 2011 . 27 ( 1 ): 17 - 28 . 21109301 DOI: 10.1016/j.dental.2010.10.023 http://doi.org/10.1016/j.dental.2010.10.023 . State of the art of self-etch adhesives C Wang , Y Li , X Wang , L Zhang , T Tang , B Fu . The enamel microstructures of bovine mandibular incisors . Anat Rec , 2012 . 295 ( 10 ): 1698 - 1706 . DOI: 10.1002/ar.22543 http://doi.org/10.1002/ar.22543 . The enamel microstructures of bovine mandibular incisors AD Wilson , BE Kent . A new translucent cement for dentistry. The glass ionomer cement . Br Dent J , 1972 . 132 ( 4 ): 133 - 135 . 4501690 DOI: 10.1038/sj.bdj.4802810 http://doi.org/10.1038/sj.bdj.4802810 . A new translucent cement for dentistry. The glass ionomer cement WA Wiltshire . Shear bond strengths of a glass ionomer for direct bonding in orthodontics . Am J Orthod Dentofacial Orthop , 1994 . 106 ( 2 ): 127 - 130 . 8059747 DOI: 10.1016/S0889-5406(94)70029-X http://doi.org/10.1016/S0889-5406(94)70029-X . Shear bond strengths of a glass ionomer for direct bonding in orthodontics D Xie , WA Brantley , BM Culbertson , G Wang . Mechanical properties and microstructures of glass-ionomer cements . Dent Mater , 2000 . 16 ( 2 ): 129 - 138 . 11203534 DOI: 10.1016/S0109-5641(99)00093-7 http://doi.org/10.1016/S0109-5641(99)00093-7 . Mechanical properties and microstructures of glass-ionomer cements GH Yassen , JA Platt , AT Hara . Bovine teeth as substitute for human teeth in dental research: a review of literature . J Oral Sci , 2011 . 53 ( 3 ): 273 - 282 . 21959653 DOI: 10.2334/josnusd.53.273 http://doi.org/10.2334/josnusd.53.273 . Bovine teeth as substitute for human teeth in dental research: a review of literature A Yelamanchili , BW Darvell . Network competition in a resin-modified glass-ionomer cement . Dent Mater , 2008 . 24 ( 8 ): 1065 - 1069 . 18243297 DOI: 10.1016/j.dental.2007.12.005 http://doi.org/10.1016/j.dental.2007.12.005 . Network competition in a resin-modified glass-ionomer cement Y Yoshida , B van Meerbeek , Y Nakayama , J Snauwaert , L Hellemans , P Lambrechts , G Vanherle , K Wakasa . Evidence of chemical bonding at biomaterial-hard tissue interfaces . J Dent Res , 2000 . 79 ( 2 ): 709 - 714 . 10728971 DOI: 10.1177/00220345000790020301 http://doi.org/10.1177/00220345000790020301 . Evidence of chemical bonding at biomaterial-hard tissue interfaces Y Yoshida , B van Meerbeek , Y Nakayama , M Yoshioka , J Snauwaert , Y Abe , P Lambrechts , G Vanherle , M Okazaki . Adhesion to and decalcification of hydroxyapatite by carboxylic acids . J Dent Res , 2001 . 80 ( 6 ): 1565 - 1569 . 11499514 DOI: 10.1177/00220345010800061701 http://doi.org/10.1177/00220345010800061701 . Adhesion to and decalcification of hydroxyapatite by carboxylic acids The authors are grateful to Prof. Yi SHEN, a statistician in the Department of Epidemiology and Health Statistic, Zhejiang University, China, who provided statistical analysis for the study. Compliance with ethics guidelines Ling ZHANG, Tian TANG, Zhen-liang ZHANG, Bing LIANG, Xiao-miao WANG, and Bai-ping FU declare that they have no conflict of interest. All institutional and national guidelines for the care and use of laboratory animals were followed. 10.1631/jzus.B1300034 Project supported by the National Natural Science Foundation of China (No. 20973152) and the Zhejiang Provincial Natural Science Foundation of China (No. Y2080045) 2013-01-29 2013-10-03 2013-05-03 2022-01-05 Copyright © Zhejiang University and Springer-Verlag Berlin Heidelberg 2013 2013 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 11 14