A Scanning Electron Microscopic Investigation of Bonding of Methacryloyloxyalkyl Hydrogen Maleate to Etched Dentin T. FUKUSHIMA and T. HORIBE Department of Dental Materials and Devices, Fukuoka Dental College, 700 Ta, Sawara-ku, Fukuoka 814-01, Japan

Three types of methacryloyloxyalkyl hydrogen maleates-2-(methacryloyloxy)ethyl hydrogen maleate (2MEM), 1-(methacryloyloxymethyl)ethyl hydrogen maleate (MMEM), and 1(methacryloyloxymethyl)propyl hydrogen maleate (MMPM)-were prepared for use as bonding agents. The relationships between the thickness of an acid-proof dentin layer and the bond strength and chemical structure of the synthesized monomers were investigated. The bond strengths of 2MEM/composite system to dentin treated with either 37% phosphoric acid solution or 10% citric acid-3% ferric chloride solution (10-3 solution) were superior to those of the others. The acid-proof dentin layer for all bonding agents containing the synthesized monomers at the resin-dentin interface was clearly visible in a scanning electron microscope, regardless of the types of dentin etching agents used. After treatment of the dentin with the phosphoric acid solution, the layers of 2MEM, MMEM, and MMPM were 5-6 tun, 3-4 pm, and 2-3 pm thick, respectively. When the dentin was treated with the 10-3 solution, the layer for each bonding agent was approximately 1 pm thick. J Dent Res 69(1):46-50, January, 1990

Introduction. Dental resins adhere less readily to acid-treated dentin than to acid-treated enamel (Rider et al., 1977; Bowen and Cobb, 1983). With recent improvements in adhesive monomers (Bowen et al., 1982; Eliades et al., 1985; Fukushima et al., 1985; Abe, 1986) and dentin treatments (Bowen, 1980; Jedrychowski et al., 1981; Nakabayashi et al., 1982; Bowen et al., 1982; Munksgaard and Asmussen, 1984), the bond strength of resin to dentin has improved considerably. In particular, the bonding agent containing 10-methacryloyloxydecyl dihydrogen phosphate (Kohyama, 1988) showed a high bond strength to dentin treated with phosphoric acid solutions (Nakajima, 1985). Dentin treatment agents containing ferric salts also greatly enhanced the bond strengths of resin composites with PolySAC, butyl acrylate-acrylic acid copolymer, NPG-GMA, and NTGGMA/PMDM promoters (Bowen, 1980; Jedrychowski et al., 1981; Bowen et al., 1982), and the MMA-TBB resin system (Nakabayashi et al., 1982) to dentin. Nakabayashi et al. (1982) and Nakajima (1985) showed that the increase in bond strength was explained by formation of an acid-proof dentin layer on treated dentin, called the resin-reinforced dentin. However, the exact mechanism by which an acid-proof dentin layer is formed is still unknown. In a previous study, Fukushima et al. (1988) prepared 2methacryloyloxyethyl hydrogen maleate (2MEM) by adding maleic anhydride to HEMA, and compared its bonding capability with that of the succinic anhydride-HEMA adduct. The results showed that the 2MEM/composite system bonded very well to dentin treated with a 37% phosphoric acid solution. It is believed that the bond strength of composite with a bonding agent containing 2MEM to the etched dentin may depend on the formation of an acid-proof dentin layer. In this study, we prepared three types of methacryloyloxyReceived for publication March 31, 1989 Accepted for publication Junc 19, 1989 46

alkyl hydrogen maleate with different hydrophobic groups (such as methyl and ethyl groups) to investigate the relationships between the thickness of acid-proof dentin layer and the bond strength and chemical structure of the synthesized monomers. The objective was to evaluate in detail the bonding effectiveness of these monomers to etched dentin.

Materials and methods. Preparation of monomers.-2-(methacryloyloxy)ethyl hydrogen maleate (2MEM), 1-(methacryloyloxymethyl)ethyl hydrogen maleate (MMEM), and 1-(methacryloyloxymethyl)propyl hydrogen maleate (MMPM) were prepared by the reaction of HEMA, 2-hydroxypropyl methacrylate, or 2-hydroxybutyl methacrylate with maleic anhydride, according to a previously described method (Fukushima et al., 1988). The structural formulae of the synthesized monomers are shown in Fig. 1. Preparation of bonding agents. -Each of the bonding agents was supplied in the form of two liquids. One contained 100 mol% synthesized monomer and 0.8 wt% benzoyl peroxide (BPO), and the other contained ethanol, 3 wt% p-toluenesulfinic acid sodium salt, and 1 wt% p-tolyldiethanol amine. Measurement of shear bond strength. -Freshly extracted bovine anterior teeth as substitutes for human teeth were used as adherents (Nakamichi et al., 1983). Enamel was removed with a diamond wheel under water coolant. The teeth were embedded in acrylic tubes with acrylic resin. The dentin surface was finely polished with 600-grit SiC paper under running water, and then etched with a 37% phosphoric acid solution or a 10% citric acid-3% ferric chloride solution (10-3 solution) for 30 s. All of the etched dentin surfaces were washed with water for 10 s after being etched. They were then dried with oil-free compressed air for 20 s. Masking tape with a six-mm-diameter hole was placed onto the dentin surface, and a plastic tube with a 5-mm internal diameter was inserted into the hole. With a small sponge pellet, a mixture of bonding agent was applied onto the dentin surface. After removal of the solvent with compressed air for 20 s, a mixture of the resin composite (Clearfil SC-II, Kuraray Co., Osaka, Japan) was poured over the bonding agents in the tube. After the resin composites were cured, the plastic tubes and tapes were removed, and all specimens were immersed in 370C water for one day and 30 days. Compressive shear bond tests (Miyairi and Fukuda, 1987) were performed on a universal testing machine (TCM-200, Minebea Co., Ltd., Miyota, Japan) at a cross-head speed of 1 mm/min, with use of an apparatus shown previously (Fukushima et al., 1988). The mean bond strength was calculated from the results from ten specimens. The results were analyzed by analysis of variance and by Scheff6's multiple comparison test at a significance level of 0.05. Scanning electron microscopic (SEM) observation. -A 2mm-thick section of etched dentin was split in the labiolingual direction. The specimen was fixed in a 5% glutaraldehyde0.25 mol/L cacodylate buffer at room temperature for 24 h and dehydrated through a graded series of ethanols. The ethanol

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was replaced with isoamyl acetate, and the specimen was dried with liquid carbon dioxide by use of a critical-point-dryer. Specimens for SEM observation of an acid-proof dentin layer were prepared in the same manner as the specimens used in the bond tests. The specimens were cut in half labiolingually with a diamond saw (ISOMET, Buehler Ltd., IL). One of the cut surfaces was finely polished with 0.5-pm alumina paste. The other was etched further with a 6 mol/L HCl solution for five s after being polished. All specimens were cleaned in water with ultrasonic agitation for one min. Subsequently, the specimens were dried in a desiccator under vacuum for 24 h. The specimens used in the shear bond test were split into 2mm-thick layers of dentin in a labiolingual direction. They were then etched with the 6 mol/L HCl solution for five s, and then washed with water. The specimens were dried in a desiccator under vacuum for 24 h. Also, the dentin surfaces were prepared for SEM observation after shear bond tests were carried out. Each surface was coated with a thin layer of gold-palladium, and was then observed with a SEM (JSM-T330, JEOL Ltd., Tokyo, Japan).

Results. Bond strength to etched dentin. -The bond strengths of the 2MEM/composite system to dentin treated with both pre-treatments were superior to those of the other resins (Tables 1 and 2). The bond strengths of composite with MMEM and MMPM were comparable, except for the bond strengths to dentin treated with the phosphoric acid solution after being immersed in 370C water for one day. The bond strengths of composite with all bonding agents increased with time when pre-treatment was with the phosphoric acid solution, while they did not change with time when pre-treatment was with the 10-3 solution

(p .0.05). SEM

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BONDING OF 2 MEM, MMEM, AND MMPM TO ETCHED DENTIN

observation. -The treatment of dentin with the phosphoric acid solution or with the 10-3 solution for 30 s resulted in the development of a porous layer. The thickness of the layers was 3-5 pLm and 1-2 pLm, respectively, and collagen was clearly visible (Fig. 2). A dentin layer that withstood the 6 mol/L HCl solution was visible at the resin-dentin interface for each specimen, as shown in Figs. 3 and 4. All bonding agents created thicker acid-proof dentin layers on dentin when it was pre-treated with the phosphoric acid solution than when it was pre-treated with the 103 solution. The layer thicknesses of 2MEM, MMEM, and

CH3 CH2-C R l COCH2CHOC CH=CHCOH

11

11

11

0

0

0

R:

-H

2MEM

-CH3 MMEM

-C2H5 MMPM Fig. 1-Structural formulae of the three synthesized

monomers.

TABLE 1 BOND STRENGTH (MPa) TO DENTIN TREATED WITH 37% PHOSPHORIC ACID SOLUTION Bonding Agent 1 day 30 days 2MEM 9.38 ± 1.61 19.77 ± 3.80 MMEM 7.50 ± 1.50 12.70 ± 6.60 MMPM 4.94 ± 1.62 9.39 ± 5.45 Bonding agents connected by a bar were not significantly different.

TABLE 2 BOND STRENGTH (MPa) TO DENTIN TREATED WITH 10-3 SOLUTION Bonding Agent 1 day 30 days 2MEM 13.02 ± 4.04 14.17 ± 3.45 MMEM 9.51 ± 2.34 9.14 ± 1.80 MMPM 7.35 ± 1.21 7.22 ± 2.26 Bonding agents connected by bars were not significantly different.

MMPM were 5-6 [Lm, 3-4 rm, and 2-3 pLm, respectively, when the dentin was treated with the phosphoric acid solution. The thickness layer for each bonding agent was approximately 1 Elm when the dentin was treated with the 10-3 solution.

Fig. 2-SEM micrographs of fractured surfaces of dentin, the top surfaces of which have been treated with phosphoric acid solution (left) and 10-3 solution (right). The bars represent 2 pum. Downloaded from jdr.sagepub.com at UNIV OF WESTERN ONTARIO on April 11, 2015 For personal use only. No other uses without permission.

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Fig. 3-SEM micrographs of the acid-proof dentin layer (A) at the interface between the resin (R) and dentin (D) treated with phosphoric acid solution. The micrographs to the left show polished surfaces, and the ones to the right show surfaces etched with 6 mol/L HCI solution (top, 2MEM; middle, MMEM; and bottom, MMPM). The bars represent 5 pLm.

Fig. 5 shows fracture patterns on the dentin surface after shear bond tests. The top, middle, and bottom micrographs show the resin-dentin interface failure, resin cohesive fracture, and dentin cohesive fracture, respectively. Specimens exhibiting high shear bond strength showed resin cohesive fracture or dentin cohesive fracture. The specimens showing low shear bond strength had failures occurring at the resin-dentin interface.

Discussion. The acid-etch technique on vital dentin is generally contraindicated because acids such as phosphoric acid and citric acid can have harmful effects on the dental pulp (Retief et al., 1974;

Stanley et al., 1975). However, Brinnstrom (1985) and Fujitani (1986) suggest that the major cause of pulpal irritation is infection. Brinnstr6m et aL (1978), Nordenvall et al. (1979), and Torstenson et al. (1982) reported that 37% and 50% phosphoric acid solutions, or a 50% citric acid solution, did not result in any appreciable pulpal reaction or inflammation. Also, caries, abrasion, and erosion often occur in areas where there is little or no enamel available for etching (Banting et al., 1985; Beck et al., 1985; Herrin and Shen, 1985). Thus, the acids are necessary for any composite systems to bond strongly to the areas, if the acid-etch technique can dramatically improve the bonding to dentin. In a previous study (Fukushima et al., 1988), the bond strength

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Fig. 4-SEM micrographs of typical acid-proof dentin layer (A) at the interface between the resin (R) and dentin (D) treated with 10-3 solution. The left micrograph shows a polished surface, and the right one shows a surface etched with 6 mol/L HCl solution. The bars represent 1 ptm.

of composite to dentin pre-treated with 37% phosphoric acid solution and 2MEM was about seven times as high as that to unetched dentin. We therefore investigated the bonding of composite with bonding agents containing methacryloyloxyalkyl hydrogen maleates to dentin treated with 37% phosphoric acid solution and 10-3 solution. The bond strengths to etched dentin ranged from 4.94 to 19.77 MPa. In particular, the monomer (2MEM), which is without branched alkyl groups, showed bond strength of more than 9.0 MPa to both etched dentins. Moreover, the SEM micrographs revealed that the specimens exhibiting high bond strength had resin cohesive fracture or dentin cohesive fracture, regardless of the types of monomers and dentin treating agents (Fig. 5). The acid-proof dentin layers for all bonding agents at the resin-dentin interface were clearly visible with SEM, regardless of the types of dentin etching agents (Figs. 3 and 4). Also, the similar acid-proof dentin layer on a labiolingually split dentin surface, which was prepared from a specimen used in the bond test, was visible with SEM (micrographs not shown). These results are in agreement with those obtained by Nakabayashi et al. (1982) and Nakajima (1985), indicating that the bond strength is dependent upon the formation of an acid-proof dentin layer. Kato et al. (1986) reported that the layer consisted of resin and dentin. The bond strength of composite with 2MEM to dentin was higher (Table 1), and the acid-proof dentin layer was thicker, compared with others (Fig. 3) when the dentin was pre-treated with the phosphoric acid solution. It therefore seems that the monomers, which are able to permeate the porous layer, as shown in Fig. 2, can readily prepare the acid-proof dentin layer and show high bond strength to etched dentin. However, there may not be a significant relationship between the bond strength and the thickness, because the bond strength of composite with each bonding agent to dentin treated with the 10-3 solution was statistically higher than that to dentin treated with the phosphoric acid solution after being immersed in 370C water for one day. We suggest that if the bonding agents do not completely cure in the porous layer, an acid-proof dentin layer with poor physical properties would be formed, so that it could not effectively play a major role in bonding. Kadoma and Imai (1988) suggested that a proper quantity of ferric chloride on the dentin shortened the curing time and promoted the polymerization for an MMA-PMMA/TBBO resin system. Their suggestions and ours may well explain the following: (1) that bond strength to dentin treated with the phosphoric acid solution increased with

time due to extended polymerization and (2) that bond strength to dentin treated with the 10-3 solution did not change with time because of complete polymerization in a short time. The bond strength was therefore dependent upon the physical properties of the acid-proof dentin layer, which may be affected by monomer types and curing systems, rather than by the layer thickness. The results clearly indicate the necessity for bonding agents to penetrate etched dentin and cure in porous dentin in a short time, if an effective acid-proof dentin layer bonded to dentin is to be formed. The final goal is the production of restorations without marginal gaps. Asmussen and Munksgaard (1985) suggest that a bonding agent showing bond strength of more than 20 MPa to dentin will result in gap-free restorations. The bond strengths of composite with 2MEM to etched dentin obtained in this study, ranging from 9.38 to 19.77 MPa, were achieved by the physical interaction of acid-proof dentin layers, because a composite with 2MEM showed very low bond strength to unetched dentin in a previous study (Fukushima et al., 1988). We therefore suggest that a bonding agent having the ability of developing an acid-proof dentin layer in etched dentin and chemically reacting with dentin will provide composites with a bond strength to dentin of more than 20 MPa. REFERENCES ABE, Y. (1986): Relation between Interpenetration and Bond Strength to Dentin with 4-META, Pheny-P, HNPM, and 4-MET/MMATBB Resins, J Jpn Dent Mater 5:839-851. ASMUSSEN, E. and MUNKSGAARD, E.C. (1985): Adhesion of Restorative Resins to Dentinal Tissues. In: Posterior Composite Resin Dental Restorative Materials, G. Vanherle and D.C. Smith, Eds., Utrecht: Peter Szulc Publishing Co., pp. 217-229. BANTING, D.W.; ELLEN, R.P.; and FILLERY, E.D. (1985): A Longitudinal Study of Root Caries: Baseline and Incidence Data, J Dent Res 64:1141-1144. BECK, J.D.; HUNT, R.J.; HAND, J.S.; and FIELD, H.M. (1985): Prevalence of Root and Coronal Caries in a Noninstitutionalized Older Population, JAm Dent Assoc 111:964-967. BOWEN, R.L. (1980): Adhesive Bonding of Various Materials to Hard Tooth Tissues. XXII. The Effects of a Cleanser, Mordant, and PolySAC on Adhesion Between a Composite Resin and Dentin, J Dent Res 59:809-814. BOWEN, R.L. and COBB, E.N. (1983): A Method for Bonding to Dentin and Enamel, J Am Dent Assoc 107:734-736. BOWEN, R.L.; COBB, E.N.; and RAPSON, J.E. (1982): Adhesive Bonding of Various Materials to Hard Tooth Tissues: Improvement in Bond Strength to Dentin, J Dent Res 61:1070-1076.

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Enamel Bond. Composite Fillings in Etched and Unetched Cavities, J Dent Res 57:3-10. ELIADES, G.C.; CAPUTO, A.A.; and VOUGIOUKLAKIS, G.J. (1985): Composition, Wetting Properties and Bond Strength with Dentin of 6 New Dentin Adhesives, Dent Mater 1:170-176. FUJITANI, M. (1986): Effects of Acid-etching, Marginal Microleakage, and Adaptation to Dentinal Wall on the Dental Pulp in Adhesive Composite Restorations, Jpn J Conserv Dent 29:228-253. FUKUSHIMA, T.; INOUE, Y.; and HORIBE, T. (1988): Synthesis of 2-Methacryloxyethyl Hydrogen Maleate and Its Bonding to Tooth Surfaces, J Jpn Dent Mater 7:675-679. FUKUSHIMA, T.; KAWAGUCHI, M.; INOUE, Y.; MIYAZAKI, K.; and HORIBE, T. (1985): Application of Functional Monomers for Dental Use (Part-9) Syntheses of Succinoxy Methacrylates and Their Adhesion to Polished and Etched Tooth Surfaces, Dent Mater J 4:33-39. HERRIN, H.K. and SHEN, C. (1985): Microleakage of Root Caries Restorations, Gerodontics 1:156-159. JEDRYCHOWSKI, J.R.; CAPUTO, A.A.; and PROLA, J. (1981): Influence of a Ferric Chloride Mordant Solution on Resin-dentin Retention, J Dent Res 60:134-138. KADOMA, Y. and IMAI, Y. (1988): Effect of Ferric Salts on Polymerization of MMA-A Model to Study the Mechanism of Adhesion of MMA Resin to Dentin, J Jpn Dent Mater 7:817-823. KATO, H.; WAKUMOTO, S.; and SUZUKI, M. (1986): Chemical Analysis of Interface Layer between Resin and Dentine by Raman Microprobe, J Jpn Dent Mater 5:232-237. KOHYAMA, H. (1988): Pulpal Response of Human Dental Pulp to an Adhesive-Light-Cured Composite Resin Restoration System and Observation of Contact Area between Composite Resin and Dentin, Jpn J Conserv Dent 31:352-387. MIYAIRI, H. and FUKUDA, H. (1987): A Shear Test Method of Dental Adhesives, J Jpn Dent Mater 6:614-620. MIZUNUMA, T. and NAKABAYASHI, N. (1986): Relationship between Bond Strength and Structure of Dentin Collagen-Adhesion to Dentin with 4-META/MMA-TBB Resin, J Jpn Dent Mater

Fig. 5-SEM micrographs of typical fracture patterns on the dentin surface after shear bond tests (top shows resin-dentin interface failure; middle shows resin cohesive fracture; and bottom shows dentin cohesive fracture). The bar represents 5 prm. BRANNSTROM, M. (1985): Composite Resin Restorations: Biological Considerations with Special Reference to Dentin and Pulp. In: Posterior Composite Resin Dental Restorative Materials, G. Vanherle and D.C. Smith, Eds., Utrecht: Peter Szulc Publishing Co., pp. 71-81. BRANNSTROM, M. and NORDENVALL, K.J. (1978): Bacterial Penetration, Pulpal Reaction and the Inner Surface of Concise

5:471-474. MUNKSGAARD, E.C. and ASMUSSEN, E. (1984): Bond Strength Between Dentin and Restorative Resins Mediated by Mixtures of HEMA and Glutaraldehyde, J Dent Res 63:1087-1089. NAKABAYASHI, N.; TAKEYAMA, M.; KOJIMA, K.; and MASUHARA, E. (1982): Studies on Dental Self-Curing Resins (19)Adhesion of 4-META/MMA-TBB Resin to Pretreated Dentine, J Jpn Soc Dent Apparatus Mater 23:29-33. NAKAJIMA, A. (1985): Bond Strength of Adhesive Composite Resins, Part 2. Adhesive Property of a New Bonding Agent to Tooth Substances and Adhesion Structures in the Dentin, Jpn J Conserv Dent 28:850-865. NAKAMICHI, I.; IWAKU, M.; and FUSAYAMA, T. (1983): Bovine Teeth as Possible Substitutes in the Adhesion Test, J Dent Res 62:1076-1081. NORDENVALL, K.J.; BRANNSTROM, M.; and TORSTENSON, B. (1979): Pulp Reactions and Microorganisms under ASPA and Concise Composite Fillings, ASDC J Dent Child 46:449-453. RETIEF, D.H.; AUSTIN, J.C.; and FATTI, L.P. (1974): Pulpal Response to Phosphoric Acid, J Oral Pathol 3:114-122. RIDER, M.; TANNER, A.N.; and KENNY, B. (1977): Investigation of Adhesive Properties of Dental Composite Materials Using an Improved Tensile Test Procedure and Scanning Electron Microscopy, J Dent Res 56:368-378. STANLEY, H.R.; GOING, R.E.; and CHAUNCEY, H.H. (1975): Human Pulp Response to Acid Pretreatment of Dentin and to Composite Restoration, JAm Dent Assoc 91:817-825. TORSTENSON, B.; NORDENVALL, K.J.; and BRANNSTROM, M. (1982): Pulpal Reaction and Microorganisms under Clearfil Composite Resin in Deep Cavities with Acid Etched Dentin, Swed Dent J 6:167-176.

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A scanning electron microscopic investigation of bonding of methacryloyloxyalkyl hydrogen maleate to etched dentin.

Three types of methacryloyloxyalkyl hydrogen maleates--2-(methacryloyloxy)ethyl hydrogen maleate (2MEM), 1-(methacryloyloxy-methyl)ethyl hydrogen male...
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