Dent Mater 8:278-282, July, 1992
Effects of surface-active resins on dentin/composite bonds G. E. Schumacher ~, F. C. Eichmiller 2,J. M. Antonucci 3 1Clinical Center/Commissioned OfficersDental Clinic, National Institutes of Health, Bethesda, MD, USA 2AmericanDental Association Health Foundation, PaffenbargerResearch Center, 3Dentaland Medical Materials Group, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD, USA
Abstract. Effectivedentin bonding systems basedon para-PMDM diadduct of pyromellitic dianhydride and 2-hydroxyethyl methacrylate (HEMA) have been developed (Bowen et al., 1982). ParaPMDM, a solid of limited solubility, is usually applied from an acetone solution to dentin that has been preconditioned with acid and N-phenylglycine. The feasibility of using a liquid, surfaceactive bonding resin to substitute for or to supplement paraPMDM was explored. Mono(2-methacryloyloxy)ethyl phthalate (MMEP), a liquid, monofunctional homolog of para-PMDM (derived from the reaction of phthalic anhydride with HEMA) was used to formulate several bonding resin systems and solutions. Dentin surfaceswere pretreatedaccording to several variationsof a three-step bonding protocol involving sequential application of 6.8w/o ferricoxalatein 2.5w/o HNO3,N_phenylglycineinacetone, and an experimental bonding resin before placement of a chemically cured composite restorative material. Tensile bond strengths were tested after 24 h storage in distilledwater at 23°C.The results suggest that solutions based on MMEP and/or para-PMDM in acetoneorinothermonomers, especiallythosecontaining HEMA, can effectively promote bonding to dentin. A new mechanism for the observed self-polymerizationof MMEP or para-PMDMwith Nphenylglycine is proposed. INTRODUCTION Strong adhesive bonds between dentin and dental composite have been achieved in vitro by the sequential application of three solutions to the dentin surface: l) ferric oxalate; 2) Nphenylglycine; and 3)a polyfunctional monomer,para-PMDM (Bowen et al., 1982; Chen and Bowen, 1989). Para-PMDM, a product of the addition reaction of two molecules of 2hydroxyethyl methacrylate (HEMA) and one molecule of pyromellitic dianhydrideis thebasis ofseveraleffectivedentin adhesive systems (Bowen et al., 1982; 1987). As a result of its symmetric chemical structure, para-PMDM has a relatively high melting point (162-163 °C) and has limited solubility in common organic solvents and other dental monomers. In most bonding protocols,para-PMDM is usually applied to preconditioned dentin from an acetone solution, Mono(2-methacryloyloxy)ethylphthalate(MMEP),the simplest homolog ofpara-PMDM, is derived from the reaction of HEMA with phthalic anhydride. As shown in Fig. 1, MMEP is monofunctional with respect to both its methacrylate and carboxylic acid groups and has a less symmetric chemical structure than para-PMDM, which probably accounts for its liquidity at ambient temperatures. Incontrast topara-PMDM, 278 Schumacher et al./Dental surface-active resins
MMEP has a wide solubility range in a number of organic solvents and dental monomers such as triethylene glycol dimethacrylate (TEGDMA), Bis-GMA, HEMA and methyl methacrylate (MMA). The object of this study was to determine the feasibility of substituting MMEP forpara-PMDM or
supplementingpara-PMDMwithMMEPinathree-stepbonding protocol previously described (Bowen et al., 1982; Chen and Bowen, 1989; Johnston et al., 1989). MATERIALS AND METHODS
All of the materials used in this study were commercially available, with the exception ofpara-PMDM (Farahani et al., 1991) and N-phenylg]ycine, NPG, (Johnston et al., 1989) which were synthesized. MMEP (Rohm Tech., Inc. Malden, MA, USA), HEMA (99.5%, Scientific Polymer Products, Inc. Ontario, NY, USA), Bis-GMA (Freeman Chemical Corp., Port Washington, WI, USA), TEGDMA (Esschem Co., Essington, PA, USA), and MMA (Aldrich Chemical Co., Milwaukee, WI, USA) were used as received. The adherend used was the chemically activated composite restorative, ADAPTIC (Johnson and Johnson, East Windsor, NJ, USA). A solution of 6.8 w/o purified ferric oxalate (FO), Fe2(C204)3, was used in a 2.5 w/o aqueous nitric acid (HNO~) solution for conditioning the cut dentin (Blosser and Bowen, 1988; Cobb et al., 1989). Extracted non-carious human molars that had been stored in distilled water were used for dentin adhesion testing. To obtain a dentin surface for bonding, the occlusal surfaces were sectioned perpendicularly to the long axis of the tooth with a slow speed diamond sectioning blade (Isomet, Buehler Ltd., Lake Bluff, IL, USA) running under water. The test assemblies usedwere described previously(Bowen, 1965). The roots of the teeth were held in the hollow end of a cylinder by imbedding them within self-curing methyl methacrylate (Formatray, Sybron Corp., Romulus, MI, USA), leaving the flattened dentin surface exposed for bonding. The rest of the testing assembly consisted of an iris with an opening in the center and a Teflon-coated surface on the side which rested against the dentin surface. The opening in the iris determined the bonding area. The chemically cured composite was mixed and immediately added to the end ofthe opposing cylinderthat was seated onto the iris where the composite made contact with the dentin bonding surface through the opening (Fig. 2). Thecylindersandirisweresurroundedbyasleevetomaintain the assembly alignment in a perpendicular attitude with respect to the bonding surface during storage and tensile testing.
o
OH2
Bonding Apparatus
H
o MMEP
II
0
Sleeve
U
(LIQUID) CH~
--
0 o
.o,
.o 0
0
CH?
- - Plunger
para PMDM
1'~ - - ~
- - Iris
(SOLID, m.p. 161 162 °C)
Fig. 1. Molecularstructuresof MMEP and PMDM
Three series of experiments involving two versions (Procedures A and B) of the FO-based three-step bonding protocol were used for bonding the composite to dentin. Approximately ten teeth were used for each bonding resin listed in Table 1 and for each bonding solution listed in Tables 2 and 3. Resin formulations and acetone solutions are expressed in weight percent. In Procedure A, each dentin surface was pretreated using the subsequently described two steps prior to adding a bonding resin (Table l) or a bonding solution (Table 2). In step one, one drop (approximately 0.05 ml) of an aqueous solution of 6.8% FO in 2.5% HN03 was placed on the dentin surface for 60 s, rinsed with distilled water for 10 s and blown dry with compressed air. Next(steptwo)a drop of 10% NPG in acetone was placed on the dentin for 1 min, followed by an acetone rinse with three or four drops of clean acetone and drying with a stream of compressed air. In step three, a drop of the bonding resin or solution was applied and left for 1 min and then blown dry with a stream of compressed air for 10 s. Adaptic, mixed according to the manufacturer's directions, was immediately applied to the opposing cylinder of the test assembly and forced through the iris onto the treated dentin surface under the load of a 2.6 kg weight for 15 s. The entire assembly was permitted to stand at room temperature in air for 15 rain before being immersed into distilled water at 23 (+_1)°C for 24 h. The bonded specimens were then tested in tension on an Instron Universal Testing Instrument, Model 1130 (Instron Corp., Canton, MA, USA), at a cross-head speed of 0.5 cm/min, In Procedure B (Table 3), the three-step bonding technique was essentially the same as Procedure A except that the NPG solution was not rinsed with acetone, but was dried with air 1 minute after application to the dentin. Two concentrations of NPG, 5% and 10%, were used in this experiment, RESULTS
The mean tensile bond strengths of the composite to dentin using the various bonding resins and bonding solutions are shown in Tables 1-3. Similarity of variance within the groups was verified by the M statistic of Bartlett and an F-test was applied to determine if differences were present between the means (Duncan, 1955; Wall, 1986). Significant differences in the means were found in all three sets of data and a Duncan's Multiple Comparison test was employed at the 95% confidence level to determine where these differences occurred
~
- - Dentin Sample
I
I
[
]
Fig. 2. Schematicof bondingapparatus,
RCO2H + :NHCH2CO,H ~ Carboxylic Acid M........ ~
CO2-"H-
H2CO2H
co,,~p~ex .~. . . . .
[(~)c'O2H
" ---:H~CO.H-
O
Complex Wran~f . .[R . . ~ ( ~
II
-
other
R--C. + p,o~.... Initiating R~d,c~
Fig. 3. Mechanism for "spontaneouspolymerization"of carboxylicacid monomers using amine accelerators.
(Duncan, 1955). The brackets in Tables 1-3 indicate groups with non-significant differences. The results in Table 1 indicate that significantly lower tensile bond strengths developed when the dentin surface was treatedwithbondingresinsHandIaswellasthecontrolJ(no bonding resin) when compared to the tensile bond strengths obtained with the other formulations. Formulations A and B, which contain both HEMA and the carboxylic acid monomers (MMEP pluspara-PMDMor MMEP), developed significantly higher tensile bond strengths than the other bonding resins tested. The results in Table 2 show that the bonding solutions 1-6, containing dissolved MMEP at varying concentrations in acetone, developed bond strengths that were significantly higher than the neat MMEP bonding resin, 7. The results shown in Table 3 indicate a significant difference in tensile bond strength of IA and the bond strengths obtained with the other bonding solutions using Procedure B. Significant differences were not found among the next group of four solutions (IB, IIB, IIA, and IIIB). A larger group of five solutions (IIA, IIIB, IIIA, IVB and IVA) also showed nonsignificant differences in their bond strengths. There was some overlap of the second and third groups of bonding solutions, as indicated by the brackets in Table 3. DISCUSSION The conditioning of dentin with certain types of acid solutions has been determined to be quite important in developing Dental Materials~July 1992 279
TABLE 1: TENSILE BONDSTRENGTHSTO DENTIN USINGBONDING
Bonding Resin Formulation A
8 C D E F G
RESINS (PROCEDUREA) Composition Tensile Bond Numberof w/o Strength Measurements (MPa) mean (s.d.) n para-PMDM 7.2 10.5 (3.7)7 10 MMEP 33.4 ~ HEMA 59.4 HEMA 16.0 9.4 (3.0)9 MMEP 38.2 Bis-GMA 45.8 HEMA 24.2 6.0 (2.3)10 Bis-GMA 75.8 TEGDMA 30.0 5.6 (2.2) 10 Bis-GMA 70.0 MMEP 11.6 5.4 (1.8) 8 MMA 88.4 TEGDMA 30.0 5.3 (2.4) 10 MMEP 70.0 HEMA 40.3 5.2 (2.2)10 Bis-GMA 59.7
H
TEGDMA 50.0 2.2 (1.8)-10 Bis-GMA 50.0 I MMEP 100.0 1.7 (1.6) 10 J None (control) 0.8 (1.3) 10 Brackets denote statistically similar groups (Duncan's Multiple Comparison testp
Effects of surface-active resins on dentin/composite bonds G. E. Schumacher ~, F. C. Eichmiller 2,J. M. Antonucci 3 1Clinical Center/Commissioned OfficersDental Clinic, National Institutes of Health, Bethesda, MD, USA 2AmericanDental Association Health Foundation, PaffenbargerResearch Center, 3Dentaland Medical Materials Group, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD, USA
Abstract. Effectivedentin bonding systems basedon para-PMDM diadduct of pyromellitic dianhydride and 2-hydroxyethyl methacrylate (HEMA) have been developed (Bowen et al., 1982). ParaPMDM, a solid of limited solubility, is usually applied from an acetone solution to dentin that has been preconditioned with acid and N-phenylglycine. The feasibility of using a liquid, surfaceactive bonding resin to substitute for or to supplement paraPMDM was explored. Mono(2-methacryloyloxy)ethyl phthalate (MMEP), a liquid, monofunctional homolog of para-PMDM (derived from the reaction of phthalic anhydride with HEMA) was used to formulate several bonding resin systems and solutions. Dentin surfaceswere pretreatedaccording to several variationsof a three-step bonding protocol involving sequential application of 6.8w/o ferricoxalatein 2.5w/o HNO3,N_phenylglycineinacetone, and an experimental bonding resin before placement of a chemically cured composite restorative material. Tensile bond strengths were tested after 24 h storage in distilledwater at 23°C.The results suggest that solutions based on MMEP and/or para-PMDM in acetoneorinothermonomers, especiallythosecontaining HEMA, can effectively promote bonding to dentin. A new mechanism for the observed self-polymerizationof MMEP or para-PMDMwith Nphenylglycine is proposed. INTRODUCTION Strong adhesive bonds between dentin and dental composite have been achieved in vitro by the sequential application of three solutions to the dentin surface: l) ferric oxalate; 2) Nphenylglycine; and 3)a polyfunctional monomer,para-PMDM (Bowen et al., 1982; Chen and Bowen, 1989). Para-PMDM, a product of the addition reaction of two molecules of 2hydroxyethyl methacrylate (HEMA) and one molecule of pyromellitic dianhydrideis thebasis ofseveraleffectivedentin adhesive systems (Bowen et al., 1982; 1987). As a result of its symmetric chemical structure, para-PMDM has a relatively high melting point (162-163 °C) and has limited solubility in common organic solvents and other dental monomers. In most bonding protocols,para-PMDM is usually applied to preconditioned dentin from an acetone solution, Mono(2-methacryloyloxy)ethylphthalate(MMEP),the simplest homolog ofpara-PMDM, is derived from the reaction of HEMA with phthalic anhydride. As shown in Fig. 1, MMEP is monofunctional with respect to both its methacrylate and carboxylic acid groups and has a less symmetric chemical structure than para-PMDM, which probably accounts for its liquidity at ambient temperatures. Incontrast topara-PMDM, 278 Schumacher et al./Dental surface-active resins
MMEP has a wide solubility range in a number of organic solvents and dental monomers such as triethylene glycol dimethacrylate (TEGDMA), Bis-GMA, HEMA and methyl methacrylate (MMA). The object of this study was to determine the feasibility of substituting MMEP forpara-PMDM or
supplementingpara-PMDMwithMMEPinathree-stepbonding protocol previously described (Bowen et al., 1982; Chen and Bowen, 1989; Johnston et al., 1989). MATERIALS AND METHODS
All of the materials used in this study were commercially available, with the exception ofpara-PMDM (Farahani et al., 1991) and N-phenylg]ycine, NPG, (Johnston et al., 1989) which were synthesized. MMEP (Rohm Tech., Inc. Malden, MA, USA), HEMA (99.5%, Scientific Polymer Products, Inc. Ontario, NY, USA), Bis-GMA (Freeman Chemical Corp., Port Washington, WI, USA), TEGDMA (Esschem Co., Essington, PA, USA), and MMA (Aldrich Chemical Co., Milwaukee, WI, USA) were used as received. The adherend used was the chemically activated composite restorative, ADAPTIC (Johnson and Johnson, East Windsor, NJ, USA). A solution of 6.8 w/o purified ferric oxalate (FO), Fe2(C204)3, was used in a 2.5 w/o aqueous nitric acid (HNO~) solution for conditioning the cut dentin (Blosser and Bowen, 1988; Cobb et al., 1989). Extracted non-carious human molars that had been stored in distilled water were used for dentin adhesion testing. To obtain a dentin surface for bonding, the occlusal surfaces were sectioned perpendicularly to the long axis of the tooth with a slow speed diamond sectioning blade (Isomet, Buehler Ltd., Lake Bluff, IL, USA) running under water. The test assemblies usedwere described previously(Bowen, 1965). The roots of the teeth were held in the hollow end of a cylinder by imbedding them within self-curing methyl methacrylate (Formatray, Sybron Corp., Romulus, MI, USA), leaving the flattened dentin surface exposed for bonding. The rest of the testing assembly consisted of an iris with an opening in the center and a Teflon-coated surface on the side which rested against the dentin surface. The opening in the iris determined the bonding area. The chemically cured composite was mixed and immediately added to the end ofthe opposing cylinderthat was seated onto the iris where the composite made contact with the dentin bonding surface through the opening (Fig. 2). Thecylindersandirisweresurroundedbyasleevetomaintain the assembly alignment in a perpendicular attitude with respect to the bonding surface during storage and tensile testing.
o
OH2
Bonding Apparatus
H
o MMEP
II
0
Sleeve
U
(LIQUID) CH~
--
0 o
.o,
.o 0
0
CH?
- - Plunger
para PMDM
1'~ - - ~
- - Iris
(SOLID, m.p. 161 162 °C)
Fig. 1. Molecularstructuresof MMEP and PMDM
Three series of experiments involving two versions (Procedures A and B) of the FO-based three-step bonding protocol were used for bonding the composite to dentin. Approximately ten teeth were used for each bonding resin listed in Table 1 and for each bonding solution listed in Tables 2 and 3. Resin formulations and acetone solutions are expressed in weight percent. In Procedure A, each dentin surface was pretreated using the subsequently described two steps prior to adding a bonding resin (Table l) or a bonding solution (Table 2). In step one, one drop (approximately 0.05 ml) of an aqueous solution of 6.8% FO in 2.5% HN03 was placed on the dentin surface for 60 s, rinsed with distilled water for 10 s and blown dry with compressed air. Next(steptwo)a drop of 10% NPG in acetone was placed on the dentin for 1 min, followed by an acetone rinse with three or four drops of clean acetone and drying with a stream of compressed air. In step three, a drop of the bonding resin or solution was applied and left for 1 min and then blown dry with a stream of compressed air for 10 s. Adaptic, mixed according to the manufacturer's directions, was immediately applied to the opposing cylinder of the test assembly and forced through the iris onto the treated dentin surface under the load of a 2.6 kg weight for 15 s. The entire assembly was permitted to stand at room temperature in air for 15 rain before being immersed into distilled water at 23 (+_1)°C for 24 h. The bonded specimens were then tested in tension on an Instron Universal Testing Instrument, Model 1130 (Instron Corp., Canton, MA, USA), at a cross-head speed of 0.5 cm/min, In Procedure B (Table 3), the three-step bonding technique was essentially the same as Procedure A except that the NPG solution was not rinsed with acetone, but was dried with air 1 minute after application to the dentin. Two concentrations of NPG, 5% and 10%, were used in this experiment, RESULTS
The mean tensile bond strengths of the composite to dentin using the various bonding resins and bonding solutions are shown in Tables 1-3. Similarity of variance within the groups was verified by the M statistic of Bartlett and an F-test was applied to determine if differences were present between the means (Duncan, 1955; Wall, 1986). Significant differences in the means were found in all three sets of data and a Duncan's Multiple Comparison test was employed at the 95% confidence level to determine where these differences occurred
~
- - Dentin Sample
I
I
[
]
Fig. 2. Schematicof bondingapparatus,
RCO2H + :NHCH2CO,H ~ Carboxylic Acid M........ ~
CO2-"H-
H2CO2H
co,,~p~ex .~. . . . .
[(~)c'O2H
" ---:H~CO.H-
O
Complex Wran~f . .[R . . ~ ( ~
II
-
other
R--C. + p,o~.... Initiating R~d,c~
Fig. 3. Mechanism for "spontaneouspolymerization"of carboxylicacid monomers using amine accelerators.
(Duncan, 1955). The brackets in Tables 1-3 indicate groups with non-significant differences. The results in Table 1 indicate that significantly lower tensile bond strengths developed when the dentin surface was treatedwithbondingresinsHandIaswellasthecontrolJ(no bonding resin) when compared to the tensile bond strengths obtained with the other formulations. Formulations A and B, which contain both HEMA and the carboxylic acid monomers (MMEP pluspara-PMDMor MMEP), developed significantly higher tensile bond strengths than the other bonding resins tested. The results in Table 2 show that the bonding solutions 1-6, containing dissolved MMEP at varying concentrations in acetone, developed bond strengths that were significantly higher than the neat MMEP bonding resin, 7. The results shown in Table 3 indicate a significant difference in tensile bond strength of IA and the bond strengths obtained with the other bonding solutions using Procedure B. Significant differences were not found among the next group of four solutions (IB, IIB, IIA, and IIIB). A larger group of five solutions (IIA, IIIB, IIIA, IVB and IVA) also showed nonsignificant differences in their bond strengths. There was some overlap of the second and third groups of bonding solutions, as indicated by the brackets in Table 3. DISCUSSION The conditioning of dentin with certain types of acid solutions has been determined to be quite important in developing Dental Materials~July 1992 279
TABLE 1: TENSILE BONDSTRENGTHSTO DENTIN USINGBONDING
Bonding Resin Formulation A
8 C D E F G
RESINS (PROCEDUREA) Composition Tensile Bond Numberof w/o Strength Measurements (MPa) mean (s.d.) n para-PMDM 7.2 10.5 (3.7)7 10 MMEP 33.4 ~ HEMA 59.4 HEMA 16.0 9.4 (3.0)9 MMEP 38.2 Bis-GMA 45.8 HEMA 24.2 6.0 (2.3)10 Bis-GMA 75.8 TEGDMA 30.0 5.6 (2.2) 10 Bis-GMA 70.0 MMEP 11.6 5.4 (1.8) 8 MMA 88.4 TEGDMA 30.0 5.3 (2.4) 10 MMEP 70.0 HEMA 40.3 5.2 (2.2)10 Bis-GMA 59.7
H
TEGDMA 50.0 2.2 (1.8)-10 Bis-GMA 50.0 I MMEP 100.0 1.7 (1.6) 10 J None (control) 0.8 (1.3) 10 Brackets denote statistically similar groups (Duncan's Multiple Comparison testp
