Polymers of Biological and Biomedical Significance - American

Chapter 12

Ascorbic Acid as an Etchant—Conditioner for Resin Bonding to Dentin 1

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2

James E. Code , Gary E. Schumacher , and Joseph M. Antonucci 1

Clinical Center/CODC, National Institutes of Health, Bethesda, MD 20892 Dental and Medical Materials Group, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899

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L-ascorbic acid (AA) was evaluated as an etchant/conditioner for dentin bonding. A solution of AA (17.6% wt.%, in H O, pH 2.0) was applied to freshly cut dentin sections for time intervals of 15-120 s. The dentin sections were thenrinsedwith distilled H O, air dried, and evaluated for smear layer removal using scanning electron microscopy (SEM). Optimal time for smear layer removal was 30-60 s. Tensile bond strengths (TBS) were measured after dentin surfaces were treated sequentially with various solutions of AA (60 s), N-phenylglycine (NPG) in acetone (60 s), an acetone solution of a surface-active monomer, SAM, (60 s), and finally with a chemically cured composite. SEM results demonstrate significant smear layer removal from dentin using aqueous AA and TBS measurements demonstrate significant dentin bonding using a NPG/SAM-resin system with aqueous AA as the dentin/etchant conditioner. 2

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Dentin, like bone, is a natural composite consisting essentially of apatitic mineral in a collagenous matrix. Because of its heterogeneous and nonuniform nature, the composition and structure of dentin is complex. Overall, the composition of dentin is 70% by weight mineral (45% by volume), and 20% by weight organic, mainly collagen, (33% by volume). A smaller but significant part of dentin is water, 10% by weight and 22% by volume. Some of this water is considered to be tightly bound, i.e., structural water. Dentinal tubule structure, both peritubular and intertubular, varies depending on the plane of the dentin surface and the distance of the tubule from the pulp. In addition, this heterogenous, vital tissue allows dentinal fluids to flow to its surface (7). Because of its complexity bonding to dentin has presented more of a challenge than bonding to enamel. Adhesion of restorative resins to dental tissues has been attributed to chemical and/or mechanical factors (2). Theoretically, chemical bonding can occur with either the inorganic (apatite) or organic (collagen)

This chapter not subject to U.S. copyright Published 1994 American Chemical Society

Shalaby et al.; Polymers of Biological and Biomedical Significance ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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POLYMERS OF BIOLOGICAL AND BIOMEDICAL SIGNIFICANCE

constituents of dentin, with the formation of ionic or covalent bonds. Mechanical adhesion is envisaged as occurring by the penetration and subsequent polymerization of monomers in intertubular dentin as well as in dentinal tubules. In many bonding procedures smear layer removal is considered critical to good bonding. Many adhesive systems use acidic treatments of dentin as thefirststep in bonding protocols to remove the smear layer. To have effective surface wetting a bonding resin must have a lower surface free energy than the dentin surface to which it is applied. Ground dentin surfaces are covered with a smear layer which presents a low energy surface. Etching the dentin surface removes the smear layer producing a cleaner, higher surface energy substrate across which resin can wet and spread (3). Effective bonding to dentin has been achieved by pretreatment with acid agents, e.g., phosphoric acid, citric acid plus ferric chloride, various forms of EDTA, etc., and acidic monomers, e.g., 2-methacryloxyethylphenylphosphoric acid (phenyl-P), 4-methacryloxyethyl trimellitic anhydride (4-META), etc. (4). An acidic solution based on ferric oxalate (FO), which both removes the smear layer and mordants the cleansed surface to give improved bonding sites for coupling agents, has been developed (5). It was anticipated that the ferric ions would form insoluble phosphates while the oxalate ions would form insoluble precipitates with calcium. The precipitated mineral would then solidify among the collagen strands and the restructured surface would be microporous, rigid and receptive to chemical agents. High bond strengths to dentin were obtained using a protocol of sequential applications of acidified FO, an N-aryl α-amino acid and a surface-active monomer (SAM) (6). Later it was found that the removal of the smear layer was primarily due to the presence of nitric acid as a contaminant in FO. Subsequently, an effective etchant/conditioner consisting of an aqueous solution of 6.8 wt. % FO and 2.5 wt. % nitric acid was developed (7). L-ascorbic acid (AA) or vitamin C (Figure 1) is a unique acid with chelating and antioxidant properties. AA's moderate acidity and solubility in water and the fact that many of its salts, including its calcium salts, are water soluble give it potential as an etchant/conditioner for enamel and dentin. Additionally, AA by virtue of its reducing properties can be used in a number of redox initiator systems for the polymerization of dental resins (8). These properties, coupled with its chelation ability, give AA many desirable features that potentially could be useful in a resin bonding system. The purpose of this study was to evaluate the use of AA as an etchant/conditioner for dentin bonding. Materials and Methods All of the materials used in this study were from commercial sources with the exception of N-phenylglycine (NPG) which was synthesized (9). The SAMs used (Figure 2) were mono(2-methacryloyloxy)ethyl phthalate, MMEP, (Rhom Tech., Inc., Maiden, MA), biphenyl dimethacrylate, BPDM, and diphenyl sulfone dimethacrylate, DSDM, (Bisco Inc., Itasca. IL), all derived by the reaction of 2hydroxyethyl methacrylate with the appropriate anhydride. Adaptic (Johnson and Johnson, East Windsor, NJ) was used as the chemically activated composite restorative material. Extracted non-carious human molars, which were stored in


12. CODE ET AL.

Ascorbic Acid as an Etchant—Conditioner

CH.OH

I*

H— C—OH

, Α Λ> Η Downloaded by CORNELL UNIV on July 19, 2016 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0540.ch012

HO

OH

Figure 1. Chemical structure of ascorbic acid (AA).

MMEP (liquid) Ο

Ο

Ο

Ο

HC 3

BPDM (solid, mixture of isomers)

CH

CH,

?

H,C

Ο Ο

CH.

Ο

HO.

Ο

DSDM (liquid, mixture of isomers) Ο

Ο

Figure 2. Chemical structures of surface-active monomers (SAM).


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distilled water, were used to test the bonding of the composite resin to dentin. The dentin surface used for bonding was obtained by removing the occlusal surface perpendicular to the long axis of the tooth with a slow speed diamond sectioning blade (Isomet, Buehler Ltd., Lake Bluff, IL) running under water. Tensile bond strength (TBS) were determined using a testing protocol and assembly previously described (6). To assess the efficacy of smear layer removal by aqueous AA the surface of 1 mm thick dentin cross sections were pretreated with one drop (0.05 mL) of AA (17.6 wt.% in distilled H 0; pH = 2.0). The durations of AA treatment were: 15, 30, 45, 60, and 120 s. Each AA treated dentin surface was rinsed with distilled water for 10 s and then was air dried. The dentin specimens were then sputter coated with gold for evaluation by scanning electron microscopy (SEM). A series of experiments were conducted using AA in a three step protocol to bond composite resin to dentin. Ten teeth were used for each bonding experiment. The bonding method employed was as follows: the dentin surface was first pretreated with one drop (0.05 mL) of AA in an aqueous solution for 60 s. The AA treated dentin surface was thenrinsedwith distilled water for 10 s and was blown dry with compressed air; a drop (0.05 mL) of either 10 wt. % or 5 wt. % NPG in acetone was placed on the dentin surface for 1 min. The acetone evaporated leaving a dry surface; one drop (0.05 mL) of the SAM in acetone was placed on the NPG treated surface for 1 min and then gently air blown to remove any excess acetone. The mixed composite paste was then applied. After 24 h of storage in distilled water at 24 °C the bonded specimens were fractured in tension on a universal testing instrument (Model 1130, Instron Corp., Canton, MA) at a crosshead speed of 0.5 cm/min.


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Results Compared to the SEM photomicrographs of an untreated dentin surface, photomicrographs of the AA treated dentin cross sections for times of 15, 30, 45, 60, and 120 s all showed significant surface changes(Figures 3-8). Optimal smear layer removal and opening of dentinal tubules occurred after 30-60 s of AA etching. The mean TBS of the composite resin to dentin using the three step protocol of applying sequential solutions of AA, NPG and MMEP are shown in Table I. ANOVA calculations found no differences for groups A-E (p< 0.05). The mean TBS of the composite resin to dentin using a three step protocol of applying sequential solutions of AA, NPG and BPDM or DSDM are shown in Table II. The use of difunctional monomers in this experiment was to demonstrate the general applicability of using A A as a dentin etchant/conditioner for dentin bonding.

Discussion SEM results clearly demonstrate removal of the smear layer from dentin using AA as a dentin etchant/conditioner. Various water soluble salts are probably formed, i.e., calcium hydrogen phosphates or calcium ascorbates, during interaction of AA



12. CODE ET AL.

Ascorbic Acid as an Etchant-Conditioner

Figure 3. SEM photomicrograph of dentin surface with smear layer untreated (3000 X).

Figure 4. SEM photomicrograph of dentin surface treated with 17.6 wt.% aqueous AA for 15 s (2000 X).


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CODE ET AL.

Ascorbic Acid as an Etchant—Conditioner




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Table I. Tensile Bond Strengths of M M E P to Dentin Composition in wt.% of solution used in 3 step protocol


Bonding Experiment

Tensile Bond Strength in MPa (Std Dev)

Number of Measurements, η

A

AA 32.0 % NPG 10.0 % MMEP 17.3 %

8.9 (1.9)

10

Β

AA 25.0 % NPG 10.0 % MMEP 20.7 %

7.7 (2.5)

10

C

AA 17.1 % NPG 10.0 % MMEP 20.7 %

7.6 (2.5)

10

D

AA 30.0 % NPG 5.0 % MMEP 23.2 %

6.2 (1.9)

10

Ε

AA 25.0 % NPG 5.0 % MMEP 20.7 %

6.8 (3.3)

10

Table Π. Tensile Bond Strength of Difunctional Aromatic Carboxylic Acid Methacrylates to Dentin

Bonding Experiment

Composition in wt.% of solution used in 3 step protocol

Tensile Bond Strength MPa (Std Dev)

Number of Measurements, η

F

AA 20 % NPG 10 % BPDM 21.5 %

10.3 (3.2)

9

G

AA 20 % NPG 10% DSDM 15 %

5.06(2.1)

10

AA 20 % 7.8 (3.9) NPG 10 % DSDM 15 % * * 2 Coats of DSDM applied to dentin

9

H



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Ascorbic Acid as an Etchant-Conditioner

with the smear layer and intact dentin. Rinsing with distilled water removes these soluble salts and leaves a clean dentin surface suitable for resin-mediated bonding. Theoretically, AA can chelate calcium mineral in the smear layer or even the calcium phosphate mineral in intact dentin. The vicinal enolic OH groups of AA (or as a partially oxidized or keto-enol form) may provide a molecular cage (analogous in structure to the vicinal dicarboxylic acid groups of hydrolyzed 4-META) that can chelate calcium ions (10). However, it has been shown that reversible adsorption of 4-META and NPG on hydroxyapatite occurs, suggesting hydrogen bonding of these molecules with hydroxyapatite rather than chemical bonding by chelation (11,12). Similar reversible, hydrogen bonding reactions have been reported to occur with hydroxyapatite and AA (13). In the presence of acidic adhesive monomers, e.g., 4-META, demineralization of the dentin surface occurs with penetration of the monomer into collagen (13). This impregnation of modified dentin by polymerized resin forms a hybrid resin reinforced layer, which also can be considered a biocomposite of modified dentin and polymerized resin. A A , whether it is demineralizing the dentin surface by virtue of its moderate acidity (pK =4.17) and/or by its ability to chelate calcium ions, effectively conditions intertubular dentin and opens dentinal tubules so that infiltration, diffusion and polymerization of the adhesive components occurs in the altered dentinal interphase, thereby promoting adhesion by micromechanical retention. TBS measurements also demonstrate consistent dentin bonding using NPG and MMEP with AA pretreatment. These values compare favorably with the TBSs, 7.4-8.7 (2.2) MPa, obtained previously for NPG-MMEP systems that used an acidified FO pretreatment (15). TBSs obtained using BPDM and DSDM as bonding resins further demonstrate the efficacy of using AA as a dentin etchant/conditioner. Significantly, DSDM appeared to give higher TBS with two coats of the monomer solution compared to one coat. Also of potential significance for resin bonding is the fact that the autooxidation of AA yields radicals that can initiate polymerization (8). Thus, both AA and NPG, acting as reducing agents, can reduce the effects of oxygen inhibition and generate free radicals that, acting synergistically, may enhance the polymerization of SAMs. A mechanism for generating initiating radicals from the interaction of aryl amines, e.g., NPG and SAMs, has been proposed (15). a

Acknowledgements We thank Dr. Agnes K. Ly for her technical assistance, Dr. Cliff Carey for assisting with the statistical analysis, and Dr. Allen D. Johnston for providing high purity NPG. Additional thanks are extended to Rohm Tech, Inc. and Bisco, Inc. for their generous contributions of the surface-active monomers. This work was partially supported by Interagency Agreement 2Y01 DE 30001 with the National Institute of Dental Research, Bethesda, MD 20892. Certain commercial materials and equipment are identified in this paper to define adequately the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, the American Dental Association Health Foundation, and


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the National Institutes of Health, or that the material or equipment is necessarily the best available for the purpose.


Literature Cited 1. Erickson, R.L. In: International Congress on Dental Materials. Joint Meeting of the Academy of Dental Materials and the Japanese Society for Dental Materials and Devices. 1989, pp 55-69. 2. Asmussen, E . ; Munksgaard, E.C. In: Posterior composite resin dental restorative materials; G. Vanherle and D.C. Smith, Eds.; Utrecht, Peter Szulc Publishing Co.: 1985, pp 217-230. 3. Beech, D.R. In: Biocompatibility of dental materials; D.C. Smith and D.F. Williams, Eds.; CRC Press: FL, 1982, Vol.II; pp 87-100. 4. Nakabayashi, N. Int. Dent. J. 1985, 35, 145. 5. Bowen, R.L. Int. Dent. J. 1978, 28, 97. 6. Bowen, R.L.; Cobb, E.; Rapson, J.E. J. Dent. Res. 1982, 61, 1070. 7. Blosser, R.L.; Bowen, R.L. Dent. Mater. 1988, 4, 225. 8. Antonucci, J.M.; Grams, C.L.; Termini, D.J. J. Dent. Res. 1979, 58, 1887. 9. Johnston, A.D.; Asmussen, E.; Bowen, R.L. J. Dent. Res. 1989, 68, 1337. 10. Birch, G.G.; Parker, K.J. In: Vitamin C, New York, Wiley, 1974, pp 136-149, 221-252. 11. Misra, D.N. J. Dent. Res. 1989, 68, 42. 12. Misra, D.N.; Johnston, A.D. J. Biomed. Mater. Res. 1987, 21, 1329. 13. Misra, D.N. Langmuir 1988, 4, 953. 14. Nakabayashi, N.; Kojima, K.; Masuhara, E. J. Biomed. Mater. Res. 1982, 16, 265. 15. Schumacher, G.E.; Eichmiller, F.C.; Antonucci, J.M. J. Dent. Mater. 1992, 8, 278. Received May 13, 1993


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