Effects of self-etching primers on dentin

Effects of self-etching primers on dentin A. Inagaki* H. Chigira K. Itoh S. Wakumoto Department of Operative Dentistry School of Dentistry Showa University 2-1-1 Kitasenzoku Ohta-ku Tokyo, Japan 145 Received March 3, 1989 Accepted August 9, 1989 *Corresponding author Dent Mater 5:403-407, November, 1989

Abstract-The effects of self-etching dentin primers on the hardness of dentin was evaluated by determination of the Vickers Hardness of both untreated dentin and dentin treated with different primers. Several experimental primers were prepared by dilution of dicarbonates or their esterified salts with different ratios of HEMA to obtain ratios of 5% and 35% by weight. Methacryloxyethyl succinate (MES) and Methacryloxyethyl phthalate (MEP) experimental primers caused less softening of dentin than did maleic acid diluted with HEMA or a commercial dentin primer (Scotchprep, 3M). A high correlation was observed between the pH of a primer and the hardness of dentin treated with the primer. A correlation was also observed between the hardness of treated dentin and the wall-to-wall polymerization contraction of a light-curing microfilled resin (Silux, 3M).

he treatment of the smear layer on the ground tooth surface has been discussed because it disturbs the bond between the resin and cavity wall. As proposed by Buonocore, the smear layer on the enamel should be removed by phosphoric acid, producing a micro-undercut to increase the interlock between the resin and the enamel (Buonocore, 1955). AIthough various dentin cleanserssuch as phosphoric acid, EDTA, citric acid containing ferric chloride, and EDTA containing benzalconium chloride-have been introduced, most of the dentin-bonding systems available on the market cannot overcome the contraction force of the resin composite, and a complete seal between the resin and dentin cavity wall has not been established (Fusayama et al., 1979; Asmussen and Bowen, 1987; N a k a b a y a s h i et al., 1982; BriinnstrSm et al., 1980, 1982). Munksgaard et al. reported that such a lack of bond by the dentin adhesives was improved significantly by the use of dentin primers composed of g l u t a r a l d e h y d e and H E M A (GLUMA) (Munksgaard and Asmussen, 1984; Munksgaard et al., 1984). Not only this dentin-bonding but also the newly developed primers were complex to utilize in the clinic, because some s t e p s - such as enameletching with phosphoric acid gel, dentin cleansing with EDTA or aiuminum oxalate, and pre-treatment with dentin p r i m e r - a r e required prior to the application of the dentinbonding agent (Bowen, 1985). Therefore, self-etching dentin primers have been introduced to simplify the procedures of the dentin-bonding system by combining dentin primer with dentin cleanser. Previously, Hasegawa and Chigira reported that glutaric acid and four acidic monomers diluted in 35% HEMA (E. Merck, Darmstadt, West Germany) solution were effective as self-etching primers (Hasegawa et al., 1988; Chigira et al., 1989). The.

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purpose of the present study was to examine the effects of the commercial and experimental acidic dentin primers on the dentin in a surface by measuring the changes in VHN and pH of the primers by observation of' the treated dentin in a field emission scanning electron microscope (SEM) (S-700, Hitachi, Tokyo, Japan). MATERIALS AND METHODS Acids or acidic monomers tested in this study are listed in Table 1. The experimental self-etching primers were prepared by dilution of these acids or acidic monomers in the HEMA solution at ratios of 5% and 35% by weight. F o r the control group, 35% HEMA solution without any acidic monomers was prepared. (1) Measurement of the V H N on the Treated Dew,t i n Su'~7?bce-The proximal surfaces of extracted human molars were ground and a fiat dentin surface prepared on wet carborundum papers ending with No. 1000. The facial or lingual half of the dentin surface was covered with adhesive tape, and one of the se]f-etching primers was applied on the dentin with a small sponge pellet t'~)r 60 s. After the excess of the primer was dried by a blast of compressed air for ten s and the adhesive tape removed, the Vickers Hardness Number CV~HN)of the covered and treated dentin surface was measured by use of the h a r d n e s s t e s t e r (MVK-E, Akashi Co., Tokyo, Japan) \~qth an indentation of 50 g for 15 s. The VHN of the dentin was determined by the mean value of ten measurements for each area, and the relative hardness between the untreated dentin and dentin after being treated with selfetching primers was presented in percent. Five specimens were tested for each primer, and the measured VHN were analyzed statistically by Student's t test. (2) p H Measurement qf P r i v n e r s The pH of the primers in a glass container of approximately 3 mL was measured by a pH meter ( M-8, Hor-

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TABLE 1 DICARBONATESAND ESTERIFIEDSALTS FOR EXPERIMENTALPRIMERS

Code CA MA PA SA MEG MEM MEP MES

Acid and Acidic Monomers Tested* 1,2-Cyclohexanedicarboxylic anhydride Maleic acid Phthalic acid Succinic acid Methacryloxyethyl hexahydrophthalate Methacryloxyethyl maleate Methacryloxyethyl phthalate Methacryloxyethyl succinate

SP Scotchprep** * These were diluted in HEMA solution at rates of 5% and 35% by weight. ** Supplied by 3M, St. Paul, MN, USA.

TABLE 2 pH OF PRIMERSAND RESIDUAL MVH AFTERTREATINGWITH PRIMERS

CODE

Residual MVH (%)*

SP MA CA MEM PA MEC MEP SA MES

66.7 68.3 76.5 81.7 83.7 84.3 85.4 85.4 87.2

_+ 3.5 __ 10.2 __ 6.5 _+ 11.4 +_ 8.0 __ 8.3 __ 5.5 _+ 6.1 __ 5.6

Control** 94.8 _+ 5.8 *Mean__ SD, n = 5. ** 35% HEMA solution without any acid or acidic monomer. *** Values joined by a line are not significantly different.

iba Co., Kyoto, Japan) at a tempera t u r e of 24 +_ 1°C, and it was determined by the mean value of three measurements. (3) S E M Observation of Treated D e n t i n - The occlusal enamel of extracted human molars was eliminated on wet silicon carbide paper, and the exposed dentin was cleansed with one of the primers by a small sponge pellet for 60 s. After being completely dried, the teeth were sectioned through the center of the surface along the tooth axis. The specimens were then dehydrated in ethanol and dried in the chamber of the critical-point-drying apparatus (HCP-2, Hitachikouki Co., Tokyo, Japan) before vacuum evaporation of gold. The section of the treated dentin was observed by the SEM (S-700, Hitachi, Tokyo, Japan) with an accelerating voltage of 15 KV.

404

pH 0.80 0.70 3.25 1.40 2.05 4.50 3.50 3.00 4.30 5.00

C

m

lpm

Fig. 1. SEM photographofthe sectioned dentin sudace (× 10,000).

RESULTS

(1) Reduction of VHN on Treated Dentin S u r f a c e - T h e changes in the VHN on the dentin treated by dentin primers are given in Table 2. The mean value of hardness compared with u n r e a c t e d d e n t i n was the smallest in the group of the commercial primers (66.7 _+ 3.5%), and it was as small as that of the maleic acid diluted in 35% HEMA solution (68.3 _+ 10.2%). More than 80% of the VHN of the untreated dentin was maintained after the application of experimental primers which were composed of esterified dicarbonates and HEMA, and these values were nearly the same when the dentin was treated with the 0.5 M EDTA (Chiba et al., 1989). (2) pH of P r i m e r s - T h e pH of the primers is presented in Table 2. Two

INAGAKIet al./EFFECTOF SELF-ETCHING ON DENTIN

m

CA

ltLm

Fig. 2. Sectioned dentin surface treated with 1,2cyclohexanedicarboxylicanhydride(CA).

primers, Scotchprep and maleic acid diluted in HEMA, showed a low pH, and a high correlation was observed between the relative hardness in percentage against that of the unreacted dentin and the pH of the primer. (3) S E M Observation of Treated D e n t i n - T h e SEM observations of the treated dentin are shown in Figs. 1-10. An amorphous layer was ob-

MA

"'"

1/.~m

Fig. 3. Sectioned dentin surface treated with maleic acid (MA).

$A Fig. 5. Sectioned dentin surface treated with succinic acid (SA).

changes were not observed compared with untreated specimens.

PA

"" l~m

Fig. ~. Sectioned dentin surface treated with phlhalic acid

(PAL

served on the superficial dentin which was treated with Scotchprep and the maleic acid diluted in 35% HEMA solution. In all specimens, the smear layer on the dentin surface was not removed completely, and the dentinal plugs remained at the tops of the dentinal tubules. In the case of the MES and MEP, morphological

DISCUSSION

Only a few dentin primers are marketed, although many researchers have claimed the importance of pretreatment by the dentin primer. This discrepancy between clinical and laboratory work may be caused by the complex handling procedures necessary for the dentin cleanser and the dentin primer. The concept of the self-etching dentin primer was to simplify the steps of the dentinbonding system by using the primer containing acid or acidic monomer.

It was reported previously that the primary requirement for the dentin cleanser was not to soften the dentin, because the efficacy of the dentin-bonding agent deteriorated with the reduction of hardness of the dentin (Chiba et al., 1989). The result obtained in this study supported this effect, because a high correlation was observed between relative hardness compared with unreacted dentin and the maximum contraction which was measured in our previous paper, as shown in Fig. 11 (r = 0.915, t = - 5.99, Pr < 0.005). The MES and MEP diluted in the HEMA solution to obtain a ratio of

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405

MEC

--l~m

Fig. 6. Sectioned dentin surface treated with

MEM

MEP

l~m

-"-

1/~m

Fig. 7. Sectioned dentin surface treated with metha¢ryloxyethyl maleate (MEM).

Fig. 8. Sectioned dentin surface treated with

methacryloxyethyl hexahydrophthalate(MEC).

5% and 35% showed a marked ability as self-etching primers because they didn't soften the dentin beneath the smear layer. The dentin-bonding agent used in this study was considered to bind to the inorganic component in the dentin structure which might be lost by the self-etching primer of low pH.

Therefore, the bond between the dentin and resin will be decreased when the dentin is decalcified and softened. Therefore, it is possible to conclude that, in order to protect not only the pulp but also the dentin structure, it is desirable that one not cause chemical and physical damage to the dentin by the cleanser or

primer. Even when the MES or MEP diluted in HEMA was employed, it was impossible to prevent contraction gap formation in a few specimens out of ten. Such a lack of bond cannot be explained by the change in the VHN by the dentin primers, and further study is required.

methacryloxyethyl phthalate (MEP).

REFERENCES ASMUSSEN,E. and BOWEN, R.L. (1987): Effect of Acidic P r e t r e a t m e n t on Adhesion to Dentin Mediated by Gluma, J Dent Res 66: 1386-1388. i~i~ ¸ ~.....

Gap% 0.20

0.15

0.10

0.05

" \ '

o m

ME S

1/.tm

Fig. 9. Sectioned dentin surface treated with methacryloxyethyl succinate (MES).

SP l/.em

Fig. 10. Sectioned dentin surface treated with Scotchprep (SP).

406 INAGAKI e~ alJEFFECT OF SELF-ETCHING ON DENTIN

J

I

60

70

I

•

"~k

8o 90 loo Residual V H N ~

F/g. 11. Correlation between the residual VHN of dentin and maximum contraction of Silux in a cylindrical dentin cavity mediated by self-etching dentin primers and Clearfil New BOnd.

BOWEN, R.L. (1985): Bonding of Restorative Materials to Dentine: The Present Status in The United States, Int Dent J 35: 155-159. BRANNSTROM, M.; GLANTZ, P.-O.; and NORDENVALL, K.J. (1982): Cavity Cleanser and Etchants, Biocompatibility of Dent Mater 11: 101-123. BRA.NNSTROM,M.; Nordenvall, K.J.; and GLANTZ, P.-O. (1980): The Effect of EDTA Containing Surface Active Solutions on The Morphology of Prepared Dentin: An in vivo Study, J Dent Res 59: 1127-1131. BUONOCORE, M.G. (1955): A Simple Method of Increasing the Adhesion of Acrylic Filling Materials to Enamel Surfaces, J Dent Res 34: 849-853.

CHIBA, M.; ITOH, K.; and WAKUMOTO, S. (1989): Effect of Dentin Cleansers on the Bonding Efficacy of Dentin Adhesive, Dent Mater J 8: 76-85. CHIGIRA, H.; KOIKE, T.; HASEGAWA, T.; ITOH, K.; WAKUMOTO, S.; and HAYAKAWA, T. (1989): Effect of the Self Etching Dentin Primers, Dent Mater Y 8: 86-92. FUSAYAMA, T.; NAKAMURA, M.; KUROSAKI, N.; and IWAKU, M. (1979): Non-pressure Adhesion Restorative Resin, J Dent Res 58: 1364-1370.

HASEGAWA, T.; MANABE, A.; ITOH, K.; and WAKUMOTO,S. (1988): The Effect of an Experimental Dentin Primer Composed of 35% HEMA and Glutaric Acid, Jpn J Conserv Dent 31:422-427

(in Japanese).

MUNKSGAARD, E.C. and ASMUSSEN, E. (1984): Bond Strength Between Dentin and Restorative Resins Mediated by Mixture of HEMA and Glutaraldehyde, J Dent Res 63: 1087-1089. MUNKSGAARD, E.C.; HANSEN, E.K.; and ASMUSSEN, E. (1984): Effect of Five Adhesives on Adaptation of Resin in Dentin Cavities, Scand J Dent Res 92: 544-548. NAKABAYASHI, N.; TAKAYAMA, M.; K(mMA, K.; and MASUHARA,A. (1982): Studies on Dental Self-Curing Resins-Adhesion of 4-META/MMA-TTB Resin to Pretreated Dentin, J Jpn Soc Dent Appar Mater 123:29-33 (in Japanese).

Dental Materials~November 1989 407