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The Influence of Phosphoric Acid Etching on Retention of Acrylic Resin to Bovine Enamel
The influence of phosphoric acid etching on retention of acrylic resin to bovine enamel
Brian D. Lee, DDS, MSD, Oakland, C alif Ralph W. Phillips, DSc Marjorie L. Swartz, MS, Indianapolis A study was made to determine whether bonding of a direct fillin g resin to bovine enamel surfaces was increased by pretreating the surfaces with a 50% phosphoric acid solution fo r 60 seconds. Tensile strength tests were used to measure the bond strength of the resin to the enamel surface. Results indicated that the pretreatment with the 50% phos phoric acid significantly increased the bonding of the direct fillin g resin, and the use of a cavity seal er in conjunction with the acid etching enhanced the retention of the resin to the enamel surface. Further laboratory tests in which stress is applied to restorations, and a well-controlled clin ica l evalu ation are needed to substantiate the findings in th is study.
It has been shown that under clinical conditions the commonly used restorative materials do not form a permanently adhesive bond to tooth struc ture.1-5 Until an adhesive restorative material is commercially available, other methods of enhanc ing the adhesion of the present materials should be explored. Research has indicated that the surface treatment of enamel or dentin, or both, with cer tain inorganic and organic acids will increase the retention o f some resin restorative dental m ateri als.6-14 Doyle15 reported that when the enamel surface is etched with 50% phosphoric acid, small incisal fractures may be restored with acrylic resins without the use o f pins for retention. These reports have been optimistic and suggest that the acid etching techniques deserve further in vestigation. Factors such as the influence of long term exposure to water, temperature, and mechani cal stress on the bond need evaluation.
M e t h o d s a n d m a te ria ls
The strength of the bond between enamel and a conventional unfilled poly (methyl methacrylate) direct filling resin as influenced by etching the enamel with phosphoric acid was investigated. In addition, the effect of the cavity sealer that was fur nished by the manufacturer was studied. Bond strength was assessed on the basis of the tensile stress required to separate the resin from tooth structure. ■ Materials: Based on the type of treatment giv en to the enamel surface before placement of the resin, the specimens were divided into the follow ing test groups: Group I— The enamel surface was etched by swabbing with a 50% solution of phosphoric acid for one minute. It was rinsed thoroughly with water and dried with air from a chip blower. The cavity sealer was then applied to the surface by means of a small camel’s hair brush. Group II— The enamel was etched for one m in ute with the 50% phosphoric acid solution, rinsed, and dried as described for group I. No sealer was used. Group III— The enamel surface was dried with air from a chip blower and then painted with the cavity sealer. Group IV— No pretreatm ent of the enamel was used. The direct filling resin in this study was Sevriton.* Immediately after the treatment of the enam el surfaces, as described earlier, the resin was ap plied to the tooth surfaces by use of the bead-brush technique. ■ Adhesion test: The method used for testing ad hesion was essentially that developed in previous studies in this laboratory.16 JADA, Vol. 82, June 1971 ■ 1381
Bovine enamel served as the test substrate since the incisor teeth are of sufficient size to perm it the preparation of a relatively large flat area for test ing. Immediately after the teeth were extracted from freshly slaughtered animals they were placed in water and frozen until used in the experiment. The roots were separated and the pulp removed from the crowns. After the crowns had been freed of all adhering tissue, a flat area, approximately 10 mm in diameter, was prepared on the labial surfaces by wet grinding on a rotary wheel with an 80 grit silicon carbide paper. The teeth were mounted in plastic in such a manner that the pre pared surface was exposed. The flat surface was placed on a glass slab and surrounded by an alumi num ring which subsequently was filled with coldcuring resin. On removal of the mounted tooth from the ring, the exposed area of enamel was giv en a final finish by hand grinding on a wet 400 grit silicon carbide paper. After the appropriate treatment of the enamel surface had been carried out, a split silicon ring, approximately 3 mm high with an inside diameter of 7 mm, was positioned over the flattened area. It was held in place by a thin coat of nail polish ap plied to the underside. A brass ring of appropriate dimensions was slipped over the outside of the split ring m atrix to maintain the dimensions dur ing placement of the resin. The bead-brush tech nique was used to flow the increments of resin on to the enamel surfaces. In this way the resin was built up to the level of the silicon ring. A mix of resin was then prepared and used to fill the cavity that was cut in a brass ball bearing. The bearing was then placed, cavity side down, on top of the resin in the matrix, and the two portions of resin were permitted to polymerize together. (This ball bearing served as a means of attaching the speci men in the testing machine.) When the resin had polymerized, the silicon matrix was removed and the specimens were stored at 37 C in distilled wa ter. A specially designed apparatus supported the specimens in the tensile testing machine. Basically, the apparatus consisted of a platform that had a hole in the center large enough to perm it passage of the brass ball but not the resin cylinder in which the tooth was mounted. This platform was attached to the top grid of the testing machine. The inverted specimen was supported on this platform, and the ball bearing was attached to a jig secured in the bottom grid. All parts o f the apparatus were freely movable to perm it alignment of the specimen dur ing application o f the load. 1382 ■ JADA, Vol. 82, June 1971
The specimens were loaded at a cross-head speed of 0.030 inches/minute, and the bond strength recorded as the pounds per square inch (psi) required to produce failure. All specimens were examined under 20-power magnification to determine whether rupture had occurred at the resin-tooth interface or through the resin itself. ■ Variables: The influence on the bond was in vestigated after storage in water; storage in water plus thermal stress; storage in water plus mechani cal stress; and storage in water plus both thermal and mechanical stress. Certain clinical dental res torations, such as the Class IV, are subjected con tinually to an aqueous environment, thermal changes, and stress induced by masticatory forces. These factors often cause degradation of mechani cal or adhesive bonds. Thus specimens of each of the four test groups were subjected to the following conditions: ■ Storage in water: Specimens were stored in distilled water at 37 C for periods of 24 hours, 30 days, and six months, after which the bond strengths were measured. ■ Thermal stress: Specimens that had been stored in water for 24 hours, 30 days, and six months were subjected to 500 cycles between two water baths, one maintained at 10 ± 5 C and the other at 50 ± 5 C. The immersion time in each bath was 30 seconds. The bond strength was mea sured on completion of the tem perature cycling. ■ Mechanical stress: After the designated stor age times, the specimens were subjected to an in term ittent tensile stress of 300 psi. (The 300 psi load was chosen since it was below the failure lev el of specimens in which the enamel had been etched.) The load was applied and released at a cross-head speed of 0.5 inch/minute. The speci mens were subjected to 60 stress cycles and then loaded to failure at a rate of 0.030 inches/minute. ■ Thermal stress and mechanical stress: Group I specimens— those treated with phosphoric acid and then the cavity sealer— were tested after being subjected to both thermal and mechanical stress. After storage in water for 24 hours, 30 days, and six months the specimens were subjected to 500 thermal cycles and then to 60 stress cycles, after which the strength of bond was measured. A t least ten specimens of each type were pre pared for each test condition. In a few instances one or two specimens were discarded because of voids at the interface or other discrepancies. In all, data for 414 specimens were recorded and includ ed in the computations.
DIRECTFILLINGRESIN 24 HRS. DISTILLEDup - 37° C
PSI Fig 1 ■ Adhesion after 24 hours' storage in water of a poly (methyl methacrylate) direct filling resin to bovine enamel as influenced by treatment of the tooth surface.
R e su lt s
The results of adhesion tests conducted on the four test groups, as related to storage time in water, thermal stress, and mechanical stress, are present ed graphically in Figures 1-3. The Newman Keul’s test17 was applied to the data to determine if a sta tistical difference existed between values obtained for the four test groups under the various testing conditions. This statistical analysis is summarized in the Table. The data obtained in the tests conducted at 24 hours appear in Figure 1. Failure of control speci mens (Group IV— no pretreatm ent of the enamel surface) occurred at a lower stress than for the spe cimens in the other test groups. The mean strength value of the resin-enamel bond for specimens stored in water for 24 hours was 64 ± 80 psi. All specimens subjected to thermal stress and to me
Fig 2 ■ Adhesion after 30 days’ storage in water of a poly (methyl mathacrylate) direct filling resin to bovine enamel as influenced by treatment of the tooth surface.
DIRECTFILLINGRESIN 6MONTHSDISTILLEDH20 -37°C
Fig 3 ■ Adhesion after six months’storage in water of a poly (methyl methacrylate) direct filling resin to bovine enamel.
chanical stress failed during storage in water, dur ing the cycling, or when being inserted into the testing machine for the adhesion test. However, the statistical analysis showed that no difference existed between the values obtained under the three testing conditions. Microscopic examination revealed that specimens in this group invariably separated at the resin-tooth interface. Use of the cavity sealer (Group III) improved the bond of the resin to the unetched enamel. Spe cimens stored in water but not subjected to either thermal or mechanical stress ruptured in tension under a force of 360 ± 132 psi. Some specimens failed to adhere at the resin-tooth interface, where as others failed partially in adhesion and partially in cohesion. There was complete degradation of the bond when the specimens were subjected to thermal and to mechanical stress. In both series of tests all specimens failed during cycling or while being mounted in the testing machine. Etching the tooth surface with phosphoric acid appreciably improved the bond of the resin to the enamel (Group II). Specimens stored in water for 24 hours withstood an average stress of 581 ± 229 psi. The strength of the bond between the resin and tooth structure was not reduced either by tem perature or by mechanical stressing of the speci mens. The highest adhesion values, 893 ± 194 psi af ter 24 hours in water, were secured with the Group I specimens, in which etching of the enamel by phosphoric acid was followed by use of the cavity sealer. Thermal cycling, mechanical cycling, and a combination of the two had no deleterious ef fect on the bond of the resin to the tooth. All breaks in this series were either cohesive or adhe sive-cohesive. Lee— P hillip s—Swartz: PHOSPHORIC ACID ETCHING ■ 1383
Table ■ Comparison among the various groups, arranged in descending order according to th e m agnitude o f group means.
G ro u p X I I I I I I I I I I I II II II II II II II II II III III III IV n i
III III IV III IV III IV IV IV III IV IV IV
S ig n if ic a n c e U nder N ew m an-K eul 1s T e s t *
30 d a y s - h 2 o 6 m o n th s - h 2 o 6 m o n th s - s . c . 30 d a y s - T .C . 30 d a y s - T .C . & S .C . 2 k h o u r s - H20 2 k h o u r s - T .C . & S .C .
30 d a y s - S .C . 6 m o n th s - T .C . 2k h o u rs -
T .C . 6 m o n th s - T .C . & S .C . 2 k h o u r s - S .C . 2 k h o u r s - T .C . _ 2 k h o u r s - S .C . 6 m o n th s - HpO 30 d a y s - T .C . 2 k h o u r s - h 2o 6 m o n th s - T .C . 6 m o n th s - S .C . 30 d a y s - S .C . 30 d a y s - h 2 o 2 k h o u r s - HgO 6 m o n th s 30 d a y s - h 28 ° 2 k h o u r s - h 2o 6 m o n th s - T .C . 6 m o n th s - S .C . 30 d a y s - T .C . 2 k h o u r s - S .C . 2 k h o u r s - S .C . 2 k h o u r s - T .C . 2 k h o u r s - T .C . 30 d a y s - T .C . 30 d a y s - ■ H2 0 30 d a y s ■ S .C . 30 d a y s ■■ s . c . 6 m o n th s - T .C . 6 m o n th s - , h 2 o 6 m o n th s - s.c.
S am p le S iz e
10 10 10 10 10 lU 10 10 10 10 10 10 10 10 10
A d h e s io n V a lu e psi S ta n d a rd M ean D e v ia tio n
1136 1030 1029 95 6
908 893 866
862 &k2
839 787 7 k6
691 651 6 30
199 179 130 272 236 19U 179 275
192 226 28k
2 70 320 275
16
595
10 10 10 10 10
581 523
175 156 219 169 185
UU6 kk2
lk 7 121
16
360
10 10
210 16k 6k 5k 5k 6 2 1 0 0 0 0 0 0 0 0 0
132 1 U5 70
16 10 10 9 10 10 10 10 10 10 10 10 10 10 8
5^7
97
69 90 11 k 3 0 0 0 0 0 0 0 0 0
* G ro u p s i n c l u d e d w i t h i n t h e sam e l i n e a r e n o t s i g n i f i c a n t l y d i f f e r e n t fr o m e a c h o t h e r a t t h e 0 .0 5 p r o b a b i l i t y l e v e l ; a n d t h o s e g r o u p s n o t i n c l u d e d w i t h i n t h e sam e l i n e a r e s i g n i f i c a n t l y d i f f e r e n t f r o m e a c h o t h e r . T .C . - T e m p e r a t u r e s t r e s s c y c l i n g S .C . - I n t e r m i t t e n t s t r e s s c y c l i n g 1384 a JADA, Vol. 82, June 1971
The effects of 30 days’ and six months’ storage in water on the adhesion of the resin to the enamel are shown in Figures 2 and 3, respectively. P ro longed immersion in water did not alter the bond strength between the resin and enamel that had been pretreated with phosphoric acid. The data obtained for Group I (acid etching plus cavity sealer) and Group II (acid etching only) after 30 days and at six months were comparable to the val ues obtained at 24 hours. With respect to control specimens, Group IV specimens failed during the storage period. Group III specimens (sealer only) that were stored in wa ter for these prolonged periods, but not subjected to either thermal or mechanical stress, showed some reduction in bond strength as compared to 24-hour specimens. Although some of the sixm onth specimens in this group survived thermal cycling and stress cycling, the adhesion values were not statistically different from those of speci mens where zero retention occurred.
D is c u s s io n
The results of this study corroborate the research of others3-4-6'11 in that the adhesion of resin to un treated enamel is either decidedly reduced or de stroyed by immersion in water. In addition, the data clearly indicate that a 50% aqueous solution of phosphoric acid applied to bovine enamel sur faces for 60 seconds significantly increases the bonding of an acrylic restorative resin; also the cavity sealer supplied by the manufacturer fur ther enhances the bonding of the resin when ap plied to enamel surfaces pretreated with phos phoric acid. However, the statistical difference be tween these two groups is small. Not only were ad hesion values appreciably higher for these groups, but there was no degradation o f the bond even af ter six months’ storage in water. Examination of each specimen in groups I and II revealed that the acrylic resin either fractured partially or wholly within the material; thus in all instances the bond strength of the acrylic resin to acid-etched enamel surfaces exceeded the tensile strength of the resin material. The improved bonding of the acrylic resin achieved by pretreating the enamel surface with 50% phosphoric acid possibly can be attributed to the fact that the etching produces a modest roughen ing of the enamel surface into which the acrylic resin could flow. Microradiographs have revealed
a honeycomb structure of the enamel surface af ter etching with acids.18 The honeycomb fram e work may provide the means by which the mechan ical retention is enhanced. Gwinnett and M atsui19 and Buonocore, Matsui, and Gwinnett20 reported filamentous “tag-like” extension of acrylic resin 15/x to 20/a in length at the interface where the resin contacted the acid-etched enamel surfaces. In addition, the phosphoric acid may improve the wettability of the surface by removing organic de bris, thereby permitting a more intimate contact between the resin and the tooth surface. The improved bonding of the acrylic resin achieved when the cavity sealer was used on acidetched enamel surfaces may be associated with the low surface tension of the liquid that permits it to flow readily into the surface irregularities of the tooth surface. The sealer conceivably adheres to the resin, thereby increasing the mechanical reten tion. In addition the sealer also may contribute to the cleaning of the tooth surface so that better wet ting of the tooth surface is achieved. The results of this study support the view of Doyle16 that the etched restoration technique for restoring small incisal fractures and hypoplastic de fects in anterior permanent teeth may be advanta geous by virtue of improving adaptation and reten tion of an unfilled acrylic resin to tooth structure. Conceivably in such restorations this improve ment could result in a reduction in marginal leak age and thus discoloration. It has been suggested that the superior retention provided by acid etch ing could modify the classical methods used for achieving retention, that is, use of pins and under cuts in certain cavity preparations. Although the laboratory test conditions used in this investiga tion parallel some o f the conditions found within the oral environment, they do not necessarily pro vide the information as to whether this improved bonding will be m aintained in the oral cavity over long periods. Therefore, further laboratory testing applying stress to restorations, followed by wellcontrolled clinical evaluation, is needed to substan tiate such a far-reaching recommendation. The results of this study may not be applicable to other resin systems, such as the composites. The wetting, and other characteristics, of such resins are decidedly different.
S u m m a r y a n d c o n c lu s io n s
The purpose of this study was to determine wheth er the bonding o f a direct filling resin to bovine Lee—P hillip s—Swartz: PHOSPHORIC ACID ETCHING ■ 1385
enamel surfaces was increased by pretreating the surfaces with a 50% phosphoric acid solution for 60 seconds. A tensile test was employed to m ea sure the bond strength of the acrylic resin to the enamel surface. A total of 414 bovine teeth were divided into four principal groups: in group I the enamel sur face was pretreated with the phosphoric acid and the cavity sealer; in group II the enamel surface was pretreated with the phosphoric acid; in group III the enamel surface was pretreated with the cav ity sealer; and in the control group IV the enamel surface was left untreated. The four groups were stored in water for 24 hours, 30 days, and six months. The effects of temperature stress cycling and mechanical stress cycling after prolonged stor age in water were also investigated. The results indicate that pretreating the enamel surface with 50% phosphoric acid for 60 seconds significantly increased the bonding of the direct filling resin, and the use of the cavity sealer in con junction with the acid-etching further enhanced the retention of the resin to enamel surface. The bond failures observed with the acid-etched specimens in groups I and II were either cohesive or adhesive-cohesive, which indicated that the bond strength of the resin exceeded the tensile strength of the material.
Portions of this paper are taken from a thesis of the senior author in partial fulfillm ent of the MSD degree, Indiana Uni versity, 1970. This research was supported in part by a grant from the USPHS, DE 00433-17, from the National Institute of Dental Research, Bethesda, Md. Doctor Lee is now practicing pedodontics at 430 61st St, Oakland, Calif 94609, and is on the staff of the University of California School of Dentistry, San Francisco. Doctor Phil lips is assistant dean and research professor of dental materi als, and Professor Swartz is professor of dental materials, In diana University School of Dentistry, 1121 W Michigan St, Indianapolis, 46202. * Sevriton Simplified, Amalgamated Dental Trade Dist., Ltd, London.
1386 ■ JADA, Vol. 82, June 1971
1. Peterson, E.A.; Phillips, R.W.; and Swartz, M.L. A com parison of the physical properties of four restorative resins. JADA 73:1324 Dec 1966. 2. Williams, J.D.; Swartz, M.L.; and Phillips, R.W. Reten tion of orthodontic bands as influenced by cementing media. Angle Orthodont 35:278 Oct 1965. 3. Swartz, M.L., and Phillips, R.W. A method of measur ing the adhesive characteristics of dental cements. JADA 50: 172 Feb 1955. 4. Paffenbarger, G.C.; Schouboe, P.J.; and Sweeney, W.T. Resin cements and posterior type direct filling resins. JADA 52:584 May 1956. 5. Hanke, G. Screening test methods and their application in measuring adhesion of selected dental resins. Thesis, In diana University School of Dentistry, 1968. 6. Newman, G.V.; Snyder, W.H.; and Wilson, C.E. Acrylic adhesives for bonding attachments to tooth surfaces. Angle Orthodont 38:12 Jan 1968. 7. Cueto, E. I., and Buonocore, M.G. Sealing of pits and fissures with an adhesive resin. Its use in caries prevention. JADA 75:121 July 1967. 8. Ripa, L.W., and Cole, W.W. Occlusal sealing and caries prevention: Results 12 months after a single application of adhesive resin. J Dent Res 49:171 Jan-Feb 1970. 9. Buonocore, M.G. A simple method of increasing the ad hesion of acrylic filling materials to enamel surfaces. J Dent Res 34:849 Dec 1955. 10. Buonocore, WI.G.; Wileman, W.; and Brudevold, F. Re port of a resin composition capable of bonding to human den tin surfaces. J Dent Res 35:846 Dec 1956. 11. Swanson, L.T., and Beck, J.F. Factors affecting bond ing to human enamel with special reference to a plastic ad hesive. JADA 61:581 Nov 1960. 12. National Institute for Dental Research. Adhesive res torative dental materials. II. Publication no. 1494. Washing ton, DC, US Govt Print Off, 1965. 13. Mulholland, R.D., and DeShazer, D.O. The effect of acidic pretreatment solutions on direct bonding of orthodon tic brackets to enamel. Angle Orthodont 38:236 July 1968. 14. Bowen, R.L. Adhesive bonding of various materials to hard tooth tissue. III. Bond to dentin improved by pretreat ment and use of surface active comonomer. J Dent Res 44: 903 Sept-Oct 1965. 15. Doyle, W.A. Operative dentistry. In Goldman, H.M., and others. Current therapy in dentistry, ed 3. St. Louis, C. V. Mosby Co., 1968, p 843-844. 16. Phillips, R.W.; Swartz, M.L.; and Rhodes, B. An eval uation of a carboxylate cement. JADA 81:1353 Dec 1970. 17. Winer, B.J. Statistical principles in experimental de sign. New York, McGraw-Hill, 1962, p 77-89. 18. Sharpe, A.N. Influence of crystal orientation in hu man enamel on its reactivity to acid as shown by high resolu tion microradiography. Arch Oral Biol 12:583 May 1967. 19. Gwinnett, A.J., and Matsui, A. A study of enamel ad hesives. The physical relationship between enamel and ad hesive. Arch Oral Biol 12:1615 Dec 1967. 20. Buonocore, M.G.; Matsui, A.; and Gwinnett, A.J. Pene tration of resin dental materials into enamel with reference to bonding. Arch Oral Biol 13:61 Jan 1968.
Brian D. Lee, DDS, MSD, Oakland, C alif Ralph W. Phillips, DSc Marjorie L. Swartz, MS, Indianapolis A study was made to determine whether bonding of a direct fillin g resin to bovine enamel surfaces was increased by pretreating the surfaces with a 50% phosphoric acid solution fo r 60 seconds. Tensile strength tests were used to measure the bond strength of the resin to the enamel surface. Results indicated that the pretreatment with the 50% phos phoric acid significantly increased the bonding of the direct fillin g resin, and the use of a cavity seal er in conjunction with the acid etching enhanced the retention of the resin to the enamel surface. Further laboratory tests in which stress is applied to restorations, and a well-controlled clin ica l evalu ation are needed to substantiate the findings in th is study.
It has been shown that under clinical conditions the commonly used restorative materials do not form a permanently adhesive bond to tooth struc ture.1-5 Until an adhesive restorative material is commercially available, other methods of enhanc ing the adhesion of the present materials should be explored. Research has indicated that the surface treatment of enamel or dentin, or both, with cer tain inorganic and organic acids will increase the retention o f some resin restorative dental m ateri als.6-14 Doyle15 reported that when the enamel surface is etched with 50% phosphoric acid, small incisal fractures may be restored with acrylic resins without the use o f pins for retention. These reports have been optimistic and suggest that the acid etching techniques deserve further in vestigation. Factors such as the influence of long term exposure to water, temperature, and mechani cal stress on the bond need evaluation.
M e t h o d s a n d m a te ria ls
The strength of the bond between enamel and a conventional unfilled poly (methyl methacrylate) direct filling resin as influenced by etching the enamel with phosphoric acid was investigated. In addition, the effect of the cavity sealer that was fur nished by the manufacturer was studied. Bond strength was assessed on the basis of the tensile stress required to separate the resin from tooth structure. ■ Materials: Based on the type of treatment giv en to the enamel surface before placement of the resin, the specimens were divided into the follow ing test groups: Group I— The enamel surface was etched by swabbing with a 50% solution of phosphoric acid for one minute. It was rinsed thoroughly with water and dried with air from a chip blower. The cavity sealer was then applied to the surface by means of a small camel’s hair brush. Group II— The enamel was etched for one m in ute with the 50% phosphoric acid solution, rinsed, and dried as described for group I. No sealer was used. Group III— The enamel surface was dried with air from a chip blower and then painted with the cavity sealer. Group IV— No pretreatm ent of the enamel was used. The direct filling resin in this study was Sevriton.* Immediately after the treatment of the enam el surfaces, as described earlier, the resin was ap plied to the tooth surfaces by use of the bead-brush technique. ■ Adhesion test: The method used for testing ad hesion was essentially that developed in previous studies in this laboratory.16 JADA, Vol. 82, June 1971 ■ 1381
Bovine enamel served as the test substrate since the incisor teeth are of sufficient size to perm it the preparation of a relatively large flat area for test ing. Immediately after the teeth were extracted from freshly slaughtered animals they were placed in water and frozen until used in the experiment. The roots were separated and the pulp removed from the crowns. After the crowns had been freed of all adhering tissue, a flat area, approximately 10 mm in diameter, was prepared on the labial surfaces by wet grinding on a rotary wheel with an 80 grit silicon carbide paper. The teeth were mounted in plastic in such a manner that the pre pared surface was exposed. The flat surface was placed on a glass slab and surrounded by an alumi num ring which subsequently was filled with coldcuring resin. On removal of the mounted tooth from the ring, the exposed area of enamel was giv en a final finish by hand grinding on a wet 400 grit silicon carbide paper. After the appropriate treatment of the enamel surface had been carried out, a split silicon ring, approximately 3 mm high with an inside diameter of 7 mm, was positioned over the flattened area. It was held in place by a thin coat of nail polish ap plied to the underside. A brass ring of appropriate dimensions was slipped over the outside of the split ring m atrix to maintain the dimensions dur ing placement of the resin. The bead-brush tech nique was used to flow the increments of resin on to the enamel surfaces. In this way the resin was built up to the level of the silicon ring. A mix of resin was then prepared and used to fill the cavity that was cut in a brass ball bearing. The bearing was then placed, cavity side down, on top of the resin in the matrix, and the two portions of resin were permitted to polymerize together. (This ball bearing served as a means of attaching the speci men in the testing machine.) When the resin had polymerized, the silicon matrix was removed and the specimens were stored at 37 C in distilled wa ter. A specially designed apparatus supported the specimens in the tensile testing machine. Basically, the apparatus consisted of a platform that had a hole in the center large enough to perm it passage of the brass ball but not the resin cylinder in which the tooth was mounted. This platform was attached to the top grid of the testing machine. The inverted specimen was supported on this platform, and the ball bearing was attached to a jig secured in the bottom grid. All parts o f the apparatus were freely movable to perm it alignment of the specimen dur ing application o f the load. 1382 ■ JADA, Vol. 82, June 1971
The specimens were loaded at a cross-head speed of 0.030 inches/minute, and the bond strength recorded as the pounds per square inch (psi) required to produce failure. All specimens were examined under 20-power magnification to determine whether rupture had occurred at the resin-tooth interface or through the resin itself. ■ Variables: The influence on the bond was in vestigated after storage in water; storage in water plus thermal stress; storage in water plus mechani cal stress; and storage in water plus both thermal and mechanical stress. Certain clinical dental res torations, such as the Class IV, are subjected con tinually to an aqueous environment, thermal changes, and stress induced by masticatory forces. These factors often cause degradation of mechani cal or adhesive bonds. Thus specimens of each of the four test groups were subjected to the following conditions: ■ Storage in water: Specimens were stored in distilled water at 37 C for periods of 24 hours, 30 days, and six months, after which the bond strengths were measured. ■ Thermal stress: Specimens that had been stored in water for 24 hours, 30 days, and six months were subjected to 500 cycles between two water baths, one maintained at 10 ± 5 C and the other at 50 ± 5 C. The immersion time in each bath was 30 seconds. The bond strength was mea sured on completion of the tem perature cycling. ■ Mechanical stress: After the designated stor age times, the specimens were subjected to an in term ittent tensile stress of 300 psi. (The 300 psi load was chosen since it was below the failure lev el of specimens in which the enamel had been etched.) The load was applied and released at a cross-head speed of 0.5 inch/minute. The speci mens were subjected to 60 stress cycles and then loaded to failure at a rate of 0.030 inches/minute. ■ Thermal stress and mechanical stress: Group I specimens— those treated with phosphoric acid and then the cavity sealer— were tested after being subjected to both thermal and mechanical stress. After storage in water for 24 hours, 30 days, and six months the specimens were subjected to 500 thermal cycles and then to 60 stress cycles, after which the strength of bond was measured. A t least ten specimens of each type were pre pared for each test condition. In a few instances one or two specimens were discarded because of voids at the interface or other discrepancies. In all, data for 414 specimens were recorded and includ ed in the computations.
DIRECTFILLINGRESIN 24 HRS. DISTILLEDup - 37° C
PSI Fig 1 ■ Adhesion after 24 hours' storage in water of a poly (methyl methacrylate) direct filling resin to bovine enamel as influenced by treatment of the tooth surface.
R e su lt s
The results of adhesion tests conducted on the four test groups, as related to storage time in water, thermal stress, and mechanical stress, are present ed graphically in Figures 1-3. The Newman Keul’s test17 was applied to the data to determine if a sta tistical difference existed between values obtained for the four test groups under the various testing conditions. This statistical analysis is summarized in the Table. The data obtained in the tests conducted at 24 hours appear in Figure 1. Failure of control speci mens (Group IV— no pretreatm ent of the enamel surface) occurred at a lower stress than for the spe cimens in the other test groups. The mean strength value of the resin-enamel bond for specimens stored in water for 24 hours was 64 ± 80 psi. All specimens subjected to thermal stress and to me
Fig 2 ■ Adhesion after 30 days’ storage in water of a poly (methyl mathacrylate) direct filling resin to bovine enamel as influenced by treatment of the tooth surface.
DIRECTFILLINGRESIN 6MONTHSDISTILLEDH20 -37°C
Fig 3 ■ Adhesion after six months’storage in water of a poly (methyl methacrylate) direct filling resin to bovine enamel.
chanical stress failed during storage in water, dur ing the cycling, or when being inserted into the testing machine for the adhesion test. However, the statistical analysis showed that no difference existed between the values obtained under the three testing conditions. Microscopic examination revealed that specimens in this group invariably separated at the resin-tooth interface. Use of the cavity sealer (Group III) improved the bond of the resin to the unetched enamel. Spe cimens stored in water but not subjected to either thermal or mechanical stress ruptured in tension under a force of 360 ± 132 psi. Some specimens failed to adhere at the resin-tooth interface, where as others failed partially in adhesion and partially in cohesion. There was complete degradation of the bond when the specimens were subjected to thermal and to mechanical stress. In both series of tests all specimens failed during cycling or while being mounted in the testing machine. Etching the tooth surface with phosphoric acid appreciably improved the bond of the resin to the enamel (Group II). Specimens stored in water for 24 hours withstood an average stress of 581 ± 229 psi. The strength of the bond between the resin and tooth structure was not reduced either by tem perature or by mechanical stressing of the speci mens. The highest adhesion values, 893 ± 194 psi af ter 24 hours in water, were secured with the Group I specimens, in which etching of the enamel by phosphoric acid was followed by use of the cavity sealer. Thermal cycling, mechanical cycling, and a combination of the two had no deleterious ef fect on the bond of the resin to the tooth. All breaks in this series were either cohesive or adhe sive-cohesive. Lee— P hillip s—Swartz: PHOSPHORIC ACID ETCHING ■ 1383
Table ■ Comparison among the various groups, arranged in descending order according to th e m agnitude o f group means.
G ro u p X I I I I I I I I I I I II II II II II II II II II III III III IV n i
III III IV III IV III IV IV IV III IV IV IV
S ig n if ic a n c e U nder N ew m an-K eul 1s T e s t *
30 d a y s - h 2 o 6 m o n th s - h 2 o 6 m o n th s - s . c . 30 d a y s - T .C . 30 d a y s - T .C . & S .C . 2 k h o u r s - H20 2 k h o u r s - T .C . & S .C .
30 d a y s - S .C . 6 m o n th s - T .C . 2k h o u rs -
T .C . 6 m o n th s - T .C . & S .C . 2 k h o u r s - S .C . 2 k h o u r s - T .C . _ 2 k h o u r s - S .C . 6 m o n th s - HpO 30 d a y s - T .C . 2 k h o u r s - h 2o 6 m o n th s - T .C . 6 m o n th s - S .C . 30 d a y s - S .C . 30 d a y s - h 2 o 2 k h o u r s - HgO 6 m o n th s 30 d a y s - h 28 ° 2 k h o u r s - h 2o 6 m o n th s - T .C . 6 m o n th s - S .C . 30 d a y s - T .C . 2 k h o u r s - S .C . 2 k h o u r s - S .C . 2 k h o u r s - T .C . 2 k h o u r s - T .C . 30 d a y s - T .C . 30 d a y s - ■ H2 0 30 d a y s ■ S .C . 30 d a y s ■■ s . c . 6 m o n th s - T .C . 6 m o n th s - , h 2 o 6 m o n th s - s.c.
S am p le S iz e
10 10 10 10 10 lU 10 10 10 10 10 10 10 10 10
A d h e s io n V a lu e psi S ta n d a rd M ean D e v ia tio n
1136 1030 1029 95 6
908 893 866
862 &k2
839 787 7 k6
691 651 6 30
199 179 130 272 236 19U 179 275
192 226 28k
2 70 320 275
16
595
10 10 10 10 10
581 523
175 156 219 169 185
UU6 kk2
lk 7 121
16
360
10 10
210 16k 6k 5k 5k 6 2 1 0 0 0 0 0 0 0 0 0
132 1 U5 70
16 10 10 9 10 10 10 10 10 10 10 10 10 10 8
5^7
97
69 90 11 k 3 0 0 0 0 0 0 0 0 0
* G ro u p s i n c l u d e d w i t h i n t h e sam e l i n e a r e n o t s i g n i f i c a n t l y d i f f e r e n t fr o m e a c h o t h e r a t t h e 0 .0 5 p r o b a b i l i t y l e v e l ; a n d t h o s e g r o u p s n o t i n c l u d e d w i t h i n t h e sam e l i n e a r e s i g n i f i c a n t l y d i f f e r e n t f r o m e a c h o t h e r . T .C . - T e m p e r a t u r e s t r e s s c y c l i n g S .C . - I n t e r m i t t e n t s t r e s s c y c l i n g 1384 a JADA, Vol. 82, June 1971
The effects of 30 days’ and six months’ storage in water on the adhesion of the resin to the enamel are shown in Figures 2 and 3, respectively. P ro longed immersion in water did not alter the bond strength between the resin and enamel that had been pretreated with phosphoric acid. The data obtained for Group I (acid etching plus cavity sealer) and Group II (acid etching only) after 30 days and at six months were comparable to the val ues obtained at 24 hours. With respect to control specimens, Group IV specimens failed during the storage period. Group III specimens (sealer only) that were stored in wa ter for these prolonged periods, but not subjected to either thermal or mechanical stress, showed some reduction in bond strength as compared to 24-hour specimens. Although some of the sixm onth specimens in this group survived thermal cycling and stress cycling, the adhesion values were not statistically different from those of speci mens where zero retention occurred.
D is c u s s io n
The results of this study corroborate the research of others3-4-6'11 in that the adhesion of resin to un treated enamel is either decidedly reduced or de stroyed by immersion in water. In addition, the data clearly indicate that a 50% aqueous solution of phosphoric acid applied to bovine enamel sur faces for 60 seconds significantly increases the bonding of an acrylic restorative resin; also the cavity sealer supplied by the manufacturer fur ther enhances the bonding of the resin when ap plied to enamel surfaces pretreated with phos phoric acid. However, the statistical difference be tween these two groups is small. Not only were ad hesion values appreciably higher for these groups, but there was no degradation o f the bond even af ter six months’ storage in water. Examination of each specimen in groups I and II revealed that the acrylic resin either fractured partially or wholly within the material; thus in all instances the bond strength of the acrylic resin to acid-etched enamel surfaces exceeded the tensile strength of the resin material. The improved bonding of the acrylic resin achieved by pretreating the enamel surface with 50% phosphoric acid possibly can be attributed to the fact that the etching produces a modest roughen ing of the enamel surface into which the acrylic resin could flow. Microradiographs have revealed
a honeycomb structure of the enamel surface af ter etching with acids.18 The honeycomb fram e work may provide the means by which the mechan ical retention is enhanced. Gwinnett and M atsui19 and Buonocore, Matsui, and Gwinnett20 reported filamentous “tag-like” extension of acrylic resin 15/x to 20/a in length at the interface where the resin contacted the acid-etched enamel surfaces. In addition, the phosphoric acid may improve the wettability of the surface by removing organic de bris, thereby permitting a more intimate contact between the resin and the tooth surface. The improved bonding of the acrylic resin achieved when the cavity sealer was used on acidetched enamel surfaces may be associated with the low surface tension of the liquid that permits it to flow readily into the surface irregularities of the tooth surface. The sealer conceivably adheres to the resin, thereby increasing the mechanical reten tion. In addition the sealer also may contribute to the cleaning of the tooth surface so that better wet ting of the tooth surface is achieved. The results of this study support the view of Doyle16 that the etched restoration technique for restoring small incisal fractures and hypoplastic de fects in anterior permanent teeth may be advanta geous by virtue of improving adaptation and reten tion of an unfilled acrylic resin to tooth structure. Conceivably in such restorations this improve ment could result in a reduction in marginal leak age and thus discoloration. It has been suggested that the superior retention provided by acid etch ing could modify the classical methods used for achieving retention, that is, use of pins and under cuts in certain cavity preparations. Although the laboratory test conditions used in this investiga tion parallel some o f the conditions found within the oral environment, they do not necessarily pro vide the information as to whether this improved bonding will be m aintained in the oral cavity over long periods. Therefore, further laboratory testing applying stress to restorations, followed by wellcontrolled clinical evaluation, is needed to substan tiate such a far-reaching recommendation. The results of this study may not be applicable to other resin systems, such as the composites. The wetting, and other characteristics, of such resins are decidedly different.
S u m m a r y a n d c o n c lu s io n s
The purpose of this study was to determine wheth er the bonding o f a direct filling resin to bovine Lee—P hillip s—Swartz: PHOSPHORIC ACID ETCHING ■ 1385
enamel surfaces was increased by pretreating the surfaces with a 50% phosphoric acid solution for 60 seconds. A tensile test was employed to m ea sure the bond strength of the acrylic resin to the enamel surface. A total of 414 bovine teeth were divided into four principal groups: in group I the enamel sur face was pretreated with the phosphoric acid and the cavity sealer; in group II the enamel surface was pretreated with the phosphoric acid; in group III the enamel surface was pretreated with the cav ity sealer; and in the control group IV the enamel surface was left untreated. The four groups were stored in water for 24 hours, 30 days, and six months. The effects of temperature stress cycling and mechanical stress cycling after prolonged stor age in water were also investigated. The results indicate that pretreating the enamel surface with 50% phosphoric acid for 60 seconds significantly increased the bonding of the direct filling resin, and the use of the cavity sealer in con junction with the acid-etching further enhanced the retention of the resin to enamel surface. The bond failures observed with the acid-etched specimens in groups I and II were either cohesive or adhesive-cohesive, which indicated that the bond strength of the resin exceeded the tensile strength of the material.
Portions of this paper are taken from a thesis of the senior author in partial fulfillm ent of the MSD degree, Indiana Uni versity, 1970. This research was supported in part by a grant from the USPHS, DE 00433-17, from the National Institute of Dental Research, Bethesda, Md. Doctor Lee is now practicing pedodontics at 430 61st St, Oakland, Calif 94609, and is on the staff of the University of California School of Dentistry, San Francisco. Doctor Phil lips is assistant dean and research professor of dental materi als, and Professor Swartz is professor of dental materials, In diana University School of Dentistry, 1121 W Michigan St, Indianapolis, 46202. * Sevriton Simplified, Amalgamated Dental Trade Dist., Ltd, London.
1386 ■ JADA, Vol. 82, June 1971
1. Peterson, E.A.; Phillips, R.W.; and Swartz, M.L. A com parison of the physical properties of four restorative resins. JADA 73:1324 Dec 1966. 2. Williams, J.D.; Swartz, M.L.; and Phillips, R.W. Reten tion of orthodontic bands as influenced by cementing media. Angle Orthodont 35:278 Oct 1965. 3. Swartz, M.L., and Phillips, R.W. A method of measur ing the adhesive characteristics of dental cements. JADA 50: 172 Feb 1955. 4. Paffenbarger, G.C.; Schouboe, P.J.; and Sweeney, W.T. Resin cements and posterior type direct filling resins. JADA 52:584 May 1956. 5. Hanke, G. Screening test methods and their application in measuring adhesion of selected dental resins. Thesis, In diana University School of Dentistry, 1968. 6. Newman, G.V.; Snyder, W.H.; and Wilson, C.E. Acrylic adhesives for bonding attachments to tooth surfaces. Angle Orthodont 38:12 Jan 1968. 7. Cueto, E. I., and Buonocore, M.G. Sealing of pits and fissures with an adhesive resin. Its use in caries prevention. JADA 75:121 July 1967. 8. Ripa, L.W., and Cole, W.W. Occlusal sealing and caries prevention: Results 12 months after a single application of adhesive resin. J Dent Res 49:171 Jan-Feb 1970. 9. Buonocore, M.G. A simple method of increasing the ad hesion of acrylic filling materials to enamel surfaces. J Dent Res 34:849 Dec 1955. 10. Buonocore, WI.G.; Wileman, W.; and Brudevold, F. Re port of a resin composition capable of bonding to human den tin surfaces. J Dent Res 35:846 Dec 1956. 11. Swanson, L.T., and Beck, J.F. Factors affecting bond ing to human enamel with special reference to a plastic ad hesive. JADA 61:581 Nov 1960. 12. National Institute for Dental Research. Adhesive res torative dental materials. II. Publication no. 1494. Washing ton, DC, US Govt Print Off, 1965. 13. Mulholland, R.D., and DeShazer, D.O. The effect of acidic pretreatment solutions on direct bonding of orthodon tic brackets to enamel. Angle Orthodont 38:236 July 1968. 14. Bowen, R.L. Adhesive bonding of various materials to hard tooth tissue. III. Bond to dentin improved by pretreat ment and use of surface active comonomer. J Dent Res 44: 903 Sept-Oct 1965. 15. Doyle, W.A. Operative dentistry. In Goldman, H.M., and others. Current therapy in dentistry, ed 3. St. Louis, C. V. Mosby Co., 1968, p 843-844. 16. Phillips, R.W.; Swartz, M.L.; and Rhodes, B. An eval uation of a carboxylate cement. JADA 81:1353 Dec 1970. 17. Winer, B.J. Statistical principles in experimental de sign. New York, McGraw-Hill, 1962, p 77-89. 18. Sharpe, A.N. Influence of crystal orientation in hu man enamel on its reactivity to acid as shown by high resolu tion microradiography. Arch Oral Biol 12:583 May 1967. 19. Gwinnett, A.J., and Matsui, A. A study of enamel ad hesives. The physical relationship between enamel and ad hesive. Arch Oral Biol 12:1615 Dec 1967. 20. Buonocore, M.G.; Matsui, A.; and Gwinnett, A.J. Pene tration of resin dental materials into enamel with reference to bonding. Arch Oral Biol 13:61 Jan 1968.