Effect of a fluoridated etching gel on enamel morphology and shear bond strength of orthodontic brackets Franklin Garcia-Godoy, DDS, MS,* Gregory W. Hubbard, DDS,** and Arthur T. Storey, DDS, PhD*** San Antonio, Texas, and Oklahoma City, Okla.
The purpose of this study was to compare the enamel morphology and shear bond strengths of orthodontic brackets bonded to enamel etched with a fluoridated or a nonfluoridated phosphoric acid gel. Ten extracted third molars were used for the enamel morphology evaluation. The buccal surfaces were divided, in two sections separated by an occlusogingival groove. One side was etched for 60 seconds with a 38% phosphoric acid gel, and the other side was etched with a 60% phosphoric acid gel containing 0.5% sodium fluoride. The specimens were then examined with a scanning electron microscope. For the shear bond strength tests, 30 extracted third molars were used. The bdccal enamel surfaces of the mesial cusp were used, and the teeth were randomly divided into two groups of 15 teeth each--group 1, 38% phosphoric acid gel for 60 seconds; group 2, 60% phosphoric acid gel with 0.5% sodium fluoride for 60 seconds. Orthodontic lingual buttons were bonded to the etched surfaces with a composite resin. The results showed that the overall morphologic etching effect was similar in both groups. The mean shear bond strength for group 1 was 11.8 MPa __ 4.2, and for group 2 it was 16.5 MPa --- 5.1. This difference was statistically significant (p < 0.01). (AMJ ORTHOD DENTOFACORTHOP 1991 ;100:163-70.)
D i r e c t bonding of orthodontic brackets to etched enamel is widely used by orthodontists and pediatric dentists. Patients who undergo orthodontic treatment are susceptible to caries and to enamel decalcification under and around orthodontic bands and brackets, ~'7 possibly because the etched enamel surface is more susceptible to caries attack 68 and because of the difficulty in maintaining good oral hygiene. 4'9:° Several methods have been proposed to increase the resistance of the enamel to caries attack; these include topical applications of fluoride before etching, n't2 incorporation of fluorides in the etching solutions, t3"t4 topical application o f fluorides to the etched enamel surfaces before bonding, tst8 and topical applications of fluorides after etching) 9 Phosphoric acid containing fluoride has shown significant enamel fluoride uptake t4,2°.2t and more resistant enamel-to-acid dissolution.:' The bond strengths of *Professor, Department of Pediatric Dentistry, University of Texas Health Science Center at San Antonio. **Former senior dental student, University of Texas tlealth Science Center at San Antonio; at present orthodontics postdoctoral student. University of Oklahoma. ***Professor and Chairman. Department of Orthodontics, University of Texas Health Science Center at San Antonio. 811123632 -
sealants-to-enamel surfaces etched with phosphoric acid containing stannous 23 or sodium t4 fluorides provided adequate bond strength in direct bonding procedures. The purpose of this study was to compare the enamel morphology and shear bond strengths of orthodontic brackets bonded to enamel etched with a fluoridated or a nonfluoridated phosphoric acid gel.
MATERIAL AND METHODS Enamel morphology Ten intact extracted third molars were cleaned with flour of pumice applied with a rubber prophylaxis cup on a lowspeed conventional handpiece and stored for 24 hours in deionized water at room temperature. The buccal surfaces were divided in half separated by an occlusogingival groove made with a high-speed fissure bur. One half was etched for 60 seconds with a 38% phosphoric acid gel (Bond-Aid etching gel, Stratford-Cookson Co., West Hempstead, N.Y.) with a pH of 0.9, applied with a disposable brush for 60 seconds according to the manufacturer's instructions. The etchant was then rinsed with a steady stream of deionized water for 20 seconds, and the teeth were dried with oil-free compressed air for 15 seconds. The other half of the buccal surface was etched with a 60% phosphoric acid gel containing 0.5% sodium fluoride (Ortho-Prep, StratfordCookson Co., West Hempstead, N.Y.) with a pH of 1.26. The etchant was applied for 60 seconds, according to the
163
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manufacturer's instructions, and rinsed with a steady stream of deionized water for 20 seconds, after which the teeth were dried with oil-free compressed air for 15 seconds. The specimens were mounted on aluminum stubs, coated with gold/palladium, and examined in a JEOL-35 scanning electron microscope (JEOL Ltd., Tokyo, Japan).
Shear bond strength test Thirty erupted intact extracted human third molars were cleaned with fine flour of pumice applied with a rubber prophylaxis cup on a low-speed conventional handpiece and stored for 24 hours in deionized water at room temperature to prevent dehydration. The buccal enamel surfaces of the mesial cusp were not physically altered (flattened or abraded); this left the natural contours to simulate clinical conditions as closely as possible and facilitated better adaptation of the lingual orthodontic bracket. An area 4.32 mm in diameter was masked with adhesive tape to control the amount of enamel surface ~rea to be subjected to the acid-etch procedure and bracket bonding. A thin layer of nail varnish was placed over the cleansed enamel and the masked area. The varnish was allowed to dry for approximately 15 minutes, and the area under the masking was revealed when the adhesive tape was removed. The area was visually inspected to make sure there were no varnish remnants left on the demarcated sites. The teeth were randomly divided into two groups of 15 teeth each. Group 1. The teeth were cleaned with a paste made of fine flour of pumice and deionized water applie d with a rubber prophylaxis cup and a low-speed conventional handpiece, rinsed for 20 seconds under deionized water, and dried with a mild continuous stream of oil-free compressed air. A 38% phosphoric acid gel with a pH of 0.9 was applied with a disposable brush for 60 seconds, according to the manufacturer's instructions. The etchant was then rinsed with a steady stream of deionized water for 20 seconds, and the teeth were dried with oil-free compressed air for 15 seconds. Group 2. The teeth were prepared as in group I but etched with a 60% phosphoric acid gel containing 0.5% sodium fluoride with a pH o f 1.26. The etchant was applied for 60 seconds, according to the manufacturer's instructions; then the teeth were rinsed with a steady stream of deionized water for 20 seconds and dried with oil-free compressed air for 15 seconds. All 30 teeth were bonded and bracketed in the same manner, according to the manufacturer's instructions. The prepared etched surfaces were coated with a thin layer of unfilled resin (Stratabond primer, Stratford-Cookson Co., West Hempstead, N.Y.). The Stratabond primer used also contained the catalyst required for the polymerization reaction of the resin system. Metal brackets (lingual button, Ormco No. 610-0001, Glendora, Calif.) were bonded to the conditioned enamel surfaces according to the manufacturer's instructions. The mesh surfaces on the enamel side of the brackets were also covered with a thin layer of Stratabond primer unfilled resin. Each bracket was bonded with equal increments of Stratabond bondin~ resin. The increment of resin was measured, by volume, to approximately 4.6 mm ~ to maintain a
Am. J. Orthod. Dentofac. Orthop. August 1991
consistent amount of bonding agent among all specimens. The increment of the filled resin was placed in the less viscous primer on the mesh back of the bracket to be bonded. The two materials, primer (catalyst and unfilled resin) and filled resin, were mixed thoroughly for 20 seconds. The conditioned enamel surface with the primer coating was approximated with the bracket containing the mixed resin bonding system. For a consistent force application among all tooth specimens during resin polymerization a mounting device was used. This force application was maintained for 2 minutes. The teeth were then stored at room temperature in a humid environment for a 24-hour period. All teeth, with brackets bonded, were thermocycled for 100 cycles. Each cycle consisted of two 30second exposures to cold deionized water (6° C) and two 30second exposures to hot deionized water (60° C). After thermocycling, the teeth were immediately mounted in a position that allowed the shearing force to be exactly perpendicular to the bonded bracket. Because of the button head on each bracket, it was difficult to approximate the tooth to the perpendicular surface of the mounting device. Therefore a grommet was slipped over the bracket that would give the best possible surface approximation I~etween the tooth and the perpendicular mounting surface. The grommet was a round cylinder that had only a slightly larger diameter than the bonding area of the bracket. Once the tooth specimen was secured on the mount with a rubber band, PVC segments of 3/4-inch diameter were cut approximately 25 mm in height to retain the molten metal used to embed the specimens. The teeth were mounted with their roots embedded in a stable molten metal material (Cerro-bend, Cerro-Metals, Pittsburgh, Pa.) that allowed the plane of the mounted brackets to be perpendicular to the shear force exerted on them by the Instron universal testing machine (Instron Corp., Canton, Mass.). The metal used had a low melting point and a fusing temperature around 85° C, enabling the mounting of teeth with temperatures lower than that of boiling water. With the specimens secured in place, the PVC retaining rings were held in place with modeling clay. The clay stabilized the ring and also formed a seal that prevented molten metal from leaking under the mount during the procedure. Because of the short time required to mount the specimens before shearing them (5 to 10 minutes), no adverse effect on polymerization or hydration state of the bonded assembly should have been expected. On completion of the mounting procedure, the rings with the teeth and brackets were placed on the Instron at a crosshead speed of 0.5 mm/minute with a knife-edged blade. Each specimen was prevented from dehydration during the testing procedure by storage in a humid environment. The results were recorded in megapascals, and a t test was used to evaluate the statistical significance.
RESULTS Enamel morphology Variations occurred in the type of etching patterns observed, but the overall etching effect was similar in both groups. Crystallites were clearly shown in the prism cores in both groups showing typical type 2 etching patterns (Figs. 1 and 2).
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Fig. 1. Conventional 38% phosphoric acid enamel conditioning displaying typical type 2 etching pattern.
Fig. 2. Fluoridated 60% phosphoric acid enamel conditioning displaying typical type 2 etching pattern.
Shear bond strength test The descriptive statistics for the shear bond strength of the orthodontic brackets bonded with the conventional and fluoridated acid etchants are presented in Table I. The results of the t test indicated that the shear bond strengths were significantly higher for the fluoridated etchant (p < 0.01). Scanning electron microscopy (SEM) observations of the orthodontic button mesh bases showed that in both groups 1 and 2, most of the specimens failed partly within the resin and at the etched enamel interface (Figs. 3 to 8). However, group 1 showed more specimens failing predominantly at the resin-tooth interface,
whereas group 2 revealed more failures occurring at the resin-bracket interface (Figs. 9 and 10). As an average, group 1 showed a failure pattern of 40% at the metalresin interface and 60% at the resin-tooth interface. In group 2, the average failure pattern was 60% at the metal-resin interface and 40% at the resin-tooth interface. The micrographs showed that in specimens failing at the metal-resin interface of both groups, the resin fully penetrated the mesh with a close relationship between the resin and the mesh. A few large air voids were observed in some specimens, presumably the result of the placement of the cement. One specimen in both groups 1 and 2 fractured
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Am. J. Orthod. Dentofac. Orthop. August 1991
Fig. 3. SEM of bracket mesh showing 0% bond failure (at bracket-tooth interface). Line = 1 mm. (Original magnification, x20.)
Fig. 4. SEM of bracket mesh showing 10% bond failure. Line = 1 mm. (Original magnification, x 20.)
Table I. Shear b o n d strengths to e n a m e l etched with the c o n v e n t i o n a l and fluoridated etching gels Group
No.
(MPa)
(MPa)
Range
Noniquoridated Fluoridated
15 15
11.8 16.5*
4.2 5.1
4.29-20. ! 1 9.90-29.93
*p < 0.01.
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Fig. 5. SEM of bracket showing 50% bond failure. Line = 1 mm. (Original magnification, x 20.)
Fig. 6. SEM of bracket showing 100% bond failure (at composite-tooth interface). Line = 1 ram. (Original magnification, ×20.)
in enamel at 10.70 MPa and 9.88 MPa, respectively.
DISCUSSION Treatment with fluorides before as well as after etching has been proposed to strengthen the enamel by reducing its solubility. The addition of fluoride to acidetching solution Was suggested as a measure to prevent white spot and carious lesions around orthodontic brackets, 24as the etched enamel surface has been shown to be a site of increased absorption of outside substances, such as glycoproteins and microorganisms, and
is more susceptible to caries attack. 3"25 The fluoride reacts with the enamel forming calcium fluoride and fluoroapatite, which act as slow-releasing agents, enhancing the remineralization of the etched enamel and making it more resistant to acid dissolution .26 However, the formation of reaction products on enamel surface as a result of acidic fluoride treatments, mainly calcium fluoride, 27"28 has been reported to reduce resin bond strength. ~6"29Other studies have shown that the incorporation of low amounts of fluoride to phosphoric acid solutions, ~'3° or application of fluoride acid solutions after acid etching before placement of resin, t9 did not
168
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Am. J. Orthod. Dentofac. Orthop. August 1991
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significantly influence the bond strength of the adhesive material to the enamel surface. In the present study, the enamel etched with 60% phosphoric acid containing 0.5% sodium fluoride had more similar morphologic characteristics than the enamel etched with the 38% phosphoric acid gel. Similar results were reported by Takahashi et al. 3° after the addition of ~odium fluoride containing less than
0.2% to 30% phosphoric acid solution and by Thornton et al. 14 with a 50% phosphoric acid solution and concentrations of sodium fluoride up to 0.9%. In this study, the 60% phosphoric acid etching gel containing 0.5% sodium fluoride provided a statistically significantly higher bond strength than the phosphoric acid gel that contained no fluoride. Other studies have shown that the addition of 1.23% fluoride to a phos-
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Fhtoridated etching gel and bracket bonding
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Fig. 9. Percent of metal Showing on brackets after conditioning with 38% phosphoric acid.
Fig. 10. Percent of metal showing on brackets after conditioning with fluoridated 60% phosphoric acid.
phoric acid solution significantly reduced the bond strength of a pit and fissure sealant to etched enamel) ~ Another study reported that the addition of 0.5% sodium fluoride (0.22% fluoride) to 50% phosphoric acid did not adversely affect the tensile bond strength of a pit and fissure sealant, but that the addition of 2.0% sodium fluoride (0.9~o fluoride) resulted in a significant decrease in bond strength when compared with the bond strengths obtained on enamel surfaces etched with 50% phosphoric acid gel that contained no fluoride)-" Takahashi et al. 3° noticed that the addition of 0.02% sodium fluoride (0.01% fluoride) to 30% phosphoric acid did not result in a significant reduction of the tensile bond strengths of a pit and fissure sealant when compared with bond strengths recorded on enamel surfaces etched with 30% phosphoric acid containing no fluoride. The addition of 0.2% sodium fluoride (0.09% fluoride), however, resulted in a significant reduction in bond strength. Our study showed that the addition of 0.5% sodium fluoride to the 60% phosphoric acid gel produced significantly higher bond strengths than the 38% phosphoric acid gel without fluoride. The reason for the discrepancy a m o n g the studies could be that different materials, techniques, and phosphoric acid concentrations with different pH were used. The higher phosphoric acid concentration in the present study could account for the finding that the 0.5% fluoride concentration did not adversely compromise the bond strength. The sodium fluoride could have acted as a buffering agent, making the phosphoric acid action milder, as the
pH of the fluoridated gel was 1.26 and that of the nonfluoridated gel was 0.9. Conditioning the enamel for 1 minut~ with 50% phosphoric acid forms monocalcium phosphate monohydrate on the surface) 3 Because of its high solubility in water, the monocalcium phosphate monohydrate coating would be completely washed away in the clinical situation, which would leave a clean surface available for sealant adhesion. 33 Chow and Brown 33 noted that a concentrated acid solution of approximately 70% dissolved considerably less apatite than a more dilute solution of 37%. When the more concentrated acid solutions were used, early formation of monocalcium phosphate monohydrate seemed to protect the teeth from excessive dissolution. The observation that concentrated phosphoric acid solutions resulted in less etching of enamel than did less concentrated solutions was also reported by Gwinnett and Buonocore. 34 In the present study, the two etching gel solutions were selected because one is a conventional conditioning agent and the other one is a new product marketed for bonding in othodontics. The phosphoric acid concentration was higher in the fluoridated gel used in our study, but the shear bond strengths were higher. Retief 35 has shown that etching the enamel with 10% to 45% phosphoric acid produced a moderate increase in surface roughness, whereas a dramatic increase in surface roughness was obtained with 50% to 60% phosphoric acid. This is of interest because an important feature in retention is not the overall depth of etch produced by the conditioning agent but the surface roughness of
170
Garcia-Godoy, Hubbard, and Store)'
the enamel surface resulting from the etching procedure. The increased roughness will create a larger surface area for bonding. The stronger bond obtained with the fluoridecontaining etching gel was also corroborated with the SEM examination of the orthodontic bracket mesh after debonding. More specimens in this group failed predominantly at the resin-bracket interface, whereas in the nonfluoridated etching gel most failures occurred at the resin-tooth interface. Further studies should be conducted with the fluoride-containing etching solutions to determine their clinical effectiveness. REFERENCES I. Noyes HJ. Dental caries and the orthodontic patient. J Am Dent Assoc 1937;24:1243-54. 2. Shannon IL. Caries risk in teeth with orthodontic bands: a review. Gen Dent 1972;20:24-8. 3. Stratemann MW, Shannon IL. Control of decalcification in orthodontic patients by daily self-administered application of a waterfree 0.4 percent stannous fluoride gel. AM J O~rHOD 1974;66: 273-9. 4. Zachrisson BU. Cause and prevention of injuries to teeth and supporting structures during orthodontic treatment. AMJ ORTHOD 1976;69:285-300. 5. Zachrisson BU. A postreatment evaluation of direct bonding in orthodontics. AM J ORa'rlOD 1977;71:173-89. 6. Lehman R, Davidson CL, Duijsters PPE. In vitro studies on susceptibility of enamel to caries attack after orthodontic bonding procedures. AM J ORTHOD 1981;80:61-72. 7. Artun J,~ Brobakken BO. Prevalence of carious white spots after orthodontic treatment with multibanded appliances. Eur J Orthod 1986;8:229-34. 8. Takahasbi Y, Matsukubo T, Takeuchi M. Effect of pretreatmeut with fluoride-containing ethanol on etched enamel surface as evaluated by electron probe microanalyzer. Bull Tokyo Dent Coll 1977;18:195-200. 9. Gwinnett AJ, Ceen F. Plaque distribution on bonded brackets. AM J ORTHOD 1979;75:667-77. i0. Ciancio SG, Cunat JJ, Mather ML, Harvey DH. A comparison of plaque accumulation in bonded versus banded teeth. J Dent Res 1985;64:359. 11. Low T, yon Fraunhofer JA, Winter GB. The bonding of a polymeric fissure sealant to topical fluoride-treated teeth. J Oral Rehabil 1975;2:303-7. 12. Brannstrom M, Nordenvall KJ, Malmgre n O. The effect of various pretreatment methods of the enamel in bonding procedures. A.~I J ORTHOD 1978;74:522-30. 13. Takahashi Y, Arakawa Y, Matsukubo T, Takeuchi M. The effect of sodium fluoride in acid etching solution on sealant bond and fluoride uptake. J Dent Res 1980;59:625-30. 14. Thornton JB, Retief DH, Bradley EL Jr, Denys FR. The effect of fluoride in phosphoric acid on enamel fluoride uptake and the tensile bond strength of an orthodontic bonding resin. AM J OR'mOP DE,'CrOFACOR'ntOP 1986;90:91-101. 15. Hirce JD, Sather AH, Chao EYS. The effects of topical fluorides after acid etching of enamel on the bond strength of directly bonded orthodontic brackets. AM J ORxrtOD 1980;78:444-52. 16. Sheykholeslam Z, Buonocore MG, Gwinnett AJ. Effect of fluorides on the bonding of resins to phosphoric acid-etched bovine enamel. Arch Oral Biol 1972;17:1037-45.
Am. J. Orthod. Dentofac. Orthop. August 1991 17. Takahashi Y, Otsuki A, Takeuchi M. Effect of pretreatment with ethanol containing fluoride on etched enamel surface as evaluated by scanning electron microscopy and tensile bond strength measurement. J Dent Res 1977;56:588-94. 18. Rowland GF, Yates JL, Hembree JH, McKnight JP. The influence of a topical stannous fluoride application on the tensile bond strengt h of pit and fissure sealants. J Pedod 1979;4:9-20. 19. Bishara SE, Chan D, Abadir EA. The effect of the bonding strength of orthodontic brackets of fluoride application after etching. AM J ORTHODDENTOFACORTHOP 1989;95:259-60. 20. Kochavi D, Gedalia I, Analse J. Effect of conditioning with fluoride and phosphoric acid on enamel surfaces as evaluated by scanning electron microscopy and fluoride incorporation. J Dent Res 1975;54:304-9. 21. Gedalia I, Baharev H, Kochavi D. X-ray diffraction and fluoride analysis of enamel powder treated with concentrated phosphoric acid containing sodium fluoride. J Oral Rehabil 1980;7:471-4. 22. Bohrer J, Gedalia I. Fluoride concentration in enamel treated with 50% phosphoric acid and NaF with subsequent decalcification in "acid gel." J Dent Res 1980;59:1022-5. 23. Freeman JE, Shannon IL. Addition of stannous fluoride to acid etchant in direct bonding procedure. Int J Orthod 1981;19:13-9. 24. Kajander KC, Uhland R, Ophaug RH, Sather AH. Topical fluoride in orthodontic bonding. Angle Orthod 1987;57:70-6. 25. Hicks MJ" Effects ° f acid etching °n caries'like lesi°ns °fenamel [Dissertation]. Iowa City, Iowa: University of Iowa, 1982. 26. Koulourides T, Keller SE, Manson-Hing L, Lilley V. Enhancement of fluoride effectiveness by experimental cari~enic priming of human enamel. Caries Res 1980;14:32-9. 27. Richardson B. Fixation of topically applied fluoride in enamel. J Dent Res 1967;46:87-91. 28. Larsen MJ, Fejerskov O. Structural studies on calcium fluoride formation and uptake of fluoride in surface enamel in vitro. Scand J Dent Res 1978;86:337..45. 29. Gwinnett AJ, Buonocore MG, Sheykholeslam Z. Effect of fluoride on etched human and bovine tooth enamel surfaces as demonstrated by scanning electron microscopy. Arch Oral Biol 1972;17:271-8. 30. Takabashi Y, Arakawa Y, Matsukubo T, Takeuchi M. The effect of sodium fluoride in acid etching solution on sealant bond ~nd fluoride uptake. J Dent Res 1980;59:625-30. 31. Wright FAC, Beck DJ. Prevention of pit and fissure caries. III. Fluoride and resin-enamel bonding. N Z Dent J 1973;69:77-84. 32. Grajower R, Glick A, Gedalia I, Kochavi D. Tensile strengths of bond between resin to enamel etched with phosphoric acid containing fluoride. J Oral Rehabil 1979;6:267-72. 33. Chow IC, Brown WE. Phosphoric acid conditioning of teeth for pit and fissure sealants. J Dent Res 1973;52:1158. 34. Gwinnett AJ, Buonocore MG. Adhesives and caries prevention: a preliminary report. Br Dent J 1965;119:77-80. 35. Retief DH. The use of 50 per cent phosphoric acid as an etching agent in orthodontics: a rational approach. AM J ORTHOD 1975; 68:165-78.
Reprint requests to: Dr. Franklin Garcia-Godoy Department of Pediatric Dentistry University of Texas Health Science Center at San Antonio 7703 Floyd Curl Dr. San Antonio, TX 78284
The purpose of this study was to compare the enamel morphology and shear bond strengths of orthodontic brackets bonded to enamel etched with a fluoridated or a nonfluoridated phosphoric acid gel. Ten extracted third molars were used for the enamel morphology evaluation. The buccal surfaces were divided, in two sections separated by an occlusogingival groove. One side was etched for 60 seconds with a 38% phosphoric acid gel, and the other side was etched with a 60% phosphoric acid gel containing 0.5% sodium fluoride. The specimens were then examined with a scanning electron microscope. For the shear bond strength tests, 30 extracted third molars were used. The bdccal enamel surfaces of the mesial cusp were used, and the teeth were randomly divided into two groups of 15 teeth each--group 1, 38% phosphoric acid gel for 60 seconds; group 2, 60% phosphoric acid gel with 0.5% sodium fluoride for 60 seconds. Orthodontic lingual buttons were bonded to the etched surfaces with a composite resin. The results showed that the overall morphologic etching effect was similar in both groups. The mean shear bond strength for group 1 was 11.8 MPa __ 4.2, and for group 2 it was 16.5 MPa --- 5.1. This difference was statistically significant (p < 0.01). (AMJ ORTHOD DENTOFACORTHOP 1991 ;100:163-70.)
D i r e c t bonding of orthodontic brackets to etched enamel is widely used by orthodontists and pediatric dentists. Patients who undergo orthodontic treatment are susceptible to caries and to enamel decalcification under and around orthodontic bands and brackets, ~'7 possibly because the etched enamel surface is more susceptible to caries attack 68 and because of the difficulty in maintaining good oral hygiene. 4'9:° Several methods have been proposed to increase the resistance of the enamel to caries attack; these include topical applications of fluoride before etching, n't2 incorporation of fluorides in the etching solutions, t3"t4 topical application o f fluorides to the etched enamel surfaces before bonding, tst8 and topical applications of fluorides after etching) 9 Phosphoric acid containing fluoride has shown significant enamel fluoride uptake t4,2°.2t and more resistant enamel-to-acid dissolution.:' The bond strengths of *Professor, Department of Pediatric Dentistry, University of Texas Health Science Center at San Antonio. **Former senior dental student, University of Texas tlealth Science Center at San Antonio; at present orthodontics postdoctoral student. University of Oklahoma. ***Professor and Chairman. Department of Orthodontics, University of Texas Health Science Center at San Antonio. 811123632 -
sealants-to-enamel surfaces etched with phosphoric acid containing stannous 23 or sodium t4 fluorides provided adequate bond strength in direct bonding procedures. The purpose of this study was to compare the enamel morphology and shear bond strengths of orthodontic brackets bonded to enamel etched with a fluoridated or a nonfluoridated phosphoric acid gel.
MATERIAL AND METHODS Enamel morphology Ten intact extracted third molars were cleaned with flour of pumice applied with a rubber prophylaxis cup on a lowspeed conventional handpiece and stored for 24 hours in deionized water at room temperature. The buccal surfaces were divided in half separated by an occlusogingival groove made with a high-speed fissure bur. One half was etched for 60 seconds with a 38% phosphoric acid gel (Bond-Aid etching gel, Stratford-Cookson Co., West Hempstead, N.Y.) with a pH of 0.9, applied with a disposable brush for 60 seconds according to the manufacturer's instructions. The etchant was then rinsed with a steady stream of deionized water for 20 seconds, and the teeth were dried with oil-free compressed air for 15 seconds. The other half of the buccal surface was etched with a 60% phosphoric acid gel containing 0.5% sodium fluoride (Ortho-Prep, StratfordCookson Co., West Hempstead, N.Y.) with a pH of 1.26. The etchant was applied for 60 seconds, according to the
163
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Garcia-Godoy, Hubbard, and Store)'
manufacturer's instructions, and rinsed with a steady stream of deionized water for 20 seconds, after which the teeth were dried with oil-free compressed air for 15 seconds. The specimens were mounted on aluminum stubs, coated with gold/palladium, and examined in a JEOL-35 scanning electron microscope (JEOL Ltd., Tokyo, Japan).
Shear bond strength test Thirty erupted intact extracted human third molars were cleaned with fine flour of pumice applied with a rubber prophylaxis cup on a low-speed conventional handpiece and stored for 24 hours in deionized water at room temperature to prevent dehydration. The buccal enamel surfaces of the mesial cusp were not physically altered (flattened or abraded); this left the natural contours to simulate clinical conditions as closely as possible and facilitated better adaptation of the lingual orthodontic bracket. An area 4.32 mm in diameter was masked with adhesive tape to control the amount of enamel surface ~rea to be subjected to the acid-etch procedure and bracket bonding. A thin layer of nail varnish was placed over the cleansed enamel and the masked area. The varnish was allowed to dry for approximately 15 minutes, and the area under the masking was revealed when the adhesive tape was removed. The area was visually inspected to make sure there were no varnish remnants left on the demarcated sites. The teeth were randomly divided into two groups of 15 teeth each. Group 1. The teeth were cleaned with a paste made of fine flour of pumice and deionized water applie d with a rubber prophylaxis cup and a low-speed conventional handpiece, rinsed for 20 seconds under deionized water, and dried with a mild continuous stream of oil-free compressed air. A 38% phosphoric acid gel with a pH of 0.9 was applied with a disposable brush for 60 seconds, according to the manufacturer's instructions. The etchant was then rinsed with a steady stream of deionized water for 20 seconds, and the teeth were dried with oil-free compressed air for 15 seconds. Group 2. The teeth were prepared as in group I but etched with a 60% phosphoric acid gel containing 0.5% sodium fluoride with a pH o f 1.26. The etchant was applied for 60 seconds, according to the manufacturer's instructions; then the teeth were rinsed with a steady stream of deionized water for 20 seconds and dried with oil-free compressed air for 15 seconds. All 30 teeth were bonded and bracketed in the same manner, according to the manufacturer's instructions. The prepared etched surfaces were coated with a thin layer of unfilled resin (Stratabond primer, Stratford-Cookson Co., West Hempstead, N.Y.). The Stratabond primer used also contained the catalyst required for the polymerization reaction of the resin system. Metal brackets (lingual button, Ormco No. 610-0001, Glendora, Calif.) were bonded to the conditioned enamel surfaces according to the manufacturer's instructions. The mesh surfaces on the enamel side of the brackets were also covered with a thin layer of Stratabond primer unfilled resin. Each bracket was bonded with equal increments of Stratabond bondin~ resin. The increment of resin was measured, by volume, to approximately 4.6 mm ~ to maintain a
Am. J. Orthod. Dentofac. Orthop. August 1991
consistent amount of bonding agent among all specimens. The increment of the filled resin was placed in the less viscous primer on the mesh back of the bracket to be bonded. The two materials, primer (catalyst and unfilled resin) and filled resin, were mixed thoroughly for 20 seconds. The conditioned enamel surface with the primer coating was approximated with the bracket containing the mixed resin bonding system. For a consistent force application among all tooth specimens during resin polymerization a mounting device was used. This force application was maintained for 2 minutes. The teeth were then stored at room temperature in a humid environment for a 24-hour period. All teeth, with brackets bonded, were thermocycled for 100 cycles. Each cycle consisted of two 30second exposures to cold deionized water (6° C) and two 30second exposures to hot deionized water (60° C). After thermocycling, the teeth were immediately mounted in a position that allowed the shearing force to be exactly perpendicular to the bonded bracket. Because of the button head on each bracket, it was difficult to approximate the tooth to the perpendicular surface of the mounting device. Therefore a grommet was slipped over the bracket that would give the best possible surface approximation I~etween the tooth and the perpendicular mounting surface. The grommet was a round cylinder that had only a slightly larger diameter than the bonding area of the bracket. Once the tooth specimen was secured on the mount with a rubber band, PVC segments of 3/4-inch diameter were cut approximately 25 mm in height to retain the molten metal used to embed the specimens. The teeth were mounted with their roots embedded in a stable molten metal material (Cerro-bend, Cerro-Metals, Pittsburgh, Pa.) that allowed the plane of the mounted brackets to be perpendicular to the shear force exerted on them by the Instron universal testing machine (Instron Corp., Canton, Mass.). The metal used had a low melting point and a fusing temperature around 85° C, enabling the mounting of teeth with temperatures lower than that of boiling water. With the specimens secured in place, the PVC retaining rings were held in place with modeling clay. The clay stabilized the ring and also formed a seal that prevented molten metal from leaking under the mount during the procedure. Because of the short time required to mount the specimens before shearing them (5 to 10 minutes), no adverse effect on polymerization or hydration state of the bonded assembly should have been expected. On completion of the mounting procedure, the rings with the teeth and brackets were placed on the Instron at a crosshead speed of 0.5 mm/minute with a knife-edged blade. Each specimen was prevented from dehydration during the testing procedure by storage in a humid environment. The results were recorded in megapascals, and a t test was used to evaluate the statistical significance.
RESULTS Enamel morphology Variations occurred in the type of etching patterns observed, but the overall etching effect was similar in both groups. Crystallites were clearly shown in the prism cores in both groups showing typical type 2 etching patterns (Figs. 1 and 2).
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Fig. 1. Conventional 38% phosphoric acid enamel conditioning displaying typical type 2 etching pattern.
Fig. 2. Fluoridated 60% phosphoric acid enamel conditioning displaying typical type 2 etching pattern.
Shear bond strength test The descriptive statistics for the shear bond strength of the orthodontic brackets bonded with the conventional and fluoridated acid etchants are presented in Table I. The results of the t test indicated that the shear bond strengths were significantly higher for the fluoridated etchant (p < 0.01). Scanning electron microscopy (SEM) observations of the orthodontic button mesh bases showed that in both groups 1 and 2, most of the specimens failed partly within the resin and at the etched enamel interface (Figs. 3 to 8). However, group 1 showed more specimens failing predominantly at the resin-tooth interface,
whereas group 2 revealed more failures occurring at the resin-bracket interface (Figs. 9 and 10). As an average, group 1 showed a failure pattern of 40% at the metalresin interface and 60% at the resin-tooth interface. In group 2, the average failure pattern was 60% at the metal-resin interface and 40% at the resin-tooth interface. The micrographs showed that in specimens failing at the metal-resin interface of both groups, the resin fully penetrated the mesh with a close relationship between the resin and the mesh. A few large air voids were observed in some specimens, presumably the result of the placement of the cement. One specimen in both groups 1 and 2 fractured
166
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Am. J. Orthod. Dentofac. Orthop. August 1991
Fig. 3. SEM of bracket mesh showing 0% bond failure (at bracket-tooth interface). Line = 1 mm. (Original magnification, x20.)
Fig. 4. SEM of bracket mesh showing 10% bond failure. Line = 1 mm. (Original magnification, x 20.)
Table I. Shear b o n d strengths to e n a m e l etched with the c o n v e n t i o n a l and fluoridated etching gels Group
No.
(MPa)
(MPa)
Range
Noniquoridated Fluoridated
15 15
11.8 16.5*
4.2 5.1
4.29-20. ! 1 9.90-29.93
*p < 0.01.
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Fig. 5. SEM of bracket showing 50% bond failure. Line = 1 mm. (Original magnification, x 20.)
Fig. 6. SEM of bracket showing 100% bond failure (at composite-tooth interface). Line = 1 ram. (Original magnification, ×20.)
in enamel at 10.70 MPa and 9.88 MPa, respectively.
DISCUSSION Treatment with fluorides before as well as after etching has been proposed to strengthen the enamel by reducing its solubility. The addition of fluoride to acidetching solution Was suggested as a measure to prevent white spot and carious lesions around orthodontic brackets, 24as the etched enamel surface has been shown to be a site of increased absorption of outside substances, such as glycoproteins and microorganisms, and
is more susceptible to caries attack. 3"25 The fluoride reacts with the enamel forming calcium fluoride and fluoroapatite, which act as slow-releasing agents, enhancing the remineralization of the etched enamel and making it more resistant to acid dissolution .26 However, the formation of reaction products on enamel surface as a result of acidic fluoride treatments, mainly calcium fluoride, 27"28 has been reported to reduce resin bond strength. ~6"29Other studies have shown that the incorporation of low amounts of fluoride to phosphoric acid solutions, ~'3° or application of fluoride acid solutions after acid etching before placement of resin, t9 did not
168
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Am. J. Orthod. Dentofac. Orthop. August 1991
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significantly influence the bond strength of the adhesive material to the enamel surface. In the present study, the enamel etched with 60% phosphoric acid containing 0.5% sodium fluoride had more similar morphologic characteristics than the enamel etched with the 38% phosphoric acid gel. Similar results were reported by Takahashi et al. 3° after the addition of ~odium fluoride containing less than
0.2% to 30% phosphoric acid solution and by Thornton et al. 14 with a 50% phosphoric acid solution and concentrations of sodium fluoride up to 0.9%. In this study, the 60% phosphoric acid etching gel containing 0.5% sodium fluoride provided a statistically significantly higher bond strength than the phosphoric acid gel that contained no fluoride. Other studies have shown that the addition of 1.23% fluoride to a phos-
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Fhtoridated etching gel and bracket bonding
Number 2
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Fig. 9. Percent of metal Showing on brackets after conditioning with 38% phosphoric acid.
Fig. 10. Percent of metal showing on brackets after conditioning with fluoridated 60% phosphoric acid.
phoric acid solution significantly reduced the bond strength of a pit and fissure sealant to etched enamel) ~ Another study reported that the addition of 0.5% sodium fluoride (0.22% fluoride) to 50% phosphoric acid did not adversely affect the tensile bond strength of a pit and fissure sealant, but that the addition of 2.0% sodium fluoride (0.9~o fluoride) resulted in a significant decrease in bond strength when compared with the bond strengths obtained on enamel surfaces etched with 50% phosphoric acid gel that contained no fluoride)-" Takahashi et al. 3° noticed that the addition of 0.02% sodium fluoride (0.01% fluoride) to 30% phosphoric acid did not result in a significant reduction of the tensile bond strengths of a pit and fissure sealant when compared with bond strengths recorded on enamel surfaces etched with 30% phosphoric acid containing no fluoride. The addition of 0.2% sodium fluoride (0.09% fluoride), however, resulted in a significant reduction in bond strength. Our study showed that the addition of 0.5% sodium fluoride to the 60% phosphoric acid gel produced significantly higher bond strengths than the 38% phosphoric acid gel without fluoride. The reason for the discrepancy a m o n g the studies could be that different materials, techniques, and phosphoric acid concentrations with different pH were used. The higher phosphoric acid concentration in the present study could account for the finding that the 0.5% fluoride concentration did not adversely compromise the bond strength. The sodium fluoride could have acted as a buffering agent, making the phosphoric acid action milder, as the
pH of the fluoridated gel was 1.26 and that of the nonfluoridated gel was 0.9. Conditioning the enamel for 1 minut~ with 50% phosphoric acid forms monocalcium phosphate monohydrate on the surface) 3 Because of its high solubility in water, the monocalcium phosphate monohydrate coating would be completely washed away in the clinical situation, which would leave a clean surface available for sealant adhesion. 33 Chow and Brown 33 noted that a concentrated acid solution of approximately 70% dissolved considerably less apatite than a more dilute solution of 37%. When the more concentrated acid solutions were used, early formation of monocalcium phosphate monohydrate seemed to protect the teeth from excessive dissolution. The observation that concentrated phosphoric acid solutions resulted in less etching of enamel than did less concentrated solutions was also reported by Gwinnett and Buonocore. 34 In the present study, the two etching gel solutions were selected because one is a conventional conditioning agent and the other one is a new product marketed for bonding in othodontics. The phosphoric acid concentration was higher in the fluoridated gel used in our study, but the shear bond strengths were higher. Retief 35 has shown that etching the enamel with 10% to 45% phosphoric acid produced a moderate increase in surface roughness, whereas a dramatic increase in surface roughness was obtained with 50% to 60% phosphoric acid. This is of interest because an important feature in retention is not the overall depth of etch produced by the conditioning agent but the surface roughness of
170
Garcia-Godoy, Hubbard, and Store)'
the enamel surface resulting from the etching procedure. The increased roughness will create a larger surface area for bonding. The stronger bond obtained with the fluoridecontaining etching gel was also corroborated with the SEM examination of the orthodontic bracket mesh after debonding. More specimens in this group failed predominantly at the resin-bracket interface, whereas in the nonfluoridated etching gel most failures occurred at the resin-tooth interface. Further studies should be conducted with the fluoride-containing etching solutions to determine their clinical effectiveness. REFERENCES I. Noyes HJ. Dental caries and the orthodontic patient. J Am Dent Assoc 1937;24:1243-54. 2. Shannon IL. Caries risk in teeth with orthodontic bands: a review. Gen Dent 1972;20:24-8. 3. Stratemann MW, Shannon IL. Control of decalcification in orthodontic patients by daily self-administered application of a waterfree 0.4 percent stannous fluoride gel. AM J O~rHOD 1974;66: 273-9. 4. Zachrisson BU. Cause and prevention of injuries to teeth and supporting structures during orthodontic treatment. AMJ ORTHOD 1976;69:285-300. 5. Zachrisson BU. A postreatment evaluation of direct bonding in orthodontics. AM J ORa'rlOD 1977;71:173-89. 6. Lehman R, Davidson CL, Duijsters PPE. In vitro studies on susceptibility of enamel to caries attack after orthodontic bonding procedures. AM J ORTHOD 1981;80:61-72. 7. Artun J,~ Brobakken BO. Prevalence of carious white spots after orthodontic treatment with multibanded appliances. Eur J Orthod 1986;8:229-34. 8. Takahasbi Y, Matsukubo T, Takeuchi M. Effect of pretreatmeut with fluoride-containing ethanol on etched enamel surface as evaluated by electron probe microanalyzer. Bull Tokyo Dent Coll 1977;18:195-200. 9. Gwinnett AJ, Ceen F. Plaque distribution on bonded brackets. AM J ORTHOD 1979;75:667-77. i0. Ciancio SG, Cunat JJ, Mather ML, Harvey DH. A comparison of plaque accumulation in bonded versus banded teeth. J Dent Res 1985;64:359. 11. Low T, yon Fraunhofer JA, Winter GB. The bonding of a polymeric fissure sealant to topical fluoride-treated teeth. J Oral Rehabil 1975;2:303-7. 12. Brannstrom M, Nordenvall KJ, Malmgre n O. The effect of various pretreatment methods of the enamel in bonding procedures. A.~I J ORTHOD 1978;74:522-30. 13. Takahashi Y, Arakawa Y, Matsukubo T, Takeuchi M. The effect of sodium fluoride in acid etching solution on sealant bond and fluoride uptake. J Dent Res 1980;59:625-30. 14. Thornton JB, Retief DH, Bradley EL Jr, Denys FR. The effect of fluoride in phosphoric acid on enamel fluoride uptake and the tensile bond strength of an orthodontic bonding resin. AM J OR'mOP DE,'CrOFACOR'ntOP 1986;90:91-101. 15. Hirce JD, Sather AH, Chao EYS. The effects of topical fluorides after acid etching of enamel on the bond strength of directly bonded orthodontic brackets. AM J ORxrtOD 1980;78:444-52. 16. Sheykholeslam Z, Buonocore MG, Gwinnett AJ. Effect of fluorides on the bonding of resins to phosphoric acid-etched bovine enamel. Arch Oral Biol 1972;17:1037-45.
Am. J. Orthod. Dentofac. Orthop. August 1991 17. Takahashi Y, Otsuki A, Takeuchi M. Effect of pretreatment with ethanol containing fluoride on etched enamel surface as evaluated by scanning electron microscopy and tensile bond strength measurement. J Dent Res 1977;56:588-94. 18. Rowland GF, Yates JL, Hembree JH, McKnight JP. The influence of a topical stannous fluoride application on the tensile bond strengt h of pit and fissure sealants. J Pedod 1979;4:9-20. 19. Bishara SE, Chan D, Abadir EA. The effect of the bonding strength of orthodontic brackets of fluoride application after etching. AM J ORTHODDENTOFACORTHOP 1989;95:259-60. 20. Kochavi D, Gedalia I, Analse J. Effect of conditioning with fluoride and phosphoric acid on enamel surfaces as evaluated by scanning electron microscopy and fluoride incorporation. J Dent Res 1975;54:304-9. 21. Gedalia I, Baharev H, Kochavi D. X-ray diffraction and fluoride analysis of enamel powder treated with concentrated phosphoric acid containing sodium fluoride. J Oral Rehabil 1980;7:471-4. 22. Bohrer J, Gedalia I. Fluoride concentration in enamel treated with 50% phosphoric acid and NaF with subsequent decalcification in "acid gel." J Dent Res 1980;59:1022-5. 23. Freeman JE, Shannon IL. Addition of stannous fluoride to acid etchant in direct bonding procedure. Int J Orthod 1981;19:13-9. 24. Kajander KC, Uhland R, Ophaug RH, Sather AH. Topical fluoride in orthodontic bonding. Angle Orthod 1987;57:70-6. 25. Hicks MJ" Effects ° f acid etching °n caries'like lesi°ns °fenamel [Dissertation]. Iowa City, Iowa: University of Iowa, 1982. 26. Koulourides T, Keller SE, Manson-Hing L, Lilley V. Enhancement of fluoride effectiveness by experimental cari~enic priming of human enamel. Caries Res 1980;14:32-9. 27. Richardson B. Fixation of topically applied fluoride in enamel. J Dent Res 1967;46:87-91. 28. Larsen MJ, Fejerskov O. Structural studies on calcium fluoride formation and uptake of fluoride in surface enamel in vitro. Scand J Dent Res 1978;86:337..45. 29. Gwinnett AJ, Buonocore MG, Sheykholeslam Z. Effect of fluoride on etched human and bovine tooth enamel surfaces as demonstrated by scanning electron microscopy. Arch Oral Biol 1972;17:271-8. 30. Takabashi Y, Arakawa Y, Matsukubo T, Takeuchi M. The effect of sodium fluoride in acid etching solution on sealant bond ~nd fluoride uptake. J Dent Res 1980;59:625-30. 31. Wright FAC, Beck DJ. Prevention of pit and fissure caries. III. Fluoride and resin-enamel bonding. N Z Dent J 1973;69:77-84. 32. Grajower R, Glick A, Gedalia I, Kochavi D. Tensile strengths of bond between resin to enamel etched with phosphoric acid containing fluoride. J Oral Rehabil 1979;6:267-72. 33. Chow IC, Brown WE. Phosphoric acid conditioning of teeth for pit and fissure sealants. J Dent Res 1973;52:1158. 34. Gwinnett AJ, Buonocore MG. Adhesives and caries prevention: a preliminary report. Br Dent J 1965;119:77-80. 35. Retief DH. The use of 50 per cent phosphoric acid as an etching agent in orthodontics: a rational approach. AM J ORTHOD 1975; 68:165-78.
Reprint requests to: Dr. Franklin Garcia-Godoy Department of Pediatric Dentistry University of Texas Health Science Center at San Antonio 7703 Floyd Curl Dr. San Antonio, TX 78284
