The effect of etch duration on the microstructure of molar enamel: An in vitro study

The effect of etch duration on the microstructure of molar enamel: An in vitro study Chris D Johnston, BDS, BSc, FDS, a David L Hussey, BDS, Donald J Burden, BDS, MSc, FDS, D.Orth, M.Orth. b Be,~ast, Northern Ireland

FDS, a

and

This in vitro study evaluated the influence of varying etch time on the microstructure of molar buccal enamel. The buccal surfaces of four groups of extracted first molar teeth were etched for 15, 30, 45, and 60 seconds. The etch patterns obtained were photographed with a scanning electron microscope and rated with a 3-grade scale. The 15-second etch time failed to produce any optimal etch patterns. The best etch patterns were found in molars etched for 60 seconds. (Am J Orthod Dentofac Orthop 1996; 109:531-4.)

T h e introduction of the acid-etch bonding technique 1 and the subsequent development of resin-bonded brackets 2 represents one of the most important advances in orthodontics. Direct bonding of orthodontic brackets to incisors, canines, and premolars is now routine. Bonding of brackets to molar teeth however, is less commonly used. Factors such as the need to use headgear and the height of the clinical crown can sometimes influence the decision to band or bond. 3 Many clinicians still prefer to use bands on molar teeth for all their patients, even though the use of bonded molar attachments favors the maintenance of periodontal health, 3 reduces chairside time, and reduces the risk of decalcification. 4 Doubts exist about the adequacy of the bond strengths attained when bonding to molar teeth. Indeed, clinical studies have found that there is a higher failure rate among brackets bonded to posterior teeth than anterior teeth. 5'6 Several theories have been put forward to explain this increased incidence of bond failure in molar teeth, including the increased masticatory forces found in the molar region, greater difficulty in maintaining a dry field, and differences in etch patterns between anterior and posterior teeth. 7 The prismless enamel often found (extending to a depth of 13 to 20 Ixm) on the cervical two-thirds of molar teeth is known to be unsuitable for the formation of an ideal etch pattern. 8-1~ It has b e e n suggested that an etching time of at least 1 minute is required to dissolve this outer prismless enamel ~'bDivision of Restorative Dentistry and Orthodontics, School of Clinical Dentistry, The Queen's University of Belfast. Copyright © 1996 by the American Association of Orthodontists. 0889-5406/96/$5.00 + 0 8/1/62007

layer, thereby exposing the underlying enamel prism structure and allowing effective etching to take place. 8 Although several investigations have examined the relationship between etching times and etch patterns for anterior teeth and premolars, 11-13 no such studies have b e e n undertaken in relation to molars. This study aimed to evaluate the influence of varying etch times on the microstructure of molar buccal enamel. MATERIALS AND METHOD Sixty recently extracted unfluoridated first molars with undamaged and unrestored buccal surfaces were used in this study. After extraction, the teeth were washed with water and stored in formalin as described by Gerbo et a l . ~4 The crowns of the molars were sectioned from their roots and then were mounted on standard scanning electron microscope aluminium stubs with epoxy resin. The molar crowns were orientated with their buccal surfaces uppermost. Each buccal surface was cleaned with a pumice/water slurry for 30 seconds, then washed vigorously with water and allowed to air dry. The area to be etched on each tooth was demarcated with adhesive tape, leaving an exposed region of enamel 3 mm in diameter. This exposed enamel was centered on the midpoint of the vertical long axis of the clinical crown, representing the area normally covered by the molar bracket base. 15 The sample of molars was randomly divided into four groups of 15 teeth each, and the exposed buccal enamel of each group was etched for 15, 30, 45, or 60 seconds with 37% phosphoric acid gel (3M Corp., St. Paul, Minn.). In each case the etchant gel was applied with a brush, and after the appropriate time interval each tooth was thoroughly washed for 30 seconds with water and then allowed to air dry. 531

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Fig. 1. SEM photomicrograph showing poor etch pattern.

American Journal of Orthodontics and Dentofacial Orthopedics May 1996

Fig. 3. SEM photomicrograph showing good etch pattern.

the examined area showing Type 1 or Type 2 etching. 3. Good etch pattern (Fig. 3). Type 1 or Type 2 etch pattern in nearly all the area examined. After a 4-week interval, the evaluations were repeated by the two observers, and the etch pattern was calculated by using the mode of the four readings. 12 The grades awarded for each molar were then collated and the significance of the differences in etch pattern between each etch-time group was assessed with the Mann-Whitney U test. RESULTS

Fig. 2. SEM photomicrograph pattern.

showing moderate etch

The etched specimens were prepared for scanning electron microscopy (SEM) by sputter coating with gold to a thickness of 10 rim. Viewing was carried out with a Jeol JSN840 SEM (JEOL UK, Ltd., Welevyn Garden City, U.K.), operated at 10 kV. One photomicrograph at 1200 times magnification was recorded from the central region of each etched area with a standardized orientation technique to ensure uniformity between specimens. Photomicrographs were then submitted to two independent assessors (D.J.B. and D.L.H.) who scored the quality of each etch pattern in a blind manner, without knowledge of the etching time, using the following 3-grade scale: 1. Poor etch pattern (Fig. 1). Smooth or amorphous surface no evidence of "Type 1" etch pattern (preferential dissolution of enamel prism cores) or "Type 2" pattern (dissolution of prism boundaries) in the area examined? 6 2. Moderate etch pattern (Fig. 2). At least 50% of

After 15 seconds of etching, none of the specimens showed a satisfactory etching pattern (Fig. 4). Nine (60%) of the specimens etched for 15 seconds showed an amorphous or smooth enamel surface, with virtually no evidence of favorable surface roughening suitable for resin bonding. The remaining six (40%) molars that were etched for 15 seconds showed a moderate etch pattern. After 30 seconds of etching, five (33%) teeth showed good etching with the classical Type 1 etching pattern evident in almost all of the region examined. However, the majority of teeth in this group only showed a moderate etch result, and two teeth showed poor etch pattern (Fig. 4). In the 45-second group, 10 (67%) teeth were found to have good enamel etch pattern, and only 5 (33%) teeth were found to have poor or moderate etch patterns. The group of molars etched for 60 seconds had nine (60%) teeth with a good etch pattern, and six (40%) teeth with moderate enamel etch pattern. None of the molars etched for 60 seconds were

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Number of teeth 12

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Etch pattern [] poor

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[] moderate

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[] good

876543210-

15 seconds

30 seconds

45 seconds

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Fig. 4. Etch patterns recorded for different etch durations.

categorized as having a poor etch pattern (Fig. 4). When all the etch-time groups were compared, the molars etched for 15 seconds were found to differ significantly from all the other etch-time groups. The quality of the etch pattern recorded in the molars etched for only 15 seconds was significantly worse than the 30-second group (p < 0.01), the 45-second group (p < 0.001), and the 60-second group (p < 0.001). Although the molars etched for 45 seconds and 60 seconds had a greater proportion of teeth showing good etch quality than the 30-second group, the differences were not statistically significant. DISCUSSION

Since tile introduction of the acid-etch technique, the recommended etch times have been gradually reduced. Brannstr6m and Nordenvall, 17 who used unspecified permanent teeth, found little difference in the microscopic appearance of enamel prepared for resin restoration after etching for 15 seconds and 120 seconds with 37% phosphoric acid solution. In vitro ls'~9 and in vivo 2°-22studies that used nonmolar teeth have shown that an etch duration of 15 seconds is adequate for orthodontic purposes. Interest in reducing etch times in orthodontics has been stimulated by the theory that minimizing etch time may decrease the amount of fluoride-rich surface enamel lost as a result of the etching process, and therefore lessen the risk of decalcification during orthodontic treatment. 23 It is possible that the use of formalin as a storage medium in the present study may have altered the enamel proteins to a small extent. However, we be-

lieve this is unlikely to significantly alter the etch patterns obtained. Formalin has been used as a storage medium in similar studies. 14 In addition, the teeth were thoroughly cleaned and washed before etching. The results indicate that an etch time of 15 seconds may not be appropriate for first molars, as this etch time is less likely to produce an etch pattern suitable for bonding orthodontic brackets. Type f or Type 2 etch patterns are considered to be the most suitable for mechanical retention. 8 When the etch time was increased to 30 seconds, a significantly higher proportion of good etch patterns was noted. Increasing the etch time for an additional 30 seconds to 60 seconds did not significantly increase the proportion of molars with good etch patterns, but no poor etch patterns were found on molars etched for 60 seconds. This finding appears to confirm McLaughlin's hypothesis 8 that the prismless enamel found on molars acts as a barrier to the development of a normal etch pattern and an etch time of at least 60 seconds is required to remove this prismless enamel to allow a more favorable etch pattern to develop. Further evidence to support the use of a longer etch time in molars can be found in the work of Knoll et al. 7 who compared the shear force required to dislodge brackets (bonded with a 60second etch time) from incisors and from molars. Although the mean shear force needed to dislodge brackets from the incisors was greater than for the molars, the failures for both anterior and posterior teeth at the resin/tooth interface occurred within the same range of loads. The authors concluded

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that the 60-second etch time used on the molars produced enamel/resin bond strengths similar to those found in anterior teeth. The lower shear bond strengths of molar brackets in the study by Knoll et al. were due to failures occurring within resin or at the resin/bracket interface. The authors attributed the reduced bond strength at these sites to the variation in thickness of the resin layer beneath the molar brackets. Knoll et a l . 7 also concluded that the differences between anterior and posterior teeth in their enamel microstructure after acid conditioning did not necessarily affect the shear bond strengths. However, these conclusions were based on a 60-second etch time. This study indicates that when the etch time is reduced to 15 seconds, the etch pattern obtained in molars may not be ideal for mechanical retention. Although the use of a longer etch time is likely to reduce the failure rate associated with molar bonds, other factors are likely to prevent molar bonding from being as successful as bonding to anterior teeth. These include difficulties in maintaining a dry field, greater masticatory forces, and the nonuniformity of the adhesive layer beneath molar brackets. CONCLUSION

This in vitro study found no poor etch patterns when a 60-second etch time was used, but the maintaining of a suitably isolated field in vivo for this period may be difficult. Nonetheless, it is recommended that the clinician use at least a 30-second etch time when bonding to molars. This etch time would appear to be sufficient since no significant differences were detected in the proportion of good etch patterns when the etch time was increased beyond 30 seconds. REFERENCES 1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34:849-53. 2. Newman GV. Bonding plastic orthodontic attachments to tooth enamel. J N J Dent Soc 1964;35:346-58. 3. Boyd RL, Baumrind S. Periodontal considerations in the use of bonds or bands on molars in adolescents and adults. Angle Orthod 1992;62:117-26. 4. Retief DH, Sadowsky P L Clinical experience with the acid etch technique in orthodontics. Am J Orthod 1975;68:64554. 5. Gorelick L. Bonding: the state of the art a national survey. J Clin Orthod 1979;13:39-53. 6. Zachrisson BU. A postoperative evaluation of direct bonding in orthodontics. Am J Orthod 1977;73:173-89.

American Journal of Orthodontics and Dentofacial Orthopedics May 1996 7. Knoll M, Gwinnett AJ, Wolff MS. Shear strength of brackets bonded to anterior and posterior teeth. Am J Orthod 1986;89:476-9. 8. McLaughlin G. Direct bonded retainers. Philadelphia: JB Lippincott, 1986:7-20. 9. Gwinnett AJ. Acid etching for composite resins. Dent Clin North Am 1981;25:271-89. 10. Gwinnett AJ. Structure and composition of enamel. Oper Dent 1992;5:10-7. 11. Arakawa Y, Takahashi Y, Sebata M. The effect of acid etching on the cervical region of the buccal surface of the human premolar, with special reference to direct bonding techniques. Am J Orthod 1979;76:201-8. 12. Oliver RG. The effects of differing acid concentrations, techniques and etch time on the etch pattern of enamel of erupted and unerupted human teeth examined using the Scanning Electron Microscope. Br J Orthod 1988;15:45-9. 13. Silverstone LM, Saxton CA, Dogon IL, Fejerskov O. Variation in the pattern of acid etching of human dental enamel examined by Scanning Electron Microscopy. Caries Res 1975;9:373-87. 14. Gerbo LR, Lacefield WR, Wells BR, Russell CM. The effect of enamel preparation on the tensile bond strength of orthodontic composite resin. Angle Orthod 1992;62:275-81. 15. "A" Company. Bracket placement guide. San Diego: "A" Company, 1986. 16. Galil KA, Wright GZ. Acid etch patterns on buccal surfaces of permanent teeth. Pediatr Dent 1979;1:230-4. 17. Brannstrm M, Nordenvall KJ. The effect of acid etching on enamel, dentin, and the inner surface of the resin restoration: a scanning electron microscope investigation. J Dent Res 1977;56:917-23. 18. Jacobs G, Kuftinec MM, Showfety KJ, von Fraunhofer JA. Bonding characteristics of impacted versus erupted permanent teeth. Am J Orthod 1986;89:242-5. 19. Wang WN, Lu TC. Bond strengths with various etching times on young permanent teeth. Am J Orthod Dentofac Orthop 1991;100:72-9. 20. Carstensen W. Clinical results after direct bonding of brackets using shorter etching times. Am J Orthod 1986;89:70-2. 21. Sadowsky PL, Retief DH, Cox PR, Hernandez-Orsini R, Rape WG, Bradley EL. Effect of etchant concentration and duration on the retention of orthodontic brackets: an in vivo study. Am J Orthod Dentofac Orthop 1990;98:417-21. 22. Kinch AP, Taylor H, Warltier R, Oliver RG, Newcombe RG. A clinical trial comparing the failure rates of directly bonded brackets using etch times of 15 or 60 seconds. Am J Orthod Dentofac Orthop 1988;94:476-83. 23. Legler LR, Retief DH, Bradley EL, Denys FR, Sadowsky PL. Effects of phosphoric acid concentration and etch duration on the shear bond strength of an orthodontic bonding resin to enamel. Am J Orthod Dentofac Orthop 1989;96:485-92.

Reprint requests to: Dr. Donald J. Burden Orthodontic Division School of Clinical Dentistry The Queen's University of Belfast Royal Victoria Hospital Grosvenor Rd Belfast BT12 6BP Northern Ireland, UK