Effects of etchant concentration and duration on the retention of orthodontic brackets: An in vivo study P. L. Sadowsky, DMD, BDS, MDent, ° D. H. Retief, MSc, BDS, PhD(Dent), DSc(Odont), b P. R. Cox, DMD, c R. Herndndez-Orsini, DMD, MPH, d W. G. Rape, DDS, ° and E. L. Bradley, PhD'
Birmhzgham, Ala. Etching of enamel before the bonding of orthodontic attachments is usually done with a solution of 37% H3PO, for 60 seconds. The purpose of this study was to evaluate the effects of etchant concentration and duration on the clinical retention of bonded orthodontic attachments. Two randomly selected groups of orthodontic patients participated in the study. In both groups, teeth in contralateral quadrants, excluding the molar teeth, were bonded. In the first group, 158 teeth were etched with a solution of 37% H3PO, for 60 seconds, and 155 teeth were etched with 37% H3PO, for 15 seconds. In the second group, 196 teeth were etched with 37% H3PO, and 196 teeth with 15% H~PO,, both for 60 seconds. Conventional edgewise mechanotherapy was used in both groups of patients. After 24 months of treatment, in the first group, 8 brackets were dislodged when etched with 37% H3PO, for 60 seconds and 9 when etched with 37% H3PO, for 15 seconds. In the second group, 6 brackets were dislodged when etched with 37% H3PO,, for 60 seconds and 13 were dislodged when etched with 15% H3PO, for 60 seconds. Reducing the etching time of 37% H3PO, from 60 seconds to 15 seconds or reducing the acid concentration from 37% to 15% H3PO, applied for 60 seconds had no significantly different effect on the retention of bonded orthodontic attachments. The results suggest that the reduction of etchant concentration and the duration of etching in orthodontic bonding should be considered. (AMJ ORTHOD DENTOFACORTHOP 1990;98:41721 .)
B o n d i n g of brackets to acid-etched enamel has become a universally accepted technique in clinical orthodontics. Buonocore, ~ in 1955, used 85% orthophosphoric acid (H3PO4) for 30 seconds and reported that the bond strength of acrylic restorative resins was significantly increased by etching of the enamel surface. Etching of enamel with H3PO~ results in a superficial etched zone and a subsurface qualitative and quantitative porous zone, according to Silverstone. z While the enamel from the superficial zone is permanently lost, the subsurface porous zones remineralize in the oral environment. 3.4 Bonding results from the flow of resin into the porous zones of the etched enamel surface, s Since the depth of etch or the amount of surface enamel lost during the etching procedure is dependent on the type of acid used, the acid concentration, the duration of etching, and the chemical composition of the
From the University of Alabama at Birmingham. =Professor, Department of Orthodontics, School of Dentistry. bProfessor, Depa~ment of Biomaterials, School of Dentistry. 'Graduate Student, Department of Orthodontics, School of Dentistry. 'k3raduate Student, Department of Orthodontics, School of Dentistry. *Graduate Student, Department of Orthodontics, School of Dentistry. tprofessor, Department of Biostatistics and Biomathematics. 8/1/17671
enamel,4 more control of these factors has been sought. In addition, fluoride is not evenly distributed in enamel but follows a negative exponential distribution, with the greatest fluoride concentration being in the surface enamel. 7 The permanent loss of enamel during the etching procedure may possibly make the enamel surface more susceptible to decalcification during and after orthodontic treatment. Several clinicians have reported that enamel decalcification beneath and adjacent to bonded attachments frequently occurs during orthodontic treatment, s'9 The increased incidence of demineralization in the teeth of patients undergoing orthodontic treatment has been widely reported. '° Sadowsky et a l . , " among others, have also shown that the resistance o f enamel to artificial carious attack increases when enamel has taken up fluoride from cements used in orthodontic bonding. It is thus reasonable to conclude that maintaining fluoride levels in the enamel is an appropriate objective. To control some of the variables related to permanent enamel loss, changes in acid-etching concentrations and etching times have been suggested. A concentration of 50% has been suggested by Retief, !2 while Silverstone ~3 and Gorelick ~4 consider a 30% to 50% concentration ideal. Most commonly available etching 417
418
Am. J. Orthod. Dentofac. Orthop. November 1990
SadowsL3' et al.
37% H3PO 4 60 Second Etch Bonded / Failed
Tooth Maxillary Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Mandibular Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Grand Totals
15 Second Etch Bonded / Failed
p Value
19 18 17 12 13
0 1 1 0 0
18 17 17 12 12
0 0 2 0 0
1.000 1.000 1.000 1.000 1.000
79
2
76
2
1.000
19 18 18 11 13
1 2 0 1 2
19 19 18 12 12
0 6 0 0 1
1.000 0.232 1.000 .478 1.000
79
6
79
7
1.000
158
8
155
9
0.808
Fig. 1. Number of teeth bonded and failures after etching with 37% H~PO, for 60 seconds and 15 seconds. Times the bonded brackets were in place ranged from 11 to 24 months.
solutions fall within the 30% to 50% range, with an etching time of 60 seconds. 4 In studies by Br~innstrfm and Nordenvall, t5 few differences in the microscopic appearance of etched enamel were noted when enamel etched for 15 seconds was compared with that etched for 120 seconds. Mardaga and Shannon 16 have shown in in vitro studies that decreasing etch time results in a reduction in tensile bond strength. They suggested a 30-second etch time for clinical use, while Barkmeier et al. t7 found no statistically significant differences in the shear bond strength for enamel etched for 15 seconds versus that etched for 60 seconds with a solution of 50% H~PO4. Legler et al. 'g carried out an in vitro study that evaluated the effects of H3PO4 concentration and etch duration on the shear bond strength between an orthodontic bonding resin and etched enamel, since dislodgement of bonded orthodontic brackets would be most likely to occur from shearing forces. Tests were carried out on enamel surfaces of extracted human maxillary canines etched with a gel containing 37% H3PO4 or a solution of 5% HaPO4 for 60, 30, and 15 seconds, respectively. They found no statistically significant differences in the strength of shear bonding to the nine differently etched enamel surfaces. In clinical, in vivo tests, Kinch et al. 19compared 15-second and 60-second etching times during a period of 6 to 13 months for bond failure. They found no significant difference be-
tween the retention of attachments bonded to enamel that had been etched for 15 seconds and that etched for 60 seconds, but there were statistical differences relative to the position of the tooth in the arch. The purpose of our study was to evaluate the effects on clinical bond retention of decreasing the acid concentrations from 37% to 15% while etching for 60 seconds and of decreasing the etching time from 60 seconds to 15 seconds while using a 37% acid concentration. The retention was evaluated over a 2-year period, which is the time usually required to complete comprehensive fixed orthodontic mechanotherapy. MATERIALS AND METHODS Forty-two patients of varying ages who had been scheduled to undergo comprehensive fixed orthodontic appliance mechanotherapy were randomly divided into groups I and II. The patients were not selected but were treated as scheduled on the clinic lists of the Department of Orthodontics at the University of Alabama School of Dentistry, University of Alabama at Birmingham. Group I consisted of 19 patients and Group II of 23 patients. Patients were treated by three clinicians. Group I In this group of 19 patients, contralateral quadrants of teeth were bonded with Concise orthodontic bonding
Volume 98 Number 5
Effects o f etchant concentration
419
60 Second EtCh 37% H3PO 4 Bonded [ Failed
Tooth Maxillary Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Mandibular Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Grand Totals
15% H3PO 4 Bonded [ Failed
p Value
22 20 21 18 15
2 0 0 0 1.
22 20 21 18 15
2 3 0 0 2
1.000 0.231 1.000 1.000 1.000
96
3
96
7
0.331
22 20 22 20 16
0 2 1 0 0
22 20 22 20 16
0 2 1 0 3
1.000 1.000 1.000 1.000 0.226
100
3
100
6
0.498
196
6
196
13
0.157
Fig. 2. Number of teeth bonded and failures after etching for 60 seconds with 37% versus 15% H~P04. Times bonded brackets were in place ranged from 6 to 24 months.
resin (Dental Products Division/3M, St. Paul, Minn.) after they had been etched with a solution of 37% phosphoric acid for 60 seconds and then for 15 seconds. An attempt was made to bond equal numbers of contralateral teeth, but the types of malocclusion and the orthodontic techniques varied, so that not all the teeth in each quadrant were bonded at the start of treatment. Preangulated, pretorqued 0.018-inch × 0.025inch brackets were bonded, and edgewise mechanotherapy was used in treatment. All the brackets were of the mesh-based variety. The range o f time that the brackets were bonded to the teeth was from 11 to 24 months. Except for one patient, who was treated for 11 months, all others were treated for periods in excess of 15 months. The reason for the shorter bonding times was that some treatments were completed during the study and the orthodontic appliances were removed by the clinician. The treatment of some patients was still incomplete after the 24-month evaluation period, so the bonded attachments remained on the teeth. In this group, 44 brackets that had been bonded after an etching time of 15 seconds and 43 brackets bonded after 60 seconds of etching were still in place, since treatment was being continued for more than 24 months. Fig. 1 indicates the teeth bonded and the failures in this part of the study.
Group II In this group of 23 patients, contralateral quadrants of teeth were bonded with Concise orthodontic bonding resin after being etched for 60 seconds with a solution of 37% H3PO4 and then one of 15% phosphoric acid (Prepared for this study by Dental Products Division/3M.). A second attempt was made to bond equal numbers of contralateral teeth, but because of the various types of malocclusion and the orthodontic techniques used, not all the teeth in each quadrant were bonded at the start of treatment. Preangulated, pretorqued 0.018-inch × 0.025-inch brackets were bonded, and edgewise mechanotherapy was the method of treatment. All brackets that were bonded were of the mesh-based variety. The range of time that they were bonded to the teeth was from 6 months to 24 months. Except for one patient whose treatment was completed in 6 months (10 brackets each bonded after etching with solutions of 37% and 15% H3PO4) and one whose treatment was completed in 10 months (10 brackets each bonded after etching with solutions of 37% and 15% H3PO4), all brackets were in place more than 15 months. When treatment was complete, bonds were removed by the clinician. Fig. 2 indicates the teeth bonded and the failures in this part of the study. In this group, 33 brackets
420
Sadowsl~yet
al.
bonded after an etching time of 60 seconds with a solution of 15% H3PO4, and 31 brackets that had been bonded after an etching time of 60 seconds with a solution of 37% H3PO4 were still in place because treatment was continuing for longer than 24 months. STATISTICAL ANALYSIS
Rates of failure were compared statistically by Fisher's exact test for proportions. 2° Results were declared significant if p < 0.05. RESULTS
At total of 705 orthodontic brackets were bonded in this project, in which 42 patients underwent comprehensive fixed appliance orthodontic mechanotherapy. In Group I, involving 19 patients, contralateral quadrants were bonded after being etched with a solution of 37% H~PO4 for 60 seconds and 15 seconds, respectively. Both the difficulty of treatment and the length of treatment were independent variables. Treatment time extended from 11 to 24 months, depending on the time required for completion of treatment. Of 158 teeth bonded after a 60-second etch, only 8 failed, while of 155 teeth bonded for 15 seconds, only 9 failed. Six of the 8 failures in the 60-second-etch group involved teeth in the mandibular arch (Fig. 1), and 7 of the 9 failures in the 15-second etch were also in the mandibular arch. Bonding failures occurred more in the mandibular than in the maxillary arch. It is of interest that among the 15-second-etch failures in the mandibular arch, 6 of 7 failures involved the mandibular lateral incisor tooth; the reasons for this finding are unknown. In Group II, involving 23 patients, contralateral teeth were bonded after being etched for 60 seconds with a solution of 37% H3PO4 and for 60 seconds with a solution of 15% H3PO4, respectively. Once again, orthodontic therapy was based on each patient's needs, and both the difficulty of treatment and the treatment time were independent variables. Treatment time extended from 6 months to 24 months, depending on the time needed for completion. Of 196 teeth bonded after being etched with a solution of 37% HzPO4 for 60 seconds, only 6 failed, 3 each from the maxillary and mandibular arches. Of 196 teeth bonded after being etched with a solution of 15% H~PO4 for 60 seconds, 13 failed, 7 from the maxillary and 6 from the mandibular arch. DISCUSSION AND CONCLUSIONS
The concentration of H3PO~ most widely used in clinical orthodontics exceeds 30%. This conclusion is partly based on the findings of Chow and Brown, 2' who
Am. J. Orthod. Dentofa¢. Orthop. November 1990
studied the phase diagram of the H3PO4-Ca(OH)2-H20 ternary system. They demonstrated that the application to enamel of solutions of H3PO4 greater than 27% resuited in the formation of monocalcium phosphate monohydrate (MCPM), while the main product of weaker H3PO4 solutions is dicalcium phosphate dihydrate (DCPD). Because MCPM is more soluble than DCPD, it will be washed more readily from the enamel surface after etching, so the more concentrated H~PO4 solutions are used clinically. A major concern in the direct bonding technique is the amount of fluoride-rich surface enamel that is lost during the etching procedure, since it is impossible to confine the etching to the bonding site. Legler et al. 22 have shown in an in vitro study that there was a stepwise decrease in the calculated and measured depths of etch with decreased acid concentration and duration of etching. They evaluated both the calculated and the measured depths of etch produced by etching enamel surfaces with solutions of 37%, 15%, and 5% H3PO4for 60, 30, and 15 seconds, respectively, and reported that the etch depths ranged from ___30 I.tm, when 37% H3PO4 was applied for 60 seconds, to ± 5 p.m, when the etching was done with a solution of 5% H3PO4 applied for 15 seconds. Previous interim reports had indicated some promise for shorter clinical etching times, but these studies had dealt with shorter treatment periods. 23,24 Reducing etching time or acid concentration would satisfy the objectives of reducing the pe rmanent loss of enamel in orthodontic etching and bonding procedures. This clinical research also validates the in vitro studies of Legler et al. TM Apart from the unexplained loss of six bonded brackets from the mandibular lateral incisor tooth after a 15-second etching with a 37% H3PO4 solution, tooth position within the arch did not seem to affect bond failure; this finding seems to be at variance with the results of Kinch et al) 9 The low failure rate in our study may relate to the clinical bonding procedures. Absolute attention to detail in the clinical bonding procedures was maintained, and the clinicians were fully conversant with the correct bonding technique. Perhaps technique and/or materials accounted for the unusually high success rate of the bonded attachments in this clinical trial. The bonded brackets that became dislodged during orthodontic treatment failed predominantly at the mesh of the bracket base and the bonding resin interface, leaving some of the bonding resin entrapped in the mesh, or islands of bonding resin on the enamel surface. Similar findings were reported by Jassem et al. ~ in an in vitro study. Fracture within the enamel was not observed visually.
Volume 98 Number 5
Effects o f etchant concentration
We plan to continue this study by further reducing the etching times and the acid concentration. REFERENCES !. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;38:849-53. 2. Silverstone LM. The acid etch technique: in ritro studies with special reference to the enamel surface and the enamel-resin interface. In: Silverstone, LM, Dogon 1L, eds. Proceedings of an international symposium on the acid etch technique. St. Paul, Minn.: North Central Publishing, 1975:13-39. 3. Lee H, Occumpaugh DE, Shaffer J, Sheblc AM. Sealing of developmental pits and fissures. IV. Measurement of in vitro fluoride pickup by electron microprobe x-ray spectrophotometry. J Dent Res 1972;51:634-9. 4. Wet SHY. Electron microprobe analysis of rcmincralization. J Dent Res 1970;49:621-5. 5. Rctief DH. The mechanical bond. Int Dent J 1978;28:!8-27. 6. Retief DH, Harris BE, Bradley EL, Denys FR. Pyruvic acid as an etching agent in clinical dentistry. J Biomed Mater Res 1985;19:335-48. 7. Brudevold F, Gardner DE, Smith FA. The distribution of fluoride in human enamel. J Dent Res 1956;35:420-9. g. Ceen RF, Gwinnett AJ. White spot formation associated with sealants used in orthodontics. Pediatr Dent 1981;3:174-8. 9. Gorelick L, Geiger AM, Gwinnett AJ. Incidence of white spot formation after bonding and banding. AM J ORTiIOD 1981;81: 93-8. 10. Sadowsky PL, Retief DH. A comparative study of some dental cements used in orthodontics. Angle Orthod 1976:171-81. I!. Sadowsky PL, Retief DH, Bradley EL. Acid resistance ofenamel exposed to fluoride containing orthodontic cements. AM J ORT~IOD 1983;83:33-8. 12. Retief DH. Effect of conditioning the enamel surface with phosphoric acid. J Dent Res 1973;52:333-41. 13. Silverstone LM. Fissure sealants. Laboratory studies. Caries Res 1974;8:2-26. 14. Gorelick L. Bonding metal brackets with a self-polymerizing sealant-composite: a 12-month assessment. AM J ORTItOD 1977;71:542-53. 15. BriinnstrOm M, Nordenvall KJ. The effect of acid etching on
16. 17.
18.
19.
20. 21. 22.
23.
24.
25.
421
enamel, dentine and the inner surface of the resin restoration: a scanning electron microscopic investigation. J Dent Res 1977;56:917-23. MardagaWJ, ShannonlL. Decreasing the depth ofetch fordirect bonding in orthodontics. J Clin Orthod 1982;16:130-2. Barkmeier WW, Gwinnett AJ, Shaffer SE. Effects of enamel etching time on bond strength and morphology. J Clin Orthod 1985;19:36-8. 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: an in vitro study. AM J ORTIIODDENTOFACORTHOP1989; 96:485-92. Kinch AP, Taylor tt, Warltier RFB, 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 ORTItOD DENTOFACORTHOP 1986;94:476-83. Brownlee KA. Statistical theory and methodology in science and engineering, 2nd ed. New York: John Wiley, 1965:163-6. Chow LC, Brown WE. Phosphoric acid conditioning of teeth for pit and fissure sealants. J Dent Res 1973;52:1158. Legler LR, Retief DH, Bradley EL. Effects of phosphoric acid concentration and etch duration on enamel depth of etch: an in vitro study. AM J ORTHOD DENTOFACORTHOP 1990;98:15460. Sadowsky PL, Retief DH, Cox PR, Hem."indezR, Rape G. Effect of etchant concentration and duration on retention of ortliodontic attachments [abstract]. J Dent Res 1988;67:361, No. 1987. Viljoen WP, Swanepoel F, Pretorius LM, Du Plessis LS, Smit HJ, De Mf~elenaereJJGG. Shorter etching times in orthodontic bonding: an in vitro study [abstract]. J Dent Res 1988;67:776, No. 19. Jassem HA, Retief DH, Jamison HC. Tensile and shear strengths of bonded and rebonded orthodontic attachments. AM J ORTHOD 1981;79:661-8.
Reprint requests to: Dr. P. Lionel Sadowsky Department of Orthodontics School of Dentistry University of Alabama University Station Birmingham, AL 35294
Birmhzgham, Ala. Etching of enamel before the bonding of orthodontic attachments is usually done with a solution of 37% H3PO, for 60 seconds. The purpose of this study was to evaluate the effects of etchant concentration and duration on the clinical retention of bonded orthodontic attachments. Two randomly selected groups of orthodontic patients participated in the study. In both groups, teeth in contralateral quadrants, excluding the molar teeth, were bonded. In the first group, 158 teeth were etched with a solution of 37% H3PO, for 60 seconds, and 155 teeth were etched with 37% H3PO, for 15 seconds. In the second group, 196 teeth were etched with 37% H3PO, and 196 teeth with 15% H~PO,, both for 60 seconds. Conventional edgewise mechanotherapy was used in both groups of patients. After 24 months of treatment, in the first group, 8 brackets were dislodged when etched with 37% H3PO, for 60 seconds and 9 when etched with 37% H3PO, for 15 seconds. In the second group, 6 brackets were dislodged when etched with 37% H3PO,, for 60 seconds and 13 were dislodged when etched with 15% H3PO, for 60 seconds. Reducing the etching time of 37% H3PO, from 60 seconds to 15 seconds or reducing the acid concentration from 37% to 15% H3PO, applied for 60 seconds had no significantly different effect on the retention of bonded orthodontic attachments. The results suggest that the reduction of etchant concentration and the duration of etching in orthodontic bonding should be considered. (AMJ ORTHOD DENTOFACORTHOP 1990;98:41721 .)
B o n d i n g of brackets to acid-etched enamel has become a universally accepted technique in clinical orthodontics. Buonocore, ~ in 1955, used 85% orthophosphoric acid (H3PO4) for 30 seconds and reported that the bond strength of acrylic restorative resins was significantly increased by etching of the enamel surface. Etching of enamel with H3PO~ results in a superficial etched zone and a subsurface qualitative and quantitative porous zone, according to Silverstone. z While the enamel from the superficial zone is permanently lost, the subsurface porous zones remineralize in the oral environment. 3.4 Bonding results from the flow of resin into the porous zones of the etched enamel surface, s Since the depth of etch or the amount of surface enamel lost during the etching procedure is dependent on the type of acid used, the acid concentration, the duration of etching, and the chemical composition of the
From the University of Alabama at Birmingham. =Professor, Department of Orthodontics, School of Dentistry. bProfessor, Depa~ment of Biomaterials, School of Dentistry. 'Graduate Student, Department of Orthodontics, School of Dentistry. 'k3raduate Student, Department of Orthodontics, School of Dentistry. *Graduate Student, Department of Orthodontics, School of Dentistry. tprofessor, Department of Biostatistics and Biomathematics. 8/1/17671
enamel,4 more control of these factors has been sought. In addition, fluoride is not evenly distributed in enamel but follows a negative exponential distribution, with the greatest fluoride concentration being in the surface enamel. 7 The permanent loss of enamel during the etching procedure may possibly make the enamel surface more susceptible to decalcification during and after orthodontic treatment. Several clinicians have reported that enamel decalcification beneath and adjacent to bonded attachments frequently occurs during orthodontic treatment, s'9 The increased incidence of demineralization in the teeth of patients undergoing orthodontic treatment has been widely reported. '° Sadowsky et a l . , " among others, have also shown that the resistance o f enamel to artificial carious attack increases when enamel has taken up fluoride from cements used in orthodontic bonding. It is thus reasonable to conclude that maintaining fluoride levels in the enamel is an appropriate objective. To control some of the variables related to permanent enamel loss, changes in acid-etching concentrations and etching times have been suggested. A concentration of 50% has been suggested by Retief, !2 while Silverstone ~3 and Gorelick ~4 consider a 30% to 50% concentration ideal. Most commonly available etching 417
418
Am. J. Orthod. Dentofac. Orthop. November 1990
SadowsL3' et al.
37% H3PO 4 60 Second Etch Bonded / Failed
Tooth Maxillary Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Mandibular Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Grand Totals
15 Second Etch Bonded / Failed
p Value
19 18 17 12 13
0 1 1 0 0
18 17 17 12 12
0 0 2 0 0
1.000 1.000 1.000 1.000 1.000
79
2
76
2
1.000
19 18 18 11 13
1 2 0 1 2
19 19 18 12 12
0 6 0 0 1
1.000 0.232 1.000 .478 1.000
79
6
79
7
1.000
158
8
155
9
0.808
Fig. 1. Number of teeth bonded and failures after etching with 37% H~PO, for 60 seconds and 15 seconds. Times the bonded brackets were in place ranged from 11 to 24 months.
solutions fall within the 30% to 50% range, with an etching time of 60 seconds. 4 In studies by Br~innstrfm and Nordenvall, t5 few differences in the microscopic appearance of etched enamel were noted when enamel etched for 15 seconds was compared with that etched for 120 seconds. Mardaga and Shannon 16 have shown in in vitro studies that decreasing etch time results in a reduction in tensile bond strength. They suggested a 30-second etch time for clinical use, while Barkmeier et al. t7 found no statistically significant differences in the shear bond strength for enamel etched for 15 seconds versus that etched for 60 seconds with a solution of 50% H~PO4. Legler et al. 'g carried out an in vitro study that evaluated the effects of H3PO4 concentration and etch duration on the shear bond strength between an orthodontic bonding resin and etched enamel, since dislodgement of bonded orthodontic brackets would be most likely to occur from shearing forces. Tests were carried out on enamel surfaces of extracted human maxillary canines etched with a gel containing 37% H3PO4 or a solution of 5% HaPO4 for 60, 30, and 15 seconds, respectively. They found no statistically significant differences in the strength of shear bonding to the nine differently etched enamel surfaces. In clinical, in vivo tests, Kinch et al. 19compared 15-second and 60-second etching times during a period of 6 to 13 months for bond failure. They found no significant difference be-
tween the retention of attachments bonded to enamel that had been etched for 15 seconds and that etched for 60 seconds, but there were statistical differences relative to the position of the tooth in the arch. The purpose of our study was to evaluate the effects on clinical bond retention of decreasing the acid concentrations from 37% to 15% while etching for 60 seconds and of decreasing the etching time from 60 seconds to 15 seconds while using a 37% acid concentration. The retention was evaluated over a 2-year period, which is the time usually required to complete comprehensive fixed orthodontic mechanotherapy. MATERIALS AND METHODS Forty-two patients of varying ages who had been scheduled to undergo comprehensive fixed orthodontic appliance mechanotherapy were randomly divided into groups I and II. The patients were not selected but were treated as scheduled on the clinic lists of the Department of Orthodontics at the University of Alabama School of Dentistry, University of Alabama at Birmingham. Group I consisted of 19 patients and Group II of 23 patients. Patients were treated by three clinicians. Group I In this group of 19 patients, contralateral quadrants of teeth were bonded with Concise orthodontic bonding
Volume 98 Number 5
Effects o f etchant concentration
419
60 Second EtCh 37% H3PO 4 Bonded [ Failed
Tooth Maxillary Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Mandibular Central Lateral Cuspid 1st Bicuspid 2nd Bicuspid
Grand Totals
15% H3PO 4 Bonded [ Failed
p Value
22 20 21 18 15
2 0 0 0 1.
22 20 21 18 15
2 3 0 0 2
1.000 0.231 1.000 1.000 1.000
96
3
96
7
0.331
22 20 22 20 16
0 2 1 0 0
22 20 22 20 16
0 2 1 0 3
1.000 1.000 1.000 1.000 0.226
100
3
100
6
0.498
196
6
196
13
0.157
Fig. 2. Number of teeth bonded and failures after etching for 60 seconds with 37% versus 15% H~P04. Times bonded brackets were in place ranged from 6 to 24 months.
resin (Dental Products Division/3M, St. Paul, Minn.) after they had been etched with a solution of 37% phosphoric acid for 60 seconds and then for 15 seconds. An attempt was made to bond equal numbers of contralateral teeth, but the types of malocclusion and the orthodontic techniques varied, so that not all the teeth in each quadrant were bonded at the start of treatment. Preangulated, pretorqued 0.018-inch × 0.025inch brackets were bonded, and edgewise mechanotherapy was used in treatment. All the brackets were of the mesh-based variety. The range o f time that the brackets were bonded to the teeth was from 11 to 24 months. Except for one patient, who was treated for 11 months, all others were treated for periods in excess of 15 months. The reason for the shorter bonding times was that some treatments were completed during the study and the orthodontic appliances were removed by the clinician. The treatment of some patients was still incomplete after the 24-month evaluation period, so the bonded attachments remained on the teeth. In this group, 44 brackets that had been bonded after an etching time of 15 seconds and 43 brackets bonded after 60 seconds of etching were still in place, since treatment was being continued for more than 24 months. Fig. 1 indicates the teeth bonded and the failures in this part of the study.
Group II In this group of 23 patients, contralateral quadrants of teeth were bonded with Concise orthodontic bonding resin after being etched for 60 seconds with a solution of 37% H3PO4 and then one of 15% phosphoric acid (Prepared for this study by Dental Products Division/3M.). A second attempt was made to bond equal numbers of contralateral teeth, but because of the various types of malocclusion and the orthodontic techniques used, not all the teeth in each quadrant were bonded at the start of treatment. Preangulated, pretorqued 0.018-inch × 0.025-inch brackets were bonded, and edgewise mechanotherapy was the method of treatment. All brackets that were bonded were of the mesh-based variety. The range of time that they were bonded to the teeth was from 6 months to 24 months. Except for one patient whose treatment was completed in 6 months (10 brackets each bonded after etching with solutions of 37% and 15% H3PO4) and one whose treatment was completed in 10 months (10 brackets each bonded after etching with solutions of 37% and 15% H3PO4), all brackets were in place more than 15 months. When treatment was complete, bonds were removed by the clinician. Fig. 2 indicates the teeth bonded and the failures in this part of the study. In this group, 33 brackets
420
Sadowsl~yet
al.
bonded after an etching time of 60 seconds with a solution of 15% H3PO4, and 31 brackets that had been bonded after an etching time of 60 seconds with a solution of 37% H3PO4 were still in place because treatment was continuing for longer than 24 months. STATISTICAL ANALYSIS
Rates of failure were compared statistically by Fisher's exact test for proportions. 2° Results were declared significant if p < 0.05. RESULTS
At total of 705 orthodontic brackets were bonded in this project, in which 42 patients underwent comprehensive fixed appliance orthodontic mechanotherapy. In Group I, involving 19 patients, contralateral quadrants were bonded after being etched with a solution of 37% H~PO4 for 60 seconds and 15 seconds, respectively. Both the difficulty of treatment and the length of treatment were independent variables. Treatment time extended from 11 to 24 months, depending on the time required for completion of treatment. Of 158 teeth bonded after a 60-second etch, only 8 failed, while of 155 teeth bonded for 15 seconds, only 9 failed. Six of the 8 failures in the 60-second-etch group involved teeth in the mandibular arch (Fig. 1), and 7 of the 9 failures in the 15-second etch were also in the mandibular arch. Bonding failures occurred more in the mandibular than in the maxillary arch. It is of interest that among the 15-second-etch failures in the mandibular arch, 6 of 7 failures involved the mandibular lateral incisor tooth; the reasons for this finding are unknown. In Group II, involving 23 patients, contralateral teeth were bonded after being etched for 60 seconds with a solution of 37% H3PO4 and for 60 seconds with a solution of 15% H3PO4, respectively. Once again, orthodontic therapy was based on each patient's needs, and both the difficulty of treatment and the treatment time were independent variables. Treatment time extended from 6 months to 24 months, depending on the time needed for completion. Of 196 teeth bonded after being etched with a solution of 37% HzPO4 for 60 seconds, only 6 failed, 3 each from the maxillary and mandibular arches. Of 196 teeth bonded after being etched with a solution of 15% H~PO4 for 60 seconds, 13 failed, 7 from the maxillary and 6 from the mandibular arch. DISCUSSION AND CONCLUSIONS
The concentration of H3PO~ most widely used in clinical orthodontics exceeds 30%. This conclusion is partly based on the findings of Chow and Brown, 2' who
Am. J. Orthod. Dentofa¢. Orthop. November 1990
studied the phase diagram of the H3PO4-Ca(OH)2-H20 ternary system. They demonstrated that the application to enamel of solutions of H3PO4 greater than 27% resuited in the formation of monocalcium phosphate monohydrate (MCPM), while the main product of weaker H3PO4 solutions is dicalcium phosphate dihydrate (DCPD). Because MCPM is more soluble than DCPD, it will be washed more readily from the enamel surface after etching, so the more concentrated H~PO4 solutions are used clinically. A major concern in the direct bonding technique is the amount of fluoride-rich surface enamel that is lost during the etching procedure, since it is impossible to confine the etching to the bonding site. Legler et al. 22 have shown in an in vitro study that there was a stepwise decrease in the calculated and measured depths of etch with decreased acid concentration and duration of etching. They evaluated both the calculated and the measured depths of etch produced by etching enamel surfaces with solutions of 37%, 15%, and 5% H3PO4for 60, 30, and 15 seconds, respectively, and reported that the etch depths ranged from ___30 I.tm, when 37% H3PO4 was applied for 60 seconds, to ± 5 p.m, when the etching was done with a solution of 5% H3PO4 applied for 15 seconds. Previous interim reports had indicated some promise for shorter clinical etching times, but these studies had dealt with shorter treatment periods. 23,24 Reducing etching time or acid concentration would satisfy the objectives of reducing the pe rmanent loss of enamel in orthodontic etching and bonding procedures. This clinical research also validates the in vitro studies of Legler et al. TM Apart from the unexplained loss of six bonded brackets from the mandibular lateral incisor tooth after a 15-second etching with a 37% H3PO4 solution, tooth position within the arch did not seem to affect bond failure; this finding seems to be at variance with the results of Kinch et al) 9 The low failure rate in our study may relate to the clinical bonding procedures. Absolute attention to detail in the clinical bonding procedures was maintained, and the clinicians were fully conversant with the correct bonding technique. Perhaps technique and/or materials accounted for the unusually high success rate of the bonded attachments in this clinical trial. The bonded brackets that became dislodged during orthodontic treatment failed predominantly at the mesh of the bracket base and the bonding resin interface, leaving some of the bonding resin entrapped in the mesh, or islands of bonding resin on the enamel surface. Similar findings were reported by Jassem et al. ~ in an in vitro study. Fracture within the enamel was not observed visually.
Volume 98 Number 5
Effects o f etchant concentration
We plan to continue this study by further reducing the etching times and the acid concentration. REFERENCES !. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;38:849-53. 2. Silverstone LM. The acid etch technique: in ritro studies with special reference to the enamel surface and the enamel-resin interface. In: Silverstone, LM, Dogon 1L, eds. Proceedings of an international symposium on the acid etch technique. St. Paul, Minn.: North Central Publishing, 1975:13-39. 3. Lee H, Occumpaugh DE, Shaffer J, Sheblc AM. Sealing of developmental pits and fissures. IV. Measurement of in vitro fluoride pickup by electron microprobe x-ray spectrophotometry. J Dent Res 1972;51:634-9. 4. Wet SHY. Electron microprobe analysis of rcmincralization. J Dent Res 1970;49:621-5. 5. Rctief DH. The mechanical bond. Int Dent J 1978;28:!8-27. 6. Retief DH, Harris BE, Bradley EL, Denys FR. Pyruvic acid as an etching agent in clinical dentistry. J Biomed Mater Res 1985;19:335-48. 7. Brudevold F, Gardner DE, Smith FA. The distribution of fluoride in human enamel. J Dent Res 1956;35:420-9. g. Ceen RF, Gwinnett AJ. White spot formation associated with sealants used in orthodontics. Pediatr Dent 1981;3:174-8. 9. Gorelick L, Geiger AM, Gwinnett AJ. Incidence of white spot formation after bonding and banding. AM J ORTiIOD 1981;81: 93-8. 10. Sadowsky PL, Retief DH. A comparative study of some dental cements used in orthodontics. Angle Orthod 1976:171-81. I!. Sadowsky PL, Retief DH, Bradley EL. Acid resistance ofenamel exposed to fluoride containing orthodontic cements. AM J ORT~IOD 1983;83:33-8. 12. Retief DH. Effect of conditioning the enamel surface with phosphoric acid. J Dent Res 1973;52:333-41. 13. Silverstone LM. Fissure sealants. Laboratory studies. Caries Res 1974;8:2-26. 14. Gorelick L. Bonding metal brackets with a self-polymerizing sealant-composite: a 12-month assessment. AM J ORTItOD 1977;71:542-53. 15. BriinnstrOm M, Nordenvall KJ. The effect of acid etching on
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Reprint requests to: Dr. P. Lionel Sadowsky Department of Orthodontics School of Dentistry University of Alabama University Station Birmingham, AL 35294
