Bond strength with various etching times on young permanent teeth Wei Nan Wang, BDS,* and Tz Chau Lu, BDS**
Taipei, Taiwan, Republic of China Tensile bond strengths of an orthodontic resin cement were compared for 15-, 30-, 60-, 90-, or 120-second etching times, with a 37% phosphoric acid solution on the enamel surfaces of young permanent teeth. Fifty extracted premolars from 9- to 16-year-old children were used for testing. An orthodontic composite resin was used to bond the bracket directly onto the buccal surface of the enamel. The tensile bond strengths were tested with an Instron machine, Bond failure interfaces between bracket bases and teeth surfaces were examined with a scanning electron microscope and calculated with mapping of energy-dispersive x-ray spectrometry. The results of tensile bond strength for 15-, 30-, 60-, or 90-second etching times were not statistically different. For the 120-second etching time, the decrease was significant. Of the bond failures, 43%-49% occurred between bracket and resin interface, 12% to 24% within the resin Itself, 32%-40% between resin and tooth interface, and 0% to 4% contained enamel fragments. There was no statistical difference in percentage of bond failure interface distribution between bracket base and resin, resin and enamel, or the enamel detachment. Cohesive failure within the resin itself at the 120-second etching time was less than at other etching times, with a statistical significance. To achieve good retention, to decrease enamel loss, and to reduce moisture contamination in the clinic, as well as to save chairside time, a 15-second etching time is suggested for teenage orthodontic patients. In the etching time over 30 seconds, some enamel fragments were found, and the amount of enamel fragments was proportional to the length of etching time. (AM J ORTHODDENTOFA(30RTHOP 1991;100:72-9.)
I n a previous study, Wang t concluded that the adhesive itself was not necessarily the major factor in determining bond strength. The topography of the etched surface enamel, the etching time, and the concentration of the etchant could also be important factors influencing bond strength. Composite resin has been used in orthodontics for more than 20 years since its first application? The accepted mechanism of bonding involves etching of the enamel surface? Silverstone4 found that phosphoric acid in concentrations between 20% and 50%, when applied to the enamel for 60 seconds, created the most retentive conditions. However, his studies did not include shorter etching times or the use of differing ages of permanent teeth. Br/innstr6m and NordenvalP found no apparent difference between 15- and 120-second etching times with 37% phosphoric acid; however, the effect of a shorter etching time was not thoroughly investigated. Norden-
From the School of Dentistry, National Defense Medical Center, *Head, Orthodontic Section, Department of Dentistry, Tri-Serviee General Hospital, and director, Orthodontic Research Laboratory. **Chief Resident, Orthodontic Department. Tri-Serviee General Hospital, and assistant, Orthodontic Research Laboratory. 8/1/22728
72
vail et al.6"8 conducted serial studies of different etching times on deciduous and young and old permanent teeth and found on young permanent teeth that 15 seconds of etching created a more retentive condition than 60 seconds. They used the degree of surface irregularities as an indicator for the quality of mechanical retention. Therefore it did not indicate the absolute bond strength. Barkmeier et al. 9 found neither qualitative differences in the enamel surface structure nor differences in bond strength after etching for 15 or 60 seconds with a 50% phosphoric acid. However, the study did not use wide ranges of etching time or count the percentages of the bond failure interface distributions. The purpose of this investigation was to compare the tensile bond strengths for 15-, 30-, 60-, 90-, or 120second etching times with 37% phosphoric acid on the buccal surfaces of young permanent teeth. Bond failure locations on tooth surfaces and bracket bases were examined by scanning electron microscope and calculated with mapping of energy dispersive x-ray spectrometry to analyze the distributive percentages. MATERIALS AND METHODS
Fifty premolars (first or second, upper or lower) were extractcd from 9- to 16-year-old patients undergoing orthodontic treatment. The criteria for tooth selection required
Vohmw I00
Nmnber I
Bond strength with various etch#tg times 73
red nail polish area
p l a s t i c cup
pencil l i n e
stone
.
hook
bracket
0,022×0,025
0.0~9x0.025 . tooth surface
bracket
tooth labial
surface
plastic cup
A
B
C
Fig. 1. A, Prebondlng surface with pencil line to delineate the bracket base. B and C, Top and lateral views of a prepared specimen for debonding.
Table I. Tensile bond strength of five various
Table II. Results of statistical test of
etching times
bond strength
Etchiltg time (secomls)
Bond strength (kg/ ram") Mean
15 30 60 90 120
O.72 0.71 0.69 0.74 0.62
SD*
Etcldngtime (seconds)
15
30
'90
0.11 0.26 0.14 0.09 0.10
15
"~'~
NS
30 60 90 120
NS NS NS A
60
NS ~ N S NS ~ _ NS NS A A
120 NS NS NS ~
A A A A
A
*SD, Standard deviation. p < 0.05.
NS: No statistical significance.
grossly perfect buccal enamel, with no cracks caused by extraction forceps, no caries, and no pretreatment with chemical agents, such as alcohol, fluoride, hydrogen peroxide, or formalin. The teeth were extracted, washed, and kept in a sealed box until use. Fifty teeth were randomly divided into five groups of 10 each. The bracket base had to fit the buccal surface grossly before testing. Fifty Dyna-Lock brackets (Unitek Corporation, Monrovia, Calif.) were selected. The bracket base was a mesh-shaped arc with a surface area of about 3.1 x 3.4 mm (10.54 mm 2) that easily fitted onto the curvature of the buccal surface of the tooth. The Concise (3M Corporation, St. Paul, Minn.) orthodontic bonding system was used. The buccal surfaces of the crowns were polished with pumice powder (Prophy-pol fine particle, Myco Industries Inc., Philadelphia, Pa.) and water paste containing no fluoride or oil for 10 seconds, then cleaned with a water spray for 10 seconds and dried. A 37% phosphoric acid liquid (3M Corp.) was applied to the buccal surface for 15-, 30-, 60-, 90-, or 120-second etching times for each group of 10 teeth. The teeth were then rinsed with a water spray for 20 seconds and dried. The buccal surfaces of the teeth appeared chalky white in color. A bracket base was adapted to the buccal surface of each tooth, the contour of the base was demarcated with a pencil, and the area outside the base was coated with
red nail polish to define the resin area shown in Fig. 1, A. The bonding agent was then applied to the central chalky white area and the bracket base. The composite resin and catalyst were homogeneously mixed, and the paste was applied immediately to the bracket base, which was then completely seated on the central area. A scaler was used to apply pressure to the bracket to assure that the bracket remained in position and also to release excess resin between the tooth surface and the bracket base. The superfluous resin was removed with a dental probe before it set. Two 5 cm long wires of 0.016 × 0.022-inch orthodontic rectangular wire (Unitek Corporation) were soldered together to form a "cross" shape. A 0.009-inch ligature wire (Unitek Corporation) was used to fix the "cross" wire in the bracket slot. The tooth was invested in a hard dental stone contained in a plastic cup, leaving the buccal surface and the bracket exposed, with the bracket in the center of the cup. After the stone set, all specimens were stored in 37 ° C water for 24 hours, then removed, and the "cross" wire was removed. Another 0.022 x 0.025-inch orthodontic wire (Unitek Corporation) with a small projecting arc was tied into the bracket slot with a 0.011-inch ligature wire (Unitek Corporation) as shown in Fig. 1, B and C. The specimens were inserted into the upper arm of an Instron machine (Instron Corporation, Model 1000, Boston, Mass.),
A: p < 0.05.
74
Wang and Lu
Am. J. Orthod. Demofac. Orthop. Jidy 1991
07-NOV-Be 1~: 04~ 13 RATE: CPS TIME IIOLSEC O0-20KEV: IOEV/CH PRST: 2OGLSEC A~ A2-METAL B~ FS3 3 " 7 0 MEN:~ A FS= 200
Ioa
IO4
Io
]oe
ix0
m
I
B Fig. 2. A, Broken interface of tooth. SEM x24. Etching for 15 seconds, sample 1. B, Broken interface of bracket base. SEM x24. with the bracket facing downward. The two ends of a 0,040inch round wire (Unitek Corporation) were shaped into a hook. One end was hooked to the projecting arc of the orthodontic wire on the specimen at the upper arm, and the other was fixed to the metal hook of the lower arm. The distance between the upper and lower hooks was 30 cm. The bracket on the upper arm and the metal hook of the plastic plate on the lower arm were placed directly in the middle of the arm, thus forming a mutually perpendicular line. The crosshead speed of the lnstron machine was 2 mm per minuteJ ''"'L"The maximum tensile bond-breaking force was recorded in kilograms. Fifty specimens were tested, and the tensile bond strengths of each were obtained and recorded. Bond failure surfaces were observed with a scanning electron microscope (SEM) (Canscan Corporation, Serial 4, Cambridge, England) and calculated with mapping of energydispersive x-ray spectrometry (EDAX) (Philips, EDAX, SW 9100, Hillegon, Holland), Test pieces were fixed on the holder of the SEM with double-sided tape and examined under × 24 magnification. The samples were coated with a thin layer of gold in the vacuum evaporator, placed in the specimen cham-
s^
/c
F
IU / R E CURSOR (KEV) =O~. 040
N I
EO^X
Fig. 3. A, Iron distribution on broken interface of bracket base (x24, Fig. 2, B). The white patches indicate the iron location and represent the broken areas between metal and resin. B, EDAX image of Fig. 3, A. The peak of iron represents the presence of metal.
ber, and evacuated (10 -5 Tort) under 20 KV. Secondary electron images were observed on the screen and recorded by energy-dispersive x-ray spectrometry for chemical elemental analysis. The bracket base was made of iron and chromium, whereas the composite resin was made of silicon, and the tooth structure consisted of calcium and phosphate, The EDAX spectrometer was used to detect chemical elements on the broken interface. From the elemental distribution on the bond failure interface records (mapping), we can cut the elemental mapping size from the paper and calculate the percentages of the metal part, of the adhesive, or of the enamel from the cnt paper by weight. The data assessed the bond strengths and percentages of bond failure interface distribution. Means and standard deviations were determined. Data were analyzed by a two-way analysis of variance, and means were ranked by a Tukey interval, L~calculated at the 95% level of confidence. Differences between two means larger than the Tukey interval were statistically significant.
Volume lO0 Number t
Boltd strength with various etching times
0 7 - N O V - B 5 Ia, Og, 17 RATE: CPS TiME tgaLSEC O0-20KEV: I O E V / C H PRET: 200L~-EC A: A 2 - E A S E 6: FS ~ 41oe MEN, B FS = 11604
Ioa
IO4
•
B CURSOR
'
(KEV),.(33. =-00
B
^ EDAX o6,,,.NOV-~,'S 0~1~OB, 1.2 RATE." CP'~ T I N E OO-~.OKEV; IOEV/C'-I FRST~ ^: TEETH-1 6,
F.==
9664
.......
ME.N: A
I~a.LEEC 2EDL~E~
.FS-
t°z
2rJc
Io." . . . . .
I
] ^
!
,-._.p v \ ..... !
E
P^
u
CURSOR (KEV) ~ .
A ~0
Er~AX
Fig. 4. A, Silicon distribution on broken interface of bracket base (x24, Fig. 2, B). The white patches indicate the resin location and represent the broken areas located either between enamel and resin or within the resin itself. B, EDAX image of Fig. 4, A. The peak of silicon represents the presence of resin. (3, Silicon distribution on broken interface of tooth surface (x24, Fig. 2, A). The white patches indicate the resin location and represent the broken areas either between resin and bracket base or within the resin itself. D, Calcium distribution on broken interface of tooth surface (x24, Fig, 2, A). The white patches indicate the enamel location and represent the broken areas between enamel and resin or enamel detachment. E, EDAX image of Fig. 4, D. The calcium peak represents the presence of enamel.
75
76
Wa/lg and Lu
Am.
J.
Orthod. Dent@~c, Orthop. Jul3' 1991
Table III. Percentages of various broken interface distributions of five various etching times Etchingtime
Metal-resin (%)
__
Within resin (%)
(seconds)
Mean
SD*
Range
CV "~*
Mean
SD
Range
CV
15 30 60 90 120
43 43 47 49 44
11.6 12.7 9.5 9.1 6.6
15--50 15--50 20~50 20--60 30--50
27.0 29.6 20.2 18.5 15.0
24 22
5.2 6.9 4.4 4.1 6.5
15-30 12~35 15~27 10-24 4-22
21.5 30,7 24.6 22.5 53.9
18
*SD = Standard deviation. **CV = Coefficientof variation. p < 0.05.
interface. Enamel fragments appeared as the etching time increased over 30 seconds. The mean, SD range, and coefficient of variation of the percentages of bond failures are shown in Table llI. The Tukey test at 95% confidence level was statistically significant. There was no statistical difference in percentage of bond failure interface distribution between bracket base and resin, resin and enamel, or the enamel detachment. However, cohesive failure within the resin itself, in the 120second etching time, was less than that for other etching times and demonstrates a statistical significance.
DISCUSSION Fig. 5. Broken interface of bracket base with calcium concentration is shown in white-spot area, The diffused white spots may not indicate tooth fragments, although calcium ions are demonstrated by EDAX image in Fig. 4, E, Only a white patch is considered as enamel detachment.
RESULTS The results of bond strength are shown in Table I. The Tukey test at the 95% level of confidence was statistically significant. There were no statistical differences among etching times of 15, 30, 60, or 90 seconds. However, etching for 120 seconds gave significantly less bond strength than others shown in Table II. Several different bond failure locations between bracket base and enamel (Fig. 2) were found: (I) interface failure between composite resin and bracket base (Fig. 3), (2) cohesive failure within the resin itself (Fig. 4, A, B, and C), (3) interface failure between resin and tooth (Fig. 4, A, D, and E), and (4) enamel fragments as a result of debonding (Figs. 5 and 4, E). Of the bond failure, 43% to 49% occurred in the bracket and the resin interface, 12% to 24% within the resin itself, and 32% to 40% in the resin and the tooth
The differences in tensile strength among the five groups of young permanent premolars, which were etched for 15, 30, 60, 90, or 120 seconds, were not statistically significant except in the 120-second group. This finding is different from the findings of two previous studies.S'l~- The etching of enamel with phosphoric acid created a retentive composite for acrylic material and, to some extent, the loss of enamel prisms and interprismatic substance. 9'~2"~3 The shorter the etching time is, the less the enamel loss in depth 9"12'~3 and the fewer enamel fragments formed in this study. In the clinical and structural aspects, 15-second etching time is adequate for the teenage patients. Enamel fragments were found as the etching time increased from 60 to 120 seconds, and the longer the etching time, the more the detached enamel fragments, as shown in Table IIl. This result coincides with the findings of a previous study. The percentages of cohesive failure within the resin interface distributions were statistically less in the 120second time etching group than in others shown in Table III. However, there was no obvious difference in the percentages of broken interface distribution between metal and resin among those five groups. Only the broken interfaces between enamel and resin and in-
Volmne I00 Number I
Bond strength with various etching times "/7
Resin-enamel (%) Mean
SD
33 35 34 32 4O
11.4 9.9 4.4 7.2 6.9
I
Enamel detachment (%)
Range
CV
20--60 22~38 25~43 20~40 32~53
34.4 28.3 13.0 22.5 17.2
traenamel fragments were revealed with higher percentages in the 120-second etching time group. The broken interfaces between resin and enamel and intraenamel fragments were 44% for the etching time of 120 seconds on the enamel surface. Tile distribution of the broken interfaces between tooth surface and bracket base is described in a model as shown in Fig. 6. Calcium or silicon fragments were revealed in the tooth interface, and iron, silicon, or calcium was observed in the bracket base interface. It is easy to calculate the percentage of broken interfaces between the bracket and resin or the tooth fragment in the bracket base, according to the distribution diagrammed in Fig. 6. However, it is difficult to determine the broken interface percentages between the enamel and resin or within the resin itself, either in the bracket base or the tooth interface. The only way to calculate it would be a combination method as described in this article. Fig. 6, A is a diagram of the tooth interface. Sir, Si,n, and SiT~ arbitrarily represent the broken areas located in the resin, whereas CaT, Car~, and Car2. arbitrarily represent the broken areas located on the enamel. Fig. 6, B represents the bracket base interface. Sin~ and Sin~ arbitrarily represent the broken areas located on the resin, and Fe~ and Fe2 arbitrarily represent the broken areas located on the metal. Can~ and Ca~2 arbitrarily represent the broken areas located on the enamel. The dotted line represents an arbitrary delineation of the fracture line that separates Si.r and Car on the tooth interface in Fig. 6, A. Fej + Fe~ in Fig. 6, B (bracket base) represents broken distribution between the metal and resin. Sir + Ser~ + SiT~ in Fig. 6, A (enamel surface) represents broken distribution within the resin itself, but it contains the possible parts of the broken distribution between the metal and resin (Fe~ + Fe2 in Fig. 6, B). Therefore interface cohesive failure within the resin itself should be calculated:
Mean
SD
Range
CV
0 0 3.2 3.2 5.3
0 0 0~10 0~[0 0~10
0 0 316.2 316.2 105.4
I. I I I
SI
B[
t
,, %,,...~ ~ S p
B
SiB2
,
Fig. 6. A, Model of tooth interface. An arbitrary delineation of the fracture line that separates Sir and Car on the tooth interface (dotted line), B, A model of bracket base interface,
Sir + Si.n + Sin - Fel - Fe2. Car + Carl + Ca~ in Fig. 6, A (tooth surface) represents broken distribution between the enamel and resin, but it contains the possible parts of enamel detachment (Ca,~ + Can,_ in Fig. 6, B). Therefore the interface located between the resin and the enamel should be calculated: CaT + Cart + Can - Ca~t - Can2. Car, + Can,. in Fig. 6, B (bracket base) represents enamel fragment. This may be summarized as follows: 1. The broken interface between bracket and resin: Fej + Fe2 (bracket base interface). 2. The broken interface within the resin itself:
78
Wang and Lu
Sir + Sir~ + Sin (tooth interface) - Fe~ Fez (bracket base interface). 3. The broken interface between resin and enamel: CaT + Carl + C a n (tooth interface) Ca~j - CaB2 (bracket base interface). 4. The tooth fragments: Ca~l + CaBe (bracket base). The EDAX spectrometer is a very sensitive machine for detecting the chemical elements. White spots are diffused generally in the broken interface of the bracket base or the enamel surface as calcium mapping shows in Figs. 3A, 4A, 4C, 4D, and 5. The diffused white spots may be the remaining calcium ions that are detached from the enamel surface after etching. These may not have been washed out cleanly and completely by the water spray before bonding; thus the presence of the white spots may not indicate tooth fragments unless a patch of white spots appears. The results of this study differ from those of Nordenvall et al. ,s as they found that the 15-second etching created better retention than the 60-second etching on young permanent teeth. They used a surface impression to detect the surface irregularities with the SEM used to indicate the quality of mechanical retention to be evaluated. The electron beam cannot penetrate deeply into the narrow depressions in the enamel; hence the surface irregularities cannot indicate the magnitude of the bond strength. In this study, orthodontic composite resin, which reaches into the deep narrow microscopic depressions of enamel, w a s u s e d . '~:4-1s Thus determination of bond strength by this study is more reliable than by judging the surface irregularities only. Barkmeier et al.9 found neither a difference in morphology nor a difference in bond strength after 15 or 60 seconds of etching with the 50% phosphoric acid. They used premolars (no mention of the ages of the teeth) and created a flat enamel surface with a lowspeed sandpaper disk before bracketing, which could have removed enamel surface layers and affected the results of bond strength. All specimens were stored in a 37 ° C water bath for 1 week and then tested, which may have caused hydrolytic degradation of the composite resin 13,~9,2° and also affected the values of the bond strength. They used only 15- or 60-second etching times with a 50% phosphoric acid for testing. In our research we used 15-, 30-, 60-, 90% or 120-second times with a 37% phosphoric acid. Bryant et al. 2~ compared the bond strength by using a 15% phosphoric acid for 30 seconds and a 5% phosphoric acid for 15 seconds. In their studies there was no significant difference between Lee Cleanse and Bond II (mixed type) (Lee Pharmaceuticals, South E1 Monte, Calif.), but there was a difference between Lee Cleanse
Am. J. Orthod. Dantofac. Orthop. July 1991
and Bond I (non-mixed type). In this study, only a 37% phosphoric acid with a mixed-type bond system was used, and varied etching times were tested. Bryant et al. 21 used the permanent canines as test teeth but did not divide the specimen into groups by age. The enamel surface hardness varies among different ages. 7 All canines were stored in a solution containing 70% ethyl alcohol, which inhibits bacterial growth. However, it may have affected the chemical consistency of the enamel and decreased the bond strength as well as increasing enamel fragments on surface. 22 It has been reported that young permanent teeth or decidious teeth have a prismless layer on the outermost enamel, which can affect etching efficiency. 23"26Therefore, to create a retentive feature on the surface enamel and to obtain a good bonding strength, prolonged etching has been recommended. 2629 In this study, however, various etching times gave the same bonding strengths, except for the 120-second etching time. The prismless layer may not resist the acid etching as previously reported, or it may have already disappeared. In our in vitro study, we recommended a 15-second etching time for young permanent teeth and agreed with the suggestions of Wolfgang 3° and Labart et al. 31 that were based on in vivo studies. However, the causative factors of bond failure at chairside are not easily controlled. Moisture contamination during bracketing, duration of bracket wear in the oral cavity, selection of bracket, disturbance during polymerization, and chewing force may all affect the results. CONCLUSION
There were no statistically significant differences in bond strength among the 15-, 30-, 60-, or 90-second etching times. However, the 120-second group showed significantly less bond strength. The enamel fragments were dramatically increased whenever the etching time was longer than 30 seconds (i.e., the amount of enamel fragments increased in proportion to the length of etching time). We propose that the optimal etching time should be 15 seconds. We thank Dr. T. M. Graber for his enthusiastic help on the revision of this article. REFERENCES
1. WangWN. Tensilebond strengthof orthodonticresins on human tooth surface. Proc Natl Sci Counc B ROC 1988;12:228-35. 2. GraberTM, Swain BF. Orthodontics,currentprinciplesand techniques. 1st ed. St. Louis: CV Mosby, 1985;485-563. 3, BuonocoreMG. A simple method of increasing the adhesion of acrylic filling materials to enamel surface. J Dent Res 1955; 34:849-53. 4. SilverstoneLM. Fissure sealants. Caries Res 1974;8:2-26. 5. Br~nnstr6mM, Nordenvall KJ. The effect of acid etching on
Volume I00 Number 1
6.
7.
8.
9.
10. 11. 12.
13.
14.
15.
16.
17.
18.
19.
enamel, dentin and the inner surface of the resin restoration: a scanning electron microscopic investigation. J Dent Res 1977; 56:917-23. Br~innstr6m M, Nordenvall KJ, Malmgren O. The effect of various pretreatment methods of the enamel in bonding procedures. AM J ORTHOD 1978;74:522-30. Br[innstrSm M, Malmgren O, Nordenvall KJ. Etching on young permanent teeth with an acid gel. AM J ORTHOD 1982;82:37983. Nordenvall KJ, Br~innstrSm M, Malmgren O. Etching of deciduous teeth and young and old permanent teeth: a comparison between 15 and 60 seconds of etching. AM J ORTHOD1980;78:99108. Barkmeier WW, Gwinnett AJ, Shaffer SE. Effects of enamel etching time on bond strength and morphology. J Clin Orthod 1985;19:36-8. DiekinsonPT, PowersJM. Evaluation offourteen direct-bonding orthodontic bases. AM J ORT~OD 1980;78:630-9. Steel RG, Torrie JH, Principles and procedures of statistics. 2nd ed. New York: McGraw-Hill, 1980:185-7. Silverstone LM. The acid etch technique: in vitro studies with special reference to the enamel surface and the enamel-resin interface. St. Paul: North Center Publishing Co., 1975. Sehutt NL, Pelleu GB. Effect of storage time and temperature on the setting times of two composite resins. J Prosthet Dent 1982;47:407-10. Diedrich P. Enamel alterations form bracket bonding and debonding: a study with the scanning electron microscope. AM J ORTHOD 1981;79:500-22, Brown CRL, Way DC. Enamel loss during orthodontic bonding and subsequent loss during removal of filled and unfilled adhesives. AM J ORTHOD 1978;74:663-71. Fitzpatrick DA, Way DC. The effects of wear, acid etching, and bond removal on human enamel. AM I OaTHOD 1977;72:67381. Lampert F. Klinische und experimentale Untersuchungen Zur schmelzoberflachenatzung. Fortschr Kieferorthop 1980;41:7783. Arakawa V, Takahashi Y, Sebata M. The effect of acid etching on the cervical region of the buccal surface of human premolar, with special reference to direct bonding techniques. AM J ORrl-roD 1979;76:201-8. McAndrew WR, Lloyd CH. Thermal diffusivity of composite restorative materials. J Dent Res 1987;66:1576-8.
Bond strength with various etching times
79
20. S6derholm K/, Zigan M, Ragan M, Fischlschweiger W, Bergman M. Hydrolytic degradation of dental composite. J Dent Res 1984;63:1248-54. 21. Bryant S, Retief DH, Russell CM, Denys FR. Tensile bond strengths of orthodontic bonding resins attachments to etched enamel. AM J ORTHOD DENTOFAC ORTHOP 1987;92:225-31. 22. Wang WN. Effects of formalin and hydrogen peroxide on the tensile bond strength of composite resin to human tooth surface. Bull Dept Dent NDMCROC 1987;18(1);1-17. 23. Ripa LW, Gwinnett A J, Buonocore MG. The "prismless" outer layer of deciduous and permanent enamel. Arch Oral Biol 1966; 11:41-8. 24. Gwinnett AJ. The ultrastmcture of the "prismless" enamel of deciduous teeth. Arch Oral Biol 1966;11:1109-15. 25. Gwinnett AJ. Human prismless enamel and its influence on sealant penetration, Arch Oral Biol 1973;18:441-4. 26. Sheykholeslam Z, Buonocore MG. Bonding of resins to phosphoric acld-etched enamel surface of permanent and deciduous teeth. J Dent Res 1972;81:1572-6. 27. Eidelman E. The structure of the enamel in primary teeth: practical applications in restorative techniques. J Dent Child i976; 43:172-6. 28. Bozalis WG, Marshall GW, Cooley RO, Mechanical pretreatment and etching of primary tooth enamel, J Dent Child 1979;46:43-9. 29. Fuks AB, Eidelman E, Shapira J. Mechanical and acid treatment of the pdsmless layer of primary teeth vs. acid etching only: a SEM study. J Dent Child 1977;44:222-5. 30. Carstensen W. Clinical results after direct bonding of brackets using shorter etching times. AM J ORT~OD 1986;89:70-2. 31. Labart WA, Barkmeier WW, Taylor MH. Bracket retention after 15 second acid conditioning. J Clin Orthod 1988;22:224-5. Reprint requests to: Dr. Wei Nan Wang Orthodontic Section Department of Td-Serviee General Hospital School of Dentistry National Defense Medical Center Taipei, Taiwan Republic of China 107
Taipei, Taiwan, Republic of China Tensile bond strengths of an orthodontic resin cement were compared for 15-, 30-, 60-, 90-, or 120-second etching times, with a 37% phosphoric acid solution on the enamel surfaces of young permanent teeth. Fifty extracted premolars from 9- to 16-year-old children were used for testing. An orthodontic composite resin was used to bond the bracket directly onto the buccal surface of the enamel. The tensile bond strengths were tested with an Instron machine, Bond failure interfaces between bracket bases and teeth surfaces were examined with a scanning electron microscope and calculated with mapping of energy-dispersive x-ray spectrometry. The results of tensile bond strength for 15-, 30-, 60-, or 90-second etching times were not statistically different. For the 120-second etching time, the decrease was significant. Of the bond failures, 43%-49% occurred between bracket and resin interface, 12% to 24% within the resin Itself, 32%-40% between resin and tooth interface, and 0% to 4% contained enamel fragments. There was no statistical difference in percentage of bond failure interface distribution between bracket base and resin, resin and enamel, or the enamel detachment. Cohesive failure within the resin itself at the 120-second etching time was less than at other etching times, with a statistical significance. To achieve good retention, to decrease enamel loss, and to reduce moisture contamination in the clinic, as well as to save chairside time, a 15-second etching time is suggested for teenage orthodontic patients. In the etching time over 30 seconds, some enamel fragments were found, and the amount of enamel fragments was proportional to the length of etching time. (AM J ORTHODDENTOFA(30RTHOP 1991;100:72-9.)
I n a previous study, Wang t concluded that the adhesive itself was not necessarily the major factor in determining bond strength. The topography of the etched surface enamel, the etching time, and the concentration of the etchant could also be important factors influencing bond strength. Composite resin has been used in orthodontics for more than 20 years since its first application? The accepted mechanism of bonding involves etching of the enamel surface? Silverstone4 found that phosphoric acid in concentrations between 20% and 50%, when applied to the enamel for 60 seconds, created the most retentive conditions. However, his studies did not include shorter etching times or the use of differing ages of permanent teeth. Br/innstr6m and NordenvalP found no apparent difference between 15- and 120-second etching times with 37% phosphoric acid; however, the effect of a shorter etching time was not thoroughly investigated. Norden-
From the School of Dentistry, National Defense Medical Center, *Head, Orthodontic Section, Department of Dentistry, Tri-Serviee General Hospital, and director, Orthodontic Research Laboratory. **Chief Resident, Orthodontic Department. Tri-Serviee General Hospital, and assistant, Orthodontic Research Laboratory. 8/1/22728
72
vail et al.6"8 conducted serial studies of different etching times on deciduous and young and old permanent teeth and found on young permanent teeth that 15 seconds of etching created a more retentive condition than 60 seconds. They used the degree of surface irregularities as an indicator for the quality of mechanical retention. Therefore it did not indicate the absolute bond strength. Barkmeier et al. 9 found neither qualitative differences in the enamel surface structure nor differences in bond strength after etching for 15 or 60 seconds with a 50% phosphoric acid. However, the study did not use wide ranges of etching time or count the percentages of the bond failure interface distributions. The purpose of this investigation was to compare the tensile bond strengths for 15-, 30-, 60-, 90-, or 120second etching times with 37% phosphoric acid on the buccal surfaces of young permanent teeth. Bond failure locations on tooth surfaces and bracket bases were examined by scanning electron microscope and calculated with mapping of energy dispersive x-ray spectrometry to analyze the distributive percentages. MATERIALS AND METHODS
Fifty premolars (first or second, upper or lower) were extractcd from 9- to 16-year-old patients undergoing orthodontic treatment. The criteria for tooth selection required
Vohmw I00
Nmnber I
Bond strength with various etch#tg times 73
red nail polish area
p l a s t i c cup
pencil l i n e
stone
.
hook
bracket
0,022×0,025
0.0~9x0.025 . tooth surface
bracket
tooth labial
surface
plastic cup
A
B
C
Fig. 1. A, Prebondlng surface with pencil line to delineate the bracket base. B and C, Top and lateral views of a prepared specimen for debonding.
Table I. Tensile bond strength of five various
Table II. Results of statistical test of
etching times
bond strength
Etchiltg time (secomls)
Bond strength (kg/ ram") Mean
15 30 60 90 120
O.72 0.71 0.69 0.74 0.62
SD*
Etcldngtime (seconds)
15
30
'90
0.11 0.26 0.14 0.09 0.10
15
"~'~
NS
30 60 90 120
NS NS NS A
60
NS ~ N S NS ~ _ NS NS A A
120 NS NS NS ~
A A A A
A
*SD, Standard deviation. p < 0.05.
NS: No statistical significance.
grossly perfect buccal enamel, with no cracks caused by extraction forceps, no caries, and no pretreatment with chemical agents, such as alcohol, fluoride, hydrogen peroxide, or formalin. The teeth were extracted, washed, and kept in a sealed box until use. Fifty teeth were randomly divided into five groups of 10 each. The bracket base had to fit the buccal surface grossly before testing. Fifty Dyna-Lock brackets (Unitek Corporation, Monrovia, Calif.) were selected. The bracket base was a mesh-shaped arc with a surface area of about 3.1 x 3.4 mm (10.54 mm 2) that easily fitted onto the curvature of the buccal surface of the tooth. The Concise (3M Corporation, St. Paul, Minn.) orthodontic bonding system was used. The buccal surfaces of the crowns were polished with pumice powder (Prophy-pol fine particle, Myco Industries Inc., Philadelphia, Pa.) and water paste containing no fluoride or oil for 10 seconds, then cleaned with a water spray for 10 seconds and dried. A 37% phosphoric acid liquid (3M Corp.) was applied to the buccal surface for 15-, 30-, 60-, 90-, or 120-second etching times for each group of 10 teeth. The teeth were then rinsed with a water spray for 20 seconds and dried. The buccal surfaces of the teeth appeared chalky white in color. A bracket base was adapted to the buccal surface of each tooth, the contour of the base was demarcated with a pencil, and the area outside the base was coated with
red nail polish to define the resin area shown in Fig. 1, A. The bonding agent was then applied to the central chalky white area and the bracket base. The composite resin and catalyst were homogeneously mixed, and the paste was applied immediately to the bracket base, which was then completely seated on the central area. A scaler was used to apply pressure to the bracket to assure that the bracket remained in position and also to release excess resin between the tooth surface and the bracket base. The superfluous resin was removed with a dental probe before it set. Two 5 cm long wires of 0.016 × 0.022-inch orthodontic rectangular wire (Unitek Corporation) were soldered together to form a "cross" shape. A 0.009-inch ligature wire (Unitek Corporation) was used to fix the "cross" wire in the bracket slot. The tooth was invested in a hard dental stone contained in a plastic cup, leaving the buccal surface and the bracket exposed, with the bracket in the center of the cup. After the stone set, all specimens were stored in 37 ° C water for 24 hours, then removed, and the "cross" wire was removed. Another 0.022 x 0.025-inch orthodontic wire (Unitek Corporation) with a small projecting arc was tied into the bracket slot with a 0.011-inch ligature wire (Unitek Corporation) as shown in Fig. 1, B and C. The specimens were inserted into the upper arm of an Instron machine (Instron Corporation, Model 1000, Boston, Mass.),
A: p < 0.05.
74
Wang and Lu
Am. J. Orthod. Demofac. Orthop. Jidy 1991
07-NOV-Be 1~: 04~ 13 RATE: CPS TIME IIOLSEC O0-20KEV: IOEV/CH PRST: 2OGLSEC A~ A2-METAL B~ FS3 3 " 7 0 MEN:~ A FS= 200
Ioa
IO4
Io
]oe
ix0
m
I
B Fig. 2. A, Broken interface of tooth. SEM x24. Etching for 15 seconds, sample 1. B, Broken interface of bracket base. SEM x24. with the bracket facing downward. The two ends of a 0,040inch round wire (Unitek Corporation) were shaped into a hook. One end was hooked to the projecting arc of the orthodontic wire on the specimen at the upper arm, and the other was fixed to the metal hook of the lower arm. The distance between the upper and lower hooks was 30 cm. The bracket on the upper arm and the metal hook of the plastic plate on the lower arm were placed directly in the middle of the arm, thus forming a mutually perpendicular line. The crosshead speed of the lnstron machine was 2 mm per minuteJ ''"'L"The maximum tensile bond-breaking force was recorded in kilograms. Fifty specimens were tested, and the tensile bond strengths of each were obtained and recorded. Bond failure surfaces were observed with a scanning electron microscope (SEM) (Canscan Corporation, Serial 4, Cambridge, England) and calculated with mapping of energydispersive x-ray spectrometry (EDAX) (Philips, EDAX, SW 9100, Hillegon, Holland), Test pieces were fixed on the holder of the SEM with double-sided tape and examined under × 24 magnification. The samples were coated with a thin layer of gold in the vacuum evaporator, placed in the specimen cham-
s^
/c
F
IU / R E CURSOR (KEV) =O~. 040
N I
EO^X
Fig. 3. A, Iron distribution on broken interface of bracket base (x24, Fig. 2, B). The white patches indicate the iron location and represent the broken areas between metal and resin. B, EDAX image of Fig. 3, A. The peak of iron represents the presence of metal.
ber, and evacuated (10 -5 Tort) under 20 KV. Secondary electron images were observed on the screen and recorded by energy-dispersive x-ray spectrometry for chemical elemental analysis. The bracket base was made of iron and chromium, whereas the composite resin was made of silicon, and the tooth structure consisted of calcium and phosphate, The EDAX spectrometer was used to detect chemical elements on the broken interface. From the elemental distribution on the bond failure interface records (mapping), we can cut the elemental mapping size from the paper and calculate the percentages of the metal part, of the adhesive, or of the enamel from the cnt paper by weight. The data assessed the bond strengths and percentages of bond failure interface distribution. Means and standard deviations were determined. Data were analyzed by a two-way analysis of variance, and means were ranked by a Tukey interval, L~calculated at the 95% level of confidence. Differences between two means larger than the Tukey interval were statistically significant.
Volume lO0 Number t
Boltd strength with various etching times
0 7 - N O V - B 5 Ia, Og, 17 RATE: CPS TiME tgaLSEC O0-20KEV: I O E V / C H PRET: 200L~-EC A: A 2 - E A S E 6: FS ~ 41oe MEN, B FS = 11604
Ioa
IO4
•
B CURSOR
'
(KEV),.(33. =-00
B
^ EDAX o6,,,.NOV-~,'S 0~1~OB, 1.2 RATE." CP'~ T I N E OO-~.OKEV; IOEV/C'-I FRST~ ^: TEETH-1 6,
F.==
9664
.......
ME.N: A
I~a.LEEC 2EDL~E~
.FS-
t°z
2rJc
Io." . . . . .
I
] ^
!
,-._.p v \ ..... !
E
P^
u
CURSOR (KEV) ~ .
A ~0
Er~AX
Fig. 4. A, Silicon distribution on broken interface of bracket base (x24, Fig. 2, B). The white patches indicate the resin location and represent the broken areas located either between enamel and resin or within the resin itself. B, EDAX image of Fig. 4, A. The peak of silicon represents the presence of resin. (3, Silicon distribution on broken interface of tooth surface (x24, Fig. 2, A). The white patches indicate the resin location and represent the broken areas either between resin and bracket base or within the resin itself. D, Calcium distribution on broken interface of tooth surface (x24, Fig, 2, A). The white patches indicate the enamel location and represent the broken areas between enamel and resin or enamel detachment. E, EDAX image of Fig. 4, D. The calcium peak represents the presence of enamel.
75
76
Wa/lg and Lu
Am.
J.
Orthod. Dent@~c, Orthop. Jul3' 1991
Table III. Percentages of various broken interface distributions of five various etching times Etchingtime
Metal-resin (%)
__
Within resin (%)
(seconds)
Mean
SD*
Range
CV "~*
Mean
SD
Range
CV
15 30 60 90 120
43 43 47 49 44
11.6 12.7 9.5 9.1 6.6
15--50 15--50 20~50 20--60 30--50
27.0 29.6 20.2 18.5 15.0
24 22
5.2 6.9 4.4 4.1 6.5
15-30 12~35 15~27 10-24 4-22
21.5 30,7 24.6 22.5 53.9
18
*SD = Standard deviation. **CV = Coefficientof variation. p < 0.05.
interface. Enamel fragments appeared as the etching time increased over 30 seconds. The mean, SD range, and coefficient of variation of the percentages of bond failures are shown in Table llI. The Tukey test at 95% confidence level was statistically significant. There was no statistical difference in percentage of bond failure interface distribution between bracket base and resin, resin and enamel, or the enamel detachment. However, cohesive failure within the resin itself, in the 120second etching time, was less than that for other etching times and demonstrates a statistical significance.
DISCUSSION Fig. 5. Broken interface of bracket base with calcium concentration is shown in white-spot area, The diffused white spots may not indicate tooth fragments, although calcium ions are demonstrated by EDAX image in Fig. 4, E, Only a white patch is considered as enamel detachment.
RESULTS The results of bond strength are shown in Table I. The Tukey test at the 95% level of confidence was statistically significant. There were no statistical differences among etching times of 15, 30, 60, or 90 seconds. However, etching for 120 seconds gave significantly less bond strength than others shown in Table II. Several different bond failure locations between bracket base and enamel (Fig. 2) were found: (I) interface failure between composite resin and bracket base (Fig. 3), (2) cohesive failure within the resin itself (Fig. 4, A, B, and C), (3) interface failure between resin and tooth (Fig. 4, A, D, and E), and (4) enamel fragments as a result of debonding (Figs. 5 and 4, E). Of the bond failure, 43% to 49% occurred in the bracket and the resin interface, 12% to 24% within the resin itself, and 32% to 40% in the resin and the tooth
The differences in tensile strength among the five groups of young permanent premolars, which were etched for 15, 30, 60, 90, or 120 seconds, were not statistically significant except in the 120-second group. This finding is different from the findings of two previous studies.S'l~- The etching of enamel with phosphoric acid created a retentive composite for acrylic material and, to some extent, the loss of enamel prisms and interprismatic substance. 9'~2"~3 The shorter the etching time is, the less the enamel loss in depth 9"12'~3 and the fewer enamel fragments formed in this study. In the clinical and structural aspects, 15-second etching time is adequate for the teenage patients. Enamel fragments were found as the etching time increased from 60 to 120 seconds, and the longer the etching time, the more the detached enamel fragments, as shown in Table IIl. This result coincides with the findings of a previous study. The percentages of cohesive failure within the resin interface distributions were statistically less in the 120second time etching group than in others shown in Table III. However, there was no obvious difference in the percentages of broken interface distribution between metal and resin among those five groups. Only the broken interfaces between enamel and resin and in-
Volmne I00 Number I
Bond strength with various etching times "/7
Resin-enamel (%) Mean
SD
33 35 34 32 4O
11.4 9.9 4.4 7.2 6.9
I
Enamel detachment (%)
Range
CV
20--60 22~38 25~43 20~40 32~53
34.4 28.3 13.0 22.5 17.2
traenamel fragments were revealed with higher percentages in the 120-second etching time group. The broken interfaces between resin and enamel and intraenamel fragments were 44% for the etching time of 120 seconds on the enamel surface. Tile distribution of the broken interfaces between tooth surface and bracket base is described in a model as shown in Fig. 6. Calcium or silicon fragments were revealed in the tooth interface, and iron, silicon, or calcium was observed in the bracket base interface. It is easy to calculate the percentage of broken interfaces between the bracket and resin or the tooth fragment in the bracket base, according to the distribution diagrammed in Fig. 6. However, it is difficult to determine the broken interface percentages between the enamel and resin or within the resin itself, either in the bracket base or the tooth interface. The only way to calculate it would be a combination method as described in this article. Fig. 6, A is a diagram of the tooth interface. Sir, Si,n, and SiT~ arbitrarily represent the broken areas located in the resin, whereas CaT, Car~, and Car2. arbitrarily represent the broken areas located on the enamel. Fig. 6, B represents the bracket base interface. Sin~ and Sin~ arbitrarily represent the broken areas located on the resin, and Fe~ and Fe2 arbitrarily represent the broken areas located on the metal. Can~ and Ca~2 arbitrarily represent the broken areas located on the enamel. The dotted line represents an arbitrary delineation of the fracture line that separates Si.r and Car on the tooth interface in Fig. 6, A. Fej + Fe~ in Fig. 6, B (bracket base) represents broken distribution between the metal and resin. Sir + Ser~ + SiT~ in Fig. 6, A (enamel surface) represents broken distribution within the resin itself, but it contains the possible parts of the broken distribution between the metal and resin (Fe~ + Fe2 in Fig. 6, B). Therefore interface cohesive failure within the resin itself should be calculated:
Mean
SD
Range
CV
0 0 3.2 3.2 5.3
0 0 0~10 0~[0 0~10
0 0 316.2 316.2 105.4
I. I I I
SI
B[
t
,, %,,...~ ~ S p
B
SiB2
,
Fig. 6. A, Model of tooth interface. An arbitrary delineation of the fracture line that separates Sir and Car on the tooth interface (dotted line), B, A model of bracket base interface,
Sir + Si.n + Sin - Fel - Fe2. Car + Carl + Ca~ in Fig. 6, A (tooth surface) represents broken distribution between the enamel and resin, but it contains the possible parts of enamel detachment (Ca,~ + Can,_ in Fig. 6, B). Therefore the interface located between the resin and the enamel should be calculated: CaT + Cart + Can - Ca~t - Can2. Car, + Can,. in Fig. 6, B (bracket base) represents enamel fragment. This may be summarized as follows: 1. The broken interface between bracket and resin: Fej + Fe2 (bracket base interface). 2. The broken interface within the resin itself:
78
Wang and Lu
Sir + Sir~ + Sin (tooth interface) - Fe~ Fez (bracket base interface). 3. The broken interface between resin and enamel: CaT + Carl + C a n (tooth interface) Ca~j - CaB2 (bracket base interface). 4. The tooth fragments: Ca~l + CaBe (bracket base). The EDAX spectrometer is a very sensitive machine for detecting the chemical elements. White spots are diffused generally in the broken interface of the bracket base or the enamel surface as calcium mapping shows in Figs. 3A, 4A, 4C, 4D, and 5. The diffused white spots may be the remaining calcium ions that are detached from the enamel surface after etching. These may not have been washed out cleanly and completely by the water spray before bonding; thus the presence of the white spots may not indicate tooth fragments unless a patch of white spots appears. The results of this study differ from those of Nordenvall et al. ,s as they found that the 15-second etching created better retention than the 60-second etching on young permanent teeth. They used a surface impression to detect the surface irregularities with the SEM used to indicate the quality of mechanical retention to be evaluated. The electron beam cannot penetrate deeply into the narrow depressions in the enamel; hence the surface irregularities cannot indicate the magnitude of the bond strength. In this study, orthodontic composite resin, which reaches into the deep narrow microscopic depressions of enamel, w a s u s e d . '~:4-1s Thus determination of bond strength by this study is more reliable than by judging the surface irregularities only. Barkmeier et al.9 found neither a difference in morphology nor a difference in bond strength after 15 or 60 seconds of etching with the 50% phosphoric acid. They used premolars (no mention of the ages of the teeth) and created a flat enamel surface with a lowspeed sandpaper disk before bracketing, which could have removed enamel surface layers and affected the results of bond strength. All specimens were stored in a 37 ° C water bath for 1 week and then tested, which may have caused hydrolytic degradation of the composite resin 13,~9,2° and also affected the values of the bond strength. They used only 15- or 60-second etching times with a 50% phosphoric acid for testing. In our research we used 15-, 30-, 60-, 90% or 120-second times with a 37% phosphoric acid. Bryant et al. 2~ compared the bond strength by using a 15% phosphoric acid for 30 seconds and a 5% phosphoric acid for 15 seconds. In their studies there was no significant difference between Lee Cleanse and Bond II (mixed type) (Lee Pharmaceuticals, South E1 Monte, Calif.), but there was a difference between Lee Cleanse
Am. J. Orthod. Dantofac. Orthop. July 1991
and Bond I (non-mixed type). In this study, only a 37% phosphoric acid with a mixed-type bond system was used, and varied etching times were tested. Bryant et al. 21 used the permanent canines as test teeth but did not divide the specimen into groups by age. The enamel surface hardness varies among different ages. 7 All canines were stored in a solution containing 70% ethyl alcohol, which inhibits bacterial growth. However, it may have affected the chemical consistency of the enamel and decreased the bond strength as well as increasing enamel fragments on surface. 22 It has been reported that young permanent teeth or decidious teeth have a prismless layer on the outermost enamel, which can affect etching efficiency. 23"26Therefore, to create a retentive feature on the surface enamel and to obtain a good bonding strength, prolonged etching has been recommended. 2629 In this study, however, various etching times gave the same bonding strengths, except for the 120-second etching time. The prismless layer may not resist the acid etching as previously reported, or it may have already disappeared. In our in vitro study, we recommended a 15-second etching time for young permanent teeth and agreed with the suggestions of Wolfgang 3° and Labart et al. 31 that were based on in vivo studies. However, the causative factors of bond failure at chairside are not easily controlled. Moisture contamination during bracketing, duration of bracket wear in the oral cavity, selection of bracket, disturbance during polymerization, and chewing force may all affect the results. CONCLUSION
There were no statistically significant differences in bond strength among the 15-, 30-, 60-, or 90-second etching times. However, the 120-second group showed significantly less bond strength. The enamel fragments were dramatically increased whenever the etching time was longer than 30 seconds (i.e., the amount of enamel fragments increased in proportion to the length of etching time). We propose that the optimal etching time should be 15 seconds. We thank Dr. T. M. Graber for his enthusiastic help on the revision of this article. REFERENCES
1. WangWN. Tensilebond strengthof orthodonticresins on human tooth surface. Proc Natl Sci Counc B ROC 1988;12:228-35. 2. GraberTM, Swain BF. Orthodontics,currentprinciplesand techniques. 1st ed. St. Louis: CV Mosby, 1985;485-563. 3, BuonocoreMG. A simple method of increasing the adhesion of acrylic filling materials to enamel surface. J Dent Res 1955; 34:849-53. 4. SilverstoneLM. Fissure sealants. Caries Res 1974;8:2-26. 5. Br~nnstr6mM, Nordenvall KJ. The effect of acid etching on
Volume I00 Number 1
6.
7.
8.
9.
10. 11. 12.
13.
14.
15.
16.
17.
18.
19.
enamel, dentin and the inner surface of the resin restoration: a scanning electron microscopic investigation. J Dent Res 1977; 56:917-23. Br~innstr6m M, Nordenvall KJ, Malmgren O. The effect of various pretreatment methods of the enamel in bonding procedures. AM J ORTHOD 1978;74:522-30. Br[innstrSm M, Malmgren O, Nordenvall KJ. Etching on young permanent teeth with an acid gel. AM J ORTHOD 1982;82:37983. Nordenvall KJ, Br~innstrSm M, Malmgren O. Etching of deciduous teeth and young and old permanent teeth: a comparison between 15 and 60 seconds of etching. AM J ORTHOD1980;78:99108. Barkmeier WW, Gwinnett AJ, Shaffer SE. Effects of enamel etching time on bond strength and morphology. J Clin Orthod 1985;19:36-8. DiekinsonPT, PowersJM. Evaluation offourteen direct-bonding orthodontic bases. AM J ORT~OD 1980;78:630-9. Steel RG, Torrie JH, Principles and procedures of statistics. 2nd ed. New York: McGraw-Hill, 1980:185-7. Silverstone LM. The acid etch technique: in vitro studies with special reference to the enamel surface and the enamel-resin interface. St. Paul: North Center Publishing Co., 1975. Sehutt NL, Pelleu GB. Effect of storage time and temperature on the setting times of two composite resins. J Prosthet Dent 1982;47:407-10. Diedrich P. Enamel alterations form bracket bonding and debonding: a study with the scanning electron microscope. AM J ORTHOD 1981;79:500-22, Brown CRL, Way DC. Enamel loss during orthodontic bonding and subsequent loss during removal of filled and unfilled adhesives. AM J ORTHOD 1978;74:663-71. Fitzpatrick DA, Way DC. The effects of wear, acid etching, and bond removal on human enamel. AM I OaTHOD 1977;72:67381. Lampert F. Klinische und experimentale Untersuchungen Zur schmelzoberflachenatzung. Fortschr Kieferorthop 1980;41:7783. Arakawa V, Takahashi Y, Sebata M. The effect of acid etching on the cervical region of the buccal surface of human premolar, with special reference to direct bonding techniques. AM J ORrl-roD 1979;76:201-8. McAndrew WR, Lloyd CH. Thermal diffusivity of composite restorative materials. J Dent Res 1987;66:1576-8.
Bond strength with various etching times
79
20. S6derholm K/, Zigan M, Ragan M, Fischlschweiger W, Bergman M. Hydrolytic degradation of dental composite. J Dent Res 1984;63:1248-54. 21. Bryant S, Retief DH, Russell CM, Denys FR. Tensile bond strengths of orthodontic bonding resins attachments to etched enamel. AM J ORTHOD DENTOFAC ORTHOP 1987;92:225-31. 22. Wang WN. Effects of formalin and hydrogen peroxide on the tensile bond strength of composite resin to human tooth surface. Bull Dept Dent NDMCROC 1987;18(1);1-17. 23. Ripa LW, Gwinnett A J, Buonocore MG. The "prismless" outer layer of deciduous and permanent enamel. Arch Oral Biol 1966; 11:41-8. 24. Gwinnett AJ. The ultrastmcture of the "prismless" enamel of deciduous teeth. Arch Oral Biol 1966;11:1109-15. 25. Gwinnett AJ. Human prismless enamel and its influence on sealant penetration, Arch Oral Biol 1973;18:441-4. 26. Sheykholeslam Z, Buonocore MG. Bonding of resins to phosphoric acld-etched enamel surface of permanent and deciduous teeth. J Dent Res 1972;81:1572-6. 27. Eidelman E. The structure of the enamel in primary teeth: practical applications in restorative techniques. J Dent Child i976; 43:172-6. 28. Bozalis WG, Marshall GW, Cooley RO, Mechanical pretreatment and etching of primary tooth enamel, J Dent Child 1979;46:43-9. 29. Fuks AB, Eidelman E, Shapira J. Mechanical and acid treatment of the pdsmless layer of primary teeth vs. acid etching only: a SEM study. J Dent Child 1977;44:222-5. 30. Carstensen W. Clinical results after direct bonding of brackets using shorter etching times. AM J ORT~OD 1986;89:70-2. 31. Labart WA, Barkmeier WW, Taylor MH. Bracket retention after 15 second acid conditioning. J Clin Orthod 1988;22:224-5. Reprint requests to: Dr. Wei Nan Wang Orthodontic Section Department of Td-Serviee General Hospital School of Dentistry National Defense Medical Center Taipei, Taiwan Republic of China 107
