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Enamel pretreatment: A critical variable in direct bonding systems
Enamel pretreatment: A critical variable in direct bonding systems Nann
A.
Lexington,
Wickwire,
D.D.S.,
M.S.D.,*
and
Daniel
Rentz,
D.D.S.,
M.S.D.**
Ky., and Coral Gables, Pla.
D
ental investigators have followed the example of industry in using acid pretreatment of enamel surfaces in order to enhance adhesion of coating materials. In 1955 Buonocorel demonstrated a markedly increased adhesion of acrylic restorative materials when their application was preceded by enamel treatment with 85 per cent phosphoric acid for 30 seconds. At present, all of the several techniques for direct bonding of orthodontic attachments employ some type of acid pretreatment. The critical nature of this step is becoming increasingly appreciated. The present article reports clinical and histologic studies of the influence of time on acid pretreatment of enamel surfaces. Methods
and
materials
(in vivo) studies.
Clinical
INVESTIGATION: BONDING TO PREMOLARS. A preliminary investigation of optimal acid etching time in vivo was conducted on patients receiving orthodontic treatment at the University of Kentucky College of Dentistry. The first study was designed to test in vivo, a range of enamel pretreatment times. Fifteen patients, 11 to 18 years of age, who needed four first premolars removed a,s part of their orthodontic trealtment were chosen as subjects. The buccal surfaces of the four first premolars of each patient were randomly selected to receive a 0-, l-, 2-, or 4-minute acid treatment. Each selected tooth was dried and isolated with cotton rolls. A cotton swab saturated with 50 per cent phosphoric acid (Ames cement liquid, pH 1.6, 6 M phosphoric PRELIMrNABY
This study was Kentucky College *Associate of Dentistry. **Present
Professor, address:
supported of Dentistry. Department 299 Alhambra
in
part
by
GRS
of
Orthodontics,
Circle,
Coral
Grant
RRO5314-11,
University Gables,
of
University Kentucky
of College
Fla. 499
Diagram
of the
strain
gauge
attached
to the
upper
member
of the
plier
beam.
acid) was continuously agitated across the enamel surface. (Agitation is important in controlling the rate of enamel dissolution and the depth of acid penetration. The nature of enamel dissolution creates a self-limiting penetration of acid as the solution quickly becomes saturated with calcium and phosphate ions.2) The etched enamel was washed with water and again isolated and dried. At no time was saliva allowed to contaminate the etched surface. The adhesive material selected for bonding was Sevriton Simplified* acrylic resin. The Sevriton was mixed rapidly according to the manufacturer’s direeCons while Sevriton cavity seal was painted on the tooth. One drop of the liquid mix was applied to the etched enamel surface and a second drop filled an undercut area on the Kentucky bracket .3 The bracket was positioned and held with light pressure on the tooth for one minute. The area was maintained in a dry and isolated state for an additional 10 minutes. The pa,Cents were instructed to record the dates if any brackets came oft-‘. Three months later the treated premolars were removed. These teeth were tagged and collected for observation with a scanning electron microscope. STRENGTH OF BONDS TO I~ISORS. The second clinical study was to relate a range of enamel acid pretreatment times in terms of strength of the bonding system in vivo. Ten patients with an average age of 13 years were selected from the first group of clinical participants. All anterior teeth of each patient received bonded Kentucky brackets, with the same bonding procedures as previously described. Each anterior quadrant containing a central incisor, a lateral incisor, and a canine was treated as one group. Following a restricted randomization design, each tooth from each group was randomly treated with an acid etching time of 2, 4, or 6 minutes. After the bonded brackets and conventional posterior bands were placed for each patient, leveling arch wires were inserted and orthodontic treatment was initiated. The experimental design stipulated that the brackets would remain in place no more than 16 weeks. If a bracket became dislodged before the termination date, the tooth and date were recorded. At the end of the sixteenth week, a bond strength determination was made for each remaining bonded tooth. The basic constituent parts of the measuring *Amalgamated
Dental Trade Distributors,
Ltd., London, England.
Enamel pretreatment
Fig.
2.
Patient to
20
Electron M. microns.
micrograph
of
S. (Magnifications: C, X2,000;
A, bar
natural X500;
is equal
surface bar to
of
is equal 10
maxillary to
40
microns.
right
first B, X1,000;
premolar bar
501
from is equal
microns.)
system were band-removing pliers with a strain gauge (micro measurement Za-xx 350DD-350) cemented to the upper member of the pliers, an amplifying system, and a. recording instrument. The band-removing pliers were calibrated to measure the shear force required to dislodge the brackets (Fig. 1). The strain gauge was linked through a Wheatstone bridge to a Beckman Type S-11 Dynograph which provided the amplifying circuitry and the direct-writing recording system. At the time of the bracket removal, the patient was seated in a Fara.day cage, which eliminated extraneous electrical interference.- The dislodged brackets and residual bonding material were labeled and placed in a sterile solution for later study with a sca,nning electron microscope (SE&I). Histologic studies. In addition to the premolar teeth from patients in the clinical study, additional freshly extracted premola.rs were selected for etching, bonding, and histologic study of acrylic and enamel surfaces as follows: A. Four first premolars from the same patient, whi.ch were treated in vitro with 0-, 2-, 4-, or 6-minute etching. Enamel surfaces were examined with the scanning electron microscope. B. Four first premolars from the same patient, which were treated in vitro with 0-, 2-, 4, or 6-minute etching. After the enamel surface had been fractured, one half was retained and the other was coated with
Fig. 3. Electron micrograph of enamel surface of maxillary ieft premolar etched for 2 minutes with 50 per cent phosphoric acid. (Magnifications: equal to 40 microns. E, X1,000; bar is equal to 20 microns. F, X2,000; 10 microns.)
of Patient D, X500; bar bar is equal
is to
Sevriton. The enamel sections with the acrylic in place were then deposited in 10 per cent hydrochloric acid, which dissolved away the enamel while leaving the acrylic intact. The acrylic subsurface was then examined with the scanning electron microscope and compared to the corresponding etched enamel surface. C. Three first premolars to which Sevriton was bonded in vitro after 2-, 4-, or 6-minute etching. The enamel-adhesive interface was examined with the scanning electron microscope after the teeth were fractured or CUtA
D. Three premolar teeth from the same patient which were coated with wax on one half of the buccal surface immediately after extraction. The exposed half of the enamel surface was etched for 2, 4, or 6 minutes and then washed with water. The teeth were embedded and sectioned to 140 microns with a Kobler hard-tissue sectioner. These sections were examined under incident light at a magnification of x1,000. The step which was present at the junction of the acid-exposed and unexposed areas of enamel surface was measured in microns by means of a calibrated eye piece. The enamel surface of premolars where bonding failures had occurred in
Volume Number
64 5
Enamel pretreatment
Fig. 4. Electron micrograph of enamel surface of mandibular Patient M. S., etched for 4 minutes with 50 per cent phosphoric X500; bar is equal to 40 microns. H, X1,000; bar is equal to is equal to 10 microns.)
vivo were examined with the scanning surfaces from bonding failures in vivo.
electron
right first premolar acid. (Magnifications: 20 microns. I, X2,000;
microscope,
503
from G, bar
as were acrylic
Results
Clinica~l sCudies. Results of the in vivo study of bonded brackets on premolar’s found that all bonds failed within 1 week without acid etching; eleven of fifteen failed within 2 days. With l-minute etching, a single bracket remained in place for 90 days, but more than half failed within the first week. With 2-minute etching, eight bonded brackets remained for 90 days and two for 60 days, with the remainder failing between 7 and 60 days. The 4-minute enamel pretreatment resulted in four bonds remaining for 60 days and nine bonds remaining the entire 90 days. Nineteen out of 12’0 bonds to anterior teeth failed prior to the testing period at 16 weeks. In each case, the bonds failed at the enamel-adhesive interface. Nine bonds failed with an enamel pretreatment time of 2 minutes, while for the 4- and 6-minute pretreatment times there were five bond failures each. Of the total bracket failures, fifteen occurred on the maxillary central or lateral incisors; seventeen of the nineteen failures occu.rred in the maxillary arch. Two of the ten patients experienced no failures. There were from one to four bracket failures in each of the remaining eight patients. It was observed that most of the bonds fractured at the enamel-adhesive
ig. 5. Electron micrograph of enamei surface of mandibuiar Patient M. S., etched for 6 minutes with 50 per cent phosphoric X500; bar is equal to 48 microns. K, X1,000; bar is equal to is equal to 10 microns.)
left first premolar acid. (Magnifications: 20 microns. L, X2,000;
from J, bar
interface. In nine of the :I01 bonds a moderate amount of bonding material remained on the toot,h following the bond fracture. In this instance, the measure was of the strength of the adhes,ive material itself. Histologic shdies. The electron micrographs in Figs. 2, 3, 4, and 5 show the premolar enamel patterns from the same patient at three magnifications following in vitro acid pretreatment times of 0, 2, 4, and 6 minutes. Fig. 2 demonstrates the control natural buccal surface of the tooth, Yerikymata are seen as troughs, each containing rows’ of small depressions. These troughs are separated by relatively smooth ridges. It appears that there is an outline of the enamel prism in the troughs and that the enamel is prism free over the ridges. Fig. 3 demonstrates the effect of a 2-minute acid etching. There is a~variation in degree of etching, ranging from no change to distinct holes 3 or 4 microns wide. The appearance of the holes suggests that the defects were in the centers of the prisms. Fig. 4 demonstrates the effect of a. 4.minute acid etching. There is a regular pattern of distinct depressions and elevations. The depressions probably correspond to the prism centers. The appearance of this etching patt.ern suggests a large surface area exposure and a regular pattern of depressions into which the adhesive could flow.
17olum,e
IFwnber
64 5
Enamel pretreatment
Fig. 6. Electron micrographs of enamel surface with its corresponding demonstrating adhesive penetration into the enam,el surface. d, Natural of maxillary left first premolar from Patient K. D. Q, Corresponding acrylic
acrylic enamel portion
505
portion surface which
was previously bonded but had the enamel dissolved away by 10 per cent hydrochloric acid. f, Enamel surface of mandibular left first premolar from Patient K. D., etched for 2 minutes. g, Corresponding acrylic portion previously bonded to a section of enamel which was etched for 2 Iminutes and then dissolved away with 10 per cent hydrochloric acid. (Magnification, X1,000; bar is equal to 20 microns.)
Fig. 5 demonstrates the effect of a 6-minute acid etching. It appears that the acid has either dissolved a,way the regular etching pattern seen in the 4-minute etching or that the acid has penetrated into a softer layer of enamel. The enamel surface appears as either wide, shallow craters measuring 10 microns in diameter or as a flattened amorphous structure surrounding the craters. Figs. 6, 7, 8, and 9 show electron micrographs of enamel etching patterns with corresponding acrylic penetration configurations. The control sample shown in Fig. 6 is similar to the natural surface shown in Fig, 2. The acrylic wetting and penetration of the enamel surface are almost nonexistent. The 2-minute acid etching of the enamel is also similar to the previous mierographs shown in Fig. 3. The acrylic penetration and wetting are improved over the control sample. The 4-minute etched surface of enamel shown in Fig. 7 does not quite resemble the 4-minute treatment in Fig. 4. However, the same type of pattern of increased surface area can be observed. The acrylic does not have any great
Fig. 7. Electron micrograph of enamel surfaces etched for 4 acrylic surfaces demonstrating adhesive penetration into the corresponding acrylic portion which was previously bonded which had been etched for 4 minutes with 50 per cent phosphoric away by 10 per cent hydrochloric acid. i and k show enamel first premolar from Patient K. D., which had been etched for
minutes and corresponding enamel. h and i show the to a section af enamel acid and then dissolved surface of right mandibular 4 minutes. (Magnifications:
h, i, and
is equal
i, X1,000;
bar
is equal
to
20 microns.
k, X5,000;
bar
to
2 microns.)
depth of penetration, but the wetting abil.ity and the surface area contact appear to be very great. The 6-minute surface enamel etching pattern of Fig. 8 somewhat resembles the pattern seen in Fig. 5. Both have large, shallow craters and a diffused flattening. The acrylic penetration appears as though there has been less wetting and surface area contact, than in the 4-minute treatment. Buonocore and his associates” have described “taglike” extensions of Sevriton penetrating up to 25 microns into an enamel surface etched for 1 minute. These “taglike” extensions at the enamel-adhesive interface could not be demonstrated with the scanning electron microscope at any pretreatment timing levels (Fig. 9). Fig. 10 shows surface topography of the acrylic backing of 0- and 4-minute enamel etching times. These bonds were placed in the mouth the same day. The 0 etching bond remained 4 days, while the enamel surface etched for 4 minutes remained bonded for 77 days. The photomicrographs of the O-minute pretreatment acrylic showed a sparse
Volume
Number
Fig. 8. phosphoric I and section 10 per premolar X500; X5,000;
64 5
Electron micrograph of enamel surface acid and acrylic surface demonstrating rn show the corresponding acrylic portion of the enamel which had been etched for cent hydrochloric acid. n and o show from Patient K. D., which had been bar is equal to 40 microns. m and n, bar is equal to 2 microns.]
Enamel pretreatment
507
etched
for 6 minutes with 50 per cent adhesive penetration into enamel. which was previously bonded to a 6 minutes and then dissolved away by enamel surface of maxillary right first etched for 6 minutes. [Magnifications: I, Xl ,000; bar is equal to 20 microns. o,
wetting of the enamel. The 4-minute acrylic backing had almost total wettability of the enamel. In a scanning electron microscope review of premaslars which had been bonded in vivo, it was determined that, following bonding failure, the amount of acrylic left on and in the enamel surface was very sparse. The enamel surface appeared to have returned to i.ts natural appearance. The cut sections of pretreated enamel viewed under the light microscope at x1,000 showed moderate with@-tooth variation. This variation caused the measurements to be more of an estimate of .enamel dissolution. The amount of enamel dissolution of the 2-minute acid-treated surface ranged from 5 to 10 microns; the 4-minute surface, 5 to 15 microns; and the 6-minute surface, 20 to 30 microns. Xtatistical results. Bond strengths for varying etching times and locations are shown in Table I. Data are pooled, ignoring individual tooth differences
ig.
9.
surface
Electron
micrograph
that
X100;
had
bar
is equal
r is equal
Tabie
to
I. Bond
of
been 20
strength
etched to
200
fractured for microns.
sections 6
of
minutes.
a,
P, X500;
enamel-adhesive Adhesive;
bar
interface
e,
is equal
enamel. to
A. Add etch 2 minutes 4 minutes 6 minutes B. Arch Right Left Qper Lower
microns.
8,
microns.]
related
to etch
time
and
arch
location
Hean bond strength Condition
40
of
iv
(PmnW
enamei
(Magnifications:
Standard
deviation Ilange
(sl
time 31 35 35
13.24 17.69 14.83
4.6-31.3 5.8-35.0 4.0-27.6
5.76 7.47 6.13
51 50 43 58
14.58 15.85 14.31 16.57
5.1-33.1 4.0-35.0 4.0-30.9 6.1-35.0
7.07 7.0 6.55 7.55
location
Xl ,000;
v01ume Number
64 5
10. Electron micrograph of acrylic backings from tooth and on a tooth etched for 4 minutes. S and T premolar from Patient B. C., which received no acid maxillary left first premolar from Patient B. C., which (Magnifications: S, X100; bar is equal to 200 microns. V, Xl 00; bar is equal to 200 microns. W, X500; bar is Fig.
Enamel pretreatment
509
bonds that failed on an unetched were bonded on maxillary right first etching. V and W were bonded on was etched with acid for 4 minutes. T, X500; bar is equal to 40 microns. equal to 40 microns.)
(the possibility that canines may react differently from central or lateral incisors). Tabulating the data in this manner, the differences between the 2- or 6-minute and 4-minute etchin.g times are statistically significant (t = 2.57; p < 0.05). Bonds in the lower arch are stronger than in the upper, but the differences are not statistically significant. Inspection of individual data, as well as our clinical experience, leads us to suspect that bonding to canines may be more successful than bonding to incisors, particularly in the maxillary arch. Analysis, of variance with a three-factor design using etch time, quadrant, and type of tooth was attempted but could’ not be done, probably because tooth type and quadrant rather than independent factors were related. Two two-factor analyses are shown in Table II. Tooth type here did not show up as a significant variable, but tooth-type quadrant interactions cannot be ruled out and may, in fact, exist. Discussion
Clinical considerations. Investigator9 have demonstrated that acid treatment greatly increases the surface area and wettability of the enamel surface. These
Table
II. Analysis
of variance
(SSi
I, Etch&g time x qua,drant A (Etching time) B (Quadrant) AB II.
Etching
cell time
x tooth
A (Etching time) B (Tooth
AB Within
type) cell
unequal
Sum of squares
Source of variation
Within
for
378.0 150.5 118.34 4,007.7
cell
,frequencies,
effects
on bond
strength
Degrees of freedom (df “)
Mean square (sm)
P
2 3 6 89
189.0 50.2 19.7 45.0
4.20 t 1.12 0.44
2 5 10 83
192.4 52.5 23.27 60.50
3.u3t 0.87 0.38
type
384.81 262.52 232.75 5.021.6
*Totall df = 100; n = 101 = number of teeth with bond remaining at end of experimental period. t Significant at 0.05 level. factors provide increased mechanical strength to an enamel-adhesive bond system. The present study found that 4-minute enamel pretreatment with 50 per cent buffered phosphoric acid gave a significantly stronger bond. The acid etching patterns observed in Fig. 7 support the interpretation of increased wettability and surface area contact. Some investigator+ 4, d consider chemical action as part of the bonding phenomenon. Chemical (polar) bonding is unlikely in vivo, since such bonds would be cancelled by the saturation of polar groups b,y water molecules in the oral environment. Van der Waals’ forces would probably produce bonds which would measure the strength of the adhesive itself instead of measuring the bond at the’ enamel-adhesive interface. The tensile strength of Sevriton is 2,000 p.s.i.7; average bond strengths in this study were 248 p.s.i. These bond strengths do not approach the strength of the adhesive itself, which gives support to the mechanical locking explanation of bonding. The superior bond strength and the photomicrographs of the 4-minute a.cid pretreatment also support the mechanical locking explanation of adhesion. At 4 minutes, there is a uniform roughness and increased surface area of enamel into which the adhesive can flow and polymerize. The surfaces etched for 2 and 6 minutes are somewhat similar in diffuse flatness, with the 6-minute etch differing only in its large shallow craters. These etching patterns were supported by the quantitative measurements of depth of enamel dissolution for the three acid pretreatment times. The 2and 4-minute treatments dissolved away 5 to 15 microns of the enamel surface, still leaving part of the outer denser layer of enamel. The 20 to 30 microns of denser enamel dissolved away with the 6-minute treatment is excessive, leaving an area of softer enamel exposed8 This can explain the large craters and flatness of the 6-minute treatment. It can be concluded, therefore, that the 4-minut.e acid treatment exposes
Volume Number
64 5
Enamel pretreatment
511
the best surface for wetting and surface area contact, that adhesion with Sevriton is due to mechanical bonding, and that the surface for bonding remains in a physically harder area of enamel. There are variables in the bonding system which can alter the concept of an “ideal” pretreatment acid time. These include the maturation of teeth with their acquired organic pellicle and fluoride incorporation which may slow down surface demineralization. Within the same patient, variability of surface enamel from tooth to tooth has been demonstrated9 In the youthful subjects of this study there were differences in time of emergence of the premolars, with a subsequent acquired organic pellicle of differing thicknesses. Physical factors, such as ‘%idedness” in brushing, could also alter the surface enamel environment prior to etching. In these instances, clinical judgment must be used to detect enamel surfaces that do not show the typical dull satin appearance of etched enamel following a 4minute acid treatment. An additional increment of 2 minutes should be considered in order to prepare the enamel to receive an adhesive. The effect of etching surface enamel which contains the fluorapatite crystalline formation, as compared to nonfluoridated enamel, was not reviewed in this study. The patients selected for this project were representative of young persons seen in orthodontic offices. Their birthplaces were geographically widespread, and only one child had lived in a fluoridated community since birth. All had at some time received topical fluoride treatments, and they usually used fluoridated dentifrices. On the basis of the well.-recognized resistance of fluoridated enamel to acid disso1ution,10 one should cons’ider the history of enamel in regard to fluorides as a variable factor in etching time. The safety of acid pretreatment has been rep0rte.d by many investigators,ll, I2 who have found that the etched enamel surface returns to its normal appearance in 2 to 48 days. Similarly, in this study of 2- and 4-minute pretreatment in vivo, no remaining enamel defects were observed in SEM scanning. Clinically, it is not unusual to expose enamel to a pH of 1.6 for several minutes when placing orthodontic bands with zinc phosphate cements.12 By comparison, the 4-minute pretreatment time is conservative. There may be the advantage of reduced solubility of enamel that has been etched and bonded. Buonocore13 reported a reduction in caries incidence on enamel surfaces where adhesives had been used and failed. There are two possibilities for this finding. First, adhesive may remain within the enamel, rendering it more resistant; second, fluoride from the adhesive becomes incorporated as calcium fluoride in the enamel. Evidence from SEM scanning supports the latter possibility, for only scant remnants of adhesive were seen within the enamel. Conclwsion
This study provided a third dimension to successful direct bracket bonding. In addition to the adhesive system which must possess strength and a thermal coefficient similar to enamel, there is the requirement for a bracket that locks mechanically to the adhesive. The third critical variable is the acid pretreatment
Wickwire
ad
Bemk
time of the enamel. The J-minute agitated application of 50 per cent phosphoric acid (pH 1.6, 6 molar, Ames cement liquid) was found clinically and histologically to best meet the requirements of strength and safety in the pretreatment of enamel for adhesion of bandless orthodontic brackets. REFERENCES
1. Buonocore, Michael G.: A simple method of increasing the adhesion of acrylic filling materials to ena,mel surfaces, J, Dent. R,es. 34: 849853, 1955. 2. Gray, J. A., Francis, M. D., and Griebstein, W. J.: Chemistry of Enamel Dissolution. In Sognnaes, R. F. (editor) : Chemistry and prevention of dental caries, Springfield, Ill., 1962, Charles C Thomas, Publisher. 3. Parker, W. T. : Thesis for certificate, University of Kentucky, 1970. 4. Buonocore, M. G., Matsui, A., and Gwinnett, A.: Penetration of resin dental materials into enamel surfaces with reference to bonding, Arch. Oral Biol. 13: 61-70, 1968. 5. Newman, George V., and Sharpe, Louis, H.: On wettability of tooth surfaces (preliminary investigation), N. J. State Dent. Sot. J. 37: 2819-293, 1966. 6. Gwinnett, A. J., and Matsui, A.: A study of enamel adhesives-The physical relationship between enamel a.nd adhesive, arch. Oral Biol. 12: 16~151620, 1967. 7. Pritchett, C. E. : Thesis for certificate, University of Kentucky, 1970. 8. Meckel, A. H. : Personal communication, September, 1970. 9. Mannerberg, F.: Changes in the enamel surface in cases of erosion: A replica study, Arch. Oral Biol., Spee. Supp. 4: 59-62, 1961. IO. Tyler, J. E.: Comparat.ive diss,olution studies on human enamel and fluorapatite, Caries Res. 4: 23-30, 1970. 11. Lenz, H., and Miihlemann, J. R.: In vivo and in vitro effects of saliva on etched or mechanically marked enamel after certain periods of time, Helvet. Odontol. Acta 7: 30-33, 1963. 12. Newman, George V.: Epoxy adhesives for orthodontic attachments : Progress report, AM. J. ORTHOD. 51: 901-912, 1965. 13. Buonocore, M.: Caries prevention in pits and fissures sealed with an adhesive resin polymerized by ultraviolet light: A two-year study of single adhesive application, J. Am. Dent. Assoc. 82: 1090-1093, 1971.
A.
Lexington,
Wickwire,
D.D.S.,
M.S.D.,*
and
Daniel
Rentz,
D.D.S.,
M.S.D.**
Ky., and Coral Gables, Pla.
D
ental investigators have followed the example of industry in using acid pretreatment of enamel surfaces in order to enhance adhesion of coating materials. In 1955 Buonocorel demonstrated a markedly increased adhesion of acrylic restorative materials when their application was preceded by enamel treatment with 85 per cent phosphoric acid for 30 seconds. At present, all of the several techniques for direct bonding of orthodontic attachments employ some type of acid pretreatment. The critical nature of this step is becoming increasingly appreciated. The present article reports clinical and histologic studies of the influence of time on acid pretreatment of enamel surfaces. Methods
and
materials
(in vivo) studies.
Clinical
INVESTIGATION: BONDING TO PREMOLARS. A preliminary investigation of optimal acid etching time in vivo was conducted on patients receiving orthodontic treatment at the University of Kentucky College of Dentistry. The first study was designed to test in vivo, a range of enamel pretreatment times. Fifteen patients, 11 to 18 years of age, who needed four first premolars removed a,s part of their orthodontic trealtment were chosen as subjects. The buccal surfaces of the four first premolars of each patient were randomly selected to receive a 0-, l-, 2-, or 4-minute acid treatment. Each selected tooth was dried and isolated with cotton rolls. A cotton swab saturated with 50 per cent phosphoric acid (Ames cement liquid, pH 1.6, 6 M phosphoric PRELIMrNABY
This study was Kentucky College *Associate of Dentistry. **Present
Professor, address:
supported of Dentistry. Department 299 Alhambra
in
part
by
GRS
of
Orthodontics,
Circle,
Coral
Grant
RRO5314-11,
University Gables,
of
University Kentucky
of College
Fla. 499
Diagram
of the
strain
gauge
attached
to the
upper
member
of the
plier
beam.
acid) was continuously agitated across the enamel surface. (Agitation is important in controlling the rate of enamel dissolution and the depth of acid penetration. The nature of enamel dissolution creates a self-limiting penetration of acid as the solution quickly becomes saturated with calcium and phosphate ions.2) The etched enamel was washed with water and again isolated and dried. At no time was saliva allowed to contaminate the etched surface. The adhesive material selected for bonding was Sevriton Simplified* acrylic resin. The Sevriton was mixed rapidly according to the manufacturer’s direeCons while Sevriton cavity seal was painted on the tooth. One drop of the liquid mix was applied to the etched enamel surface and a second drop filled an undercut area on the Kentucky bracket .3 The bracket was positioned and held with light pressure on the tooth for one minute. The area was maintained in a dry and isolated state for an additional 10 minutes. The pa,Cents were instructed to record the dates if any brackets came oft-‘. Three months later the treated premolars were removed. These teeth were tagged and collected for observation with a scanning electron microscope. STRENGTH OF BONDS TO I~ISORS. The second clinical study was to relate a range of enamel acid pretreatment times in terms of strength of the bonding system in vivo. Ten patients with an average age of 13 years were selected from the first group of clinical participants. All anterior teeth of each patient received bonded Kentucky brackets, with the same bonding procedures as previously described. Each anterior quadrant containing a central incisor, a lateral incisor, and a canine was treated as one group. Following a restricted randomization design, each tooth from each group was randomly treated with an acid etching time of 2, 4, or 6 minutes. After the bonded brackets and conventional posterior bands were placed for each patient, leveling arch wires were inserted and orthodontic treatment was initiated. The experimental design stipulated that the brackets would remain in place no more than 16 weeks. If a bracket became dislodged before the termination date, the tooth and date were recorded. At the end of the sixteenth week, a bond strength determination was made for each remaining bonded tooth. The basic constituent parts of the measuring *Amalgamated
Dental Trade Distributors,
Ltd., London, England.
Enamel pretreatment
Fig.
2.
Patient to
20
Electron M. microns.
micrograph
of
S. (Magnifications: C, X2,000;
A, bar
natural X500;
is equal
surface bar to
of
is equal 10
maxillary to
40
microns.
right
first B, X1,000;
premolar bar
501
from is equal
microns.)
system were band-removing pliers with a strain gauge (micro measurement Za-xx 350DD-350) cemented to the upper member of the pliers, an amplifying system, and a. recording instrument. The band-removing pliers were calibrated to measure the shear force required to dislodge the brackets (Fig. 1). The strain gauge was linked through a Wheatstone bridge to a Beckman Type S-11 Dynograph which provided the amplifying circuitry and the direct-writing recording system. At the time of the bracket removal, the patient was seated in a Fara.day cage, which eliminated extraneous electrical interference.- The dislodged brackets and residual bonding material were labeled and placed in a sterile solution for later study with a sca,nning electron microscope (SE&I). Histologic studies. In addition to the premolar teeth from patients in the clinical study, additional freshly extracted premola.rs were selected for etching, bonding, and histologic study of acrylic and enamel surfaces as follows: A. Four first premolars from the same patient, whi.ch were treated in vitro with 0-, 2-, 4-, or 6-minute etching. Enamel surfaces were examined with the scanning electron microscope. B. Four first premolars from the same patient, which were treated in vitro with 0-, 2-, 4, or 6-minute etching. After the enamel surface had been fractured, one half was retained and the other was coated with
Fig. 3. Electron micrograph of enamel surface of maxillary ieft premolar etched for 2 minutes with 50 per cent phosphoric acid. (Magnifications: equal to 40 microns. E, X1,000; bar is equal to 20 microns. F, X2,000; 10 microns.)
of Patient D, X500; bar bar is equal
is to
Sevriton. The enamel sections with the acrylic in place were then deposited in 10 per cent hydrochloric acid, which dissolved away the enamel while leaving the acrylic intact. The acrylic subsurface was then examined with the scanning electron microscope and compared to the corresponding etched enamel surface. C. Three first premolars to which Sevriton was bonded in vitro after 2-, 4-, or 6-minute etching. The enamel-adhesive interface was examined with the scanning electron microscope after the teeth were fractured or CUtA
D. Three premolar teeth from the same patient which were coated with wax on one half of the buccal surface immediately after extraction. The exposed half of the enamel surface was etched for 2, 4, or 6 minutes and then washed with water. The teeth were embedded and sectioned to 140 microns with a Kobler hard-tissue sectioner. These sections were examined under incident light at a magnification of x1,000. The step which was present at the junction of the acid-exposed and unexposed areas of enamel surface was measured in microns by means of a calibrated eye piece. The enamel surface of premolars where bonding failures had occurred in
Volume Number
64 5
Enamel pretreatment
Fig. 4. Electron micrograph of enamel surface of mandibular Patient M. S., etched for 4 minutes with 50 per cent phosphoric X500; bar is equal to 40 microns. H, X1,000; bar is equal to is equal to 10 microns.)
vivo were examined with the scanning surfaces from bonding failures in vivo.
electron
right first premolar acid. (Magnifications: 20 microns. I, X2,000;
microscope,
503
from G, bar
as were acrylic
Results
Clinica~l sCudies. Results of the in vivo study of bonded brackets on premolar’s found that all bonds failed within 1 week without acid etching; eleven of fifteen failed within 2 days. With l-minute etching, a single bracket remained in place for 90 days, but more than half failed within the first week. With 2-minute etching, eight bonded brackets remained for 90 days and two for 60 days, with the remainder failing between 7 and 60 days. The 4-minute enamel pretreatment resulted in four bonds remaining for 60 days and nine bonds remaining the entire 90 days. Nineteen out of 12’0 bonds to anterior teeth failed prior to the testing period at 16 weeks. In each case, the bonds failed at the enamel-adhesive interface. Nine bonds failed with an enamel pretreatment time of 2 minutes, while for the 4- and 6-minute pretreatment times there were five bond failures each. Of the total bracket failures, fifteen occurred on the maxillary central or lateral incisors; seventeen of the nineteen failures occu.rred in the maxillary arch. Two of the ten patients experienced no failures. There were from one to four bracket failures in each of the remaining eight patients. It was observed that most of the bonds fractured at the enamel-adhesive
ig. 5. Electron micrograph of enamei surface of mandibuiar Patient M. S., etched for 6 minutes with 50 per cent phosphoric X500; bar is equal to 48 microns. K, X1,000; bar is equal to is equal to 10 microns.)
left first premolar acid. (Magnifications: 20 microns. L, X2,000;
from J, bar
interface. In nine of the :I01 bonds a moderate amount of bonding material remained on the toot,h following the bond fracture. In this instance, the measure was of the strength of the adhes,ive material itself. Histologic shdies. The electron micrographs in Figs. 2, 3, 4, and 5 show the premolar enamel patterns from the same patient at three magnifications following in vitro acid pretreatment times of 0, 2, 4, and 6 minutes. Fig. 2 demonstrates the control natural buccal surface of the tooth, Yerikymata are seen as troughs, each containing rows’ of small depressions. These troughs are separated by relatively smooth ridges. It appears that there is an outline of the enamel prism in the troughs and that the enamel is prism free over the ridges. Fig. 3 demonstrates the effect of a 2-minute acid etching. There is a~variation in degree of etching, ranging from no change to distinct holes 3 or 4 microns wide. The appearance of the holes suggests that the defects were in the centers of the prisms. Fig. 4 demonstrates the effect of a. 4.minute acid etching. There is a regular pattern of distinct depressions and elevations. The depressions probably correspond to the prism centers. The appearance of this etching patt.ern suggests a large surface area exposure and a regular pattern of depressions into which the adhesive could flow.
17olum,e
IFwnber
64 5
Enamel pretreatment
Fig. 6. Electron micrographs of enamel surface with its corresponding demonstrating adhesive penetration into the enam,el surface. d, Natural of maxillary left first premolar from Patient K. D. Q, Corresponding acrylic
acrylic enamel portion
505
portion surface which
was previously bonded but had the enamel dissolved away by 10 per cent hydrochloric acid. f, Enamel surface of mandibular left first premolar from Patient K. D., etched for 2 minutes. g, Corresponding acrylic portion previously bonded to a section of enamel which was etched for 2 Iminutes and then dissolved away with 10 per cent hydrochloric acid. (Magnification, X1,000; bar is equal to 20 microns.)
Fig. 5 demonstrates the effect of a 6-minute acid etching. It appears that the acid has either dissolved a,way the regular etching pattern seen in the 4-minute etching or that the acid has penetrated into a softer layer of enamel. The enamel surface appears as either wide, shallow craters measuring 10 microns in diameter or as a flattened amorphous structure surrounding the craters. Figs. 6, 7, 8, and 9 show electron micrographs of enamel etching patterns with corresponding acrylic penetration configurations. The control sample shown in Fig. 6 is similar to the natural surface shown in Fig, 2. The acrylic wetting and penetration of the enamel surface are almost nonexistent. The 2-minute acid etching of the enamel is also similar to the previous mierographs shown in Fig. 3. The acrylic penetration and wetting are improved over the control sample. The 4-minute etched surface of enamel shown in Fig. 7 does not quite resemble the 4-minute treatment in Fig. 4. However, the same type of pattern of increased surface area can be observed. The acrylic does not have any great
Fig. 7. Electron micrograph of enamel surfaces etched for 4 acrylic surfaces demonstrating adhesive penetration into the corresponding acrylic portion which was previously bonded which had been etched for 4 minutes with 50 per cent phosphoric away by 10 per cent hydrochloric acid. i and k show enamel first premolar from Patient K. D., which had been etched for
minutes and corresponding enamel. h and i show the to a section af enamel acid and then dissolved surface of right mandibular 4 minutes. (Magnifications:
h, i, and
is equal
i, X1,000;
bar
is equal
to
20 microns.
k, X5,000;
bar
to
2 microns.)
depth of penetration, but the wetting abil.ity and the surface area contact appear to be very great. The 6-minute surface enamel etching pattern of Fig. 8 somewhat resembles the pattern seen in Fig. 5. Both have large, shallow craters and a diffused flattening. The acrylic penetration appears as though there has been less wetting and surface area contact, than in the 4-minute treatment. Buonocore and his associates” have described “taglike” extensions of Sevriton penetrating up to 25 microns into an enamel surface etched for 1 minute. These “taglike” extensions at the enamel-adhesive interface could not be demonstrated with the scanning electron microscope at any pretreatment timing levels (Fig. 9). Fig. 10 shows surface topography of the acrylic backing of 0- and 4-minute enamel etching times. These bonds were placed in the mouth the same day. The 0 etching bond remained 4 days, while the enamel surface etched for 4 minutes remained bonded for 77 days. The photomicrographs of the O-minute pretreatment acrylic showed a sparse
Volume
Number
Fig. 8. phosphoric I and section 10 per premolar X500; X5,000;
64 5
Electron micrograph of enamel surface acid and acrylic surface demonstrating rn show the corresponding acrylic portion of the enamel which had been etched for cent hydrochloric acid. n and o show from Patient K. D., which had been bar is equal to 40 microns. m and n, bar is equal to 2 microns.]
Enamel pretreatment
507
etched
for 6 minutes with 50 per cent adhesive penetration into enamel. which was previously bonded to a 6 minutes and then dissolved away by enamel surface of maxillary right first etched for 6 minutes. [Magnifications: I, Xl ,000; bar is equal to 20 microns. o,
wetting of the enamel. The 4-minute acrylic backing had almost total wettability of the enamel. In a scanning electron microscope review of premaslars which had been bonded in vivo, it was determined that, following bonding failure, the amount of acrylic left on and in the enamel surface was very sparse. The enamel surface appeared to have returned to i.ts natural appearance. The cut sections of pretreated enamel viewed under the light microscope at x1,000 showed moderate with@-tooth variation. This variation caused the measurements to be more of an estimate of .enamel dissolution. The amount of enamel dissolution of the 2-minute acid-treated surface ranged from 5 to 10 microns; the 4-minute surface, 5 to 15 microns; and the 6-minute surface, 20 to 30 microns. Xtatistical results. Bond strengths for varying etching times and locations are shown in Table I. Data are pooled, ignoring individual tooth differences
ig.
9.
surface
Electron
micrograph
that
X100;
had
bar
is equal
r is equal
Tabie
to
I. Bond
of
been 20
strength
etched to
200
fractured for microns.
sections 6
of
minutes.
a,
P, X500;
enamel-adhesive Adhesive;
bar
interface
e,
is equal
enamel. to
A. Add etch 2 minutes 4 minutes 6 minutes B. Arch Right Left Qper Lower
microns.
8,
microns.]
related
to etch
time
and
arch
location
Hean bond strength Condition
40
of
iv
(PmnW
enamei
(Magnifications:
Standard
deviation Ilange
(sl
time 31 35 35
13.24 17.69 14.83
4.6-31.3 5.8-35.0 4.0-27.6
5.76 7.47 6.13
51 50 43 58
14.58 15.85 14.31 16.57
5.1-33.1 4.0-35.0 4.0-30.9 6.1-35.0
7.07 7.0 6.55 7.55
location
Xl ,000;
v01ume Number
64 5
10. Electron micrograph of acrylic backings from tooth and on a tooth etched for 4 minutes. S and T premolar from Patient B. C., which received no acid maxillary left first premolar from Patient B. C., which (Magnifications: S, X100; bar is equal to 200 microns. V, Xl 00; bar is equal to 200 microns. W, X500; bar is Fig.
Enamel pretreatment
509
bonds that failed on an unetched were bonded on maxillary right first etching. V and W were bonded on was etched with acid for 4 minutes. T, X500; bar is equal to 40 microns. equal to 40 microns.)
(the possibility that canines may react differently from central or lateral incisors). Tabulating the data in this manner, the differences between the 2- or 6-minute and 4-minute etchin.g times are statistically significant (t = 2.57; p < 0.05). Bonds in the lower arch are stronger than in the upper, but the differences are not statistically significant. Inspection of individual data, as well as our clinical experience, leads us to suspect that bonding to canines may be more successful than bonding to incisors, particularly in the maxillary arch. Analysis, of variance with a three-factor design using etch time, quadrant, and type of tooth was attempted but could’ not be done, probably because tooth type and quadrant rather than independent factors were related. Two two-factor analyses are shown in Table II. Tooth type here did not show up as a significant variable, but tooth-type quadrant interactions cannot be ruled out and may, in fact, exist. Discussion
Clinical considerations. Investigator9 have demonstrated that acid treatment greatly increases the surface area and wettability of the enamel surface. These
Table
II. Analysis
of variance
(SSi
I, Etch&g time x qua,drant A (Etching time) B (Quadrant) AB II.
Etching
cell time
x tooth
A (Etching time) B (Tooth
AB Within
type) cell
unequal
Sum of squares
Source of variation
Within
for
378.0 150.5 118.34 4,007.7
cell
,frequencies,
effects
on bond
strength
Degrees of freedom (df “)
Mean square (sm)
P
2 3 6 89
189.0 50.2 19.7 45.0
4.20 t 1.12 0.44
2 5 10 83
192.4 52.5 23.27 60.50
3.u3t 0.87 0.38
type
384.81 262.52 232.75 5.021.6
*Totall df = 100; n = 101 = number of teeth with bond remaining at end of experimental period. t Significant at 0.05 level. factors provide increased mechanical strength to an enamel-adhesive bond system. The present study found that 4-minute enamel pretreatment with 50 per cent buffered phosphoric acid gave a significantly stronger bond. The acid etching patterns observed in Fig. 7 support the interpretation of increased wettability and surface area contact. Some investigator+ 4, d consider chemical action as part of the bonding phenomenon. Chemical (polar) bonding is unlikely in vivo, since such bonds would be cancelled by the saturation of polar groups b,y water molecules in the oral environment. Van der Waals’ forces would probably produce bonds which would measure the strength of the adhesive itself instead of measuring the bond at the’ enamel-adhesive interface. The tensile strength of Sevriton is 2,000 p.s.i.7; average bond strengths in this study were 248 p.s.i. These bond strengths do not approach the strength of the adhesive itself, which gives support to the mechanical locking explanation of bonding. The superior bond strength and the photomicrographs of the 4-minute a.cid pretreatment also support the mechanical locking explanation of adhesion. At 4 minutes, there is a uniform roughness and increased surface area of enamel into which the adhesive can flow and polymerize. The surfaces etched for 2 and 6 minutes are somewhat similar in diffuse flatness, with the 6-minute etch differing only in its large shallow craters. These etching patterns were supported by the quantitative measurements of depth of enamel dissolution for the three acid pretreatment times. The 2and 4-minute treatments dissolved away 5 to 15 microns of the enamel surface, still leaving part of the outer denser layer of enamel. The 20 to 30 microns of denser enamel dissolved away with the 6-minute treatment is excessive, leaving an area of softer enamel exposed8 This can explain the large craters and flatness of the 6-minute treatment. It can be concluded, therefore, that the 4-minut.e acid treatment exposes
Volume Number
64 5
Enamel pretreatment
511
the best surface for wetting and surface area contact, that adhesion with Sevriton is due to mechanical bonding, and that the surface for bonding remains in a physically harder area of enamel. There are variables in the bonding system which can alter the concept of an “ideal” pretreatment acid time. These include the maturation of teeth with their acquired organic pellicle and fluoride incorporation which may slow down surface demineralization. Within the same patient, variability of surface enamel from tooth to tooth has been demonstrated9 In the youthful subjects of this study there were differences in time of emergence of the premolars, with a subsequent acquired organic pellicle of differing thicknesses. Physical factors, such as ‘%idedness” in brushing, could also alter the surface enamel environment prior to etching. In these instances, clinical judgment must be used to detect enamel surfaces that do not show the typical dull satin appearance of etched enamel following a 4minute acid treatment. An additional increment of 2 minutes should be considered in order to prepare the enamel to receive an adhesive. The effect of etching surface enamel which contains the fluorapatite crystalline formation, as compared to nonfluoridated enamel, was not reviewed in this study. The patients selected for this project were representative of young persons seen in orthodontic offices. Their birthplaces were geographically widespread, and only one child had lived in a fluoridated community since birth. All had at some time received topical fluoride treatments, and they usually used fluoridated dentifrices. On the basis of the well.-recognized resistance of fluoridated enamel to acid disso1ution,10 one should cons’ider the history of enamel in regard to fluorides as a variable factor in etching time. The safety of acid pretreatment has been rep0rte.d by many investigators,ll, I2 who have found that the etched enamel surface returns to its normal appearance in 2 to 48 days. Similarly, in this study of 2- and 4-minute pretreatment in vivo, no remaining enamel defects were observed in SEM scanning. Clinically, it is not unusual to expose enamel to a pH of 1.6 for several minutes when placing orthodontic bands with zinc phosphate cements.12 By comparison, the 4-minute pretreatment time is conservative. There may be the advantage of reduced solubility of enamel that has been etched and bonded. Buonocore13 reported a reduction in caries incidence on enamel surfaces where adhesives had been used and failed. There are two possibilities for this finding. First, adhesive may remain within the enamel, rendering it more resistant; second, fluoride from the adhesive becomes incorporated as calcium fluoride in the enamel. Evidence from SEM scanning supports the latter possibility, for only scant remnants of adhesive were seen within the enamel. Conclwsion
This study provided a third dimension to successful direct bracket bonding. In addition to the adhesive system which must possess strength and a thermal coefficient similar to enamel, there is the requirement for a bracket that locks mechanically to the adhesive. The third critical variable is the acid pretreatment
Wickwire
ad
Bemk
time of the enamel. The J-minute agitated application of 50 per cent phosphoric acid (pH 1.6, 6 molar, Ames cement liquid) was found clinically and histologically to best meet the requirements of strength and safety in the pretreatment of enamel for adhesion of bandless orthodontic brackets. REFERENCES
1. Buonocore, Michael G.: A simple method of increasing the adhesion of acrylic filling materials to ena,mel surfaces, J, Dent. R,es. 34: 849853, 1955. 2. Gray, J. A., Francis, M. D., and Griebstein, W. J.: Chemistry of Enamel Dissolution. In Sognnaes, R. F. (editor) : Chemistry and prevention of dental caries, Springfield, Ill., 1962, Charles C Thomas, Publisher. 3. Parker, W. T. : Thesis for certificate, University of Kentucky, 1970. 4. Buonocore, M. G., Matsui, A., and Gwinnett, A.: Penetration of resin dental materials into enamel surfaces with reference to bonding, Arch. Oral Biol. 13: 61-70, 1968. 5. Newman, George V., and Sharpe, Louis, H.: On wettability of tooth surfaces (preliminary investigation), N. J. State Dent. Sot. J. 37: 2819-293, 1966. 6. Gwinnett, A. J., and Matsui, A.: A study of enamel adhesives-The physical relationship between enamel a.nd adhesive, arch. Oral Biol. 12: 16~151620, 1967. 7. Pritchett, C. E. : Thesis for certificate, University of Kentucky, 1970. 8. Meckel, A. H. : Personal communication, September, 1970. 9. Mannerberg, F.: Changes in the enamel surface in cases of erosion: A replica study, Arch. Oral Biol., Spee. Supp. 4: 59-62, 1961. IO. Tyler, J. E.: Comparat.ive diss,olution studies on human enamel and fluorapatite, Caries Res. 4: 23-30, 1970. 11. Lenz, H., and Miihlemann, J. R.: In vivo and in vitro effects of saliva on etched or mechanically marked enamel after certain periods of time, Helvet. Odontol. Acta 7: 30-33, 1963. 12. Newman, George V.: Epoxy adhesives for orthodontic attachments : Progress report, AM. J. ORTHOD. 51: 901-912, 1965. 13. Buonocore, M.: Caries prevention in pits and fissures sealed with an adhesive resin polymerized by ultraviolet light: A two-year study of single adhesive application, J. Am. Dent. Assoc. 82: 1090-1093, 1971.