Effect of Temperature and Humidity on the Adhesive Strength of Orthodontic Direct Bonding Materials M. A. KHOWVASSAH, S. E. BISHARA, T. C. FRANCIS, and W. HENDERSON College of Dentistry, University of Iowa, Iowa City, Iowa 52242, USA, and Burlington, Iowa 52240
The effects of temperature and humidity on the adhesive strength of two direct bonding orthodontic materials were investigated. Tensile testing showed that the adhesive bond was much stronger than the polycarbonate brackets. There is a significant increase in tensile and shearlike strength after 15-, 22-, and 30-day incubation at 37 C and 100% humidity, as compared with initial strength (after 30 minutes) at room temperature. In the past several years, considerable interest has been generated in testing the application of several adhesives used in dentistry. Barnacle cement1 is currently the object of extensive research and at present has no clinical (or industrial) application. Epoxy resins24 are quite satisfactory adhesives in many respects; however, they generally are extremely sensitive to moisture contamination and tend to have prolonged setting time. Acrylic resins5-9 and polycarboxylate adhesives10-13 have been tested in the laboratory and are widely accepted for clinical use. Acid etching of enamel surfaces before application of acrylic resin adhesives improved the adhesion considerably5 and increased the bonding surface area without discernible damage to these surfaces.9'14'15 Some investigators theorize that in addition to the mechanical adhesion resulting from acid etching, the organic components of enamel and dentin may offer a means for chemical bonding with adhesives.9'16 Other investigators This investigation was supported, in part, by USPHS Grant No. IR23DE03380-02 and USPHS General Research Support Grant No. 5SO -RRO531 3- 11, from the National Institute of Dental Research, National Institutes of Health, Bethesda, Md. Received for publication November 12, 1973. Accepted for publication June 5, 1974.
146
concluded that the organic phase of the tooth structure contributes little to the bond strength of the adhesive resins.17 The literature offers no conclusive evidence of clhemical adhesion.18 Enamel surfaces were made wettable to methacrylate resin adhesives after treatment with 85% phosphoric acid.'9 Several other chemicals known as "surface active co-monomers" improved the wettability of enamel surfaces.9,20,2' It was shown that definite deterioration of the bond strength of acrylic resin adhesives occurred after an extended period of water immersion,8,9,22,23 yet some investigators contend that no appreciable change can be seen.24,25 At the present time the acrylic resin adhesives have potential use in orthodontics for the cementation of brackets directly to the tooth surface.9"14,26 Direct bonding of orthodontic brackets to teeth offers several clinical advantages: no separation of teeth is needed, band spaces are eliminated, soft tissue irritation is reduced, oral hygiene is improved, and decalcification that may be observed under loose bands is eliminated.4 Direct bonding procedures have some disadvantages, such as complexity of application, possible failure with the use of edgewise wire and torquing, and difficulty of application to posterior teeth.9'14,26 Therefore, the purpose of this investigation was to test, in vitro, the bond strength of two commercially available direct bonding adhesives under the following environmental conditions: 30 minutes after initial polymerization of the adhesives at room temperature; and after incubating the bonds for 15, 22, and 30 days at 37 C and 100% humidity.
Vol 54 No. I
ORTHODONTIC DIRECT BONDING MATERIALS
Materials and Methods One hundred sixty extracted human teeth (maxillary central and lateral incisors, and mandibular molars) were stored in 10% buffered Formalin until used. This solution had no effect on the adhesive-enamel interface.7,27 The labial surfaces of maxillary central and lateral incisors, and the lingual surfaces of mandibular molars offered desirable bonding sites for the purpose of this investigation. The Rocky Mountain direct bonding system "DBS,"a and the Unitek direct bonding systemb were used in this study. Both adhesive systems use acid etching procedures. The Rocky Mountain direct bonding polycarbonate bracketsc with a constant bonding surface area of 0.0297 square inch were used throughout this study. Rocky Mountain Company and Unitek Corporation indicated that the plastic brackets were of similar material and could be interchanged with various adhesives.28 The root of each tooth was embedded in cold-curing acrylic blocks to facilitate accurate gripping in the Instron machine.d All teeth were rinsed thoroughly in water, and the selected enamel surfaces were pumiced to remove any adherent debris, rinsed again, and blotted dry before bonding. Bonding of the brackets to the tooth surfaces was performed at room temperature (24 ± 4 C) following manufacturers' instructions. In an attempt to maintain a uniform film thickness of the bonding materials, brackets were contoured to the tooth surface whenever needed and excess adhesive was removed from the margins before setting. All specimens were allowed to set for 30 minutes at room temperature to complete the initial polymerization of the adhesive. For each of the two adhesives, 20 teeth (10 for tensile and 10 for shear strength) were tested under each of four conditions. Both adhesives were tested for tensile and sbearlike bond strength under the following conditions: 30 minutes at room temperature after initial polymerization, to be used as a baseline to compare the data (condition 1); Rocky Mountain Dental Products Co., Denver, Colo. Unitek Corp., Monrovia, Calif. e No. A-3023, Rocky Mountain Dental Products Co.,
a
b
Denver, Colo. d Instron Universal Testing Machine, Instron Corp., Canton, Mass
147
15 days at 37 C and 100%, humidity (condition 2); 22 days at 37 C and 100% humidity (condition 3); and 30 days at 37 C and 100% humidity (condition 4). In the last three conditions the specimens were placed in an incubator adjusted to the proper temperature and humidity. Incubation was done after initial polymerization had taken place. The time periods were arbitrarily selected in this experiment as periods of time in which one might expect to observe the effect of moisture and temperature on bond strengths. The table model Instron testing machine was used to break the bond. Each specimen was positioned in one grip in such a way as to allow for testing either tensile or shearlike forces and a 0.018 x 0.025 inch stainless steel wire attached to the bracket was secured in the other grip. Curved tooth surfaces do not lend themselves to the lap-shear test. Therefore, shearlike tests are more applicable. To break the bond, the load was applied at a constant crosshead speed of 0.05 inches per minute, and the load required to break each bond was recorded on a strip chart recorder moving at a speed of 1 inch per minute. Data were calculated for each specimen and expressed in pounds per square inch (psi). A two-factor analysis of variance was used to statistically evaluate the experimental data.29 The F-ratios of the analysis of variance were used to test for significant effects caused by adhesives, conditions, and interaction between adhesives and conditions. The F-ratios measure the statistical significance of the differences between the sample group means.
Results Table 1 illustrates the data for the tensile test. There was no statistical analysis of this data because after the adhesive bonds had completely set, the brackets consistently distorted or fractured before the adhesive bond ruptured. The exception in this case was in condition 1 (30 minutes after initial polymerization), where bond failure occurred before polymerization of the adhesive was completed (Fig 1). Table 2 illustrates the data for the shearlike test. Both adhesives tested at condition 2 approximately doubled their initial bond strength obtained from condition 1. In both Tables I and 2 some of the groups
c04
tGo
r.
.o_
0C
Q
C~C
+1
.n
*0 +1
dz 0 m 04 Co 10 -C"
v
Z
.
Q CO)
00
tl +1
Cr.
in
0 00
_-0
z
o Co O~ +1
00 10
0
.
Uce
10
O e t
o~10 z
O~.04d
t- 001
C,- Cp.
+1 tl +1
C%
CC
dz
0
r.
a
10 0'
U
CO 000 GQ
0 U
-
:.
Q)
0 0
r10
C)
+1Q0
CA
.°
VOO
v
0
Q
oo . Uo
0
44
00 -_
w
0_IV4
xr
0
n44 C
0 or0
m 0
+1 +1+1t _ p.
tl- +I
z Z
0
04
tl +1
-4
an
~0
p.
14 ~.
0 P.
4 0
0
00
tC
z
cn _
0 c
"t r. l:t
m o= C CC~ 004C +1 tl +1 +1 0
E44
_" C'4
-
H
0 . CO 1004 s
0
U
44
:1.
c
n4
- t- V Zo t
0
1-
._
CO 0
.
-4
U4
Q
+1 +1 + CO CCG
lz
rA
z 0
ur1
+l 04- 04 0
CC
+l +1 tl +1 C < 04cu
--0Z .
om oc-
rC-0
C: 00
00 V_ 10rxi VD
u
z .O
I
o
cr
Lp. -04 _0 C
00 0000
z
4 -10 il,
t.
00.
rl.
P. 10 # #M L- +1 lld +1 X t. t4 .-
=
10
0
-
+1 0
:6
r
..4"d
o
t tq
4
0
't GI -4
o00
j
-
+1 tl +1 "1m
on
0
u
u
0-IXI00 0
_lC>
8 .)-'
C
Co
1.
._ 'C
.0
In
t)
'C
o =-
'A
CZ O J
0= 0
148
+-
.C .44
00o
Vol 54 No. I
ORTHODONTIC DIRECT BONDING MATERIALS
contain less than ten specimens each. This is because some teeth fractured under the loading forces applied and a few brackets fell off during the incubation period. The analysis of variance of the shearlike data (Table 3) revealed no significant difference in bond strength between the Rocky Mountain adhesive and the Unitek adhesive at each condition tested. This is indicated by the F-ratio, 2.97. There was also no significant interaction between the two adhesive systems, as indicated by the F-ratio, 0.23. The behavior of both adhesives paralleled one another under all test conditions. There was, however, a highly significant difference between conditions for each adhesive, as shown by the F-ratio, 11.29. This ratio does not indicate whether the significant difference lies between conditions 1 and 2, conditions 2 and 3, or so forth. However, examination of the shearlike test data in Table 2 shows a large difference in the means of both adhesives between condition I and conditions 2, 3, and 4. This indicates that the significant difference in conditions, shown by the analysis of variance, lies at this level, and corroborates our impression that the bond strength was weakest under condi tion 1.
Discussion In this investigation the polycarbonate bracket failed consistently during the tensile test procedure. This does not imply that these brackets are not suitable for clinical use because the brackets failed at forces well above those generally required to move teeth.26 Since the brackets did withstand the forces of the shearlike testing procedure, it was possible to make comparisons of bond strength under all of the environmental conditions. There was a significant difference seen in the shearlike bond strength of both adhesives between condition 1 and conditions 2, 3, and 4. This implies that in both adhesive systems the manufacturer's recommended time for initial polymerization (15 to 30 minutes) was not long enough for sufficiently strong adhesive bonds to develop. The manufacturer's instructions for both of these direct bonding systems state that light forces can be applied to the bracket after initial polymerization has occurred (15 to 30 minutes). Clinical data9 suggest that after 5 to 10 minutes sufficient setting of the adhesive bond had occurred to enable the insertion of an arch wire in the brackets. However, Newman, Snyder, and Wilson's8 study of the copolymers of the methyl methacrylate
Rocky Mountain
700i C,)
a. C
0
C,)I cn 0
a.
-0CL
CL 0
a.
-Ann
Condition I
149
Condition 2
Condition 3
Condition 4
FIG 1.-Mean and standard deviation of tensile test of both adhesives. # indicates adhesive bond failure; other bars indicate failure of bracket either by distortion or fracture.
KHOWASSAH ET AL
150
J
Dent Res January-February 1975
slow. No specific explanation can be offered for the apparent discrepancy in bond strength for the Unitek adhesive under condition 3 (Fig 2). It is also of interest that both adhesive systems behaved similarly under all conditions, as evidenced by the fact that there were no statistically significant differences caused by adhesives or interaction as seen in Table 3. However, because both materials are chemically similar, this observation
Condition
Condition2
Condition3
Condition4
FIG 2.-Mean and standard deviation of shear-
like bond strength of both adhesives.
resins indicate they require one hour setting time to attain a maximum shear strength. This perhaps indicates that for light orthodontic forces (3 to 4 ounces),26 maximum bond strength is not required. Although this study did not investigate the time involved for complete setting of the adhesive bond after the initial polymerization period, caution should be exercised in applying any force, such as a wire, to the bracket until complete setting of the adhesive has occurred. There was no significant weakening effect on the shearlike bond strength between conditions 2, 3, and 4. This is consistent with the finding of Newman, Snyder, and Wilson,8 and others9,23-25 that the breakdown in bond strength of these materials after long exposure to moisture, is reasonably
COMPUTED ANALYSIS Source of Variation
Adhesive Condition Interaction Within cell variation Total 0
Significant at a = 0.05.
OF
is
not surprising.
In every specimen the bond failure occurred within the bond and not at the tooth interface or the bracket interface. No attempt was made to accurately standardize the film thickness, so Buonocore's'6 suggestions that thick or uneven adhesive layers give weaker joints could be a possible explanation for this observation. A number of uncontrollable variables, such as tooth surface topography, chemical variation of enamel structture, and difficulty of precisely duplicating the exact bonding procedure could have contributed to the large variation seen in this data. Conclusions The results of the shearlike testing suggest that under condition 1 the adhesive bonds were not completely polymerized and had gained only about 50% of the bond strength at condition 2. Analysis of variance showed that there was a significant difference in bond strength between condition 1 and conditions 2, 3, and 4. There was, however, no significant effect on bond strength over the 15-, 22-, or 30-day period for either adhesive. There was no significant difference between the two adhesives at any of the test conditions. The direct bonding systems currently are being investigated for extended periods of time and under different thermal conditions
TABLE 3 OF SHEARLIKE TEST DATA
VARIANCE
AND
F-RATIOS F-Ratio
SS
MS
1
15,132,566.82
3 3 65
172,961,223.16
15,132,556.16 57,653,741.06
11.29* (3,65)
3,408,055.12 332,110,797.12 518,572,552.72
1,109,396.88
0.23 (3,65)
5,109,396.88
df
72
. .
with df
...
2.97 (1,65)
ORTHODONTIC DIRECT BONDING MATERIALS
Vol 54 No. I
to closely simulate orthodontic clinical application. The authors suggest that the direct bonding procedures have a place in the orthodontic armamentarium and should be considered an adjunct to full banded therapy. However, at their present state of technological development, it is doubtful that they will replace full banded appliances.
References 1. Barnacle Cement as a Dental Restorative Adhesive, National Institutes of Health Publication No. 151, 1968. 2. RoSE, E.E.; LAL, J.; WILLIAMs, N.B.; and FALcETrI, J.P.: The Screening of Materials for Adhesion to Human Tooth Structure, J Dent Res 34: 577-588, 1955. 3. NEWMAN, G.V.: Epoxy Adhesives for Orthodontic Attachments, Am J Orthod 51: 901912, 1965. 4. RETIEF, D.H.; DREYER, C.J.; and GAURON, G.: The Direct Bonding of Orthodontic Attachments to Teeth by Means of an Epoxy Resin Adhesive, Am J Orthod 58: 21-40, 1970. 5. BUONOCORE, M.G.: A Simple Method of Increasing the Adhesion of Acrylic Filling Materials to Enamel Surfaces, J Dent Res 34:
849-853, 1955. 6. SWANSON, L.T., and BECK, J.F.: Factors Affecting Bonding to Human Enamel with Special Reference to Plastic Adhesives, JADA 61: 581-586, 1960. 7. BUONOCORE, M.G.; MATGUI, A.; and GWINNETT, A.J.: Penetration of Resin Dental Materials into Enamel Surfaces with Reference to Bonding, Arch Oral Biol 13: 61-70, 1968. 8. NEWMAN, G.V.; SNYDER, W.H.; and WILSON, C.G.: Acrylic Adhesives for Bonding Attachments to Tooth Surfaces, Angle Orthod 38: 12-18, 1968. 9. MIuRA, F.; KAZUHIKO, N.; and MASUHARA, E.: New Direct Bonding System for Plastic Brackets, Am J Orthod 59: 350-361, 1971. 10. SMITH, D.C.: A New Dental Cement, Br Dent J 125: 381-384, 1968. 11. MIZRAHI, E., and SMITH, D.C.: The Bond Strength of a Zinc Polycarboxylate Cement, Br Dent J 127: 410-441, 1969. 12. MIZRAHI, E., and SMITH, D.C.: Direct Cementation of Orthodontic Brackets to Dental Enamel, Br Dent J 127: 371-375, 1969. 13. MORTIMER, K.V., and TRANTER, T.C.: A Preliminary Laboratory Evaluation of Polycarboxylate Cements, Br Dent J 127: 365-370, 1969. 14. NEWMAN, G.V., and FAcQ, J.M.: The Effects
151
of Adhesive Systems on Tooth Surfaces, Am J Orthod 59: 67-75, 1971. 15. POOLE, D.F.G., and JOHNSON, N.W.: The Effects of Different Demineralizing Agents on Human Surfaces Studied by Scanning Electron Microscopy, Arch Oral Biol 12:16211634, 1967. 16. BUONOCORE, M.G.: Adhesive Retention and Adhesive Restorative Materials, JADA 67: 382-391, 1963. 17. BOWEN, R.L.: Adhesive Bonding of Various Materials to Hard Tooth Tissues: V. The Effects of a Surface Active Comonomer on Adhesion to Diverse Substrates, J Dent Res 44: 1369-1373, 1965. 18. GWINNETT, A.J., and BUONOCORE, M.G.: Adhesives and Caries Prevention, Br Dent J 119: 77-80, 1965. 19. NEWMAN, G.V., and SHARPE, L.H.: On the Wettability of Tooth Surfaces: Preliminary Investigation, J NJ Dent Assoc March, 1966. 20. MULHOLLAND, R.D., and DESHAZER, D.D.: The Effect of Acidic Pretreatment Solutions on the Direct Bonding of Orthodontic Brackets to Enamel, Angle Orthod 38: 236243, 1968. 21. BOWEN, R.L.: Adhesive Bonding of Various Materials to Hard Tooth Tissues: Bonding to Dentine Promoted by a Surface Active Comonomer, J Dent Res 44: 895-902, 1965. 22. KHOWASSAH, M.A., and SHIPPY, R.L.: In Vitro Investigation of the Adhesive Strength of Cyanoacrylate Bonds to Human Hard Tooth Structures, J Biomed Mater Res 5:
159-169, 1971. 23. MiuRA, F.: Direct Bonding of Plastic Brackets, J Clin Orthod 6: 446-454, 1972. 24. BOWEN, R.L.: Adhesive Bonding of Various Materials to Hard Tooth Tissues: IV. Bonding to Dentine Enamel, and Fluoroapatite Improved by the Use of a Surface Active Comonomer, J Dent Res 44: 906-911, 1965. 25. COHL, M.E.; GREEN, L.J.; and EICK, J.D.: Bonding of Clear Plastic Orthodontic Brackets Using an Ultraviolet Sensitive Adhesive, Am J Orthod 62: 400-411, 1972. 26. THUROW, R.C.: Edgewise Orthodontics, 2nd ed, St. Louis: C. V. Mosby Co., 1966, p 104. 27. RETIEF, D.H., and DREYER, C.J.: Epoxy Resins for Bonding Orthodontic Attachments to Teeth, J Dent Assoc S Afr 22: 338-
346, 1967. 28. MILANO, M.A., and BARNGOVER, CA.: Personal communication. 29. REMINGTON, R.D., and ScHoRK, MA.: Statistics with Application to the Biological and Health Sciences, Englewood Cliffs: PrenticeHall Inc., 1970, pp 288-301.
The effects of temperature and humidity on the adhesive strength of two direct bonding orthodontic materials were investigated. Tensile testing showed that the adhesive bond was much stronger than the polycarbonate brackets. There is a significant increase in tensile and shearlike strength after 15-, 22-, and 30-day incubation at 37 C and 100% humidity, as compared with initial strength (after 30 minutes) at room temperature. In the past several years, considerable interest has been generated in testing the application of several adhesives used in dentistry. Barnacle cement1 is currently the object of extensive research and at present has no clinical (or industrial) application. Epoxy resins24 are quite satisfactory adhesives in many respects; however, they generally are extremely sensitive to moisture contamination and tend to have prolonged setting time. Acrylic resins5-9 and polycarboxylate adhesives10-13 have been tested in the laboratory and are widely accepted for clinical use. Acid etching of enamel surfaces before application of acrylic resin adhesives improved the adhesion considerably5 and increased the bonding surface area without discernible damage to these surfaces.9'14'15 Some investigators theorize that in addition to the mechanical adhesion resulting from acid etching, the organic components of enamel and dentin may offer a means for chemical bonding with adhesives.9'16 Other investigators This investigation was supported, in part, by USPHS Grant No. IR23DE03380-02 and USPHS General Research Support Grant No. 5SO -RRO531 3- 11, from the National Institute of Dental Research, National Institutes of Health, Bethesda, Md. Received for publication November 12, 1973. Accepted for publication June 5, 1974.
146
concluded that the organic phase of the tooth structure contributes little to the bond strength of the adhesive resins.17 The literature offers no conclusive evidence of clhemical adhesion.18 Enamel surfaces were made wettable to methacrylate resin adhesives after treatment with 85% phosphoric acid.'9 Several other chemicals known as "surface active co-monomers" improved the wettability of enamel surfaces.9,20,2' It was shown that definite deterioration of the bond strength of acrylic resin adhesives occurred after an extended period of water immersion,8,9,22,23 yet some investigators contend that no appreciable change can be seen.24,25 At the present time the acrylic resin adhesives have potential use in orthodontics for the cementation of brackets directly to the tooth surface.9"14,26 Direct bonding of orthodontic brackets to teeth offers several clinical advantages: no separation of teeth is needed, band spaces are eliminated, soft tissue irritation is reduced, oral hygiene is improved, and decalcification that may be observed under loose bands is eliminated.4 Direct bonding procedures have some disadvantages, such as complexity of application, possible failure with the use of edgewise wire and torquing, and difficulty of application to posterior teeth.9'14,26 Therefore, the purpose of this investigation was to test, in vitro, the bond strength of two commercially available direct bonding adhesives under the following environmental conditions: 30 minutes after initial polymerization of the adhesives at room temperature; and after incubating the bonds for 15, 22, and 30 days at 37 C and 100% humidity.
Vol 54 No. I
ORTHODONTIC DIRECT BONDING MATERIALS
Materials and Methods One hundred sixty extracted human teeth (maxillary central and lateral incisors, and mandibular molars) were stored in 10% buffered Formalin until used. This solution had no effect on the adhesive-enamel interface.7,27 The labial surfaces of maxillary central and lateral incisors, and the lingual surfaces of mandibular molars offered desirable bonding sites for the purpose of this investigation. The Rocky Mountain direct bonding system "DBS,"a and the Unitek direct bonding systemb were used in this study. Both adhesive systems use acid etching procedures. The Rocky Mountain direct bonding polycarbonate bracketsc with a constant bonding surface area of 0.0297 square inch were used throughout this study. Rocky Mountain Company and Unitek Corporation indicated that the plastic brackets were of similar material and could be interchanged with various adhesives.28 The root of each tooth was embedded in cold-curing acrylic blocks to facilitate accurate gripping in the Instron machine.d All teeth were rinsed thoroughly in water, and the selected enamel surfaces were pumiced to remove any adherent debris, rinsed again, and blotted dry before bonding. Bonding of the brackets to the tooth surfaces was performed at room temperature (24 ± 4 C) following manufacturers' instructions. In an attempt to maintain a uniform film thickness of the bonding materials, brackets were contoured to the tooth surface whenever needed and excess adhesive was removed from the margins before setting. All specimens were allowed to set for 30 minutes at room temperature to complete the initial polymerization of the adhesive. For each of the two adhesives, 20 teeth (10 for tensile and 10 for shear strength) were tested under each of four conditions. Both adhesives were tested for tensile and sbearlike bond strength under the following conditions: 30 minutes at room temperature after initial polymerization, to be used as a baseline to compare the data (condition 1); Rocky Mountain Dental Products Co., Denver, Colo. Unitek Corp., Monrovia, Calif. e No. A-3023, Rocky Mountain Dental Products Co.,
a
b
Denver, Colo. d Instron Universal Testing Machine, Instron Corp., Canton, Mass
147
15 days at 37 C and 100%, humidity (condition 2); 22 days at 37 C and 100% humidity (condition 3); and 30 days at 37 C and 100% humidity (condition 4). In the last three conditions the specimens were placed in an incubator adjusted to the proper temperature and humidity. Incubation was done after initial polymerization had taken place. The time periods were arbitrarily selected in this experiment as periods of time in which one might expect to observe the effect of moisture and temperature on bond strengths. The table model Instron testing machine was used to break the bond. Each specimen was positioned in one grip in such a way as to allow for testing either tensile or shearlike forces and a 0.018 x 0.025 inch stainless steel wire attached to the bracket was secured in the other grip. Curved tooth surfaces do not lend themselves to the lap-shear test. Therefore, shearlike tests are more applicable. To break the bond, the load was applied at a constant crosshead speed of 0.05 inches per minute, and the load required to break each bond was recorded on a strip chart recorder moving at a speed of 1 inch per minute. Data were calculated for each specimen and expressed in pounds per square inch (psi). A two-factor analysis of variance was used to statistically evaluate the experimental data.29 The F-ratios of the analysis of variance were used to test for significant effects caused by adhesives, conditions, and interaction between adhesives and conditions. The F-ratios measure the statistical significance of the differences between the sample group means.
Results Table 1 illustrates the data for the tensile test. There was no statistical analysis of this data because after the adhesive bonds had completely set, the brackets consistently distorted or fractured before the adhesive bond ruptured. The exception in this case was in condition 1 (30 minutes after initial polymerization), where bond failure occurred before polymerization of the adhesive was completed (Fig 1). Table 2 illustrates the data for the shearlike test. Both adhesives tested at condition 2 approximately doubled their initial bond strength obtained from condition 1. In both Tables I and 2 some of the groups
c04
tGo
r.
.o_
0C
Q
C~C
+1
.n
*0 +1
dz 0 m 04 Co 10 -C"
v
Z
.
Q CO)
00
tl +1
Cr.
in
0 00
_-0
z
o Co O~ +1
00 10
0
.
Uce
10
O e t
o~10 z
O~.04d
t- 001
C,- Cp.
+1 tl +1
C%
CC
dz
0
r.
a
10 0'
U
CO 000 GQ
0 U
-
:.
Q)
0 0
r10
C)
+1Q0
CA
.°
VOO
v
0
Q
oo . Uo
0
44
00 -_
w
0_IV4
xr
0
n44 C
0 or0
m 0
+1 +1+1t _ p.
tl- +I
z Z
0
04
tl +1
-4
an
~0
p.
14 ~.
0 P.
4 0
0
00
tC
z
cn _
0 c
"t r. l:t
m o= C CC~ 004C +1 tl +1 +1 0
E44
_" C'4
-
H
0 . CO 1004 s
0
U
44
:1.
c
n4
- t- V Zo t
0
1-
._
CO 0
.
-4
U4
Q
+1 +1 + CO CCG
lz
rA
z 0
ur1
+l 04- 04 0
CC
+l +1 tl +1 C < 04cu
--0Z .
om oc-
rC-0
C: 00
00 V_ 10rxi VD
u
z .O
I
o
cr
Lp. -04 _0 C
00 0000
z
4 -10 il,
t.
00.
rl.
P. 10 # #M L- +1 lld +1 X t. t4 .-
=
10
0
-
+1 0
:6
r
..4"d
o
t tq
4
0
't GI -4
o00
j
-
+1 tl +1 "1m
on
0
u
u
0-IXI00 0
_lC>
8 .)-'
C
Co
1.
._ 'C
.0
In
t)
'C
o =-
'A
CZ O J
0= 0
148
+-
.C .44
00o
Vol 54 No. I
ORTHODONTIC DIRECT BONDING MATERIALS
contain less than ten specimens each. This is because some teeth fractured under the loading forces applied and a few brackets fell off during the incubation period. The analysis of variance of the shearlike data (Table 3) revealed no significant difference in bond strength between the Rocky Mountain adhesive and the Unitek adhesive at each condition tested. This is indicated by the F-ratio, 2.97. There was also no significant interaction between the two adhesive systems, as indicated by the F-ratio, 0.23. The behavior of both adhesives paralleled one another under all test conditions. There was, however, a highly significant difference between conditions for each adhesive, as shown by the F-ratio, 11.29. This ratio does not indicate whether the significant difference lies between conditions 1 and 2, conditions 2 and 3, or so forth. However, examination of the shearlike test data in Table 2 shows a large difference in the means of both adhesives between condition I and conditions 2, 3, and 4. This indicates that the significant difference in conditions, shown by the analysis of variance, lies at this level, and corroborates our impression that the bond strength was weakest under condi tion 1.
Discussion In this investigation the polycarbonate bracket failed consistently during the tensile test procedure. This does not imply that these brackets are not suitable for clinical use because the brackets failed at forces well above those generally required to move teeth.26 Since the brackets did withstand the forces of the shearlike testing procedure, it was possible to make comparisons of bond strength under all of the environmental conditions. There was a significant difference seen in the shearlike bond strength of both adhesives between condition 1 and conditions 2, 3, and 4. This implies that in both adhesive systems the manufacturer's recommended time for initial polymerization (15 to 30 minutes) was not long enough for sufficiently strong adhesive bonds to develop. The manufacturer's instructions for both of these direct bonding systems state that light forces can be applied to the bracket after initial polymerization has occurred (15 to 30 minutes). Clinical data9 suggest that after 5 to 10 minutes sufficient setting of the adhesive bond had occurred to enable the insertion of an arch wire in the brackets. However, Newman, Snyder, and Wilson's8 study of the copolymers of the methyl methacrylate
Rocky Mountain
700i C,)
a. C
0
C,)I cn 0
a.
-0CL
CL 0
a.
-Ann
Condition I
149
Condition 2
Condition 3
Condition 4
FIG 1.-Mean and standard deviation of tensile test of both adhesives. # indicates adhesive bond failure; other bars indicate failure of bracket either by distortion or fracture.
KHOWASSAH ET AL
150
J
Dent Res January-February 1975
slow. No specific explanation can be offered for the apparent discrepancy in bond strength for the Unitek adhesive under condition 3 (Fig 2). It is also of interest that both adhesive systems behaved similarly under all conditions, as evidenced by the fact that there were no statistically significant differences caused by adhesives or interaction as seen in Table 3. However, because both materials are chemically similar, this observation
Condition
Condition2
Condition3
Condition4
FIG 2.-Mean and standard deviation of shear-
like bond strength of both adhesives.
resins indicate they require one hour setting time to attain a maximum shear strength. This perhaps indicates that for light orthodontic forces (3 to 4 ounces),26 maximum bond strength is not required. Although this study did not investigate the time involved for complete setting of the adhesive bond after the initial polymerization period, caution should be exercised in applying any force, such as a wire, to the bracket until complete setting of the adhesive has occurred. There was no significant weakening effect on the shearlike bond strength between conditions 2, 3, and 4. This is consistent with the finding of Newman, Snyder, and Wilson,8 and others9,23-25 that the breakdown in bond strength of these materials after long exposure to moisture, is reasonably
COMPUTED ANALYSIS Source of Variation
Adhesive Condition Interaction Within cell variation Total 0
Significant at a = 0.05.
OF
is
not surprising.
In every specimen the bond failure occurred within the bond and not at the tooth interface or the bracket interface. No attempt was made to accurately standardize the film thickness, so Buonocore's'6 suggestions that thick or uneven adhesive layers give weaker joints could be a possible explanation for this observation. A number of uncontrollable variables, such as tooth surface topography, chemical variation of enamel structture, and difficulty of precisely duplicating the exact bonding procedure could have contributed to the large variation seen in this data. Conclusions The results of the shearlike testing suggest that under condition 1 the adhesive bonds were not completely polymerized and had gained only about 50% of the bond strength at condition 2. Analysis of variance showed that there was a significant difference in bond strength between condition 1 and conditions 2, 3, and 4. There was, however, no significant effect on bond strength over the 15-, 22-, or 30-day period for either adhesive. There was no significant difference between the two adhesives at any of the test conditions. The direct bonding systems currently are being investigated for extended periods of time and under different thermal conditions
TABLE 3 OF SHEARLIKE TEST DATA
VARIANCE
AND
F-RATIOS F-Ratio
SS
MS
1
15,132,566.82
3 3 65
172,961,223.16
15,132,556.16 57,653,741.06
11.29* (3,65)
3,408,055.12 332,110,797.12 518,572,552.72
1,109,396.88
0.23 (3,65)
5,109,396.88
df
72
. .
with df
...
2.97 (1,65)
ORTHODONTIC DIRECT BONDING MATERIALS
Vol 54 No. I
to closely simulate orthodontic clinical application. The authors suggest that the direct bonding procedures have a place in the orthodontic armamentarium and should be considered an adjunct to full banded therapy. However, at their present state of technological development, it is doubtful that they will replace full banded appliances.
References 1. Barnacle Cement as a Dental Restorative Adhesive, National Institutes of Health Publication No. 151, 1968. 2. RoSE, E.E.; LAL, J.; WILLIAMs, N.B.; and FALcETrI, J.P.: The Screening of Materials for Adhesion to Human Tooth Structure, J Dent Res 34: 577-588, 1955. 3. NEWMAN, G.V.: Epoxy Adhesives for Orthodontic Attachments, Am J Orthod 51: 901912, 1965. 4. RETIEF, D.H.; DREYER, C.J.; and GAURON, G.: The Direct Bonding of Orthodontic Attachments to Teeth by Means of an Epoxy Resin Adhesive, Am J Orthod 58: 21-40, 1970. 5. BUONOCORE, M.G.: A Simple Method of Increasing the Adhesion of Acrylic Filling Materials to Enamel Surfaces, J Dent Res 34:
849-853, 1955. 6. SWANSON, L.T., and BECK, J.F.: Factors Affecting Bonding to Human Enamel with Special Reference to Plastic Adhesives, JADA 61: 581-586, 1960. 7. BUONOCORE, M.G.; MATGUI, A.; and GWINNETT, A.J.: Penetration of Resin Dental Materials into Enamel Surfaces with Reference to Bonding, Arch Oral Biol 13: 61-70, 1968. 8. NEWMAN, G.V.; SNYDER, W.H.; and WILSON, C.G.: Acrylic Adhesives for Bonding Attachments to Tooth Surfaces, Angle Orthod 38: 12-18, 1968. 9. MIuRA, F.; KAZUHIKO, N.; and MASUHARA, E.: New Direct Bonding System for Plastic Brackets, Am J Orthod 59: 350-361, 1971. 10. SMITH, D.C.: A New Dental Cement, Br Dent J 125: 381-384, 1968. 11. MIZRAHI, E., and SMITH, D.C.: The Bond Strength of a Zinc Polycarboxylate Cement, Br Dent J 127: 410-441, 1969. 12. MIZRAHI, E., and SMITH, D.C.: Direct Cementation of Orthodontic Brackets to Dental Enamel, Br Dent J 127: 371-375, 1969. 13. MORTIMER, K.V., and TRANTER, T.C.: A Preliminary Laboratory Evaluation of Polycarboxylate Cements, Br Dent J 127: 365-370, 1969. 14. NEWMAN, G.V., and FAcQ, J.M.: The Effects
151
of Adhesive Systems on Tooth Surfaces, Am J Orthod 59: 67-75, 1971. 15. POOLE, D.F.G., and JOHNSON, N.W.: The Effects of Different Demineralizing Agents on Human Surfaces Studied by Scanning Electron Microscopy, Arch Oral Biol 12:16211634, 1967. 16. BUONOCORE, M.G.: Adhesive Retention and Adhesive Restorative Materials, JADA 67: 382-391, 1963. 17. BOWEN, R.L.: Adhesive Bonding of Various Materials to Hard Tooth Tissues: V. The Effects of a Surface Active Comonomer on Adhesion to Diverse Substrates, J Dent Res 44: 1369-1373, 1965. 18. GWINNETT, A.J., and BUONOCORE, M.G.: Adhesives and Caries Prevention, Br Dent J 119: 77-80, 1965. 19. NEWMAN, G.V., and SHARPE, L.H.: On the Wettability of Tooth Surfaces: Preliminary Investigation, J NJ Dent Assoc March, 1966. 20. MULHOLLAND, R.D., and DESHAZER, D.D.: The Effect of Acidic Pretreatment Solutions on the Direct Bonding of Orthodontic Brackets to Enamel, Angle Orthod 38: 236243, 1968. 21. BOWEN, R.L.: Adhesive Bonding of Various Materials to Hard Tooth Tissues: Bonding to Dentine Promoted by a Surface Active Comonomer, J Dent Res 44: 895-902, 1965. 22. KHOWASSAH, M.A., and SHIPPY, R.L.: In Vitro Investigation of the Adhesive Strength of Cyanoacrylate Bonds to Human Hard Tooth Structures, J Biomed Mater Res 5:
159-169, 1971. 23. MiuRA, F.: Direct Bonding of Plastic Brackets, J Clin Orthod 6: 446-454, 1972. 24. BOWEN, R.L.: Adhesive Bonding of Various Materials to Hard Tooth Tissues: IV. Bonding to Dentine Enamel, and Fluoroapatite Improved by the Use of a Surface Active Comonomer, J Dent Res 44: 906-911, 1965. 25. COHL, M.E.; GREEN, L.J.; and EICK, J.D.: Bonding of Clear Plastic Orthodontic Brackets Using an Ultraviolet Sensitive Adhesive, Am J Orthod 62: 400-411, 1972. 26. THUROW, R.C.: Edgewise Orthodontics, 2nd ed, St. Louis: C. V. Mosby Co., 1966, p 104. 27. RETIEF, D.H., and DREYER, C.J.: Epoxy Resins for Bonding Orthodontic Attachments to Teeth, J Dent Assoc S Afr 22: 338-
346, 1967. 28. MILANO, M.A., and BARNGOVER, CA.: Personal communication. 29. REMINGTON, R.D., and ScHoRK, MA.: Statistics with Application to the Biological and Health Sciences, Englewood Cliffs: PrenticeHall Inc., 1970, pp 288-301.
