Bond strength of ceramic brackets under shear stress: An in vitro report Anthony D. Viazis, DDS,* Gerald Cavanaugh, DDS, PhD,** and Richard R. Bevis, DDS, PhD** Minneapolis, Minn. The shear bond strength and the potential enamel damage on debonding of various currently available ceramic and stainless steel brackets were examined in vitro using extracted premolar teeth. The brackets were divided into two groups, one bonded with a new light-cured orthodontic adhesive and the other with a conventional chemically cured system. An Instron Universal testing machine was used to apply the shear stress. Mean, standard deviation, and extreme values were calculated for each group. Statistical analysis showed that the mean shear bond strength of the silane chemical bond provided by some ceramic brackets is significantly higher (p < 0.05) than the mean of the mechanical bond of other ceramic and stainless steel brackets. There was no statistically significant difference between the mean shear bond strength of the two adhesives used. Mechanical bonds failed primarily within the adhesive itself, whereas chemical bonds failed predominantly at the adhesive-bracket interface. Single-crystal ceramic brackets tend to be more brittle than the polycrystalline ones. Strong chemical bonds can potentially lead to enamel failure on debonding. (AM J ORTHODDENTOFACORTHOP1990;98:214-21.)
R e c e n t controversy over the high bond strength of the ceramic bracket to the enamel surface and the potential enamel damage on debonding requires that the effects of debonding be thoroughly investigated. Since ceramic brackets do not have the ductility of stainless steel, they can present problems during debonding with failures within the bracket or the enamel itself. This depends greatly on the kind of ceramic bracket base design and its adhesion through a chemical or mechanical bond. 1 Currently there are two types of ceramic bracket bases.t'2 One type is formed with undercuts or grooves that provide a mechanical interlock to the adhesive. Such brackets are Allure (GAC International, Central Islip, N.Y.) and Gem (Ormco Corp., Glendora, Calif.). The other configuration has a smooth surface and relies on a chemical coating to enhance bond strength. As explained by Swartz, 2 since direct adhesion to aluminum oxide with any of the dental bonding adhesive resins is not possible, the aluminum oxide bracket base is coated with silica (glass) and then is silane treated. One end of the silane molecule is a "silanol" group that can bond tenaciously to silica and its other end bonds to the acrylic resin. The manufacturers of such brackets (Transcend, Unitek/3M Company, Monrovia, Calif. In partial fulfillment of the requirements for the degree of master of science. From the Department of Orthodontics, School of Dentistry, University of Minnesota. *Resident. **Professor. 811112932
and Starfire, "A"-Company, San Diego, Calif.) report that they achieve higher bond strength when compared to mechanical retention. Their unpublished data stating that enamel is twice as strong in a shear direction than in a tensile direction suggest that special debonding instruments that apply a torsional shear stress be used instead of the conventional debonding pliers. According to Swartz, 2 the rate, or speed, at which materials are stressed has a great influence on their resulting strain and fracture mode. This is certainly true of rigid materials like ceramics and enamel. In vitro bond failure modes conducted at slow stress rates may not be indicative of failure modes when subjected to much faster, sudden loads. Several clinical reports of fractured enamel, using chemically treated ceramic brackets on lower teeth, seemed to have occurred in instances of sudden loading (biting and trauma). Therefore debonding of ceramic brackets at a very low rate is recommended. Diedrich 3 was the first to report the risk of severe enamel loss caused by debonding of metal brackets when he found enamel detachments of up to 160 l-tm. Very recently 0degaard and Segner4 reported that the shear bond strength of the chemical-base ceramic bracket was found to be superior to the metal brackets using a no-mix and paste/paste adhesive. They stated that bond failure with the ceramic bracket occurred predominantly in the enamel/adhesive interface; the failure site for the metal bracket was mainly in the bracket/adhesive interface. It was concluded that the bond strength between the ceramic bracket and the ad-
Bond strength of ceramic brackets under shear stress
Table I. Brackets of group I
AI B1 C~ D= E~
Name Transcend Allure Starfire Gem
American Unitek / 3M GAC "A"-Company Ormco
Stainless steel Polycrystalline Polycrystalline Single crystal Single crystal
Bond Mechanical Chemical Mechanical Chemical Mechanical
Table II. Brackets of group II
American Unitek/3M GAC
Stainless steel Polycrystalline Polycrystalline
A2 B2 C2
hesive in shear mode is stronger than that between the adhesive and the enamel. This investigation aims to: 1. Compare the bond strengths of various currently available ceramic brackets under shear stress 2. Examine the potential enamel damage on debonding of the ceramic appliances under shear stress 3. Compare the adhesive properties of a new lightcured resin to a conventional two-phase, chemically cured adhesive system by testing the resistance of the bond METHODS AND MATERIALS
Eighty fresh human premolar teeth along with ten canines were obtained from the Department of Oral Surgery, School of Dentistry, University of Minnesota, and from local oral surgeons in private practice. Immediately after extraction, the teeth were stored in plastic bottles containing a solution of 0.9% sodium chloride (normal saline), which was changed periodically to prevent bacterial growth. Each tooth was examined for surface irregularities under a Zeiss 40-power stereomicroscope. Any tooth with damaged enamel or restoration was excluded from the study. The light-cured orthodontic adhesive Transbond (Unitek/3M) used in this study consisted of an orthodontic bonding paste and an enamel bond sealing resin. After the etching procedure, the sealing resin was applied to the enamel surface for 10 seconds according to the manufacturer's instructions, whereas the bonding paste was applied to the bracket base. The bracket then was firmly placed on the tooth. Any excess paste was removed. Immediately afterward the light source Was placed directly over the bracket for a total of 10 seconds
Bond Mechanical Chemical Mechanical
for all ceramic brackets. As suggested by the manufacturer, the metal brackets were exposed to the light source for 30 seconds to ensure complete polymerization of the adhesive under the metal bracket base. A total of 50 teeth were bonded with this adhesive (group I). The chemically cured adhesive system Concise (Unitek/3M) is composed of two orthodontic bonding pastes (A and B). According to the manufacturer's instructions, equal volumes of the resin were mixed for I0 seconds and applied to the etched enamel surface with a sponge. Immediately afterward equal amounts of pastes A and B were mixed for 20 seconds with a spatula on a pad provided by the manufacturer. The mixed paste was then applied to the bracket base, which then was firmly seated on the tooth. Again, any excess adhesive was removed in time before polymerization. Thirty teeth were bonded with the chemically cured adhesive (group II). The 50 teeth in group I were divided into five subgroups of 10 teeth (ArEl). Five groups of ten brackets from one stainless steel manufacturer and four different ceramic bracket manufacturers were used (Fig. 1, Table I). The teeth in subgroup Dl were canines because the manufacturer provided canine brackets for this experiment. Since there is a marked difference in size between the ceramic bracket bases of all the companies, the use of canine brackets was not considered an additional variable in this study. The 30 premolar teeth in group II were divided into three subgroups of 10 teeth (A2-C2). Three groups of I0 brackets from one stainless steel manufacturer and two different ceramic bracket manufacturers were used (Table II).
Am. J. Orthod. Dentofac. Orthop. September 1990
Viazis, Cavanaugh, and Bevis
Fig. 1. Brackets used in the study. Fig. 2. Bracket failure of Starfire single-crystal brittle ceramic bracket.
Each tooth was embedded in self-curing acrylic, which was added to a metal ring that would hold the tooth in an aluminum jig positioned vertically for the shear stress testing. The exposed enamel of each tooth received a 15-second prophylaxis with wet flour of pumice on a slowly rotating cup, followed by a water spray rinse, and dried with an air-water syringe. The teeth were etched with a 37% orthophosphoric acid gel for a period of 15 seconds, followed by a 30-second water spray rinse, and then dried with an air syringe. Any tooth that did not display a uniformly etched surface was reetched as described above. An Instron Universal testing machine, model number 1123, (Instrom Corp., Canton, Mass.), located in
the laboratory of the Dental Products Division, 3M Company, St. Paul, Minn., was used for stress testing. The machine consisted of a lower jaw mounted to the base. The jaws were hydraulic vises with manual controis. The crosshead moved at a fixed rate of 1 mm/min. A recording chart recorded the force applied at all stages until the bond failure. The recording chart was maintained at the speed of 20 mm/min, with the scale set at 50 kg. To apply a shear stress to the bond, the jig was positioned in the lower jaw so that the bracket base of the sample was parallel to the direction of force. Both ends of a 10 cm long piece of 0.030 inch diameter stainless steel wire were placed in the upper jaw to form
Bond strength of ceramic brackets under s/tear stress 217
Table III. Shear bond strength values with Transbond adhesive
Force noted at momentof bond or bracket failure
Mean SD Min. Max.
Ai (Metal) (kg)
Bi (Transcend) (kg)
C, (A/lure) (kg)
23.8 20.1 22.8 18.4 17.5 22.7 26.7 16.3 14.2 24.5 20.7 4.027 14.2 26.7
36.3 34.1 44.2 38.2 37.3 31.6 36.2 48.4 41.5 34.7 38.25 5.077 31.6 48.4
24.2 26.7 21.7 31.6 26.2 21.8 36.1 42.5 40.7 28.7 28.833 6.897 21.7 42.5
D, (Starfire) (kg) 16.3 17.2 14.1 16.4 13.0 178.0 13.1 21.4 23.0 15.4 16.79 3.316 13.0 23.0
E, (Gem) (kg) 10.5 8.2 12.1 10.9 7.1 8.8 6.2 10.8 13.0 I1.1 9.87 2.203 6.2 13.0
Table IV. Shear bond strength values with Concise adhesive
I Force noted at moment of bond failure
Mean SD Min. Max.
Az (Metal) (kg)
B2 (Transcend) (kg)
Cz (Allure) (kg)
22.2 19.6 24.6 22.5 19.0 17.6 23.7 27.2 19.4 26.0 22.16 3.19 17.6 27.2
38.3 32.3 41.7 30.9 38.5 39.6 48.0 46.4 35.4 45.5 39.66 5.826 30.9 48.0
25.7 27.5 26.0 22.2 31.1 29.7 34.3 23.2 26.5 31.4 27.76 3.825 22.2 34.3
a suspended loop. The loop was positioned just under the body of the bracket by adjusting the crosshead manually. The Instron machine was activated and the shear stress at bond failure was recorded. A tabulation was made of the shear bonding strength in kilograms for each group. Mean, standard deviation, and extreme values were calculated. Statistical analysis of the data was carried out with the BMDP package of statistical programs? '6
RESULTS The results of shear bond strength tests using the Transbond and Concise adhesives for the various
bracket groups are given in Tables III and IV, respectively. The figures represent the amount of force in kilograms at the point of bond or bracket failure. One-way analysis of variance (ANOVA) was used to evaluate the differences for the means of all the various bracket groups when bonded with the Transbond and Concise adhesives. 5'6 The overall test was significant at p < 0.05. All groups were different on a statistically significant level with a p value of at least 0.05 according to the Bonferroni t tests? '6 Two-way analysis of variance also was used to evaluate the differences for the means of shear strength when comparing the Transbond samples with their Con-
Am. J. Orthod.Dentofac.Orthop. September1990
Viazis. Cavanaugh, attd Bevis
Table V. Fracture sites after debonding
Group A, Bz C~ Di El A2 B2 Cz
Enamel failure 1
4 1 4 1
Intraadhesive failure 4 2 6
2 6 I
9 4 l 5
5 1 5
Each of the above groups comprised a total of 10 brackets. The percentages can be calculated from the above figures by a factor of 10.
cise equivalents. 5 No statistically significant difference was found between the adhesive properties of these two systems. A Zeiss 40-power stereomicroscope was used to examine the bracket base and the enamel surface after debonding. Five sites of bond failure were noted: (1) within the enamel itself, (2) at the enamel/adhesive interface, (3) intraadhesive, (4) at the adhesive/bracket interface, and (5) within the bracket (bracket failure). Table V shows the results and the percentage values of the failure sites reported. DISCUSSION
The shear bond strengths recorded in this investigation are the highest ever reported in the literature for the chemically treated ceramic brackets. 4'7 The Transcend ceramic brackets demonstrated a mean shear strength of 39.660 kg when bonded with Concise and 38.250 kg using the Transbond light-cured adhesive. These values were significantly greater than those obtained with the mechanical retention of the Allure ceramic brackets (27.760 and 28.833 kg for the adhesives, respectively) and those of the foil mesh stainless steel brackets (22.160 and 20.7 kg, respectively). The maximum reported value of the Transcend bracket samples was 48.4 kg, bonded with the Transbond light-cured adhesive. This exceeds the equivalent of 100 lb of force. This is such a heavy force that it might require care in removal with specialized instruments if damage to the enamel and the tooth surface is to be avoided. 4 The very low shear bond strength values obtained for the single-crystal Gem ceramic appliances with both adhesives should be attributed solely to the poor mechanical bond offered by their wide grooved bracket base design. On the other hand, the strong chemical adhesion of the single-crystal Starfire ceramic brackets to the enamel surface allowed bracket failure (Fig. 2)
instead of bond failure in 80% of the samples. The looped wire used in this experiment to exercise the shear force most likely introduced surface scratches and immediate reduction in the fracture toughness of these materials, s The two previous investigations published recently4'7 stated that after debonding the bulk of the adhesive remained on the ceramic bracket, indicating that the bond between the etched tooth surface and the adhesive must be weaker than the bond of the adhesive to the silane-treated ceramic bracket base. In one report less than 10% of the adhesive remained on the enamel s u r f a c e . 4 The present study demonstrated quite different findings. In 60% of the samples bonded with the lightcured adhesive and 80% of those bonded with the chemically cured adhesive, there was a pure failure between the bonding surface of the bracket and the adhesive (Fig. 3). The bonding surface of the bracket appeared as if no bonding had taken place, whereas the whole bulk of the adhesive remained on the tooth, demonstrating a clear, shiny surface. This could be attributed to the fact that shattering of the monolayers of the bracket base silane coating had occurred. Thus we can explain the glossy surface of the adhesive after debonding and the negative impression of the bracket base left on the adhesive in Fig. 3. The mean bond strength of the foil mesh metal brackets and the grooved based ceramic brackets (mechanical retention) was significantly less than the mean bond strength of the chemical bond as expressed by the Unitek brackets. On the other hand, mechanical bonds failed primarily within the adhesive itself (ranging from 50% to 90% regardless of the adhesive used), whereas the chemical bond, although higher, left the adhesive intact. One would expect the adhesive to either stay on the ceramic bracket, as supported by previous investigators, 4"7 or to fracture within itself.
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The reason for the differential behavior of the two bonds lies in the way that the stress concentration is distributed over the bonding surfaces. Ceramic brackets that offer a mechanical bond with the adhesive have retentive grooves in which the edge angles are 900. 4 Furthermore, there are crosscuts to prevent the brackets from sliding along the undercut grooves. These also have sharp edge angles. This leads to high localized stress concentrations around those sharp edges, which results in brittle failure of the adhesive. Thus, on application of the shear debonding force, part of the adhesive is left on the tooth and part on the grooved bracket. On the other hand, the shiny surface of the ceramic bracket with a chemical bond allows a much greater distribution of stress over the whole adhesive interface without the presence of any localized stress areas. Therefore the shear bond must be much greater to cause debonding and pure adhesive failure, in contrast to brittle failure with mechanical bonds, leaving all of the bonding agent intact on the tooth. In one of the samples of this experiment, there was enamel failure in which approximately one half of the enamel cusp (down to the pulp chamber) broke off the rest of the tooth during debonding and remained bonded to the chemically treated ceramic bracket (Unitek/3M) (Fig. 4). Although this phenomenon raises serious questions about the clinical application of such high chemical bonds, one should take into account the conditions that led to enamel failure of this sample. Extracted teeth used during in vitro experiments, no matter how well preserved, are in a much drier state than those in the mouth. The contraction of the resin and the rise of its temperature during polymerization of the material are also factors that promote a drier sample. In addition, the application of the shear debonding force used in this investigation differs from the torsional peeling type of movement applied by the specialized tools for bracket removal recommended by the manufacturer. On the other hand, enamel failure might occur even under clinical conditions. Endodontically treated teeth are more brittle and fracture easier. The low fracture toughness values of enamel, 0.7-1.2 M N / m 3, as compared with the polycrystalline alumina (2.4-4.5 MN/m 3) and to the single-crystal ceramics (3.0-5.3 MN/m 3) promote enamel failure when a shear load is exercised between these rigid, nonflexible materials, s In addition, the inherent high variability in the properties of enamel (including hardness) adds to the unpredictability of the occurrence of enamel failure. 9 Bracket failure occurred in 80% of the samples of
Bond strength o f ceramic brackets under s/tear stress
Fig. 3. SEM photograph of pure adhesion failure after debonding of Starfire ceramic bracket with chemical bond.
the single-crystal ceramic brackets of "A"-Company and in one sample of the Ormco product. No bracket failures occurred within the polycrystalline groups. As explained by Scott, 8 any kind of scratch or cracking of the surface of the ceramic materials causes immediate brittle failure, which is expressed clinically by breakage of the wings of these appliances. Obviously the looped wire used in this experiment to induce the shear force introduced such minute crackings. CONCLUSIONS
The following conclusions can be stated from this investigation: 1. The shear bond strengths recorded in this investigation are the highest ever reported in the literature for the chemically treated ceramic brackets. The maximal value exceeded the equivalent of 100 lb of force, which stresses the need to examine whether use of specialized instruments is necessary to avoid enamel damage. 2. The mean shear bond strength of the silane chemical bond provided by some ceramic brackets is significantly higher (p < 0.05) than the mean shear
Am. J. Orthod. Dentofac. Orthop. September 1990
Viazis, Cavanaugh, and Bevis
Fig. 4. A, Chemical bond of Transcend ceramic bracket with tooth enamel. B, Tooth failure (enamel and dentin) after the attempt to debond the ceramic bracket.
bond strength of the grooved mechanical bond of various other ceramic appliances and the foil mesh base of the stainless steel brackets. 3. There is no statistically significant difference between the mean shear bond strength of the new lightcured orthodontic adhesive tested and the conventional chemically cured system. 4. Single-crystal ceramic appliances tend to be more brittle than the polycrystalline ones, especially when there is a chemical bond with the adhesive media (bracket failure instead of bond failure). 5. Mechanical bonds (metal foil mesh and groovedbased ceramic bracket bases) under shear stress fail primarily within the adhesive itself (brittle failure of
the adhesive from localized stress areas), whereas chemical bonds (silane-treated ceramic bracket bases) fail mostly at the adhesive-bracket interface (pure failure caused by wider stress distribution over the whole interface). 6. Enamel failure at the time of debonding can occur, especially if a tooth is endodontically treated, has a low fracture toughness value, and the bond of the ceramic brackets reaches maximum values. REFERENCES 1. Phillips HW. The advent of ceramics: the editor's comer. J Clin Orthod 1988;22:69-70, 2. Swartz ML. Ceramic brackets. J Clin Orthod 1988;22:82-8. 3. Diedrich P. Enamel alterations from bracket bonding and de-
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5. 6. 7.
Bond strength o f ceramic brackets under shear stress
bonding: a study with the scanning electron microscope. AM J ORntOD 1981;79:500-22. Eidegaard J, Segner D. Shear bond strength of metal brackets compared with a new ceramic bracket. AM J OR'naOD DENTOFAC OR'mOP 1988;94:201-6. Dixon WJ. BMDP statistical software 1981. Berkeley, California: University of California Press, 1981. Sokal RR, Rohlf FJ. Biometry. 2rid ed. San Francisco: WH Freeman, 1981. Gwinnett AJ. A comparison of shear bond strengths of metal and ceramic brackets. A.~t J ORTHOD DE.VI'OFACORTHOP 1988;93: 346-8. Scott GE. Fracture toughness and surface cracks--the key to understanding ceramic brackets. Angle Orthod 1988;1:3-8.
9. Lambrechts P, Braem M, Vanherle G. Quantitative in vivo wear of human enamel as acceptance standard for posterior composites. J Dent Res 1987:IADR Abstract 605. Reprint requests to: Dr. Anthony Viazis Department of Orthodontics School of Dentistry University of Minnesota Moos Tower, 6th Hoor 515 Delaware St. S.E. Minneapolis, MN 55455
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