shear, torsional, or tensile bond strength of each ceramic bracket type in relation to ... in other studies, and the failure site was at the bracket-adhesive interface.
Shear, torsional, and tensile bond strengths of ceramic brackets using three adhesive filler concentrations Alan J. Ostertag, DDS, MS, a Virendra B. Dhuru, BDS, MSc, a Donald d. Ferguson, DMD, = and Ralph A. Meyer, Jr., PhD d Marshfield and Milwaukee, Wis. The effect of changes in adhesive filler concentration on the shear, torsional, and tensile bond strength of a chemical, a mechanical, and a chemical/mechanical retained ceramic bracket was evaluated. Two hundred ten bovine teeth were bonded with one of three ceramic brackets using a 30%, 55%, or 80% filled adhesive. The brackets were debonded with a shear, torsional, or tensile force to test the bond strength and the site of bond failure. No significant difference was found in the shear, torsional, or tensile bond strength of each ceramic bracket type in relation to changes in the adhesive filler concentration. However, there was a trend toward increased bond strength with increasing filler concentration. Combining the data according to adhesive type revealed that the 80% filled adhesive displayed a significantly greater shear bond strength than the 30% or 55% filled adhesive and a greater torsional bond strength than the 30% filled adhesive. This supports the hypothesis of increased bond strength with increased adhesive filler concentration. The mechanically retained ceramic bracket showed greater shear bond strength and maximum shear bond strength in torsion than the chemical or chemical/mechanical retained ceramic bracket. The tensile bond strength of the mechanically retained ceramic bracket was similar to that of metal brackets reported in other studies, and the failure site was at the bracket-adhesive interface. (AM J ORTHOD DENTOFAC ORTHOP 1991 ;100:251-8.)
C e r a m i c brackets were created when the demand for "esthetic braces" increased. Along with their improved esthetics, ceramic brackets introduced some unique clinical characteristics: abrasiveness to enamel, brittleness leading to fracture, and increased bond strengths leading to enamel fracture. Once these characteristics were acknowledged, manufacturers introduced elastic "cushions" to cover the bracket occlusal surface to reduce abrasion, altered bracket design to help reduce bracket fracture, created new debonding instruments, and issued more detailed instructions to assist in bracket removal. Since ceramic brackets were introduced, there have been several bonding studies using ceramic brackets. t-5 These studies have relied on commercially availFrom Marquette University School of Dentistry, Based on a thesis in partial fulfi]lment of the requirements for the degree of Master of Science, =In private practice in Marshfield, Wis. bActing chairman and associate professor of the Department of Dental Materials and associate professor of operative dentistry. ~Assistant professor and chairman of the Department of Orthodontics, dProfessor of physiology. 8/1],24328
able adhesive systems to test bond strength and have concentrated on differences in bracket base characteristics or major changes in adhesive filler sizeconcentration combinations. With the recent concern expressed about the debonding difficulties encountered with ceramic brackets, a logical question to be explored would be whether there is a specific resin-bracket combination that will aid in bracket removal and yet maintain adequate bond strength. The purpose of this study was to evaluate and compare the shear and torsional bond strength and the site of bond failure of a chemical, a mechanical, and a chemical/mechanical retained ceramic bracket to specific changes in adhesive filler concentrations. In addition, the tensile bond strength and the site of bond failure of the mechanically retained bracket were tested. METHODS
Three commercially available ceramic orthodontic brackets, one single crystalline, the others polycrystalline, were used in this study (Table I). All the brackets were standard edgewise 0.022 × 0.028-irtch slot mandibular incisor brackets. The Starfire single :crystalline bracket base contained no 251
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al.
Am, J, Orthod. Dentofac. Orthop. September 1991
Table I. C e r a m i c brackets used in the study
Name
Crystalline form
Retention
Bracket base dimensions
Manufacturer
Allure IIl Starfire Transcend 2000
Polycrystalline Single crystalline Polyerystalline
Chemical/mechanical* Chemical** Mechanical***
2.40 x 3.40 mm 3.04 x 3.82 mm 2.60 x 3.34 mm
GAC International, Central Islip, N.Y. "A" Company, San Diego, Calif. Unitek Corp., Monrovia, Calif.
*Silane-treated bracket base containing four rectangular retention slots. **Silane-treated bracket base. ***Particles fused to bracket base.
Table II. A d h e s i v e s used in the study
Compositlon*
l Batchnumber"l
Manufacturer
Silica-filled BIS-GMA Paste A: 145-49-C All adhesives were resin (30% by Paste B: 145-47-A specially formuweight) luted by Reliance Orthodontic Products, Inc., Itasca, Ill. Silica-filled BIS-GMA Paste A: 145-47-B resin (55% by Paste B: 145-49-D weight) Silica-filled BIS-GMA Paste A: 145-47-C resin (80% by Paste B: 145-49-E weight) *All fillers were 5 to 15 Ixm in size.
mechanical retention but was treated with silane by the manufacturer to enhance retention. The polycrystalline brackets consisted of two types, Transcend 2000 and Allure UI. Their bracket bases differed in that the Transcend 2000 bracket base had fused particles to provide retention, whereas the Allure III bracket base had four rectangular retention slots and had been treated with silane. Three specially formulated BIS-GMA type, autopolymerizing composite orthodontic adhesives were used in this study (Table II). The size of the fillers in the adhesives ranged from 5 v m to 15 ~m. The filler concentration was the only variable that changed among the adhesives. Filler concentrations selected were 30%, 55%, and 80% by weight. The sealant used was Phase II sealant (Reliance Orthodontic Products, Inc., ltasca, I11.), an unfilled BIS-GMA adhesive. Two hundred ten extracted bovine teeth were obtained and stored at room temperature in distilled water. 6 The teeth were mandibular incisors taken from approximately 15 month-old Holstein steers, raised in the same locale in central Wisconsin. The teeth were randomly assigned to one of 21 treatment groups containing 10 teeth per group. The bracket, adhesive, and force mode combinations are listed in Table III. All the teeth were prepared and the brackets were bonded by a similar technique in a random order. The facial surface of each tooth was cleaned with a slurry of pumice for 30 seconds with a rubber cup driven by a low-speed handpiece.
The surface was rinsed with water to remove any pumice or debris and dried for 15 seconds with an air stream. A 37% orthophosphoric acid etching solution was applied with a sponge pellet to the facial surface of each tooth for 20 seconds. The tooth was then rinsed with water for 30 seconds and air dried for 15 seconds. The unfilled sealant was mixed in equal parts, and a thin coat was applied to the enamel surface. The adhesive base and catalyst pastes were mixed on a paper mixing pad according to the manufacturer's instructions. The pastes were mixed until an even color was obtained and then placed on the bracket base. The bracket was then positioned on the facial surface of the tooth with a dental explorer. A level area of the enamel surface was chosen to provide an area of best fit. Pressure was applied to the bracket, simulating clinical chairside procedures, to express any excess adhesive from between the bracket and the enamel surface. The excess was then removed from the periphery of the bracket base with an explorer. To prevent any reduction in bond strength, the tooth was left undisturbed for 20 minutes at room temperature. After this time, the bonded tooth was embedded in a cylinder of acrylic and stored in 37 ° C distilled water for 48 hours. Each specimen was tested in one of three force modes: shear, torsion, or tensile. The shear bond strengths were measured with an Instron testing machine (Instron universal testing instrument, Model TTC, Instron Corp., Canton, Mass.). Each shear test specimen was placed into a Vise that positioned the tooth surface parallel to the direction of load application. A shear load was applied at a crosshead speed of 0.50 nun per minute, and the maximum load necessary to debond the bracket was recorded. The torsional bond strengths were measured with a torque meter (Tohnichi torque gauge, Model BTG120Z, Tohnichi America Corp., Skokie, Ill.). Each specimen was fixed in a vise, and the torque meter containing a custom-fabricated torquing wrench was placed over the bracket. The load was applied manually in such a manner as to simulate clinical chairside conditions. The maximum torque necessary to debond the bracket was recorded. The tensile bond strengths were measured with the Instron universal testing machine. Only the Transcend 2000 brackets were tested under a tensile force because no other manufacturer recommended that their brackets be debonded under tensile force conditions. It was believed that intrabracket fractures might occur if the other brackets were tested in tension. Each specimen was suspended from the upper member of the
Volume tO0 Number 3
Shear, torsional, and tensile bond strengths of ceramic brackets
253
Table III. Mean bond strength and torque values
Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
1--
Bond strength Bracket
Adhesive
Transcend 2000 Transcend 2000 Transcend 2000 Starfire Starfire Starfire Allure III Allure III Allure III Transcend 2000 Transcend 2000 Transcend 2000 Starfire Starfire Starfire Allure III Allure Ill Allure III Transcend 2000 Transcend 2000 Transcend 2000
30 55 80 30 55 80 30 55 80 30 55 80 30 55 80 30 55 80 30 55 80
Force mode
MPa
]
Torque
(SD)
N" m
_
(SD)
Shear Shear Shear
17,02 17.79 25.92
(8.78) (11.08) (6.83)
----
--
Shear Shear Shear Shear Shear Shear Torsion Torsion Torsion Torsion Torsion Torsion Torsion Torsion Torsion Tensile Tensile
10.60 12.76 18.19 8,17 10.50 11,07 56.31 57.40 58.69 28.00 36.79 38.05 41,65 41.43 48,46 5.25 6.04
(3.49) (5.65) (9.03) (2.57) (4.64) (5.25) (8.69) (12.45) (9.52) (3.24) (14.16) (9.56) (6,03) (7,12) (6.24) (0.92) (1,30)
-~ ---~ 0,29 0.29 0.30 0,22 0,29 0,30 0.19 0.19 0.22 ---
---
Tensile
6,34
(1.14)
--
Instmn machine with a custom-made fixture allowing selfalignment of the tooth surface perpendicular to the line of force, A custom-fabricated tensile debonding instrument, fixed to the lower member of the Instron machine, was secured to the mesial and distal sides of the bracket base. A tensile load was applied at a crosshead speed of 0.50 mm per minute, and the maximum tensile force necessary to debond the bracket was recorded. All test specimens were viewed under a stereomicroscope to determine the site of bond failure. The bracket base dimensions were measured to the nearest 0.01 mm with a traveling microscope (Gaertner Scientific Corp. No. 234P, Chicago, II1.). The mesiodistal curvature of the bracket base was not considered in the determination of bracket base area. The radius of curvature of the mandibular incisor bracket base was minimal; in addition, the adhesive thickness minimized the influence of the radius on the bracket base area,
RESULTS The mean force and the standard deviation (SD) required to induce bond failure in each treatment group are shown in Table III. The means of the torque values are reported as force times distance and also as the maximum shear stress generated during torsion to demonstrate the influence of bracket base dimensions on the force levels generated. 7 The bond strength and torque values shown in Table III are graphically illustrated in Figs. 1 through 4. Any brackets that fractured during testing were omitted from the calculation of the
(0.04) (0.06) (0,05) (0.03) (0,1 I) (0.08) (0.03) (0.03) (0,03)
treatment group means. A summary of the bond failure pattern is listed in Table IV, A 3 × 3 factorial analysis of variance (ANOVA) and a Tukey multiple comparisons test were employed in the statistical analysis of the shear and torsionaltreatment groups, The tensile treatment groups were subjected to a single-factor analysis of variance. The factorial ANOVA and the Tukey test revealed a significant difference in the shear force levels among the three bracket types, with the Transcend 2000 bracket being significantly greater than the Starfire or Allure III bracket. The three adhesives also show a significant difference in shear force values, with the 80% filled adhesive displaying a significantly greater bond strength than the 30% or 55% filled adhesive. No statistically significant interaction between adhesive and bracket was found. A factorial ANOVA and a Tukey test for the torsion data revealed a significant difference in the N . m torque levels among the bracket types, with the Transcend 2000 and Starfire brackets being significantly greater than the Allure III bracket. Conversion of the torque values into maximum shear stress levels revealed a significantly greater stress for the Transcend 2000 bracket than for the Starfire or Allure III bracket and a greater stress for the Allure III bracket than for the Starfire bracket. The adhesives also showed a significant difference in torque levels, with the 80% filled adhesive displaying a significantly greater bond strength
254
Ostertag el al.
4m. J. Orthod. Demofac. Orthop. September 1991 30-
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