Bruce M. Oliver, DDS, MS. / Ariel R. Dujovne, Ing., M.Sc.
Abstract
This study compares bond strengths of two bracket/bonding systems and evaluates whether or not different priming agent or shelf life affects bonding strength of adhesive precoated brackets. Pilot studies involved 9 bovine teeth and 13 extracted, human permanent molars. Different bracket designs and lot numbers of varying shelf lives were tested. Specimens were randomly assigned to enamel surfaces. Polymerization of visible light-cured adhesives was accomplished using 20 seconds of indirect irradiation. Specimens were stressed to failure using a shear/peel force. In all cases the site of failure occurred as mixed adhesive-cohesive phenomena. A statistically significant difference was noted between bond strengths of the two bracket/bonding systems, as well as between bond strengths of adhesive precoated brackets (A.P.C.) tested at 2 months and 10 months form date of manufacture. No significant difference existed between bond strengths of the two adhesive priming agents used with APC brackets.
INTRODUCTION
Clinical trials have evaluated performance of conventional metal brackets bonded with either light cured or chemically cured adhesives (references:1,2,3,4,5,6). An 18 month clinical trial showed 23% failure rate for brackets placed using light cure adhesive as compared to 16% for chemically cured adhesives.(reference 1) Researchers have found a 4.7% bond failure3 when using a direct bond technique and a 6.5% bond failure(reference 4) when using indirect bonding technique in 6 month clinical trials involving metal brackets with light-cure adhesives. Except for a study by Cooper and Sorenson (1993), there are no long term evaluations of the clinical performance of adhesive precoated brackets.(reference 7) The 24 hour study conducted by Cooper and Sorenson (1993) showed a 1.4% failure rate.(reference 7) Literature points out the inconveniences of bond failures causing longer treatment times and greater risks of decalcification. (reference 8)
Clinical observations of bracket failure rates appear to be related to in vitro testing of bonding strength. ( reference 9) In vitro shear bond strengths (reference 10) and in vivo clinical trials (reference 1,3,4) demonstrate a tendancy for lower bond strength and higher initial failure rates for brackets placed with light-cured adhesives compared to chemically-cured adhesives. Early shear bond strengths of brackets suggest that visible light cured resin may not polymerize as completely as that cured chemically (reference10). While some researchers report shear bond strengths for visible light cure resin less than one half that of chemically cured resin (reference 10), others investigators suggest in vitro bond strengths are comparible (reference 11) for both mechanisms of polymerization. Eliades (1995) found the degree of cure (D.C.) for chemically cured adhesive bonded to metal brackets to be significantly greater than the degree of cure for Transbond visible light-cured adhesive bonded to metal brackets when using 20 seconds of direct irradiation. (reference 12) The degree of cure was also greater for chemically cured adhesive bonded to metal brackets compared to Transbond visible light-cured adhesive bonded to metal brackets when using 20 seconds of indirect irradiation. (reference 12)
Assuming no deficiencies in clinical technique, bond strengths are largely dependent on adequate incorporation of the cured resin into the bracket base. (references13,14,15) Most in vitro physical test on bond strength of metal brackets to enamel indicate that failure sites are mainly at the bracket/adhesive interfaces for both chemical (references15,16) and light cure adhesives.(reference17) Higher bond strengths could result if greater polymerization of resin occured within the retentive grooves of the base. (reference17) Depth of cure, hardness pattern and curing pattern can be increased through use of low viscosity coupling agents.(reference18) Bracket base design,(references 3,13,16,19) duration of curing and setting times (references 4,10,17) affect bond strengths, amount of transillumination being a factor for light cure adhesives.(reference 20)
In 1992 3M/Unitek marketed an adhesive precoated metal bracket (A.P.C.). These brackets were designed to decrease the number of variables in the bonding procedure and were advertised as providing predictable bond strength during the critical 0 to 24 hour phase. APC brackets were laboratory tested, patient proven and clinical studies were said to demonstrate a bond failure rate of less than 1% following initial arch wire insertion. Claims regarding performance of APC bracket/bonding system have been challenged (reference 17), establishing a need to further test their validity. In a 6 month study, considerably higher rates of bond failure than 1% on initial arch wire insertion were noted. (reference 21) Anecdotal reports from a survey of orthodontists, suggest a significantly higher incidence of bond failure occurs when using precoated brackets compared to conventional brackets bonded with light-cure adhesive.(reference 22) This points out a discrepancy between the manufacturers experienced level of product performance and that observed clinically.(references 21,22) The majority of operators were informed by the manufacturer of APC that the bonding failures experienced might be due to operator technique.(reference 22)
Initial testing was carried out on bovine teeth due to the facility of collecting samples, research has suggested a similarity between bond strengths of human and bovine enamel. (reference 23) Results from initial tests on bovine teeth led to more detailed testing of APC product with varying shelf life on human teeth. The decision to examine the effect of Scotchbond MP in the human study was carried out as a courtesy to the manufacturer who, subsequent to receiving initial results suggested substitution of Scotch Bond MP for the Transbond Primer. Brackets differed slightly in design between the bovine and human study, in the bovine study the brackets had no vertical slot. This reflected the preference of technique used by the orthodontist who donated the brackets. This introduced different variables for size of the bracket base, however, values of shear bond strength provide a normalized magnitude of the bonding strengths per unit area which allows direct comparisons between brackets of unequal sizes.
A shear/peel force test was used in this study as the literature states that a shear/peel force test is a severe test of adhesion which is most likely to correlate with clinical performance. (reference11) Researchers suggest a minimun value of 60 to 80 kg/cm2 (6 to 8 MPa) or (870 to 1160 lbs/in2) would appear to be necessary for successful clinical bonding.( reference 24)
The purpose of this study was to test the hypothesis that an adhesive precoated metal bracket/bonding system (APC) is comparable to or better than other bracket/bonding systems.
MATERIALS AND METHODS
In a pilot study nine extracted bovine teeth were mounted in plastic molds using dental plaster. The specimens were positioned so buccal, lingual and proximal surfaces were perpendicular to the base of the mounting molds. Plaster was poured to the level of the cementoenamel junction and allowed to set for 45 minutes. Twenty-four sample sites were randomly assigned to proximal buccal and lingual surfaces with each tooth receiving one of each bracket adhesive system and assigned B1 and B2 groupings. Enamel surfaces were polished for 60 seconds with a slurry of pumice, etched for 20 seconds with 37% phosphoric acid, rinsed for 20 seconds under running tap water and blown dry with clean oil free air. Sample sizes for the two systems differ reflecting available stock of material for testing.
Two bracket/adhesive systems were used in the first pilot study using bovine teeth. In group B1* an 80 guage metal mesh bracket served as a control. These brackets were bonded with Light Bond** a visible light-cured resin with similar properties to the precoat resin, both adhesives have approximately the same percent filler content by weight. Group B2*** consisted of APC brackets dated within one year of their expiration date, stored at room temperature and out of direct light exposure (Lot #01460877Y and 01460692X). The brackets used in B1 and B2 were .022 X .028 stainless steel premolar brackets without a vertical slot. A description of bracket types is given in Table 1 (below).
All teeth were bonded by one experienced operator. Firm pressure was used to seat each bracket and excess material was removed with an explorer before polymerization. Polymerization was achieved using an Ortholux XT curing unit*** to supply indirect irradiation (2X10 seconds from about 2mm distance on both the mesial and distal edges of the bracket) this has been demonstrated to manifest significantly higher depth of cure values than direct irradiation.12 Adequate light intensity was verified for each bracket using a light meter**** (Cure Rite Tester, Model 8000). Sites were cured in a clockwise fashion beginning with the buccal surface. An opaque shield was used to limit light transmittance to adjacent brackets. After bonding the samples were stored at room temperature in water.
* "A" Company, 11436 Sorrento Valley Road, San Diego, California, USA, 92121-1393 ** Reliance Orthodontics Products Inc., P.O. Box 678, Itasca, Illimois, USA, 60143 *** 3M Unitek Corporation/3M Dental Products Division, 2724 South Peck Road, Monrovia, California, USA, 91016-7118 **** Engineer Fiber Optics Systems Inc., 2260 Argentia Road, Mississauga, Ontario, Canada, L5N 6H7
In the second study, thirteen extracted human molars, stored in saline at room temperature, were mounted on acrylic formatray blocks.* The human teeth were mounted similar to the bovine teeth keeping buccal, lingual and proximal surfaces aligned perpendicular to the base of the mounting blocks, this was accomplished through visual inspection.
Four different systems (bracket/adhesive combinations) were used. These groups are referred to as H1**, H2***(Lot# 0145 4722X), H3***(Lot# 10483080F) or H4***(Lot# 10483080F). System H1 served as a control and consisted of non precoated stainless steel premolar brackets with vertical slot bonded with Light Bond**** adhesive. Systems H2, H3, and H4 consisted of premolar precoated stainless steel brackets with vertical slot. Description of bracket types are shown in Table 1. The difference between systems H2 and H3 is the manufacturing lot and shelf-time before use. System H2 was manufactured in January 1994 and used after 10 months, whereas system H3 was manufactured in October 1994 and used after 2 months. In the H4 group, Scotchbond MP*** dental adhesive was substituted for the Transbond Primer*** to test the manufacturers claim that altering the primer would alter bond strength. Test sites were assigned so each tooth had at least one control and one of each APC systems bonded to it. During bonding an opaque light shield was used to shield adjacent bonds from exposure. Curing was done in a clockwise direction so as not to favour any bracket/bonding system. Up to four brackets were bonded to each tooth, making a total of 47 brackets.
* Kerr Manufacturing Co., 28200 Wick Road, Romulus, MI, USA, 48174 ** "A" Company, 11436 Sorrento Valley Road, San Diego, California, USA, 92121-1393 *** 3M Unitek Corporation/3M Dental Products Division, 2724 South Peck Road, Monrovia, California, USA, 91016-7118 **** Reliance Orthodontics Products Inc., P.O. Box 678, Itasca, Illinois, USA, 60143
A digital caliper* accurate to 10 micrometers was used to calculate projected surface areas of the bracket base in millimeters. System H1 consisted of pentagonal pads with a projected surface area of approximately 9.20 mm˛ (~0.0143 sq.in.). All three adhesive precoated bracket systems (H2, H3, and H4) included rectangular pads with a projected surface area of approximately 8.80 mm˛ (~0.0137 sq.in.). Out of the 47 specimens, twelve belonged to system H1, thirteen to system H2, thirteen to system H3 and nine to system H4. The testing operator was blinded with respect to type of bracket/adhesive system used at each site at time of testing.
Bovine and human specimens were subjected to biomechanical testing after thirty hours. Specimens were fixed in a Chantillon uniaxial testing machine (Model TCM 201-SS) and a non stretched 0.020 inch stainless steel debonding clip** (Number 444-780) was placed under the gingival tie wings to reduce peeling moment. A self aligning, flexible mechanism linked the cross head of the testing machine to the debonding clip. Force in a gingivoincisal direction subjected each bracket to a shear/peel stress as shown in Figures 1 through 3 at a ramp speed of 1.27 mm/min until bonding failure occured. Maximun peak forces at failure were recorded in Newtons (N) divided by the projected area of the base in square millimeters (mm2) to obtain bond strength values in MegaPascals (MPa). Bond strengths for the bracket/adhesive systems were analysed statistically by analyses of variance with a factorial design. Means were ranked by a Scheffe interval.
Fracture modes were determined by examination of the debonded enamel surfaces and bracket bases under a stereomicroscope. As in other studies (references 10,25) a sharp explorer was used to verify curing under
* Mitutoyo Manufacturing Company, 965 Corporate Blvd, Aurora, Illinois, USA, 60504 ** 3M Unitek Corporation/3M Dental Products Division, 2724 South Peck Road, Monrovia, California, USA, 91016-7118
the bracket. Representative samples of each bracket/bonding system were examined by Scanning Electron Microscopy (SEM). Location and site of residual adhesive on bracket and enamel surfaces was recorded at 30X and 50X magnification.
RESULTS
Bovine teeth, premolar brackets without vertical slots Results for 23 tested specimens of 24 available specimens are represented in Bar graphs in Fig.4. Twelve specimens belonged to group B1 and eleven to group B2. One specimen failed due to plaster fracture under loading. Group B1 had a mean bond strength of 8.88 MPa with values ranging from 5.62 MPa to 15.68 MPa. Eighty three percent of this sample achieve bond strengths of 6 MPa or greater. Group B2 had a mean bond strength of 6.77 MPa with values ranging from 2.21 MPa to 10.09 MPa. Fifty five percent of this sample achieved bond strengths of 6 MPa or greater. There was no statistically significant difference between mean bond strengths of groups B1 and B2. Mean strength values and standard deviations are listed in Table 2.
Human teeth, premolar brackets with vertical slots Results for 42 tested specimens of 47 available specimens are shown in Fig.5. Eleven specimens belonged to system H1, 13 to system H2, 9 to system H3 and 9 to system H4. Testing of the remaining 5 specimens was not possible due to the teeth de-mounting from the formatrey block under loading. Mean strength values and standard deviations are listed in Table 2.
System H1 had a mean bond strength of 12.50 MPa the highest of all bracket/bonding systems tested. The range was from 4.57 to 17.61 MPa. Ninety one percent of the samples tested in this group achieved bond strengths of 6 MPa or greater. In contrast the mean bond strength for System H2 was 6.24. The range was from 4.07 to 10.06 MPa. Fifty four percent of this sample achieved bond strengths of 6 MPa or greater. The mean bond strengths for systems H3 was 10.68 MPa and for H4 10.22 MPa. H3 and H4 showed no statistical difference for mean bond strengths but did show a statistically significant difference compared to bond strength of system H2. There was a statistically significant difference between mean bond strengths of groups H1 and H2.
Two ANOVAS were performed to determine if the test systems differed in bonding strength. APC bonding systems H2, H3, H4 were examined in the first ANOVA. System H2, H3 and H4 differed in bonding strength, F(2,28)=14.80, (p < .01). Scheffe post hoc comparisons revealed system H2 was significantly different from system H3 and system H4; (p < .01).
The second ANOVA, a planned comparison used to determine if system H1 differed from APC systems (H2, H3, H4) in bonding strength, revealed a difference between system H1 and the APC systems F(1,38) = 14.42; (p < .01). As an alternative to the planned comparison, an independent t-test was done in which system H2, H3 and H4 are treated as one system since they are all APC bracket/bonding systems. System H1 differed significantly from the APC systems H2, H3 and H4 in bonding strength, T(40) = 3.25, (p < .01).
Site of bond failure Failure sites for the specimens of each bracket/adhesive system is shown in Table 3. SEM photomicrographs for the APC and control bracket/bonding systems are shown in Fig.6 to 9. When stressed to the failure point 90% of the control group failed at the bracket/adhesive interface. The APC bracket/bonding systems failed cohesively within the resin more often than at the bracket/adhesive interface. The SEM (Figs.7 and 9) indicate that the APC adhesive fails in different areas leaving voids in the adhesive especially at the bracket edges. The distribution of residual adhesive (Figs.6 and 8) is more uniform on the control brackets which have mesh pads.
DISCUSSION
Mean bond strengths for APC (6.24 ± 1.79 MPa) reported in this study are higher than those recorded by Oesterle et al (1995) who reported mean bond strengths of APC (4.5 ± 0.49 MPa) for metal brackets cured for similar lengths of time.(reference 17) The mean shear bond strength (12.50 ± 4.09 MPa) for the metal mesh brackets bonded with Light Bond* adhesive, is similar to the mean shear bond strength (11.03 ± 3.05 MPa) reported by Newman et al (1994) for metal mesh brackets bonded with Contacto-Lite** light-cured adhesive under a 60 second exposure with the Optilux 400*** light. (reference 26)
It is interesting to note that bond strengths for group H3 and H4 (APC brackets) performed statistically different (p < .01 level) from other APC lots tested. The same operator was involved in bonding procedures for all test specimens and each bracket/bonding systems tested was carried out on different surface of the same tooth. It would not seem that the different bond strengths observed could be explained by operator technique or condition of the tooth surface as these variables were held constant. One possible explanation for the differences in bond strengths observed, may be that the adhesive shelf life is much shorter than previously suggested by the manufacturer. Adequate bond strengths for APC were achieved by this operator when the product was used within 2 months of date of manufacture.
* Reliance Orthodontics Products Inc., P.O. Box 678, Itasca, Illimois,
USA, 60143
** General Orthodontic Supply Inc., P.O. Box 298, West Orange, NJ 07052.
*** Demetron Research Corp., 5 Ye Olde Road, Danbury, CT 06810.
Although shelf life could be a factor, normal shelf life for similar light-cure adhesives (Ex: Light Bond) is 2 years and all products tested were well within the expiry date indicated by the manufacturer. The 40% increase in bond strength noted in group H3 and H4 which were provided by the manufacturer for testing are difficult to explain. Silanation can increase bond strengths of metal brackets on the range of 28%.(references 19,27) A chemical surface treatment, carried out on the brackets to obtain true mechanical retention between the metal pad and the adhesive could be accomplished by laying down an angstron - layer of inorganic compound, typically SiO2, followed by hydrolyzed silane applied to the bracket base. Paciorek (1995) has carried out spectrometry analysis of APC lot numbers H2, H3 and H4.(reference 28) A trend for increased Si was found in systems H3 and H4 compared to H2 but sample size did not permit any statistically significant difference to be noted between the systems, so no definitive conclusion can be drawn at this point.(reference 28) Research has shown changes of bases due to surface treatment modifications are not apparent at 80 X magnification so researchers and clinicians who evaluate bond strengths of different new products are dependant on manufacturers to provide this information.(reference 19) Some authors report that silanes that are stored (especially for periods longer than 6 months) are chemically unstable.(reference 29)
The manufacturer has suggested that replacement of Transbond Primer by Scotchbond MP may inprove the adhesion of the APC bracket. This study indicates the replacement of Transbond sealant by Scotchbond does not significantly alter the bond strength of APC.
A critical factor in achieving successful bonding with the APC bracket/bonding system is maintaining good penetration of the adhesive in the integral groove base.(reference 30) The adhesive tends to pull away from the APC bracket during removal from the packaging blister.(reference31) The APC bracket and adhesive are shipped on a celluloid liner which must be in contact with the adhesive to prevent premature exposure of adhesive to oxygen which could inhibit polymerization. There is a force required to separate the APC system from the celluloid liner and this can pull adhesive away from the bracket base when the APC bracket is removed from the blister.(reference 31) Adhesive material should be " Buttered in " to the base not just padded on, this is important for maximal incorporation of the resin into the mesh or mechenical grooves of the base.(reference 8) Incorporation of the adhesive into the mesh base could be a very important factor in explaining the different bond strengths observed for the two systems tested.
The data shows a statistically significant difference among bracket/adhesive systems tested. Several factors could influence bond strengths making comparisons amongst in vitro tests and between in vitro and in vivo studies difficult. Firstly the reader must remember that in vitro bond failures occur under static loading where as in vivo bond failures occur under cyclic loading. One can not extrapolate directly from in vitro bond strengths to conclusions regarding bond failure rates under clinical conditions. However, in vitro tests still provide an insight on the performance of two different systems intended for the same application.
The reader should be aware, the use of two different types of bracket base design with potentially different failure thresholds built in to the design may explain the different shear bond strengths observed. Previous research has shown higher bond strengths for mesh pads versus mechanical groove bases.(references 3,13,16) This study corroborates those findings. This study was not undertaken to confirm the above but rather to test bond strengths of precoated brackets compared to a system with proven clinical performance. Although base area, type of mesh, and adhesives used differ for the two systems as shown in Table 1, it should be remembered that the bonding configuration for APC brackets/adhesive system was designed to "prevent patients from losing brackets sooner than they should". Any bias as to study design might favor the new APC bracket/bonding system, since the manufacture has suggested superior clinical performance of APC over other bracket/bonding systems. If the manufacturing process of precoating the bracket with adhesive is responsible for this then comparison to a well tested control is justifiable. In the present study variability in bond strengths were less for the adhesive precoated brackets but the bond strengths were also lower, indicating design of the bracket base may be more important than whether the bracket is precoated with a specific thickness of adhesive.
Condition of enamel surface alters with increasing age, amount of fluoride in diet, enamel cracking or damage during extraction, all can influence bond strengths.( references 32,33) Whittaker (1982) has reported larger amounts of aprismatic enamel on molars and premolars.(reference 34) This may affect the strength of the micromechanical bond and lead to poorer adhesion of posterior brackets.(reference34) Certain researchers avoid grinding or reducing enamel surfaces during in vitro tests as it affects the nature of the enamel surface, in vitro results may be irrelevant to the clinical situation when outer aprismatic layers of enamel are removed.(reference 35) In this study, enamel surface were not ground as tooth curvature aided in providing good adaptation to curved premolar bases. Morphology of tooth surfaces vary and can affect bond strengths. (reference 36) If there is poor adaptation between tooth morphology and contour of the base then the bracket is forced into contact at the highest point of curvature to the enamel surface, thereby displacing most of the adhesive resulting in a starved joint. If in vitro testing wishes to mimic clinical situations then the bracket/bond system must deal with the fact that different morphologies of enamel surfaces exist.
Extracted teeth used during in vitro experiments, no matter how well perserved, are much drier then those in the mouth.(reference 37) Clinically adhesives must perform under numerous deleterious conditions in the oral cavity, such as constant moisture, relatively high ambient temperature, and adherent contamination that is difficult to remove completely.(reference 36) Further it must be capable of withstanding considerable masticatory stress as well as applied orthodontic forces. Previous studies conflict on need for thermocycling as no significant effect has been demonstrated on shear bond and rebond strengths.(reference 38) This study investigates the manufacturers claims regarding bond failures at initial arch wire insertion therefore thermocycling was not considered an issue. Certainly further studies which address bonding failures during treatment should be performed on the precoated bracket system and in these situations thermocycling becomes an important factor.
Mounting teeth in plaster by visual or optical inspection is similar to methodology used by other investigators.(reference 2) However, in future studies use of a mounting jig that accurately assures placement of bracket base and uniform thickness of adhesive for each sample should be considered. Third molars have short and often converging roots making retention in the mounting blocks difficult an improvement in fixation of the test specimen is required.
In vitro bond failure modes conducted at slow stress rates may not be indicative of in vivo failure modes where much faster, sudden loads may occur.(reference 37) The ramp speed of 1.27 mm/min used in this study is about average for testing speeds used in other research.(references 26,37)
CONCLUSIONS
Similar to other research (reference17) the findings of this study indicate that 20 seconds of curing for APC bracket/bonding system would not always produce the 6 to 8 MPa of bond strength which is considered necessary to achieve clinically acceptable levels of bond strength.(reference 24) Present results do not substantiate the suggestion that bond failures are due to operators' technique. In all groups tested, higher mean bond strengths were consistently found for conventional brackets bonded with light-cure adhesives compared to the precoated bracket/bonding system. The purpose of this investigation was to verify the manufacturer claims that use of APC bracket/bonding system may lead to fewer bond failures than other conventional bracket/bonding systems. In order to test this claim the in vitro bond strength of APC was compared to another commercially available bracket/bonding system under similar test conditions. Findings from this study do not support the manufacturers claims that APC may cause less bond failures then other bracket/bonding systems. All bracket/bonding systems in this study achieved mean shear/peel strengths between 6-8 MPa which previous researchers suggest are reliable bond strengths for clinical situations,(reference 24) however, a large variance was observed within APC lot numbers. Forty five percent of the APC samples tested on bovine teeth and 46% of the APC samples from the human teeth used after 10 months from manufacturing date did not obtain 6 to 8 MPa of bond strengths. In contrast all APC samples for the group which had a shelf life of 2 months tested above the 6-8 MPa level of bond strength, placing in question the shelf life of the adhesive precoated bracket.
Figure 1: Shear test configuration.
FIGURE 2: Chantillon uniaxial testing machine (Model TCM 201-SS)
FIGURE 3: Three-way angle tilting vise attached to base of Chantillon testing machine to assure shear/peel force.
| Bracket Type | Base Design | Projected Surface Area in mm2 |
|---|---|---|
| "A" Company Premolar 022 X 028 with vertical slot | Mesh pad | 9.20 |
| "A" Company Premolar 022 X 028 without vertical slot | Mesh pad | 9.08 |
| APC Premolar 022 X 028 with vertical slot | Integral groove base | 8.84 |
| APC Premolar 022 X 028 without vertical slot | Integral groove base | 8.13 |
TABLE 2 : (Editor's note: These appear as tables in Netscape 2.0, other browsers may display them diffently.)
| GROUP | N | AVERAGES (S.D.) [MPa].........[lbs/sq.in.] | % OF SAMPLE TESTING BELOW 6 MPa |
|---|---|---|---|
| B1 | 12 | 8.88(2.96) 1295(431.71) | 17% |
| B2 | 11 | 6.77(2.40) 982.40(347.44) | 9% |
| H1 | 11 | 12.5(4.09) 1812.73(592.45) | 9% |
| H2 | 13 | 6.24(1.78) 904.70(257.57) | 46% |
| H3 | 9 | 10.68(2.10) 1549.36(304.40) | 0% |
| H4 | 9 | 10.22(2.29) 1482.21(331.47) | 11% |
FIGURE 4: Bond strengths of control (Group B1) and APC (Group B2) brackets bonded to bovine teeth.
FIGURE 5: (55 Kb) Bond strengths of control (H1) and APC (H2*,H3**,H4***) brackets bonded to human teeth.
TABLE 3 : FAILURE SITE FOR THE DIFFERENT GROUPS
| Bracket/Adhesive System | Bracket/Resin | Resin/Enamel | Within Resin | TOTAL | % Failure at Bracket/Adhesive Interface |
|---|---|---|---|---|---|
| B1 | 11 | 0 | 1 | 12 | 92% |
| B2 | 2 | 0 | 9 | 11 | 18% |
| H1 | 10 | 0 | 1 | 11 | 91% |
| H2 | 5 | 0 | 8 | 13 | 38% |
| H3 | 4 | 0 | 5 | 9 | 44% |
| H4 | 2 | 0 | 7 | 9 | 22% |
FIGURE 6: Scanning electron micrographs of metal mesh bracket base. Original magnification X30.
FIGURE 7: Scanning electron micrograph of APC bracket base.
FIGURE 8: Scanning electron micrographs of metal mesh bracket base.
FIGURE 9: Scanning electron micrograph of APC bracket base.
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