The shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated composite resins V. P. Joseph, BDS, MSc (Dent.),* and E. Rossouw, BSc, BChD, BChD (Hons.), MChD** Tygerberg, Republic of South Africa Since the introduction of ceramic brackets to orthodontic therapy, a need has arisen to test the manufacturer's claims regarding these brackets. Forty-eight noncarious human canine teeth were divided equally into groups A to D. Brackets were bonded to these teeth with the use of the acid-etch technique and a composite resin according to the manufacturer's instructions. The combination within each group was as follows: A = stainless steel brackets and chemically cured resin; B = ceramic brackets and chemically cured resin; C = ceramic brackets and light-cured resin; D = stainless steel brackets and light-cured resin (via transillumination). After curing, the teeth were stored for 1 week in distilled water at 37 ° C. The Instron machine was used to test the shear bond strengths of the brackets to the teeth. The brackets were individually tested to failure of the bond, which was recorded along with the site of fracture. The conclusions are as follows: (1) all combinations produced shear bond strengths that were greater than those that are considered clinically acceptable, (2) the ceramic groups exhibited a significantly higher bond strength than that of the stainless steel group, and (3) enamel fractures occurred among the B group in 40% of the samples tested in that group. It is thus apparent that a fracture of enamel is a real possibility during therapy or at debonding of the ceramic brackets, especially if the tooth is nonvital. (AM J ORTHOD DENTOFAC ORTHOP 1990;97:121-5.)

B u o n o c o r e t demonstrated that the bond strength of polymeric dental resins to enamel could be increased significantly by etching the enamel surface with 85% orthophosphoric acid. An epoxy resin system that proved to be of sufficient strength to withstand orthodontic forces has been described. 2 Diacrylate resins, commonly referred to as Bowen's reshz or bisglyceral methacrylate (bisphenol A glycidyl dimethacrylate), were designed to improve bond strength and increase dimensional stability by cross linking, s Stainless steel orthodontic brackets can be secured to teeth with this resin. The predominantly weak link in this bonding chain is at the resin/bracket base interface: It was not until 1977 that the first detailed posttreatment evaluation of direct bonding in orthodontics during a full treatment time (mean of 17 months) in a large sample of patients was published: This study concluded that acid etching and bonding with filled composite resins would make a major impact on the orthodontic world. *Department of Conservative Dentistry, Oral and Dental Teaching Hospital, University of the Westem Cape. **Department of Orthodontics, Oral and Dental Teaching Hospital, University of Stellenbosch. 8/1/10370

In a survey in the United States, it was found that 93% of orthodontists used chemically cured resin bonding for bracket placement: A major drawback of this system is the inability of the practitioner to manipulate the setting time of the composite resin. The clinician must position the brackets correctly on the teeth to assure a functional end result. 7 This, however, must be done rapidly when the chemically cured resins are used, because polymerization starts immediately on mixing. If left around the bracket, the excess composite resin will lead to plaque accumulation and resultant enamel decalcification: -~° However, the clinician must wait until final setting of the composite resin to remove any excess from around the bracket." The incorporation of air in mixing the composite resin could lead to a weakening of the bond strength of the resins and to increased surface porosity. The fact that light-cured composite resins exhibit markedly less porosity than chemically cured resins has been reported by numerous authors) 2 The polymerization of light-activated resins under metal brackets by transillumination has been shown to be successful, because the tooth conducts visible light well. "a3 Claims have been made that light polymerization (command curing) improves the accuracy of bracket positioning and thus minimizes the need for

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Joseph attd Rossottw

Am. J. Orthod. Dente~w. Orthol,. February 1990

Table I. Recorded mean shear bond strengths, standard deviations, and Duncan's multiple range test groupings Group

J Mean SBS

SD

I

I Duncan's MRT I

A B C D

17.34 28.27 24.25 17.80

2.93 7.91 7.88 3.54

~ ~

SBS. Shear bond strength m meganewtons per square meter; SD. standard deviation; Duncan's MRTo Duncan's multiple range test.

MATERIALS AND METHODS

Fig. 1. Photograph of apparatus ready for testing shear bond strengths of brackets.

realigning of teeth after debonding.~4 Excess composite material can also be removed before light polymerization because any accidental movement of the bracket will not affect the bonding capacity of the resin. Once the bracket position has been correctly determined, the resin can be command cured. The advent of ceramic brackets in the orthodontic field has led to a trend of more "cosmetic" orthodontics. The manufacturer* claims that these brackets bond to composite resins by chemical bonds and exhibit a higher bond strength than stainless steel brackets. A minimum value of 60 to 80 kg/cm'- (6 to 8 M N / M 2) would appear to be necessary for successful clinical bonding.~5 There was obviously a need to test the manufacturer's claims in regard to the higher bond strength of these ceramic brackets, with the mesh-backed stainless steel brackets used as a control. Two different types of orthodontic composite resins were also tested. The purpose of this study was to establish the difference, if any, between the shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated resins.

*Unitek Corporation. Monrovia, Calif.

Forty-eight noncarious extracted human canine teeth were collected over a period of months. After extraction, the teeth were cleaned on a dental lathe with pumice and stored in 70% ethyl alcohol as has been reported in the literature. ,6 They were divided into two groups of 24 teeth each. One group was used in the testing of the chemically cured resin Concise,* while the other was used in the testing of the light-cured resin Heliosit.i The chemically cured group was further subdivided in two groups of 12 teeth each. Group A had stainless steel brackets~ bonded to the teeth, and group B had ceramic brackets ~ bonded to the teeth. The light-cured group of 24 teeth was similarly subdivided in two groups of 12 teeth each. Group C had ceramic brackets ~ bonded to the teeth with a light-cured microfilled resin,t which was handled according to the manufacturer's instructions. Group D had stainless steel brackets~ bonded to the teeth with transillumination of visible light. These teeth were light-cured at the bracket's gingival and incisal margins for 10 seconds initially and then exposed to a further 2 minutes of transillumination of visible light through the palatal side of the tooth. The bonding of these brackets was according to accepted techniques. 17 All teeth were polished with pumice before a 60-second etch of 37% orthophosphoric acid. They were washed for 20 seconds under running tap water to remove the orthophosphoric acid and blown dry with clean uncontaminated air. During all stages of bonding of the brackets, all materials were *Chemically activated concise orthodontic bonding system (large-particle macrofilledresin). Manufacturedby 3M Dental Products, St. Paul, Minn. Batch No. 1960, i'Light-activated tteliosit Ortho (microfilledresin). Manufacturedby Vivadent AG. Schaan, Liechtenstein. Batch No. 392401. :~Standard stainless steel springwing brackets with mesh backing for direct bonding. Manufactured by Unitek Corporation, Monrovia, Calif. §Transcend ceramic brackets. Manufactured by Unitek Corporation.

Shear bond strengths of various orthodontic brackets 123

Volume 97 Number 2

14

Samples

12 10 8 6 4 2 0

A

B

C

D

Groups R/E

~

R/B

~

In E

Fig. 2. Graph of fracture sites after debonding. RIB, Between resin and bracket; R/E, between resin and enamel; in E, in enamel.

handled in accordance with manufacturer's instructions. The teeth were then prepared for mounting in brass cups by sectioning their roots and undercutting the pulp chambers as described in the literature) 8 All the teeth protruded above the lip of the cup by 1 mm and were secured in the cup by means of an acrylic resin.* All forty-eight teeth were stored at 37 ° C in an incubator in distilled water for 1 week. The specimens were secured in a clamp device and a 0.20-inch gauge wire was placed under the wings of the bracket with the ends of the wire clamped to a selfcentering device (Fig. 1). The specimens were then stressed in a gingivo-incisal direction to failure with a shear bond test on the Instron universal testing machine.'[" The procedure was similar to one that was described in the previous literature. + All the recordings were calculated as a force measured in newtons, which was subsequently converted to stress per unit area (meganewtons per square meter) by dividing the force by the unit area of the base of the bracket. The bonding surface area of the brackets was determined by means of a reflex microscope and computer program. The sample sequence was determined by means of a computer-generated random order table. The sites of fractures of all the teeth were examined with a light optical stereoscopic microscope and recorded. The *Fastcure. Manufactured by Kerr F,ldnufacturing Co., Romulus, Mich. Batch No. 64248. tlnstron testing machine. Manufactured by lnstron Corp., Canton, Mass.

brackets were also examined for damage after debonding. RESULTS Shear bond strengths

A difference in the shear bond strengths was recorded between the different samples in each group and from group to group (Table I). When the mean was calculated for each group, it was apparent that thetwo groups that included the ceramic brackets exhibited the highest values (Table I). Fracture sites

The sites of fracture are represented in the graph in Fig. 2. These fracture sites were divided among those that were between resin and bracket, between resin and enamel, and totally in enamel. More than two thirds of the fracture had to be evident in any one of these sites to classify it as a particular type of fracture. The.only group that displayed fracture of enamel on debonding was group B (chemically cured resin and ceramic brackets). Bracket fracture

A fracture in the body of the ceramic bracket type was observed in 6.66% of the ceramic samples from group B. None of the stainless steel brackets was observed to fracture. One specimen in group C and D failed when a fracture of the retaining resin in the brass cup allowed the whole tooth to dislodge. This is reflected in the results.

124 Joseph and Rossouw

Am. J. Orthod. Dentofac. Orthop. February 1990

cohesive fracture strength to create a debonding of the bracket. This influence was likely to be the deformation of the metal of the bracket when the shearing forces were applied. This deformation could create a fracture plane, which would propagate through the union. Thus the properties of the bonding composite resins (brittle or elastic) were masked by this deformation in the brackets when the brackets were placed under stress. No enamel or bracket fractures were noted in groups A and D. Ceramic bracket groups

Fig. 3. Photograph of fracture of enamel and ceramic bracket.

Statistical analysis The mean and standard deviation of each group was calculated (Table I). An analysis of variance was used to test the hypothesis that there was no significant difference between the results of the four groups. The analysis of variance showed a significant difference between the groups at the p = 0.0001 level of significance. The grouping of these differences by Duncan's multiple range test showed that groups A and D were not significantly different and were grouped together as were groups B and C. However, groups A and D were significantly different from groups B and C (Table 1). DISCUSSION Stainless steel bracket groups

The results obtained from both the light-cured and chemically cured groups were in accordance with the bond strengths quoted in the literature when stainless steel brackets were tested. ~9When the mean of the bond strengths of the two groups A and D of stainless steel brackets were compared, they were found to be very similar (17.34 M N / M 2 and 17.80 M N / M z) even though the resins used were different (chemically activated macrofilled and light-cured microfilled resins). Chemically cured macrofilled resins are reported to have higher bond strengths (more elastic) than light-cui'ed microfilled resins (more brittle).'8 It can therefore be assumed that some extra influence was exerted on the resins before they reached their

The shear bond strengths of both ceramic bracket groups B (28.27 M N / M z) and C (24.25 M N / M ~) were signifcantly higher than those of stainless steel bracket groups A (17.34 M N / M 2) and D (17.80 MN/M2), whether the chemically cured or light-cured resins were used (Table I). This difference may be explained as follows: It can be assumed that because the ceramic brackets are so rigid, no deformation occurred when shearing forces were applied to the brackets. Thus chemically cured composite resin (macrofilled and more elastic) fractured at higher bond strengths than the light-cured composite resin samples (microfilled and more brittle). These groups exhibited the differences between the two types of resins as reported in the literature. ~8 However the increased bond strength resulted in enamel fractures in four of the samples (Fig. 3). This disturbing feature must be emphasized, inasmuch as the use of ceramic brackets on nonvital teeth could cause a higher incidence of enamel fractures during therapy and at debonding. Testing for vitality before bonding and debonding would seem to be a necessity. Special debonding procedures are described by the manufacturer of the ceramic brackets. Because of the high bond strength obtained with the rigid ceramic brackets, the safety margin of the stresses that could be withstood by the cohesive strength of enamel is reduced. This in turn could lead to an increase in enamel fractures. The incidence of fractures of the ceramic brackets themselves (6.66%) is of concern. Pieces of bracket could be ingested or inhaled inadvertently if the fracture occurred in the mouth during function. It appears imperative that informed consent be obtained before ceramic bracket therapy is begun, along with testing of the vitality of teeth before both bonding and debonding. Fracture sites

When the fracture sites were examined by means of a stereoscopic microscope, the sites could be divided into resin/bracket, resin/enamel, and in enamel (Fig.

Voh,me 97 Number 2

Shear bond strengths o f various orthodontic brackets

2). Group B had the highest mean shear bond strengths (Table I), but 40% of samples exhibited enamel fractures (Fig. 2). Many of the ceramic brackets after debonding exhibited a smooth and shining debonded surface. This suggests that the union to these brackets is chemical and not mechanical.

2. Retief DH, Dreyer CJ. Epoxy resins for bonding orthodontic attachments to teeth. J Dent Assoc South Afr 1967;22:338-46. 3. GraberTM, Swain BF. Orthodontics:currentprinciplesand techniques. St. Louis: CV Mosby, 1985. 4. Knoll M, Gwinnett AJ, Wolff MS. Shear strength of brackets bonded to anterior and posterior teeth. AM J ORTtlOD 1986; 89:476-9. 5. ZachrissonBU. A postoperative evaluationof direct bondingin orthodontics. AM J ORTHOD1977;73:173-89. 6. Gorelick L. Bonding: the state of the art--a national survey. J Clin Orthod 1979;13:39-53. 7. Andrews LF. Six keys to normal occlusion. AM J ORTHOD 1972;62:296-309. 8. StratemanMW, ShannonIL. Controlof decalcificationin orthodontic patients by daily self-administeredapplicationof a water free 0.4% stannous fluoridegel. AM J ORTHOD1974;66:273-9. 9. ZachrissonBU. Fluoride applicationprocedures in orthodontic practice, current concepts. Angle Orthod 1975;45:72-81. I0. GwinnettJA, Ceen RF. An ultravioletphotographic technique for monitoringplaque during direct bonding procedures. AM J OR'nIOD1978;73:178-86. 11. KingL, Smith RT, Wendt SL, Behrents RG. Bond strengths of lingual orthodontic brackets bonded with light-curedcomposite resins cured by transillumination. AM J ORTHODDENTOFAC ORTHOP1987;91:312-5. 12. Joseph VP, Cohen J. Porosities of chemicallycured and light cured composite resins. Delivered at the annual meeting of the InternationalAssociationfor Dental Research in Johannesburg, 1986. InternationalAssociationfor DentalResearchAbstractNo. 17. 13. TavasMA, WattsDC. Bondingof orthodonticbrackets by transilluminationof a light activated composite: an in vitro study. Br J Orthod 1979;6:207-8. 14. Read MJF. The bondingof orthodontic attachments using a visible light cured adhesive. Br J Orthod 1984;11:16-20. 15. ReynoldsIR. A reviewof direct orthodonticbonding.BrJ Orthod 1975;2:171-8. 16. Bryant S, Retief DH, Russell CM, Denys FR. Tensile bond strengthsof orthodonticbondingresinsand attachmentsto etched enamel. AM J ORTHODDENTOFACORTtIOP1987;92:225-31. 17. Daft KS, LugassyAA. A preliminarystudy of orthodontic treatment with the use of directly bonded brackets. AM J ORTItOD 1974;65:407-18. 18. Joseph VP. A laboratoryevaluationof conventionalchemlcally activated and microfilled light activated composite restorative resins [Thesis]. Cape Town, Republicof South Africa: University of the WesternCape; 1987. 19. Lopez JJ. Retentive shear strengths of various bonding attachment bases. AM J ORTtIOD1980;77:669-78.

CONCLUSIONS 1. The most important feature of this study was the observation of the fracture sites. Enamel fractures occurred in 40% of the chemically cured resin and ceramic bracket group (group B) and this must be noted when teeth that are nonvital or have been endodontically treated are to be bonded with a ceramic bracket. Debonding, either intentionally or unintentionally, may lead to fractures of enamel. 2. The need for vitality testing before bonding and before debonding appears to be essential in the light of this information. 3. The brackets of groups A and B (bonded with the heavily macrofilled resin, Concise) and the brackets of groups C and D (bonded with the lighter microfilled resin, Heliosit) produced a shear bond strength that is greater than that considered clinically acceptable. 4. Groups A and D (stainless steel brackets) exhibited no significant difference in bond strengths, because the metal deforms and produces a fracture plane before the cohesive fracture strength of the composite is reached. 5. Groups B and C (ceramic brackets) both had significantly higher bond strengths than groups A and D (stainless steel), but were not significantly different from each other (Table I). This could be the influence of the different types of resins used. 6. The incidence of fracture of the ceramic brackets was 6.66%. 7. It is advised that informed consent regarding the complications listed above be obtained. We acknowledge Miss Gina Joubert of Medical Research Council for her help with the statistics and Dr. C. Jooste for research advice.

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REFERENCES 1. BuonocoreMG. A simple method of increasingthe adhesion of acrylic filling material to enamel surfaces. J Dent Res 1955; 34:849-53.

Dr. E. Rossouw Department of Orthodontics Oral and Dental TeachingHospital Universityof Stellenbosch Private Bag XI Tygerberg, 7505, Republicof South Africa

125

The shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated composite resins.

Since the introduction of ceramic brackets to orthodontic therapy, a need has arisen to test the manufacturer's claims regarding these brackets. Forty...
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