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Scientific Section |
1 Royal Aberdeen Children’s Hospital, UK
2 Dundee Dental Hospital and School, UK
C. J. Larmour, Orthodontic Department, Royal Aberdeen Children’s Hospital, Cornhill Road, Aberdeen AB25 5ZG, UK. Email: Colin.Larmour{at}arh.grampian.scot.nhs.uk
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Abstract |
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Setting: An ex vivo study.
Method: Three groups of 20 extracted premolar teeth were bonded with metal orthodontic brackets. Group 1 was bonded with Transbond using the conventional technique (control). Group 2 was bonded using the new Transbond-PlusTM combined etch/primer system to wet enamel and Group 3 to dry enamel. The teeth were debonded using an Instron Universal Testing Machine. The mean debond force was calculated for each group and compared statistically. The teeth were examined under the stereomicroscope to assess the site of debond and adhesive remnant index.
Results: Group 2 (etch/primer on wet enamel) had the lowest mean debond value at 5.2 MPa. ANOVA and Tukey tests confirmed that the bond strength results of Group 2 were significantly lower than Groups 1 (P < 0.01) and 3 (P < 0.05). The enamel/resin interface was the commonest site of bond failure for both etch/primer groups (Groups 2 and 3). They had less retained resin and significantly (P < 0.001) lower ARI scores compared with Group 1 (control).
Conclusions: The results of this ex vivo study suggest that the self-etch primer should achieve adequate bond strengths when applied to dry enamel surfaces. In addition there should be less retained resin requiring removal at debond.
Key words: Adhesive remnant index, orthodontic bonding, self-etching primer
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Introduction |
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Its use in orthodontics was pioneered by Newman2 and later refined by Miura et al.3 Typically, current techniques involve applying phosphoric acid to dry tooth enamel for approximately 15–30 seconds prior to thoroughly washing and drying the enamel surface.4 This etching causes dissolution of inter-prismatic material in the enamel producing an irregular enamel surface facilitating the retention of an orthodontic attachment via its bonding adhesive. Any etch procedure is therefore technique sensitive, and requires adequate isolation to prevent moisture contamination of the etched enamel surface and resulting reduced bond strength.5 Modern orthodontic bonding adhesives in routine use are Bis-GMA based composite resin systems with variable amounts of filler. Polymerization can be initiated chemically (chemically cured) or with a visible blue light source (light cured) or a combination (dual cured) depending on the system. Most systems require the application of a layer of unfilled resin or primer onto the etched enamel surface prior to bonding. Due to this the bonding process has an added step, which adds to the clinical time required for fixed appliance placement.
Recently, to overcome this problem, the manufacturers of a light-cured bonding system, TransbondTM (3m Unitek, Moravia, California, USA) have introduced a combined etch primer system, Transbond-PlusTM. This system by combining the etching and priming steps in the bonding process aims to reduce the clinical time required for appliance placement. It comprises methacrylated phosphoric acid esters, which will both etch and prime the enamel surface prior to bonding. The manufacturers also claim that it can be applied to a wet enamel surface and achieve adequate etching and priming in a 3-second period. Obviously, from a clinical perspective this would be very advantageous since isolation should be less of a problem and enamel preparation would be less technique sensitive, and could be achieved more rapidly compared to a conventional etch/priming procedure.
The present ex vivo study aims to assess the bond strength of brackets bonded with the new self-etch/ priming system compared with those bonded using a conventional etch/priming technique.
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Methods |
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The teeth were bonded with pre-adjusted 0.022-inch 3M minitwinTM brackets using the following bonding techniques:
The materials were all mixed and applied according to the manufacturers instructions by one operator. Group 1 (control) was bonded conventionally with a 30-second acid etch time and separate application of conventional TransbondTM primer. Groups 2 and 3 were bonded with the new self-etching primer system, which was applied for less than 5 seconds as recommended by the manufacturer. Light curing for all groups was carried out with a 60-second exposure to a blue light source (Visilux 2 3M, St Paul, Mn, USA). This was split into two 30-second durations from the mesial and distal of each specimen. A longer than conventional curing time was used to ensure complete polymerization of the specimens.
The bonded teeth were stored in distilled water at 37°C. The teeth were debonded using the Instron Universal Testing Machine (Instron Ltd, High Wycombe, U.K.) as recommended previously.6. The debonding technique involved the application of a force to the bracket via a wire loop located under the gingival aspect of the bracket. The steel wire and polyester blocks were mounted on universal joints to ensure perpendicular pull to the bracket. The force was applied by the Instron with a cross-head speed of 1 mm/minute. The maximum force applied to produce bond failure was measured in Newtons and recorded. The force per unit area was then calculated and recorded in MPa as the shear bond strength.
Following debond each tooth was examined under the stereomicroscope and the site of bond failure recorded along with the Adhesive Remnant Index.7 This index consists of the following scoring : 0 = no retained resin, 1 = <50 per cent retained resin, 2 = >50 per cent retained resin, and 3 = all resin retained with bracket imprint.
The data was assessed using summary statistics before being analysed using analysis of variance and Tukey tests. Weibull analysis was also carried out which relates the probability of bracket failure to the applied load. This analysis has been advocated previously.6,8 The ARI data was assessed using chi-square tests.
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Results |
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. The control Group 1 (conventional etch/bond technique) and Group 3 (etch/primer on dry enamel) had similar mean debond values at 7.1 and 7.2 MPa, respectively. Group 2 (etch/primer on wet enamel) had the lowest mean debond value at 5.2 MPa. ANOVA and Tukey tests confirmed that the bond strength results of Group 2 were significantly lower than Groups 1 (P < 0.01) and 3 (P < 0.05).
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demonstrates the Weibull analysis of the test groups. The reliability of the material is a function of the Weibull modulus and normalizing parameter (characteristic strength). The correlation coefficient describes how closely the data fits the curve produced by the Weibull equation. The data is presented graphically in Figure 1 and consists of the cumulative probability of bond failure plotted against applied load. The probability of bond failure at 75 N (7.1 Mpa) was calculated for each group as this approximated to the mean debond force level required to debond the control group. The probability of bond failure at 75 N (7.1 Mpa) was calculated at 52 per cent for Groups 1 and 3, and 86 per cent for group 2.
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along with the adhesive remnant index (ARI) scores. The bracket/resin interface was the commonest site of failure for Group1(control). However, the enamel/resin interface was the commonest site for both etch/primer groups (Groups 2 and 3). Chi-square testing demonstrates that the etch/primer groups (Groups 2 and 3) had significantly (P < 0.001) lower ARI scores and therefore, less retained resin compared with Group 1 (control).
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Discussion |
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and used in previous bond strength studies.9,10 At present, there is not a universally accepted minimum clinical bond strength. However previous studies11,12 suggest that orthodontic bond strength should be in the order of 3–7 Mpa.
The results of the present ex vivo study suggest that adequate bond strengths can be achieved with the new self-etching primer system when bonding is carried out to a dry enamel surface. (7.2 Mpa) No significant differences in bond strength measurements were found compared to the conventionally bonded control group (7.1 Mpa). A previous study13 utilizing a different self-etch primer system reported a similar mean bond strength (7.1 Mpa).
The manufacturers of the new self-etching primer system suggest that adequate bond strengths can be achieved bonding to a wet enamel surface. This would obviously be very advantageous from a clinical point of view. However, the results of the present study suggest that brackets bonded in this way have significantly lower bond strengths (5.2 MPa) compared with a conventionally bonded control. Weibull analysis relating probability of failure to applied load suggests that 86 per cent of brackets will fail at 75 N (7.1 Mpa) compared with 52 per cent of the control. A previous study14 using a different self-etch primer reported similar results bonding to wet enamel with a mean bond force of 4.8 MPa. Interestingly, the reported reduced bond strength occurred irrespective of whether the contamination of the enamel surface occurred before or after application of the etch primer.
In the present study, when assessing the site of bond failure, the percentage of brackets failing at the enamel/ resin interface was increased with the new self-etch primer (72 and 63 per cent) compared with the conventionally bonded control (15 per cent). They also had correspondingly lower adhesive remnant scores. This would be an advantage in the clinical situation as less time will be required at the end of treatment removing retained resin from enamel surfaces.
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Conclusions |
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Acknowledgments |
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References |
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2 Newman GV. Epoxy adhesives for orthodontic attachments. Am J Orthod 1965; 51: 901–912.[CrossRef][Medline]
3 Muira F, Nakagawa K, Mashura E. New direct bonding system for plastic brackets. Am J Orthod 1971; 59: 350–360.[CrossRef][Medline]
4 Olsen ME, Bishara SE, Boyer DB, Jakobsen JR. Effect of varying etch times on the bond strength of ceramic orthodontic brackets. Am J Orthod Dentofacial Orthop 1996; 109: 403–409[CrossRef][Medline]
5 Hobson RS, Ledvinka J, Meechan JG. The effect of moisture and blood contamination on bond strength of a new orthodontic bonding material. Am J Orthod Dentofacial Orthop 2001; 120: 54–57.[CrossRef][Medline]
6 Fox NA, McCabe JF, Buckley JG. A critique of bond strength testing in Orthodontics. Br J Orthod 1995; 21: 33–43.
7 Artun J, Bergland S. Clinical trials with crystal growth conditioning as an alternative to acid etch pre-treatment. Am J Orthod 1984; 85: 333–340.[CrossRef][Medline]
8 Millet DT, McCabe JF, Gordon PH. The role of sandblasting on the retention of metallic brackets applied with glass ionomer cements. Br J Orthod 1993; 20: 117–122.[Abstract]
9 Larmour CJ, McCabe JF, Gordon PH. An ex vivo assessment of a resin modified glass ionomer bonding system in relation to ceramic bracket debond. J Orthod 2000; 27: 329–332.
10 Larmour CJ, Stirrups DR An ex vivo assessment of a resin modified glass ionomer bonding system in relation to bonding technique. J Orthod 2001; 28: 207–210.
11 Reynolds IR. A review of direct orthodontic bonding. Br Dent J 1975; 2: 171–178.
12 Keizer S, Tencate JM, Arends J. Direct bonding of orthodontic brackets. Am J Orthod 1976; 69: 318–327.[CrossRef][Medline]
13 Bishara SE, VonWald L, Laffoon JF, Warren JJ. Effect of a self-etch primer/adhesive on the shear bond strength of orthodontic brackets. Am J Orthod Dentofac Orthop 2001; 119: 621–624.[CrossRef][Medline]
14 Bishara SE, Oonsombat C, Ajlouni R, Denehy G. The effect of saliva contamination on the shear bond strength of orthodontic brackets when using a self-etch primer. Angle Orthod 2002; 72: 554–557.[Medline]
Received July 5, 2002; accepted January 29, 2003
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