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Scientific Section |
Department of Child Dental Health, Bristol Dental Hospital, Bristol, UK
Department of Dental Biomaterials Science, Guys Hospital, London, UK
Address for correspondence: Dr A. J. Ireland, Department of Child Dental Health, Bristol Dental Hospital, Lower Maudlin Street, Bristol BS1 2LY, UK. E-mail: tony.ireland{at}bristol.ac.uk
Received 25 January 2005; accepted 1 February 2006
| Abstract |
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Design: In vitro laboratory study
Setting: Bristol Dental Hospital, UK. Sept 2003-Sept 2004
Material and Methods: Nine groups of 20 premolars were bonded using metal orthodontic brackets using three protocols: (1) 37% o-phosphoric acid etch and TransbondTM XT adhesive; (2) 37% o-phosphoric acid and Ideal 1 adhesive; (3) Ideal 1 SEP and Ideal 1 adhesive. Force to debond and locus of bond failure were determined at three time intervals.
Results: Enamel pre-treatment prior to bonding, namely SEP versus conventional etching had no significant effect on the median force to debond with the Ideal 1 adhesive. Similarly, when the enamel was conventionally etched, the adhesive type, namely Ideal 1 or TransbondTM XT, had no significant effect on the measured force to debond. However, there appeared to be differences in the locus of bond failure: failure predominated at the enamel/adhesive interface for the TransbondTM XT conventional etch group and at adhesive/bracket interface for the Ideal 1 SEP and adhesive group and the Ideal 1 adhesive conventional etch group.
Conclusion: These results suggested that the complete Ideal 1 SEP and adhesive system might be successful in vivo leading therefore to a clinical trial. However, implications for clean up time are discussed and improvements to in vitro study designs are advised.
Key words: Brackets, bond strength, orthodontics, self-etching primers
| Introduction |
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In an attempt to reduce the number of procedural steps when bonding to dentine and enamel the self-etching primers (SEPs) were developed.10
These materials combine etching and priming into one stage for simultaneous use on enamel or dentine. SEPs are aqueous mixtures of acidic monomers and hydroxyethyl methacrylate (HEMA), the resultant methacrylated phosphoric acid ester being the active component of the SEP. It is this active molecule that etches and primes simultaneously. Unlike conventional acid etch methods; the SEP is not rinsed away after application. Instead, the calcium dissolved from the hydroxyapatite, forms a complex with the phosphate group and is then incorporated into the resin network during polymerisation.
The purported advantage of SEPs is that by combining etching and priming into a single procedure, enamel preparation for diacrylate bonding is streamlined, reducing clinical application time, improving cost effectiveness and improving comfort for the patient.
SEPs were originally developed for restorative use and as such these systems required higher bond strengths than those desirable for orthodontic purposes. They were also required to bond to hydrophilic dentine. There have been many in vitro studies to investigate bond strengths following their use in the restorative field11
13
and more recently in orthodontics; initially using products such as Prompt-L-Pop (3M/UnitekESPE) designed for restorative dentistry.14
16
To date there appears to be no consensus on the attainable shear bond strength following their use, with conflicting results even being reported by the same authors. Interestingly however, in response to concerns about possible enamel fracture at debond, the manufacturer of Prompt-L-Pop advised against using this product for orthodontic bonding.14
17
It has subsequently been modified for orthodontic use and is marketed as TransbondTM Plus SEP (3M/Unitek, Dental Products Division, Monrovia, CA, USA). This material comes in a single-use foil package containing 3 blister pack compartments, which are pressed and folded to mix the components before application. TransbondTM SEP, like most of the SEPs on general release at the present time is a 2-component system that requires mixing in some way prior to use. In vitro experiments have shown there to be no significant difference in bond strength following the use of this SEP and a conventional 37% o-phosphoric acid etchant.18
,19
Indeed some laboratory based experiments have shown TransbondTM SEP to produce significantly greater shear bond strength than that achieved by etching with 37% o-phosphoric acid gel for 30 seconds.20
,21
Nonetheless if the ultimate aim of any adhesive system is to reduce the number of clinical steps required during the bonding procedure, a true 1-component no-mix self-etching primer would be an advantage. The Ideal 1 adhesive system (GAC International Inc., Bohemia, NY, USA) is such a product and is supplied as a no-mix 1 component SEP solution along with Ideal 1 composite adhesive. To date only one other study has been performed investigating the same self-etching primer. In this in vitro study, the authors compared the shear bond strength of the Ideal 1 system with a 2-component self-etching primer system. Their findings were encouraging in that both systems produced similar bond strengths,22
but human molars were used and bonded with central incisor brackets. Furthermore, the 2 component SEP was applied for 10 seconds longer than recommended by the manufacturer.
However, whilst Fox et al., (1994) highlighted the need for standardisation in laboratory bond strength studies,23
another problem associated with such studies is that little or no attempt has been made to establish whether the studies are actually appropriately powered i.e. whether the sample sizes are sufficiently large to demonstrate a clinically and statistically significant difference should one exist.16
22
This is essential if laboratory studies are to have any relevance to the clinical situation. The purpose of the present study was therefore fourfold: to undertake a pilot study to determine the force to debond (N) and locus of bond failure when bonding stainless steel orthodontic brackets with the complete Ideal 1 self-etching and adhesive system. This was then compared with the conventional acid etch and rinse regimen of 37% o-phosphoric acid solution and using either TransbondTM XT adhesive or the adhesive from the Ideal 1 system. The effect of time was also investigated on both force to debond and locus of bond failure. In addition, the results of this pilot study were intended to be helpful for future sample size calculations for other in vitro studies. This would not only help to improve the relevance of laboratory studies to the clinical situation but would also (in the event) help address some of the recent ethics approval changes. These relatively new arrangements now affecting the UK mean that studies require statistical input in order to demonstrate that they are adequately powered.
| Material and Methods |
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Vacuum formed polythene moulds were made using a Drufomat thermopressure machine (Dreve, Germany) over a plaster of Paris block measuring 15 x 15 x 35 mm. Using these moulds each premolar tooth was embedded horizontally in self-curing acrylic, so that the buccal surface of the crown was proud of the block and parallel to it. The 180 teeth in their acrylic blocks were then randomly subdivided into 9 groups of 20 teeth and the base of each block was engraved with an experimental group reference code. The exposed enamel surface was then polished using a slow speed handpiece and slurry of pumice and water in a rubber cup.
0.022-inch lower premolar stainless steel orthodontic brackets (Omni, GAC International, Bohemia, NY, USA) were bonded to the teeth according to one of three protocols and the force to debond (N) and locus of bond failure were determined at three different time intervals, 10 minutes, 1 week and 6 months. All experimental procedures were performed by 1 operator (KH). The enamel preparation and materials were as follows:
Group 1 - Conventional etch and TransbondTM XT adhesive
The enamel was etched with 37% o-phosphoric acid for 15 seconds followed by rinsing with copious amounts of water and then dried with oil free compressed air until frosty white in appearance. TransbondTM XT adhesive was then applied to the bracket base before positioning the bracket on the tooth. TransbondTM XT primer was not applied to the enamel surface. Firm pressure was applied to the bracket using a Mitchells trimmer and any excess adhesive removed from around the margins using a probe.
Group 2 Conventional etch and Ideal 1 adhesive
Enamel preparation and subsequent bonding in this group was identical to Group 1, the only difference being the adhesive used, namely Ideal 1 adhesive.
Group 3 Ideal 1 self-etching primer and Ideal 1 adhesive
Self-etching primer (Ideal 1 GAC International, Bohemia, NY, USA) was applied to the enamel and rubbed gently for 20 seconds per tooth using the applicator provided and according to the manufacturers instructions. A gentle 5 second blast of air, from an oil free triple syringe, was then applied to each tooth in order to thin the material. Ideal 1 adhesive was then applied to the bracket base, the bracket positioned on the tooth and any excess once again removed using a probe.
In the case of all groups, once the bracket was positioned on the tooth it was then light-cured for 20 seconds (10 seconds mesially and 10 seconds distally). using a halogen curing light (Optilux 501 curing lamp, Kerr, 21 Commerce Drive, Danbury, CT, USA). The efficiency of the lamp was tested after each curing cycle using the meter within the unit. The teeth from each group were then stored in separate beakers of distilled water in a water bath, at a constant temperature of 37°C until bond testing at one of three time periods, 10 minutes, 1 week or 6 months. The distilled water was changed weekly.
Shear bond testing to failure was performed using a custom-made jig in a Lloyd Universal testing machine (Series 2000R, Lloyd Instruments, Southampton, UK) and with a crosshead speed of 0.5mm/minute (Figure 1
). The measurements recorded were force to debond and locus of bond failure. Locus of bond failure was recorded using the Adhesive Remnant Index (ARI)24
following examination of the failure surfaces using a low power binocular microscope at x10 magnification. The ARI system was developed to grade the amount of adhesive remaining on the enamel surface following debond and thereby act as a means of helping define the mode of bond failure between the enamel, adhesive and bracket base. The amount of residual adhesive adhering to the enamel surface is scored by visual inspection and is allocated to one of four groups: 0 (where no adhesive is left on the tooth) to 3 (where all the adhesive left on the tooth, with a distinct impression of the bracket mesh.
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| Results |
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= 0.05. The median force to debond (N) and 95% confidence intervals of the medians were determined. Summary statistics are illustrated in Table 1
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For all the enamel pre-treatments and bonding material combinations under test, except that is for the 1-week results using the 37% o-phosphoric acid and TransbondTM XT adhesive, there appeared to be a trend for the median force to debond to increase with time. However, the increase in each case is probably not statistically significant in view of the overlap in the confidence intervals (Figure 2
).
The effect of time on the ARI scores was analysed using the Kruskal-Wallis non-parametric one-way analysis of variance (Table 2
). The results show that the ARI is unaffected by time in the case of each adhesive. However, the stacked bar charts (Figures 3
, 4
and 5
) show that for the TransbondTM XT conventional etch group, failure was predominantly interfacial at the enamel/adhesive interface. With the Ideal SEP and Ideal 1 adhesive group, and the conventional etch and Ideal 1 adhesive group, failure was predominantly interfacial at the adhesive/bracket interface. Perhaps somewhat alarmingly the missing category represented teeth where the enamel fractured during testing and more of these were seen in the SEP group.
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| Discussion |
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In order to fully evaluate each component of the Ideal 1 adhesive system under test, the effect of enamel pre-treatment and adhesive were tested separately and then in combination, with the TransbondTM XT adhesive and conventional acid-etching specimens acting as the control. Looking firstly at the effect of enamel pre-treatment it would seem there is little difference in the observed force to debond (N) between the SEP group and the conventional etch group certainly at both 1 week and 6 months (Figure 2
). These results are in agreement with other in vitro investigations comparing SEPs designed for orthodontic use with conventional acid etch procedures.17
21
The median force to debond at 10 minutes for the SEP was slightly lower than at the other two time periods when using the same Ideal 1 adhesive, although this difference may not be statistically significant as there is some overlap of the confidence intervals.
When considering the effect on force to debond of the two different adhesives tested, the conventional etch and Ideal 1 adhesive combination provided a comparable if not slightly greater median force to debond than the TransbondTM XT adhesive and at all three time periods. The reason for this is unclear, although bracket/adhesive combination has been shown to have an effect on observed in vitro force to debond and it is possible that it may also be the case in this study.25
,26
The present study ultimately aimed to compare the force to debond of a new adhesive system, Ideal 1 consisting of a one component SEP and light-cured adhesive, with TransbondTM XT and a conventional acid etch regimen. The findings would indicate that both systems provide a similar observed force to debond (Figure 2
and Table 1
) and in both cases the median force to debond increased slightly with time (Table 1
).
Analysis of the ARI scores using the Kruskal-Wallis one-way analysis of variance (Table 2
) showed that within each of the three main treatment groups there was no statistically significant difference between the ARI scores at the three time periods under test. However, consideration of the raw data in the stacked bar charts (Figures 3
, 4
and 5
) shows there to be a significant difference in the locus of bond failure between the TransbondTM XT conventional etch specimens and the other two treatment groups namely the Ideal 1 SEP adhesive system group and the conventional etch and Ideal 1 adhesive group. This is despite the similar force to debond results. With the Ideal 1 system the majority of the ARI scores were 3, namely at the bracket/adhesive interface. Therefore the adhesive bond to the enamel and the cohesive strength of the adhesive were higher than the adhesive bond to the bracket base. The converse was true for the conventional etch TransbondTM XT group. Other in vitro investigations into the locus of bond failure when comparing the use of SEPs and conventional etching have not produced a consensus view. The majority of studies have shown bond failure to occur most frequently at the enamel/ adhesive interface, with less residual adhesive remaining on the enamel at debond than if prepared by the conventional acid etch and prime regimen.14
,17
,19
,27
29
Other investigators have demonstrated that SEPs produce a similar locus of bond failure to conventional etching20
,30
,31
Locus of bond failure is determined by a complex combination of contributory factors including the direction of applied force, enamel pre-treatment, the adhesive and the bracket type.32
The reason for primarily adhesive/bracket interfacial bond failure when using the Ideal 1 SEP and adhesive system in the current experiment is unclear. Certainly the discrepancy between the Kruskal-Wallis analysis and the raw data as illustrated by the stacked bar charts might indicate there is a lack of statistical power. Therefore the correct interpretation should be that there is perhaps a lack of evidence in the light of the current investigation as to the likely effect on the enamel in the clinical setting.
It would seem then from the observed force to debond results that the single component SEP system of Ideal 1 is comparable to the use of conventional acid etching and a light-cured composite bonding agent. However, for all the SEPs currently on sale, the manufacturers instructions advise that the teeth are pumiced, rinsed and dried before application in order to remove any salivary pellicle that may inhibit the bonding process. For the conventional etching and rinsing regimen, current best evidence suggests that pumice prophylaxis is unnecessary.33
,34
Therefore some of the potential time-saving with a SEP is negated by this necessity to pumice the enamel prior to use. Despite the evidence suggesting pumicing is not required prior to conventional etching, pumice prophylaxis was included for all groups in this study in an attempt to limit the number of variables.
In addition, the high number of adhesive/bracket interfacial failures seen with the SEP in this study would suggest that at debond more time must be spent cleaning the residual adhesive from the enamel surface. Of some concern were the large number of missing values (Figure 4
and 5
) seen with the Ideal adhesive, both with the SEP system and particularly conventional etching. These missing values denote enamel fracture at debond. However, as always laboratory findings should be extrapolated to the clinical situation with some caution.
| Conclusion |
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| Contributors |
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Dr AJ Ireland is the guarantor and as such accepts full responsibility for the conduct of the study, has access to the data and controlled the decision to publish.
| Acknowledgments |
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| References |
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27 Bishara SE, VonWald L, Olsen ME, Laffoon JF. Effect of time on the shear bond strength of glass Ionomer and composite orthodontic adhesives. Am J Orthod Dentofacial Orthop 1999; 116: 61620.[CrossRef][Medline]
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29 Cacciafesta V, Sfondrini MF, De Angelis M, Scribante A, Klersy C. Effect of water and saliva contamination on shear bond strength of brackets bonded with conventional, hydrophilic, and self-etching primers. Am J Orthod Dentofacial Orthop 2003; 123: 63340.[CrossRef][Medline]
30 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 Dentofacial Orthop. 2001; 119: 62124.[CrossRef][Medline]
31 Zeppieri IL, Chung CH, Mante FK. Effect of saliva on shear bond strength of an orthodontic adhesive used with moisture-insensitive and self-etching primers. Am J Orthod Dentofacial Orthop 2003; 124: 41419.[CrossRef][Medline]
32 Katona TR. Stresses developed during clinical debonding of stainless steel orthodontic brackets. Angle Orthod 1997; 67: 3946.[Medline]
33 Barry GRP. A clinical investigation of the effects of omission of pumice prophylaxis on band and bond failure. Br J Orthod 1995; 22: 24548.[Abstract]
34 Ireland AJ, Sherriff M. The effect of pumicing on the in vivo use of a resin modified glass poly (alkenoate) cement and a conventional no-mix composite for bonding orthodontic brackets. J Orthod 2002; 29: 21720.
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