Abstract
The purpose of this
References
Miura F, Nakagawa K, Masuhara E New direct bonding system for plastic brackets. Am J Orthod. 1971; 59:350-361
Eliades T Orthodontic materials research and applications: Part 1. Current status and projected future developments in bonding and adhesives. Am J Orthod Dentofacial Orthop. 2006; 130:445-451
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Tavas MA, Watts DC A visible light-activated direct bonding material: an in vitro comparative study. Br J Orthod. 1984; 11:33-37
Graenlaw R, Way DC, Galil KA An in vitro evaluation of a visible light-cured resin as an alternative to conventional resin bonding systems. Am J Orthod Dentofacial Orthop. 1989; 96:214-220
Chamda RA, Stein E Time-related bond strengths of light-cured and chemically cured bonding systems: an in vitro study. Am J Orthod Dentofacial Othop. 1996; 110:378-382
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Achieving early bond strength: a solution
From Volume 5, Issue 4, October 2012 | Pages 122-124
Article
At present, the bonding of attachments, such as brackets and tubes, on the enamel surface is a routine clinical procedure, typically utilizing a resin composite adhesive.1,2 Since Buonocore first described the use of phosphoric acid for creating irregularities in the enamel surface to enhance mechanical locking, the effects of etching time3,4 and the phosphoric acid concentrate5 have been investigated to seek the most suitable method of enamel preparation.
Although the highest possible bond strength to tooth structure is desirable in restorative dentistry, orthodontic bond strength must satisfy two separate requirements; it must be sufficient to retain the brackets, but low enough to allow the easy clean-up of adhesives when the brackets are removed. Otherwise, greater bond strength may increase the risk of enamel fracture.6
Etching of enamel with an acid, such as 30–40% phosphoric acid, results in selective dissolution of the enamel prisms and creates a surface with a high surface energy that allows effective wetting by a low viscosity resin. Microporosities are created, within and around the enamel prisms, that can be infiltrated with resin and polymerized in situ.7
The use of 5.2% sodium hypochlorite (NaOCI) as a deproteinizing agent may be a possible strategy to optimize adhesion by removing organic elements of both the enamel structure and the acquired pellicle before acid etching. The purpose of this in vitro study is to investigate the effect of the deproteinizing agent NaOCI on sheer bond strength of orthodontic brackets.
Material and methods
A total of 60 freshly extracted maxillary human premolars, that had been extracted for orthodontic purposes, were collected and stored in distilled water at room temperature, with thymol crystals (1% wt/vol) added to inhibit bacterial growth. The criteria for tooth selection included intact buccal enamel with no pretreatment with chemical agents or any visible cracks or caries. The teeth were cleaned and polished with a fluoride-free pumice slurry and rubber cups for 10 seconds and then thoroughly washed and dried with an oil-free airstream. They were examined under a light stereomicroscope at 10x magnification to ensure the absence of caries and enamel cracks. The teeth were embedded in cold-curing acrylic (Orthocryl, Dentaurum, Ispringen, Germany) by using metal ring moulds. The specimens were randomly divided into two groups (each group contained 30 specimens).
In group A: the buccal surfaces of the 30 extracted human premolars were acid-etched for 30 seconds. Subsequently, the enamel was rinsed and dried and orthodontic brackets (Gemini, 3M Unitek, Monrovia, CA) were bonded with self-cure composite resin (Model LRX-plus, Lloyd Instruments Ltd, Fareham, UK).
In group B: the buccal surfaces of the 30 extracted human premolars were deproteinized with 5.25% sodium hypochlorite (NaOCI) for 1 minute followed by rinsing, drying and acid etching for 30 seconds. Subsequently, the enamel was dried and orthodontic brackets were bonded with the brackets and materials as in group A.
After bonding, each group was divided into three equal groups to test the shear bond strength at three time intervals: immediately after bonding, 1 hour and 24 hours after bonding.
Each sample was mounted on the lower fixed compartment of a computer-controlled materials testing machine (Orthocryl), with a load cell of 5kN, and data were recorded using computer software (Nexygen-MT, Lloyd Instruments). It was then subjected to compressive loading in an occluso-gingival direction at a crosshead speed of 0.5mm/min via a mono-bevelled chisel attached to the upper moveable compartment of the testing machine. The chisel tip was positioned to touch only the base of the bracket. The maximum failure load was recorded in N and the bond recorded in MPa by dividing the maximum load by the bracket base surface area (mm2).
Failure was manifested by displacement of the bracket and confirmed by a sudden drop along the load-deflection curve recorded by computer software.
The shear bond strength data of the groups was subjected to two-way analysis of variance (ANOVA) for the descriptive statistics of both groups.
Results
The two-way ANOVA results shows that the least significant difference (LSD) is 0.894, meaning that, if the difference between any two mean values is more than the least significant difference (LSD), it is considered statistically significant at p=0.01 (Tables 1 and 2).
| Source | Sum of squares | DF | Mean square | F Ratio | Probability |
|---|---|---|---|---|---|
| Columns | 21.301 | 1 | 21.301 | 46.565 | 7.96E-09 |
| Rows | 530.674 | 2 | 265.337 | 580.035 | 0.00E-00 |
| Interaction | 2.364 | 2 | 1.182 | 2.584 | 0.0848 |
| Error | 24.702 | 54 | 0.547 | ||
| Total | 579.041 | 59 | |||
| LSD (0.01) | 0.894 |
| Mean ± SD | |||
|---|---|---|---|
| Immediate | One hour | 24 hours | |
| Etch | 1.59 ± 0.24 | 2.22 ± 0.40 | 4.78 ± 0.59 |
| Clorox + Etch | 2.20 ± 0.30 | 4.78 ± 0.16 | 6.09 ± 0.46 |
On measuring the shear bond strength mean difference between different intervals within group A, the results showed no significant difference between immediate and after 1 hour of debonding. There was a significantly higher bond strength difference between both immediate and 24 hours and between 1 hour and 24 hours of debonding (Table 3).
| Time Intervals | Difference | Significance |
|---|---|---|
| Immediate and 1 hour | 2.58 | NS |
| Immediate and 24 hours | 3.19 | S |
| 1 hour and 24 hours | 2.65 | S |
On measuring the shear bond strength mean difference between different intervals within group B, the results showed significant difference between different time intervals of debonding (Table 4).
| Time Intervals | Difference | Significance |
|---|---|---|
| Immediate and 1 hour | 2.58 | S |
| Immediate and 24 hours | 3.89 | S |
| 1 hour and 24 hours | 1.31 | S |
On measuring the mean difference shear bond strength between different intervals within both groups A and B, the results showed no significant difference after immediate bonding in both groups. While there was significant difference after 1 hour and 24 hours of bonding between both groups (Table 5).
| Time Intervals | Difference | Significance |
|---|---|---|
| Immediate | 0.61 | NS |
| 1 hour | 2.65 | S |
| 24 hours | 1.31 | S |
Discussion
There are a number of factors that can potentially contribute to the bond strength between the enamel and the orthodontic bracket to include:
Adhesion to enamel depends on achieving the maximum retentive capacity of the surface from the effect of acid etching. This retentive morphology should be homogeneous over the entire treated surface.16
It is identified that the action of etching acid on the enamel surface occurs mostly on mineralized tissues (inorganic matter). The morphological changes generated vary from tooth to tooth, with a prevalence of a type 3 etching pattern, which decreases significantly the ability of materials to bond effectively to enamel.17
The use of 5.2% sodium hypochlorite (NaOCI) as a deproteinizing agent may optimize adhesion by removing the organic elements of both the enamel structure and the acquired pellicle.
In this study, on applying acid etching alone there was a significant low initial bond strength that increases more (100%+) within 24 hours.
In comparison, on applying 2.25% NaOC1 for 50 seconds prior to enamel etching, there was a significant stronger initial bond strength that increases (about 70%) within 24 hours.
On using NaOCl, the shear bond strength reached immediately the same bond strength that was achieved after 1 hour of bonding on using etch alone, and also with NaOCl the shear bond strength after 1 hour was the same as when using etch alone after 24 hours of bonding.
The bond strength of a setting composite increases with time due to continued polymerization of the resin under the bracket base.18,19,20
Chamda and Stein21 compared bond strength of light- and chemically-cured composites and found higher bond strengths with light-cured resin two and five minutes after curing. For longer time intervals after bonding (10 Minutes, 60 minutes, 24 hours), no difference in bond strengths between the two types of composite was found.
In previous studies, in vitro bond strength testing is commonly performed 24 hours after the bonding procedure.22 At this time, polymerization will be completed, and only minor changes in bond strength can be expected.18,21
On using 2.25% NaOCl for 60 seconds, prior enamel etching will increase the chairside time by only 1 minute. However, using high-intensity light curing units can avoid this time restraint.23,24,25 The plasma arc curing lights are high-intensity units that can cure composite in as little as 3–5 seconds.
From a clinical perspective, the present findings suggest that, on using acid etch alone, the clinician should apply fairly light forces to the bracket during the ligation of the first archwire after bonding in order to minimize the incidence of bracket failure at this initial appointment. Bond failure at this stage results in a delay and extends chair time considerably. It might be even more prudent to tie the archwires after 24 hours from the time of bonding because, by that time, bond strength would have reached an optimal level to withstand the orthodontic forces that will be applied during ligation and tooth movement.
On the other hand, on adding 5.25% NaOCl for 60 seconds prior to enamel etching, the first hour after bonding may be considered a reasonable time for stress induced on the bond when tying in the initial archwire.
Conclusion
Based on the above findings of this study, it would appear that application of NAOCl significantly increases the initial bond strength of orthodontic brackets.