An in vitro comparison

An in vitro comparison of shear bond strength, adhesive remnant cleanup and microleakage of a flash-free and conventional orthodontic adhesive
Rachel Soyland 1 2, Jing Ye 3, Thyagaseely Premaraj 3 and Sundaralingam Premaraj 3

1.Former dental student, College of Dentistry, University of Nebraska Medical Center, Lincoln, NE, USA 

2.Orthodontic resident, College of Dentistry, The University of Oklahoma, Oklahoma City, OK, USA 

3.Department of Growth and Development, College of Dentistry, University of Nebraska Medical Center, Lincoln, NE, USA

ABSTRACT

Objectives: The purpose of a recently introduced flash-free adhesive coated appliance system (APC™ FlashFree Adhesive Coated Appliance System, 3M Unitek, Monrovia, CA) is to eliminate the need for adhesive flash (AF) clean up during bonding procedure and reduce the bonding time. In this in vitro investigation, shear bond strength (SBS), the amount of adhesive remained on the tooth surface after bracket debonding, time required for the removal of adhesive remnants and microleakage were compared between brackets bonded using flash-free and conventional adhesives. 

Materials and methods: A total of 124 extracted human teeth were used in this study. Teeth were bonded either with ceramic brackets pre-coated with flash-free adhesive or ceramic brackets coated with conventional adhesive (Transbond XT Light Cure Adhesive, 3M Unitek). Thirty teeth were subjected to shear bond strength test. Nine sets of teeth (n=72) were debonded by orthodontic residents. Adhesive remnants were scored using adhesive remnant index (ARI) and the time took for adhesive removal was recorded. Microleakage was investigated using microcomputed tomography. 

Results: There were no statistically significant differences in SBS, ARI, adhesive removal times or microleakage between the flash-free and the conventional adhesive. 

Conclusions: Material properties of the flash-free adhesive are comparable and satisfactory to that of the conventional adhesive in shear bond strength, adhesive remnant index, adhesive removal time and quality of the bond at the enamel-bracket interface. 

Key words: shear bond strength, orthodontic adhesive, adhesive flash, orthodontic bracket 

INTRODUCTION

Treatment with fixed orthodontic appliance depends on successful bonding of orthodontic brackets to tooth enamel. Bonding of an orthodontic bracket using composite resin depends on acid etching of enamel to allow the mechanical retention of the adhesive. Conventionally, composite resin is coated onto each bracket base and placed on the tooth with pressure, resulting in certain amount of adhesive flash (AF) is expressed alongside the boundary between the bracket and surface enamel. If AF is not removed adequately during the bonding procedure before polymerization, these surfaces could potentially act as a plaque retentive factor and leads to mechanical irritation of gingiva and development of white spot lesions. Removal of AF with instruments prior to light-curing of the resin may cause dislodgment of the bracket and result in an improperly placed bracket. Even though, complete removal of AF during orthodontic bonding is desirable, clinicians frequently leave AF behind during bracket bonding. The amount of AF remains after bonding could be reduced either by minimizing amount of flash produced during the bonding procedure by modifying properties of the adhesive or incorporating a visual marker into the adhesive to facilitate AF clean-up. However, incorporation of a coloring agent to orthodontic adhesive to assist visualization of the AF during bonding procedure failed to demonstrate a reduction in the amount of AF around the brackets.

The purpose of a recently introduced flash-free adhesive coated appliance system (APC™ FlashFree Adhesive Coated Appliance System, 3M Unitek, Monrovia, CA) is to eliminate the need for AF clean up during bonding procedure and reduce the bonding time. This is achieved through a non-woven form-fitting fiber mesh on the bracket base, which conforms to the tooth surface during bonding, ensuring uniform and reliable contact with the tooth surface while leaving no AF to cleanup.

Brackets bonded with flash-free adhesive have been shown to produce minimal AF with a smooth surface at the microscopic level.

An ideal orthodontic adhesive should have adequate bond strength and ease of adhesive remnant cleanup following removal of brackets. Shear bond strength (SBS) is a critical factor to ensure survival of the bond for the duration of orthodontic treatment and the adhesive must be able to withstand masticatory forces as well as forces applied on the brackets during orthodontic treatment. . The recommended bond strength for orthodontic brackets is a minimum of 5.9-7.8MPa [9]. When a bonded bracket is removed, remnants of the composite adhesive may remain on the enamel surface. Removal of adhesive remnants using a high speed hand-piece require longer appointments and increased chair time on the day of removal of orthodontic appliances.

The purpose of the current study was to determine and compare the SBS of brackets bonded using flash-free adhesive with conventional adhesives. In addition, microleakage, the amount of adhesive remained on the tooth surface after bracket debonding and the time required for the removal of adhesive remnants were compared between flashfree and conventional adhesives.

MATERIALS AND METHODS

Specimen Preparation

This study was approved by the Institutional review board (IRB) of the University of Nebraska Medical Center. A total of 124 extracted human teeth were collected and utilized for this research project. Teeth were stored in a 0.1% aqueous solution of sodium hypochlorite at room temperature. Selection criteria included maxillary and mandibular anterior teeth with sound labial enamel and no damage from the extraction process. All teeth were mounted in silky rock dental stone in sets of eight teeth to resemble a dental arch. The teeth were embedded in stone to the height of the cemento-enamel junction and then bonded with ceramic brackets using the procedure described below. 

The obtained teeth were selected randomly and divided into two equal-sized groups. Teeth in the group 1 were bonded using the flash-free adhesive and teeth in the group 2 were bonded using the conventional adhesive. Briefly, buccal surface of every tooth was cleaned and polished using pumice on a rubber cup attached to a low-speed handpiece for 5 seconds, rinsed with water, and dried with air. Thirty-five percent orthophosphoric acid gel (Ultra-Etch Etchant, Ultradent Products, Inc., South Jordan, UT) was then applied for 30 seconds, rinsed thoroughly with water, air-dried until a chalky appearance is observed, and primed using a light cure adhesive primer (Transbond XT Primer, 3M Oral Care, St. Paul, MN) following the manufacturer’s instructions. The teeth were bonded with ceramic maxillary central incisor brackets. Teeth in the group 1 were bonded with ceramic maxillary incisor brackets (ClarityTM ADVANCED, 3M Oral Care) using APC Flash-Free Adhesive Coating Appliance System (3M Oral Care). Teeth in the group 2 were bonded with ceramic maxillary incisor bracket using Transbond XT Light Cure Adhesive (3M Oral Care). Both groups received identical brackets, differing only in the adhesive coated on the brackets. All brackets were bonded under a constant pressure of 3N, which was calibrated with a pressure gauge. A scaler was used to remove excess adhesive around the brackets bonded with conventional adhesive. All brackets were light-cured for 10 seconds with a new light-emitting diode (LED) polymerization device (Ortholux Luminous Curing Light, 3M Oral Care). The distance between the exit window and the bracket was maintained at less than 5mm in order to achieve optimum polymerization. After completion of bonding, the teeth specimens were stored in distilled water at 37°C for 24 hours to allow bond maturation, before further testing. 

Shear bond strength

Thirty teeth were randomly selected to undergo testing to evaluate the shear bond strength. The teeth were divided into two equal-sized groups and brackets were bonded with the flash-free adhesive to the teeth in group 1 and conventional adhesive to the teeth in group 2 using the procedure detailed above. Each tooth was mounted with silky rock in a 1-inch phenolic ring. An inciso-gingival load was applied to produce a shear force at the brackettooth interface using an Instron testing machine (Instron, Norwood, MA) (Figure 1). A mounting jig was used to align the labial surfaces of the teeth to be parallel to the applied force during the shear bond strength test. The bond strengths were measured at a crosshead speed of 1mm/min and the load applied at the time of fracture was recorded in Newton and then calculated by dividing the debonding force by the bracket base surface area yielding megapascals (MPa) as a unit.

 
An in vitro comparison01

                                              Figure 1. Determination of shear bond strength in Instron: Frontal view (A) and Lateral view (B)

Debonding and adhesive remnant cleanup

Nine sets of teeth were utilized for the remnant cleanup part of the study. Nine orthodontic residents were asked to debond the teeth using a bracket debonding instrument (Unitek selfligating bracket debonding instrument (804-170)). The orthodontic residents were blinded as to the adhesive type used on each bracket, being told only that one side was ‘Product 1’ and the other was ‘Product 2.’ Following debonding of the brackets, a single calibrated operator (RS) scored the adhesive remnant index (ARI) under x2.5 magnification using dental loupes. The ARI was scored as follows: 0=no adhesive left on the tooth; 1=less than half of the adhesive left on the tooth; 2=more than half of the adhesive left on the tooth; 3=all adhesive left on the tooth. Intra-examiner reliability was determined by rescoring 4 sets of teeth after a period of one month. Orthodontic residents were then asked to remove the remaining adhesive from the tooth using a tungsten carbide finishing bur in a highspeed hand piece. Adhesive removal was considered complete when the tooth surface felt smooth and was visibly free of composite under inspection using a dental operating light and x2.5 magnification loupes. Adhesive removal was timed to the nearest second using a digital stopwatch and verified under x2.5 magnification dental loupes.

Microcomputed tomography

Twenty four teeth were used for the microleakage part of the study. The teeth prepared for microcomputed tomography were submerged in a 50% aqueous solution of silver nitrate(Sigma Aldrich, St. Louis, Missouri, USA) at room temperature for 24 hours for further micro-CT imaging and analysis (Bruker SkyScan1172, Kontich, Belgium). Each tooth was scanned and reconstructed into a 3D-structure with a voxel size of 7.0 μm. The X-ray tube voltage was 89 kV and the current was 90 μA, with a 0.5 mm thick aluminum filter. Exposure time was 580 ms. 3D reconstructions were performed using NRecon software (Bruker, Kontich, Belgium). For the quantitative analysis, the focus was on the site of adhesive on the teeth. The position of each tooth was corrected with Data Viewer and then saved on a volume of interest (VOI) of 500 slices. For better analysis, this was followed by careful placement of a constant square region of interest (ROI) in the tooth with 200 slices above and below the bracket. To standardize analysis within the teeth, special care was taken so as to keep the constant ROI in the center for each sample, with the horizontal and vertical edges of the bracket serving as horizontal and vertical points of reference, respectively. All scans were scrutinized, slice-by-slice, for silver nitrate microleakage into the adhesive layer which was indicated by presence of the radiopaque dye between the bracket base and the enamel surface. For each image, the adhesive layer was selected as another irregular region of interest (ROI).The ROIs were measured to discriminate silver nitrate from background using optimum threshold, which was visually determined by gradual variation and comparison of the outcome with silver nitrate deposits on the bracket surface in the original scan. In each image, only images above the threshold, which represented the silver nitrate, kept the original gray value. However those images below the threshold were seen as transparent. All the analysis process were performed with the constant threshold settings in CT Analysis software (Micro Photonics Inc., Allentown, PA) for all samples. The following parameters were measured and calculated for each adhesive: adhesive volume (TV), which stood for the volume of adhesive; sliver nitrate volume (BV), which represented the volume of silver nitrate leaked into the adhesive. The ratio of silver nitrate microleakage was then calculated using BV/TV.

STATISTICAL ANALYSIS

Mean values, standard deviation and range of the SBS were calculated for each adhesive. Difference in the SBS was tested for statistical significance using one-way ANOVA. Mean values, standard deviations and coefficients of variations of the time required for adhesive remnant cleanup were calculated for each adhesive. Differences in adhesive removal time were tested for statistical significance using MannWhitney rank sum test. Mean values and standard deviation of the ratio of silver nitrate microleakage were calculated for each adhesive. Differences in microleakage was tested for statistical significance using student T test. Statistical analysis were performed by SAS 9.4 (SAS Institute Inc., Cary, NC). P values of less than 0.05 considered statistically significant.

RESULTS

Shear bond strength

Mean shear bond strength and standard deviation values are shown in Table 1. There was no statistically significant difference in shear bond strength between the flash-free adhesive and the conventional adhesive (f=0.83, p=0.37).

Table 1. Mean and standard deviation values for shear bond strength

Amount of adhesive remaining on the tooth following bracket debonding

Percentage of ARI after removal of brackets are shown in Table 2. The weighted kappa statistic for intra-examiner agreement for ARI scoring was 0.89 indicating excellent agreement. An ARI score of 2 or 3 indicates that all or most of the adhesive remained on the tooth following bracket debonding. Such a score is suggestive of bracket failure at the bracketadhesive interface. For brackets bonded with the flash-free adhesive, 91 percent of subjects failed with all or most of the adhesive remaining on the tooth. Coincidentally, 91 percent of subjects bonded with conventional adhesive also failed in the same manner. Statistical analysis revealed that there was no significant difference in the amount of remaining adhesive following debonding between the flashfree adhesive and the conventional adhesive.

Table 2. Adhesive Remaining index (ARI) after debonding

The ARI was scored as follows: 0=no adhesive left on the tooth; 1=less than half of the adhesive left on the tooth; 2=more than half of the adhesive left on the tooth; 3=all adhesive left on the tooth

Adhesive Remnant cleanup time

On average, the time required for adhesive remnant cleanup per quadrant for the flash-free adhesive was 52 seconds whereas the time required for the conventional adhesive was 60 seconds. The results are shown in Table 3. Although the flash-free adhesive remnants were removed 8 seconds faster, on average, than the conventional adhesive, the result was not statistically significant (p=0.101).

Time required for adhesive remnant cleanup
Table 3. Time required for adhesive remnant cleanup

Results are mean values (standard deviations) COV, coefficient of variation No statistically significant differences between adhesives (P>0.05).

Microleakage

Images of microcomputed tomography scans of brackets bonded with the flash-free and the conventional adhesive are shown in Figure 2. The flash-free adhesive showed a smooth, well-formed fillets on the enamel surface with the adhesive spread out. In contrast, bonding with conventional adhesive, produced an irregular edge from adhesive to the teeth surface with a rough surface. The ratios of silver nitrate penetration into the adhesive are shown in table 4. The ratio of silver nitrate microleakage for the flash-free and conventional adhesives are 0.449 and 0.574 (%), respectively. Although there was slightly more microleakage observed in the conventional adhesive group, this difference was not statistically significant (p=0.589).

Figure 2. Images of microcomputed tomography scans of brackets bonded with flash-free (A) and conventional (B) adhesives. 

The results show a smooth surface in flashfree sample in contrast of an irregular surface in conventional adhesive group. No statistically significant difference (P˃0.589)

DISCUSSION

Success of fixed orthodontic appliance treatment depends on brackets remain bonded to the teeth for the duration of treatment. Orthodontic adhesive should be capable of providing adequate bond strength to withstand the forces generated by the orthodontic appliances and mastication. In addition, orthodontic adhesives should allow easy removal of brackets at the completion of treatment without damage to the enamel surface. 

Even though conventional orthodontic adhesives have proven to be adequate in providing necessary bond strength and ease of bracket of removal, there are still weaknesses associated with their use which could be improved to provide better outcomes for the patient and orthodontist. A major drawback of the conventional adhesive is adhesive flash (AF) that is expressed alongside the boundary between the bracket and surface enamel. Removal of AF during bonding procedure before curing of the adhesive increases chair-time. Inadequate removal of AF before polymerization promotes plaque retention and gingival irritation

A newly developed flash-free adhesive (APC FlashFree adhesive coated appliance System, 3M Oral Care) has been shown to reduce the bonding time through elimination of flash cleanup after placement of bracket. In addition, this flash-free adhesive produced clinically insignificant amount of AF with smooth surface. The present in vitro study was carried out to compare the shear bond strength, amount of adhesive remnants on the tooth surface after debonding and microleakage of brackets bonded with flash-free adhesive with that of conventional adhesive. However, in vivo studies are the ideal way to evaluate the clinical performance of orthodontic adhesives where all the causes that can contribute to the bond failure are present. 

The results of the shear bond strength test indicate that the conventional adhesive demonstrated higher SBS, averaging 14.28MPa. Average SBS for the flash-free adhesive was 12.58MPa. This is comparable to the SBS reported for the flash-free adhesive by Lee and Kanavakis. The minimum SBS of orthodontic adhesives that is clinically acceptable for use in patients is 5.9-7.8MPa. In the present study, differences in the SBS between the two adhesives were not statistically significant. In contrast, Lee and Kanavakis have shown that SBS of flash-free adhesive is significantly higher than conventional adhesive. However, both the conventional adhesive and the newly developed flash-free adhesive demonstrate sufficient SBS to be clinically successful in the treatment of orthodontic patients.

The amount of adhesive remaining on the tooth following debonding is very comparable between the conventional adhesive and the flash-free adhesive. Our results are in line with the findings reported by Foersch et al. In the present study, all or most of the adhesive was left on the tooth surface following debonding 91 percent of the time with both adhesives. In contrast to our finding, previous studies reported that more adhesive left on the tooth when the flash-free adhesive was used. ARI scores are subjective, which could lead to lack of agreement among studies. Having all or most of the adhesive remaining on the tooth following debonding may be beneficial to the patients because it reduces the chance of damage to the enamel during debonding procedure. However, more remnant adhesive remaining on the tooth surface necessitate more chair time for cleanup.

Although the results of the study showed that on average, the conventional adhesive took 8 seconds longer to remove from the tooth, this difference was not statistically significant. This is predictable because the conventional adhesive had an ARI score of 3 more often than the flash-free adhesive (80 percent of cases compared to 72 percent of cases) whereas the flash-free adhesive had an ARI score of 2 more often (19 percent of cases compared to 11 percent of cases). Our results are in agreement with previously reported findings. Overall, the time required to remove both of the adhesives was very comparable and difference would be insignificant in a clinical setting.

One of the main disadvantages of composite resin is polymerization shrinkage and microleakage. Microleakage around orthodontic brackets can lead to development of white spot lesions. In the present investigation using micro CT, we found very minimal microleakage for both adhesives and could not demonstrate any difference in the amount of microleakage between flash-free and conventional adhesive. Our results are in agreement with a previous study that showed no difference in microleakage between adhesives. In contrast, Foersch et al. reported more microleakage in conventional adhesive than flash-free adhesive.

 

CONCLUSIONS

Both the flash-free and the conventional adhesives demonstrated adequate shear bond strength that will outlast the therapeutic and functional forces applied to the bracket during clinical use. The amount of adhesive remaining on the tooth surface following bracket debonding is comparable between flash-free adhesive and conventional adhesive. The difference in adhesive remnants removal times is not statistically significant between the flash-free and conventional adhesive. Both flash-free and conventional adhesives produced very minimal microleakage at the enamel-adhesive interface.

Funding

This study was supported by UNMC College of Dentistry Student summer research fellowship to Rachel Soyland. Orthodontic adhesives and brackets used in this study were donated by 3M Oral Care. 

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