EFFECT OF REMINERALIZING AGENTS WITH/WITHOUT LASER IRRADIATION ON ENAMEL PROPERTIES AND SHEAR BOND STRENGTH OF ORTHODONTIC BRACKETS

Headnote

Abstract

Introduction: Remineralizing agents can be used for treating WSL. This study aimed at assessing the effect of two remineralizing agents with/ without laser irradiation. Materials and methods: The study evaluated 72 premolars in 6 groups (n=12) (I) sound enamel, (II) demineralized enamel, (III) Fluoride, (IV) Fluoride with laser, (V) CPPACP, and (VI) CPP-ACP with laser. The agents were applied to enamel after their immersion in a demineralizing agent. In groups IV and VI, the laser irradiated after applying the agents. 48 premolars were divided into 4 groups (n=12; F, F/L, C, C/L) and their microhardness was measured. SEM and EDS were utilized to assess the formation of hydroxyapatite. Results and discussion: The SBS significantly decreased in group II (P < 0.001). This finding was similar to microhardness results. Conclusions: Remineralizing agents can significantly improve the microhardness and structural properties of demineralized enamel, similarly to that of sound enamel, with no adverse effect on SBS of orthodontic brackets.

Keywords: laser irradiation, enamel propertie, orthodontic brackets.

1.INTRODUCTION

Orthodontic treatment aims at improving the function, aesthetics and stability. Despite the favorable functional and aesthetic results of the orthodontic treatment with fixed appliances, this treatment may have unfavorable consequences, such as periodontal disease and dental caries, particularly in patients with a poor oral hygiene. Furthermore, the presence of archwires, brackets and bands can complicate oral hygiene practice in orthodontic patients [1].Organic acids caused by acidogenic bacteria can dissolve the enamel minerals and cause porosities in the tooth surface, leading to initial carious lesions, known as white spot lesions (WSLs) [2].

Most studies have reported a prevalence of 23%73% for WSLs in orthodontic patients, depending on treatment duration [3-5]. Several strategies, such as patient education to maintain a good oral hygiene [6], application of adhesives containing remineralizing agents [7] and the prophylactic use of fluoride varnishes [8] have been suggested to minimize the development of WSLs. In addition to these preventive methods, the use of remineralizing agents is recommended before and during the orthodontic treatment, especially for patients with demineralized enamel, prior to the onset of treatment. Some studies have reported a reduction in bond strength when fluoride or (CPP-ACP) were applied as a remineralizing agent before acidetching, while some others did not confirm the adverse effects of remineralizing agents on the bond strength of brackets [9-13].

Many laser types are also used for preventive purposes in dentistry. The laser is suitable for use on enamel surfaces, because its wavelength is within the infrared spectrum [14]. It plays an important role in caries prevention, with insignificant adverse effects on the tooth structure [15]. Laser irradiation generates heat, which modifies the carbonated hydroxyapatite by the fusion of hydroxyapatite crystals, and the reduction of interprismatic spaces in the enamel structure [16,17].

Decreased shear bond strength (SBS) can lead to bracket debonding, causing problems for both clinician and patient, increasing the treatment cost, and prolonging the course of treatment. Therefore, it is essential to find out whether the remineralizing agents have the ability to treat the demineralized enamel, with no adverse effect on their SBS of orthodontic brackets. A safe material should not decrease the bond strength lower than 6 to 8 MPa, which is the minimum bond strength required for orthodontic bracket bonding [18,19]. This study aims at evaluating the effect of two remineralizing agents, with and without laser irradiation, on the structural characteristics of demineralized enamel and its SBS to orthodontic brackets.

2.MATERIALS AND METHODS

1. Sample preparation

For the [SBS] test, 72 human premolars (6 groups, 12 sample for each group) and, for microhardness, 48 samples (4 groups, 12 sample for each group) were utilized, all teeth being extracted for orthodontic purposes after taking a valid consent. This study was approved by the Ethical Research committee of the Faculty of Dentistry, Mansoura University (M09160321)

2. Microhardness

2.1.Teeth preparation for the microhardness test

48 premolars were decoronated buccally and lingually, by using a water-cooled, low speed device to obtain buccal and lingual enamel samples with 2-4 mm thickness and smooth surfaces. 48 enamel samples were mounted in a custom-made mold and polished with silicon carbide discs under water coolant, to obtain flat enamel surfaces. Two layers of nail varnish were applied to cover the enamel surface, leaving a window of about 4x4 mm for demineralization and remineralization processes.

2.2 Microhardness measurement

48 enamel samples, divided into 4 groups, underwent demineralization and remineralization processes.

Vickers surface hardness was evaluated at room temperature by the micro-Vickers hardness tester.

5 day immersion in demineralizing solution was chosen as the proper time to demineralize the enamel surface [20] (a decrease in microhardness value by less than 50%). To evaluate the remineralization ability, the samples were treated with the respective remineralizing agents, according to manufacturers' instructions.

3.Demineralization and remineralization processes

3.1. Demineralization solution

[CaCl2], [NaCl], [NaH2PO4], [NaN3], Acetic acid.

3.2. Remineralization solution

[CaCl2], [NaCl], [NaH2PO4], [NaN3].

3.3. Artificial saliva

3.4. Remineralizing agents group

(I) Fluoride varnish (F)

(II) Fluoride varnish and laser (F/L)

(III) CPP-ACP (C)

(IV) CPP-ACP and laser (C/L)

(F) and (F/L) groups: Fluoride varnish was applied to demineralized enamel surfaces using a micro-brush for 1 minute.

(C) and (C/L) groups were applied to enamel surfaces by a 2 mm thick brush, for 30 seconds, and left to dry .

3.5. Laser groups

After applying the remineralizing agents, the pulsed laser 650 nm red Diode laser 2mw maxadjustable was irradiated with a 10.6 pm wavelength, 10 ms pulse duration, a 50 Hz repetition rate, 0.3 mm beam diameter and 0.7 W power in a scanning motion for 10 seconds at a 5 mm distance from the enamel surface.

Before immersion in the remineralizing solution, a thin layer of a 7th generation bonding agent (Transbond XT) was applied passively over the remineralizing agents to protect them from being washed-out.

The adhesive was applied for 20 seconds and cured using a light-curing unit for 10 seconds. The samples of each group were individually stored in glass containers containing artificial saliva, as a remineralizing agent for 7 days.

At the end of the storage period, the adhesive layer covering the remineralizing agents was gently removed with a tweezer. The samples were then rinsed with deionized water.

The composition of materials and solutions used for demineralization and remineralization processes is presented in Table 1.

Enamel Characterization

(SEM, EDS)

In order to evaluate hydroxyapatite formation, scanning electron microscopy and energydispersive spectrometry were utilized. The enamel samples of premolars were prepared according to the method described for the microhardness test.

For SEM, the enamel samples were cleaned with acetone, dehydrated with ethanol and left to dry at room temperature for 24 hours. The samples were then sputtered with a Coated Evaporator with gold.

Shear Bond Strength

72 premolars were randomly divided into 6 groups (sound enamel, demineralized enamel, F, F/L, C, and C/L). The remineralizing agents were applied as explained earlier. After the demineralization and remineralization processes, orthodontic brackets were bonded to premolars. Stainless steel brackets (Dentaurum) were positioned at the center of the buccal surface of the anatomic crown.

Bonding procedure

Enamel surfaces were polished with a rubber polishing cups and a non-fluoridated pumice. Then, the surfaces were rinsed with water spray and dried with compressed air.

The enamel was etched for 30 seconds using 37% phosphoric acid etch gel, rinsed with water for 15 seconds and dried, then a layer of Bond Adhesive was applied to the etched surfaces using a microbrush. Bond Adhesive was applied on the base of the bracket and the bracket was placed in the correct position on the enamel and composite surfaces and pressed firmly. The excess adhesive was removed from the bracket base periphery using dental probe, and the resin was light-cured for 40 seconds with a dental light-curing unit with 1,200 mW / cm2 intensity.

Thermocycling

Straight stainless steel wire (0.016·0.016 inch) was used to perpendicularly align the buccal surface of each tooth by their bonded brackets. Thousand cycles were performed in deionized water between 5-55°C with 30 seconds of dwell time to simulate the oral environment.

For SBS testing, a cylindrical mold was made of stone to act as a mounting jig, the samples being secured in the lower jaw of a universal testing machine (Zwiek/ Roell Z050, Ulm, Germany).

The samples were stressed in an occlusogingival direction with a cross-head speed of 1 mm/min.

Surface area of bracket was measured with a digital caliper.

After debonding, the teeth were inspected under a stereomicroscope (SMZ 800, Nikon, Japan) at ·10 magnification, to quantify the remaining adhesive, according to the modified adhesive remnant index (ARI). The ARI scores ranged from 1 to 5, as described in Table (2).

3. RESULTS

Surface microhardness

Table 3 revealed that the demineralization process caused a significant reduction in the mean microhardness after the application of remineralizing agents in all groups (P < 0.05). The ability to improve the condition from demineralization to remineralization was not significant in different groups, the results showing no statistically significant difference among the remineralization groups. However, the microhardness values were higher in groups (C and F/L) compared to groups (F and C/L).

Similar superscripted letters denote a nonsignificant difference between groups by Post Hoc Tukey test.

Enamel Characterization

SEM/EDS

Fig. 1 (A-F) show the SEM patterns of the groups. For the sound enamel, a homogeneous smooth appearance with no irregularity was seen. The demineralized enamel surface was rough and eroded. In group F, porosities were covered with a mineral deposit. However, the underlying pattern was still visible in some areas. In group F/L, the underlying pattern could not be seen, while a melted and welded appearance caused by laser irradiation was observed. In group C, the enamel surface was entirely covered with granular-shaped deposits while, in group C/L, the enamel surface was covered with a relatively smooth and more homogeneous crystalline structure compared to group C, due to the typical melting effect of the laser, also showing the chemical composition and atomic percentages of C, O, P, Ca, Na, Si, and F in the groups. The hydroxyapatite deposit was confirmed by EDS analysis in all groups. The effect of demineralization and remineralization protocols on the enamel surface composition was evaluated by measuring the Ca/P ratio, which was the highest for the Fluoride and (CPP-ACP) group and the lowest for the demineralized group.

Shear bond strength

Table (4) shows that SBS was significantly lower in the demineralized group compared to all other groups (P < 0.001), while there was no significant difference among other groups (P > 0.05). The laser groups showed SBS values higher than in those without laser. Table 1 lists the ARI scores for the 6 groups. The demineralized groups showed an ARI score of 5 in 4 samples out of 12, which agreed with the results of the SBS test.

Similar superscripted letters denote a nonsignificant difference between groups by the Post Hoc Tukey test

4.DISCUSSION

White spot lesions (WSLs) are subsurface enamel porositystructures resulted from carious demineralization. They appear as milky white opacities when located on smooth surfaces [21].

Overall, WSL management includes prevention and treatment methods, such as fluoride toothpastes, gels, varnishes, mouth rinses, antimicrobials, xylitol gum, diet counseling, and casein derivate [22].

Daily use of caseinphosphopeptide (CPP)amorphous calcium phosphate (ACP) has been shown to increase enamel remineralization, while mineralization of artificial subsurface enamel lesions increased by more than 78% compared to the control samples [23,24].

The mean surface microhardness values showed a significant decrease in the microhardness of the enamel surface after demineralization. Our result was in accordance with the elements leached out from the enamel, as determined by EDS chemical composition results.

The significant increase in microhardness after the remineralization procedure indicates a direct correlation between the Ca/P ratio and the subsequent surface microhardness of the remineralized enamel. According to Amaechi et al., remineralization of eroded enamel by saliva can take 28 days but, as shown by our results, applying remineralizing agents can significantly accelerate this process [25]. It has been demonstrated that materials with high amounts of calcium and phosphate have a high penetration into enamel lesions and, as a result, increased remineralization occurs. Due to the porous and fragile structure of the initial carious lesions, only some studies support the use of lasers alone, without the use of remineralizing agents. Furthermore, it was proposed that the heat generated during laser irradiation can effectively fuse the loosely attached layer formed on the enamel surface [26]. Despite the higher microhardness values obtained in the laser groups, the differences were not significant. As a result, the synergistic effect of the laser with remineralizing agents was not evident in our findings, which is similar to those of Farhadian et al [27]. Melting and recrystallization of the enamel surface may explain higher microhardness values. Contrary to our findings, Khamverdi et al. and Niyazi et al. showed a synergistic effect of the laser with CPP-ACP on improving enamel microhardness, which confirms the facilitating role of lasers in the penetration of remineralizing agents into deeper layers of HA [28,29].

Different laser settings, the type of remineralizing agent and its frequency and duration of application can explain different results in the literature. The lower bond strength values of the demineralized samples agreed with the ARI scores.

5.CONCLUSIONS

1. Demineralization significantly reduces the SBS of orthodontic brackets.

2. Remineralizing agents can significantly improve the microhardness and structural properties of demineralized enamel to a level similar to that of sound enamel, with no adverse effect on SBS of orthodontic brackets.

References

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