INTRODUCTION

In orthodontic practice, the bond strength between the enamel surface and bracket base must be enough to withstand the mechanical and thermal effects of the oral environment.1 Bonding of brackets is based on alteration of the enamel surface, and the standard protocol for this procedure is acid etching.2 Enamel etching alters the surface from a low-energy hydrophobic surface to a high-energy hydrophilic surface and increases the surface area.3 The most commonly used chemical is 37% phosphoric acid. Etching with phosphoric acid has the advantage of a high level of bracket bond strength.4 One potential disadvantage of acid etching is the demineralization of the most superficial layer, which makes the enamel surface more susceptible to long-term acid attack and caries, especially around orthodontic attachments.5

After Maiman6 submitted the four-ruby laser in 1960, Stern and Sognnaes7 reported on the use of lasers in dentistry in 1964. Compared with other lasers, the erbium-doped yttrium aluminum garnet (Er:YAG) laser can more effectively alter enamel and dentin surfaces because of its 2.94-mm wavelength emission, which is coincident with the main absorption band of water (wavelength 3.0 mm) and OH− groups in hydroxyapatite (wavelength 2.8 mm).8 The absorbed laser energy is converted to heat and boils water in the tooth. This forms high-pressure steam, and the explosive vaporization of water alters the smooth tooth surface and creates disorganized and microretentive morphology.4 Laser etching of an enamel surface leads to a fractured and uneven surface with open dentinal tubules (when bonding to dentin) that is ideal for adhesion.9 Moreover, the surface produced by laser etching is reported to be resistant to carious attacks.10 Consequently, laser irradiation was considered as an alternative to etch enamel for orthodontic bonding.11

Pulse duration is an important factor in the ability of a given laser to modify enamel surfaces.12 Active electronic control of laser pulse duration and amplitude is now possible with the improvement of Variable Square Pulse Technology (Fotona, Ljubljana, Slovenia). Duration of the pulses could be adjusted from 50 microseconds (super-short pulse) to 100 microseconds (very short pulse), 300 microseconds (short pulse), 600 microseconds (long pulse). or 1000 microseconds (very long pulse). Because of its higher energy in the shorter pulse, the energy loss with heat is less. As a result of this, ablation becomes more effective, and thermal effect is not evident on the tissue.13

Quantum-square pulse (QSP) mode has recently been introduced in Er:YAG laser technology. In the QSP mode, each pulse is split into several shorter ones following each other at an optimum fast rate. In this way, absorption and scattering of the laser beam is avoided, and undesirable thermal effects on the tissues are decreased.14 Lasers operating at this mode are reported to provide fast and precise hard dental tissue preparation.15

An electronic search of the literature presented that there are no data about the effects of laser pulse duration on orthodontic retention. Therefore, the aim of this study was to compare shear bond strength (SBS), enamel surface characteristics, and the adhesive remnant index (ARI) scores of bonding after QSP- or medium-short pulse mode (MSP)-mode laser irradiation and phosphoric acid etching of enamel surfaces. For the purposes of the study, the null hypothesis assumed that different pulse settings and acid etching did not cause differences in the parameters studied.

MATERIALS AND METHODS

Thirty-six human premolars were collected, cleaned, and stored in distilled water at room temperature. The teeth were those extracted for orthodontic purposes. The selected teeth had intact enamel and no caries, cracks, restorations, or infections. The teeth were embedded in phenolic rings using autopolymerizing polymethylmethacrylate, with the buccal surface parallel to the load direction under SBS testing. The teeth were pumiced and rinsed.

Teeth were randomly divided into three groups of 12 premolars each. In group 1, the enamel surfaces were etched with 37% phosphoric acid gel for 20 seconds, rinsed with air-water spray for 15 seconds, and dried to a chalky-white appearance. In group 2, the enamel surfaces were etched with an Er:YAG dental laser (2970-nm wavelength; LightWalker, Fotona, Slovenia) for 15 seconds (120 mj, 10 Hz, 1.2 W, water [50 ml/min]) in MSP mode. In group 3, the enamel surfaces were etched with the same dental laser and same power settings, but in QSP mode. In both laser-etching groups, the area to be bonded was scanned for 15 seconds with horizontal movements perpendicular to the enamel at a distance of 1 mm with a contact-type handpiece. The laser irradiation of enamel was performed manually, and no special setup was used.

Scanning electron microscope (SEM) photographs of one representative tooth from each group were taken (1000×; JSM-5310; JEOL, Peabody, Mass) to observe alterations in enamel surfaces. The surfaces of the enamel were evaluated according to the enamel damage index (EDI),16 which is a modification of surface roughness index described by Howell and Weekes.17 The EDI includes the following categories: grade 0, smooth surface without scratches, and perikymata might be visible; grade 1, acceptable surface, with fine scattered scratches; grade 2, rough surface, with numerous coarse scratches or slight grooves visible; and grade 3, surface with coarse scratches, wide grooves, and enamel damage visible to the naked eye.

To evaluate the surface roughness, one tooth from each group was evaluated using an atomic force microscope (AFM; Ntegra Solaris; NT-MDT, Moscow, Russia). Then the surfaces of the remaining roughened enamels were examined using a surface roughness profilometer (MarSurf M 300 C+RD 18 C; Mahr GmbH, Göttingen, Germany). This instrument has a stylus moving on the surface for the prescribed length, and it quantifies roughness as an average surface roughness (Ra) value. All enamel surfaces were evaluated by using the profilometer, and average Ra values were recorded.

On the remaining 10 premolars in each group, stainless steel brackets (American Orthodontics, Sheboygan, Wis) with an average bracket base surface area of 10.55 mm2 were bonded to upper premolars using Transbond XT primer and resin (3M Unitek, Monrovia, Calif) according to the manufacturer's instructions. The adhesive was light cured (Flashlite 1401; Discus Dental, Los Angeles, Calif) on each proximal side for 10 seconds.

After bracket bonding, the teeth were stored in distilled water for 24 hours at room temperature and later subjected to thermocycling 5000 times in distilled water between 5°C and 55°C, with a dwell time in each bath of 30 seconds and a transfer time of 15 seconds.

The SBS was tested using a universal testing machine (Shimadzu AG-X, Tokyo, Japan) operating at speed of 0.5 mm/min. The specimens were stressed in an occluso-gingival direction under the occlusal wings of the bracket and parallel to the long axis of the tooth. The values were obtained in Newtons and converted into megapascals (MPa) by dividing the value by the surface area of the bracket base.

After debonding, the bracket bases and the enamel surfaces were examined under 30× magnification using a stereomicroscope (SMZ 1000 Nikon; Nikon Corporation, Tokyo, Japan) to determine the amount of residual adhesive remaining on each tooth. The ARI, ranging from 0 to 3, was used to assess the amount of adhesive left on the enamel surfaces. A score of 0 indicates no adhesive remained on the enamel surface, 1 indicates less than half of the adhesive remained on the tooth, 2 indicates more than half of the adhesive remained on the tooth, and 3 indicates all adhesive remained on the tooth structure.

RESULTS

The surface roughness measurement results (in Ra) were presented in Table 1. There were statistically significant differences among the surface roughness values of the groups (P < .001). The QSP group had the highest surface roughness values (P < .001), followed by the MSP group, then the acid-etched group (P < .001).

Table 1. The Surface Roughness Measurements and SBS Values of the Groups^a

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The mean SBSs were summarized in Table 1. There were statistically significant differences among the SBS values of the groups (P < .01). The QSP group produced the highest SBS values, followed by the MSP and then acid-etched groups. The QSP group demonstrated significantly higher SBS values compared to the acid-etched group (P < .01), whereas the differences were not statistically significant between the acid-etched and MSP laser groups and between the two laser groups (P > .05). The null hypothesis was thus rejected in part.

SEM images of each enamel surface were presented in Figures 1–3. The sample from the acid-etched group demonstrated a rough surface, with numerous coarse scratches or slight grooves visible (EDI grade 2). Both laser modes seemed to produce coarse scratches, wide grooves, and enamel damage visible to the naked eye (EDI grade 3).

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The views obtained by the AFM scans are presented in Figures 4–6. Differences between the surface roughnesses of the acid etched group and the laser groups were in accordance with images obtained by via SEM; topographic irregularities were observed in all samples, but the acid-etched sample demonstrated a more regular surface, whereas rougher surfaces were visible in the laser groups.

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With regard to ARI scores (Table 2), the laser groups demonstrated that the failure rate in general was mixed in nature, with less than 50% of adhesive remaining on enamel. In the acid-etched group, ARI scores were mainly adhesive in nature, demonstrating less than 50% or no adhesive remaining on enamel.

Table 2. ARI Scores of Groups^a

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DISCUSSION

In the present investigation, the effect of phosphoric acid etching or MSP mode or QSP mode laser irradiation on surface roughness characteristics, SBS values of brackets, and ARI scores was evaluated. It has been shown that composites that were thermocycled absorb more water than those that were not thermocycled18 and, accordingly, SBSs of brackets are affected by thermocycling.19 Therefore, the samples were thermocycled before the testing procedure in this study.

Etching with phosphoric acid is commonly used to create resin tags on the enamel surface.6,20 The process of acid etching is effective but leads to demineralization of the enamel surface, which is undesirable for its clinical implications because acid attack is the main cause of dental caries. With the advent of dental lasers, the effects of these devices on surface treatment of enamel have been investigated, and laser ablation has become an alternative to acid etching.21 Research has shown that laser irradiation produces an amount of surface roughness comparable22 or similar23 to that seen with acid etching.

Er:YAG laser conditioning has been proven effective for hard-tissue ablation without thermal side effects.24,25 Moreover, Kim et al.4 reported that the Er:YAG laser-treated enamels are resistant to acid attack compared to phosphoric acid-etched enamels. Several studies have demonstrated that the SBS of Er:YAG laser-conditioned surfaces is variable.26–30 In some studies, Er:YAG laser was reported to interact well with dental hard tissue and to promote increased SBS in comparison with acid etching.26,27 On the other hand, higher bond strength following acid etching was reported in other studies.28

Er:YAG pulse duration and pulse energy play a decisive role related to laser ablation ability and the surface conditioning for adhesion.11 In order to reduce thermal deposition and scattering effects during surface modification, Er:YAG laser pulses should be of short duration and low pulse energy.14 However, Er:YAG lasers are inefficient when operating in the short duration, low-pulse-energy regime. By the development of variable square pulse (VSP) pumping technology, control of the Er:YAG pulse duration is possible.12 The QSP technology extended the range of treatment parameters of VSP Er:YAG lasers. The QSP mode reduces the undesirable effects of laser beam scattering and absorption in the debris cloud during hard-tissue ablation.14

Reynolds3 reported that adequate bond forces in orthodontics range from 6 to 8 MPa. The results of this in vitro study demonstrated that mean bond strength values of all groups remained within this range. MSP mode and QSP mode yielded statistically similar bond strength values. This result is in accordance with the findings of Tanji et al.27 and Başaran et al.,21 while conflicting with the findings of Martinez-Insua et al.28 and Usumez et al.,29 probably because of the different power and irradiation settings of laser devices used.

In this study, the SEM and AFM scans were acquired to make visual evaluations of the groups. AFM is a very high-resolution type of scanning probe microscopy with demonstrated resolution on the order of fractions of a nanometer. AFM is accepted as a suitable method for analysis of hard surfaces that exhibit microirregularities.31 The acid-etched sample had regular and slight grooves visible, whereas both laser modes created uneven and heterogeneous surface characteristics with microcracks. The AFM scans supported the findings of SEM. The acid-etched sample had a regular and slightly rough surface, whereas both of the laser samples had irregular and severely rough surfaces. Moreover, the MSP sample had deeper grooves than did the QSP sample. However, the SEM evaluations and AFM scans were made on single specimens from each group, providing only visualization of surface morphology. Therefore, there is a need for further research with larger samples to determine the extent of surface morphology and to obtain quantitative data.

Considering the surface roughness data, profilometric examination showed that both laser groups produced rougher surfaces than that of acid etching. Moreover, the QSP mode produced the roughest enamel surfaces.

The ARI scores indicate that failure sites were mainly at the enamel-adhesive interface in the acid-etched group, causing minimal risk for enamel fractures. Some authors suggested that bond failure within the adhesive or at the bracket-adhesive interface is more desirable than failure at the enamel-adhesive interface, because it might lead to enamel fracture and crazing while debonding.32 On the other hand, the time spent removing adhesives from the enamel depends largely on the amount of remnant adhesive33 and, accordingly, the fewer adhesive remnants reduces the chair time. Although the failure sites were also mainly at the enamel-adhesive interface, there was more adhesive left on the enamel surface in both laser irradiation groups, suggesting a somewhat safer debonding, which is in accordance with the results of Usumez et al.29

Er-YAG laser is a promising technology with various applications in the orthodontic field. It is evident that comparable and higher SBS can be achieved with laser irradiation. However, further investigations are required to determine the effect of different Er:YAG laser settings on the adhesive interface micromorphology and the alterations of enamel chemistry. Ability to restore the enamel surfaces to original gloss after laser etching may also be a concern and should be further evaluated. The SEM, AFM, and surface profilometry used in this study demonstrated successful alteration of the enamel surface with this technology. However, long-term clinical studies are required to verify clinical success.

CONCLUSION

• Within the limitations of this study, the results suggest Er:YAG laser etching with MSP and QSP modes presents a successful alternative to acid etching by providing higher or comparable SBS values.