Phosphoric acid etching is a good method of preparing tooth enamel for bonding resins and orthodontics attachments. However, a potential disadvantage is the possibility of decalcification, which leaves the enamel susceptible to caries attack, especially under orthodontic attachments. A technique that produces clinically useful bond strength, without leaving the enamel susceptible to caries, would be advantageous.

There has been extensive research to find such alternative conditioning methods to overcome this main disadvantage of phosphoric acid etching. Some researchers have worked on conditioning enamel with polyacrylic acid, crystal growth conditioning, and pretreatment of the enamel surface with sandblast of aluminum oxide to reduce the enamel loss during etching. ¹ , ² , ³ However, these methods failed to achieve adequate bond strength to resist intraoral forces. ⁴

One alternative to acid etching is laser etching. Various commercially available laser systems have been introduced for dental use erbium, chromium-doped: Yttrium scandium-gallium-garnet (Er. Cr: YSGG) laser, which uses a pulsed-beam system has been shown to be effective for soft-tissue surgery and for cutting enamel, dentine, and bone. ⁵ , ⁶ Enamel and dentin surfaces etched with Er. Cr: YSG lasers show micro irregularities and no smear layer. Kim et al. found that laser-etched enamel was more resistant to acid attack compared to phosphoric acid etching and this could be of great importance in orthodontics. ⁷ Since water spraying and air drying are not needed with laser etching, procedural errors can be reduced and time saved.

Er. Cr: YSGG laser irradiation might be a suitable technique to etch enamel for orthodontic bonding. Therefore, the purpose of the study was to compare the surface characteristics, shear bond strength (SBS), and adhesive remnant index (ARI) of the samples treated with conventional acid etching and Er, Cr: YSGG laser etching.

In this the present analytical/descriptive study was performed on 90 premolars extracted for orthodontic purpose.

Criteria for selection of tooth:

Teeth without enamel defects

All teeth were stored in distilled water immediately after extraction, and the distilled water changed every week. These teeth were washed in water to remove any traces of blood. Teeth were cleaned off from adherent tissue tags and debris with ultrasonic scaler. The samples were stored in saline solution until ready for use.

Out of the total sample of 90 premolars, 15 premolars were used for surface roughness evaluation. The rest 75 premolars were used for SBS evaluation. The 15 premolars for surface evaluation were further divided into five groups of three teeth each, and the 75 premolars for SBS were randomly divided into five groups of 15 teeth in each group.

For surface roughness, the 15 premolars were divided into five groups of three teeth each

Phosphoric acid etching

For SBS, the 75 premolars were divided into five groups of 15 teeth each

Phosphoric acid etching

All these teeth were mounted vertical in self-cure acrylic resin block so that only the crown was exposed Figure 1. Figure 1

Premolar teeth mounted on acrylic blocks.

The ARI score of 3 was obtained in acid etching group for 80% of the samples. Group 5 (laser etching 1.5 W/20 s) showed ARI score of 3 in 66. Nearly 7% samples and ARI score of 2 in 33.3% samples. The other three groups (laser etching 1.5 W/10 s, 1 W/20 s, and 1 W/10 s) showed ARI scores of 2, 1 and 0 in 93.3%, 73.3%, and 80% of the samples, respectively. The difference between all the groups was highly significant and more amount of adhesive was left on acid etched followed by the 1.5 W/20s laser-etched group.

The difference between these two groups was not found to be statistically significant. All the other three groups showed average roughness which was less than the above two groups, the difference being statistically significant.

The statistical analysis of the values obtained for SBS showed that acid etching (Group 1) showed the highest bond strength (10.48 MPa) followed by laser etching at 1.5 W/20 Hz (10.46 MPa). The difference between these two groups was not statistically significant. This was followed by laser etching at 1.5 W/10 s (8.38 MPa), 1 W/20 s (6.89 MPa), and 1 W/10 s (4.79 MPa), respectively. The difference between these three groups and the other two groups (acid etching and laser etching for 1.5 W/20 s) was statistically significant Table 2and Table 3.{Table 2}{Table 3}

Laser systems are more commonly used in dentistry in recent years. The first commercially available lasers, such as carbon-dioxide and neodymium-doped: YAG, were suitable only for soft-tissue treatments, especially in periodontics. The main disadvantage for application on dental hard tissues was their thermal side effects. Er doped: YAG laser systems are capable of ablation in both soft and hard tissues without thermal side effects. ⁹ , ¹⁰ , ¹¹ In orthodontics, lasers are mainly used for etching the enamel surface, curing, and debonding the brackets.

This study was designed to determine whether laser etching can be an alternative to conventional acid etching. Out of the total samples of 90 premolars, 15 premolars were used for surface roughness evaluation, three for each of the five groups of treated enamel. The remaining 75 premolars were randomly divided into five groups of 15 each and bonded with a conventional acid etching method using phosphoric acid etching and Er, Cr: YSGG laser systems at different power outputs and duration namely 1 W/10 s, 1 W/20 s, 1.5 W/10 s, and 1.5 W/20 s. Enamel etching pattern of phosphoric acid etching and laser were studied using a 3D profiler. The Instron Universal Testing Machine (UNITEK) was used to test the SBS of each tooth followed by the assessment of ARS index according to the criteria put forward by Reynolds. ⁸

The Er, Cr: YSGG laser used in the present study has an average power output that can vary from 0 to 6 W. For cutting enamel, high irradiation outputs from 2.5 to 6 W can be used. ¹² , ¹³ In a previous study by Basaran et al., ¹⁴ three power outputs were used (0.5, 1, and 2 W). However, the 0.5 W setting resulted in inadequate bond strength. The usage of different power outputs causes different effects. Ozer et al. compared Er, Cr: YSGG laser irradiation at 0.75 and 1.5 W with phosphoric acid etching and self-etching primer (SEP) for orthodontic bonding. They stated that varying power outputs of laser irradiation made different etching patterns: 0.75 W laser irradiation had lower SBS, whereas 1.5 W power output showed comparable SBSs with phosphoric acid and SEP. ¹² In the present study, 1 W and 1.5 W were used because these were the minimum power outputs which could produce acceptable bond strength.

The study by Basaran et al. ¹⁴ used 15 s for laser etching. However, in this study, both 10 and 20 s were used to evaluate the effect of time on the surface characteristics and the SBS. Acid-etching times can vary from 15 to 60 s.

Surface roughness evaluation

A quantitative measurement of surface roughness was obtained using a noncontact 3D optical profiler. The highest surface roughness was obtained for acid etching (0.491 μm) followed by laser etching at 1.5 W/10 s (0.487 μm). The other three groups showed surface roughness values of 0.157 μm (1 W/10 s), 0.222 μm (1 W/20 s), and 0.222 μm (1.5 W/10 s), respectively. The statistical analysis, ANOVA, and Tukey HSD tests showed that the acid etching and laser etching at 1.5 W/20 Hz have statistically significant higher surface roughness when compared to the other three groups.

The results of this study were in concordance with the study done by Ozer et al., who evaluated the surface characteristics of enamel etched with phosphoric acid, SEP, 0.75 W, and 1.5 W Er, Cr: YSGG laser using scanning electron microscope. They found that enamel etched with 1.5 W Er, Cr: YSGG laser had similar surface roughness to acid-etched enamel. ¹²

Shear bond strength evaluation

SBS was evaluated for the conventional acid etch group, and the laser etch groups with different power outputs and duration. The mean SBS of conventional acid etch was found to be 10.48 Mpa and was the highest. This was followed by the laser etch (1.5 W/20 Hz) group with a value of 10.46 Mpa. The other three laser-etched groups showed lower SBS values of 4.79Mpa (1W/10 s), 6.89Mpa (1 W/20 s), and 8.38 (1.5 W/10 s), respectively. The difference between acid etching and 1.5 W/20 Hz laser etching was not statistically significant.

Basaran et al. reported that the mean SBS and enamel surface etching obtained with an Er, Cr: YSGG laser (operated at 1W or 2W for 15 s) was comparable to that obtained with acid etching. ¹⁴ In another study by Ozer et al., almost similar bond strength was achieved with acid etching and laser irradiation at 1.5W indicating that the mean bond strengths in this study were similar to these studies. ¹²

Acid-etching results in chemical changes that may modify the organic matter and decalcify the inorganic component. As a result of this demineralization, enamel becomes more susceptible to caries attack, which is induced by plaque accumulation around the bonded orthodontic attachments. ¹⁵ Laser etching of enamel creates microcracks that are ideal for resin penetration. Hossain et al. reported an increase in the calcium to phosphorus ratio achieved during laser irradiation, which leads to caries inhibition. ¹⁶ Therefore, laser irradiation might have an advantage as an etchant for orthodontic bonding. Laser etching can also save chairside time because of the lesser number of steps required and this maybe an important factor from a clinical point of view. ¹⁷

The findings obtained from this study were also in agreement with von Fraunhofer et al., who reported that adequate bond strength of laser etching was obtained only with higher power output. ¹⁸

Although the mean bond strength of the three laser groups 1 W/10 Hz, 1 W/20 Hz, and 1.5 W/10 Hz showed a statistically significant difference from the other two groups, the bond strength was in the acceptable range as suggested by Reynolds, ⁸ Maijer and Smith. ¹⁹

High adhesive strength between bracket and tooth are an essential factor in any treatment concept, also increased bond strength is always necessary in certain clinical situations. Frequent debonding can lead to prolonged treatment time and patient burn out.

The SBS values also correlated with the surface roughness values wherein, those groups which had a higher surface roughness value had a correspondingly higher SBS value. This indicates that increased surface roughness translates to a better micromechanical bond.

Adhesive remnant index

In this study, the adhesive remnant index was carried out as given by Reynolds in 1975. ⁸ The samples were graded from 0 to 3, where 0 indicates no adhesive left on the tooth and 3 indicates all adhesive left on the tooth, with distinct impression of bracket mesh. The mean of ARI scores for acid-etched and laser-etched (1.5 W/20 Hz) groups ranged from score 2–3 indicating that the bond between the enamel surface and adhesive was much stronger than that between the adhesive and the bracket base. This could be either an advantage or disadvantage. Less chair time is needed with less adhesive left on the enamel after debonding, but it might cause enamel fracture while debonding.

The Chi-square test showed that difference between all the groups were highly significant statistically, but more amount of adhesive (score 3) was left on acid etched and 1.5 W/20 Hz laser-etched groups. This result correlates with a study done by Ozer et al. in 2008. It was found that laser etching at higher output showed more amount of adhesive left on the tooth surface. ¹²

In recent years, orthodontists deal with an ever-increasing number of adolescents and young adults who lack the seriousness of proper maintenance of fixed appliance in comparison with adults; the necessity for increasing the bond strength for such patients is beneficial. In case of lingual orthodontics, bracket breakage and rebonding are a tedious and laborious process. Thus, in all such cases, acceptable level of bond strength will be comfortable for both clinician and patient.

At that same time, there are some clinical situations such as bonding of ceramic brackets which frequently cause enamel fracture because of high SBS. In these cases, a lower power output and duration of laser etching may bring down the SBS values to acceptable levels, thereby preventing enamel damage during debonding and facilitate easier clean up. This versatility of lasers where the power output and duration can be altered to suit the clinical situation can also be an advantage.

To inculcate lasers in the armamentarium, the clinician should have a comprehensive understanding of the principles and fundamentals of laser and its helpful abilities. The numerous advantages of Er, Cr: YSGG laser, make it a viable alternative to conventional acid etching. The main disadvantage is the capital expenditure the clinician has to spend for Er, Cr: YSGG lasers, but this type of lasers can be used for vast variety of procedures in all fields of dentistry, thereby giving a better cost/benefit ratio.

Further in vivo studies using a split-mouth design comparing conventional acid etching and laser etching should be conducted to know its effect on a clinical setting. In addition, future in vivo studies can concentrate on the caries resistance of laser-etched enamel.

Overall, from this study it can be concluded that laser etching at 1.5W/20Hz achieved similar surface roughness and shear bond strength compared to conventional acid etching and could be a viable alternative for surface preparation of enamel. Lower power outputs can be used in those clinical situations which demand lesser bond strength.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, and financial or nonfinancial in this article.