The Use of Laser Energy for Etching Enamel Surfaces in Dentistry—A Scoping Review

Background: In dental practice, different situations require etching the enamel layer. Acid etching, the present golden standard, may be replaced by other methods, such as laser etching. The main focus of our scoping review is to assess the existent literature regarding the effectiveness of different types of lasers, to identify the main aspects studied so far, and to understand where new search strategies are needed. Methods: The search was conducted in several databases focusing on the laser etching of human definitive enamel. We included English language articles published between January 2000 and December 2021. Results: The 34 articles reviewed showed that hard lasers, Er:YAG, Er,Cr:YAG, may represent an alternative etching method on enamel surfaces. They create a fractured, irregular surface and open dentin tubules, highly suitable for adhesion but with a lower risk of cavity formation. Nd:YAG, CO2, and Diode lasers do not help in creating sufficient shear bond strength. There is, however, evidence suggesting that microcracks in the enamel layer may appear after thermomechanical ablation using laser energy. Conclusions: While the use of acid etching is still successfully used for enamel conditioning, some researchers have emphasized the role played by saliva in the enamel-remineralization process a few days after the procedure. In this context, laser energy can be used, especially for bonding ceramic brackets in the case of orthodontic treatments. However, as thermomechanical ablation can generate microcracks, further research is required in order to establish clear findings concerning the use of laser energy on enamel etching.


Introduction
Miaman [1] pioneered the application of lasers in dentistry in 1960, and, up until now, their applications have continued to expand. Based on the active medium, several types of lasers are available: (1) gas lasers, such as carbon dioxide (CO 2 ); (2) solid-state lasers, such as neodymium yttrium aluminum garnet (Nd:YAG), the erbium-doped yttrium aluminum garnet laser (Er:YAG); (3) liquid (dye) lasers, such as Rhodamine G6 (containing liquid colorant as the medium); (4) semiconductor lasers, such as GaAs or GaAIAs lasers, having a semiconductor as the medium, also known as Diode lasers and (5) 'free-electron' lasers, which use an electron accelerator, but are not available for dental applications [1]. Diode lasers, also known as soft lasers, are considered in low-level laser therapy or 'bio-stimulation' [1,2]. Lasers can be used to perform a variety of dental treatments, including frenectomies, crown shaping, composite polymerization, and control of hemorrhaging, caries detection and removal, pain and hypersensitivity treatments, gingivectomy, gingivoplasty, and soft-tissue lesions' treatment [3].
The CO 2 laser wavelength has an important high affinity for water, thus rapidly removing soft tissue and providing homeostasis without penetrating tissues. However, its disadvantages refer to its large size, high price, and its capacity to interfere with and Materials 2022, 15,1988 2 of 13 destroy hard tissues [4]. Erbium lasers have two distinct wavelengths, Er,Cr:YSGG (yttrium scandium gallium garnet) and Er:YAG (yttrium aluminum garnet), with the most important absorption of water in any dental laser wavelength and a high affinity for hydroxyapatite. They can be used for treating dental hard tissues [5]. The Nd:YAG wavelength is used in surgery for removing and coagulating dental soft tissues and in periodontal treatments [6].
The Diode laser has several applications in dental practice, being used frequently for crown reshaping through gingivoplasty, frenectomies, exposure of superficially impacted teeth, removal of inflamed and hypertonic tissues, and photostimulation of the aphthous and herpetic lesions [7].
There are four different possible interactions of lasers with a target tissue: reflection, transmission, scattering, and absorption. Through absorption, the laser elevates the temperature and creates photochemical effects varying depending on the water content of the tissues. Depending on the temperature reached, ablation-vaporization of the water in the tissues, denaturing of the proteins, or dehydration and burning of the tissuecarbonization-can occur. Absorption requires a molecule that absorbs light, known as chromophores, with an affinity for specific wavelengths of light. In the soft tissue present in the oral cavity, chromophores are melanin, hemoglobin, and water, and in hard tissues of dental origin, they are water and hydroxyapatite. There are different absorption coefficients depending on the wavelengths the lasers have [8,9].
In clinical practice, different situations (composite fillings, adhesive techniques in restorative dentistry, bracket bonding in orthodontics, etc.) imply etching the enamel layer. Acid etching involves a selective dissolution of the enamel, causing microporosities, resulting in bonding via mechanical retention. This classic technique of acid etching of the enamel layer was introduced by Buonocore [10], while Newman used this technique for bracket bonding in orthodontics by using composite resins on the etched dental surfaces [11].
Ever since the application of laser energy in dentistry, various laser types have been applied when etching the enamel and dentin layer. It is known that laser irradiation on the enamel layer produces melting and recrystallization, which leads to a surface roughness comparable to the one obtained after acid etching at a microscopic level [11,12].
The wavelength, power, mode of operation (pulsed, continuous wave), and exposure duration all affect the results of laser applications. Irradiation was tested as a viable way to produce etching effects on hard dental tissue, as there have been various research papers discussing the best laser parameters to optimize etching applications ( Figure 1) [11][12][13][14].
disadvantages refer to its large size, high price, and its capacity to interfere with and destroy hard tissues [4]. Erbium lasers have two distinct wavelengths, Er,Cr:YSGG (yttrium scandium gallium garnet) and Er: YAG (yttrium aluminum garnet), with the most important absorption of water in any dental laser wavelength and a high affinity for hydroxyapatite. They can be used for treating dental hard tissues [5]. The Nd: YAG wavelength is used in surgery for removing and coagulating dental soft tissues and in periodontal treatments [6].
The Diode laser has several applications in dental practice, being used frequently for crown reshaping through gingivoplasty, frenectomies, exposure of superficially impacted teeth, removal of inflamed and hypertonic tissues, and photostimulation of the aphthous and herpetic lesions [7].
There are four different possible interactions of lasers with a target tissue: reflection, transmission, scattering, and absorption. Through absorption, the laser elevates the temperature and creates photochemical effects varying depending on the water content of the tissues. Depending on the temperature reached, ablation-vaporization of the water in the tissues, denaturing of the proteins, or dehydration and burning of the tissue-carbonization-can occur. Absorption requires a molecule that absorbs light, known as chromophores, with an affinity for specific wavelengths of light. In the soft tissue present in the oral cavity, chromophores are melanin, hemoglobin, and water, and in hard tissues of dental origin, they are water and hydroxyapatite. There are different absorption coefficients depending on the wavelengths the lasers have [8,9].
In clinical practice, different situations (composite fillings, adhesive techniques in restorative dentistry, bracket bonding in orthodontics, etc.) imply etching the enamel layer. Acid etching involves a selective dissolution of the enamel, causing microporosities, resulting in bonding via mechanical retention. This classic technique of acid etching of the enamel layer was introduced by Buonocore [10], while Newman used this technique for bracket bonding in orthodontics by using composite resins on the etched dental surfaces [11].
Ever since the application of laser energy in dentistry, various laser types have been applied when etching the enamel and dentin layer. It is known that laser irradiation on the enamel layer produces melting and recrystallization, which leads to a surface roughness comparable to the one obtained after acid etching at a microscopic level [11,12].
The wavelength, power, mode of operation (pulsed, continuous wave), and exposure duration all affect the results of laser applications. Irradiation was tested as a viable way to produce etching effects on hard dental tissue, as there have been various research papers discussing the best laser parameters to optimize etching applications ( Figure 1) [11][12][13][14].  [11,15]. Among the advantages of using laser energy for enamel etching, reducing the probability of enamel damage with the reduction in the debonding force needed has clinical importance [11,16,17]. However, some disadvantages, such as undesired thermal side effects and the developments of microcracks (which might represent a starting point for carious attacks), have been reported [11,18]. The purpose of this scoping review was to assess the existent literature on the effectiveness of different types of lasers (Er:YAG, Er,Cr:YAG, Nd:YAG, CO 2 ) on the enamel layer, to identify the main aspects studied so far regarding the topic, as well as to understand where new search strategies are needed. Our research examines the current state of science regarding lasers for different clinical situations necessitating the etching of the enamel layer and investigates any advantages of using laser energy compared to classical methods. The research question was: 'to what extent can laser energy can be used for enamel etching in dental practice?'

Materials and Methods
This study is a scoping review, considering the fact that outcomes and methodologies of studies regarding laser use for etching of the enamel layer are heterogeneous. Our research was performed according to the recommendations provided by Arkey and O'Malley in 2005, as well as the protocol guidelines provided by the Joanna Briggs Institute [19][20][21]. The search strategy was performed in accordance with the PRISMA-ScR guidelines ( Figure 1) [19].

Search Strategy
The search included several databases-PubMed Central, Scopus, Medline via Ovid in December 2021-focusing on laser etching. Search for additional literature was completed via Google Scholar and through additional research of references from the included publications. All databases were searched between January 2000 and December 2021. The terms 'laser', 'etch', 'enamel layer', and their combinations were used together using 'AND' to build the search strategies. All references were imported and organized in the bibliographic software Mendeley ® .

Selection of Articles
The inclusion criteria were as follows: all study types systematic reviews and metaanalysis, experimental studies performed on human definitive teeth, articles written in English, and articles for which full text is available. Studies performed on bovine and human temporary teeth, studies concerning the adhesion specifically on dentin layer, or research focusing on laser preparation of cavities instead of laser etching were excluded (Table 1). A total of 118 papers were discovered by using the search method. After the duplicates were eliminated, 78 articles were considered. The authors individually screened the abstracts in order to identify the papers that were relevant to the aims of the research, resulting in 43 studies. A total of 7 records were additionally excluded based on the outcomes, which did not match the aims of this research. After full-text reading of the resulting studies, 16 publications were eliminated because they did not meet the inclusion criteria, and a total of 34 publications were eventually included in the study (Figure 2).
A total of 118 papers were discovered by using the search method. After the duplicates were eliminated, 78 articles were considered. The authors individually screened the abstracts in order to identify the papers that were relevant to the aims of the research, resulting in 43 studies. A total of 7 records were additionally excluded based on the outcomes, which did not match the aims of this research. After full-text reading of the resulting studies, 16 publications were eliminated because they did not meet the inclusion criteria, and a total of 34 publications were eventually included in the study (Figure 2).

Data Collection
From each publication, the data that were extracted included the authors, year of publication, journal, aim of study, and methodology. In addition, key findings and conclusions were also extracted. In order to organize the data, Excel spreadsheets (Microsoft Office 2019 ®, MS, Redmond, WA, USA) were used.

Results
A selection of 34 articles was included in this scoping review. All publications investigated one of the four types of lasers-Er:YAG, Er,Cr:YAG, Nd:YAG, CO2-for conditioning the enamel surface. All publications are presented in three tables, according to laser types and their outcomes, in the order of publication (Tables 2-4).

Data Collection
From each publication, the data that were extracted included the authors, year of publication, journal, aim of study, and methodology. In addition, key findings and conclusions were also extracted. In order to organize the data, Excel spreadsheets (Microsoft Office 2019 ®, MS, Redmond, WA, USA) were used.

Results
A selection of 34 articles was included in this scoping review. All publications investigated one of the four types of lasers-Er:YAG, Er,Cr:YAG, Nd:YAG, CO 2 -for conditioning the enamel surface. All publications are presented in three tables, according to laser types and their outcomes, in the order of publication (Tables 2-4).

Discussion
The classic method used for enamel etching is acid etching, a method that uses 37% phosphoric acid for the selective dissolution of the enamel layer, causing microporosities and resulting in a bonding mechanism via mechanical retention (the penetration of the resin tags into the microporous substrate) [11]. While this method is used successfully in different dental domains, there are some disadvantages, such as the possibility of decalcification, which leaves the enamel layer susceptible to caries attacks, as well as the discoloration caused by resin tags [11]. Although there are few studies published on enamel remineralization after acid etching using orthophosphoric acid, there is some evidence suggesting that a few days after conditioning, thanks to the role played by saliva, conditioned enamel cannot be distinguished from untreated enamel [54]. This is why novel technologies, such as laser irradiation and laser-etching techniques, have been developed as promising alternatives to acid etching. However, the use of lasers to condition the enamel surface could be even more aggressive, as it results in the ablation of similar tissue. The shear bond strength (SBS) has been investigated, especially in the context of orthodontic brackets bonding, and while some studies suggest that using self-etching primers reduces the bond strength and therefore the risk of the enamel layer fracture [55] when debonding, laser irradiation (especially Er,Cr:YSGG laser) shows promising results, especially for producing less-important adhesion forces to the enamel [56].
When using laser energy on the enamel surface, a melting and recrystallization process is initiated, creating a porous surface similar to the type III pattern produced by orthophosphoric acid via acid etching, thus providing an alternative to traditional etching [57]. On dental tissue, laser etching seems to create a fractured, uneven surface and open dentin tubules, highly suitable for adhesion [58].
Laser irradiation of dental hard tissues modifies the proportion of minerals in the tissues, reduces water and organic component content, and helps form stable, less-acidsoluble compounds [59]. Through this mechanism, the surface becomes less susceptible to cavity formation. Groth and collaborators found that, when combined, laser and acid conditioning increased etching depth, and laser-only etched enamel showed a small reduction in mineral concentration and higher porosity, revealing a greater penetration of acid [22].

Er:YAG Lasers
Most studies identified by the reviewers focused on Er:YAG lasers, a type of laser with application in cavity preparations in dental practice. Studies compared laser conditioning with conventional 37% phosphoric acid, revealing that the two methods can provide similar shear bond strengths [28,31,32,35,36]. There are certain studies revealing even a higher bond strength for laser etching [30,33]. The laser helps in improving shear bond strength values when bonding orthodontic brackets to the enamel surfaces by using a self-etching adhesive system [33]. Nd:YAG and Diode lasers also help improve the adhesion of selfetching systems in cavities [15]. When comparing Er:Yag to Nd:Yag for laser etching, the latter showed significantly lower results [25,37]. When combining acid and laser etching, the fissure sealant retention was improved [34].
However, three pieces of research revealed that laser conditioning of the enamel layer was less effective than acid etching [23,26,27]. These articles seem to be biased by the protocol used, lower power of lasers, or misuse of the systems. Compared to the nine studies showing improvement of adhesion when using laser, the outcome of the latter studies may be caused by technical inaccuracies.
Other studies compared different adhesive systems with significantly different results. Prime & Bond NT completely sealed dental hard tissue margins, while Etch & Prime 3.0 has shown the poorest overall results, which are statistically significant [24]. Additionally, additional laser conditioning after phosphoric acid etching might be beneficial to generation V, total etching in 2 steps [29].
These findings show that Er:YAG laser etching may function as a less-aggressive, high-efficacy method to create micro retention in total or self-etching adhesive systems, minimizing thermal damage. Nevertheless, thermomechanical ablation caused by these lasers can generate microcracks in the enamel layer [11].

Er,Cr:YSGG Laser
An Er,Cr:YSGG laser also helps increase surface roughness and eliminate the smear layer without cracking, as shown by scanning electron microscopy [38]. Similarly, 1.5-and 2-W laser irradiation may be an alternative to conventional acid etching [39,41]. Similar results between conventional acid etching and laser etching with this laser type were proven by several studies [42,[44][45][46][47]. There is, however, the risk of enamel damage due to thermomechanical ablation, which can lead to microcracks [11].
Lower adhesion when using a laser was found in some studies, but the laser output was generally lower, as the main cause for such findings [40,43,48].
For this type of laser, the efficacy of etching seems slightly lower, but the advantages shown in the elimination of acid etching side effects show that it may be a viable alternative.

Other Lasers
Goswami and his team studied the Nd:YAG-laser-etched enamel surface, finding a surface that is similar in aspect to other laser types but with a lower shear bond strength than acid etching [50]. One study found similar effects of bond strength when comparing Nd:YAG to acid etching [52]. Contrary to this, Fuhrmann and collaborators found CO 2 and Nd:YAG lasers produced sufficient modification of enamel for bracket bonding [49]. However, the CO 2 laser was proven to produce lower adhesion in most studies [51,53].
As shown by the research included in this review, Nd:YAG and CO 2 lasers may produce similar results and are not a viable alternative to conventional etching. Their effects on hard tissue are limited, and acid etching is preferred.

Conclusions
While classic acid enamel conditioning provides suitable results in order to assure proper adhesion in dental procedures, laser use has also been considered in this matter. While there are few pieces of research on enamel remineralization after acid etching, there is evidence suggesting that, due to saliva, the conditions of enamel cannot be distinguished from untreated enamel a few days after the procedure. On the enamel surface, hard lasers such as Er:YAG and Er,Cr:YSGG seem to create a surface suitable for composite materials' adhesion. Studies testing this method on orthodontic brackets, dental sealing, and composite fillings show mostly similar or higher adhesion than the golden standard, orthophosphoric acid. Laser irradiation of dental hard tissues helps form stable, less-acidsoluble adhesion, also lowering the risk of cavities' formation and eliminating the smear layer. When combined, laser and acid conditioning increase etching depth. Additionally, when laser etching is prior to a self-etching adhesive, studies have shown higher shear bond strength values. Differences between findings may be caused by the laser output and power-or user-related inconsistencies. There are, however, some concerns regarding laser etching related to thermomechanical ablation, which might generate microcracks in the enamel layer. CO 2 , Diode, and Nd:YAG lasers have not been researched enough to provide us with a definitive conclusion.
While surface modifications have been thoroughly researched through scanning electronic microscopy, new-generation materials' interaction with laser-etched surfaces may need further research. Therefore, based on the limitations of this scoping review, the results suggest that laser energy is beneficial in addition to acid etching in order to increase shear bond strength in dental-adhesive techniques.
Based on the information provided by the studies included in this research, a possible use of laser etching in dental practice could invovle bonding ceramic brackets in the case of orthodontic treatments. As the procedure for debonding the brackets (at the end of the treatment) can produce damage to the enamel layer, the situation may require a lesser shear bond strength. Institutional Review Board Statement: Submission for ethical approval was not required as the study relied on secondary data analysis of publicly available scientific literature.