Erbium Lasers for the Prevention of Enamel and Dentin Demineralization: A Literature Review

Abstract

Objective: The aim of this article is to review the current literature concerning Erbium lasers: Er:YAG (λ=2.94μm), Er:YSGG (λ=2.79μm), and Er,Cr:YSGG (λ=2.78μm) for the prevention of enamel and dentin demineralization. Methods: Features such as laser parameters, reported outcome, inhibition of demineralization, and mechanisms of laser action were analyzed. A total of 55 publications were found in four electronic databases and were complemented by hand searching. Results: Evidence regarding the potential of laser-induced prevention of demineralization (LIPD) was demonstrated in the literature, especially when subablative parameters were applied. Although ablation parameters have shown partial positive results in LIPD, some studies have shown severe morphological alterations in enamel and dentin. Until now, there are still no reports on the long-term effects of laser treatment. Additionally, it is unclear if there is a best combination of mechanisms that leads to the optimal LIPD. Other variables that are poorly investigated in the literature and have an important role in LIPD include pulse width, water irrigation, and air flow. Conclusions: This review demonstrates the current knowledge concerning the use of erbium lasers in LIPD, and brings forth essential questions that should be further addressed.

Introduction

Although the first report describing the use of lasers in the oral cavity was published in 1977, the field of lasers in dentistry only began in 1989 with the United States Food and Drug Administration (FDA) clearance of the first laser for general dentistry (Nd:YAG laser).¹ Nowadays, a range of different laser wavelengths for dental applications is available for soft tissue surgery, decontamination, tissue photobiomodulation, and hard tissue ablation. Examples of different laser wavelengths used in dentistry are: Argon (488, 515 nm), Helium-neon (633 nm), diode (635, 670, 810, 830, 980 nm), Nd:YAG (1.064 μm), Er,Cr:YSGG (2.78 μm), Er:YSGG (2.79 μm), Er:YAG (2.94 μm), and CO₂ (9.6 μm, 10.6 μm).² Among these wavelengths, Erbium and CO₂ lasers (2.9 μm, 10.6 μm) are indicated for dental hard tissue management.³

In dentistry, Erbium lasers operating at a wavelength of ∼λ=3 μm were introduced specifically for the removal of dental hard tissue.⁴ The good absorption of the radiation wavelength in the region of λ=3μm in water is responsible for this application.⁴ Instantaneous evaporation of the water inside the tissue leads to microexplosions that blast away tiny particles of hard substance.⁴ Er:YAG laser was the first dental laser approved to be used for dental hard tissue ablation by the FDA in 1997.^5,6 Depending on the energy and pulse width applied during erbium lasers irradiation, it is expected thermal changes in enamel, which can result inchemically and/or morphologically structure alteration andhard tissue ablation.^5,7,8 Erbium lasers offer several advantages over the high-speed drill for the removal of dental hard tissue.³ The laser procedure is well tolerated by the patient and there is reduced or no pain, because of the absence of vibration and contact.³ Moreover, carious tissue can be preferentially removed because of the high content of water and protein that are present in carious tissue.³ Lasers can also be used to modify the chemical composition of the mineral phase of enamel.³ This fact has been investigated and confirmed with regard to preventing or reducing tooth demineralization that may occur in dental caries and erosion.⁵

The mechanism by which tooth demineralization occurs differs when considering caries or erosion lesions. Regarding dental caries, fermentable carbohydrates such as glucose, sucrose, fructose, or cooked starch can be metabolized by acidogenic bacteria and create organic acids as products from metabolism.⁹ The acids diffuse through the plaque into the porous subsurface enamel (or dentin, if exposed), dissociating to produce hydrogen ions.^10,11 When the pH reaches below the critical pH (5.5), the hydrogen ions readily dissolve the mineral, which can diffuse out of the tooth.¹² As the pH is lowered, acids diffuse rapidly into the underlying enamel or dentin.¹²

Dental erosion, however, consists on the tooth demineralization that occurs because of intrinsic and extrinsic acids, without the involvement of bacteria.¹³ Extrinsic factors include acidic foods, carbonated beverages, sports drinks, red and white wines, citrus fruits, and, to a lesser degree, occupational exposure to acidic environments.¹⁴ The most common intrinsic factors include chloride acid, caused by chronic gastrointestinal disorders such as gastroesophageal disease, as well as health issues such as eating disorders (anorexia and bulimia), in which regurgitation and frequent vomiting are common.^14
–16 When a solution, with a pH<5.5, comes in contact with the mineral phase of the tooth, the acid with its hydrogen ion will start to dissolve the crystal. The unionized form of the acid will then diffuse into the interprismatic areas of enamel and dissolve mineral in the subsurface region.¹¹ This will lead to an outflow of tooth mineral ions (calcium and phosphate). With the aim of comparison, the pH produced by acidogenic bacteria is ≤pH 5.5 and the pH of acid beverages varies from 2.6 to 4.08.^14,17 The remineralization process occurs when the pH rises again and calcium, phosphate, and fluoride from saliva enter the subsurface region of the lesion and form a new veneer on the existing crystal remnants in the lesion.¹⁸ When there is an imbalance between demineralization and remineralization process, caries or erosion process may end up with tooth decays/cavities in instances of caries, or tooth wear, in instances of erosion.

For the prevention of tooth demineralization, laser light must be strongly absorbed and converted efficiently to heat without damage to the underlying or surrounding tissues, with the advantage of changing the composition or decreased solubility of dental hard tissues.¹⁹ Therefore, wavelengths must be chosen according to their higher absorption to components of dental hard tissues. Anatomically, the mineralized part of a tooth is composed by enamel, dentin and in a minor part by cementum. Enamel contains by volume, 12% water, 85% mineral (carbonated hydroxyapatite), and 3% protein and lipid. Dentin is composed of an interwoven network of water, small mineral crystals, and collagen fibers. Intertubular dentin is ∼47% by volume mineral (carbonated hydroxyapatite), 33% protein (mostly collagen) and 20% water.³ The efficiency of laser action basically depends on wavelength absorption by the target tissue.⁷ The use of wavelengths highly absorbed by water and hydroxyapatite together is expected to generate thermal changes in enamel and dentin, which may be able to alter its structure chemically and/or morphologically.^5,7,8 Among the high power lasers that are used on dental hard tissues, erbium lasers are indicated because of the great absorption by water and hydroxyapatite, which are components highly presented in the tooth structure.³

Commercially, Er,Cr:YSGG (2.78 μm) and Er:YAG (2.94 μm) lasers are available for dental use. The effect of Er,Cr:YSGG (2.78 μm) and Er:YSGG (2.79 μm) irradiation slightly differs from that of Er:YAG laser (2.94 μm).³ Er,Cr:YSGG and Er:YSGG laser irradiation emission is coincident with the narrow apatite absorption band at λ=2.8 μm, whereas Er:YAG emission overlaps the broad water absorption centered at λ=2.94 μm.³ This may result in aslightly better absorption of the Er:YSGG laser in the hydroxyl ion (OH^-) on the enamel apatite, in comparison with the Er:YAG laser.⁴

Although laser-induced demineralization prevention has been intensively studied worldwide, throughout the past few decades, the clinical impacts and the mechanistic details remain largely unknown. The development of long-lasting demineralization prevention treatment continues to be a target for the field of dentistry, especiallyfor patients with poor dietary habits and oral hygiene, or with compromised motility.²⁰ Additionally, dental caries and dental erosion may be a problem among the elderly, especially considering the increasing prevalence of root caries^21,22and dental erosion.²³

The possibility of increasing the acid resistance of enamel after laser irradiation was first reported in 1965 with a ruby laser.^24,25 Laser irradiation has demonstrated the potential in inhibiting or reducing phosphate and calcium loss from enamel or dentin. Several studies have already described the effect of laser-induced prevention of demineralization (LIPD) with different wavelengths.^26

–31 For an effective laser treatment, optimal interactions between the laser light and tissue must occur. The laser interaction with dental hard tissues depends on irradiation parameters such as wavelength, pulsed or continuous emission, pulse duration, energy, frequency, spot size, delivery method, laser beam features, and optical properties of tissue such as the refractive index, scattering coefficient (μs), and absorption coefficient (μa).³² Considering its potential in LIPD, laser light must be strongly absorbed with the aim of altering the composition or solubility of dental hard tissues.¹⁹ As Erbium lasers are currently found in dental offices worldwide for multiple applications, the focus of this review is on Er:YAG (2.94 μm), Er:YSGG (2.79 μm), and Er,Cr:YSGG (2.78 μm) lasers, bringing into discussion the findings in the literature concerning their potential use of LIPD in enamel and dentin. Features such as laser parameters and reported outcome, as well as the mechanisms of laser action will be discussed. The goal of this review is to describe the state of the art concerning Erbium laser irradiation in LIPD, and also to bring important questions that should be further investigated. The association of fluoride with erbium laser irradiation will be addressed in a further revision.

Materials and Methods

Studies were searched up to July 15, 2014 in four electronic databases: PubMed, Embase, Scopus, and Cochrane Library, and they were complemented by hand searching. The main terms used were as follows: “Er:YAG laser and caries,” “Er:YAG laser and enamel demineralization,” “Er:YAG laser and dentin demineralization,” “Er:YAG laser and caries prevention,” “Er:YAG laser and dental erosion,” “Er,Cr:YSGG laser and caries prevention,” “Er,Cr:YSGG laser and enamel demineralization,” “Er,Cr:YSGG laser and dentin demineralization,” “Er,Cr:YSGG laser and caries prevention,” “Er,Cr:YSGG laser and dental erosion,” “Er:YSGG laser and caries prevention,” “Er:YSGG laser and dental erosion,” “Erbium laser and caries prevention,”and “Erbium laser and dental erosion.”

The included studies met the following criteria: they were original scientific studies that used Er:YAG (λ=2.94μm), Er:YSGG (λ=2.79μm), and/or Er,Cr:YSGG (λ=2.78μm) laser treatment in human or bovine teeth, on enamel and/or dentin surfaces in vitro, in situ, or in vivo. Studies were published in a refereed scientific journal, in English, with no restrictions regarding the year of publication. Studies reported in languages other than English, conference abstracts, case reports, and single-case studies were excluded. The studies' details were organized in four tables according to the groups: (1) in vitro/enamel (Table 1); (2) in situ/enamel (Table 2); (3) in vitro/dentin (Table 3); and (4) in vitro/enamel plus dentin (Table 4). As there were no studies developed in situ with dentin samples and in vivo studies, the present article does not address those groups.

Table 1.

Details of the Selected Studies Concerning Enamel: in Vitro Model

#	Author (year)	Sample	Λ (μm)	Irradiation parameters	Design (groups)
1	Dos Reis Derceli et al. (2015)⁵⁴	60 HS (n=12)	2.94	Beam diameter: NI/mode: prefocused/irradiation distance: 4 mm/average power: NI/power density: NI/frequency: 2 Hz/pulse width: 250 μs/duration of irradiation: 10 sec/energy per pulse 60 mJ/energy density: 3.92 J/cm²/water cooling: no	Fluoride(F)/Er:YAG laser/Er:YAG+F/F+Er:YAG laser/laser+F applied simultaneously
2	Zamudio-Ortega et al. (2014)⁷⁸	80 HS (n=10)	2.94	Beam diameter: 1.3; 1; 0.8 mm/mode: noncontact/irradiation distance: 1 mm/average power: NI/power density: 1.88 x 10⁴; 3.18 x 10⁴; 9.95 x 10⁴/frequency: 7 Hz/pulse width: 400 μs/duration of irradiation: 13 sec/energy per pulse 100–200 mJ/energy density: 7.5 – 12.7 – 39.8 J/cm²/water cooling: yes (5 mL/min)	Control (C)/Er:YAG (7.5 J/cm²)/Er:YAG (12.7/cm²)/Er:YAG (39.8/cm²)/fluoride (F)/Er:YAG (7.5J/cm²)+F/Er:YAG (12.7/cm²)+F/Er:YAG (39.8/cm²)+F
3	Mathew et al. (2013)¹⁰⁶	60 HS (n=10)	2.94	Beam diameter: NI/mode: noncontact/irradiation distance: 2.5 mm/average power: 1.4 W/power density: NI/frequency: 7 Hz/pulse width: NI/duration of irradiation: NI/energy per pulse: 200 mJ/energy density: NI/water cooling: no	Control (C)/APF/Er:YAG laser/CO₂/Er:YAG+APF/CO₂+APF
4	Zamataro et al. (2013)²⁰	272 BS (n=34)	2.78	Beam diameter: 750 μm/mode: noncontact/irradiation distance: 1 mm/average power: 0.75 W/frequency: 20 Hz/pulse width: 140 μs (total 64 pulses)/duration of irradiation: 7.5 mm/sec/energy per pulse 32.5 mJ/energy density: 8.5 J/cm²/water cooling: no	G1-untreated/G2-fluoridated dentifrice/G3-APF 4 min/G4-Er,Cr:YSGG/G5-Er,Cr:YSGG+dentifrice/G6-Er,Cr:YSGG+APF/G7-dentifrice+Er,Cr:YSGG/G8-APF+Er,Cr:YSGG
5	Geraldo-Martins et al. (2013)⁶²	40 HS (n=10)	2.78	Beam diameter: 600 μm/mode: focused mode/irradiation distance: 1 mm/average power: 0.25 or 05 W/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 20 sec/energy density per pulse: 4.48 or 8.94 J/cm²/pulse/energy density: 62.5; 125 J/cm²/water cooling: 5.0 mL/min water flow in G2 and G4	G1(0.25 W, 62.5 J/cm², no water cooling)/G2(0.25 W, 62.5 J/cm², 5.0 mL/min)/G3(0.5 W, 125 J/cm², no water cooling)/G4(0.5 W, 125 J/cm², 5.0 mL/min)
6	Liu et al. (2013)³⁸	12 HS (n=12)	2.94	Diameter focal spot: 0.5 mm/mode: noncontact: NI/irradiation distance: NI/average power: NI/frequency: 2 Hz/pulse width: 100 μs/duration of irradiation: 5 sec/energy per pulse: 40; 100 mJ/energy density: 2.0; 5.1 J/cm²/water cooling: no	Control/L1 (5.1 J/cm²−100 mJ)/L2 (2.0 J/cm²−40 mJ)
7	Liu et al. (2013)⁶⁴	10 HS	2.94	Diameter spot: 0.5 mm/mode: noncontact/irradiation distance: NI/average power: NI/frequency: 5 Hz/pulse width: 100 μs/duration of irradiation: 5 sec/energy per pulse: 100 mJ/energy density 5.1 J/cm²/water cooling: no	Control/fluoride/fluoride+Laser
8	Fekrazad et al. (2013)¹⁰⁷	60 HS (n=15)	2.78	Spot size or fiber: 600 μm/mode: NI/irradiation distance: 1–2 mm/average power: 0.25 W/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 10 sec/energy per pulse: 12.5 mJ/energy density: NI/water cooling: no	Control/laser/fluoride/laser+fluoride
9	Ahrari et al. (2012)⁵⁰	50 HS (n=25)	2.94	Spot size: 1 mm/mode: NI/irradiation distance: 1 mm/average power: NI/frequency: 10 Hz/pulse width: 180 μs/duration of irradiation: 10 sec/energy per pulse: 300 mJ/energy density: NI/water cooling: 5 mL/min	G1: 37% phosphoric acid gel/G2: laser irradiation
10	Anakari et al. (2012)¹⁰⁸	120HS (n=20)	2.78	Spot size: 600 μm diameter/mode: noncontact/irradiation distance: 1–2 mm/average power: 0.25 W (0.5 W in EL/F group)/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 10 sec/energy per pulse: 12.5 mJ/energy density: NI/water cooling: no	Control (C)/fluoride (F)/Er,Cr:YSGG laser (EL)/Er,Cr:YSGG laser+fluoride (EL/F)/CO₂ laser (CL)/CO₂ laser+fluoride (CL/F)
11	Ana et al. (2012)⁴⁰	264 HS (n=33)	2.78	Fiber optic diameter: 430 μm/mode: noncontact/irradiation distance: 1 mm/power output range: 0–6 W/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 4 mm/sec/energy per pulse: NI/energy density: 2.8; 5.6; 8.5 J/cm²/water cooling: no	G1: untreated/G2: APF/G3: laser (2.8 J/cm²)/G4: laser (5.6 J/cm²)/G5: laser (8.5 J/cm²)/G6: laser (2.8 J/cm²)+APF/G7: laser (5.6 J/cm²)+APF/G8: laser (8.5 J/cm²)+APF
12	Correa-Afonso et al. (2012)⁴⁵	45 HS oclusal (n=15)	2.94	Spot size or fiber: 0.63 mm/mode: noncontact/irradiation distance: 4 mm/average power: 0.16 W/frequency: 2 Hz/pulse width: 250–500 μs/duration of irradiation 30 sec/energy per pulse 80 mJ/energy density 1.26 J/cm²/water cooling: 2 mL/min	G1: Er:YAG/G2: Nd:YAG laser/G3: CO₂ laser
13	Lasmar et al. (2012)⁴⁹	60 HS (n=15)	2.94	Spot size or fiber: 0.63 mm/mode: noncontact/irradiation distance: NI/average power: NI/frequency: 4 Hz/pulse width: 250–400 μs/duration of irradiation: 40 sec/energy per pulse: 80 mJ/energy density: NI/water cooling: yes	Control/acid (37% phosphoric acid, 30 sec)/laser/laser+acid
14	Altinok et al. (2011)⁴⁴	50 HS (n=10)	2.94	Spot size or fiber: 3 00 μm/mode: noncontact/irradiation distance: NI/average power: NI/frequency: 10 Hz/pulse width: NI μs/duration of irradiation: NI/energy per pulse: NI/energy density 1.2 J/cm²/water cooling: yes	G1 (negative control), G2 (APF), G3 (APF+laser), G4 (laser+APF), G5 (laser)
15	Correa-Afonso et al. (2010)⁵³	84 HS (n=12)	2.94	Spot size or fiber: NI/mode: noncontact/irradiation distance: prefocused (4 and 8 mm) or unfocused (16 mm)/average power: NI/frequency: 2 Hz/pulse width: 250–500 μs/duration of irradiation: NI/energy per pulse: 80 mJ/energy density: NI/water cooling: 2 mL/min	G1: control/G2-G7: laser irradiation with different distances with or without cooling/G2: laser 4 mm–2 mL/G3: laser 4 mm–no cooling/G4: laser 8 mm–2 mL/G5: laser 8 mm–no cooling G6: 16 mm–2 mL/G7: 16 mm–no cooling
16	De Freitas et al. (2010)²⁸	45 HS (n=9)	2.78	Beam diameter: 750 μm/mode: noncontact, focused mode (1 mm from surface)/average power: 0.25–0.75 W/frequency: 20 Hz/pulse width: 140–200 μs/duration of irradiation: NI/energy per pulse: NI/energy density: 2.8–5.7 and 8.5 J/cm²/water cooling: no	G1(0.25 W, 20 Hz, 2.8 J/cm²)/G2(0.50 W, 20 Hz, 5.7 J/cm²)/G3(0.75 W, 20 Hz, 8.5 J/cm²)/G4-dentifrice/G5-control
17	Zezell et al. (2010)⁴¹	A:21 HS (n=7) B:240 (n=30)	2.78	Beam diameter sapphire with 750 μm diameter, 6 mm long/mode: noncontact, focused mode (1 mm from surface)/average power: 0.75 W/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 4 mm/sec/energy/pulse: 32 mJ/pulse/energy density: 8.5 J/cm²/water cooling: NI	A: (G1-unlased/G2-Er,Cr:YSGG laser G3-Nd:YAG laser) B: (G1-untreated/G2-fluoride/ G3-Nd:YAG laser/G4-Nd:YAG laser+fluoride/G5-fluoride+Nd:YAG laser/G6-Er,Cr:YSGG laser/G7-Er,Cr:YSGG laser+fluoride/G7-fluoride+Er,Cr:YSGG laser)
18	Rabelo et al. (2010)⁸³	Bovine powder	2.78	Beam diameter: 600 μm/mode: NI/average power: NI/frequency: 20 Hz/pulse width: 140–200 μs/duration of irradiation: NI/energy/pulse: 21.30; 30.96; 38.84 mJ/pulse/energy density: 7.53; 10.95; 13.74 J/cm²/water cooling: NI	G1(21.30 mJ; 7.53 J/cm²)/G2 (30.96 mJ; 10.95 J/cm²)/G3 (38.84 mJ; 13.74 J/cm²)
19	Perito et al. (2009)⁵⁵	72 HS	2.94	Beam: 0.63 mm spot size/mode: focused mode (distance of 12 mm)/average power: NI/frequency: 6 Hz/pulse width: NI/duration of irradiation: NI/energy per pulse: 300 mJ/pulse/energy density: 47 J/cm²/water cooling: 5 mL=min	Er:YAG laser+GI/Er:YAG laser+RM/Er:YAG laser+CR/diamond bur+GI/diamond bur+RM/diamond bur+CR
20	Bachmann et al. (2009)⁹⁷	3 HS	2.79	Beam diameter: 750 μm/mode: NI/average power: 0.25 W/frequency: 20 Hz Pulse width: 140 μs/duration of irradiation: NI/speed of irradiation: 4 mm/sec/energy/pulse: 12.5 mJ/pulse/energy density: 2.8 J/cm²/water cooling: NI	Control/Er:Cr,YSGG laser
21	Moslemi et al. (2009)¹⁰⁹	85 HS (n=17)	2.78	Beam diameter: 600 μm/mode: 1–2 mm distance/average power: 0.25 W/frequency: 20 Hz/pulse width: 140 ms/duration of irradiation: 10 sec/speed of irradiation: NI/energy per pulse: 12.5 mJ/pulse/energy density: NI/water cooling: no	G1: control/G2: Er,Cr:YSGG laser/G3: APF/G4: Er,Cr:YSGG+APF/G5: APF+Er,Cr:YSGG
22	De Freitas et al. (2008)²⁹	15 HS (n=5)	2.78	Beam diameter: 750 μm/mode: focused (1 mm distance)/average power: 0.25 W; 0.50 W; 0.75 W/frequency: 20 Hz/pulse width: 140–200 μs/duration of irradiation: 20 sec/speed of irradiation: NI/energy per pulse: NI/energy density: 2.8 J/cm², 5.68 J/cm² and 8.52 J/cm²/water cooling: no	G1 (0.25 W, 2.84 J/cm²)/G2 (0.50 W, 5.68 J/cm²)/G3 (0.75 W, 8.52 J/cm²)
23	Bevilacqua et al. (2008)³⁵	110 BS	2.94	Beam diameter: 1 mm/mode: contact and distance (2.6 and 3 mm)/average power: NI/frequency: 7 Hz/pulse width: 400 μs/duration of irradiation: NI/speed of irradiation: NI/energy per pulse: 100; 150; 200; 250 mJ/energy density: 0.9; 1.8; 2.08; 19.10; 25.47; 31.84 J/cm²/water cooling: no	Control/phosphoric acid (37%; 15 sec)/Er:YAG: (A) 250 mJ; 31.84 J/cm²; contact (B) 200 mJ; 25.47 J/cm²; contact (C) 150 mJ; 19.10 J/cm²; contact (D) 250 mJ; 2.08 J/cm²; 3 cm distance (E) 200 mJ; 1.8 J/cm²; 2.6 cm distance (F) 100 mJ; 0.9 J/cm²; 2.6 cm distance/fluoride applied after treatments
24	Castellan et al. (2007)²⁷	40 ES (n=10)	2.94	Fiber diameter: 320 μm/mode: focused/distance: NI/average power: NI/frequency: 2 Hz/pulse width: NI/duration of irradiation: NI/energy per pulse: 60 mJ/energy density: 40.3 J/cm²/water cooling: no (informed by author–personal communication)	G1: control/G2:fluoride/G3:Er:YAG laser/G4:Nd:YAG laser
25	Andrade et al. (2007)⁶⁰	13 HS (n=25)	2.94	Fiber diameter: NI/mode: 13 mm distance/average power: NI/frequency: 10 Hz/pulse width: NI/duration of irradiation: 3 sec/energy per pulse: 100, 200, 300, and 400 mJ/energy density: 21; 43; 64; 86 J/cm²/water cooling: yes–volume NI	G1: control/G2: Er:YAG irradiation: (area 1) 100 mJ; 10 Hz; 21 J/cm² (area 2) 200 mJ; 10 Hz; 43 J/cm² (area 3) 300 mJ; 10 Hz; 64 J/cm²/(area 4) 400 mJ; 10 Hz; 86 J/cm²/G3: the same parameters of G2 followed by artificial caries simulation
26	Ana et al. (2007)²⁶	81 HS	2.78	Fiber diameter: 750 μm/mode: 1 mm distance/average power: 0–6 W/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 30 sec/energy per pulse: NI/energy density: 2.8; 5.6; 8.5 J/cm²/water cooling: no Photosensitizer (PHS): triturated coal deionized water, ethanol	G1 (2.8 J/cm²)/G2 (PHS+2.8 J/cm²)/G3 (5.6 J/cm²/G4: PHS+5.6 J/cm²)/G5 (8.5 J/cm²)/G 6 (PHS+8.5 J/cm²)
27	Maung et al. (2007)⁸⁷	11 HS	2.94	Spot size diameter: 500 μm/mode: distance: NI/average power: NI/frequency: 5 Hz/pulse width: NI/duration of irradiation: 5 sec/energy per pulse: 50 mJ/energy density: 26.45 J/cm²/water cooling: 10 mL/min	Control group/laser group
28	Maung et al. (2007)⁸⁷	20 HS (n=10)	2.94	Spot size diameter: 500 μm/mode: 1 mm distance/average power: NI/frequency: 5Hz/pulse width: NI/duration of irradiation: 5 sec/energy per pulse: 50 mJ (FRAP) and 80 mJ (FTS)/energy density: 26.45 J/cm² (FRAP); 4.08 J/cm² (FTS)/water cooling: 10 mL/min	Control/laser treated/organic matrix extracted/Laser+organic matrix extracted
29	Liu and Hsu (2007)⁸⁴	6HS (n=12)	2.94	Spot size: 1 mm/mode: distance: NI/average power: NI/frequency: 2 Hz/pulse width: 100 ms/duration of irradiation: 5 sec/energy per pulse: NI/energy density: 5.1 J/cm²/water cooling: yes (10 mL/min)	Before laser irradiation/after laser irradiation
30	Liu et al. (2006)⁴²	21 HS (n=NI)	2.94	Spot size diameter: 1 mm/mode: distance: NI/average power: NI/frequency: 10 Hz/pulse width: NI/duration of irradiation: 16 sec/energy per pulse: 100, 200, and 300 mJ/energy density: 12; 25.5; 38.2 J/cm²/water cooling: no	GA: control/GB (100 mJ, 12.7 J/cm²)/GC (200 mJ, 25.5 J/cm²)/GD (300 mJ, 38.2 J/cm²)
31	Kim et al. (2006)⁵⁷	BS (n=47)	2.94	Spot size diameter: 1.2 mm/mode: noncontact mode/average power: NI/frequency: 2Hz/pulse width: NI/duration of irradiation: 1 pulse per spot/energy per pulse: 380 mJ energy density: 33 J/cm²/water cooling: yes (volume NI)	G1: control/G2: laser (33 J/cm²)/G3: phosphoric acid (33%)
32	Checcini et al. (2005)¹¹⁰	100 HS (n=10) (n=5) SEM	2.94	Spot size diameter: 0.63 and 0.32 mm/mode: contact/noncontact 12 mm distance/average power: NI/frequency: 2 Hz/pulse width: NI/duration of irradiation: NI/energy per pulse: 60; 64; 80; 86.4; 120; 135 mJ/energy density: 33.3; 44.4; 66.6; 20; 26.9; 42.2 J/cm²/water cooling: yes (5 mL/min)	G1: 60 mJ, 33.3 J/cm² (handpiece# 2051, noncontact)/G2: 80 mJ, 44.4 J/cm² (# 2051, noncontact)/G3: 120 mJ, 66.6 J/cm² (#2051, noncontact)/G4: 64 mJ, 20 J/cm² (#2055, contact)/G5: 86.4 mJ, 26.9 J/cm² (#2055, contact)/G6: 135 mJ, 42.2 J/cm² (#2055, contact)/G7: control
33	Kwon et al. (2005)⁹⁹	BS (n=10)	2.94	Spot size diameter: 1.2 mm mode: noncontact/average power: NI/frequency: 2 Hz/pulse width: NI/duration of irradiation: 0.5 sec (1 laser pulse/spot)/energy/pulse: 380 mJ/energy density: 33 J/cm²/water cooling: yes (volume NI)	G1: control/G2: Er:YAG laser/G3: Er:YAG laser+fluoride/G4: Er:YAG laser+fluoride+CO₂ laser
34	Ying et al. (2004)⁸⁶	HE powder (n=5)	2.94	Spot size diameter: 2 mm/mode: NI/average power: NI/frequency: 4 Hz/5 pulses/spot laser pulses applied/pulse width: NI/duration of irradiation: NI/energy per pulse: NI/energy density: 6.25 J/cm²/water cooling: NI	Laser/laser+NaClO
35	Delbem et al. (2003)⁹¹	120 HS (n=30)	2.94	Spot size diameter: 0.63 mm/mode: defocused (40 mm distance)/average power: NI/frequency: 1 Hz/pulse width: 250–500 μs/duration of irradiation: 10 sec/energy per pulse: 60 mJ/energy density: 0.95 J/cm²/water cooling: no	Control group/Er:YAG laser/APF/Er:YAG laser+APF
36	Apel et al. (2003)⁵⁸	10 HS (n=5)	2.94 2.79	Spot size diameter: 0.7 and 0.9 mm sapphire tip/mode: contact/quasicontact/average power: NI/frequency: 20 Hz/pulse width: NI/duration of irradiation: NI/energy per pulse: 305; 260 mJ/energy density: 41; 104 J/cm²/water cooling: 33 and 74 mL/min	Control #1/Er:YAG(260 mJ; 41 J/cm²; 74 mL per min water cooling)/control #2/ Er,Cr:YSGG (305 mJ; 104 J/cm²33 mL per min water cooling)
37	Apel et al. (2002)⁴	294 BS (n=21)	2.94 2.79	Spot size diameter: 610 μm–0.7 mm sapphire tip; mode: NI/average power: NI; frequency: 5Hz (125 pulses/sample); pulse width: 150 μs/duration of irradiation: 25 sec/energy per pulse: NI/energy density: 4; 6; 8 J/cm²/water cooling: NI	G1-CG (control)/G2(Er:YAG 4 J/cm²)/G3 (Er:YAG 6 J/cm²)/G4 (Er:YAG 8 J/cm²)/G5 (Er:YSGG 4J/cm²) G6-Er:YSGG 6 J/cm²)/ G7(Er:YSGG 8 J/cm²)/G8 (CG+F)/G9 (F+Er:YAG 4 J/cm²)/G10 (F+Er:YAG 6 J/cm²)/G11 (F+Er:YAG 8 J/cm²)/G12 (F+Er:YSGG 4 J/cm²)/G13 (F+Er:YSGG 6 J/cm²)/ G14 (F+Er:YSGG 8 J/cm²)
38	Apel et al. (2000)³⁹	30 BS (n=10)	2.78	Sapphire tip diameter: 0.7 mm/mode: NI/average power: 0.21; 0.5 W/frequency: 20 Hz/pulse width: NI/duration of irradiation: NI/energy per pulse: NI/energy density: 2.7; 6.5 J/cm²/water cooling: NI	Control/laser1 (2.7 J/cm²)/laser2 (6.5 J/cm²)
39	Young et al. (2000)⁵⁶	30 HS (n=10)	2.94 2.79	Spot diameter: 516 μm, 734 μm/mode: NI/average power: NI/frequency: 2 Hz/pulse width: 200 μs/duration of irradiation: 15 pulses/spot/energy/pulse: 135; 454 mJ/energy density: 64; 105 J/cm²/water cooling: yes (volume NI)	Er:YAG (135 mJ; 64 J/cm²)/Er:YSGG (454 mJ; 105 J/cm²)/CO₂ laser/carbide bur (control)
40	Fried et al. (1996)⁵	NI (n=10)	2.94 2.79	Spot diameter: NI/mode: NI/average power: NI/frequency: 5 Hz/pulse width: 150 μs/duration of irradiation: 25 pulses/energy per pulse: NI/energy density: 1–50 J/cm²/water cooling: NI	Er:YAG/Er:YSGG
41	Morioka et al. (1991)³³	NI	2.94	Spot diameter: NI/mode: defocused at 3 mm diameter/average power: NI/frequency: 1; 2 and 10 Hz/pulse width: 0.2 ms/duration of irradiation: NI/energy per pulse: 0.39–0.92 J/energy density: NI/water cooling: NI	Laser (0.39 J/pulse; 10 Hz; 10 shots-black ink, white ink, nil ink)/laser (0.92 J/pulse1;2 Hz; 10 shots-black ink, white ink, nil ink)/laser (0.92 J/pulse; 1 shot-black ink, white ink, nil ink)

NI, not informed; HS, human samples; BS, bovine samples; F, fluoride; SDF, silver diamine fluoride; APF. acidulated phosphate fluoride; GI, glass-ionomer cement; RM, resin modified glass-ionomer; CR, composite resin.

Table 2.

Details of the Selected Studies Concerned Enamel: in Situ Model

#	Author (year)	Sample	Λ (μm)	Irradiation conditions described	Design (groups)
1	Scatolin et al. (2014)⁴⁶	56 BS (n=28)	2.94	Spot diameter: 0.9 mm/mode: noncontact/defocused/irradiation distance: 25 mm/average power: 0.15 W/frequency: 2 Hz/pulse width: NI/duration of irradiation 10 sec/energy/pulse: 85 mJ/energy density: 5.2 J/cm²/water cooling: 3 mL/min	Unirradiated+eroded in intraoral phase Unirradiated+uneroded in intraoral phase Irradiated+eroded in intraoral phase Irradiated+uneroded in intraoral phase Sound area–initial erosion area Sound area–treatment area Sound area–final erosion area
2	Colucci et al. (2015)⁵²	100 BS (n=10)	2.94	Spot size or fiber: NI/mode: noncontact/focused/irradiation distance: 12 mm/average power: NI/frequency: 2, 4, 6 Hz/pulse width: NI/duration of irradiation NI/energy/pulse: 300 mJ/energy density: NI/water cooling: 2, 5, and 8 mL/min	High speed handpiece (positive control); Er:YAG laser groups: 2 Hz, 2.0 mL/min; 2 Hz, 5.0 mL/min; 2 Hz, 8.0 mL/min; 4 Hz, 8.0 mL/min; 6 Hz, 2.0 mL/min; 4 Hz, 5.0 mL/min; 4 Hz, 2.0 mL/min; 6 Hz, 5.0 mL/min; 6 Hz, 8.0 mL/min
3	Correa-Afonso et al. (2013)⁷⁵	52 HS (with pits/fissures)	2.94	Spot size or fiber: 0.63 mm/mode: noncontact/defocused/irradiation distance: 4 mm/average power: 0.16W/frequency: 2Hz/pulse width: 250–500 μs/duration of irradiation 30 sec/energy/pulse: 80 mJ/energy density: 1.26 J/cm²/water cooling: 5 mL/min	G1: control/G2: Er:YAG/G3:Nd:YAG/G4:CO₂
4	Chimello et al. (2008)¹⁰⁴	91 HS (n=13)	2.94	Beam focal area of 0.4 mm²/mode: focused; 12 mm distance/average power: NI/frequency: 2, 3, and 4 Hz/pulse width: NI/duration of irradiation: 20 sec/energy/pulse: 250; 350 mJ/energy density: 62.5; 87.5 J/cm²/water cooling: 1.5 mL/min	GI−250 mJ, 62.5 J/cm², 2 Hz/GII−250 mJ, 62.5 J/cm², 3 Hz/GIII–250 mJ, 62.5 J/cm², 4 Hz/GIV– 350 mJ, 87.5 J/cm², 2 Hz/GV–350 mJ, 87.5 J/cm², 3Hz/GVI–350 mJ, 87.5 J/cm², 4 Hz /GVII-high-speed handpiece
5	Chimello et al. (2008)¹⁰⁵	84 HS (n=12)	2.94	Beam focal area of 0.4 mm²/mode: focused; 12 mm distance/average power: NI/frequency: 2, 3, and 4 Hz/pulse width: NI/duration of irradiation: 20 sec/energy/pulse: 250; 350 mJ/energy density: 62.5; 87.5 J/cm²/water cooling: 1.5 mL/min	GI−250 mJ, 62.5 J/cm², 2 Hz/GII−250 mJ, 62.5 J/cm², 3Hz/GIII–250 mJ, 62.5 J/cm², 4 Hz/GIV–350 mJ, 87.5 J/cm², 2 Hz/GV–350 mJ, 87.5 J/cm², 3 Hz/GVI–350 mJ, 87.5 J/cm², 4 Hz/GVII-high-speed handpiece
6	Apel et al. (2005)³⁴	12 HS (n=6)	2.94 2.79	Spot size diameter: 610 μm 0.7 mm sapphire tip/mode: contact/average power: NI/frequency: 5 Hz/pulse width: 200–250 μs/duration of irradiation: 25 sec/energy per pulse: NI/energy density: 6 and 8 J/cm²/water cooling: no	Control 1/Er:YAG: 6 J/cm²/control 2/Er:YSGG: 8 J/cm²
7	Apel et al. (2004)¹⁰³	18 HS (n=9)	2.94 2.79	Spot size diameter: 610 μm–0.7 mm sapphire tip/mode: contact/average power: NI Frequency: 5 Hz–total of 125 laser pulses applied/pulse width: 150 μs/duration of irradiation: 25 sec/energy per pulse: NI/energy density: 6 and 8 J/cm²/water cooling: No	Control 1/Er:YAG: 6 J/cm²/control 2/Er:YSGG: 8 J/cm²

NI, not informed; HS, human samples; BS, bovine samples.

Table 3.

Details of the Selected Published Studies Concerned with Dentin: in Vitro Model

	Author (year)	Sample	Λ(μm)	Irradiation conditions described	Design (groups)
1	Mei et al. (2014)⁹⁰	24 HS (n=24)	2.94	Spot size or fiber: NI/mode: NI/irradiation distance: 1 s/mm²/average power: 2 W/frequency: NI/pulse width: 100 μs/duration of irradiation: NI/energy per pulse: 100 mJ/energy density: NI/water cooling: NI	SL–fluoride+Er:YAG/S-fluoride/L-Er:YAG/W-negative control
2	Botta et al. (2011)⁷⁰	8 BS (n=4)	2.78	Spot size or fiber: fiberoptic system with a sapphire terminal of 750 mm diameter/mode: noncontact/irradiation distance: 1 mm/average power: 0.25 W/frequency: 20 Hz/pulse width: 140 μs/duration of irradiation: 4 mm/sec/energy per pulse: 12.5 mJ/energy density: 2.8 J/cm²/cooling: cooling: NI	C–unlased group/L–laser group
3	Manesh et al. (2009)⁷²	45 HS (n=15)	2.94	Beam diameter: 400 μm, focal length of 75 mm/average power: NI/frequency: 5 Hz/pulse width: 35μs/duration of irradiation: 1 min/energy per pulse: 10 mJ/pulse/energy density: 8 J/cm²/scan rate: 0.25 mm/sec/water cooling: yes (volume NI)	Er:YAG-Nd:YAG-CO₂: lesion, lesion plus laser, sound, sound plus laser, fluoride, and fluoride plus laser
4	Manesh et al. (2008)⁷¹	HS (n=11)	2.94	Beam diameter: 400 μm/average power: NI/frequency: 5 Hz/pulse width: 35 μs/duration of irradiation: NI/energy per pulse: NI/energy density: 8 J/cm²/scan rate: 0.25 mm/sec/water cooling: yes (volume NI)	Er:YAG lesion, lesion plus laser, sound, sound plus laser, fluoride, and fluoride plus laser

NI, not informed; HS, human samples; BS, bovine samples.

Table 4.

Details of the Selected Published Studies Concerned with Enamel and Dentin: in Vitro Model

#	Author (year)	Sample	Λ (μm)	Irradiation conditions described	Design (groups)
1	Ceballos et al. (2001)⁵¹	10 HS (n=5)	2.94	Spot size diameter: NI/mode: noncontact/unfocused (15 mm distance)/average power: NI/frequency: 2 Hz/pulse width: 250 μs/duration of irradiation: NI/energy per pulse: 300 mJ (E); 250 mJ (D)/energy density: NI/water cooling: yes (volume NI)	Enamel (E): -35% phosphoric acid (15 sec)/-Er:YAG laser etching Dentin (D): -35% phosphoric acid (15 sec)/-Er:YAG laser etching
2	Hossain et al. (2001)⁶¹	30 HS (n=NI)	2.78	Spot size: 0.442 mm² with the use of a 750 μm diameter fiber/mode: noncontact: 2 cm distance/average power: 5; 6 W/frequency: 20 Hz/pulse width: 140–200 μs/duration of irradiation: 12 sec/energy/pulse: NI/energy density: 56.6 J/cm²/water cooling: 32% water cooling and no water cooling	Enamel control/enamel laser (67.9 J/cm²; 6W)/enamel laser+water cooling/dentin control/dentin laser (56.6 J/cm²; 5W)/dentin laser+water cooling
3	Hossain et al. (2000)⁶	20 HS (n=40)	2.94	Spot size diameter: 0.63 mm/mode: no contact: 2 cm distance/average power: NI/frequency: 2 Hz/pulse width: NI/duration of irradiation: 9 sec/energy/pulse: 400 mJ/energy density: NI/water cooling: depend on the group	Enamel control/enamel laser/enamel laser+water cooling/dentin control/dentin laser/dentin laser+water cooling

NI, not informed; HS, human samples; BS, bovine samples.

Results

A total of 55 original research publications were found. Among them, 7 were developed in situ, and 48 were developed in vitro. Forty-eight publications exclusively analyzed the enamel, four exclusively analyzed dentin, and three analyzed both enamel and dentin. Sixteen studies applied the Er,Cr:YSGG laser (λ=2.78 μm) and/or the Er:YSGG laser (λ=2.79 μm) for irradiation, 33 applied the Er:YAG laser (λ=2.94 μm), and 6 studies applied both wavelengths. Regarding erosion prevention, three studies tested the Er:YAG laser. The details of each publication are described in Tables 1 –4.

Er:YAG laser on the prevention of tooth demineralization

In enamel, Er:YAG laser irradiation showed more positive results in caries prevention than negative results. From the 36 studies, 24 showed positive results and 13 showed negative results. In contrast, when considering only the in situ studies, six studies described negative results and one described positive results. In dentin, five studies tested the Er:YAG laser on the prevention of demineralization, and three of them showed positive results.

Er,Cr:YSGG and Er:YSGG lasers on the prevention of demineralization

In enamel, Er,Cr:YSGG and Er:YSGG laser irradiation showed more positive than negative results. From a total of 17 studies, 12 showed positive results and 5 showed negative results. Two publications testing the effect of Er:YSGG laser on caries prevention using an in situ model were found, but both failed to reach statistical significance. In dentin, two studies tested Er,Cr:YSGG in demineralization prevention, and both showed positive results.

For discussion, study analysis was divided according to the following topics: (1) influence of laser parameters on outcomes, (2) influence of tissue factors on outcomes, (3) mechanisms of action, (4) clinical studies, and (5) final considerations.

Discussion

In 1991, Morioka and Tagomori³³ investigated the effect of Er:YAG laser on the acid resistance of tooth enamel using different laser parameters such as energy/pulse (0.39 and 0.92 J) and frequency (1, 2, and 10 Hz). Results showed positive results in LIPD³³. According to the study, the higher energy per pulse and frequency tended to confer higher acid resistance. The publication did not cite details of the laser parameters, such as beam diameter, average power, duration of irradiation, energy density, and the use of water cooling during irradiation. However, this first study brought forth new concepts for further investigations. Subsequently, in 1996, Fried et al.⁵ described important information about both Er:YAG and Er:YSGG lasers, concerning the ideal temperature that led to LIPD without thermal damage. The results suggested that the high temperature excursions may not be necessary for effective LIPD.⁵ Authors showed that irradiation with λ=2.94 μm and λ=2.79 μm produces a more resistant surface to acid dissolution caused by thermally induced chemical and structural changes in the intrinsic mineral. After these first reports, several studies analyzed the effect of Erbium lasers in LIPD, revealing different hypotheses and methodologies.

Influence of laser parameters on outcomes (energy threshold, repetition rate, pulse width, water/air irrigation, temperature rise)

Energy threshold, repetition rate, focus mode, and pulse width for enamel demineralization prevention (in vitro studies)

The optimal energy range for LIPD using Erbium laser wavelengths has been a controversial point. Some authors described that subablative energy densities are required,³⁴ but other authors showed that acid resistance was greater when higher fluences were applied on the surface.³⁵ The ablation threshold regarding the applied fluences of Erbium lasers was determined by several authors, but without consensus among them (Table 5). The ablation threshold is linked to the absorption depth and heat diffusion into the tissue during the laser pulse.³⁶ However, the influence of the pulse duration on ablation threshold is especially important for clinical procedures. In addition to leading to less thermal transfer to the surrounding tissue, shorter pulse durations also decrease the threshold for ablation.³⁷ In this article, the considered threshold for ablative fluences was based on the studies of Fried et al. (1996),⁵ Apel et al.,⁴ Seka et al.,³⁶ and da Ana et al. (2007),²⁶ as described in Table 5.

Table 5.

Ablation Threshold Described in Literature Regarding Erbium Lasers

Wavelength	Author (year)	Ablation threshold
Er:YAG (λ=2.94 μm)	Li et al. (1992)¹¹¹	7.2–18.6 J/cm²
	Fried et al. (1996)⁵	7–9 J/cm²
	Apel et al. (2002)⁴	9–11 J/cm²
	Seka et al. (1996)³⁶	≤7 J/cm²
Er:YSGG (λ=2.79 μm)	Fried et al. (1996)⁵	<18 J/cm²
	Apel et al. (2002)⁴	10–14 J/cm²
	Seka et al. (1996)³⁶	≤18 J/cm²
Er,Cr:YSGG (λ=2.78 μm)	Ana et al. (2007)²⁶	>2.8 J/cm²

Concerning Er,Cr:YSGG laser, subablative fluences and frequencies of 2 J/cm²–2 Hz,³⁸ 5.1 J/cm²–2 Hz,³⁸ 6.5 J/cm²–20 Hz,³⁹ 8.5 J/cm²–20 Hz^28,40,41 lead to significant increases in LIPD. Concerning the Er:YAG laser, subablative fluence of 12 J/cm² (10 Hz, 100 mJ) resulted in a significant increase in LIPD.⁴² A positive correlation between an increase in laser-induced enamel acid resistance and temperature rise was cited.^8,38,43 In LIPD caused by erosion challenge, Er:YAG laser irradiation (1.2 J/cm², 10 Hz) could significantly prevent demineralization.⁴⁴ However, the same energy parameter adopted in another study (1.2 J/cm²)⁴⁵ could not verify any significant LIPD effect. Despite the differences in methodology applied between both studies, one difference that could interfere with the results could be the frequency variation found between both studies (2 and 10 Hz). Most likely with 1.2 J/cm² and 2 Hz,⁴⁵ a sufficient temperature rise to increase acid resistance was not reached. A recent study tested for the first time the effect of Er:YAG laser irradiation in controlling the progression of enamel erosion (5.2 J/cm², 2 Hz), but its results showed that the Er:YAG laser with the applied parameters was not able to reduce the in situ progression of erosive lesions in enamel.⁴⁶

According to the literature, the repetition rate needs to be sufficiently slow to allow for cooling between pulses, but high enough to allow for clinically useful irradiation time.⁴⁷ The number of pulses also must be sufficient to provide the desired effect, to affect the immediate subsurface positively, but to minimize the total energy delivered to avoid overheating the pulp.⁴⁷ For CO₂ lasers, the suggestion of 10 Hz as a repetition rate that fulfills both the aforementioned conditions can be found in the literature.⁴⁷ For erbium lasers, further studies need to be conducted in order to reach consensus regarding the repetition rate that fulfills these conditions. Additionally, the choice of pulsed width allows for short bursts of energy with beneficial temperature excursions and enough time between pulses for the enamel to cool sufficiently.⁴⁷ The ideal pulse width will be similar to the thermal relaxation time of the tissue being treated.⁴⁷ The thermal relaxation time of a 10 μm enamel layer was calculated to be ∼60 μs.⁴⁸ According to Featherstone et al.,⁴⁷ based on this calculation for enamel, a pulse width in the range of 50–100 μs should be effective.

Some studies applied Er:YAG irradiation with energy parameters varying from 80 to 300 mJ, and evaluated any effect in LIPD.^49
–51 The parameters of 200⁵²–300 mJ⁵¹and 2 Hz showed the best results in demineralization. However, 300 mJ and 10Hz failed to stimulate LIPD.⁵⁰ Considering that both studies used the same energy of 300 mJ/pulse,^50,51 the difference in frequency (2 and 10 Hz) could have influenced the results. Curiously, the study from Liu et al.⁴² also failed to obtain any preventive effect in LIPD with the parameters of 300 mJ and 10 Hz. Among the variations in the studies, the pulses per second seem to be an important parameter to be considered in LIPD.

Most studies employed the Er:YAG laser in the focused mode, obtaining maximum energy density. The knowledge of the Erbium laser's effects in the unfocused mode and the ideal irradiation distance are important factors to be considered in LIPD. Considering that the Er:YAG laser focus is obtained with 12 mm distance from the target tissue, one study considered the irradiation distance of 4 and 8 mm in a prefocused mode, and 16 mm in a defocused mode.⁵³ Results showed greater LIPD in enamel at a 4 mm irradiation distance with water cooling, and no morphological changes were detected.⁵³ The group of 4 mm irradiation distance, without water cooling, did not result in LIPD because of overheating, exceeding the ideal temperature range.⁵³ In addition to the results described, another study applied irradiation with the same prefocused irradiation mode (4 mm distance), without water irrigation, with 60 mJ and 2Hz, and showed no beneficial results.⁵⁴ At 8 and 16 mm distances, tissue loss areas were observed.⁵³ These groups presented the same distance from the focus point, thus producing similar results on the irradiated enamel.⁵³ A study that applied Er:YAG in an unfocused mode at 25 mm irradiation distance showed no beneficial results of laser irradiation in controlling the progression of enamel erosion.⁴⁶ The irradiation distance (mode of focus) seems to be an interesting variable that should be further investigated.

Concerning the ablative parameters, studies showed that heat is spread to the cavity margins.⁵⁵ The heat accumulation would be enough to thermally modify the enamel during cavity preparation and improve its acid resistance.⁵⁵ Some studies tested whether Er:YAG laser cavity preparation was able to reduce the susceptibility of the prepared enamel to demineralization, achieving a potential for a secondary LIPD effect.^55

–58 The studies applied the following fluences: 64, 33, 47, and 41 J/cm²; the following energy/pulses: 135, 380, 300, and 260 mJ; and the following frequencies: 2, 2, 6, 20 Hz.^55

–58 The three first sets of parameters showed positive results in LIPD.^55
–57 The parameters of 41 J/cm², 260 mJ, and 20 Hz did not result in LIPD.⁵⁸ The main difference found in comparing the studies was the repetition rate.^55

–58 In this case, the higher frequency (20 Hz) seemed to interfere in the results found.⁵⁸ Other studies tested a wide range of Er:YAG ablation parameters and their influence on the prevention of enamel demineralization.^42,59,60 The parameters of 20 J/cm² (64 mJ), 33 J/cm² (60 mJ), and 44.4 J/cm² (80 mJ) with a frequency of 2 Hz presented decreased demineralization.⁵⁹ Another study showed a positive result with Er:YAG used at 100 mJ, 10 Hz, and 21 J/cm².⁶⁰ Moreover, one investigation showed that with the Er:YAG laser (10 Hz), irradiation up to 200 mJ (25.5 J/cm²) showed significant results in the prevention of demineralization.⁴² The application of 300 mJ (38.2 J/cm²) failed to show positive results.⁴² Concerning Er,Cr:YSGG, increased acid resistance was obtained with 67.9 J/cm² (20 Hz),⁶¹ 62.5 J/cm² (20 Hz).⁶² The fluence of 67.9 J/cm² was shown to increase acid resistance through molten enamel.⁶¹ Higher fluences, such as 105 J/cm² (2 Hz), were tested in pit and fissure irradiation and 72% caries inhibition was achieved.⁵⁶ On the contrary, other authors showed no advantages in terms of resistance to secondary caries by applying an Er:YSGG laser at 20Hz, 305 mJ, 104 J/cm².⁵⁸ However, laser parameters applied in pits and fissures should be analyzed separately, because anatomical particularities in these cases can modify the photothermal effect and results will differ from a smooth surface.

The mechanisms of LIPD after cavity preparation may be caused by the absorption of heat and penetration into the nonablated enamel layers at the cavity wall.³⁵ It is known that the effects of laser irradiation on tissue will decrease from the center to the periphery of the laser pulse.⁶³ Therefore, it is speculated that residual heat caused by the laser during the ablation process is restricted to the superficial layers of the cavity walls, which can be a limiting factor for obtaining high acid resistance at some extended distances from the restoration margins.⁵⁵

Influence of water cooling during irradiation on acid resistance outcomes

The knowledge of the use of water cooling during Erbium laser irradiation is an important factor to be considered in LIPD. For Erbium lasers, the water plays an important role in the laser's interaction with dental hard tissues, increasing this interaction and, consequently, increasing the ablative process.³⁶ For LIPD, some studies suggested performing the irradiation with low energy densities and without water cooling to avoid ablation of the target tissue.^20,38,64 Some studies showed that the presence of water in large amounts during irradiation would increase the chance of ablation as well as the porosity of the dental surface, which would facilitate the diffusion of acids into the enamel structure, increasing the depth of demineralization.^65,66 However, satisfactory results have been achieved with or without the use of cooling, although its role in the mechanism of LIPD is yet to be clearly established.⁶ Among the studies that showed the benefits of the use of Er:YAG laser irradiation on LIPD, some used water cooling and others did not (Table 6).

Table 6.

Studies that Investigated the Use of Water Cooling During Erbium Laser Irradiation

Author (year)	Parameter (best parameters in studies with positive outcomes) (frequency/energy per pulse/energy density)	Prevention	Water cooling	Volume (mL/min)
Young et al. (2000)⁵⁶	2 Hz/135 mJ; 64 J/cm²	Yes	Yes	NI
Ceballos et al. (2001)⁵¹	2 Hz/300 mJ (E); 250 mJ (D)/energy density: NI	Yes	Yes	NI
Apel et al. (2003)⁵⁸	20 Hz/(260 mJ; 41 J/cm²) (305 mJ; 104 J/cm²	No	Yes	75 and 33
Delbem et al. (2003)⁹¹	1 Hz/60 mJ/0.95 J/cm²	Yes	No	-
Kwon et al. (2005)⁹⁹	2 Hz/380 mJ/33 J/cm²	Yes	Yes	NI
Cecchini et al. (2005)⁵⁹	2 Hz/(60 mJ, 33.3 J/cm²) (80 mJ, 44.4 J/cm²) (64 mJ, 20 J/cm²)	Yes	Yes	5
Kim et al. (2006)⁵⁷	2 Hz/380 mJ/33 J/cm²	Yes	Yes	NI
Liu et al. (2006)⁴²	10 Hz/(100 mJ, 12.7 J/cm²) (200 mJ, 25.5 J/cm²)	Yes	No	-
Castellan et al. (2007)²⁷	2 Hz/60 mJ/40.3 J/cm²	Yes	No	-
Maung et al. (2007)⁸⁷	5 Hz/50 mJ/26.45 J/cm²	Yes	Yes	10
Maung et al. (2007)⁸⁷	5 Hz/50 and 80 mJ, 26.45 and 4.08 J/cm²	Yes	Yes	10
Andrade et al. (2007)⁶⁰	10 Hz/(100 mJ, 21 J/cm²)	Yes	Yes	NI
Bevilacqua et al. (2008)³⁵	7 Hz/250 mJ; 31.84 J/cm²; contact 200 mJ; 25.47 J/cm²; contact 150 mJ; 19.10 J/cm²; contact 250 mJ; 2.08 J/cm²; 3 cm distance 200 mJ; 1.8 J/cm²; 2.6 cm distance 100 mJ; 0.9 J/cm²; 2.6 cm distance	Yes	No	-
Perito et al. (2009)⁵⁵	6 Hz/300 mJ/47 J/cm²	Yes	Yes	5
Lasmar et al. (2012)⁴⁹	4 Hz/80 mJ/energy density: NI	Yes	No	NI
Liu et al. (2013a)³⁸	2 Hz/(100 mJ; 5.1 J/cm²) (2.0 J/cm²−40 mJ)	Yes	No	-
Liu et al. (2013b)⁶⁴	5 Hz (100 mJ; 5.1 J/cm²)	Yes	No	-
Correa-Afonso et al. (2010)⁵³	2 Hz/80 mJ/energy density: NI	Yes	Yes	2
Correa-Afonso et al. (2012)⁴⁵	2 Hz/80 mJ/1.26 J/cm²	No	Yes	2
Ahrari et al. (2012)⁵⁰	10 Hz/300 mJ/energy density: NI	No	Yes	5
Zamudio-Ortega et al. (2014)⁷⁸	7 Hz/100–200 mJ/7.5J/cm², 12.7/cm², 39.8/cm²	No	Yes	5
Colucci et al. (2015)⁵²	2 Hz/300 mJ	Yes	Yes	2
Mathew et al. (2014)¹⁰⁶	7 Hz/200 mJ/1.4 W	No	No	-
Dos Reis Dercelli et al. (2015)⁵⁴	2 Hz/60 mJ/3.92 J/cm²	No	No	-
Scatolin et al. (2014)⁴⁶	2 Hz/85 mJ/5.2 J/cm²	No	Yes	3

NI, not Informed; D, dentin; E, enamel.

Few publications included the use and absence of water cooling as study groups. At the moment, there is no consensus regarding the importance of water cooling during laser irradiation with the aim of achieving LIPD. It was shown that the absence of water cooling did not achieve positive results because of overheating, exceeding the ideal temperature range to inhibit the enamel demineralization. When water cooling was considered, there was a significant LIPD with parameters of 1.26 J/cm², 80 mJ, and 2 Hz.⁵³ On the other hand, other studies also obtained positive results when higher energies without water cooling were applied. Enamel and dentin treated with the Er:YAG laser (400 mJ, 2 Hz) revealed a statistically significant preventive effect in calcium loss, which was greater without water mist (67%) than with water mist (15%).⁶ In another study, the reduced water flow (2 mL/min) showed better results in LIPD in comparison with a 5 or 8 mL/min water flow rate when the Er:YAG laser at 300 mJ was applied.⁵² According to the authors, when a higher water flow rate was applied, an increased water film was formed and the energy achieved at the tooth surface was decreased.⁵²

Er,Cr:YSGG-lased enamel and dentin treated with 67.9 and 56.6 J/cm², respectively, with or without a water mist, exhibited significantly reduced calcium loss. However, the lowest calcium loss was recorded for the tooth samples irradiated without water mist.⁶ In another study, the Er,Cr:YSGG laser was also able to increase the enamel acid resistance in 23% compared with the control surface at 62.5 J/cm² without air/water cooling.⁶² It was concluded by that the presence of water, even in minimal amounts, was not favorable for LIPD, in all groups tested.⁶²

The effectiveness and safety of erbium lasers in LIPD and tissue ablation is directly related to the adequate setting of the working parameters.⁶⁷ Depending on the energy applied, some authors recommend cooling with a fine water spray to be indispensable for clinical application, in order to avoid damage caused by overheating as well as the irritation of the dental pulp.⁶⁸ Other authors suggest that although water cooling seems to be important to avoid the increase of temperature,⁶⁵ the continued exposure of enamel to water during irradiation can absorb part of the energy delivered from the laser and lead to enamel ablation.⁶⁵ In the above-cited studies,^6,56,61,62 despite the positive results in LIPD using high fluences without water cooling, additional studies should be performed in order to analyze the increased temperature of pulp chamber during irradiation and also the possible morphological damage that can occur. It should also be noted that the type of surface – occlusal or smooth – may also interfere in the choice of using or not using water cooling during laser irradiation, as well as in the temperature increase during irradiation,^6,53,61 and this difference should be investigated in further studies. Based on the current review of the literature (Table 6), high and low fluences with and without water cooling can lead to positive results in LIPD. Therefore, it is not possible to come to conclusions as to whether the use of water cooling during Erbium laser irradiation is needed or not to achieve the optimal prevention of demineralization. However, it is clear that water cooling will reduce the surface temperature and potentially enhance ablation during the laser treatment.

Temperature evaluation during laser irradiation

Temperature increase is an important condition to be considered during laser irradiation. The enamel surface temperature during Er,Cr:YSGG was measured using a thermographic camera.⁶⁹ Authors detected temperature rises of 79.6°C when samples were irradiated with 2.8 J/cm², 184.1°C when they were irradiated with 5.6 J/cm², and 247.6°C when they were irradiated with 8.5 J/cm².⁶⁹ However, authors described that the temperatures were most likely higher considering the morphological changes promoted at the enamel surfaces.⁶⁹ The increase of the pulp chamber temperature was evaluated during Er,Cr:YSGG laser irradiation with subablative parameters without water cooling (energy density up to 8.5 J/cm²) in two studies using thermocouples.^29,69 In the first study, the temperature was recorded at a sampling rate of 20 Hz and in the second, the temperature was captured every 2 sec. The results of both studies indicated that intrapulpal temperature increased below the threshold for pulp damages. However, taking into account that the pulse width of Er,Cr:YSGG laser is 140 μs, the 900 Hz recording rate of the thermographic camera,⁶⁹ the 20 Hz recording rate of thermocouples,⁶⁹ and the 2 sec interval in temperature capture²⁹ seemed to be unable to detect the highest temperature peaks during laser irradiation.⁶⁹ Fried et al.⁵made measurements with a time resolution of 1 μs by an elliptical mirror and a HgCdZnTe detector, and found ∼400°C using 8.5 J/cm², different from the 247.6°C found by Ana et al., which applied a 900-Hz recording rate of the thermographic camera.⁶⁹Although the risks of the pulp overheating with Er,Cr:YSGG parameters applied in the cited studies seems to be low, a more accurate system is required to precisely determine the maximum temperature peaks with proper temporal resolution. With respect to the Er:YAG laser, there is no report of pulp temperature analysis with the parameters used for the prevention of tooth demineralization.

Additionally, there are some variations in enamel/dentin thicknesses and the presence of carious tissue that may influence the increase of the intrapulpal temperature during laser irradiation. Carious tissue presents a great amount of water; therefore, the heat transfer to the pulp may be more excessive in decayed teeth.⁶⁹ Therefore, future studies should investigate the effect of these conditions regarding intrapulpal temperature increases during laser irradiation before the clinical application can be achievable.

Influence of tissue factors on outcomes

The outcomes of LIPD and the best parameters to be applied may vary according to different substrates (enamel or dentin), surface morphologies (smooth surfaces or occlusal surfaces), and structure (deciduous or permanent tooth). LIPD related to these specific variations is described in the following sections.

LIPD in dentin

As root caries may be a problem among the elderly,^21,22 the discovery of a long-lasting preventive method would be significant. Only seven publications tested the prevention of demineralization with Erbium lasers on dentin. Considering the heterogeneity in dentin composition in comparison with enamel, the energy absorption and distribution during laser irradiation should behave differently. The root dentin irradiation with the lowest energy density obtainable with a commercial Er,Cr:YSGG laser equipment (20 Hz, 12.5 mJ, 2.8 J/cm²) revealed evidence of ablated and rough surfaces as correlated with each laser pulse incidence, which demonstrated the strong interaction of Er,Cr:YSGG lasers with dentin.⁷⁰ When higher energies were applied, Er,Cr:YSGG laser irradiation (56.6 J/cm², 20 Hz) appeared to be as effective in increasing dentin acid resistance,⁶¹ as a lower frequency (2Hz, 400 mJ).⁶ Regarding the Er:YAG laser, negative^71,72 and positive results⁵¹ were described concerning improvements in dentin acid resistance. Most of the studies showed positive results for Erbium lasers applied on dentin with the aim of inhibiting the demineralization process. Further studies are necessary to verify temperature increases in surrounding tissues and morphological alterations, especially when higher energies are applied, as well as the best parameters for this use.

LIPD in pits and fissures

The narrow morphology of pit and fissure sites provides a perfect niche for pathogenic carious organisms to proliferate, and because this niche is not easily cleansed by traditional methods, these become highly cariogenic sites.⁵⁶ On the occlusal surface, some risk factors, such as the tooth morphology, difficulties in maintaining proper hygiene, the eruption period, and the patient's age need to be taken into account.⁷³ Pit and fissure sealants have been developed to overcome this problem. However, studies have demonstrated that complete or partial sealant loss is common, which can result in secondary caries.⁷⁴ Therefore, the need for new strategies and preventive measures for caries on occlusal surfaces has been observed. The irradiation of pits and fissures with Erbium lasers and the analysis of acid resistance were analyzed by two in vitro and one in situ study.^45,56,75 The first in vitro study applied ablation parameters on occlusal surfaces using Er:YAG (2 Hz, 135 mJ, 64 J/cm²) and Er:YSGG (2 Hz, 454 mJ, 105 J/cm²) with water cooling,⁵⁶ and the other studies performed in vitro and in situ studies and applied lower energy parameters (2 Hz, 80 mJ, 1.26 J/cm²) with water cooling.^45,75 Only the first study cited that applied higher energy densities showed positive results, consisting of a 50% inhibition of caries progression for the Er:YAG laser (64 J/cm²) and 72% caries inhibition for the Er:YSGG laser (105 J/cm²) when compared with the control group.⁵⁶ According to the authors, as a beneficial side effect of peripheral heat deposition during the laser procedure, the walls of the prepared cavity may acquire an increased resistance to decay.⁵⁶ In contrast, the other studies reported that the results of Er:YAG laser in occlusal fissure caries prevention were similar to those achieved in the control portion of the substrate.^45,75 One hypothesis given by the authors for these findings could be related to the use of water cooling during irradiation. The irregularity of the occlusal surface could promote water buildup at the bottom of the fissures, and the necessary heating temperature was not reached.^45,53 However, the first study that applied higher energy densities (64–105 J/cm²)⁵⁶ also used water cooling during irradiation. Most likely, when water cooling is applied during irradiation, higher energies should be used in order to achieve the necessary heating, as indicated in the previous paragraph.

LIPD in deciduous teeth

Two studies tested the Erbium laser in deciduous enamel to achieve LIPD. The first study used Er:YAG at 2 Hz, 60 mJ, and 40.3 J/cm² without water cooling.²⁷ Lower mineral loss (35.7%) compared with the unirradiated samples was found in the Er:YAG laser group. The authors concluded that Er:YAG presents a potential to inhibit the demineralization process of deciduous enamel subjected to an acid challenge, comparable with the use of acidulated phosphate fluoride. Considering that carbonate was found to be significantly higher in deciduous teeth (2.23%) than in permanent enamel (2.15%),⁷⁶ it would, therefore, be expected to see greater LIPD in primary teeth than in permanent teeth, because more carbonate enamel may have been removed.⁷⁷ The second study applied 2 Hz, 100–200 mJ, and a threshold of energy density from 7.5 to 395 J/cm².⁷⁸ The authors described mild to severe morphological damage in the irradiated enamel, according to the energy applied, and considered conditions employed that were not recommended for deciduous LIPD.⁷⁸ Other studies should be developed in order to enhance the knowledge of potential LIPD in deciduous teeth.

Mechanisms of Erbium lasers in caries prevention

Decreased carbonate, crystalline water, organic matrix decomposition

Among the main mechanisms leading to a less-soluble enamel structure are the decreased carbonate and crystalline water, the decomposition of organic matrix, and increased structural OH.^8,79 The level of carbonate will influence the enamel susceptibility to demineralization, because it fits less well in the lattice and, therefore, gives rise to a less stable and more acid-soluble apatite phase.⁸⁰ This substance could be considered to be an impurity that may lead to high solubility and less resistance to demineralization. Laser irradiation reduces the degree of dissolution when the carbonate is removed.⁷ It has been suggested that the compositional changes of enamel heated in the range of 100°–200°C include the reduction of water content by 12–27%,⁷⁹ and a decrease in carbonate ion (CO₃ ^2-) content by ∼13–23%.^64,81 Studies reported a substantial loss of carbonate and water at temperatures between 100°C and 400°C, which were sufficient to change the crystallinity of the enamel.^8,79 Other studies, however, indicated that the decomposition of carbonate began from 400°C onward.⁸²

Significant loss of adsorbed water and structural OH⁻ was observed at temperatures >400°C in oven heating, and 600°C for Er,Cr:YSGG laser irradiation (13.74 J/cm²).⁸³ It was suggested that enamel should be irradiated with Er,Cr:YSGG at energy densities >10.55 J/cm² in order to obtain the possible desired effects for caries prevention. A significant laser-induced reduction of carbonate and the modification of organic matrix content was obtained after Er:YAG laser irradiation with a fluence of 5.1 J/cm² and a temperature rise might be in the range of 200–400°C on the enamel surfaces with approximately one third carbonate removed.⁸⁴ Other authors revealed a decrease of water in enamel, carbonate, and organic contents after Er,Cr:YSGG irradiation at 8.5 J/cm².⁴¹

Concerning organic matrix, the hypothesis proposed that the partial decomposition of the organic matrix that occupies inter- and intraprismatic spaces of the enamel during irradiation leads to a blockage of these spaces. Consequently, acid influx and ion diffusion out of enamel may be compromised.⁸⁵ The organic matter that caused a statistically significant decrease in the pore volume in the enamel after laser irradiation may be one of the key players in the laser-induced blocking of the diffusion pathway and the subsequent prevention of enamel demineralization.⁸⁶ The contribution of organic matrix in laser-treated enamel has been demonstrated to be at least 25% in inhibiting mineral loss and 57% in lesion progression.⁸⁵ Other studies showed that the organic matrix contributions to the laser-induced reduction of lesion depth and mineral loss were 55% and 25%, respectively.⁸⁷ The temperature range of 60°–200°C was associated with the denaturation of enamel organic matrix.^88,89 Increase in temperature may result in the removal of the enamel organic matrix and high-energy laser treatment may potentially burn away the organic matrix >400°C.⁷⁹ It was suggested that the organic blocking effect may reach a maximum and decrease after the decomposition of organic matrix >400°C.⁸⁵ It was cited that the diffusion-inhibiting effect of a laser treatment on enamel demineralization might be preserved by the use of lower energy laser therapy,⁶⁴ and that high temperature laser treatment may not be necessary for effective LIPD.⁵

On dentin, laser irradiation might have led to increased mechanical properties because of water and organic component vaporization together with higher calcium and phosphate weight percentages.⁹⁰

Physical alteration

Studies showed that the enamel surface does not necessarily need to be morphologically changed to reduce tooth solubility; it is possible that the chemical alteration is more important than the changes in surface topography.^91,92 However, other studies showed that enamel's acid resistance is related to morphological changes,^61,93,94and even low energies can promote small morphological alterations that may contribute to acid resistance. Laser irradiation can promote the formation of microspaces on the enamel surface,^57,59 and the size of these spaces will vary depending on laser energy.⁵⁹ Some authors suggest that these microspaces can act as an area of ion precipitation, contributing to remineralization of the irradiated enamel. On the other hand, some authors suggest that these spaces can also act as open channels, facilitating the acid attack of the subsurface, thus enhancing the mineral loss.^34,57 Concerning Erbium laser irradiation, it seems that even low energies will lead to morphological changes of enamel and dentin. Controversial results were found in the current literature concerning the beneficial effects of these alterations after Erbium laser irradiation. Furthermore, detrimental effects may arise if a proper set of laser parameters is not employed.

Exposed enamel prisms, a rough surface, and different crater sizes were described as a result of the high energy irradiation.⁵⁹ However, in the study, there was no evidence of a denaturing or disruption of the enamel structure resulting from the increase in surface temperature during irradiation with fluences of: 20, 33.3, and 44.4 J/cm².⁵⁹ The exposition of the hydroxyapatite rods and, as a consequence, increasing the roughness of enamel were also reported with Er,Cr:YSGG laser irradiation (8.5 J/cm²).²⁰ It was reported that subablative Er:YAG and Er:YSGG irradiation produced fine cracks in the enamel surface, and the depth of the cracks could be observed over 100 μm depth.³⁴ According to the authors, these cracks acted as starting points for acid attack and favored deep demineralization; therefore, these changes would reduce or eliminate the positive effect of LIPD.³⁴ Additionally, the clod-like breakup of the enamel surface was attributable to the dehydration of the dental enamel.³⁴ One cause could possibly be that dehydration and carbonate loss leads to the contraction in the α-axis of the apatite crystal, which results in a reduction in volume.⁹⁵

Another possible explanation described by the authors is that the free water present in the dental enamel undergoes expansion as a result of heating, thus causing the enamel structure to give way.³⁴ Both theories are supported by the observation that the cracks run along the line of “weaker” structures: the prism boundaries. The authors ruled out the possibility of thermal stress cracks.³⁴ Er,Cr:YSGG laser irradiation at 2.8, 5.6, and 8.5 J/cm² led to a slight degree of ablation, with the removal of the outer surface of enamel and the exposition of enamel rods.⁶⁹ Er:YAG laser irradiation at 21, 43, 64, and 86 J/cm²created craters with rough surfaces that became more evident as the energy per pulse was increased, and no thermal changes or melting signs were visualized.⁶⁰ Scanning electron microscopy analysis revealed molten enamel and dentin after Er,Cr:YSGG laser irradiation at 56.6 J/cm² with or without water mist.⁶¹ However, after acid demineralization, the molten enamel or dentin was almost unchanged.⁶¹ Therefore, it could be considered that there might be little effect of acid dissolution on the molten enamel or dentin and that they might play a major role in acid resistance. The same irregular patterns after Er:YAG laser irradiation were described on dentin,⁹⁰ Also, after Er,Cr:YSGG laser irradiation at 2.8 J/cm², evidence of ablation, and, consequently, rough surfaces in dentin were observed, which correlated with each laser pulse incidence.⁷⁰

The morphological alterations found in the literature after Erbium laser irradiation were consequences of the higher absorption of Erbium lasers by water and hydroxyapatite,³⁶ promoting a microablation process even when low energy densities were used. Some studies agreed that this morphological alteration might contribute to caries prevention, and others reported that these alterations might be harmful to the enamel/dentin tissue. Therefore, there is still no consensus among the existing publications. Additionally, future investigations should also verify whether this alteration in morphology, especially an increase of surface roughness, could contribute to bacterial adhesion to the enamel.

Improvement of crystal planes and formation of new phases

The increase in the enamel acid resistance after laser irradiation is attributed to photothermal effects that occur when the temperature in the enamel surface rises from 100° to 650°C and promotes chemical alterations in the dental hard tissue as described. At 650°–1100°C, the main changes are thermal recrystallization and crystal size growth, and pyrophosphate reacts with apatite to form PO₄ along with the formation of the beta phase of tricalcium-phosphate (β-TCP) (β-Ca₃[PO₄]²).⁹⁶ At temperatures >1100°C, the main change is that the β-TCP is converted to the alpha phase (α-TCP) (α-Ca₃[PO₄]2) and tetracalcium phosphate (Ca₄[PO₄]2O) (TetCP), and when the temperature reaches 1430°C, this compound changes into a high temperature polymorph. The detection of new crystallographic phases after irradiation is considered by some authors to be one possible explanation for the results described in the literature for caries resistance.^60,97 In an aqueous solution, TCP is unstable and is more soluble than hydroxyapatite under acidic conditions.⁹⁸ Two studies described an improvement in crystallinity in some crystal planes after Er:YAG irradiation (33 J/cm²), but whole crystal planes did not show improvement in crystallinity, and the formation of a new phase was not found.^57,99 Enamel irradiation with Er,Cr:YSGG (8.5 J/cm²) maintained a major phase corresponding to the hydroxyapatite (Ca₅[PO₄]₃OH); however, the formation of a single additional phase was observed, which was considered to be TetCP (Ca₄[PO₄]2O).^41,97 It was demonstrated that enamel irradiation with Er,Cr:YSGG (2.8 J/cm²) promoted the formation of a minor phase of TetCP embedded in a hydroxyapatite matrix.⁹⁷ The appearance of the mentioned new phases and the induced changes in the chemical composition of Er,Cr:YSGG irradiated enamel is expected to improve the overall caries resistance, as suggested in others' investigations.^4,19,34 In dentin, the presence of morphological changes was found under atomic force microscopy analysis after Er,Cr:YSGG laser irradiation (2.8 J/cm²), described by the authors as “nodule formation.”⁷⁰ These “nodules”could signify a morphological change in hydroxyapatite size,¹⁰⁰ or the formation of new crystallographic phases, which corroborates the study of Bachmann et al.⁹⁷ It is important to note that the formation of TCP and TetCP on laser-irradiated dentin reported in the literature^101,102 is speculated to be a result of temperature increase, which can occur with Er,Cr:YSGG laser irradiation on dentin, even at the low energy density of 2.8 J/cm².⁷⁰

Clinical studies

Six studies that applied an in situ model failed to show any significant preventive effect of Erbium laser in demineralization prevention, and one study showed a positive result. The lower energy density of 8 J/cm² for the Er:YSGG laser and 6 J/cm² for the Er:YAG¹⁰³were tested in situ, and despite missing statistical significance, the results of the study revealed a tendency for acid solubility to be reduced after subablative Erbium laser irradiation.¹⁰³ The irradiation of pits and fissures with an Er:YAG laser (2 Hz, 80 mJ) submitted to an in situ model, revealed no significant difference in demineralization prevention.⁷⁵ Er:YAG laser irradiation (2 Hz, 80 mJ) also failed to control the progression of enamel erosion in situ. ⁴⁶ Higher energy densities (62.5 and 87.5 J/cm²)^104,105 were also tested in the in situ model and showed that Er:YAG laser presented a similar performance of the high-speed handpiece with regard to the demineralized area, the inhibition zone width, the crack occurrence, and the demineralization score in enamel adjacent to composite resin restorations. The score analysis of the inhibition zone could suggest a lower degree of demineralization at the restoration margin of the irradiated samples.¹⁰⁴ However, according to the authors, this result seemed to be inconclusive, and further studies on this aspect would be required. Recently, following this line of thought, Er:YAG laser irradiation employed in cavity preparations in situ significantly influenced the acid resistance of the irradiated substrate, and Er:YAG laser irradiation was capable of controlling the development of the caries lesion around composite resin restorations.⁵² The authors applied 300 mJ at different frequencies (2, 4, and 6 Hz) and water flow rates (2, 5, and 8 mL/min). Despite the fact that all groups prepared with the Er:YAG laser demonstrated microhardness values higher that those prepared with a high speed handpiece, the Er:YAG laser at 2 Hz–2 mL/min water flow rate showed the highest microhardness values, followed by 2 Hz–5 mL/min and 2 Hz–8 mL/min. The Er:YAG irradiation at 4 and 6 Hz and water flow rates of 2, 5, or 8 mL/min showed microhardness values lower than those of the groups cited, and they were statistically similar.

Final considerations

Several studies have investigated the use of Erbium lasers in the prevention of demineralization, including a large range of irradiation conditions. However, there are still discrepancies in the literature concerning the potential of Erbium laser irradiation that leads to acid resistance on enamel and dentin. It is difficult to compare the numerous publications because of the heterogeneous parameters and conditions tested, as well as the differences in sample characteristics, such as hardness, thickness, and hydration, which can also lead to different results when the sample interacts with lasers or even with a drill. Higher energies applied have been shown to cause severe morphological alterations in enamel and dentin. Lower fluences lead to slighter morphological alterations, which seem to be important in the mechanism of demineralization prevention, especially in fluoride retention, which will be discussed in the second part of the review. The major unsolved issues regarding the Erbium laser in LIPD are as follows.

1. Optimal set of parameters including pulse width, frequency, fluences, and those mentioned previously.

2. Thermal impact on the pulp tissue

3. Morphological alterations and possible side effects that these alterations could provoke

4. Influence of water irrigation during irradiation in LIPD

5. Differences in outcomes regarding deciduous and permanent teeth

6. Mechanistic details related to LIPD

7. Sustainability of LIPD

It is important to note that there are many unanswered questions that should be further investigated concerning Erbium lasers in the prevention of demineralization caused by caries or erosion. As new equipment with a range of laser parameters is commercially available, future studies may be able to provide pertinent information paving the way for the clinical application of LIPD.

Conclusions

Although there is much controversial information in the literature concerning the potential of the Erbium laser in the prevention of enamel and dentin demineralization, a large portion of the publications showed positive results, as well as the investigation of chemical/morphological alterations that could result in positive outcomes. Therefore, there are indications of the beneficial effect; however, there is no consensus in the literature on several critical issues. Future investigations are necessary to fulfill the information gaps concerning Erbium lasers in the prevention of demineralization. At the present moment, it is not possible to formulate a specific protocol for the prevention of enamel and dentin demineralization using Erbium lasers.

Footnotes

Acknowledgments

This work was supported by São Paulo Research Foundation (Fundação de Amparo a Pesquisa do Estado de São Paulo -FAPESP) and Post-Doctoral Fellowship # 2013/12317-9.

Author Disclosure Statement

No competing financial interests exist.

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