The Er:YAG laser uses a solid-state active medium of yttrium and aluminum garnet doped with Erbium ions (Er³⁺). The Er:YAG laser operates via an optical pumping system characterized by a luminous flash corresponding to an absorption band of the Erbium ion (Er³⁺). It has a wavelength of 2940 nm corresponding to the absorption peak of water but also of hydroxyapatite. This results in a very good absorption by enamel, dentin and soft tissues. The Er:YAG laser is a very shallow penetrating laser (a few microns), which avoids heating of the peripheral tissues. It has photo-ablative and photo-acoustic effects [6] .

The parameters of the Er:YAG laser are:

Operating mode: Pulsed.

Pulse duration: 50 to 200 μs.

Pulse frequency: 15 to 20 Hz.

Energy per pulse: 20 to 1500 mJ.

Average power: 0.3 to 20 W.

The Er, Cr:YSGG laser has as active medium a crystal of yttrium scandium gallium garnet doped with Erbium ions (Er³⁺). The pumping system is also obtained by a light flash corresponding to an absorption band of the Erbium ion. As for its wavelength, it is 2780nm. The Er,Cr:YSGG and Er:YAG lasers have similar characteristics, they both have a high affinity for hydroxyapatite crystals and water. These two lasers have similar properties, however the Er,Cr:YSGG laser has a penetration of about 15 microns which is 3 times more than for the Er:YAG laser. This difference in penetration is explained by the fact that the wavelength of the Er,Cr:YSGG laser is slightly less absorbed by water at 2780 nm than the wavelength of the Er:YAG laser which operates in the water absorption peak; the difference in absorption coefficients leads to a difference in the penetration depths of the two Erbium laser wavelengths in the dental tissues [21] .

1) Infiltration studies

Several studies have shown that cavity preparation with diamond bur followed by acid treatment of the enamel surface and application of an all-in-one adhesive system had the lowest infiltration values compared to laser prepared surfaces [1] [12] [23] [24] [25] . However, other studies have shown that there was no statistically significant difference in infiltration values between these two preparation methods [26] [27] [28] [29] [30] .

2) Mechanical tests

Shadman et al. [19] found that the shear strength of dentin prepared with a rotary bur was higher than if it was prepared with a 4 W and 5 W laser. It is the changes in the hydroxyapatite with areas of carbonization, amides and proteins as well as decomposition and disproportion of minerals that causes more microcracks. The denatured matrix proteins thus prevent proper penetration of the adhesive into the collagen matrix, preventing the formation of a proper hybrid layer at the tooth/restorative interface thus decreasing the shear strength [20] [31] [32] [33] . A decrease in bond strength has been shown on surfaces prepared with the Er:YAG laser (2 Hz, 300 mJ for enamel preparation, 250 mJ for dentin preparation) following the formation of a laser-modified layer [20] .

However, with the Er,Cr:YSGG laser “6 W, 15 Hz, 80% water spray and 50% air spray” the shear strength is significantly higher than with bur preparations. This could be due to the irregularities and interlocking of the laser-irradiated hard tissue, which increases the contact surface and improves the bonding of the resin to the prepared tooth surface. In addition, laser irradiation creates a surface free of dentin sludge, with open dentin tubules that allow infiltration of the adhesive resin into the tubules to form bonds between the dentin surfaces and the resin, making the surface more favorable for adhesion [10] [20] .

Other studies have shown no significant difference in bond strength between the two preparation techniques (laser/diamond bur) when a 35% phosphoric acid etch is used on the dentin surface [12] [34] [35] .

3) Evaluation of the thickness of the hybrid layer

According to Kallis et al. [17] , cavity preparation with the Er,YAG laser “3.75 W, 15 Hz, 250 mJ, water spray 20 ml/min, QSP mode” produces a rough dentin surface, not demineralized but with open dentin tubules, which can improve micromechanical retention. In contrast, in bur-prepared dentin, the surface is smoother, and the dentinal tubules are covered with dentinal sludge that prevents resin infiltration. These morphological characteristics may explain the greater thickness of the hybrid layers formed in laser-treated surfaces.

However, other studies using a “126 mj and 180 mj” Er:YAG laser have reported low laser efficacy for the preparation of dental surfaces [36] [37] . These contradictions can be attributed to various parameters such as different laser irradiation parameters, dental substrates, experimental design, methodology, etc.

The universal adhesive containing MDP improves the bonding quality, and this is due to the ability of 10-MDPmonomer to create a chemical bond with hydroxyapatite resulting in the formation of a durable nanolayer and consequently an increase in mechanical strength [10] . The universal adhesive in M&R mode significantly increases the adhesion values to laser-irradiated dentin compared to the universal adhesive used in self-etch mode [38] [39] . In fact, the shear strength of laser-prepared dentin increases when surface treatment with M&R adhesive is performed [19] .

Surface treatment with Er,Cr:YSGG laser irradiation prior to bonding with a self-etching adhesive system (Clearfil SE Bond) significantly increases the bond to eroded dentin [3] . Laser irradiation removes the eroded dentin layer, revealing a dent in surface that is more favorable for adhesion, without adversely affecting the dentin substrate [33] [40] [41] .

The use of the M&R adhesive after Er:YAG laser preparation allows for a greater hybrid layer thickness than those treated with the self-etching adhesive [17] . However, the laser-prepared surfaces showed an increase in calcium and phosphate ions and a reduction in carbonate and water after thermal effects and crystallographic changes [18] .

The carboxyl groups in the self-etching adhesive can chemically bind to hydroxyapatite and calcium and thus form stable calcium salts that enhance resin adhesion through the formation of strong ionic interactions between the substrate and the adhesive layer [42] .

Comparison between self-etching systems

Cavities prepared with the Er:YAG laser at 490 mJ and 15 Hz showed that the tensile strength with a HEMA-containing adhesive was better than that with an adhesive without HEMA [11] . This is because the hydrophilic nature of HEMA promotes adhesion, improves wetting of the dentin and thus the bond strength. The better bond strength obtained with the HEMA-rich and ethanol-water-based self-etch adhesiveskept exposed collagen moist and does not collapse as much as air-dried dentin [43] .

In addition, moist dentin may provide a more porous collagen network, allowing greater infiltration of adhesive monomers than on surfaces that are air-dried and in which the collagen collapses.

According to Chen M.L et al. [13] , pretreatment with phosphoric acid or low-frequency Er:YAG laser irradiation “150 mJ; 10 Hz; short-pulse mode (SP, 300 μs); average power of 1.5 W; 19.10 J/cm² energy delivered/pulse;10 ml/min water spray” significantly improved the bond strength between self-etching adhesives and laser-prepared dentin “P = 4 W, Pulse duration = 100 μs, E = 200 mJ, F = 20 Hz, Energy density: 25.46 J/cm²”.

However, in the study by Ceballo et al. [37] , laser preparation of dental surfaces combined with 35% phosphoric acid etching resulted in decreased shear strength values for a two-step total-etch adhesive. Their results also demonstrated that 35% phosphoric acid can only remove the surface of the laser-modified dentin layer, leaving partially denatured collagen fibrils that likely interfere with adhesive resin infiltration.

Preconditioning with phosphoric acid or phosphoric acid followed by sodium hypochlorite increases the bonding strength of the composite resin to the enamel and dentin prepared with an Er,Cr:YSGG laser [15] . This is due to the ability of phosphoric acid and/or sodium hypochlorite to remove the denatured dentin layer produced by the laser irradiation resulting in the opening of the dentinal tubules which subsequently facilitates the infiltration of the adhesive resin. However, the application of sodium hypochlorite after the acid etches dissolves not only the collagen in the heat-denatured dentin, but also the collagen in the healthy dentin directly underneath the denatured layer. Excessive pretreatment of the laser-prepared dentin could induce embrittlement of the hybrid layer at the adhesive interface and thus decrease the bond strength values [15] .

According to Jhingan et al. [20] , pre-etching of Er,Cr:YSGG laser-prepared tooth surfaces with phosphoric acid has a negative effect on the shear strength of self-etching adhesives.

This can be explained by the increased loss of calcium from the collagen network on the conditioned surface due to the strongly acidic action of phosphoric acid (pH = 0.5 to 1), resulting in a decrease in chemical bonds between the tooth surface and the self-etching adhesive.

In contrast to these results, Ergücü et al. [44] showed that surface treatment with an acid primer or phosphoric acid after laser preparation of the dentin significantly increased the adhesion values by removing the laser irradiation modified dentin layer that appeared to be resistant to acid attack [45] .