1.

Introduction

Dental erosion has been a major concern of dentists, as its prevalence has increased significantly in the population, even among children and youths.^1–3

The etiology of tooth erosion is multifactorial, and after diagnosis, measures must be taken to prevent its progression or, regarding severe lesions with great surface loss, to restore the teeth’s function or aesthetics, and/or treat dentin hypersensitivity.⁴ When tooth restoration becomes necessary, a direct restorative technique with composite resin is considered a useful conservative procedure.^4,5

However, the eroded dentin (ED) is a challenging substrate to bond to.^5,6 The dissolution of hydroxyapatite crystals by erosion causes the exposure of long collagen fibrils and the increase of the inner diameter of dentinal tubules.⁷ This fibril network is very resistant to mechanical impact, such as brushing,⁸ and can compromise the permeability of adhesive systems and hybrid layer formation. Consequently, to reach a greater bond strength, it is necessary to prepare the tooth surface before the use of adhesive systems^5,9 in order to selectively remove the outer demineralized dentin.

Techniques to treat the ED surface prior to bonding procedures have been investigated,^5,9 and high-power lasers have been recently reported for that purpose.¹⁰ As the wavelength of the Er:YAG laser ($\lambda =2.94\mu \mathrm{m}$) coincides with the absorption band of water and hydroxyapatite, specific parameters can conservatively remove dental hard tissue (ablation process) and promote the selective removal of carious dentin, microbial reduction, and pretreatment (prior to adhesive procedures) of the tooth surface.^11–14

Most studies on the use of Er:YAG lasers for tooth preparation have used equipment with a pulse width in the range from 100 to $350\mu \mathrm{s}$. However, there is already commercially available equipment with shorter pulse widths that are reported to cause less or no thermal damage to the tooth surface.^15–17 Although there are many studies on the use of Er:YAG laser on dentin prior to restorative procedures,^{12,13,16,18–22} little is known about its effect on ED.¹⁰ To date, no studies about the effects of short pulse Er:YAG laser on the bond strength of ED to composite resin have been conducted.

Given the information above, this in vitro study aimed to evaluate the effect of the dentin surface’s (sound and eroded) irradiation with a short pulse width Er:YAG laser on the adhesion to composite resin.

2.

Materials and Methods

3.

Results

3.1.

Dentin Surface Morphology

Morphological analysis of the specimens showed different characteristics according to the surface treatment.

The SD appeared homogeneous, with grooves caused by the abrasion of the silicon carbide polishing paper and a smear layer covering the whole surface with the obliteration of the dentinal tubules [Figs. 1(a) and 1(b)]. The sound dentin submitted to the laser irradiation (SID) revealed a microretentive pattern with an irregular surface, absence of smear layer, open dentinal tubules, and prominent peritubular dentin [Figs. 1(c) and 1(d)].

Fig. 1

Electromicrographs obtained by SEM: morphological characteristics of the dentin surfaces after different surface treatments: (a) and (b) SD, (c) and (d) SID, (e) and (f) ED, and (g) and (h) EID.

The ED samples showed a homogeneous porous surface with large and open dentinal tubules, demineralized peritubular dentin, and a rough intertubular dentin, showing the exposed network of collagen fibers [Figs. 1(e) and 1(f)]. Samples of ED that were treated with the Er:YAG laser (EID) revealed a regular flat surface with obliterated tubules, highlighting a collapse aspect of the collagen fibers [Figs. 1(g) and 1(h)].

3.2.

Microshear Bond Strength

The microshear bond strength values found for the SD samples, regardless of being irradiated ($\mathrm{SID}\text{-}12.77\pm 5.09\mathrm{A}$) or not ($\mathrm{SD}\text{-}9.76\pm 3.39\mathrm{B}$), were higher than the bond strength values of the eroded samples ($\mathrm{ED}\text{-}5.12\pm 1.72\mathrm{D}$; $\mathrm{EID}\text{-}7.62\pm 3.39\mathrm{C}$). Also, the irradiated samples resulted in higher bond strength values than their respective untreated controls (nonirradiated) as described in Fig. 2.

Fig. 2

Microshear bond strength values obtained for all experimental groups.

In all groups, the failure mode was predominantly adhesive (between adhesive and dentine), followed by mixed failure and cohesive failure as depicted in Fig. 3.

Fig. 3

Prevalence (in percentage) of failure mode for each experimental group.

4.

Discussion

Without control of the etiologic factors, the loss of dental mineralized structure by erosion can result in dentin exposure.²⁸ In such cases, restorative treatments are indicated to reduce the thermal sensitivity, to prevent injuries to the pulp, and to rehabilitate the contour, function, and aesthetics of the affected elements.²⁹ In order to understand the behavior of restorative materials in different dentinal substrates, some authors seek to simulate dental erosion in in vitro protocols, although laboratorial studies present limitations to accurately replicate the in vivo conditions.³⁰ In in vitro studies, however, isolated conditions in different tooth substrates exposed to erosive challenges can be investigated under destructive/nondestructive methods.

In the case of dentin exposed to erosive challenges, the acids may remove the plugs that seal the dentinal tubules and demineralize both inter- and peritubular dentin, exposing its organic matrix and increasing the diameter of the tubules.^31,32 Regarding this aspect, the erosive cycling considered for the present study succeeded in reproducing the characteristics of eroded dentin as seen in SEM images, with open tubules, without perituburar dentin, and having intertubular dentin with a decalcified and fibrous appearance.^7,33

As adhesive materials must adhere to different surfaces other than those for which they were originally created and tested, studies on ED are mandatory, but adhesion to this substrate has been poorly investigated. Studies have so far found lower bond strength to this substrate compared to SD,^6,9,10,34,35 corroborating our findings.

The high level of demineralization on ED submitted to etching results in a softened surface layer that, after the penetration of the adhesive, leads to the formation of a thin hybrid layer when compared to an etched SD. These layers are structurally imperfect and contain pores that generate predominantly hydrophilic areas and demineralized zones without resin reinforcement.³⁶ These factors can contribute to the low bond strength values found for ED, because resinous monomers’ penetration into the demineralized substrate is compromised.^34,36,37

The irradiated ED showed higher bond strength values than ED, even though their bond strength values were lower than those found for SD (irradiated or not). These findings are consistent with the morphological analysis of the ED submitted to irradiation. The Er:YAG laser irradiation of the ED resulted in a homogeneous demineralized surface with occluded tubules and collapsed collagen fibrils. These characteristics are possibly a result of the residual heat generated during the ablation process, which could have negatively influenced the penetration of the adhesive and the formation of the hybrid layer.³⁸ Therefore, treating the surface with the laser parameters tested in the present study was not effective in removing the softened outer dentin layer and improving the adhesion to composite resin.

Ramos et al.¹⁰ reported that the dentin surface treatment with the Er:YAG laser (60 mJ, 2 Hz, 0.12 W, $19.3\mathrm{J}/{\mathrm{cm}}^2$, 150 to $250\mu \mathrm{s}$) was not able to increase the bond strength values to ED or SD. Their results contrast with our findings that showed that irradiated dentin reached higher bond strength values than those of its nonirradiated counterparts. Even though the energy density considered by Ramos et al.¹⁰ and the one used in the present study were similar, the higher pulse width used by the authors may have led to thermal damages at the target tissue³⁹ and influenced the adhesion to dentin.

Among the tested groups, the irradiated SD was the one that reached the highest shear bond strength values. SEM images showed that, unlike SD (nonirradiated, uniform smear layer, occluded dentinal tubules), SID showed wide opened tubules, irregular surface, and prominent peritubular dentin, creating the microretentive surface that has been suggested by some authors.^25,40 For Moretto et al.,²⁰ although the resulting microretentive aspect of the irradiated dentin surface apparently favors the bond strength, it did not actually improve it. The authors reported microcracks on the dentin subsurface and suggested that the decrease in bond strength was due to these changes. According to Esteves-Oliveira et al.,¹³ the thermal degradation of collagen may also be a contributing factor to the reduction of bond strength values.

It is important to highlight that the pulse width of the laser equipment used in previous studies^13,20,21,41 might have negatively affected the properties of the irradiated tissue, due to the heat diffusion into the inner dentin.³⁹ According to Fried et al.,⁴² the higher the pulse duration, the higher the residual heat in the irradiated tissue. Bodrumlu et al.⁴³ compared the temperature increase during root apicectomy using Er:YAG at three different pulse widths (50, 100, and $300\mu \mathrm{s}$) and reported the lowest temperature increase for the $50\mu \mathrm{s}$ pulse width.

Staninec et al.⁴⁴ suggested that shorter pulse widths, as used in the present study, might minimize the thermal damage to the dentin surface and subsurface. Their results showed that the SD irradiated with a super short pulse Er:YAG laser ($35\mu \mathrm{s}$) resulted in bond strength similar to the nonirradiated control group. Bahrami et al.⁴⁵ also reported a significant increase in bond strength of the composite resin to dentin when irradiating the substrate with an Er:YAG super short pulse laser (80 mJ, 10 Hz, $50\mu \mathrm{s}$, $12.58\mathrm{J}/{\mathrm{cm}}^2$).

In the current study, microcracks on the surface of irradiated SD and ED were not expected.⁴⁶ The treatment with the Er:YAG laser ($50\mu \mathrm{s}$ pulse width) with low energy densities appears to be an alternative for tooth pretreatment, without compromising the adhesion to composite resin.

Given the exposures above, we conclude that, regardless of the substrate (SD or ED), the Er:YAG laser with super short pulses favored the dentin bond strength to composite resin. It was the first time that Er:YAG laser was shown to enhance bond strength to ED. Future studies should be conducted in order to assess different laser parameters, the long-term effect of the laser treatment on SD and ED bond strength, and the quality of the bonding interface prior to adhesive procedures.

5.

Conclusion

The short pulse Er:YAG laser ($50\mu \mathrm{s}$), when used prior to the restorative treatment, is able to promote positive changes on both SD and ED surface and contribute to the adhesion to composite resin.