Shear bond strength of acid and laser conditioned enamel and dentine to composite resin restorations: An in vitro study

Abstract Objective To compare the shear bond strength of enamel or dentin conditioned with either Er,Cr:YSGG (erbium, chromium: yttrium–scandium–gallium–garnet) laser or phosphoric acid to composite resin restoration. Material and methods Forty posterior human extracted teeth were used. After mesiodistal sectioning of the teeth crowns, the samples were randomly divided into two groups—in the first group (E), bonding was performed on the enamel after roughening and in the second group (D), the enamel was removed and bonding was performed on the dentin. These groups were further randomly divided into two subgroups according to the type of etching (n = 20 each). In the acid‐etched groups (EA and DA), the surfaces were etched with 37% phosphoric acid. In the laser‐conditioned groups (EL and DL), the surfaces were conditioned with Er,Cr:YSGG laser. Total‐etch adhesive system was used to bond all the 80 specimens resin composite. The composite was vertically light‐cured, and the specimens were subjected to a shear bond strength test. Modes of bond failure were determined with a stereomicroscope. Results The highest shear bond strength was observed for the DA group (16.25 ± 1.10 MPa, p < 0.0001), whereas the lowest was observed for the DL group (8.56 ± 0.67 MPa). The adhesive failure mode was the most frequently observed in all groups. Conclusions The shear bond strength of composite resin bonded to enamel and dentin etched with phosphoric acid was higher than when conditioned with Er,Cr:YSGG laser. Thus, laser conditioning is not recommended.

Composite resin restorations are commonly used to restore dental structures, but they typically illustrate lower bond strength when used on dentin compared with enamel (De Munck et al., 2005). Progressing to increase the strength of this bond, several adhesive systems have been introduced (Nasseri, Majidinia, & Sharbaf, 2017). Per the technique used and also the mechanism of adhesion, adhesive systems are broadly categorized into two main categories: total-etch and self-etch adhesive systems (Gupta et al., 2017). Many companies produce total-etch adhesive systems as either a three-step system (acid etchant, primer, and adhesive) or a two-step system (acid etchant, and a combination of primer and adhesive in a single bottle) (Rechmann, Bartolome, Kinsel, Vaderhobli, & Rechmann, 2017). Selfetch adhesive systems are composed of a self-etching primer and an adhesive resin that's either provided in two separate bottles (two-step system) or combined in a single bottle (one-step system). Three-step total-etch adhesives are believed to be the gold standard in enamel bonding thanks to the effective bond formed after the utilization of the solvent-free, neutral pH, hydrophobic, and adhesive resin layer as a separate step (Raposo & Santana, 2012).
Several studies have investigated the likelihood of replacing the use of acid with newer techniques such as laser etching (Nelson, Wefel, Jongebloed, & Featherstone, 1987). The evolution of lasers in dentistry has facilitated the development of various soft and hard tissue procedures including soft tissue surgeries, dental bleaching, restorative curing, and painless caries removal and tooth preparation (Turkmen et al., 2010). Several kinds of lasers are utilized in dental practice such as the Nd:YAG laser, which is not well-absorbed by hard dental tissues, and the carbon dioxide laser, which might cause an a rise in pulpal temperature (van As, 2004). These limitations have been eliminated by the introduction of the erbium (Er) family of lasers, which were approved in 1998 by the US FDA for irradiating tooth surfaces (Ustunkol, Yazici, Gorucu, & Dayangac, 2015).
There are two known wavelengths of Er lasers in the dental field: Er,Cr:YSGG (Er, chromium: yttrium-scandium-gallium-garnet) lasers (2780 nm) and Er:YAG (Er: yttrium-aluminum-garnet) lasers (2940 nm). These wavelengths show high absorbability by both water and hydroxyapatite compared with any other dental laser wavelengths. Therefore, Er lasers are considered optimal for to be used on hard dental tissues. Successful dental ablation can be achieved with Er,Cr:YSGG laser because of its shorter wavelength, high absorption by water and enamel, and also the laser's water-cooled system allows control of the pulpal temperature (Kumar, Dhillon, & Rehman, 2016).
Several studies have also proven that enamel and dentin surfaces conditioned with Er,Cr:YSGG laser lead to the removal of the smear layer and formation of micro-irregularities along the dental surface (Ustunkol et al., 2015). Laser conditioning alters the calcium/phosphorus ratio on the dental surface, and these changes provide the enamel with resistance against caries attacks (Kumar et al., 2016).
The use of lasers for enamel conditioning is controversial, as some investigations have shown that lasers do not seem to be always fully effective for this purpose. Despite their advantages of being heatless and painless, laser were found by some investigators to create uneven enamel surface with a lot of fracture areas (Usumez & Aykent, 2003;von Fraunhofer, Allen, & Orbell, 1993). Usumez et al. in 2002 stated that "enamel conditioning with an Er,Cr:YSGG laser cannot be considered a successful alternative to the conventional methods of increasing bond strengths to enamel" (Usumez, Orhan, & Usumez, 2002). In contradictory, other researchers have reported satisfactory results and increased bonding strength between the enamel and composite resin after laser conditioning (Basaran, Ayna, Basaran, & Beydemir, 2011;Hossain et al., 2003;Visuri, Gilbert, Wright, Wigdor, & Walsh Jr., 1996). They reported that Er,Cr:YSGG laser increase enamel acid resistance by altering calcium to phosphorus ratio and carbonate to phosphorus ratio within enamel structure (Fowler & Kuroda, 1986;Keller & Hibst, 1990). In addition, it was found that laser ablation with a power of 2 W (5.6 J/cm 2 ) produces an etch pattern that resembles type III acid etching pattern with surface roughness similar or less to that produced by conventional acid etching (Silverstone, Saxton, Dogon, & Fejerskov, 1975). There's also debate concerning the subject of dentin bonding; many studies have found that the utilization of phosphoric acid for etching dentin before composite resin restorations yields increased shear bond strength in comparison with laser conditioning (Armengol, Jean, Weiss, & Hamel, 1999;Dunn, Davis, & Bush, 2005;Jaberi Ansari et al., 2012).
On the other hand, other studies reported that equal or higher shear bond strength after laser conditioning of dentin (Bertrand et al., 2006;Visuri et al., 1996). Therefore, this study was aimed to evaluate the shear bond strength enamel or dentin conditioned with either Er,Cr: YSGG (erbium, chromium: yttrium-scandium-gallium-garnet) laser or phosphoric acid to composite resin restorations.

| Specimen preparation
In this study, posterior human extracted teeth were used. The teeth were thoroughly inspected for caries, cracks, fluorosis, abrasion facets, and damage from extraction and 40 teeth were selected. Samples were thoroughly washed and stored in dark glass containers in 1% (v/v) thymol solution at 4 C after extraction and used within 2 months.  Table 1.

| Shear bond strength testing
The specimens were subjected to shear bond strength testing with a universal testing machine (Instron 5965; Instron, England) with a load cell of 5 kN operated by a single operator. A knife-edged rod with a width of 0.5 mm was applied at the interface of the resin composite disk with the enamel/dentin at a crosshead speed of 0.5 mm/min.

| Statistical analysis
One-way analysis of variance was utilized to compare the mean values of shear bond strength among all four groups, followed by Tukey's test for pairwise comparisons. p-values of <0.05 were considered statistically significant.

| RESULTS
The descriptive statistics (minimum, maximum, mean, and standard deviation) of the shear bond strength (MPa = N/mm 2 ), which was measured at maximum load (N), are given in Table 2.
Comparison of the mean shear bond strength among the four study groups (EL, EA, DL, and DA) showed a statistically significant difference (p < 0.0001). Tukey's pairwise comparison between each of the four study groups showed that the mean shear bond strength of the DA group was statistically significantly higher than that of the other three groups (DL, EL, and EA), whereas the mean for the DL group was statistically significantly lower than that of the other three groups. The statistical results are summarized in Tables 3 and 4 (Basaran, Ayna, et al., 2011;Jaberi Ansari et al., 2012;Usumez et al., 2002). For instance, Usumez and Aykent (2003) and Usumez et al. (2002) adjusted the laser wavelength and irradiated the enamel surface with Er,Cr:YSGG laser at a power output of (2 W, 20 Hz, 100 mJ) or (1 W, 20 Hz, 50 mJ). Decreasing the power to half decreased the bond strength of the irradiated surface, although variable results were recorded. Within the current study, a power of 4.50 W with energy density of 90 mJ was utilized to irradiate the enamel and dentin groups and higher shear bond strength was achieved compared with that obtained in previous studies (Usumez et al., 2002;Usumez & Aykent, 2003 (Oilo, 1987;Swift, Perdigão, & Heymann, 1995). Therefore, shear bond strength test were used in this study to evaluate composite restoration bond strength to laser etched and acid etched enamel and dentine.

| DISCUSSION
Several studies have proven that laser conditioning of enamel surfaces is useful (Basaran, Hamamci, & Akkurt, 2011;Hossain et al., 2003;Turkmen et al., 2010); however, others have produced contradictory results (Dunn et al., 2005 (Ustunkol et al., 2015). These results can be explained by the effect of Er,Cr:YSGG laser irradiation on enamel surfaces, which show a chalky surface when viewed with scanning microscope. This surface provides increased retention of composite filling material, which is valuable in the restorative procedure (Hibst, 2002;Hoke, Burkes Jr., Gomes, & Wolbarsht, 1990 Gurgan et al. (2008). This outcome can be illustrated by the changes noticed in the composition and conformation of the organic matrix that might result in collagen degradation and deterioration of adhesive penetration (Bachmann, Diebolder, Hibst, & Zezell, 2005). Furthermore, Erbium laser irradiation on dentin causes odontoblastic tubules to open up, and dentin shows surface scaling after the application of laser, and this often results in flaking and peritubular cuffing. This odd manifestation of dentin is explained by Lin et al. (1999) as micro-explosions within the inorganic structures in the teeth that appear after Er,Cr:YSGG laser irradiation. It had been also proposed by Sennou, Lebugle, and Gregoire (1999) that laser conditioning of dentin binds collagen fibrils together, which results in the absence of interfibrillar space and thus prevention of resin penetration into the intertubular dentin happens. This result might explain the low shear bond strength of laser conditioned dentin in the current study.
The failure modes of the samples were also tested during this study, and therefore the most frequent failure mode among all the four study groups was the adhesive failure mode. It is notable that the group with the lowest shear bond strength (DL) was also the group demonstrating only the adhesive failure mode. This results in agreement with a study done by Lee et al. (2007) who found that Er,Cr: YSGG laser irradiation adversely affect dentin adhesion to resin composite because laser irradiation produce scaly, irregular surface with no smear layer and open dentinal tubule.
The diverse and often contradictory results of previous studies could be due to the application of different technical parameters, including the physical parameters of the laser or the kind of restorative material used. Further studies are therefore required to verify the main conclusions of our study.

| CONCLUSIONS
Within the limitation of this in vitro study, it can be concluded that laser conditioning of enamel and dentin is not recommended because: 1. The shear bond strength of enamel and dentin groups etched with phosphoric acid was higher than that of groups ablated with Er,Cr: YSGG laser.
2. Adhesive failure was the foremost predominant failure mode for all the tested groups, and it had been the sole failure mode apparent in a laser-irradiated dentin group.

ACKNOWLEDGMENTS
The authors would like to acknowledge the Institutional Review Board, College of Medicine, King Saud University and the College of Dentistry Research Center, King Saud University for facilitating this project.

CONFLICT OF INTEREST
The authors declare that they have no conflict of interest.

AUTHOR CONTRIBUTION
All authors have contributed to study design and consecution, writing, revision, and proofing the manuscript.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.