Influence of Er,Cr:YSGG Laser Dentin Conditioning on the Bond Strength of Bioactive and Conventional Bulk-Fill Dental Restorative Material

Abstract

Objective:

The aim of the study was to evaluate shear bond strength (SBS) and modes of failure for bioactive (BA) material when bonded to Er,Cr:YSGG lased dentin in comparison with other conditioning regimes.

Materials and methods:

One hundred twenty extracted teeth were mounted and allocated into eight groups (n = 15) according to the surface conditioning [group 1: Er,Cr:YSGG laser + BA; group 2: Er,Cr:YSGG laser + multi-core (MC); group 3: Er,Cr:YSGG + Ketac Conditioner (polyacrylic acid) + BA; group 4: Er,Cr:YSGG + Ketac Conditioner + MC; group 5: conventional etch and rinse + BA (Control); group 6: conventional etch and rinse + MC (Control); group 7: self-etch + BA; group 8: self-etch + MC]. Based on the type of bulk-fill material, samples from groups 1, 3, 5, and 7 were bonded to BA and groups 2, 4, 6, and 8 were bonded to conventional MC bulk-fill material. All specimens were tested for SBS using universal testing machine. Ten samples from each group were assessed for modes of failure. Data were assessed using analysis of variance and Tukey multiple comparisons test.

Results:

The maximum SBS value was in group 6 (18.96 ± 0.315) with etch and rinse dentin conditioning and bonded to MC. The minimum SBS value was exhibited by group 7 (16.04 ± 0.854) self-etch conditioning bonded to BA. SBS among group 6 specimens were significantly higher than that of specimens in group 7 (p < 0.05). Among all groups the most common type of observed failure was adhesive.

Conclusions:

Dentin surface treatment with Er,Cr:YSGG phototherapy for the adhesive bonding of BA and conventional bulk-fill resin composite MC showed comparable bond strength outcomes to conventional conditioning techniques. Er,Cr:YSGG laser dentin conditioning of dentin for bonding of bulk-fill BA materials has potential to be used clinically as it shows potential for adequate adhesive bond integrity.

Introduction

The primary objective of restorative dentistry is to eliminate a carious lesion and to restore function, anatomy, and esthetics of a tooth with an appropriate restorative material.¹ Undeniably, dual cure restorative composites are considered the material of choice for restorations; however, they have shown limitations in polymerization stresses resulting in fracture, cuspal deflection, and low bond strength between tooth composite interface.^2,3 Alternatively, bulk-fill composites of different viscosities according to their filler content have tried to overcome these shortcomings with better mechanical properties, marginal integrity, and low polymerization stresses.^4,5 Yet, bulk-fill composites are found to generate contraction stresses, creating gap formation resulting in increased microleakage scores despite improved depth of cure.⁶ Recently, a new biodentin replacement material has been introduced in the form of bioactive (BA) materials, for posterior teeth with good dimensional stability, esthetics, antimicrobial properties, marginal fit, and bond strength.^7,8 These restorative material constitutes of BA resin and glass respond to change within the oral environment by diffusion of fluoride, calcium, and phosphate ions playing an active role in prevention and remineralization.^7,9

Apart from the restorative material properties, the other factor playing a critical role for clinical success is surface treatment of dentin. Conventionally, dentin is conditioned with 37% phosphoric acid for eliminating smear layer and creating resin tags, followed by application of dentin bonding agent.¹⁰ However, evidence suggests traditional method of dentin etching to be time consuming, that is, involving many clinical steps, prone to salivary contamination, and its overuse causing irreversible demineralization.¹¹ Alternatively, “all-in-one” adhesive has been introduced, that is, a single component self-etch dental adhesive combining etching, priming, and bonding altogether.¹² This conditioning method is found to be simpler to use, it impregnates the smear layer, and exhibits gentle etch pattern.^13,14

Recently, use of laser as phototherapy in treating dental and medical conditions have shown mounting interest.^15

–19 Laser therapy in the form of Er,Cr:YSGG working at a wavelength of 2.78 mm is well absorbed by water and hydroxyapatite and hence results in surface irregularities and ablates the dentin and enamel effectively.²⁰ At present, the available evidence of Er,Cr:YSGG phototherapy on dentinal surface in removing smear layer and free radicals and improving bond strength is controversial. Studies by Alkhudhairy et al.¹⁷ and Vohra et al.²¹ proclaim conditioning of dentinal surface with Er,Cr:YSGG phototherapy has improved bond strength compared with conventional dentinal etching technique. However, work by Kameyama and Kawadavmasakazu²² and Attrill et al.²³ reports decreased bond strength compared with traditional etching.

There is limited evidence on bond integrity of BA bulk-fill restorative material (Activa) (BA) to dentin conditioned with Er,Cr:YSGG in comparison with conventional bonded bulk-fill material.²⁴ Consequently, it is assumed that dentin treated with Er,Cr:YSGG at (30 Hz 4.5 W) and bonded to BA will display comparable bond strength to conventional acid etching of dentin bonded to traditional bulk-fill composite multicore (MC). Therefore, the purpose of this in vitro study was to evaluate shear bond strength (SBS) and modes of failure for BA bulk-fill material when bonded to Er,Cr:YSGG lased dentin in comparison with other dissimilar dentin conditioning regimes.

Materials and Methods

The present in vitro study was permitted by the ethical committee of King Saud University under (ethical approval number project no. E-18-3345). The study followed the Check List for reporting In-vitro Study guidelines.

One hundred twenty extracted noncarious, nonfractured, restoration-free molars were collected and stored in 0.01% of thymol solution for 1 week. Calculus, plaque, tissue remnants, and other organic debris were removed before tooth preparation with the help of periodontal scaler (Sonic flex 2000; Biberach, Germany). The specimens were transferred to distilled water 4°C until preparation. All the sample were mounted vertically in acrylic resin (Meliodent; Kulzer, Hanau, Germany) within the segments of polyvinyl pipe (4 mm radius) equal to cementoenamel junction revealing only the clinical crown. To homogenize dentinal depth and to expose fresh dentinal tubules the buccal surface of all molars were ground to a depth of 2 mm with an area of 3 mm using Isomet saw (Buehler, IL). The surfaces were polished with a 300–500 grit silicon carbide paper (Buehler) on a rotary polishing machine (AROPOL 2V; Arotec) (6.98 g) under water irrigation for 20 sec. Specimen were randomly allocated into eight groups (n = 15) according to the type of surface conditioning.

Groups 1 and 2 were surface treated with Er,Cr:YSGG laser. Group 3 and group 4 surfaces were conditioned with Er,Cr:YSGG + Ketac Conditioner. Groups 5 and 6 surfaces were treated with conventional etch and rinse and finally the specimens of groups 7 and 8 were surface conditioned by self-etch dental adhesive. These experimental groups followed the subsequent bonding protocol.

Groups 1 and 2: Dentinal surface of the samples were conditioned by Er,Cr:YSGG (Waterlase C100; BioLase Tech, Inc., CA) laser power 4.5 W and frequency 30 Hz in a noncontact mode from a distance of 2 mm using tip [mizoram (MZ) = 8, 6 mm] for a duration of 60 sec. After laser procedure, the specimens were bathed in artificial saliva (NeutraSal; Orapharma, North America LLC) for 60 sec.¹⁵ The samples were rinsed, dried using compressed air removing moisture but were not desiccated. A universal bonding agent (Tetric N-Bond Universal; Ivoclar-Vivadent) was applied and light cured (Bluephase G2; Ivoclar-Vivadent) for 10 sec.

Groups 3 and 4: Dentinal surface of the samples were conditioned by Er,Cr:YSGG laser power 4.5 W and frequency 30 Hz in a noncontact mode from a distance of 2 mm using tip (MZ = 8, 6 mm) for a duration of 60 sec. After laser procedure, the specimens were bathed in artificial saliva (NeutraSal) for 60 sec. The samples were rinsed dried using compressed air removing moisture but were not desiccated. Ketac conditioner (3M ESPE; Dental Products, Seefeld, Germany) was used on the dentinal surface for 10 sec and then washed with copious water for 10 sec and air dried.

Groups 5 and 6 (control): Dentin was surface treated through conventional method that is, etch and rinse. Thirty-seven percent phosphoric acid was applied for 20 sec (Harward, Etch) and washed for 30 sec. The samples were dried using compressed air removing moisture but were not desiccated. A universal bonding agent (Tetric N-Bond Universal; Ivoclar-Vivadent) was applied and light cured (Bluephase G2; Ivoclar,Vivadent) for 10 sec.

Groups 7 and 8: All specimens received two applications of All-In-One (OptiBond; KaVo Kerr, West Collins, Orange, CA) with a brushing motion for 20 sec each (as per manufacturer's instructions). The adhesive was dried for 5 sec and light cured (Bluephase G2; Ivoclar-Vivadent) for 10 sec. Table 1 provides the material used in the study.

Table 1.

Material Used in the Study

Product	Manufacturer
MultiCore Flow bulk-fill composite	Ivoclar-Vivadent, Schaan, Liechtenstein
Bioactive restorative material Activa, Pulpdent Cooperation	Watertown, MA
Ketac conditioner (polyacrylic acid)	3M ESPE, Dental Products, Seefeld, Germany
Tetric N Bond	Ivoclar-Vivadent
37% Phosphoric acid	Harward, Etch
All-In-One (OptiBond)	KaVo Kerr, West Collins, Orange, CA

Now based on the type of bulk-fill composite, samples from groups 1, 3, 5, and 7 (n = 15) were bonded to BA materials. A BA restorative material (Activa; Pulpdent Cooperation, Watertown, MA) was dispensed in a polyether rubber mould having standardized dimensions (3 mm diameter and 3 mm height) already placed on the dentin samples using a cement plunger and was light cured for 20 sec. The moulds were removed carefully. Similarly, specimens from groups 2, 4, 6, and 8 (n = 15) were bonded to conventional bulk-fill resin material (MC Flow; Ivoclar-Vivadent, Schaan Liechtenstein). MC was used for core build-up using standardized rubber moulds and dispensing methods as explained earlier.

All the specimens were kept in an incubator (Memmert Universal Oven, Germany) at 37°C in a humid environment at 55% for 2 days. Further, the samples were thermocycled between 5°C and 60°C for 8000 cycles (Automated Thermal Cycler; Applied Biosystems, CA) for 45 sec each, before assessing the SBS testing.

All specimens from each group were positioned in a universal testing machine (Instron 8500 Plus; Canton) for SBS testing. The samples were mounted in a metallic mould and were exposed to increasing load at a crosshead speed of 1 mL/min at the dentin restorative material interface. The shear strength that separated the test material was calculated. Similarly, 10 samples from each group were assessed for modes of failure (40 × magnification) using a stereomicroscope (SZX7; Olympus, Hamburg, Germany) and classified into cohesive, adhesive, and admixed failure.

Data obtained through bond strength testing was tabulated using Statistical Package for the Social Sciences (SPSS version 21, Inc., Chicago, IL). Normality of data obtained was assessed using Kolmogorov–Smirnov test. Mean values and standard deviations (SDs) were compared using analysis of variance (ANOVA) and Tukey's post hoc test at a significance level of p = 0.05

Results

Table 2 provides SBS values and SD of each interventional group. The maximum SBS value was demonstrated in group 6 (18.96 ± 0.315) using conventional etch and rinse dentin conditioning when bonded to MC. Similarly, the minimum SBS value was exhibited by group 7 (16.04 ± 0.854) self-etch conditioning bonded to BA. Further, SBS values among group 1 (18.31 ± 4.272—Er,Cr:YSGG + BA), group 2 (18.11 ± 3.114—Er,Cr:YSGG + MC), group 3 (18.05 ± 4.892—Er,Cr:YSGG + Ketac + BA), group 4 (18.66 ± 3.521—Er,Cr:YSGG + Ketac + MC), group 5 (18.84 ± 0.216—etch and rinse + BA), and group 6 (18.96 ± 0.315—etch and rinse + MC) were found to be comparable (p > 0.05). Moreover SBS values among specimens of group 7 (16.04 ± 0.854—self-etch + BA) and group 8 (16.21 ± 0.524—self-etch + MC) were found to be comparable (p > 0.05). For bond strength values, ANOVA showed significant difference among the study groups (p < 0.05).

Table 2.

Mean and Standard Deviation for Bond Strength Values Among Study Groups Using Analysis of Variance and Tukey Multiple Comparisons Test

Experimental groups	Mean ± SD (Mpa)	Variance	p Value^*
Group 1 Er,Cr:YSGG laser + BA	18.31 ± 4.272^A	1.063	<0.05
Group 2 Er,Cr:YSGG laser + MC	18.11 ± 3.114^A	1.057
Group 3 Er,Cr:YSGG + Ketac + BA	18.05 ± 4.892^A	1.131
Group 4 Er,Cr:YSGG + Ketac + MC	18.66 ± 3.521^A	1.024
Group 5 Conventional etch and rinse + BA (Control)	18.84 ± 0.216^A	1.214
Group 6 Conventional etch and rinse + MC (Control)	18.96 ± 0.315^A	1.089
Group 7 Self-etch + BA	16.04 ± 0.854^B	1.541
Group 8 Self-etch + MC	16.21 ± 0.524^B	1.217

A,B

Different superscript capital alphabets denote statistical significant difference (Tukey multiple comparison test).

Showing significant difference among study group (analysis of variance).

Er,Cr:YSGG, Erbium Yttrium Scandium Gallium Garnet; BA, bioactive restorative material (Activa); MC, multicore; SD, standard deviation.

Table 3 shows mode of failure among experimental groups. Dentin surface lased by Er,Cr:YSGG displayed a majority of (60–70%) admixed type of failure, whereas 60–80% of specimens surface conditioned with self-etch technique presented adhesive type of failures. Moreover, among conventional etch and rinse group specimens were evenly distributed among failure type groups. Among all groups the most common type of observed failure was adhesive.

Table 3.

Modes of Failure Among Study Groups Specimens

Study groups	Adhesive (%)	Cohesive (%)	Admixed (%)
Group 1 Er,Cr:YSGG laser + BA	30	—	70
Group 2 Er,Cr:YSGG laser + MC	40	—	60
Group 3 Er,Cr:YSGG + Ketac + BA	20	—	80
Group 4 Er,Cr:YSGG+ Ketac + MC	25	—	75
Group 5 Conventional etch and rinse + BA	30	40	30
Group 6 Conventional etch and rinse + MC	40	15	45
Group 7 Self-etch + BA	60	40	—
Group 8 Self-etch + MC	80	20	—

Discussion

This study was based on the hypothesis that dentin treated with Er,Cr:YSGG at (30 Hz 4.5 W) and bonded to BA will display comparable bond strength to conventional acid etching of dentin bonded to traditional bulk-fill composite (MultiCore bulk fill). Of interest, SBS values were comparable among the majority of groups and the hypothesis was partly accepted. In this in vitro study, SBS values were evaluated using universal testing machine to follow standardization, homogeneity, and consistency. The test measures specific variables keeping other variables constant.²⁵

Er,Cr:YSGG belonging to the Erbium family, ablates both hard and soft tissues without thermal side effects. Histological evidence reveals no pulpal damage of tooth, conditioned with Er,Cr:YSGG. The laser is found to be bactericidal in nature and makes dentin resistant to carious attack.²⁶ In the existing study, dentin lased with Er,Cr:YSGG showed comparable bond strength values to conventional acid etch treatment that is known to be a gold standard in restorative dentistry.²⁷ Multiple explanations can be presented in this regard. Primarily, thermomechanical ablation by Er,Cr:YSGG phototherapy causes evaporation of water and organic component making dentinal surface scaly, flaky, and more receptive to bonding. The intertubular dentin because of its less mineral and water content is ablated more by Er,Cr:YSGG causing protrusion of the dentinal tubules and generating cuff-like appearance. Second, improved adhesion can be attributed to increase energy of the dentinal surface and removal of smear layer by Er,Cr:YSGG making the lased dentinal surface mechanically stable and forming resin tags resulting in easy permeation of adhesive hence improving bond strength.^20,28

Subsequently, in this study, authors used 4.5 W and 30 Hz (laser parameters) settings to condition dentin surface. These power and frequency settings were in coexistence with protocol followed by Hossain and colleagues,²⁹ Chou et al.,²⁸ and Alkhudhairy et al.¹⁶ that surface roughness surges at high power and frequency improving bond strength values. Therefore, it can be inferred that dentin ablated at low frequency and power does not create abundant craters and roughness hence compromising bond integrity.²⁸ Further, in this study it was observed all lased specimens treated with Er,Cr:YSGG exhibited substantive SD values, whereas SD values were smaller than those observed for specimens treated with conventional etch and rinse. A possible explanation of this finding is because of manually controlled sweeping motion of laser beam over dentin surface during conditioning displaying weak etching patterns.³⁰ Further, the author speculates that contradictory results as reported previously of low bond strength scores in lased dentinal groups can be accredited to type of dentin specimens (bovine vs. human), different laser parameters (power and frequency), irrigation, duration of laser treatment, distance, and type of adhesive used.^15,17

In this study, polyacrylic acid (Ketac conditioner) was applied on lased dentinal surface. Polyacrylic acid is considered a weak acid, widely used to remove smear layer, facilitate hybridization, and improve adhesion.³¹ At present there is no evidence to compare the findings of lased dentin conditioned by polyacrylic acid. Most of the literature currently available is on conditioning of dentin with polyacrylic acid when bonded to glass ionomer cements.^32,33 In this study, it is assessed that there is an ionic diffusion between lased surface of dentin and BA between the initial intermediary layer displaying higher bond strength values (18.05 ± 4.892).³⁴ Similarly, lased dentinal surface is free of smear layer increasing dentinal surface roughness and forming craters, making bonding for the bulk composite more viable and receptive.³⁵ Of importance, the effect of polyacrylic acid is already performed by Er,Cr:YSGG laser phototherapy on dentin itself. Therefore the authors do not recommend the clinical use of Ketac conditioner (polyacrylic acid) in the surface conditioning of lased dentin when bonded to BA and MC.

The least bond strength values were observed in “all-in-one” self-etch adhesive-treated specimens. All-in-one adhesives are also called multi-mode or universal adhesives. These adhesives consist of hydrophilic and hydrophobic monomers, organic solvent, water, and functional monomer in a single paste.^12,36 Low SBS values in this group can be accredited to limited conditioning ability of self-etch adhesive creating a gentle etching pattern, thin hybrid layer, impregnation of smear layer, water sorption, and solubility of resin compromising mechanical properties and bond strength.^37,38 A study by De Munck et al. stated three-step adhesive technique to be gold standard and durable and suggested that any simplification in etching by “all-in-one” adhesive results in loss of bond integrity, effectiveness, and bond durability.³⁹

In this study, the longevity and adhesive capacity of the bulk-fill material was evaluated quantitatively through SBS testing. The method is relatively easy to use, widely acceptable, and does not require expensive or sophisticated protocols.²⁵ Two different types of bulk-fill material were used in this study, that is, BA and MC. There is no available evidence to compare the results of the existing study in comparison with these materials (MC and BA). It is hypothesized by the authors that better bond strength between BA with dentinal surface and conventional etch and rinse is because of ionic exchange mechanism, better bonding of exposed surface free of smear layer, better penetration of adhesive, improved micro-porosities for mechanical bonding on dentinal surface, and strong affinity of bioactive glass with exposed dentinal tubules and collagen.^20,40,41 Similarly, MC displayed better bond integrity because of presence of silane coupling agent and high filler content. It has been noted that high filler content of material encourages better bond strength and elastic modulus.^24,42 Although evidence advocates that multiple factors are involved, altogether that may influence resin bond strength, that is, quality of material, aging process, curing time, dentinal surface, bonding agent, and so on.⁴²

In relation to modes of failure, admixed type of failure were dominant in lased dentinal surface. Thermomechanical ablation of Er,Cr:YSGG on dentinal surface compromising the physical properties may be a reason for this type of failure. Further, lateral forces, type of material, and debonding method may influence admixed type of failure. Similarly, the validity of adhesive failure was confirmed through low bond scores in group 7 and group 8 in which dentin was conditioned with “all-in-one” adhesive, whereas mix mode of failure and cohesive failure type was dominant in conventional etch and rinse specimen groups, authenticating high bond score values.⁴³

Within the limitations of this study, more in vitro and in vivo studies are essential to confirm and substantiate the results of the existing study. Subsequently, the standardization of dentinal surface was difficult, because of sclerotic dentin, ethnic variations, morphology, and thickness of dentinal tubules. Moreover, future research should be directed on the surface profilometry and surface assessment of fracture patterns on lased dentin. The findings of this study are only applicable to the type of laser parameters, material used, and shear bond parameters. Finally, to recommend Er,Cr:YSGG phototherapy on dentinal surface at 4.5 W and 30 Hz for adhesive bonding of BA materials, further clinical trials are recommended.

Conclusions

Dentin surface treatment with Er,Cr:YSGG phototherapy for the adhesive bonding of BA and conventional bulk-fill resin composite MC showed comparable bond strength outcomes to conventional conditioning techniques (Etch and rinse) and has potential to be recommended in clinical settings.

Footnotes

Acknowledgments

The authors extend their asppreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group number (RGP-1438-028).

Author Disclosure Statement

The authors declare that they have no conflict of interest and all authors have read and approved the final draft.

Funding Information

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group NO (RGP-1438-028).

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