Effects of different Er,Cr:YSGG laser parameters on dentin shear bond strength and interface morphology: an in vitro study
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The aim of this in vitro study is to evaluate the effect of Er,Cr:YSGG laser parameters, specifically power (Watt) and frequency (Hertz), throughout both, shear bond strength (SBS) and dentin-composite interface morphology, when using laser etch as an alternative to acid etching with a “two-step” total etch adhesive.
Thirty-six human sound molars were mounted on acrylic blocks, randomly divided into six groups, six teeth into each group, where five teeth were selected for the SBS test, and one sectioned for the scanning electron microscope (SEM) analysis. Teeth were sliced occlusally to expose a flattened dentin surface, then grouped according to various treatments: control group, only treated with phosphoric acid etchant; and groups 1–5, only treated with Er,Cr,:YSGG laser irradiating at 3 W/25 Hz, 3 W/50 Hz, 4 W/25 Hz, 4 W/50 Hz, and 5 W/75 Hz respectively. The adhesive was applied followed by composite build up. SBS test was carried out using a universal testing machine. The resin-dentin interface was analyzed utilizing a SEM.
The control group showed the highest SBS values (11.38 ± 2.03 Mpa). Group 5 (5 W/75 Hz) was the second highest (8.46 ± 1.82 Mpa), yet the highest among the entire laser-irradiated groups, showing a marginal/borderline significance with the control group (P = 0.044). Group 3 (4 W/25 Hz) came second through the laser samples (7.41 ± 0.97 Mpa). SEM analysis manifested premier retentive composite dentin interface among the control group and groups 5 and 3.
When applying Er,Cr,:YSGG laser solely for dentin etching, adjusting the settings to a power of 5 W and frequency at 75 Hz will lead to optimum results regarding both SBS and interface morphology.
KeywordsEr,Cr:YSGG laser Parameters Dentin Shear bond strength Dental etching Dental adhesive
It has been always inevitable for every dentist to search for the, so called, optimum adhesion with the best durability. And since Dr. Michael Buonocore developed the phosphoric acid as a primary enhancement for dental adhesion in 1955, the bonding effectiveness of conventional adhesive systems is shown to represent good results until nowadays. A successful adhesive system primarily depends on the surface morphology of the treated dental tissue . The complex structure of dentin makes it more challenging than enamel to bond as the bond between dentin and composite resin filling material is more micromechanical than chemical [2, 3]. Acid etching’s main effect is primarily through demineralizing the peritubular and intertubular dentin, where resin monomers would infiltrate, as well as removing the smear layer created through conventional bur leaving dentin al tubules open [3, 4]. Yet, this might have a few complications such as variability of penetration depth, saliva contamination, and strong washing and drying effects on the bond strength.
After Maiman developed laser in 1960, it has been widely used in medicine and dentistry ever since. One of the most recent advances to enhance adhesion is by using the dental laser as a prime method to overcome the acid etching’s complications such as CO2 (carbon dioxide), Nd:YAG (neodymium-doped yttrium aluminum garnet), Er:YAG (erbium-doped yttrium aluminum garnet), and Er,Cr:YSGG (erbium, chromium-doped yttrium scandium gallium garnet) lasers. The laser beam modifies the morphology of irradiated surface and produces surface roughness. This ablation procedure occurs as the high energy absorbed by water micro droplets in the hard tissue causes micro explosions eventually removing the tissue . Advantageous to the conventional bur cavity preparation, there is a slight 2 °C increase in the pulp temperature allowing the dentist to operate on the patient without a local anesthesia needle. It has a disinfection effect reducing bacteria on the dentin surface as well [6, 7].
However, when it comes to lasers with the highest absorption coefficient in water and hydroxyapatite such as Er:YAG and Er,Cr:YSGG, it has not been yet able to drastically be proven that it has the upper hand when it comes to bond strength values. Working with fixed parameters, Lee et al. , Yildirim et al. , Hassoon , Bahrololoomi et al. , and Chimello-Sousa et al.  concluded from their results that laser had adverse effects on bond strength. Whereas, Gutknecht et al. , Sung et al. , Ergücü et al. , and Yazici et al.  revealed that the results were comparable to the conventional methods. Divergently, de Carvalho et al.  and Başaran et al.  experimented with different parameters and detected equivalent outcomes to acid etching but only when operating at certain settings.
Therefore, it seems to be of value to study the effect of adjusting the laser parameters. The aim of this in vitro study is to evaluate the effect of changing Er,Cr:YSGG laser parameters, specifically power in watt (W) and frequency in hertz (Hz), through both SBS and dentin-resin interface morphology and, additionally, to negotiate the best available beam settings to achieve the highest bond strength and most retentive morphology when using an Er,Cr:YSGG laser solely. The hypothesis to be tested is whether different Er,Cr:YSGG laser parameters would affect dentin bond strength and morphology and if there would be maneuver in order to reach the optimum adhesion results comparable to these of conventional acid etching methods.
Sample size calculation
Based upon the results of Ayar et al. , the effect size for SBS values was found to be 1.6, using alpha (α) level of 5% and beta (β) level of 10%, i.e., power = 90%; the minimum estimated sample size was 5 specimens per group for a total of 30 specimens. Sample size calculation was performed using G*Power Version 126.96.36.199.
A total of thirty-six human sound molars were selected for this study; all teeth were scaled and polished with rubber cup 1 day after extraction. Teeth were placed in phosphate buffered saline plus 0.02% thymol to control bacterial growth. All teeth were mounted on acrylic resin blocks and the occlusal surfaces of teeth were ground using a precision saw (IsoMet 4000™; BUEHLER, USA) (speed 2500 rpm, feeding rate 10 mm/min) to expose dentin. The exposed teeth were randomly distributed into six groups, six teeth into each group, where five teeth were selected for the SBS test (n = 5) × 6, and one sectioned for the scanning electron microscope (SEM) analysis (n = 1) × 6.
Group 1: Er,Cr:YSGG laser (3 W/25 Hz).
Group 2: Er,Cr:YSGG laser (3 W/50 Hz).
Group 3: Er,Cr:YSGG laser (4 W/25 Hz).
Group 4: Er,Cr:YSGG laser (4 W/50 Hz).
Group 5: Er,Cr:YSGG laser (5 W/75 Hz).
During setting up this study’s conditions, as a standard of operations for all measurements prior to obtaining results and analysis, group 5 was prepared using different parameters (5 W/75 Hz) to examine the difference in results when abruptly increasing the power and frequency settings compared to groups 1–4 that were measured in parallel but respectively under different conditions for reach. This approach was applied to check if the outcome would differ from measurements in groups 1–4.
The Chemical Etchant-32% Phosphoric acid (Scotchbond™ Universal Etchant; 3M ESPE, USA) was applied only in the control group, on exposed dentin surfaces, according to manufacturer’s instructions. It was applied for 15 s, rinsed with water for 15 s then the surface was dried using a cotton pellet.
Er,Cr:YSGG laser device
Shear bond strength test: The universal testing machine (Instron 3345; England) was used to measure the SBS by a chisel-shaped blade. Compression mode of force with head speed of 0.5 mm/min. Data was recovered using Bluehill 3 software version 3.3 (Fig. 2).
SEM: The specimens were sectioned perpendicular to the bonded interface using a precision saw (IsoMet 4000™; BUEHLER, USA), polished with wet silicon carbide papers, immersed in hydrochloric acid (6 mol/L) for 30 s, then in sodium hypochlorite (1% NaOCl) for 10 min, and washed in ultrasonic bath of distilled water for 5 min. The sections were mounted on aluminum stubs and sputter coated with a fine film of gold to get it ready for examination (K550X sputter coater; EMITECH, England). Specimens were examined using SEM (Quanta 250 FEG; FEI, Netherlands). Field emission gun is attached with EDX unit (energy dispersive X-ray analysis), with accelerating voltage 30 kV and magnification ×14 up to 1,000,000.
Shear bond strength test
Descriptive statistics and results of one-way ANOVA test for comparison between shear bond strength values (MPa) of the six groups (n = 5)
Control group (acid etch)
Group 1 (3 W/25 Hz)
Group 2 (3 W/50 Hz)
Group 3 (4 W/25 Hz)
Group 4 (4 W/50 Hz)
Group 5 (5 W/75 Hz)
Scanning electron microscope analysis
Control group: It showed uniform hybrid layer with even thickness of the adhesive material. There is no gap formation, neither between composite resin and the adhesive nor between the dentin and the adhesive. Well-defined and interdigitated resin tags extending into the dentin are also recognized (Fig. 4).
Group 1 (3 W/25 Hz): There are gaps with variable thicknesses along the dentin-composite interface, precisely between the adhesive layer and the dentin surface. Besides, non-uniform hybrid layer formation with non-obvious resin tags (Fig. 5).
Group 2 (3 W/50 Hz): Both the hybrid and the adhesive layers have a non-even thickness with gap formations. Though there are very minor resin tags, there is still intimate interdigitation between all three, the composite resin, the adhesive material, and dentin (Fig. 5).
Group 3 (4 W/25 Hz): Well-defined and prominent resin tags. Continuous, uniform hybridization indicating a firm attachment between the filling material and the dentin surface. No gaps formation at all (Fig. 4)
Group 4 (4 W/50 Hz): Gap formation between the adhesive and dentin but with a relatively smaller extent than gaps formed in the previous specimens. The adhesive material has an even thickness. Little or no resin tags or hybridization through the interface (Fig. 5)
Group 5 (5 W/75 Hz): As in the control group, there is no gap formation neither between composite resin and the adhesive nor between the dentin and the adhesive. A very distinct and obvious tight and firm interlock between all three layers, composite, adhesive, and dentin. Numerous resin tags are also observed projecting from the resin into the dentin (Fig. 4).
As stated by the results of this study, the first part of the hypothesis tested “whether different Er,Cr:YSGG laser parameters would affect dentin bond strength and morphology” was accepted, yet, as for the second element “if there would be maneuver in order to reach the optimum adhesion results comparable to these of conventional acid etching methods” still requires further investigations.
Through research in this particular subject, there always seems to be a controversy whether the effect of laser etching as an exclusive substitute to acid etching is favorable or not. Furthermore, what would be the major factors controlling the resulting laser etch. Upon going through literature, three genres of studies were found. In the first group, results advocated that laser has an adverse effect on bond strength values. When Lee et al. ablated dentin using an Er,Cr:YSGG laser applying the following settings: 3.5 W, 20 Hz, there was no major difference between the bur cut/acid etched and Er,Cr:YSGG ablated/acid etched dentin while laser ablated dentin had the lowest tensile bond strength values .
Hassoon employed Er,Cr:YSGG laser [2.5 W, 20 Hz] to etch dentin, and mean values of SBS were yet lower than both groups, those treated with acid etch and also the ones treated with acid etch + laser . Furthermore, Yildirim et al. utilized Er,Cr:YSGG laser [3 W, 20/35/50 Hz] and [6 W, 20/35/50 Hz]; still the laser-irradiated groups treated with different frequencies yielded significantly lower initial dentin bond strength as compared to the control group . The lower values in the laser etched groups were attributed to the pulsing nature of the laser beam creating less undercuts, as well as the dentinal tubules blocked due to high calcium-phosphorus ratio and low carbonate-phosphorus ratio making the dentin surface more resistant to etching and therefore less penetration of composite resin into them. Moreover, it was mentioned that laser causes fusion of collagen fibrils and so decreases the interfibrillar space letting less resin diffusion in the intertubular space, besides the fact that dentin has high water and low mineral content and so more ablation occurs leaving more protruding dentinal tubules increasing the adhesive area, but no or less demineralization of the peritubular dentin might be causing a slight decrease in bond strength [13, 20, 21, 22, 23, 24, 25]. Throughout the second group of trials, there lie studies with conclusions in support of using laser regarding obtaining equivalent outcomes compared to the conventional acid etching, such as Ergücü et al. treating sound and carious dentin surfaces with an Er,Cr:YSGG laser, where the resultant bond strength values were either higher or not significantly different from those treated with traditional methods. In this particular experiment, the peritubular dentin, which was once seen as a mean of degenerating bond strength, is observed as reinforcing it, as the peritubular dentin was found protruding from the surrounding intertubular dentin due to its high mineral and low water contents, favoring enhancement of bond strength . Başaran et al. negotiated the optimum settings to laser etch enamel at six different power outputs, and it was figured out that the mean SBS to enamel surface after etching using an Er,Cr:YSGG laser operated at 1.5 and 1.75 W for 15 s is similar to that obtained with acid etching . In another unique study, Lin et al. compared SBS between Er,Cr,:YSGG laser etched and bur-prepared dentin, where both groups received no acid etching in order for the results to reflect the change in surface topography only. Results were not significantly different between laser-treated and bur-prepared dentin . Over the third group of studies, they used different laser beam’s settings trying to approach the best parameters to obtain similar results to acid etch. Ayar et al. assessed the effects of Er,Cr:YSGG laser on dentin’s microtensile bond strength; they used different power and frequency settings. Control/acid etched groups showed higher values than laser-ablated groups, and despite that, SEM microphotographs provided evidence suggesting that the selection of high pulse frequency values might reduce those thermo-mechanical damages of laser irradiation . Sun et al. treated dentin with Er,Cr:YSGG laser at different power settings (from 1 to 6 W) at a frequency of 20 Hz. Results showed that the 4 W, 20 Hz settings are the most successful. It was concluded that by increasing power, the more open dentinal tubules will be but at 5 and 6 W cracks and lower roughness values were found decreasing the bond strength . Chou et al. operated Er,Cr:YSGG laser, not only under various power settings, but also changing exposure times as well. The SBS scores were not significantly different between Er,Cr:YSGG laser and chemical etching .
Based on all the outcomes mentioned above and when trying to define the similarities between the results of the current study with results of the studies concurring the use of Er,Cr:YSGG laser for dentin etching purposes, it would be found that Chou et al. agreed upon the fact that the 5 W power setting/irradiation for 30 s combination had the highest roughness value, even higher than that conventionally etched, making it suitable for dental restoration applications regarding SBS , where through this study, group 5 [5 W, 75 Hz] power/frequency combination scored the highest SBS values among the entire laser-irradiated groups. Besides, Ergücü et al. concluded that using the 4 W, 25 Hz settings for preparing sound dentin did not have any negative consequence on microtensile bond strength , consenting with the fact that group 3 [4 W, 25 Hz] along this in vitro study resulted in the second highest SBS among the laser-treated specimens. Furthermore, Sun et al. concluded that the 4 W, 20 Hz settings are the most favorable in respect of surface roughness and bond strength results . Needless to say that when Lin et al. irradiated dentin using 4 W, 20 Hz, there was no major difference in SBS values between the group prepared conventionally and another prepared using an Er,Cr:YSGG laser . On the other hand, there are also studies acknowledging the settings that lead to lower and adverse bond strength values. Where Ayar et al. presumed that that 3 W, 20 Hz group had the lowest microtensile bond strength values among all groups laser treated using the [3 W] power setting, also gaps were shown along the resin dentin interface , and this almost goes along with the figures resulting from this study’s SBS test that using the 3 W, 25 Hz settings resulted in a remarkable lower bond strength values.
In order to try and legitimize the results of this study, reviewing former studies evaluating SBS between different adhesives/bonding agents was undertaken, and a closer look was taken at groups using the same adhesive material as the one used in this study, the “two-step” total etch adhesive system (Adper™ Single Bond 2 Adhesive; 3 M ESPE, USA). Villela-Rosa et al. examined different bonding systems at various dentin depths; they found that the mean bond strength value for superficial dentin is 13.67 ± 7.4 Mpa . Additionally, Hegde et al. and Nassar et al. recorded mean results of 15.7 ± 1.93 and 11.38 ± 3.14 Mpa respectively [31, 32]. When Numan was studying the effect of eugenol on SBS of composite resin restorations to dentin, through the control group where the adhesive was applied directly after etching followed by the resin material, the SBS mean was 11.11 ± 1.3 Mpa . All these outcomes are not considerably different from those obtained in the control group of this study (11.38 ± 2.03 Mpa), falling within the same range, when using the same “two-step” total etch adhesive (Adper™ Single Bond 2) on a dry dentin surface. However, when Montagner et al. were examining the effect of sodium hypochlorite dentin pre-treatment on SBS, values through the untreated control group on the surface occlusal, deep occlusal, and proximal dentin surfaces were 11.5 ± 2.4, 7.7 ± 2.8, and 14.9 ± 4.5 Mpa respectively . Equally, Ravikumar et al. investigated the effect of two dentin bonding agents, other than (Adper™ Single Bond 2), the SBS of composite to dry dentin surfaces was 7.68 ± 0.96 Mpa when employing a total etch adhesive and 6.28 ± 0.75 Mpa with a self-priming adhesive . Results from Ravikumar et al.’s study, along with the deep occlusal mean from Montagner et al.’s study, are less than the highest mean (8.46 ± 1.82 Mpa) among the laser-irradiated groups in this study, group 5 [5 W, 75 Hz], yet approximate to the second highest, group 3 [4 W, 25 Hz], with a mean of 7.41 ± 0.97 Mpa.
According to the SBS values of this study, scanning electron microscope micrograph analysis of the composite resin-dentin interface, and corresponding outcomes from previous trials, it would be fair to admit the possibility of acquiring an etched dentin surface using an Er,Cr:YSGG laser, that is indistinguishable and analogous to that of phosphoric acid etching, but only when employing the right specifications regarding both, the laser beam’s parameters/settings, and also the fair working conditions with the proper materials’ protocol. Furthermore, two limitations of this in vitro study have to be taken into consideration including the employment one adhesive system, “two-step” total etch adhesive system, along with the fact that no thermocycling was executed.
Consequently, it is recommended that there is still a need to conduct further studies regarding the use of Er,Cr:YSGG laser as an exclusive and absolute alternative to acid etching of dentin. The research shall incorporate changeable laser beam parameters such as power, repetition rate/frequency, irradiation time, and distance to target tissue and also variable adhesive protocols using different adhesive/bonding agents and composite resin filling materials.
When using Er,Cr:YSGG laser solely for dentin surface etching along with a“two-step”total etch adhesive, it is proposed to use the following irradiation parameters: power = 5 W, repetition rate/frequency = 75 Hz, and an average exposure time of 30 s.
SBS values of the acid etching/control group are marginally higher than the highest group among the lased specimens, group 5 [5 W, 75 Hz] taking into consideration the restrictions and limitations through the working conditions, as well as attempts to standardize many variable elements.
The increased demand for using dental lasers necessitates holding further investigations concerning adhesion of composite resin filling materials to various dental tissues.
The author would like to thank the following:
Professor Dr. med. dent. Norbert Gutknecht. Clinic for Conservative Dentistry, Periodontology and Preventive Dentistry, University Hospital Aachen, RWTH University. Scientific Director of Lasers in Dentistry Programs.
Mohamad Medhat Abdelsalam and Soha Negm El-din (financial and morale support).
Professor Dr. Rami Maher Ghali/Faculty of Dentistry-Ain Shams University’s Dental Laser Center.
Faculty of Dentistry-Misr International University’s Dental Laser Center.
Associate professor Dr. Emad abo El-azm. Operative Dentistry Department. Faculty of Dentistry-Suez Canal University (specimens’ preparations and measurements).
Dr. Khaled Keraa (statistical analysis).
Dr. Khaled El-Hadad, lecturer at Oral Biology Department. Faculty of Dentistry. Ain Shams University (SEM micrographs interpretation).
Mai Hussein (dental photography).
Compliance with ethical standards
Conflict of interest
The author declares that there is no conflict of interest.
This study does not include any human participants or animals. It was conducted on anonymous extracted teeth.
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