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Lasers in Dental Science

, Volume 2, Issue 4, pp 213–219 | Cite as

Effect of duration of Er,Cr:YSGG laser etching on dentin morphology: an in vitro study

  • F. Mahdisiar
  • A. Mirzaei
  • A. Fallah
  • N. Gutknecht
  • S. Akhoundan
Original Article

Abstract

Introduction

The aim of this study was to evaluate the effect of different durations of Er,Cr:YSGG laser irradiation on dentin morphology using scanning electron microscopy (SEM).

Materials and methods

Twenty-five extracted human-impacted permanent third molars were cut below the occlusal pit and fissure level, perpendicular to the long axis of the tooth. An occlusal area measuring 5 mm in length and 5 mm in width was prepared on each tooth for laser irradiation. The teeth were randomly divided into five groups for different durations of laser irradiation: T1, 5 s; T2, 10 s; T3, 20 s; T4, 40 s; T0, no laser irradiation. The effects of laser application on dentin surfaces were evaluated using SEM at × 80 and × 500 magnifications. Also, the presence/absence of smear layer was scored according to SEM findings. Score 0 indicated the absence and score 1 indicated the presence of smear layer.

Results

Forty seconds of irradiation resulted in an irregular dentin surface without smear layer, with open dentinal tubules and without enlargement.

Conclusion

Laser preparation of dentin creates a retentive surface for composite restoration, without the problems caused by smear layer after conventional preparation. Also, the disadvantages of acid etching can be avoided as such.

Keywords

Er,Cr:YSGG laser Dentin conditioning SEM Dentin morphology Phosphoric acid etching 

Introduction

The conventional method of cavity preparation by rotary instruments is not favored by many patients. On the other hand, dentin prepared as such is covered with smear layer, which is composed of dental hard tissue, carious debris, and residual bacteria. This decreases the dentin surface energy and prevents adequate adhesion to dentin [1, 2].

Dentin is a major component of teeth. Dentin is a complex substrate for bonding due to its heterogeneous composition, mainly organic structure, hydrophilic nature, and morphological variations. Introduction of adhesive primers with enhanced hydrophilicity for dentin surface conditioning and providing a stronger bond to more hydrophobic adhesive resins largely resolved this issue [3, 4, 5, 6]. The conventional method of forming a strong bond to dentin is via phosphoric acid etching and removal of the mineral content to create microporosities within the collagen network. Upon removal of the hydroxyapatite crystals of the outer layer of dentin, about 50% unfilled space and about 20% of water remain in the dentin surface. In order to obtain a strong bond, resin should infiltrate into the collagen scaffold and form a hybrid layer. The primer also penetrates into the dentinal tubules concurrent with the formation of the hybrid layer. This results in formation of quite large resin tags. After etching, the tooth should be rinsed with air and water spray to thoroughly remove the acid and stop the etching process [7, 8, 9]. Otherwise, cysteine cathepsins, which can be activated in mildly acidic environments, may also activate matrix-bound matrix metalloproteinases and destabilize the hybrid layer in long term [10, 11]. If the etching time is too long and the etched zone is too deep, decalcified dentin may not be fully impregnated. The etched but not impregnated space may serve as a mechanically weak zone. After rinsing, drying of dentin must be performed cautiously. Even a short air blast from an air–water spray can inadvertently dehydrate the outer surface and cause the remaining collagen scaffold to collapse. Once it happens, the collagen mesh prevents the penetration of primer and bonding will fail. On the other hand, excess moisture tends to dilute the primer and interfere with resin penetration [7, 8, 9]. Excessive acid conditioning causes incomplete infiltration of resin monomers and creates a gap between resin tags and dental structure that decreases the bond strength by creating a weak zone [12]. In conventional surface treatment, the primer penetrates into the fluid-filled dentinal tubules. It is generally under-cured and forms soft flexible tags [7, 8]. Today, laser system, as a novel modality, has been suggested for use as an alternative to dentin surface etching. Among laser systems, the erbium family of lasers is believed to be the most successful. There are several studies that have explored various parameters such as laser power and frequency for dentin etching and surface conditioning for proper bonding [3, 4, 5]. But no study has investigated the effects of duration of Er,Cr:YSGG laser etching on dentin surface morphology.

The aim of this study was to evaluate ultrastructural morphological changes in dentin following different durations of Er,Cr:YSGG laser irradiation using scanning electron microscopy (SEM).

Materials and methods

Sample preparation

Twenty-five extracted human-impacted permanent third molars were used in this study. Soft tissue residues were completely removed from the tooth surfaces with a dental scaler. All teeth were then stored in distilled water containing 0.4% thymol for 1 week for disinfection.

Then, samples were stored in distilled water at room temperature until the experiment. Each tooth was cut below the occlusal pit and fissure level, perpendicular to the longitudinal axis of the tooth by means of a high-speed handpiece and silicon carbide disc to remove the occlusal enamel and expose the superficial dentin surface. Next, an area measuring 5 mm in length and 5 mm in width was prepared on the occlusal surface of each tooth for laser irradiation.

Laser application

The marked occlusal area was irradiated with Er,Cr:YSGG laser (Biolase, USA) at a wavelength of 2780 nm. The laser parameters were as follows:

Output power

4.5 W

Peak power

1500 W

Energy density per pulse

8.57 J/cm2

Energy per pulse

0.09 J

Frequency

50 Hz

Water

80

Air

60

Pulse duration

60 μs

Tip diameter

600 μm

Cross section of tip

0.028 cm2

Angle of radiation

8

Irradiation surface

1.16 mm

Distance

2 mm

The teeth were randomly divided into five groups according to the duration of laser irradiation:
  • T1, 5 s

  • T2, 10 s

  • T3, 20 s

  • T4, 40 s

  • T0, no laser irradiation

After laser irradiation, the samples were stored in distilled water.

SEM analysis

The effects of laser irradiation on dentin surfaces were evaluated using SEM at × 80 and × 500 magnifications. Prior to SEM analysis, the samples were vacuum-dried and sputter-coated with gold for 180 s. SEM observations were carried out at an accelerated voltage of 20 kV with 25 mm working distance. SEM findings were scored to evaluate the effect of duration of laser irradiation on the smear layer as follows:
  • Score 0 = absence of smear layer

  • Score 1 = presence of smear layer

More SEM images were obtained from sample number 4 at × 1.00 K, × 3.00 K, × 5.00 K, × 10.00 K, and × 20.00 K magnifications.

Results

Analysis of the results with the Mann–Whitney U test showed that 40 s of irradiation in T4 group caused significant removal of the smear layer compared to T0 group (P = 0.008). Other durations of radiation did not completely remove the smear layer (P = 1, Table 1).
Table 1

Effect of duration of irradiation on the smear layer

Duration of irradiation

T1

T2

T3

T4

T0 = control

Exposure 1

1

1

1

0

1

Exposure 2

1

1

1

0

1

Exposure 3

1

1

1

0

1

Exposure 4

1

1

1

0

1

Exposure 5

1

1

1

0

1

0 = smear layer was not observed

1 = smear layer was observed

T1–T4 = different durations of irradiation: T1, 5 s; T2, 10 s; T3, 20 s; T4, 40 s; T0, no irradiation

Exposure 1–5 = number of irradiated areas by Er,Cr:YSGG laser

SEM morphological analysis of the specimens showed different characteristics according to the surface pretreatment, as described below:

Control group (T0): The surface was covered with smear layer (Fig. 1).
Fig. 1

SEM micrographs of the dentin surfaces pretreated only with silicon disc (control group). a × 80. b × 500

Er,Cr:YSGG laser irradiation for 5, 10, and 20 s: The dentin surface in these groups revealed different amounts of the smear layer (Figs. 2, 3, and 4).
Fig. 2

SEM micrograph of the dentin surfaces pretreated with Er,Cr:YSGG laser irradiation for 5 s. a × 80. b × 500

Fig. 3

SEM micrograph of the dentin surfaces pretreated with Er,Cr:YSGG laser irradiation for 10 s. a × 80. b × 500

Fig. 4

SEM micrograph of the dentin surfaces pretreated with Er,Cr:YSGG laser irradiation for 20 s. a × 80. b × 500

Er,Cr:YSGG laser irradiation for 40 s: Dentin surface in this group showed an irregular pattern without the smear layer, with open dentinal tubules and no enlargement. A prominent peritubular dentin appearance suggested greater removal of intertubular dentin due to its higher water sorption. There were no evident signs of melting or microcracks (Fig. 5).
Fig. 5

SEM micrograph of the dentin surfaces pretreated with Er,Cr:YSGG laser irradiation for 40 s. a × 80. b × 500

Among the different time durations of Er,Cr:YSGG laser irradiation, only 40 s of laser irradiation caused smear layer removal from the dentinal tubules. According to the results in group 4, further SEM analyses at × 80 K, × 500 K, × 1.00 K, × 3.00 K, × 5.00 K, × 10.00 K, and × 20.00 K magnifications were performed in this group (Fig. 6).
Fig. 6

SEM micrograph of the dentin surfaces pretreated with Er,Cr:YSGG laser irradiation for 40 s. a × 1.00 k. b × 3.00 k. c × 5.00 k. d × 10.00 k. e × 20.00 k

Discussion

The quality of the dentin-resin interface plays an important role in achieving a high quality and durable composite restoration [13]. Dentin preparation by rotary instruments creates smear layer on dentin surface that causes problems in obtaining suitable bond between the adhesives and dentin. On the other hand, the conventional method of smear layer removal includes the use of phosphoric acid on dentin for 15 s. This method has limitations such as (1) demineralization that occurs with the removal of dentin mineral content, (2) over-etching since by increasing the duration of etching, greater depth of dentin is demineralized, (3) inadequate washing of the etchant results in unwanted continuation of the etching process, and (4) over-drying causes the collagen network to collapse and under-drying dilutes the primer.

After the application of bonding agent, resin tags form by penetration of primer into the fluid-filled dentinal tubules. These resin tags are generally under-cured, soft, and flexible. In addition, the interface is prone to nanoleakage because of gap formation between tags and dentin due to incomplete penetration of adhesive [3, 7].

In the 1990s, erbium lasers were introduced for preparation of hard tissue as an alternative to rotary instruments. Er,Cr:YSGG laser (emitting at a wavelength of 2.79 μm) is an effective tool for removal of dental hard tissues [14, 15]. This wavelength is absorbed by the hydroxyapatite and water. The hydroxyl radicals and water in hydroxyapatite crystals receive most of the laser energy. By water evaporation in the tooth mineral components, a large volumetric expansion occurs [1, 2]. Next, microexplosions occur that remove the hard tissue from the irradiated regions [16]. It has minimal side effects on the sound tooth structure [1].

Dentin conditioning with laser has advantages. As reported in some studies, the laser settings can be adjusted to physically etch the dentin surface. Power, frequency, and other parameters can be adjusted to prevent smear layer formation on the dentin surface. Laser does not cause dentin demineralization. It does not have the risk of over-etching or over/under-drying. The erbium laser-treated dentin is dehydrated prior to priming and bonding; thus, the resin tags are more likely to be long and strong [1, 17, 18].

Of studies on the effect of different laser parameters on dentin morphology, no study investigated the effect of various durations of Er,Cr:YSGG laser irradiation on dentin surface morphology.

Dentin irradiated with Er,Cr:YSGG laser shows a microscopically rough surface [19, 20] without demineralization, open dentinal tubules [21, 22, 23], no smear layer, and satisfactory sterilization of the cavity [24]. These characteristics are considered as an advantage of laser preparation if composite resins are to be applied as the filling materials [25].

The Er,Cr:YSGG laser setting used in this study included 4.5 W average power, 1500 W peak power, 0.09 J energy per pulse, 50 Hz frequency, 8.57 J/cm2 energy density, 80% water and 60% air, pulse duration of 60 μs, and distance of 2 mm above the surface. The energy density used in our study was not within the ablation range. Only dentin surface was etched and conditioned for the bonding process. Five, 10, and 20 s of laser irradiation caused different amounts of smear layer. The applied Er,Cr:YSGG laser setting with 40 s of duration caused a scaly-like appearance on the surface with less homogenous and less regular surface creating a microretentive pattern on dentin without heat injury or melting, which is favorable for bonding process. The dentin surface showed no smear layer; dentinal tubules were open; and the subsurface was not demineralized. Open tubules and absence of smear layer are additional factors that enhance bonding to laser-treated dentin [14]. This can be explained by microexplosions at the tissue surface, resulting from the sudden boiling of water within the tissue (thermo-mechanical ablation) [26]. The results obtained from this study can be used in further studies to evaluate the composite bond strength with different bonding systems.

Conclusion

Forty seconds of laser irradiation with the aforementioned parameters eliminated the smear layer from the dentin surface, and the obtained surface had microretentive pattern on dentin and open tubules without heat injury or melting and demineralization which was suitable morphology for bond to composite resin.

Laser irradiation for less than 40 s could not completely remove the smear layer from the surface.

Each one of these surfaces could have optimum bonding with composite by applying different adhesives systems which should be investigated in further studies.

Notes

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Informed consent

This article was done on extracted human third molars, and it does not include any human participant. For this type of study, formal consent is not required.

Supplementary material

41547_2018_38_MOESM1_ESM.docx (34 kb)
ESM 1 (DOCX 33 kb)

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Copyright information

© Springer Nature Switzerland AG 2018

Authors and Affiliations

  • F. Mahdisiar
    • 1
  • A. Mirzaei
    • 2
  • A. Fallah
    • 3
  • N. Gutknecht
    • 3
  • S. Akhoundan
    • 1
  1. 1.Dental Branch, Department of Restorative DentistryIslamic Azad UniversityTehranIran
  2. 2.Laser Research Center of Hamadan Medical Science UniversityHamadanIran
  3. 3.Clinic of Conservative Dentistry, Periodontology and PreventionUniversity Hospital of the RWTH AachenAachenGermany

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