Evaluation of the effect of diode laser used for dental desensitization on dentin shear bond strength of resin cement

Introduction

Laminate veneers (LV) are an alternative method for restoring anterior teeth, and are considered conservative compared with full-coverage options [1]. However, when performing a 0.3-0.9 mm buccal surface reduction for LV, there is a risk of dentin exposure, particularly in cervical area where the enamel thickness is thin [4, 5]. It has been found that approximately a quarter of buccal surface may be susceptible to dentin exposure after LV preparation [3], and the exposure of dentin tubules is a major cause of dentin hypersensitivity [6].
Dentin hypersensitivity (DH) is a significant issue that can negatively impact patients’ quality of life [7]. At present, there are no universally accepted guidelines for selecting an effective desensitizing treatment for this condition. Stimulation of nerves endings by the influx of chemical, thermal, or osmotic stimuli through the open dentin tubules leads to DH [6].
Diode laser, a form of low-level laser therapy (LLLT), has been suggested as a treatment for DH [8]. The typical wavelength range for diode laser is between 630 and 904 nm [9]. LLLT works by stabilizing nerve membranes, thus reducing DH [10], and this effect can be clinically observed within 15-30 minutes after laser irradiation [11, 12]. Additionally, diode laser treatment stimulates dentin odontoblasts and promotes gradual formation of secondary dentin, which can be observed 14 days after laser irradiation; it is effective in reducing DH over time by occluding dental tubules internally [12-15].
The success and longevity of LV restorations depends on bonding strength to enamel tissue [16, 17]. Advances in resin cements and dentin bonding agents have led to the enhancement of LV cementation when dentin is exposed [18]. The openings of dental tubules play a significant role in dentin bonding, and there are two concepts of dentin bonding that involve removal or conditioning of the smear layer [19]. The etch and rinse system, which involves the removal of the smear layer using 37% phosphoric acid, is considered the gold standard for etching [20, 21].

Objectives

The objective of this study was to evaluate the impact of diode laser irradiation at wavelengths of 635, 660, and 808 nm on the dentin shear bond strength of resin cement. The null hypothesis was that the application of diode laser at these wavelengths on exposed dentin does not affect the dentin shear bond strength of resin cement.

Material and methods

A total of forty intact extracted human maxillary third molars were obtained from patients aged between 20 and 30 years, who were seeking treatment at the Department of Maxillofacial Surgery, Faculty of Dentistry, Damascus University. All samples were stored at room temperature in 10% formalin after removal of dental plaque and periodontal tissue.

Sample preparation technique

Acrylic resin blocks were used to embed the teeth, with the buccal surface of each molar facing outward. The enamel was completely removed by employing a cylinder diamond bur (Torpedo diamond bur, Komet Dental, Gebr. Brasseler, Lemgo, Germany), which exposed the dentin. Flat areas with a diameter greater than 6 mm were created in the cervical and middle thirds of the teeth. The dentin was then polished using a fine diamond bur (#8868 314, Komet Dental, Gebr. Brasseler, Lemgo, Germany). Forty cylindrical specimens with a diameter of 3 mm and a height of 2 mm were produced by light polymerizing composite (Tetric N-ceram, Ivoclar Vivadent) using a metal matrix.
The specimens were randomly divided into four groups based on the application of diode laser (n = 10). Group 1 – the control group, in which the teeth were not exposed to diode laser irradiation. Group 2 – the dentin was subjected to 660 nm diode laser, with an output power of 250 mW in continuous wave mode CW. Energy density was set at 4 J/cm2 (Figure 1A). Group 3 – the dentin was exposed to 635 nm diode laser, with an output power of 50 mW in continuous wave mode CW. Energy density was set at 4 J/cm2. Group 4 – the dentin was treated with 808 nm diode laser using an output power of 50 mW in continuous wave mode CW. Energy density was set at 4 J/cm2. Laser was applied with a 0.7 cm2 optic fiber tip at a distance of 0.5 mm from the dental outer surface.

Cylinder cementation

The dentin was etched with 37% phosphoric acid (N-Etch, Ivoclar Vivadent) for 15 seconds (Figure 1B). Afterwards, the acid was rinsed off with a water spray (Figure 1C). Following drying of the dentin, a bonding agent (Excite F, Ivoclar Vivadent) was applied (Figure 1D). The cylinders were then cemented using light cured luting cement (Variolink, Ivoclar Vivadent, Schaan, Liechtenstein) (Figure 1E). Finger pressure was consistently applied to ensure realistic conditions for the process of cementation. Excess cement was removed with a micro-brush, and a light cure was applied (Figure 1F).

Shear bond strength test

The samples were subjected to a shear bond strength test at a crosshead speed of 0.5 mm/min until failure using a universal testing machine (TesT, 114.200kN). Mean values of shear bond strength were calculated for each group. Normality of independent variables was assessed with Shapiro-Wilk test. Subsequently, data were statistically analyzed using one-way ANOVA test to determine differences in shear bond strength among the four groups (SPSS v. 26, IBM, USA) (p = 0.05).

Results

The mean values and standard deviations of shear bond strength for the control group and laser groups using 660, 635, and 808 nm diode laser were 26.1 ± 7.2 Mpa, 23.0 ± 7.0 Mpa, 22.3 ± 7.0 Mpa, and 22.4 ± 6.5 Mpa, respectively (Figure 2). One-way ANOVA test revealed no statistically significant differences between the control group and laser groups (p = 0.347) (Table 1).

Discussion

Dentin hypersensitivity after application of laminate veneers is a common issue that can negatively impact patients’ well-being [7, 22]. Various treatments have been proposed to alleviate patient discomfort and improve their quality of life. Previous research has shown that laser applications yield better results compared with other chemical or physical treatments [23]. However, no study has explored the effects of laser irradiation prior to the placement of laminate veneers, and there are a few existing studies, which investigated the influence of dio­de laser irradiation on bond strength of composite to exposed dentin. Therefore, the objective of this in vitro study was to assess if using diode laser affects the bond strength of resin cement applied to dentin in laminate veneer cementation. In the cause of simulating the process of LV cementation, the cylindrical specimens were cemented to dentin to mimic real-world conditions. Parameters of a laser, such as wavelength, output power, energy density, irradiation time, and irradiation mode, are crucial factors in determining the efficacy of desensitizing treatments [24, 25]. In this experimental study, diode lasers with wavelengths of 660, 635, and 808 nm were selected based on previous research recommending using red and near-infrared wavelengths to reduce dentin hypersensitivity, particularly in patients aged 25-35 years [11, 12, 25, 26]. Low output powers of 50 mW and 250 mW were utilized in this study, as low-level laser therapy (LLLT) reduces hypersensitivity by desensi­tizing nerves without causing any changes in dentin structure through overheating and melting of dental surfaces [10]. An energy density of 4 J/cm2 was applied according to a previous research recommending energy densities between 2-5 J/cm2 for reducing dentin hypersensitivity [27].
The present study did not observe a significant difference in the shear bond strength of resin cement between the laser groups and the control group. The main reason for this outcome is the use of low-level laser therapy (LLLT), which reduces dentin hypersensitivity by altering permeability of nerve fiber membranes to Na+ and K+ ions, thereby inhibiting pain without constricting dentin tubules [10]. On the contrary, high output power leads to narrowing and sealing of dental tubules through melting of dentin surfaces, resulting in dentin morphology changes and blockage of dentin tubules [28-31]. Blocked dentin tubules impede the penetration of adhesive systems, preventing the formation of resin tags, which is critical during laminate veneer cementation.
Moreover, the absorption of diode laser wavelengths by dentin is minimal, signifying that diode laser irradiation does not interact with the dentin surface, and therefore does not affect dentin morphology or occlude dentinal tubules [9, 13]. In contrast, a Nd-Yag laser, which is considered the most effective desensitizing laser, has a highly absorbable wavelength for dental tissue, but it can alter dentin morphology and have a negative impact on the bond strength of adhesive systems [32-36].
The irradiation mode is also a significant factor. High output powers above 80 mW in continuous mode can lead to dental tubule occlusion within 10 seconds [28]. Similarly, high output powers above 50 mW in pulse mode can result in dentin melting and narrowing of tubules [37, 38].
study by Can-Karabulut reported no significant difference in the bond strength when dentin was treated with a 650 nm wavelength; the mean values were recorded at 10.52 Mpa compared with 11.73 Mpa for the control group [39]. However, the current study achieved higher bond strength, which may be attributed to the use of etch-and-rinse adhesive system compared with the universal adhesive system used in the Can-Karabulut study without pre-etching. Studies have shown that pre-etching with 37% phosphoric acid is an important step even with self-etch systems [20, 40]. Furthermore, pre-etching has been found to increase the shear bond strength of dentin [21].
A study by Aranha et al. [41] reported significantly lower bond strength of composite when dentin was treated with a 660 nm wavelength, with mean values recorded at 7.21 ± 4.6 Mpa. This result may differ from the current study’s findings due to the use of bovine teeth. The morphology of bovine dental tissue is different, with higher tubule density [42]. However, other studies have recommended the use of bovine dentin in adhesion studies instead of human dentin [42, 43], and reported no difference in chemical composition, number of tubules per mm, and tubule diameter between bovine and human teeth [42, 44].
A scanning electron microscopic study is required to identify the failure position and failure mode. A clinical study involving teeth indicated for extraction should be considered to assess the shear bond strength after a delay of 14 days post-laser irradiation. Clinical studies are essential to determine the ideal parameters for diode laser irradiation in reducing dentin hypersensitivity during laminate veneer treatment. Although this approach adds further steps to laminate veneer cementation, the results are promising.

Conclusions

The utilization of diode laser with low output power and wavelengths of either 660 nm, 635 nm, or 808 nm for dentin desensitization does not affect the bonding strength during the cementation of laminate veneers.

Disclosures

1. Institutional review board statement: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: This research was funded by the Damascus University, grant No. 501100020595.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References