The effects of Er:YAG laser pre-treatment on the surface characteristics of dentin and the corresponding SBS to resin composite were investigated in the current study. For this purpose, dentin specimens prepared from extracted teeth were treated with conventional acid etching or Er:YAG laser under variable energy and frequency and the surface features and SBS were characterized. The experimental and theoretical results support the adoption of Er:YAG laser pretreatment during the clinical treatment of primary teeth among pediatric patients. Er:YAG laser pre-treatment remarkably changed the morphology of the dentin surface. The application of specific pre-treatment parameters such as energy level (50–200 mJ) and frequency (5–20 Hz) clearly opened the dentin tubules and induced the protrusion of the intertubular and peritubular dentin (Figs. 2 and 3). Beyond this range, the dentin surface developed crack formations and structural disorientation. Similarly, the SBS increased significantly following Er:YAG laser pre-treatment, but decreased gradually beyond a certain range of energy and frequency values (Figs. 4 and 5).
Air turbine hand-pieces are frequently used in clinical dentistry [10–11]. Although these devices are simple to use and efficient for decay removal [12], the heat produced by mechanical friction may cause insult to pulpal tissues and pain. In addition, mechanical vibration and noise may aggravate stress and fear among young children, reducing their compliance to treatment and the quality of restorations [13]. Lasers are used for various restorative dentistry procedures and may potentially replace or minimize the use of air turbine hand-pieces [14–15]. The advantages of lasers for clinical dental applications include low noise, no vibration, and less heat production [15]. In addition, there is almost no pain and discomfort to children during caries excavation and cavity preparation, making them useful tools for the treatment of carious primary teeth [15].
The Er:YAG laser is a solid-state laser that uses erbium, yttrium, aluminum, and garnet as the medium. The wavelength is 2940 nm, which is the absorption peak for water and hydroxyapatite [16]. The water and hydroxyapatite present in dental hard tissues absorb energy to form a "micro explosion", enabling the removal of water-containing tissues through energy absorption [17]. Effective tooth cutting can be selectively carried out to retain the maximum amount of healthy tooth tissues possible and to remove carious tissues. The laser used for treatment may have a substantial influence on the dentin surface morphology. The Er:YAG laser can enhance the bonding and interface between dentin and restorative materials [18–19]. However, a few studies have reported that Er:YAG laser treatment reduces the bonding strength of dentin to resin composite [20–21].
During cavity preparation, the dental pulp is sensitive to temperature changes; however, cavity temperature fluctuations of less than 5.5⁰C degrees do not cause irreversible damage [22]. In addition, the ablation of dental hard tissues is affected by the intensity of the irradiation [23]. Lijun Ye et al. [23] reported that irradiation with a maximum power of 20 Hz and 6 W for 20 s increased the pulp cavity temperature by 3.05 ± 0.50 °C, which is well below the threshold temperature required for irreversible pulp damage. Therefore, a small dose and a short period of laser irradiation may prevent excessive removal and melting of healthy tooth tissues [23]. The power values selected in the current study were lower than 6 W, which ensured no excessive rise in the temperature during the treatment process and can be considered safe for pulp tissues. Referring to the parameter table for clinical applications of Fotona dental lasers and previous studies [8, 18, 23], laser frequency values of 5–30 Hz and energy values of 50–300 mJ were selected in the current study for the surface modification of dentin.
The SEM images showed obvious changes in the dentin surface morphology following Er:YAG laser pre-treatment with variable energy and frequency. Within a specific energy (50–200 mJ) and frequency (5–20 Hz) range, the dentin tubules showed enhanced opening and protrusion of the intertubular surface dentin, which may promote the penetration of adhesive resin tags into the dentinal tubules and improve adhesion [24]. Further increases to the energy (250–300 mJ) and frequency (25–30 Hz) led to the formation of surface cracks and disintegration of the structural dentin. These findings are in agreement with previous studies [25–26], which demonstrated that the Er:YAG laser at a certain energy and frequency range promotes the opening of dentinal tubules by effective removal of the smear layer [27]. Beyond this optimal range of energy and frequency, laser pre-treatment leads to crack formation and disintegration of dentin due to excessive energy and heat to the tissues.
Corresponding to the surface changes, bond strength testing showed significantly increased SBS for the dentin slices bonded to resin composite following Er:YAG laser treatment. As the Er:YAG laser treatment was non-contact, no smear layer was formed on the dentin tubules, peritubular dentin, or intercellular dentin [25]. The Er:YAG laser treatment improved the dentin surface roughness and surface area by opening the dentinal tubules and facilitated the penetration of adhesive into the dentin tubules, resulting in improved bonding [24]. In contrast, drilling dentin with an air turbine hand-piece leads to the formation of a smear layer on the surface of the dentin, blocking the dentinal tubules as well as interfering with the penetration of the resin adhesive and bonding with restorative materials. Therefore, to achieve better adhesion with tooth tissues, it is vital to reduce the formation of a smear layer and to remove the formed smear layer from the tooth surface. These findings are consistent with previous studies. Yazici et al. [19] reported that the application of the Er:YAG laser using a self-etching bonding system can increase the dentin SBS. In contrast, Ferreira et al. [28] indicated that dentin surfaces treated with the Er:YAG laser exhibited SBS values below that obtained with the traditional phosphoric acid etching treatment. This result may be related to the existence of a wide range of microcracks under the dentin surface after laser treatment. Acid etching is a technique-sensitive process and a minor variation in the etching time may change the surface and structural properties of the dentin [29]. Dunn et al. [30] suggested that the bonding strength of enamel following laser irradiation was significantly lower than that obtained with acid etching alone, possibly due to the high laser energy density (140 mJ/30 Hz). These variations in the bonding test results reported by different scholars suggest a lack of consensus, which is related to the use of variable parameter settings for laser treatment, laser brand selection, and type of adhesive [21, 31–32].
In the present study, the SBS increased gradually within a specific range of Er:YAG laser energy (50–200 mJ). However, increasing the energy further gradually reduced the SBS. This outcome is associated with the disintegration and structural changes in dentin tubules corresponding to the excessive energy. Ceballo et al. [33] and Koliniotou-Koumpia et al. [20] reported denaturing of the tooth surface collagen fiber following laser irradiation. The denaturing of collagen fibers reduces surface porosity, which is not conducive to the penetration of adhesive [34].
The formation of a fracture can be influenced by the bonding area. A larger bonding area may lead to the formation of more stress concentration points in the sample matrix, which facilitates internal fracture [35]. On the other hand, a smaller bonding area results in more uniform stress distribution, reducing the likelihood of cohesive fracture [36], which can reflect the true bond strength of the material. In the current study, 77.34% of the observed specimens had interface fractures (adhesive failures), followed by mixed fractures, which reflected the actual fracture modes for the dentin and resin. The data suggested that Er:YAG laser treatment (for all groups) made no significant difference in the failure mode compared to acid etching.
There are a few limitations worth noting. This in vitro study was carried out using healthy extracted teeth; therefore, the findings may be different for the oral environment during actual clinical applications. Our findings are based on healthy dentin; however, the effect of the Er:YAG laser on carious dentin may differ from that of normal dentin. Further experiments are required with different conditions and parameters.