Effects of Pulsed Nd:YAG Laser on Tensile Bond Strength and Caries Resistance of Human Enamel

The field of adhesive dentistry has achieved remarkable progress over the past decades. This development is largely attributed to the major advancements in enamel surface pretreatments, which roughen the enamel surface and offer more microspace for resin monomers to penetrate and form resin tags. The microspace significantly increases the bond strength of resin to the enamel.^1,2  The acid-etching technique that usually uses 35% phosphoric acid gel is one of the most common enamel surface pretreatments for resin bonding.³  However, the possibility of decalcification that leaves the enamel at the resin restoration margin or around orthodontic attachments more susceptible to caries attack cannot be ignored because it ultimately leads to treatment failures.^4-6  Therefore, enamel surface pretreatment needs to be improved to maintain clinically useful bond strengths while minimizing enamel loss; enamel surface pretreatment also needs to be simplified to reduce the bonding steps.^7,8 

Various lasers have recently elicited considerable interest in the field of dentistry because of their function in the detection and removal of enamel caries, diagnosis of dental caries, cavity preparation, etching of teeth for resin bonding, demineralization prevention, root canal sterilization, and so on.^9-11  Lasers have received significant attention in resin bonding, especially in terms of their roughness and demineralization prevention effect on dental hard tissues.^11,12  Laser etching eliminates the need for separate steps of water spraying and air drying, thereby reducing procedural errors and saving time.

Pulsed Nd:YAG laser emits a wavelength of 1.064 μm. It is capable of promoting the fusion and recrystallization of dental hard tissue surfaces and causing an irregular honeycomb or crater morphology, which increases surface microhardness and contributes to resin bonding.^12,13  In our previous studies, pulsed Nd:YAG laser was found to contribute significantly to root canal sterilization and to enhance the bond strength/microleakage of resin to human dentin.^14,15  However, the enamel is less pigmented than the dentin, and the Nd:YAG laser wavelength is preferentially absorbed in pigmented tissues. Therefore, the effects of pulsed Nd:YAG laser on the bond strength of human enamel may differ from its influence on human dentin.

This study aims to investigate the effects of pulsed Nd:YAG laser on the bond strength and acid resistance of human enamel to provide a potential enamel surface pretreatment for resin bonding.

A total of 201 human premolars that are caries-free and freshly extracted for orthodontic treatment were selected after approval from the Institutional Review Board. From the 201 human premolars, 21 teeth were used for morphology observations, 100 for tensile bond strength (TBS) testing, and 80 for acid resistance evaluation. The roots were removed and the buccal surfaces were ground using a low-speed cutting machine (MF-PERFECTA, Bürmoos, Austria) under a water coolant. A flat surface more than 4 × 4 mm in enamel was prepared and polished with 200-, 400-, and 600-grit silicon carbide abrasive papers.

The prepared specimens were conditioned with the treatments according to the different experiments in this study. The surface-conditioning methods used in this study are presented in Table 1.

The 21 prepared specimens were divided into seven groups (three specimens for each group). They received no treatment, acid etching, or laser irradiation with five different laser-parameter combinations (Table 2). The treated specimens were cleaned with distilled water, dehydrated in a graded series of alcohol solutions (70%, 90%, and 100%) for 10 minutes at each concentration, and then sputter-coated with gold. The morphologic changes were examined using a scanning electron microscope (SEM) (Jeol JMS 5200, Tokyo, Japan).

The 100 prepared specimens were randomly divided into 10 groups (n=10). Five groups were used to optimize laser-parameter combinations, whereas the other five were utilized to compare pulsed Nd:YAG laser with 35% phosphoric acid gel (Table 2).

The specimens were embedded in self-cured acrylic resin (Shanghai Dental Factory, Shanghai, China). The prepared buccal enamel surfaces were maintained in a face-up position and about 1 mm higher than acrylic resin to keep them unaffected. A thin paper with a 4 × 4 mm hole was fixed on the enamel surface of each specimen. The treatments were performed as shown in Table 2. A two-step etch-and-rinse adhesive system (Adper Single Bond 2, 3M/ESPE, Irvine, CA, USA) was used according to the manufacturer's instructions. The adhesive was applied with disposable microbrush tips and light-cured for 10 seconds with a halogen curing light (Elipar 2500, 3M ESPE, St Paul, MN, USA) under a light intensity of 800 mW/cm² after the excess solvent was evaporated with a gentle air stream for 5 seconds. A 4-mm high composite buildup that entirely covered the prepared enamel surface was made with a micro-hybrid resin composite (Filtek Z250, A2, 3M/ESPE) and light-cured for 80 seconds (20 seconds for each side). All specimens were immersed in distilled water at 37°C for 24 hours. The TBS values were tested using a universal testing machine (WDW-10, Panasonic, Osaka, Japan). Each specimen was mounted on the machine, after which tensile force was applied at a constant speed of 0.5 mm/min through the self-made clamps. The force values (N) at the failure point were recorded and converted to MPa by dividing them with the exposed enamel surface (16 mm²).

The fractured surfaces of the specimens were examined under stereomicroscope (10×) and SEM. The fracture patterns were assessed by adhesive remnant index (ARI), which was based on the remaining resin amount on the enamel surface (ARI score: 0, no resin left; 1, little resin left; 2, resin covering less than half of the bonding surface; 3, resin covering more than half of the bonding surface; and 4, resin covering the whole bonding surface).¹⁶ 

The 80 prepared specimens were randomly divided into eight groups (n=10). Before undergoing the treatments listed in Table 3, the specimens were coated by an acid-resistant varnish (nail polish) with a 4-mm square window on the enamel surface left uncoated. The specimens were then subjected to an artificial demineralization solution (0.1 M lactic acid) for 48 hours, as described in a previous study.¹⁷  The demineralization solutions were analyzed using a calcium-selective electrolyte analyzer (ISE-trol AVL 9180, AVL Scientific Corp, Roswell, GA, USA) to evaluate the concentration of calcium ion (Ca²⁺) dissolved from the treated enamel surfaces in each group. The fully dried specimens were longitudinally sectioned and examined by SEM. An electron probe microanalyzer was used to determine the mean demineralization depths in the enamel surfaces of each group.

The data were analyzed with SPSS 13.0 software. One-way analysis of variance (ANOVA) was used to determine whether a significant difference existed between the experimental groups on TBS, Ca²⁺ concentrations, and artificial caries depths. Pearson chi-square test was used to analyze the frequencies of the specimens in each ARI score.

SEM images showed that the untreated enamel surfaces (Figure 1A) were coated with a smear layer that was replaced by a typical acid-etched pattern with a regular rough surface after 35% phosphoric acid gel treatment (Figure 1B). The laser irradiation caused uneven surface melting and resolidification in the enamel surface and the formation of numerous irregular honeycombs or craters (Figure 1C through G). The honeycombs or craters surrounded by abundant bubble-like cavities and numerous micropores were scattered on the enamel surface irradiated with low frequency (15 Hz) (Figure 1C,E), but became relatively equally distributed with an increase in frequency (20 Hz) (Figure 1D,F,G). The diameter of the honeycombs or craters was enlarged with a change in power from 1.0 W to 2.0 W. The enamel surfaces treated with 1.5 W/20 Hz and 2.0 W/20 Hz showed more etch-like appearance than those with other laser-parameter combinations.

The mean TBS values are shown in Table 2. In the five laser irradiation groups, TBS values increased and subsequently decreased with an increase in laser output energy. The mean TBS values significantly increased (p<0.01) when the power increased from 1.0 W to 1.5 W, and reached the highest value of 13.87 ± 1.91 MPa at 1.5 W/20 Hz. However, the values significantly decreased at a power of 2.0 W. The data indicate that the optimal laser-parameter combination for enamel pretreatment before bonding is 1.5 W/20 Hz.

Compared with the 35% phosphoric acid group whose TBS mean was 21.50 ± 3.02 MPa in this study (Table 2), the pulsed Nd:YAG laser group with optimized laser-parameter combination showed less efficient effects on TBS value, though its TBS mean (14.45 ± 1.67 MPa) was significantly higher than that of the untreated group (3.48 ± 0.35 MPa) (p<0.01). The combination of acid etching and laser irradiation (12.42 ± 0.39 MPa) showed no significant decrease compared with the laser group (p>0.05), whereas the combination of laser irradiation and acid etching significantly increased the TBS value. The TBS mean of the latter combined methods (26.64 ± 5.22 MPa) was significantly higher than that of the acid etching group (p<0.01), indicating that the irradiation of pulsed Nd:YAG laser before etching significantly contributed to the TBS of resin to human enamel.

The frequencies of the specimens in each ARI column are shown in Table 4. The ARI scores for each group are calculated as follows: 34, group D; 30, group B; 23, group C; 20, group E; and 7, group A. These scores were positively related to the TBS values. The SEM images of the fractured enamel surfaces showed that the remnant resin covered almost the whole bonding surface in most specimens of groups B and D (Figure 2B,D). By contrast, only a small amount of remnant resin was observed in group A (Figure 2A). The remnant resin covered more than half of the bonding surface in most specimens in groups B and E (Figure 2C,E). The SEM images of resin/enamel interfaces showed the thinnest gaps in groups B (Figure 3A) and D (data not shown), followed by those in groups C (Figure 3B) and E (data not shown). A clear gap was observed in group A (Figure 3C). The morphologic data enhanced understanding of the TBS results.

The effects of laser-parameter settings on enamel surface demineralization were tested (Table 3). Group III showed the lowest Ca²⁺ concentration dissolved from enamel surfaces, followed by group IV. No significant difference was observed between the two groups (p>0.05), but they were significantly lower than those of the other three groups (p<0.01). The Ca²⁺ concentration in group V was significantly higher than those in all other groups (p<0.05). The data showed that the moderate laser-parameter combinations of 1.5 W/15 Hz and 1.5 W/20 Hz significantly contributed to enamel acid resistance.

In the comparison experiments, the pulsed Nd:YAG laser showed the lowest Ca²⁺ concentration (Table 3), whereas the 35% phosphoric acid showed the highest Ca²⁺ level dissolved from enamel surfaces, which was even significantly higher than that in the untreated group (p<0.05). SEM investigations showed that several shallow demineralization islands (Figure 4A) were found in the enamel surfaces of the laser irradiation group, whereas whole demineralization defects were found in the acid-etching (Figure 4B) and untreated groups (Figure 4C). Demineralization depth measurements (Figure 5) showed that the mean demineralization depth was only 47.78 ± 6.31 μm in the laser irradiation group, whereas it was 236.42 ± 11.88 μm in the acid etching group. This value was also significantly higher than that in the untreated group (179.06 ± 13.25 μm) (p<0.05). The data indicate that enamel acid resistance was significantly enhanced by the pulsed Nd:YAG laser, but adversely affected by 35% phosphoric acid.

Enamel pretreatment is critical for the application of composite resin in the direct bonding of orthodontic attachments or the esthetic restoration of anterior tooth defects.^18,19  Evidence has shown that the Nd:YAG laser can cause an irregular honeycomb or crater morphology on the enamel surface by irradiation, and thus, presents a potential use in enamel pretreatment.¹³  To establish an alternative enamel pretreatment method, we investigated the effects of pulsed Nd:YAG laser on the TBS and caries resistance of human enamel, given that acid etching increases the risk of caries.

Previous studies have indicated that lasers produce morphologic modifications on dental hard tissue by melting and solidification.^20,21  These alterations include honeycombs or craters and numerous microcavities or micropores at their periphery, which may offer microspace for the sealant or composite anchorage.^22-24  However, previous studies have shown conflicting results because of the high number of variables involved in the lasing process, such as power, pulse frequency, and duration of irradiation.²⁵  Laser-parameter combinations should be optimized to maximize the potential of pulsed Nd:YAG laser. We found that laser output energy level is crucial in the effects of pulsed Nd:YAG laser on enamel morphologic modifications. Low power and frequency were insufficient to alter enamel surface, and the formed honeycombs or craters were scattered. An increase in laser output energy resulted in relatively equally distributed honeycombs or craters and etching-like appearances on the enamel surfaces. This finding was consistent with the report of Bedini and others²⁶  that a high-energy level of Nd:YAG laser creates a retentive surface suitable for sealant or composite anchorage.

The Nd:YAG laser is less efficient in ablating the dental hard tissues compared with the Er:YAG laser, but it is superior in inducing morphologic alterations that are essential in providing microspaces for resin monomer penetration and in increasing enamel acid resistance.²⁷  In this study, we investigated the different output energy levels of pulsed Nd:YAG laser and found that neither low nor high laser-parameter combinations contributed to the TBS of resin to enamel. On one hand, significantly low laser-parameter combinations were proven inadequate in altering enamel surfaces and in offering sufficient microspace for resin monomers to penetrate. On the other hand, significantly high laser-parameter combinations induce excessive melting and recrystallization, as well as decrease the enamel surface free energy and microhardness that seriously damage the bond strength of resin to enamel. Moderate energy levels are more efficient in increasing the bond strength of resin to enamel. These findings were inconsistent with the study of Kwon and others²⁴  who reported that a higher laser power produces higher bond strength than a lower laser power. This phenomenon is possibly caused by the different laser scan speeds and irradiation times used in this study, which resulted in the different energy levels in the unit area.

Although considerable TBS values were achieved with pulsed Nd:YAG laser treatment, they remained significantly lower than that of 35% phosphoric acid treatment, as presented in previous reports.^21,28  However, the difference in the current study is that laser irradiation, followed by etching, significantly increased the TBS of resin to enamel. We speculate that the subsequent treatment of 35% phosphoric acid further roughened the lased enamel surfaces and simultaneously eliminated the weak structures caused by the Nd:YAG laser.

Among the physical and chemical alterations of lasers on enamel surfaces, the enhancement of acid resistance has immense advantages in adhesive dentistry. A number of studies have demonstrated that lasers significantly alter the permeability, crystallinity, and acid-solubility of the enamel surface, thus promoting its resistance to demineralization.^29-31  In addition, lasers promote fluoride uptake and prolong the fluoride-releasing time in an oral environment.³²  However, the influence of laser output energy parameters on enamel acid resistance has not been fully discussed. In this study, the effect of pulsed Nd:YAG laser on Ca²⁺ concentration dissolved from human enamel surfaces was significantly affected by the laser-parameter combinations. Moderate, not low or high, laser-parameter combinations were proven efficient in preventing enamel surface demineralization. Previous studies have indicated that the thermal effect of a laser is the main factor that prevents the demineralization of enamel.^33,34  A lased enamel surface is subjected to water loss from 80°C to 120°C, to decomposition of organic substance at 350°C, to initial loss of carbonate hydroxyapatite from 400°C to 660°C, and to enamel melting at more than 800°C to 1000°C with the formation of β- and α-TCP (tri-calcium-phosphate), which is potentially soluble in an acid environment.^30,33  The foregoing are the reasons why the moderate laser-parameter combinations were tested for efficiency in the acid resistance of enamel surface in this study. Low laser-parameter combinations were insufficient to induce melting and recrystallization, which are essential factors in the acid resistance of enamel surfaces. However, high laser-parameter combinations led to the formation of acid-soluble compounds, such as β- and α-TCP, which increased the demineralization of enamel surfaces.

Unlike laser ablation, phosphoric acid etching modifies enamel surface morphology by the selective removal of interprismatic mineral structure; the organic materials are less affected.³⁵  This regularly rough and extensively microfissured structure is very useful in increasing the retention of resin composites for adhesion but is very vulnerable to acid attack. Acid etching does not improve the crystalline structure and block the ion diffusion channel, but it removes the acid-protecting superficial enamel layer.³⁶  In this study, 35% phosphoric acid significantly increased the mean Ca²⁺ concentration dissolved from enamel surfaces and the demineralization depth. Data and SEM observations suggest that the pulsed Nd:YAG laser-treated enamel subsurface presents fewer soluble compounds and may be more acid-resistant during pH cycling.

Previous studies have reported that a minimum bond strength of 5.9 MPa to 7.9 MPa could result in successful clinical bonding.^37,38  The mean TBS value of 14.45 ± 1.67 MPa in the pulsed Nd:YAG laser group in this study is adequate to meet the clinical need for dental adhesion, although this value is significantly lower than that in the 35% phosphoric acid group. Pulsed Nd:YAG laser has numerous advantages when used as an enamel surface pretreatment method. Pulsed Nd:YAG laser can eliminate the need for the separate steps of water spraying and air drying, which are essential in the acid-etching technique. In contrast to acid-etching that increased the risk of caries attack, pulsed Nd:YAG laser enhanced the acid resistance of enamel surfaces in this study. In addition, the Nd:YAG laser has been reported to be more efficient than the acid-etching technique in reinforcing the decreased bond strength of resin composite to recently bleached enamel.^39,40  Therefore, the pulsed Nd:YAG laser can be used as an alternative enamel pretreatment method, especially in the caries-susceptible population or in individuals who recently underwent 35% hydrogen peroxide bleaching of the teeth. However, the proposed method needs further investigation in our future studies.

The effects of pulsed Nd:YAG laser on morphologic modification, TBS, and acid resistance of human enamel were influenced by laser output energy parameters. The moderate laser parameters significantly enhanced the TBS and acid resistance of human enamel, although high-energy level laser parameters were proven efficient in altering the enamel surface. The pulsed Nd:YAG laser was less efficient in increasing the TBS of resin to enamel compared with the acid-etching technique, but the proposed method significantly contributed to the acid resistance of human enamel. The combinations of pulsed Nd:YAG laser and 35% phosphoric acid showed the highest mean TBS value in this study.

This work was supported by grants from the National Natural Science Foundation of China (Grant 31070863), the Natural Science Foundation Projects of Chongqing (Grant CSTC2010BB5161), and the Project of State Key Laboratory of Trauma, Burns, and Combined Injury (Grant SKLKF201212), China.

The authors have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.