Effect of Nd:YAG Laser Pulse Energy on Mercury Vapor Release from the Dental Amalgam

Abstract

Objective: The aim of this study was to evaluate the effect of different pulse energies of Nd:YAG laser on the amalgam ablation, and its effect on the amount of mercury vapor release from amalgam. Background data: Toxic vapor release from amalgam restorations at the laser focus site is possible. Materials and methods: Forty-five amalgam samples (4 mm in diameter and 5 mm in height) were placed in sealed containers and underwent Nd:YAG laser irradiation with pulse energies of 50, 150, and 250 mJ at a distance of 1 mm from the amalgam surface for 4 sec. Subsequently, 150 mL of air was collected from the inside of the container using an Apex Pump to analyze the amount of mercury vapor in the air samples using a mercury vapor analyzer. Data were analyzed using Kruskal–Wallis and Mann–Whitney U tests (p<0.05). Results: The amount of mercury vapor release significantly increased with an increase in the pulse energy of Nd:YAG laser (p<0.001). In addition, the amount of mercury vapor release with 250 mJ pulse energy was significantly higher compared with the standard mercury vapor concentration (50 μg/m³) (p<0.001). Nd:YAG laser produced cavities on the amalgam surface, which increased in size with an increase in the energy of the laser beam. Conclusions: The amount of mercury vapor significantly increased with an increase in the pulse energy of the laser beam, and was significantly higher than the standard mercury vapor concentration with 250 mJ pulse energy.

Introduction

T he use of ruby laser (693.4 nm) for hard tissue ablation was first described in 1965.¹ After that, different types of lasers (Nd:YAG [1.065 μm], CO₂ [9.6 μm], Ho:YAG [2.12 μm]) were examined. However, their use was eventually abandoned for hard tissue preparations, because these lasers caused increased temperatures in the dental pulp, microcracks, and carbonization.^2
–4 During the late 1980s, new lasers (Excimer and Er lasers) were developed, which had improved temperature control and smaller preparation depths.^5,6 With the present lasers, it is possible to incise and re-contour soft tissues efficiently and predictably with immediate homeostasis and minimal postoperative pain during periodontal and oromaxillofacial surgeries.^7,8 The application of lasers to hard tissues includes detection,⁹ prevention,¹⁰ and removal of carious lesions,¹¹ cavity preparation,^12,13 alleviation of tooth hypersensivity,¹⁴ bleaching procedures,¹⁵ and root canal treatment procedures.¹⁶

The idea of replacing a drill with a laser has led to its introduction in restorative dentistry. Apart from being more acceptable to patients, any laser type used to replace routine drills in restorative dentistry should be able to remove tooth hard structures and restorative materials such as composite resins.¹⁷ Recently, lasers have been used to ablate ceramic¹⁸ and amalgam restorations, also.^19,20 Hibst and Keller examined the effects of Er:YAG laser on cements, composites and amalgam, and concluded that ablation efficacy was sufficient for clinical applications, but pointed out that knowledge about the composition of the released toxic vapors was essential to evaluate potential risks.¹⁹ According to Pioch and Matthias, Nd:YAG, Er:YAG, and Nd:YLF lasers can ablate amalgam if they come into contact with amalgam, with the highest ablation ability being with the Nd:YAG laser.²¹

Furthermore, as a result of an increase in the application of Nd:YAG laser in various restorative, endodontic, and periodontal treatments, in which many of these treatments involve application of the laser beam in areas adjacent to existing amalgam restorations of dentition, inadvertent contact of laser beams with amalgam restorations in the oral cavity is a distinct possibility.

Therefore, the present study was an attempt to evaluate the effect of different pulse energies of Nd:YAG laser on amalgam ablation, and its effect on the amount of mercury vapor released from amalgam in vitro.

Materials and Methods

Sample preparation

Forty-five cylindrical molds, measuring 4 mm in diameter and 5 mm in height, were made of acrylic resin to prepare amalgam samples from encapsulated amalgam (World Work Sri, Vicenza, Italy). Amalgam mixing was performed by a Softly automatic amalgamator (de Gtzen^® S.R.L. Via Roma, Italy) at 4000 rpm for 8 sec, based on manufacturer's instructions. Subsequently, using the standard technique, amalgam was packed into the cylindrical molds and after 24 h, the amalgam surfaces were abraded using silicon carbide paper to remove the superficial layer with a thickness of 1 mm, which might be rich in mercury. Then the amalgam surfaces were wet-polished to create smooth surfaces.

Laser application

According to setting provided by manufacturer of the laser, different parameters were suggested for various procedures including pulpotomy, canal sterilization, hemostasis, abscess, tissue retraction, gingival incision, periodontal treatment, frenectomy, and operculectomy. Among them, three parameters, including 50 mJ, 40 Hz, 2 W; 150 mJ, 40 Hz, 6 W; and 250 mJ, 30 Hz, 7.5 W, which could be representative of low, moderate, and high intensity of the laser beam, were selected for the purpose of this study. The samples were randomly divided into three groups of 15, based on laser pulse energy. Each specimen was separately placed inside a box that was devoid of any orifices, and underwent Nd:YAG laser irradiation (Nd:YAG Dental Laser System, Lambda Scientifica Srl, Vicenza, Italy) with different pulse energies of 50, 150, and 250 mJ. There was a distance of 1 mm between the laser tip and the specimen surfaces and irradiation duration was 4 sec for each sample.

Mercury analysis

Immediately after laser irradiation, ∼150 mL of the air inside each container was pumped out using an Apex Pump (Apex Pump, Casella Cell, UK) and analyzed for mercury vapor using a mercury analyzer (Mercury Analyzer System, Shimdazu, MAS 10, Tokyo, Japan), which uses cold vapor atomic absorption technique.

Scanning electron microscope (SEM)-energy dispersive x-ray (EDXA) observations

At the next stage, three additional amalgam specimens were prepared for each study group. Then each specimen was separately placed in a container that was completely sealed and had no orifices; a quartz slab was placed parallel to the amalgam surface to absorb particles released from amalgam during laser irradiation. Subsequently, the specimens underwent Nd:YAG laser irradiation with pulse energies of 50, 150, and 250 mJ. It should be pointed out that these specimens were prepared in a manner similar to that for the specimens mentioned previously. The amalgam surfaces were evaluated by an SEM (VEGA II/TESCAN, Czech Republic) in relation to the dimension of the cavities produced. In addition, particles released from amalgam as a result of laser irradiation, which had deposited on the quartz slab surface, were evaluated in relation to their elemental composition by an EDXA microanalyzer connected to the SEM.

Data analysis

Considering lack of equality of variances among the test groups in the Levene's test (p<0.001), nonparametric Kruskal–Wallis and Mann–Whitney tests were used for data analysis at a significance level of p<0.05 using SPSS (version 15.0, SPSS, Chicago, IL) for Windows. In addition, the means of mercury vapor released in each group were compared with the permissible standard level of 50 μg/m³ using a one sample t test.

Results

Table 1 presents the means, standard deviations, and standard errors of mercury vapor released from dental amalgam specimens after laser irradiation with pulse energies of 50, 150, and 250 mJ.

Table 1.

Means, Standard Deviations (SD), and Standard Errors of Mercury Vapor Released (μg/m ³) from Dental Amalgam Specimens After Laser Irradiation

					Mann–Whitney test results
Groups	Pulse energy of laser beam	(n)	Mean±SD	Std. error	1	2	3
1	50 mJ	15	0.08±0.03^A	0.00		<0.001	<0.001
2	150 mJ	15	36.51±1.27^B	0.32			<0.001
3	250 mJ	15	190.46±2.08^C	0.53

Values followed by different capitals were statistically different by Mann–Whitney test at a significance level of p<0.05.

The results of the Kruskal–Wallis test showed statistically significant differences in the amount of mercury vapor release among the study groups (p<0.001). Two-by-two comparisons of the groups with Mann–Whitney U test showed statistically significant differences in the amount of mercury vapor release among all the study groups (p<0.001), with a significant increase in the amount of mercury vapor release with an increase in laser pulse energy (p<0.001).

In addition, comparison of the amount of mercury vapor released in each group with the permissible standard amount of 50 μg/m³ using a one-sample t test indicated that the amount of mercury vapor released during amalgam ablation with 50 and 150 mJ pulse energies of Nd:YAG laser was significantly lower than the permissible standard level of 50 μg/m³ in the dentist's office (p<0.001). However, mercury vapor released after ablation with the pulse energy of 250 mJ was significantly higher than the standard level (p<0.001).

SEM-EDXA observations

SEM observations showed an increase in the size of amalgam surface cavities with an increase in the pulse energy of Nd:YAG laser (Fig. 1).

FIG. 1.

Scanning electron microscopic (SEM) image of amalgam specimens after irradiation with Nd:YAG laser beam with pulse energies of (A) 50, (B) 150, and (C) 250 mJ.

In addition, the results of SEM-EDXA evaluations showed that the particles deposited on the quartz slab contained mercury, copper, and tin, which had been released from the amalgam surface during ablation by Nd:YAG laser. An important consideration was the fact that there was a significant increase in the amount of Hg deposited on the quartz slab surface with an increase in the Nd:YAG laser pulse energy (Fig. 2).

FIG. 2.

Scanning electron microscope (SEM)-energy dispersive x-ray (EDXA) evaluation of the quartz slabs after irradiation with a laser beam with pulse energies of (A) 50, (B) 150, and (C) 250 mJ.

Discussion

Amalgam has been used in dentistry as a restorative material for almost 150 years.^22,23 However, its allergic potential or toxicity resulting from its mercury content has long been reported and discussed in the literature.^24
–26 Some studies have indicated a positive relationship between the number of amalgam restorations and the concentration of mercury vapor in the oral cavity.^27

–30 In addition, some factors influence the release of mercury vapor from the dental amalgam, such as masticatory function,^30,31 various restorative procedures,³² contact of amalgam with bleaching agents,³³ and the oral cavity conditions such as heat, pH, or oxidants.^34,35

As lasers are now extensively used in various restorative, endodontic, and periodontal procedures,^{7

–20} the aim of the present study was to evaluate the amalgam ablation capacity of different pulse energies (50, 150, and 250 mJ) of Nd:YAG laser, and the effect of increasing pulse energies on the amount of mercury vapor released after ablation, and also to compare the values determined in the present study with the standard mercury vapor concentration of 0.05 mg/m³.

In the present study, Nd:YAG laser beams with the pulse energies of 50, 150, and 250 mJ were used for amalgam ablation. The results showed a significant increase in mercury vapor release with an increase in Nd:YAG laser pulse energy. Based on the principles of thermodynamics, the vapor pressure of all the materials increases with an increase in temperature, and mercury is not an exception.³⁶ The release of mercury vapor from amalgam restorations cannot be influenced by any variable except for temperature. The interaction between laser and material can reach high temperatures, causing almost instantaneous melting in time scales of <1 ps once the melting point has been exceeded.³⁷ It is clear that when an Nd:YAG laser beam with various energies of 50, 150, and 250 mJ is irradiated on the amalgam surface, the surface temperature rises to different degrees, which might justify the finding described. To confirm this finding, SEM-EDXA observations showed that there was an increase in the amount of mercury vapor released from the amalgam surface, which is deposited on the surface of the quartz slab, with an increase in the intensity of Nd:YAG laser beam.

Pioch et al. applied Nd:YAG laser with a pulse energy of 100 mJ to ablate amalgam, and showed that Nd:YAG laser resulted in the highest amount of amalgam ablation and mercury vapor release compared with Er:YAG and Nd:YLF lasers. However, after irradiation of the amalgam surface with CO₂ laser, no signs of amalgam ablation or mercury vapor release were observed. They reported a mercury vapor release of 0.999 mg/m³ with beam energy of 100 mJ, which is higher than that released with 50 mJ and less than that released with 150 and 250 mJ in the present study.²¹ Furthermore, similar to findings by Cernavin and Hogan,³⁸ SEM observations showed that there was an increase in amalgam ablation with an increase in the pulse energy of Nd:YAG laser, and the size of the craters on the amalgam surface increased.

Another noteworthy finding of the present study was the fact that the amount of mercury vapor released during amalgam ablation with 50 and 150 mJ pulse energies of Nd:YAG laser was significantly lower than the permissible standard level of 50 μg/m³ in the dentist's office. However, mercury vapor released after ablation with a pulse energy of 250 mJ, was significantly higher than the standard level. It should be pointed out that these values are the result of 4 sec of exposure, and, therefore, multiple exposures or any attempt to remove amalgam with Nd:YAG laser could have cumulative effects on mercury vapor release, especially in the absence of sufficient air conditioning, which can be hazardous for individuals who are in contact with mercury, including patients, dentists, and office personnel. As a primary hazard, Nd:YAG laser with the parameters used in this study is categorized as class 4 according to International Laser Safety Standard (IES 60825-1) when its power output is >0.5 W. Therefore, skin and eyes in particular are endangered even at diffuse reflection. As a secondary hazard, interaction of this laser with special materials such as dental amalgam, would produce vapors and dust, which are toxic. Therefore, special precautions were considered for the personnel involved in this study.³⁹

According to Engle et al.,³² the greatest amount of mercury was released during dry polishing of amalgam (44 μg). Removal of amalgam from a class I cavity under water spray and high volume evacuation also generated large amounts of mercury as expected (15–20 μg). The total amount of mercury generated during placement (6–8 μg), wet polishing (2–4 μg), and trituration (1–2 μg) were also measured. These authors concluded that the total amount of mercury released during any procedure was far below the total exposure level calculated from the daily threshold limits established by regulatory agencies for occupational exposure. Therefore, it can be recommended that, in as far as possible, Nd:YAG laser should not be used for ablation of amalgam. During the application of Nd:YAG laser in other dental procedures, attempts should be made to avoid inadvertent contact of the laser beams with amalgam restorations, and all the dental facilities where Nd:YAG laser is used for dental procedures should be equipped with strong and efficient air conditioning systems.

Regarding limitations for this in vitro study, it should be pointed out that this study was performed on freshly mixed amalgam, and that the buffering effect of saliva and the presence of dental plaque on amalgam restorations were ignored. Furthermore, at present, erbium lasers (Er:YAG and Er,Cr:YSGG) are the most efficient lasers used for caries removal, cavity preparation, and cavity pretreatment (laser etching) for adhesive procedures and, with the right parameters, especially in conjunction with water spray, the thermal side effects are small. Therefore, there is substantial need for further studies using different types of lasers, especially in vivo, to establish "gold standards" in this regard.

Conclusions

Considering the limitations of this in vitro study, it can be concluded that:

1. As the pulse energy of Nd:YAG laser irradiation increased, the amount of mercury vapor released from the amalgam surface significantly increased;

2. As the pulse energy of Nd:YAG laser irradiation increased, the size of ablation craters increased; and,

3. The amount of mercury vapor released after irradiation with 250 mJ pulse energy of Nd:YAG laser was significantly higher than the permissible standard level of 50 μg/m³.

Footnotes

Acknowledgments

The authors thank the vice chancellor for research at Tabriz University of Medical Sciences for financial support. Furthermore, the authors thank Dr. M Ghojazadeh for statistical analysis of the data and Dr. M Abdolrahimi, who edited the English version of this article.

Author Disclosure Statement

No financial interests exist.

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