Comparison of Different Energy Levels of Er:YAG Laser Regarding Intrapulpal Temperature Change During Safe Ceramic Bracket Removal

Abstract

Objective: This study was done to compare the intrapulpal temperature change generated by different energy levels of Er:YAG laser used during debonding of ceramic brackets and find the most suitable level for clinical use. Material and methods: Eighty polycrystalline alumina brackets were bonded on bovine incisor teeth, which were randomly divided into 4 groups of 20. One group was assigned as control. In the study groups, after laser exposure with 2, 4, or 6 Watt energy levels, brackets were debonded using an Instron Universal Testing machine. Adhesive remnant index (ARI) scores were recorded to evaluate the site of debonding. To assess intrapulpal thermal increase, 60 human premolar teeth that were prepared in the same way, at the same energy levels, by a thermocouple were used. Results: When the debonding forces, intrapulpal temperature increases, and ARI of the groups were examined, statistically significant difference was observed between the groups. Mean temperature increases of 0.67°C ± 0.12°C, 1.25°C ± 0.16°C, and 2.36°C ± 0.23°C were recorded for the 2, 4, and 6 Watt laser groups. The mean shear bond strength was 21.35 ± 3.43 megapascals (MPa) for the control group, whereas they were 8.79 ± 2.47, 3.28 ± 0.73, and 2.46 ± 0.54 MPa for the 2, 4, and 6 Watt laser groups, respectively. Conclusions: Four watts is the most efficient and safe energy level to be used, utilizing Er:YAG laser with water cooling spray for 6 sec by scanning method during debonding of polycrystalline alumina brackets without any carbonization effects and detrimental temperature changes at debond sites.

Introduction

Orthodontists prefer ceramic brackets to their stainless-steel alternatives to meet the increasing esthetic demand and cosmetic concern of patients seeking orthodontic treatment. Currently, various types of ceramic brackets are available in the market by all major bracket manufacturers who have tried to overcome the inherent disadvantages of these esthetic brackets to a certain degree like brittleness, unfavorable frictional characteristics, and debonding problems.^1,2

One of the main problems with ceramic brackets emerges at the debonding phase due to their high bonding strength to enamel, together with their low fracture toughness and brittleness.² The main drawback during debonding is the risk of damage on enamel and the long operation duration, besides pain.³ To overcome these problems, thermally softening of the adhesive resin was carried out in several studies to decrease the bond strength through heat conductivity.^4

–7

The most recently introduced debonding technique is laser-aided debonding. This method was found to be efficient for debonding ceramic brackets without enamel tear-outs or bracket fractures, reducing the shear bond strengths of ceramic brackets from high values to levels for safe removal from the teeth. However, in laser debonding, as is carefully monitored in other dental disciplines where laser is used, probable intrapulpal temperature increase should be taken into account to prevent any iatrogenic damage to the pulpal tissue.

The amount of energy delivered can be efficiently controlled when using lasers.^8

–15 This energy emitted by a laser is absorbed, reflected, or scattered after interacting with the target material. One of the main variables that affect energy absorption by a target tissue is the laser light wavelength. Since CO₂ laser (wavelength of 10,600 nm) and Er:YAG laser (wavelength of 2940 nm) are highly absorbed in water, they seem to be more appropriate for debonding, whereas diode (wavelength of 800–980 nm) and Nd:YAG (wavelength of 1064 nm) lasers have a high absorption coefficient in pigmented tissues.¹⁶

In the study by Strobl et al.,⁸ the results revealed that during removal of ceramic brackets by using Nd:YAG laser, 69–75% of light was not absorbed by the brackets or the adhesive material, thus reached the enamel surface, and possibly caused a risk of pain and damage. On the other hand, Er:YAG lasers emit a wavelength of 2940 nm that closely approximates the absorption peak of water that is present in the adhesive resin as water or residual monomer.¹⁶ Most of the reports on ceramic debonding have used CO₂ laser as the choice of laser wavelength; however, in recent years, a few studies have also reported on the efficiency of Er:YAG laser.^14

–17

Besides wavelengths, there are other factors that affect energy absorption during ceramic bracket debonding like pulsed or continuous waveform, power (watts), duration of exposure, optical properties of the tissues, peak power of pulse, and pulse duration.¹⁸ These parameters also have an influence on the possible thermal change on the target material and the surrounding tissues. To protect pulp tissue and prevent melting of tooth tissue, laser irradiation should always be accompanied by water cooling to prevent cracking of dental substrates. To meet these specifications, Er:YAG lasers have a free-running pulse emission that provides target tissue with thermal relaxation time in which to cool.¹⁶ Also, it provides a water-cooling spray that can be operated on demand. Recently, the effects of different application durations of Er:YAG laser were evaluated by studies,^14,17 which reported that debonding with this laser was a safe and effective method without causing any damage on pulpal tissues.

The purpose of this study was to determine the optimal power setting of Er:YAG laser application in order not to have any detrimental thermal effects on dental pulp tissues during debonding of ceramic brackets. Shear bond strengths, adhesive remnant index (ARI) scores, and intrapulpal temperature increases were measured and compared. The null hypothesis is that there is no difference between 2, 4, and 6 Watts (W) power of Er:YAG laser application, from the point of thermal changes created in the pulp.

Materials and Methods

The study was performed in two parts. Part 1 was carried out on bovine teeth to determine the shear bond strengths and debond characteristics; part 2 was carried out on human premolars to determine the thermal changes induced in the pulp chamber.

Part 1

Eighty freshly extracted, non-carious bovine permanent mandibular incisors were used in this study. The teeth were preserved in 0.1% thymol solution until the roots were cut away and soft tissue debris and coronal pulps were removed. Eighty polycrystalline alumina brackets (Transcend series 6000; 3 M Unitek, Monrovia, CA) were bonded by one operator (M.O.O.) on the buccal surfaces of these teeth using Transbond XT (3 M Unitek) orthodontic adhesive system, after conditioning the enamel surface with 37% phosphoric acid for 15 sec. The composite resin was light cured for 20 sec with a halogen light-curing unit (Optilux, Kerr, Orange, CA) on the mesial and distal sides, as recommended by the manufacturer. Subsequently, the samples were embedded in an autopolymerizing epoxy resin (Fortex Cold Curing Dental Polymer 2000) with the buccal surfaces and the brackets exposed. During embedding, each tooth was adjusted with a guiding device to ensure reproducibility in the position of the bracketed teeth during testing. All samples were stored in distilled water at 37°C for 48 h before testing.

The prepared samples were then randomly divided into 4 groups of 20 samples. The first group was assigned as the control group and no laser application was performed. In the study groups, Er:YAG laser (VersaWave, Hoya ConBio, Fremont, CA) with a wavelength of 2940 nm was used for lasing before debonding of the brackets. The laser parameters were as follows: pulse repetition rate of 20 Hz, pulse duration of 300 msec, water spray of 40–50 mL/min, tip diameter of 1 mm, and the laser irradiation for the three study groups were set at a power of 2 W (100 mJ at 20 Hz), 4 W (200 mJ at 20 Hz), and 6 W (300 mJ at 20 Hz). Calibration to ensure the consistent output power was performed before and after each step of an experiment using Trutest^® fiber calibration port integrated to the device. The samples were scanned with the horizontal movements positioned perpendicularly 2 mm from each bracket surface for 6 sec as described by Oztoprak et al.¹⁵

The shear test was carried out immediately after the laser exposure, using a universal testing machine (Instron, Canton, MA) with a shear blade placed at the bracket base-enamel interface. Occlusogingival force was applied to the bracket at a crosshead speed of 1 mm/min and the load at failure was recorded. Shear bond strengths were calculated in megapascals (MPa) and the results of each test were recorded with a computer connected to the testing machine. After debonding, any residual adhesive on the tooth or on the bracket surface was examined under a light microscope at × 20 magnification, and the ARI scores were assigned according to the following scale: “0” indicates that no adhesive was left on the enamel, “1” indicates that less than half of the adhesive was left on the enamel, “2” indicates that more than half of the adhesive was left on the enamel, and “3” indicates that all of the adhesive was left on the enamel.¹⁹

Part 2

Sixty extracted non-carious human premolars were collected and stored in 0.1% thymol solution until use. The apical third of the roots were cut off and the pulpal tissue was removed with an endodontic file to facilitate the placement of a K-type thermocouple (Ishifuku Metal Industry, Tokyo, Japan). The preparation of the enamel surface and bonding of the polycrystalline alumina brackets were done in the same way as Part 1. The samples were then randomly divided into 2, 4, and 6 W of energy level groups, each consisting of 20 specimens. Before the insertion of the thermocouple, a silicone heat-conducting medium was injected into the pulp chamber. A 0.2 mm diameter K-type thermocouple (Ishifuku Metal Industry) was inserted in the pulp chamber and the temperature change was continuously monitored with a recorder (XY Recorder WX2400, Graphtec Corp., Tokyo, Japan) as each group was being lased sequentially, while the room temperature was kept at 25°C.

Statistical calculations were performed with Statistical Package for the Social Sciences software (SPSS for Windows 15.0, SPSS, Chicago, IL) for Windows. Standard descriptive statistical calculations (mean, standard deviation, median, minimum, and maximum) were carried out. The Shapiro–Wilk test was used to inquire the normal distribution of the data. Welch test was used for the equality of means of shear bond groups that could not be assumed to have the same variances. To evaluate intergroup differences, Dunnett T3 test was used. Chi-square test was performed to analyze the differences between ARI scores. In the detection and comparison of temperature changes of the groups, ANOVA and Tukey test were used. Spearman correlation test was used to assess the relationship between ARI scores and laser power (W). Statistical significance level was established at p < 0.05.

Results

The mean and standard deviation of shear bond strength for the non-lased brackets was 21.35 ± 3.43 MPa, whereas they were 8.79 ± 2.47, 3.28 ± 0.73, and 2.46 ± 0.54 MPa for the 2, 4, and 6 W laser groups, respectively (Table 1). Welch test pointed to differences between the groups (p = 0.000) (p < 0.05). The shear test pointed out significantly lower shear bond strengths in the lased groups compared with the control group (p < 0.001). A statistically significant difference was seen between all groups except 4 and 6 W groups.

Table 1.

Descriptive Data and Comparison of the Shear Bond Strengths in Megapascals with Welch Test and Pairwise Comparisons By Dunnett T3 Test are Shown

	n	Mean ± SD	Minimum	Median	Maximum
Control	20	21.35 ± 3.43^a	12.20	21.3	29.10
2 W	20	8.79 ± 2.47^b	4.90	9.10	15.50
4 W	20	3.28 ± 0.73^c	1.01	3.42	5.61
6 W	20	2.46 ± 0.54^c	0.50	2.65	4.40

Means with the same superscript letter are not significantly different.

SD, standard deviation.

Chi-square analysis revealed statistical difference between the ARI scores of the control and the 6 W laser-irradiated group (Table 2). However, there was no statistical difference among experimental groups, excluding the control group.

Table 2.

Frequency of Distribution of Adhesive Remnant Index Scores

	ARI scores
	0, n (%)	1, n (%)	2, n (%)	3, n (%)	χ²	Fisher's exact
Control	2 (10)	4 (20)	11 (55)	3 (15)	6 W	0.037
2 W	0 (0)	3 (15)	7 (35)	10 (50)	—
4 W	0 (0)	2 (10)	8 (40)	10 (50)	—
6 W	0 (0)	2 (10)	4 (20)	14 (70)	Control

ARI, adhesive remnant index.

Statistical difference between the groups was observed when data for the temperature increase were evaluated by ANOVA test (p < 0.05). The temperature increases were 0.67°C ± 0.12°C, 1.25°C ± 0.16°C, and 2.36°C ± 0.23°C for the 2, 4, and 6 W groups, respectively (Table 3). The results of the Tukey test revealed that the pulpal temperature increase of 2 W group (0.67 ± 0.12) was lower than 4 (1.25 ± 0.16) (p = 0.006) (p < 0.01) and 6 (2.36 ± 0.23) (p = 0.000) (p < 0.001) W groups. The temperature increase in the 4 W group (1.25 ± 0.16) was lower than the 6 W group (2.36 ± 0.23) (p = 0.000) (p < 0.001). The results of the Spearman correlation test showed that as power (W) was increased, ARI scores were higher, which revealed that the amount of adhesive left on the enamel was also increasing, respectively.

Table 3.

Comparison of Mean Increase in Intrapulpal Temperature for the Laser Groups By Using ANOVA and Tukey Tests

	n	Mean ± SD
2 Watt	20	0.67 ± 0.12^a
4 Watt	20	1.25 ± 0.16^b
6 Watt	20	2.36 ± 0.23^c

Means with the same superscript letter are not significantly different.

Discussion

The major aim of debonding by the aid of lasers is to eliminate the probability of fracturing enamel without causing any iatrogenic injury to the pulp tissue. Our results revealed that, as the laser power increased, the force needed to debond the brackets was lower. Since the adequate bond strength for orthodontic attachments was reported to be 8 MPa by Maijer and Smith,²⁰ we may consider this value as a threshold for easy removal of brackets. There was no statistical difference between the shear bond strength means of 4 and 6 W groups in this study, and clinically, both results were way below the threshold of adequate bonding. In these two groups, where the shear bond strength values were low, ARI scores were high, indicating that more amount of the adhesive was left on the enamel.

If we assess these results in terms of bond failure, investigations showed that ceramic brackets with chemically retained bases failed most of the time at the resin–enamel interface, with the increased risk of enamel fractures and cracks.^21,22 When our results are compared, it can be stated that bond failure at the bracket–resin interface or within the resin is more favorable during debonding of ceramic brackets.

After the brackets were debonded and the ARI scores were assessed, the remnant adhesive would have been removed from the buccal surfaces and enamel surfaces would have been assessed according to the enamel damage index (EDI). In previous studies, in which the ceramic brackets and dental hard tissues were evaluated by light microscopy after the debonding procedure, it was revealed that the bond between the ceramic bracket and the tooth was disrupted mainly at the bracket/cement interface, leaving the majority of the adhesive on tooth surface. In addition, it was shown that there were no ablation craters or any marks of ablation on the tooth surface. As the ARI scores increased, EDI scores decreased, reducing the risk of any enamel damage.^23
–25 Since the ARI scores were found high in this study, further tests to measure EDI scores were not needed.

Previously, other studies have shown that Er:YAG laser had less thermal effect than Nd:YAG or CO₂ lasers and it could be used to safely debond ceramic brackets.^4,14,17,26 These studies also revealed that using Er:YAG laser to assist debonding would reduce the bond strength significantly; thereby aiding the easy removal of ceramic brackets.

There is a risk of thermally induced pulpal damage during the thermal degradation of the resin material when high temperatures are reached while using laser.^27,28 According to Zach and Cohen,²⁷ the threshold value for intrapulpal temperature increase in order not to generate any pulpal damage is 1.8°C. This threshold value is exceeded by the temperature increase generated by lasing with 6 W (2.36°C ± 0.23°C) in this study. However, the value for 6 W stayed below the 5.5°C benchmark, which is the reported temperature increase level where 85% of teeth stayed healthy,²⁷ and very near, but below a 3°C temperature increase, which is the benchmark reported by animal experiments.²⁹

The results of this study for the temperature change in pulp chamber is higher than the results of the study by Suh et al.,¹⁷ where the authors investigated the temperature change of dental tissues induced by Er:YAG laser debonding of the ceramic brackets. The authors applied laser on a bonded ceramic bracket at two points, 1 pulse each. Their results revealed that 1 Hz, 450 Mj of laser energy was safe for polycrystalline brackets. The temperature change in the pulp chamber had a mean of 0.90°C ± 0.15°C. The result of that study regarding optimum energy level is similar to this study; however, there were ablated volcano-like hollows on the tooth and adhesive surfaces under the irradiated brackets in their study.

In another study by Ma et al.,¹¹ where 20 W maximum output continuous wave carbon dioxide laser with a waveguide of length 1 m and spot size 1 mm in diameter was used, the mean increase in intrapulpal temperature was 1.10°C ± 0.81°C at 2 sec, when the brackets were debonded at a tensile load of a quarter of non-lased group. On the other hand, Obata et al.¹⁰ have reported an increase of 1.4°C with 2 W and 2.1°C with 3 W when using super-pulse CO₂ laser. Hayakawa¹³ has used Nd:YAG laser for debonding of ceramic brackets and stated that maximum value of temperature increase in the pulp chamber was 5.1°C with the amount of energy used (2.0 and 3.0 J), and reported carbonization at the debonding site, which was held as an evidence of higher temperature at those areas. The parameters of the laser were a pulse duration of 1.2 ms and 1 pulse per location.

This study had given results that the temperature increases were reasonable and measured in an order of 2 W < 4 W < 6 W groups. These results were as expected; however, since there was no difference between the groups lased with 4 and 6 W from the point of clinical efficiency, 4 W (with the parameters used in this study) was the most efficient and safe energy level to be used to avoid any detrimental temperature increase on pulpal tissue.

The lack of carbonization and volcano-like hollows in this study shows that the energy used was not concentrated in any certain area. In another study¹⁵ using scanning method, any evidence of ablation or explosion was not seen as noted in most of the studies,^7,13,19 but only thermal softening of the adhesive, which enabled the operator to remove the brackets easily. The difference between the studies in this regard may be due to the application method of the laser. Oztoprak et al.¹⁵ stated that, to lessen the heat conduction to the pulp, the effect of the energy is reduced by scanning through the surface of the bracket, which could avoid the probable temperature rise at just one point causing explosions and carbonization.

On the other hand, Tocchio et al.⁷ have reported that thermal ablation was an event that required time for the material to boil and temperature rises during debonding would be greater than those occurring during photoablation. They stated that when a high energy pulse was absorbed, the rate of energy deposition into a specific atom or molecule could exceed its thermal relaxation time, and negligible heat diffusion could occur during debonding. The bracket would be blown off the surface, and because of the short time, which could be less than the thermal relaxation time, the bracket and tooth would be cool. However, in this study, we have seen that with the optimal amount of power used, both the temperature rise would be under control and the debonding surfaces would not be injured/carbonized as seen in other studies.

Laser-aided debonding mechanism is explained by thermal softening, thermal ablation, or photoablation of the adhesive. In this study, the debonding force was not applied at the same time with lasing. However, the debonding force was still significantly lower compared with the control. If thermal softening was the only mechanism, we would not get reduced shear strength. In this study, using high level of water spray (40–50 mL/min) helped minimize pulp temperature increase, while it did not affect the decomposition of the composite. The reason that adhesive resin was not resistant as before when cooled down, might have been due to decomposition of the resin matrix, in addition to thermal softening.

Even though the study was designed to reflect the normal conditions of the oral cavity, in-vitro studies hold some limitations due to their character; the method of measuring the temperature increase may be misleading because water content of the dental tissues in the samples would not be the same as the living tissue, which would affect heat conductivity. Besides the type of the laser and the energy levels, parameters of laser beam, whether pulsed lasers (also the duration of the pulse) or continuous lasers, affect the energy flow coming out of the beam. Thus, the readers of this article should keep this in mind while interpreting the results.

Conclusions

With the laser parameters used in this study, 4 W is the most efficient and safe energy level to be used when debonding polycrystalline alumina brackets utilizing Er:YAG laser.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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