Intrapulpal Temperature Increases Caused by 445-nm Diode Laser-Assisted Debonding of Self-Ligating Ceramic Brackets During Simulated Pulpal Fluid Circulation

Abstract

Objective: This study investigated temperature increases in dental pulp resulting from laser-assisted debonding of ceramic brackets using a 445-nm diode laser. Materials and methods: Eighteen ceramic brackets were bonded in standardized manner to 18 caries-free human third molars. Pulpal fluid circulation was simulated by pumping distilled water at 37°C through the pulp chamber. The brackets were irradiated with a 445-nm diode laser. Temperatures were measured using a thermal camera at points P1 (center of the pulp) and P2 (in the hard dental tissue) at the baseline (T0), at the start and end of laser application (T1 and T2), and the maximum during the sequence (T_max). Results: Significant differences in the temperatures measured at P1 and P2 were observed among T0, T1, T2, and T_max. Significant increases in temperature were noted at points P1 and P2, between T1 and T2, T1 and T_max, and T2 and T_max. The maximum P2 values were significantly higher than at P1. The maximum temperature increase measured in the pulp was 2.23°C, lower than the critical threshold of 5.5°C. Conclusions: On the basis of the laser settings used, there is no risk to the vitality of dental pulp during laser-assisted debonding of ceramic brackets with a 445-nm diode laser.

Introduction

The requirements for barely visible orthodontic treatment approaches are very high. Ceramic brackets are esthetically more discreet than metallic brackets, and these bracket systems are therefore extremely popular and have been discussed in several recent publications on the subject.^1

–4

In addition to their strong esthetic advantages, ceramic brackets have one critical disadvantage in comparison with metallic brackets. Ceramic materials have a small modulus of elasticity, as they are brittle and hard. This leads to a greater risk of fractures in the ceramic bracket and enamel during the debonding procedure, in comparison with metallic brackets.⁵ The upper anterior teeth are of critical importance esthetically, and ceramic brackets are therefore often used in this area. Damage to the enamel of these teeth may result in a need for costly restorative treatment. In addition to the risk of aspiration, ingestion, and injury to the eyes from ceramic fragments, the fragments of the bracket base left on the tooth after a bracket fracture have to be removed using a diamond bur on a high-speed handpiece. This leads to longer chair time and substantially increases the risk for enamel damage.⁶

Various methods have been developed to modify the bracket failure mode and simplify the debonding procedure with ceramic brackets, such as special bracket removal pliers for specific bracket systems⁷ and electrothermal debonding devices.^8
–10 Various high-energy laser systems have also been investigated to simplify the debonding procedure with ceramic brackets, such as ytterbium fiber lasers,¹¹ thulium-doped yttrium aluminum perovskite (Tm:YAP),¹² neodymium-doped yttrium aluminum garnet (Nd:YAG),¹³ diode laser,⁸ CO₂ laser,⁶ and erbium-doped yttrium aluminum garnet (Er:YAG).¹⁴ Recent studies have shown that the risk of ceramic bracket fractures during the debonding procedure can be reduced by influencing the bracket failure mode and decreasing the shear bond strength with a novel diode laser with a wavelength of 445 nm.^15,16 The effect of lasers on ceramic bracket debonding results from thermal reactions and leads to a temperature increase in the dental hard tissue.^17
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In vivo investigations of dental pulp exposed to heat at temperatures increasing to 39°C–42°C have shown that there is a response with higher circulation (hyperemia).²⁰ When a temperature of 46°C–50°C was reached for several seconds, the circulation was arrested and thrombosis occurred.¹⁹ Another study showed that an increase in the temperature of dental pulp to 42.2°C led to necrosis of the pulp in some cases, with a further rise in temperature causing necrosis in more than half of the teeth investigated in an animal model.²¹ A critical benchmark of 5.5°C was defined as the damage threshold for dental tissue.²¹ It is therefore necessary to know the extent of the temperature increase in the pulp caused by different laser systems before they are used in vivo. The temperature changes occurring during laser-assisted bracket debonding have already been described for almost all the laser systems mentioned.^6,11,12,22 During testing, a thermal sensor is generally incorporated into the pulp to measure the effect of the laser irradiation. With this study design, however, most investigations ignore the complex anatomy of the tooth and the cooling effect of the microcirculation in the pulp. Studies investigating the influence of simulated pulp microcirculation on thermal effects during the use of curing units²³ and during application of a neodymium-doped yttrium orthovanadate (Nd:YVO₄) laser¹⁹ confirmed that the microcirculation has a significant influence on thermal changes in the pulp. Simulated pulp microcirculation for pulp temperature measurements has therefore become an established method.

No data are currently available on the heat generated in the pulp chamber when the 445-nm diode laser system is used to influence the bracket failure mode and decrease the shear bond strength of ceramic brackets^15,16 during debonding procedures. The aim of the present study was therefore to investigate whether using a 445-nm diode laser for laser-assisted bracket debonding, with the same parameters as in earlier studies by our group,^15,16 leads to an increase in temperature in dental pulp with simulated microcirculation. The temperature increase in the pulp resulting from laser-assisted debonding with a 445-nm diode laser was also to be investigated, with a special focus on the critical threshold of a 5.5°C increase in the pulp.

Materials and Methods

The study included 18 freshly extracted human molars with two roots, from different patients. Immediately after extraction, the teeth were stored in 0.9% isotonic NaCl solution with 0.001% sodium azide. The study was conducted in full accordance with established ethical principles (World Medical Association Declaration of Helsinki, version VI, 2002). All the patients were informed that their teeth were to be used in an in vitro research project.

Tooth preparation

All the teeth were cut cross-sectionally along the long axis using a water-cooled diamond cutting disc (SuperDiaflex G; Horico, Berlin, Germany). The two root canals had to be exposed. In a second step, the pulp chamber and root canals were cleared of soft tissue and were also shaped with hand files (ISO size 45; VDW GmbH, Munich, Germany). A piece of Plexiglas (2 cm × 2 cm × 0.1 mm) was glued (with Superglue; Kentzler-Kaschner Dental GmbH, Ellwangen/Jagst, Germany) onto the opened fields on the tooth. The space of the pulp chamber and roots was now enclosed by the hard tissue of the tooth and the Plexiglas. A rinsing cannula (Capillary Tip; Ultradent Products, Inc., South Jordan, UT) was inserted into each root through the apical foramen. The tips of the roots, the surroundings of the apical foramen, and the cannula were encased in silicone impression material (Contrast; VOCO GmbH, Cuxhaven, Germany) to secure the position of the cannula and to seal the microcirculation system (Fig. 1). On the basis of the setup used in previous studies with the 445-nm diode laser, the polycrystalline, active, and self-ligating bracket system In-Ovation C was used (GAC, Gräfelfing, Germany). Before bracket bonding, the teeth were dried with a dental air syringe. This was followed by conditioning of the enamel surfaces using 37% phosphoric acid for 20 sec. The etching gel was removed by water spraying and the enamel surface was dried with a dental air syringe. Subsequently, the enamel surface was coated with the bonding agent Transbond XT (3M Unitek, Monrovia, CA). The brackets were bonded to the tooth in the standard way with Transbond XT adhesive paste (3M Unitek). Transbond XT is a commonly used adhesive system for bracket bonding and has a high adhesive strength.^14,18 The coronal curing took 40 sec for each bracket, using a light-emitting diode with a light intensity of 1200 mW/cm² (Elipar FreeLight 2; 3M ESPE, Neuss, Germany).

FIG. 1.

Example Probe. An example probe with cannulas inserted for simulated pulpal blood circulation.

Simulated pulpal fluid circulation

The experimental setup was adapted from that described by Braun et al.¹⁹ Distilled water was warmed to 37°C (mean 37.39°C ± 0.45°C) in a heat bath (type 3042; Köttermann, Hänigsen, Germany). A water pumping device (RCS mgw; Lauda, Lauda-Königshofen, Germany) with a dedicated tubing system was used to transfer the water into the pulp chamber. To ensure minimal temperature loss during the transfer of water into the tooth, most of the tubing system was placed in the heat bath. The pumping device was located outside the heat bath. The tubing system was directly connected to one cannula and passed the water into the pulp via one root. The water left the pulp chamber via the other root and cannula. The flow rate was set to 6 mL/min, representing a physiological value (Fig. 2).¹⁹

FIG. 2.

Thermal Image. Example thermal images at T1 and T_max for one sample. P1 marks the center of the pulp and P2 marks the measurement point in the hard dental tissue.

Laser system, laser parameters and laser application

In accordance with the parameters used in earlier studies, a 445-nm class IV diode laser device (SiroLaser Blue; Dentsply Sirona, Bensheim, Germany) was used for laser-assisted debonding procedures in this study. The laser was set to 2 W/cm² (2 J/cm²) in continuous-wave mode, resulting in an effective output 1.76 W/cm² (±0.05 W/cm²) at the fiber tip. These parameters have been found to be appropriate for debonding with the 445 nm diode laser, as shown in previous studies.^15,16 The setting is capable of influencing the bracket failure mode and decreasing shear bond strength values. The handpiece was used with a 320 μm (core diameter) fiber and was activated using a finger switch. According to previous studies laser energy was applied to three sides of the bracket bases for 5 sec each (lateral–coronal–lateral, a total of 15 sec) using the laser tip in contact mode in a 90° angle to the bracket surface, resulting in a total dose of 30 J/cm² (Table 1).^15,16

Table 1.

Laser Parameters

Lasersystem	Wavelength	Power	Dose total	Time	Wavemode	Fiber
SIROLaser Blue	445 nm	2 W/cm²	30 J/cm²	3 × 5 sec	CW	320 μm

Definition of the laser parameters in the present study; W, Watt, SI = [W]; J, Joule, SI = [J]; CW, continuous-wave mode.

Temperature measurement

A thermal camera (VarioCAM HD head; ESW GmbH, Jena, Germany) was used for temperature assessment. The camera was located at right angles to the opened surface of the tooth and Plexiglas. The camera's dedicated software recorded 15 images per second (IRBIS 3 plus; InfraTec GmbH, Dresden, Germany). The recording process was started 3 sec before laser application for each sample. The whole recording process (sequence) for one sample was set to 120 sec. On the basis of the images from each sequence, two measurement points for temperature assessment were defined. Point 1 (P1) was set at the center of the pulp chamber. Point 2 (P2) was set at the point of the highest temperature increase in the hard dental tissue during the sequence. Temperature was measured in degrees centigrade at P1 and P2 at the start of the sequence (T0), at the start of the laser application (T1, 5 sec after T0), and at the end of the laser application (T2). The maximum temperature values (T_max) at P1 and P2 during each sequence were also assessed (Fig. 2).

Statistical analysis

Statistical calculations were carried out using the software programs SPSS Statistics, version 17.0 (SPSS, Inc., Chicago, IL) and G*Power (Franz Faul, University of Kiel, Germany). The Friedmann test was performed to determine whether there were any statistically significant differences in the P1 (T0, T1, T2, T_max) and P2 (T0, T1, T2, T_max) groups. The following pairwise Wilcoxon tests with Bonferroni correction were used to analyze changes in the relevant intragroup temperature measurements: T0 versus T1; T1 versus T2; T1 versus T_max; and T2 versus T_max for P1 and P2. The same test was used to compare temperature measurements between P1 and P2 at T_max.

A power analysis was carried out before the study, and the effect size was accordingly set to 0.8 in accordance with Cohen's criteria.²⁴ For an α error of 0.05 and a power of 0.8, a sample size of at least 15 specimens in each group was calculated.

Results

The Friedmann test showed statistically significant differences between the temperature values measured at T0, T1, T2, and T_max at P1 (p < 0.05). No statistically significant differences (p > 0.05) were observed at P1 between the temperature values at T0 and T1. Statistically significant increases in temperature (p < 0.05) were noted when the temperature values at T1 versus T2, T1 versus T_max, and T2 versus T_max were compared (Fig. 3, Table 2).

FIG. 3.

Temperature Values. A box plot diagram for temperature values at P1 (center of the pulp) and P2 (in the hard dental tissue) at different time points: at the start of the sequence (T0), at the start of laser application (T1, 5 sec after T0), and at the end of laser application (T2), along with the maximum temperature value during the whole sequence (T_max). Statistically significant differences are marked with asterisks (*).

Table 2.

Temperature Values

Sample	P1—T0	P2—T0	P1—T1	P2—T1	P1—T2	P2—T2	P1—T_max	P2—T_max
Median	37.43	32.34	37.57	32.31	37.78	36.00	38.07	39.74
Mean	37.39	31.74	37.49	31.86	37.67	35.99	38.15	39.58
SD	0.45	2.56	0.50	2.58	0.61	1.94	0.51	2.38
Minimum	36.70	25.78	36.73	25.85	36.66	31.61	37.09	35.28
Maximum	38.34	35.43	38.25	36.05	38.54	38.94	38.98	45.23
Interquartile	0.71	2.38	0.89	2.12	0.87	2.09	0.58	2.34

Temperature values (in degrees centigrade) at measurement points P1 (center of the pulp) and P2 (in the hard dental tissue) at different points of time: at the start of the sequence (T0), at the start of laser application (T1, 5 sec after T0), and at the end of the application (T2), along with the maximum temperature value during the whole sequence (T_max).

SD, standard deviation.

With regard to the values measured at P2, the Friedmann test showed statistically significant differences in the temperature values measured at T0, T1, T2, and T_max (p < 0.05). No statistically significant differences (p > 0.05) were observed at P2 between the temperature values at T0 and T1. Statistically significant increases in temperature (p < 0.05) were observed when the temperature values T1 versus T2, T1 versus T_max, and T2 versus T_max were compared (Fig. 3, Table 2).

When the temperature values assessed at P1 and P2 were compared at T_max, the values for P2 (median 39.74, minimum 35.28, maximum 45.23, and interquartile range 2.34) were statistically significantly higher than the values for P1 (median 38.07, minimum 37.09, maximum 38.98, and interquartile range 0.58; p < 0.05; Fig. 3, Table 2).

Discussion

The aim of this study was to investigate temperature increases in dental pulp induced by a novel 445-nm diode laser during laser-assisted debonding of ceramic brackets. The results show that there was a statistically significant increase in temperature in the dental pulp (at P1) between T1 and T_max. The maximum temperature increase measured in the pulp was 2.23°C. The mean temperature increase between T0 and T_max was 0.76°C. When these values are compared with the critical threshold of 5.5°C for dental tissue,²¹ it can be concluded that although there is a statistically significant increase in temperature in the pulp chamber, the temperature rise does not represent any risk for the vitality of the dental pulp, on the basis of the settings used during laser-assisted debonding of ceramic brackets with the 445-nm diode laser. The hypothesis of the study was therefore confirmed.

Another study investigated the temperature in dental pulp during laser-assisted debonding of polycrystalline brackets using a 980-nm diode laser with an energy level of 2.5 W and an irradiation time of 10 sec. The study reported a mean temperature increase of 1.46°C in the pulp. This value is still lower than the critical threshold of 5.5°C, but it is already twice as high after 3 sec of irradiation in comparison with the results of the present study.¹⁸ Another study that investigated the influence on temperature increases in dental pulp of a diode laser during debonding of polycrystalline brackets described a mean increase of more than 2.5°C after 3 sec of irradiation at an energy level of 2 W. Unfortunately, the study was based on bovine teeth and did not report the wavelength of the laser used or the type of laser system itself. The mean value is lower than the critical benchmark of 5.5°C, but is already 3.3 times higher after 3 sec of irradiation in comparison with the present results.⁸ Conventional diode laser systems work with wavelengths of around 810–980 nm. On the basis of the absorption characteristics of water for wavelengths around 810–980 nm, the absorption of the energy produced by these conventional diode laser systems is very high. This leads to interactions with water in the enamel and results in a temperature increase. By contrast, the laser energy of the 445-nm diode laser is not well absorbed by water, and the interaction with the water content of the enamel is nearly negligible.²⁵ This behavior of the novel 445-nm diode laser may partly explain the lower temperature increase in the dental pulp observed in the present study. However, the two other studies^8,18 were based on experimental setups for temperature assessment that did not simulate microcirculation in the pulp. In addition to the different behavior of the 445-nm diode laser in comparison with other wavelengths, the physiological microcirculation has a strong cooling effect on the temperature in the pulp.^19,23 The present study supports these findings.

A significant difference has been reported between the temperature increases observed in enamel and dentin after ultrashort-pulse laser (USPL) ablation of dental hard tissue. The highest temperature increase was measured in thin enamel samples, with values up to 67 Kelvin (K).²⁶ Another study examined the heat generated during removal of restorative dental material when a USPL system is used. The maximum temperature increase was 17 K.²⁷ Although these temperature increases were higher than the biological threshold, they did not necessarily lead to injury to the pulp tissue. One explanation for this might be that dental hard tissue is a good thermal isolator, significantly reducing temperature increases during resin composite photocuring.²⁸ In the present study, the temperature increase in the dental hard tissue (at P2) from T1 to T_max was found to be statistically significant. The maximum temperature increase was 13.66°C, with a mean temperature increase of 7.84°C. When the T_max values at P2 were compared with those at P1, the P2 values were statistically significantly higher than the P1 temperature values. In addition, the T_max values in the hard tissue were distinctly higher than the threshold of 5.5°C. Despite these high temperatures in the hard tissue and their close vicinity to the pulp tissue, the temperature in the pulp chamber only increased by 0.76°C. This once again emphasizes the cooling effect of the physiological pulpal microcirculation, and this should not be overlooked in studies focusing on temperature effects in the pulp resulting from dental interventions.

To simulate physiological conditions during the experimental procedure, the pulp was heated to 37°C. As the temperature of the surroundings was lower than 37°C, the exterior parts of the tooth were also cooler than 37°C. The patient's mouth is not closed during dental treatment, and the lower temperature in the exterior parts of the tooth corresponds to natural conditions; as can be seen from Table 2, the temperature in the exterior parts is very stable (±0.36°C). There were no statistically significant differences between T0 and T1 at P1 and P2.

Although the experimental setup is close to physiological conditions, it still represents an in vitro simulation. The values measured do not correspond to the precise temperatures in the pulp chamber that can be expected in vivo; that would not be possible with an in vitro design in the absence of physiological pulp tissue. On the contrary, this setup provides the closest possible simulation of in vivo conditions, since conducting in vivo tests with real human teeth would not be possible without injuring the patient's teeth.

Other studies that have investigated the effects of a diode laser on increases in temperature in dental pulp during bracket debonding have used different wavelengths, energy levels, and irradiation times.^8,18 The question of whether higher energy levels and irradiation times—and if so, which—might lead to higher temperature increases in dental pulp has not yet been resolved and remains an interesting issue for further research.

Conclusions

Based on the used laser settings in this study, there is no risk to the vitality of dental pulp tissue during laser-assisted debonding of ceramic brackets with a 445 nm diode laser. Consequently, pulpal temperature increase as consequence of laser irradiation by 445 nm diode laser during ceramic bracket debonding is no exclusion criteria for further in vivo tests in this context.

Footnotes

Acknowledgments

The authors thank Densply Sirona for providing the laser device.

Author Disclosure Statement

No competing financial interests exist.

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