Intra pulp chamber temperature variation caused by Nd : YAG and Diode LASER irradiation

Objetivo: A irradiação com LASER de Neodímio (Nd:YAG) e de Diodo sobre sistemas adesivos pode aumentar a resistência de união em dentina, no entanto a existe a preocupação com a variação de temperatura intracâmara pulpar. O objetivo deste trabalho foi avaliar a variação da temperatura intracâmara pulpar após a aplicação dos sistemas adesivos e subsequente a irradiação com LASER de diodo e Nd:YAG. Material e Métodos: Este estudo apresentou um desenho experimental fatorial com dois fatores: LASER em dois níveis (Diodo [D] e Nd:YAG [N]) e sistema adesivo em quatro níveis (AdperTM ScotchbondTM Multi-Purpose [MP], AdperTM SingleBond 2 [SB], ClearfilTM SE Bond [CSE] e AdperTM Easy One [EO]). A variável de resposta quantitativa foi a variação da temperatura (∆t) em oC .Utilizou-se 40 dentes bovinos, separados em 8 grupos (n=5), a superfície vestibular foi desgastada até 1 mm de espessura. Depois da aplicação de cada sistema adesivo, mas previamente a fotopolimerização, os espécimes eram irradiados com LASER de diodo e Nd:YAG com parâmetro padronizados. A ∆t foi obtida com um termopar inserido intracâmara pulpar. Os valores obtidos foram submetidos ao teste ANOVA a 2 critérios seguido de Tukey para comparações individuais(p<0,05). Resultados: A média e o desvio padrão da ∆t obtidos foram: N-MP: 12.60 (± 2.51), N-SB: 10.40 (± 5.03), N-CSE: 11.80 (± 5.12) e N-EO: 10.20 (± 2.39), D-MP: 4.4 (± 1.82), D-SB: 5.20 (± 1.54), D-CSE: 4.60 (± 1.14), D-EO: 3.60 (± 1.52). Conclusão: O tipo de sistema adesivo não foi significante no aquecimento gerado pela irradiação com LASER, mas o LASER de Nd:YAG pode fornecer um potencial maior para causar danos a pulpar. ABSTRACT

Intra pulp chamber temperature variation caused by Nd:YAG and Diode LASER irradiation Ishikiriama SK et al.

INtRoDuctIoN
T he use of LASER in dentistry has increased in many areas [1][2][3][4], especially in restorative dentistry, due to growing knowledge of LASER interaction with dental structures [5][6][7][8].Despite these advances, when new LASER irradiation technique is proposed, temperature variation in pulp chamber should be considered, and some investigations are needed prior to clinical use.[9,[10][11][12] Recently, a new technique of LASER irradiation was proposed to improve adhesion obtained between dentin and adhesive systems [13].This technique, which consists of Nd:YAG LASER irradiation on adhesive systems already applied on dentin, however prior to light curing, has obtained favorable results with increased bond strength [14,15] and reduced dentin microleakage [16].Maenosono et al. [17], using similar technique, using a Diode LASER (970 nm) instead, also observed significant increase in bond strength of simplified adhesive systems to dentin.
Despite the favorable results, the parameters used for LASER irradiation are quite different among other reported techniques and again, the concern with temperature variation to the pulp organ should be considered.The present study aimed to assess the intra pulp chamber temperature variation caused by irradiation of different adhesive systems with Nd:YAG and Diode LASERs.

II. SPECIMEN PREPARATION
The sample consisted of 40 bovine incisors, cleaned and stored in thymol solution (0.1%) until specimen's preparation.A coronary opening in the palatal surface was initially performed with high-speed handpiece and a diamond tip (1016 HL, KG Sorensen Medical Burs, Cotia, SP, Brazil).The buccal surface was flattened in a polishing machine (Aropol-2V, Arotec, Cotia, SP, Brazil) with silicon carbide sandpaper, with 320 and 600 grit (Carbimet Paper Discs, Buehler, Lake Bluff, IL, USA) at low speed and water cooling, obtaining a simulated smear layer.The remaining buccal dentin thickness was standardized in 1 mm.
In order to standardize the area of LASER irradiation, a 6 x 6 mm silicone matrix (Zetalabor, Zhermack, São Paulo, Brasil) was positioned in the center of flattened surface and the test area was delimited with nail polish, comprising 36 mm2.The teeth were then stored in deionized water at room temperature until the LASER irradiation protocol.

III. TEMPERATURE VARIATION TEST
The specimens were treated in accordance to respective adhesive system of their groups, and prior to light curing they were irradiated with Nd:YAG LASER (Smartfile, Deka, Calenzano, Italy) or Diode LASER (SiroLaser, Sirona Dental Systems, Benshein, Germany).The irradiation was carried out with the optic fibers in contact mode, perpendicular to flattened surface, using same parameters for both LASERs (Table 2).After each irradiation, a small portion of optic fiber endpoint was cut to eliminate residues of adhesive system.
The intra pulp chamber temperature variation (∆t) was recorded with digital thermometer (MT401A, Minipa, Shanghai, China) linked to a universal type K thermocouple (MTK-01, Minipa, Shanghai, China), positioned internally to pulp chamber, in contact to inner  buccal dentin (Figure 1).The remaining pulp chamber spaces were filled with a white thermal paste (Implastec, Votorantim, São Paulo, SP, Brazil).After the thermocouple positioning, the lingual surface was isolated with black eletrical tape (3M Brazil, SP, Brazil).LASER irradiation was performed by a single operator.

IV. STATISTICAL ANALYSIS
The values of ∆t were submitted to two-way ANOVA, followed by Tukey test for individual comparisons (p < 0.05).

Result
The average and standard deviation of ∆t observed during LASER irradiation for each adhesive system are listed in table 3.No differences were found between adhesive systems submitted to the same LASER type.On the other hand, differences were found for LASER factor, without interaction between the variables.The use of Nd:YAG LASER resulted in higher ∆t than Diode LASER for all adhesives systems.

DIscussIoN
Thermal injuries to the pulp have always been studied by extensive research since excessive heat might lead to pulpal necrosis.Classical studies [18] demonstrated that temperature variation (∆t) greater than 5.5 °C has a high potential for pulp damage, while more recent studies suggest that tolerable ∆t is 11.2 °C [19].The present study evaluated the According to the results, no differences were observed in ∆t obtained between the adhesive systems.Some authors [20] observed differences in ∆t when different dyes were applied to dentin surface, which may absorb the energy in the spectrum of LASER irradiation (1064 nm and 970 nm) and determine increased temperatures.The results of the present study suggests, based on the absence of significant differences between the adhesive systems, that besides the different chemical composition, all presented similar interaction with tested LASERs.A a near infrared spectroscopy of the adhesive systems is however necessary to confirm this hypothesis.
When LASER factor was analyzed, it was observed that Nd:YAG LASER caused an increased ∆t when compared to Diode LASER, rejecting the second null hypothesis.Once the same parameters were used for both groups, the difference in ∆t can be attributed to the wavelength, which is inherent to the type of LASER and can determine greater or lesser energy absorption by the substrate, generating different patterns of heating.
Gutknecht et al. [21] noted that water has a higher absorption coefficient to Nd:YAG LASER when compared to Diode LASER, which may have determined the higher heating observed for Nd:YAG LASER.Other authors [22] claim that Diode LASER depth of action is lower than Nd:YAG LASER, [14] which can also contribute to ∆t lower values observed.Also some authors do not recommend the use of Nd:YAG LASER on continuous mode within dental substrate, because higher temperature might lead to protein denaturation [23].
The low temperature obtained with Diode Laser is in accordance to classic studies of Zach and Cohen.Despite higher values of ∆t determined by Nd:YAG LASER, the average ∆t can be considered tolerable, since more recent studies demonstrated clinicanly and histologically that increase of 11.2 °C does not cause problems [19].There is no evidence that heating compromises adhesion due to increase in temperature; on the other hand, studies using similar parameters found increase in bond strength of different adhesive systems to dentin [17,15].
The present study has the objective of contributing to the possible clinical use of the technique.Complementary studies are still needed, since the oral environment and the presence of pulp may influence energy absorption.Studies evaluating the longevity of adhesion obtained by employing the technique, and better understanding of mechanism of lasers over adhesive systems should still be performed prior to clinical use.Standard parameters are still needed to ensure greater safety of LASERs.

coNclusIoN
Within the limitations of this study, it can be concluded that temperature variation is related to LASER type, reagardless the adhesive system used.Nd:YAG LASER might provide a higher potential to cause a pulp damage.
Intra pulp chamber temperature variation caused by Nd:YAG and Diode LASER irradiation Ishikiriama SK et al.

Table 1 -
Chemical composition of the used adhesive systems, according to the manufacturers.

Table 2 -
Parameters used for the irradiation test area with LASER

Table 3 -
Mean ± standard deviation of variation in intra pulp chamber temperature (ºC).
*Different capital letters indicate differences between rows of the same column (p > 0.05).**Different lowercase letters indicate differences between the columns of the same row (p > 005).