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Brazilian Journal of Oral Sciences, Vol. 4, No. 12, Jan./Mar. 2005, pp. 685-688 Pulp chamber temperature changes during resin composite photoactivation Luis Felipe Jochims Schneider 1 Larissa Maria Assad Cavalcante 2 Rubens Nisie Tango 1 Simonides Consani 3 Mário
Alexandre Coelho Sinhoreti 4 Lourenço Correr Sobrinho 3
1 DDS Graduate student, Dental Materials, School
of Dentistry of Piracicaba, State University of Campinas –Brazil 2 DDS
Graduate student, Operative Dentistry, School of Dentistry of Piracicaba,
State University of Campinas –Brazil 3 DDS, MS, PhD Titular Professor of Dental Materials, Department of Restorative Dentistry, School of Dentistry of Piracicaba, State University of Campinas - Brazil 4 DDS, MS, PhD Associate Professor of Dental Materials, Department of Restorative Dentistry, School of Dentistry of Piracicaba, State University of Campinas - Brazil Received for publication: June 01, 2004 Code Number: os05009 Abstract The aim of the present study was to verify if there is any difference in the pulp chamber temperature rise during the photo-activation of two resin composites with different viscosities. Eighteen extracted bovine incisors were divided into two groups (n=9) that were restored with two different resin composites – Filtek Z250 and Filtek Flow (3M/ESPE Dental Products, St Paul, MN 55144, USA). During the photo-activation, with a conventional halogen light curing unit (XL 2500, 3M/ESPE), a type-K thermocouple registered the temperature rise peak in the pulp chamber. The temperature rise data were submitted to Student-t test at 5% significance. The resin composite Filtek Flow shows statically higher mean temperature rise (p<0.05). We conclude that the resin composites with different viscosities produce different temperature changes during the photo-activation. Key Words: resin composites, photo-activation, pulp chamber, temperature rise, dental materials Introduction
Dental composites are materials that unite filler particles and resinous matrix with a silane agent1-2. The matrix consists basically of organic monomers, a polymerization inhibitor, and an activator/initiator system3. The resinous matrix is a fluid component that, when polymerized, becomes rigid4. This occurs due to free radical formation, which induces covalent unions between organic molecules, thus, generating macromolecules called polymers5. For this free radical formation, an activator/ initiator system that generates enough energy to break the benzoyl peroxide molecule is necessary1,6. In light cure units (LCUs), a specific wavelength of visible light (468 nm) excites the camphorquinone molecule that, together with a tertiary amine, induces the benzoyl peroxide molecule break7. The polymerization starts with an addition reaction, in which the required energy to start the reaction is set free in the form of heat, i.e., an exothermic reaction8. The LCUs that use the halogen light bulb require the use of a filter that hinders infrared rays, which is the main irradiation produced by the halogen light bulbs of quartz and tungsten and the responsible for the pulp temperature increase. However, the absorption capacity of these filters decreases with time7. Thus, it has been reported that the, besides heating the dental element promoted by the LCUs9-12, an exothermic reaction happens into the resinous mass, which is polymerized during the dental composite photoactivation13-14. Since pulp alterations can be induced by the pulp chamber temperature increase6,15-16 the aim of this study was to verify whether there is any difference in the pulp chamber temperature changes, during the light curing of two composites with different viscosities. Material and Methods
Eighteen extracted mandibular bovine incisors had their roots cut off with a flexible diamond disc n.7020 (KG Sorensen, Brazil). Standardized cavities were then prepared in the bucal face, using cylindrical diamond bur n.1098 (KG Sorensen, Brazil) in a high-speed hand piece (Kavo do Brasil, Joinvile, SC, Brazil), adapted to a microscope base. The cavities walls were 3.0 mm in length and the gingival wall was located 13 mm above the enamelcementum junction so that a uniform leftover dentine was obtained (Figure 1). The measurements of the cavities were conferred with a digital calliper (Mitutoyo, Japan), with a 0.01mm precision. The prepared teeth were stored in absolute humidity at 25°C for 48 hours. Before acid etching, a thermocouple, adapted to a digital thermometer (Iopetherm 46, IOPE, São Paulo, Brazil), was introduced into the pulp chamber. Phosphoric acid (37 %; MagicAcid, Vigodent, Brazil) was applied for 15 seconds, before washing copiously for 30 seconds and drying with absorbent paper. The Single Bond one-bottle adhesive system (3M/ESPE Dental Products, St Paul, MN 55144, USA) was applied and light cured for 10 seconds. The dental composites were then randomly bulk inserted in the cavity. Nine teeth (n=9) had been restored with the resin composite Filtek Flow (3M/ESPE) shade A2 and nine (n=9) with the resin composite Filtek Z250 (3M/ESPE) of the same shade. The compositions of these resin composites and their filler contents, in volume, are shown in Table 1. The resin composites were light cured for 20 seconds in accordance with the manufacturers’ instructions with a conventional LCU (XL 2500, 3M/ESPE Dental Products, St Paul, MN 55144, USA), perpendicular to the preparation. The light intensity was verified with a radiometer Curing Radiometer model 100 (Demetron/Kerr, Danburg, CT, USA), which registered 700 mW/cm². All restorative procedures were carried out in a controlled environment, with a mean temperature of 25ºC and relative humidity of 40 %. For measurements of the temperatures in the pulp chamber the thermocouple’s tip was placed inside the pulp chamber, in contact and under the dentine of the axial wall. Following temperature stabilization, the initial temperature was measured and the composite was then light cured for 20 seconds. The temperature peak was then registered and the initial temperature was deducted from the final value. The restored teeth were sectioned in the bucco-lingual direction, with a flexible diamond disc n. 7020 (KG Sorensen, Brazil) in the centre of the restoration. After sectioning, the leftover dentine thickness was measured with a digital calliper (Mitutoyo, Japan) with a precision of 0.01 mm. The cavities should present a pulpal wall with 1.0 mm ± 0.2 mm remaining dentine. The teeth that presented pulpal walls with different thickness from those of the standard were excluded from the research. The results, in Celsius degree, were submitted to Student-t test at 5% significance. Results Table 2 shows the mean temperature rises observed in the pulp chamber. The difference in temperature increase between the two resin composites was found to be statistically significant (p<0.05), in that the resin composite Filtek Flow demonstrates a higher mean temperature rise. Discussion External heat applied to the tooth can increase the pulp temperature, thus resulting in irreversible damage to pulp tissues6,15-16. This thermal trauma can be induced by cavity preparation, exothermal cure reaction of cements and restorative materials, and LCUs14,17. Thus, during resin composite polymerization, the LCU and the exothermal process, generated by the polymerization by the addition reaction, can increase the pulp temperature8,13,18. The leftover dentine thickness plays an important role in the thermical insuling of the pulp, since it contributes to the thermal diffusivity due to its low thermal conductivity coefficient19. However, this property diminishes as the thickness of dentine decreases20-21. Thus, the damaging potential to pulp is greater in deeper cavities where the leftover dentine is thinner and the tubular area is larger18. For this reason, our intention was to standardize the leftover dentin thickness so that it could not interfere in the results. For these motives, shade A2 was used for both resin composites, since this factor can also affect the temperature increase22. With conventional halogen LCUs, darker composites generate higher temperatures compared to lighter ones, as the latter are likely to show reduced light absorption23. In the present study, the temperature increase generated by the light curing energy in association with the composite curing reaction was evident. As the objective of this study was to verify if the resin composite type (related to the composition) could interfere with the temperature rise, the same LCU was used for both resin composites. Thus, a standardised light intensity was maintained, because the higher the light intensity, the greater the temperature increase12. Moreover, the speed of the exothermal cure reaction of visible-light activated resin composites increases with the light intensity rise24. Therefore, the difference in the temperature rise between resin composites Z250 and Filtek Flow (Table 2) is probably due to the composition of these materials. According to Harrington et al.25, this fact may be related to the difference in the composition of the monomers, which could modify the light transmission of the product. Asmussen26 verified that composites that use Bis-GMA associated with TEGDMA presented greater conversion degree when the amount of TEGDMA was increased. The resin composite Filtek Flow contains Bis-GMA and TEGDMA in its composition. Since this resin is “flowable”, this material may possess a greater amount of diluent monomers than “conventional”resin composites with the same monomer association. Therefore, besides viscosity, conductivity and thermal diffusivity can be modified. According to Lloyd and Brown9, the cure reaction is exothermal and it is directly related to the resin volume. Thus, the lower the load content, the greater the exothermal reaction will be. This may be the main factor to explain why the restorations made with the Filtek Flow resin composite (47% of filler, in volume) induced a significantly higher temperature increase than those restored with the Filtek Z250 resin composite (60% of filler, in volume). Moreover, Anusavice3 affirmed that the extenuating coefficient of light can vary considerably for the different composites depending on the particle size and on the filler concentration, i.e., resin composites with greater filler amount tend to diminish the light absorption, because part of this light is reflected by these particles. As the amount of absorbed light and the heat generation are directly related, the Z250 resin composite tends to conduce a lower amount of thermal energy because it possesses a significantly higher amount of filler particles. Attempting to decrease the temperature generated during dental resin composites photo-activation, Uhl et al.8 verified that a reduced amount of composite and/or a LCU of low intensity may be necessary to reduce the thermal stress on pulp tissue. With the same objective, Masutami et al.24 recommended the incremental insertion technique. However, Lloyd et al.27 reported that the temperature increased with the resin composite thickness reduction. Moreover, Shortall and Harrington18 demonstrated that the temperature rise was higher when the light was applied to an empty cavity, i.e., the temperature was lower in the presence of the resin composite. Therefore, the authors observed that the capacity of the resin composite to attenuate the heat generated by light was higher than the exothermal reaction during polymerization. According to Loney and Price28, the best form of reducing the temperature generated during dental composite photoactivation is the application of shorter light activation times and the maintenance of a thicker dentine layer after cavity preparation. The studies that have tried to accurately determine the temperature rise capable of damaging the pulp tissues are controversial. The classic study by Zach and Cohen16 suggested that a rise of 5.5ºC in the pulp chamber could result in irreversible damage causing inflammatory process. However, Baldissara et al.29, through clinical and histological studies with human teeth, affirmed that a pulp temperature increase of even 11.2ºC does not cause damage to pulp tissues. In the present study, no specimen presented a temperature rise of greater than 5.0ºC. This is an in vitro study; therefore its results could predict what may be happening in a clinical situation. References
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