In-vitro Study on Temperature Changes in the Pulp Chamber Due to Thermo-Cure Glass Ionomer Cements

Abstract

The application of the Glass Ionomer Cements in clinical dentistry is recommended due to properties such as fluoride release, chemical adhesion to tooth, negligible setting shrinkage, and coefficient of thermal expansion close to tooth, low creep, and good color stability. However, the cement is vulnerable to early exposure to moisture due to slow setting characteristics. The uses of external energy such as ultrasound and radiant heat (Thermo-curing) have been reported to provide acceleration of the setting chemistry and enhance physical properties. Aim: The aim of this in vitro study was to analyze temperature changes in the pulpal chamber when using radiant heat to accelerate the setting of GICs. Material and Methods:The encapsulated GIC Equia Forte was used for this study. The temperature changes in the pulp were measured using thermocouple in the cavities which were 2,6 and 4,7mm deep with and without filling. Results:The results showed that a temperature rise (ΔT) in the pulp chamber was 3,7°C. ΔT for the 2.6mm and 4.7mm deep cavity and without placing any restoration the temperature was 4,2°C and 2,6°C respectively. After the restoration has been placed, the ΔT range in the pulp chamber was lower ranging from 1.9°C to 2.4°C. Conclusion: It could be concluded that Thermo-curing of the GIC during the setting is safe for the pulp and can be recommended in clinical practice.

Introduction

Glass Ionomer Cements have been used for several years. Their application in clinical dentistry has been driven by several properties such as fluoride release, chemical adhesion to tooth, negligible setting shrinkage, and coefficient of thermal expansion close to tooth, low creep, and good colour stability. However, the cement is vulnerable to early exposure to moisture due to slow setting characteristics (1, 2). Resin-based materials were better in this respect and the use of external radiation (u.v. or visible light) to initiate their polymerization improved this and gave the clinician greater control over the setting process (“command set”). However, the addition of resins produced adverse effects on abrasion resistance, dimensional stability, colour stability and certainly biocompatibility. The uses of external energy such as ultrasound and radiant heat have been reported to provide acceleration of the setting chemistry of GICs. (3, 4). Both are reported to enhance physical properties (5, 6).

Commercial GICs have recently become available which advocate the use of “Thermo setting” via the use of radiant heat from a portable LED lamp. However, there have been concerns that exposure to heat from these sources could lead to damage to tooth pulp. Therefore, the aim of this in vitro study was to analyze temperature changes in the pulpal chamber when using radiant heat to accelerate the set of GICs

Materials and Methods

Encapsulated commercial GIC Equia Forte (GC Corporation, Japan) was used in this study. Thermo-curing was performed using the LED light (D-Light Duo, GC Corporation, Japan) recommended by the manufacturer.

The heat output of the lamp was measured by touching the tip of the lamp with the thermocouple and recording the maximum heat after 90 seconds. Thereafter, a small entrance was made in the facial surface of non-carious molars procured from Queen Mary University London (QMUL) tissue bank.

The tooth was placed above a water bath (without immersing it into water) in a plastic tent to ensure humid environment at 37°C (Figure 1, 2). Radiant heat was applied for 90 seconds to the occlusal surface using the LED lamp and temperature changes in the pulp chamber were recorded using the thermocouple. Following this, a 2.6mm deep cavity (measured from the cusp tips) was cut into the same molar using a high speed handpiece and a diamond bur. The tooth was placed back in the plastic tent and temperature changes in the pulp chamber were recorded while using radiant heat for 90 seconds.

Figure 1.

Experiment set- up

Figure 2.

Experiment set- up

To measure the temperature changes while thermo-curing a GIC restoration, the tooth was then restored with Equia Forte A3 shade following the manufacturer’s instructions. The GIC was packed into the cavity using the “finger press” technique. The GIC was thermo-cured for 90 seconds and the temperature changes in the pulp chamber were recorded. First, the temperature changes in the pulp chamber were recorded at the level of 2,6mm. Subsequently, the cavity in the tooth was extended to 4.7mm (measured from the cusp tip). Temperature changes in the pulp chamber with and without GIC filling were measured as described above (Figures 1, 2).

This process was performed 3 times to ensure the reliability and reproducibility of research.

Results

The results showed that the maximum temperature from the LED thermocuring light used in this study was 60°C. The light reached its maximum temperature after 60 seconds. After that, there was no further rise in temperature.

On applying the light to the tooth from the coronal surface without any cavity preparation, the temperature rise (ΔT) in the pulp chamber was 3,7°C. ΔT for the 2.6mm and 4.7mm deep cavity without placing any restoration was 4,2°C and 2,6°C respectively. However, after the restoration has been placed, the ΔT range in pulp chamber was lower ranging from 1.9°C to 2.4°C (Figure 3, 4, 5).

Figure 3.

Temperature changes in the cavities with and without filling after 90s on the depth of 4.7mm

Figure 4.

Temperature changes in the 4.7mm deep cavity without filling

Figure 5.

Temperature changes in the 4.7mm deep cavity with filling

Discussion

The acid base neutralization reaction of Glass Ionomer Cements can be accelerated through the use of external energy such as ultrasound (2-4) and heat (5-7). This is particularly useful in overcoming the moisture sensitivity which adversely affects the properties of GIC (8-20). Although ultrasound accelerated the setting, its use was clinically difficult. Consequently, there has been less commercial interest in the technique. Heat on the other hand can be applied through LED lamps such as those marketed by GC corp with its Equia Forte system.

As with the use of light activated bleaching systems (21), thermo-setting GIC can lead to intra pulpal temperature rises. However, the current study showed that the application of heat for up to 90 seconds using GC D-light Duo leads to 2-3oC rise in temperature in the pulp chamber even though the surface temperature rose as high as 50.2oC. This happened because GICs are thermal insulators and as previously noted by Gavic et al. there is low thermal conductivity in GIC restorations with the thickness of 4mm and 2mm (22). However, their study was not performed in a tooth. The results also showed a lower rise of temperature with the GIC applied in the cavity compared with those that were not applied in the cavity. Water is formed during the reaction, which possibly cools the tooth. Also, the drop in temperature can be explained by a lower initial temperature of the filling material which is around 23oC when placed in the cavity.

An extensive debate has been ongoing as to adverse effects of temperature rises on the pulp. Some research was carried out to examine whether temperature rise can be lethal to the pulp. Zach and Cohen (23) investigated the effect of temperature rise on the pulp of rhesus monkeys. Their study reported that a temperature rise of 5.6oC (above body temperature, meaning above 42.6oC) led to irreversible damage in 15% monkeys. It was found that the temperature rise of 16.6oC led to pulp damage in 100% monkeys. In contrast, some studies (24-26) found that the rise in temperature in human pulp between 8.9°C and 14.7°C does not produce any pathology. Although this may suggest that human pulp is more resilient to heat damage, the thresholds reported by these studies are higher than the intrapulpal increase observed in the current study. Moreover, these temperature rises are comparable and in some cases lower than the intrapulpal temperature rises reported with the use of the light activated bleaching lamps (24).

Conclusion

Based on the obtained results it could be concluded that the use of external heat during the setting of GIC material does not lead to harmful overheating of the pulp tissue, hence it does not cause any pathological conditions. The application of external heat (Thermo-curing) as a “Command set” method and technique for improving mechanical properties and adhesion of GIC materials can be part of regular clinical practice.