Abstract

INTRODUCTION

The restoration and maintenance of dental health through adequate restorative treatment in order to protect pulp function is the main purpose of restorative dentistry. Pulp is a highly vascularized tissue. Pulpal temperature is critical and must not exceed normal values in dental restorative procedures.[1,2] The pulp must be protected not only against the thermal/electrical conductivity of the restorative material, but also against head irradiation of dental LCUs throughout curing of restorative materials.[3]

The curing of composite resins produces a temperature rise, caused by exothermic reaction and the energy absorbed during polymerization.[4] The temperature rise throughout curing is influenced by factors such as intensity of light, composition and transmission properties of composite resins, depth of the cavity and restoration, duration of light exposure, and type of light source.[5]

Protection of the dentin-pulp complex is an important factor in pulp vitality during operative procedures. This involves the avoidance of thermal stimuli caused by operative procedures, toxicity of restorative materials, and bacteria penetration. Liners have long been used for this purpose. One of the most important effects of liners is thermal insulation, although this varies significantly depending on their composition. The thermal insulation effect of materials with relatively low thermal conductivity may be small even when these are thickened. A thermal insulation effect is, therefore, required in pulp capping agents when a cavity is very deep and very close to the dental pulp.[6,7] The materials available for this purpose are calcium hydroxide (Ca[OH]₂) base cements, glass ionomer cements, resins, and adhesive systems.[8]

Pulp capping materials which contain Ca(OH)₂ accelerate the formation of reparative dentin, protect the pulp from chemical irritants from the restorative material, and sterilize the remaining soft dentin by increasing the pH of the underlying dentin.[9] Resin-modified glass ionomer cements (RMGICs) are indicated for pulp capping.[10] RMGICs provide adequate protection for the dental pulp, preventing the occurrence of postoperative sensitivity. Adhesion of RMGICs to dental substrates together with their ability to release fluoride, which may act to prevent secondary caries, makes these materials an excellent option in clinical procedures. Adhesive systems may be used as liners. When applied to dentin, they act as a cover and protect the pulp from thermal stimuli by sealing the dentinal tubules with a hybrid layer.[11]

The purpose of this study was to evaluate the thermal insulating properties of different cavity liners and composite resins. The null hypothesis was that the thermal insulating properties of cavity liners and composite resins would not differ significantly.

MATERIALS AND METHODS

Sixty-four dentin discs were prepared from extracted sound human third molars. Teeth were embedded in cylindrical acrylic blocks. The occlusal surfaces were first cut horizontally, exposing the dentin. Dentin discs, 1 mm thick and 8 mm in diameter, were produced using a universal slow speed saw (Isomet, Buehler, Evanston, IL, USA) under water cooling. The discs were sectioned from just above the pulp chamber. The specimens were then divided into four groups (n = 16). Three light curing liners and two adhesive systems were applied to the dentin discs according to the manufacturers’ instructions [Table 1]. The four groups established were:

Table 1

Brand, material type, matrix, filler type, curing time, manufacturer, and batch number of pulp capping cements and adhesives used in this study

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1. Group 1: Flowable composite (Grandio Flow, VOCO, Germany).

2. Group 2: RMGICs (Vitremer, 3M ESPE Dental Products, St. Paul, MN, USA).

3. Group 3: Light curing Ca(OH)₂ cement (Calcimol LC, VOCO, Germany).

4. Group 4: Two adhesive systems — Filtek Silorane Adhesive System (3M ESPE Dental Products, St. Paul, MN, USA) and Futurabond adhesive (Grandio, VOCO, Germany).

After curing of liners and adhesive systems, all groups were further divided into two subgroups (n = 8). Two composite resins, a silorane-based composite resin (Filtek Silorane, 3M ESPE Dental Products, St. Paul, MN, USA) and a methacrylate-based composite resin (Grandio Universal Nano-Hybrid, VOCO, Germany) were applied at 2 mm in each subgroup.

RESULTS

Table 2 shows temperature rise values in liners and adhesive systems. There were significant differences in mean temperature rises among different materials at different curing times (P < 0.05). Adhesive systems were not effective in protecting pulp from thermal stimuli. The highest temperature rise was determined in Silorane adhesive. Temperature rises from highest to lowest were Silorane adhesive > Grandio adhesive > Flowable composite > RMGIC > Ca(OH)₂ based cement. Ca(OH)₂ base cement best protected pulp capping cement against temperature rise.

Table 2

Temperature rise (°C) and standard deviation of liners and adhesives at different time intervals

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The second phase of this study compared temperature rise between composite resins. The results showed that silorane-based composite resin exhibits a greater temperature rise than methacrylate-based composite resin (P < 0.05).

As shown in Table 3, Group 4 (Silorane adhesive and Futurabond adhesive) exhibited a greater temperature rise. These results show that applying liners under composite resin reduces temperature rise during polymerization.

Table 3

The maximum temperature rise (°C) in each group throughout curing (20 s)

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DISCUSSION

External heat can increase the temperature in the pulp chamber. It causes inflammatory reactions and resulting damage to the pulp tissue.[12] Clinical research has shown irreversible damage to pulp tissues, at levels of 60% at 5.5°C and 100% at 11°C.[13] Two important factors cause temperature rise during polymerization of composite resins; heat from LCU and exothermic reaction during polymerization. Strang et al.[14] reported that light source is more important than exothermic reaction, while Masutani et al.[3] maintained the opposite. Temperature rise may be measured with a thermocouple connected to dentin discs. In the literature, the K-type thermocouple is described as effective for measuring temperature rise under dentin discs during polymerization.[15]

Materials with low thermal conductivity exhibit good thermal insulation properties. The thermal conductivity of a restorative material is one of the important factors in the protection of pulp from temperature rise during polymerization. Thermal conductivity shows the time of the temperature change between one side of a specimen and the other. Low thermal conductivity means that pulp tissues will be less harmed during polymerization of restorations with LCUs.[4] The thermal conductivities of organic materials are generally lower than those of inorganic materials. This means that aromatic sulfonamide, hydrocarbon resin, methyl salicylate polymers, and monomers have lower thermal conductivity than calcium phosphate, calcium oxide, and barium sulfate. Therefore, differences in the composition of inorganic components significantly affect thermal conductivity, and thermal conductivities of high organic filler content cements are generally lower than those of inorganic ones.[16]

Our study compared the thermal insulating properties of four liners and two composite resins. Applying liner under composite resin is effective in reducing thermal stimuli due to LCU and exothermic reaction during polymerization of composite resin. Adhesive systems exhibited low thermal insulation. This was probably due to adhesive systems’ high inorganic filler load. Silorane adhesive exhibited more thermal conductivity than the Futurabond adhesive system because of its chemomechanical properties. Futurabond contains organic silica particles, and siloranes’ silanized inorganic matrix results in greater thermal conductivity. The flowable composite used in this study has a high filler particle content (80.2% w/w). However, like HEMA, monomers in this composite resin matrix caused a smaller temperature rise than adhesive systems.[17]

Silorane-based composite resin exhibited more temperature rise than methacrylate-based composite resin. Due to their chemical structure and matrices. In the context of conventional dimethacrylate-based composite resins, it has been reported that temperature rise decreases as the resin filler content of a dental material increases because less resin is available for polymerization. Fillers are chemically inert and do not contribute to an increase in the heat of a reaction.[18] However, the filler temperature increases along with that of the matrix. Therefore, a considerable proportion of the energy which might otherwise raise the temperature of the resin matrix is absorbed by the filler. In this context, fillers are active phases with a moderate role.[19]

The term “silorane” is derived from its composition of siloxanes and oxiranes, which polymerize via a cationic ring-opening reaction.[20] This reaction occurs in the oxiranes component and is induced by a photochemical event in which Camphorquinone, excited by light energy, interacts with iodonium salts and electron donors to produce cations as propagating active centers.[21] According to Miletic et al.,[22] Silorane-based composite resin has a different temperature curve with significantly higher temperatures compared with dimethacrylate-based composite resin. This was confirmed during polymerization of a silorane-based system in another study. This implies that the cationic ring-opening polymerization reaction of Filtek Silorane has a different heat generation pattern.[23] Moreover, optical pyrometry studies have shown that cationic ring-opening polymerization of oxiranes is a highly exothermic reaction.[24] It has also been suggested that the rate of heat generated in a cationic polymerization is directly related to the number of photo-generated initiating species present in the resin system.

Ca(OH)₂ base cement exhibited significantly lower thermal conductivity than the others in our study. It is important to apply a thin layer to protect the pulp from thermal stimuli. In addition to their organic component and low thermal conductivity, Ca(OH)₂ base cements can be used as a pulp capping agent in deep cavities due to their therapeutic effects and stimulation of the dentin bridge.[25] RMGIC has some important advantages, in that it bonds chemically to the dentin, and so functions as a great tubule seal. In this way, RMGIC prevents hydrodynamic fluid flows and protects the pulp from thermal stimuli.[10] Its low thermal conductivity and tubule sealing property make it an important pulp capping agent. However, polymerization with a light emitting diode curing unit caused a small temperature rise in dentin discs. In our study, flowable composites were not effective as Ca(OH)₂ and RMGIC in eliminating thermal stimuli.

In the light of the above findings, the restoration procedure in deep cavities may cause dangerous temperature rises in pulp. Adhesive systems are not enough by themselves to eliminate this effect. Ca(OH)₂ and RMGIC have lower thermal conductivity and are effective in reducing thermal stimulation during composite resin polymerization.

CONCLUSION

Within the limitations of the study, the thermal insulating properties of different cavity liners and composite resins invalidated the null hypothesis. The findings from this study are that:

1. There are significant differences among temperature rise during polymerization of different liner materials.

2. Adhesive systems were not as effective as Ca(OH)₂ and RMGIC in protecting pulp from thermal stimuli.

3. The thermal conductivity of silorane-based composite resins is greater than that of Methacrylate-based composite resin.

4. Ca(OH)₂ is the most effective liner for protecting pulp from thermal stimuli.

REFERENCES