Polymerization of dual cured composites by different thickness

Yun Ju Kim; Myoung Uk Jin; Sung Kyo Kim; Tae-Yub Kwon; Young Kyung Kim

doi:10.5395/JKACD.2008.33.3.169

Abstract

The purpose of this study was to evaluate the effect of thickness, filling methods and curing methods on the polymerization of dual cured core materials by means of microhardness test.

Two dual cured core materials, MultiCore Flow (Ivoclar Vivadent AG, Schaan, Liechtenstein) and Bis-Core (Bisco Inc., Schaumburg, IL, USA) were used in this study. 2 mm (bulky filled), 4 mm (bulky filled), 6 mm (bulky and incrementally filled) and 8 mm (bulky and incrementally filled)-thickness specimens were prepared with light cure or self cure mode. After storage at 37℃ for 24 hours, the Knoop hardness values (KHN) of top and bottom surfaces were measured and the microhardness ratio of top and bottom surfaces was calculated. The data were analyzed using one-way ANOVA and Scheffe multiple comparison test, with α = 0.05.

The effect of thickness on the polymerization of dual cured composites showed material specific results. In 2, 4 and 6 mm groups, the KHN of two materials were not affected by thickness. However, in 8 mm group of MultiCore Flow, the KHN of the bottom surface was lower than those of other groups (p < 0.05). The effect of filling methods on the polymerization of dual cured composites was different by their thickness or materials. In 6 mm thickness, there was no significant difference between bulk and incremental filling groups. In 8 mm thickness, Bis-Core showed no significant difference between groups. However, in MultiCore Flow, the microhardness ratio of bulk filling group was lower than that of incremental filling group (p < 0.05). The effect of curing methods on the polymerization of dual cured composites showed material specific results. In Bis-Core, the KHN of dual cured group were higher than those of self cured group at both surfaces (p < 0.05). However, in MultiCore Flow, the results were not similar at both surfaces. At the top surface, dual cured group showed higher KHN than that of self cured group (p < 0.05). However, in the bottom surface, dual cured group showed lower value than that of self cured group (p < 0.05).

I. INTRODUCTION

The recent development of direct core materials enables dental clinicians to restore non-vital teeth by replacing the tooth structure that was lost due to endodontic treatment1). Core materials can be provided as self cured, light cured or dual cured system. Self cured composites can build up the lost tooth structure at one time, and have better marginal adaptation and present less damage to the integrity of the restored tooth2). However, they have limited working time and long setting time. On the contrary, light cured composites offer a longer working time than self cured ones, but there is a possibility of incomplete polymerization especially in a deep cavity due to the limited depth of light transmission3). For those reasons, dual cured systems, that combined favorable properties of both self cured and light cured systems, have been widely used as core build-up resin materials2-4).

Adequate polymerization of a resin composite is a critical factor to obtain adequate physical and biological properties1,3-7). The effectiveness of polymerization depends on not only the chemistry of the material and concentration of the initiator, but also the filler particle type, size, and loading1,5,8-10). In a light cured or dual cured version, it is also affected by the curing light irradiance, exposure time and light transmission9).

Dual cured composites by different manufacturers have different handling characteristics, compositions, mixing types and properties from each other. The dual cured composite at the top surface is mainly polymerized through photo-initiated chemical reactions, while at the bottom surface it is done via chemically initiated polymerization. However, the deeper region of dual cured systems may not polymerize fully, when chemical polymerization is not sufficient4).

There are several methods to evaluate degree of polymerization of resin materials1,5,6,8). Although direct method, such as Fourier-transformed infrared (FTIR) spectroscopy or Raman spectroscopy, has been widely used and most accurate method, it is complex, expensive and time-consuming1,5,6). Therefore, microhardness test is considered as a simple and, at the same time, effective method to evaluate the degree of conversion1,5,8). Moreover, a positive correlation has been reported between the results of hardness value and FTIR spectroscopy or Raman spectroscopy5,8-11).

The purpose of this study was to examine the effect of the thickness, filling methods and curing methods on the polymerization of dual cured composites by means of microhardness test.

II. MATERIALS AND METHODS

Two dual cured core materials, MultiCore Flow (Ivoclar Vivadent AG, Schaan, Liechtenstein) and Bis-Core (Bisco Inc., Schaumburg, IL, USA) were used in this study. MultiCore Flow is auto-mixed type and Bis-Core is hand-mixed type of two pastes. Their components and concentrations are presented in Table 1.

Specimen Preparation

Each composite was packed into 2 mm (bulky filled), 4 mm (bulky filled), 6 mm (bulky and incrementally filled) and 8 mm (bulky and incrementally filled)-thickness Teflon mold, respectively. The mold cavity was confined between opposing 0.05 mm transparent polyester films (Hawe Striproll, KerrHawe SA, Bioggio, Switzerland). A glass slide was covered on top of the resin composite and pressed, permitting the excess material to extrude from the mold. The material was irradiated for 10 sec per 1 mm using a light curing unit (Optilux 501, Kerr, Danbury, USA), providing a light intensity of 500 mW/cm² as evaluated by a hand-held radiometer, or self cure mode (waiting for 30 min in dark at room temperature). And then the samples were removed from the mold and the upper surfaces (closer to the light source) were marked with a pen. Seven samples were assigned to each group. Samples were stored in the distilled water at 37℃ for 24 hours. The top and bottom surfaces of samples were polished with a #2000 abrasive paper and PoGo system (Dentsply, Konstanz, Germany) to remove the oxygen inhibited layer ( Table 2).

Microhardness Measurement

The Knoop hardness values (KHN) of the top and bottom surfaces were measured at 50 gf load and a dwell time of 10 seconds with a digital microhardness tester (FM-7, Future-Tech Corp., Tokyo, Japan). Indentations were made at five points on each surface. The microhardness ratio of two surfaces (hardness ratio) was defined as KHN of the bottom surface/KHN of the top surface.

Statistical Analysis

Statistical evaluation of the data was performed by one-way analysis of variance (ANOVA). Following the ANOVA, Scheffe multiple comparison test (α= 0.05) was used to identify pairwise differences. All statistical analysis was performed using SPSS 12.0 for Windows (SPSS Inc., Chicago, IL, USA).

III. RESULTS

The mean KHN and the hardness ratio of MultiCore Flow are shown in Table 3 and Figure 1. At the top surface, M8S group showed significantly lower hardness value than those of the other groups (p < 0.05). At the bottom surface, M8 group showed the lowest hardness value, 31.8 ± 3.3. For the hardness ratio, M8 group showed the lowest value. However, the hardness ratios of all the other groups were over 0.8.

The mean KHN and the hardness ratio of Bis-Core are shown in Table 4 and Figure 2. At the top and bottom surfaces, B8S group showed significantly lower hardness values than those of the other groups (p < 0.05). The hardness ratios in all groups were higher than 0.8.

Figure 3 shows the hardness ratios of MultiCore Flow and Bis-Core by different thickness when the bulk technique was used. The hardness ratio of M8 group was significantly lower than those of another three groups (p < 0.05). In Bis-Core, there was no significant difference between groups.

IV. DISCUSSION

Dual cured version of resin composite was introduced to combine favorable properties of both self cured and light cured systems3,4). However, it is still unclear whether polymerization of dual cured composites is consistent or not throughout the depth of a cavity, because of the complicated polymerization reaction and various formulation of the materials. Therefore, the aim of this study was to examine the effect of thickness, filling methods, curing methods on the polymerization of two dual cured core products by using a microhardness test.

In all groups except M8, the microhardness value of the cured surface was not affected by the thickness. The KHN of the bottom surface of M8 group showed lower value than those of the other groups. This implies that the polymerization of the material was not enough in the deep portion probably due to the insufficient chemical polymerization reaction. Therefore, although dual cured version has the incorporation of chemical and light curing modes in the same material, the two types of polymerization may not complement each of the other. Another explanation may be possible. Initial low intensity light curing accelerated change of the dual cure composite matrix from the gel to post-gel phase, thus the free movement of the radical might be inhibited2,13). On the other hands, B8 group showed no significant difference with another groups in KHN. This result indicates that the polymerization of dual cured composites especially in a deep cavity seems to be material dependent.

There have been many of studies addressing the effect of curing mode on a variety of properties of dual cured luting composites2,13). Some researchers proposed that the dual cured composites had inferior mechanical and physical properties when the material was only chemically cured2,13,14). In the present study, the microhardness value of M8S group was lower than that of light cured group. This suggests that light curing is needed to obtain good mechanical properties in the curing of dual cured materials.

The ideal hardness ratio of resin composites would be 1.01). That is, the hardness of the bottom surface should be similar to that of the top surface1,5). However, it is not always possible to obtain such a value practically. In clinical conditions, the hardness ratio ranging from 0.80 ~ 0.90 has been employed as criteria for adequate conversion at a specific sample thickness1,5,7,15). In MultiCore Flow, the hardness ratio of M8 group was lower than 0.8, which means that polymerization at the bottom surface was not sufficient to provide optimal mechanical properties. On the other hand, the hardness ratios in all groups of Bis-Core showed higher than 0.8, which means that the polymerization of Bis-Core was not affected by thickness. When the bulk technique is used in a deep cavity up to 8 mm depth, the material should be carefully chosen because the polymerization of dual cure version is material specific.

However, other factors, such as filler load, filler type, filler size, or resin matrix types, shall be taken into consideration when dual cured version is used4). Therefore, it is difficult to compare the degree of conversion between the different brands of composites only using microhardness test. Light transmission can also affect the microhardness. If light transmission of Bis-Core to the bottom surface is better than that of MultiCore Flow, the polymerization of the bottom surface of Bis-Core in a deep cavity can be better and enough to provide optimal mechanical properties. Different mixing methods were employed for two dual cured composites. MultiCore flow is auto-mixed type and Bis-Core is hand-mixed type. Bis-Core might contain more voids (porosity) than the MultiCore Flow, as the result of incorporating air while mixing the two pastes. Because the presence of oxygen in the voids inhibits polymerization, the degree of conversion can not be enough2). Nevertheless, in the present study, the microhardness value of Bis-Core was higher than that of MultiCore Flow. This result suggests that characteristics of material may affect more than mixing type on the microhardness although the microhardness value can be affected by their mixing types.

Within the limitations of the present study, the degree of polymerization of dual cured composite evaluated by means of microhardness test was not consistent throughout all the depth of a cavity. The incremental filling method and sufficient light curing to the materials may be recommended especially in a deep cavity to obtain adequate polymerization of a dual cured composite. However, the degree of polymerization of dual cured composites also seems to be material specific. Further researches are needed to elucidate polymerization reaction of dual cured composites.

V. CONCLUSION

This study evaluated the effect of thickness, filling methods and curing methods on the polymerization of two dual cured core materials, MultiCore Flow and Bis-core by means of microhardness test.

The effect of thickness and curing methods on the polymerization of dual cured composites showed material specific results. In 2, 4 and 6 mm groups, the KHN of two materials were not affected by thickness. However, in 8 mm group of MultiCore Flow, the KHN of bottom surface was lower than those of the other groups. In Bis-Core, the KHN of dual cured group were higher than those of self cured group at both surfaces. However, in MultiCore Flow, dual cured group showed higher KHN than that of self cured group at top surface, while the opposite at bottom surface. The effect of filling methods on the polymerization of dual cured composites was different by their thickness or materials. In 6 mm thickness, there was no significant difference between bulk and incremental filling groups. In 8 mm thickness, Bis-Core showed no significant difference between groups. However, in MultiCore Flow, the microhardness ratio of bulk filling group was lower than that of incremental filling group.