Measurement of thermal expansion characteristic of root canal filling materials: Gutta-percha and Resilon

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Abstract

I. INTRODUCTION

One of the most important objectives of nonsurgical root canal therapy for success is to completely obturate the prepared canal1). Several studies have revealed that an incomplete sealing of the canal is one of the most important causes of failure2,3). In order to obtain a three-dimensional obturation of root canal systems, various materials have been used. Above all, gutta-percha is still the material most favored by endodontists for the obturation of root canals due to its thermoplastic behavior and its biocompatibility3-5). For a long time this material has been used successfully with a variety of nonadhesive root canal sealers for obturation of root canal.

Improving the quality of root canal obturations should logically be accomplished by an alternative to gutta-percha. Resilon (Pentron Clinical Technologies, Wallingford, CT, USA), has recently been introduced as an another root canal filling material that has the potential to challenge the use of gutta-percha6,7). It is a polyester based thermoplastic polymer which contains a bioactive glass and some radiopaque fillers. It performs like gutta-percha, and has the same handling properties, and for retreatment purposes may be softened with heat or dissolved with solvent like chloroform. Furthermore, it is able to bond to resin-based systems, such as dentin bonding agents and resin-based luting cements, so a monoblock is formed without the gaps typical in gutta-percha filling. Shipper et al.6) evaluated the resistance to bacterial penetration in roots filled with this new material and found that Resilon was superior to gutta-percha groups. They also reported that the Resilon "Monoblock" System was associated with less apical periodontitis, which may be due to its superior resistance to coronal microleakage8). Teixiera et al.9) reported that filling the root canal with Resilon increased the in vitro resistance to fracture of single-canal extracted teeth. However, Tay et al.10-12) doubted the bonding of Resilon with root canal wall dentin, and reported that Resilon is susceptible to enzymatic hydrolysis13,14). In any case there have been a few studies about the properties of this material because it is a relative new one.

Various techniques have been introduced for hermetic root canal sealing and all of these have been clinically successful3,15,16). The lateral condensation technique which has been used for a long time successfully takes advantage of the dimensional stability of gutta-percha. However, it cannot fill lateral canals and makes the obturation mass held together only by friction and the sealer. Homogeneity occurs only at the point where the coronal excess is removed with a hot instrument. The warm vertical condensation technique provides a homogenous filling and greater density obtained in the apical portion of the canal using gutta-percha in a plastic state without solvent3,17). Above all, continuous wave of condensation technique using system B and Obtura II makes warm vertical condensation more easier and faster, so it has been used increasingly by many clinicians today. However, every root canal filling materials show dimensional change by temperature change18).

Warm vertical condensation technique must have needs of some heat, it means that there would be some dimensional change of canal filling material during canal obturation procedure. Especially it is expected that thermoplasicized injectable instruments like Obtura II show more dimensional change because they extrude very hot filling material. As gutta-percha doesn't have the ability to bond to dentin wall of a root canal, it is possible to doubt that canal leakage is started if some dimensional shrinkage would happen in the canal space after canal obturation was completed19). Although several researchers reported thermal expansion properties of gutta-percha18-20), there is still insufficient information available on the effect of heat on the thermal expansion properties of injectable thermoplasticized canal filling materials including Resilon, newly introduced material. The comparison of thermal expansion properties of these materials are valuable.

The purpose of this study was to evaluate the thermal expansion characteristics of injectable thermoplasticized gutta-perchas and Resilon.

II. MATERIALS AND METHODS

Materials

The materials investigated are listed in Table 1. Epiphany is Resilon, and the others are injectable thermoplasticized gutta-perchas.

Table 1

Root canal filling materials used in this study

Measurement of the thermal dimensional change

A cylindrical ceramic mold was made for thermal expansion test, which was 27 mm long, with an internal bore diameter of 3 mm and an outer diameter of 10 mm. And the ceramic plunger with a diameter of 3 mm was also made. The mold was filled with a material of each groups, and both barrel ends were sealed with two ceramic plungers.

The samples in the ceramic molds were loaded in a dilatometer (Thermal Dilatometer, DIL 402C, TASC 414/3A Controller, Netzche, Germany). Thermal expansion curves were obtained at a temperature range from 25℃ to 75℃ with a heating rate of 1℃/min under a load of 30N. From dimensional change of samples as a function of temperature, the coefficients of thermal expansion were determined at the range 35 - 45℃, 45 - 55℃, and 55 - 75℃. The measurement was repeated six times for each material. In addition, the percentages of volume change at the temperature range of 35 - 55℃ and 35 - 75℃ were calculated by the following equation.

(λ : the percentage of volume change, Lo : sample initial length, dL_A, dL_B : the length change of sample at temperature A, B)

The data were analyzed by the One-way ANOVA and the Tukey's post-hoc test at 95% significant level using a statistical software (SPSS Release 12.0 for Windows, SPSS, Chicago, USA).

III. RESULTS

Measurement of the temperature of extruded filling material

The mean temperatures of the extruded materials and heating chamber at different setting temperature are shown in Table 2. The measured temperatures increased in the order of 23-G, 20-G needle, and the heating chamber for all setting temperature of Obtura II, and were similar among the materials (Table 2).

Table 2

Temperature of each materials, when extruded at the tip of application needles and existed in the heating chamber (℃)

The extruded temperature of the Obtura with a 23-G at the setting temperature of 200℃, which is the temperature recommended by the manufacturer, was about 76℃.

Measurement of the thermal dimensional change

There was almost no dimensional change of materials below 45℃. And at the temperature range of 45℃ - 55℃ the volume of materials increased rapidly, then slightly increased at over 55℃ (Figure 1). The coefficients of thermal expansion were calculated in the range of 35 - 45℃, 45 - 55℃, and 55 - 75℃ (Table 3). The coefficients at the range of 45 - 55℃ were larger than those of other temperature range, and Epiphany showed some larger coefficient than other materials at the range of 45 - 55℃ (603 ± 113.8 × 10^-6). However, there was no statistically differences between the materials.

Figure 1

The mean dimensional volume change of four materials as a function of temperature at the range of 25℃ - 75℃.

Table 3

The linear thermal expansion coefficient (10^-6/℃) of each tested material at different temperature range

The percentages of dimensional change between materials at the temperature range of 25℃ - 75℃ were shown in Figure 1 and 2. There was no statistical difference between four materials at the range from 35℃ to 55℃. However, Obtura gutta-percha showed smaller thermal expansion than Diadent and Metadent ones from 35℃ to 75℃ (p < 0.05, Table 4).

Table 4

The percentage of thermal expansion of each material at different temperature range (unit: Vol %)

IV. DISCUSSION

Resilon, a thermoplastic synthetic polymer-based root canal filling material, has recently been introduced. It has the similar properties of gutta-percha, so it is considered as the potential substitute for gutta-percha as a root filling material. Based on synthetic polymers of polyester, it contains bioactive glass, bismuth oxychloride, and barium sulfate. The overall filler contents is approximately 65% by weight. The thermoplasticity of Resilon is attributed to the incorporation of polycarprolactone, a synthetic, biodegradable, semi-crystalline aliphatic polyester13). And its bonding ability with resin cement and self-etching primers is derived from the inclusion into the resin of difunctional methacryloxy groups.

The composition of gutta-percha cones is approximately 19% to 22% gutta-percha, 59% to 75% zinc oxide, with the remaining small percentages of the components various waxes, coloring agents, anti-oxidants, and metallic salts. The particular percentage of the components vary by manufacturer, with resultant variations in the several physical properties of the individual cones primarily because of the gutta-percha and zinc oxide percentages3,21-23).

In spite of different chemical composition between gutta-percha and Resilon, they showed similar thermal expansion in this study and there was no statistical significant difference between them. This result means that Resilon have a similar thermal expansion property to gutta-percha which has been widely used for a long time as a root canal filling material, and can substitute it successfully at the point of thermal expansion characteristic like other favorable properties (radiopacity, plasticity and ability to flow at elevated temperatures, solubility in certain organic solvents etc).

Previous measurement of thermal volumetric change of dental gutta-percha has been performed dilatometrically with mercury. This technique has its advantages and disadvantages. Mercury based dilatometer has potential health wizards, is very sensitive to thermal fluctuations and very labour intensive24,25). In this study we used a ceramic mold which has a relatively very low thermal expansion coefficient. Generally the thermal expansion coefficient of ceramic is 8 × 10^-6/℃ (20 - 50℃)26), and it seems to be almost zero compared with that of gutta-percha and Resilon. The dilatometer used in this study can measure the dimensional change over temperature by micrometer unit.

In this study, the thermal coefficient of materials were larger than those of other studies20,27). The mean value of this study was more double than the one reported by Cohen et al.20). In addition, Tsukada et al.19) showed that the percentage of specific volume change at cooling between 75 and 24℃ was 4-5%, which was much larger than our results although the equation for calculation and experimental condition were slight different. These differences are possibly due to the different materials used in experiment or sensitivities of the different thermal expansion recording systems. However they are worthwhile from a standpoint of comparing to each materials under the same condition. It is possible to say that the thermal expansion of Resilon was not different from the other gutta-percha materials.

The results of this study seem to indicate that concerns about the high temperature of thermoplasticized injectable gutta-percha are not justified. When the temperature was measured, there was evidently a rapid continuous heat loss from the time the heated gutta-percha leaved the injection needle and reached the thermocouple due to the effect of room temperature air causing a rapid cooling of the exposed experimental materials. Weller and Koch28) measured the temperature produced with the Obtura II heated gutta-percha system and found intracanal temperatures ranging from 40℃ to 57℃ and external root surface temperatures ranging from 36℃ to 42℃. Sweatman et al29) reported that with the Obtura II, the lowest mean internal temperature change was 5.22℃ at the 0 mm level, whereas the highest mean internal temperature change was 26.63℃ at 6 mm level. We did not measure the intracanal temperature during the root canal filling procedure, postulated that the intracanal temperature range during the canal filling procedure was 35-55℃ according to previous studies28-33).

The thermal expansion of Obtura group appeared smaller than those of Diadent and Metadent groups (p < 0.05) and showed no difference with that of Resilon group at the range of 35℃-75℃. However there was no statistical difference between four material groups at the range of 35℃-55℃, the clinical meaningful temperature.

It is unfortunate that tests could not be carried out on the material as supplied by the manufacturer, and only during the heating procedure. Tsukada G et al.19) showed that, on cooling, gutta-percha will shrink to its original length, although not at the same rate as it expanded on heating. However it may be assumed that the shrinkage on cooling from working temperature to intracanal temperature will be of an equal magnitude to the expansion over that temperature range. And, it is reasonable to assume that the shrinkage occurs in the range from the softening temperature down to intracanal temperature (55-35℃).

Schilder et al.34) identified two major transitions, or phase changes, when dental gutta-percha was heated. They concluded that, on heating material, two transformations occurred at temperatures between 42-49℃ and 53-58℃. These appear to agree with the results calculated for phase-change temperature in another studies20). We found that there was the change of thermal expansion rate at near 45℃ and 55℃, a fiducial point, it suggested that the phase transition occurred at these temperatures. During the setting process, the crystallization of gutta-percha is thought to adversely affect the sealing ability in the root canal space. The root canal treatment failure is mainly caused by incomplete sealing of the root canal, and accordingly it is important to obturate the root canal hermetically. A marked specific volume change was observed at the temperature change of 45℃ to 55℃ in this study. On the basis of our findings, it seems to be better that thermoplasticized canal filling material is condensed with plugger until the intracanal temperature decreased at least under the temperature of 45℃ during the back-filling procedure.

V. CONCLUSION

Therefore this results showed that Resilon had a potential to substitute for gutta-percha from a thermal expansion characteristic point of view. However there still remain many subjects to investigate this material compared with gutta-percha. More comparison about thermal properties of Resilon and Gutta-percha will be needed in the future.

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