Parameters Affecting Degree of Conversion of Dual-Cure Resin Cements in the Root Canal: FTIR Analysis

Abstract

Objective: To evaluate the degree of conversion of dual-cured resin cements applied for luting of translucent and opaque fibre posts.

Methods: Two dual-cured resin cements, RelyX ARC and Nexus 2, were used to cement 2 types of tooth-coloured fibre posts, D.T. Light-Post (translucent) and D.T. White-Post (opaque). The degree of conversion for each resin cement was measured. Post-curing polymerization and chemical curing of the cements were also measured. The degree of conversion was measured at various distances (4, 6 and 8 mm) from the tip of the light-curing unit by Fourier transform infrared spectroscopy. The data were analyzed with analysis of variance and post hoc tests (α = 0.05).

Results: The degree of conversion of the dual-cure cements was significantly higher with the D.T. Light-Posts than with the D.T. White-Posts (p < 0.05). No significant difference was observed in the degree of conversion at different depths for the RelyX ARC resin cement in conjunction with the D.T. Light-Posts (p > 0.05). The overall degree of conversion decreased linearly with increasing distance from the light-curing tip. Monitoring of post-curing polymerization and chemical curing revealed no further increase in degree of conversion after 5 minutes (p > 0.05).

Conclusion: The translucent fibre posts (D.T. Light-Posts) allowed a significantly higher degree of conversion with dual-cure resin cements than the opaque fibre posts (D.T. White-Posts) because of the light-transmitting property of their optical fibres.

Excessive loss of coronal tooth structure and internal root canal dentin decreases tooth strength and increases the risk of root fracture.¹ When the remaining coronal tooth structure is insufficient to support and retain a coronal restoration, placement of a post system is recommended.²

Metallic posts do poorly at distributing stresses to the tooth structure.³ These posts also exhibit esthetic problems when used with semitransparent ceramic cores, especially in the anterior region. Ceramic posts have esthetic benefits but are associated with increased risk of root fracture because of their stiffness and brittleness.⁴

Therefore, the use of polymeric fibre posts is increasing.⁵ Fibre posts have mechanical properties similar to those of dentin. Furthermore, they bond well to the tooth structure, which increases retention of the post, reinforces root structure internally and improves resistance to tooth fracture. This type of post also represents a good choice when esthetics are important.⁴ The most important aspect in cementation of a fibre post in the root canal is reaching an adequate degree of cure at depth.⁶ Although selfcuring resins exhibit uniform polymerization, applying this type of resin in the canal is difficult because polymerization occurs quickly after the 2 pastes are mixed.^5,6 Application of lightcuring resins is also limited at depths beyond 2 mm because transmission of light through the resins is limited.⁵ Therefore, dual-cure cements may be the adhesive of choice in the cementation of fibre posts. However, numerous studies have shown that initiation of polymerization of dualcure resin composites requires exposure to light^.7,8 Therefore, translucent fibre posts were introduced to overcome the problem of lack of curing in deep locations with limited penetration of light.⁹ The efficacy of these light-transmitting posts has been supported by several studies.^1,9

Various methods are employed to determine the degree of conversion (DC%) of resin composites. Fourier transform infrared (FTIR) spectroscopy is a powerful and reliable method, based on molecular vibrations, which can be used to study the degree of polymerization of dental resins.^10,11

The objective of this study was to investigate, using FTIR spectroscopy, the polymerization conversion of 2 dual-cure resin composite cements applied to 2 types of fibre posts. Post-curing polymerization (i.e., polymerization occurring after light exposure has been stopped) and chemical curing of the cements were also monitored. The null hypothesis was that there would be no significant difference in DC% of dual-cured resin cements applied with fibre posts in coronal, middle and deep areas of the root canal.

Methods

Two dual-cured resin cements (RelyX ARC and Nexus 2) and 2 types of fibre posts (D.T. Light-Post and D.T. White-Post) were used in this study (Table 1).

Preparation of Specimens

Cylindrical moulds with a height of 2, 4 or 6 mm and a diameter of 8 mm were prepared and filled with a self-curing dental composite (Fig. 1). The composite was mixed with black pigment to prevent lateral transmission of light through the composite during evaluation of conversion of the test cements. The tapered end of specimens of each type of fibre post was removed, and the cylindrical portion was cut into lengths of 4, 6 or 8 mm from the top. Each post was then inserted vertically into the centre of a mould of appropriate height, as described below, before the composite became set. To simulate realistic conditions, the 4-, 6- and 8-mm posts were inserted into moulds with height of 2, 4 or 6 mm, respectively, such that 2 mm of each post extended out beyond the composite. The upper and lower ends of each post were ground with sandpaper to remove any contamination by the composite. Each type of post was then tested with each of the 2 cements. For controls, the same type of moulds were used, except a cylindrical hole with 8-mm diameter (equal to the diameter of the posts) was left in the centre of the composite, to allow evaluation of conversion in the absence of posts.

Measurement of DC%

For FTIR spectroscopic analysis, equal amounts of base and catalyst pastes of each dualcure cement were mixed and placed between 2 layers of transparent polyethylene film, which was then pressed to form a thin film of cement (40–70 µm) (Fig. 1). This "sandwich" was placed into the sample holder of the FTIR spectrometer (Equinox 55, Bruker, Germany) (Fig. 2), and absorbance peaks were recorded by transmission mode at a resolution of 4 cm^–1, with 32 scans in the range of 4000–400 cm^–1.

After the FTIR spectra of the uncured cement films had been recorded, each film was placed under a specified cylindrical mould, which was set on a white background. The samples were then light-cured for 60 seconds with a quartz–tungsten– halogen dental light source having irradiance of about 700 mW/cm² (Optilux 501, Kerr Corporation, Orange, CA). Absorbance peaks were then recorded for the cured samples. For irradiation, care was taken to place the resin cement film exactly beneath the post. The DC% was determined from the ratio of absorbance intensities for aliphatic carbon–carbon double bonds (peak 1637 cm^–1) and the internal reference of aromatic carbon–carbon double bonds (peak 1608 cm^–1) before and after curing of the specimen, according to the following equation:

To evaluate the efficiency of the reduction–oxidation system of dual-cure cements, the resin specimens were kept in the spectrometer (at room temperature) after light irradiation, and additional FTIR spectra were subsequently recorded. These post-curing measurements were performed only for resin specimens that had been irradiated in conjunction with posts 8 mm in length. To evaluate the effect of chemical curing, without light irradiation, on the promotion of polymerization in dual-cured resin cements, uncured specimens for which FTIR spectra had been recorded were placed in a dark location at room temperature, and additional FTIR spectra were collected every 5 minutes up to 15 minutes.

For each specimen, 5 sets of spectroscopic data were recorded. The data were analyzed and compared using one-way analysis of variance (ANOVA) or univariate ANOVA (UNIANOVA) and Tukey's post hoc test at a significance level of α = 0.05.

Results

Table 2 indicates that the length and type of the post and the type of cement all affected the degree of conversion of the 2 dual-cured resin cements tested in this study, with irradiation at the same light intensity for the same period of time.

Comparison of films prepared from RelyX ARC cement irradiated at different depths revealed the following findings. For specimens irradiated at 4 mm distance from the tip of the light-curing unit, DC% was significantly lower with the D.T. White-Posts than with controls or D.T. Light-Posts (p < 0.05). For specimens irradiated at 6 mm distance, DC% was significantly higher with the D.T. Light-Posts than with controls or D.T. White-Posts (p < 0.05), while there was no significant difference between specimens irradiated under control conditions and those irradiated with D.T. White-Posts (p > 0.05). At 8 mm distance, there were significant differences in DC% among the 3 groups of specimens, with the following order: D.T. Light-Posts > control > D.T. White-Posts (p < 0.05).

The following results were obtained from comparisons of films prepared from the Nexus 2 cement irradiated at different depths. For specimens irradiated at 4 mm distance, D.T. White-Posts had the lowest DC% (p < 0.05). Among specimens irradiated at 6 and 8 mm distance, those cured under the D.T. Light-Posts had the highest DC% (p < 0.05).

In comparisons between the 2 post types, the D.T. Light-Posts yielded higher DC% than the D.T. White-Posts (p < 0.05). For irradiation in conjunction with the D.T. Light-Posts, there was a trend of decreasing DC% with increasing post length (from 4 to 8 mm) for Nexus 2 specimens (p < 0.05), whereas the degree of conversion was the same with different post lengths for RelyX ARC specimens (p > 0.05). For irradiation in conjunction with the D.T. White-Posts, there was a significant difference between 8 and 4 mm distance for both cements (p < 0.05). In the control groups, there was a trend toward decreasing DC% with increasing distance between the light tip and the cement specimen for both types of cement (p < 0.05).

Multiple comparisons followed by Tukey's HSD test revealed that, regardless of cement type, DC% values had the following order: D.T. Light-Post > control > D.T. White-Post. The comparison also showed that the DC% of the cements decreased with increasing distance between the light-curing tip and the cement film. This correlation, illustrated in Fig. 3, was linear (R² > 0.99).

Polymerization of the cement specimens progressed to some extent after light irradiation was stopped (Table 3). In particular, RelyX ARC cement specimens stored in the dark at room temperature (with no irradiation) had low levels of conversion (small DC% values) after 5 minutes and no further significant increase up to 15 minutes (p > 0.05) (Fig. 4). Following photopolymerization of the RelyX ARC cement specimens for 60 seconds under an 8-mm D.T. Light-Post, DC% was 52.0% (standard deviation [SD] 5.3%), which then increased to 62.8% (SD 4.5%) by 10 minutes (p < 0.05) and remained unchanged with 5 minutes of additional storage (p > 0.05) (Fig. 5). Storage of RelyX ARC cement in the dark at 37°C showed that polymerization did not proceed at this temperature without light irradiation (Fig. 6). Performing the same test on Nexus 2 cement specimens after curing for 60 seconds with the D.T. Light-Post (8 mm length) revealed no significant increase in DC% 5 minutes after irradiation was stopped (Fig. 7).

Discussion

Inadequate polymerization of resin cement may result in problems such as microleakage; recurrent caries; degradation of cement; reduction in retention and properties such as elastic modulus, compressive strength and stiffness; and increase in the release of unreacted monomers leading to toxic effects.^12,13 To overcome these problems, light-transmitting posts have been introduced. When such posts are used, light is transmitted through their optic glass fibres, which results in greater DC% in deep areas of the root canal.⁵ Various factors affect the DC% of any resin cement, including the intensity of light received by the resin and the type of resin itself. FTIR spectroscopy is a precise and reliable method for determining the DC% of dental resins.^10,11,14 For this study, a method was developed to simulate photopolymerization of cement specimens under fibre posts of different lengths, with measurement of DC% by FTIR spectroscopy.

Effect of Post Length

The degree of conversion of resin cement decreases with increasing distance from the light source because of a dramatic decrease in the intensity of light reaching the cement. According to the Beer Lambert law, light intensity is exponentially related to the length of the light path:

where I is the intensity of light after it passes through the sample, I₀ is the initial light intensity, ε is the molar absorptivity of the absorber (the medium through which the light passes [in this study, either a post or air]), b is the path length, and c is the concentration of the absorber.¹⁵ The following considerations are also pertinent: first, measurement of DC% by FTIR spectroscopy is based on the percentage of carbon–carbon double bonds converted to single bonds, not the output intensity of the light; and second, there is a lower limit of light intensity (about 400 mW/cm²) above which the total DC% of resin cements may not be significantly affected by light intensity.

Under various conditions (excluding RelyX ARC cement with D.T. Light-Posts), DC% decreased with increasing post length or (for the control group) increasing distance between the light-irradiating window and the specimen (Table 2). Comparison of controls with specimens irradiated with D.T. White-Posts revealed that this type of post significantly decreased DC% (p < 0.05). D.T. White-Posts are made of quartz fibres that are not translucent; the fibres therefore absorb some of the light passing through the post. Under control conditions, DC% decreased with increasing distance between the light window and the specimens, but this decline was not as great as observed with the D.T. White-Posts, which indicates that these fibre posts had higher absorptivity than air (the term ε in the Beer Lambert law). The overall DC% of the cements showed linear correlation with distance from the light source (Fig. 3). The linear equation fitted to the experimental data represented good correlation (R² > 0.99), predicting a DC% of about 66% for each cement at a distance of 0 mm, which is in the range of reported DC% values for the cements.

Because of the dramatic drop in light intensity with increasing distance, dual-cure resin cements have been introduced to compensate for the lack of adequate light intensity in deep areas through chemical initiation of the polymerization reaction.¹⁶ It seems that using a dual-cure cement in combination with translucent posts may overcome the problem of inadequate polymerization conversion, but the results show that the type of cement is also a factor in determining DC% (Table 2).

Although inadequate curing negatively affects the properties of the cement in the tooth canal,¹² it has been reported that a gradual decrease in DC% within the root canal provides flexibility in the apical part of the cement–post assembly, which may improve the distribution of stress.¹⁷ However, this explanation remains controversial.

Effect of Cement Type

For tests involving the D.T. Light-Posts, higher DC% was observed with RelyX ARC cement than with Nexus 2 cement (p < 0.05), probably because of differences in their chemical compositions. Factors such as chemical formulation, type and amount of resin, initiator system and size of filler particles have all been shown to influence the curing process.^18-20

The detailed formulation of each of the commercially available cements used in this study is unknown, so conclusions cannot be drawn regarding specific reasons for differences in DC%. However, it is known that the proportion of filler in each cement is similar (67% for RelyX ARC and 70% for Nexus 2; see Table 1), so the difference is more likely due to differences in monomers and initiator systems.

Effect of Fibre Post Type

The cements cured under the D.T. Light-Posts had higher DC% values than those cured under the D.T. White-Posts (p < 0.05), which indicates that light was more effectively transmitted through the translucent posts, leading to a higher degree of polymerization. It has previously been reported that light-transmitting posts allow greater curing of light-curing resin composites.^1,5 The optic fibres can transmit incident light without any significant reduction in intensity. Mallmann and colleagues²¹ showed that this type of post was associated with higher DC% values for light-activated resin cements in middle and apical areas. Faria e Silva and colleagues²² showed that translucent posts exhibited higher DC% at cervical and medium depth. In contrast, when the same authors studied the effect of applying adhesive on the push-out bond strength to dentin, they found no significant difference between a translucent post and a carbon fibre post.²³ Goracci and colleagues²⁴ used spectrophotometry to study the light-transmitting capability of 14 types of commercially available posts, suggesting that light transmission along the translucent fiber reinforced composite posts might be affected by the refractive indices of the glass fibres and resin matrix. Radovic and colleagues² also studied light transmission through fibre posts and the effect of post type on elastic modulus and hardness of dual-cured resin cement. They suggested that light transmission along D.T. Light-Posts was not significantly different at the coronal and middle levels. Furthermore, they found no difference between the coronal and middle thirds in terms of elastic modulus and Vicker's hardness measurements for with D.T. Light-Posts, whereas both elastic modulus and Vicker's hardness were significantly lower in the apical third.² When Goracci and colleagues²⁴ measured the light-transmitting capability of 10 fibre posts, they found that transmission of light with the D.T. Light-Post was significantly higher at the coronal level and decreased at the middle and apical levels of the post. Therefore, we suggest that for the D.T. Light-Post, differences in refractive indices among the component quartz fibres and epoxy matrix are such that light rays passing through the fibres cross the cladding boundary (matrix) at an angle greater than the critical angle of the matrix, allowing light to be reflected into the fibres, which results in greater overall transmission of light.

Chemical Polymerization during Curing

Following initial polymerization of the dualcure resin cements by irradiation, no further progress in polymerization was observed after 5 minutes of storage in the absence of light (Table 3). Furthermore, in the complete absence of light after mixing, polymerization was minimal: DC% values after 5 minutes of storage in the absence of light were 2.9 (SD 1.8) for RelyX ARC and 0.9 (SD 0.5) for Nexus 2, which are in the range of deviation for FTIR measurements (Figs. 4 and 8). Storage in the dark was continued for up to 15 minutes with RelyX ARC specimens (Fig. 4), but no significant increase in polymerization was observed. This indicates that exposure to light is necessary to start the curing process of dual-cure resin cements. To examine the effect of higher (i.e., body) temperature on chemical curing of the cements, RelyX ARC specimens were kept at 37°C for 5 minutes, but there was no significant increase in DC% (Fig. 6).

These results are in agreement with previous reports that the polymerization of dual-cure resin cements is dependent on initial exposure to light. Without light irradiation or with inadequate light exposure, chemical polymerization of cement proceeds very slowly, and adequate polymerization is not achieved. Initial exposure to light produces active species that may activate and/or accelerate the chemical curing process in the polymerization of dual-cure resins.^14,25-27 Numerous studies have shown that the photopolymerization and chemical curing initiation systems have synergistic effects.^{12,25,26,28,29}

Curing after Light Exposure

After light irradiation was stopped, specimens were kept in the FTIR chamber to allow monitoring of subsequent curing. After 5 minutes, there was some increase in DC% (Figs. 5 and 7, Table 3), but the changes were not statistically significant (p > 0.05). When RelyX ARC specimens were monitored for up to 15 minutes (Fig. 8), polymerization was observed to continue for up to 10 minutes (p < 0.05) but further progress of polymerization was not significant (p > 0.05). Over time, the activity of macroradicals declines dramatically through vitrification of the cured cements. Propagation of polymerization after vitrification is controlled by diffusion of the unreacted monomers and macroradicals through the highly cross-linked polymer network. This dramatically decreases the polymerization rate.¹⁰ Although radicals and unreacted monomers remain, which should be able to react and promote curing, the resulting changes in DC% were not detectable with FTIR spectroscopy.

Conclusions

Within the limitations of this study, 3 main conclusions can be drawn. First, resin cements used with D.T. Light-Posts had greater polymerization (indicated by DC%) because of the translucency of this type of post. Second, under the same curing conditions, the degree of conversion of RelyX ARC resin cement was greater than that of Nexus 2 resin cement, which indicates a significant effect of the type of resin cement on DC%. Third, light irradiation is necessary to start and/or accelerate the polymerization process for dual-cured resin cements.

THE AUTHORS

	Dr. Shadman is assistant professor in the department of operative dentistry, oral and dental diseases research center, faculty of dentistry, Kerman University of Medical Sciences, Kerman, Iran.
	Dr. Atai is associate professor at the Iran Polymer and Petrochemical Institute, Tehran, Iran.
	Dr. Ghavam is associate professor in the department of operative dentistry, Tehran University of Medical Sciences, Tehran, Iran.
	Dr. Kermanshah is assistant professor in the department of operative dentistry, Tehran University of Medical Sciences, Tehran, Iran.
	Dr. Farzin Ebrahimi is assistant professor in the department of operative dentistry, oral and dental diseases research center, faculty of dentistry, Kerman University of Medical Sciences, Kerman, Iran.

Correspondence to: Dr. Mohammad Atai, Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran. Email: m.atai@ippi.ac.ir

The authors have no declared financial interests in any company manufacturing the types of products mentioned in this article.

This article has been peer reviewed.