Radiopacity and Filler Content of Dual-Cure Resin Core Material, Dual-Cure Resin Cement, and Bulk-Fill Resin Composites Used for Restoring Endodontically Treated Teeth Open Access

The restoration of endodontically treated teeth continues to be a challenge because of the variability of clinical protocols involving different tooth conditions and the diversity of resin-based materials.^1,2  Often, the amount of coronal structure is insufficient to provide retention for direct or indirect restoration.^1,3  In these cases, to provide adequate retention, utilizing fiberglass posts associated with core reconstruction becomes necessary.^1,4  Until now, dual-cure resin cement has been the first option to cement the fiberglass post associated with coronal reconstruction using conventional or bulk-fill resin composites.^1,5  Simplifying the restorative procedure, minimizing the number of materials used, and reducing the number of interfaces are desirable goals in the restoration of endodontically treated teeth. Dual-cure resin core materials have been promoted to accomplish both procedures. Thus, using a single material reduces the number of interfaces and saves time.^5,6  The in vitro evidence suggests that resin-based core build-up materials that have higher filler content showed the highest elastic modulus, which indicates stiffness, thereby improving the fracture resistance of the endodontically treated teeth compared to conventional resin composites.^7,8 

Secondary caries located at the gingival margin restoration is a frequent cause of restoration failure.⁹  Adequate radiopacity aids the diagnosis of recurrent caries at the gingival margin, and can also facilitate the identification of other potential issues such as poor adaptation, contact with adjacent teeth,¹⁰  interfacial gaps, and voids present in the restoration.^11–13 

The difficulty in discriminating the radiopacity levels among fiberglass posts, resin cements, and core materials may contribute to the complexity of evaluating the restorative procedures of endodontically treated teeth.^14–18  The radiopacity of dental materials has been established according to ISO 4049 and is generally expressed in terms of equivalent aluminum thickness (mm).¹⁹  Restorative materials must have a radiopacity value that is equal to or greater than the radiopacity of an aluminum step wedge of the same thickness.^20,21  Digital radiographic images can measure the gray values on a scale of 0 to 255 through software, which provides a quantitative analysis of the material's radiopacity.²¹  In this study, ImageJ software has been used to evaluate the mean gray values of the digital radiography of different resin-based materials.^17,18 

The radiopacity level of resin-based materials can be influenced by various parameters such as composition, polymer matrix content, filler particle size, filler concentration, and thickness variations.^19,22  Individual filler fraction analyses using thermogravimetric analysis (TGA) may better explain the physical-mechanical performance of resin-based materials.²³  Filler particle characterization using X-ray dispersive energy (EDS) microanalysis and scanning electric microscopy (SEM)²⁴  can produce a combination of methodologies to characterize and explain the radiopacity levels of different resin-based materials.

Although the radiopacity of resin-based materials has been investigated extensively,^{12,17,18,25,26}  few studies have evaluated the relationship between the filler characteristics and the radiopacity of current resin-based materials used for restoring endodontically treated teeth. Furthermore, to the best of our knowledge, no study has evaluated the radiopacity of different resin core materials. Therefore, this in vitro study aimed to evaluate the radiopacity of different types of resin-based materials using a digital radiography system, determine the relationship between filler composition and radiopacity, and compare the radiopacity values expressed as the equivalent thickness of aluminum. The null hypotheses tested are as follows: (1) the tested dual-cure resin core materials, dual-cure resin cements, and bulk-fill resin composites would exhibit radiopacity levels that meet the requirements of ISO 4049; and (2) the filler content, and consequently, the radiopacity levels, of the tested materials would not differ from one another.

Nine resin-based materials used for restoring endodontically treated teeth were tested in this study, which includes: four dual-cure resin core materials: (1) Allcem Core (FGM, Joinville, Brazil), (2) Clearfil DC Core Plus (Kuraray, Tokyo, Japan), (3) LuxaCore Z (DMG, Hamburg, Germany), and (4) Rebilda DC (VOCO, Cuxhaven, Germany); three automix dual-cure resin cements: (1) RelyX Universal (3M Oral Care, St Paul, USA), (2) RelyX U200 (3M Oral Care), and (3) Allcem Dual (FGM); and two bulk-fill resin composites: (1) OPUS Bulk Fill APS (FGM) and (2) Filtek One Bulk Fill (3M Oral Care). The composition and light-curing procedures provided by the manufacturers are listed in Table 1. All the materials were tested for filler content (%) through TGA, SEM, and EDS to evaluate the morphology of the inorganic filler.

TGA was used to obtain the filler mass fractions (%) of all the tested materials (n=3).²³  The change in mass of the materials was measured as a function of temperature. Approximately 0.005 mg of each sample was light-activated for 40 seconds. Subsequently, the specimen was placed in a thermogravimetric analyzer (model TGA 55, TA Instruments, New Castle, Delaware, USA) and immersed in standard NETZSCH alumina 85 mL crucibles attached to the thermo-analytical unit (TGA-50, Netzsch-Thermische Analyse, Selb, Germany) with a TA System Controller (TASC 414/2, New Castle, DL, USA) at a temperature range of 10-900°C and a heating rate of 10°C per minute for approximately 80 minutes under synthetic air dynamics (50 mL/minute). Thereafter, only the filler remained, and the filler content was calculated by considering the % of the remaining mass. The data were graphically analyzed at the onset of the E/T curve.²³ 

The morphology and size of the fillers were evaluated using scanning electron microscopy. A unique paste of the resin-based material, 0.3 gram of each base and catalyst, was inserted into a conical bottom centrifuge tube in polypropylene (Falcon, Vitchlab, Wertheim, Germany) in 5 mL of acetone and centrifuged for two minutes at 1,000 rpm; this process was repeated three times. The remaining material mass was immersed thrice in 5 mL of chloroform P.A. (Alphatec, Rio de Janeiro, Brazil) and centrifuged as described above to further wash and eliminate the matrix.²⁷  The filler content was then smeared over the aluminum stubs and sputter-coated with gold/palladium in high vacuum (QR 150ES, Quorum, Laughton, East Sussex, United Kingdom). The specimens were examined under a scanning electron microscope (VEGA 3 LMU; TESCAN, Brno, Czech Republic) operating at 20.00 kV. The images were obtained at 5,000×, 10,000×, and 20,000× magnification. The particle composition was determined using the obtained powder and observed using an SEM/EDS integrated analysis system (INCA X-ACT model 51-ADD0007, Oxford, Abingdon, Oxfordshire, United Kingdom). The analyses were performed at working distance of 14.9 mm, 20 kV, and variable spot sizes to obtain a 30-second lifetime and 20% dead time. For each specimen, one 4-second scan analysis of was performed at a magnification of 5000×, 10,000×, and 20,000×.

Digital radiographs were obtained using standardized specimens measuring 5 mm in length, 2 mm in width, and 2 mm in thickness. The specimens were positioned over a phosphor plate (size 2, Dürr Dental, BietigheimBissingen, Germany), and radiographically exposed using radiography equipment (Timex 70 E, Gnatus, Ribeirão Preto, Brazil) exposing the specimens for 0.07 seconds at 70 kV and 7.0 mA. The aluminum scale, with a thickness ranging from 1 to 10 mm was increased every 1 mm (Odeme, Santa Catarina, Brazil) and placed on the matched plate having a focal length of 20 cm. Radiopacity was determined using the ImageJ software (National Institutes of Health, Bethesda, MD, USA).^11,28–30  Five locations were determined for each specimen, proportional to its length, and the mouse cursor was positioned to collect the radiopacity value. The average of five calculated values was used as the radiopacity level for each specimen. A value was assigned to each pixel to represent a specific gray shade. These pixel values range from 0 to 255, where 0 represents the darkest gray or black and 255 represents the lightest gray or white.

Radiopacity and TGA data were tested for normal distribution (Shapiro-Wilk) and equality of variances (Levene's test), followed by parametric statistical tests. One-way analysis of variance (ANOVA) was performed to identify the differences in the tests. A Tukey post hoc test was used for multiple comparisons between the tested materials. All the tests were performed at a significance level of α = 0.05. The SEM and EDS analyses were performed descriptively.

The mean (± standard deviation) filler content (weight %) values of the resin-based materials are shown in Figure 1. One-way ANOVA revealed significant differences among the materials (p<0.001). Tukey's post hoc test showed that both bulk-fill resin composites and three of the four dual-cure resin core materials had significantly higher filler contents than the dual-cure resin cements. Both bulk-fill resin composites (Filtek One Bulk Fill and OPUS Bulk Fill) and two of the dual-cure core materials (Clearfil DC Core Plus and Rebilda DC) had the highest filler content, while RelyX Universal dual-cure resin cement had the lowest filler content (p<0.001).

The mean (± standard deviation) radiopacity values of all the materials are shown in Figure 2. One-way ANOVA showed significant differences among the materials (p<0.001). Tukey's post hoc test showed that both bulk-fill resin composites (Filtek One Bulk Fill and OPUS Bulk Fill) and two of the dual-cure core materials (Clearfil DC Core Plus and Rebilda DC) had significantly greater radiopacity than the three resin cements and the other two core materials (Luxacore Z and Allcem Core). All materials exceeded the radiopacity levels recommended by ISO 4049-2019 (Figure 3).

The morphologies of the filler particles in the dual-cure resin core materials, dual-cure resin cements, and bulk-fill resin composites are shown in Figures 4, 5, and 6, respectively. The evaluated materials contained irregularly agglomerated particles of different sizes. Allcem Core presented more homogeneous fillers without agglomerates (Figure 4), whereas RelyX Universal exhibited only nanoparticles in the catalyst paste (Figure 5). Furthermore, Clearfil DC Core Plus, RelyX U200, and LuxaCore Z had the largest particle sizes (Figures 4 and 5). Filtek One Bulk Fill resin composite contained rounded nanocluster particles (Figure 6), unlike the resin cements (RelyX Universal and RelyX U200, Figure 5) from the same manufacturer. In contrast, the OPUS Bulk Fill APS resin composite (Figure 6) presented a size, shape, and distribution of filler content similar to the dual-cure resin cement (Allcem Dual) and dual-cure resin core material (Allcem Core) from the same manufacturer.

The EDS analysis (Table 2) showed that oxygen (O, 8) and silicon (Si, 14), followed by carbon (C, 6), aluminum (Al, 13), and barium (Ba, 56), were the prevalent elements present in the filler content of all the tested materials. RelyX Universal also contained bromine (Br, 35) fluorine (F, 9), ytterbium (Yr, 70), and strontium (Sr, 38). RelyX U200 contained sodium (Na, 11), phosphorus (P, 15), calcium (Ca, 20), lanthanum (La, 57), Sr, and F. Filtek One Bulk Fill contained zirconium (Zr, 40), and ytterbium. The radiopacity of the materials is related to the presence of high atomic number elements such as Yr (70), La, (57), Ba (56), Zr (40), Sr (38), and Br (35) in the filler composition.

The first null hypothesis that all the tested dual-cure resin core materials, dual-cure resin cements, and bulk-fill resin composite would have radiopacity levels meeting the ISO 4049 standard was accepted. However, the second null hypothesis that all materials would have similar filler content and radiopacity levels was rejected. In general, the dual-cure resin cements had significantly lower filler content and radiopacity levels than those of the dual-cure resin core materials and bulk-fill resin composites.

Resin-based materials possess different radiopacities; consequently, they can perform differently in radiographic monitoring.^9–11  The intrinsic radiopacity of resin-based materials depends, in part, on the atomic numbers of their constituent elements. Resin-based materials tend to be more radiopaque if they contain high atomic number elements such as Zr (40), Ba (56), and Yr (70). The presence of elements such as Sr (38), Y (39), and La (57) can also contribute to the radiopacity.^12,31  Barium was the most commonly used elemental filler in the tested materials. However, materials produced by 3M Oral Care contained Sr, La, and Yr. All materials exceeded the ISO 4049-2019 standard for radiopacity of restorative materials.²⁰ 

The radiopacity of resin-based materials is crucial for clinically visualizing potential issues, such as interfacial gaps, to assess material adaptation to the root canal space. Radiopacity is also helpful in diagnosing marginal gaps, secondary caries, and marginal overhangs extending to subgingival regions, and differentiating tissues and structures such as dentin, enamel, restorative materials.^9,20  Fiberglass posts, cement, and core build-ups can be better monitored when using radiopaque materials.^11,28,30  Additionally, material radiopacity can aid in identifying any excess that needs to be removed to prevent periodontal inflammation or other complications.⁵  Using radiopaque cement helps identify the contour of fiberglass posts during long-term follow-up.^11,19  The materials selected to restore endodontically treated teeth must be discernable in the radiographs.³¹ 

The higher filler content of the resin-based luting cements is associated with increased rigidity and mechanical resistance.⁵  Moreover, the higher filler content is associated with elevated viscosity, which hinders injection of the material into the root canal, causing voids and gaps.³¹  To overcome this potentially significant clinical problem, specific strategies can be employed, such as using specialized applicator tips (typically supplied with many systems) for better canal access and being careful and deliberate to minimize voids when injecting the cement.² 

The dual-cure resin core materials, predominantly based on methacrylate monomers, exhibited higher filler content, endowing superior mechanical properties compared to conventional resin dual-cements.⁷  Our TGA analysis suggests that dual-cure resin core materials exhibited filler content similar to bulk-fill resin composites.²⁹  However, the accuracy of the TGA method itself can be questioned. The sample size of n = 3 per group is relatively small. While acceptable for the specific parameter measured, the percentage of filler content by weight verified in the present study is very close to the values specified by the manufacturers. Moreover, the higher percentage of high atomic number fillers significantly influences radiopacity; resin cements contain a greater percentage of radiopaque fillers, aiding in clinical detection.^19,29  We observed that Clearfill DC Core Plus and Rebilda DC exhibited higher filler content, and barium particles, resulting in higher radiopacity values. In contrast, RelyX U200 and RelyX Universal lack barium, but contain zirconium, strontium, and ytterbium. Ensuring that these materials exhibit radiopacity values within the standard range is essential for clinicians to diagnose interfacial gaps, voids, defects in adaptation, discrepancies in restoration contours, and lack of contact with neighboring teeth.^10,11 

Filler content plays a critical role in determining the performance of dental materials, particularly when used to lute fiberglass posts and fixed prostheses, and restore core build-ups.⁴  The higher the percentage of filler, the better these materials can support stresses, which is crucial to sustaining the loads placed on them during function.³¹  Additionally, higher filler percentages can influence post-gel shrinkage and subsequently increase fracture resistance.⁵  Comprehensive analyses of the physical and mechanical properties of dual-cure resin core materials highlight their potential for supporting the rehabilitation of endodontically treated teeth.²⁴  These materials facilitate the cementing of fiberglass posts, reconstruction of core build-up, and help to perform crown cementation with reduced number of interfaces, thereby offering a convenient and efficient solution.³  Moreover, the presence of higher filler content may lead to an increase in the fracture resistance of weak teeth.^7,23  Ultimately, understanding the composition of resin-based materials through methodologies such as TGA provides valuable insights into their mechanical properties and performance.²³ 

The composition of the monomers significantly influences the rheological and viscoelastic properties of the resin-based materials, impacting their flow characteristics and overall performance.⁷  The present study demonstrated that dual-cure core materials may have content similar to resin composites.²⁴  The addition of monomers with low molecular weight enhances the flexural properties and reduces viscosity in these materials.^5,24  The use of dual-cure resin core materials with high modulus elasticity can enhance the fracture resistance of endodontically treated teeth and serve as an alternative for one-stage fiberglass post-placement and core build-up.²³ 

Within the limitations of this in vitro study the following conclusions were drawn:

• The radiopacity of all the tested resin-based materials exceeded the minimum requirement according to the ISO Standards 4049.

• The presence of high atomic number elements in all the tested resin-based materials contributes to the higher radiodensity.

• The filler content and radiopacity of the dual-cure resin core materials varied among the tested products. LuxaCore Z and Allcem Core exhibited lower values compared to Filtek One Bulk Fill and OPUS Bulk Fill APS.

The present study was supported by the National Council for Scientific and Technological Development — CNPq grant INCT Saúde Oral e Odontologia 406840/2022-9; 140615/2021-0, 311001/2021-1, and 422603/2021-0; and by the Research Support Foundation of the State of Minas Gerais – FAPEMIG grant number APQ-02105-18.