Comparison of Radiopacity of Dentin Replacement Materials

Introduction

Protection of dentin-pulp tissues in deep cavities is essential to preserve pulp vitality. Pulp-capping procedure include use of protective material over deep pulp floor or open pulp tissue after caries removal (1). Base or liner is been employed for dentin replacement and pulp capping. These materials supply both an outward dentinal tube seal and a therapeutic effect for pulp against microorganisms, irritants and thermomechanical stimulus(2). The weak bonding of capping materials to dentin, adaptation failures, and polymerization shrinkage of such products may lead to micro‑gaps at the dentin bonding interface. This would increase risk of secondary caries as a common cause of restoration failures (3, 4).

Various materials have been used for pulp capping, some of which contain calcium hydroxide–based materials, tricalcium silica–based materials, and glass-ionomer cements. These materials may release calcium, which is useful for pulp structure and supplies a pioneer for apatite construction and supports dentin remineralization. Glass ionomer cement may promote interfacial conservation as it release fluoride and have a buffering capacity (5). Pulp capping materials must have enough radiopacity for accurate diagnosis of secondary caries (6, 7). In addition, sufficient radiopacity allows clinicians to determine defects in margins, gaps between the material and tooth structure and contact with adjacent dental substrates (8). The application of visually non-radiopaque materials can easily go unnoticed, which may result in incorrect diagnosis and treatment planning. Radiopaque components are generally added to dental materials (e.g., composite resins, capping materials, and cements) to supply adequate radiopacity. These added components are composed of elements with higher atomic numbers, such as ytterbium, zirconium, barium, and strontium. An ideal radiopacifier should be nonhazardous and inert to structures, thus providing an adequate visualization of radiographic images without any adverse effects (9, 10). However, because of commercial protectionism, adequate data concerning chemical components of a dental material are usually either missing or not completely explained by manufacturers.

                International organizations have suggested methods for determining radiopacity of composite resins using an aluminum step wedge as a reference. The ISO standard (4049) for radiopacity test requires preparation of cylindrical material samples with a 1-mm thickness. Samples and a highly-purified aluminum wedge with thicknesses from 0.5 mm to 5 mm in increments of 0.5 mm are exposed together to standardized X-ray beam at a determined distance. This standard calls for material to be at least as radiopaque as an equal thickness of aluminum (11). Dentin has shown to have similar radiopacity to an equal thickness of pure aluminum (4, 12). A reasonable alternative method to detect radiolucency along restorations is to prepare a standardized cavity using an extracted tooth. It would allow for a radiopacity assessment of dental materials in clinically relevant areas (13, 14).

A previous study has recommended routine analyses of radiopacity of recent dental materials to confirm they meet minimum requirements (6). There are numerous commercially available pulp-capping materials with various ingredients that provide different levels of radiolucency. Although most manufacturers have stated their materials to be radiopaque, no studies have examined the degree of radiopacity to facilitating caries diagnosis and making changes adjacent to restorations. The aim of this in vitro study was to assess radiopacity of various pulp-capping materials applied over the pulp floor in a standardized cavity.

Materials and Methods

Specimen preparation

Seventy non-carious and unrestored human third molars were collected. The crown dimensions of the teeth measured  by a digital caliber were within similar range (0.5 ± 0.01 mm). The roots were embedded on a silicon mold filled with a self-curing acrylic resin up to 2 mm under the cemento-enamel junction. Class I cavities were prepared (4-mm mesio-distal width, 5-mm bucco-lingual width, and 4-mm depth) using a cylindrical diamond bur (836 G 014; Wilofa Diamant, Germany) in a high-speed turbine cooled by air/water. Each diamond bur was changed with a new bur after five teeth were prepped. A digital micrometer (Tchibo Caliper, Hamburg, Germany) was used to verify preparation dimensions. The specimens were divided into seven groups consistent with test materials: TheraCal LC, Biodentine, Calcimol LC, Ultra-Blend Plus, Equia Forte, Ionoseal, and ApaCal ART. The properties of materials used are shown in Table I. Each material was applied over the cavity floor up to a 2-mm thickness, under manufacturer’s recommendations. Single Bond Universal adhesive (3M ESPE, Deutschland GmbH, Seefeld, Germany) was applied to all remaining cavity surfaces according to manufacturer instructions and then Filtek Z250 (3M ESPE, St Paul, MN, USA) was used as the restorative material to fill the cavity.

Table I: Compositions and manufacturers of materials used in this study

Radiopacity measurements

The specimens were placed over a phosphor plate on the lingual surface where the lingual surface was on the plate and the buccal side was in the tube side position. Later,  radiographic exposure was applied using dental X-ray unit (Acteon Group, X-mind DC, Rome, Italy) set at 70 kV and 8.0 mA. The exposure time was 0.25 seconds and the focal spot to the object distance was 50 cm. Three exposures were implemented for each specimen. Immediately after the exposure, all phosphor plates were scanned using a Vistascan device (Durr Dental, Bietigheim-Bissingen, Germany) and the radiographs were transferred to a personal computer. The radiopacity (in pixels) of specimens were measured by resident software (RadiAnt DICOM Viewer) supplied by the manufacturer. To standardize opacity measurements, three hypothetical vertical lines were described: One line 1 mm apart from inner side of distal and one line 1 mm apart from mesial axial walls of the cavity, and third line is in the center of mesial-distal distance (Figure 1). All measurements were made in 1-mm increments apart from interface on the vertical plane of restorative surfaces and teeth. The mouse cursor was placed under this measurement line on dentin, capping material, or on Filtek Z250 to determine the radiopacity values for each specimen. Radiographic measurements for enamel were taken from the tip of each specimen cusp. Mean values of each group was calculated, and each material group was compared with dentin, enamel, and composite values.

Scanning electron microscope/energy-assisted X-ray spectroscopy system analysis

One disc-shaped specimen (2-mm thickness and 5-mm diameter) from each material was fabricated by a Teflon mold. To ensure conductivity of all samples, specimen surface (Quorum brand, SC-7620 model) was coated with gold thickness of 200 angstroms. Later, a composition analysis and elemental mapping of each sample was performed by an energy-assisted X-ray spectroscopy (EDS) system based on scanning electron microscopy (SEM; Oxford Instruments Brand, Inca X-act model).

Statistical analysis

The radiopacity data confirmed assumptions of parametric analyses. Therefore, statistical analyses were performed using one-way ANOVA and post-hoc Tukey HSD test. All data were evaluated using SPSS 23.0 software (IBM Corp, Chicago, IL). The statistical significance level was set at p < 0.05.

Results

The radiopacity means of different materials, dentin, Filtek Z250, and enamel are presented in Table II. The radiopacity values revealed significant differences among tested materials (p < 0.0001). Ultra‑Blend Plus had the lowest radiopacity values. Calcimol LC, Equia Forte, and Ionoseal had significantly higher radiopacity values compared to other materials, but were not significantly different from each other. Biodentine had a statistically similar radiopacity value to both TheraCal LC (p = 0.153) and ApaCal ART (p = 0.638). All materials had significantly higher radiopacity levels than dentin (p < 0.05). Ionoseal, Calcimol LC, and Equia Forte had higher radiopacity than enamel, while Ultra-Blend Plus had a significantly lower radiopacity than enamel (p = 0.001). ApaCal ART, Biodentine, and TheraCal LC had statistically similar radiopacity values to enamel. Filtek Z250 had significantly greater radiopacity than all other materials except Ionoseal. Filtek Z250 opacity was even higher than  dentin and enamel. Figure 2 shows radiographic images of cavities filled with tested materials.

Table II: Radiopacity of replacement materials, dentin, composite resin, and enamel Mean (SD)

Different uppercase letters represent significant difference in horizontal line. Different lowercase letters represent significant difference in vertical line (One-way ANOVA and Tukey HSD tests; P < 0.05).

Table III shows elemental analyses on materials surface. Figure 3 shows EDS layered images of each material in backscatter mode. All materials had different compositions with various sized particles and radiopacifiers within the matrix. ApaCal ART, Calcimol LC, Ionoseal, and Ultra-Blend Plus included barium, while Filtek Z250 and Biodentine contained zirconium as a radiopacifier. Equia Forte contained aluminum, silicon, flour, and strontium as a radiopacifier. Cracks were observed on the surface of glass ionomer (Equia Forte), which probably were due to drying effect of SEM machine vacuum.

Table III: Mean (SD) of atomic percentage (wt. %) of elements on material surfaces

Discussion

The relative radiopacity of a restorative material is essential for identifying secondary caries and defects, and distinguishing the restorative margin from tooth structure. Insufficient radiopacity of a restorative material may lead to visual challenges in providing a satisfactory restoration in clinics. The techniques used for radiopacity analyses may involve using an aluminum step wedge and a tooth specimen along with a test material located over a radiographic sensor. In the current study, tooth selection and cavity preparation were standardized considering that recognition of radiopacity around restorations also relies on thickness and density of remaining tooth structure. A crucial issue in radiopacity evaluation technique used in this study was systematic analysis of the material placed inside prepared tooth, which allowed clinicians to investigate material radiopacity that was clinically vital for restorations closer to pulp (13).

In the present study, radiopacity values showed significant differences among materials. Radiopacity of resin-based dental materials depends on percentage and type of fillers (15). Filtek Z250 showed significantly higher radiopacity than other materials, except Ionoseal. This can be explained by higher weight of silica (silicon dioxide) and zirconia fillers. Inorganic fillers in the resin matrix caused a higher attenuation of X-rays. High atomic number elements such as zirconium and barium, and their high concentration result in more radiopacity in images (10). On the other hand, Ionoseal showed similar radiopacity values to Filtek Z250; however, Ionoseal contains a lower proportion of fillers. This might be attributed to have higher proportion of barium (wt% = 12.2). Although Ionoseal, a resin-modified glass cement, has fluoroaluminosilicate glass particles fluorine element could not be detected in its composition by EDS analysis. This event could be interpreted because of lower amount of fluorine. Ionoseal, Calcimol LC, and Equia Forte had statistically similar radiopacity values despite their different compositions. This may be associated with different proportions and types of radiopacifying agents and fillers (16). The radiopacity values of Apacal ART, Biodentine, and TheraCal LC were significantly higher than that of Ultra-Blend Plus. Ultra-Blend Plus has barium in similar proportion (wt% = ±6.2) to that of Apacal ART, but higher than that of TheraCal LC (wt% = 2.6); however, it does not contain silica particles (silicon dioxide). This may explain why it had the lowest radiopacity value. Moreover, it can be difficult to distinguish Ultra-Blend Plus from dentin in certain areas (Figure 2). Biodentine contains tricalcium silicate powders and zirconium oxide as radiopacifiers. TheraCal LC is also a methacrylate-based resin incorporated with tricalcium silicate and barium zirconate as a radiopacifier (17). Farrugia et al. (18) reported that Biodentine has a significantly higher radiopacity than TheraCal LC, which conflicts our findings. This difference could be explained by different methodologies used in evaluations. Thickness of restorative material, remaining tooth structure, and X-ray angulation can affect radiopacity (19).

The optimal radiopacity of a dental material is still debatable. Some authors have reported that an ideal radiopacity of a resin composite should be close to that of dentin for secondary caries detection and that the material with a higher radiopacity presents the worst accuracy for caries diagnoses (4). Others have determined that radiopacity of resin composites should be higher than that of enamel to accurately discern the margin between restoration and tooth (12), or equal to that of enamel to detect recurrent caries (20). In the present study, all materials had higher radiopacity values compared to dentin. However, three (Ionoseal, Equia Forte, and Calcimol LC) of seven pulp-capping materials evaluated presented significantly higher radiopacity levels than enamel. Materials with lower radiopacity than enamel are not suitable in deep areas of cavities when these materials are used as an initial increment because of difficulty to detect marginal integrity or overhang between tooth structure and restorative material (21). Also, a previous study has suggested using materials with similar radiopacity to make radiographic examinations easier when cavities are restored with different materials (22). In the current study, pulp-capping materials, except for Ionoseal, were less radiopaque than conventional resin composite.  Consequently, they attributed to a diagnostic challenge on radiographs when used under restorative materials with a minimal thickness of 0.5 to 1 mm.