Radiopacity Evaluation of Gutta-Percha Points in Thinner Samples than the ANSI / ADA Recommendation

(Endo Points®, Dentsply®, Tanari®, Meta®, Roeko® and Odous®) in samples of 1 mm thick as established by ANSI/ADA Specification #57 and ISO 6876/2001, in comparison with thinner samples. Twelve test specimens for each material, four for each thickness (0.3, 0.6, and 1 mm and diameter of 8 mm), were laminated and compressed between two polished glass plates until the desirable thickness. Digital radiographs were obtained along with a graduated aluminum stepwedge varying from 1 to 10 mm in thickness. The X-ray unit was set at 70 kVp, 10 mA and 0.4 s exposure time, at a focal distance of 36 cm. One calibrated observer quantified the average values of pixels with Adobe Photoshop® software. Data were analyzed using ANOVA and Tukey tests, at 5% significance level. At 0.6 and 1 mm thickness, all the tested materials showed radiopacity higher than 3 mm of aluminum (reference value). At 0.3 mm thickness, Odous and Tanari presented significantly less radiopacity than the reference, and the other materials showed similar radiopacity to the reference. The study concluded that the materials demonstrated different radiopacities and all had values above the minimum recommended by ANSI/ ADA specification #57, being Odous and Tanari less radiopaque than the reference value in thinner samples (0.3mm). Radiopacity Evaluation of GuttaPercha Points in Thinner Samples than the ANSI/ADA Recommendation


Introduction
International standards require that gutta-percha points be sufficiently radiopaque to permit evaluation of the density of the root canal filling and to be distinguished from the tooth structures (1,2).The dependence of dental diagnosis on radiographs makes it necessary for all materials used in the oral cavity to be radiopaque.So, in all commercial brands of gutta-percha points, different radiopacifying agents are added in several concentrations (3).
The ANSI/ADA Specification #57 for endodontic sealing materials establishes the minimal value of radiopacity, stating that 1 mm thick of gutta-percha must have a radiopacity not less than the equivalent to 3 mm of aluminum (1).However, the specification does not state the maximal radiopacity value, giving autonomy to manufacturers to choose the radiopacifying agents and their concentrations.Due to root canal filling aesthetic appreciation, gutta-percha points of different commercial brands have been produced with two to three times more radiopacity than the minimal required (4)(5).
Considering that, during the endodontic therapy, after biomechanical preparation, the diameter of the root canal at the apical region is frequently less than 1 mm (6), the investigation of gutta-percha radiopacity in a thickness smaller than 1mm is critical.According to ANSI/ADA, the minimal radiopacity value for gutta-percha must be determined at 1mm thinness in vitro samples and do not consider the superposing of anatomical structures and different thick of filling along the root canal.In a recent investigation, a new in vitro method was proposed to evaluate radiopacity of endodontic materials using a tissue simulator to approximate radiopacity evaluation to the clinical reality.It was observed that the superimposition of anatomical structures increases the filling material radiopacity (7).So, since most root canal fillings are distinguished from anatomical structures during radiographic analysis, even in smaller diameters than 1 mm, is important to investigate the gutta-percha radiopacity in thinner samples than the recommendation.
Currently, there are no studies in the literature concerning the radiopacity of gutta-percha brands at thicknesses lower than that determined by the ANSI/ ADA specification #57 (1) and that have discussed its implications in clinical aspects.Therefore, this study aimed to evaluate the radiopacity of different gutta-percha points (Endo Points®, Dentsply®, Tanari®, Meta®, Roeko® and Odous®) using digital radiography and the aluminum stepwedge method, in samples of 1 mm thick as established by ANSI/ADA Specification #57 (1) and ISO 6876/2001 (2), in comparison with thinner samples.

Sample Preparation
Six gutta-percha brands were evaluated in this study: Endo Points®, Dentsply®, Tanari®, Meta®, Roeko®, and Odous® (Table 1).Twelve test specimens with thicknesses of 0.3, 0.6 and 1 (±0.02)mm and diameter of 8 (±0.02) mm were made from each material (n=4).For specimen confection, three gutta-percha points, placed on a glass plate, were laminated on the flame of a lamp in order to obtain a homogeneous malleable mass that was compressed manually between two smooth glass plates.The plastification and compression procedures were repeated until obtaining gutta-percha thicknesses of 0.3±0.02mm, 0.6±0.02mm or 1.00±0.02mm.The specimens' thicknesses were confirmed at three different points of the sample using a digital caliper (ME 727, Starrett Co., Athol, MA, USA).After, the edges of the circular gutta-percha discs were cut out circumferentially, resulting in standardized specimens with eight millimeters in diameter.

Radiographic Procedures
The specimens were positioned over an occlusal radiographic sensor (Digora Soredex, Digora Soredex, Tuusula, Finland) along with an aluminum stepwedge (1100 alloy) with variable thickness (from 1 to 10 mm in increments of 1 mm).Four sets of each material, containing three specimens of different thickness, were positioned and radiographed using a radiographic unit (Timex 70C, Gnatus, Ribeirão Preto, SP, Brazil) operating at 70 kVp, 10 mA, 0.4 s exposure time and a focal distance of 36 cm (Fig. 1).Specimens with radiographic image voids or defects were replaced.

Radiopacity Assessment
The generated images (TIFF format) were encoded and analyzed by one blinded and calibrated examiner (ICC=1), as described by Malka et al. (7).Briefly, the examiner evaluated the images at a 50 cm distance from a 24-inch liquid crystal display monitor under dimmed light.The digital images were analyzed with Adobe Photoshop® software v. 10.0 (Adobe Systems, San Jose, CA, USA).A standard-size circle (400 pixels) was drawn in the center of the disc and another circle was drawn in the sixth step of the aluminum stepwedge, equivalent to 3 mm of aluminum (reference value).The average and standard deviation of the gray scale pixel values in each area selected were measured using the histogram tool and were recorded.
Then, the mean radiopacity values were calculated for each group and used for the statistical comparisons.

Statistical Analysis
Data were submitted to statistical analysis at SPSS® software, v. 10.0 (SPSS Inc., Chicago, IL, USA ), using oneway analysis of variance (ANOVA), followed by Tukey test, for comparing the radiopacity among thicknesses, materials and the reference value.The correlation between the specimens' thickness and the greyscale pixel values was determined by Pearson test (α=0.05%).

Results
Table 2 summarizes data for radiopacity comparison.At 0.6 and 1 mm thickness, all the tested materials showed radiopacity higher than 3 mm of aluminum (reference value).At 0.3 mm thickness, Odous and Tanari presented significantly less radiopacity than the reference and the other materials showed similar radiopacity to the reference.

B.L. Petry et al.
Pearson test showed a positive correlation between gutta-percha thickness and greyscale pixel values (r=+0.4943).

Discussion
This study evaluated the radiopacity of different gutta-percha brands using a digital radiographic method.Although revised in 2012, ANSI/ADA #57 specification (1) does not provide parameters for the use of more sensitive digital receptors and software for measuring radiopacity as pixel intensity, which is the reality of modern radiology.However, the use of the aluminum scale, X-rayed along with the specimen (8)(9)(10)(11)(12)(13)(14), is intended to expose both objects to the same parameters.
Several studies have evaluated the radiopacity of guttapercha using an aluminum stepwedge as a comparison standard (3,4), since the filling materials must have a minimum degree of radiopacity to be distinguished from soft and mineralized tissues during radiographic analysis (1,15).For root canal filling materials, it was established that the minimal radiopacity value for 1 mm thick of guttapercha must be equal or higher than that generated by 3 mm of aluminum (1,2).Also, the ANSI/ADA Specification #57 (1) and ISO 6876/2001 (2) establishes that the filling materials radiopacity must be measured in specimens with 1 or 2 mm thick.
The evaluation of gutta-percha radiopacity in a thickness smaller than 1 mm can be justified by the fact that root canal diameters in the apical portions frequently is less than 1 mm thicknesses after chemo-mechanical procedures completion (6).For curved root canals, the last instrument used at working length is usually 0.25 or 0.30 diameter.Consequently, the gutta-percha point used for filling has the same diameter at its tip, that is, less than 1 mm (6).However, even being thinner, most root canal fillings are supposed to be distinguished from adjacent anatomical structures during radiographic analysis.The results of this research support this statement, since most trademarks tested, except Odous® and Tanari® at 0.3 mm, showed values of radiopacity greater or equal to the reference value.At 1 mm thickness, the radiopacity level detected for all brands were adequate, according to the ANSI/ADA Specification #57 (1) and did not differ from those found in other studies (3,5,12).At 0.6 mm thickness, the six gutta-percha brands evaluated reached the minimum radiopacity recommended.This result suggests that possibly the root canal fillings with equivalent or larger diameter than 0.6 mm, regardless of the brand of gutta-percha employed, are suitably distinguished from adjacent structures during radiographic analysis.
However, at 0.3 mm thickness, Odous® and Tanari® presented lower radiopacity than the minimum recommended by ANSI/ADA Specification #57 (1).This fact could be considered useful, once the radiopacity excess could hide flaws present in the filling material, as observed by Gurgel-Filho et al. (20) for Tanari® group.The absence of barium sulfate in the gutta-percha cones of these trademarks may be related to the lower radiopacity (20,21).Barium is a chemical element with atomic number greater than Zinc and, thus, barium sulfate (BaSO4) absorbs X-rays faster than zinc oxide, providing high radiopacity to the material (22).According to Maniglia-Ferreira et al. (21), Endo Points also has no barium sulfate in its composition.However, as in this study, the authors observed that the guttapercha radiopacity was above the minimum recommended value for all investigated thicknesses, showing that the absence of this component did not compromise its radiopacity.Tanomaru et al. (23) suggested that this fact probably is due to the addition of other radiopacifying agents not reported by the manufacturer, such as Different lowercase letters represent a significant difference between gutta-percha and aluminum (P<0.05).Different capital letters represent a significant difference between the gutta-percha brands with the same thickness (p <0.05).

Radiopacity of thinner gutta-percha samples
bismuth subcarbonate and bismuth oxide, commonly present in the composition of some sealers.The Pearson correlation test demonstrated that the average gray level increased as the filling material became thicker.This finding is important since it is known that the root canals become tapered after cleaning and shaping (24), occurring different radiopacity levels along the root canal filling.However, the perception of these differences in radiographic images by the human eye and its influence on the diagnostic skills are still unknown.Furthermore, besides the radiopacity generated by the gutta-percha points, the radiopacity produced by the sealer should be considered, especially in the apical part of the canal, wherein the amount of gutta-percha is lower (15,25).
Some investigations revealed radiographic detection of defects in the apical third of fillings was easier than in the medium and cervical thirds (16,24).It is important to point out that, from a clinical perspective, as gutta-percha is associated with sealer and with the superposition of anatomic structures that influence its radiopacity, points with less radiopacity could facilitate failures identification, mainly in thicker portions of the root canal (medium and cervical thirds).Therefore, the clinical advantage of gutta-percha points with higher radiopacity than the recommended is questionable, since it can hide root canal filling defects.
Due to the importance of the filling material radiopacity in defects identification (16), it is suggested to discuss the possibility of including, in the ISO 6876/2001 standards (2) and the ANSI/ADA specification #57 (1) parameters for gutta-percha radiopacity evaluation in smaller thickness specimens.Besides, it would be interesting to adapt the standards considering the digital systems (17) and to standardize methods to address the radiopacity generated by the association of gutta-percha and sealers, to approach the relationship between the laboratory findings with those produced during clinical practice.So, further studies are suggested to evaluate the behavior of the gutta-percha radiopacity associated with endodontic sealers.
Considering the limitations of this in vitro laboratory study, it can be concluded that all gutta-percha brands evaluated presented appropriate radiopacity, above the minimum recommended by the ISO, being Odous® and Tanari® less radiopaque than the reference value in thinner samples (0.3 mm).

Figure 1 .
Radiographic image of the specimens in 0.3 mm thickness (A), 0.6 mm (B), 1 mm (C), next to the aluminum scale (D).

Table 2 .
Mean values and standard deviations of pixels observed for each gutta-percha brand in 0.3, 0.6 and 1mm thickness and for 3mm thickness of the aluminum stepwedge