Discoloration and radiopacity of white mineral trioxide aggregate with various radiopacifiers

Introduction

Mineral trioxide aggregate (MTA) is mainly composed of calcium silicate-based cement, Portland cement, and metallic oxide, which has been used for apexification, vital pulp therapy, root canal surgery, and perforation sealing (1-4).

In terms of the esthetic point of view, the discoloration of MTA has been reported as an inconvenient property for the earlier product, which may even cause a clinical misdiagnosis, and hence the color of MTA has been changed from gray to white. Still, the discoloration of MTA through various mechanisms has been reported(5-8).

The discoloration of both gray MTA (GMTA) and white MTA (WMTA) has been caused by metal oxides (iron, aluminum, and magnesium oxides) (9), blood contact (7), and the effect of light and oxygen (9, 10). The interaction of MTA with sodium hypochlorite (NaOCl) which is regularly used for irrigation in endodontics, resulted in a black MTA surface (6, 12, 13). Such incidence is common with MTA that uses the bismuth oxide as the radiopacifier, as it reacts with sodium hypochlorite, forming bismuth carbonate (13).

The sodium hypochlorite was used for root canal disinfection, smear layer removal, and the dissolution of pulp tissue, however it was reported to cause discoloration when coming into contact with WMTA (6, 13).

Materials and Methods

1. Materials

Bismuth oxide (Bi₂O₃, MW: 465.96, CAS No. 1304-76-3), calcium tungstate (CaWO₄, MW: 287.392, CAS No. 7790-75-2), barium oxide (BaO, MW: 153.33, CAS No. 1304-28-5) and zirconium oxide (ZrO₂, MW: 253.81, CAS No. 7553-56-2) were purchased from Sigma Aldrich (St. Louise, MO, USA). White Portland cement (Union Cement, Seoul, Korea) and calcium sulfate hemihydrates (Sam Woo Co. Ltd., Ulsan, Korea) were also purchased.

The radiopacifiers considered in this study are listed in Table 1. Each radiopacifier (20 wt%) was mixed with White Portland cement (75 wt%) and calcium sulfate hemihydrates (5 wt%). ProRoot MTA (PR, Dentsply, York, PA, USA) was used as a control product, and the other compositions are listed in Table 1(11). The test materials were mixed with a ball milling system using a zirconia ball and sieved. Then, each mixture was collected in a glass vial and stored in a desiccator before the tests. The material with no radiopacifier (NR) was used as the negative control.

Table 1.

Radiopacifiers of white mineral trioxide used in this study

Results

1. Color change

The color changes and images of the test specimens before and after immersion in the solution are shown in Figure 1. The PR, which contained bismuth oxide, showed the most significant color change, which was significantly higher than the rest of the samples (p<0.05). BM also showed a significantly higher color change than BO, CT, ZO, and NR (p<0.05). The BO changed to a light-gray color in which the change was significantly higher than CT, ZO, and NR (p<0.05). Finally, the CT and ZO showed no color changes as there were no significant difference in color changes between CT, ZO and NR (p>0.05).

Figure 1

Color change before and after immersion in sodium hypochlorite solution. White MTA with four different radiopacifiers; bismuth oxide (BM), calcium tungstate (CT), barium oxide (BO), and zirconium oxide (ZO). The group with no radiopacifier (NR) was used as a negative control and ProRoot MTA (PR) was used as the commercial control. The same lowercase letters indicate no statistical significant differences (p>0.05).

2. Radiopacity

The mean radiopacity values (equivalent to thickness of Aluminum in mm, Almm) and images of the specimens are presented in Figure 2. The BM had the highest value compared to other samples (p<0.05). This was followed by BO = PR > CT > ZO > NR (p<0.05). All samples, except NR (which had no radiopacifier) showed radiopacity greater than 3 Almm, which is the radiopacity recommended by ISO 6876 (14).

Figure 2.

Radiopacity of specimens in comparison with thickness of the aluminum step wedge in mm (Almm). White MTA with four different radiopacifiers; bismuth oxide (BM), calcium tungstate (CT), barium oxide (BO), and zirconium oxide (ZO). The group with no radiopacifier (NR) was used as a negative control and ProRoot MTA (PR) was used as the commercial control. The same lowercase letters indicate no statistical significant differences (p>0.05).

Discussion

Despite its advantages, crown discoloration of MTA in the pulp chamber has been reported in the depth of the material. There have been various reasons for this discoloration, including blood contamination and contact with the solutions routinely used in endodontics (15). Among these various solutions, the hydrogen peroxide used for bleaching and lidocaine used as an anesthetic did not affect the tooth color (16). On the other hand, the hypochlorite solution used for canal irrigation produced a color change when contacting bismuth-containing MTA. When it contacted sodium hypochlorite, it lost oxygen, which led to a dark brown color. It seems that this dark color change could indicate a change from oxide to bismuth metal (6).

The radiopacity of an endodontic material is also one of the most important characteristics for the observation of the radiographic interface between the material and tooth substrate, and it is tested based on the international standard in comparison to an aluminum wedge according to standard. It is quantitatively assayed by comparing a graduated aluminum step wedge with equivalent thickness to determine the optical radiographic density under X-ray exposure (14).

To overcome the discoloration, we replaced the bismuth oxide with other radiopacifiers and investigated the color change and radiopacity after immersion in a hypochlorite solution. Among the radiopacifiers, radio-contrast agents used to enhance the visibility of internal structures under X-ray exposure have been reported to produce allergic reactions. Thus, we selected FDA-approved radiopaque formulations, including barium oxide, bismuth oxide, calcium tungstate, and zirconium oxide (17, 18). The selection of an appropriate radiopacifier in the proper amount requires a thorough understanding of its characteristics and how various radiopaque compounds could affect MTA.

Barium compounds such as barium sulfate have been used as X-ray radiocontrast agents. However, barium oxide reacts violently with water, which makes it unsuitable for water-based cement. The barium compound had a lower density than bismuth oxide (5.72 g/cm³), which showed a lower radiopacity for the same volume and it is an insoluble additive for oil well drilling fluids. In addition, in a purer form, it is used as an X-ray radiocontrast agent for imaging the human gastrointestinal tract. However, soluble barium ions and soluble compounds are poisonous, and water-soluble barium compounds are poisonous, with low doses of barium ions acting as a muscle stimulant and affecting the nervous system (18).

Bismuth is used in various medical devices, although it is more expensive than a barium-based material, it is twice as dense (8.90 g/cm³). Because of its density, a 40% bismuth compound contains only approximately half the volume ratio of a 40% barium sulfate or barium oxide compound. Because bismuth produces a brighter, sharper, higher-contrast image on an X-ray film or fluoroscope than barium, it is commonly used whenever a high level of radiopacity is required (17).

Tungsten has a higher specific gravity than bismuth, and a loading of 60% tungsten has approximately the same volume ratio as a 40% bismuth compound. Because of its density, a device with calcium tungstate can be made highly radiopaque with low loading. White calcium tungstate (density: 6.06 g/mL) showed the potential to replace bismuth oxide, however, it showed a lower radiopacity than bismuth oxide in same percentage. Zirconium oxide (density: 6.52 g/cm³) showed a radiopacity result similar to that of calcium tungstate, with no discoloration.

In terms of discoloration, calcium tungstate and zirconium oxide showed no color change. However, the barium oxide group showed change in color to light gray. The bismuth oxide containing groups (BM and PR) showed the highest color change, turning dark brown. As previously mentioned, the bismuth oxide showed a change by oxidation.

From the results, bismuth oxide showed the highest radiopacity result and the calcium tungstate and zirconium oxide also showed distinguishable results. All the test materials showed radiopacity except the negative control group.

Various alternative radiopaque contrast agents were identified in this study, and the potentials of some materials were confirmed. The further tests are required for confirmation of optimal radiopacifier in terms of both radiopacity and discolorations. Still, it was evident from this study that the calcium tungstate followed by the zirconium oxide could be a choice of radiopacifier.

Conclusion

Within the limits of the present study, the following conclusions can be drawn

1. Color changes of White MTA containing bismuth oxide was significantly greater than White MTA containing other radiopacifier.

2. Color changes of White MTA containing calcium tungstate and barium oxide was significantly lower than White MTA containing bismuth oxide.

3. White MTA containing calcium tungstate and barium oxide showed reduced but adequate level of radiopacity than MTA containing bismuth oxide.

Acknowledgments

This study was supported by a new faculty research seed money grant of Yonsei University College of Dentistry (2019-32-0017).

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