In vitro investigations into the etiology of mineral trioxide tooth staining.

AIM: To investigate the role of bismuth oxide, a constituent of contemporary mineral trioxide aggregate (MTA) materials, and its response to various solutions that may contribute to the potential discoloration that occurs following MTA applications within the scope of endodontics. SETTING AND
DESIGN: Laboratory assessment of chemical reactions with white ProRoot(®) MTA and white Portland cement (WPC).
MATERIALS AND METHODS: Set specimens and freshly mixed specimens of white ProRoot(®) MTA and white ProRoot(®) MTA powder, along with specimens of WPC were exposed to distilled water, phosphate buffered saline (PBS), 10% formalin, hydroxyethylmethacrylate (HEMA), sodium hypochlorite, sodium hydroxide (NaOH) base, and hydrochloric acid (HCl) acid. Specimens were visually inspected periodically for color changes.
RESULTS: All forms of ProRoot MTA showed discoloration when exposed to 10% formalin within 30 min, as opposed to WPC, and were completely blackened at 4 days. Bismuth oxide alone and with calcium oxide also turned black within 30 min after exposure to 10% formalin. No discoloration was seen when exposed to the other solutions.
CONCLUSIONS: Exposing MTA in various forms to a variety of liquids has determined that bismuth oxidein combination with other chemical moieties is the prime cause of staining observed by clinicians.

INTRODUCTION

The introduction of mineral trioxide aggregate (MTA) to dentistry has enhanced treatment procedures and provided excellent outcomes within the scope of endodontic applications.[1] This includes vital pulp therapy, surgical intervention, and root reparative procedures.[2] The predictable healing of pulpal and periodontal tissues following the placement of this material has enabled the retention of many teeth that may have either undergone more extensive procedures or that may have been extracted. MTA has been shown to induce healing through the release of calcium hydroxide, followed by the formation of hydroxyapatite (HA)[34] and with an ultimate hard tissue response, whether it be reparative dentin in vital pulp therapy or cementum in surgical and root reparative procedures.[56]

In the past few years, however, there has risen a global clinical concern with the use of MTA, particularly when used in the coronal portion of the tooth, that is, pulp capping or pulpotomy.[789101112131415] Observations have indicated that there is dark staining of the tooth structure following the use of MTA in these situations that is impacting tooth esthetics and which may have deterred many clinicians from using this material. Whilst some studies have considered this outcome as a “mystery”, it is believed to be material provoked and triggered by a chemical reaction upon setting, particularly in response to a white MTA product (ProRoot® MTA, Dentsply Tulsa Dental Specialties, Tulsa OK, USA).[12] Many studies have speculated on the cause of this staining with the prime etiologies being leakage around the coronal restoration, presence of residual pulp tissue, contamination with blood, dentinal fluid, exposure to hydrogen peroxide and sodium hypochlorite, use of anesthetic solution and even sterile water as the mixing media, and exposure of the material to an acidic environment.[711131516]

To counteract or prevent this observed discoloration, some studies have proposed various solutions, such as application of a bonding agent over the MTA,[17] internal bleaching when it did occur,[18] or the use of modified MTA materials.[19] Whilst one exact mechanism for the occurrence of the discoloration has yet to be identified as being the prime suspect, one possibility lies in the bismuth oxide component of the MTA. Bismuth oxide is added to enhance the radiopacity of MTA, and has been reported to be present only at the 8.4% level in set MTA, as against the 21.6% in the unset material.[6] The bismuth forms a part of the structure of the calcium silicate hydrate gel and also affects the precipitation of calcium hydroxide in the hydrated paste.[5] Both bismuth and calcium are leached out from MTA over time, with the calcium decreasing over a 5-week period, whilst the bismuth oxide levels increased during that time frame.[6]

The purpose of this study was to explore and clarify the chemical reactions that may occur within the MTA itself upon setting, with a focus on the bismuth component of the material when it is in contact with specific agents that may be used in clinical treatment protocols.

MATERIALS AND METHODS

The evaluation of chemical reactions with MTA was divided into three studies:

Set specimens soaking in liquid

Included in this part of the study were the following liquids; distilled water (distilled H2O) (Ozarka; Oklahoma City, OK, USA), phosphate buffered saline (PBS) (×1 without calcium and magnesium, Mediatech Inc, Manassas, VA, USA), sodium hypochlorite (bleach) (8.25% Clorox, The Clorox Company, Oakland, CA, USA), and formalin (10% formaldehyde in water) (MP Biomedicals, LLC, Solon, OH, USA). These liquids were used to evaluate the following set of specimens: White ProRoot (Dentsply International, Tulsa, OK, USA) mixed 3:1 with distilled water and white Portland cement (WPC) (Type 1 Lehigh, Allentown, PA, USA), mixed 3:1 with distilled water (H2O).

The set white ProRoot specimen was produced by blending together 2.25 g of white ProRoot powder with 0.75 g of distilled water on a glass slab until fully hydrated and uniform. Then the mixture was placed on a glass microscope slide with two 1 mm spacers. A second microscope slide was placed on top and clamped to the bottom microscope slide. An additional WPC cement specimen was produced in a similar manner. All specimens were allowed to set in a 37°C and 95% relative humidity chamber for 7 days. The set specimens were then removed from the microscope slides.

The specimens were then placed in plastic vials and 5 ml of liquid was added to each vial. The vials were sealed and stored in ambient conditions (23 ± 3°C). The specimens were inspected periodically to determine if there were any changes in color.

Freshly mixed specimens soaking in liquid

Included in this part of the study were the following liquids; formalin, bleach, and 15% hydroxyethylmethacrylate (HEMA) in glycol (Aldon Corporation, Avon, NY, USA). These liquids were used to evaluate the following fresh mixes: White ProRoot mixed 3:1 with distilled water and WPCmixed 3:1 with distilled water. The specimens were prepared as detailed above and once mixed, covered with the solutions indicated above, and stored in ambient conditions (23 ± 3°C). All specimens were carefully scraped from the glass slab and transferred to a vial that was inspected periodically to determine if there were any changes in color.

Specimen powders suspended in liquids

This portion of the evaluation included the following liquids; PBS, sodium hydroxide (NaOH) (32% Sigma Aldrich, Buchs, Switzerland), hydrochloric acid (HCl) (25% Sigma Aldrich, Laborchemikalien, Germany), formalin, and HEMA. These liquids were used to evaluate the following powders: Bismuth oxide (Bi2O3) (Varistor grade, MCP group Tilly, Belgium), calcium oxide (CaO) (Sigma Aldrich, St. Louis, MO, USA), WPC, and white ProRoot.

Each paste was produced by stirring 3 g of liquid with 3 g of powder in a small, sealable vial. Stirring continued until a smooth uniform paste was produced. The vial was sealed and stored in ambient conditions (23 ± 3°C). The paste was inspected periodically to determine if there were any changes in color.

The PBS, formalin, and HEMA were determined to be pH neutral and were used as received from the supplier. The NaOH was added to PBS to produce a liquid with a pH of 11. The HCl was added to PBS to produce a liquid with a pH of 6. Table 1 summarizes the groups studied for color change.

Table 1

Summary of groups studied

RESULTS

Set specimens of white ProRoot were soaked forup to 43 days, Group 1 in distilled H2O, Group 2 in formalin, and Group 3 in bleach. The specimens were inspected at 30 min and 4, 11, and 43 days after soaking began. The set specimens in Group 1 started white and did not visually change color. The set specimens in Group 2 began turning black after 30 min. The pH was measured to be approximately 11.0. The specimens were completely black after 4 days. The specimens were broken in half and the interior of the specimen was also black. Figure 1 shows photographs taken of the specimens from Group 2. The set specimens in Group 3 started white and did not visually change color.

Figure 1

(a) Group 2 set specimen of white ProRoot after soaking for 30 min in formalin. (b) Group 2 set specimen of white ProRoot after soaking for 4 days informalin

Groups 4 and 5 included set WPC soaking in PBS and formalin. The specimens were inspected after 4, 11, and 43 days. The Group 4 specimens started white and after 43 days remained white. However, a white precipitate had formed over the specimen. This is expected to be a calcium phosphate compound produced from the CaO being released into the SBF. After 4 days the pH measured approximately 11.0. The Group 5 specimens were white at the beginning of the soak and retained that color at 43 days.

Fresh mix soaking in liquid

The freshly mixed specimens of white ProRoot that were soaked in Formalin were inspected after 30 min and 4, 11, and 43 days. The fresh mixed specimen in Group 6 started white and visually changed color to black after 30 min. The pH was measured to be approximately 11.0. The specimen remained black for the remainder of the 43-day soak. The fresh mix specimens of white ProRoot, which soaked in HEMA was inspected after 16 days. The fresh mixed specimen in Group 7 started white and did not change color during the evaluation period. The fresh mixed specimens of white ProRoot, which soaked in bleach was inspected after 30 min and 4 and7 days. The fresh mixed specimen in Group 8 started white and did not change color; however, the bleach turned pink after 30 min and remained pink.

The freshly mixed specimens of WPC that were soaked in Formalin were inspected after 7, 15, 22, and 50 days. The freshly mixed specimens in Group 9 started white and visually did not change color after soaking for 50 days. The pH was measured after 7 days to be approximately 12. The freshly mixed specimens of WPC that were soaked in HEMA were inspected after 16 days. The fresh mixed specimen in Group 10 started white and did not change color during the evaluation period.

Powders suspended in liquids

White ProRoot specimens mixed into a paste and suspended in PBS, Group 11, were inspected after 30 min and 4, 11, and 43 days. The white ProRoot paste started white and remained white for 43 days. The pH was measured to be approximately 12 after 30 min. WPC specimens mixed with PBS, Group 12, were inspected over the same time frame and remained white for 43 days. The pH was measured after 30 min to be approximately 12. Bi2O3 + CaO (2:1) specimens mixed with PBS, Group 13, were inspected over the same time frame and remained yellow for 43 days. The pH was measured after 6 days to be approximately 13.

Bi2O3 was mixed in a basic solution and an acidic solution (Group 14: PBS + NaOH, pH = 11; Group 15: PBS + HCl, pH = 6). The pastes were inspected after 7, 11, 18, and 50 days. The Bi2O3 started yellow and after 50 days remained yellow.

White ProRoot, WPC, Bi2O3, and Bi2O3 + CaO (2:1) were mixed into formalin (Groups 16, 17, 18, and 19). These suspensions were inspected after 30 min and 4, 11, and 43 days. Group 16, white ProRoot, had turned black after 30 min and had a pH of about 13. The powder remained black for 43 days. Group 17, WPC, was white and remained white for 43 days. Group 18, Bi2O3, started yellow and remained yellow until day 4. It was yellow when inspected after 30 min. On day 4 it was turning black and continued to be black for 43 days. Group 19, Bi2O3 + CaO (2:1), started yellow but turned black after 30 min. The suspension remained black for 43 days.

Bi2O3 and Bi2O3 + CaO (2:1) were mixed in HEMA (Groups 20 and 21). The suspensions were inspected after 16 days. The paste began yellow and remained yellow. The results of all specimens are summarized in Table 2. Unless noted, all solutions remained clear, whilst only the specimens turned color.

Table 2

Summary of discoloration results

DISCUSSION

The discoloration of teeth following application of MTA in vital pulp therapy has been disconcerting to the clinician. Obvious reasons for this occurrence have not been readily identified and this occurrence has created reluctances on the part of clinicians to use this material, especially in anterior teeth with significant amounts of intact tooth structure. This in vitro study was designed to ascertain the causes of this discoloration.

These findings indicate that the discoloration of MTA-type materials maybe produced by exposing one of its components, bismuth oxide to formalin-based substances. The formalin used in this study was 10% formaldehyde. All forms of the white, commercially available MTA, ProRoot; whether set, freshly mixed, or just powder; discolored in the presence of formalin within 30 min. The presence of bismuth oxide was determined to initiate and drive this discoloration from a white mix to one that was black. Bismuth oxide exposed to formalin discolored, but required 4 days.

A previous study indicated that calcium oxide is produced during the setting reaction of Portland cement.[20] In the present study, a mixture of bismuth oxide and calcium oxide in formalin discolored in 30 min.

During experiments exposing powdered bismuth oxide and set white ProRoot to bleach for 24 h, the specimen discolored.[16] The powder bismuth oxide was dried after soaking and had a sodium chloride precipitate. The white ProRoot was dried after soaking and also had turned black.

During experiments addressing the compression strength of white MTA, specimens that had been loaded into Delrin molds (Sabri Dental Enterprises, Downers Grove, IL) were discolored during their setting whilst being soaked in PBS.[21] Delrin is the trade name from Dupont for acetal resin or polyformaldehyde. The surface of the white MTA in contact with these molds was discoloring during this experiment, whereas the surface in contact with the PBS was not. This finding led to the methodology and research design in the present study.

Whilst the tissue response to White MTA is positive for tissue repair and regeneration, it has been identified as causing staining deeply into the dentin upon the evaluation of MTA removal during root canal revision procedures.[22] The majority of staining has been noted clinically following the use of MTA for pulpotomies in primary teeth,[232425] with percentages as high as 94% when the teeth were restored with composite materials following the placement of MTA.[12]

Marciano et al.,[26] analyzing the potential impact that bismuth oxide may have in color changes in a calcium silicate cement, could not find a direct correlation with the bismuth oxide alone and indicated that components in the dentin and the cement may, along with the bismuth oxide may induce an adverse chemical reaction resulting in the darkness seen. Furthermore, in further studies by Marciano et al.,[27] when assessing color changes in response to a white MTA (WMTA Angelus; Angelus, Londrina, PR, Brazil), alterations in color were attributed to contact of the MTA with the collagen in the dentinal matrix and reactions with bismuth oxide. To counteract this adverse outcome, an alternative radiopacificer was recommended for use in MTAreplace bismuth in white MTA. Moreover, Vallés et al.,[28] suggested that an association between oxygen supply and light curing of some restorative materials may alter the color of MTA cement.

In an attempt to prevent discoloration following the use of MTA, Akbari et al.,[17] recommended the use of a dentin-bonding agent to seal the dentinal tubules. In effect, the bonding agent was preventing contact of the MTA with the collagen matrix. However, this approach was speculated to interfere with the seal of the MTA and release of calcium from the dentinal tubules.

Chemistry of discoloration

Formalin is a 10% mixture of formaldehyde in water. Formaldehyde is a gas, but is soluble in water. Once dissolved in the water it exists in two metastable states; formaldehyde and methylene glycol.[29] The methylene glycol over time exists in a metastable state with methylene glycolate anions. The methylene glycolate anion is a reducing agent.

Once the bismuth oxide is exposed to the formaldehyde (and thus the reducing agent), it is reduced tometallic bismuth powder; thus, producing the discoloration.

Formaldehyde in dental materials

Formaldehyde can be released from methacrylate-based resins during the polymerization and/or as a degradation product of the oxygen-inhibited surface layer.[30] The formaldehyde may be an additive or formed after chemical reactions that occur during the set of the material.[2831] One mechanism of formation of formaldehyde is primary oxidation of unsaturated methacrylate groups. A second mechanism would be oxygen copolymerized with methacrylate groups during the initial phase of polymerization. It can be released from other adhesives as well, as some zinc oxide eugenol sealers are known to include formaldehyde or lead to the formation of formaldehyde in tissues.[31]

CONCLUSION

The investigation in the etiology of MTA staining was conducted as an in vitro study. Exposing MTA in various forms to various liquids has determined that bismuth oxide is the prime cause of the staining observed by clinicians. Once bismuth oxide is exposed to formaldehyde it begins to undergo a transformation to a dark color. The chemical reaction appears to be accelerated by the presence of calcium oxide. A blend of bismuth oxide and calcium oxide exposed to formaldehyde can stain in as little as 30 min at room temperature. The staining of the tooth appears to be caused by the stained bismuth oxide leaching from the MTA.