MTA shows improved outcomes in endodontics as a result of its excellent biocompatibility (4, 23), good sealing ability (30), low leakage (24), and promotion of hard tissue formation (31). However, the tooth discoloration induced by MTA is one of its drawbacks. For the prevention and treatment of discoloration, previous studies have suggested placing a double layer of DBA over the dentine in an access cavity before applying MTA (14) and internal bleaching after removing MTA (11). To the best of our knowledge, only one previous study (14) has investigated the application of adhesive materials prior to use of MTA inside the pulp chamber. Moreover, no study has compared the degree of tooth discoloration following placement of ProRoot MTA and other recently developed tricalcium silicate cements or investigated the effect of prior application of DBA on these materials. Therefore, our findings provide information useful for preventing the discoloration induced by MTA.
Following the 12-week treatment period, tooth shade changed in nearly all teeth, including the control teeth. Teeth from the MTA group displayed the most extreme color change to gray, while the MTA-B teeth exhibited only marginal changes. Additionally, the degree of discoloration was significantly less in the RMTA group than in teeth in the MTA group. Analyses of the CIE L*a*b* parameters revealed that tooth lightness decreased in all groups after 12 weeks, with the greatest reduction observed in the MTA group (ΔL*=10.44). Notably, the large reduction in ΔL for the MTA group was due to its substantial reduction in gray discoloration. The a* and b* values were indicative of redness and yellowness, respectively. The a* was reduced in all groups, with the greatest reduction in the MTA group (Δa*=2.49). Moreover, b* increased in all groups except MTA, in which it decreased (Δb*=1.14). These data were consistent with the observation of gray and dark blue discolorations in the MTA group.
One previous study evaluating the discoloration of GMTA and WMTA revealed that discoloration occurred within the MTA directly rather than in the dentin. In that study, crown shade also improved when MTA was removed [16, 17]. However, discolored byproducts remained in the dentin following the removal of MTA, which may have contributed to coronal discoloration. Another study revealed that the addition of two layers of DBA prior to MTA treatment prevented discoloration . In cases when discoloration occurred, removal of MTA and bleaching required caution and additional operational procedures. Thus, we hypothesized that the pre-application of DBA could achieve the same effect as the removal of MTA and bleaching. Therefore, we compared the pre-application of DBA to post-bleaching in ProRoot MTA-treated teeth.
In the MTA group, the ΔE calculated on the basis of the L*, a*, and b* parameters continuously increased. The magnitude of the increase in ΔE was significantly smaller in the MTA-B group than the MTA group. This result was consistent with the findings of a previous study that suggested that DBA may prevent discoloration by ProRoot MTA . DBA may seal the dentinal tubules and prevent MTA penetration. In the RMTA group, ΔE increased following the 12-week treatment period, but was not significantly different from that in the control group. Additionally, pre-application of DBA did not prevent tooth discoloration. The lower discoloration by RetroMTA than by ProRoot MTA was due to their differences in composition. RetroMTA does not contain metal oxides, which cause the discoloration by ProRoot MTA. RetroMTA also includes calcium zirconia complex as a radiopacifier instead of bismuth oxide. Thus, it could be suggested that, because RetroMTA-treated teeth had low discoloration, the DBA pre-application was less effective.
The removal of composite materials in the MTA and RMTA groups confirmed that the MTA itself darkened over time. This is particularly true for ProRoot MTA. Following the removal of MTA, dark discoloration spreading to the dentin was observed at the MTA-dentin interface. This finding is consistent with that of a previous study where staining of the complete dentin wall of the pulp chamber was observed in both WMTA and GMTA with staining penetrating into the dentinal tubule . Recent in vitro studies showed that by-products of MTA hydration accumulate on the surface of the material or on the MTA-dentin interface and intratubular dentin [18, 19]. Another study suggested that calcium released from the MTA reacted with phosphate ions in the tissue fluid, causing precipitation of the carbonated apatite . It was hypothesized that MTA constituents bound phosphate ions or plasma proteins in the dentinal fluid and that the byproducts were oxidized and transformed into pigmented byproducts . Pre-application of DBA protects dentin from MTA powders, which induce contamination or discoloration. Additionally, this process prevents the MTA from reacting with the ions in the tissue fluid and prohibits byproducts from penetrating the dentinal tubules.
The MTA and RMTA groups were used to test the effects of bleaching on MTA discoloration. In the MTA group, the removal of the discolored MTA prior to the application of the bleaching agent resulted in the reversal of discoloration. Upon bleaching of the remaining discolored dentin in teeth from the MTA group, the ΔE values were smaller than those of the MTA-B group. However, this difference was not significant. This comparison is also limited since the results depended on the timing and duration of the bleaching procedure in the MTA group. Thus, DBA pre-application is recommended for esthetic purposes; however, bleaching can also be used if DBA pre-application is not performed.
One recent study revealed that bleaching of MTA may destroy the MTA surface due to the acidic pH . Therefore, pre-application of DBA may be a superior treatment option prior to treatment with ProRoot MTA. Conversely, the use of DBA may be limited since the cytotoxicity of the DBA monomer may affect pulp tissue . DBA may also interfere with the calcium releasing capacity of MTA or its sealing ability. Further studies are required to evaluate the potential limitations, and although DBA may influence the pulp and MTA, detailed studies are lacking. Therefore, to prevent complications associated with DBA, methods should be devised to protect pulp before the application of DBA.