δ-Bismuth Oxide Responsible for Tooth Discolouration-A Laboratory Investigation

To investigate the phase changes of bismuth oxide in contact with sodium hypochlorite responsible for tooth discolouration.

Bismuth oxide (monoclinic α-phase; C) was mixed with sodium hypochlorite at 20°C, 37°C, and 60°C (B20, B37, B60) for a period of 24 weeks with weekly refreshing of solutions. The products were imaged by scanning electron and optical microscopy and characterized by thermographic analysis (TGA), phase analysis by X-ray diffraction (XRD) using Bragg Brentano geometry and Pilatus detector, infrared spectroscopy (FT-IR), and X-ray absorption fine structure (XAFS).

The interaction of bismuth oxide with sodium hypochlorite resulted in a change in microstructure and colour. The thermographic assessment showed a change in mass (5%-10% weight change) and colour reversal to the yellow bismuth oxide at ~450°C. Phase changes dependent on temperature were demonstrated with δ-bismuth oxide, sodium bismuthate and bismuth oxychloride formed as by-products at the different temperatures.

The δ-phase bismuth oxide formation led to the material darkening which will cause tooth discolouration in a clinical setting. Due to the phase changes, the material chemistry after the interaction is different from that of the material placed in the tooth. The by-products of the reaction have not been tested for use in patients. It is recommended to ban the use of bismuth oxide from dental materials and other clinical use due to its instability. The clinical guidance for endodontic treatment needs to be changed to reflect this.