An in vitro study of an alternative endodontic bleaching system

Abstract

Current techniques for intra-coronal bleaching of root filled teeth employ oxidative bleaching with hydrogen peroxide. However, concern over the potential for external cervical resorption in bleached root-filled has been expressed and recommendations have been made to limit the use of this agent (Friedman et al, 1988; Madison and Walton, 1990). In the textile industry conventional bleaching of wool and cellulose pulp also employs oxidative bleaching with hydrogen peroxide. There are many reports however of a combined oxidative-reductive bleaching process using hydrogen peroxide and thiourea, which produces improved whiteness of wool and cellulose pulp than hydrogen peroxide alone. In biological systems thiourea is a highly cell permeable scavenger of hydrogen peroxide and hydroxyl radicals (Stahl, Pan, and Longhurtst, 1993). The generation of hydroxyl radicals during bleaching of root-filled teeth has been demonstrated recently (Dahlstrom, Bridges and Heithersay, 1993). Hydroxyl radicals are extremely reactive and can destroy connective tissue products and may be a causative factor in bleaching related invasive cervical resorption (Dahlstrom, Bridges and Heithersay, 1993). The development of a combined oxidative-reductive-bleaching protocol for rootfilled æeth using hydrogen peroxide and thiourea offers the possibility of producing superior bleaching and potentialty may have the added benefit of removing damaging hydroxyl radicals. The aim of the present study was to investigate the effectiveness of combining thiourea and hydrogen peroxide to bleach root-filled teeth discoloured by blood In the first part of the study the effectiveness of various amine bleaching agents was compared wíth30Vo hydrogen peroxide by quantitative and qualitative analysis of the abso¡ption spectra of haemoglobin and methaemoglobin. The amine bleaching agents studied were 0.1 M aqueous thiourea, 0.1 M acidified thiourea and 0.1 M thiourea dioxide. The controls for the spectrophotometric studies were 0.01 M hydrochloric acid and 30Vo w/v hydrogen peroxide. The changes in the absorption spectra of haemoglobin and methaemoglobin produced by these agents were evaluated. In summary, 30Vo hydrogen peroxide rendered the haemoglobin and methaemoglobin colourless whereas the amine bleaching agents only partially decolourised these pigments in solution. In the second part of the study extracted premolar teeth were discoloured by blood using an adaptation of the technique of Freccia and Peters (1982). Stained dentin samples were then subjected to 23 different bleaching regimens. The change in the colour of the blood-stained dentin samples was measured at each stage of the bleaching process with a photometer and a reflection densitometer. Comparisons of the different bleaching treatments was made using a method of Least Significant Difference and/or Analysis of Variance. These methods were applied separately to the data obtained using the photometer and that obtained using the reflection densitometer. It was concluded that the bleaching regimens which employed the sequential use of acidified thiourea or aqueous thiourea and 307o hydrogen peroxide were at least as effective at bleaching blood-stained dentin as 307o hydrogen peroxide alone. Furthermore the reducing agent thiourea dioxide alone was as effective at bleaching blood-stained dentin as 30Vo hydrogen peroxide. The recognition that bleaching discoloured teeth is a chemical process which can be achieved by both reducing agents and oxidising agents offers the possibility of developing new clinical bleaching protocols. products.