The ability of thiourea to scavenge hydrogen peroxide and hydroxyl radicals during the intra-coronal bleaching of bloodstained root-filled teeth

Abstract

Background: Hydrogen peroxide, an agent used in the intra-coronal bleaching of root-filled teeth for over a century, has been shown to diffuse from the pulp chamber to the outer root surface. Furthermore, it has been demonstrated that destructive hydroxyl radicals, the by-products of the bleaching process, have been detected on the external root surface. The control of such diffusion may be of importance in minimizing the risk of invasive cervical resorption (ICR) which has been linked to intra-coronal bleaching of discoloured root-filled teeth using hydrogen peroxide. The aims of the present in vitro study are to quantify the diffusion of hydrogen peroxide and hydroxyl radicals to the outer root surface following intra-coronal bleaching, and to evaluate the ability of thiourea incorporated into the bleaching protocol to scavenge residual hydrogen peroxide and hydroxyl radicals. Methods: Thirty-five single rooted premolar teeth with intact cementum at the cemento-enamel junction were used in this project. Thirty teeth were stained with red blood cells and root-filled with gutta-percha and AH26. The five unstained teeth were root-filled and constituted a negative control (Group 1). The stained teeth were divided equally into the following experimental groups and subjected to various intra-coronal bleaching regimes: Group 2 – ‘walking bleach’ with 20μl 30 per cent w/w hydrogen peroxide; Group 3 – 20μl 30 per cent w/w hydrogen peroxide and thermocatalytically activated; Group 4 – 20μl acidified thiourea; Group 5 – 20μl acidified thiourea and 20μl 30 per cent w/w hydrogen peroxide; Group 6 – 20μl acidified thiourea and 20μl one per cent sodium hypochlorite; Group 7 – 20μl acidified thiourea, 20μl one per cent sodium hypochlorite and 20μl 30 per cent w/w hydrogen peroxide. The reaction products of the bleaching process were quantified at the outer root surface using high performance liquid chromatography and electrochemical detection (HPLC-ECD).Results: Results showed that hydrogen peroxide used alone in Groups 2 and 3 was able to be detected at the outer root surface in 100 per cent of the samples, and that the presence of the hydroxyl radical generated in both groups was detected in equal amounts (P<0.05). When thiourea was incorporated into the bleaching protocols in Groups 5–7, it was shown to scavenge both hydrogen peroxide and hydroxyl radicals to a significant degree (P<0.05). Conclusions: Acidulated thiourea is an effective scavenger of residual hydrogen peroxide and hydroxyl radicals generated during the intra-coronal bleaching of bloodstained root-filled teeth.