Review of the current status of tooth whitening with the walking bleach technique

Bleaching agents for the whitening of root-filled teeth.
Reports on the bleaching of discoloured non-vital teeth were first described in the middle of the 19th century. Chlorinated lime was recommended for the whitening of non-vital teeth (Dwinelle 1850). Later, oxalic acid (Atkinson 1862, Bogue 1872) and other agents such as chlorine compounds and solutions (Taft 1878/1879, Atkinson 1879, Harlan 1891), sodium peroxide (Kirk 1893), sodium hypochlorite (Messing 1971) or mixtures consisting of 25% hydrogen peroxide in 75% ether (pyrozone) (Atkinson 1892, Dietz 1957) were used to bleach non-vital teeth.
An early description of hydrogen peroxide application was reported by Harlan (1984/1885), Superoxol (30% hydrogen peroxide, H2O2) was also mentioned by Abbot (1918). Some authors proposed using light (Rosenthal 1911, Prinz 1924), heat (Brininstool 1913, Merrell 1954, Brown 1965, Stewart 1965, Caldwell 1967, Hodosh et al. 1970, Lemieux & Todd1981, Leendert et al.1984) or electric current (Kirk1889,Westlake 1895) to accelerate the bleaching reaction by activating the bleaching agent. In these cases, the bleaching medium was applied in the dental surgery so that the effect on the tooth was restricted to a relatively short period of time.
Prinz (1924) recommended using heated solutions consisting of sodium perborate and Superoxol for cleaning the pulp cavity. The first description of the walking bleach technique using a mixture of sodium perborate and distilled water was mentioned in a congress report by Marsh and published by Salvas (1938). In this procedure, the mixture was left in the pulp cavity for a few days and the access cavity was sealed with provisional cement. This concept of application of a mixture of sodium perborate and water to the tooth for a few days was re-considered again by Spasser (1961) and modified by Nutting & Poe (1963) who used 30% hydrogen peroxide instead of water to improve the bleaching effectiveness of the mixture. The use of an intracoronal filling of sodium perborate mixed with water or H2O2 continued till today, and has been described many times as a successful technique (Nutting & Poe 1967, Serene & Snyder 1973, Boksman et al. 1983, Rotstein et al. 1993, Attin & Kielbassa1995).
In addition, some authors described the successful clinical use of external bleaching of non-vital root-filled teeth with carbamide peroxide gels (Putter & Jordan 1989, Swift 1992, Frazier 1998). The whitening gel can be applied by a bleaching tray without an access opening, other reports recommended that the pulp cavity should be open during this bleaching therapy to enable the penetration of the gel into the discoloured tooth (Liebenberg1997, Carillo et al.1998). However, it should be taken into consideration that an unsealed access cavity enables bacteria and staining substances to penetrate into dentine, and that even with a sound root filling the passage of bacteria through the tooth can be observed (Barthel et al. 1999). Therefore, a restorative material such as glass-ionomer cement or composite should be used to seal the root filling at the orifice.
The decomposition of H2O2 into active oxygen is accelerated by application of heat, addition of sodium hydroxide or light (Hardman et al.1985, Chen et al.1993). H2O2-releasing bleaching agents are therefore chemically unstable. Only fresh preparations should be utilized, which must be stored in a dark, cool place. The thermocatalytic technique was proposed for many years as the best way of whitening non-vital root-filled teeth because of the high reactivity of H2O2 upon application of heat (Grossman1940, Brown1965, Ingle1965, Abramson et al.1966,Tewari & Chawla1972, Kopp1973, Howell 1980,Weine 1982, Boksman et al. 1984, Grossman et al. 1988, Hulsmann 1993). In this thermocatalytic procedure, 30-35% H2O2 is applied to the pulp cavity and heated by special lamps or hot instruments. In addition to this, cotton pellets impregnated with 30-35% H2O2 were often used as temporary fillings (Weisman 1963, Lowney1964, Cohen1968).
Sodiumperborate (sp.) in the form of mono-, tri- or tetrahydrate is used as a H2O2-releasing agent. Since 1907, sodium perborate has been employed as an oxidizer and bleaching agent especially in washing powder and other detergents. In1990, the world-wide consumption of sodium perborate was 600 000 tones. New formulae (Rompp Lexikon Chemie 1991) characterize sodium perborate in the solid aggregate state as a cyclic peroxoborate (Table 1).The whitening efficacy of sodium perborate mono-, tri- or tetrahydrate mixtures with either water or hydrogen peroxide is not different (Ari & Ungor 2002). H2O2 is released during the decomposition of perborate (Fig.1).The releasedH2O2 can generate different radicals or ions depending on pH value, light influence, temperature, existence of co-catalysts and metallic reaction partners (Feinman et al.1991, Goldstein & Garber 1995). Thus, perhydroxy radicals preferably arise in an alkaline environment resulting in effective bleaching agents (Goldstein & Garber 1995). These products are formed after the cleavage of H2O2 and are responsible for the oxidative and reductive and therefore the bleaching properties of H2O2.The radicals can crack unsaturated double bonds of long, coloured molecules or reduce the coloured metallic oxides like Fe2O3 (Fe3) to colorless FeO(Fe2). It should be appreciated that free radicals can cause oxidative effects to lipids, proteins and nucleic acids (Floyd 1997). This means that important cellular enzymatic reactions can be influenced (Moore et al.1989) and therefore radicals are suspected of being mutagenic and cancinogenic. The sensitivity of tissue increases with age and existence of inflammation, or high concentration of H2O2 and a long contact period of H2O2 to tissue (Floyd & Carney 1992, Li 1998). However, H2O2 is also synthesized by the human body itself, e.g. by neutrophil granulocytes for destruction of bacteria or by the human liver (Nathan 1987, McKenna & Davies1988). Some bacteria also produce H2O2 (Ryan &Kleinberg1995). The reare variety of human regulatory mechanisms that provide protection from oxidative reagents, e.g. peroxidases in saliva and plasma, glutathione peroxidase, catalases or the glutathione redox system (Tenovuo&Pruitt1984,Maddipati et al.1987, Gaetani et al. 1989, Bowles & Burns 1992, Sinensky et al. 1995,Tipton et al.1995, Floyd 1997). It can be concluded that no cancinogenic or cytotoxic risk results from appropriate use of H2O2 in bleaching therapy (Li1998).