T. Attin, F. Paque, F. Ajam & A. M. Lennon
Department of Operative Dentistry, Preventive Dentistry and Periodontology, Georg-August Universitat Gottingen, Gottingen, Germany.
Discoloured teeth, especially in the anterior region, can result in considerable cosmetic impairment. Besides invasive therapies, such as crowning or the placement of veneers, the whitening of teeth is an alternative therapeutic method. In contrast to crowning or veneering, whitening of teeth is relatively non-invasive and conserves dental hard tissue. Vital teeth can be whitened by the nightguard vital bleaching technique utilizing carbamide peroxide gels as the bleaching medium (Fasanaro 1992, Haywood 1992a, b, Attin & Kielbassa 1995, Attin1998). The whitening of root-filled teeth can be carried out by internal whitening treatment (walking bleach technique) (Weisman 1968,Vernieks & Geurtsen 1986, Arens 1989,Weiger 1992, Bose & Ott 1994, Beer 1995, Ernst et al.1995, Glockner et al.1997). This review of the walking bleach technique describes the recommended procedures to help reduce the risks of complications and to ensure a successful bleaching therapy.
Indications for the walking bleach technique.
Dissemination of blood components into the dentinal tubules caused by pulp extirpation or traumatically induced internal pulp bleeding is a possible reason for discolouration of non-vital teeth (Arens 1989, Goldstein & Garber1995). A temporary colour change of the crown to pink can often be detected initially. Then, blood degradation products such as haemosiderin, haemin, haematin and haematoidin release iron during haemolysis (Guldener & Langeland1993).The iron can be converted to black ferric sulphide with hydrogen sulphide produced by bacteria, which causes a grey staining of the tooth. Apart from blood degradation, degrading proteins of necrotic pulp tissue may also cause discolouration. If the access cavity is prepared inappropriately, pulp tissue can be left in the pulp chamber in the pulp horns (Brown 1965, Faunce 1983, Kielbassa & Wrbas 2000), which maybe another reason for discolouration. Coronal discolouration of root-filled teeth can also be caused by some root-filling materials (van der Burgt & Plaesschaert 1985, 1986, van der Burgt et al. 1986a,b, Davis et al. 2002) or tetracycline-containing medicaments (e.g. Ledermix, Lederle Pharmaceuticals, Wolfrathausen, Germany) (Kim et al. 2000). These discolourations arise when remnants of root-filling materials or medicaments are left in the pulp chamber and the staining substance infiltrates the dentinal tubules. Although no penetration of the dental enamel takes place, there is an observable difference of colour on the tooth (Vogel 1975). Discolouration caused by root-filling materials can be treated by bleaching depending on the staining substance (van der Burgt & Plaesschaert 1986). Discolouration caused by metallic ions (silver cones, amalgam) cannot be removed by whitening treatments (Glockner & Ebeleseder1993).
Internal discolouration of teeth represents the primary indication for whitening of root-filled teeth (Arens 1989,Werner1989, Glockner & Ebeleseder1993). In addition, there are reports and studies on the successful use of the walking bleach technique for correction of severely discoloured teeth caused by incorporation of tetracycline in the dental hard tissue during pre-eruptive maturation of teeth (Hayashi et al. 1980, Abou-Rass 1982, 1998, Fields 1982, Walton et al. 1983, Lak eet al. 1985, Anitua et al. 1990, Aldecoa & Mayordomo 1992). This procedure starts with intentional devitalization and root-canal treatment of the tooth in order to enable application of the bleaching agent into the pulp chamber. As the methods of intentional devitalization and root canal treatment have risks, the advantages and disadvantages of this therapy should be assessed. Restorative treatment options such as ceramic veneers should be considered as an alternative procedure.
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).
Table 1. Old and new formulae (as cyclic peroxoborate) of sodiumperborate.
Figure 1. After adding water to sodium peroxoborate, H2O2 is formed that is further decomposed into different radicals or ions.
The chemical reaction mentioned above emphasizes that release of H2O2 by mixing sodium perborate and water is achieved without supplementary addition of H2O2. Several studies have reported bleaching effectiveness by comparing mixtures of sodium perborate with distilled water or H2O2 in different concentrations. Rotstein et al. (1991d, 1993) and Weiger et al. (1994a) did not report any significant difference in the effectiveness between sodiumperborate mixed with 3-30%H2O2 and the sodiumperborate-distilled water mixture. However, the whitening effect of the second mixture can take longer, so that more frequent changes of the bleaching agent may be necessary. The shade stability of teeth treated by a mixture of perborate and water is as high as the shade stability of teeth in which a mixture of sodium perborate with 3 or 30% H2O2 was used (Rotstein et al. 1993, Ari & Ungor 2002). Other surveys found that mixing sodiumperborate with 30%H2O2wasmore effective than mixing with water (Ho & Goerig1989,Warren et al. 1990). Freccia et al. (1982) showed that the walking bleach technique with a mixture of 30% H2O2 and sodiumperborate was as effective as the thermocatalytic technique.
Complications of the walking bleach technique are contributed to an acidic pH of the bleaching reagent; 30% H2O2 has a pH value between 2 and 3.When 30% H2O2 is mixed with sodium perborate in a ratio of 2 :1 (g mL_1), the pH of this mixture is alkaline. If further 30% H2O2 is added, it becomes acidic (Kehoe 1987, Rotstein & Friedman 1991). Weiger et al. (1993) tested the pH value of mixtures consisting of 2 g sodiumperborate and 1mL of 10-30% H2O2 or distilled water. Initially, a neutral or weak alkaline pH for all compositions was apparent, e.g. the mixture of 30% H2O2 and perborate showed an initial pH value of 7.0-8.7 depending on the perborate used (mono-, tri- or tetrahydrate). It was also shown that the pH significantly in creased with decreasing concentration of H2O2. The highest initial pH was observed when sodiumperborate was mixed with water. Within a day, a distinct increase of the pH value of 9- 11was achieved. This is true for a sodiumperborate suspension mixed with water or H2O2. This increase in pH is desirable because the whitening effectiveness of buffered alkalineH2O2 is significantly higher than the effect of unbuffered H2O2 (Frysh et al.1995).
Other H2O2-separating agents such as sodium percarbonate (2Na2CO3_H2O2) can be used to bleach discoloured teeth. Suspensions consisting of sodium percarbonate and water or 30%H2O2 had a good bleaching effect on teeth which were artificially stained in vitro by iron sulphide (Kaneko et al. 2000). However, clinical studies using sodium percarbonate have not been reported.
Aldecoa & Mayordomo (1992) described good clinical success rates when using a mixture consisting of sodium perborate and10%carbamide peroxide gel. This suspension was used as a temporary intracoronal filling after application of a regular walking bleach paste with sodium perborate and H2O2. The authors claimed that this procedure led to long-term stability of the tooth whitening therapy.