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Review of the current status of tooth whitening with the walking bleach technique
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Published on 02/17/2010
 

T. Attin, F. Paque, F. Ajam & A. M. Lennon
Department of Operative Dentistry, Preventive Dentistry and Periodontology, Georg-August Universitat Gottingen, Gottingen, Germany.

Internal bleaching procedures such as the walking bleach technique can be used for whitening of discoloured root-filled teeth. The walking bleach technique is performed by application of a paste consisting of sodium perborate-(tetrahydrate) and distilled water (3% H2O2), respectively, in the pulp chamber. Following a critical review of the scientific literature, heating of the mixture is contra-indicated as the risk of external cervical resorption and the formation of chemical radicals is increased by application of heat. An intracoronal dressing using 30% H2O2 should not be used in order to reduce the risk of inducing cervical resorption. This review provides advice based on the current literature and discusses how the walking bleach technique can lead to successful whitening of non-vital root-filled teeth without the risks of side-effects.


Introduction.

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.

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.

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.


Influence of bleaching agents used for the walking bleach technique on tooth tissue.

A 30% H2O2 irrigation at both 37 and 50C temperature leads to a reduction of the surface microhardness of enamel and dentine. However, the microhardness of teeth was not influenced when treated by a mixture of sodium perborate and 30% H2O2 carried out under the same temperature conditions (Lewinstein et al. 1994). Apart from an attack on the inorganic components of teeth, the denaturation of collagen is presumed to be the mode of action of bleaching agents (Lado et al.1983, Ramp et al. 1987, Rotstein et al. 1992a, 1996). Generally, changes of composition or structure of the inorganic components of teeth correlate with a shift of the calcium: phosphate ratio of apatite. Therefore, Rotstein et al. (1996) determined the calcium:phosphate ratio in enamel, dentine and cementum of teeth. They observed no significant change in the calcium:phosphate ratio in enamel, dentine and cementum after application of a suspension containing sodium perborate and water. In contrast to this, the calcium:phosphate ratio in all the three components of teeth reduced significantly when 30% H2O2 was used. These findings do not correspond to the results of Ruse et al. (1990) who found no change in the calcium and phosphate content in enamel after application of 35% H2O2. Further research on dentine and cementum showed that loss of calcium was significantly higher after use of 30% than after use of 3% H2O2, or a mixture of sodium perborate and 3% H2O2 or distilled water, respectively. There was no difference between a pure 30% H2O2 solution and a suspension consisting of sodiumperborate and 30%H2O2 (Rotstein et al. 1992a). However, when mixing sodium perborate with 3% instead of 30% H2O2, a 10-12-fold reduction of dentine and cementum solubility is achieved (Rotstein et al.1992a).
Scanning electron microscope photographs show a precipitate formed on the surface of enamel specimens after contact with a 35% H2O2 solution for several minutes. The precipitate is intensified and the enamel surface becomes more porous by subsequent acid etching with 37% H3PO4. According to these authors, these changes to enamel could have an influence on the adhesion of composite restorations (Titley et al.1988a).
It was hypothesized that alterations in dentine permeability owing to whitening therapies may result in pronounced bacterial contamination of dentine. This contamination may contribute to the occurrence of external resorptions (Cvek& Lindvall1985). Heling et al. (1995) showed that the dentine permeability of Streptococcus faecalis was significantly higher after application of 30% H2O2 than after use of a mixture of sodium perborate and water. The latter suspension did not change the dentine permeability for the microorganisms in comparison to the control group treated with water. The authors concluded that the low pH value of the 30% H2O2 solution led to an acid-induced enlargement of the dentinal tubules.
Whether bleaching increases the brittleness of teeth is of importance to the clinical outcome. Unfortunately, there are only a few reports on this topic. Seghi & Denry (1992) observed a 30% reduction in fracture resistance of enamel when 10% carbamide peroxide gel (3.6% H2O2) was applied in the vital bleaching therapy. However, according to another study, no increase in the brittleness of dentine could be detected by using a mixture of sodium perborate with 30% H2O2 (Glockner et al. 1995). In particular, 30% hydrogen peroxide had detrimental effects on the biomechanical properties of dentine, such as tensile and shear strength (Chng et al. 2002). These adverse effects are significantly lower for mixture of sodium perborate with either water or 30% hydrogen peroxide. Generally, no fractures of whitened teeth were reported in studies on internal bleaching (Brown1965, Howell1980,1981, Feiglin1987,Holmstrup et al. 1988, Anitua et al. 1990, Aldecoa & Mayordomo 1992, Glockner et al. 1995, 1999, Abou-Rass 1998). However, it should be appreciated that teethcanbeweakened by removal of stained dentine. An increased risk of fracture may be expected when the tooth is already weakened by tooth tissue loss (Geurtsen & Gunay 1995). Therefore, severely discoloured dentine should be removed cautiously to prevent further weakening.

Clinical performance of the walking bleach technique.
Preliminary treatment.

It is important to determine whether discolouration of the tooth is caused by internal staining. The surface of the tooth should be cleaned thoroughly to estimate the degree of external discolouration. The patient should be informed that the results of bleaching therapies are not predictable and that complete recovery of colour is not guaranteed in all cases (Baratieri et al. 1995). Moreover, information should be given about the different treatment stages, possible complications and the fact that application of the bleaching agent often needs to be repeated for obtaining optimal results.

Examination of root fillings, existing restorations and tooth substance.
Prior to treatment, a radiograph should be taken to check the quality of the root filling. A thoroughly cleaned root canal and application of a dense root filling are prerequisites for a successful outcome of root-canal treatment. A root-canal filling should also prevent coronal-apical passage of microorganisms or other substances, such as bleaching agents, which might have detrimental effects on the apical tissue. Therefore, a deficient root filling should be replaced prior to bleaching therapy and the filling material should be completely set before the beginning of the bleaching therapy. Deficient restorations should be identified before bleaching therapy and should be replaced, carious lesions should be restored. If the restorations are only discoloured, they should be renewed at the end of treatment with materials matching the whitened tooth colour. Colour of the tooth resulting from bleaching cannot be reliably predicted and this makes it difficult to select the correct shade of filling material prior to bleaching. Therefore, it is advisable to either apply temporary materials (for carious lesions or replacement of deficient fillings) before treatment or to replace restorations after completion of bleaching. Generally, it is important that the tooth is restored with high quality fillings in order to ensure the effectiveness of the bleaching agent and to avoid leakage of the agent into the oral cavity.

Preparation of the pulp cavity.
Before preparation of the access cavity, a rubber dam should be applied to protect the adjacent structures. The access cavity should be shaped in such a way that remnants of restorative materials, root-filling materials and necrotic pulp tissue are removed completely. Additional cleaning of the cavity with 1-3% sodium hypochlorite for removal of difficult, accessible remnants of pulp tissue is recommended (Attin & Kielbassa 1995). In some reports, conditioning of the dentinal surface of the access cavity with 37% H3PO4 is suggested in order to remove the smear layer (Hulsmann 1993, Beer 1995). Others advise cleaning the pulp cavity with alcohol before application of the bleaching agent so that the dentine becomes dehydrated (Werner 1989, Ernst et al. 1995). It is assumed that bleaching agents are able to penetrate more easily into the dentine and therefore are more effective following pretreatment. However, studies have shown that removal of the smear layer with H3PO4 does not improve the bleaching effectiveness of either sodium perborate or of high concentrated H2O2 (Casey et al.1989, Horn et al.1998).However, the pretreatment of dentine with acid may lead to an increased diffusion of bleaching agents into the periodontium, as these agents are able to penetrate the dentine easily (Fuss et al.1989). Therefore, it may not be advisable to remove the smear layer from the dentine of the pulp chamber prior to bleaching.

Cervical seal.
The root filling should be reduced 1-2 mm below the enamel-cementum junction. This can be controlled by using a periodontal probe placed into the pulp cavity. Root fillings do not effectively prevent diffusion of bleaching agents from the pulpal chamber to the apical foramen (Costas & Wong 1991, Smith et al.1992). Therefore, sealing the root filling with glass-ionomer cement or composite is essential. Rotstein et al. (1992b) demonstrated that a 2 mm layer of glass-ionomer cement was required to prevent penetration of a 30% concentrated H2O2 solution into the root canal. The seal material should reach the level of the epithelial attachment or the cemento-enamel junction, respectively, to avoid leakage of bleaching agents in the periodontium (Steiner & West 1994). The proximal cemento-enamel junction curves in an incisal direction. A flat barrier, level with the labial cemento-enamel junction, leaves a large portion of the proximal dentinal tubules unprotected. The barrier location should be determined by probing the level of the epithelial attachment at the mesial, distal and labial aspect of the tooth. The intracoronal level of the barrier is placed 1mm incisal to the corresponding external probing of the attachment. With this method (Steiner &West1994), the coronal outline of the attachment defines an internal pattern for the shape and location of the barrier. However, the impact of the bleaching agents on the discoloured dentine should not be hampered by the cervical seal. If bleaching of the cervical region of the tooth is required a stepwise reduction of the labial part of the seal and use of a mild bleaching agent is recommended for the final dressings (Rotstein et al.1992b).

Application of the bleaching agent.
Sodium perborate (tetrahydrate) mixed with distilled water in a ratio of 2 :1 (g mL_1) is a suitable bleaching agent as mentioned above (Weiger1992). Incase of severe discolouration,3%H2O2 can be applied in place of water. The use of 30%H2O2 is not appropriate because of possible risks such as cervical resorptions (Friedman et al. 1988, Kinomoto et al. 2001). The bleaching agent can be applied with an amalgam carrier or plugger and should be changed every 3-4 days. Successful bleaching becomes apparent after one to four visits. The patients should be instructed to evaluate the tooth colour on a daily basis and return when the bleaching is acceptable in order to avoid over-bleaching (Geurtsen & Gunay 1995).

Temporary filling.
Before application of the bleaching agent, the enamel margins of the cavity should be etched with 37% H3PO4 in order to enable an adhesive temporary filling. The walking bleach technique requires a sound seal around the access cavity with composite or compomer restorative to ensure its effectiveness and to avoid leakage of the bleaching agent into the oral cavity. This cannot be guaranteed if temporary filling materials are used (Waite et al.1998). In addition, a good seal prevents re-contamination of the dentine with microorganisms and staining substances.
It is often difficult to insert filling material on to soft sodium perborate mixture or a cotton pellet. A cotton pellet, that is covered with a bonding material, placed on the sodium perborate mixture and then light-cured, simplifies the application of the temporary filling material.
The temporary filling is only attached to the enamel margins of the access cavity. In this phase of treatment, the pulp chamber is filled with the sodium perborate mixture and not with an adhesively attached restorative material, so that no internal stabilization of the tooth is provided. Therefore, the patient should be informed about the increased risk of fracture (Baratieri et al. 1995), and occlusal adjustment may be required in order to avoid overloading the tooth.


Restoration of the access cavity and postoperative radiograph.

Following bleaching, the access cavity should be restored with a composite which is adhesively attached to enamel and dentine. This avoids re-contamination with bacteria and staining substances and improves the stability of the tooth. A sound restoration with sealed dentinal tubules is a prerequisite to create a successful bleaching therapy (Baratieri et al. 1995, Abou-Rass 1998). Some authors (Glockner et al. 1997, Abou-Rass 1998) recommend the use of composites with light colours, in case the bleaching therapy does not result in complete success. The adhesion of composites to bleached enamel and dentine is temporarily reduced (Titley et al. 1988b, 1992, Murchison et al. 1992, Garc|. a-Godoy et al. 1993, Toko & Hisamitsu 1993, Dishman et al. 1994, Josey et al. 1996, Swift & Perdigao1998). Glass-ionomer cement also has a reduced adhesion to bleached dentine (Titley et al. 1989). It is assumed that remnants of peroxide or oxygen on the surface or pores of the tooth inhibit the polymerization of composite (Torneck et al.1990, Dishman et al. 1994). It is less likely that changes in the enamel structure may influence composite adhesion (Ruse et al. 1990, Torneck et al. 1990). The formation of composite tags in bleached enamel is less regular and distinct than in unbleached enamel (Titley et al. 1991). This could explain why access cavities of bleached teeth, which are filled with composite, occasionally show marginal leakage (Barkhordar et al.1997). The negative influence of H2O2-containing bleaching agents on adhesion can be clearly reduced by moderate beveling of the cavity before acid etching (Cvitko et al.1991). The same can be achieved by pretreatment of enamel with dehydrating agents such as alcohol and the use of acetone-containing adhesives (Kalili et al.1993, Barghi & Godwin 1994). To dissolve remnants of peroxide, the cavity can also be cleaned with catalase or sodium hypochlorite (Rotstein 1993, Attin & Kielbassa 1995). A contact time of at least 7 days with water is recommended to avoid the reduction of adhesion of composites to enamel (Torneck et al. 1991, Adibfar et al.1992,Titley et al.1993). Optimal bonding to prebleached dental hard tissue could be achieved after a period of about 3 weeks (Cavalli et al. 2001, Shinohara et al. 2001). During this period, the colour stability of the bleached tooth should be controlled and a calcium hydroxide dressing should be placed in the pulp cavity for buffering the acidic pH which can occur on cervical root surfaces after intracoronal application of bleaching agents (Kehoe 1987, Baratieri et al. 1995). The calcium hydroxide suspension temporarily placed into the pulp chamber after completion of the bleaching procedure does not interfere with the adhesion of composite materials used for final restoration of the access cavity (Demarco et al. 2001).
A radiograph of the bleached tooth should be taken after treatment in order to diagnose cervical resorption as early as possible. No information is available in the literature regarding the time intervals for taking postoperative radiographs after bleaching. In accordance with the recommendations for postoperative radiographic controls of endodontically treated teeth, given by the European Society of Endodontology (1994), a radiographic inspection within the first year after bleaching is suggested.

Prognosis and complications during internal bleaching of non-vital root-filled teeth.
Colour stability.

Despite many clinical reports, there are few evidence based studies on aesthetic dentistry (Niederman et al. 1998). Most reports present initial results following bleaching (Table 2). Complete colour matching of the bleached tooth with the adjacent teeth is regarded as an optimal result. However, darkening after internal bleaching can be observed occasionally (Friedman 1997) that is caused presumably by diffusion of staining substances and penetration of bacteria through marginal gaps between the fillings and the tooth. It is worth noting that the opinion of the patient regarding the success of the therapy is often more positive than the opinion of the dentist (Anitua et al. 1990, Glockner et al. 1995,1999).
Some have presumed that teeth with a discolouration existing for several years do not respond as well to bleaching therapy as teeth that are stained for a short period of time (Brown1965, Howell1980). Howell (1981) could not confirm this claim. Furthermore, it is uncertain whether darkening after bleaching is more probable when the tooth is heavily or mildly discoloured (Brown 1965, Howell 1980, 1981). Discolouration caused by restorative materials has a dubious prognosis (van der Burgt & Plasschaert 1986). Certain metallic ions (mercury, silver, copper, iodine) are extremely difficult to remove or alter by bleaching. No scientific study has yet directly compared the bleaching efficacy in differently (for example grayish or yellowish) discoloured non-vital teeth. However, Brown (1965) reported that trauma or necrosis induced discolouration can be successfully bleached in about 95% of cases compared to lower values for teeth discoloured as a result of medicaments or restorations (Brown 1965). There are differing opinions on whether teeth that respond rapidly to bleaching have a better long-term colour stability (Howell 1981, Feiglin 1987, Holmstrup et al. 1988). Some studies report that stained teeth can be more easily bleached in young patients than in older patients (Chandra1967, Hodosh et al.1970, Feiglin1987, Glockn er et al.1996), as the wide open dentinal tubules of young teeth enable a better diffusion of the bleaching agent. However, not all studies confirm the age dependency of bleaching success (Brown1965,Howell1981).Teeth with internal discolouration caused by root-canal medicaments, root-filling materials or metallic restorations such as amalgam have a poor prognosis regarding bleaching success (Brown1965).Anterior teeth with several approximal restorations occasionally show a less optimal result than teeth with a palatal access cavity only (Glockner et al.1996,1999). This may be because of the fact that composites cannot be bleached (Monaghan et al.1992). In these cases, replacing the existing restorations after finishing the whitening treatment is recommended in order to get an optimal result.

Table 2. Studies concerning the success rate of internal whitening treatment of non-vital root-filled teeth.

Studies concerning the success rate of internal whitening treatment of non-vital root-filled teeth


Complications.

Occurrence of external cervical resorption is a serious complication following internal bleaching procedures (MacIsaac & Hoen 1994, Friedman 1997). Cervical resorption is mostly asymptomatic and is usually detected only through routine radiographs (Trope 1997). Heithersay (1999) analysed 257 teeth in 222 patients with cervical resorptions and discovered that in24.1%of cases the resorption was caused by orthodontic treatment, 15.1% by dental trauma, 5.1% by surgery (e.g. transplantation or periodontal surgery) and 3.9% by intracoronal bleaching. A combination of internal bleaching procedures with one of the other causes is responsible for13.6% of cervical resorption cases.

Table 3. Occurrence of cervical resorption after internal bleaching procedures in clinical studies and case reports.

Occurrence of cervical resorption after internal bleaching procedures in clinical studies and case reports

Table 3 provides an overview of clinical studies and case reports in which the occurrence of cervical resorption was observed, partly diagnosed many years after internal bleaching was applied (Harrington & Natkin 1979, Lado et al. 1983). However, experimental animal studies showed histological signs of resorptions only 3 months after internal thermocatalytic bleaching therapy with heated 30% H2O2 (Rotstein et al.1991a, Heller et al. 1992). One month after bleaching, no changes in the tooth substance could be detected. Cervical resorptions often proceed in an asymptomatic way, however, sometimes swelling of the papilla or percussion sensitivity of the bleached teeth can be observed (Harrington & Natki n 1979, Lado et al. 1983). Table 2 shows that teeth that were root-filled as a result of trauma often show cervical resorption. Furthermore, the studies and case reports indicate that application of heat (thermocatalytic method), lack of a cervical seal and the use of 30% H2O2 are associated with the occurrence of cervical resorption. In an experimental animal study, Madison & Walton (1990) showed that the thermocatalytic technique supported the development of external resorption. However, the walking bleach technique applied in that study with a sodium perborate-H2O2 solution did not cause cervical resorption even 1 year after bleaching. This observation may be explained by the fact that sodium perborate inhibits the function of macrophages, because macrophages stimulate both osteoclastic bone resorption and destruction of dentine and cementum induced by lytic processes in periodontal tissue (Jime. nez-Rubio & Segura1998).
The mechanisms responsible for resorption in bleached teeth have not yet been adequately explained. It has been proven that formulations using either 30% H2O2 alone or in combination with sodium perborate are more toxic for periodontal ligament cells as compared to a perborate-water suspension (Kinomoto et al. 2001). Lado et al. (1983) presumed that application of bleaching agents led to denaturation of dentine in the cervical region of tooth. According to the authors, this denatured dentine induces a foreign body reaction, although this could not be verified. Other authors claim that diffusion of H2O2 via dentine causes irritation in the periodontium, which later results in bacterial colonization of the open dentinal tubules (Cvek& Lindvall 1985). This could trigger inflammation of adjacent tissues and external resorption. Harrington & Natkin (1979) suspected that H2O2 diffuses into the periodontium via dentinal tubules and directly induces an inflammatory resorptive process. It is known that H2O2 can diffuse through dentine (Pashley & Livingston 1978, Wang & Hume 1988, Hanks et al.1994) and radicals and the low pH value of highly concentrated H2O2 can be considered as tissue damage inducing factors (Friedman et al.1988).Table 3 reveals that patients who had bleaching therapy at a young age often have external resorption. A possible explanation is that H2O2 can more easily penetrate into the periodontium because of wide dentinal tubules in young teeth (Schroder1992).Increasing permeability of dentine is associated with both decreasing dentine thickness and high surrounding temperature (Outhwaite et al.1976). Application of heat leads to widening of dentinal tubules and facilitates diffusion of molecules in the dentine (Pashley et al. 1983). This explains the increasing dissemination of H2O2 into dentine with rise in temperature (Rotstein et al. 1991c). Moreover, application of heat resulted in generation of hydroxyl radicals from H2O2 that are extremely reactive and have been shown to degrade components of connective tissue (Dahlstrom et al. 1997). Diffusion of H2O2 to cervical tissues is also increased after pretreatment of dentine in the pulp chamber with 5% NaOCl (Barbosa et al. 1994). In addition, the penetration of H2O2 into the cervical region can be facilitated by cervical defects or special morphological patterns at the enamel-cementum junction (Rotstein et al. 1991b, Koulaouzidou et al. 1996, Neuvald & Consolaro 2000). According to Rotstein (1991), lack of root cementum resulted in diffusion of up to 82% of the H2O2 (30% concentration) which was applied in the pulp chamber. However, dissemination of H2O2 into dentine cannot be totally prevented using mixtures of sodium perborate with 30% H2O2 or water. The amount of H2O2 diffusion is significantly lower when a mixture of sodium perborate-tetrahydrate and water is used, than in case of application of 30% H2O2 mixed with different sodium perborates (Weiger et al. 1994b). Even if there is low-H2O2 diffusion into adjacent tissues when sodium perborate solutions are applied, a sound cervical seal should be assured in order to prevent penetration of H2O2 through dentine (see above).
Tooth extraction is often inevitable in cases of severe external root resorption (Goon et al. 1986, Latcham 1986). Inflammatory osteolytic lesions have a low-pH value that is optimal for hard tissue resorption (McCormick et al.1983). Root-canal dressings consisting of calcium hydroxide are able to induce a higher pH in dentine (Tronstad et al. 1981, Webber 1983). Tronstad et al. (1981) assumed that reparative formations of hard tissue are supported by this treatment. Cases have shown that an intracoronal dressing with calcium hydroxide can sometimes prevent progression of external resorption (Montgomery 1984, Gimlin & Schindler 1990). However, on the radiograph, only osseous regeneration of the defect and no dental hard tissue regeneration could be detected.
Another possible therapy for external cervical resorption is orthodontic tooth extrusion, followed by restoration of the tooth with a post-retained crown (Latcham 1991, Emery1996). Cervical resorption can also be treated by direct restorations after gaining surgical access to the defect (Meister et al. 1986, Friedman 1989, Al-Nazhan1991).


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