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.
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.
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).
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.