Results - References.
The pH values of the surrounding medium became acidic in all groups. The mean pH values for the four groups are presented in Table 2. The base line pH values for the controls were 7.21, 7.20, 7.19 and 7.21, respectively. One day (24 h) following the bleaching procedure a pH decrease was recorded in all groups of teeth, which continued during days 2 and 4. At days 10 and 15, the pH values in groups B and D in which Ca(OH)2 was placed as a supplementary barrier increased. However, the two-way anova test revealed that no statistically significant differences existed between the four experimental groups at all experimental days (P ј 0.790) with respect to the measured pH. The mean values, standard error and 95% confidence intervals for the measured pH according to anova are shown in Table 3. Discussion The results of this in vitro study showed that the pH value of the surrounding medium in all groups of teeth treated with the thermocatalytic technique became more acidic. Our findings are in agreement with the report by Kehoe (1987). The acidic pH conditions that have been found to exist in cervical root surfaces following bleaching could create a favourable environment for osteoclastic activity, as polymorphonuclear leucocytes and osteoclasts function best at an acidic pH. If these conditions occurred in vivo, inflammatory root resorption could be initiated. It should be noted that Kehoe (1987) did not use an intracoronal isolating barrier, and also did not explain how he was able to determine the exact position of the CEJ in order to control the seepage of the bleaching materials to the surrounding medium. In an attempt to minimize the variations in dentine permeability and cementum intergrity owing to ageingo rmechanical injuries during tooth extraction, the teeth used in our study were a traumatically extracted from young adults for orthodontic reasons. Furthermore, artificial defects were created on the cervical root surface, whilst all other surfaces were covered with wax and nail polish. In this way, we simulated conditions where the radicular dentine is devoid of cementum, and prevented the leakage of the bleaching agent through any undetected defects of the CEJ.
Table 2. Summary of the mean pH values for the four groups at the times recorded.
Table 3. Mean values, standard error and 95% confidence intervals for all experimental groups according to ANOVA.
The presence of Ca(OH)2 as an additional barrier did not significantly affect the pH values recorded. In the two groups of teeth in which Ca(OH)2 was placed as a supplementary barrier, the pH values showed a slight increase, late in the experiment (10 and 15 days). However, this increase was not statistically significant (P ј 0.790) at any of the experimental periods. This suggests that placement of a Ca(OH)2 paste adjacent to the protective base material would not reverse the acidic pH created in the cervical periodontium during bleaching procedures. Dentine has been shown to be permeable to calcium ions in vitro (Foster et al. 1993, Calt et al. 1999), especially in the absence of smear layer (Foster et al.1993).However, Fuss et al. (1989) found that Ca(OH)2 did not leak from the canal through patent dentinal tubules or changed the pH of the periodontal tissues adjacent to the resorption area, although the cervical cementum was removed from the teeth used in their experimental model. Wang & Hume (1988) observed that little movement of hydroxyl ions occurred in vitro across a layer of dentine when an aqueous paste of Ca(OH)2 was placed in a cavity in dentine. Based on thesefindings, Ca(OH)2 is not likely to play a role in arresting the initiation of cervical root resorption following intracoronal bleaching. The intracoronal placement of Ca(OH)2 following completion of bleaching procedures has been shownto increase the pHof the dental hard tissues in vitro (Tronstad et al.1981). It was proposed that this alkaline environment may inhibit osteoclastic activity and activate alkaline phosphatase which plays a key role in hard tissue formation. Although the Ca(OH)2 barrier was placed during and not after the bleaching procedure in the present study, the pH increase recorded was not sufficient enough to support the above-mentioned hypothesis.
The location of the isolating barrier (GIC) in relation to the CEJ was also shown not to affect the pH created on the external root surface in the present study. All the tested materials have been shown to be equally effective in preventing H2O2 penetration when the thickness of the layer exceeds 1mm (Kehoe 1987). The thickness and location of the barrier are considered to be the critical factors for the reduction of H2O2 penetration, rather than the type of the material used (Rotstein et al. 1992b). However, this was not shown in our study. The limitation that exists regarding the type of barrier used in this study is that glass-ionomer cement is a sensitive material, which may suffer composition alterations during setting, or when in contact with H2O2 molecules owing to its complex setting reaction.
Most of the previous studies involved the use of the walking bleach technique and results were dependent upon the mixing ratio of walking bleach pastes, as dilutions of sodium perborate exceeding 2 g mL_1 would result in an acidic solution (Kehoe 1987). The thermo catalytic bleaching procedure that was used in our study was preferred to the walking bleach technique on the basis that it presented a greater challenge to the effectiveness of the isolating barrier. The pulp chamber was fluid filled and the procedure involved what amounts to thermal cycling (Brighton et al. 1994). The diffusion capacity of various substances through dentine and cementum depends on factors such as the nature of the penetrating agent, the nature of the dental tissue, the surface area exposed and its location, the remaining dentine thickness, previously applied agents, the presence of smear layer and temperature (Outhwaite et al. 1976, Pashley & Livingston 1978, Pashley et al. 1983). The application of heat during the thermocatalytic procedure results in increased dentine permeability owing to thermal expansion, which induces an increase in tubular diameter (Outhwaite et al.1976, Pashley et al. 1983).
It is recognized that the addition of 30% H2O2 to sodium perborate in the ‘walking bleach’ technique enhances the bleaching efficacy of the solution (Nutting & Poe1963). Although cervical root resorption has been reported following bleaching of endodontically treated teeth with the ‘walking bleach’ technique (Friedman et al.1988), it remains one of the most widely used nonvital bleaching techniques. The amount of H2O2 leakage to the periradicular tissues during these procedures has been shown to depend, amongst other factors, on the form of sodium perborate used (Weiger et al. 1994). Therefore, using aqueous solutions of sodium perborate would seem as a more reasonable treatment option because cervical root resorption needs to be prevented. No cases of cervical root resorption have been reported with the use of sodiumperborate and water. Holmstrup et al. (1988) failed to find any signs of root resorption in teeth bleached with sodium perborate and water, after a 3-year follow-up. In addition, equal bleaching results can be accomplished without the use of H2O2, just b y increasing the number of treatment sessions (Rotstein et al.1993).
Baratieri LN, Ritter AV, Monteiro S, Andrada MAC, Vierra LCC (1995) Nonvital tooth bleaching: guidelines for the clinician. Quintessence International 26, 597-603.
Brighton DM, Harrington GW, Nicholls JI (1994) Intracanal isolating barriers as they relate to bleaching. Journal of Endodontics 5, 228-32.
Calt S, Serper A, Ozcelik B, Dalat MD (1999) pH changes and calcium ion diffusion from calcium hydroxide dressing materials through root dentin. Journal of Endodontics 25, 329-31.
Dahlstrom SW, Heithersay GS, Bridges TE (1997) Hydroxyl radical activity in thermo-catalytically bleached root-filled teeth. Endodontics and Dental Traumatology 13, 119-25.
Foster KH, Kulild JC, Weller RN (1993) Effect of smear layer removal on the diffusion of calcium hydroxide through radicular dentin. Journal of Endodontics 19,136-40.
Friedman S, Rotstein I, Lib?eld H, Stabholtz A, Heling I (1988) Incidence of external root resorption and esthetic results in 58 bleached pulpless teeth. Endodontics and Dental Traumatology 4, 23-6.
Fuss Z, Szajkis S, Tagger M (1989) Tubular permeability to calcium hydroxide and to bleaching agents. Journal of Endodontics 8, 362-4.
Goon WW, Cohen S, Borer RF (1986) External cervical root resorption following bleaching. Journal of Endodontics 112, 414-8.
Harrington GW, Natkin E (1979) External resorption associated with bleaching of pulpless teeth. Journal of Endodontics 5, 344-8.
Holmstrup G, Palm AM, Lambjerg-Hansen H (1988) Bleaching of discolored root-filled teeth. Endodontics and DentalTraumatology 4, 197-201.
Kehoe JC (1987) pH reversal following in vitro bleaching of pulpless teeth. Journal of Endodontics1, 6-9.
Lado EA, Stanley HR, Weisman WI (1983) Cervical resorption in bleached teeth. Journal of Oral Surgery 55, 78-80.
Nutting EB, Poe GS (1963) A new combination for bleaching teeth. Journal of South California Dental Association 31, 289-91.
Outhwaite WC, Livingston MJ, Pashley DH (1976) Effects of changes in surface area, thickness, temperature and postextraction time on human dental permeability. Archives of Oral Biology 121, 599-603.
Pashley DH, Livingston MJ (1978) Effect of molecular size on permeability coefficients in human dentine. Archives of Oral Biology 23, 391-5.
Pashley DH, Thompson SM, Stewart FP (1983) Dentin permeability: effects of temperature on hydraulic conductance. Journal of Dental Research 62, 956-9.
Rotstein I (1991a) In vitro determination and quantification of 30% hydrogen peroxide penetration through dentin and cementum during bleaching. Oral Surgery, Oral Medicine and Oral Pathology 72, 602-6.
Rotstein I, Torek Y, Misgav R (1991b) Effect of cementum defects on radicular penetration of 30% hydrogen peroxide during intracoronal bleaching. Journal of Endodontics 5, 230-3.
Rotstein I, Friedman S, Mor C, Kantzelson J, Sommer M, BabI (1991c) Histological characterization of bleaching-induced external root resorption in dogs. Journal of Endodontics 9, 436-41.
Rotstein I, Lehr Z,Gedalia I (1992a) Effect of bleaching agents on inorganic components of human dentin and cementum. Journal of Endodontics 6, 290-3.
Rotstein I, Zyskind D, Lewinstein I, Bamberger N (1992b) Effect of different protective base materials on hydrogen peroxide leakage during intracoronal bleaching in vitro. Journal of Endodontics 3, 114-7.
Rotstein LL, Mor C, Friedman S (1993) Prognosis of intracoronal bleaching with sodium perborate preparations in vitro:1-year study. Journal of Endodontics 19, 10-2.
Steiner DR, West JD (1994) A method to determine the location and shape of an intracoronal bleach barrier. Journal of Endodontics 6, 304-6.
Tronstad L, Andreasen JO, Hasselgren G, Kristerson L, Riis I (1981) pH changes in dental tissues after root-canal filling with calcium hydroxide. Journal of Endodontics 7, 17-21.
Wang J-D, Hume WR (1988) Diffusion of hydrogen ion and hydroxyl ion from various sources through dentin. International Endodontic Journal 21, 17-26.
Weiger R, Kuhn A, Lost C (1994) Radicular penetration of hydrogen peroxide during intra-coronal bleaching with various forms of sodium perborate. International Endodontic Journal 27, 313-7.