Internal bleaching of teeth: an analysis of 255 teeth

Transcription

1 SCIENTIFIC ARTICLE Australian Dental Journal 2009; 54: doi: /j x Internal bleaching of teeth: an analysis of 255 teeth P Abbott,* SYS Heah* *School of Dentistry, The University of Western Australia. ABSTRACT Background: Studies about bleaching have not analysed factors that affect the outcome. This aim of this study was to analyse the outcome of, and the factors associated with bleaching. Methods: Internal bleaching was done on 255 teeth in 203 patients. Colour was assessed pre-operatively, postoperatively and at recalls. The cause and type of discolouration, number of applications, bleaching outcome, and colour stability were assessed. Results: The most common teeth were upper central (69 per cent) and lateral (20.4 per cent) incisors. Trauma was the most common cause (58.8 per cent), followed by previous dental treatment (23.9 per cent), pulp necrosis (13.7 per cent) and pulp canal calcification (3.6 per cent). Dark yellow and black teeth required more applications of bleach than light yellow and grey teeth. Colour modification was good (87.1 per cent) or acceptable (12.9 per cent). Teeth restored with glass ionomer cement composite resin had good colour stability, but this was less predictable with other restorations. No teeth had external invasive resorption. Conclusions: Bleaching endodontically treated teeth was very predictable, especially for grey or light yellow discolourations. Glass ionomer cement composite restorations were effective at preventing further discolouration. Patient age and tooth type did not affect treatment outcome and no cases of external invasive resorption were observed. Keywords: Internal bleaching, outcome, discolouration. Abbreviations and acronyms: EIR = external invasive resorption; GIC = glass ionomer cement; PCO = pulp canal calcification; PN = pulp necrosis. (Accepted for publication 19 April 2009.) INTRODUCTION Internal bleaching is a conservative means of managing discoloured root-filled teeth. The primary indication for internal bleaching is intrinsic (internal) discolouration. Such discolourations have a number of different causes which can be either local or systemic, and they are distinct from those resulting in extrinsic (external) staining. 1 Blood products and their subsequent derivatives can disseminate into the dentinal tubules after trauma to the pulp or when the pulp is removed. As haemolysis continues, various iron compounds are produced which can be converted into black ferric sulphide. This is the most common cause of intrinsic discolouration according to Grossman, 2 but degrading proteins, as is the eventual situation with pulp necrosis, can also cause discolouration. 3,4 In 1967, Nutting and Poe 5 reported that pronounced discolouration was more likely to accompany pulp haemorrhage than pulp degeneration without haemorrhage. Dental restorative materials, including root filling materials, can also cause intrinsic discolouration. 1 If remnants of root filling materials and some root canal medicaments are left in the pulp chamber, the substance can infiltrate into the surrounding dentinal tubules and cause staining. Unfortunately, the discolouration caused by metal ions such as staining due to amalgam generally cannot be removed by internal bleaching. 4 Systemic causes of intrinsic discolouration include tetracycline-containing medicines and high levels of fluoride consumption. 1 Pulp canal calcification is another form of intrinsic tooth discolouration. 1 It is normal for odontoblasts to continuously form dentine throughout the life of the tooth, but bacterial challenge or other stimuli such as trauma to a tooth can accelerate this protective mechanism. Hence, it is not uncommon to observe pulp canal calcification in elderly patients or in traumatized teeth. Accompanying pulp canal calcification is a decrease in the translucency of 326 ª 2009 Australian Dental Association

2 Outcome of internal bleaching the tooth combined with a yellowing or dark discolouration of the tooth. 1 The advantages of internal bleaching have been well reported in the literature. Conservation of tooth structure and achievement of good aesthetics is possible while the procedure is inexpensive and simple to perform. 1 However, the process requires a root-filled tooth, into which a bleaching paste can be placed in the pulp chamber. Hence, discoloured teeth with normal pulps, as is usually the case with tetracycline staining and often with pulp canal calcification, must undergo elective endodontic treatment and root canal filling prior to internal bleaching. Other restorative dental treatment options such as porcelain veneers or crowns may be considered in such cases in order to avoid the possible risks and disadvantages of elective endodontic treatment. Bleaching has been a topic of some concern in the past for several reasons. Colour stability after bleaching has been reported in the literature by many authors with varying results. 6 9 There have also been concerns about the low ph levels that are caused by hydrogen peroxide (H 2 O 2 ) when used alone 4 and bond strength of the final restoration has been shown to be negatively affected by residual bleaching solution. 10 Therefore, it is recommended that a period of 7 10 days be allowed before the final restoration is placed to overcome this latter problem. 1 External invasive resorption (EIR) has been reported to have an association with internal bleaching, both with and without the application of heat. 11 In 1979, Harrington and Natkin reported seven cases of external invasive resorption after internal bleaching using a combined technique. 12 Another case series of 11 teeth with EIR after internal bleaching was reported by Cvek and Lindvall in Currently, it is thought that the cause is the passage of H 2 O 2 through the dentinal tubules and cementum to irritate the periodontal tissue. 14 The true incidence of external invasive resorption following internal bleaching has yet to be determined although Heithersay et al. 15 reported that only 1.9 per cent of approximately 200 cases developed EIR over a 19-year follow-up period after bleaching. Chlorinated lime was first used for internal bleaching in the early 1900s. 4 Since then many other solutions such as oxalic acid, chlorine compounds, sodium peroxide and sodium hypochlorite, have been used to improve the efficacy and outcome of internal bleaching. 4 A thermocatalytic technique was introduced in 1924 by Prinz, 4,16 who used 20 25% H 2 O 2 applied to the pulp chamber and activated by heat lamps or hot instruments. This was proposed by many as the best method of internal bleaching due to the high reactivity of H 2 O 2 in response to heat. Another popular technique, known as the walking bleach technique was described in 1938 by Marsh and published by Salvas. 17 Hydrogen peroxide by itself has an acidic ph value between 2 and 3 but when combined with sodium perborate in the ratio of 2:1 g ml, the ph becomes alkaline. 18,19 The increase in ph buffers the H 2 O 2 and this significantly improves the whitening efficiency. 20 Currently, 35% H 2 O 2 and sodium perborate are commonly used in the walking bleach technique. 1 There is little doubt that a short-term improvement in tooth colour can be predictably achieved in most cases. However, the long-term prognosis of intracoronal bleaching has been reported by several authors with variable results. Three possible causes of colour regression have been postulated, namely: (1) chemical reduction of oxidation products; (2) marginal breakdown of the final restoration; and (3) the inherent permeability of the enamel and dentine to extrinsic substances. 9 The variation in results may be due to inadequate numbers of cases, differing bleaching techniques and final restorations, as well as varying definitions of colour regression. Howell 9 studied the longevity of intracoronal bleaching in 43 teeth and found that short-term outcome was good, but colour regression occurred in 53 per cent of the teeth after one year. Chandra and Chawala 21 reported that only 17 out of 239 teeth showed colour deterioration after one year and that there was evidence of restorative margin deterioration in all of these cases. 21 Brown 22 reviewed 80 teeth in 1965 by comparing pre- and post-treatment photographs. He reported that of those teeth that responded to bleaching, 46.3 per cent showed some colour regression over 1 5 years. Howell concluded that those teeth that are more difficult to bleach have a greater tendency to discolour again after treatment. 9 Currently, there is little information in the literature that relates the outcome of internal bleaching to the pre-bleaching colour and the cause of the discolouration. Hence, the main aim of this study was to assess the initial outcome of internal bleaching relative to the original colour, the cause and degree of discolouration. The teeth were also reviewed after bleaching to monitor the long-term stability of the colour and whether EIR developed. MATERIALS AND METHODS This study was a retrospective review of 255 consecutive teeth that were bleached by a single operator in 203 patients and followed up for up to five years. The bleach technique as described by Abbott 1 was used following root canal filling of the teeth. Prior to bleaching, the tooth colour was assessed and recorded along with the cause of the discolouration. As part of completing the root canal filling, the gutta percha was removed to a level 2.5 mm below the cemento-enamel junction and then a 2.5 mm thick base of Cavit (ESPE 3M; Norristown, PA, USA) was placed as a base over the gutta percha to ensure that none of the bleaching ª 2009 Australian Dental Association 327

3 P Abbott and SYS Heah solution would enter dentinal tubules in the root portion of the tooth. Cotton wool and Cavit, used as a temporary restoration, were placed in the endodontic access cavity and left for one week to allow the Cavit base to set completely before bleaching. At the first bleaching appointment, the temporary restoration (Cavit) and cotton wool were removed. The Cavit base was checked to ensure it followed the cemento-enamel junction contours. Liquid orthophosphoric acid (37.5%) was used to etch the entire access cavity for 30 seconds and then this acid was thoroughly washed out of the access cavity. A thick paste consisting of fresh 35% hydrogen peroxide and sodium perborate powder was prepared immediately prior to use and then it was packed into the cavity. Some of the bleaching paste was removed from the proximal and palatal aspects to provide retention for temporary restoration of the access cavity which was again done with Cavit. The patients were then reviewed after 5 7 days to assess the colour modification. If further colour modification was required, then a fresh mix of the bleaching paste was placed and the access cavity was again filled with Cavit. Where more than one application of the bleach mixture was needed, the teeth were reviewed at 5 7 day intervals until the bleaching was judged to be complete. When completed, the bleaching paste was rinsed from the access cavity and a further temporary restoration (Cavit) was placed. In the early stages of the study, a cotton pellet was placed in the cavity before the Cavit was placed but in the later stages the cavity was completely filled with Cavit and no cotton pellet was used. This variation was done because a small number of the early cases had some colour regression during the review period and all of these teeth were found to have had the cotton pellet left in the cavities when they were restored by the referring dentists. Once the bleaching had been completed, the patient was referred back to his her general dentist for restoration of the access cavity after a minimum of two weeks. A letter was sent to the referring dentist requesting that they restore the access cavity using the glass ionomer cement (GIC) composite resin sandwich (laminate) technique. 23 Instructions regarding the procedure for placing this type of restoration were also enclosed. The particular brands of GIC, composite resin and resin bonding agent used were recorded by the referring dentist and returned to the author for analysis. Pre-treatment and post-treatment photographs were taken of each patient on Kodachrome 35 mm colour transparencies. The patients were reviewed, initially after six months and then at regular intervals for up to five years wherever possible. During these review appointments, the patients and in some cases where relevant their parents, were questioned about their perception of the colour of the tooth and whether it had changed since the bleaching had been done. The tooth was assessed clinically and photographs were taken for comparison with previous photographs. In all cases, there was consensus agreement about the colour of the tooth between the operator and the patient parent. Factors assessed in this study included: patient age, gender, tooth type, causes of the discolouration, original tooth colour, initial outcome of bleaching, number of applications of bleaching paste required, colour stability over six months to five years, factors related to subsequent colour changes, and the incidence of external invasive resorption. Colour modification achieved by internal bleaching was assessed by the author and the patient (and in young patients, also by the parents) using the classifications of good, acceptable or no change. Good colour modification was recorded when the colour of the bleached tooth matched the colour of the adjacent teeth and the patient (and or parent) was entirely pleased with the outcome. Acceptable colour modification was recorded when the colour of the bleached tooth was similar to the colour of the adjacent teeth although not an exact match but the patient (and or parent) were pleased with the result. In addition, the operator assessed that further colour modification was unlikely to improve the outcome any further. No change in colour was recorded if there had been no modification of the tooth colour following bleaching this was assessed clinically and by comparing the pre- and post-bleaching photographs. Any colour changes noted at review appointments were classed as either acceptable or unacceptable. An acceptable change was one that was minor, did not concern the patient (and or parent) and did not require further bleaching or other management. In contrast, an unacceptable colour change was one that required further bleaching, veneering or crowning of the tooth. Tooth discolouration was classified according to its cause and colour. The causes were grouped as trauma, pulp necrosis (PN), pulp canal calcification (PCO) or dental materials used during previous endodontic treatment. The discolourations were classified as grey, black, light yellow and dark yellow. All comparisons were tested for statistical significance at the 5% level using Pearson s chi square, oneway ANOVA and the Scheffe post hoc test. RESULTS There were 203 patients with a total of 255 teeth included in this study. Of the 203 patients, 46 per cent were male and 54 per cent were female (Table 1). Just one tooth was bleached in 162 patients, whereas there were two teeth bleached in 30 patients and three teeth in 11 patients (Table 1). The most common age range of the patients was years (Table 2) although age was not significantly related to the cause 328 ª 2009 Australian Dental Association

4 Outcome of internal bleaching Table 1. Number of patients and number of teeth treated 1 tooth 2 teeth 3 teeth Total no. of teeth Males (46%) Females (54%) Total no. of patients patients 255 teeth Table 2. Age distribution of the patients Age range (years) Number of patients Per cent of patients < % % % % % % Total % Table 3. Tooth types bleached Tooth Maxillary Mandibular Central incisor 176 (69%) 17 (6.6%) Lateral incisor 52 (20.4%) 4 (1.6%) Canine 5 (2%) 0 1st premolar 1 (0.4%) 0 Total 234 (91.3%) 21 (8.2%) of discolouration, the colour or the number of applications of bleach required. Of the teeth that required bleaching, 91 per cent were maxillary teeth, predominantly central (69 per cent) and lateral (20.4 per cent) incisors (Table 3). Only 8.2 per cent of the teeth were mandibular teeth, and there was only one posterior tooth included in this study. Chi-square analysis indicated that there were significant differences (p = 0000) for the cause of the discolouration and the initial colour of the teeth. Table 4 shows that trauma to the teeth was significantly more likely to cause grey or light yellow discolouration, whereas previous dental materials used during endodontic treatment, PN and PCC generally caused dark yellow discolouration. The initial colour had a significant effect on the outcome of bleaching (p = 0.000). Teeth that were initially grey were the most predictable ones to bleach with good colour modification being achieved in all grey teeth (Table 5). Light yellow and black discolourations showed slightly less favourable results (94.9 per cent and 86.1 per cent, respectively, were good ) while a good result was only achieved for 67.5 per cent of the dark yellow teeth. It was notable that all teeth in this study had either good or acceptable colour modification following bleaching and there were no teeth that had no improvement in their colour. The number of applications of bleaching paste required for the various pre-operative discolourations (Table 6) increased in order from light yellow, grey, black to dark yellow. Approximately half the teeth required only one application, 30 per cent required two applications while about one-quarter needed 3 5 applications to achieve a good or acceptable result. Typically (i.e., approximately three-quarters of each discolouration), the light yellow and grey Table 4. Initial colour of the discoloured teeth and the cause of the discolouration Pre-operative discolouration Trauma Cause of the discolouration Previous dental treatment Pulp canal calcification Pulp necrosis Total No. % No. % No. % No. % No. % Dark yellow Light yellow Grey Black Total Table 5. The outcome of the bleaching procedure according to the pre-operative discolouration Outcome of bleaching Pre-operative discolouration Dark yellow Light yellow Grey Black Total No. % No. % No. % No. % No. % Good Acceptable No change Total ª 2009 Australian Dental Association 329

5 P Abbott and SYS Heah Table 6. The number of applications of the bleaching mixture required to modify the various pre-operative discolourations No. of applications required Pre-operative discolouration Dark yellow Light yellow Grey Black Total No. % No. % No. % No. % No. % Total Table 7. The number of applications of the bleaching mixture related to the cause of the discolouration No. of applications required Trauma Cause of the discolouration Previous dental treatment Pulp canal calcification Pulp necrosis Total No. % No. % No. % No. % No. % Total Table 8. Number of patients due for, and who attended, recall appointments plus the number of teeth due to be, and actually, reviewed. Note: the number of teeth at each recall appointment was less than the original starting number shown in the Table Recall interval Due for review Number attended review No. of patients No. of teeth No. of patients (% of those due to attend) No. of teeth (% of those due for review) 6 months (78.3%) 141 (68.4%) 1 year (100%) 58 (100%) 2 years (64.6%) 50 (54.3%) 3 years (81.1%) 36 (78.3%) 4 years (70.8%) 20 (74.1%) 5 years (81.8%) 14 (82.4%) discolourations required only one application of the bleach paste. There were significant differences between the following pairs: black and grey; black and light yellow; dark yellow and light yellow; dark yellow and grey; dark yellow and black. Teeth that were discoloured by trauma required significantly less applications (usually only one) than those discoloured by other causes (Table 7). There were no significant differences between the other causes and the number of applications. Patients were reviewed at intervals ranging from six months up to five years, depending on their clinical need and availability (Table 8). Of those due for a sixmonth recall, 78.3 per cent attended while 100 per cent of those due for a one-year recall attended. Recall attendance of those patients scheduled from two to four years varied from 64.6 per cent to 81.1 per cent. A fiveyear recall was attended by 81.8 per cent of those who were due for the appointment. At the review appointments, the total percentage of teeth that had discoloured again, to both acceptable and unacceptable levels, was 3.9 per cent (Table 9). Four teeth (originally two dark yellow, one light yellow and one grey) had an acceptable colour change, while six teeth (originally two dark yellow, two light yellow, one black and one grey) had unacceptable colour regression. Of those that were acceptable, one tooth discoloured after two years, another after three 330 ª 2009 Australian Dental Association

6 Outcome of internal bleaching Table 9. Colour stability at recall appointments according to the pre-operative colour and the outcome of the bleaching procedure Pre-operative colour Initial result Original no. of teeth No. (%) with acceptable colour change [recall time interval when colour change noted] No. (%) that had unacceptable discolouration again [recall time interval when discolouration noted] Dark yellow Good 52 2 (3.8%) [2 yrs, 3 yrs] 2 (3.8%) [6 mths, 3 yrs] Dark yellow Acceptable (8%) [6 mths, 1 yr] Light yellow Good 56 1 (1.8%) [4 yrs] 0 Light yellow Acceptable Black Good (3%) [3 yrs] Black Acceptable Grey Good 83 1 (1.2%) [4 yrs] 1 (1.2%) [5 yrs] Grey Acceptable years and two teeth after four years. Of those that were unacceptable and needed to be re-bleached, two had discoloured after six months and the access cavity had not been restored in both of these cases. One tooth discoloured after one year, another two teeth after three years and one tooth after five years. Six teeth required re-bleaching due to restoration breakdown and re-staining of the tooth structure. Five of the teeth had been extracted for various reasons such as periodontal issues, root fractures (horizontal, vertical and crown root) and inability to replace an unsatisfactory restoration. Four teeth required full coverage crown restorations due to crown fractures, and three teeth had veneers placed, one due to over-bleaching and the other two were to replace old veneers that had been placed prior to the bleaching. No cases were found to have external invasive resorption at the review appointments. DISCUSSION The results of this study reinforce that internal bleaching is a predictable, simple, quick and cheap procedure. It is conservative of tooth structure, and maintains the natural contour, occlusion, form and function of the tooth. Potential problems associated with dental prostheses are avoided, such as periodontal problems, changes in occlusion, root fractures, opposing tooth wear, and aesthetic concerns. The procedure is safe if adequate precautions are taken, such as those described by Abbott. 1 A thorough understanding of the chemistry of the materials and the procedure is essential to ensure patient and staff safety since the materials are caustic. The year old age group has been reported to have a higher incidence of dental trauma than other age groups 24 and dental trauma is a common cause of discolouration requiring internal bleaching. This study supported these reports since just over half of the patients belonged to this age group and over half of the teeth in this study were discoloured as a result of trauma. A typical endodontic practice has a demographic distribution of females to males of 2:1. 25,26 Contrary to this, the per cent gender distribution in this study was almost equal (i.e., 46 per cent males:54 per cent females). This may be due to a higher incidence of dental trauma in boys when compared to girls 24 which is reflected in the distribution of teeth that required bleaching, and the cause of the discolourations. The distribution of specific teeth types encountered in this study is consistent with the distribution of teeth requiring internal bleaching reported in other studies. 22,27,28 It also correlates to the typical distribution of teeth reported in dental trauma studies. 24 The majority of teeth had either grey or dark yellow discolouration. The most common cause of discolouration was trauma to the teeth (58.8 per cent). Approximately half of these teeth had grey discolouration and approximately one-third were light yellow. Previous endodontic and restorative dental treatment led to discolouration of about one-quarter of all teeth in the study and the most common discolouration in this group was dark yellow. Pulp necrosis was most likely to cause dark yellow discolouration as was pulp canal calcification but this latter condition was not a common cause of discolouration overall. The outcomes obtained with the internal bleaching procedure used in this study support previous reports 3,5,6,21,28 that have found internal bleaching to be a predictable procedure. In this study, all teeth had either good or acceptable colour change and, notably, there were no cases that were considered to have had no change at all. All of the grey cases and almost all of the light yellow and black cases had good outcomes. The dark yellow teeth were harder to bleach as about one-third of this group had only acceptable changes rather than good changes. However, overall, the results of the bleaching were very encouraging and predictable. Overall, almost half of the teeth (but up to threequarters of the grey and light yellow cases) required only one application of the bleaching mixture in order to achieve good or acceptable colour modification. Another quarter required two applications and 15 per cent required three applications. Hence, the ª 2009 Australian Dental Association 331

7 P Abbott and SYS Heah bleaching procedure was relatively quick in most cases with little chair-side time involved approximately minutes per application. The majority of teeth that had discoloured as a result of trauma had colour modification after just one application of the bleaching mixture. This was not surprising since most of the traumatized teeth had either grey or light yellow discolouration and these two colours were the quickest to change. Teeth with discolouration as a result of pulp canal calcification were the slowest to be modified with up to almost half of them requiring three applications of the mixture. Not all patients were recalled at the same interval. The recall interval depended on the initial reason for endodontic treatment and the patient s availability. Attendance at recall examinations is a limiting factor for all clinical review or follow-up studies. An overall recall attendance of 50 per cent is considered to be excellent with most endodontic studies only achieving about per cent attendance. In this study, the majority of patients were recalled within the first year. Although only a few patients attended their recall appointments at five years (only 11 patients were due for a five-year follow-up, of which nine attended), only one tooth was found to have discoloured at the fiveyear recall appointment. In contrast, Feiglin 6 reported that of 20 teeth followed for six years after internal bleaching, 45 per cent remained the same colour, or were similar in colour to the adjacent tooth. A larger cohort of patients followed up to five years may be required to validly examine the long-term stability of internal bleaching procedures. All teeth reviewed in this study that had further discolouration after internal bleaching were deemed to have had an unsatisfactory restoration of the access cavity. Cotton wool was found in the pulp chambers of all cases that required further bleaching. The GIC composite resin laminate technique when used to restore the access cavity was found to produce less cases with further discolouration compared to other restorative techniques used. These findings reinforce the earlier stated concept that further discolouration is likely to be a result of the restoration breakdown and uptake of food stains into the tooth rather than being due to chemical reduction of oxidation products produced by the bleaching itself. Although no cases of EIR were found in this study, not all teeth were reviewed at five years, and more longterm studies are required to assess the true incidence of EIR. 29 However, at least in the short term, EIR was not found to be associated with the internal bleaching technique used in this study. CONCLUSIONS Internal bleaching is a predictable procedure. Colour modification was usually good while the rest were acceptable. Teeth stained due to trauma, and with grey or light yellow discolourations were easier and quicker to bleach than darker teeth which required more applications of the bleaching paste. Dark yellow discolouration was the most difficult to modify and stains from previous dental materials were also difficult to remove. Some teeth discoloured again over 2 5 years and this appeared to be related to breakdown of the access cavity restoration. There were no cases of external invasive resorption noted in the five-year follow-up period. REFERENCES 1. Abbott PV. Aesthetic considerations in endodontics: internal bleaching. Pract Periodontics Aesthet Dent 1997;9: Grossman L, Oliet S, Del Rio C. Endodontic Practice. 11th edn. Philadelphia: Lea & Febiger, Howell RA. Bleaching discoloured root-filled teeth. Br Dent J 1980;148: Attin T, Paque F, Ajam F, Lennon AM. Review of the current status of tooth whitening with the walking bleach technique. Int Endod J 2003;36: Nutting EB, Poe GS. Chemical bleaching of discolored endodontically treated teeth. Dent Clin North Am 1967;Nov: Feiglin B. A 6-year recall study of clinically chemically bleached teeth. Oral Surg Oral Med Oral Pathol 1987;63: Glockner K, Hulla H, Ebeleseder K, Stadtler P. Five-year followup of internal bleaching. Braz Dent J 1999;10: Amato M, Scaravilli MS, Farella M, Riccitiello F. Bleaching teeth treated endodontically: long-term evaluation of a case series. J Endod 2006;32: Howell RA. The prognosis of bleached root-filled teeth. Int Endod J 1981;14: Attin T, Hannig C, Wiegand A, Attin R. Effect of bleaching on restorative materials and restorations a systematic review. Dent Mater 2004;20: Rotstein I, Zalkind M, Mor C, Tarabeah A, Friedman S. In vitro efficacy of sodium perborate preparations used for intracoronal bleaching of discolored non-vital teeth. Endod Dent Traumatol 1991;7: Harrington GW, Natkin E. External resorption associated with bleaching of pulpless teeth. J Endod 1979;5: Cvek M, Lindvall AM. External root resorption following bleaching of pulpless teeth with oxygen peroxide. Endod Dent Traumatol 1985;1: Heller D, Skriber J, Lin LM. Effect of intracoronal bleaching on external cervical root resorption. J Endod 1992;18: Heithersay G, Dahlstrom S, Marin P. Incidence of external root resorption in bleached root-filled teeth. Aust Dent J 1993;39: Ho S, Goerig AC. An in vitro comparison of different bleaching agents in the discolored tooth. J Endod 1989;15: Salvas C. Perborate as a bleaching agent. J Am Dent Assoc 1938;25: Rotstein I. In vitro determination and quantification of 30% hydrogen peroxide penetration through dentin and cementum during bleaching. Oral Surg Oral Med Oral Pathol 1991;72: Kehoe JC. ph reversal following in vitro bleaching of pulpless teeth. J Endod 1987;13: ª 2009 Australian Dental Association

8 Outcome of internal bleaching 20. Frysh H, Bowles WH, Baker F, Rivera-Hidalgo F, Guillen G. Effect of ph on hydrogen peroxide bleaching agents. J Esthet Dent 1995;7: Chandra S, Chawla TN. Clinical evaluation of various chemicals and techniques of bleaching of discolored root filled teeth. J Indian Dent Assoc 1972;44: Brown G. Factors influencing successful bleaching of the discolored root-filled tooth. Oral Surg Oral Med Oral Pathol 1965;20: McLean JW. Glass-ionomer cements. Br Dent J 1988;164: Bastone E, Freer T, McNamara J. Epidemiology of dental trauma: a review of the literature. Aust Dent J 2000;45: Abbott PV. Analysis of a referral-based endodontic practice: Part 1. Demographic data and reasons for referral. J Endod 1994;20: Waldman HB, Feigen ME. Endodontists in a period of improving dental economics and changing realities of practice. J Endod 1990;16: Holmstrup G, Palm AM, Lambjerg-Hansen H. Bleaching of discoloured root-filled teeth. Endod Dent Traumatol 1988;4: Amato M, Scaravilli M, Farella M, Riccitiello F. Bleaching teeth treated endodontically: long-term evaluation of a case series. J Endod 2006;32: Szajkis S, Tagger M, Tamse A. Bleaching of root canal treated teeth and cervical external resorption: review of the literature. Refuat Hashinayin 1986;4: Address for correspondence: Winthrop Professor Paul Abbott School of Dentistry The University of Western Australia 17 Monash Avenue Nedlands WA paul.v.abbott@uwa.edu.au ª 2009 Australian Dental Association 333