RESEARCH ARTICLE
Effect of Bleaching Treatment on Fatigue Resistance and Flexural Strength of Bovine Dentin LAURA E. TAM, DDS, MSC*, WOOHYUN CHO†, BRIDGET Y. WANG†, GRACE DE SOUZA, DDS, MSC, PHD‡
ABSTRACT Purpose: To determine the effects of bleach on dentin fatigue resistance and flexural strength. Materials and Methods: Eighty bovine dentin specimens (2 × 2 × 17 mm) were treated with: placebo or 10% carbamide peroxide bleach. Treatment was applied for 6 hours/day for 2 or 8 weeks. After treatment, 10 specimens per group were subjected to fatigue testing (106 cycles) whereas the other 10 were stored in artificial saliva as fatigue controls. The specimens undergoing fatigue were checked daily for visible signs of fracture and excluded from subsequent flexural strength tests if fractured. Fatigue control and surviving fatigued specimens were subjected to flexural strength testing. Chi-square, Kruskal–Wallis, factorial analysis of variance (p < 0.05) and Mann–Whitney (p < 0.002) tests were performed. Results: There were significant differences in fatigue resistance (p = 0.003) and flexural strength rank scores (p < 0.0001) among the groups. None of the specimens in the “8-week bleach” group survived the fatigue testing. Fatigue (p = 0.005) and interaction of time and treatment (p = 0.039) were significant factors in the flexural strength results. Fatigued specimens had lower flexural strength than nonfatigued and “8-week bleach” had lower flexural strength than placebo and “2-week bleach” groups. Conclusions: Prolonged direct bleaching of bovine dentin reduces its fatigue resistance and flexural strength in vitro. Further research is needed in this area.
CLINICAL SIGNIFICANCE It remains prudent to advise patients to limit their exposure to tooth bleaching materials by avoiding direct application of bleach to exposed dentin and by minimizing the duration of bleach treatment. (J Esthet Restor Dent 27:374–382, 2015)
INTRODUCTION Tooth bleaching is primarily performed using hydrogen peroxide or carbamide peroxide as the active ingredient. Primary methods of bleach treatment include at-home bleach application using custom-made bleaching trays, in-office bleach application with or without light activation, and at-home over-the-counter bleach application using strips. Tooth sensitivity has been identified as a common but transient side effect of bleach treatment1–4.
Long-term effects of bleach treatment are unknown. Bleach procedures can be prone to overuse in an attempt to achieve a whiter tooth color result, either by using too high a concentration of bleach or by bleaching for a prolonged period of time. Many studies have investigated the effects of tooth bleaching on enamel and dentin surface properties such as hardness,5–7 surface morphology,8–10 bonding,5,6,11–13 surface demineralization,14–16 and abrasion/erosion.17 Relatively fewer studies have investigated the effects of tooth bleaching on enamel and dentin mechanical
*Professor, Restorative Dentistry, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, Toronto, Canada † Undergraduate Research Student, Restorative Dentistry, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, Toronto, Canada ‡ Assistant Professor, Restorative Dentistry, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, Toronto, Canada
374
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
properties such as strength, fatigue, or fracture toughness. The ultimate tensile strength and fracture toughness of enamel was shown to be reduced significantly by different bleaching regimens.18,19 Significant reductions in tensile and shear strengths of dentin were reported after an in vitro direct intracoronal bleach application of 30% hydrogen peroxide.20 The in vitro fracture strength of bovine incisor crowns were reduced after intracoronal bleaching procedures.21 The flexural strength of bovine dentin was significantly reduced by an application of 20% carbamide peroxide bleach for 2 weeks.22 In another in vitro study, the fracture toughness of dentin was significantly reduced by the indirect (through intact enamel) application of peroxide bleaching agents (this represents a simulation of clinical bleaching of teeth with full enamel coverage), as well as by a direct bleach application method to dentin, and that it was reduced more for the longer application time period (8 versus 2 weeks) and for the higher (16% versus 10%) bleach concentration.23 The clinical relevance of these in vitro studies is uncertain and in vivo studies are lacking. One in situ study did report a decrease in dentin fracture toughness as a result of bleach application in a clinical setting.24 However, the decrease was not significant. It was speculated that the variability associated with the in situ setting contributed to a wide variation that is not present in in vitro studies. Fatigue tests are used to mimic the clinical situation where teeth are subject to cyclic loads during mastication and should be a more important test of long-term structural integrity than a test such as microhardness. Subcritical flaws can grow and coalesce with cyclic loading. Although the cyclic forces of mastication are generally insufficient to cause failure in perfect dentin, normal dentin contains pre-existing flaws that can propagate to catastrophic size under small cyclic loads.25 Tooth fracture is a significant problem that is predicted to become more prevalent as people retain their teeth longer. Data on the incidence of complete, incomplete, cuspal, and root fractures are extensive with the risk of noncarious tooth fracture at 72.7 per 1,000 person-years.26,27 Fatigue leads to a weakened tooth which then fractures under a minimal load. The advent of widely available tooth bleaches was
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
only 10–20 years ago. It is possible that the incidence of tooth fractures attributable to tooth bleaching will manifest after a lifetime of chewing as a result of crack propagation from initial microscopic flaws introduced by tooth bleaching. Therefore, fatigue tests may be useful to show the effects of microscopic alterations to dentin which are not initially apparent at early testing periods without fatigue loading. A fatigue test may be important to help clarify the biomechanical consequences of changes to the mineral-collagen complex of bleached dentin. The objective of this study is to determine the effects of dental bleach on the fatigue resistance and flexural strength of bovine dentin in vitro. The null hypothesis for this study is: bleaching has no effect on the fatigue resistance and flexural strength of bovine dentin in vitro.
MATERIALS AND METHODS Bovine incisors (extracted within 6 months of the experiment, frozen until use) were collected to provide the dentin for testing. One dentin specimen was obtained from each tooth except occasionally, two dentin specimens were obtained from larger teeth. Flexural strength test rectangular beams, approximately 2 × 2 × 17 mm, were prepared from the dentin on the facial surface using a water-cooled low-speed diamond saw (Buehler Ltd., Lake Bluff, IL, USA), keeping the location and orientation of the dentin as standardized as possible. A micrometer (Digimatic Caliper, Mitutoyo Corporation, Kanagawa, Japan) was used to measure specimen dimensions to the nearest 0.01 mm. The dentin specimens were stored in freshly made artificial saliva28 in individual containers until testing. The test groups are outlined in Table 1. The specimens were randomly divided into two groups of 40: placebo or bleach (Opalescence, Lot No. B81FR & B8295, Ultradent Products Inc, South Jordan, UT, USA). The placebo (Placebo, Ultradent Products Inc, South Jordan, UT, USA) contained glycerine, carbopol, and water whereas the bleach gel contained the same components with the addition of 10% carbamide peroxide. A bleach
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
375
EFFECT OF BLEACH ON DENTIN Tam et al.
TABLE 1. Outline of test groups Treatment
Placebo (glycerine, carbopol, water)
Bleach (10% carbamide peroxide, glycerine, carbopol, water)
Time
2 weeks (6 hours × 14 days)
8 weeks (6 hours × 48 days)
2 weeks (6 hours × 14 days)
8 weeks (6 hours × 48 days)
Fatigue*
N-F
F
N-F
F
N-F
F
N-F
F
Initial sample size
N = 10
N = 10
N = 10
N = 10
N = 10
N = 10
N = 10
N = 10
*N-F = not fatigued (storage in 37°C artificial saliva only prior to flexural strength testing); F = fatigued (fatigue loading at 20 MPa and 2 Hz for 106 cycles in 37°C artificial saliva prior to flexural strength testing).
treatment consisted of an application of a 1–2 mm-thick layer of fresh bleach gel to completely surround the tooth specimen surface for six continuous hours. The specimens were stored at 37°C, >80% relative humidity for the duration of the bleach treatment. Bleach was applied either daily for 2 weeks or 6 days a week for 8 weeks to simulate a typical and a prolonged at-home overnight bleaching regimen, respectively. At the end of each bleach treatment, the specimens were rinsed with tap water to remove all external traces of bleach and the specimens were stored in 37°C artificial saliva until the next bleach treatment. At the end of the last bleaching session, 10 specimens from each group were randomly selected for fatigue testing whereas the other 10 specimens remained in artificial saliva as control specimens. The dentin specimens for fatigue testing were mounted in custom-designed holders with a span distance of 8 mm, immersed into 37°C artificial saliva, and subjected to fatigue loading at 20 MPa and 3 Hz for 106 cycles using a chewing simulator (CS 4.4 Chewing Simulator, SD Mechatronik) (Feldkirchen-Westerham, Bayern, Germany). The artificial saliva was changed daily. The specimens were checked daily for visible signs of fracture and fractured specimens were excluded from subsequent flexural strength tests. Specimens that survived the fatigue loading were subjected to the flexural strength test. Ten days after the last bleach application, control and fatigued specimens were mounted on an Instron universal testing machine (Model 4301, Instron Corp, Canton, MA, USA) for three-point bending flexural strength test using a custom-designed mounting jig with a span distance of
376
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
14 mm. During testing, the mounting apparatus was immersed in a 37 ± 2°C water bath. Flexural loading as applied at a rate of 0.5 mm/minute until specimen fracture. The force recorded at fracture was used to calculate flexural strength using the following equation:
FS = 3Pf L 2WH2 where Pf is the measured maximum load at the time of specimen fracture, L is the distance between the supports on the tension surface, W is the mean specimen width, and H is the mean height of the specimen between the tension and compression surfaces. For statistical analyses, the issue of missing flexural strength values in the fatigued groups was handled using nonparametric statistical analyses. The specimens that did not survive the fatigue test were assigned the lowest rank (lowest rank = 1) and the Kruskal–Wallis was applied to evaluate the flexural strength results (p < 0.05). Pairwise Mann–Whitney tests with a Bonferroni correction were used for post hoc analyses (p < 0.002). The chi-square test was used to compare the proportion of survivors among the groups that underwent fatigue testing (p < 0.05). For additional information, a mean flexural strength value for each group was determined from the specimens that did survive the fatigue test and a factorial analysis of variance was conducted to evaluate the significance of the factors treatment (placebo versus bleach), time (2 versus 8 weeks), and fatigue (fatigued versus nonfatigued).
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
RESULTS All specimens in the bleached groups were noticeably whiter than the specimens in the placebo groups. Two specimens broke accidentally, leaving 78 specimens for mechanical testing.
Fatigue Resistance Figure 1 shows the percentage of specimens that survived the fatigue loading. Specimen failure was observed in all groups during the fatigue test and these failures all occurred within the first 250,000 cycles. The “8-week bleach” group had a 100% failure rate. There were significant differences in fatigue resistance among
% Survival
Fagued specimens 90 80 70 60 50 40 30 20 10 0
Fracon survival
the groups (p = 0.003). The adjusted residuals for the “2-week placebo” and “8-week bleach” group suggested a greater incidence of survival and failure, respectively, than the null hypothesis predicts.
Flexural Strength The flexural strength rank scores derived from the Kruskal–Wallis test are presented in Figure 2. There were significant differences in flexural strength among the groups (p < 0.0001). The flexural strength rank results for the “8-week placebo” and for the “2- and 8week” bleach treatment groups were significantly decreased by the fatigue loading test. The flexural strength rank results were not significantly changed by a “2-week bleach” treatment but they were significantly decreased in the “8-week bleach” treatment groups, with or without fatigue loading. The mean flexural strength results of the nonfatigued groups and the survivors of the fatigue test in the fatigued groups are presented in Table 2. Fatigue was a significant factor in the flexural strength results (p = 0.005). The interaction of time and treatment was also a significant factor in the flexural strength results (p = 0.039).
2-week placeboa 2-week bleachb 8-week placebob 8-week bleachc
8/10
6/10
5/9
0/10
FIGURE 1. Percentage of fatigued dentin specimens that survived the fatigue loading. There were significant differences in fatigue resistance among the groups (p = 0.003). Groups denoted by the same letter indicate no significant difference (p < 0.05).
DISCUSSION Bovine incisors were selected over human teeth as the source of dentin primarily because human teeth are too small to provide the size of dentin needed for the flexural strength tests. The fatigue properties of bovine
FIGURE 2. Flexural strength rank scores (derived from the Kruskal–Wallis test). There were significant differences in fatigue resistance among the groups (p < 0.0001). Groups denoted by the same letter indicate no significant difference (p < 0.05).
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
377
EFFECT OF BLEACH ON DENTIN Tam et al.
TABLE 2. Flexural strength results (mean, standard deviation, and sample size) derived from all the specimens that were tested for flexural strength in the nonfatigued groups and from the survivors of the fatigue test that made it to the flexural strength test in the fatigued groups Treatment
Placebo
Bleach
Time
2 weeks
Fatigue*
N-F
F
N-F
F
N-F
F
N-F
F
Mean (MPa)
95
84
84
67
100
72
60
—
Standard deviation (MPa)
23
20
18
17
18
40
12
—
N
10
8
9
5
10
6
10
0
8 weeks
2 weeks
8 weeks
Fatigue (p = 0.005) and the interaction of time and treatment (p = 0.039) were significant factors in the flexural strength results. *N-F = not fatigued (storage in 37°C artificial saliva only); F = fatigued (fatigue loading at 20 MPa and 2 Hz for 106 cycles in 37°C artificial saliva).
dentin29 and human dentin30 have been shown to be similar. Ultimate tensile strength and elastic modulus values for bovine and human dentin have also been shown to be similar.31 Previous studies have shown that frozen storage did not significantly affect the tensile strength of or the shear bond strengths to bovine dentin.29,32 Furthermore, the reported adverse effects to the flexural strength of bovine dentin33 were similar to the reported adverse effects to the fracture toughness of human dentin.23 Therefore, we believe that the effect that was observed on bovine dentin in this study would be comparable with one on human dentin. The specimens were all loaded in a parallel direction relative to the dentinal tubules. Nalla and colleagues34 reported no significant differences between the “in-plane parallel” and “anti-plane parallel” fracture toughness of dentin. A unique aspect of this paper is that it specifically addresses the structural properties of bleached dentin using fatigue loading in addition to a flexural test. Typical fatigue tests of dentin indicate that failure is caused by the initiation and growth of a single, dominant crack and that dentin is designed to not fail from normal mastication loads of around 20 MPa.30 The load parameter for our fatigue test was selected to be below the fatigue endurance limit for normal dentin. However, there were specimen failures in all the groups that underwent fatigue testing, including the placebo groups. The placebo treatment did appear to have some effect on dentin as the flexural strength and fatigue
378
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
resistance results were generally lower, albeit not statistically, for the “8-week placebo” groups compared with the “2-week placebo” groups. We speculate that the placebo treatment led to reduced fatigue resistance and flexural strength as a result of some tooth dehydration, demineralization, or dentin matrix degradation that occurred during the placebo treatment time. Although all the treatments were conducted in >80% relative humidity, it is possible that the treatment gel acted as a barrier to the humidity in the test chamber. In the mouth, both the treatment gel and the bleaching tray could act as barriers that lead to transient tooth dehydration. Placebo materials have been associated with other negative effects. They caused tooth sensitivity in clinical bleaching studies.2,35,36 They reduced enamel and dentin microhardness and this reduction was attributed to a demineralization or a barrier effect of the placebo.37,38 The results show large standard deviations but they are not out of line with biological specimens. Despite the large standard deviations, the differences were great enough to show statistically significant differences using less sensitive nonparametric statistics. The significant interaction effect of time and treatment can be seen in the significantly lower flexural strength rank scores for the “8-week bleach” treatment groups, with and without fatigue. A decrease in dentin flexural strength was not significant after 2 weeks but was significant after 8 weeks of bleaching. The decrease in dentin flexural strength in the nonfatigued “8-week bleach” treatment
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
group was similar to the decrease observed caused by the fatigue testing on the placebo and “2-week bleach” groups. In other words, the in vitro effect of an 8-week bleach treatment (without fatigue) on flexural strength was comparable with the in vitro effect of 1 million fatigue cycles. This would be equivalent to at least 1 year of clinical functional aging as it has been estimated that a normal adult would perform an average of 500,000 to 1,000,000 loading cycles per year.25,39,40 The mechanism for the reduction in dentin fatigue resistance and flexural strength by bleaching agents is not fully understood. Nanoindentation tests led to the conclusion that both intertubular and peritubular dentin are affected by carbamide and hydrogen peroxide.41 Bleaching agents may adversely affect dentin properties by affecting the inorganic mineral component of dentin. It has been suggested that hydrogen peroxide increases the release of calcium and phosphorus from bovine enamel and dentin.14,42 There may have been more dentin demineralization in the bleach groups than in the placebo groups as a result of a relative difference in pH (bleach pH: 6.4, placebo pH: 6.8). Alternatively, it has been suggested that the dentin demineralization can be attributed more to the oxidizing ability than to the acidity of hydrogen peroxide.43 Data are emerging to suggest that the organic component of the tooth is the primary target of hydrogen peroxide oxidation. This suggests that dentin would be more susceptible than enamel to adverse bleach effects. A study on the effect of hydrogen peroxide bleaches on the stiffness of demineralized dentin suggested that dental bleach interacted specifically with the organic dentin matrix.44 Recent interesting studies have reported that hydrogen and carbamide peroxide increased matrix metalloproteinase-mediated collagen degradation and cathepsin B proteolytic activity in dentin with home bleaching inducing the highest collagen degradation.45,46 Furthermore, the mechanism for enamel whitening was determined to be caused by changes to the organic component of enamel and not the inorganic component of enamel, and the authors of this paper suggested that accordingly, hydrogen peroxide could have a similar or even stronger effect on dentin as it contains a higher organic content.47
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
Fatigue loading was a significant factor that caused a reduction in flexural strength. The flexural strength rank results were significantly decreased by the fatigue loading test for all treatments except for the “2-week placebo” group (Figure 2). The mean flexural strength results of the surviving specimens in the fatigued groups were also lower than in the nonfatigued groups (Table 2) and it can be assumed that the mean flexural strength results of the fatigued groups should be even lower than what was reported because only the surviving specimens were used for the mean calculation. The effect of fatigue appeared to be a more significant factor than the interaction effect of time and treatment. This confirms the importance of the effect of fatigue on dentin longevity. An initially small adverse effect on dentin structure, potentially caused by bleach treatment, could magnify and have a great impact over the lifetime of a tooth as a result of fatigue and crack propagation. The combination of “8-week bleach” treatment and fatigue loading was particularly destructive as all of the dentin specimens in this group did not survive the fatigue test. An effect of such magnitude would not be expected in vivo. The intraoral environment will dilute the bleaching agent much more than the in vitro situation. The small cross-section of the dentin specimens in this study would allow for greater permeation of the bleaching agent into dentin than a bulky tooth with an enamel covering. Furthermore, the usual intended substrate for bleach application is enamel, not dentin. We have focused our attention on dentin because it is the foundational material for tooth structure and it is unquestionably exposed to tooth bleach, both directly and indirectly. There would be many clinical situations in which tooth bleach is applied directly to the dentin (cases of root exposure, occlusal attrition, and in cases of nonvital tooth bleaching). That being said, enamel is not considered an impermeable barrier to hydrogen peroxide or carbamide peroxide.48–50 The ability of hydrogen peroxide and carbamide peroxide bleaching agents to readily penetrate through intact enamel and dentin is evident from both the significant amounts of bleach measured in the pulp chambers after bleach application in vitro51–53 and from reported tooth sensitivity during
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
379
EFFECT OF BLEACH ON DENTIN Tam et al.
indirect bleach application in vivo. The diffusion data for hydrogen peroxide and carbamide peroxide suggested that bleaching agents were capable of diffusing through enamel and dentin within 15 and 25 minutes.52,53 An in vitro comparison of a direct application of bleach to dentin compared with an indirect application of bleach to dentin through an intact enamel surface (the latter represents a simulation of clinical bleaching of teeth with full enamel coverage) showed a decreased but still significant adverse effect on human dentin fracture toughness with the indirect application method.23 The “8-week bleach” group was devised to test the effect of a prolonged or exaggerated at-home bleach treatment regimen. Tooth bleaching treatment was originally only administered under the supervision of a dentist and the bleach concentration and application were controlled. Currently, tooth bleaches are available at significantly higher concentrations, in a greater variety of products and in a less controlled manner (over-the-counter), so it is possible that there are patients that will bleach their teeth for an accumulated time of 8 weeks. However, patients will use shorter bleach application times with over-the-counter products and generally will not bleach their teeth on a daily basis for such a prolonged period. We have subsequently performed further fatigue testing (with an over-the-counter product) to confirm the findings of this study (manuscript in preparation). Longer intervals between bleaching treatments may provide greater opportunities for tooth remineralization and restoration of tooth mechanical properties. The use of water, artificial saliva, fluoride, and casein phosphopeptide-amorphous calcium phosphate has been investigated with equivocal results for recovery of mechanical properties.44,54,55
380
significance of this study can be questioned. There have been little or no long-term adverse effects associated with tooth bleaching, even after prolonged periods of time.56 However, the property of fatigue resistance has never been specifically investigated and the issue of bleach effects on long-term structural integrity remains an important issue, especially when you consider the widespread usage of bleach products. The observed significantly reduced fatigue resistance and flexural strength of dentin and in particular, the premature fracture of all “8-week bleach” specimens reported in this first in vitro study using fatigue suggest that further research is needed in this area.
DISCLOSURE AND ACKNOWLEDGEMENTS The study was supported by a Dental Research Institute, Faculty of Dentistry, University of Toronto grant number 13-14-4. The Opalescence 10% carbamide peroxide was provided by Ultradent Products Inc.
REFERENCES 1.
2.
3.
4. 5.
CONCLUSIONS
6.
The results of this study suggest that fatigue resistance and flexural strength of bovine dentin are compromised by the direct application of a prolonged home bleaching procedure using 10% carbamide peroxide. The clinical
7.
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
Cardoso PC, Reis A, Loguercio A, et al. Clinical effectiveness and tooth sensitivity associated with different bleaching times for a 10 percent carbamide peroxide gel. J Am Dent Assoc 2010;141:1213–20. Haywood VB, Leonard RH, Nelson CF, et al. Effectiveness, side effects and long-term status of nightguard vital bleaching. J Am Dent Assoc 1994;125:1219–26. Leonard HJ. Efficacy, longevity, side effects, and patient perceptions of nightguard vital bleaching. Compend Contin Educ Dent 1998;19:766–81. Li Y. Safety controversies in tooth bleaching. Dent Clin North Am 2011;55:255–63. Bitter NC, Sanders JL. The effect of four bleaching agents on the enamel surface: a scanning electron microscopic study. Quintessence Int 1993;24:817–24. de Freitas PM, Basting RT, Rodrigues AL Jr, et al. Effects of two 10% peroxide carbamide bleaching agents on dentin microhardness at different time intervals. Quintessence Int 2002;33:370–5. Zantner C, Beheim-Schwarzbach N, Neuman K, et al. Surface microhardness of enamel after different home bleaching procedures. Dent Mater 2007;23:243–50.
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
Bitter NC. A scanning electron microscope study of the long-term effect of bleaching agents on the enamel surface in vivo. Gen Dent 1998;46:84–8. Hosoya N, Honda K, Iino F, et al. Changes in enamel surface roughness and adhesion of Streptococcus mutans to enamel after vital bleaching. J Dent 2003;31:543–8. Lopes GC, Bonissoni L, Baratieri LN, et al. Effect of bleaching agents on the hardness and morphology of enamel. J Esthet Restor Dent 2002;14:24–30. Titley K, Torneck C, Ruse N. Adhesion of composite resin to bleached and unbleached bovine enamel. J Dent Res 1988;67:1523–8. Titley KC, Torneck CD, Ruse ND. The effect of carbamide-peroxide gel on the shear bond strength of a microfil resin to bovine enamel. J Dent Res 1992;71:20–4. Torneck CD, Titley KC, Smith DC, et al. Adhesion of light-cured resin to bleached and unbleached bovine dentin. Endod Dent Traumatol 1990;6:97–103. Al-Salehi SK, Wood DJ, Hatton PV. The effect of 24 hour non-stop hydrogen peroxide concentration on bovine enamel and dentine mineral content and microhardness. J Dent 2007;35:845–50. Efeoglu N, Wood DJ, Efeoglu C. Thirty-five percent carbamide peroxide application cause in vitro demineralization of enamel. Dent Mater 2007;23:900–4. Yeh S-T, Su Y, Lu Y-C, et al. Surface changes and acid dissolution of enamel after carbamide peroxide bleach treatment. Oper Dent 2005;30:507–15. Sulieman M, Addy M, Macdonald E, et al. A safety study in vitro for the effects of an in-office bleaching system on the integrity of enamel and dentine. J Dent 2004;32:581–90. Seghi RR, Denry I. Effects of external bleaching on indentation and abrasion characteristics of human enamel in vitro. J Dent Res 1992;71:1340–4. Silva AP, Oliveira R, Cavalli V, et al. Effect of peroxide-based bleaching agents on enamel ultimate tensile strength. Oper Dent 2005;30:318–24. Chng HK, Palamara JEA, Messer HH. Effect of hydrogen peroxide and sodium perborate on biomechanical properties of human dentin. J Endod 2002;28:62–7. Kuga MC, Reis JMSN, Fabricio S, et al. Fracture strength of incisor crowns after intracoronal bleaching with sodium percarbonate. Dent Traumatol 2012;28:238–42. Ghavamnasiri M, Abedini S, Mehdizadeh Tazangi A. Effect of different time periods of vital bleaching on flexural strength of the bovine enamel and dentin complex. J Contemp Dent Pract 2007;8:21–8. Tam LE, Noroozi A. Effect of direct and indirect bleaching on dentin fracture toughness. J Dent Res 2007;86:1193–7.
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
24. Tam LE, Bahrami P, Oguienko O, et al. Effect of 10% and 15% carbamide peroxide on fracture toughness of human dentin in situ. Oper Dent 2013;38:142–50. 25. Kinney JH, Marshall SJ, Marshall GW. The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature. Crit Rev Oral Biol Med 2003;14:13–29. 26. Bader JD, Martin JA, Shugars DA. Incidence rates for complete cusp fracture. Community Dent Oral Epidemiol 2001;29:346–53. 27. Ellis SGS, McCord JF, Burke FJT. Predisposing and contributing factors for complete and incomplete tooth fractures. Dent Update 1999;26:150–8. 28. Ablal MA, Kaur JS, Cooper L, et al. The erosive potential of some alcopops using bovine enamel: an in vitro study. J Dent 2009;37:835–9. 29. Tonami K, Takahashi H, Nishimura F. Effect of frozen storage and boiling on tensile strength of bovine dentin. Dent Mater J 1996;15:205–11. 30. Nalla RK, Kinney JH, Marshall SJ, et al. On the in vitro fatigue behavior of human dentin: effect of mean stress. J Dent Res 2004;83:211–5. 31. Sato H, Ciucchi B, Matthews WG, et al. Tensile properties of mineralized and demineralized human and bovine dentin. J Dent Res 1994;73:1205–11. 32. Titley KC, Chernecky R, Rossouw PE, et al. The effect of various storage methods and media on shear-bond strengths of dental composite resin to bovine dentin. Arch Oral Biol 1998;43:305–11. 33. Tam LE, Lim M, Khanna S. Effect of direct peroxide bleach application to bovine dentin on flexural strength and modulus in vitro. J Dent 2005;33:451–8. 34. Nalla RK, Kinney JH, Ritchie RO. Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms. Biomaterials 2003;24:3955–68. 35. Jorgensen MG, Carroll WB. Incidence of tooth sensitivity after home whitening treatment. J Am Dent Assoc 2002;133:1076–82. 36. Leonard RH, Bentley C, Eagle JC, et al. Nightguard vital bleaching- a long term study on efficacy, shade retention, side effects and patients’ perception. J Esthet Restor Dent 2001;13:357–69. 37. Basting RT, Rodrigues AL Jr, Serra MC. The effects of seven carbamide peroxide bleaching agents on enamel microhardness over time. J Am Dent Assoc 2003;134:1335–42. 38. Basting RT, Rodrigues AL Jr, Serra MC. The effect of 10% carbamide peroxide, carbopol and/or glycerin on enamel and dentin microhardness. Oper Dent 2005;30:608–16. 39. Teixeira EC, Piascik JR, Stoner BR, et al. Dynamic fatigue and strength characterization of three ceramic materials. J Mater Sci Mater Med 2007;18:1219–24.
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
381
EFFECT OF BLEACH ON DENTIN Tam et al.
40. Braem M. Materials for minimally invasive treatments. In: Roulet JF, Vanherle G, editors. Adhesive technology for restorative dentistry. Berlin: Quintessence; 2005, pp. 87–95. 41. Forner L, Salmeron-Sanchez M, Palomares M, et al. The use of atomic force microscopy in determining the stiffness and adhesion force of human dentin after exposure to bleaching agents. J Endod 2009;35:1384–6. 42. Rotstein I, Dankner E, Goldman A, et al. Histochemical analysis of dental hard tissues following bleaching. J Endod 1996;22:23–6. 43. Jiang T, Ma X, Wang Y, et al. Effects of hydrogen peroxide on human dentin structure. J Dent Res 2007;86:1040–5. 44. Berger SB, Pavan S, Vidal CM, et al. Changes in the stiffness of demineralized dentin following application of tooth whitening agents. Acta Odontol Scand 2012;70:56–60. 45. Sato C, Rodrigues FA, Garcia DM, et al. Tooth bleaching increases dentinal protease activity. J Dent Res 2013;92:187–92. 46. Toledano M, Yamauti M, Osorio E, et al. Bleaching agents increase metalloproteinases-mediated collagen degradation in dentin. J Endod 2011;37:1668–72. 47. Eimar H, Siciliano R, Abdallah M-N, et al. Hydrogen peroxide whitens teeth by oxidizing the organic structure. J Dent 2012;40S:e25–33. 48. Fuss Z, Szajkis S, Tagger M. Tubular permeability to calcium hydroxide and to bleaching agents. J Endod 1989;15:362–4. 49. McCaslin A, Haywood V, Potter B, et al. Assessing dentin color changes from nightguard vital bleaching. J Am Dent Assoc 1999;130:1485–90.
382
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
50. Oliver T, Haywood V. Efficacy of nightguard vital bleaching technique beyond the borders of a shortened tray. J Esthet Dent 1999;11:95–102. 51. Cooper JS, Bokmeyer TJ, Bowles WH. Penetration of the pulp chamber by carbamide peroxide bleaching agents. J Endod 1992;18:315–7. 52. Hanks CT, Fat JC, Watah JC, et al. Cytotoxicity and dentin permeability of carbamide peroxide and hydrogen peroxide vital bleaching materials, in vitro. J Dent Res 1993;72:931–8. 53. Thitinanthapan E, Satamanont P, Vonsavan N. In vitro penetration of the pulp chamber by three brands of carbamide peroxide. J Esthet Dent 1999;11:259–64. 54. Khouroushi M, Mazaheri H, Manoochehri A. Effect of CPP-ACP application on flexural strength of bleached and dentin complex. Oper Dent 2011;36:372–9. 55. Tam L, Abdool R, El-Badrawy W. Flexural strength and modulus properties of carbamide-peroxide treated bovine dentin. J Esthet Restor Dent 2005;17:359–68. 56. Haywood VB, Leonard RH, Dickinson GL. Efficacy of six months of nightguard vital bleaching of tetracycline-stained teeth. J Esthet Dent 1997;9:13–29.
Reprint requests: Laura E. Tam, DDS, MSc, Restorative Dentistry, Room 352B, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON M5G 1G6, Canada; Tel.: 416-979-4934 X4420; Fax: 416-979-4936; email: [email protected]
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
Effect of Bleaching Treatment on Fatigue Resistance and Flexural Strength of Bovine Dentin LAURA E. TAM, DDS, MSC*, WOOHYUN CHO†, BRIDGET Y. WANG†, GRACE DE SOUZA, DDS, MSC, PHD‡
ABSTRACT Purpose: To determine the effects of bleach on dentin fatigue resistance and flexural strength. Materials and Methods: Eighty bovine dentin specimens (2 × 2 × 17 mm) were treated with: placebo or 10% carbamide peroxide bleach. Treatment was applied for 6 hours/day for 2 or 8 weeks. After treatment, 10 specimens per group were subjected to fatigue testing (106 cycles) whereas the other 10 were stored in artificial saliva as fatigue controls. The specimens undergoing fatigue were checked daily for visible signs of fracture and excluded from subsequent flexural strength tests if fractured. Fatigue control and surviving fatigued specimens were subjected to flexural strength testing. Chi-square, Kruskal–Wallis, factorial analysis of variance (p < 0.05) and Mann–Whitney (p < 0.002) tests were performed. Results: There were significant differences in fatigue resistance (p = 0.003) and flexural strength rank scores (p < 0.0001) among the groups. None of the specimens in the “8-week bleach” group survived the fatigue testing. Fatigue (p = 0.005) and interaction of time and treatment (p = 0.039) were significant factors in the flexural strength results. Fatigued specimens had lower flexural strength than nonfatigued and “8-week bleach” had lower flexural strength than placebo and “2-week bleach” groups. Conclusions: Prolonged direct bleaching of bovine dentin reduces its fatigue resistance and flexural strength in vitro. Further research is needed in this area.
CLINICAL SIGNIFICANCE It remains prudent to advise patients to limit their exposure to tooth bleaching materials by avoiding direct application of bleach to exposed dentin and by minimizing the duration of bleach treatment. (J Esthet Restor Dent 27:374–382, 2015)
INTRODUCTION Tooth bleaching is primarily performed using hydrogen peroxide or carbamide peroxide as the active ingredient. Primary methods of bleach treatment include at-home bleach application using custom-made bleaching trays, in-office bleach application with or without light activation, and at-home over-the-counter bleach application using strips. Tooth sensitivity has been identified as a common but transient side effect of bleach treatment1–4.
Long-term effects of bleach treatment are unknown. Bleach procedures can be prone to overuse in an attempt to achieve a whiter tooth color result, either by using too high a concentration of bleach or by bleaching for a prolonged period of time. Many studies have investigated the effects of tooth bleaching on enamel and dentin surface properties such as hardness,5–7 surface morphology,8–10 bonding,5,6,11–13 surface demineralization,14–16 and abrasion/erosion.17 Relatively fewer studies have investigated the effects of tooth bleaching on enamel and dentin mechanical
*Professor, Restorative Dentistry, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, Toronto, Canada † Undergraduate Research Student, Restorative Dentistry, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, Toronto, Canada ‡ Assistant Professor, Restorative Dentistry, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, Toronto, Canada
374
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
properties such as strength, fatigue, or fracture toughness. The ultimate tensile strength and fracture toughness of enamel was shown to be reduced significantly by different bleaching regimens.18,19 Significant reductions in tensile and shear strengths of dentin were reported after an in vitro direct intracoronal bleach application of 30% hydrogen peroxide.20 The in vitro fracture strength of bovine incisor crowns were reduced after intracoronal bleaching procedures.21 The flexural strength of bovine dentin was significantly reduced by an application of 20% carbamide peroxide bleach for 2 weeks.22 In another in vitro study, the fracture toughness of dentin was significantly reduced by the indirect (through intact enamel) application of peroxide bleaching agents (this represents a simulation of clinical bleaching of teeth with full enamel coverage), as well as by a direct bleach application method to dentin, and that it was reduced more for the longer application time period (8 versus 2 weeks) and for the higher (16% versus 10%) bleach concentration.23 The clinical relevance of these in vitro studies is uncertain and in vivo studies are lacking. One in situ study did report a decrease in dentin fracture toughness as a result of bleach application in a clinical setting.24 However, the decrease was not significant. It was speculated that the variability associated with the in situ setting contributed to a wide variation that is not present in in vitro studies. Fatigue tests are used to mimic the clinical situation where teeth are subject to cyclic loads during mastication and should be a more important test of long-term structural integrity than a test such as microhardness. Subcritical flaws can grow and coalesce with cyclic loading. Although the cyclic forces of mastication are generally insufficient to cause failure in perfect dentin, normal dentin contains pre-existing flaws that can propagate to catastrophic size under small cyclic loads.25 Tooth fracture is a significant problem that is predicted to become more prevalent as people retain their teeth longer. Data on the incidence of complete, incomplete, cuspal, and root fractures are extensive with the risk of noncarious tooth fracture at 72.7 per 1,000 person-years.26,27 Fatigue leads to a weakened tooth which then fractures under a minimal load. The advent of widely available tooth bleaches was
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
only 10–20 years ago. It is possible that the incidence of tooth fractures attributable to tooth bleaching will manifest after a lifetime of chewing as a result of crack propagation from initial microscopic flaws introduced by tooth bleaching. Therefore, fatigue tests may be useful to show the effects of microscopic alterations to dentin which are not initially apparent at early testing periods without fatigue loading. A fatigue test may be important to help clarify the biomechanical consequences of changes to the mineral-collagen complex of bleached dentin. The objective of this study is to determine the effects of dental bleach on the fatigue resistance and flexural strength of bovine dentin in vitro. The null hypothesis for this study is: bleaching has no effect on the fatigue resistance and flexural strength of bovine dentin in vitro.
MATERIALS AND METHODS Bovine incisors (extracted within 6 months of the experiment, frozen until use) were collected to provide the dentin for testing. One dentin specimen was obtained from each tooth except occasionally, two dentin specimens were obtained from larger teeth. Flexural strength test rectangular beams, approximately 2 × 2 × 17 mm, were prepared from the dentin on the facial surface using a water-cooled low-speed diamond saw (Buehler Ltd., Lake Bluff, IL, USA), keeping the location and orientation of the dentin as standardized as possible. A micrometer (Digimatic Caliper, Mitutoyo Corporation, Kanagawa, Japan) was used to measure specimen dimensions to the nearest 0.01 mm. The dentin specimens were stored in freshly made artificial saliva28 in individual containers until testing. The test groups are outlined in Table 1. The specimens were randomly divided into two groups of 40: placebo or bleach (Opalescence, Lot No. B81FR & B8295, Ultradent Products Inc, South Jordan, UT, USA). The placebo (Placebo, Ultradent Products Inc, South Jordan, UT, USA) contained glycerine, carbopol, and water whereas the bleach gel contained the same components with the addition of 10% carbamide peroxide. A bleach
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
375
EFFECT OF BLEACH ON DENTIN Tam et al.
TABLE 1. Outline of test groups Treatment
Placebo (glycerine, carbopol, water)
Bleach (10% carbamide peroxide, glycerine, carbopol, water)
Time
2 weeks (6 hours × 14 days)
8 weeks (6 hours × 48 days)
2 weeks (6 hours × 14 days)
8 weeks (6 hours × 48 days)
Fatigue*
N-F
F
N-F
F
N-F
F
N-F
F
Initial sample size
N = 10
N = 10
N = 10
N = 10
N = 10
N = 10
N = 10
N = 10
*N-F = not fatigued (storage in 37°C artificial saliva only prior to flexural strength testing); F = fatigued (fatigue loading at 20 MPa and 2 Hz for 106 cycles in 37°C artificial saliva prior to flexural strength testing).
treatment consisted of an application of a 1–2 mm-thick layer of fresh bleach gel to completely surround the tooth specimen surface for six continuous hours. The specimens were stored at 37°C, >80% relative humidity for the duration of the bleach treatment. Bleach was applied either daily for 2 weeks or 6 days a week for 8 weeks to simulate a typical and a prolonged at-home overnight bleaching regimen, respectively. At the end of each bleach treatment, the specimens were rinsed with tap water to remove all external traces of bleach and the specimens were stored in 37°C artificial saliva until the next bleach treatment. At the end of the last bleaching session, 10 specimens from each group were randomly selected for fatigue testing whereas the other 10 specimens remained in artificial saliva as control specimens. The dentin specimens for fatigue testing were mounted in custom-designed holders with a span distance of 8 mm, immersed into 37°C artificial saliva, and subjected to fatigue loading at 20 MPa and 3 Hz for 106 cycles using a chewing simulator (CS 4.4 Chewing Simulator, SD Mechatronik) (Feldkirchen-Westerham, Bayern, Germany). The artificial saliva was changed daily. The specimens were checked daily for visible signs of fracture and fractured specimens were excluded from subsequent flexural strength tests. Specimens that survived the fatigue loading were subjected to the flexural strength test. Ten days after the last bleach application, control and fatigued specimens were mounted on an Instron universal testing machine (Model 4301, Instron Corp, Canton, MA, USA) for three-point bending flexural strength test using a custom-designed mounting jig with a span distance of
376
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
14 mm. During testing, the mounting apparatus was immersed in a 37 ± 2°C water bath. Flexural loading as applied at a rate of 0.5 mm/minute until specimen fracture. The force recorded at fracture was used to calculate flexural strength using the following equation:
FS = 3Pf L 2WH2 where Pf is the measured maximum load at the time of specimen fracture, L is the distance between the supports on the tension surface, W is the mean specimen width, and H is the mean height of the specimen between the tension and compression surfaces. For statistical analyses, the issue of missing flexural strength values in the fatigued groups was handled using nonparametric statistical analyses. The specimens that did not survive the fatigue test were assigned the lowest rank (lowest rank = 1) and the Kruskal–Wallis was applied to evaluate the flexural strength results (p < 0.05). Pairwise Mann–Whitney tests with a Bonferroni correction were used for post hoc analyses (p < 0.002). The chi-square test was used to compare the proportion of survivors among the groups that underwent fatigue testing (p < 0.05). For additional information, a mean flexural strength value for each group was determined from the specimens that did survive the fatigue test and a factorial analysis of variance was conducted to evaluate the significance of the factors treatment (placebo versus bleach), time (2 versus 8 weeks), and fatigue (fatigued versus nonfatigued).
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
RESULTS All specimens in the bleached groups were noticeably whiter than the specimens in the placebo groups. Two specimens broke accidentally, leaving 78 specimens for mechanical testing.
Fatigue Resistance Figure 1 shows the percentage of specimens that survived the fatigue loading. Specimen failure was observed in all groups during the fatigue test and these failures all occurred within the first 250,000 cycles. The “8-week bleach” group had a 100% failure rate. There were significant differences in fatigue resistance among
% Survival
Fagued specimens 90 80 70 60 50 40 30 20 10 0
Fracon survival
the groups (p = 0.003). The adjusted residuals for the “2-week placebo” and “8-week bleach” group suggested a greater incidence of survival and failure, respectively, than the null hypothesis predicts.
Flexural Strength The flexural strength rank scores derived from the Kruskal–Wallis test are presented in Figure 2. There were significant differences in flexural strength among the groups (p < 0.0001). The flexural strength rank results for the “8-week placebo” and for the “2- and 8week” bleach treatment groups were significantly decreased by the fatigue loading test. The flexural strength rank results were not significantly changed by a “2-week bleach” treatment but they were significantly decreased in the “8-week bleach” treatment groups, with or without fatigue loading. The mean flexural strength results of the nonfatigued groups and the survivors of the fatigue test in the fatigued groups are presented in Table 2. Fatigue was a significant factor in the flexural strength results (p = 0.005). The interaction of time and treatment was also a significant factor in the flexural strength results (p = 0.039).
2-week placeboa 2-week bleachb 8-week placebob 8-week bleachc
8/10
6/10
5/9
0/10
FIGURE 1. Percentage of fatigued dentin specimens that survived the fatigue loading. There were significant differences in fatigue resistance among the groups (p = 0.003). Groups denoted by the same letter indicate no significant difference (p < 0.05).
DISCUSSION Bovine incisors were selected over human teeth as the source of dentin primarily because human teeth are too small to provide the size of dentin needed for the flexural strength tests. The fatigue properties of bovine
FIGURE 2. Flexural strength rank scores (derived from the Kruskal–Wallis test). There were significant differences in fatigue resistance among the groups (p < 0.0001). Groups denoted by the same letter indicate no significant difference (p < 0.05).
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
377
EFFECT OF BLEACH ON DENTIN Tam et al.
TABLE 2. Flexural strength results (mean, standard deviation, and sample size) derived from all the specimens that were tested for flexural strength in the nonfatigued groups and from the survivors of the fatigue test that made it to the flexural strength test in the fatigued groups Treatment
Placebo
Bleach
Time
2 weeks
Fatigue*
N-F
F
N-F
F
N-F
F
N-F
F
Mean (MPa)
95
84
84
67
100
72
60
—
Standard deviation (MPa)
23
20
18
17
18
40
12
—
N
10
8
9
5
10
6
10
0
8 weeks
2 weeks
8 weeks
Fatigue (p = 0.005) and the interaction of time and treatment (p = 0.039) were significant factors in the flexural strength results. *N-F = not fatigued (storage in 37°C artificial saliva only); F = fatigued (fatigue loading at 20 MPa and 2 Hz for 106 cycles in 37°C artificial saliva).
dentin29 and human dentin30 have been shown to be similar. Ultimate tensile strength and elastic modulus values for bovine and human dentin have also been shown to be similar.31 Previous studies have shown that frozen storage did not significantly affect the tensile strength of or the shear bond strengths to bovine dentin.29,32 Furthermore, the reported adverse effects to the flexural strength of bovine dentin33 were similar to the reported adverse effects to the fracture toughness of human dentin.23 Therefore, we believe that the effect that was observed on bovine dentin in this study would be comparable with one on human dentin. The specimens were all loaded in a parallel direction relative to the dentinal tubules. Nalla and colleagues34 reported no significant differences between the “in-plane parallel” and “anti-plane parallel” fracture toughness of dentin. A unique aspect of this paper is that it specifically addresses the structural properties of bleached dentin using fatigue loading in addition to a flexural test. Typical fatigue tests of dentin indicate that failure is caused by the initiation and growth of a single, dominant crack and that dentin is designed to not fail from normal mastication loads of around 20 MPa.30 The load parameter for our fatigue test was selected to be below the fatigue endurance limit for normal dentin. However, there were specimen failures in all the groups that underwent fatigue testing, including the placebo groups. The placebo treatment did appear to have some effect on dentin as the flexural strength and fatigue
378
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
resistance results were generally lower, albeit not statistically, for the “8-week placebo” groups compared with the “2-week placebo” groups. We speculate that the placebo treatment led to reduced fatigue resistance and flexural strength as a result of some tooth dehydration, demineralization, or dentin matrix degradation that occurred during the placebo treatment time. Although all the treatments were conducted in >80% relative humidity, it is possible that the treatment gel acted as a barrier to the humidity in the test chamber. In the mouth, both the treatment gel and the bleaching tray could act as barriers that lead to transient tooth dehydration. Placebo materials have been associated with other negative effects. They caused tooth sensitivity in clinical bleaching studies.2,35,36 They reduced enamel and dentin microhardness and this reduction was attributed to a demineralization or a barrier effect of the placebo.37,38 The results show large standard deviations but they are not out of line with biological specimens. Despite the large standard deviations, the differences were great enough to show statistically significant differences using less sensitive nonparametric statistics. The significant interaction effect of time and treatment can be seen in the significantly lower flexural strength rank scores for the “8-week bleach” treatment groups, with and without fatigue. A decrease in dentin flexural strength was not significant after 2 weeks but was significant after 8 weeks of bleaching. The decrease in dentin flexural strength in the nonfatigued “8-week bleach” treatment
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
group was similar to the decrease observed caused by the fatigue testing on the placebo and “2-week bleach” groups. In other words, the in vitro effect of an 8-week bleach treatment (without fatigue) on flexural strength was comparable with the in vitro effect of 1 million fatigue cycles. This would be equivalent to at least 1 year of clinical functional aging as it has been estimated that a normal adult would perform an average of 500,000 to 1,000,000 loading cycles per year.25,39,40 The mechanism for the reduction in dentin fatigue resistance and flexural strength by bleaching agents is not fully understood. Nanoindentation tests led to the conclusion that both intertubular and peritubular dentin are affected by carbamide and hydrogen peroxide.41 Bleaching agents may adversely affect dentin properties by affecting the inorganic mineral component of dentin. It has been suggested that hydrogen peroxide increases the release of calcium and phosphorus from bovine enamel and dentin.14,42 There may have been more dentin demineralization in the bleach groups than in the placebo groups as a result of a relative difference in pH (bleach pH: 6.4, placebo pH: 6.8). Alternatively, it has been suggested that the dentin demineralization can be attributed more to the oxidizing ability than to the acidity of hydrogen peroxide.43 Data are emerging to suggest that the organic component of the tooth is the primary target of hydrogen peroxide oxidation. This suggests that dentin would be more susceptible than enamel to adverse bleach effects. A study on the effect of hydrogen peroxide bleaches on the stiffness of demineralized dentin suggested that dental bleach interacted specifically with the organic dentin matrix.44 Recent interesting studies have reported that hydrogen and carbamide peroxide increased matrix metalloproteinase-mediated collagen degradation and cathepsin B proteolytic activity in dentin with home bleaching inducing the highest collagen degradation.45,46 Furthermore, the mechanism for enamel whitening was determined to be caused by changes to the organic component of enamel and not the inorganic component of enamel, and the authors of this paper suggested that accordingly, hydrogen peroxide could have a similar or even stronger effect on dentin as it contains a higher organic content.47
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
Fatigue loading was a significant factor that caused a reduction in flexural strength. The flexural strength rank results were significantly decreased by the fatigue loading test for all treatments except for the “2-week placebo” group (Figure 2). The mean flexural strength results of the surviving specimens in the fatigued groups were also lower than in the nonfatigued groups (Table 2) and it can be assumed that the mean flexural strength results of the fatigued groups should be even lower than what was reported because only the surviving specimens were used for the mean calculation. The effect of fatigue appeared to be a more significant factor than the interaction effect of time and treatment. This confirms the importance of the effect of fatigue on dentin longevity. An initially small adverse effect on dentin structure, potentially caused by bleach treatment, could magnify and have a great impact over the lifetime of a tooth as a result of fatigue and crack propagation. The combination of “8-week bleach” treatment and fatigue loading was particularly destructive as all of the dentin specimens in this group did not survive the fatigue test. An effect of such magnitude would not be expected in vivo. The intraoral environment will dilute the bleaching agent much more than the in vitro situation. The small cross-section of the dentin specimens in this study would allow for greater permeation of the bleaching agent into dentin than a bulky tooth with an enamel covering. Furthermore, the usual intended substrate for bleach application is enamel, not dentin. We have focused our attention on dentin because it is the foundational material for tooth structure and it is unquestionably exposed to tooth bleach, both directly and indirectly. There would be many clinical situations in which tooth bleach is applied directly to the dentin (cases of root exposure, occlusal attrition, and in cases of nonvital tooth bleaching). That being said, enamel is not considered an impermeable barrier to hydrogen peroxide or carbamide peroxide.48–50 The ability of hydrogen peroxide and carbamide peroxide bleaching agents to readily penetrate through intact enamel and dentin is evident from both the significant amounts of bleach measured in the pulp chambers after bleach application in vitro51–53 and from reported tooth sensitivity during
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
379
EFFECT OF BLEACH ON DENTIN Tam et al.
indirect bleach application in vivo. The diffusion data for hydrogen peroxide and carbamide peroxide suggested that bleaching agents were capable of diffusing through enamel and dentin within 15 and 25 minutes.52,53 An in vitro comparison of a direct application of bleach to dentin compared with an indirect application of bleach to dentin through an intact enamel surface (the latter represents a simulation of clinical bleaching of teeth with full enamel coverage) showed a decreased but still significant adverse effect on human dentin fracture toughness with the indirect application method.23 The “8-week bleach” group was devised to test the effect of a prolonged or exaggerated at-home bleach treatment regimen. Tooth bleaching treatment was originally only administered under the supervision of a dentist and the bleach concentration and application were controlled. Currently, tooth bleaches are available at significantly higher concentrations, in a greater variety of products and in a less controlled manner (over-the-counter), so it is possible that there are patients that will bleach their teeth for an accumulated time of 8 weeks. However, patients will use shorter bleach application times with over-the-counter products and generally will not bleach their teeth on a daily basis for such a prolonged period. We have subsequently performed further fatigue testing (with an over-the-counter product) to confirm the findings of this study (manuscript in preparation). Longer intervals between bleaching treatments may provide greater opportunities for tooth remineralization and restoration of tooth mechanical properties. The use of water, artificial saliva, fluoride, and casein phosphopeptide-amorphous calcium phosphate has been investigated with equivocal results for recovery of mechanical properties.44,54,55
380
significance of this study can be questioned. There have been little or no long-term adverse effects associated with tooth bleaching, even after prolonged periods of time.56 However, the property of fatigue resistance has never been specifically investigated and the issue of bleach effects on long-term structural integrity remains an important issue, especially when you consider the widespread usage of bleach products. The observed significantly reduced fatigue resistance and flexural strength of dentin and in particular, the premature fracture of all “8-week bleach” specimens reported in this first in vitro study using fatigue suggest that further research is needed in this area.
DISCLOSURE AND ACKNOWLEDGEMENTS The study was supported by a Dental Research Institute, Faculty of Dentistry, University of Toronto grant number 13-14-4. The Opalescence 10% carbamide peroxide was provided by Ultradent Products Inc.
REFERENCES 1.
2.
3.
4. 5.
CONCLUSIONS
6.
The results of this study suggest that fatigue resistance and flexural strength of bovine dentin are compromised by the direct application of a prolonged home bleaching procedure using 10% carbamide peroxide. The clinical
7.
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
Cardoso PC, Reis A, Loguercio A, et al. Clinical effectiveness and tooth sensitivity associated with different bleaching times for a 10 percent carbamide peroxide gel. J Am Dent Assoc 2010;141:1213–20. Haywood VB, Leonard RH, Nelson CF, et al. Effectiveness, side effects and long-term status of nightguard vital bleaching. J Am Dent Assoc 1994;125:1219–26. Leonard HJ. Efficacy, longevity, side effects, and patient perceptions of nightguard vital bleaching. Compend Contin Educ Dent 1998;19:766–81. Li Y. Safety controversies in tooth bleaching. Dent Clin North Am 2011;55:255–63. Bitter NC, Sanders JL. The effect of four bleaching agents on the enamel surface: a scanning electron microscopic study. Quintessence Int 1993;24:817–24. de Freitas PM, Basting RT, Rodrigues AL Jr, et al. Effects of two 10% peroxide carbamide bleaching agents on dentin microhardness at different time intervals. Quintessence Int 2002;33:370–5. Zantner C, Beheim-Schwarzbach N, Neuman K, et al. Surface microhardness of enamel after different home bleaching procedures. Dent Mater 2007;23:243–50.
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
EFFECT OF BLEACH ON DENTIN Tam et al.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
Bitter NC. A scanning electron microscope study of the long-term effect of bleaching agents on the enamel surface in vivo. Gen Dent 1998;46:84–8. Hosoya N, Honda K, Iino F, et al. Changes in enamel surface roughness and adhesion of Streptococcus mutans to enamel after vital bleaching. J Dent 2003;31:543–8. Lopes GC, Bonissoni L, Baratieri LN, et al. Effect of bleaching agents on the hardness and morphology of enamel. J Esthet Restor Dent 2002;14:24–30. Titley K, Torneck C, Ruse N. Adhesion of composite resin to bleached and unbleached bovine enamel. J Dent Res 1988;67:1523–8. Titley KC, Torneck CD, Ruse ND. The effect of carbamide-peroxide gel on the shear bond strength of a microfil resin to bovine enamel. J Dent Res 1992;71:20–4. Torneck CD, Titley KC, Smith DC, et al. Adhesion of light-cured resin to bleached and unbleached bovine dentin. Endod Dent Traumatol 1990;6:97–103. Al-Salehi SK, Wood DJ, Hatton PV. The effect of 24 hour non-stop hydrogen peroxide concentration on bovine enamel and dentine mineral content and microhardness. J Dent 2007;35:845–50. Efeoglu N, Wood DJ, Efeoglu C. Thirty-five percent carbamide peroxide application cause in vitro demineralization of enamel. Dent Mater 2007;23:900–4. Yeh S-T, Su Y, Lu Y-C, et al. Surface changes and acid dissolution of enamel after carbamide peroxide bleach treatment. Oper Dent 2005;30:507–15. Sulieman M, Addy M, Macdonald E, et al. A safety study in vitro for the effects of an in-office bleaching system on the integrity of enamel and dentine. J Dent 2004;32:581–90. Seghi RR, Denry I. Effects of external bleaching on indentation and abrasion characteristics of human enamel in vitro. J Dent Res 1992;71:1340–4. Silva AP, Oliveira R, Cavalli V, et al. Effect of peroxide-based bleaching agents on enamel ultimate tensile strength. Oper Dent 2005;30:318–24. Chng HK, Palamara JEA, Messer HH. Effect of hydrogen peroxide and sodium perborate on biomechanical properties of human dentin. J Endod 2002;28:62–7. Kuga MC, Reis JMSN, Fabricio S, et al. Fracture strength of incisor crowns after intracoronal bleaching with sodium percarbonate. Dent Traumatol 2012;28:238–42. Ghavamnasiri M, Abedini S, Mehdizadeh Tazangi A. Effect of different time periods of vital bleaching on flexural strength of the bovine enamel and dentin complex. J Contemp Dent Pract 2007;8:21–8. Tam LE, Noroozi A. Effect of direct and indirect bleaching on dentin fracture toughness. J Dent Res 2007;86:1193–7.
© 2015 Wiley Periodicals, Inc.
DOI 10.1111/jerd.12157
24. Tam LE, Bahrami P, Oguienko O, et al. Effect of 10% and 15% carbamide peroxide on fracture toughness of human dentin in situ. Oper Dent 2013;38:142–50. 25. Kinney JH, Marshall SJ, Marshall GW. The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature. Crit Rev Oral Biol Med 2003;14:13–29. 26. Bader JD, Martin JA, Shugars DA. Incidence rates for complete cusp fracture. Community Dent Oral Epidemiol 2001;29:346–53. 27. Ellis SGS, McCord JF, Burke FJT. Predisposing and contributing factors for complete and incomplete tooth fractures. Dent Update 1999;26:150–8. 28. Ablal MA, Kaur JS, Cooper L, et al. The erosive potential of some alcopops using bovine enamel: an in vitro study. J Dent 2009;37:835–9. 29. Tonami K, Takahashi H, Nishimura F. Effect of frozen storage and boiling on tensile strength of bovine dentin. Dent Mater J 1996;15:205–11. 30. Nalla RK, Kinney JH, Marshall SJ, et al. On the in vitro fatigue behavior of human dentin: effect of mean stress. J Dent Res 2004;83:211–5. 31. Sato H, Ciucchi B, Matthews WG, et al. Tensile properties of mineralized and demineralized human and bovine dentin. J Dent Res 1994;73:1205–11. 32. Titley KC, Chernecky R, Rossouw PE, et al. The effect of various storage methods and media on shear-bond strengths of dental composite resin to bovine dentin. Arch Oral Biol 1998;43:305–11. 33. Tam LE, Lim M, Khanna S. Effect of direct peroxide bleach application to bovine dentin on flexural strength and modulus in vitro. J Dent 2005;33:451–8. 34. Nalla RK, Kinney JH, Ritchie RO. Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms. Biomaterials 2003;24:3955–68. 35. Jorgensen MG, Carroll WB. Incidence of tooth sensitivity after home whitening treatment. J Am Dent Assoc 2002;133:1076–82. 36. Leonard RH, Bentley C, Eagle JC, et al. Nightguard vital bleaching- a long term study on efficacy, shade retention, side effects and patients’ perception. J Esthet Restor Dent 2001;13:357–69. 37. Basting RT, Rodrigues AL Jr, Serra MC. The effects of seven carbamide peroxide bleaching agents on enamel microhardness over time. J Am Dent Assoc 2003;134:1335–42. 38. Basting RT, Rodrigues AL Jr, Serra MC. The effect of 10% carbamide peroxide, carbopol and/or glycerin on enamel and dentin microhardness. Oper Dent 2005;30:608–16. 39. Teixeira EC, Piascik JR, Stoner BR, et al. Dynamic fatigue and strength characterization of three ceramic materials. J Mater Sci Mater Med 2007;18:1219–24.
Journal of Esthetic and Restorative Dentistry
Vol 27 • No 6 • 374–382 • 2015
381
EFFECT OF BLEACH ON DENTIN Tam et al.
40. Braem M. Materials for minimally invasive treatments. In: Roulet JF, Vanherle G, editors. Adhesive technology for restorative dentistry. Berlin: Quintessence; 2005, pp. 87–95. 41. Forner L, Salmeron-Sanchez M, Palomares M, et al. The use of atomic force microscopy in determining the stiffness and adhesion force of human dentin after exposure to bleaching agents. J Endod 2009;35:1384–6. 42. Rotstein I, Dankner E, Goldman A, et al. Histochemical analysis of dental hard tissues following bleaching. J Endod 1996;22:23–6. 43. Jiang T, Ma X, Wang Y, et al. Effects of hydrogen peroxide on human dentin structure. J Dent Res 2007;86:1040–5. 44. Berger SB, Pavan S, Vidal CM, et al. Changes in the stiffness of demineralized dentin following application of tooth whitening agents. Acta Odontol Scand 2012;70:56–60. 45. Sato C, Rodrigues FA, Garcia DM, et al. Tooth bleaching increases dentinal protease activity. J Dent Res 2013;92:187–92. 46. Toledano M, Yamauti M, Osorio E, et al. Bleaching agents increase metalloproteinases-mediated collagen degradation in dentin. J Endod 2011;37:1668–72. 47. Eimar H, Siciliano R, Abdallah M-N, et al. Hydrogen peroxide whitens teeth by oxidizing the organic structure. J Dent 2012;40S:e25–33. 48. Fuss Z, Szajkis S, Tagger M. Tubular permeability to calcium hydroxide and to bleaching agents. J Endod 1989;15:362–4. 49. McCaslin A, Haywood V, Potter B, et al. Assessing dentin color changes from nightguard vital bleaching. J Am Dent Assoc 1999;130:1485–90.
382
Vol 27 • No 6 • 374–382 • 2015
Journal of Esthetic and Restorative Dentistry
50. Oliver T, Haywood V. Efficacy of nightguard vital bleaching technique beyond the borders of a shortened tray. J Esthet Dent 1999;11:95–102. 51. Cooper JS, Bokmeyer TJ, Bowles WH. Penetration of the pulp chamber by carbamide peroxide bleaching agents. J Endod 1992;18:315–7. 52. Hanks CT, Fat JC, Watah JC, et al. Cytotoxicity and dentin permeability of carbamide peroxide and hydrogen peroxide vital bleaching materials, in vitro. J Dent Res 1993;72:931–8. 53. Thitinanthapan E, Satamanont P, Vonsavan N. In vitro penetration of the pulp chamber by three brands of carbamide peroxide. J Esthet Dent 1999;11:259–64. 54. Khouroushi M, Mazaheri H, Manoochehri A. Effect of CPP-ACP application on flexural strength of bleached and dentin complex. Oper Dent 2011;36:372–9. 55. Tam L, Abdool R, El-Badrawy W. Flexural strength and modulus properties of carbamide-peroxide treated bovine dentin. J Esthet Restor Dent 2005;17:359–68. 56. Haywood VB, Leonard RH, Dickinson GL. Efficacy of six months of nightguard vital bleaching of tetracycline-stained teeth. J Esthet Dent 1997;9:13–29.
Reprint requests: Laura E. Tam, DDS, MSc, Restorative Dentistry, Room 352B, Department of Clinical Sciences, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON M5G 1G6, Canada; Tel.: 416-979-4934 X4420; Fax: 416-979-4936; email: [email protected]
DOI 10.1111/jerd.12157
© 2015 Wiley Periodicals, Inc.
