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Micromorphology of Enamel Surface After Vital Tooth Bleaching
Basic Research—Technology
Micromorphology of Enamel Surface After Vital Tooth Bleaching Ljubisa Markovic, DDS,* Rainer Andreas Jordan, DDS,* Nebojsa Lakota,† and Peter Gaengler, PhD, DDS* Abstract The aim of the present study was to assess microroughness changes of enamel surfaces after bleaching procedures with 10% and 16% concentrations of carbamide peroxide by confocal laser scanning microscopy. Twenty caries-free incisors, extracted for periodontitis reasons, were sectioned into two halves of experimental and control specimens. The teeth were divided into two groups, and the experimental specimens were exposed to either 10% or 16% carbamide peroxide for 4 hours per 7 days. Measurements were made at three randomly selected 140 ⫻ 100 m areas of 10 experimental and control specimens per group at the same crown level. Microroughness was measured in “total roughness” (Rt) and “average roughness” (Ra) descriptor values. The statistical analysis showed significantly higher microroughness according to Rt values and Ra values for both groups of carbamide peroxide exposed enamel surfaces. (J Endod 2007;33:607– 610)
Key Words Bleaching, carbamide peroxide, confocal microscopy, enamel
P
resent tooth-bleaching techniques are based on hydrogen peroxide as the active agent (1). It may be applied directly or released by a chemical reaction from carbamide peroxide (2). The latter acts as a deposit material releasing hydrogen peroxide and urea in liquid solution. Hydrogen peroxide liberates in further reactions free radicals, reactive oxygen molecules, and hydroxyl anions (3). Enamel and dentin have shown high permeability to hydrogen peroxide (4), although the process of molecular permeation has not been identified yet. The whitening effect is supposedly because of degradation of complex organic structures of high molecular weight, reflecting a specific wavelength of light. The resulting degradation leads to a reduction or elimination of the discoloration (5). However, there is no in vitro optical assessment of changing enamel translucency after bleaching. To date, there is no agreement concerning the severity of morphologic alteration or changes of biomechanical properties of the enamel (6 –10). Most of the studies in the present literature used a two-dimensional analysis by scanning electron microscopy (11). Therefore, the purpose of this investigation was to assess the effect of carbamide peroxide on human dental enamel by three-dimensional documentation of superficial micromorphology. The microstructural alterations caused by 10% and 16% concentrations of carbamide peroxide were studied by confocal laser scanning microscopy (CLSM), which is known in dental hard tissue research as a nondestructive technique of microscopic tomography (12). These bleaching formulations are accepted by the American Dental Association as “safe” and “effective” products to be used at home (13).
Materials and Methods From the *Department of Conservative Dentistry, University of Witten/Herdecke, Witten, Germany; and †Institute of Materials/Material Testing, Ruhr University of Bochum, Germany. Address requests for reprints to Dr. Ljubisa Markovic, Department of Conservative Dentistry, Faculty of Dental Medicine, University of Witten/Herdecke, Alfred Herrhausen-Straße 50, 58445 Witten, Germany. E-mail address: Ljubisa.Markovic@ uni-wh.de. 0099-2399/$0 - see front matter Copyright © 2007 by the American Association of Endodontists. doi:10.1016/j.joen.2007.01.011
JOE — Volume 33, Number 5, May 2007
Twenty sound human incisors, extracted for periodontitis reasons, were stored in saline containing 0.1% thymol. Teeth exhibiting any visible cracks or hypoplastic defects were excluded. The specimens have been thoroughly cleaned by using a fluoridefree prophylaxis paste (Miraclean, Hager und Werken, Duisburg, Germany) and were randomly subdivided into two groups. Tooth crowns were sectioned cervicoincisally into two equal halves using a diamond saw, representing the experimental and the control specimen per tooth. Group 1 was treated with 10% carbamide peroxide, pH 6.4, releasing ⬵3.6% H2O2. Group 2 was treated with 16% concentration of carbamide peroxide, pH 6.2, releasing ⬵5.8% H2O2. Bleaching agents were applied for 7 days, for 4 hours each day, and were removed by rinsing the specimens under running tap water for 30 seconds. During nontreatment times, all teeth were placed in artificial saliva containing, 150 mmol/L KCl, 1.5 mmol/l CaCl2, and 0.9 mmol/L KH2PO4 per 100 mL aqua bidest., 6.9 to 7.0 pH, in an incubator at 37°C. The samples were examined by CLSM (Leica ICM 1000, Wetzlar, Germany) using an Argon-LASER at 635 nm excitation and ⫻500 magnification. Measurements were made at three randomly selected 140 ⫻ 100 m areas of each tooth halved at the same crown level. Microroughness (in micrometers) was measured in two ways: (1) “total roughness” Rt (ISO 4287), which is made up of the sum of the “maximum peak” (Rp) and the “deepest valley” (Rv) of the roughness profile, and (2) “average roughness” Ra (ISO 4287), which reflects average condition of the roughness profile, given by the sum of the absolute values of all areas above and below a tomographic mean line. Confocal laser scanning microscopy (CLSM) parameters Rt and Ra were determined for the experimental and the control tooth halves and statistically analyzed by using a Wilcoxon signed rank test for paired comparisons within a group, and a Mann-
Enamel Surface and Vital Tooth Bleaching
607
Basic Research—Technology
Figure 1. 3D-CLSM appearance of enamel surfaces of control and bleached regions of human incisors: (A) control region exhibiting a smooth structure, average roughness Ra 0.19 m (magnification ⫻500); (B) bleached region after exposure to 10% carbamide peroxide showing increased surface roughness, Ra 0.29 m (magnification ⫻500); (C) control region exhibiting a smooth structure, average roughness Ra 0.25 m (magnification ⫻500); and (D) bleached region after exposure to 16% carbamide peroxide showing increased surface roughness, Ra 0.34 m (magnification ⫻500).
Whitney U test for unpaired analysis between groups. The level of significance accepted for all parameters was p ⱕ 0.05.
Results The CLSM appearance differs in all bleached enamel areas of both experimental groups compared with the individual nonbleached control areas of each tooth. Figure 1 shows examples of the three-dimensional structure of the assessed enamel surface of both bleached and unbleached regions at same crown level. Increasing brighter yellow color fractions indicate an inhomogeneous texture with higher Ra and Rt values. Optical darker control areas impress with a smoother surface structure, in comparison to the lighterappearing bleached regions. Analysis showed that the microroughness, in particular Rt and Ra, of the treated halves was significantly higher than in control regions and was independent from bleaching agent concentrations (p ⬍ 0.04). For teeth treated with 10% carbamide peroxide, the Rt and the Ra statistical values were significantly different to untreated regions. The mean Rt value of the treated surface was 3.91 m in contrast to the control side 608
Markovic et al.
with 3.17 m (p ⬍ 0.01)(Fig. 2A). Mean Ra difference between the treated and control side was 0.09 m (p ⬍ 0.02) (Fig. 2E). For teeth treated with 16% carbamide peroxide, mean Rt value was 6.3 m in the treated regions and 4.27 m in the controls (p ⬍ 0.02) (Fig. 2C). Ra parameters were significantly different between the treated and control sides (p ⬍ 0.04) (Fig. 2D). Comparison between the treated regions of both concentrations showed a statistically significant higher value for Rt in the 16% group (p ⬍ 0.01) (Fig. 2B), whereas Ra was not significantly different (p ⫽ 0.9) (Fig. 2F). The microroughness in all control regions of both groups was statistically not different (p ⫽ 0.07).
Discussion The CLSM was first suggested and used in dentistry by Watson to visualize tooth-restoration interface (14 –16). Its principle is based on the elimination of nonfocal portions of the image by confocal apertures and to achieve in this way a more compact point response and higher image fidelity in comparison with conventional methods of light microscopy. The visualization is carried out in pseudocolors, yellow and red,
JOE — Volume 33, Number 5, May 2007
Basic Research—Technology
Figure 2. Overview of Ra and Rt statistical values: (A) total roughness Rt (in micrometers) of the 10% carbamide peroxide group, bleached versus control regions; (B) comparison between the treated regions of both concentrations showed a statistically significant increase of the total roughness (Rt), 10% bleached versus 16% bleached; (C) total roughness Rt (in micrometers) of the 16% carbamide peroxide group, bleached versus control regions; (D) average roughness Ra (in micrometers) of the 16% carabmide peroxide group, bleached versus control regions; (E) average roughness Ra (in micrometers) of the 10% carbamide peroxide group, bleached versus control regions; and (F) comparison between the treated regions of both concentrations showed no statistically significant increase of the average roughness (Ra), 10% Ra bleached versus 16% bleached.
showing high intensities of reflected light. Zones, from which no light is emitted, appear black. In this way, CLSM can be used for example to describe microroughness on dental hard tissues (17). Because it was the aim of the present study to assess clinically relevant micromorphologic alterations of enamel surfaces after bleaching procedures, the results showed a statistically significant increase in Rt and Ra in general, after bleaching agent exposure with two different carbamide peroxide concentrations. These findings correspond with other results showing a semiquantitative increase of microroughness after exposure to different concentrations of carbamide peroxide bleaching agents (18). The Rt indicate unusual enamel surface conditions that could be causative associated with microcracks because of the excessive physicochemical interactions. In this respect, higher Rt values can be explained for both bleaching groups compared with the untreated controls, in particular for the 16% carbamide peroxide group. In contrast, no significant different values of Ra could be shown after 10% and 16% carbamide peroxide treatment. The increase in the hyperreflexible areas indicates enamel morphologic alterations as shown in the three-dimensional CLSM pictures. This mechanism has been suggested in former studies as a preferential loss either in prism cores or in their periphery, which is characteristic for acid-etched enamel (17). Our investigation confirms the presence of hyperreflexible areas already after bleaching agent application at nondemineralizing pH levels. These results are supported by our preliminary observa-
JOE — Volume 33, Number 5, May 2007
tions of stable Ca/P ratios with no signs of primary demineralization after long-term application of bleaching products (19). Effects of bleaching agents on human enamel are controversially discussed; some findings document slight morphologic alterations and the loss of calcium and phosphorus (6, 8, 11, 20, 21), whereas other investigations did not show surface changes (9, 22–25). Our earlier results showed dissolution of the organic enamel surface layers (pellicle and cuticle) and loss of the aprismatic enamel layer after exposure to simulated long-term application of high-percentage hydrogen peroxide agents (19). Detectable decrease in microhardness as well as reduced fracture toughness of bleached enamel and dentin (26 –28) is seemingly caused by the oxidizing attack on the organic matrix of dental hard tissues. These alterations in the organic matrix of enamel (and dentin) is leading to the “frosted-glass effect” supposedly, whereby ideal natural translucency of enamel appears more opaque, thus masking subjacent dentin layers.
Conclusion Bleaching with 10% and 16% carbamide peroxide results in increased microroughness of human dental enamel shown by higher Ra and Rt values. That is supposed to be a result of loss of organic matrix after exposure to hydrogen peroxide. As a result of increased roughness, possible recurrent extrinsic discoloration as well as the “frostedglass effect” need to be investigated in the future to minimize clinical risks of repeated bleaching procedures.
Enamel Surface and Vital Tooth Bleaching
609
Basic Research—Technology References 1. Dahl JE, Pallesen U. Tooth bleaching—a critical review of the biological aspects. Crit Rev Oral Biol Med 2003;14:292–304. 2. Budavari S, O’Neil M, Ann S, Heckelman PE. The Merck index. An encyclopedia of chemical drugs, and biologicals. Rahway, NJ: Merck and Co., Inc., 1989. 3. Cotton F, Wilkinson G. Oxygen. In: Cotton FA, Wilkinson G, eds. Advanced in Inorganic Chemistry. A Comprehensive Text. New York: Interscience Publisher, 1972:403–20. 4. Bowles W, Ugwuneri Z. Pulp chamber penetration by hydrogen peroxide following vital bleaching procedures. J Endod 1987;13:375–7. 5. Flaitz C, Hicks M. Effects of carbamide peroxide whitening agents on enamel surfaces and caries-like lesion formation: an SEM and polarized light microscopic in vitro study. ASDC J Dent Child 1996;63:249 –56. 6. Bitter N. 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. 7. Ben Amar A, Liberman R, Gorfil C, Bernstein Y. Effect of mouthguard bleaching on enamel surface. Am J Dent 1995;8:29 –32. 8. Josey A, Meyers I, Romaniuk K, Symons A. The effect of a vital bleaching technique on enamel surface morphology and the bonding of composite resin to enamel. J Oral Rehabil 1996;23:244 –50. 9. Gultz J, Kaim J, Scherer W, Gupta H. Two in-office bleaching systems: a scanning electron microscope study. Compend Contin Educ Dent 1999;20:965– 8. 10. White D, Kozak K, Zoladz J, Duschner H, Gotz H. Effect of tooth-whitening gels on enamel and dentin ultrastructure: a confocal laser scanning microscopy pilot study. Compend Contin Educ Dent 2000;29:29 –34. 11. Ernst C, Marroquin B, Willershausen-Zönnchen B. Effects of hydrogen peroxidecontaining bleaching agents on the morphology of human enamel. Quintessence Int 1996;27:52– 6. 12. Engelhardt J, Knebel W. Leica TCS—the confocal laser scanning microscope of the latest generation; technique and applications. Scientific Technical Info 1993;10: 159 – 68. 13. Al-Qunaian T. The effect of whitening agents on caries susceptibility of human enamel. Oper Dent 2005;30:265–70.
610
Markovic et al.
14. Watson T. A confocal optical microscope study of the morphology of the tooth/ restoration interface using Scotchbond 2 dentin adhesive. J Dent Res 1989; 68:1124 –31. 15. Watson T. A confocal microscopy study of some factors affecting the adaptation on a lightcured glass ionomer to tooth tissue. J Dent Res 1990;69:1531– 8. 16. Watson T. Application of confocal scanning optical microscopy to dentistry. Br Dent J 1991;171:287–91. 17. Zentner A, Duschner H. Structural changes of acid etched enamel examined under confocal laser scanning microscope. J Orofac Orthop 1996;57:202–9. 18. Pinto C, Oliveira R, Cavalli V, Ganini M. Peroxide bleaching agent effects on enamel surface microhardness, roughness and morphology. Pesqui Odontol Bras 2004; 18:306 –11. 19. Markovic L, Heine S, Gaengler P, Arnold W. Bleaching systems: a scanning electron microscopic and micro analytic study. J Dent Res 2004;83(Spec Iss B):336. 20. Baumgartner J, Reid D, Picket A. Human pulpal reaction to the modified McInnes bleaching technique. J Endod 1983;9:527–9. 21. Shannon H, Spencer P, Gross K, Tira D. Characterization of enamel exposed to 10% carbamide peroxide bleaching agents. Quintessence Int 1993;24:39 – 44. 22. Nucci C, Marchionni S, Piana G, Mazzoni A, Prati C. Morphological evaluation of enamel surface after application of two ‘home’ whitening products. Oral Health Prev Dent 2004;2:221–9. 23. Haywood V, Leech T, Heymann H, Crumpler D, Bruggers K. Nightguard vital bleaching: effects on enamel surface texture and diffusion. Quintessence Int 1990;21:801– 4. 24. Murchison D, Charlton D, Moore B. Carbamide peroxide bleaching: effect on enamel surface hardness and bonding. Oper Dent 1992;17:181–5. 25. White D, Kozak K, Zoladz J, Duschner H, Gotz H. Peroxide interactions with hard tissues: effects on surface hardness and surface/subsurface ultrastructural properties. Compend Contin Educ Dent 2002;23:42– 8. 26. Attin T, Mueller T, Patyk A, Lennon A. Influence of different bleaching systems on fracture toughness and hardness of enamel. Oper Dent 2004;29:188 –95. 27. Chng H, Palamara J, Messer H. Effect of hydrogen peroxide and sodium perborate on mechanical properties of human dentin. J Endod 2002;28:62–7. 28. Lewinstein I, Hirfschfeld Z, Stabholz A, Rotstein I. Effect of hydrogen peroxide and sodium perborate on the microhardness of human enamel and dentin. J Endod 1994;20:61–3.
JOE — Volume 33, Number 5, May 2007
Micromorphology of Enamel Surface After Vital Tooth Bleaching Ljubisa Markovic, DDS,* Rainer Andreas Jordan, DDS,* Nebojsa Lakota,† and Peter Gaengler, PhD, DDS* Abstract The aim of the present study was to assess microroughness changes of enamel surfaces after bleaching procedures with 10% and 16% concentrations of carbamide peroxide by confocal laser scanning microscopy. Twenty caries-free incisors, extracted for periodontitis reasons, were sectioned into two halves of experimental and control specimens. The teeth were divided into two groups, and the experimental specimens were exposed to either 10% or 16% carbamide peroxide for 4 hours per 7 days. Measurements were made at three randomly selected 140 ⫻ 100 m areas of 10 experimental and control specimens per group at the same crown level. Microroughness was measured in “total roughness” (Rt) and “average roughness” (Ra) descriptor values. The statistical analysis showed significantly higher microroughness according to Rt values and Ra values for both groups of carbamide peroxide exposed enamel surfaces. (J Endod 2007;33:607– 610)
Key Words Bleaching, carbamide peroxide, confocal microscopy, enamel
P
resent tooth-bleaching techniques are based on hydrogen peroxide as the active agent (1). It may be applied directly or released by a chemical reaction from carbamide peroxide (2). The latter acts as a deposit material releasing hydrogen peroxide and urea in liquid solution. Hydrogen peroxide liberates in further reactions free radicals, reactive oxygen molecules, and hydroxyl anions (3). Enamel and dentin have shown high permeability to hydrogen peroxide (4), although the process of molecular permeation has not been identified yet. The whitening effect is supposedly because of degradation of complex organic structures of high molecular weight, reflecting a specific wavelength of light. The resulting degradation leads to a reduction or elimination of the discoloration (5). However, there is no in vitro optical assessment of changing enamel translucency after bleaching. To date, there is no agreement concerning the severity of morphologic alteration or changes of biomechanical properties of the enamel (6 –10). Most of the studies in the present literature used a two-dimensional analysis by scanning electron microscopy (11). Therefore, the purpose of this investigation was to assess the effect of carbamide peroxide on human dental enamel by three-dimensional documentation of superficial micromorphology. The microstructural alterations caused by 10% and 16% concentrations of carbamide peroxide were studied by confocal laser scanning microscopy (CLSM), which is known in dental hard tissue research as a nondestructive technique of microscopic tomography (12). These bleaching formulations are accepted by the American Dental Association as “safe” and “effective” products to be used at home (13).
Materials and Methods From the *Department of Conservative Dentistry, University of Witten/Herdecke, Witten, Germany; and †Institute of Materials/Material Testing, Ruhr University of Bochum, Germany. Address requests for reprints to Dr. Ljubisa Markovic, Department of Conservative Dentistry, Faculty of Dental Medicine, University of Witten/Herdecke, Alfred Herrhausen-Straße 50, 58445 Witten, Germany. E-mail address: Ljubisa.Markovic@ uni-wh.de. 0099-2399/$0 - see front matter Copyright © 2007 by the American Association of Endodontists. doi:10.1016/j.joen.2007.01.011
JOE — Volume 33, Number 5, May 2007
Twenty sound human incisors, extracted for periodontitis reasons, were stored in saline containing 0.1% thymol. Teeth exhibiting any visible cracks or hypoplastic defects were excluded. The specimens have been thoroughly cleaned by using a fluoridefree prophylaxis paste (Miraclean, Hager und Werken, Duisburg, Germany) and were randomly subdivided into two groups. Tooth crowns were sectioned cervicoincisally into two equal halves using a diamond saw, representing the experimental and the control specimen per tooth. Group 1 was treated with 10% carbamide peroxide, pH 6.4, releasing ⬵3.6% H2O2. Group 2 was treated with 16% concentration of carbamide peroxide, pH 6.2, releasing ⬵5.8% H2O2. Bleaching agents were applied for 7 days, for 4 hours each day, and were removed by rinsing the specimens under running tap water for 30 seconds. During nontreatment times, all teeth were placed in artificial saliva containing, 150 mmol/L KCl, 1.5 mmol/l CaCl2, and 0.9 mmol/L KH2PO4 per 100 mL aqua bidest., 6.9 to 7.0 pH, in an incubator at 37°C. The samples were examined by CLSM (Leica ICM 1000, Wetzlar, Germany) using an Argon-LASER at 635 nm excitation and ⫻500 magnification. Measurements were made at three randomly selected 140 ⫻ 100 m areas of each tooth halved at the same crown level. Microroughness (in micrometers) was measured in two ways: (1) “total roughness” Rt (ISO 4287), which is made up of the sum of the “maximum peak” (Rp) and the “deepest valley” (Rv) of the roughness profile, and (2) “average roughness” Ra (ISO 4287), which reflects average condition of the roughness profile, given by the sum of the absolute values of all areas above and below a tomographic mean line. Confocal laser scanning microscopy (CLSM) parameters Rt and Ra were determined for the experimental and the control tooth halves and statistically analyzed by using a Wilcoxon signed rank test for paired comparisons within a group, and a Mann-
Enamel Surface and Vital Tooth Bleaching
607
Basic Research—Technology
Figure 1. 3D-CLSM appearance of enamel surfaces of control and bleached regions of human incisors: (A) control region exhibiting a smooth structure, average roughness Ra 0.19 m (magnification ⫻500); (B) bleached region after exposure to 10% carbamide peroxide showing increased surface roughness, Ra 0.29 m (magnification ⫻500); (C) control region exhibiting a smooth structure, average roughness Ra 0.25 m (magnification ⫻500); and (D) bleached region after exposure to 16% carbamide peroxide showing increased surface roughness, Ra 0.34 m (magnification ⫻500).
Whitney U test for unpaired analysis between groups. The level of significance accepted for all parameters was p ⱕ 0.05.
Results The CLSM appearance differs in all bleached enamel areas of both experimental groups compared with the individual nonbleached control areas of each tooth. Figure 1 shows examples of the three-dimensional structure of the assessed enamel surface of both bleached and unbleached regions at same crown level. Increasing brighter yellow color fractions indicate an inhomogeneous texture with higher Ra and Rt values. Optical darker control areas impress with a smoother surface structure, in comparison to the lighterappearing bleached regions. Analysis showed that the microroughness, in particular Rt and Ra, of the treated halves was significantly higher than in control regions and was independent from bleaching agent concentrations (p ⬍ 0.04). For teeth treated with 10% carbamide peroxide, the Rt and the Ra statistical values were significantly different to untreated regions. The mean Rt value of the treated surface was 3.91 m in contrast to the control side 608
Markovic et al.
with 3.17 m (p ⬍ 0.01)(Fig. 2A). Mean Ra difference between the treated and control side was 0.09 m (p ⬍ 0.02) (Fig. 2E). For teeth treated with 16% carbamide peroxide, mean Rt value was 6.3 m in the treated regions and 4.27 m in the controls (p ⬍ 0.02) (Fig. 2C). Ra parameters were significantly different between the treated and control sides (p ⬍ 0.04) (Fig. 2D). Comparison between the treated regions of both concentrations showed a statistically significant higher value for Rt in the 16% group (p ⬍ 0.01) (Fig. 2B), whereas Ra was not significantly different (p ⫽ 0.9) (Fig. 2F). The microroughness in all control regions of both groups was statistically not different (p ⫽ 0.07).
Discussion The CLSM was first suggested and used in dentistry by Watson to visualize tooth-restoration interface (14 –16). Its principle is based on the elimination of nonfocal portions of the image by confocal apertures and to achieve in this way a more compact point response and higher image fidelity in comparison with conventional methods of light microscopy. The visualization is carried out in pseudocolors, yellow and red,
JOE — Volume 33, Number 5, May 2007
Basic Research—Technology
Figure 2. Overview of Ra and Rt statistical values: (A) total roughness Rt (in micrometers) of the 10% carbamide peroxide group, bleached versus control regions; (B) comparison between the treated regions of both concentrations showed a statistically significant increase of the total roughness (Rt), 10% bleached versus 16% bleached; (C) total roughness Rt (in micrometers) of the 16% carbamide peroxide group, bleached versus control regions; (D) average roughness Ra (in micrometers) of the 16% carabmide peroxide group, bleached versus control regions; (E) average roughness Ra (in micrometers) of the 10% carbamide peroxide group, bleached versus control regions; and (F) comparison between the treated regions of both concentrations showed no statistically significant increase of the average roughness (Ra), 10% Ra bleached versus 16% bleached.
showing high intensities of reflected light. Zones, from which no light is emitted, appear black. In this way, CLSM can be used for example to describe microroughness on dental hard tissues (17). Because it was the aim of the present study to assess clinically relevant micromorphologic alterations of enamel surfaces after bleaching procedures, the results showed a statistically significant increase in Rt and Ra in general, after bleaching agent exposure with two different carbamide peroxide concentrations. These findings correspond with other results showing a semiquantitative increase of microroughness after exposure to different concentrations of carbamide peroxide bleaching agents (18). The Rt indicate unusual enamel surface conditions that could be causative associated with microcracks because of the excessive physicochemical interactions. In this respect, higher Rt values can be explained for both bleaching groups compared with the untreated controls, in particular for the 16% carbamide peroxide group. In contrast, no significant different values of Ra could be shown after 10% and 16% carbamide peroxide treatment. The increase in the hyperreflexible areas indicates enamel morphologic alterations as shown in the three-dimensional CLSM pictures. This mechanism has been suggested in former studies as a preferential loss either in prism cores or in their periphery, which is characteristic for acid-etched enamel (17). Our investigation confirms the presence of hyperreflexible areas already after bleaching agent application at nondemineralizing pH levels. These results are supported by our preliminary observa-
JOE — Volume 33, Number 5, May 2007
tions of stable Ca/P ratios with no signs of primary demineralization after long-term application of bleaching products (19). Effects of bleaching agents on human enamel are controversially discussed; some findings document slight morphologic alterations and the loss of calcium and phosphorus (6, 8, 11, 20, 21), whereas other investigations did not show surface changes (9, 22–25). Our earlier results showed dissolution of the organic enamel surface layers (pellicle and cuticle) and loss of the aprismatic enamel layer after exposure to simulated long-term application of high-percentage hydrogen peroxide agents (19). Detectable decrease in microhardness as well as reduced fracture toughness of bleached enamel and dentin (26 –28) is seemingly caused by the oxidizing attack on the organic matrix of dental hard tissues. These alterations in the organic matrix of enamel (and dentin) is leading to the “frosted-glass effect” supposedly, whereby ideal natural translucency of enamel appears more opaque, thus masking subjacent dentin layers.
Conclusion Bleaching with 10% and 16% carbamide peroxide results in increased microroughness of human dental enamel shown by higher Ra and Rt values. That is supposed to be a result of loss of organic matrix after exposure to hydrogen peroxide. As a result of increased roughness, possible recurrent extrinsic discoloration as well as the “frostedglass effect” need to be investigated in the future to minimize clinical risks of repeated bleaching procedures.
Enamel Surface and Vital Tooth Bleaching
609
Basic Research—Technology References 1. Dahl JE, Pallesen U. Tooth bleaching—a critical review of the biological aspects. Crit Rev Oral Biol Med 2003;14:292–304. 2. Budavari S, O’Neil M, Ann S, Heckelman PE. The Merck index. An encyclopedia of chemical drugs, and biologicals. Rahway, NJ: Merck and Co., Inc., 1989. 3. Cotton F, Wilkinson G. Oxygen. In: Cotton FA, Wilkinson G, eds. Advanced in Inorganic Chemistry. A Comprehensive Text. New York: Interscience Publisher, 1972:403–20. 4. Bowles W, Ugwuneri Z. Pulp chamber penetration by hydrogen peroxide following vital bleaching procedures. J Endod 1987;13:375–7. 5. Flaitz C, Hicks M. Effects of carbamide peroxide whitening agents on enamel surfaces and caries-like lesion formation: an SEM and polarized light microscopic in vitro study. ASDC J Dent Child 1996;63:249 –56. 6. Bitter N. 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. 7. Ben Amar A, Liberman R, Gorfil C, Bernstein Y. Effect of mouthguard bleaching on enamel surface. Am J Dent 1995;8:29 –32. 8. Josey A, Meyers I, Romaniuk K, Symons A. The effect of a vital bleaching technique on enamel surface morphology and the bonding of composite resin to enamel. J Oral Rehabil 1996;23:244 –50. 9. Gultz J, Kaim J, Scherer W, Gupta H. Two in-office bleaching systems: a scanning electron microscope study. Compend Contin Educ Dent 1999;20:965– 8. 10. White D, Kozak K, Zoladz J, Duschner H, Gotz H. Effect of tooth-whitening gels on enamel and dentin ultrastructure: a confocal laser scanning microscopy pilot study. Compend Contin Educ Dent 2000;29:29 –34. 11. Ernst C, Marroquin B, Willershausen-Zönnchen B. Effects of hydrogen peroxidecontaining bleaching agents on the morphology of human enamel. Quintessence Int 1996;27:52– 6. 12. Engelhardt J, Knebel W. Leica TCS—the confocal laser scanning microscope of the latest generation; technique and applications. Scientific Technical Info 1993;10: 159 – 68. 13. Al-Qunaian T. The effect of whitening agents on caries susceptibility of human enamel. Oper Dent 2005;30:265–70.
610
Markovic et al.
14. Watson T. A confocal optical microscope study of the morphology of the tooth/ restoration interface using Scotchbond 2 dentin adhesive. J Dent Res 1989; 68:1124 –31. 15. Watson T. A confocal microscopy study of some factors affecting the adaptation on a lightcured glass ionomer to tooth tissue. J Dent Res 1990;69:1531– 8. 16. Watson T. Application of confocal scanning optical microscopy to dentistry. Br Dent J 1991;171:287–91. 17. Zentner A, Duschner H. Structural changes of acid etched enamel examined under confocal laser scanning microscope. J Orofac Orthop 1996;57:202–9. 18. Pinto C, Oliveira R, Cavalli V, Ganini M. Peroxide bleaching agent effects on enamel surface microhardness, roughness and morphology. Pesqui Odontol Bras 2004; 18:306 –11. 19. Markovic L, Heine S, Gaengler P, Arnold W. Bleaching systems: a scanning electron microscopic and micro analytic study. J Dent Res 2004;83(Spec Iss B):336. 20. Baumgartner J, Reid D, Picket A. Human pulpal reaction to the modified McInnes bleaching technique. J Endod 1983;9:527–9. 21. Shannon H, Spencer P, Gross K, Tira D. Characterization of enamel exposed to 10% carbamide peroxide bleaching agents. Quintessence Int 1993;24:39 – 44. 22. Nucci C, Marchionni S, Piana G, Mazzoni A, Prati C. Morphological evaluation of enamel surface after application of two ‘home’ whitening products. Oral Health Prev Dent 2004;2:221–9. 23. Haywood V, Leech T, Heymann H, Crumpler D, Bruggers K. Nightguard vital bleaching: effects on enamel surface texture and diffusion. Quintessence Int 1990;21:801– 4. 24. Murchison D, Charlton D, Moore B. Carbamide peroxide bleaching: effect on enamel surface hardness and bonding. Oper Dent 1992;17:181–5. 25. White D, Kozak K, Zoladz J, Duschner H, Gotz H. Peroxide interactions with hard tissues: effects on surface hardness and surface/subsurface ultrastructural properties. Compend Contin Educ Dent 2002;23:42– 8. 26. Attin T, Mueller T, Patyk A, Lennon A. Influence of different bleaching systems on fracture toughness and hardness of enamel. Oper Dent 2004;29:188 –95. 27. Chng H, Palamara J, Messer H. Effect of hydrogen peroxide and sodium perborate on mechanical properties of human dentin. J Endod 2002;28:62–7. 28. Lewinstein I, Hirfschfeld Z, Stabholz A, Rotstein I. Effect of hydrogen peroxide and sodium perborate on the microhardness of human enamel and dentin. J Endod 1994;20:61–3.
JOE — Volume 33, Number 5, May 2007