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Effects of Direct and Indirect Bleach on Dentin Fracture ToughnessRestorative Dentistry, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON, Canada M5G 1G6 Correspondence: * corresponding author, laura.tam{at}utoronto.ca
There are concerns that tooth-whitening procedures irreversibly damage tooth structure. We investigated the hypothesis that dental bleaches significantly affect dentin structural integrity. The objective was to evaluate the effects of peroxide bleaches on dentin fracture toughness. Compact test specimens, composed of human dentin, were used (n = 10/group). Bleach (16% or 10% carbamide peroxide or 3% hydrogen peroxide) or control material, containing 0.1% sodium fluoride, was applied directly or indirectly to dentin through enamel (6 hrs/day) for 2 or 8 weeks. Fracture toughness results were analyzed by ANOVA and Fisher’s LSD test (p < 0.05). There were significant decreases in mean fracture toughness after two- and eight-week direct (19–34% and 61–68%, respectively) and indirect (up to 17% and 37%, respectively) bleach application. The in vitro reduction in dentin fracture toughness caused by the application of peroxide bleaches was greater for the direct application method, longer application time, and higher bleach concentration.
Key Words: dentin • fracture toughness • peroxide bleach
The main active ingredient in tooth-bleaching materials is hydrogen peroxide or carbamide peroxide. The most common side-effect of the vital tooth-bleaching technique is tooth sensitivity. In an effort to reduce the incidence and severity of tooth sensitivity, fluoride is added to some proprietary tooth-bleaching materials. Damage to the structural integrity of the tooth is not usually considered a significant problem associated with tooth bleaching. However, tooth-bleaching treatments have been associated with negative effects on dental hard tissues, including decreased bonding ability (Torneck et al., 1990; Titley et al., 1992), surface hardness (de Freitas et al., 2002; Basting et al., 2003) and abrasion resistance (Seghi and Denry, 1992), and changes to enamel and dentin surface morphology (Shannon et al., 1993; Ernst et al., 1996; Josey et al., 1996; Bitter, 1998; Chen et al., 2002). Bonding and hardness studies reflect changes to the surface only, and do not characterize changes to dentin structure beyond the surface. The structural integrity of teeth is better determined by strength and fracture toughness studies. The flexural strength and modulus of bovine dentin were reported to decrease after a direct daily application of carbamide peroxide (Tam et al., 2005b). Significant reductions in tensile and micropunch shear strengths of dentin were reported after an intracoronal bleach application of 30% hydrogen peroxide (Chng et al., 2002). In these studies, bleach was applied directly to the dentin surface. In the clinical situation, however, bleach is applied to enamel and, in cases of gingival recession, to cementum, and the exposure of dentin to bleach would be indirect, through either enamel cracks or diffusion processes. We hypothesized that a fracture mechanics study would provide the data to show that dental bleaches significantly affect dentin structural integrity. This study used the fracture toughness parameter, K1C, to investigate the effects of tooth bleach on dentin in a direct application and, in a more clinically relevant experiment, in an indirect application. The objective was to determine the effects of direct and indirect dental bleaches that contain fluoride on the fracture toughness of human dentin.
Human molar teeth, extracted within 3 mos of the study and stored in a 1% chloramine solution at 4°C, were collected in accordance with a patient informed consent protocol for the use of teeth that was approved by the institutional Research Ethics Board (Protocol Reference #18028). The teeth were randomly divided into bleach and control groups. These groups were further divided into time (2 and 8 wks) and application mode (direct or indirect) groups (Table  ) (n = 10/group).
The bleach materials included 16% carbamide peroxide, 10% carbamide peroxide, or 3% hydrogen peroxide (Pola Night or Day, Southern Dental Industries, Victoria, Australia) gels. Placebo gels, containing the same ingredients as the bleach gels except for the active hydrogen peroxide or carbamide peroxide, were used as control materials (Southern Dental Industries). All bleach and control materials contained 0.1% sodium fluoride (NaF). A bleach treatment consisted of a 1- to 2-mm-thick application of bleach to the tooth surface for 6 hrs, either daily for 2 wks or 5 days/wk for 8 wks, to simulate a typical or prolonged at-home bleaching regimen, respectively. The specimens were stored at 37°C, > 80% relative humidity for the duration of bleach treatment. After each daily bleach treatment, the specimens were rinsed with tap water to remove all external traces of bleach and stored in 37°C artificial saliva (Söderholm et al., 1996) until the next bleach treatment. Compact tension test specimens, described previously (El Mowafy and Watts, 1986), based on an ASTM standard specimen geometry (American Society for Testing and Materials, 2001), were prepared from coronal dentin for fracture toughness testing. We used a water-cooled low-speed diamond saw (Buehler Ltd., Lake Bluff, IL, USA) to cut a rectangular slice with approximate dimensions of 4.6 x 4.5 x 1.6 mm from the dentin below the occlusal enamel. The orientation of the slice was parallel to the occlusal surface. One slice was obtained from each tooth. The central notch was made with diamond discs (ThinFlex, Premier Products Co., Plymouth Meeting, PA, USA). Specimen and notch dimensions were measured by means of a micrometer or traveling microscope. For direct dentin bleaching, treatment materials were applied directly onto dentin that was already prepared as compact tension specimens. For indirect dentin bleaching, treatment materials were applied to the molars prior to compact tension specimen preparation. The molar specimens were suspended through a 1-mm-thick sheet of wax that was adapted to each molar around the approximate cemento-enamel junction. Because of the scalloped nature of the cemento-enamel junction, a small area of root cementum on the mesial and distal surfaces remained above the wax lid, and served to represent the exposed root surface in cases of gingival recession. No attempt was made to measure this area of exposed root for each molar, and there would have been small daily variations in the size of this area, depending on the location of the wax placement. The roots were immersed in artificial saliva beneath the wax while treatment materials were applied to the coronal enamel and exposed root dentin. Preliminary investigations with dye showed that the wax served to prevent inadvertent contact and mixing of the viscous bleach material with artificial saliva. Compact tension test specimens were prepared from the indirectly treated molars approximately 20 hrs after the last bleaching session. Twenty-four hours after the last bleaching session, the specimens were mounted on an Instron testing machine (Model 4301, Instron Corp., Canton, MA, USA) equipped with a specially designed mounting jig. Tensile loading was applied at 10 mm/min until specimen fracture. The fracture toughness results were analyzed by ANOVA and Fisher’s LSD test (p < 0.05). Two specimens were randomly selected from each group for evaluation of the fracture surface by scanning electron microscopy (Hitachi S-2500, Hitachi, Tokyo, Japan). These specimens were stored immediately after fracture in 100% ethanol for 2 wks, critical-point-dried (Polaron CPD-7501, Fisons Instruments, Hertfordshire, England), and sputter-coated with platinum (Polaron SC515 SEM Coating Systems, Fisons Instruments, England).
There was a significant interaction among the treatment factors: material, time, and application mode (p < 0.0001).
Direct Application Mode Results
Indirect Application Mode Results There were no significant differences in mean fracture toughness among the indirect control groups (Fig. 2 ). Compared with their control groups, there were significant decreases in mean fracture toughness for 16% carbamide peroxide after both two- and eight-week indirect treatment, and for 10% carbamide peroxide after eight-week indirect treatment. With a 37% reduction in mean fracture toughness, the eight-week 16% carbamide peroxide group showed the lowest indirect significant result.
Scanning Electron Microscopy Results In general, the surface morphology of the dentin fracture surfaces of the bleach and control dentin groups appeared similar (Fig. 3 ). There was localized evidence of increased demineralization, particularly in the tubule walls of bleached specimens, that was not present in control specimens.
The use of a fracture toughness test better quantifies the tooth’s resistance to fracture than do conventional strength tests (Kelly, 1995), and measures an intrinsic property that has relevance to material failure in the presence of flaws. In this study, the in vitro fracture toughness of dentin was significantly reduced by direct and indirect applications of peroxide bleaches over 2 and 8 wks. The observed reduction in dentin fracture toughness could relate to changes in dentin water content, mineralization, collagen, or non-collagenous proteins. Changes to the level of dentin hydration have been shown to affect dentin mechanical properties (Jameson et al., 1994; Kinney et al., 2003; Kishen and Asundi, 2005). It has been reported that strain-at-fracture and fracture energy were significantly greater for hydrated and rehydrated dentin than for dehydrated dentin (Jameson et al., 1994). In the present study, although some dentin dehydration might have occurred during the time of bleach application, it was expected that rehydration would occur during the time of artificial saliva storage. Both the bleach and control groups were subjected to the same potential dehydration and rehydration processes. Therefore, changes in water content cannot readily explain the difference in dentin fracture toughness between the bleach and control groups. The slightly acidic pH of the bleach and control materials for the prolonged time of application could have caused dentin demineralization. Studies have reported chemical and physical evidence of enamel and dentin demineralization to some degree following bleach application (McCracken and Haywood, 1996; Rotstein et al., 1996; Perdigão et al., 1998). The pH values of the bleach materials were similar to those of the control materials. Therefore, it is questionable whether demineralization alone could account for the difference in fracture toughness values between the bleach and control groups. However, a slightly greater degree of demineralization was observed in the bleached groups under scanning electron microscopy. It is speculated that this demineralization increased the access of peroxide bleach to dentinal collagen. Collagen fibrils contribute significantly to the fracture toughness of mineralized biological tissues. Preliminary investigations using immunohistology specifically to recognize degraded (but not native) dentinal collagen have shown evidence of collagen degradation occurring in dentin after daily direct peroxide bleach applications, but not after daily control applications (unpublished observations). Other studies have reported evidence of collagen and matrix alteration after the application of bleach to dentin, enamel, and bone, and have postulated that hydrogen peroxide caused oxidative destruction or denaturation of collagen and other matrix proteins (Rotstein et al., 1992; Seghi and Denry, 1992; Chen et al., 2002). Collagen degradation could then increase the access of peroxide bleach to intrafibrillar mineral. Loss of mineral would accompany the loss of matrix to which it was bound. The proposed mechanism for fracture toughness reduction in the bleached specimens of this study, therefore, relates to reduced mineral content coupled with collagen or protein breakdown. Fluoride is used in caries prevention because it reduces demineralization and enhances remineralization (Featherstone et al., 1982; ten Cate, 1999). On the assumption that some demineralization occurs during bleach exposure, fluoride could play a beneficial role with respect to dentin fracture toughness during tooth bleaching. It has been reported that fluoride decreased the degree of surface hardness reduction in enamel and dentin during bleach treatment (Attin et al., 1997; Lewinstein et al., 2004). However, other studies suggested that fluoride treatment either did not restore the structurally bound fluoride or the erosion resistance of bleached enamel to the levels of unbleached enamel (Burgmaier et al., 2002), or had no significant effect on the hardness of bleached enamel and dentin (Joiner et al., 2004). In a previous study, daily topical fluoride treatment for 2 wks following bleach treatment did not help to recover the flexural strength and modulus of bleached specimens (Tam et al., 2005a). In this study, the inclusion of 0.1% NaF, a concentration shown to retard dentin mineral loss at pH = 5 (Arends et al., 1987), in the bleach material did not prevent a significant reduction in dentin fracture toughness. It is possible that the effects of fluoride were limited to the dentin surface and were not sufficient to have an impact on dentin structural integrity. A fluoride application method or concentration different from that used in this study might have a different effect on bleached dentin fracture toughness. The limited role of fluoride in preventing the reduction in dentin fracture toughness during bleaching could also be explained by the suggestion that the effect of bleach related primarily to changes to the organic, rather than the inorganic, component of dentin. The direct bleach application mode caused a significantly greater degree of dentin weakness than did the indirect mode. The indirect bleach application mode differed from the direct mode because the treatment material was applied to a surface that was remote from the site of dentin crack initiation. This is the first study to show a significant reduction in the structural integrity of dentin after indirect bleach treatment. A previous study showed no significant decrease in bovine dentin flexural strength after an indirect application of bleach (Tam et al., 2005a). In that study, bleach was strictly withheld from the root surface and thin cervical enamel in an effort to avoid inadvertent bleach application to the root surface. In this study, no special effort was made to prevent the exposure of small areas of the root to bleach. It is speculated that the exposed root surface provided access for bleach entry toward the coronal dentin that was used for indirect fracture toughness testing. In the clinical situation, bleach also has access to root and coronal dentin in areas of gingival recession and occlusal attrition. The thickness of the overlying enamel, the presence of enamel cracks, and the permeability and direction of the dentinal tubules are factors that could affect the degree of bleach penetration into the dentin. The clinical relevance of the in vitro results is uncertain. In vital teeth, there is an outward movement of fluid through dentinal tubules, which would tend to expel and buffer the applied bleach. The authors are unaware of clinical tooth fractures that are attributable to bleach treatment, despite the estimated large numbers of bleaching cases that have been performed thus far. However, the fracture toughness test gives an indication of damage-tolerance, and a reduced dentin fracture toughness result is highly suggestive of a greater likelihood for clinical tooth fracture over a lifetime, especially in an already-susceptible tooth. The reduction in dentin fracture toughness in this study appeared to be cumulative, or at least related to the length of bleaching time. The "rebound" potential—demonstrated for bonding by a restoration of bond strengths to enamel and dentin (Torneck et al., 1991; Rotstein, 1993; Lai et al., 2001), and for surface hardness (de Freitas et al., 2002)—after a period of time is questionable for the mechanical property of dentin fracture toughness. Therapies for the prevention of dentin weakening or for the recovery of dentin fracture resistance after bleaching are unknown. The inclusion of fluoride in the bleach formulations used in this study did not prevent a reduction in dentin fracture toughness. Until the specific effects of tooth bleaches on dentin are clarified, bleaching concentrations and times should be kept to a minimum, and direct application of bleach to areas of exposed dentin, such as in gingival recession or occlusal attrition cases, should be avoided whenever possible.
We thank Southern Dental Industries for their contribution of materials. This study was supported by a Dentistry of Canada Fund Research Grant. Received for publication August 9, 2006. Revision received August 8, 2007. Accepted for publication September 4, 2007.
Journal of Dental Research, Vol. 86, No. 12,
1193-1197 (2007) This article has been cited by other articles:
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