2022 Volume 22 Issue 1 Pages 13-19
Purpose: The purpose of this study was to evaluate the extension of bleaching effect by linearly measurement on the crosscut surfaces of bleached teeth in vitro. Methods: Thirty extracted bovine incisors were stained by black tea and were divided three groups in each of ten. The labial surface was bleached using an in-office bleaching material for 0, 3, and 6 times respectively. Then, teeth were crosscut and color was measured in bleached and not bleached area linearly from enamel surface to pulpal dentin to obtained CIE L*a*b* values using a digital camera with CIE XYZ color gamut (RC500). And whiteness index (WID) was calculated from those values. Results: Both 3-times and 6-times bleaching showed the bleaching effect and 6-times bleaching extended the effect deeper than 3-times bleaching. Three samples of 3-times bleaching group and one sample of 6-time bleaching group showed less bleaching effect at enamel dentin junction (EDJ). In 3-times and 6-times bleaching groups, WID of enamel was statistically higher than those of shallower and deeper dentin. Conclusion: It was concluded that bleaching effect was extended deeper along with dentinal tubules beyond EDJ.
The importance of aesthetic treatments in dentistry, and the need for whiter teeth, have been increasing due to the growing awareness of beauty [1]. It is also believed that there is a strong association between a beautiful and healthy appearance and social status such as better job and social acceptance [2]. Tooth bleaching is the most common form of elective dental treatment and has become an important means for patients to obtain a beautiful smile. It is widely recognized for its proven safety and efficacy, as well as for its cost-effectiveness in treating tooth discoloration and preserving tooth [3,4].
There are several factors which influence tooth discoloration. Tooth staining is the result of exogenous and endogenous chromogenic pigments. The endogenous discoloration is caused by the binding of pigments within the tooth structure during tooth formation or after tooth eruption. Extrinsic discoloration, on the other hand, is caused by the deposition of dye on the tooth surface. In addition, as physiological age changes, tooth color darkens due to the uptake of exogenous dyes, and enamel wear that greatly affects the color of the dentin [5,6]. Colorants are found in most foods and have a significant impact on tooth staining. Beverages such as tea, coffee, and wine are very popular all over the world as colorants. Staining is caused by these familiar items [7]. As for the cause of exogenous coloration, such beverages have an impact on the coloration. And smoking is another source of staining.
Bleaching methods for vital teeth are classified into in-office bleaching and at-home bleaching, which bleach the discolored teeth from the surfaces. Both methods use carbamide peroxide or hydrogen peroxide as the active ingredient. The mechanism of bleaching is that the free radicals liberated from the peroxide in the bleaching agent oxidize and decompose exogenous and endogenous colorants. In-office bleaching is quicker and more effective than at-home bleaching. There are some reports on the factors of bleaching effect such as concentration of bleaching material, light intensity of irradiation, and treatment time [8,9]. Methods of evaluating tooth color include visual comparisons using shade guides, spectrophotometry, and objective measurements using image analysis. Those methods are generally evaluated tooth shade from the surface. The internal color of the tooth needs to be measured because the enamel is transparent and the dentin color was shown through the enamel. However, only few reports have confirmed the bleaching effect inside the teeth [10,11]. The purpose of this study was to evaluate the extension of bleaching effect by linearly measurement on the crosscut surface of bleached teeth in vitro .
Figure 1 shows the experimental procedure. Thirty extracted bovine incisors which were obtained from a slaughterhouse were used in this study. The teeth were stored frozen after extraction and were thawed by running tap water before the experiment. They were cleaned by removing soft tissue remnants using a scalpel. The center of the labial surface was ground using silicon carbide (SiC) papers (Sankyo-Rikagaku, Okegawa, Japan) from #280 to #1,200 leaving 1 mm in thickness of enamel. The roots were removed from the crown by a diamond disc (SummaDisk, Shofu, Kyoto, Japan) with a straight-type micromotor handpiece. A staining solution was prepared by immersion of two black teabags (Lipton Yellow Label tea, Unilever Japan, Tokyo, Japan) in 100 mL of boiled water for 5 min. Specimens were immersed in the solution and stored in the incubator at 37˚C for 7 days. The solution was changed after third day to avoid decantation of the solution. A piece of adhesive tape with hole of 3 mm in diameter was placed on the center of the test surface to determine the bleaching area. Stained teeth were divided in three groups in each of ten ( n = 10). The teeth of the first group were not bleached (Group 1) and those of other two groups were bleached by an in-office bleaching agent (Shofu HiLite, Shofu) according to the manufacturer's instruction. The composition of the bleaching agent used in this study was shown in Table 1. The powder and liquid were mixed in the ratio of 0.05 g: 0.15 g and the mixed paste was applied on the test surface for 5 min, then the light irradiation was performed for 3 min using a light unit (Optilux 501, Kerr, CA, USA). After light irradiation, the bleaching agent was left for 10 min, then the paste was wiped off using a piece of damp gauze. Bleaching treatments were repeated 3 times (Group 2) or 6 times (Group 3). Each sample was crosscut at the center of the test surface longitudinally by a rotary diamond saw (Mini Lab‐cutter MC-110, Maruto, Tokyo, Japan). And crosscut surface was polished by ascending the SiC papers (#800 - #2,000). Then, they were fixed parallel to the glass slide by a cyanoacrylate glue (Model Repair 2 Blue, Dentsply Sirona, Tokyo, Japan).
Fig. 1 Experimental procedures
Fig. 2 A CIE XYZ camera
Fig. 3 Color measurements
White arrow shows bleached area. Color was linearly measured at bleached and non-bleached area.
L*, a*, b* values at middle of enamel, one-third depth of the dentin, and two-third depth of the dentin were selected as representative data.
Table 1 Material used in this study
| Material | Composition | Manufacturer | Lot number |
|---|---|---|---|
| Shofu Hi-lite | powder: potassium peroxohydrogen sulphate, potassium hydrogen sulphate, manganese sulphate, hydrated amorphous silica, Gantrez Ms-955, Guinea green dye | Shofu, Kyoto, Japan | 31720 |
| liquid: 35% hydrogen peroxide | 81701 |
Classification and photograph
The prepared crosscut surface of each specimen was carefully observed and classified into four categories by the depth of bleaching effect as follows:
Score 0: The bleaching effect was not obvious on the crosscut surface.
Score 1: The bleaching effect was observed up to 1/3 depth of the dentin from the bleached surface.
Score 2: The bleaching effect was observed up to 2/3 depth of the dentin from the bleached surface.
Score 3: The bleaching effect was observed beyond 2/3 of the dentin from the bleached surface.
Photographs of these samples were also taken with a digital camera (D80 and Medical Nikkol, Nicon, Tokyo, Japan).
Color measurementsA digital camera with CIE XYZ color gamut (RC500, PaPaLaB, Hamamatsu, Japan) was used as the colorimetric camera. The colorimetric method was performed at distance of 20 cm, an exposure duration time of 0.2 s between the specimen and the lens of the camera, spotted with D65 standard illuminant from 45˚/0˚ geometry on both sides in a black box (Fig. 2) [12,13,14]. The XYZ color information captured by the RC500 was converted to CIE L*a*b* values using a software (RC View, PaPaLaB). The color data of crosscut surface was measured straight linearly from the center of bleached enamel surface to pulpal wall of the dentin along with the orientation of dentinal tubules (Fig. 3). The pixels of the image are 0.05 × 0.05 mm per pixel. The linear color measurements were done for 1 mm in width (20 pixels). Average values of those 20 pixels at each depth were obtained as representative data. Another liner color measurement at no bleached area was obtained parallel to the above measurement apart from 4 mm to apical direction.
The whiteness (WID) was calculated from obtained CIE L*a*b* values as following equation [15,16,17,18].
WID = 0.511L* − 2.324a* − 1.100b*
The subtractions of WID of the bleached area from WID of the unbleached area were obtained and those values were used for evaluation of the bleaching effect.
Statistical analysisWID at the middle of the enamel, one-third depth of dentin, and two-thirds depth of dentin were selected and statistically analyzed. As the distribution of the data fitted the presumption of normality (Kolmogorov-Smirnov test, P = 0.15) and the equality (Levene’s test, P = 0.391) of parametric analysis, the WID data were evaluated by two-way analysis of variance (ANOVA). The differences within groups were evaluated by Tukey’s multiple comparison test ( α = 0.05). A global significance level of 95% was applied. All statistical analyses were performed using a standard statistical software package (SPSS Ver.26 for Windows, IBM, Armonk, NY, USA).
Table 2 Bleaching times and effects
| Bleaching time | Bleaching effect | |||
|---|---|---|---|---|
| Score 0 | Score 1 | Score 2 | Score 3 | |
| 0 | 10 | 0 | 0 | 0 |
| 3 | 0 | 2 | 3 | 5 (3)* |
| 6 | 0 | 0 | 3 | 7 (1)* |
*No bleaching effect in EDJ
Figure 4 shows the typical bleached surface, crosscut surface, and CIE L*a*b* values in each group. In Group 1, liner L*, a* and b* values were almost same at same depth of bleached and nonbleached areas from enamel surface to pulpal side of dentin (Fig. 4a). In Group 2, the bleaching effect of enamel surface was obvious. The linear L* values of the bleached area was higher than those of the nonbleached area from enamel surface to pulpal side of dentin. The linear b* values of bleached area in enamel was lower than those of nonbleached area (Fig. 4b). In Group 3, the linear L* values of bleached area were higher than those of nonbleached area. The linear b* values of nonbleached area were lower than those of nonbleached area. The liner a* values of bleach area were slightly lower than those of nonbleached area. Those differences were observed not only in enamel but also in dentin (Fig. 4c).
The score distribution of bleaching extension in each group is shown in Table 2. In Group 2, two samples showed bleaching effect which was found only in enamel as Score 1. Eight samples showed the bleaching effect in dentin as Score 2 or 3. In Group 3, all sample showed bleaching effect in the dentin as Score 2 or 3. For three samples in Group 2 and one sample in Group 3, the stain was remained at enamel-dentin junction (EDJ), although the bleaching effect was found in the dentin beyond EDJ.
Typical images of the specimen in each category are shown in Fig. 5. And Fig. 6 shows whiteness index (WID) of those specimens. In Score 0, both linear WID of bleached area and nonbleached area were almost same (Fig. 6a). In Score 1, linear WID of the bleached area was higher than that of the nonbleached area at enamel (Fig. 6b). In Score 2, linear WID of the bleached area was higher than that of the nonbleached area between enamel surface and two thirds of dentin (Fig. 6c). In Score 3, linear WID of the bleached area was higher than that of the nonbleached area through whole enamel and dentin (Fig. 6d). Some samples showed low WID at EDJ (Fig. 6e).
Whiteness index at middle of enamel (E1/2), a third of dentin (D1/3), and two thirds of dentin (D2/3) in each group is shown in Fig. 7. In E1/2 area, WID of Group 2 and 3 showed statistical higher than that of Group 1 ( P < 0.05). In D1/3 area, WID of Group 3 showed statistical higher than that of Group 1 ( P < 0.05). For WID at D2/3 area, there was no statistical difference among three groups. In Group 2 and 3, WID at E1/2 area showed statistical higher than that at D1/3 and D2/3 area ( P > 0.05).
The left side of the figure is a photograph of the surface and crosscut surface, and the right side is a graph of L*a*b*.
L*, a*, b* values of crosscut surface was measured liner from the center of bleached surface to pulpal along the orientation of dental tubules with 1 mm of width. Another liner measurement apart from 4 mm of apical direction was performed.
(a) Typical sample in Group 1: The L*a*b* values are similar between bleached and nonbleached area.
(b) Typical sample in Group 2: Bleaching effect of enamel surface was obvious. Linear L* values of the bleached area was higher than those of the nonbleached area.
(c) Typical sample in Group 3: Linear L* values of bleached area were higher than those of nonbleached area. Linear b* values of nonbleached area were lower than those of nonbleached area.
Fig. 5 Images of tooth bleaching
→ ← Bleached area
(a) A sample from Score 0
(b) A sample from Score 1
(c) A sample from Score 2
(d) A sample from Score 3
(e) A sample in which less bleaching effect was observed at EDJ.
Fig. 6 Typical pattern of the whiteness index (WID) at bleached and nonbleached area of each scored sample
(a) A sample from Score 0: WID of both area was similar.
(b) A sample from Score 1. WID of bleached area was higher than that of nonbleached area in enamel and a third of dentin.
(c) A sample from Score 2. WID of bleached area was higher in enamel and two thirds of dentin.
(d) A sample from Score 3. WID of bleached area was higher in both enamel and dentin.
(e) A sample in which less bleaching effect was observed at EDJ: WID at DEJ in bleached and nonbleached area was similar.
Fig. 7 Results of whiteness index ( n = 10)
Values with different symbols are significantly different within the same depth group ( P < 0.05).
The line indicates a significant difference in the same bleaching times.
Values with different symbols are significantly different within the same group ( P < 0.05).
Tooth color is determined by the combined effects of endogenous and exogenous color. Intrinsic tooth color is related to the light scattering and absorption properties of enamel and dentin [5]. Exogenous color is associated with the absorption of substances into the enamel surface, especially the pellicle coating, ultimately causing exogenous staining. [5,15] The tooth bleaching affected not only tooth surface, but also inside tooth structure [11]. In this study, artificial discolored tooth sample was prepared by staining from enamel surface and pulpal dentin in order to examine the effect of both external and internal staining. The black tea extract was used for staining the teeth, based on the discoloration model [19].
Generally, the bleaching effect was evaluated from the bleached surface [20,21] and there are very few reports evaluated on the crosscut surface [10]. In this study, bleaching effect was evaluated on the crosscut surface from enamel to pulpal dentin continuously. There are two methods for evaluation of tooth shades before and after tooth bleaching. One is subjective method; visual analysis by comparison with a standard tooth shade guide, and another is objective method; the use of a spectrophotometers or a calorimeter occasionally combines with an image analysis technique with software [1]. Although the evaluation of tooth shade by color guide matching is a simple method to use, it is not very reliable, because it is influenced by the observer’s experience, eye fatigue, and variation of the ambient light, among other factors [5]. The esthetic aspects of tooth color are difficult to quantify and color perception is highly subjective and prone to individual variation [21].
The camera used in this study (RC500) is equipped with 3 filters (S1, S2, and S3) with spectral transmittance derived from the CIE color-matching functions in XYZ color system. In the computer, a set of S1, S2, and S3 digitized signal captured by CCD (active pixel of 4008(H) by 2672(V) and 43.3 mm diagonal (3:2 ratio)) are treated as an image and converted to XYZ color information by a 3×3 color conversion matrix, which is derived from a set XYZ measured values of the reference color samples and their equivalent imager outputs with regard to the filter of the camera by multiple regression analysis. Additionally, the camera incorporates an embedded color calibration system with a spectrophotometer mounted inside the optical system Therefore, the outputs of the camera are linearly transformed into device- independent XYZ tristimulus values [12]. In addition, this system can be set up to measure any area of the desired length and width, and the color measurement area from the start point to the endpoint can be measured continuously. The pixels of the image are 0.05 mm x 0.05 mm per pixel. The color tone from the enamel surface to the pulpal was defined and measured to be 1 mm in width (20 pixels). The value of each depth is the average value of the pixel width data of the measurement area. In this study, RC500 and an equipped software were used for the evaluation of the color. These could evaluate the color on the crosscut surface of bleached teeth linearly expressing L*a*b* values.
Whiteness is defined as the attribute of color perception by which an object color is judged to approach the preferred white, whereas whiteness index is a number that indicates the degree of departure of an object color from that of a preferred white [16]. For this reason, whiteness was used in this evaluation. It is important to point out that color measuring devices currently used in dentistry almost exclusively use the CIE-Lab color space for measurements. Moreover, chromatic changes in clinical practice situations and research are usually assessed by means of color differences associated with this color space. Some whiteness indices are based on the CIE 1931 XYZ color notation system, such as the CIE whiteness index WIC and WIO. WIO maintains the functional form of the CIE WIC index and has been proven adequate for tooth whitening research and monitoring [22]. Develop and assess a new whiteness index (WID), designed specifically for dentistry and based on the CIE-Lab color notation system. WID would correlate with visual findings, and that it would perform well compared with other whiteness indices based on both CIE-Lab and the CIE 1931 XYZ color notation systems [13,15]. WID outperforms previous indices, being the only CIE-Lab based index developed for evaluation of whiteness in dentistry. Different whiteness indices are being used to measure tooth whiteness. WIC (CIE recommended whiteness index) is widely used in the textile, paint, and plastic industries. In order for a whiteness index to be valid, it must be used on the type of material for which it is intended. Studies were carried out to optimize the coefficients of the WIC formula according to visual results and a new optimized formula was developed which is the WIO index for teeth [23,24]. Since the CIE-Lab color space is used in both lab and clinical practice in esthetic dentistry, and since the CIE-Lab color space is also used in clinical dentistry, the newly proposed white index (WID) has become important as a method for evaluating the whiteness of many used methods and materials. WID is shown to be superior to CIE XYZ based WIC, the CIE recommended whiteness index. It is shown that WID is superior to other whiteness indices in both laboratory and clinical validation experiments [16,25]. WIO and WIC are also excellent in whiteness evaluation, but for these reasons, WID was used in this study [26].
Both 3-times and 6-times bleaching (Groups 2 and 3) showed the bleaching effect not only in enamel but also in dentin, and 6-times bleaching extended the bleaching effect deeper than 3-times bleaching. By repeating bleaching procedure, bleaching may show the effect for discolored teeth by intrinsic stain. And bleaching evaluation on crosscut surface is thought to be useful for better understanding of bleaching effect. However, 3 samples of 3-times bleaching group and one sample of 6-time bleaching group showed less bleaching effect at EDJ. EDJ and the structure of the dentin in the shallow part are considered to be less effective in bleaching [27,28]. Less bleaching effect at DEJ may due to the anatomical structure of DEJ. Further study is necessary for this matter.
Conflict of Interest
The authors declare that they have no conflict of interest.
