1452014年 4月

Tooth Surface Observation after Bleachingby Optical Coherence Tomography

MATSUO Ryoko, MANABE Atsufumi,HOSOKAWA Mayumi＊ and KAKUMA Hideo＊

Department of Conservative Dentistry, Division of Aesthetic Dentistry and Clinical Cariology,Showa University School of Dentistry

＊Yoshida Dental Mfg.

Abstract　Purpose: To observe the tooth structure after bleaching by using experimental swept-source optical coher-ence tomography（SS-OCT, Yoshida Dental）.　Methods: This study was approved by the Showa University Institutional Review Board（Approval num-ber 2011‒035）. Fourteen extracted intact human incisors were used. Commercial in-office bleaching gel con-taining 35％ hydrogen peroxide（Shofu Hi-Lite, Shofu）was applied to the labial enamel surface according to the manufacturer’s instructions. The bleaching gel was applied to the surface for 3 min once a week for 4 weeks, and the 4-week bleaching regimen was repeated three times. Before and after bleaching, the tooth structure was observed by SS-OCT, which constructs images by ultrahigh-speed scanning of the converted wavelength of a near-infrared laser and allows rapid noninvasive construction of tomographic images of teeth.　Results: In the SS-OCT images, light reflection from the enamel surface became strong after bleaching. The light reflection intensity deep in the tooth tended to be lower than that at the enamel surface. The light reflection intensity in the SS-OCT images of the control before and after storage in water at 37℃ either decreased or remained the same. The tooth appeared whiter after bleaching to the naked eye. The value of luminosity（L＊）increased, and the values of color parameters（a＊ and b＊）decreased. Significant differences in the color difference（ΔE＊ab）before and after bleaching were found by Student’s t-test（p＜0.05）.　Conclusion: The light reflection intensity in the SS-OCT images increased for the enamel surface bleached with 35％ hydrogen peroxide, indicating that small structural changes may occur that are not visible by SEM. SS-OCT can be used to provide patients with information in a clinical setting and may be a promising new tool for tooth bleaching.

Key words: SS-OCT, enamel, bleaching

　Corresponding author: Dr. MATSUO, Department of Conservative Dentistry, Division of Aesthetic Dentistry and Clinical Cariology, Showa University School of Dentistry, 2‒1‒1, Kitasenzoku, Ohta-ku, Tokyo 145‒8515, Japan　TEL: ＋81‒3‒3787‒1151, FAX: ＋81‒3‒3787‒1229, E-mail: 12102078d@grad.showa-u.ac.jp　Received for Publication: January 15, 2014/Accepted for Publication: January 22, 2004　DOI: 10.11471/shikahozon.57.145

日歯保存誌 57（2）：145～153，2014

146 日　本　歯　科　保　存　学　雑　誌 第 57巻　第 2号

Introduction

　Optical coherence tomography（OCT）is a diagnostic technology for cross-sectional imaging of internal bio-logical structures. The technique, which was first reported by Huang et al. in 1991, is comparable to X-ray radiography, computed tomography, magnetic resonance imaging, and sonography, although OCT imaging is used for more detailed analyses than the other imaging techniques1）. OCT has proven to be use-ful in ophthalmology and other fields of medicine.　Fujimoto et al.2） published a review of OCT technol-ogy and described its biomedical and medical applica-tions. Fourier domain（FD）-OCT was first used for in vivo imaging in 1995 by Fercher et al.3）. The principles of FD-OCT differ from those of time domain（TD）-OCT in some respects, although there are many similarities. FD-OCT is 100- to 1000-fold more sensitive than TD-OCT for the same data acquisition period4）, although FD-OCT measurements take about 10 times longer than TD-OCT. FD-OCT has the advantage of providing phase information in addition to a strong OCT signal. Spectral domain（SD）-OCT uses an interferometer and spectroscope with a broadband light source, a wave-length scanning light source, and a point photodetector to measure and resolve the interference signals of the wavelengths. In contrast, swept source（SS）-OCT achieves dislocation imaging by measuring a spectrum interference fringe, and is different from SD-OCT. Real-time FD-OCT5） and video-rate spectral-domain OCT6） have also been developed. Feldchtein et al.7）. reported the in vivo OCT imaging of hard and soft tissues of the oral cavity and that the technique can accurately image dental tissue structures and detect small anomalies. Colston et al.8）. discussed the dental OCT system for diagnosing periodontal disease, detecting caries, and evaluating dental restoration. Otis et al.9）. used OCT to image teeth and locate soft and hard tissue boundaries in the periodontium to evaluate restoration margins.　Tooth bleaching is one of the most popular aesthetic dental procedures. Hydrogen peroxide or carbamide peroxide is commonly used to bleach discolored teeth, although there are many materials and techniques available. Tooth bleaching products fall into three cate-gories: professional in-office agents; dentist-prescribed at-home agents; and over-the-counter agents. In-office

bleaching techniques typically use 30‒35％ hydrogen peroxide and are monitored directly by the dentist. The most common home bleaching method is Nightguard vital bleaching10）, which offers an apparently safe and effective means of bleaching mildly discolored teeth. The technique depends on a complicated oxidative pro-cess starting with free radicals released from hydrogen peroxide11）. However, Rotstein et al.12）. showed that the calcium concentration and calcium/phosphorous ratio（Ca/P ratio）in hard dental tissues were decreased sig-nificantly when 30％ hydrogen peroxide was applied to the tooth surface over 7 days. Attin et al.13）. investi-gated the hardness of bleached enamel and showed that bleached and unfluoridated specimens underwent a much greater reduction in hardness compared with flu-oridated specimens. No significant difference was observed between the two fluoridated groups, and rem-ineralization of the bleached enamel was improved by applying concentrated fluorides. Hegedüs et al.14）. observed enamel surface changes by atomic force microscopy after 28　h of bleaching with three peroxides containing bleaching agents and indicated that perox-ides affect both the enamel surface and the inner struc-ture of enamel; the low molecular weight of hydrogen peroxide means that it can penetrate the enamel. Bit-ter15） observed surface grooves on tooth enamel by scanning electron microscopy（SEM）and suggested that the bleaching agents alter the enamel surface, the depth of the enamel rods, and possibly the dentin. McGuckin et al.16）. used profilometric analysis to compare the effect of three bleaching products and found that they all increased the roughness and waviness of the enamel surface.　The differences between the office- and home-treated surfaces were unrelated to the pH of the bleaching agents. Several studies have concluded that the bleach-ing effect depends on irregularities in the enamel sur-face17）. High concentrations of carbamide peroxide can alter the enamel surface. The surface roughness and staining susceptibility can also depend on the bleaching agent. However, many studies18） suggest that tooth color is primarily determined by the dentin, which can be altered by bleaching. In one study19）, 35％ hydrogen peroxide caused color changes in both the enamel and the dentin. Other studies dispute the conjecture of color change in dentin and suggest that the color changes occur in the enamel, thereby masking the unchanged

1472014年 4月 Tooth Surface Observation after Bleaching Using OCT

dentin20）. However, the successful bleaching of tetracy-cline-stained teeth and those with dentinogenesis imperfecta adds weight to the argument that the color change takes place primarily in dentin21）. Chiappinelli et al22）. reported that discoloration from tetracycline might not be limited to tooth development in children, but might also affect adult dentition. Croll and Sasa23） reported that carbamide peroxide altered the tooth color of a teenager with dentinogenesis imperfecta. McCaslin et al24）. showed that the color change caused by 10％ carbamide peroxide in dentin occurred at a uniform rate and a further study demonstrated that 35％ hydrogen peroxide in-office bleaching gel bleached dentin to a uniform depth. In the present study, we used experimental SS-OCT（Yoshida Dental）to observe the difference between teeth before and after bleaching and investigated the clinical usefulness of SS-OCT.

Materials and Methods

1 ．Specimens　This study was approved by the Showa University Institutional Review Board（approval number 2011‒035）. Fourteen extracted non-carious intact human inci-sors were used. Discolored teeth, such as tetracycline- or fluorosis-stained teeth, were excluded. The prepara-tion of specimens is shown in Fig.　1. After removing calculus and the periodontal membrane using a dental

hand scaler, the labial enamel tooth surface was pol-ished using polishing paste（Pressage, Shofu）with a polishing cup（Merssage-cup, Shofu）for 10　s. The tooth roots were removed with a diamond rotary cutting（Shofu Diamond Point FG 1104,）instrument at the cer-vical line, and the remaining crown was sectioned longi-tudinally into two parts along the tooth axis. The right half of the crown surface was bleached（treated）and the left half was not bleached and served as a control（control）. The specimens were polished and cut, then ultrasonically cleaned for 15 min. The specimens were fixed in silicone rubber impression material（Exafine putty type, GC）.

2 ．Bleaching　The bleaching procedures are shown in Fig.　2. The bleaching was carried out according to the manufac-turer’s instructions. The powder and liquid were mixed for 30　s, and then the mixture was applied to the enamel surface for 5 min. The mixture was irradiated with a high-power LED visible light-curing unit（G-light, GC）for 3 min and allowed to rest for 1 min. The bleach-ing agent was removed, the enamel surface was wiped with a cotton swab, and the bleached surface was washed with water. This procedure was repeated once a week for 4 weeks, and the 4-week bleaching regime was repeated three times. During the study, all speci-mens were stored individually in water at 37℃ and 100％ humidity.

Fig.　1　Specimen preparation

Tooth polishing Removingtooth roots

Sectioning longitudinally along the tooth axis into two parts

Embedding in siliconerubber impression material

Fig.　2　Bleaching procedure

Apply bleaching paste Irradiate for 3 min Wash out and removepaste Store in water

148 日　本　歯　科　保　存　学　雑　誌 第 57巻　第 2号

3 ．SS-OCT　The tooth was observed by the experimental SS-OCT before and after bleaching. Fig.　3 shows the fiber-optic Mach-Zehnder interferometer configuration of the OCT system. The light from the source is split into the sample and reference arms by coupler 1. In the sample arm, the light illuminates the sample and the reflected light is collected. In the reference arm, light is reflected

off the reference mirror. The light from each arm is combined by coupler 2, which outputs the interference signal. The signal is detected by the balanced receiver, which sends it to the computer to process the images. The experimental setup is shown in Fig.　4. The OCT system consists of an optical unit, a probe, and a com-puter. Fig.　5 shows the sample stage, which consists of an XYZ stage that can tilt and rotate to position the sample correctly. Table　1 shows the specifications of the OCT system.

4 ．Colorimetric measurement　Colorimetric measurements of the enamel surface were performed before and after bleaching with a den-tal chroma meter（ShadeEye NCC, Shofu）against a black background. The diameter of the color measure-ment area was 3.0 mm. The color measurements were performed at three points on the experimental enamel surface and the mean values were calculated. The cor-responding values of luminosity（L＊）and color param-eters（a＊ and b＊）were measured, and the color differ-ence（ΔE＊ab）before and after bleaching was calculated. To investigate the effect of bleaching, Student’s t-test analysis was performed.

5 ．SEM observations　The enamel surface was observed by SEM（s-4700,

Table　1　Specifications of the OCT system

Center wavelength 1,310 nmScan range 140 nmLine rate 50 kHzAxial resolution 13μmLateral resolution 35μm

Fig.　3　Fiber-optic Mach-Zehnder interferometer configuration of OCT system

Sample Objective lens 1Galvano mirror

Collimator lens 1

Circulator 1 Polarization controller 1

Balanced receiver

Coupler 2

Polarization controller 2Circulator 2

Computer

Sample arm

Reference armCoupler 1

Objective lens 2

Mirror Collimator lens 2

Laser source

Fig.　4　Experimental setup

PC

Monitor

Optical unit

Stage

Probe

Fig.　5　Setup around the probe and sample stage

Sample stage

Probe

X

YZ

1492014年 4月 Tooth Surface Observation after Bleaching Using OCT

Hitachi）. The specimens were embedded in epoxy resin and polished with a wet silicon carbide paper（#1500）and then with a linen cloth and alumina slurry（grain size 0.03μm）. A 0.5 mol/l neutralized EDTA solution（pH 7.4）was used for 60　s to remove only the smear layer on all specimens. The specimens were dehy-

drated in ethyl alcohol solution in which the concentra-tion was increased from 70％ to 95％ in 5％ increments for 30 min at each concentration and in 99％ alcohol solution for two 15-min periods. The specimens were critical point-dried（HCP-2, Hitachi）and sputter-coated（E-1030, Hitachi）with palladium and platinum.

Before bleaching After bleachingFig.　6　OCT image of treated specimen

Light reflectionfrom enamelsurface Enamel

Dentin

Light reflectionintensity deep in the tooth

Before storage in water at 37℃ After storage in water at 37℃Fig.　7　OCT image of control specimen

Enamel

Dentin

Before bleaching After bleachingFig.　8　SEM image of enamel surface

150 日　本　歯　科　保　存　学　雑　誌 第 57巻　第 2号

Results

　Fig.　6 shows OCT images of the enamel before and after bleaching. The light reflection from the enamel surface became strong after bleaching. The light reflec-tion intensity deep in the tooth tended to be lower than that at the enamel surface. Fig.　7 shows the OCT

images of the control before and after it was stored in water at 37℃. The light reflection intensity either decreased or remained the same. The tooth appeared whiter after bleaching to the naked eye. Fig.　8 shows SEM images of the enamel surface. The images show little difference between before and after bleaching. The mean values of the color parameters for the treated teeth（L＊, a＊, and b＊）are shown in Table　2

Table　2　Color parameters of treated section

Before After Change in color dimension

L0 L1＊ a1

＊ b1＊ L2

＊ a2＊ b2

＊ ΔL＊ Δa＊ Δb＊ ΔE＊ab

1 76.7 　1.4 20.0 80.1 －0.2 8.4 　3.4 －1.6 －11.6 12.22 75.4 　0.6 11.9 77.8 －0.6 7.0 　2.4 －1.2 －4.9 5.63 73.2 　1.2 14.6 75.0 　0.5 9.0 　1.8 －0.7 －5.6 5.94 68.4 　1.4 26.3 74.1 －0.9 14.6 　5.7 －2.3 －11.7 13.25 74.6 　1.4 11.9 68.6 　0.0 12.9 －6.0 －1.4 　 1.0 6.26 71.4 　0.2 13.5 73.6 －0.6 5.9 　2.2 －0.8 －7.6 8.07 77.4 　1.7 13.1 75.0 　0.6 10.2 －2.4 －1.1 －2.9 3.98 76.2 －0.9 14.7 71.8 －2.1 11.0 －4.4 －1.2 －3.7 5.99 73.5 　0.6 12.7 74.1 －0.2 6.9 　0.6 －0.8 －5.8 5.910 76.9 －0.8 12.0 81.3 －0.1 8.4 　4.4 　0.7 －3.6 5.711 71.6 －0.7 14.7 76.7 　0.1 8.3 　5.1 　0.8 －6.4 8.212 70.3 　3.7 19.7 74.4 　1.2 11.2 　4.1 －2.5 －8.5 9.813 76.2 －0.9 19.4 83.2 －0.2 10.0 　7.0 　0.7 －9.4 11.714 71.0 　0.6 15.7 79.9 　0.9 9.6 　8.9 　0.3 －6.1 10.8

ΔL＊＝L2＊－L1

＊，Δa＊＝a2＊－a1

＊，Δb＊＝b2＊－b1

＊，ΔE＊ab＝（ΔL＊2＋Δa＊2＋Δb＊2）1/2

Lo location

Table　3　Color parameters of the control

Before After Change in color dimension

L0 L1＊ a1

＊ b1＊ L2

＊ a2＊ b2

＊ ΔL＊ Δa＊ Δb＊ ΔE＊ab

1 75.4 　1.5 20.7 75.5 　0.6 20.8 　0.1 －0.9 　0.1 12.52 72.3 　0.7 11.9 74.3 －0.2 10.4 　2.0 －0.9 －1.5 2.73 72.8 　1.4 14.0 72.7 －0.3 13.8 －0.1 －1.7 －0.2 7.04 68.5 　1.5 25.7 68.8 －1.0 24.9 　0.3 －2.5 －0.8 19.05 66.3 　2.0 16.8 67.4 　0.3 12.2 　1.1 －1.7 －4.6 8.26 66.8 　0.6 13.8 66.3 －0.7 13.9 －0.5 －1.3 　0.1 10.97 73.5 　2.8 13.7 73.8 －0.6 13.6 　0.3 －3.4 －0.1 7.98 69.3 －0.5 14.9 71.6 －0.7 14.8 　2.3 －0.2 －0.1 5.69 72.2 　0.7 14.2 73.7 －0.1 13.0 　1.5 －0.8 －1.2 8.410 77.7 －1.3 9.3 75.3 －0.1 7.0 －2.4 　1.2 －2.3 8.211 69.5 　0.8 14.0 68.6 　0.8 12.8 －0.9 　0.0 －1.2 7.612 69.4 　3.6 20.8 70.8 　1.9 18.9 　1.4 －1.7 －1.9 10.913 75.5 －0.7 19.6 76.3 －0.2 19.8 　0.8 　0.5 　0.2 12.514 66.5 　0.9 18.3 66.4 　1.8 17.4 －0.1 　0.9 －0.9 14.4

ΔL＊＝L2＊－L1

＊，Δa＊＝a2＊－a1

＊，Δb＊＝b2＊－b1

＊，ΔE＊ab＝（ΔL＊2＋Δa＊2＋Δb＊2）1/2

Lo location

1512014年 4月 Tooth Surface Observation after Bleaching Using OCT

and the control values are shown in Table　3. The value of L＊ increased and a＊ and b＊ decreased. In other words, an increase of the value of L＊ indicated that a light area became a brighter color, while a decrease of the values of a＊ and b＊ showed that the color phase became green and blue. Significant differences in ΔE＊a＊b＊ were found by Student’s t-test（p＜0.05）.

Discussion

　The Hi-Lite bleaching system was developed by Fre-idman and coworkers, and it was released in 1991 by Shofu Dental, USA. However, it was approved for use in Japan in only 1998 by the Ministry of Welfare, Labour and Welfare25）. In this bleaching system, 35％ hydrogen peroxide and powder are mixed and applied to the enamel surface. The mixture is irradiated with a visible light-curing unit for 3 min. This dual activated bleach-ing system indicates activation of the mixture by color. The systems remove coloration of the enamel surface caused by organic colorants, including food and drink, color changes caused by aging, and light tetracycline staining. This bleaching system causes minute cavities and cave-in in the enamel surface by partly dissolving the enamel substrate26）. The pellicle of the enamel sur-face is removed and colored areas around the enamel rods are dissolved by the hydrogen peroxide27）. This is masked by the white appearance of the surface of the tooth, which is caused by the decalcification of the enamel surface changing the refractive index28）. The bleaching mechanism involves the redox reactions of hydrogen peroxide although the details remain un- known. In the present study, OCT images showed that the intensity of light reflected off the enamel surface increased after bleaching, although the SEM images did not show substantial changes. We suggest that the OCT images may capture small structural changes that cannot be observed by SEM. Thus, in addition to colori-metric measurement, OCT may be a useful new evalua-tion method after bleaching that yields information on the enamel surface and internal structure.

Conclusions

　The light reflection intensity in the SS-OCT images increased for the enamel surface bleached with 35％ hydrogen peroxide, indicating that small structural

changes may occur that are not visible by SEM. SS-OCT can be used to provide patients with information in a clinical setting and may be a promising new tool for tooth bleaching.

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1532014年 4月 Tooth Surface Observation after Bleaching Using OCT

　責任著者連絡先：松尾涼子　〒 145‒8515　東京都大田区北千束 2‒1‒1　昭和大学歯学部歯科保存学講座美容歯科学部門　TEL：03‒3787‒1151, FAX：03‒3787‒1229, E-mail：12102078d@grad.showa-u.ac.jp　受付：平成 26年 1月 15日/受理：平成 26年 1月 22日

Optical Coherence Tomographyによるホワイトニング前後のエナメル質観察

松　尾　涼　子　　真　鍋　厚　史細　川　真　弓＊ 　鹿　熊　秀　雄＊

昭和大学歯学部歯科保存学講座美容歯科学部門＊吉田製作所

抄録　緒言：近年，白くて美しい歯は人々の QOLの向上には欠かせない要因となり，審美歯科に対する関心の高まりとともにホワイトニングを希望する患者も増加してきている．ホワイトニング剤の歯質に対する影響はいまだ不明な点が多く，ホワイトニング剤によりエナメル質を保護するペリクルは除去され，エナメル質表面になんらかの影響を及ぼしていると考えられている．そこで，本研究では，オフィスホワイトニング処置前後の歯質の変化を経時的に観察し，ホワイトニング後の評価項目として測色だけでなく，非侵襲性の光干渉断層装置（Swept‒Source Optical Coherence Tomography，以下，SS‒OCT）を用い，新たな評価法の検討を目的とした．　材料と方法：試料はヒト抜去歯を合計 14本用いた．歯冠部歯質を近遠心中央部で二等分し近心側を Treat-ment側，遠心側を Control側とした．なお，本研究で使用した抜去歯は本学歯学部医の倫理委員会の承認を得たものである（承認番号 2011‒035号）．歯面清掃後 ShadeEye NCC（松風）にて測色し，吉田製作所製試作 SS‒OCTを用いてホワイトニング処置前の試験面を唇側から撮影した．オフィスホワイトニング剤には 35％過酸化水素を主成分とする Shofu Hi-lite（松風）を使用し，使用説明書の術式に準じ Treatment側のホワイトニング操作を行った．Control側は試験期間中 37℃の水中に保管した．ホワイトニング処置後，ShadeEyeNCCにて試料の測色と，SS‒OCTを用いてホワイトニング処置前と同部位の試料撮影を行った．また SEM（s‒4700，Hitachi）による観察を行った．　成績：SS‒OCTを用いたホワイトニング処置前後のエナメル質の観察において，ホワイトニング処置後の光反射強度はエナメル質表層で強くなる傾向を示し，エナメル質内部では光透過性が亢進する傾向が得られた．ホワイトニング前後の ShadeEye NCCを使用して得られた L＊ a＊ b＊からΔE＊abを算出したところ，ホワイトニング前後で有意差が認められた（p＜0.05）．また，SEM像ではホワイトニング前後で変化が認められなかった．　結論：ホワイトニング前後の光反射強度に変化がみられたことから，SS‒OCTは，ホワイトニング後の評価法として測色に加え，歯質表層ならびに内部の構造など多くの情報が得られる可能性が示唆された．

キーワード：SS‒OCT，エナメル質，ホワイトニング