日大歯学 , 94（2）, 65-71, 2020

Original article

Nondestructive measurement of remaining dentin thickness using time-domain
optical coherence tomography
Koji Komoriya1,2, Hiroyasu Kurokawa1,2, Toshiki Takamizawa1,2, Akimasa Tsujimoto1,2,
Ryo Ishii1,2, Yoshiyuki Amari1, Taketo Okuwaki1, Masashi Miyazaki1,2
1 Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan
2 Division of Biomaterials Science, Nihon University School of Dentistry, Tokyo, Japan

Abstract: Purpose: The ability of optical coherence tomography (OCT) to detect the remaining dentin
thickness (RDT) in cases of erosive tooth wear was evaluated.
Methods: The labial surfaces of extracted bovine teeth were ground with wet silicon carbide papers
ranging from #240-grit to #2,000-grit to create different RDTs. The time-domain (TD)-OCT imaging system
was projected onto the specimen surface, and the areas of interest were scanned using a probe attached to amounting device. A depth profile of backscattering along a line perpendicular to the object surface was
generated (A-scan), and the RDT was obtained. After TD-OCT scanning, the specimens were sectioned
longitudinally (mesial-distal), and the thicknesses were measured using laser scanning microscopy (LSM). The
data obtained from the different measurements were subsequently compared. Bland-Altman comparison andthe paired t-test were used at a significance level of 0.05.
Results: The agreement between the different measurement methods was analyzed to assess theintermethodology variation, with strong and positive correlations found for each group of measurements.
The results indicated that the mean difference between the TD-OCT and LSM measurements was 7.857 ±
68.284 µm, with the 95% Bland-Altman limits of agreement ranging from −125.980 to 141.694 µm.
Conclusion: Although TD-OCT scanning has several limitations, it is a promising, nondestructive method
for assessing RDT under the conditions of this study.
Keywords: dentin thickness measurement, optical coherence tomography, erosive tooth wear

Time-domain 型光干渉断層画像法を用いた残存象牙質厚径の非破壊測定
小森谷
石 井

康

司 1,2
亮 1,2

黒
甘

川
利

弘
佳

康 1,2
之1

髙見澤 俊 樹 1,2
奥 脇 岳 人1

辻 本 暁 正 1,2
宮 崎 真 至 1,2

1 日本大学歯学部保存学教室修復学講座
2 日本大学歯学部総合歯学研究所生体工学研究部門

要旨：目的：非破壊観察が可能な光干渉断層画像法（OCT）を用いて酸蝕歯の残存象牙質厚径（RDT）について評価した。
方法：ウシ下顎前歯の唇側面を耐水性 SiC ペーパーの #240 から #2,000 まで順次研磨することで，象牙質の残存
厚径が異なる測定用試片を製作した。OCT については，Time-domain 型 OCT 装置からの照射光線が，これら測定用試片表面に垂直に照射されるよう調整し，装置付属のプローブを用いてスキャンした。測定は，A-scan mode
で行い，測定範囲内の信号強度を得ることで試片の残存象牙質厚径の測定を行った。また，OCT 計測後の測定用試片を，近遠心方向に縦断し，縦断面の象牙質残存厚径を，レーザ走査型顕微鏡を用いて計測することで，OCTで得られた RDT と比較した。OCT およびレーザ走査型顕微鏡で得られた RDT について，Bland-Altman 分析お
よび paired t-test を用いて，有意水準 5% の条件で統計学的検定を行った。
成績：異なる測定方法で得られた RDT の一致度を比較した結果，測定値間に強い正の相関が見られた。すなわち，
OCT およびレーザ走査型顕微鏡で得られた RDT の平均の差は 7.857 ± 68.284μm であり，Bland-Altman 分析の結果，95% 一致限界の−125.980 から 141.694μm の範囲内であることが示された。

（受付：令和 2 年 5 月 13 日）
Corresponding author: Dr. Hiroyasu Kurokawa
Department of Operative Dentistry, Ninon University School of
Dentistry
1-8-13 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan

TEL：+81-3-3219-8141
FAX：+81-3-3219-8347
E-mail：kurokawa.hiroyasu@nihon-u.ac.jp

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第 94 巻第 2 号

結論：OCT を用いた測定はいくつかの制限があるものの，非破壊的に象牙質の残存歯質厚径の測定が可能である
ことが示された。
キーワード：象牙質厚径測定，光干渉断層画像法，酸蝕歯is often overlooked because there are a few signs andIntroduction

fewer, if any, symptoms. Therefore, determining the

Erosive tooth wear has been defined as the loss of

possible clinical course and progressive nature of ero-

hard dental tissue caused by nonbacterial chemical

sive tooth wear is of importance for identifying theattack 1). In the early stages, erosive tooth wear ispredisposing and exacerbating factors to enable bet-

limited to the surface softening of the enamel andter monitoring, prognoses, and prevention of further

dentin. However, an extensive surface loss can occur

damage to teeth. Although the patientʼs clinical

as a result of demineralization and associated physi-

appearance is the most important feature used by

cal forces, such as brushing and attrition 2). Results

dental professionals for the diagnosis of erosive toothfrom longitudinal studies have shown that anwear, it is difficult to visually assess the remaining

increased number of teeth are affected by erosive

dentin thickness (RDT). A number of methods havetooth wear and that the severity of erosive toothbeen developed to assess RDT, including cone-beamwear increases with increasing age 3). Erosive agentscomputed tomography 8) , electrical resistance mea-

can be classified as intrinsic (gastric acid) or extrinsic

surement 9), and ultrasonic measurement technique 10).

(foods and drinks), with increased attention paid to

Optical coherence tomography (OCT) has been usedacidic diets, as there has been a dramatic growth infor providing cross-sectional images of biologic struc-the consumption of acidic beverages in the past few

tures based on the differences in tissue optical prop-

decades. A link between erosive tooth wear and theerties 11) and has also been employed for the determi-

intake of sweet, carbonated drinks was found, andnation of enamel remineralization 12) . Time-domain

other dietary elements have been reported to be(TD) OCT can generate a depth scan (A-scan) by

associated with erosive tooth wear, including fruit

mechanically scanning the coherence gate along thejuices, sports drinks, fresh fruits, vitamin C, vinegar,

depth range. The TD-OCT imaging system is able toand herbal tea 4).

generate in vivo optical cross-sections of the samples
with a typical axial resolution of tens of micrometers

There are variabilities in the prevalence data for
erosive tooth wear in the general population for bothand low-acquisition states of a few images per second

primary and permanent teeth. The results of a(i.e., a few hundred A-scan/s). Therefore, TD-OCT

review paper 5) indicated that the mean number ofmight be beneficial for applications that require ateeth with facets of moderate to severe wear was 5.4,

wide scanning range or a high lateral resolution 13).

and 51% of the participants had four or more teeth

The purpose of this study was to evaluate thewith facets of moderate to severe wear. The accept-

accuracy of RDT measurements using the TD-OCT

able amount of tooth wear is dependent on theimaging system compared with measurements byexpected life span of the teeth, which is different for

laser scanning microscopy (LSM) after grinding of the

primary teeth compared with permanent teeth.

bovine tooth surface. We tested the hypothesis that

However, erosive damage to the permanent teeth

there would be significant differences between

that occurs in childhood may compromise the grow-

TD-OCT and LSM measurements.

ing child’s dentition over their entire lifetime andMaterials and Methods

may require repeated and increasingly expensive restorations 6) . Therefore, early diagnosis of the tooth

1. Specimen preparation

wear process in children and adults is important inFive freshly extracted bovine incisors, without

enabling the initiation of appropriate preventive mea-

cracks or erosion, were cleaned and stored in physio-sures. In addition, early intervention can commence

logical saline for up to 2 weeks. Teeth were mounted

only if risk factors, as well as biological and behav-

in cold-curing acrylic resin (Tray Resin II; Shofu,

ior-modifying factors, are considered .

Kyoto, Japan) to expose the labial surface and then7)

ground with wet #240-grit silicon carbide (SiC) paper

Diagnosis in the early stages of erosive tooth wear

66

Dentin thickness measurement using TD-OCT

令和 2 年 12 月

(Fuji Star Type DDC, Sankyo Rikagaku, Saitama,

object surface was generated (A-scan). These signals

Japan) using water coolant to expose the dentin sur-were amplified and demodulated by an amplifier; sub-face. Then, a sequence of SiC papers (ending withsequently, the voltage from the lock-in amplifier was

2,000- g r i t ) w a s u s e d t o c r e a t e d i f f e r e n t R D T s .

converted to a digital signal using a data acquisition

Specimens were cleaned with an ultrasonic bathboard and then processed on a personal computer(Quantrex 360; L&R Mfg., Kearmy, NJ, USA) in dis-using Origin 9 analysis software (OriginLab,

tilled water for 3 min.

Northampton, MA, USA).
Three specific positions on the specimen were

2. OCT measurement
A focused light beam from the TD-OCT imaging

assigned to obtain the RDT measurement bysystem (J. Morita Mfg., Saitama, Japan) was projected

TD-OCT for five different bovine teeth with three dif-

onto locations of the specimen surface and scanned

ferent measurement points (Fig. 1). Air was gently

across the areas of interest using a probe attached toblown over the specimen surface for 10 s, and thena mounting device.

the specimen was positioned under the probe of theSuperluminescent diodes (DL-CS3184 B; DensLight

TD-OCT, with the notch oriented toward the probe

Semiconductors, Singapore) with a central wavelength

handle. The scanning probe connected to the TD-OCTof 1310 ± 50 nm, spectral bandwidth of 40 nm, anddevice was fixed 2 mm from the sample surface. The

optical output power of 7.5 mW were used as the

thickness of the sample was determined by subtract-

light source. The emission light was coupled to a sin-

ing the vertical position of the refl ected light of thegle-mode fiber-optic Michelson interferometer andpulp–dentin junction from the vertical position of thedelivered to the reference mirror and samples. The

sample surface in the TD-OCT image, using an aver-reference mirror was mounted on a linearly translat-

age refractive index (RI) of 1.540 14).

ing galvanometer, which was driven by a triangular

3. LSM measurement

voltage waveform with a fringe modulation frequency

After performing the TD-OCT measurements, theof 1 kHz. The light was reflected off the mirror backspecimens were sectioned longitudinally (mesial-distal)

onto the retroreflector and reimaged on the reference

using a precision sectioning saw that was equipped

arm fiber. The light beam scanned the sample sur-with a diamond-impregnated blade (IsoMet 1000;

face point-by-point and line by line. A depth profile ofBuehler, Lake Bluff, IL, USA). The sectioned surfaces

backscattering along a line perpendicular to thewere subsequently ground with wet SiC papers end-

Fig. 1. TD-OCT two-dimensional tomography B-scan mode (lower) and the corresponding signal intensity profi les of the
A-scan mode (upper) for the measurement of the remaining dentin thickness.

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日大歯学

第 94 巻第 2 号

ing with 2,000-grit SiC paper. Specimens from approx-

ison method was used to determine the agreement inimately the same positions were observed using LSM

RDT between the TD-OCT and the LSM measure-(VK-8700 3 D; Keyence, Osaka, Japan). The excitation

ments (Fig. 3). The results indicated that the meanlight had a maximum wavelength of 658 nm, and thedifference between the TD-OCT and the LSM mea-

excitation light intensity and photomultiplier amplifi -s u r e m e n t s w e r e 7.857 ± 68.284 µ m , w i t h a 95%

cation remained constant. The recorded image size

Bland-Altman limit of agreement ranging fromwas 81.5 × 71.5 µm , with a resolution of 1,024 ×−125.980 to 141.694 µm.

2768 pixels. Images were obtained for four different
sites from each sample to determine the RDT.

Discussion

4. Statistical analysis
The significance level of the normality test was set

Determining the progression of pathological toothat α = 0.05. Data for each group were tested forwear over time is essential for the initiation of thehomogeneity of variance (Bartlett’s test) and normal

most appropriate treatment and to ensure a good

distribution (Kolmogorov-Smirnov test). If the test’s

prognosis for the teeth 15) . Surveillance involves ap-value was <0.05, the data were not normally dis-series of tests and measurements that are repeatedtributed, and a nonparametric test, such as the
Wilcoxon signed-rank test, was used instead of thepaired t-test. However, if the p-value was >0.05, thedata were deemed normally distributed, and thepaired t-test was used. We used the Bland-Altman
comparison method to evaluate the agreement in
RDT values between the TD-OCT measurement anddirect measurement using LSM. All statistical analyses were performed using a commercial statisticalsoftware package (SigmaPlot 11.2, Systat Software,
Chicago, IL, USA).

Results
The RDT values obtained using TD-OCT and LSM
are shown in Table 1. The correlations in specimen
thicknesses between the TD-OCT and LSM measurem e n t s a r e p r e s e n t e d i n F i g . 2. T h e a g r e e m e n t
between the different measurement methods was ana-

Fig. 2. Comparison of methods using a linear regression
analysis of the data presented in Table 1. The solid line
denotes the linear regression line, and the dashed line
denotes the line of identity.

lyzed to assess the variation between methodologies,
and we found strong correlations for each group of
measurements (R = 0.996). The Bland-Altman compar-

Table 1 Comparison of RDT values measured by TD-OCT and LSM methods

68Dentin thickness measurement using TD-OCT

令和 2 年 12 月

after a specified time period to assess whether a par-average value of the measured pairs on the x-axis.

ticular phenomenon is progressive. This is the only

The reason for plotting the average value obtained

method to determine whether tooth wear is progres-from the two methods on the x-axis is that the true

sive or stationary, and monitoring is essential forvalue is closer to the average value, and the relation-

treating tooth wear. If the wear process is aggres-ship between the differences between the measured

sive, immediate treatment is needed, with a focus onvalues and the magnitude of the measured value canits main cause. In the present study, we evaluatedbe evaluated more precisely 18) . The results of thethe reliability of TD-OCT measurements to determine

present study showed good agreement between thethe RDT and then correlated the results with LSM

TD-OCT and LSM measurements for determining

measurements. Because all TD-OCT measurements

RDT and also demonstrated that TD-OCT was accu-

confirmed the surface of the specimen and the den-rate and effective for measuring dentin thickness.

16)

tin–pulp junction, we determined that all of the mea-

The imaging depth used for OCT depends not only

surements using ground dentin specimens in theon the device’s attributes and specifications but alsopresent study were acceptable. Because of the differ-on the optical properties of the tooth substrate. In

ent RDTs in the ground-tooth specimens, TD-OCT

this context, the RI is an important parameter of

could also be used to determine a wide range of den-light propagation in tooth specimens, and scattering

tin thicknesses from 345 µm to 1,415 µm.

is the final result of the local variation in RI 19). The

We examined the differences between the mea-

signal attenuation through dentin is greater than that

sured values obtained as pairs from the TD-OCT andof enamel because dentin contains approximately 55

LSM measurement methods using the Bland-Altman

vol% mineral, 30 vol% organic material, and 15 vol%method 17). The mean difference between the paired

w a t e r 20) , a l l o f w h i c h s c a t t e r a n d a b s o r b t h emeasurement values is known as “bias (systemic

near-infrared light. In addition, the presence and ori-

error),” and the standard deviation of the difference

entation of dentinal tubules have a great impact on

between the measurement values is known as “preci-

light scattering and attenuation 21). Although the RDT

sion.” One frequently used method to represent themeasured by LSM is assumed to represent the actual

results of this analysis is the Bland-Altman plot, also

dimension of the dentin, the depth measured by

known as the difference against the mean plot

TD-OCT is affected by the optical path length in the(Fig. 3). This graph is obtained by plotting the differ-

dentin, and thus, it is not possible to directly show

ences between the methods on the y-axis and thethe physical dimension unless the RI values are given

Fig. 3. Bland-Altman plot of the different measurement methods for RDT (bias: mean ofdifference, SD: standard deviation, CI: confidence interval).

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日大歯学

第 94 巻第 2 号

in advance 22) . Previous studies that calculated theservice, and/or company that is presented in thisRIs of dentin using OCT 23) used an RI value of 1.540

article.

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Acknowledgments
This work was supported in part by Grants-in-Aid
for Scientific Research (grants 19 K10158, 19 K10159)
from the Japan Society for the Promotion of Scienceand Foundation Nakao Grant 2020. This project wasalso supported in part by the Sato Fund and by agrant from the Dental Research Center of the Nihon
University School of Dentistry, Japan.

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