Dental Materials Journal 2012; 31(3): 409–417

INTRODUCTION

Marginal adaptation of tooth restoration is a clinical requirement for success of treatment. Thus, dental students need to be educated on the subject through their pre-clinical training. Although the external morphology of the tooth, including the contour, surface roughness, and presence or absence of a gap at the cavity margin of restorations can be evaluated for dental training, it is extremely difficult to evaluate the internal adaptation and void formation non-invasively. Especially, most of the composite restorations suffer from volumetric polymerization shrinkage that leads to gap formation at tooth-restoration interface1,2). It was reported that gaps mainly occurred at the line-angles or cavity base, and range from less than a micrometer up to several tens of micrometers in size3).

Optical coherence tomography (OCT) developed by Fujimoto et al. in 1991 can construct images through wave interference that occurs when light from two sources is used4). This can selectively detect reflected light and is unaffected by scattering, thereby allowing retrieval of detailed images from in vivo biological tissue. Conventional OCT combines light from a low coherence light source with a Michelson interferometer to produce cross-sectional images of tissue structures generated as a result of interaction between a partially coherent beam of optical radiation and tissue component. Consequently, OCT is a non-invasive imaging technique as it does not

require ionizing radiation. In particular, swept-source OCT (SS-OCT), which can construct images by ultra-high-speed scanning of the converted wavelength of near-infrared laser and allows non-invasive construction of tomographic images of natural teeth and restorations in a short time4-10). Thus, its application to dental practice, including treatment of caries and adaptation of restorations, is expected in future4-10).

However, since OCT including SS-OCT utilizes optical interference, the observation area is limited to the depth penetrable by light. In addition to restorative composite materials, melamine resin tooth used for pre-clinical training is composed of homogeneous polymer materials and has a certain degree of transparency. Therefore, SS-OCT should be a suitable method for evaluation of restorations during practical training. The purpose of this study was to investigate the possibility to utilize SS-OCT for teaching the importance of cavity adaptation at dental school pre-clinical training. In this study, composite resin restorations of melamine resin teeth were evaluated using SS-OCT during practical training for dental students to determine the advantages of SS-OCT.

MATERIALS AND METHODS

SS-OCT systemFigure 1 shows an Dental OCT System (Prototype 2, Panasonic Healthcare Co., Ltd., Ehime, Japan) used in the present study, and Fig. 2 schematically illustrates the fundamental components of the system. The system

3D evaluation of composite resin restoration at practical training using swept-source optical coherence tomography (SS-OCT)Yasushi SHIMADA1, Alireza SADR2, Amir NAZARI1, Hisaichi NAKAGAWA1, Masayuki OTSUKI1, Junji TAGAMI1,2 and Yasunori SUMI3

1Cariology and Operative Dentistry, Department of Restorative Sciences, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan2Global Center of Excellence (GCOE) Program; International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan3Division of Oral and Dental Surgery, Department of Advanced Medicine, National Hospital for Geriatric Medicine, National Center for Geriatrics and Gerontology, Gengo 35, Morioka, Obu-city, Aichi 474-8511, JapanCorresponding author, Yasushi SHIMADA; E-mail: shimada.ope@tmd.ac.jp

Internal adaptation of restorations to the cavity wall is one of the important topics in clinical dentistry. The purpose of this study was to investigate the possibility to utilize the non-invasive tomographic imaging system for teaching the importance of cavity adaptation at dental school pre-clinical training. Swept-source optical coherence tomography (SS-OCT) was used for detection of marginal and internal defects in the composite resin restorations as an educational device. Class 1 and Class 2 composite restorations to melamine resin molar tooth were assigned to the students and prepared during the skill test, and SS-OCT imaging was performed to evaluate students’ works. SS-OCT could detect the internal gaps and voids within the restorations in tomography images synthesized based on the backscatter signal from within the restoration. It is suggested that the SS-OCT is promising diagnostic modality, as well as educational imaging device for the detection of internal gaps in adhesive restorations.

Keywords: SS-OCT, 3D evaluation, Composite resin, Internal adaptation, Practical training

Color figures can be viewed in the online issue, which is avail- able at J-STAGE.Received Nov 28, 2011: Accepted Feb 9, 2012doi:10.4012/dmj.2011-244 JOI JST.JSTAGE/dmj/2011-244

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Fig. 1 Dental OCT System (Prototype 2, Panasonic Healthcare Co., Ltd.) used in the present study.

OCT laser light source uses a bandwidth in the near-infrared with wavelengths ranging from 1,280 to 1,380 nm and a sweep frequency of 30 KHz.

Fig. 2 Basic principle of SS-OCT. SS-OCT uses an interferometer with a narrow band, frequency swept laser and detectors. Light emitted

from the light source is separated using a coupler, with one light beam directed toward the sample and the other directed toward the reference mirror. Light directed toward the sample is backscattered toward a detector in proportion with the difference in the refractive indices of the internal structures. Light directed toward the reference mirror merges and interferes with light returning from the sample. Fringe response versus frequency is detected with a balanced detector. The signal is Fourier transformed, and depth-reflectivity profile is obtained. Cross-sectional image is then reconstructed.

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is a frequency domain technique with a tunable light source. The SS-OCT system used in this study incorporates a hand-held probe of which power is less than 10.0 mW within the safety limits defined by American National Standards Institute5). The spectral bandwidth of the laser is over 100 nm centered at 1,330 nm at a 30-kHz sweep rate. The light beam from the laser source is projected onto the restoration and scanned across the area of interest using the handheld probe. Backscattered light carrying information about the microstructure of the sample is collected, returned to the system, digitized in time scale and then analyzed in the Fourier domain to reveal the depth information of the subject.

Cavity preparationThe practical training test was performed as part of the syllabus for dental students from the School of Dentistry, Tokyo Medical and Dental University, who completed the course requirements for operative dentistry (39 h coursework and 59 h practical training). The basic theory and clinical method of cavity preparation and restorative technique for composite resin restoration were instructed to the dental students at both the coursework and practical training before the test.

The test was performed using artificial teeth arranged to the occlusal articulator model attached to the manikin. Upper right melamine resin first molar teeth models (Nissin Dental Products Inc., Kyoto, Japan) were used in preclinical operative dentistry practical exam for 67 fourth-year dental students. Students were asked to perform composite restorations under the

assumption of mesial surface caries and occlusal pit caries directly beneath the contact point on the first molar tooth, while the mesial surface of the adjacent upper right second premolar had been cut by creating an MOD (mesio-occluso-distal) inlay cavity, which allowed cavity preparation and restorative manipulation on the first molar under direct observation. The caries lesion assumed is illustrated in Fig. 3.

The students were informed that the internal adaptation of restorations will be evaluated on tomographic images. They were also told that participation was voluntary and that they could opt out of the study, in which case they would not be penalized in terms of grading for non-participation. It was also disclosed that the results would be published as a study.

Restorative procedure and evaluation A two-step self-etching adhesive system (Clearfil SE Bond; Kuraray Medical Inc., Tokyo, Japan) as a surface treatment agent and a hybrid-type universal composite resin (Estelite Σ, Tokuyama Dental Corp., Tokyo, Japan) as a filling material were used in this study. Aspects related to the cavity preparation technique, including cavity contour, depth, shade selection of the composite resin, use of flowable composite as a cavity liner, and use of the layering technique were left to the student’s discretion11).

The time limit for all procedures was 90 min. At the end of test, the restored melamine resin teeth were collected from the students, and the composite resin restoration was evaluated by the instructor with 25 years of clinical experience, who had previously

Fig. 3 Practical training of dental students. The theme assigned to the 4th grade undergraduate dental students at Tokyo Medical and Dental

University as a practical training test of restorative dentistry. The time limit was 90 min. Composite resin restoration was performed under the assumption of occlusal and mesial caries in the upper right melamine resin tooth number 6. However, the mesial surface of the same tooth is cut by creating an MOD cavity in the upper tooth number 5, allowing cavity preparation and filling of composite resin under direct observation.

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Fig. 4 Evaluation of cavity counter, final polished surface, internal adaptation, and scores. a: Excellent adaptation without gap or babble (score 3). Backscattered OCT signal from the composite is slightly

intensified compared to the surrounding melamine resin tooth; however, transition from the melamine resin tooth to composite resin is smooth and no increase is observed in the brightness at the interface because of the no gap (arrow). b: Poor adaptation with significant gap at the cavity floor (score 1). A composite resin restoration with a large gap at the cavity floor. Composite resin was not filled up to the cavity floor, thereby leaving a large gap. The interface between the cavity floor of the melamine resin tooth and composite resin can be clearly recognized as a dark space (arrow). c: A restoration with mixtures of gaps and air bubbles (score 2). A bubble introduced in the region of the cavity floor appears to have developed into a gap. The region with increased brightness shows an irregular shape (arrow). d: A restoration containing several bubbles (score 2). A restoration containing several bubbles. Several bubbles were introduced within composite resin, and an oval bright contour can be seen (arrow).

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conducted studies using SS-OCT. The evaluation took into account the margins, shape and extension of cavity for the contour, and smoothness and quality of surface for the polishing with 1 as lowest, and 3 as the highest scores, as defined in Fig. 4. The cavity contour and final polished surface were evaluated using a magnifying glass (SurgiTel, General Scientific Co., Ann Arbor, MI, USA) at a magnification of ×2.5. Considering the minimally invasive concept, those who excessively removed sound tissue to joint the two lesions and those who did not preserve the mesial marginal ridge of the molar despite direct access to the caries lost 1 point in this section. Score 3 was as good, and 1 was considered as poor result.

The internal quality of each restoration, including the presence or absence of interfacial gaps and voids was evaluated using SS-OCT (Fig. 4). The 3D scan of each restoration was carried out with the probe held almost perpendicular to the composite resin restoration. Three cross-sectional 2D tomographic images, at the locations of lingual third, center, buccal third were chosen from the 3D scan and were used to evaluate the internal adaptation of each restoration, including the presence or absence of large gaps and voids.

To assure the presence of gap and void within the

restoration, ten representative cavities were chosen for direct cross-sectional observation using conforcal laser scanning microscopy (CLSM, 1LM21H/W, Lasertec Co., Yokohama, Japan). The proximal surface of melamine teeth were removed to expose the same location as the middle slice of SS-OCT images and polished using 600 grid silicone carbide paper and diamondpaste with particle size down to 3 µm. The polished surface was then observed under CLSM at a magnification level of 500.

RESULTS

All the students completed and submitted the composite resin restoration within the time limit. Of the 67 students, 27 obtained good results in all 3 evaluation parameters—cavity contour, adaptation of each restoration, and final polished surface. Two students obtained poor results in all 3 evaluation parameters, while 12 obtained poor results in 2 of 3 evaluation parameters. Twenty-six obtained poor results in 1 of 3 evaluation parameters. The distribution of students in each evaluation parameters are summarized in the histograms (Fig. 5).

Fig. 5 Histogram of cavity contour, final polished surface, and internal adaptation.

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Cavity contourOf the 67 students, 39 prepared and restored two separate cavities for the caries lesions that were assumed to involve the mesial surface and the occlusal pit (Fig. 6). Twenty-eight students prepared a Class 2 complex cavity (MO) by joining 2 different caries; out of which 8 extended the cavities distally.

Internal adaptation of restorationsThe use of SS-OCT enabled internal observation of the composite resin restorations of melamine resin teeth. Large gaps or voids were detected in 23 composite resin restorations. Of these, 14 contained bubbles, 16 contained gaps, and 7 contained both air bubbles and gaps (Figs. 7, 8). The direct CLSM observation on cross-sectioned cavities could confirm the presence of gaps and bubbles within the cavities at the same location where SS-OCT visualized the defects in tomograms (Fig. 9).

Final polished surface of restorationsA highly polished surface could be achieved in most of restorations. Only 6 restorations had a poor polished surface (Fig. 5).

DISCUSSION

As light is reflected from the interface of media with different refractive indices, it is suggested that OCT images show heterogeneity of refractive indices produced by the subtleties of compositions and structures in living dental tissue. We reported that use of SS-OCT allows detection of caries because the increase in signal intensity in the decalcified area of enamel or dentine results increased brightness of the affected area in the reconstructed image6,7). In addition, the internal structure of composite resin restorations can be non-invasively examined by OCT. Makishi et al. demonstrated the effectiveness of SS-OCT through 3D imaging for detecting gaps around the composite resin filled in extracted teeth8). Bakhsh et al. processed SS-OCT 2D images to verify that an increase in brightness observed at resin restorative margins was because of reflection at the gap9).

Gaps around the composite resin restorations are likely to be created due to the setting contraction and incomplete filling of cavity, particularly in areas of difficult access1-3,8-10,12,13). The formation of gaps or voids is believed to lead to microleakage that has been suggested to contribute to secondary caries development. Several lines of evidence indicated that presence of large wall gaps significantly associated with secondary caries12,13). Resent study reported that gap space at dentinal wall could lead to the development of wall lesions if it was accompanied with more than 500 µm enamel space13).

In the present study, composite resin restorations of melamine resin teeth performed by 4th year dental students were non-invasively observed using SS-OCT that allowed the evaluation of internal adaptation of the restorations. The model teeth used in this study are comprised of two layers in terms of appearance, which play the role of enamel and dentine. The outer enamel-like layer is more translucent compared to the inner dentine-like structure. Despite the differences in the appearance, the constitutent material of both layers is an almost homogeneous polymer, which differs from enamel that comprises apatite crystals and dentine that comprises crystals incorporated into an organic collagen matrix. SS-OCT showed a slight increase in brightness

Fig. 6 Melamine resin tooth before (A) and after the preparation (B, C). Two separate cavities for the caries lesions that were assumed to involve the mesial surface and the occlusal pit (B, arrows). Class 2 complex cavity (MO) by joining 2 different caries (C, arrow). Considering the minimally invasive concept, excessively removed sound tissue to joint the two lesions despite direct access to the caries lost 1 point in “cavity contour”.

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Fig. 7 3D construction of occlusal composite restoration was performed using SS-OCT (upper left). Three dotted-lined locations (yellow, light green, green) were chosen where 2D cross-sectional images were visualized. Distinct gap at the cavity floor was visualized along the yellow as a dark zone (upper right). Light green also found the presence of distinct gap at the bottom (lower right). Green showed big bubble at the deep part of the restoration (lower left).

Fig. 8 3D construction of the composite restoration using SS-OCT (upper left). This case, two separate caries at occlusal and mesial surfaces were connected and joint-filled. Three dotted-lined locations along yellow, light green and green were chosen where 2D cross-sectional images were visualized. Small superficial crack was observed within the composite, but almost fine adaptation along the yellow (upper right). Marginal fracture was detected along the light green (lower right). Marginal gap was found along the green (lower left).

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of the melamine resin structure surrounding the composite resin compared what has been seen for enamel or dentine (Fig. 4, 9). In this regard, it was relatively easy to differentiate between the melamine resin tooth and the composite resin in SS-OCT images, since the filled composite resin showed deeper penetration of right and lower signal attenuation, accompanied by a higher brightness owing to a difference in refractive indices between the matrix resin and filler, thereby resulting in increased scattering of light (Fig. 4, 7–9).

OCT imaging depth depending on the optical properties of the structure and light penetration; the depth of imaging on the composite restorations in this study was around 2.5 mm, allowing in depth assessment of restorations made by students according to the examination theme4,5). If a gap was present at the adhesive interface between the melamine resin tooth and composite resin, laser light emitted from the light source in a SS-OCT system was believed to be reflected from detached surfaces of the composite resin and melamine resin tooth, resulting in higher brightness at the margin8,9). Results were, therefore, similar to those for composite resin filled in an extracted tooth reported by Bakhsh et al., where a slight increase in the brightness of the image would verify the presence of a gap (Fig. 4b and c) 9). In addition, a large bubble present in the restoration was often observed as an oval-shaped dark shadow within composite resin with a slight increase in brightness of the borders; similar to interfacial gaps, the bright outlines of voids was due to increased reflection as a result of a difference in refractive indices of air and composite (Fig. 4d). These findings were well in

accordance with the previous study that demonstrated voids within acrylic dentures14). While the study reported that void of 30 µm in diameter could be visualized in SS-OCT, further research seems necessary to investigate the accuracy of SS-OCT for the detection of void within composite resin restorations. Although filling cavities with composite resin without introducing any air bubbles is difficult, 44 cavities showed good structural integrity (Fig. 4a).

In addition, many restorations had gaps within the composite resin while the cavity contour and final polished surface were relatively in good appearance (21 restorations with internal defects and 10 with good cavity contour and final polished surface; Fig. 4b). This suggested that the students needed to be reminded of the importance of filling manipulation in adhesive restoration in educational settings.

OCT imaging systems, by which tomograms can be obtained without exposure to ionizing radiation, are expected to be useful as a clinical diagnostic imaging device that can be safely used for children and pregnant woman, as well as in educational settings including practical training for dental students2,7). Compared to X-ray micro-computed tomography, SS-OCT creates less motion artifacts and can obtain real-time images at high speed2-8), which helps the students immediately observe the adaptive condition of restoration during training. In addition, since the use of SS-OCT allows non-invasive observation, the student can be guided or instructed, if required, on how to refill composite resin in the cavity contour and final polished surface are visually excellent. Consequently, the importance of various factors involved

Fig. 9 SS-OCT image (left) and CLSM direct observation (right). SS-OCT could image the gap (1) and bubbles (2,3) within the restoration. The results of CLSM observation on the cross-sectioned restoration could confirm the presence of gap (1) and bubbles (2,3) at the same location.

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in composite restorations including the placement technique and composite manipulations can be emphasized. Such information cannot be obtained in conventional educational settings where such imaging technology is not available. Further studies are required in future for the effective use of SS-OCT as a tool to improve practical training for dental students.

CONCLUSION

The internal adaptation of composite resin restorations of melamine resin teeth performed by 4th year dental students using SS-OCT could be non-invasively evaluated on tomograms. Therefore, an SS-OCT imaging system is highly useful as a device for visualizing the adaptive condition of adhesive restorations, such as composite resin restoration.

ACKNOWLEDGMENTS

This work was in part supported by the Grant-in-Aid for Scientific Research (No. 23390432) from Japan Society for the Promotion of Science, in part by the Research Grant for Longevity Sciences (21A-8) from Ministry of Health, Labor and Welfare, and in part from the Japanese Ministry of Education, Global Center of Excellence (GCOE) Program, International Research Center for Molecular Science in Tooth and Bone Diseases.

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