Ⅰ.INTRODUCTION
Dental composite resins are one of the restorative materials most world widely used in dentistry fields. Synthetic composite resins developed as a restorative materials due to its insoluble property, adhesion to tooth structure, the improvement in mechanical properties, easy manipulation and reasonably inexpensive cost. The main advantage of a dental composite resin is highly improved esthetics. It can be provided in a wide range of white tooth colors allowing nearly invisible existing tooth.1-2)
However, many resin restorations are proved to be clinically unsatisfactory. Failure of a restoration may be due to fracture, debonding, gap formation, porosities, loss of the supporting tooth restoration, marginal deficiencies, staining and microleakage. Especially, dental composite resins also suffered from significant central shrinkage after light curing. It causes microleakage which, if not found early, can cause secondary caries, the most significant failure of resin restoration.3~4) For restorations, risk of secondary caries in the composite group was 3.5 times risk of secondary caries in the amalgam group.5)
Though there are many researches about failure and longevity of the resin restorations, many of these can not provide useful visual information. Conventional imaging method such as X-ray can not provide accurate microscopic visual information.6-7) So, we will clinically apply a new optical method for evaluation of dental composite resin restoration.
Resin filler can determine the mechanical strength and clinical longevity of dental composite resin according to its usage. Adding of filler into resin matrix greatly influences and improves resin’s physical properties or otherwise it may actually weaken the resin. Benefits of filler content presence are increased hardness, strength, radiopacity and decrease in polymerization shrinkage, thermal expansion and contraction, water sorption, softening, staining and finally improved workability.8~9) There is no method, as far as we know, to determine the properties of composite resin, especially for resin filler.
Optical coherence tomography (OCT) is an real-time optical technique for medical imaging which employs non-ionizing light, possibly imaging 3 dimensional way with high-resolution micrometer resolution and several millimeters depth. The OCT is based on low-coherence interferometry unit, typically employing near-infrared non-ionizing light(around 1300nm wavelength).10~11) The application of relatively long wavelength light can allow it to penetrate into the millimeter depth of biological tissue. It can be applied to image about 1 to 7 mm under the tissue surface, because at greater depths the proportion of light that escapes without scattering.12) An light projected to the sample, and a small portion of this light that reflects from sub-surface image is collected. The OCT is effectively imaging reflected light from within tissue to provide 3 dimensional images.13)
OCT is attracting interest in the medical diagnostic fields because it can provide microscopic tissue morphology at much higher resolution than other conventional imaging techniques such as X-ray or ultrasound.14~15) Special preparation of a biological specimen is not required, and images can be obtained non-contact, non-ionizing and real-time. It is also important to note that the laser output from the OCT is low and no damage to the sample because eye-safe near infra red light is used.16)
In present study, OCT can be applied clinically the evaluation of dental composite resin restoration and resin filler distribution. The aim of this study was to evaluate the quality of composite resin restorations and resin filler particle distribution by using the OCT.
Ⅱ.MATERIALS & METHODS
1.Instrumentation
This OCT system applied for cross sectional tomographic imaging of in-vitro resin restoration. This optical devices consist of optic fiber interferometery unit, low coherence 1350nm wavelength laser and intra-oral OCT dental probe. This system integrates a high frequency laser, the intra-oral OCT dental probe that is less than 5 mW , and the light source ranges the wavelength from 1260 nm to 1360 nm at a 50-kHz rate. (Fig. 1.) A focused laser light is projected to the resin restoration tooth sample and reflected from the sample and scanned through the area of interest in two dimensions using a dental handheld probe.(Fig. 2.) Reflected light from the sample goes through optic fiber interferometry unit, and interacts with the reference light. An interference light signals are converted by a time and space scale and transformed to the Fourier transform analysis to show the depth visual information of the tooth sample, making cross-sectional images. The OCT imaging of resin restorations was used with in vitro handheld OCT dental probe for clinical application.(Fig. 2.)
2.Specimen preparation
Ten extracted resin restored teeth were selected. The specimens were taken as an image by OCT. OCT imaging of resin restorations was performed with a handheld in vitro OCT dental hand-hold probe for application in chair-side.
Four commercial resin products were prepared for measurement resin filler distribution. These commercial resin products are 3M: ESPS Filtek Z350 XT, GC: G-ænial Flo, Kuraray: CLEARFIL MAJESTY Flow, GC: Gradia Direct.
3.Viewing conditions
All resin restoration tooth samples were dried and the sample tooth examined the resin restorations by using OCT for 25 seconds. During the OCT scan, the dental probe is connected to the OCT instrument that was at a fixed distance(1cm) over the surface of resin restorations. The cross-sectional OCT image was edited by Amira software which processed and collected the data generating the OCT three dimension images. The OCT images were collected and visually inspected for the absence of gap formation, porosities, gap formation, marginal adaptation.
Ⅲ.RESULTS
In order to compare the OCT image with another conventional image technique, we take a picture for the X-ray image and micro-CT image. Figs. 3. show gross image, X-ray, Micro-CT and sliced image of resin restorations. Figs. 3. (B) show the radio-opacity of resin restoration part. However we cannot evaluate the delicate structure of resin restoration only through X-ray. Figs. 3. (C) show the Micro-CT image. Micro-CT can recognize inside of hard tissue. However, Micro CT is time consuming, and has limition of ionizing exposure. We can see air trapped because of inappropite restoration.(Fig. 3.(D))
Figs. 4. show representative pictures and OCT surface images of resin restorations. The OCT surface images are expressed in orange color scale. OCT is the tomography and has improved penetration depth, so we can see inside microstructure of tooth. On the occlusal plane, we can see the surface of the resin restoration tooth. And we can see the OCT image from every direction since it is 3D image.
Figs. 5 show inside of the resin restoration. We evaluate the quality of resin restoration by using OCT. In conventional X-ray image, We can not check inside miroscopic defect image. However, This OCT image can show the defect restorations of marginal adaptation, large porosity, and gap formation between the resin material and the tooth tissue. Fig. 5. (A) show the defective restorations of gap formation, large porosities, and marginal adaptation in the resin material. Fig. 5. (B) OCT image has poor contour on the resin restoration. This contour is an under-fill of the cavity of the resin material. Since the resin restoration is not perfect, this restoration shows a poor contour. Fig. 5. (C) shows large air trapped porosity in the resin, measuring approximately 1.0 mm in diameter, in the resin material. Fig. 5. (D) is XY plane cutting picture, or coronal view. This picture also reveals a gap formation and pour contour under the resin material. Fig. 5. (A) and (D) reveals a gap formation under the resin material. In this way, using OCT imaging, we can evaluate the quality of resin restoration and clinicians’ restoration skill.
Figs. 6. show resin filler distribution. It can be compared with different commercial products, depending on the resin filler distribution, fill particle size and regularity. The biggest in resin filler size is Ganial Flo, followed by Gradia Direct, Filtek, majesty flow. How regularly distributed the filler particles are evaluated by majesty flow, Filtek, Gradia, Ganial and Gania flo. The content and size of resin filler and will be different depending on the usage of resin. To observe this kind of resin filler directly using OCT will be useful in evaluating the usage of resin.
Ⅳ.DISCUSSION
In this research, the OCT system was used to image visualization of microscopic defect such as large porosities, marginal adaptation, internal integrity, and gap formation. The result indicates that the OCT can be a promising clinical tool to provide visual information of the microstructure of resin restorations.
The inappropriate resin restoration such as gap formation and porosities, marginal adaptation and cracks are the most frequent restoration defects. It can be related to internal deficiencies, which is dependent on the clinician’s finishing restoration technique. These restoration defects can provide a negative effect on the its mechanical properties of the resin material and should be prevented as possible. They may also increase the possibility of secondary caries and failure of resin restorations.8)
For these reason, real-time clinical evaluation of these restoration defects is needed. However, conventional X-ray and visual inspection can not provide enough diagnostic information about inadequate gap formation, large porosity, and marginal adaptation.14) Although some defeat can be noticed only through a visual inspection or sharp dental explorer in the conventional evaluation method, inside of resin restoration can not be detected. Furthermore, if secondary caries occurs around the resin restorations, the use of a sharp explorer to detect these secondary caries should be avoided and is neither reliable nor valid.7)
Because the OCT can visualize cross-sectional, high-resolution, tomographic imaging of the resin restorations, these anatomically inappropriate contoured restorations can be detected in detail.13~14) The result indicated that the OCT can detect inadequate gap formation, marginal adaptation, and large porosities with very high sensitivity.
The OCT can provide a real-time visual information of the overall resin restoration, which can perform chair side diagnosis. Development of an hand-hold intra-oral OCT dental probe would enable detection of the restoration structures in real-time, and become a potential method for planed treatment, non-ionizing, non-contact to the patient.14) The OCT can provide a promising diagnostic information to evaluate the longevity of resin materials and internal integrity of restorations in vitro and in vivo.
Dental composite resin improves the resin property with dental filler added. The resin filler will be various depending on the usage of resin filler. Until now, we can not check resin filler. However, OCT can directly distinguish resin filler.8~9)
In this research, we measured the resin filler distribution by using OCT. We prepared 4 commercial resin product. OCT can distinguish resin filler because resin filler has different Refractive index. The content and size of resin filler and will be different depending on the usage of resin. To observe this kind of resin filler by using OCT will be useful information in evaluating the usage of resin.17)
The OCT system is available as a fast and convenient, easy to manipulate chair-side method for evaluation of resin restorations. It also can provide real-time visual feedback, thus helping them to detect the defects of resin restoration. This new optical approach will improve the diagnosis, feedback resin restorations, provide the most proper treatment, and conform a best clinical outcome. In the future, advanced OCT system will allow more accurate evaluation and further contribute to the diagnosis and feedback of resin restoration.
Ⅴ.CONCLUSION
The OCT system is a new and advanced clinical method to diagnose the defects of resin restoration. The OCT has the potential to provide high-resolution and several millimeter penetration depth sub-surface information which are not available with current imaging method.
Because the OCT system can be directly used on the chair-side, clinical evaluation of a dental composite resin restoration will be used in the near future using this method.
