Outline
Research at the Cariology and Operative Dentistry involves a broad range of subjects ranging from the basic science study on the caries process and to the development of advanced clinical techniques in restorative dentistry. Main areas include the evaluation of dental adhesives and other materials, studies on oral biofilm and hard tissue remineralization, and development of aesthetic dental treatments with regard to the “Minimal Intervention” concepts. The recent trend in dentistry is the move from surgical restoration to biomedical healing approaches, as well as patient-centered oral health care. Our group has devoted strong efforts in the past years to realize non-surgical dentistry. These effort include developing and evaluating new products for this purpose as well as enhancing methodologies required to provide evidence on the effectiveness of these approaches.
Subjects of Interest:
-Oral Biofilm
-Saliva Buffer Capacity
-Caries Mechanism
-Caries Inhibition
-Hard-tissue Remineralization
-Pulpal Healing
-Laser Treatment
-Ozone Treatment
-Effects of Fluoride
-Resin Coating Technique
-Interfacial Nanoleakage
-Acid-Base Resistant Zone
-Cavity Configuration Factor
-Durability of Adhesion
-Mechanical Characterization of Adhesives
-Micro-tensile Bond Strength
-Micro-shear Bond Strength
Laboratory Equipment:
-Scanning Electron Microscope (SEM)
-Optical and Stereo Microscopes
-Confocal Laser Scanning Microscope (CLSM)
-Argon Ion Etching
-Sputter Coater
-Thermo-cycling Device
-Low Speed Diamond Saws
-Microtome Saw
-Ion-selective Potentiometer
-Knoop Microhardness Tester
-Micro CT (Computed Tomography) System
-Nanoindentation System
-Universal Testing Machines (conventional and micro)
-Incubator
-pH meters (conventional and hand-held)
-Fluorescence Microscope
-Oral Biofilm Reactor (OBR)
-Lasers (CO2 and Er-YAG)
-Ozone Generator
Research Topics:
Acid-base Resistant Zone
Good adhesion is thought to enhance long-term sealing of the cavity margin, resulting in protection of the restoration against secondary caries. Artificial secondary caries around adhesive restorations placed in bovine root dentin has been observed using a scanning electron microscope (SEM). It was found that the change in ultrastructure of the cavity margins after acid-base challenge was adhesive material dependent. The hybrid layer was defined as a layer that the monomer penetrated into the dentin and cured in situ. In order to distinguish the hybrid layer dentin and resin, acid treatments, such as hydrochloric acid and phosphoric acid, have been routinely used in previous studies. Therefore, the hybrid layer was defined as an acid-base resistant zone with respect to its characteristics against acid treatment. Using a self-etching primer adhesive system, the so-called an “acid-base resistant zone” was observed beneath the hybrid layer after acid-base challenge. The adhesive resin had impregnated the exposed collagen bundles and became entangled with them to create a hybrid layer, which was distinguished by argon-ion etching and resistant to acid-base challenge. Also, using different self-etching primer systems, an acid-base resistant zone was detected after acid-base challenge, but there was a difference morphologically and mechanically between the systems.
Fluorosed Tooth Bonding
Dental fluorosis is a malformation of tooth enamel and dentin, which is believed to be caused by chronic ingestion of fluoride during tooth development. The structure of human fluorosed dentin shows nanomorphological differences at the apatite crystallite and collagen fibrillar level. These structural aberrations play a major role in remarkable caries prone potential of the tissue, which is quite obviously the opposite effect of fluorosed enamel. The currently available adhesive systems do not offer the same bonding efficacy with this pathological state of enamel and dentin tooth tissue.
Image: FE-SEM of fluorosed dentin treated with the enchant of an adhesive system.
Resin Coating Technique
Tooth−colored indirect restorations have become widely accepted. To overcome the lackluster bonding performance to dentin, a resin coating technique was developed. By means of a combined use of a dentin adhesive system and a low-viscosity flowable resin composite or simply use of a dentin adhesive system, the prepared dentin is immediately protected. Therefore, this technique has the potential to minimize pulp irritation and postoperative sensitivity and provide good bonding performance to dentin. Further, resin coated dentin has resistance to caries because of “Super Dentin” is formed on the dentin surface.
Bonding to Root Dentin
Composite core systems can prevent root fracture of non-vital teeth; a good dentin bonding system has been shown to reinforce the remaining tooth structure. For successful adhesive restorations of non-vital teeth, material selection is a key to obtaining good bonding to coronal, root and pulpal floor dentin. In addition, it has been found that a tight-sealing reduce the incidence of coronal leakage.
Self Adhesive Resin Cements
Self-adhesive resin cements are the latest generation of materials in the adhesive dentistry. These materials may simplify clinical procedures of cementation and reduce the technique sensitivity of multi-step cement systems. Those luting agents maybe applied in a single step and require no pretreatment of the substrate dental tissue.These dual-curing materials take advantage of the functional adhesive monomer technology developed for self-etching primer adhesive systems.
Image: A self-adhesive resin cement has sealed the margins and the dye penetration test showed no microleakage.
Zirconia Bonding
Porcelain fused to metal crowns and fixed partial dentures have been clinically successful. Nevertheless, metal structures, even when covered with ceramics, may represent an esthetic problem. On the other hand, conventional ceramic materials used for all-ceramic restorations were prone to failure due to their insufficient physical properties; therefore, their use is limited for a single crown or short-span fixed partial denture. Zirconia is used in biomedical applications and has been recently introduced in restorative dentistry. The use of the zirconium-oxide all ceramic material provides several advantages, including biocompatibility, chemical stability, esthetic appearance and high mechanical properties. Presently, zirconia is the only ceramic material that has the potential to substitute the metal used in the porcelain-fused-to-metal technology. Zirconia can be used for long-span bridges. The only problem related to its performance is that adhesion of resin cements to such ceramics is inferior. The long-term bonding durability is inevitable for clinical success. Therefore, we work on the bonding of resin cements to zirconia using veneering porcelain for zirconia in order to obtain better bonding.
Polymerization Shrinkage
Light-cured resin composites are now widely used in clinical practice on metal free restorations, because of the tooth colored esthetic advantages. However, the polymerization reactions of light-cured composites lead to volumetric shrinkage. This contraction stress has been shown to lead to marginal gaps, enamel and dentin cracks, postoperative sensitivity and secondary caries. Recent research has shown it may be possible to reduce the stresses within a bulk of resin composite using Slow-start curing method, and flowable resin composite. Slow-start curing method can be done, by prepolymerization at low light intensity followed by final cure at high light intensity.
Saliva Buffering
The micro pH sensor was developed for the clinical usage with support by HORIBA, Ltd. (Kyoto, Japan), and it is used for quantitative assessments for caries activity and reminelarization process. We also developed the quantitative saliva buffer capacity test using a hand-held pH meter (checkbuf®, J Morita, Tokyo, Japan), and use it at the chair-side for caries risk assessments.
Biofilm Research
Oral biofilm formation is a key factor in the process of caries. For prevention of caries, in particular, new and effective prevention methods were proposed including biofilm removal by alkaline water and ozone disinfection of water.
Micro-shear Bond Strength
We evaluate bonding performance and durability of adhesive materials using newly developed bond test method. The new method enables to detect the difference among the variation of tooth structure, and is widely recognized as a precise bond test in dentistry. We also evaluate the leakage of adhesive restorations under electron-microscopic level.
Biocompatibility
We evaluate biocompatibility and bioadaptability of restorative materials using in vivo pulpal response test and so on. Especially, pulpal response tests of composite resin restorations are continued and renewed since the development of the material, of which results over many years are recognized as valuable information in dental clinic.
Mechanical Behavior of Biomaterials
Dental adhesive layer is a thin polymer film. Understanding and exploring the mechanical properties of adhesive materials is necessary for advancement in the adhesive dentistry. Besides the traditional mechanical testing and informative bond strength tests, we have an interest in exploring the specific mechanical behavior and relevant contributing factors of dental adhesives, through alternative test techniques such as nanoindentation. In addition to adhesives and composites, the technique has found a great utility in characterization of enamel and dentin. We have plotted enamel nanohardness at the 1um resolution.
Root Caries Prevention
The prevention of root caries is the most important theme in aging society. The root dentin is low resistant to caries. However, resistance to the acid is given from a past research by spreading and using the bonding material, and the prevention of the root caries. Resin coating technique is available for inhibition of root dentin demineralization, however prevention of root dentin demineralization varies in coating materials.
Optical Coherence Tomography
Dental hard tissues and biomaterials can be non-destructively assessed using optical coherence tomography (OCT), an emerging diagnostic tool. Swept Source (SS)-OCT has an improved imaging resolution and speed. Data obtained in B-scans can be used to assess the quality and depth-resolved information on the tissues. We are working to develop new methods and devices to take advantage of SS-OCT both in the research and the practice of cariology and operative dentistry, in collaborative projects. OCT has enabled objective evaluation in clinical trials.
De/Remineralization Research
Using such methodologies as our custom setup of transverse microradiography (TMR), nanoindentation hardness and various microscopes, we demonstrate the efficacy of novel agents containing bioactive ingredients such as nanohydroxyapatite, calcium and fluoride. We have demonstrated the feasibility of complete repair of early caries lesions in vitro.
Effects of a chewing gum containing phosphoryl oligosaccharides of calcium (POs-Ca) and fluoride on remineralization
The aim of this study was to assess the effect of a chewing gum containing phosphoryl oligosaccharides of calcium (POs-Ca) and fluoride on remineralization of enamel subsurface lesions, in a double-blind, randomized controlled in situ trial. 36 volunteer subjects wore removable buccal appliances with three different. For 14 days the subjects chewed one of the three chewing gums (placebo, POs-Ca, POs-Ca+F). After each treatment period, the insets were removed from the appliance, embedded, sectioned, polished and then subjected to laboratory tests; mineral level determined by transverse microradiography and hydroxyapatite (HAp) crystallites assessed by synchrotron radiation wide-angle X-ray diffraction. Chewing POs-Ca and POs-Ca+F gums resulted in over 20 percent mineral recovery, which was significantly higher than that of placebo gum. Chewing POs-Ca+F gum resulted in some 25 percent HAp crystallites recovery, which was significantly higher compared to POs-Ca or placebo. Although POs-Ca+F gum was not superior in TMR recovery rate when compared with POs-Ca gum, other results highlighted the importance of fluoride ion 
bioavailability in formation of HAp crystallites and reinforcement of enamel subsurface lesions in situ.