Fundamental and clinical study for application of titanium in dentistry

Titanium has been used for dental implants because of its excellent biocompatibility and corrosion resistance, while application in general dentistry has been limited. The reason is that casting, grinding and polishing of titanium are not so easy compared to the conventional dental alloys, because it has a high melting point and high ability to reduce various oxides, or refractory and abrasive materials at elevated temperatures. The low viscosity of molten titanium tends to cause internal defects of casting. The surface layer formed by reactions with investment materials have bad influence on the mechanical properties of casting. Reactions with abrasive materials not only hinder the efficient finishing work but also deteriorate the quality of finished surface.
Accordingly, we have conducted the following research projects for the application of titanium in general dentistry: 1) Development of investments for titanium casting; 2) Observation of molten titanium flow in the mold cavity by means of "tracer element molten method" and clarification of the internal defect formation mechanism; 3) Establishment of dental casting planning; 4) Reactivity of titanium with abrasive materials and chemical characteristics of finished surfaces; 5) Establishment of grinding and finishing techniques; 6) Development of easy-to-grind titanium alloys with low melting point; 7) Adaptability of cast titanium crowns; and 8) Soldering of titanium.

How should dental implants be biomechanically designed?

Since dental implants transfer, without the stress-buffering distributor, or soft tissue, the occlusal and masticatory force directly to bone, bone stress tends to be concentratedly localized. Therefore, the success or failure of implant is highly dependent on the biomechanical consideration in design.
Accordingly, we have analyzed stress in surrounding bone as well as implant, using the three-dimensional finite element method, and proposed guidelines for bio-mechanically reasonable design. Since the loosening and fracture of implant-abutment joint are also serial problems to the success of implant subject to repeated force, the fatigue behavior of the connection screw has been investigated.

In vivo stability of implant materials

Since the artificial tooth inserted in bone contacts directly with the hard tissue, the used material should be stable in bone for a long period.
Therefore, we have conducted the following researches: 1) In vivo long-term stability and changes of hydroxyapatite (HAP) and 2) In vivo stability of titanium polished under various conditions. Furthermore, in order to investigate the chemical and mechanical stability of a HAP-coated implant, morphological and chemical aspects of the HAP-titanium interface were elementally, chemically, and crystallographically characterized.

Establishment of qualitative and quantitative analysis technique of trace metals in bio-tissue sections: for the histopathological diagnosis of metal-surplus and deficient diseases and dust disease

For the histopathological diagnosis of metal-surplus and deficient diseases, an analysis technique using an electron probe microanalyzer has been developed to qualify and quantify metals of small amounts in thin bio-tissue sections, and successfully applied to analyze the sections of liver and kidney from Wilson's disease patients. This technique also has been applied to detect metallic elements involved in lung tissue of dust disease suspected patients.

Establishment of technique to identify easily and instantly metallic elements in the oral cavity: for the clinical diagnosis of dental metallic allergy

For the clinical diagnosis of dental metallic allergy, an analysis technique using X-ray fluorescent analyzer has been developed to identify easily and instantly metallic elements in the oral cavity of allergy suspected patients. From the sample collection to the completion of analysis it takes only about ten minutes. The amount of sample powder collected is in order of ng.

Fundamental study of metal-porcelain bonding: high temperature oxidation of alloys, interface reactions and improvement of fusing technique

Ni-Cr alloys for porcelain-fusing are cheap in cost as well as comparable to noble alloys for porcelain-fusing in the mechanical properties. However, since the major constituent components have intrinsically high ability to be oxidized, the pre-oxidation treatment has to be made carefully as well as the temperature control during firing. In this series of experiments, to clarify the interfacial reactions of porcelain with various Ni-Cr alloys at higher temperatures, the porcelain-side interface layers have been successively analyzed in the depth direction, using electron probe microanalysis, X-ray diffraction, and X-ray fluorescent analysis.

Synthesis of fine ceramic powders by sol-gel method and their application in dentistry

The sol-gel processing is a new technique of producing oxides by heating gels obtained through the hydrolysis and subsequent polymerization of solution of inorganic metal salts or alcoholates. Compared to the conventional processes, the synthesized ceramics are higher in purity and can be sintered at lower temperatures, since the gels are prepared from uniform solutions of extremely pure compounds. According to this processing, biocompatible ceramics of calcium phosphate have been successfully obtained. Further research aims to the application of the sol-gel processed materials in the form of powder, film, and composite to biomaterial.

Setting Expansion Pressure of Dental Plaster and Stone (Calcium Sulfate Hemihydrate)

Plaster or stone (calcium sulfate hemihydrate) is used as a model material or binder of molding material in fabricating fixed denture or retainer. When mixed with water, the hemihydrate changes into the dihydrate and the mixed expands seemingly. This expansion influences significantly the dimensions of fixed denture or retainer, which has urged many researchers to investigate the setting expansion rate. On the other hand, setting expansion pressure is also an important factor, since certain restraints are apt to reduce the setting expansion rate. However, there are very few researches on the setting expansion pressure. In our laboratory, the setting expansion pressure has been measured to investigate the relationship between setting expansion and restraint.

Study of Cast-On-Technique

Fracture mechanics study of bonding strength of metal-resin composites

Rigidity and fatigue resistance of cast occlusal rests using Co-Cr, Ni-Cr and Ag-Pd-Cu-Au alloys