The application of composite resin crowns by CAD/CAM has been expanded to molar restorations. However, problems of crown detachment and fracture remain, especially in the second molars with less height for metal-allergic patients. Since titanium and titanium alloy show high mechanical strength, corrosion resistance, and biocompatibility, a group in the Japanese Society for Dental Materials and Devices investigated titanium and titanium alloy cast crowns as alternative prosthodontic devices in such cases. As the result of a proposal made with the Japan Prosthodontic Society, the grade 2 titanium cast crown has been listed in the Japanese health insurance system since June 2020. Because of the necessity of special casting machines and investment materials, it was approved as a new technology. Since the price of Ag-Pd-Au dental casting alloy keeps increasing, titanium and titanium alloys are also considered promising candidates as substitutive materials.

Properties of commercially pure titanium (CP Ti) as restorative material are summarized and overviewed based on the property of CP Ti itself and published research results: Changes in mechanical properties of CP Ti through dental casting, workability and deformability, soldering and welding, polishing and grinding, porcelain-fused-to-metal and adhesion to resins, cement luting, and corrosion and tarnish. The majority of research on CP Ti for dental restoratives has been performed by Japanese researchers. The mechanical properties are changed by the solid solution of impurity elements and formation of a reaction layer with investment materials after dental casting. A diamond wheel is effective for grinding CP Ti. Laser welding under an argon atmosphere is effective for jointing CP Ti. In addition, the removal of the reaction layer with investment materials by sandblasting and chemical treatment increases the bonding strength with porcelains and resins. Resin cements are effective for the luting of CP Ti. CP Ti may be corroded or tarnished by fluoride contained in toothpastes and mouth washes.

Following the revision of Japan’s medical insurance in FY2020, cast titanium crowns for molars became covered insurance. Here, research findings collected over 30 years are discussed in relation to dental casting titanium and application to fixed prostheses.

Titanium has established a solid reputation in medical and dental fields as an alloy with excellent biocompatibility. In our department, it is considered that titanium should be used not as a substitute for conventional dental alloys but as a safer material for prosthetic devices placed in the oral cavity. For the past 25 years, we have positively applied cast titanium frameworks and delivered more than 1,000 titanium dentures. The problems with titanium dentures have been investigated in the laboratory and by follow-up observation, and have been resolved based on basic research and technical and clinical ideas. As a result, greater usefulness of titanium as a metal for denture frameworks has been confirmed, and clinical applications and new possibilities are actively being pursued.
In this paper, the clinical problems and solutions are summarized from the standpoint of a removable denture specialist. Titanium dentures fabricated not only by casting but also by milling and metal additive manufacturing are also discussed.

Titanium cast crowns have become covered by Japanese insurance. We are delighted that this provides an opportunity to expand the dental applications of titanium. The insurance coverage is the result of the efforts of researchers who have conducted extensive studies at the Japanese Society for Dental Science and Engineering. In this report, we review the casting characteristics of titanium and examined prostheses that make use of these characteristics.

Teeth play a critical functional role in the body. Thus, it is important to understand the mechanical properties of teeth in order to preserve them. The mechanical properties of teeth according to their structure have been investigated for almost a century. However, because of the limitations of specimen size, many structural features require further investigation.
Teeth consist of a crown and root or roots. In both sections, dentin is the most abundant mineralized tissue by both weight and volume. The dentin is covered by enamel, and the dentin-enamel junction (DEJ) is the biological interface between the two tissues. Enamel, DEJ, and dentin all exhibit anisotropic tensile properties when miniaturized specimens are tested. Dentin contains more organic material (mainly collagen) than enamel, while enamel is predominantly mineral in composition. Thus, dentin strength is affected by the orientation of not only dentinal tubules but also collagen fibers. Dental tissue, particularly enamel and dentin, also changes with aging：aged tissue is weaker than young tissue. Variations in dental tissue make teeth a difficult substrate to manage in clinical dentistry；therefore, the anisotropic structure should be considered when determining methods of strengthening tooth tissue and devising adaptable clinical procedures.

The corrosion resistance of a Co-Cr-Mo alloy containing higher levels of N and Cr than conventional Co-Cr-Mo alloys was evaluated with electrochemical impedance and anodic polarization techniques in a lactic-acid-containing NaCl solution, and the results were compared with those from conventional and commercial Co-Cr-Mo alloys. In addition, surfaces of the alloys after anodic polarization were observed using an optical microscope. The resistance to corrosion until 72 h of Co-33Cr-5Mo-0.3N alloy was the strongest among Co-Cr-Mo alloys employed in this study. The corrosion resistance of Co-31.3Cr-6Mo until about 10 h was the strongest, while that at 72 h was moderate. Therefore, Co-33Cr-5Mo-0.3N alloy shows sufficient corrosion resistance, especially after 40-50 h. Elements C, Si, and Mn decrease the corrosion resistance of Co-Cr-Mo alloys；high concentrations of Cr and N increase it. The significance of evaluation using electrochemical impedance was clearly revealed by this study.