Yasuhiro Hotta

Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes.

This paper studied the surface quality (damage, morphology, and phase transformation) of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) in CAD/CAM milling, and subsequent polishing, sintering and sandblasting processes applied in dental restorations. X-ray diffraction and scanning electron microscopy (SEM) were used to scan all processed surfaces to determine phase transformations and analyse surface damage morphology, respectively. The average surface roughness (Ra) and maximum roughness (Rz) for all processed surfaces were measured using desk-top SEM-assisted morphology analytical software. X-ray diffraction patterns prove the sintering-induced monoclinic-tetragonal phase transformation while the sandblasting-induced phase transformation was not detected. The CAD/CAM milling of pre-sintered Y-TZP produced very rough surfaces with extensive fractures and cracks. Simply polishing or sintering of milled pre-sintered surfaces did not significantly improve their surface roughness (ANOVA, p>0.05). Neither sintering-polishing of the milled surfaces could effectively improve the surface roughness (ANOVA, p>0.05). The best surface morphology was produced in the milling-polishing-sintering process, achieving Ra=0.21±0.03µm and Rz=1.73±0.04µm, which meets the threshold for bacterial retention. Sandblasting of intaglios with smaller abrasives was recommended as larger abrasive produced visible surface defects. This study provides technical insights into process selection for Y-TZP to achieve the improved restorative quality.

Bending properties of Ce-TZP/A nanocomposite clasps for removable partial dentures.

PURPOSE Ceria-stabilized zirconia/alumina nanocomposite (Ce-TZP/A) has excellent fracture toughness and bending strength that could be useful for partial denture framework application. The aim of this study was to investigate the effects of three-dimensional (3D) geometry on the bending and fatigue properties of a model simulation of Ce-TZP/A clasps. MATERIALS AND METHODS Half oval-shaped Ce-TZP/A rods were prepared in six 3D designs. Specimens were either of standard (width divided by thickness: 2.0/1.0 mm) or flat type (2.5/0.8 mm) cross-sectional areas with taper ratios of 1.0, 0.8, or 0.6. As a comparison, cobalt-chromium (Co-Cr) alloy rods of the same shape as the Ce-TZP/A standard shape rod were prepared. All specimens were subjected to the cantilever test and loaded until fracture. They were also cyclically loaded 106 times with various constant displacements, and the maximum displacement prior to fracture was determined for each specimen. Three-dimensional finite element analysis (3D FEA), simulating the cantilever test, was performed to determine the stress distribution during loading. RESULTS Specimens with the standard cross-sectional shape exhibited higher rigidity and higher fracture loads than the flat specimens by the cantilever test. In addition, lower taper ratios were consistently associated with larger displacements at fracture. Fatigue tests revealed that the maximum displacement prior to fracture of Ce-TZP/A specimens was comparable to that of Co-Cr alloy specimens. The 3D FEA showed that specimens with a taper ratio of 0.6 had the least stress concentration. CONCLUSIONS Ce-TZP/A clasp specimens with a standard cross-sectional shape and a 0.6 taper ratio exhibited the best bending properties among those tested.

A study of biocomposite resins for creating orthodontic appliances using a 3D printer

Abstract Purpose This study is to investigate the physical and mechanical properties of various biocomposite resins used with 3D printer for fabrication of orthodontic appliances used for cleft lip and palate patients and compare the results to the properties of self-curing resin used orthodontic appliances. Material and methods Density, flexural strength, elastic modulus, hardness, and water absorption and solubility dimensional change ratio, were performed on each sample of the biocomposite resins used in this study and the self-curing resin. For self-curing resin and the other samples, the difference in the average values were tested to determine whether there were significant differences in the average values. Result The results of the study showed that a photocurable resin: The elastic modulus and flexural strength were low. The hardness was equivalent. A Polyjet photopolymer: After water absorption, the flexural strength and elastic modulus were significantly lowered. The hardness was equivalent. Acrylic-type biocomposite resin (ABR): The elastic modulus and flexural strength were significantly lower. Furthermore, the amounts of water absorption and solubility were extremely large. When the material dried after water absorption, cracking occurred. Epoxy-type biocomposite resin (EBR): There were no significant differences in the elastic modulus and flexural strength. Hardness was equivalent. EBR showed strength greater than ABR. Conclusion The domestically produced prototype EBR tested in this study showed greater strength than ABR and did not contain toxic substances such as Bisphenol A. This suggests that it has a potential to be used as a photocurable resin in the field of dentistry in future.