No abstract

The design of removable partial dentures (RPDs) is an important factor for good prognostication. The purpose of this study was to clarify the effectiveness of denture designs and to clarify the component that had high rates of failure and complications. A total of 91 RPDs, worn by 65 patients for 2-10 years, were assessed. Removable partial dentures were classified into four groups: telescopic dentures (TDs), ordinary clasp dentures (ODs), modified clasp dentures (MDs) and combination dentures (CDs). The failure rates of abutment teeth were the highest and those of retainers were the second highest. The failure rates of connectors were generally low, but they increased suddenly after 6 years. Complication and failure rates of denture bases and artificial teeth were generally low. Complication and failure rates of TDs were high at abutment teeth and low level at retainers. Complication and failure rates of ODs were high at retainers.

Five types of removable partial dentures (two attachment dentures, two telescopic dentures and one clasp denture) were designed. The two attachment dentures were retained by the rigid-precision attachments with or without a stabilizing arm, and the two telescope dentures were retained with cone telescope crowns with or without cross-arch stabilization. The stresses acting on abutment teeth and denture bases and the movements of denture bases were investigated, and the influences of denture design were clarified. The stress acting on a terminal abutment tooth retained by a rigid-precision attachment or cone telescopic crown was larger than that acting on a terminal abutment tooth retained by a clasp. The attachment dentures tended to concentrate more stress at the terminal abutment tooth than did the telescopic dentures. The stress of denture base of an attachment denture and a telescopic denture was less than that of a clasp denture. There was no difference between the stresses of attachment and telescopic dentures. The displacement of the denture base tended to be less when the denture was designed with a rigid connection for the retainer and with cross-arch stabilization.

Hardness of glass is known to be related to the resistance to permanent deformation. However, the mechanism of permanent deformation of glass under a sharp diamond indenter is not clear yet. One of the deformation modes of oxide glass at room temperature is permanent densification. In this study, the indentation-induced densification of soda-lime glass under diamond indenters was evaluated from the volume recovery of indentation imprint by thermal annealing. The volume change of the indentation imprint by annealing corresponds to the densified volume under the indenter. Using some kinds of diamond indenters, which have different inclined face angles, the ratios of densified volume to the total “lost” volume under the indenters were determined. With an increase in the inclined face angle, the densification contribution decreased and the shear-flow contribution increased. This indenter-shape dependence of densification in glass is discussed in terms of the stress dependence of the deformation mechanisms in glass.

It is known that indentation using a diamond indenter induces permanent densification to result in plastic or inelastic deformation of glass. However, it is still unclear whether a high tensile stress causes such a structural change or not. In this study, fracture‐ and indentation‐induced structural changes of glasses with the compositions of 20 Na2O − 40x B2O3 – (80−40x) SiO2 (in mol%, x = 0, 0.5, 1, 1.5) are investigated. Two‐point bending tests of the glass fibers are performed to apply a high tensile stress to the glasses, and Vickers indentation tests of the glasses are also carried out for comparison. The structural change of the glass is evaluated by using Raman spectroscopy. It is elucidated that a tensile side of the fractured fiber for every composition shows a permanent structural change, which is characterized by a lower wave number shift of Raman peak assigned to the Si‐O‐Si bending vibration mode. It is also found that the behaviors of Raman peaks of the fractured fiber are opposite to those of the indented glass under a high compressive stress.