Tadashi Jimi

Basic Study of Sintered Metal Restoration. Part 1. Fitness Accuracy and Surface Roughness.

Sintered metal restorations, made from powder metallurgy developed by Degussa Co., Ltd., and conventional cast restorations were prepared by using a metal modelfor MOD inlay. A comparative study was carried out, and the results were as follows:1. As for surface roughness, no significant difference of Rz could be observed both on the polished surface and cavity floor. In the graphs of Rz on the cavity floor, the picture was rough for cast restorations and sink hole-like gas cavities were locally recognized on the surface of sintered metal restorations with a roughness of 3 μm on average. As the results of the corrosion test, roughness of the polished surface of both restorations rose and the increase was larger in cast restorations. Sintered restorations showed better corrosion resistance.2. The fitness test showed no significant difference in the margin, butthe fitness of sintered metal restorations was inferior to cast restorations on the inner surface.3. The surface hardness of sintered metal restorations was lower than the values indicated by the manufacturer.Based on these results, it was considered that powder metallurgy was not suitable for the preparationof a thick abutment tooth with sharp angles such as MOD.

A Study on Porcelain Laminate Veneer Crown. Part 2. Setting Characteristics of Light-Curing Luting Materials.

On the assumption that porcelain laminate veneer crown had been bonded to the abutment tooth with the light-curing luting material, setting characteristics of four light-curing luting materials were evaluated for linear shrinkage from the start of exposing, water absorption, Vickers hardness and compressive strength after exposing the materials to visible light. The results were as follows : 1. The light-curing luting materials rapidly shrank as soon as exposed to light passing through a cover glass or a plate made of porcelain. When exposed to light through the laminate veneer plate, the shrinkage of materials was less than that of materials exposed to light through a cover glass. The result suggested that the curing process was delayed because of a shade effect of the laminate veneer plate. 2. The compressive strength was shown to have a tendency to increase with the elapse of time. The compressive strength of luting materials stored in air at 37•Ž was larger than that of materials immersed in distilled water at 37•Ž, and such a tendency was remarkable from 1 to 4 weeks after exposing. 3. The values of Vickers hardness of light-curing luting materials were almost the same from 1 to 4 weeks after exposing, and the hardness values of the materials in air at 37•Ž was larger than those of materials immersed in distilled water at 37•Ž. 4. The amounts of water absorption (wt%) increased rapidly after the luting materials were immersed in distilled water at 37•Ž, but were hardly increased 8 weeks after exposing the materials to visible light.

A Study on Adhesion of Dental Porcelain to Castable Glass Ceramics. Part II Fabrication of Tensile Test Pieces and Tensile Test.

Castable glass ceramics-porcelain jacket crown, in which castable glass ceramics are used as a coping and dental porcelain is partially or completely fired, is recommended from an aspect of esthetics. The adhesive strength between two ceramic materials is considered to be important for this jacket crown. The purposes of this study were to evaluate the adhesion of two ceramic materials under tensile load and to establish fabrication of tensile test pieces.The following results were obtained.1. Established method for fabricating tensile test pieces and tensile test are adequate to know the behavior of ceramic materials under a tensile load.2. Dental porcelain used in this study showed favorable adhesion to castable glass ceramic, without detracting the inherent physical properties of castable glass ceramic.3. The white layer, which shows the formation at the surface of castable glass ceramic after the ceraming process, does not affect the adhesion of dental porcelain to castable glass ceramic.4. Reinforcement of the white layer is expected to be obtained by firing dental porcelain.

3/4冠・陶材焼付冠支台の補綴物が歯牙の動揺度と支持組織の負担力に与える影響

A mechanical simulation model of tooth and alveolar bone in the application of 3/4 crown and porcelain-fused crown restorations was made and analyzed using three-dimensional finite element technique. The results obtained were as follows. 1. When the physiological tooth mobility was less than 150 microns, the teeth were sufficiently fixed using the splint and the bridge connected with more than two teeth, in the case of 20 percent absorption of the alveolar bone, and more than three teeth in the case of 40 percent absorption. In the case of 60 percent absorption, the teeth were fixed with the splint of six teeth connections, and were fixed with the bridge of five teeth connections. 2. The periodontium supporting force decreased markedly according to the increase of the number of the abutment teeth. 3. The teeth accompanied with the absorption of the alveolar bone were connected, in order to inhibit the tooth mobility and the periodontium supporting force. In this application, it had the inhibitory action when the replacing ratio of metal and ceramics to a crown of the tooth was less than 35 percent. 4. According to the finite element analysis, the design bases of fixed restorations applied in the teeth accompanied with the absorption of the alveolar bone were preferred.

Studies on Mechanical properties of Soft Lining Materials

Viscoelastic properties of silicone material (code-named MDX-4-4210) and 4 commercial soft liners and related materials (Kurepate, Neo-Snugger, Super-Soft and Viscogel) were examined using a stress relaxation testing instrument. All measurements were preformed in water at a temperature of 37±0.5°C.In the case of other 4 materials, except Visco-gel, the decreasing rates of relative stress {f(t)/f(0)} with time were relatively small. Furthermore, the decreases of stress relaxation modulus {Er(t)} of these materials with period soaked in water at 37°C were also very small, with exception of Visco-gel. Code-named MDX-4-4210 showed the most superior elastic properties in materials used. The relative stress in Visco-gel diminished markedly with time.These results indicate that all materials, excluding Visco-gel, have possessed the favourable properties as soft liners and related materials since it is necessary for these materials to remain in oral cavity for long period of month.

Studies on the retentive strength of polycarboxylate cements and pulp reaction.

In order to determine the clinical assessment of the polycarboxylate cements, some test were made on four commercially available polycarboxylate cements and one zinc phosphate cement as control.The findings are as follows:1. Tensile strength of the polycarboxylate cements obtained with the crown model was similar or stronger than that of zinc phosphate cement. Tensile strength with taper 1/10 crown model was significantly stronger than that with taper 2/10 crown model.2. In case of the shear strength at dentine-polycarboxylate interface on smooth surface was stronger than on rough surface, and the result showed that adhesion was due to chelate bond. While the shear strength at dentinecement interface obtained with polycarboxylate cements was variable, it was certainly superior to that obtained with zinc phosphate cement.3. The shear strength of polycarboxylate cements was more than twice that of zinc phosphate cement.4. The polycarboxylate cements were not irritant to dental pulp.5. The properties obtained with these test made the polycarboxylate cement the material of choice for cementing full crown restorations and bridge work on vital teeth.