To evaluate the cushioning effects of casting ring liners impregnated with petrolatum(petrolatum-liner)on the setting expansion of investment materials, cushioning ability was determined for 15 ceramic liners(A:rockwool type, B:ceramic fiber low temperature type, C:ceramic fiber standard type, D:ceramic fiber low temperature standard type, E:kaolin type)and 3 asbestos ribbons(AS). Petrolatum-liners were seated inside the sliding frame of an appratus for expansion measurement, and then the setting expansion was measured for 120 min. Cushioning effects were calculated by the difference between the setting expansion without liner and the setting expansion with liner. The cushioning effect of petrolatum-liners was classified into three groups.(1)Prtrolatum impregnation treatment improved cushioning effect of two products of A, one product of B, two products of C, one product of D, one product of E and one product of AS.(2)Petrolatum impregnation treatment worsened the cushioning effect of one product os AS.(3)The other nine liners slightly changed the cushioning effect.

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The possibility of using a water-jet technique for removing softened dentin or for deleting adhered dental cements and restored composite resins was investigated. High-speed water was jetted to healthy tooh substances, softened artificial dentin, dental cements and compositeresins using a compact water-jet device. The diameter and depth of the pit performed by high-speed water on the surface of each material were measured and the load applied to each material by high-speed water was measured using a load cell. When cylinder pressure was 300 kgf / cm², all test specimens were weighted at about 45gf. As a result, it was found that water-jet technique is a method of selectively removing dental caries and the cement or composite resin adhering to teeth without over-grinding the healthy tooth substance when the proper cylinder pressure(under 300 kgf / cm²)is chosen.

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To re-examine the stress-strain diagram of dental casting alloy, that is, type 1 and type 4 gold alloy, Ag-Pd-Au alloy, and Co-Cr alloy, tensile test of the cast specimens was performed. The strain evaluated by a video extensometer and that estimated from the movement of the actuator of the test machine were compared. As a result, the former was effective in evaluating strain, while the latter should not be used because the shape to the yield point of the diagram. The elastic moduli evaluated by a video extensometer were significantly larger than those estimated from movement of the actuator, mainly owing to sliding between the grip wedge / specimen during the early stage of the tensile test. Furthermore, continuation of the sliding affects the stress-strain diagram and elongation. In conclusion, the extensometer was thought to be indispensable for working out the stress-strain diabram of dental materials.

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Using three kinds of porcelain for metal ceramics(PFM), the effect of subcritical crack growth(SCG)on mechanical properties was investigated by measuring fracture toughness K_IC and four points bending strength in both dry and corrosive surroundings. The K_IC in corrosive solutions(water, 0.9wt% NaCl and 50wt% acetic acid solution)and the bending strength in distilled water were decreased remarkably compared with dry condition. From these facts, it was considered that the SCG of PFM would be promoted by water, and would also take place in oral conditions. Furthermore, in order to elucidate the relations between SCG and the PFM textures, the load-displacement curve in the midst of SCG was measured, and the polishedetched surfaces and the failure surfaces were observed. As a result, it was found that although the PFM with large lucite crystals had a low bending strength even when dry because of cracks generated around the crystals, the K_IC was greater than that of other PFM. This indicates that these cracks may provide resistance against SCG.

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Fracture toughness of commercial Ag-Pd-Cu-Au dental alloy, where the precipitation of β phase was controlled systematically, was evaluated. The relationship between fracture toughness and volume fraction or size of precipitated β phase was quantitatively investigated in this study. Volume fraction and diameter of precipitated β phase increased as aging time and aging temperature increased. Fracture toughness of Ag-Pd-Cu-Au alloy decreased with increasing volume fraction of precipitated β phase under aging conditions when the diameter of precipitated β phase is constant. While the fracture toughness of Ag-Pd-Cu-Au alloy increases with the increasing diameter of precipitated β phase in under aged conditions when volume fraction of precipitated β phase is constant. The fracture toughness increases with increasing width of slip area observed on the side surface of the specimen. The fracture toughness increased with the degree of crack deflection.

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This study evaluated a newly developed orthodontic closed coil spring delivering a constant force in the oral environment. First, the influence of oral temperature on the orthodontic force of super-elastic Ti-Ni alloy coil springs was investigated. As a result, the loads of the original coil springs, measured at several displacement positions were markedly increased as the temperature changed during both martensitic(loading)and reverse(unloading)transformation. After being applied with a two step heat treatment, the load hysteresis of the new coil springs were reduced to 47〜60% of the original one. Therefore the amount of the increased loads during temperature change were also less 12% of the respective original loads. These results indicate that the super-elastic Ti-Ni alloy coil spring made by this two step heat treatment demonstrates appropriate properties in the oral environment.

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Dissolution and cell viabilith of gold-silver-palladium alloy were examined by gyrating specimens weighing 1.5g and 4.8g on zirconia balls for 7 and 14 days at 200, 230, and 260 rpm, as well as under static conditions. The amount of dissolved metals increased with the increasing weight of specimens, gyration speed, and extraction time. Copper was most abundantly found in extracts, followed by silver, palladium, and gold. Copper was the only element found in the filtrates in the same amount as in the extracts. The specimens weighing 1.5g did not have effects on cell viability. However, heavier specimens depressed cell viability to 60〜70% in the extracts that were obtained after gyration for 14 days at 230 or 260 rpm, while the filtrates exerted fewer effects on cell viability compared to the extracts. These results indicate that dynamic extraction might be useful in evaluating the dissolution and cytotoxicity of metallic biomaterials that mimic stress-bearing restorations.

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The creep and the compressive force(resistance to compression)of orthodontic open-type coil springs made of stainless steel and Ni-Ti alloy were investigated to explore the time for the replacement of these springs. The coil springs were compressed under loading times(7, 30 and 90 days)and compression rate(20% and 40%)that were chosen to simulate clinical use. After compression, the creep and the compression force(chosen to resistance to compression)of the coil springs were measured. No creep was induced in the Ni-Ti coil springs regardless of the laoding time and compression rate. On the other hand, the stainless steel coil spring showed creep with increases in the loading time and creep after 90 days of 4.2% and 7.6% under 20% and 40% compression, respectively. The compressive force at 20% or 40% compression of the original length did not significantly change for the Ni-Ti coil spring at any loading time, whereas that of the stainless steel coil spring significantly decreased after 90 days. This suggests that the stainless steel coil spring needs to be replaced after 90 days of application, while the Ni-Ti coil spring does not.

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