The effects of the molecular weight of poly(methyl methacrylate)(PMMA) used in PMMA/MMA-tributylborane(TBB) resin cement on working time, setting time, film thickness and adhesion to dentin were studied using PMMA beads with the same mean diameter and five different molecular weights. A PMMA powder used in a commercial MMA-TBB-based resin(SB powder) was also included in the study as a control. The results showed that working time, setting time, and film thickness had good linear correlations with the inverse molecular weight and that the adhesive strength was practically independent of the molecular weights. SB powder provided shorter working time, shorter setting time and greater film thickness compared with the values extrapolated from the correlations obtained in the present study.

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To obtain good-fitting crown restorations made from pure titanium, the present study compared the fitness of crowns fabricated using a machining process in a dental CAD/CAM system with that using a casting procedure. Crown specimens were prepared using two different types of models(abutment tooth model prepared for full crown and the standard master die) by CAD/CAM system or by casting procedures with two different investment materials. The fitting test for crown specimens was carried out to examine the effects of fabrication methods on the fitness of a pure titanium crown restoration. The following results and conclusions were obtained. 1.Crown specimens made using dental CAD/CAM showed adequate machining accuracy. 2.There was no remarkable effect of crown thichness on the fitness of crowns made using dental CAD/CAM. 3.The fitness of cast crown specimens was influenced by differences in thickness of the crown and types of mold materials. 4.The line angles of the abutment tooth affected the fitness of crown specimens made using either dental CAD/CAM or casting.

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This study examined changes in shear bond strength of glass ionomer cements for luting to dentin from initial setting using three different mixing ratios. Conventional and resin-modified glass ionomer cements for luting were used. The cast dental alloy rod was cemented on the bovine dentin surface with a 20 μm-thick cement layer. The longitudinal bond strength from initial setting up to 24 hours was determined. A significant increase in the bond strength of the conventional cement was observed from 10 min to 20 min at all the mixing ratios. For all the mixing ratios the bond strength of the resin-modified cement increased with time. Slight differences in the bond strength were seen between the two different types of glass ionomer cement. In the initial setting, changes in the mixing ratios affected the bond strength of both cements.

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The possible correlation between the expansion of fillings made from fluoride-releasing glass polyalkenoate cement and water sorption of the materials was examined. Resin-modified glass ionomer materials had larger amounts of water sorption than conventional glass ionomer cements(acid-base type). And resin-modified glass ionomer materials had larger amounts of expansion by water sorption than conventional glass ionomer cements after immersion in distilled water kept at 37℃. There were positive correlations between the amounts of water sorption and expansion of each material. A semicircular profile was observed on the expanded surface of the resin-modified glass ionomers filled in the pre-treated bovine cavities, while the specimens filled in Teflon cavities generally revealed a trapezoidal shape. The absorption of water influence the surface profiles of glass ionomers filled in cavities, particularly for the resin-modified glass ionomers. The adhesion of glass ionomers to the cavity walls may influence the direction of expansion by water sorption.

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