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We examined the abrasiveness of glycine powders with particle diameters of 63 and 100 mum by measuring the depth and volume of defects produced during air polishing of human dentin. A total of 36 extracted human teeth were embedded in acrylic resin. The resin blocks were polished until the dentin surfaces were exposed. The nozzle of an air polisher was mounted 4 mm from the dentin surface, and the dentin surface was treated for 5 s at one of two angles of incidence (45 degrees or 90 degrees). Three materials were used in the polishing process: NaHCO(3) powder with a mean particle diameter of 100 microm (Handy Jet Powder), glycine powder with a mean particle diameter of 63 microm (Handy Jet Powder PMTC), and glycine powder with a mean particle diameter of 100 microm (Handy Jet Powder Recall). The defect depth at both angles was significantly deeper after treatment with Handy Jet Powder or Handy Jet Powder PMTC. The defect volume was the greatest with Handy Jet Powder, followed by Handy Jet Powder PMTC, and Handy Jet Powder Recall. The larger diameter glycine powder resulted in less damage to the dentin.

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The diamond burs of two dental CAD/CAM systems (GN and CD) were examined if they could be used to fabricate up to 21 ceramic full crowns without fracture.After one, 11, and 21 machining times, the surfaces of the diamond burs were observed and the number of particles captured on SEM pictures was counted.The average surface roughness of the crowns was also measured.All diamond burs could be used to fabricate 21 ceramic crowns without fracture. A significant decrease in the number of diamond particles was found on the surfaces of GN burs after 11 and 21 machining times, but not on those of CD burs. The average surface roughness of GN crowns significantly increased with increase in the number of machining times. A significant positive correlation was found between the average surface roughness and the number of diamond particles.

The purpose of this study was to investigate the effect of repeated machining up to 51 times using the same diamond bu: on machining accuracy of inner and outer surfaces of CAD/CAM (computer-aided designing and computer -aided manufactur ing) machined ceramic crowns. The surface topography of machined crowns was examined using photographs . It was fount that machining accuracy was not affected by the number of machining times . In all measuring points, the inner surface wa; machined to a dimension larger than the die model (i.e., increased gap) , whereas the outer surface was machined to a dimen sion smaller than the crown model (i.e., smaller crown) . Photo observation showed that cervical contour was machined it a clear, rounded form from 1st to 11th crowns.

Recently, the use of all-ceramic restorations has increased and been applied for many types of clinical treatment such as bridges which can be made with high strength ceramic material: yttria partially stabilized zirconia (Y-PSZ) employing the CAD/CAM system. Consequently, points that can grind high strength ceramic material effectively are necessary for modifying the shape of restorations or removing them. So in our research, we fabricated new diamond points used to grind Y-PSZ. These points were made by electrodeposition of diamond grains. The diameter of the diamond grains are larger (200 µm) than that of traditional points (100 µm). High strength ceramic material was ground to assess the grinding weight and grinding depth various types of points. As a result, Y-PSZ could be ground efficiently with the diamond point which has a larger diameter and employs lager grains (200 µm).

INTRODUCTIONWear of restorative materials is a very complex phenomenon that is affected by several extrinsic and intrinsic factors. Wear of restorative materials in the oral environment results from direct contact between the tooth and the restoration during mastication, oral parafunctions, toothbrushing with abrasive particles, as well as chemical effects caused by dietary and regurgitated acids 1） . To date, many studies 2-9） have attempted to evaluate the wear behavior of restorative materials using several testing methods. However, these methods evaluated only a part of the wear process of restorative materials. It is absolutely necessary to integrate main wear processes such as abrasion and fatigue for a thorough and holistic evaluation of the wear behavior of restorative dental materials. To this end, a combined wear test system -which carried out two different types of wear test -was developed 10） . In the combined wear test, a simulated occlusal wear test and a toothbrush wear test repeated alternately on the same position of one specimen. This testing system provided a means for clinical simulation and a basic wear model which consisted of different wear mechanisms induced in the oral environment. Occlusal force and brushing force are important factors for characterizing the wear behavior of restorative dental materials, especially when the two different wear processes are combined.The purpose of this study was to investigate the effects of different occlusal forces and brushing forces on the wear of composite resins in the combined wear test as well as in the simulated occlusal wear test and toothbrush wear test. Table 1 shows four commercially available light curing composite resins used in this study. APX （KURARAY, Okayama, Japan）and Z100（3M, MN, USA）were intended for both anterior and posterior restorations, while SRE （GC, Tokyo, Japan） and SDX（Shofu, Kyoto, Japan）were for posterior restorations. Preparation of the wear specimens and wear tests were carried out following the procedure reported in a previous study 11） . Wear specimens were prepared using a metal mold （diameter: 15 mm, depth: 8 mm） . Thickness of the specimens was adjusted to 2 mm using a spacer in the mold. Each composite resin was injected into the mold and pressed with a glass plate（1.4 mm thickness） . The specimen was polymerized using a curing light MATERIALS AND METHODS Specimen preparation and wear testEffects of occlusal and brushing forces on the wear of composite resins were investigated using three different wear tests: simulated occlusal wear test, toothbrush wear test, and combined wear test which carried out toothbrush wear test and occlusal wear test alternately. Test specimens were prepared from four commercial composite resins. Worn volume and maximum worn depth were measured under different occlusal forces（40 N and 80 N）and brushing forces（1.5 N and 3 N） in the three wear tests. Worn surfaces were observed using a SEM.In all the three wear tests, both higher occlusal and brushing forces resulted in significantly ...