Showing papers by "Osamu Miyakawa published in 2003"

Influence of implant design and bone quality on stress/strain distribution in bone around implants: a 3-dimensional finite element analysis.

Abstract: Purpose A 3-dimensional finite element analysis was performed to evaluate the influence of implant type and length, as well as that of bone quality, on the stress/strain in bone and implant. Materials and methods Two types (screw and cylinder) and 4 lengths (9.2, 10.8, 12.4, and 14.0 mm) of titanium implants were buried in 4 types of bone modeled by varying the elastic modulus for cancellous bone. Axial and buccolingual forces were applied to the occlusal node at the center of the abutment. Results Regardless of load direction, maximum equivalent stress/strain in bone increased with a decrease in cancellous bone density. Under axial load, especially in the low-density bone models, maximum equivalent strain in cancellous bone was lower with the screw-type implant than with the cylinder-type implant. It was also lower with the longer implants than with the shorter implants. Under buccolingual load, equivalent stress/strain was influenced mainly by bone density. Discussion This study confirms the importance of bone quality and its presurgical diagnosis for implant long-term prognosis. Implant length and type can also influence bone strain, especially in low-density bone. Conclusions The results of this study suggest that cancellous bone of higher rather than lower density might ensure a better biomechanical environment for implants. Moreover, longer screw-type implants could be a better choice in a jaw with cancellous bone of low density.

Fatigue resistance of cast occlusal rests using Co-Cr and Ag-Pd-Cu-Au alloys.

Abstract: Statement of problem Fatigue failure in a removable partial denture framework includes fracture of the occlusal rest at the rest-minor connector angle. Purpose This in vitro study evaluated the fatigue resistance of 0.8-mm–thick occlusal rests cast with Co-Cr and Ag-Pd-Cu-Au alloys. Material and methods The specimen consisted of occlusal rest (0.8 × 2.0 × 10 mm), vertical minor connector (0.9 × 2.0 × 5.0 mm), and denture base connector (1.5 × 5.0 × 25 mm). Twenty-five specimens each were cast with Co-Cr and Ag-Pd-Cu-Au alloys. From each group, 5 specimens were subjected to a load-deflection test conducted to determine the amount of deflection to be used in fatigue test. The fatigue test was performed such that the occlusal rest component was deflected by displacing the denture base component in a tissueward direction. Predetermined denture base displacement values of 0.25 and 0.49 mm for Co-Cr and Ag-Pd-Cu-Au specimens, respectively, were repeated at a rate of 500 cycles/min by use of a displacement-controlled fatigue testing machine, until occlusal rest failure occurred or a preset limit of 2 million cycles was achieved. The survival rates of the occlusal rests were calculated assuming a chewing cycle of 2 × 10 5 per year. Scanning electron microscopy (SEM) photographs were made to examine the fracture surfaces and to identify casting defects. The relationship between fatigue cycles and number of casting defects was determined by Spearman rank correlation analysis ( P Results All 20 Co-Cr specimens outran the preset limit, whereas 18 Ag-Pd-Cu-Au specimens fractured. After 3 years of simulated clinical use, only 50% of the Ag-Pd-Cu-Au occlusal rests survived. Statistical analysis showed that the fatigue cycles and number of casting defects were inversely related ( P Conclusions This study suggests that cast Co-Cr rests are more rigid and fatigue resistant than Ag-Pd-Cu-Au rests. The fatigue resistance of 0.8-mm–thick occlusal rests may be adequate if cast with Co-Cr alloy. An increased number of casting defects may hasten the fatigue failure of occlusal rests.