PubMed · 4230436
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[Chromium-cobalt technic--preheating and casting].. https://pubmed.ncbi.nlm.nih.gov/4230436/
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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 x 2.0 x 10 mm), vertical minor connector (0.9 x 2.0 x 5.0 mm), and denture base connector (1.5 x 5.0 x 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 x 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<.01). 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<.01). Fatigue crack initiation occurred at the inner-curvature surface of the rest-minor connector angle. The typical Ag-Pd-Cu Au fatigue fracture surface consisted of smooth propagation and dimpled and smooth final fracture areas. 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.
Surface characterization of passive film formed on different NiCr-based dental casting alloys in an acidic artificial saliva was investigated by using electrochemical impedance spectroscopy (EIS) measurement and potential decay test. The polarization resistance (R(p)), which is inversely proportional to corrosion rate, of passive film was obtained from the EIS data. The stable passive current density, passivating rate, and passivity decay rate of passive film were evaluated by potential decay test. Surface chemical analyses of passive film were performed by using X-ray photoelectron spectrometer and Auger electron spectrometer. Results showed that the R(p), stable passive current density, passivating rate, and passivity decay rate, respectively, were significantly different among the various NiCr-based alloys (p<0.001). The presence of higher Cr (maximum 31-37 at%; mainly as Cr(2)O(3)) and Mo (maximum 10-11 at%; mainly as MoO(3)) contents in the passive film led to a higher R(p), lower stable passive current density, faster passivating rate, and lower passivity decay rate. The presence of Be (>20 at%; mainly as BeO) in the passive film was detrimental to both the corrosion resistance and the stability of passive film. The presence of small amount of Ti (<6 at%; mainly as TiO(2)) in the passive film did not show any influence on the characterization of passive film.
OBJECTIVES: This study was undertaken to evaluate the shear bond strength of four materials used as aesthetic material bonded to Ni-Cr alloy. METHODS: Sixty-eight alloy discs were prepared and divided equally into four groups, and received four treatments for veneering: conventional feldspathic porcelain (Noritake EX-3) and three light-cured prosthodontic composite resins (Artglass, Solidex and Targis). The aesthetic materials were applied after metal structure conditioning in accordance with the manufacturers' recommendations. The specimens were stored in distilled water at 37 degrees C for 7 days. A universal testing machine was used to measure the shear bond strength of the specimens at a cross head speed of 0.5 mm/min. Fractured specimens were examined by using both optical and scanning electron microscope. RESULTS: The analysis of variance and Tukey's test showed that the strongest mean shear bond was obtained with Noritake EX-3 (mean shear bond strength 42.90+/-7.82 MPa). For composites, the highest mean shear bond strength was observed for Targis (12.30+/-1.57 MPa); followed by Solidex (11.94+/-1.04 MPa) and Artglass (10.04+/-0.75 MPa). Optical analysis of the fractured surfaces indicated that for Targis and Noritake EX-3 all failures were a mixture of both cohesive and adhesive patterns. As for Artglass and Solidex, the fractures were mainly adhesive in nature. CONCLUSIONS: The Solidex system was equivalent to the Targis system in bond strength and exhibited greater strength than the Artglass system. The porcelain fused-to-metal showed considerably higher shear bond strength than the three metal-resin bonding techniques.