title: Fatigue resistance of cast occlusal rests using Co-Cr and Ag-Pd-Cu-Au alloys.
abstract: BACKGROUND
  Fatigue failure in a removable partial denture framework includes fracture of the occlusal rest at the rest-minor connector angle.
  
  
  OBJECTIVE
  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.
  
  
  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.
authors.affiliation: Division of Removable Prosthodontics, Graduate School of Medical and Dental Sciences, Niigata University, 2-5274 Gakkocho, Niigata City 951-8514, Japan.
authors.forename: Cynthia G
authors.initials: CG
authors.lastname: Gapido
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kobayashi
authors.forename: Osamu
authors.initials: O
authors.lastname: Miyakawa
authors.forename: Shoji
authors.initials: S
authors.lastname: Kohno
mesh_terms: D002858:Chromium Alloys; D003735:Dental Casting Technique; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D003779:Denture Design; D003832:Denture, Partial, Removable; D006047:Gold Alloys; D008422:Materials Testing; D018583:Pliability; D016019:Survival Analysis
chemical_list: D002858:Chromium Alloys; D006047:Gold Alloys