title: Effect of burn-out temperature on magnetic keepers in cast noble metal alloy posts.
abstract: BACKGROUND
  Early loss of magnetic keepers in cast gold posts is seen in clinical practice.
  
  
  OBJECTIVE
  The purpose of this vitro study was to investigate the effect of burn-out temperature on the retention of magnetic keepers in cast gold alloy and on the thickness of the oxide layer at the keeper-alloy interface.
  
  
  METHODS
  One hundred and five keepers (Magfit EX) were cast with gold alloy (Protor 3) at 3 different burn-out temperatures: 500 degrees C, 600 degrees C, or 700 degrees C (n=35). To test for retention of keepers, 60 specimens (n=20) were tested to failure in tension mode in a universal testing machine (UTM); 45 specimens (n=15) were sectioned, and the interface was evaluated under a scanning electron microscope (SEM). Tensile force data were analyzed with 1-way ANOVA, and SEM data were analyzed with 1-way repeated measures ANOVA. Bonferroni multiple comparisons were performed for post hoc analysis (alpha=.05).
  
  
  RESULTS
  Retention of the keepers was significantly higher at lower burn-out temperatures (P<.001). The retention of the keepers became unpredictable and immediate failures were observed at a burn-out temperature of 700 degrees C. Oxide formation at the keeper-alloy interface was significantly less at lower burn-out temperatures (P<.001).
  
  
  CONCLUSIONS
  A burn-out temperature of 500 degrees C for casting the Magfit EX keeper with type IV high-strength gold alloy is recommended.
authors.affiliation: Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, China. ehnpow@hkusua.hku.hk
authors.forename: Edmond H N
authors.initials: EH
authors.lastname: Pow
authors.forename: Kingsley H C
authors.initials: KH
authors.lastname: Chan
authors.forename: Henry W K
authors.initials: HW
authors.lastname: Luk
authors.forename: Andrew P L H
authors.initials: AP
authors.lastname: Dias
authors.forename: Anne S
authors.initials: AS
authors.lastname: McMillan
mesh_terms: D002133:Calcium Sulfate; D003722:Dental Alloys; D003734:Dental Casting Investment; D003735:Dental Casting Technique; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D003781:Denture Retention / Q000295:instrumentation*; D003828:Denture, Overlay; D006047:Gold Alloys / Q000737:chemistry; D006358:Hot Temperature; D008280:Magnetics / Q000295:instrumentation*; D010087:Oxides; D011176:Post and Core Technique; D013193:Stainless Steel; D013718:Tensile Strength
chemical_list: D003722:Dental Alloys; D003734:Dental Casting Investment; D006047:Gold Alloys; D010087:Oxides; D013193:Stainless Steel; D002133:Calcium Sulfate