Effect of burn-out temperature on magnetic keepers in cast noble metal alloy posts.

Statement of Problem: Early loss of magnetic keepers in cast gold posts is seen in clinical practice.
Purpose: 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.
Material and 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.