Ring liner and burn-out temperature affect the clinical time required to fit a cast post

Statement of problem: A cast post that is made with an unlined metal casting ring may exhibit anisotropic shrinkage which could result in distortion and consequently require more chair time for fitting in the root canal. Purpose: The purpose of this study was to investigate whether the presence of a ring liner and the burn-out temperature of the investment material affect the clinical time required to fit a cast post. Material and methods: Sixty identical post patterns were fabricated from a customized stainless steel root form. They were divided into 3 groups (n=20) as follows: group LH used a cellulose ring liner and a final burn-out temperature of 815°C; group NH used no liner with the same temperature; group NL used no liner with a final burn-out temperature of 600°C. All groups were invested in stainless steel casting rings with the same phosphate-bonded investment (Hi Temp) and cast with same alloy (Protocol). Two blinded investigators, with different levels of experience, adjusted and placed the posts in the experimental root form until a subjective acceptable fit was achieved. Each investigator fit 10 cast posts for each group, and the time required to achieve a subjective acceptable fit was measured in seconds. Two-way ANOVA was used to test the significance of the investigator, group, and their interaction. One-way ANOVA and a post hoc Tukey HSD test were used to find significant differences among the groups (α=.05). Results: The 2-way ANOVA showed a significant effect of the investigator (P=.001) and group (P<.001), but not interaction (P=.15). The 1-way ANOVA showed significant differences among the 3 groups (P<.001), and the Tukey HSD test showed significant differences between the following pairs: LH and NH (P=.009), and LH and NL (P<.001). NH and NL did not demonstrate a significant difference. Conclusions: Investing a post pattern without a ring liner and setting the final burn-out temperature at 600°C allow the fabrication of cast posts that require significantly less adjustment time for clinical placement than those invested with a ring liner and burned out at 815°C. (J Prosthet Dent 2009;102:224-228).