Ultrasound efficacy in removing of posts luted with different cements

Objectives: One of the most challenging parts of a root canal retreatment procedure is the safe removal of an existing post. For this purpose, ultrasonic instruments are widely recommended. However, different cements behave differently in response to the ultrasonic vibration. The aim of this study was to evaluate the effect of different duration of ultrasonic application on the post retention cemented with zinc phosphate or glass inomer. Methods: 96 intact premolars extracted due to orthodontic reasons were selected. All specimens were decoronated and then root canal therapy was performed. After one week a 9 millimeter post space was prepared using a no. 4 parapost drill. Specimens were mounted with self-curing resin, and then half of the samples were cemented with glass ionomer (Ketac-Cem) and the rest with zinc phosphate cement (Fleck's). After 3 weeks storage in tap water, each group was divided in to 4 subgroups. In control subgroups, no ultrasonic vibration was applied. However, in experimental subgroups of both cements, 4, 8 or 12 minutes of ultrasonic application were performed. The force required to dislodge each post was determined using an Instron machine. One-way ANOVA and post hoc tukey's tests were used for statistical analysis. Results: In zinc phosphate group, 8 and 12 minutes subgroups had significant differences with control and 4 minute subgroups and also with one another (p < 0.05). However, in glass ionomer group only after 12 minutes of ultrasonic application a significant reduction in retention were observed (p < 0.05). Conclusion: ultrasonic vibration can efficiently use for post removal. However, it seems compared to glass ionomer, zinc phosphate cement is more susceptible to ultrasonic vibration.