Background: Ultrasonic devices have a lot of dental application specially in hygiene and surgery. However there are some disadvantages like cut slowness and heat production. Prosthodontics application is less investigated. In particular the heat production could be an important factor for the clinician. In fact more than 5.5 °C temperature increasing could determine pulp necrosis during tooth preparation. The aim of this study is to investigate the increasing of temperature during finishing line repositioning and polishing phases using ultrasounds inserts and to verify if it remains within the limit of 5.5 °C.
Methods: A sample of 32 human molars (extracted for periodontal reasons) was selected. The teeth were endodontically treated and prepared with burs in order to obtain a prosthetic round chamfer preparation, leaving the apical portion pervious for inserting the thermocouple probe. Then, they were inserted in plaster cubes up to the cement-enamel junction. A wall has been selected for each tooth for margin repositioning and finishing and prepared with the piezoelectric instrument (Multipiezo Touch with TipHolder DB2, Mectron, Carasco, Genoa, Italy). A mechanical arm was used to standardize the operator-dependent parameters. These parameters were: the pressure exerted on the dental wall, the cutting length and the time required for margin repositioning and finishing. The test phase consisted in a first stage of margin repositioning using an regular ultrasound tip with a diamond grain of 120 micron (DB120, Mectron, Carasco, Italy) (group 1), followed by a second finishing step conducted by a extra fine ultrasound tip with a diamond grain of 60 micron (DB60, Mectron, Carasco, Italy) (group 2). Each test lasted 60 seconds: this was the time that the mechanical arm needed to accomplish 32 rides. During these stages the intrapulpal temperature has been recorded thanks to a thermocouple. The obtained temperature data were analyzed by Kruskal-Wallis test and Mann-Withney post-hoc test, without Bonferroni correction (P<0.001).
Results: The average pulpal temperature increase was 4.65 °C with a standard deviation of 0.99 °C for the DB 120 ultrasonic tip and 5.40 °C with a standard deviation of 0.84 °C for the DB 60 ultrasonic tip. However, neither of the instruments reach the medium critical level of 5.5°C reported in the literature, there are some single values who exceed it. There is statistically significant difference using tips with different granulometries within the two groups (P value =0.013).
Conclusions: Ultrasonic tools are very performing to achieve results in repositioning and polishing of prepared tooth. The in-vitro analysis show that the pulp temperature increasing remains within the safe limits literature shows. It is important underline the polishing phase is the most critical and the clinicians have to pay attention to irrigation and pression to avoid pulp damages. Additional clinical studies have to be performed to confirm these results.