Methods: Odontoblast-like MDPC-23 cells were grown as an adherent monolayer. Prior to ultrasound treatment, the odontoblastic cells were detached and taken up into a cell suspension. Low frequency ultrasound was then administered to the cells by a D-tip dental scaler probe powered by a 30 kHz piezoelectric generator. Three power settings were used which produced a linear increasing tip oscillation displacement correlating to increasing ultrasonic output. Cell viability was assessed by the trypan blue exclusion assay. Subsequently, control and ultrasound-treated cells were cultured and cell adhesion to the culture plastic was analysed after 4 and 24 hours. BrdU incorporation after 24h was determined immuno-cytochemically to measure cell proliferation. Analysis of gene expression was performed using semi-quantitative RT-PCR.
Results: Ultrasound induced approximately ~ 25% cell death at the highest power tested, but only a marginal level of cell death (3-4%) was observed at the two lower amplitudes. The latter was reflected in a 10-15% decrease in cell adhesion after 4-24 hours of culture, while cell attachment was reduced by approximately 30% in the highest power group. No significant changes were observed in cell proliferation 24h after ultrasound treatment. Analysis of gene expression showed a significant increase in the GAPDH-normalised expression of the small heat shock protein 25 in all treatment groups. Furthermore, ultrasound resulted in a mild increase in the expression of collagen type I, transforming growth factor-beta1, osteopontin, and heat shock protein 70. Gene expression of alkaline phosphatase and CBFA1/Runx2 were not affected by ultrasound.
Conclusions: These findings show that ultrasound may modulate odontoblast viability and function. Ultrasound may be considered as a non-invasive therapeutic application in tooth repair.