Effect of low-intensity pulsed ultrasound on the activity of osteoclasts: An in vitro study

The objective of this in vitro study was to evaluate the effect of low-intensity pulsed ultrasound on the resorption activity of osteoclast cell cultures.

RAW 264.7 cells were cultured and seeded over plates that were pre-coated with a synthetic carbonate apatite, and marked with fluoresceinamine-labeled sodium chondroitin polysulfate. Plates were randomly divided into 4 groups according to the treatment assigned to each one of them: NO RANKL (no RANK-L addition and no ultrasound application), NO LIPUS (addition of RANK-L and no ultrasound application), LIPUS 10 (addition of RANK-L and 10min of ultrasound application per day), and LIPUS 20 (addition of RANK-L and 20min of ultrasound application per day). The ultrasound device produced 1.5MHz pulses with a repetition rate of 1kHz and intensity of 30mW/cm². The experiment extended for one week and afterwards, resorption activity was evaluated according to the fluorescence intensity analysis and pit resorption measurements (number of pits and mean area).

Our experiment consistently demonstrated that low-intensity pulsed ultrasound application enhanced osteoclasts resorptive activity. In addition, it was demonstrated that when daily ultrasound application lasted longer (20min) the resorption was the highest. Results obtained from both evaluation methods were reasonably coherent.

Low-intensity pulsed ultrasound increases osteoclast resorptive activity in the absence of osteoblasts. This effect seems to be influenced by ultrasound treatment time. Future research might be directed to investigate osteoclast response to different ultrasound application protocols (frequencies and intensities) and potential cellular mechanisms.