Tarek El-Bialy (editor) - Therapeutic Ultrasound in Dentistry: Applications for Dentofacial Repair, Regeneration, and Tissue Engineering

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Low-intensity pulsed ultrasound (LIPUS) is a form of an acoustic wave that produces micromechanical strain in the tissue through which it passes, leading to biochemical events [].

The exact mechanism of LIPUS effects on biological tissues is still not completely understood. The biophysical effects of ultrasound are mostly studied in the in vitro studies, and these results are extrapolated to the in vivo studies. Since different studies use different ultrasound parameters and various experimental setups, it is very difficult to completely understand the mechanism of action. The potential biological response of therapeutic ultrasound can be divided into thermal and nonthermal effects.

Therapeutic ultrasound produces vibrational forces as it passes through the cell culture or the tissue. These vibrational forces cause a rise in temperature due to absorption of the energy. Heat generation is dependent on the intensity applied and the waveform. Pulsed ultrasound causes lower heat generation as compared to continuous waveform. Lehman et al. []. The amount of energy absorbed is dependent on the tissue density, making bone responsive to heat. This effect is considered useful in physiotherapy where ultrasound is used for muscle stiffness. Thermal effect of ultrasound also includes increased blood flow, decreased muscle pain and spasm, and increased collagen extensibility.

LIPUS produces a nonthermal effect which causes cavitation and micro-streaming and affects the cell membrane. Cavitation is the growth and oscillation of gaseous cavities which grow rapidly and collapse leading to change in the cell membrane. These changes increase the cell membrane permeability and hence increase the cellular uptake of the extracellular ions, growth factors, and drugs [], it explains the changes in cell membrane bilayer with ultrasound application. In positive pressure, the cell membrane layers are pushed together, while in negative pressure, the membranes are pulled apart. These movements also cause the activation of mechanosensitive elements. The process by which cell senses the mechanical load and transduces it into a cascade of cellular and molecular events is called mechanotransduction. Mechanosensitive elements, including cytoskeleton, stretch-activated ion channels, integrins, and G-protein receptors, have been extensively explained in the literature.