Ultrasound with frequencies in the kilohertz (kHz)-range has been demonstrated to promote biological effects and has been suggested as a non-invasive tool for tissue healing and repair.
Developing ultrasound for therapy
However many challenges exist to characterize and develop kilohertz ultrasound for therapy. In particular there is limited evidence based guidance and standard procedure in the literature concerning the methodology of exposing biological cells to ultrasound in vitro.
Previous studies have showed that low-intensity ultrasound is considered an effective non-invasive therapy to stimulate hard tissue repair, in particular to accelerate delayed non-union bone fracture healing.
More recently, ultrasound has been proposed as a therapeutic tool to repair and regenerate dental tissues. Our recent work suggested that low-frequency kilohertz-range ultrasound is able to interact with dental pulp cells which could have the potential to stimulate dentine reparative processes and hence promote the viability and longevity of teeth.
Genetic expression stimulated by ultrasound
Our work addresses the question as to whether ultrasound can be used as a non-invasive biomechanical therapy to promote dental health and tissue repair. Oral health is essential for human health and well-being. Dental disease affects the quality of life for individuals worldwide imposing an immense burden on healthcare systems as reported by the World Health Organization.
Despite advances in restorative materials, traditional dental treatments using filling materials are relatively inefficient with approximately 50% of cases requiring revision within 5–10 years after treatment.
We investigated the effects of low frequency (kilohertz-range) ultrasound, generally used in dentistry for dental scaling, on odontoblast-like cells. These studies have shown that a single exposure of odontoblast-like cell lines to low-frequency ultrasound resulted in distinct effects on cell vitality and cell behavior.
Ultrasound was able to stimulate the expression of genes and production of growth factors, such as transforming growth factor β1 and vascular endothelial growth factor, believed to be important for odontoblast activity and dentine repair.
These promising data highlight the significant potential for the exploitation of ultrasound in novel dental regenerative therapies.
Backed by Finite Element analyses completed at the School of Engineering and Applied Sciences at Hofstra University, we were able to model ultrasonic waves travelling through the different layers of mineralized dental tissues (enamel, dentine) and their interaction with the dentine–pulp complex (please see video below).
This was used to explore and thus confirm the potential theory that with the correct combination of frequency and intensity, a tooth can be repaired using small doses of ultrasound.
The risks of multiwell plates
Our recent study at the School of Dentistry, College of Medical and Dental Sciences at the University of Birmingham, aimed to characterize ultrasound fields and bioeffects in multiwell culture plates using a similar odontoblast-like cell line (MDPC-23) model.
However the treatment of these cells with ultrasound was modified to investigate the effects on non-treated cells cultured in adjacent wells of multiwell plates. A spatial beam plot identified the risks to adjacent wells when a multiwell plate is used for experiments involving in vitro cell culture.
We showed that low-frequency ultrasound has a beam profile with significant lateral spread reaching and affecting cell cultures in adjacent wells of a multiwell culture plate.
Cells from culture wells directly exposed to ultrasound demonstrated both a change in temperature and a biological effect. This is in contrast to findings from culture wells not directly treated with ultrasound where a biological effect was reported without a temperature rise.
This adds to the evidence of a mechanical effect of ultrasound on biological cells. This study demonstrates the importance of characterizing the ultrasonic output from equipment, and questions the suitability of multiwell culture plates for low-frequency ultrasound application.
This opens the door to further studies of the biological effects of kilohertz low-frequency ultrasound on adherent cell cultures.
Despite this, we believe that ultrasound-based dental therapy is just around the corner. Our data suggest that ultrasound can be harnessed to propagate to the dental pulp region where it can interact with the living cells to promote dentine repair.
Further research is definitely required to analyze the precise physical and biological interactions of low-frequency ultrasound with the dental pulp to develop this novel non-invasive tool for dental tissue regeneration.
Once this is done, it will be simply a matter of time; first to get it accepted by the dental community; and second to make its way to the dental office.