940 and 980 nanometer Diode Laser Applications in Endodontics

Abstract Endodontic therapy aims to disinfect the root canal and its 3-dimensional dentine tubular network. This is commonly achieved by instrumentation, irrigation with antimicrobial rinses such as NaOCl and EDTA, and placement of temporary intra-canal dressings. Effective treatment requires the combination of physical and chemical agents to eradicate soft tissue debris, smear layer, and microorganisms both in planktonic forms and in multi-layered biofilms. During conventional root canal therapy, biofilms, infected dental pulp tissue and contaminated dentine are only removed to a limited extent. The cutting action of mechanical instrumentation on the dentine walls of the root canal results in the formation of a layer of debris (the smear layer) composed of dentine chips, remnants of soft tissues, and bacteria. This layer prevents irrigants and medicaments from penetrating into the dentinal tubules, and if not removed it may slowly disintegrate, increasing the risks of re-infection. In recent years, middle infrared Er:YAG and Er,Cr:YSGG lasers have been used to assist with disinfection and cleaning of the root canal system, however, neither of these can completely disinfect the root canal or remove the smear layer. Laser energy from near infrared Nd:YAG and diode lasers penetrates deeply into tooth structure. There is a range of diode laser wavelengths available, including 810, 830, 940, and 980 nm. The 940 and 980 nm wavelengths are of particular interest because their absorption in water is higher than the other available near infrared wavelengths. Shockwaves can be generated in water using water absorbing laser wavelengths. These shockwaves may be able to disrupt biofilms. Generating shockwaves using near infrared lasers would also exploit their deeply penetrating disinfecting capabilities. During laser activation, shockwaves can be generated in aqueous media through the formation of water vapour. In addition to this, direct release of oxygen free radicals from materials containing hydrogen peroxide or ozone is also known during laser irradiation. The use of ozone as a disinfectant is well established in industry, and in dentistry ozone has been used in the treatment of dental unit water lines and in the bio-oxidation and disinfection of incipient root surface carious lesions. It is known from industrial applications that the antimicrobial actions of ozonated water are increased by sonic and ultrasonic energy, and it is likely that laser-induced shockwaves (which are known to also create cavitations) will have a similar activating effect in terms of disinfection and physical flushing. This study examined the capabilities of pulsed near infrared lasers, 940 and 980 nm wavelengths, in forming cavitation and shockwave generation in root canals by using distilled water inside a capillary-glass tube and conventional fibers. It also determined shockwave dynamics of oxygen based liquid preparations, such as hydrogen peroxide and ozonated water, while activated by the 940 and 980 nm pulsed near infrared lasers. Moreover, the thermal safety using two types of temperature measuring devices, the thermocouple and a blackbody near infrared thermometer, was determined. The suitability of the thermocouple in measuring temperature changes during diode laser application was also assessed. The results demonstrated the ability of both the 940 and 980 nm near infrared wavelengths to induce cavitation and shockwave generation in less than 5 seconds. This timing was similar when using ozonated water but was greatly improved with the use of hydrogen peroxide at both 3 and 6 % concentrations. The 980 nm wavelength performed better than the 940 nm wavelength with distilled water. However, when hydrogen peroxide was used, the 940 nm wavelength was faster at cavitation induction than the 980 nm, marking different absorption curves between water and hydrogen peroxide. Both of these wavelengths produce minimal temperature increase on the external root surface during laser activation. In the period of temperature change examination, it was found that the transmitted laser energy through tooth structure is absorbed by the thermocouple giving a higher temperature reading than the real tooth surface temperature. Therefore, it is recommended that a non-contact, infrared temperature measuring device is used with near infrared laser wavelengths rather than the thermocouples. This study shows the potential benefit of the 940 and 980 nm near infrared wavelengths in the cleaning and disinfecting of the root canal system through the formation of cavitation and shockwave generation. Additional research examining smear layer removal and the disinfecting capabilities of these wavelengths, using both plain and modified fibers, is required to further evaluate their role in endodontics