In the great majority of teeth requiring root canal treatment, the goal is either prevention or elimination of a microbial infection in the root canal system (Haapasalo et al. 2005). Traditionally, the cleaning and shaping of root canals has been achieved with chemo-mechanical methods, which aim at removing infected pulp tissue and dentine as well as shaping the canals to prepare for the obturation. Even when chemo-mechanical treatment is meticulously performed in infected root canals, healing does not always occur. Antimicrobial irrigants are reported to reduce bacterial load (Johal et al. 2007), although studies with clinical outcome measures such as a decrease in the frequency of apical periodontitis are lacking. One disadvantage with irrigants, however, might be their inability to penetrate the deeper parts of the dentinal tubuli where microorganisms may reside (Berutti et al. 1997). Failures manifested as persisting or newly developing apical periodontitis could be due to residual microorganisms; it has been shown that a residual root canal infection present at the time of root filling adversely affects the outcome (Sjögren et al. 1997). As stated by Ng et al. (2011), it would be beneficial to develop adjunctive antibacterial therapeutic strategies to chemo-mechanical methods to target residual microorganisms and thus enhance the healing rates of teeth with infected root canals.
Various laser techniques have been considered as alternative methods for root canal disinfection (Gordon et al. 2007) and are suggested to more effectively affect bacteria located deep in the dentine than traditional chemo-mechanical methods (Klinke et al. 1997). Laser techniques are also reported to substantially reduce bacterial load when used as an adjunct to conventional treatment (Garcez et al. 2007). The bactericidal effects when lasers are used in conjunction with root canal treatments depend on the type of laser used, but the exact killing effects are not fully understood. Nd:YAG lasers are thought to eradicate bacteria mainly by thermal effects, whereas the suggested bactericidal mechanism of action for Er:YAG lasers is linked to the strong water absorption of the laser output. Lasing parameters such as pulse length, fluence and irradiance are also suggested to be involved in the anti-bacterial effect (Meire et al. 2011).
Lasers are also used in techniques that employ photoactivated substances or photosensitizers; however, the mode of action is completely different from the ones described above. This technique, photodynamic therapy, was developed as a cancer therapy but has been used to target bacteria and viruses. It requires three basic elements: a photosensitizer, a light source and tissue oxygen. A light source with a suitable wavelength, for instance a diode laser, excites the photosensitizer to produce highly cytotoxic singlet oxygen, which causes the chemical destruction of a limited area of tissue or bacteria that either have selectively taken up the photosensitizer or have been locally exposed to light (Dougherty et al. 1998). Obviously, these laser applications each have their own drawbacks and thus varying risks when used. Regardless of mode of action, commercially available laser systems are considered to improve clinical performance of root canal disinfection when used as an adjunct to conventional chemo-mechanical disinfection.
The aim of this systematic review was to evaluate the clinical efficacy of lasers as an adjunct to chemo-mechanical disinfection with the outcome measures ‘normal periapical condition’ or ‘reduction of microbial load’ in infected root canals.