Introduction

The outcome of root canal treatment of teeth with periapical periodontitis depends on efficient disinfection of the root canal system and prevention of reinfection (1, 2). Traditionally, it is accomplished by a combination of mechanical instrumentation and the use of disinfecting solutions for irrigation. Although instrumentation alone may reduce bacterial load by mechanical removal of microorganisms and infected dentine tissue, it does not provide a bacteria free root canal (3). Therefore, much has been expected from various combinations of disinfecting solutions. Sodium hypochlorite (NaOCl) at 0.5 to 5.25% is still considered the gold standard for root canal irrigation (4) due to its wide antimicrobial spectrum of action and ability to dissolve organic tissue (5). Other antimicrobial irrigants such as chlorhexidine, potassium iodine, MTAD (a mixture of tetracycline, citric acid and a detergent) and QMix (a mixture of ethylenediaminotetraacetic acid, chlorhexidine and detergent) have been investigated, but still not proven to be more effective than NaOCl (6).

Another important aim of the root canal irrigation is the removal of a smear layer (1 to 2 μm thick) (Fig. 1), which is formed on the root canal walls due to the preparation, and consists of dentine debris, pulpal remnants, bacteria, endotoxins (7). The smear layer is removed by rinsing the root canals with 10 or 17% aqueous ethylenediamine tetraacetic acid (EDTA) (Fig. 2) or 10% citric acid at the end of mechanical instrumentation (4), thus achieving a closer interface between obturation materials and dentine walls (8, 9). Since EDTA and citric acid reduce the antimicrobial effect of NaOCl (10), a final rinse with a disinfecting solution after the smear layer removal is beneficial (11).

Figure 1 A SEM photograph (x 1000) of root canal wall after mechanical instrumentation and irrigation with 2.5% NaOCl, showing remaining smear layer.

Figure 2 A SEM photograph (x 3500) of root canal wall after final rinsing the root canal with 17% EDTA following mechanical instrumentation and irrigation with 2.5% NaOCl. A root canal wall with open dentinal tubules is seen.

Due to the complex intracanal anatomy and the limitations of the syringe/needle irrigation technique (12, 13), new adjunctive antibacterial therapeutic strategies have been recommended to target residual microorganisms and thus enhance the healing rates of teeth with periapical perodontitis (14). Over the last two decades, different irrigant agitation techniques have been introduced as final irrigation protocols in endodontic treatment (15, 16). Passive ultrasonic irrigation (PUI) and sonic activated irrigation have been reported to be efficient in the removal of the intracanal smear layer and debris (17, 18), and to facilitate the disruption of endodontic biofilms (19, 20).

Relatively new approaches to disinfecting and cleaning the root canals include lasers (21). The effects of several laser wavelengths on the root canal walls and endodontic microorganisms have already been investigated (21, 22). However, the acceptance of laser technology by clinicians still remains limited (23). The aim of this review is to explain basic principles of the interaction between laser and biological tissue in root canals and to present current knowledge and scientific status of the laser efficacy in root canal disinfection and cleanliness.

Laser tissue interaction

Biomedical applications of lasers have been under investigation since Theodore Maiman used the first Ruby laser in 1960. Commercially available medical laser wavelengths cover a short band in the electromagnetic spectrum, ranging from ultraviolet (UV) to mid-infra-red (IR) (approximately 200 nm to 10 μm). The laser beam is, due to its monochromaticity (single wavelength), coherence (photons in phase), collimation (very low beam divergence) and intensity (24), highly precise and selective in interaction with biological tissues (24, 25). When a laser beam interacts with tissue, it can be reflected, transmitted, scattered and absorbed in varying proportions (26). The absorbed laser energy is particularly important because it transforms into thermal energy and causes changes inside the tissue. The total amount of laser energy, which will be absorbed, depends on the laser wavelength and the optical characteristics of the target tissue, such as pigmentation (chromophores) and water content (26). The effect of the absorbed laser energy in the target tissue can be controlled by the irradiation parameters: 1) irradiance or pulse energy; 2) mode of energy emission (continuous wave (cw) or pulse irradiation); 3) laser beam size on the tissue; 4) laser pulse length and repetition rate; 5) the use of water spray. In addition, any change in the physical properties of the tissue, as a result of laser irradiation with certain parameters, can also influence the effect of the laser tissue interaction (27).

Potential benefit of lasers in medicine and dentistry depends on the particular properties of each type of laser and the specific target tissue. The physical properties of the tissue (absorption and scattering coefficient, thermal conductivity, mechanical strength, heat capacity) and laser irradiation parameters govern the course of laser-tissue interactions (28). At low irradiances and/or energies, laser tissue interactions are either purely optical, or a combination of optical and photochemical or photobiomodulative effect. When laser power is increased, photothermal interactions start to dominate. Finally, photomechanical (sometimes referred to as photoacoustic) effects become apparent when repetitive and very short laser pulses with high pulse energy are delivered to the tissue (27).

It can be concluded that laser light irradiation of relatively low intensity during longer time is less destructive than those applied for short-duration at high intensities. Moreover, there is a minimal level or threshold of energy required for specific types of interaction to occur (28).

Lasers in endodontics

The first use of laser in endodontics was reported by Weichman and Johnson in 1971 (29) who attempted to seal the apical foramen in vitro with a high power carbon dioxide (CO₂) laser. Since then, many papers on laser use in endodontics have been published (30). However, the clinical application of lasers in endodontics started in the late 90s when the new delivery systems, including thin and flexible fibres and endodontic tips, were developed. Today, lasers can be used in various endodontic procedures such as: pulp capping/pulpotomy, cleaning and disinfecting the root canal system, obturation, endodontic retreatment, and apical surgery (31).

The laser wavelengths described for cleaning and disinfecting the root canal system are: erbium: yttrium aluminium garnet (Er:YAG), 2940 nm; erbium, chromium: yttrium scandium galium garnet (Er,Cr:YSGG), 2780 nm; neodimium:yttrium aluminium garnet (Nd:YAG), 1064 nm; diode, 635 to 980 nm; potassium titanyl phosphate (KTP), 532 nm; carbon dioxide (CO₂), 9600 and 10 600 nm. The physical effect of these lasers in root canals depends on the absorption of their wavelengths in biological components and chromophores such as water, apatite minerals, and various pigmented substances (microorganisms). Wavelengths of the visible and near-infrared electromagnetic radiation (Nd:YAG, diode, KTP lasers) are poorly absorbed in water and hydroxyapatite and have deeper bactericidal effects in dentine. On the contrary, mid-infrared erbium lasers, whose wavelengths are highly absorbed in water and hydroxyapatite, have a superficial effect on dentine walls and can be used for removal of the layer and disruption of intracanal biofilms (32).

When using lasers inside the root canal, several limitations have to be taken into consideration. Firstly, the laser light is emitted in a straight line from the tip of an optical plain-ended fibre or a laser guide with a divergence angle of only 18 to 20 degrees (22). With such unidirectional laser beam, it is difficult to gain equal irradiation of the whole root canal dentine surface (22, 33). Moreover, the root canal preparation as well as retreatment procedures with laser and plain fibres is dangerous in curved root canals because of the risk of creating ledges and perforations (34, 35). To improve the surface area of the root canal dentine being irradiated, a helicoidal withdrawing motion from apical to coronal part is proposed when using fibre tips (36). Besides, new conical side-firing fibre tips with 80% lateral and 20% forward radiation provide complete coverage of intra-canal walls (37) (Figure 3, 4). Another limitation is the safe use of lasers in the root canal, especially thermal damage of periradicular tissues through the open apical foramen may occur when using the erbium lasers at ablative settings (38).

Figure 3 Handpiece for the Er,Cr:YSGG laser with conical side firing fibre tip for endodontic use (200 μm in diameter).

Figure 4 Radial firing fibre tip with stripped side wall (600 μm in diameter) which is used in the coronal part of root canal for photon initiated photoacoustic streaming (PIPS).

Nd:YAG

Nd:YAG laser (1064 nm) has been the most widely investigated laser for endodontic disinfection. Antimicrobial effect of the Nd:YAG is based on thermal heating of the bacterial environment and local heating inside bacteria (through chromophores inside bacteria sensitive to the laser light) (39). The advantage of the Nd:YAG laser in root canal disinfection is its significant bactericidal effect up to 1 mm into the dentine (40). Moritz et al. (41) found 99.16% reduction of bacterial numbers (Enterococcus faecalis and Escherichia coli) in inoculated root canals after Nd:YAG irradiation. Gutknecht et al. (42) achieved an average of 99.92% reduction in the number of intracanal Enterococcus faecalis using the Nd:YAG laser at 15 Hz and 100 mJ. In order to provide even irradiation of the dentine walls and to prevent thermal damage to the periradicular tissues, a thin glass fibre tip of the Nd:YAG laser (diameter of approximately 200 μm) has to be placed within 1-2 mm of the apex and moved in slow circular movements to the crown (36).

Since Nd:YAG laser irradiation is well absorbed in melanin and dark pigmented tissues and poorly in water, it is not as effective against nonpigmented bacteria (such as Enterococcus faecalis) (37) and bacterial biofilms (38, 39) and therefore higher energy densities are required to induce a lethal thermal effect. At present, safety parameters for the Nd:YAG laser are 15 Hz, 100 mJ, and 1.5 W, four times for 5 to 10 seconds, with an interval of 20 s (43). Antibacterial effect of Nd:YAG laser has never been shown to be superior to conventional NaOCl irrigation (38). Bergman et al. (44) concluded that Nd:YAG laser irradiation is not an alternative but a possible adjunct to existing protocols for root canal disinfection.

After Nd:YAG laser irradiation of intracanal dentine walls, morphological changes such as melting and recrystallization with open or closed dentinal tubules can be observed (45) (Figure 5). If the smear layer has not been removed, Nd:YAG irradiation will cause contraction, evaporation and glazing effects on its surface (45, 46). At higher power settings (3W and above), total removal of tissue remnants, structural changes, carbonisation and cracks are found as a result of thermal damage (47).

Figure 5 A SEM photograph (x 3000) of a Nd:YAG-lased dentine surface. A melted bubbled area with signs of recrystallization is observed.

Diode laser

Diode lasers emit radiation within the visible (mostly 660 nm) and infrared (810 to 980 nm) range of the electromagnetic spectrum. Due to the higher absorption coefficient in water (0.68 cm-1), diode lasers have lower penetration depth into the dentine (up to 750 µm) compared to Nd:YAG laser (48). There are only few data about the antimicrobial effectiveness of diode lasers in root canal treatment available in the literature so far (49). Moritz et al. (50) showed the bactericidal effect of a diode laser (810 nm) at 3 W during 5 x 5 s against intracanal Escherichia coli and Enterococcus faecalis in extracted teeth. Irradiation at 4 W was even more effective although associated with a temperature rise of 6°C. The same result with a diode laser (810 nm) at 3 W during 30 s was reported by Gutknecht et al. (36) against intracanal Enterococcus faecalis. In a study by Bago et al. (20) the effect of a diode laser (985 nm, 2 W, 3x20 s) against E. faecalis biofilm was similar to the use of 2.5% NaOCl for 60 s.

Morphological changes in the dentine walls after diode laser irradiation are similar to those obtained with a Nd:YAG laser (disruption and melting of the smear layer, closed and opened dentinal tubules) (50) (Figure 6).

Figure 6 A SEM photograph (x 3000) of a dentine surface after irradiation with the diode laser. The root canal was previously rinsed with 2.5% NaOCl during mechanical preparation. Open dentinal tubules are found next to a melted surface area and some remaining debris.

Antimicrobial photodynamic therapy (aPDT)

Antimicrobial photodynamic therapy (aPDT) or photoactivated disinfection (PAD) is a laser induced photochemical disinfection or sterilization of hard and soft tissues which is based on the activation of a nontoxic photosensitizer by low laser energy. As a result of the interaction between the phothosensitizer and the laser light, singlet oxygen (1O₂) is formed out of molecular oxygen (3O₂) which causes damage to the bacterial membrane and to its DNA (51). The photosensitizers are selected to have a specific affinity to the bacterial membranes, without affecting the host cells viability (52).

The effectiveness of aPDT depends on several factors: the type of the photosensitizer or dye, its concentration, the type of bacteria, the light source and the irradiation parameters (53). Various combinations of light sources (diode laser at 630nm, 660nm and 670nm; Helium: Neon laser) and dyes (methylene blue, tolonium chloride) have been investigated and are commercially available.

Many in vitro (38, 39) and ex vivo (20, 54) studies have shown that aPDT has potential to improve root canal disinfection after classical instrumentation and rinsing. Fonesca et al. (55) reported a high bacterial reduction rate (99.9%) after treating intracanal Enterococcus faecalis with toluidine blue and 50 mW diode laser (660 nm). The same reduction rate was achieved by phenothiazine chloride or toluidine blue activated by a diode laser (660 nm, 100 mW) for 1 min in the study of Bago et al. (20). In an in vivo study of Garcez at al. (56), a combination of polyethylenimine chlorine and a diode laser (40 mW, 4 min, energy: 9.6 J), was used successfully for the eradication of multi-drug resistant microorganisms. Ng et al. (14) found 86.5% of root canals without bacteria after endodontic therapy followed by methylene blue mediated aPDT (665 nm, 1 W, 30J/cm²) for 2.5 min twice. The results of other in vivo studies recommended aPDT as an alternative, or a supplement an adjunct to currently used root canal disinfection methods (52, 57).

Souza et al. (58) did not find a significant additional effect of the aPDT compared to chemomechanical instrumentation with either methylene blue or toluidine blue and 660 nm diode laser (40 mW). Meire et al. (38) found a higher efficacy of 2.5% NaOCl compared to PAD. The contradictory results may be caused by the different light parameters and wavelengths; by the concentration, type and volume of the photosensitizers; light delivery techniques; interaction time of the dye with the medium and whether or not a mature biofilm was used (53). Furthermore, limited diffusion of the photosensitizer into intracanal irregularities, dentinal tubules and into the biofilm with restricted production of reactive oxygen species (ROS) will interfere with the efficacy of a PDT in root canal disinfection (58).

Er:YAG and Er,Cr:YSGG lasers

The wavelengths of the Erbium lasers (Er:YAG, 2940 nm; Er,Cr:YSGG, 2790 nm) are well absorbed in water and hydroxyapatite and are therefore mostly used for the ablation of dental hard and soft tissues. In endodontics, the Erbium lasers are very effective in the removal of the intra-canal smear layer (59, 60) (Figure 7) and have the potential to destroy biofilms on dentine walls (39). The energy of the Erbium lasers is almost completely absorbed in the first 300 to 400 μm of dentine tissue so that the bactericidal effect is superficial (32).

Figure 7 A SEM photograph (x 3000) of a Er,Cr:YSGG lased dentine. The surface is without the smear layer and open dentinal tubules are seen.

Over the last few years, there has been an increasing interest in the use of Erbium lasers for the agitation of intracanal water-based fluids. Laser activated irrigation (LAI) is based on the creation of specific cavitation phenomena and acoustic streaming in intracanal fluids as a result of photothermal and photomechanical effects. The strong absorption of the Erbium laser energy (at low settings of 50-75 mJ) in water and NaOCl causes vaporization and formation of large elliptical vapour bubbles. The vapour bubbles cause a volumetric expansion of up to 1,600 times the original volume of an irrigant with high intracanal pressure which drives the fluid out of the canal. The bubbles implode after 100 to 200 microseconds, creating pressure which sucks fluid back into the canal: inducing secondary cavitation effect (61, 62). This technique was demonstrated to be effective in the removal of intracanal dentine debris and smear layer (63). De Moor et al. (64) and De Groot et al. (61) showed a higher efficiency of LAI with Er,Cr:YSGG and Er:YAG (75 mJ, 20 Hz, 1.5 W, 4 x 5 s) and 2.5% NaOCl in the removal of dentine debris from the apical part of the root canal compared to conventional irrigation or PUI. If the Er:YAG laser is used at low settings (20 mJ, 15 Hz) and ultra-short laser pulses (50 µs), intracanal cavitations and shockwaves are created as a result of photoacoustic and photomechanical effects. This phenomenon is called photon induced photoacoustic streaming (PIPS). Compared to the LAI, where intracanal conical side-firing fibre tips are positioned 5 mm from the apex, PIPS uses a tapered 600 µm wide side-firing stripped tip which is kept at the entrance of the root canal and used with continuous irrigation (65).

Conclusion

There is consensus that laser irradiation has the potential to kill microorganisms and to remove debris and smear layer from root canals. In root canal disinfection, there is insufficient evidence to suggest that any specific laser is superior to the traditional endodontic treatment. Erbium and Nd:YAG wavelengths can be used for the root canal debridement and cleaning; the Erbium lasers for the laser activated irrigation and photon initiated photoacoustic streaming, and the Nd:YAG laser for the evaporation and contraction of the smear layer. Finally, the use of laser is recommended not as an alternative to NaOCl but as an adjunct to the traditional disinfection and debridement protocols. More clinical randomized studies are necessary to evaluate endodontic treatment outcome following the use of laser.