Is it Right Time for Lasers in Endodontics ?

I am currently finishing my PhD which is about lasers in endo. I am definitely not the reference in the laser field but at least I can tell you what I know. I understand more or less how lasers work, I have read quite some lit about it and I used a number of lasers for some microbio studies in the lab, that's my background. Here are some critical points about lasers in endo.

Transmission of the laser beam to the root canal is mostly done with a fibre optic. The smallest optical fibres are 200µm, so they fit well in most prepared root canals. They are fairly flexible, although not as flexible as NiTi, so they won't take severe curvatures. More importantly, the distal end of most fibres is flat, hence the laser beam is emitted straight forward. The beam diverges a little bit (the exit angle is about 15°), but not enough to fully cover the canal walls. To overcome this, the manufacturers propose to bring the fibre to the apical area, activate it while withdrawing in a helicoidal motion in order to irradiate the canal walls. Maybe that's possible in a short central incisor but in long, thin or curved roots, the majority of the canal will not be irradiated.

So using conventional fibre optics, coverage of the canal walls with a laser is problematic.

In recent years, modifications of the fiber ends have been introduced in dentistry (eg etching with strong acids). They result in cone-shaped fibre ends that emit most irradiation laterally. This is a major improvement because they enable better irradiation of root canal walls. As a drawback, energy densities are tremendously reduced because the laser light is spread over a much larger surface. Often a minimum energy density is required to yield a significant interaction with the target (eg to be antimicrobial). An equivalent increase of the laser power (in order to raise energy density again) is not always possible because the fibre is limited in the amount of energy it can transfer.

There are many lasers out there. They differ basically in the wavelength they emit. With regard to disinfection, it seems that the lasers in the visible and near infrared spectrum (lasers emitting visible light, most diode lasers and Nd:YAG laser) are not the best choice. Their wavelength is rather poorly absorbed in dentin and in most bacteria. Their antimicrobial action is a photothermal one, which means that they kill through (localized) heating of the substrate.

Lasers in the mid infrared spectrum (like Er; Cr:YSGG or Er:YAG laser), on the other hand, have more interesting properties. They are well absorbed in hydroxyapatite, which allows cutting of dental hard tissues. Maybe more importantly, they are also very well absorbed in water. This results in very good antimicrobial action since micro-organisms (and biofilms!!) consist for the most part of water. (for the sake of completeness, we have to mention here that the drawback of high absorption is limited penetration depth so bacteria deep below the surface of dentin or water escape the laser action). Another benefit that this high absorption in water offers is the ability to induce cavitation and acoustic streaming in irrigating solutions (known as LAI, 'laser assisted irrigation'). PIPS tip make use of this principle. The action of erbium lasers on NaOCl is very powerful and the primary and secondary cavitation bubbles that expand and implode result in impressive fluid movement in the canal. It has been shown in a number of in vitro studies that LAI results in better debridement. The disinfection part has yet to be investigated.

For LAI, it is not necessary to bring the fiber all the way down to the apical 1/3 (PIPS for example are held at the canal entrance). That is because the action is so powerful. The above-mentioned problems with canal coverage are hereby overcome.

Stating that a laser kills all the bugs in the canal is nothing but unfounded commercial talk. If it were true, I'd buy that machine immediately.

In terms of disinfection, visible and near infrared lasers will probably not add much to current root canal disinfection procedures. The combination of erbium lasers with side-firing tips seems promising, as well as the activation of irrigants with these lasers (see for example the interest of an increasing number of research centers, e.g. at ESE 2011). Future studies will have to point out if lasers are able to positively affect endodontic treatment outcome.

by Dr Maarten

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