Bite - In

This months Rewind takes a look at one of the most exciting areas of dentistry today i.e. the Laser. Even though lasers have been around for quite some time it is only of late that they are becoming part of the regular armamentarium with a dentist.Dr. Ajay Kakar takes a look at the technology today.

The LASER in Dentistry

A Conceptual Introduction

Great strides in the field of equipment technology, has been one of the biggest boons of the industrial and electronic eras of modern science. The same has touched our field of dentistry in a large way. Gone are the days of old belt driven motors to remove caries from teeth. Motors gave way to air rotors, which today, seem to be giving way to the air abrasion devices available. The Radiograph is one of the most outstanding examples of how technology change touched the way dental therapy was conceived and carried out.

One very exciting technology making great inroads into a lot of areas of dentistry today is the LASER technology. Lasers seem to be all prevalent, right from the smallest laser pointer used by youngsters to have fun, to complex lasers used in medicine to lasers used in stage shows and programs. Lasers have been tried out in dentistry for over two decades but has come into the fore front as an everyday tool only in the last 3 or 4 years. Refinements in the technology have found more and more applications in dentistry.

The LASER is an acronym for the rather long term Light Amplification by Stimulated Emission of Radiation. The LASER beam is essentially a beam of light comprising of excited photons. The uniqueness of this beam of light is that the light is coherent and monochromatic vis a vis regular light which is nor coherent nor monochromatic. The laser is generated by applying a source of energy to a substance is contained in a sealed chamber. The substance contained in the sealed chamber is known as the "Exciting Medium".

When energy is applied to this medium photons get excited and create a sort of chain reaction. The sealed chamber has highly reflective mirrors on both ends of the chamber. The photons bounce back and forth between the two mirrors and continue the reaction. This reaction continues till the external source of energy is available. This energy is usually as electrical energy or heat energy applied with the help of tungsten arc lamps. One of the two mirrors of the sealed chamber has a small opening, which lets out the laser beam. This beam is then focused with the help of mirrors and applied to the oral tissues or it is carried to the oral tissues with fibre optic cables.

In concept all lasers work with the above mentioned principle. Even though similar in principle, all lasers have quite differing modes of action and ultimate end results. This variation is due to the difference in the quality of the laser beam emitted. The quality of the laser beam is a dependant of the exciting medium to which the energy is applied. The most singularly important difference is the wavelength of the laser beam generated. Wavelengths of lasers tried out in dentistry vary from as low as 0.157 microns generated by the Excimer laser to 10.6 microns generated by the Co2 laser. The other lasers like the Ruby laser, the Argon laser the ND:YAG laser and the Erbium:YAG laser all fall within this range of wavelengths. The wavelengths falling between 0.315 and 32 are the visible spectrum and can be seen by the human eye.

When a laser beam strikes an object there are four possible reactions. The beam may be reflected of the object, it may be absorbed by the object, it may be transmitted by the object or it may be scattered by the object. As far as the oral cavity is concerned, laser beams will be transmitted or absorbed depending on the object and the wavelength of the beam. None of the oral tissues will reflect a laser beam.

On complete or partial absorption of the laser beam, four different kinds of biological reactions can take place. These effects are photo chemical, photo thermal, photo mechanical and photo electrical. The photo chemical effect is mainly the biostimulation of tissues that takes place and enhances the healing cycle. Such effects are most useful in cases of apthous ulcers etc. The most widely used effect of lasers is the photo thermal effect i.e. the ablation of tissues by vaporization and photopyrolysis or the burning away of the tissues. The photo mechanical effects cause the breaking apart of the tissue structures by the laser light and also the breaking of tissue cells due to shock waves generated by the laser. The last action is the photo electrical effect in which tissue is removed by the formation of electrically charged ions.

The different responses take place dependent upon the power of the laser beam and duration of time. From a low power to high power the effect changes from chemical to thermal to mechanical to electrical. This is concurrent with an inverse proportion to the time of application of the laser beam. Generally a laser beam will create a crater on soft tissue as well as hard tissue. A laser crater usually as a central zone of ablation wherein all the tissue is vaporized. Surrounding the zone of ablation is a zone of irreversible damage. The extent of this zone depends on the kind of laser used as well as the duration and power of the laser pulse. Further from this zone is the zone of reversible damage where hypothermia and edema develops which is subsequently resolved.

The resultant strength of a laser beam is measured in terms of Power Density (PD). The power density is directly proportional to the power output of the laser unit and inversely proportional to the square of the focused area. The general formula for calculating the PD is as follows PD = W/cm2. The laser beam is usually focussed at a certain point on the tissues. As the beam is moved away from the tissue the area increases and the Power Density drops considerably. This way of using the laser beam is known as using it in the defocused mode versus the standard method of using it in the focused mode.

Lasers are also applied to tissues in what is known as the continuous mode or the pulsed mode. In the pulsed mode the beam is applied intermittently and then the tissues are given rest. The variations are done depending upon the desired end result. Lasers with a wavelength of less than 2.5 microns can be delivered with a fibreoptic system. These lasers are carried to the operating area along a fiber tip. When the tip is n contact with the tissues it is called as the contact mode i.e. the focussed mode. When the tip is moved away it is known as the non-contact mode or the de-focussed mode. Lasers with greater wavelengths than 2.5 microns like the Erbium:YAG and the CO2 laser are generally delivered via series of reflective mirrors or a hollow tube. The beam is directly focussed with the help of lenses on the tissues. The area of focus of the beam can be visualised with a concurrently delivered soft beam of light which is non reactive to the tissues. This beam of light is usually a HE-Neon beam of extremely low wattage. The sole purpose of this beam is to act as a guide for positioning the handpiece. One error which should always be avoided in the use of such non contact lasers is pre-focusing. If the laser handpiece is held at a distance whereby the focal point of the actual laser will be below the tissue surface, damage can be caused which may not be immediately visualised. Care should be taken to avoid such situations.

Lasers have three different kinds of applications, hard tissue applications, soft tissue applications and applications on dental materials. Lasers can be used to achieve a near instant very high percentage of polymerization of resin based materials. They can also be used to activate bleaching agents for in office bleaching of stained teeth. Hard tissue applications include removal of caries, re-orientation of the crystalline structure of the enamel and dentine thereby making it more resistant to caries and pit and fissure sealing. Lasers have also shown good promise in increasing the bond strength of resin based materials to lased tooth surface. Lasers can be used as an add on to acid etching to improve this bond strength. The soft tissue applications of lasers in dentistry are wide spread and productive in multiple situations.

Lasers can be used for any situation requring bloodless removal of soft tissue. It is of great use in overgrown gingiva during restorations. Crown lengthening procedures is another very usefull application of lasers in soft tissue. They can also be very effectively used for removal of any kind of gingival growths and lesions, especially ones having severe inflmmation and a tendency to bleed. Gingivectomies and gingivoplasties are very easy to achieve with a laser. Removal of soft tissue from third molar areas which is other wise difficult with a knife due to access problems is easily facilitated with the help of a laser. Lasers can also be used for disinfecting periodontal lesions as well as osteotomy sites while doing implants.

The laser is a very vesatile tool in the armamentarium of the dentist. Applications range over a wide spectrum of situations. The one negative aspect of the laser is the still high cost which will presumably drop as the technology gets more widespread.