Therapeutic lasers made easy for the dentist

Jan Tunér DDS

My colleagues often tell me that the use of laser phototherapy (LPT) is so complicated and that they do not understand all the difficult terms and parameters involved. But if you take the courage of looking closer at these seemingly difficult matters, you (I hope) will find that there is not much that you did not already know.

The laser light has very unique properties, that is why it is so useful, but it is still ”only” light. And we are already using light daily in our clinics – the curing light. So let us compare the two lights – the well-known curing light and the therapeutic laser, and see what we already know!

First we have the wavelength. The curing light is broad banded, whether from a light emitting diode (LED) or a filtered halogen lamp. The peak of this blue light is around 470 nanometres, where the optimal curing property of the camphorquinone is. Some filling materials need light peaks at slightly different wavelengths but they are all in the visible blue range. I am sure you already knew this, maybe not the exact wavelength peak, but that is not important in daily practice. So the curing light is blue, the beginning of the colour of the rainbow: blue, green, yellow and red. These are the colours that we can see and they are in the range of about 400-800 nanometres (nm), after which the invisible infrared comes. The lasers commonly used in laser phototherapy are in the red and infrared spectrum, 650-904 nm.

Now let us look at the power of our lights. The output of the curing lamp differs depending on its design: filtered halogen lamp or blue LED. Generally the power of curing lights is in the range 200-250 mW. But what comes out of the curing wand is not measured in watts like in the lamps used in your ceiling. The intensity (power density) is rather given in the specifications from the manufacturer, and it is expressed in mW per cm^2. OK, you remember that you used to have a curing lamp of ”500” but upgraded to ”1200” something. And yes, the upgrading was the milliwatts per square cm (mW/cm^2) range. This tells you exactly this: the number of mW irradiated over each square cm, measured at the opening of the wand. Maybe you own some sort of ”turbo” probe to obtain faster cure? If so, it is a probe with a thinner ending. The power (number of photons) stays the same but now they are forced into a smaller area and the intensity (mW/cm^2) increases. Just like we do with a magnifying glass where the rays from the sun are focused into a very small spot and the intensity of the pleasant sun suddenly can start a fire. The same goes for the lasers. If the probe opening is small, the power density is higher than if the probe has a larger diameter. This is an important aspect of laser therapy. The output of one laser may be high, but the intensity of the beam also depends upon the size of the “laser eye”.

The curing depth of our curing devices is generally considered to be 2 mm for light shades. So if 10 seconds is fine for A2, you would probably irradiate for 20 seconds when using A4. The same is true for lasers: fat and mucosa are rather transparent, bone less transparent and muscles even less so. The main absorber of the light is the haemoglobin. The obvious fact that different materials have different penetration indices is obvious and we have to consider this when using lasers. We must think of dose at target, not just the dose. Because of the gradual loss of light intensity in the tissue, we must make a slight calculation, based upon the depth of our target. To give the same dose at target to the root of a lower molar we must use a higher dose than for an upper lateral incisive.

Now back for a while to the wavelength. The wavelength of the curing light is set once and for all to fit the material that you are using. With lasers the wavelength is also fixed for each instrument, but there are different instruments with various wavelengths. Which wavelength is ”best”? There is no such thing as an ”optimal” wavelength, so now things become a bit more complex that the curing light. Red light is best for superficial tissues such as mucosa and skin. Teeth crowns are very transparent, too. The penetration rate increases as the wavelength increases and around 800 nm we are very ”transparent”. So infrared lasers have a better ability to reach areas such as the inferior alveolar nerve maxillary sinuses and apices. But this does not mean that infrered is not useful for superficial tissues, only that it is not optimal and dosage has to be slightly adapted.

Now we have reached ”the hot spot” – dosage! With the curing light we count seconds and that is very convenient. We can do so because the materials we are using are very similar and have the same photoacceptors. With the laser we can also use seconds, but first we have to make a little calculation, because the ”material” that we are irradiating differs from point to point: mucosa, bone, muscles etc. I will not bother you with the basics of such calculations. As with the curing light it is up to the manufacturer to supply you with a precise manual, indicating the number of joules given per each second of irradiation and the dose then applied per second, when the laser probe is held in contact with tissue. Such an easy-to-read manual should be part of any laser investment in your clinic. Anyway, the problem is to understand the difference between “joule” and “dose”. Joule (J) means energy. If the output of the laser is 50 mW and you irradiate for 10 seconds, the energy applied is 50 x 10 = 500 millijoules (mJ) = 0.5 J. A laser of 100 mW used for 5 seconds produces 100 x 5 = 500 mJ = 0.5 J. This calculation is easy. But joule is not the dose! The dose is the energy in joules, divided by the irradiated area. So if the energy is 4 J and the area is 1 cm^2 , we have 4/1 = 4 J/cm^2. But if the area is only 0.25 cm^2, then the calculation is 4/0.25 = 16 J/cm^2! This is because the intensity increased; all those photons were added to a smaller area. This is a little complicated, admittedly. If you are going to perform a scientific study, these matters are very important. In the clinical situation, however, we can use the less exact term “energy per point”. A “point” is then about 0.5 cm^2 and we only count the energy in joules added to that imaginary point. In your clinical notes you can just add “J per point”.

Now, what about the mechanisms behind laser phototherapy? Well, I cannot say that they are easy to understand. But on the other hand, do you know the mechanisms behind composite curing, ultrasound and electrosurgery? If you do, congratulations, but as for myself I only have faint knowledge about these things but I find no problems in applying these techniques daily. A deeper understanding of the mechanisms behind everything we do is an advantage but not always necessary for having good results. Many of my colleagues using therapeutic lasers started with a rather limited understanding of the mechanisms but along a successful road of therapies later on had a reborn interest in looking into these matters in detail.

Summing up, the confusing laser parameters were not so confusing, after all. Old knowledge is still applicable. If you later on would be interested in having a closer look at what is really happening in the laser-tissue interaction, you will see that it is indeed a very complicated field. But in the meantime, the clinical application is pretty straightforward.