Lasers in medical practice
Advantages of medical lasers
In recent years dentistry (following the trend of the whole medical field) took advantage from technological improvements aiming to make these treatments minimally invasive while granting the best possible results to the patient.
Dental laser systems gives to the dentist a great help giving him several invaluable tools to ease different precision surgical procedures. Compared to traditional tools, dental lasers can drastically reduce patient pain, bleeding, and contamination of the treated area. Some of the operations can be conducted even without anesthetics, and, when properly used, anxiety of the patient is reduced and healing times are shorted.
Before choosing a dental laser and a specific treatment, it’s important to know how the light interacts with the tissues and some basic concepts about medical usage of radiant energy.
Information provided here is indicative only and it can contain mistakes or omissions. Correct device usage is dictated in product manual and reference clinic literature.
Absorbed and Luminous Power
Each light source, from light bulbs to medical lasers, emit a certain quantity of luminous energy, lower than the energy it is supplied with. The ratio between luminous emitted power and input power is called efficiency. Laser diodes have different efficiency that mostly depends on wavelenght.
Laser power is expressed in Watt. In simple terms, about 70 Watts are needed increase the temperature of 1 liter of water of 1°C. Watts are units of power, or energy emitted on every second. So, a 1W laser source releases 1J (Joule) of energy per second. In order to emit a 1W laser beam, a diode with 50% efficiency would require 2W of electrical energy. The balance is wasted as heat. Thanks to technological evolution it is possible to build new laser types able to emit light with better efficiency.
Laser power is mostly defined by two parameters: continuous mode power and pulsed mode power. The former tells the power that the system is able to withstand when emitting a continuous light beam. The latter, much higher, is the maximum power that the device can emit in a very short pulse.
Wavelength and tissue absorption
As already stated in previous article, each tissue can absorb, reflect or transmit received laser light in a different manner. The graph below shows an approximated logarithmic curve of the absorption of the light spectrum of oxyhemoglobin, melanin and water.
White bands show typical dental laser wavelengths. Each wavelength is used for a specific application. It’s obvious that water (transparent to visible spectrum) has a very low absorption of wavelength between 400 and 700nm. By contrast, melanin and hemoglobin have higher absorption in the spectrum band between 400 and 900nm.
When a tissue receives a large amount of light it heats up until it gets vaporized. This opens a clean and sterilized cut. Since tissues are not incised, most structures are preserved, thus bleeding is limited and nerve endings are minimally stimulated. Each tissue has its own properties, a choice of five parameters is essential for success: treatment time, light beam power, angle of incidence, distance between source and tissue, and wavelength.
Wavelenght: 808 or 980nm?
When choosing a dental laser, the first parameter to decide is the wavelength. Most lasers for dental surgery available today emit light with 808 or 980nm. As stated in previous section, emitted wavelength depends on laser diode construction (mostly, junction materials). Each semiconductor has specific construction implications, including laser diode structure, efficiency and cost.
A typical 980nm with the same power consumption of an 808nm, emits about twice the light (it has nearly twice the efficiency of the latter). Manufacturers can build smaller and cheaper lasers with high declared output power. However, when laser diode is used for surgery, tissue light absorption (similar to hemoglobin absorption) is about 5 times higher for 808nm when compared to 980nm. This is the reason why, for soft tissues, an 808nm grants better results with lower power. A third wavelength that is widely used in dental lasers is near water light absorption peak (2940nm). This light is emitted by Erbium lasers (YAG). Very short pulses warm up the superficial water to the evaporation in a fraction of a second. Expanding water molecules causes disintegration of a thin superficial layer of the tissue without touching anything below. Heating of the nearby areas is not noticeable and the results are limited to the outer layer. This property allows this laser type to be used to treat hard tissues (such as caries), mostly without anesthetics and without drilling (avoiding micro cracks in the tooth).
Most dental laser emitters are uncollimated. This means that, after a distance of some millimeters, the beam opens with an angle of about 30°. When it is used for surgery, the laser spot should be kept as small as possible (contact application or with minimal distance). For aesthetics, regeneration or whitening, light beam has to be diffused on the surface under treatment. This is the reason why there are different handpieces with optics and lenses for different applications.
Angle of incidence changes the amount of absorbed and reflected light. Maximum absorption is possible when light emission is perpendicular to the surface. By tilting the light source, instead, light power touching the surface is decreased.
Application time considers both pulse type and total treatment time. A very short pulse with high power and a long pulse with lower power could carry the same energy but give very different results (similar to the difference of emptying a water bucket in small drops or all at once).
Laser systems are designed to allow the user to choose both parameters. Manufacturer or clinical literature, together with the experience, allows the operator to set each parameter to obtain the best results for the treatment of choice.
When evaluating laser emitted radiation a reference parameter is the fluency. It is the emitted light energy divided by the unit of surface. It is automatically calculated by the laser device as the product between emitted light power and emission time, divided by the surface intercepted by the beam.
Fiber and Handpiece
Handpiece and fiber type are essential choices too. Some examples in the images below:
TIP Handpiece. It is the usual handpiece for intraoral surgery and other surgical applications. Termination is composed by an interchangeable fiber tip with bendable head. Tips are designed also for decontamination purposes, when using spherical emission tips.
It is used for whitening, dermatology, and aestetics.
It maintains the spot diameter when the distance changes. It is used when the surface has to be treated in the same way on an area that is larger than the beam.