Lasers in Dentistry Part 2: Laser-tissue interactions

Laser Dentistry

Dr Ilay Maden BDS MSc PhD

Part 2

Laser-tissue interactions

Tissues can be affected by laser energy in different ways. With low levels of laser energy, the tissues' biomodulation is possible, which is a photochemical interaction. With more power, coagulation, vaporization and ablation of tissues are realized as photothermal interactions with tissues.  With too much power and too much heat accumulation, even the tissue's carbonisation, which is a misuse of laser, may be observed. Biomodulation has many synonymous terms, including: “biostimulation”, ‘‘low power laser therapy”, ‘‘low-level laser therapy’’, ‘‘low-intensity laser irradiation’’, “low-intensity laser therapy”, ‘‘photobiomodulation’’, ‘‘low energy laser irradiation’’ “soft laser therapy” and “laser therapy’’ (including the laser power output between 0.005 and 0.5W) carried out by a ‘‘non-thermal laser’’, also known as ‘‘cold laser’’ or a ‘‘soft laser’’. While the aforementioned laser devices do not produce laser powers more than 0.5W, other “surgical” lasers, which can, can be set to produce this low-intensity laser to provide biomodulation. The biomodulation effect of laser was first observed in 1967 by Endre Mester (Mester et al. 1968) at Semmelweis University in Budapest, Hungary, a few years after the first working laser was invented. While applying lasers to the backs of shaven mice, experimenting with lasers' effects on skin cancer, he noticed that the shaved hair grew back quicker on the treated group than the untreated group. Low-level laser therapy (LLLT) seems to stimulate mitochondrion and increase adenosine triphosphate production (ATP), increasing reactive oxygen, affecting cells' proliferation. Increased proliferation affects the process of wound healing(Mester et al., 1972). Additionally, laser therapy has positive effects like regeneration, inflammation reduction, analgesia, and enhanced immune function  (Da Silva et al. 2010).

Photodynamic therapy (PDT) is another indication of laser use. This technique also has many names, such as antimicrobial photodynamic therapy (aPDT), photodynamic antimicrobial chemotherapy (PACT), photodynamic disinfection (PDD) and photoactivated disinfection (PAD). PDT is a form of phototherapy using nontoxic light-sensitive compounds that are exposed selectively to light, and they kill bacteria, fungi, parasites and viruses. PDT applications involve three components: a photosensitizer, a light source and tissue oxygen. In air and tissue, molecular oxygen occurs in a triplet state and oxygen is relatively non-reactive at physiological conditions. A photosensitizer is a chemical compound that can be transferred into the excited state upon absorption of light and produce singlet oxygen. Singlet oxygen rapidly attacks any organic compounds it encounters, thus being highly antimicrobial. In addition to some cancer treatments, PDT is used to treat acne, wounds and other infections. In dentistry, it used to disinfect periodontal (de Oliveira et al. 2007) and peri-implant pockets, root canals, apicoectomy sites and in the treatment of lichen planus (Agha-Hosseini and Pour 2014) and stomatitis.

Thermal effects apply to higher output powers; >1W. Example of these effects are haemostasis at 60^oC and vaporization at 100^oC (Coluzzi 2004). Due to increased temperatures on the tissue, blood and lymphoid vessels are sealed while the tissue is vaporised. The bactericidal effect is also simultaneous. Ablation is the two-step process of removal of tissue through vaporization and mechanical disruption. Vaporisation creates a negative pressure around the molecules, which causes the deportation of surface substance. If this is water-mediated, the thermal effect is on this mediator covering the tissue, protecting the tissue from the harmful temperature rise. To have “thermo-mechanical ablation”, a high amount of energy is needed to be delivered in a short time, hence the need for pulse durations of erbium lasers to be as short as possible to be able to remove hard tissue. Carbonisation is the “burning” of the tissue at 200^oC (Coluzzi 2004), which causes a delay in healing due to the thermal-side effects in a wide area and post-operative discomfort, so it should be avoided.

The first prerequisite to have these effects on tissues is to interact between the target tissue and the chosen laser. Their wavelengths differentiate the lasers. As a part of the electromagnetic wave spectrum (Fig 6), lasers propagate in the form of sinusoidal waves, which repetitively oscillate and the distance over which the wave's shape repeats itself is called the wavelength of the laser. This wavelength gives us the energy of the photon, which is the energy-package of light. Depending on the compatibility of this photon's energy and the tissue's optical properties, an interaction may or may not occur (Franzen 2010). That is why no one laser can carry out all possible indications with dental lasers. Some lasers can only remove soft tissue but have some indications on hard tissues, while some other lasers can remove both hard and soft tissues, but they lack some effects that other kinds of lasers have.

If there is no absorption taking place, there is mainly transmission, leading to penetration of the energy. This is very important because if we cannot visualise the effect we would like to see on the tissue, that means the laser energy is not enough to make a visible change on the tissue. Still, the energy will accumulate to have some side effects on deeper tissue. Other possibilities of light-matter interactions are reflection and scattering. Reflection means the energy is bounced back without entering the matter and is the reason we must always protect our eyes which are sensitive to light, with filtering goggles. Scattering usually takes place at a neglectable proportion.

The most used lasers in dentistry are erbium lasers; erbium-doped yttrium aluminium garnet [Er:YAG (Er:Y₃Al₅O₁₂)] with a wavelength of 2940 nm, and erbium, chromium: yttrium-scandium-gallium garnet [Er,Cr:YSGG (Y_2.93Sc_1.43Ga_3.64O₁₂)] with a wavelength of 2790 nm, diode lasers with wavelengths of   ̴635nm, 810nm, 940nm and 980nm, neodymium-doped yttrium aluminium garnet (Nd:YAG [Nd:Y3Al5O12]) and carbon dioxide lasers (CO₂) with 10600nm.

The chromophores of oral tissues are water, hydroxyapatite, haemoglobin/oxy-haemoglobin or melanin. That means the presence or absence of these will determine if there will be an interaction (absorption of the optical energy to be transferred into chemical or thermal energy) or not. Near-infrared lasers, 635nm, 810nm, 940nm, 980nm diodes and 1064nm Nd:YAG,  are well-absorbed in melanin and haemoglobin. This feature allows us to work with haemostasis. 980nm diodes are absorbed about 10 times, and Nd:YAG laser about 3 times more than 810nm diodes. This makes 980nm diodes interact more with the tissue's water content (Cecchetti et al. 1996). Both erbium lasers and CO₂ lasers are absorbed in water and hydroxyapatite significantly more than the near-infrared lasers. Er:YAG  (µ_a = 13000 cm^-1)is absorbed in water almost double the amount as Er,Cr:YSGG (µ_a = 7000 cm^-1) is absorbed, making the interaction more intense with the same parameters.

The ability of laser light energy to vaporize tissue depends on the absorption level of the wavelength by the target tissue chromophores, the spatial concentration (fluence; J/cm²) and temporal concentration (the amount of time the energy is being emitted into the tissue, which is the pulse duration) of energy. An amount of energy can provide biomodulation if spread to a large area and cause carbonisation if concentrated to a tiny spot. Likewise, an amount of energy may even remove enamel if transported to the tissue in 50µs, while it can only heat-up if transferred in 1000µs.  Knowing these parameters and their effects prepares a practitioner to be a successful laser dentist and experience lasers’ benefits.

Classification of lasers

There are a few different classifications for dental lasers. One of them classifies lasers according to their location on the electromagnetic spectrum. That shows that all dental lasers except for the ones used for Photodynamic Therapy and Low-Level Laser Therapy (  ̴ 635-665nm) are infrared lasers. That means they have a wavelength of more than   ̴700nm (which would be red) and are invisible. Our eyes can only see the light with wavelengths roughly between 400 and 700nm. One may say, “I can swear that I saw the red laser while it was used!” however that actually is the additional “guide beam” used to show where the actual laser is aimed at, not the real laser at work. If you dig deeper, there are subgroups of infrared near, mid and far, but this information alone does not help dentists who want to use lasers much.

There is a classification according to the potential damage lasers can cause. All surgical lasers have an average power of more than 0.5W, placing them in Class IV and making it mandatory to wear protective goggles for anyone in the same room when they are active.

Another “classification” of dental lasers is grouping them as “hard” or “soft”. Soft lasers are used in biomodulation and Photodynamic Therapy (PDT). So they are soft because they can’t be used surgically. They are “too soft”, so they are non-invasive, while “hard” lasers are used surgically to remove or cut through tissue.

One method to classify dental laser is according to their target tissue. As an example, many of you may know that diode lasers are called soft tissue lasers. Indeed, they can only remove or cut through soft tissue; however, they are also used to treat hard tissues, like disinfection of root canals and deep dentin for endodontics, de-sensitization of sensitive teeth, disinfection of infected bone defects etc. Erbium lasers are called hard tissue lasers, whereas they are fully capable of removing soft tissues too.

Another widely used classification is hot/cold lasers. This concept is based on the temperature rise in the tissues. Again this is not very accurate because, to remove any tissue, we need to have optical laser energy to transfer into thermal energy. Diode and Nd:YAG lasers are already known to be hot lasers since they interact with the haemoglobin and melanin in the tissue, vaporising the tissue. On the other hand, erbium lasers interact with water, so when we have the water spray on, the energy is absorbed by that water, resulting in energy not reaching the tissue but removal of tissue by a series of mechanisms called thermo-mechanical ablation. Hence the tissue is left cold. However, if we were to turn off the water spray and use erbium lasers on hard tissue, we could easily observe that erbium lasers are hot on calcified tissues as the energy is now absorbed in the hard tissue surface, making them hot lasers.

Two of the less encountered classifications are the delivery systems of laser devices; fibre, articulated-mirror arm or hollow waveguide and operation mode; contact or non-contact.

Lasers are also grouped according to the state of the medium used to produce the laser light; solid - semiconductor/crystal, liquid or gas.

The last way of grouping may be done through the lasers' active mediums, where they also get their names from. We may say Er:YAG (2940nm) and Er,Cr:YSGG (2780nm) lasers are erbium lasers, 810nm, 940nm, 980nm lasers are diode lasers. Although there is nothing wrong with this in general terms, clinically speaking, this is as good as saying I have a house. Whether it is a penthouse, country house, the urban apartment is vague. This is because even though they are built similarly, each wavelength interacts uniquely with different tissues.

There is not a best way to group dental lasers, exactly summarizing their clinical use. It is the best practice for practitioners to know the wavelength – oral tissue constituent interaction and think more specifically on a laser wavelength instead of classifying.

Table 3: Terminology

Ablation: The two-step process of removing tissue with a laser through vaporization and disruption.

Absorption: Conversion of light energy into other forms of energy - mainly thermal or chemical - affects the tissues differently. Chromophores are the determiners of the optical properties of tissue to absorb the photons of light.

Active medium: The part of the laser machine that the laser light is produced in. Dental lasers have crystals (Er:YAG. Er,Cr:YSGG, Nd:YAG), gas (CO₂), or semiconductors (Diodes) as their active media.

Beam transfer: Mirrors (in articulated arms), optical fibers, or hollow waveguide are the options to carry the laser beam from the machine to the handpiece and tissue.

Chromophore: The tissue component that absorbs the laser energy. Like water, hydroxyapatite, haemoglobin or melanin.

Coherence: Symmetrical alignment of laser light waves’ peaks and valleys.

Continuous-wave (CW) mode: Continuous laser emission.

Divergence: The laser beams spreading, defocusing; depends on the specific laser and optics used.

Energy density: The concentration of energy in an area.

Er,Cr:YSGG: Erbium, Chromium doped Yttrium Scandium Gallium Garnet crystal.

Er:YAG: Erbium-doped Yttrium Aluminium Garnet crystal

Free running pulsed mode: Laser emission in isolated pulses with specific temporal characteristics.

Gated wave mode: Continuous wave laser light emission with the beam being blocked intermittently by a shutter or electronic device creating a pulsed laser emission.

Joule: Energy unit                

Laser: Light Amplification by Stimulated Emission of Radiation

LSO: Laser Safety Officer

Monochromatic: Having a single wavelength - colour - of light.

Nd:YAG: Neodymium-doped Yttrium Aluminium Garnet crystal

Optical pumping: Excitement of an active medium by flash lamps.

Power: Energy per time.

Resonator: The laser's enclosed part where the active medium produces laser light, which is amplified in between two mirrors.

Stimulated emission: Light emission by stimulation of an electron in its excited state by a photon to return to lower levels.

Thermal relaxation: Cooling time – heat dissipation time - of the tissue.

Transmission: Penetration of light into the tissue.

Watt: Power unit.

References

Agha-Hosseini F, Pour NM (2014) Photodynamic treatment of oral lichen planus. Oral surgery, oral medicine, oral pathology and oral radiology 117:257.

Cecchetti W, Guazzieri S, Tasca A (1996) 980-nm diode laser and fiber optic resectoscope in endourological surgery. BiOS

Coluzzi DJ (2004) Fundamentals of dental lasers: science and instruments. Dental clinics of North America 48:751-70, v. Da Silva JP, Da Silva MA, Almeida APF, et al. (2010) Laser therapy in the tissue repair process: a literature review. Photomedicine and laser surgery 28:17-21.

Franzen R (2010) Principles of Medical and Dental Lasers.

da Silva JP1, da Silva MA, Almeida AP, Lombardi Junior I, Matos AP. Laser therapy in the tissue repair process: a literature review. Photomed Laser Surg. 2010 Feb;28(1):17-21.

Mester E, Szende B, Gärtner P (1968) [The effect of laser beams on the growth of hair in mice]. Radiobiologia, radiotherapia 9:621-6.

Mester E, Szende B, Spiry T, Scher A (1972) Stimulation of wound healing by laser rays. Acta chirurgica Academiae Scientiarum Hungaricae 13:315-24.

de Oliveira RR, Schwartz-Filho HO, Novaes AB Jr, Taba M Jr (2007) Antimicrobial photodynamic therapy in the non-surgical treatment of aggressive periodontitis: a preliminary randomized controlled clinical study. Journal of Periodontology 78:965-73.