5.1 Introduction

Lasers have gained immense use due to the optimization of the wavelength for human tissue interactions. Initially, dental lasers were primarily used for soft tissue surgical procedures, such as excisions and incisions. Instruments then evolved for use on both hard and soft oral structures [1, 2]. Photobiomodulation (PBM) is now universally used as a substitute for low-level laser therapy, has been shown to have a positive effect on living tissues, and as such have been used as a noninvasive alternative in the treatment of various pathological conditions [3, 4].

The biomedical and therapeutic application of the photonic energy is based on the light-tissue interactions leading to laser absorption by cellular components such as mitochondria cytochromes and endogenous chromospheres, which give rise to phenomena such as fluorescence, chemical reactions, and thermal effects [5].

PBM is NOT a thermal interaction in the sense of significantly raising the tissue temperature. These laser therapies often involve red blood cells due to the high optical absorbance of hemoglobin; thus, have proved to be the “most versatile” [6, 7]. The red blood cells are also found in many organs in high concentration. Therefore, the damaging effect of these irradiations occurs at the cell level since hemoglobin is located in red blood cells [8]. The damage is thermal and is related to mechanical stress, vapor bubbles, and overheating because of irradiation [9]. These physiological phenomena stimulate other processes in the cell such as enzyme inactivation, alteration of metabolic rate, and coagulation among other structural changes in the cell [10].

Important effects of low-energy laser light have been highlighted in the scientific works of literature, which are tissue regeneration, reduction of inflammation, pain relief, and immune system enhancement [11]. The low-energy intensity lasers have in recent years being used in the noninvasive treatment of musculoskeletal and cutaneous complications and have also gained increased use in wound healing, nerve repair, and pain management, among other clinical applications. In diabetic conditions where wound healing is compromised, low-energy intensity lasers have shown beneficial effects of healing impaired wounds [12, 13].

Laser used in clinical dentistry has spanned over 30 years. Prior to that, carbon dioxide lasers (CO₂, wavelength 10.6 μm) were used in general medical surgeries and subsequently were applied to the soft tissue surgical procedures within the mouth [14]. Neodymium YAG (1.064 μm) was the first dental laser which was launched in 1989, and from there the next 10 years witnessed the emergence of different major wavelengths Er:YAG and Er,Cr:YSGG and diode semiconductor-based technology [15, 16].

5.2 Laser and its Components

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation, which is based on theories and principles first put forth by Einstein in the early 1900s.The first actual laser system was introduced by Maiman in 1960 [17].

Laser energy is a man-made product and consists of photons of a single wavelength. The process of lasing occurs when an excited atom is stimulated to remit a photon before it occurs spontaneously; spontaneous emission of light results in unorganized light waves similar to light emitted by a light bulb [18]. Stimulated emission of photons generates a very coherent, collimated, monochromatic ray of light that is found nowhere else in nature. As that light is so concentrated and focused, it can have a decided effect on target tissue at a much lower energy level than natural light. The effect of laser energy on target tissue is dependent on its wavelength, which is determined by the lasing medium inside the laser device [19].

Laser irradiation is a type of electromagnetic radiation which exhibits the properties of a wave, and discrete energy packages referred to as photons. The short wavelengths such as high-energy ionizing irradiation cause the ionization of molecules in an indiscriminate manner with the far-infrared heating biological tissues [20, 21].

Gas discharge lasers include the common type: helium neon lasers, carbon dioxide lasers, noble gas lasers. Semiconductor lasers include the high-power diode laser. The optically pumped laser uses photons of light to pump directly the lasing medium to higher energy levels [22, 23]. Lasers have their basic components, which are the power source, which provide energy, lasing medium (which is the active medium and can be solid or gas), and reflecting mirrors (two or more in number that forms an optical cavity or a resonator) with production of light of particular wavelength, which defines the type of laser as shown in Fig. 5.1.

The optical cavity contains the active medium and reflective surfaces. Initial energy is provided by an excitation source. Photons within the active medium are reflected, amplified, and then collimated. Those are emitted into a lens assembly, which then produces the useful laser beam [24].

In a diode laser, the semiconductor medium is a wafer that contains layers of positively and negatively charged compounds, bounded by reflective coating. Several of these wafers are grouped together to produce a useful laser beam [25, 26].

Laser radiation is specific wave generated and is a highly focused directional beam as opposed to the visible light which is mainly white light and nondirectional and nonfocused. Commercially available dental laser instruments all have emission wavelengths in the range of 488 nm to 10,600 nm [27, 28]. All are of nonionizing radiation and span the visible, near, mid, and far-infrared portions of the electromagnetic spectrum as shown in Fig. 5.2.

5.3 Laser Delivery Systems and Laser Emission Modes

Laser energy can be delivered to the surgical site by various means that are accurate and precise. These include the fiber-optic system, hollow fiber, articulated arm delivery system, and handpieces [29, 30]. See Figs. 5.3 and 5.4 below.

Lasers in the visible (445 and 532 nm) and near-infrared (from 810 nm to 1064 nm) range use fiber-optic strands by means of a handpiece with straight and precise tips that deliver the laser energy to the target tissue. This type of delivery system can degrade with time but is lightweight with a good tactile sensation and is easy to use and sterilize. Some newer models have more rugged fibers [31].

Erbium and CO₂ devices are lasers with more rigid glass fibers, semiflexible hollow waveguides, or articulated arms. However, there is a loss of energy over time with lack of control due to internal reflection, with these systems [5]. For hard tissue procedures, a water spray is used for cooling, and that can be switched off for soft tissue surgery. In addition, lasers with articulated arm delivery systems utilize a progression of verbalized mirrors (generally 7) associated one to each other, prompting transmission of vitality [32, 33].

5.4 Types of Tissue Interactions

When laser light is exposed to the tissue, it can reflect, scatter, be absorbed, or be transmitted to the surrounding tissues. Different wavelengths have different absorption coefficients, and this property accounts for their variable effect on human tissue as shown in Fig. 5.5. The curves represent hemoglobin, melanin, water, and tooth enamel (carbonated hydroxyapatite.) Above the light green line, there is absorption, and below it is transmission of the photonic energy [34, 35].

The hydroxyapatite crystals of teeth and bone absorb carbon dioxide, and the erbium family’s photons. The visible and near-infrared wavelengths have virtually no interaction with either enamel or water. The erbium family is very highly absorbed in water, while carbon dioxide is about 10 times less absorbed [21]. All wavelengths have different depths of penetration through soft tissue because of the different water absorption characteristics. Diode lasers can reach deeper into the tissue approximately several thousand layers, whereas the erbium lasers are absorbed on the surface [33, 36].

Lasers can have various effects on the target tissue. One effect is photothermal: simply stated, light is converted to heat. When the tissue absorbs the photonic energy used during surgical procedure, it elevates the temperature. When a temperature of 100 °C is reached, vaporization of the water within the tissue occurs. This process is called ablation. At temperatures between 60 and 100 °C, denaturization of the proteins starts occurring, without dehydrated and then burned, resulting in an undesirable effect called carbonization or vaporization [37, 38].

Nonsurgical, low-level power laser applications include photo-bio modulation (PBM), diagnostics, photo-activated antibacterial processes, laser tooth whitening, and laser scanning of tooth cavity preparations [39]. Another effect is photochemical effects such as curing of the composite resin [40]. Lastly, some lasers can also produce tissue fluorescence, which is used as a diagnostic method for caries detection [41, 42].

5.5 Lasers in Dentistry

The use of LASER technology in the dental industry is a subject of continuous researches and advancements. In Fig. 5.6, a systematic review on how the laser interacts with the dental tissue is shown.

5.6 Lasers Generally Practiced in Dentistry

5.6.1 Carbon Dioxide Laser

CO₂ laser’s photonic energy has a very high affinity for water; it causes rapid soft tissue removal and hemostasis. The penetration of depth is relatively shallow. A newly available model of CO₂ with ultrashort pulse duration can be used for hard and soft dental tissue.

5.6.2 Neodymium Yttrium Aluminum Garnet Lasers

Absorption of this wavelength is high in pigmented tissue; therefore, it is sufficient for cutting and coagulating dental soft tissues, with high-grade hemostasis. Other uses include nonsurgical periodontal therapy.

5.6.3 Erbium Laser

The family of erbium lasers has two distinct wavelengths, Er, Cr: YSGG lasers and Er: YAG lasers. Erbium wavelengths have a secondary affinity for hydroxyapatite crystals and the most powerful absorption of water in any dental laser wavelength. It is the laser of selection for treatment of dental hard tissues, and can be used for soft tissue ablation because the dental soft tissue comprises a large percentage of water.

5.6.4 Diode Laser

The diode laser is a solid-state semiconductor containing various combinations of aluminum, gallium, arsenide, and occasionally indium producing different laser wavelengths, varying from approximately 445 to 1064 nm. These wavelengths are absorbed principally by tissue pigment melanin and hemoglobin, and inadequately absorbed by hydroxyapatite and water.

5.7 Soft Tissue Application

5.7.1 Photobiomodulation

It is also known as “soft laser therapy” and is based on the concept that low level of doses of specific coherent wavelengths can turn on or turn off certain cellular components or functions. At low laser doses (2 J/cm²), laser utilization stimulates proliferation, while at large doses (16 J/cm²) it is suppressive [43, 44]. This technique induces analgesic, anti-inflammatory, and biomodulation effects at molecular level improving tissue healing processes and less postoperative discomfort for patients, without any negative effects [10, 45]. This technique is most useful in medically compromised patients where wound closure and tissue healing is of prime importance.

LLLT (low level laser) results in vasodilation of the cells, which causes increase in local blood flow, relaxation of smooth muscles, thus bringing in oxygen and further migration of immune cells to the targeted tissue. This in turn results in rapid maturation and regeneration [9, 46]. Accurate effects of PBM on the healing of lesions of recurrent aphthous stomatitis in humans have been recorded. Research has shown that LPL encourages healing and dentinogenesis following pulpotomy. This makes it useful in the healing of mucositis and oropharyngeal ulceration in patients enduring radiotherapy for head or neck cancer [47, 48].

Photostimulation of aphthous ulcers and recurrent herpetic lesions, with low power laser energy (He-Ne), can provide pain relief and accelerate the healing process. In recurrent herpes simplex labialis lesions, lesions get arrested before painful vesicles form, thus expediting the overall healing time, and minimizing the frequency of recurrence (Figs. 5.7 and 5.8) [49–51].

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