Laser–Hard Tissue Interaction

Fig. 4.1

Laser–tissue interaction: laser light can be reflected, scattered, absorbed, or transmitted
  • Reflection is an optical phenomenon resulting from the lack of affinity of the light with the target tissue, which will repel it. The proportion of laser radiation reflected is generally low (5 % of the emitted radiation) and represents that aspect of laser therapy that involves the safety measures (safety glasses with filter for each specific wavelength). Laser radiation is in fact potentially harmful to the structures of the eye (retina, cornea, lens, aqueous humor) (see Appendix).
  • Absorption is, on the opposite, the expression of high affinity between the light and the target tissue, which will keep the light in the point of interaction. The portion of the laser energy absorbed by the tissues is responsible for the majority of the therapeutic effects: it is converted locally into photochemical energy, photothermal energy, and photomechanical–photoacoustic energy, depending on the type of laser, the parameters, and the emission mode used.
  • Scattering, typical of a limited spectrum laser light (in the visible and especially near-infrared spectrum), is the ability of the light to spread more in depth and in a disorderly manner in the target. The portion of light energy diffused in the tissues is responsible for the photochemical and/or photothermal effects of these laser wavelengths, which allow, for example, biostimulation and decontamination effects deeply in the tissue, at a distance from the point of interaction.
  • Transmission is the passage of light through a non-affine tissue or body without interaction on it and therefore without producing physical or biological effects.

4.1 The Wavelength

The first laser to be studied for application to the hard dental tissues was the ruby laser (visible red at 694.3 nm) [1]. The lack of cooling effect and the low affinity of the ruby laser for hard dental tissues led to discouraging results for the ablation of dental tissues (charring, melting, and cracking), and this wavelength was soon abandoned in restorative dentistry.
Subsequently, the carbon dioxide (CO2) laser (in the far-infrared spectrum of light, at 10,600 nm) [2], the excimer (XeCl) laser (in the ultraviolet spectrum of light at 308 nm) [3], and the neodymium:yttrium–aluminum–garnet (Nd:YAG) laser (in the near-infrared spectrum of light, at 1,064 nm) [46] have been studied for use in restorative dentistry, with poor clinical outcomes because of the poor absorption in the enamel and dentin and the excessive damage caused by heat [7, 8].
Due to the absorption of Nd:YAG laser in pigmented areas, it was proposed in the past to apply pigment or dark ink to the enamel of pits and fissures of posterior teeth for sealant or class 1 cavity preparation. The unwanted thermal damage on the irradiated surface (cracks, melting, bubbles, and recrystallization of mineral dental structures) discouraged the continued research on the use of Nd:YAG laser in restorative dentistry [9, 10].
The lasers in the near-infrared light spectrum (from 810 to 1,340 nm) have high affinity for hemoglobin and melanin but minimal or absent affinity for hard dental tissues. When irradiating a tooth with safe clinical parameters, there is no ablative interaction with the enamel and dentin, but only the release of thermal energy that diffuses deeply toward the pulp tissue, rich in their absorbing chromophore, the hemoglobin (Fig. 4.2).

Fig. 4.2

Visible and near-infrared laser wavelengths in the electromagnetic spectrum of light and relative absorption in soft tissue chromophores. Mel melanin, Hb Hemoglobin, HbO2 Oxyhemoglobin, H2O water
The deep thermal effect of these wavelengths produces decontamination of a deep cavity [11] and also melting of the superficial dentin that can be carefully used for the treatment of dentinal hypersensitivity (see Fig. 4.67). Its interaction with hemoglobin also allows its possible use in vital pulp therapy (see Chap. 8).
The other wavelengths used in dentistry also have a limited utility when used in restorative dentistry.
In the visible light spectrum (from 532 to 675 nm), we find other lasers with high affinity for hemoglobin and melanin. The green light of the KTP laser (532 nm) is mainly used in surgery for its excellent ability to cut and for coagulation; in esthetic dentistry, it can be used for dental bleaching [13, 14] (see Fig. 4.2).
The lasers in the red spectrum of visible light (630–675 nm) are used for pain therapy, biostimulation, and anti-inflammatory therapy (low-level laser therapy, LLLT); for their affinity with specific photosensitizers with bactericidal activity, they are also used for the deep decontamination of the endodontic system and of periodontal pockets (photodynamic therapy (PDT) or photoactivated disinfection (PAD)) and have also been proposed for possible conservative use in deep dentin decontamination [15].
After the first experimental studies on CO2 laser at 9,300 nm [16, 17], in recent years, new studies have revived this wavelength for the ablation of enamel and dentin [18, 19].

4.1.1 Medium-Infrared Lasers

Due to the specific affinity of water with medium-infrared wavelengths [20], specifically the erbium, chromium:yttrium–scandium–gallium–garnet laser (Er, Cr:YSGG at 2,780 nm) and the erbium:yttrium–aluminum–garnet laser (Er:YAG at 2,940 nm), now called erbium family laser, these are currently the only two wavelengths capable of being greatly absorbed within the dental tissues when used with safe and accepted clinical parameters [2124] (Fig. 4.3).

Fig. 4.3

Relative absorption of erbium wavelengths in hard tissue chromophores: hydroxyapatite (orange line) and water (blue line)
The Er:YAG laser wavelength operates into the peak of absorption of water, at 2,940 nm, while the Er, Cr:YSGG laser wavelength is absorbed slightly less by water (300 % less) at 2,780 nm [23, 25] (Fig. 4.4).

Fig. 4.4

Different absorption in water of different erbium lasers
The difference in the absorption coefficients leads to a difference in the penetration depths of the two erbium laser wavelengths in dental tissues. The Er:YAG laser wavelength penetrates approximately 7 μm in the enamel and 5 μm in the dentin; the Er, Cr:YSGG laser wavelength penetrates three times deeper, 21 μm in the enamel and 15 μm in the dentin [26] (Fig. 4.5). As a result of the very superficial absorption and due to the specific optical properties of these wavelengths, the diffusion phenomenon is negligible.

Fig. 4.5

Different penetration depth of erbium and erbium, chromium laser in dentin
Furthermore, the wavelength 2,780 nm falls into the second peak of the absorption curve of the hydroxyl group of hydroxyapatite in dental hard tissues [2731], but its role in the ablation of hard tissues is secondary (see Fig. 4.3); it is the stronger water absorption of 2,940 nm (Er:YAG) and 2,780 nm (Er, Cr:YSGG) that plays the dominant role in dental laser ablation [10, 31]. Clinically, the different absorption in water of the two wavelengths is scarcely relevant in quantitative terms of speed of ablation of the hard tissues.

4.2 The Target Tissue

Substances present in most tissues of the human body, such as water, hydroxyapatite, collagen protein substances, melanin, and hemoglobin, are widely represented in the hard and soft oral tissues.
In restorative dentistry, the main target tissues are the dental hard tissue (enamel, dentin, and decayed tissue), composed of different percentages of hydroxyapatite, water, and collagen matrix. From the point of view of optical physics, these components are defined as “chromophores” and have selective affinity for the wavelengths 2,780 nm and 2,940 nm of the medium-infrared spectrum [23, 2630].
It is important to know the difference in water content of the various hard tissues (enamel, dentin, and carious tissue) and to consider the differences in composition between primary and permanent teeth to understand the different absorption coefficients and thresholds of ablation of different tissues when considering the energy setting of laser for dental ablation [23].
Because of the dental–periodontal relationship and its involvement in the proximal subgingival carious lesions, in the carious and non-carious lesions of the neck of the tooth (class 5 cavities) and in dental trauma, this chapter will also discuss the periodontal soft tissue (gum), which is also composed of a different percentage of water, protein fibrous matrix, melanin, and hemoglobin.

4.2.1 Water and Hydroxyapatite Content of Dental Tissues

The enamel is the hard and white external covering of human teeth, characterized by a prismatic structure with radial orientation (orthogonal) to the tooth surface. The enamel has a thickness ranging from 2 to 3 mm at the tip of the cusps and 1–1.3 mm on buccal and lingual surfaces. In deciduous teeth, enamel has a thickness of 1 mm or less.
A healthy enamel is a highly mineralized tissue composed of 93–96 % hydroxyapatite, 3–5 % water, and 1 % organic tissue by weight and 85 % hydroxyapatite, 12 % water, and 3 % organic tissue by volume [32] (Fig. 4.6). Some authors reported a different value for the hydroxyapatite 96–98 % by weight or 89–91 % by volume and the remaining proportion of tissue volume being occupied by the organic matrix and water [3337].

Fig. 4.6

Enamel ultrastructural morphology and erbium laser target: hydroxyapatite crystals are the core of the enamel prism (orange line); water (blue line) and organic tissues (yellow line) are on the periphery of the prism
The dentin is a mixture of mineralized collagen fibers that determine the architecture of the dentinal tubules, which extend from the pulp to the enamel (see Chap. 1).
A healthy dentin is less mineralized than enamel; it is composed of only 65–70 % of mineral, with a higher content of organic tissue (18–20 %) and water (10 %) by weight. In volume, the proportions of mineral are 45–47 %, 30–33 % of organic tissue, and 20–24 % of water [3234] (Fig. 4.7).

Fig. 4.7

Dentin ultrastructural morphology and erbium lasers target: the peritubular dentin is more mineralized (hydroxyapatite, orange line); the intertubular dentin is less mineralized and richer in water (blue arrows)
The water content of carious tissue is higher than in healthy tissues (from 27 to 54 %) depending on the stage of the caries lesion [38] and highly and selectively absorbs the medium-infrared wavelengths, resulting in a faster ablation (Fig. 4.8).

Fig. 4.8

Carious tissue and erbium laser target: occlusal face and assial section show the distribution and penetration of pit and fissure decay in enamel and dentin; the carious tissue is demineralized and richer in water (blue line)
The percentage composition in hydroxyapatite, water, and organic tissue also varies from individual to individual, depending on the age of the tooth, in case of parafunctions (grinding, bruxism), in vital teeth compared to non-vital teeth. Also, intrinsic factors, such as the fluoride content of the hydroxyapatite crystals in the enamel, influence the hardness of hard tissue and thus the speed of ablation.
The ultrastructure of the deciduous teeth is different from that of permanent teeth. In primary dentition, the enamel structure appears disorganized and the prisms are large and irregular, with a superficial layer of aprismatic tissue, explaining the whiter and more opaque aspect of primary teeth. Furthermore, deciduous teeth have higher water content, and the thickness of the enamel is thinner, with widely expanded pulp chamber. The dentinal tubules are smaller in diameter and more widely spaced with a lower number of dentinal tubules per unit area compared to permanent teeth [33, 34].
Since water is the main chromophore that absorbs the laser wavelengths of 2,780 and 2,940 nm, the different water content of dental tissues should be taken into account when adjusting the setting for laser ablation of dental tissues. Even the pulp tissue has a higher water content than hard tissues; therefore, care must be taken in deep cavity ablation close to the pulp chamber.
In addition to the absorption of erbium laser energy by water, there is a small amount of energy that is absorbed by the hydroxyl group of hydroxyapatite found in dental hard tissue [25], which is considered negligible for the purposes of clinical trials, in comparison to the higher water absorption [10, 31].

4.2.2 Water and Hemoglobin Content of Soft Tissues

Oral soft tissues are mainly composed of water, hemoglobin, melanin, and organic tissues (collagen and elastic fibers). Gingiva has a variety of types, such as keratinized and nonkeratinized gingiva and gingiva with thick or thin biotype; other differences are dependent on tissue location (free or attached gingiva), on the health of the tissue (healthy or inflamed), on vascularization and hydration, and on pigmentation [39] (Figs. 4.9, 4.10, and 4.11).

Fig. 4.9

Healthy gingiva with thin biotype
Fig. 4.10

Inflamed, highly keratinized gingiva with thick biotype
Fig. 4.11

Inflamed, keratinized gingiva
Best results will occur when the appropriate wavelength is matched to the main chromophore (water or hemoglobin or melanin) within the target tissue, maximizing the absorption.
Inflamed tissue that contains more blood and therefore more hemoglobin will react favorably (easily and faster) with wavelengths in the visible and near-infrared regions (see Fig. 4.2). Therefore, it is necessary to consider that the use of local anesthesia with a vasoconstrictor affects the vascularization, causing ischemia of the tissue. The distribution of melanin pigment varies from individual to individual, depending on the skin phototype (according to Fitzpatrick) and on race (Figs. 4.12 and 4.13).

Fig. 4.12

Fitzpatrick skin phototype 4 (moderate brown skin) shows dark pigmentation of the gingiva and mucosa
Fig. 4.13

Fitzpatrick skin phototype 6 (brown to black skin) shows black adherent and keratinized gingiva and pink mucosa
Healthy or minimally vascularized tissue, where water is the principal component, is efficiently vaporized by medium- and far-infrared wavelengths [12, 39] (see Fig. 4.3).

4.3 Mechanism of Interaction of the Erbium Family Lasers on Hard Tissues

The interaction of the erbium laser with the hard tissue is the result of a complex mechanism, which involves primarily the photothermal effect and, secondarily, the photomechanical and photoacoustic effects that occur rapidly.

4.3.1 Thermal Effect

The first effect that determines the ablative action of the erbium family laser is a direct thermal effect on the water molecules within the dentin and enamel.
The rapid temperature increase up to the boiling point of water (100 °C), trapped within the dental interstitial structure, causes an increase of pressure when it exceeds the structural tension of the surrounding tissue, leading to a micro-explosion within the tissue [29, 30, 4043]. The richer the tissue is in water, the more quickly it reacts with laser energy [23].

4.3.2 Photomechanical and Photoacoustic Effects

The phenomenon that follows the primary thermal effect and the explosion of the water molecules inside the dental tissues is a secondary photomechanical effect, with a rapid shock wave that causes an expansion of the volume of the disrupted tissue, which results in the destruction of the surrounding mineral matrix that explodes and is removed from the irradiated surface, thus removing the tooth structure [23, 25, 4143] (Fig. 4.14). The micro-explosion of the water molecules of the spray coaxial to the laser beam generates a pressure so high that it mechanically removes the hard tissues already irradiated and exploded by the effect of thermomechanical laser, thus participating in the ablative mechanism, with a cooling and cleansing effect [23, 44]. The products of the ablation of hard tissue, vaporized, go to form a suspension of microparticles (cloud), which in turn interferes with the ablation itself [45] (Fig. 4.15).

Fig. 4.14

Interaction of the erbium laser beam with the target creates a cloud of debris
Fig. 4.15

Scattering of the laser beam after the interaction with a cloud of debris, which is formed following the ablation of dental tissues (Courtesy of M. Lukac, Slovenia)

4.4 Role of the Water in Hard Tissue Ablation

The water participates in the ablative process of the erbium family laser not only as a target chromophore but also by its cleansing and cooling action, for rehydration, which affects the quality of the ablation of the hard tissue [23, 46].
The water spray appears to be important for its action on the tissue, because it removes the products of micro-explosion (cleaning effect) [47], modulates the laser energy directly absorbing it before interacting with the tissue (reducing the photothermal effect) [46], and cools the tissue (cooling effect) [46, 47]; this enables undesirable structural thermal changes of the enamel to be avoided.
An ablative “hydrokinetic” model has also been proposed; however, it is not widely accepted [4852].

4.4.1 Water Within the Dental Tissue as Absorbent Chromophore

The primary role of water in the ablation of hard and soft tissues is as target chromophore. The greater the water content, the greater the absorption of the wavelengths 2,780 and 2,940 nm.
Studies of [53] reported that only the water content of dentin significantly influences the volume of ablation (p < 0.0001) of the Er:YAG laser. On the other hand, the ablative efficiency of the Er:YAG laser on the enamel is not affected by the few (minimal) water content of the enamel. The ablation volume of the Er, Cr:YSGG laser also would not be influenced by the water content of the dentin enamel [53].
This result can be explained by the low water content of the enamel and the ablative mechanism of the Er, Cr:YSGG laser (2,780 nm) that could also involve the interaction with the hydroxyapatite rather than with the water (see Fig. 4.3) and/or even greater possible hydrokinetic effect of the water spray energized by the laser [4850].
The concept of selectivity of the interaction of the laser with the tissue or material richer in water (enamel, dentin, decayed tissue) allows the management both of the mini-invasiveness of the procedure and its speed, with the modulation of the applied energy. In this regard, Lizarelli et al. (2003) compared also the ablation rate between composite resins and dental hard tissues (enamel and dentin) after Er:YAG laser irradiation, to distinguish the energy used to remove the composites so as to protect the dentin and enamel from unwanted ablation; while the idea of ultraconservative dentistry seems to be fully applicable for the enamel, it is not applicable for the dentin, because the dentin’s composition and water content make the Er:YAG laser ablation equal or superior in rate compared with the used resins (nano- or microfilled, microhybrid, and condensable) [52].

4.4.2 Water Spray as a Cleaner and Cooler

The water spray is important not only for being the main absorber and for its possible ablative action [48, 51] but also, above all, for its ability to remove the products of micro-explosion (cleaning effect), cool the tissue (cooling effect), and modulate the laser energy acting on the tissue (reducing the photothermal effect) [46]; this enables to avoid undesirable structural thermal changes in the dentin and enamel.
Many studies demonstrated the importance of the water spray to avoid micro- and macrostructural damage to the enamel, dentin (cracks, melting, and bubbles produced by thermal damage, with melting and recrystallization) [5460], and pulp tissue [61].

4.4.3 Water Spray’s Effects on Pulp Temperature

The interaction of erbium lasers with the water content of dental tissues and the instantaneous rise in temperature (up to 100 °C) within the tooth itself during the laser ablation process may be a concern, and unanimous consent considers water cooling to be mandatory for the safety of the pulp during the ablation of dental tissues.
One of the first studies to evaluate the safety of Er:YAG laser ablation of dental tissues was carried out by Dostálová et al. (1997), which evaluated, in vivo, on human premolars scheduled for extraction during orthodontic therapy and the pulpal response to Er:YAG laser cavity preparation. After extraction, the teeth were processed for light microscope observation that revealed no inflammatory reaction in the pulp and showed normal vascularity with the odontoblasts presenting the usual starlike cell shape [54].
Eversole et al. (1997) found no pulpal inflammatory responses either immediately or 30 days after Er, Cr:YSGG cavity preparation [62], and also Rizoiu et al. (1998) showed that pulp temperature did not increase and even decreased by 2 °C during tooth preparation with an erbium, chromium:yttrium–scandium–gallium–garnet (Er, Cr:YSGG) laser system. As a comparison, conventional burr preparation resulted in a 3–4 °C rise [63].
Glockner et al. (1998) confirmed the temperature drop after a few seconds of erbium:YAG laser preparation, from 37 to 25 °C to 30 °C, due to the water spray’s cooling effect. In comparison, conventional preparation showed a higher rise in pulp temperature [64].
However, Armengol et al. (2000), Louw et al. (2002), and Cavalcanti et al. (2003) found no significant difference in the Er:YAG laser and high-speed handpiece groups when water spray was used to prepare class 5 cavities [6567].
Other studies investigated the in vitro intrapulpal temperature variation during Er:YAG laser ablation. Oelgiesser et al. (2003) reported a rise in temperature that was lower than 5.5 °C (degrees Celsius) which is considered as the critical value for pulp vitality [68], while Attrill et al. (2004) reported a rise in temperature that was lower than 4.0 °C [69].
Other studies compared the intrapulpal temperature increases produced by a high-speed turbine and Er:YAG laser and concluded that Er:YAG laser generated a lower temperature rise but without statistical differences with both low- and high-torque handpieces groups [70, 71].
Krmek et al. (2009) examined the temperature variations in the pulp chamber during cavity preparation with an Er:YAG laser (2,940 nm) using a very short pulse duration (100 μs), at different depths (enamel and dentin) and different settings with a 1-mm-diameter tip. The highest rise in temperature in the pulp was achieved after enamel irradiation with 400 mJ and 15 Hz (2 °C) and the lowest was after irradiation with 320 mJ and 10 Hz (0.7 °C).
In dentin, the highest temperature increase was achieved with 340 mJ and 10 Hz (1.37 °C) and the lowest was with 200 mJ and 5 Hz (0.43 °C). It appears evident that both energy level and pulse frequency affected the temperature rise; however, the two-way analysis of both enamel and dentin showed that the influence of energy on temperature increase was stronger than that of frequency [72].

4.4.4 Water Spray’s Influence on Dental Ablation

The effect of water spray on dental hard tissue ablation efficiency using the erbium lasers is still a subject of study.
Rizoiu and DeShazer (1994) and Kimmel et al. (1996) have suggested the role of a hydrokinetic effect as a fundamental ablative mechanism of hard dental tissues [48, 49].
Freiberg and Cozean (2002), comparing the effect of a water spray with that of a superficial film of water in mediating the ablative action of the erbium laser, concluded that if a hydrokinetic effect exists, it does not cause a volumetric increase in tissue ablation [73].
Kim et al. (2003) reported that when using Er:YAG laser, effective hard tissue ablation requires that the appropriate water flow rate corresponds properly to irradiation conditions. They found that at 250 mJ, the most effective ablation resulted from a water flow rate of 1.69 mL/min in both the enamel and dentin. At 400 mJ/pulse, a different water flow rate (6.75 mL/min) is required for enamel ablation, while dentin does not require more water for better ablation [74].
Meister et al. (2006) reported that the external supplied water always has a significant influence on the effectiveness of the ablation process and that only the water content in dentin influences the efficiency of Er:YAG laser ablation. He found no significant relation between dentin and enamel’s water content and Er, Cr:YSGG ablation efficiency [53].
Kang et al. (2007, 2008) found a 60 % higher ablation threshold for spray-associated irradiation due to water spray absorption during irradiation. The enhanced acoustic peak pressures were six times higher, and the ablation volume of the spray-assisted process was up to two times larger compared to dry ablation, as a result of rapid water vaporization, material ejection with recoil stress, interstitial water explosion, and possibly liquid-jet formation [47, 51]. In both studies, they concluded that dry ablation exhibited severe carbonization due to excessive heat accumulation while spray induced slightly reduced efficiency but also provided significant beneficial effects, such as clean-cutting with augmented material removal and cooling effects during laser ablation.
A study by Olivi et al. (2010) described the role of water spray as modulator of the laser energy to avoid the undesirable structural thermal changes to dental tissues; a safer and more effective irradiation of the enamel was found at high percentages of air and water (Er, Cr:YSGG 92 and 80 %: 56 mL/min). The authors reported the important role of the water flow rate to obtain a qualitatively better ablation, both reducing the thermal effect of the laser interaction and increasing the tissue, cooling, and cleaning action [46]. Different percentages of the air/water spray, with wider range between air and water, appeared to slightly increase the ablative action by increasing the photothermal effect of the laser beam, but to the detriment of the quality of the ultrastructural morphology [46] (Figs. 4.16, 4.17, and 4.18).

Fig. 4.16

Enamel ablation with Er, Cr:YSGG laser at 5.5 W, 20 Hz, 225 mJ, 140-μ pulse duration, 600-μm tip, with 92/80 % air/water spray presented a more regular aspect of the prismatic structure (type I or type II Silverstone) (Reprinted with permission from Olivi et al. [46])
Fig. 4.17

Enamel ablation with Er, Cr:YSGG laser at 5.5 W, 20 Hz, 225 mJ, 140-μ pulse duration, 600-μm tip, with 95/70 % air/water spray showed a more disordered prismatic structure, due to prevalent destruction of the superficial prismatic structure (type III Silverstone) (Reprinted with permission from Olivi et al. [46])
Fig. 4.18

Enamel ablation with Er, Cr:YSGG laser, at 5.5 W, 20 Hz, 225 mJ, 140-μ pulse duration, 600-μm tip, with 82/70 % air/water spray presented an intermediate outcome, with moderate presence on the surface of exploded products of ablation and few areas of melting (type II or type III Silverstone) (Reprinted with permission from Olivi et al. [46])
Lately, Kuščer and Diaci (2013) studied the efficiency of the erbium laser ablation of hard tissues under different water cooling conditions. They found that the use of a continuous water spray during laser irradiation of hard dental tissues reduced the laser ablation efficiency in comparison with laser irradiation in dry mode. The phenomenon of ablation stalling can primarily be attributed to the blocking of laser light by the loosely bound and recondensed desiccated minerals that collect on the tooth surface during laser ablation. Also no evidence of the influence of the water absorption shift on the hypothesized increase in the ablation efficiency of the Er, Cr:YSGG wavelength was observed. Another positive function of the water spray during erbium laser irradiation is that it rehydrates the minerals within the tooth, thus sustaining the subsurface expansion ablation process [45].

4.5 Mechanism of Interaction of Different Lasers on Soft Tissues

The different composition of the gum tissue in melanin, hemoglobin, water, and protein matrix (non-operator-dependent factors) determines the different interaction with the selected wavelength.
Therefore, the choice of wavelength is the most important operational factor (operator-dependent factor): visible, near-, medium-, or far-infrared lasers all interact with the soft tissues, but with different modalities (scattering or absorption), different target chromophores (hemoglobin and melanin or water), and different penetration depths (deeper or superficial) (see Fig. 4.2).
The lasers in the visible and the near-infrared spectrum are absorbed predominantly by melanin and hemoglobin. The lasers in the visible spectrum (532-nm KTP) have an optical behavior of absorption–diffusion to 50 %, with less deep penetration in the soft tissue. The lasers in the near-infrared spectrum are spread more in depth with the increase of their wavelength.
The medium-infrared (Er, Cr:YSGG and Er:YAG) and far-infrared lasers (CO2) are absorbed by the water within tissues; the CO2 laser has a moderate surface absorption in tissue.
The erbium:YAG laser is much more shallow, having a maximum absorption in the aqueous component of the gingiva, mucosa, and dental pulp. The absorption wavelength of 2.78 μm is lower, with greater penetration into the soft tissue; this translates, for the same energy emitted, in a higher ablative efficiency for the soft tissues. It is the author’s experience a lower power usage for the Er, Cr:YSGG laser (from 50 to 75 mJ) compared to the Er:YAG (from 100 to 150 mJ) during the incision and/or vaporization of soft tissues; this clinical experience supports the physical basis of the higher absorption in water of the Er:YAG laser compared to the Er, Cr:YSGG laser (Figs. 4.19 and 4.20). Also to be considered is the proportion of energy absorbed by the water spray, which limits the efficiency of Er:YAG soft tissue vaporization.

Fig. 4.19

Laser gingivectomy to expose subgingival class V decay
Fig. 4.20

Incision performed with an Er:YAG laser at 130 mJ, 20 Hz, 300-μ pulse duration, 600-μm conical tip, air/water spray
Whatever are the wavelengths, the laser energy absorbed by the target chromophore produced a photothermal effect on the target that generates incision and vaporization of the soft tissues. However, a stable coagulation is only obtained after visible or near-infrared lasers’ interaction with hemoglobin (Figs. 4.21 and 4.22).

Fig. 4.21

Laser gingivectomy to expose subgingival class V decay
Fig. 4.22

Incision performed with a 810-nm diode laser at 1 W in CW, 400-μm tip: near-infrared laser in continuous wave performed an incision with a very good coagulation and few carbonization

4.6 Laser Parameters

The laser–tissue interaction depends on the wavelength and the target tissue. The consequent effects on tissue are closely influenced by the parameters used [23, 41].
In this chapter, we consider only the use of erbium lasers in restorative dentistry.
Erbium lasers are called “free-running pulsed” lasers because they emit pulses that have a specific beginning, peak, and end; pulsed emission concentrates the amount of energy and time in a defined space (temporal and spatial profile), at defined intervals.
The parameters of laser used that influence the effects on the tissue are:

  • The energy emitted and its density (fluence)
  • The frequency of pulses in the time unit
  • The average power emitted and its density (power density)
  • The pulse duration and peak power
Also important are the temporospatial characteristics of the single pulse (temporal and spatial pulse profile) and the modality and operative technique of the clinician (distance, angle, speed, and time of irradiation) because they also influence the parameters and they will be discussed later (see Sect. 4.7).

4.6.1 Energy and Threshold of Ablation

Energy is the ability of the system to perform a task. The term comes from the Greek word “energheia” (ένέργεια), used by Aristotle to express effective action: it is composed of “en” (έν) which means “intensive particle” and “ergon” (έργον), meaning “ability to act.” The term therefore expresses the ability of a laser to emit particles of energy (quantum) that can perform a given work (job), in our case the ablation of dental tissues.
The energy density (fluence) is the amount of energy emitted per unit of irradiated surface in a unit of time (expressed in J/cm2). It is a value affected by the amount of irradiated surface covered in the unit of time, which also is closely related to the speed of hand movement when using the laser. This value is difficult to assess clinically and is best suited for experimental evaluations and for various applications for therapeutic purposes (LLLT). It is more useful, clinically, to consider the energy density in relation to the diameter of the fiber tip to use. At the same amount of energy emitted, the smallest fibers emit energy at a higher density; to have the same energy density, a larger-diameter tip requires a greater amount of energy, while less energy is needed for a smaller tip. Other parameters that affect the fluence are focusing or defocusing the laser beam, which, respectively, increases or decreases the density of the energy. As the distance between the laser tip and the target tissue increases, the fluence decreases precipitously. At 2-mm tip-to-tissue distance, fluence is calculated to decrease by 68 % from its level at the tip surface. At 3 mm, it decreases by 78 % [23, 32] (Fig. 4.23).

Fig. 4.23

Fluence and working distance; using the same fiber diameter and the same energy, the fluence decreases with the working distance
The minimum energy required to generate a clinical effect, ablation or vaporization, is called the “threshold of ablation.”
The energy that does not reach the ablation threshold is called sub-ablative.
When considering an erbium laser and water, its target chromophore contained in the dental tissues, the ablation threshold of the enamel was approximately calculated by Apel et al. (2002) in values of 9–11 J/cm2 for the Er:YAG laser and slightly higher at 10–14 J/cm2 for the Er, Cr:YSGG laser [75].
Lin et al. (2010) have calculated that the threshold values for the ablation of dentin are approximately 2.97–3.56 J/cm2 for the Er:YAG and 2.69–3.66 J/cm2 for the Er, Cr:YSGG laser [76].
Also, Majaron and Lukac (1996) have calculated the values of the ablation threshold for the dentin in 4 J/cm2 for the Er:YAG laser [77].
The ablation threshold also depends on pulse duration, and it decreases toward shorter pulse duration. Experiments by Apel et al. (2002) revealed that when pulses of shorter duration are used, the limit at which ablation starts is reduced by up to approximately 3 J/cm2. This expands the ablation threshold range of Er:YAG laser radiation to between 6 and 10 J/cm2 [78]. This is due to the fact that for shorter durations, the energy has little time to escape from the ablated volume and so less heat is diffused into the surrounding tissue [79, 80]. However, although the ablation threshold of the dental enamel can be changed by varying the pulse duration of the Er:YAG laser, no clinical consequences can be expected, as the shift is only slight [78].
So, a good knowledge of the energy values to use is necessary for a selective ablation of the enamel, dentin, and decayed tissue, bearing in mind the individual variability of mineral composition of the tooth. The more energy applied, the greater the effect produced on the tissue. Only energy above what is needed to reach the threshold is used for ablation. Lower ablative energy (just above the threshold of ablation) can be used to smooth and condition the hard tissue surface through macroroughening and cleansing of the enamel and dentin (often incorrectly called laser etching; see Sect. 4.8).
Here, it is important to recall one of the basic concepts of laser therapy:
Apply the minimum effective energy, that is, the energy capable of causing the desired clinical effect, limiting the undesirable ablative effects related to higher energy used.

4.6.2 Pulse Repetition Rate

Also called pulse frequency or improperly frequency, expressed in Hz and/or more correctly in pulses per second (pps), it is an expression of the number of pulses emitted per unit of time. Numerous pulses per second increase the speed and power of the interaction; the more numerous the pulses in the time unit, the smaller the interval between one pulse and the other, with less time for tissue cooling.

4.6.3 Power

Power expresses the speed with which a certain amount of work is produced. The average power of the laser is determined by the energy emitted in the unit time (second). It is determined by the value of the energy of each single laser pulse (expressed in J) multiplied by the number of pulses in a second (pulse repetition rate or pulse frequency, expressed in Hz or pps).
Power (W) = energy (J) × pulse repetition rate (Hz or pps)
The greater the power applied, the faster the effect on the tissue.
Power density is determined by the power emitted per unit of surface area of the fiber tip or tip (expressed in Watts/cm2).
Furthermore, other parameters influence the result of laser irradiation:

4.6.4 Pulse Duration and Peak Power

The peak power of each pulse is calculated by the energy emitted by a single pulse divided by its duration (pulse duration); it determines the effectiveness of the pulse output. The shorter the pulse duration, the more energy is concentrated in the unit time and the more effective is the ablative action with minimum thermal effect (Fig. 4.24). Short pulses cause a high peak power and lead to better efficiency for ablation of hard tissues. Pulse duration is the duration of each pulse that determines the thermal effect and the ablative efficiency of the pulse. Usually, the pulse is not variable in its length, being determined by the hardware components used in the pulse forming network (PFN) [25, 82]. Long pulses have a higher emission of thermal energy on the tissue and are more effective for the vaporization of soft tissues (see Fig. 4.24). Short pulses have better efficiency for hard tissue ablation.

Fig. 4.24

Ablation rate (in mm3/s) of caries in dentin for different Er:YAG pulse duration modes of Fotona Fidelis laser in comparison with a steel burr. Shorter pulse durations result in lower heat deposition and higher ablation rate (Reprinted with permission from Lukac et al. [81])
Consequently, for an erbium laser, the possibility to vary and control the pulse duration is critical for the success of laser dental treatments [82].
Table 4.1 summarizes the main operating parameters of the erbium laser.

Table 4.1

Main operating parameters of the erbium laser
Energy (E): J
Fluence or energy density (Fl): J/cm2
Pulse repetition rate or frequency (F): Hz o pps
Average power (P): Watt = E (J) × F (Hz o pps)
Power density (Pd): W/cm2
Peak power (PP): W = E (J) ÷ pulse duration (s)
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Nov 22, 2015 | Posted by in Dental Materials | Comments Off on Laser–Hard Tissue Interaction
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