Argon is a noble and inert gas and extremely stable chemical element. Argon gas is used as active medium inside the optical cavity; when it is bombed by the pumping system, its ionized molecules produce different kinds of wavelengths, among them two in the blue spectrum (470–488 nm) and one in the green spectrum (514 nm).

The possibility of using the blue argon laser (470 nm) in dentistry has been extensively studied by a Japanese group at Showa University School of Dentistry, Tokyo, Department of Endodontics, in 1998 and 1999 [24–26]. The investigations verified the ability of argon laser for smear layer and debris removal from the root canals, prepared in a conventional way and irradiated at 1 W, with 0.05 s pulse duration and 5 Hz pulse frequency [24, 25] and at 0.3 W in continuous wave [26]. However, as of today, the use of the argon lasers remains limited only to ophthalmology in retinal pathology for superficial photocoagulation (see Fig. 5.1).

In the spectrum of visible light, in the green band, the wavelength 532 nm was chosen for the construction of the KTP laser (a duplicate neodymium of 532 nm), with high affinity with hemoglobin (see Fig. 5.1). This laser in fact was introduced to dentistry in the 1990s (nineties), due to its elevated capacity for soft tissue cutting and coagulation [27, 28], and it is used mainly in oral surgery and periodontics. Nowadays however, its diffusion remains limited because of its cost.

Effectively its system of construction is complex. The active medium is a solid: a crystal of Nd:YAG is excited by a diode laser pumping system; a crystal of KTiOPO₄ is located between the active medium and the semi-reflecting mirror and duplicates the vibration frequency of photons, splitting the wavelength of the Nd:YAG (1,064 nm) and emitting a radiation of 532 nm in the green visible spectrum.

KTP laser emits energy continuously. Alternatively, a mechanically interrupted mode (chopped or gated) can be used to control the thermal rise in tissue during the irradiation. The delivery system is a flexible optic fiber that exits through a terminal handpiece. Fibers of different diameters (200, 300, and 600 μm) permit different uses in dentistry including endodontics.

The effective use in endodontics was reported by several studies [28–33], at power ranging from 2.25 to 3 W, at 5 Hz, per 2–5 s, and repeated for 4–5 cycles, depending on the study; no damaging thermal rise for the periodontal tissue was observed at these parameters [28, 29]. Also the root canal decontamination ability after conventional endodontic instrumentation was studied, at different power output (at 1 and 1.5 W), in dry mode or wet model [30–34].

Some diode lasers are built with wavelengths in the range of visible light. Lasers at 635–675 nm are used for antibacterial photodynamic therapy (aPDT or photoactivated disinfection, PAD) [35–40] because the light of these lasers, emitted in the red spectrum of light, is absorbed by particular photoactive colored substances, which when activated release singlet oxygen with elevated lytic capacity on many species of bacteria. For this reason, LED lights or diode lasers in this spectrum of light have been used in recent years with success in root canal, periodontal, and implant decontamination. This laser application in endodontics will be analyzed in Chap. 7. Recently, a new diode emitting laser radiations in both visible and near-infrared spectrum of light is promisingly introduced at 445, 660, and 970 nm for several application in dentistry, including endodontics; the optical characteristics of the blue wavelength are similar to those of blue argon laser (see Fig. 5.1).

Following the first studies with excimer lasers, near-infrared lasers were the first to be studied and then largely used for root canal decontamination, in particular the Nd:YAG (1,064 nm) and then diode lasers (from 810 to 1,064 nm), thanks to the optic fiber technology, introduced at the end of the 1980s (eighties) [13] that allow the introduction of the laser light deeply in the root canal.

Target chromophores of near-infrared lasers are the hemoglobin, the melanin, and the pigment of some type of bacteria (Fig. 5.2). Near-infrared lasers are mainly scattered in the target tissues; the absorption is negligible and reflection minimal. Shorter wavelengths have less deep diffusion than the longer Nd:YAG laser (1,064 nm), both in soft tissues and in radicular dentin. Near-infrared lasers of different wavelengths have slight different optical behavior, depending on the affinity of the specific wavelength with the target chromophore (see the absorption curve). Different affinities with the target correspond to different energies necessary for the clinical use.

This paragraph analyzes only the constructive characteristics and the interaction with tissues typical of these lasers. Chapter 6 will analyze diffusely the effects of these lasers in endodontics.

At the end of the 1980s (eighties) to the beginning of the 1990s (nineties), the erbium YAG laser (2,940 nm) [49–52] first and then the erbium, chromium YSGG laser (2,780 nm) [53, 54] were introduced in dentistry with the idea of substituting the drill for caries removal.

Studies in endodontics started in 1998 for the erbium YAG laser and in 2001 for the erbium chromium YSGG laser [55, 56] for the availability of specific thin tips to access to the root canals (Fig. 5.7).

The Er:YAG laser has a solid active medium, a crystal of yttrium, aluminum, or garnet, doped with erbium atoms. Erbium is a silver metal of the group of rare heart elements (Er, atomic number of 68). When stimulated, the YAG crystal emits photons of a wavelength of 2,940 nm. The laser energy is delivered to a terminal handpiece through different systems, among which the optic fiber and the articulated arm are still in use.

The Er,Cr:YSGG laser also has a solid active medium, a crystal of yttrium, scandium, gallium, and garnet (YSGG), doped with atoms of erbium and chromium. When stimulated, the crystal emits radiations in the medium-infrared spectrum at 2,780 nm. This energy is transferred to a terminal handpiece through an optic fiber.

The term erbium family lasers combines the two wavelengths and is frequently used when referring to both the wavelengths without any specific connection to one another.

Erbium lasers use a visible aiming beam, red or green, to provide good visibility and control of interaction with tissue.

These lasers can develop high power, and for this reason, the energy is emitted in free-running pulsed mode, with pulses of duration and frequency of variable repetition depending on the different equipment used.

The wavelengths of the erbium family fall on the curve of water absorption (principal target chromophore). Absorption in hydroxyapatite is also high, but it has unimportant value for clinical purposes [57–60] (Fig. 5.8).

The high absorption in water explains the effects on dentin, more pronounced in the intertubular zone, that is, more rich in water than in mineral.

The photothermal effect of erbium lasers produces vaporization of the smear layer and a certain grade of ablation and decontaminating effect that are however limited to the dentin canal surface (up to 250–300 μm in depth), where laser energy is mainly absorbed and where it produces a direct photothermal effect.

The dentin ablation happens when the energy used exceeds the threshold of dentin ablation (see Sect. 4.​6.​1). The ability of medium-infrared lasers to ablate radicular dentin encouraged research in the past years, to explore the possibility of preparing the canal with lasers, with quite poor results.

The affinity of erbium lasers with water explains the activation and agitation of the irrigant solution (NaOCl ed EDTA), their tridimensional streaming through the endodontic system (LAI and PIPS techniques), and their effect on bacteria decontamination, on debris and smear layer removal, via an indirect laser action.

The great presence of water in the oral soft tissues; in the hard dental tissue, such as the enamel and the dentin; and much more in the carious tissue explains the extensive use of erbium lasers in dentistry. Water is also present in the bone. For this reason, erbium lasers are considered “all tissue” lasers, very versatile for clinical use in dentistry, in periodontics, restorative, endodontics, and bone surgery, and also recommended for soft tissue therapy, especially on tissues with low vascularization and more evident fibrous components (fibroma removal, labial and lingual frenectomy, gingivectomy, gingivoplasty, operculectomy).

Clinically, the two wavelengths work in overlapping mode. However, the higher absorption in water tissues for the Er:YAG laser makes its tissue interaction more superficial when compared to Er,Cr:YSGG laser (Fig. 5.9). Under equal parameter used, also less energy is necessary for the Er:YAG laser, compared to the Er,Cr:YSGG, to ablate hard dental tissues (see ablation threshold, Chaps. 4 and 7) [61, 62]. However, is the different technology of construction and control (analogic or digital) of different laser units that produces variable performances of different lasers.

Different erbium lasers can have differences in terms of delivery system (optical fiber or articulated arm), handpiece (in close contact with the tip or no contact using a mirror), straight or angled, and big or small (see Figs. 4.​3, 4.​4, 4.​5, 4.​6, 4.​9, and 4.​10); also tips are of different shape and size, as well as the efficacy and modulability of the air-water spray. Technological characteristics include the temporal characteristic of the pulse (pulse duration) which varies from 50 to 1000 μs and the spatial characteristics of the pulse (pulse shape). Different laser systems also offer different choice of a variety of pulse frequency from 2 to 100 pps and different power output from 0.1 to 20 W, all characteristics that allow greater possibility of managing the interaction with tissues, with better clinical application and better control of pain stimulation.

CO₂ far-infrared lasers (9,300–10,600 nm) were the first to be studied and used in endodontics for the decontamination and fusion of the apex in endodontic surgery [7–9]. Nowadays, they are no longer used in endodontics, except for the vital pulp therapy (decontamination, vaporization, and coagulation in pulp capping and pulpotomy).

Together with KTP and near-infrared lasers, the CO₂ laser is still one of the best systems for incision, vaporization, and coagulation of soft tissues (gingiva, mucosa, and pulp) but is not very widely used because of its cost and the range of clinical applications limited to soft tissues.

The main target tissue is water that greatly absorbs the wavelengths.

The active medium is a gas, the carbon dioxide (CO₂), and the emitted laser radiation is delivered through an optic fiber or an articulated arm.

The radiation is emitted, with different wavelengths (9,300 nm, 9,600 nm, or 10,600 nm), in the invisible far-infrared spectrum. Native CO₂ laser at 10,600 nm is only useful on soft tissue. New laser technology introduced some modifications and an oxygen isotope (O₁₈) to emit a radiation at 9,300 nm, matching the peak absorption of hydroxyapatite and making also the interaction with hard tissue applicable.

Being invisible in the far-infrared spectrum, also these wavelengths are supported by a visible coaxial aiming beam (red or green, of a few mW of power).

In the past, the modality of continuous emission (CW) created problems of thermal damage, especially in the hard tissues. Today, pulsed and super-pulsed technology make this laser one of the best systems for oral surgery, due to its elevated coagulating ability via photothermal effect. The thermal effect of CO₂ lasers is able to seal small (0.3–0.5 mm of diameter) blood or lymphatic vessels, realizing an excellent coagulating effect.

Lasers are used with different techniques in endodontics (Table 5.1).

They can be used to directly irradiate the dentin walls or to irradiate/activate photoactive substances or irrigants, so performing the clinical action on the endodontic system indirectly (Table 5.1) (Fig. 5.10).

The interaction between laser light and the target tissue follows the rules of physics. Laser radiation can be:

Absorption on the contrary is the expression of high affinity between light and the target which retains light in the point of irradiation. The portion of laser energy absorbed is responsible for the majority of therapeutic effects, and it is locally converted into photochemical energy, photomechanical energy, and photo-acoustic energy depending on the type of laser used, the modality of emission, and the parameters used.

Besides intrinsic optical properties of different wavelengths, the interaction depends also on the optical characteristics of the target and, in particular, on the absorption coefficient of the specific chromophore of the target tissue for a specific wavelength (optical affinity). Accordingly, different interactions happen with the target tissues: dentin, smear layer, debris, organic remnants, bacteria, and irrigant fluids.

In summary, three groups of laser-tissue interaction can be identified:

When the laser light interacts with the target tissue, for diffusion or absorption, it generates on it the biological effects responsible for the desired therapeutical results and for the unwanted collateral effects.

The laser light, absorbed and/or diffused in the target, generates different effects:

Depending on the wavelength and on the technique used, the laser interaction will target with different modalities different substances: