The smear layer consists of two parts: a superficial layer on the surface of the root canal wall approximately 1 to 2 μm thick and a deeper layer packed into the dentinal tubules to a depth of up to 40 μm.⁴⁰ It contains inorganic and organic substances that include microorganisms and necrotic debris.⁴¹ In addition to possible infection of the smear layer itself, it also can protect the bacteria already present deeper in the dentinal tubules by preventing intracanal disinfection agents from penetrating into the tubules.⁴² Pashley⁴³ also has postulated that a smear layer containing bacteria or bacterial products might provide a reservoir of irritants. Thus, complete removal of the smear layer would be consistent with elimination of irritants from the root canal system.⁴⁴

In addition, Peters et al.⁴⁵ clearly demonstrated that more than 35% of the canals’ surface area remained unchanged after instrumentation of the root canal using four different nickel-titanium (NiTi) preparation techniques. Because most current intracanal medicaments have a limited antibacterial spectrum and limited ability to diffuse into the dentinal tubules, development of newer treatment strategies designed to eliminate microorganisms from the root canal system should be considered. Such strategies must include use of agents that can penetrate the dentinal tubules and destroy the microorganisms located in an area beyond the host defense mechanisms, where they cannot be reached by systemically administered antibacterial agents.⁴⁶

Numerous studies also have documented that CO₂,⁴⁷ Nd:YAG,^47–49 argon,^47,50 Er,Cr:YSGG,⁵¹ and Er:YAG^52,53 laser irradiation have the ability to remove debris and the smear layer from the root canal walls after biomechanical instrumentation.

However, the intracanal use of lasers has several limitations.⁵⁴ The emission of laser energy from the tip of the optical fiber or the laser tip is directed along the root canal and not necessarily laterally along the root canal walls.⁵⁵ Thus, it is almost impossible to obtain uniform 360-degree coverage of the internal aspect of the root canal system surface using a laser.^54,55 An important additional consideration, in view of the risk of thermal damage to the periapical tissues, is safety.⁵⁵ Direct emission of laser radiation from the tip of the optical fiber in the vicinity of the apical foramen of a tooth may result in transmission of the energy beyond the foramen. This in turn may adversely affect the supporting tissues of the tooth, which can be hazardous in teeth close to the mental foramen or to the mandibular nerve.^55,56

Matsumoto³ emphasized the possible limitations of laser use in root canal systems, suggesting that “removal of smear layer and debris by laser is possible, however it is difficult to clean all root canal walls, because the laser is emitted straight ahead, making it almost impossible to irradiate the lateral canal walls.” These workers researchers strongly recommended improving laser endodontic tips to enable irradiation of all areas of the root canal walls.

Erbium lasers have gained increasing popularity among clinicians after their clearance by the U.S. Food and Drug Administration (FDA) for use on hard dental tissues.⁵⁷ Stabholz et al.^55,56 described a newer endodontic tip that can be used with an erbium laser system. The beam of the erbium laser is delivered through a hollow tube, permitting development of an endodontic tip that allows lateral emission of the laser radiation (side-firing) rather than direct emission through a single opening at the far end. This new endodontic side-firing spiral tip was designed to fit the shape and the volume of root canals prepared by NiTi rotary instrumentation. It emits the erbium laser radiation laterally to the walls of the root canal through spiral slits located all along the length of the tip. The tip is sealed at its far end, preventing the transmission of radiation to or through the apical foramen of the tooth. In an evaluation of the efficacy of the endodontic side-firing spiral tip in removing debris and smear layer from distal and palatal root canals of freshly extracted human molars, scanning electron microscopy (SEM) of the treated root canal walls revealed clean surfaces, free of smear layer and debris (Figures 13-2 to 13-5).⁵⁶

The dentinal tubules in the root run a relatively straight course between the pulp and the periphery, in contrast to the typical S-shaped contours of the tubules in the tooth crown.⁴¹ Studies have shown that bacteria and their byproducts, present in infected root canals, may invade the dentinal tubules. Also, the presence of bacteria in the dentinal tubules of infected teeth was noted at approximately one half the distance between the root canal walls and the cementodentinal junction.^58,59 These findings justify the rationale and need for developing effective means of removing the smear layer from root canal walls after biomechanical instrumentation. This cleansing step would allow disinfectants and laser energies to reach and destroy microorganisms within the dentinal tubules.

In various laser systems used in dentistry, the emitted energy can be delivered into the root canal system by a thin optical fiber—as in Nd:YAG, potassium titanyl phosphate (KTP)–Nd:YAG, Er:YSGG, argon, and diode lasers—or by a hollow tube—as in CO₂ and Er:YAG lasers. Thus, the potential bactericidal effect of laser irradiation can be effectively used for additional cleansing and disinfecting of the root canal system after biomechanical instrumentation. This effect was extensively studied using CO₂,^60,61 Nd:YAG,^62–65 KTP-Nd:YAG,⁶⁶ excimer,^67,68 diode,⁶⁹ and Er:YAG^70–72 lasers.

The apparent consensus is that laser irradiation performed using dental laser systems has the potential to kill microorganisms. In most cases the effect is directly related to the amount of irradiation and to its energy level. Case Study 13-1, with Figure 13-6, illustrates the use of erbium laser energy to clean and disinfect a root canal system.

DeVito et al. have described a new method of removing the smear layer and disinfecting root canals using an Er:YAG laser: These researchers have patented a procedure called Photon Induced Photoacoustic Streaming (PIPS). According to their work, at subablative parameters, when an Er:YAG laser is activated in a root canal, the strong absorption of the wavelength in water creates a shockwave, which removes the smear layer and disinfects the canals. An in vitro study of 80 single-rooted extracted human teeth demonstrated pronounced smear layer removal with no thermal damage to the dentinal walls, and thermocouple studies have shown that the root surface temperature increases were well within an acceptable level.⁷³

Lloyd et al. used 14 extracted human mandibular molars to compare single-needle irrigation of the root canal systems with photon-induced photoacoustic streaming. They concluded that the laser technique eliminated debris better than single-needle irrigation.⁷⁴ Peters et al. studied the efficacy of canal disinfection using sodium hypochlorite irrigation, sodium hypochlorite ultrasonically activated; and Er:YAG laser. These researchers concluded that all three techniques disinfected the root canal systems, with the laser treatment the most effective modality.⁷⁵ Other in vitro studies^76,77 have replicated the results of the various investigations. These in vitro studies have established proof of principle for this technique; however, longitudinal in vivo studies evaluating the long-term success rate of teeth treated endodontically with this new technique versus more conventional techniques need to be published to validate this treatment.

The concept of thermoplasticized compaction is not new and covers any technique that is based entirely on the heat softening of gutta-percha combined primarily with vertical compaction. The technique has many different names, including warm sectional technique, vertical compaction with warm gutta-percha, and the Schilder technique. Schilder⁷⁹ first described the technique to fill root canals in three dimensions more than 40 years ago. At present, some dental practitioners still use this technique, whereas others use newer, warm gutta-percha techniques such as thermomechanical compaction, thermoplasticized gutta-percha, and injection of softened gutta-percha, which have been introduced to simplify the root canal filling procedure.

The first laser-assisted root canal filling procedure used the wavelength of the 488-nm argon laser. This wavelength, which is transmitted through dentin, was used to polymerize a resin placed in the main root canal. Testing the ability of this biomaterial to penetrate into accessory root canals showed that the resin in the lateral canals was readily polymerized at low energy levels (30 mW). Further use of this wavelength became irrelevant because of its poor properties for most other dental procedures.⁸⁰

Anic and Matsumoto⁸¹ were the first to compare different root canal filling techniques used to fill single-rooted teeth. Lateral condensation, vertical condensation, low-temperature gutta-percha (Ultrafil), and laser-cured resin with different wavelengths (argon, CO₂, Nd:YAG) were used. The apical sealing ability achieved by the various filling techniques was compared by measuring apical dye penetration after placement of the samples in a 1% solution of methylene blue. Gutta-percha softened with an argon laser created an apical seal similar to that obtained with the lateral condensation and Ultrafil techniques.

Maden et al.⁸² used the dye penetration method to measure apical leakage by comparing lateral condensation, System B technique (using thermoplasticized compaction; SybronEndo, Orange, California), and Nd:YAG-softened gutta-percha. No statistically significant differences among the different treatment groups were reported. Anic and Matsumoto⁸³ measured a temperature elevation on the outer root surface of 14.4° C with use of the Nd:YAG and of 12.9° C with the argon laser. Such a temperature increase may be detrimental to the tissues of the periodontal ligament and attachment apparatus of the teeth, so the implications of using such treatment methodologies remain in question.

To examine whether laser irradiation does improve the adhesion of endodontic materials to the dentinal walls of the root canal and reduce apical leakage, Park et al.⁸⁴ used different sealers and two root canal filling techniques. These investigators concluded that Nd:YAG laser irradiation at the end of the root canal preparation (at 5 W, 20 Hz) reduced the apical leakage regardless of the sealer or the technique used. Kimura et al.⁸⁵