Systematic Review on the Role of Lasers in Endodontic Therapy: Valuable Adjunct Treatment?

(1) Background: Adjunctive instruments, such as lasers have been investigated to address the risk of failure of endodontic therapy due to the complexity of the root canal system. Lasers have been used therapeutically, in direct irradiation of the root canals or adjunct to irrigants placed into the canals, in combination with a photosensitizer (antimicrobial photodynamic therapy) and in pain management (photobiomodulation). The purpose of this systematic review was to evaluate the evidence in clinical use within these three areas of therapy. (2) Methods: PubMed, Cochrane and Scopus search engines were used to identify human clinical trials regarding the use of lasers in endodontic therapy. (3) Results: After applying the keywords and additional filters, inclusion and exclusion criteria, the initial number of 1486 articles was reduced to 17. It was revealed that almost all studies (14/17) presented a statistically significant improved outcome in laser-assisted endodontic therapy, with the remaining three not showing any adverse effects. (4) Conclusions: The use of laser photonic energy of appropriate delivered parameters can be proposed as useful adjunctive when considering optimal treatment modalities in orthograde endodontics. Additionally, a tendency of research towards pain modulation in this field is developing.


Pulpitis-Apical Periodontitis
In deep carious lesions where microorganisms have invaded the dental pulp, a substantial inflammation known as pulpitis may take place. Usually, the inflammatory reaction remains localized even after the microorganisms have invaded the pulp cavity. Notably, as long as the pulp tissue is vital only a limited number of microorganisms reside inside the root canal, and hence the infection does not diffuse into root dentine. In this case, the treatment of choice is endodontic therapy, and its prognosis is excellent as far as microorganisms are concerned.
However, as a persisting infection it can potentiate the development of apical periodontitis, which is an inflammatory process of the peri-radicular tissues caused by microorganisms inside the root canal system. In apical periodontitis, the lesion contains phagocytes and other defense cells, which can, in turn prevent further proliferation of the microbial infection [1].

Endodontic Therapy
The goal of endodontic therapy (also known as endodontic treatment, or root canal therapy, or root canal treatment) is the elimination of diseased dental pulp tissue residue and the prevention of inflammation of the periapical tissue, or the control and elimination of microbial infection with the further aim of promoting healing in the case of persistent lesions [1].
The accumulation and persistence of microorganisms inside a necrotic root canal depends on the availability of oxygen and nutrients, along with the host's immune defense; this leads to differential microfloral compositions. Despite the consequent variants of micro-organisms, the dominant species are anaerobes. Specifically, the proportion of the anaerobic microbial load is reported to lie between 70 and 100%. The main pathogen found in the event of persistent periapical infection is Enterococcus faecalis, which in view of its ability to form complex biofilms, and to survive without nutrients for months, belongs to one of the most resistant species [2]. The penetration of microorganisms into the surrounding dentine has been shown to occur via the dentinal tubules and contamination can reach a depth of approximately 1000 µm [3].
In order to combat the microbial challenge, the use of local antimicrobial irrigating solutions with tissue-dissolving ability, such as sodium hypochlorite (NaOCl), combined with mechanical instrumentation has represented the "gold standard" in endodontic therapy [4]. The penetration of NaOCl into root dentine is estimated to be ca. 130 µm [5]. Additionally, the complex three-dimensional anatomy of root canals has a limiting effect on the penetration of irrigants within this multiplex system.
According to Haapasalo et al. [6], the successful elimination of endodontic infection depends on the following factors: (i) host defense system; (ii) systemic antibiotic therapy in some cases; (iii) chemo-mechanical preparation and irrigation; (iv) local root canal disinfecting medicaments; (v) permanent root canal obturation; and (vi) permanent restoration to achieve an effective coronal seal.
Systemic antibiotics are not predictable outcome treatments and should only be prescribed in cases of spreading infection and compromised hosts [6].
Unfortunately, a risk of failure has been described in the scientific literature of between 7 and 16%, and mainly through the complexity of the root canal system [7]. Following such failure, very often a re-treatment has to be performed and in this case the respective reported rate varies between 11 and 24% [8]. Hence, additional methods have been investigated to improve the cleansing and disinfecting action of irrigant solutions.

Lasers in Endodontic Therapy
Apart from ultrasonic irrigation techniques, laser use in endodontics has been a major field of research since their introduction in dentistry. The interaction of near-infrared (NIR) wavelengths (810-1064 nm) with host tissue is predominately through a photothermal effect. In view of their high penetration depth into dental tissue, their penetration into dentine is shown to reach >1000 µm through scattering and transmission along the dentinal tubules, which in turn act as "light guides" [9]. Since NIR-laser photonic energy can be absorbed by chromophores such as melanin, only pigmented microorganisms will be susceptible to direct inactivation [10]. Additionally, an indirect microbicidal effect will occur from photothermal damage [11]. In vitro studies have been performed both in wet and dry root canals, with promising antimicrobial results. However, overheating and melting of the root canal walls is one drawback of laser use in dry environments [12].
In view of their strong broad absorption band from O-H bond stretching and inter-molecular associations through hydrogen (H)-bonding, lasers available in dentistry which operate within the mid-infrared region (2780-2940 nm, equivalent wavenumbers 3597-3401 cm −1 ), have been investigated more recently, not least because of their ability to cause a "cavitation" effect in such media. This depends on a rapid fluid motion in the root canal arising from expansion and implosion of bubbles at the laser tip, which are caused by the high peak power experienced during pulsed emission along with the high absorption in irrigating solutions. During laser-activated irrigation (LAI), as well as PIPS TM (Photon Induced Photoacoustic Streaming) and SWEEPS TM (Shock Wave Enhanced Emission Photoacoustic Streaming) techniques which rely on the same phenomenon, the movement of the irrigant is extremely turbulent, leading to improved removal of microorganisms and their biofilms from the root canal system. This action is based on a merely physical effect [4]. In addition, a chemical effect also appears to play a significant role, with an increased reaction rate of NaOCl upon activation by erbium lasers [13].
Laser use in endodontic therapy has further developed through antimicrobial photodynamic therapy (aPDT) and has been adopted as an adjunct alternative. This technique is based on a photosensitizer (PS) that is applied inside the root canal and after a particular incubation time, is irradiated by a light source, the wavelength of which coincides with the absorption band maximum of the photosensitizer. In the presence of oxygen, a reaction takes place which leads to the production of reactive oxygen species (ROS) and singlet oxygen ( 1 O 2 ), a process leading to microbial cell damage [14]. The benefits of this technique are the wide-spectrum of affected microorganisms, the absence of photo-resistant species after multiple applications and minimal damage to host tissue. Moreover, the treatment outcome is independent of the antibiotic-resistant pattern of the micro-organisms. The selectivity of this approach is dependent on the incubation time: microorganisms require minutes of exposure, but host cells require hours [15]. Nevertheless, inside the infected root canal system, the only existing cells are microorganisms. Most research has been performed with methylene blue (MB), toluidine blue (TBO), and indocyanine green (ICG) as photosensitizers and lasers as light sources with corresponding wavelengths of 660, 635 and 810 nm, respectively.

Post-operative-endodontic Pain
Except for microorganism infection, another major concern of endodontic treatment is the post-operative-endodontic pain (POP) experienced by a high number of patients. This has been described in the scientific literature as having a prevalence of between 3 and 58%, where the range in these reports can be explained by the use of differential criteria to assess POP [16]. When pain occurs after endodontic treatment, patients may consider the treatment per se as the causative factor and may question the clinician's skills. Hence, the management of pain is of critical importance.
The actual cause of post-operative-endodontic pain is considered to be an irritation of periradicular tissues associated with microorganisms, or a mechanically-or chemically-induced injury to the radicular area. Specifically, apical extrusion of tooth debris or irrigants, intra-canal dressings and micro-organisms might occur, resulting in inflammation and pain [17]. This irritation contributes to nociceptor activation and local inflammation, leading to a release of prostaglandins, bradykinin, leukotrienes, serotonin and cytokines from the injured tissues, which therefore potentiates peripheral sensitization [18,19].
Typically, the duration of pain lasts between 24 and 48 hours, with reports of pain persisting for three days following root canal treatment [20]. The suggested management of POP includes administration of non-steroidal anti-inflammatory drugs, paracetamol or corticosteroids. Recent studies have proposed pain management to be successful with photobiomodulation therapy (PBMT) [21][22][23].

Photobiomodulation (PBM)
PBM therapy (PBMT), through the application of photonic energy at specific wavelengths within the optical window of 650-1350 nm, works on the principle of inducing a biological response through energy transfer. Such non-ablative photonic energy delivered into tissues modulates biological processes within that tissue, and also within the biological system of which that tissue is a component part. In this context, cellular metabolism can be modulated, leading to secondary effects which modify cellular behavior [24]. The benefits of this approach can be described as anti-inflammatory, analgesic and therapeutic and with a correct incident dose applied, PBM therapy has no appreciable thermal effects in irradiated tissue [25].
In vivo studies have shown that PBM can inhibit nerve function. Other alterations include local conduction blockage, disruption of axonal flow, and specific nociceptor inhibition. All these changes give rise to pain relief and are reversible without side effects [23].

Aims of the Study
In this systematic review of randomized controlled clinical trials on lasers used as an adjunct in non-surgical endodontic treatment, three major fields have been explored:

1.
Conventional laser use inside the root canal as an additional disinfection method; 2.
Lasers combined with a photosensitizer inside the root canal in antimicrobial photodynamic therapy (aPDT); 3.
The aim of this systematic review was to evaluate which field is most strongly supported by clinical evidence, and if so, which shows more favorable results than application of the gold standard (endodontic) treatment alone.

Search Strategy
The search engines PubMed, Cochrane and Scopus were used with following keywords and combinations: Titles and abstracts of the above articles were independently screened by two reviewers, and in the case of disagreements, this was resolved by discussion. The following inclusion/exclusion criteria were applied: After screening and implementation of the eligibility criteria, 17 articles were included in total, which were categorized in terms of:

•
Conventional laser-assisted endodontic treatment (4 articles); • aPDT in endodontics (5 articles); • PBM in endodontics (8 articles) The search was performed from April 08 to April 15, 2020. The following flow-chart ( Figure 1) which was prepared in accordance with the PRISMA guidelines [26], indicates the study selection process.

Data Extraction
Data extraction of the selected studies was based on the following factors: • Citation (first author and publication year); • Type of study/number of sampling participants; Outcome.

Quality Assessment
Furthermore, studies were analyzed through a risk of bias assessment. The Cochrane Risk of Bias tool [27] was modified according to the requirements of this systematic review.
The risk of bias was determined according to the number of "yes" or "no" answers to the following questions allocated to each study: Correct interpretation of data?
The classification was performed according to the total number of "yes" answers to the above questions. The degree of bias was calculated as follows: • High risk: 0-4 • Moderate risk: 5-7 • Low risk: 8-10.

Primary Outcome
The aim of this systematic review was to evaluate the outcome of the studies and detect and analyze the missing parameters of their protocols.

Data Presentation
The extrapolated data of each laser application category are presented in Tables 1-3.
The most common negative answers concerned the questions (a) use of a power-meter, (b) sample size calculation and required sample number included, and (c) correct description of the protocol.

Analysis of Data
Regarding the treatment outcomes, 14/17 articles (82.3%) presented a positive therapeutic result, with significant differences observed between the laser treatment and their respective control group, whilst 3/17 articles (17.7%) showed no significant differences between these classifications.
From these studies, 11/14 showed only a low risk of bias, whilst 3/14 showed a moderate risk.
Hence, it is clear that the dominant research area was pain evaluation (14/17). Moreover, two studies [30,33] examined additional factors. One [30] analyzed pain and bacterial counts, whilst the second [33] analyzed pain, bacterial counts and radiographic healing.
For the studies with incomplete parameter descriptions (5/17), the following deficiencies were found: In addition to the above deficiencies, the spatial beam profile was not mentioned in any of the 17 studies examined.
Analysis of the correctly described protocols (12/17) has been performed for each laser application category as shown in Tables 5-7.

Discussion
Endodontic treatment conforming to the "state-of-the-art" demands the availability of sufficient chemo-mechanical instrumentation, with the adjunctive use of various irrigants. The most widely used and investigated of these are ethylenediamine tetra-acetate (EDTA) and sodium hypochlorite (NaOCl). The former is a chelating agent with no antibacterial effect per se, but which facilitates cleansing and the removal of infected tissue. However, the latter is a strong antimicrobial agent with the capacity to 'dissolve' the organic part of pulp residues and dentinal walls. It is used in various concentrations between 0.5 and 5.25% (w/v), a range in which a concentration higher than 2.5% (w/v) has not been clinically proven to be more effective [6].
Throughout the scientific literature, it is suggested that lasers cannot substitute such traditionally accepted endodontic therapies. The use of laser photonic energy within the root canal system of any tooth may be affected to a greater or lesser extent, by the consequences of thermal conduction, direct beam irradiation and the effects of refracted energy consequent to dentinal structure, root morphology, patency of access along the canal and the existence of multiple (accessory) root canals.
In vitro trials could not prove that the application of LAI with saline could efficiently replace NaOCl [4]. Specifically, De Meyer et al. showed that LAI applied by a 2940 nm laser system with saline could only reduce the viable counts of a dual-species biofilm by approximately 1 log 10 unit, whereas LAI with NaOCl diminished these levels by >2.2 log 10 units [4].
Similarly, Kreisler et al. showed that laser irradiation alone with an 809 nm diode laser in vitro was no more effective than the simultaneous use of the laser with NaOCl (1.49 log 10 versus 2.84 log 10 unit differences were observed, respectively). They concluded that the potential application of this diode laser should not be a substitute for conventional treatment, but should be regarded as a possible adjunctive treatment [45]. This was also supported by more recent studies [46]. Sohrabi et al. also noted that the use of a 980 nm laser system in a dry canal was significantly less effective than conventional chemomechanical treatment alone [47].
Both aPDT and PBM are essentially non-photothermal applications of laser photonic energy. The range of wavelengths for aPDT is currently 450-810 nm with a complementary application of photosensitizers. PBM effects in relation to this study may have direct application in pain modulation through the sub-ablative use of visible and near infra-red wavelengths, or may be an indirect benefit of surgical laser use at similar wavelengths within the canal and along a scatter gradient through the apex or dentinal tubules to the surrounding living tissue [12,24].
Chiniforush et al. outlined the fact that aPDT should be applied together with conventional chemo-mechanical techniques in order to further reduce the number of microorganisms, or alternatively modify their virulence factors, leading to a limited ability for them to form biofilms [12].
PBMT, in view of its sub-ablative action, clearly cannot replace a complete root canal treatment and hence this represents a purely adjunctive treatment modality.
Therefore, only clinical studies using lasers as an adjunct to the established, traditional methods were included in this systematic review.
Regarding the three different fields of laser application in non-surgical endodontic treatment explored in this review (conventional laser use, aPDT and PBMT), a tendency towards PBMT is clearly observable from the number of articles which could be included (i.e., 7/17).
With regard to the investigational objectives of the studies, pain, bacterial count and radiographic healing were examined. Specifically, conventional laser-assisted endodontic studies explored pain and bacteria, aPDT studies examined pain, bacteria and radiographic healing, and PBMT studies evaluated pain only.
It was also evident that most of the studies (14/17), independent of the fields considered (conventional, aPDT or PBMT), evaluated the effect of laser use in pain management as a major concern in the delivery of endodontic therapy. However, as a result of the subjective nature of pain perception, studies evaluating pain intensity were found to be highly heterogenous [19]. Consequently, in two of the studies [31,40], in addition to the visual analog scale (VAS)-pain evaluation, a quantification of calcitonin gene-related peptide (CRGP) in gingival crevicular fluid (GCF) and its correlation with pain, was assessed.
An interesting approach was taken by Arslan et al. in 2018 [40]. Using a 970 nm diode laser (with parameters listed in Table 3) with PBM as the second test group and a placebo laser system serving as a control, a sample size of 39 patients were tested for CGRP levels in GCF along with pain levels, after endodontic treatment and intracanal laser application in one of the groups [40]. CGRP is a pro-inflammatory mediator triggering neurogenic inflammation, during which pain sensitivity increases and pain threshold decreases [48]. These researchers concluded that conventional intra-canal laser application as well as PBM, exert an immunomodulatory effect. This was supported by the observation that in both laser treatment groups, the modification in CGRP levels for experimental teeth was closer to that of healthy contralateral teeth, than with endodontically-treated and contralateral teeth in the placebo group. Additionally, they were able to show a positive correlation of VAS scores of pain on percussion with both the pre-and post-operative total amount of CGRP in GCF [40].
Similarly, Yoo et al., using a 1440 nm Nd:YAG laser source (parameters available in Table 1), demonstrated that laser irradiation was significantly more effective in reducing pain on percussion (p = 0.003), and also in its ability to decrease substance P levels (p = 0.002), CGRP (p= 0.049), and MMP-8 (p = 0.002). VAS on percussion was positively correlated with substance P, CGRP, and MMP-8 concentrations [31].
It is plausible that for both studies, with protocols involving higher fluences being delivered, there was still a photobiomodulatory effect observed; although possibly lying outside the "biphasic dose response" range, this hypothesis is in agreement with Cronshaw et al., in that the dosimetry associated with pain relief applied in contemporary clinical practice lies within a higher range than that required for biostimulation [49].
Concerning the aPDT studies included in this review, de Miranda et al., found that using pain and bacterial count evaluations and radiographic healing criteria, they could show a significant difference in the periapical index score (PAI) at a six-month follow-up time [33]. This could be attributed to an improved healing axis with photodynamic therapy. This observation was primarily based on the antimicrobial action of this approach, and secondly on the ability of the laser photonic energy to effectively scatter and diffuse beyond the strict limits of the target tissue, hence photobiomodulating the area involved [50].

Conclusions
State-of-the-art, conventional endodontic therapy techniques continue to be the acknowledged as a "gold standard" treatment. Notwithstanding, the variety in reported failure rates and post-operative pain has prompted the requirement for adjunctive alternatives. In this context, the use of lasers has been thoroughly investigated. This systematic review aimed to explore the evidence of this technology's clinical value. It was revealed that almost all studies (14/17) presented a statistically significant improved outcome in laser-assisted endodontic therapy. The remaining three did not show any differences over that of their corresponding control groups, but neither did they demonstrate any adverse effects. Therefore, lasers can be suggested as useful adjunctive treatment modalities.
As far as the safety of this treatment is concerned, a lack of parameter reporting, which especially in PBMT as well as in aPDT is of major importance, complicates a near-flawless conclusion. Irradiation protocols should be interpreted with special care regarding the thermal increase in the root canal system and that of the surrounding tissues.
A tendency of research towards pain modulation in this field is developing. For future directions, more studies with clear and standardized protocols should be performed in order to further confirm the evidence base of this approach.