Laser dentistry in daily practice during the COVID-19 pandemic: Benefits, risks and recommendations for safe treatments

The COVID-19 pandemic forced dental professionals to cope with an unexpected challenge and caused an abrupt cessation of conventional care practices. The high degree of contagiousness as well as the diffusion of the virus through the air and droplets via respiratory transmission placed dental professionals at top-level risk of contracting and spreading the disease. General recommendations were announced in different countries, including patient distancing, air ventilation, surface and instrument sanitization, and the wearing of suitable masks and shields. However, many dental treatments are performed using lasers, and some specific precautions must be added to conventional procedures to ensure the advantages of this technology to patients because of the particular tissue–matter interaction effects of laser wavelengths. Based on the literature, the authors evaluated all of using laser wavelengths to analyze the risk and the benefits of using lasers in daily dental practice, and to provide safety recommendations during pandemic. An unrestricted search of indexed databases was performed. Laser use effects were categorized into: 1) explosive processes that produce tissue ablation and aerosol formation; 2) thermal actions that create vaporization and smoke plume; 3) photobiomodulation of the cells; and 4) enhanced chemical activity. and plume forma-tion, and the application of suction systems with high flow volume and good filtration close to the surgical site will avoid virus dissemination during laser use. In the categories that involve low energy, the beneficial effects of lasers are available and sometimes preferable during this pandemic because only conventional precautions are required. Lasers maintain the potential to add benefits to dental practice even in the COVID-19 era, but it is necessary to know how lasers work to utilize these advantages. The great potential of laser light, with undiscovered limits, may provide a different path to face the severe health challenges of this pandemic.


Background
Coronaviruses are a large family of viruses that cause illness in animals and humans. Several coronaviruses cause respiratory infections in humans and range from the common cold to more severe diseases, such as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS). Acute respiratory diseases caused by the new coronavirus SARS-CoV-2 (formerly known as 2019-nCoV) spread across China by the end of 2019 and gained worldwide attention. On January 30, 2020, the World Health Organization (WHO) officially declared the coronavirus 2019 (COVID-19) outbreak a public health emergency of international concern. 1 The coronavirus pandemic has challenged the activities of health professions around the world in the last 4 months.
The role of dentists in the management of patients and simultaneous prevention of the transmission of CO-VID-19 is crucial. A continuation of normal dental procedures is not adequate because of the social distancing measures introduced by governments in countries affected by this pandemic. Therefore, routine dental care was suspended, and only emergency dental care is provided. However, patients suffering from systemic diseases, such as diabetes and cardiovascular disease, are more sensitive to this viral infection and its life-threatening consequences. Therefore, the return-to-work care for dental professionals is urgent and fundamental for the population. 2,3 All healthcare professionals have the moral duty to treat patients in the safest manner to avoid the spread of the virus. The use of lasers in medical and dental practices has become widespread in the past decades, and the benefits of lasers in dental practice are universally recognized and have been reported in the literature for more than 30 years. These advantages should be emphasized and discussed in this particular moment because the adaptation of the laser clinical protocols during this pandemic is essential to ensure the best prevention and safety for patients and practitioners.
All dental laser professionals in the affected countries are involved in preventing the spread of CO-VID-19. Many countries are gradually de-confining and renewing their normal activities. The need to restart regular dental care is rising. Consequently, practitioners who use lasers in their daily practices need clear recommendations.

Objectives
This short article focuses on the risks and benefits of using some laser wavelengths and provides safety recommendations for various laser treatments in daily practice during the COVID pandemic.

Characteristics of the wavelengths in dentistry: general considerations
Several laser wavelengths are used in different medical and dental fields. The interaction between laser beams and tissues or matter depends on the wavelength and its absorption parameters. We limit our analysis to the most commonly used lasers on oral hard and soft tissues, such as erbium family lasers (Er:YAG, Er,Cr:YSGG), Nd:YAG (yttrium aluminium garnet) and Nd:YAP (yttrium aluminium perovskite doped with neodymium crystal) lasers, diode lasers or KTP (potassium-titanyl-phosphate) and CO 2 lasers, as well as red and near infrared (NIR) lights used for photobiomodulation.
We divided the main tissue and matter interaction effects of the laser wavelengths into the following categories: 1. Explosive process leading to tissue and matter ablation. The high absorption of some wavelengths produces an explosive process in the target hard and soft tissues, which leads to photothermal and photomechanical ablation. Many lasers are in this category, including Er:YAG, Er,Cr:YSGG, Nd:YAP and Nd:YAG lasers action on dark matter and metals.
2. Heat generation leading to the vaporization of soft tissues and fusion followed by the melting of hard tissues. This group includes CO 2 lasers (10.6 µm), diode lasers (445 nm and 810-980 nm) and Nd:YAP and Nd:YAG lasers action on soft tissues. The vaporization process of soft tissues produces smoke.
3. Biological photobiomodulation in tissues. Low energy red and NIR light irradiation produces a harmless interaction with cells, improves cell viability, and reduces pain and inflammation. 4. Chemical reaction enhancement. The absorption of laser light enhances chemical reactions and causes the release of free radicals (e.g., antimicrobial photodynamic therapy (aPDT), photoactivated disinfection (PAD), photodynamic inactivation (PDI), and teeth bleaching). Visible light wavelengths are primarily included in this category because of its ability to react with electrons.
Based on those 4 simplified categories, we analyzed the benefits and risks during dental treatments and suggested recommendations for safe clinical protocols of the use of lasers in different fields of dentistry.

Photothermal and photomechanical laser ablation
An aerosol is a heterogeneous system that consists of solid or/and liquid particles dispersed and suspended in the air. The particle size is between 0.001 μm and 100 μm, and aerosols can contain dust, industrial dust particles, bacteria, microorganisms, and plant spore dust. For example, atmospheric smog, which contains PM 2.5 dust particles, is a "huge" aerosol compared to a virus. PM 2.5 particulate matter is 2.5 μm or less, and the diameter of the SARS coronavirus particles is about 0.08-0.12 μm. Splatter is a mixture of air, water and/or solid substances that range from 50 μm to several millimeters in diameter. Obviously, viruses with these small dimensions easily float in the air.
SARS-CoV-2 persists in aerosols and splatter for up to 3 h, and it has a relatively long half-life of about 1.1-1.2 h. 4 Aerosols and droplets generated during speech have been implicated in the person-to-person transmission of viruses, and there is great interest in understanding the mechanisms responsible for the spread of COVID-19 in this manner. Speaking generates different sizes of oral fluid droplets that may contain infectious virus particles. Large droplets quickly fall to the ground, but small droplets dehydrate and linger in the air as "droplet nuclei", where they act as an aerosol and expand the spatial extent of the emitted infectious particles. 5 The transmission of SARS-CoV-2 in humans occurs through 4 different ways: 1) inhalation of infected droplets; 2) close contact with infected person; 3) contact with contaminated surfaces followed by face contact and dissemination; and 4) aerosol transmission of pathogens in closed spaces. During the earlier SARS outbreak, emergency dental care was provided with advice on strict personal protection, and protocols were recommended to reduce or avoid the production of droplets and aerosols using high volume and power suction. 6 Several publications reported that dental professionals were at high risk of COVID-19 infection due to close face-toface contact. 7 Some studies also suggested that COVID-19 was suspended in the air in aerosols that formed during medical procedures or indirectly through saliva in asymptomatic patients. 8 COVID-19 is about 0.12 μm in diameter, and aerosol particle sizes range from 0.001 μm to 100 μm. Therefore, it is mandatory to consider the risk of viral diffusion in aerosols produced by some laser wavelengths.
The Er:YAG and Er,Cr:YSGG lasers induce bubble dynamics, a fragmentation process and tissue ablation through photothermal and photomechanical/photoacoustical mechanisms. 9 At the beginning of irradiation with erbium lasers, the rapid expansion of the layer of evaporated water leads to an increase in pressure. The fragmentation process occurs with and without plasma formation. In both cases, ablation occurs shortly after the beginning of laser irradiation, and a plume of fine, emulsified material is expelled into the surrounding water. 10 The ablation process accelerates with increased laser energy, which reduces the thermal side effects. When the energy is increased, the pressure and speed of the ejected material also increases, which creates a larger plasma formation. 11 The lasers used for oral hard and soft tissue ablation and skin resurfacing are pulsed for short exposures of 1-10 ms (or even less than 1 ms). These fast pulses produce a more explosive effect with greater tissue ablation and, consequently, a greater spread of viral particles. 12 Some authors using erbium lasers demonstrated the dissemination of viable cells, viral genes and infectious viruses using in vitro methods. 13 Due to the proven potential of COVID-19 transmission, it is even more relevant to maintain safety precautions during dental laser applications, including limiting the use of laser-producing aerosols, except when their use has a strong therapeutic advantage compared with other modalities. The choice of the lowest parameters that allow good clinical effects and a reduction of air-flow is important. An accurate adaptation of the water-air spray during laser ablation to reduce aerosols and viral diffusion must be utilized. The suction system of the dental chair must have a high flow volume. The use of rubber dam isolation is strictly recommended. All laser handpieces and the entire device must be disinfected before and after the treatment. The protection of skin, eyes, mouth, and respiratory tract is essential.

Laser-generated heat leads to vaporization of soft tissues and smoke production
Some lasers offer several advantages during surgery compared to the scalpel. Due to the nature of the pathological tissue and the anatomical district, better results may be obtained by selecting the most suitable laser device. For example, a CO 2 laser (10,600 nm) is optimal for the excision of very hydrated lesions, such as fibrous lesions, while diode lasers (445 nm, 810 nm and 980 nm) and Nd:YAG (1064 nm) lasers, which have a better coagulating action, may be used for very vascularized lesions or tissues that are rich in vessels with a high amount of hemoglobin and risk of hemorrhage. 14,15 The factor that unites all of these wavelengths is the utilization of the photothermal effect, because the absorbed laser energy is converted to heat. Therefore, considerable amounts of smoke plume are generated, which necessitate constant suction away from the surgical site. 12 Several studies suggested that laser-derived vapors contained infectious particles or viral DNA. 13 Garden et al. detected the presence of human and bovine papilloma virus DNA (HPV and BPV) in laser vapors from warts treated with a CO 2 laser, 16 and Sawchuk et al. showed that laser vaporderived from the excision of bovine papilloma contained infectious particles. 17 A single case of a laryngeal papillomatosis (HPV type 6 and 11) was detected in a surgeon who previously treated patients with anogenital warts using a Nd:YAG laser. 18 Garden et al. 12 presented the possibility that BPV maintained its vitality and potential to infect tissue in the smoke plume produced by the laser beam. Several CO 2 laser settings were evaluated, and the laser plume at each laser setting was collected and inoculated into animals. Typical BPV lesions containing BPV developed at all laser settings. These viral tumors confirmed the ability of the laser plume to produce infection. Aside from viable viruses, the laser smoke may also contain partially inactive or incompetent viruses. Virus-infected cells and viral genes as far as 6.3 cm from the point of laser impact and laser debris at distances of up to 100 cm were described. 13 However, the lack of HPV DNA in the smoke plume after warts treatment with an Er:YAG laser was reported in 1998. 19 The literature confirmed that lasers generating smoke during surgical procedures may have infectious potential and pose a significant risk to healthcare providers. Because the presence of 2019-nCoV on the oral mucosa, tongue and saliva was confirmed, the use of photothermal lasers that produce smoke must be performed with caution, using the same precautions that were adopted for treatments that produce aerosols. The concern for laser surgeons and their assistants to inhale the SARS-CoV-2 virus via the smoke plume produced by thermal lasers is only theoretic, but likely. Considering several advantages of laser technology in oral soft tissue treatment, it is important to continue its use with the adoption of strict safety precautions for practitioners and assistants, including skin protection with gloves and gowns, eye protection, use of masks (FFP2, FFP3 if possible), and the use of a smoke suction system with a high flow volume and good filtration, positioned at the closest distance from the surgical site to avoid nosocomial viral dissemination. 12

Lasers inducing biological photobiomodulation in tissue
Photobiomodulation (PBM) is the medical way to use red or NIR laser light to achieve a chemical-and thermal-free beneficial interaction with tissues and cells. Many studies support its properties of tissue regeneration, wound healing, anti-inflammation, analgesia, and neuroregeneration. [20][21][22] Moreover, PBM may also act against microorganisms, such as yeasts, bacteria and viruses. The literature includes several interesting studies about the potential efficiency of PBM directly against microorganisms and indirectly to reinforce the immune response or reduce the damage induced by the inflammatory process. 23 Tsen et al. 24 demonstrated damage of the viral capsid caused by the Raman resonance induced by the irradiation of a femtosecond 425-nm low level laser. This particular kind of damage modified the protein structure of viral capsid and induced viral aggregation. The damage to the capsid did not prevent the penetration of viruses into the host cells, but it did not allow the viral envelope to dissolve and transmit the viral genetic message to the nucleus. Therefore, the virus was blocked and remained ineffective in the host cell cytoplasm.

investigated the direct and indirect antiviral actions of PBM and demonstrated the efficiency of 2 different PBM protocols using a 445-nm blue laser and a 970-nm NIR on neurological microglia cells infected with Zika virus (ZIKV). In both cases, the PBM irradiation enhanced the vitality of the infected cells and reduced the viral load.
Great interest should be focused on the interesting potential of the so-called transdermal PBM. 26 This technique displaced the old intravenous laser irradiation and consists of the PBM irradiation of blood via laser application over the surface of the radial artery. 27 The irradiation of blood induces the production of super oxide dismutase (SOD), which is fundamental in the protection of the cells against the toxic overproduction of reactive oxygen species (ROS). The possibility to vehiculate the beneficial effects of this positive and proven action of PBM to all organs, such as the lungs, which are highly damaged by the COVID-19 infection because of the immune system overreaction, could be very helpful and should open a path of research and clinical applications of unexpected potential.
The application of PBM in daily clinical practice is slowly increasing, with great benefits for clinicians and patients.
The PBM protocols involve the use of red and near-infrared lasers emitting in the so-called "therapeutic window" wavelength between 600 nm and 1400 nm. 20 The lasers most frequently used for the PBM are the red visible or NIR diode lasers and the more penetrating Nd:YAG lasers. It is important to emphasize that the PBM protocols do not generate heat, smoke or aerosol droplets, which is different from the traditional high-energy surgical laser applications.
Dental professionals are at high risk for nosocomial infections, and they may become potential carriers of the disease. These risks are attributable to the unique nature of dental interventions, which include aerosol generation, the handling of sharp implements, and the proximity of the practitioner to the patient's oropharyngeal region. 28 Therefore, in the present pandemic condition, the welldocumented and effective anti-inflammatory, regenerative or analgesic applications of PBM may represent an essential tool to face the various emergencies of our patients, although the crucial step of correctly identifying the proper and standardized dosages is not resolved at the moment. However, the wide consensus in the literature concerning its clinical benefits encourages the spread of its use as much as possible in daily clinical practice. The PBM may be used as an adjuvant or alternative therapy in a wide variety of situations, without side effects or drug interactions. 29 Therefore, practitioners may treat acute diseases, such as recurrent aphthous stomatitis and traumatic ulcers, temporomandibular disorders, recurrent intraoral and labial herpes, oral edema, postoperative pain following periodontal treatments, oral surgery, and endodontic procedures in emergencies using PBM, even in suspected or confirmed cases of COVID-19 infection. Similarly, endodontists may be called upon for the assessment and management of odontogenic pain, swelling and dental alveolar traumas, and they may find the support of PBM helpful. 28 All of the aforementioned considerations should lead researchers and clinicians to dedicate increasing efforts in the identification of precise and standardized PBM protocols to test the impact that this proven and biologically safe therapeutic method could have in treatment during the present COVID-19 pandemic.

Lasers enhancing chemical reactions
One of the objectives during this pandemic is to decrease the viral load in our patients during dental treatments. There are antecedents in which the use of photodynamic antimicrobial chemotherapy significantly decreased the viral load in different types of oral viruses. [30][31][32][33][34][35] Antimicrobial photodynamic therapy (aPDT) is a form of phototherapy that is performed using nontoxic, lightsensitive compounds with selectively exposed/absorbed laser light, which become toxic to target pathogens. The efficacy of PDT was demonstrated in recent clinical research on periopathogens, 36-38 carious bacteria, 39,40 onychomycosis, 41,42 and paracoccidiodomycosis. 43 Antiviral therapy using laser light, known as photodynamic inactivation (PDI), is based on the same mechanisms as photodynamic antimicrobial chemotherapy (PACT), i.e., non-thermal viral inactivation. We can differentiate 2 types of virus structural damage caused by light -type I and type II photoreactions. Both mechanisms may be active at the same time, but that depends on the photosensitizer (PS), its concentration and the concentration of oxygen. 44 The PDI can use many compounds with potential photodynamic antiviral activity, ranging from plant extracts (psoralen), eosin, curcumin, and hypericin (St. John's wort), to synthetic compounds. 45 The sensitivity of viruses to photodynamic therapy was reported for herpes infections, 30,31 HPV infections (papillomatosis), [32][33][34] cytomegalovirus CMV, 35 and other viruses. 46 Many studies showed that enveloped viruses are significantly more sensitive to photodynamic destruction than non-enveloped ones. 47 To evaluate the sensitivity of the viral photoinactivation process, molecular quantitative methods, such as nucleic acid amplification, including real time polymerase chain reaction (RT-PCR), are needed. This method is sensitive and fast, but it only detects viral nucleic acid, and it does not determine infectivity. Other methods using bacteriophages as surrogates of mammalian viruses may be useful for the quantification of infectivity. 25,44 Some studies reported reactivation of viruses, such as herpes simplex virus (HSV), after photodynamic treatment as a side effect of PDT. Photo-inactivated viruses still trigger an immune response that may be unwanted, but the treatment is still useful to generate a competent immune response and vaccination. 45,48 Schikora et al. reported that aPDT mediated by methylene blue and 660-nm red laser light applied to nasal and oral cavities of patients with COVID-19 reduced mortality and comorbidities. 49 There is no evidence from randomized controlled trials for any specific anti-nCoV treatment. One of the most frequent recommendations during this COVID-19 pandemic is the use of mouth rinses prior to dental treatments with 0.5-1% H 2 O 2 or 0.1% sodium hypochlorite for 1 min. 50,51 Other antimicrobial agents, such as 0.02% chlorhexidine digluconate, are less effective. 51 Different studies showed that the combination of H 2 O 2 with a laser produced benefits, especially related to bacterial load. 52,53 This reduction of microbial viability was demonstrated when combined with diode, neodymium or erbium lasers. Some studies demonstrated that the photoactivation of 3% H 2 O 2 with a diode laser increased the bactericidal effect due to the acceleration of hydroxyl radical generation. 53,54 Laser light activates H 2 O 2 release of free radicals and singlet oxygen. The stabilized H 2 O 2 and presence of oxygen allow the reactions of photoactivation and singlet oxygen release, which induce microbial death through the destruction of the bacterial membrane, degradation of the lysosomal membrane, alteration of mitochondrial function, and denaturation of DNA molecules. Caccianiga et al. 55 reported the successful reduction of periopathogens from implant surfaces in a clinical study using high-level laser therapy (HLLT). These studies demonstrated that the combination of different laser wavelengths (405 nm, 940 nm, 980 nm, 1064 nm, and 2940 nm) with the H 2 O 2 was much more effective than laser or antioxidant alone. 52-57 Nammour et al. 57 performed a randomized clinical study using 980-nm wavelength and 3% H 2 O 2 , and showed significant clinical improvement and microbiological reduction in a twelve-month follow-up. Grzech-Leśniak et al. 56 evaluated the effectiveness of treatments with Nd:YAG laser irradiation and 0.5% NaOCl and 0.5% H 2 O 2 solutions as single or combined treatment modalities. The authors demonstrated that combined treatment with the Nd:YAG laser at low level laser settings and a low concentration of H 2 O 2 (0.5%) or NaOCl (0.5%) solution effectively reduced the microorganisms. The combination of lasers and H 2 O 2 may be helpful for viral reduction. The good efficacy of photodynamic viral inactivation has been well-known since its early application in 1973 for the treatment of HSV infections performed by Felber. 58 The development of medicine increased the treatment options used in dental practice, which now include a wide range of antiviral application portfolios.
Future research on the possibility of destroying SARS COVID-19 using PDI is necessary.

Specific recommendations for laser dentistry in the SARS-CoV-2 pandemic
All universal recommendations for general dentistry must be respected, and practitioners must follow their own national recommendations. Additional specific care must be used for lasers, according to the following guidelines: 1. Choose the lowest laser parameters that permit efficient clinical effects and a reduction of air flow. A correct water spray during laser ablation may reduce the aerosol production and viral spread.
2. Face and eyes protection: wear safety glasses/google for each laser wavelength to avoid eye contamination. It is also necessary to use a face shield for splash protection. Use a face mask (e.g., FFP3) to protect the respiratory tract from droplets and smoke plume. 59 3. Use a rubber dam to avoid viral contamination from the mouth and saliva.
4. Use a high-volume saliva ejector/volume suction system placed at the closest distance to the treated site.
5. If a smoke plume is produced, use a vacuum system with a high flow volume positioned at the closest distance to the surgical site. The ventilation of the room and disinfection of the dental operating area can effectively limit the transmission of SARS-CoV-2 aerosols.
6. For PBM treatments: The handpieces of the devices are easily protected through the application of transparent removable plastic films to avoid any contact with the skin, mucosa and oral fluids, and permit superficial or deep penetration of the Red/NIR laser beams.
7. Special care must be dedicated to the disinfection of laser devices before and after their use. Handpiece and tips must be autoclaved.

Conclusions
Even during the present pandemic, all lasers retained their well-known advantages and benefits. However, it is mandatory to know the specific functions and properties of lasers in order to utilize their potential and adapt to the present situation using previously adopted safety recommendations. Moreover, laser light may provide a different path to face the severe health challenges of the current COVID-19 pandemic.