Effect of Different Laser Wavelengths on Periodontopathogens in Peri-Implantitis: A Review of In Vivo Studies

Nowadays, many studies are examining the effectiveness of dental lasers in the treatment of peri-implantitis; however, most of them only report periodontal parameter changes. The authors of this review tried to address the question: “What is the effect of different laser wavelengths on oral bacteria that cause peri-implantitis?” An electronic search of PubMed and Cochrane Central Register of Controlled Trials was performed. The following search terms were used: (peri-implantitis OR periimplantitis) OR/AND (microbial OR microbiologic) AND (laser OR Er:YAG OR erbium OR diode OR Nd:YAG OR neodymium-doped OR Er,Cr:YSGG OR chromium-doped). Initially, 212 studies were identified. After screening the titles and abstracts and excluding studies according to predefined inclusion criteria, seven publications were included in the review. Three studies about the effect of aPDT (antimicrobial photodynamic therapy) reported a decrease in the different bacterial strains associated with peri-implantitis, e.g., A. actinomycetemcomitans, P. gingivalis, P. intermedia, T. denticola, T. forsythia, F. nucleatum, and C. rectus. Two studies showed that the high-power diode laser may have some effect on peri-implant pathogens. Two articles about the Er:YAG laser reported a lowering in the count of oral pathogens; however, it was hard to determine if this was due to the use of the laser. aPDT has the ability to decrease the count of peri-implant pathogens, whereas Er:YAG laser application shows no significant effect on oral bacteria in the long term.


Introduction
Endosseous implants are a widely chosen prosthetic rehabilitation treatment for missing teeth [1,2]. However, studies report that up to 56% of implant patients and even 43% of implant sites can be ailed by the peri-implant inflammatory process, known as peri-implantitis [3]. Peri-implantitis is described as inflammation of an implant supporting tissues in association with bone loss, which if left untreated, can result in the destruction of the bone [3].
The etiology of peri-implant disease is diverse. Peri-implantitis is reported to occur more frequently in nicotine smokers [4] and in patients with periodontitis [5,6]. Nonetheless, the presence of microorganisms is fundamental for the development of the infection [7,8]. As studies have shown, [7,8] peri-implantitis harbors high levels of diverse periodontal pathogens, predominantly gram-negative species. This is also because peri-implant tissue is more complex then periodontal tissue. Peri-implant tissue is characterized by a lack of the periodontal ligament, cementum, and has altered connective tissue fibers (Figure 1). The current principles of peri-implantitis treatment are based on results established from periodontal disease therapies, and they can be divided into mechanical and/or chemical treatments [9]. Mombelli et al. [8] suggested that decontaminating the implant surface, reducing or removing periodontal pockets, re-osseointegration, and proper oral hygiene should be employed in periimplantitis treatment. In mechanical modalities (subgingival debridement, conventional and ultrasonic scaling, carbon fiber curettes), [8,9] the main goal is the removal of biofilm from implant surfaces. However, these methods can damage the surface of implants, which can be seen in scanning electron microscopy, without effectively decontaminating the surface [10,11] or producing a significant pocket depth improvement [11]. In addition, various systemic and local chemical antimicrobial agents (citric acid, chlorhexidine, hydrogen peroxide application) [9,12] and antibiotics have been introduced for the treatment of peri-implantitis [8,13,14]. These agents complement mechanical treatment and result in more effective eradication of periodontopathogens. Nonetheless, antibiotic modalities are not free from disadvantages and can cause increases in antibiotic resistant periodontopathogens, negative systemic reactions, and immunosuppression in patients [8], whereas chemical agents can cause tissue damage or irritation [10,13].
An alternative antimicrobial treatment without the complications listed above is laser-assisted therapy. Laser irradiation targets pathogens locally; thus, no systemic reactions exist. Various in vitro studies report that some laser wavelengths can not only eliminate bacteria but also bacterial toxins and lipopolysaccharides (LPS) [15,16]. In periodontal diseases, lasers can be used as a separate treatment or in addition to conventional scaling and root planning [17][18][19].
Nowadays, a variety of lasers are used in dental procedures as innovative therapeutic techniques that reduce bleeding, swelling, and pain [20][21][22][23][24][25]. The most used lasers are the diode and the Er:YAG laser [20]. They can be employed in both non-surgical and surgical procedures [21]. The diode laser is characterized by a hemostatic effect, the promotion of healing, and its use in sulcular debridement [21]. The Er:YAG laser has the capability to be absorbed by water molecules and therefore results in minimal thermal damage and tissue carbonization [21]. Thus, bacteria are eliminated without overheating the surrounding tissues. Additionally, some studies report that the Er:YAG laser is capable of removing plaque and calculus even from rough implant surfaces [21,26,27].
The aim of the present study was to evaluate the bactericidal potential of laser application in patients with peri-implantitis. The current principles of peri-implantitis treatment are based on results established from periodontal disease therapies, and they can be divided into mechanical and/or chemical treatments [9]. Mombelli et al. [8] suggested that decontaminating the implant surface, reducing or removing periodontal pockets, re-osseointegration, and proper oral hygiene should be employed in peri-implantitis treatment. In mechanical modalities (subgingival debridement, conventional and ultrasonic scaling, carbon fiber curettes), [8,9] the main goal is the removal of biofilm from implant surfaces. However, these methods can damage the surface of implants, which can be seen in scanning electron microscopy, without effectively decontaminating the surface [10,11] or producing a significant pocket depth improvement [11]. In addition, various systemic and local chemical antimicrobial agents (citric acid, chlorhexidine, hydrogen peroxide application) [9,12] and antibiotics have been introduced for the treatment of peri-implantitis [8,13,14]. These agents complement mechanical treatment and result in more effective eradication of periodontopathogens. Nonetheless, antibiotic modalities are not free from disadvantages and can cause increases in antibiotic resistant periodontopathogens, negative systemic reactions, and immunosuppression in patients [8], whereas chemical agents can cause tissue damage or irritation [10,13].

Focused Question
An alternative antimicrobial treatment without the complications listed above is laser-assisted therapy. Laser irradiation targets pathogens locally; thus, no systemic reactions exist. Various in vitro studies report that some laser wavelengths can not only eliminate bacteria but also bacterial toxins and lipopolysaccharides (LPS) [15,16]. In periodontal diseases, lasers can be used as a separate treatment or in addition to conventional scaling and root planning [17][18][19].
Nowadays, a variety of lasers are used in dental procedures as innovative therapeutic techniques that reduce bleeding, swelling, and pain [20][21][22][23][24][25]. The most used lasers are the diode and the Er:YAG laser [20]. They can be employed in both non-surgical and surgical procedures [21]. The diode laser is characterized by a hemostatic effect, the promotion of healing, and its use in sulcular debridement [21]. The Er:YAG laser has the capability to be absorbed by water molecules and therefore results in minimal thermal damage and tissue carbonization [21]. Thus, bacteria are eliminated without overheating the surrounding tissues. Additionally, some studies report that the Er:YAG laser is capable of removing plaque and calculus even from rough implant surfaces [21,26,27].
The aim of the present study was to evaluate the bactericidal potential of laser application in patients with peri-implantitis.

Focused Question
A growing body of researchers is examining the clinical effectiveness of dental lasers in the treatment of peri-implantitis. However, most of the studies mainly engage with changes in periodontal parameters, less often in oral periodontal pathogens, and most them are in vitro studies. Therefore, the addressed focused question in this paper was "What is the effect of different laser wavelengths on oral bacteria that cause peri-implantitis?"

Protocol
The text of the review was structured in accordance with guidelines from PRISMA [28] and the Cochrane Handbook of Systematic Reviews of Interventions [29]. A detailed description of the study protocol is included in this paper (Table 1).

Eligibility Criteria
Studies were considered acceptable for inclusion in the review if they met the following criteria: • Studies involving human subjects; • Patients with peri-implantitis; • Surgical or non-surgical use of dental lasers in the treatment of peri-implantitis; • Evaluated changes in specified oral bacterial profiles before and after the laser treatment; • Prospective case series; • Non-randomized controlled clinical trials (NRS); and • Randomized controlled clinical trials (RCT).
The exclusion criteria the reviewers agreed upon were as follows ( No restrictions were applied with regard to the year of publication or to the language of the study.

Information Sources, Search Strategy, and Study Selection
An electronic search of PubMed and the Cochrane Central Register of Controlled Trials (CENTRAL) databases was conducted on 5 April, 2019. To review the data available on the subject of interest, the following search terms were used: (peri-implantitis OR periimplantitis) OR/AND (microbial OR microbiologic) AND (laser OR Er:YAG OR erbium OR diode OR Nd:YAG OR neodymium-doped OR Er,Cr:YSGG OR chromium-doped) ( Table 1). The search was limited to human subjects and studies that adhered to other eligibility criteria. The references of all selected full-text articles and related reviews were scanned. Only papers with available or accessible full-text versions were considered. If required, an attempt was made to contact the corresponding authors of unpublished or missing data. Screening was performed by each author and the acquired data were compared.

Data Collection Process, Data Items
Two reviewers independently extracted data from papers that met the inclusion criteria. The following data were used: first author, title, year of publication, study design, laser type, laser parameters, and changes in specified oral bacterial profiles before and after treatment. Extracted data were entered into a standardized Excel file.

Risk of Bias in Individual Studies
In the initial study selection, to minimize the potential for reviewer bias, each author screened titles and abstracts independently. The level of agreement between reviewers was determined by the Cohen k test. Any disagreement about the inclusion or exclusion of a study was resolved by discussion.

Quality Assessment
The methodological quality of each included study was evaluated independently by two blinded reviewers. The study design, implementation, and analysis were based on the following criteria: population representativeness in the treatment group (average of the population), demonstration that the outcomes were not present at the start of the treatment, comparability of the baseline and the outcome parameters, accuracy of the microbial genome evaluation technique, randomization, adequate follow-up (for outcomes to occur), and acceptable follow-up loss (complete follow-up, subjects lost to follow-up unlikely to introduce bias) ( Table 2). The descriptive information about the studies was graded. The score range was from 0 to 6 points, with a higher score indicating a higher study quality. Any disagreements were resolved through discussion until reaching a consensus. Persson et al. [32] Arisan et al. [33] Yoshino et al. [34] Bassetti et al. [14] Population representativeness in the treatment group (average of the population) Comparability of the baseline and the outcome parameters Accuracy of the microbial genome evaluation technique Adequate follow-up (for outcomes to occur) Acceptable follow-up loss (complete follow-up, subjects lost to follow-up unlikely to introduce bias)

Risk of Bias Across Studies
After the scores of each study were calculated, an overall estimate of the risk of bias (low, moderate, or high) was made for each publication, as recommended in the Cochrane Handbook for Systematic Reviews of Interventions [29].

General Characteristics of the Included Studies
Seven articles reporting four randomized controlled trials [14,30,32,33] and three prospective case series [31,34,76] were included in this review (Table 4). Two studies evaluated the photothermal effect of the diode laser [30,77], two evaluated the Er:YAG laser [32,34] and three evaluated aPDT (antimicrobial photodynamic therapy) [14,31,76] on peri-implant pathogens. Various bacterial profiles were assessed in the articles. Also, the follow-up period differentiated from right after the therapy to up to 2 years (Table 5).

Results of Individual Studies
In the study by Dörtbudak et al. [76], the authors reported that the aPDT reduced the bacterial counts of A. actinomycetemcomitans (A.a.), P. gingivalis (P.g.), and P. intermedia (P.i.). Also, Bassetti et al. [14] found that mechanical debridement and additional aPDT (TBO dye) seem to be able to decrease the total bacterial count, but the difference was reported to be of no significance. Caccianiga et al. [31] reported a medium decrease of most periodontal pathogens, including A.a., P.g., T. denticola (T.d.), T. forsythia (T.f.), F. nucleatum (F.n.), and C. rectus (C.r.) but excluding E. corrodens (E.c.), that increased in count after aPDT (3% hydrogen peroxide). Moreover, in the research by Birang et al. [30], diode laser irradiation of the implant site decreased the counts of P.g. and A.a. The authors also reported significantly decreased A.a., T.f., and P.g. in the test group that used a photosensitizer. These findings correspond to the study by Arisan et al. [33] who concluded that diode laser application reduced the total bacterial count, but the difference was stated to be of no significance.
Yoshino et al. [34], in their study on the Er:YAG laser, reported eradication of oral pathogens. Additionally, Persson et al. [32] showed lower bacterial counts in the laser-treated group for F. nucleatum naviforme and F. nucleatum nucleatum at 1 month after treatment and in comparison to baseline levels. However, at 3 months and at the final examination at 6 months, counts of bacteria increased (Table 6).

Synthesis of Results
The studies included in the review varied in terms of the following features: laser parameters, evaluated periodontal pathogens (Table 4), follow-up period, and microbial genome evaluation (Table 5). Collected features were analyzed and put into an Excel file ( Table 6).
The authors of the review paid special attention only to significant microbiological changes in the bacterial counts of each study at each follow-up period. It was impossible to analyze each bacterial count since some studies did not include that information. Therefore, the authors decided to focus on the changes (lower, higher) in relation to the baseline, accentuating significant variables (Table 6).

Risk of Bias Across Studies
The articles by Persson et al. [32] and Birang et al. [30] reported blinding of the patients and examiner and random allocation of the patients by a computer program; therefore, the studies were considered to have a low risk of bias. Another two were RCTs and had some above information missing and were reported to be at a medium risk of bias [14,33]. Since three studies were prospective case series [31,34,76], they were considered to be at a high risk of bias (Table 7).

Discussion
The findings of this review show that laser application in peri-implantitis may decontaminate the implant surface and eradicate periodontal pathogens in some cases. The seven papers included to this review can be divided according to the laser treatment modalities used. Two studies evaluated the photothermal effect of the diode laser [30,33], two evaluated the Er:YAG laser [32,34] and three evaluated antimicrobial photodynamic therapy [14,31,76] on the eradication of peri-implant pathogens.
In the studies of Arisan et al. [33] and Birang et al. [30], the authors used high-power diode lasers to eradicate the bacterial biofilm from the implant titanium surface due to the photothermal effect. The use of high-power diode lasers on tissues is inextricably linked with the increase of their temperature, which, in turn, causes several changes in the structure of tissues through protein denaturation, microbial elimination, water evaporation, coagulation, and even melting [17,18]. The first changes in the structure of the oral soft tissue begin when its temperature was increased to 42 • C (∆T = 5 • C), and then changes occurred within the cell membranes [78]. In the study by Arisan et al. [33], mechanical therapy and additional diode laser application decreased the total bacterial count on the affected peri-implant zone. The author measured insignificant differences in the results of decreasing of A.a., P.g., T.d., T.f., C.r., E.c. species after a 1 month follow-up period in contrast to the control group. In turn, the research by Birang et al. [30] showed that diode laser (810 nm, 300 mW, 30 s) irradiation of the implant site decreased the count of P.g., and differences in A.a. reached the significance threshold. Moreover, the authors reported significant decreases in the A.a., T.f., and P.g. strains after 3 months in the test group that additionally used infracyanine green. However, there were no significant differences between the laser modalities (with or without infracyanine green).
Antimicrobial photodynamic therapy was applied for peri-implantitis treatment in the studies of Dörtbudak et al. [76], Bassetti et al. [14], and Caccianiga et al. [31]. During this process, a combination of nontoxic photosensitizing dyes with appropriate wavelengths of light led to the production of reactive oxygen species. The reactive oxygen species produced with aPDT had high killing potential against bacteria, fungi, and viruses [33]. In the study by Dörtbudak et al. [76], the authors evaluated the culture counts for A. actinomycetemcomitans, P. gingivalis, and P. intermedia. It was found that aPDT reduced the bacterial counts measured immediately after therapy by 2 logs. These findings correspond to Caccianiga et al. [31], who reported a medium decrease in most periodontal pathogens (A.a., P.g., T.d., T.f., F.n., C.r.) excluding E.c. which increased in count after a 6 month follow-up period. Also, in the study by Bassetti et al. [14], mechanical debridement and additional aPDT application reduced the total bacterial counts on affected peri-implant sulcus. The counts of P.g. and T.f. decreased significantly from baseline to 6 months following aPDT and after 12 months in the local antibiotics group. The authors concluded that aPDT seems to be an alternative approach to local antibiotics for non-surgical therapy of peri-implantitis.
Two articles assessed the influence of the Er:YAG laser on oral cavity microbes associated with peri-implantitis [32,34]. The Er:YAG laser has the highest absorption coefficient in water, which is the main component of vital organisms. This physical property influences the ability of the wavelength to damage of the water-rich cells and constitutes the significant killing potential of this laser. [79] Yoshino et al. [34], in their study, reported on the eradication of oral pathogens; however, the laser application was preceded by a systemic antibiotic protocol (amoxycilin, metronidazol); therefore, it is impossible to conclude the real reason for the bacteria count decrease. In turn, Persson et al. [32] showed lower bacterial counts in the laser-treated group for F. nucleatum naviforme and F. nucleatum nucleatum at 1 month after treatment and in comparison to baseline levels. However, at 3 months and at the final examination at 6 months, the counts of bacteria increased. It should be highlighted that the study conducted by Person et al. [32] was a non-surgical approach to the treatment of peri-implantitis. The threads and rough surfaces of implants are complicated to manage by non-flap methods; thus, this could explain the lack of reduction in the total bacterial amount in the study.
More studies with bigger populations of patients are needed to determine the use of dental lasers in bacteria decontamination.

Conclusions
It can be concluded that a high-power diode laser may have some effect on peri-implant pathogens causing peri-implantitis, whereas Er:YAG laser application shows no significant effect on oral bacteria in the long term.
aPDT has the ability to reduce the total count of the different bacterial strains associated with peri-implantitis, e.g., A. actinomycetemcomitans, P. gingivalis, P. intermedia, T. denticola, T. forsythia, F. nucleatum, and C. rectus. Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest:
The authors declare no conflict of interest.