Assessing the Viability of Laser-Activated Dental Bleaching Compared to Conventional In-Oﬃce Bleaching Methods: A Systematic Review of Clinical and In Vitro Studies

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Introduction
In recent years, a high demand for an a ractive appearance and brighter smiles has led to an improvement in oral health, with marked increases in consultations at the dental office [1].
Amongst methods available for improvement, dental bleaching is a minimally invasive procedure to enhance the shade of the teeth, which can be performed at home following instruction by dental professionals, or as an in-office procedure [1,2].In-office bleaching offers the benefit of providing immediate results, while requiring less effort on the part of the patient [3].
The mechanism of action can be described as follows: Bleaching agents such as hydrogen peroxide (HP-H2O2) or carbamide peroxide (CP-a 1:1 addition complex of urea with H2O2), of varying concentrations are applied to the tooth surface [4].In view of its low molecular mass, HP is capable of permeating via diffusion through the interprismatic spaces in the enamel and the dentinal tubules in the dentin [5].As HP penetrates the tooth structure, it has the ability to generate reactive oxygen species (ROS).Such ROS may directly engage with organic stain molecules or induce a chemical reaction that modifies the molecular structures of tooth pigments located inside the dentinal tubules, resulting in the formation of chemical species with altered optical characteristics [4,[6][7][8][9].In addition, the oxidation of the organic enamel matrix can contribute to the treated teeth acquiring a more "opaque" appearance [10].
The whitening effects outlined above may be accelerated through the implementation of various methods, including the use of higher concentrations of HP, an extended bleaching duration, the application of heat, and activation through light or laser systems [11].By employing these techniques, it becomes possible to achieve higher levels of HP penetration into the dental tissue, a process consequently leading to an improved bleaching outcome [11].In particular, the application of light or lasers during bleaching episodes has been described in the literature as a means to decrease patients' chair time, and, more importantly, to minimize the contact time of these potentially toxic agents with dental tissues [1].The main distinction between broad-band light and laser applications lies in the fact that ordinary light sources emit a broad spectrum, multi-wavelength photonic energy, compared to the monochromatic light characteristic of lasers [4,12].However, this difference may constitute a higher risk of potential thermal damage with ordinary light sources [12].
In view of the transparency of HP to visible and near-infrared laser light commonly used in clinical practices, coloring agents are incorporated as absorbers within the bleaching gel to induce specific photothermal effects, resulting in increased ROS generation and improved color outcome [4].This approach also mitigates heat-related risks to dental pulp [4,11,13].Notably, a 10 °C temperature increase within the bleaching agent approximately doubles the rate of chemical reactions, leading to faster ROS reactions, and hence more rapid interaction with pigmented molecules within the teeth [1,4,6].
Furthermore, in terms of the photochemical effects (photodissociation), an activation of HP through direct excitation is exclusively achievable through irradiation with highenergy photons, since its dissociation energy is determined to be 2.02 eV [4,14].Photonic energy and emission wavelength are inversely related, and therefore laser systems in wavelength ranges below 614 nm are capable of inducing this reaction [4,14,15].
Additionally, a physicochemical action may take place when specific absorbers, such as titanium dioxide (TiO2) or carotenes (a range of photosynthetic pigments), are incorporated within the bleaching gel [14].When exposed to light irradiation, they have the capacity to undergo modifications, resulting in changes in their electrical charges [16].Consequently, this can lead to the destabilization of peroxides, or disruption of the pH balance of the whitening gel and hence to the generation of more reactive ROS, such as hydroxyl radical ( • OH) and/or mono-deprotonated peroxide (HO2 − ), the la er only formed at significantly elevated pH values [14,16,17].
Evidently, laser-activated bleaching primarily relies on a photothermal reaction [1,15].While optimizing the reaction rate and minimizing the risk of sensitivity reactions, it is crucial to exercise control over the heat transmi ed to the dental pulp [15].In consequence of the above, laser-activated bleaching may be considered a safe and effective option for achieving enhanced results [1,4,12,15].
However, it is important to emphasize that laser-activated bleaching may have the potential to alleviate the adverse effects commonly associated with this procedure [18,19].
The elements investigated in bleaching studies refer mainly to the color outcome and the sensitivity appearing coincident to, or following, the bleaching treatment [20].The color outcome is a measure of the effectiveness of the procedure, and it remains the most critical factor for the applied treatment's success, because of the growing aesthetic expectations of patients and dental practitioners [21].The change in shade (ΔΕ) as defined by the Commission lnternationale de I'Eclairage (CIE) L*a*b* color system is commonly used to evaluate shade changes objectively.The shade change value ΔΕ is calculated using the equation: where L* represents lightness, a* corresponds to the red-green, and b* corresponds to the yellow-blue axes [22,23].In this regard, a mean color change value (ΔΕ) exceeding 3.3 is regarded as clinically significant.This threshold underscores the importance of achieving substantial color improvements to meet the demands of modern dental care [22,23].The evaluation by instruments such as spectrophotometers, in addition to subjective visual assessments, is supported by the scientific literature since they offer a reliable and repeatable method for measurement [24].
Tooth sensitivity is a main concern, however, and may arise through selected pathways: (1) as a direct activation of neuronal receptors, or (2) through fluid shifts within the odontoblasts caused by desiccation or penetration of free radicals [25].The exposure of pulp tissue to HP remains a subject of consideration.This occurs by inward diffusion through the odontoblasts [26].The presence of HP and, further, more reactive ROS, may induce oxidative stress in the dental pulp tissue [25,26].Oxidative stress is associated with various cellular alterations, which have the potential to induce apoptosis, and can result in cell death [27].Fortunately, defense mechanisms such as enzymes within the pulp and the positive pulp pressure may reduce these phenomena [25].The quantity of HP identified in the pulp chamber typically shows a tendency to rise proportionately with both the duration of exposure, and the concentration of HP in the bleaching gel [25][26][27].
Additionally, direct overheating of the pulp tissue may play a crucial role in the occurrence of bleaching sensitivity [25,28,29].The critical threshold for a temperature increase is 5 °C for a duration of one minute, after which pulp vitality may be jeopardized [30].The extended application of heat, even at temperatures of around 40 °C (as experienced during lamp-activated bleaching), has the potential to impact the health of the dental pulp [29,31].During laser-activated bleaching, however, the temperature increase is brief, typically lasting around 30 s [31][32][33].This allows sufficient time for the pulp tissue to relax, and when the irradiation parameters are appropriately combined with suitable absorbers in the bleaching gels, it is unlikely to give rise to any adverse events [15,34].
Furthermore, direct contact between the bleaching products and oral soft tissues can cause irritation.The "caustic" nature of the bleaching agents can result in soft tissue irritation, including gingival ulcers, erosions, and changes in the periodontal tissue [35].By implementing a gingival barrier, and carefully isolating the soft tissues, this risk can be significantly reduced, ensuring a safer and more comfortable bleaching procedure for the patient [25,36].Application of antioxidant gels (i.e., those containing Vitamin E) or neutralizing agents (sodium bicarbonate) has been proposed for an immediate alleviation of these symptoms [36].
Further concerns arise regarding potential side effects on dental hard tissue.The main alterations observed in enamel during the bleaching process include modifications in morphology, surface porosity, and roughness, alterations in organic content, and a decrease in microhardness [37].These adverse effects are dependent on the composition of the bleaching gels, the concentration of peroxide, pH value, bleaching technique protocols, and the length of application.Scientific evidence is controversial, typically reporting mild-to-moderate changes where the majority of the studies are conducted in vitro [38,39], The presence of saliva and fluoride in the oral cavity plays a significant role on remineralization [40].Thus, in situ studies reveal minimal enamel alterations in view of saliva's beneficial role in the remineralization process [41,42].Implementing shorter contact times and less acidic gels in bleaching protocols can effectively reduce potential side effects on tooth structure [43].
Moreover, there is a concern regarding the possibility of systemic exposure to HP and its by-products [25].Inherent defense mechanisms at the cellular and tissue levels offer protection against potential damage caused by HP during oxidative reactions, and also facilitate the repair of any resulting damage [28].These defense mechanisms rely on the action of enzymes such as catalase, superoxide dismutase (SOD), and peroxidase, which are naturally present in saliva, tissues, and organs [25,28].By promoting the conversion of HP into water and oxygen, these enzymes help maintain a balance of ROS within the human body [25].This balance is essential for cellular and tissue health, since excessive levels of ROS may lead to oxidative stress, and cell and tissue damage arising therefrom [25].Nevertheless, a systemic exposure during in-office bleaching with reduced contact times and proper isolation techniques is rather insignificant [25].
The potential advantages of laser activation of bleaching materials have not been widely embraced by the scientific community, even though numerous studies have been performed over many years [11,12,[44][45][46].Acceleration of the bleaching procedure, shortening of the gel-tissue interaction time, and a more efficient and targeted production of ROS, along with reductions in side effects, are some of the possible benefits declared [1,47].There is inconsistency of study findings which can be a ributed to the diverse study designs, variations in whitening materials utilized, and the usage of differential activating lights [40].With regard to laser-activated bleaching, it should be noted that, even with the use of similar devices, there can be discrepancies in parameter selection and power output, which further contribute to the inconsistencies observed [48].In order to develop suitable protocols, it is essential to have a thorough understanding of laser-tissue interaction fundamentals, knowledge of laser safety, and comprehensive clinical training.These prerequisites may ensure the proper application of lasers in dental procedures [49].
Consequently, the aim of this systematic review is to critically scrutinize the protocols applied in laser-activated, compared with non-laser-activated, bleaching methods, and to evaluate their outcomes.This involved the exploration of several key factors, including the comparison of laser application with non-activated methods, evaluating the influence of photonic fluence on outcomes and potential sensitivity issues, defining a safe fluence range, assessing the impact of specific handpieces, studying the differences between "high-power" and "low-power" groups in terms of color and sensitivity, investigating the effect of irradiation wavelength applied, understanding the relationship between higher concentrations of active ingredients (HP) and outcome, and considerations of the significance of HP interaction times.These elements are crucial for the pursuit of an optimized dental bleaching protocol.

Protocol and Registration
The protocol of the present study was registered with the International Prospective Register of Systematic Reviews (PROSPERO-CRD42022306443) and followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement for reports [50].
The Review questions are:  Is laser-activated dental bleaching a favorable alternative compared to conventional in-office bleaching? Which laser wavelengths, parameters, and gel concentrations are applied during these applications?An evaluation of both clinical and in vitro studies will be performed to explore this.

Search Strategy
An electronic search was performed relating to the effects of laser-activated dental bleaching.Databases used were PubMed, Cochrane, Scopus, ScienceDirect, and Google Scholar with the following MeSH terms and keywords: (bleaching OR whitening) AND laser AND (tooth OR dental OR enamel OR dentine) published after 2010.The last search for possible recently published papers was performed in July 2023.
The initial articles retrieved numbered 1001 and were allocated per database as follows: After removing 764 duplicate reports and ineligible clinical trial registrations, 237 studies remained.
Subsequently, following the exclusion of 114 irrelevant studies, titles and abstracts of the articles were independently screened by two reviewers (E.A. and V.M.) via application of the inclusion and exclusion criteria listed below.In the case of any disagreements arising, these were resolved by discussions between the researchers involved.After implementation of these criteria, the remaining 123 articles were further reduced to 38.
Additionally, one study was included via manual citation searching from the above articles.
Eventually, 39 studies were included in this systematic review.
Regarding the nature of the articles, these studies could be further sub-divided as shown:
In accordance with the PRISMA 2020 statement [50], details of the selection criteria are presented in Figure 1.

Data Extraction
After having reached a consensus on included studies, the full paper files were transferred into the reference manager software Mendeley desktop version 1. 19 In the case of missing data, the principal investigators/authors of the study reports concerned were contacted for clarification or additional details.
The extracted data were inserted into two tables (clinical and in vitro) for further evaluation, and in case of any disagreements, these were resolved by discussions between the researchers involved.

Quality Assessment
Risk of bias assessment of all included articles was performed following data extraction by the same two reviewers (E.A. and V.M.) independently.The Cochrane risk of bias tool [52] was modified according to the requirements of this systematic review.Two tables of risk of bias were created concerning the nature of the studies (clinical and in vitro).In both cases, the risk of bias was determined according to the number of "yes" or "no" responses to the questions provided below, which were allocated to each study.
Regarding the clinical studies the variables evaluated were: In both cases, the classification was performed according to the total number of "yes" answers to the above questions.The degree of bias was computed according to the score limits provided below:
In case of any disagreements arising, these were resolved by discussions between the researchers involved.

Strategy for Data Analysis
Qualitative and quantitative statistical analyses were performed for both study categories, as explained below.Evaluation of gel and laser application protocols from the studies showing a positive outcome.

Statistical Analysis
The objective was to employ suitable statistical analysis models to determine the significance and magnitude of variable contributions towards differences observed between standard (control) and laser-enhanced tooth-whitening processes for the aims of tooth-whitening efficacy and tooth sensitivity in the clinical, and tooth-whitening efficacy and enamel/dentine morphology alterations, in the in vitro studies.For both categories, data originating from the included studies were inserted into a Microsoft Excel software module spreadsheet, version 16.71 for a Mac (Microsoft Corporation, Redmond, Washington, DC, USA), and further evaluated by statistical analysis using XLSTAT2020 software version 22.4 (Addinsoft, New York, NY, USA; www.xlstat.com,accessed on 28 August 2023).

Clinical Studies
For the analysis performed on the potential quantitative "predictor" variables, primarily one-way (completely randomized design) analysis-of-variance (ANOVA) was performed in a univariate context in order to test the significance of outcome-mediated differences between the three "predictor" variables, specifically: (1) the above (test group-control group) difference in median H2O2 dose levels (w/v%), ( 2) the (test groupcontrol group) difference in gel contact times (minutes), and (3) the fluence (J/cm 2 ) applied, with a total of three separate ANOVA models applied for each of these predictor variables.This approach was applied for both Aims 1 and 2 (tooth-whitening efficacy and tooth sensitivity respectively for the clinical studies).However, for the qualitative "predictor" variables evaluated, the above model was adapted for use as its non-parametric version, i.e., the Kruskal-Wallis test, again with the above dependent variable representing outputs of +1, 0, and −1.These qualitative variables were wavelength and handpiece size, and were only relevant to the laser treatment groups.
All study variables were weighted according to the total sample size of the study conducted.

In Vitro Studies
Data processing and transformations, together with statistical analyses of these datasets, were performed as described for the clinical studies above.However, the outcomes evaluated were tooth-whitening efficacy and enamel/dentine morphology alterations, where morphology alterations include or are indicated by changes in microhardness, roughness, permeability, adhesion, chemical stability, and caries susceptibility.As for the sensitivity parameter, the exploration of this outcome is not applicable, in view of the in vitro nature of the studies considered here.

Primary Outcomes
The primary goals of this systematic review were: (a) to evaluate the efficacies of laser-activated dental bleaching compared to those of conventional in-office bleaching procedures, both clinically and in vitro, and (b) to critically appraise the laser bleaching protocols.

Data Presentation
The extrapolated data appraised for the clinical and in vitro studies are presented in Tables 1 and 2, respectively.

Quality Assessment Presentation
The risk of bias (ROB) considered for the included studies is presented in Tables 3  and 4.  None of the studies finally included exhibited a high risk of bias.
A power meter was not used in any of the studies to calibrate the laser devices before their application.The other most common negative answers found concerned (a) the sample size calculations of studies and their required number included; and (b) blinding of the researchers.

In Vitro Studies
The classification revealed the following results: Similarly, none of the studies exhibited a high risk of bias.Overall, the mean ± standard error (SEM) Cochrane risk of bias score parameter was 8.10 ± 0.14 out of a perfect value of 10 (95% confidence intervals 7. 81-8.39).
In this category, a power meter was used in only two of the studies [84,86] to calibrate the laser devices before their performance.The other most common negative answer concerned the sample size calculation and the required number of these required for inclusion.
There was no significant difference between the two mean values of the ROB score parameter for clinical and in vitro studies (two-sample t-test), and nor did the intrasample variances differ between these two groups.

Clinical Studies (a) Analysis of aims of studies and respective evaluation methods
The various aims of the studies and respective evaluation methods are provided below:

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Proteolytic and ROS activities in dentin-pulp complex: 1/1 applied a spectrofluorometric evaluation.
Regarding the primary outcome (a), 30% of the studies presented a positive outcome, 62% showed no difference between test and control groups, while only 8% showed a negative outcome for the test group.
Specifically, for each examined aim, the number of studies and the respective outcome are exhibited in Figure 2: Aims of clinical studies and their respective outcome significance.Codification: green: test group significantly be er, yellow: no significant difference, red: test group significantly worsenote: the total number differs from the total number of studies, as some studies had more than one aim.
(c) Evaluation of gel and laser application protocols for studies showing a positive outcome Regarding the primary outcome (b), the different laser bleaching protocols that exerted a beneficial effect are displayed in Table 5.
Table 5.The laser bleaching protocols that exerted a beneficial effect in the clinical studies conducted.In the majority of the studies mentioned above, a hybrid LED/laser device was used in conjunction with varying H2O2 concentrations, resulting in observed inhomogeneity.However, the common irradiation parameters were 18/18 and 24/42 J/cm 2 , repeated 3-4 times, with a total gel contact time of 24 min.For the lower concentrated bleaching gel, the duration extended up to 48 min.As for the sole use of one wavelength, there are two studies where an 810 nm laser was applied.However, these two protocols were completely different with regard to the spot size, irradiation time, and fluence parameters employed.

In Vitro Studies
(a) Analysis of aims of studies and respective evaluation methods Respective evaluation methods concerning the various aims of the studies are shown below:  Color change: 7/10 studies used an objective (spectrophotometric) method. Mineral content: 2/2 applied energy dispersive X-ray spectrometry (EDX) and Scanning Electron Microscopy (SEM). Adhesion: 1/1 used SEM. Microhardness: 1/3 used Vickers and 2/3 Knoop hardness method. Chemical stability of dentin: 1/1 used confocal microscopy and Raman spectroscopy. Morphology of dentin: 1/2 used confocal microscopy and Raman spectroscopy, while 1/2 used SEM. Microroughness: 2/2 used a profilometer. Permeability: 1/1 used a staining technique. HP penetration: 1/1 used a spectrophotometer to monitor this. Pulp ΔΤ: 4/4 used a thermocouple connected to a digital thermometer and data logger. Gel ΔΤ: 1/1 used a thermocouple connected to a digital thermometer and data logger.
(b) Outcome assessment Regarding the primary outcome (a), 43% of the studies presented a positive outcome, 10% were negative, while 47% showed no difference between the test and control groups.Specifically, for each examined aim, the number of studies and the respective outcome are shown in Figure 3, where the abovementioned categories: mineral content, adhesion, microhardness, chemical stability of dentin, morphology of dentin, microroughness, permeability are grouped together under the term "hard tissue alterations".

Figure 3.
Aims of in vitro studies and their respective outcome significance.Codification: green: test group significantly be er, yellow: no significant difference, red: test group significantly worsenote: the total number differs from the total number of studies, as some studies had more than one aim.
(c) Evaluation of gel and laser application protocols for the studies showing a positive outcome.
Regarding the primary outcome (b), the different laser bleaching protocols that exerted a beneficial effect are displayed in Table 6.Table 6.The laser bleaching protocols that exerted a beneficial effect in the in vitro studies.It was also revealed that, besides the green light and blue hybrid LED/laser device, the activation sources most commonly used are the near infrared (NIR) wavelengths ranging from 810 to 980 nm.

Spot
Except from the LED/laser hybrid, which was used in only one of the aforementioned studies, the other parameters applied are listed below:

Statistical Analysis
The objective was to provide results arising from the application of statistical analysis models to determine the significance and magnitude of variable contributions towards differences observed between standard (control) and laser-enhanced tooth-whitening processes for the aims of tooth-whitening efficacy and tooth sensitivity in the clinical, and tooth-whitening efficacy and enamel/dentine morphology alterations, for the in vitro studies.

Clinical Studies
The results from the statistical analysis among the study groups showing significantly be er, indifferent, or significantly worse color and sensitivity outcomes (groups +1, 0, and −1 respectively) are shown in Table 8.The difference in median dose levels applied was found to be significantly lower for the −1 group than it was for the 0 group (p = 0.0032), with a mean ± 95% confidence interval (CI) difference in (w/w)% dose of −24.15 ± 10.15% H2O2.From this, it can be concluded that the non-laser-treated control group required a H2O2 dose level which was, on average, 24.15% (w/w) higher in order to achieve a status which is equivalent to that of the laseractivated bleaching group.
Nevertheless, for the contact time, this did not show any significant difference in terms of the color outcome (p = 0.069).However, on examining the contact times of each group (test and control), the values were significantly different (p = 0.002).Hence, it can be stated that, for no difference in whitening effect to be observed (score 0), a mean contact time of only 31.92 min was required for the test group, whereas 41.00 min for the control group was necessary, and this led to a clinically significant difference.
The other examined parameters (gel thickness, fluence, wavelength, and handpiece type) did not exert a statistically significant effect on the color outcome (Table 7).
(b) Sensitivity One-way ANOVA or its non-parametric equivalent (Kruskal-Wallis test) showed that no significant differences were detected in any of the weighted parameter values examined on the sensitivity outcome.9.As noted in Table 9, no significant differences between the −1, 0, and +1 groups were detected for any of the examined parameters.The difference in median dose level could not be tested since these values were exactly the same (30-39%) for all papers evaluated.
However, there was a very highly significant difference (p = 0.0004) between gel contact time for the test and control groups, with the mean test-control group difference being −10.59 min.Therefore, in order to achieve a 0 score value of no difference between these two sets of samples, we may suggest that a contact time difference of >10 min is required.
(b) Hard tissue alterations The results from the statistical analysis are shown in Table 10.The difference in median dose level could not be tested since these values were exactly the same (30-39%) for all papers evaluated.The difference in gel contact time was close to statistical significance (p = 0.088), and indicated an increase in outcome score (−1, 0, or +1) with increasing time.

Discussion
The primary goals of this systematic review were (a) to evaluate the efficacy of laseractivated dental bleaching compared to conventional in-office bleaching methods, and (b) to critically appraise the laser bleaching protocols employed to date.This systematic literature review encompassed a total of 19 clinical and 20 in vitro studies, incorporating both qualitative and quantitative assessments.The results indicated that clinical studies predominantly focused on evaluating color outcome and sensitivity, while in vitro studies primarily emphasized color outcome and hard tissue morphology alterations.Within the pool of included studies, there were 39 investigations where the bleaching gel was activated by a laser device.Specifically, a "hybrid" LED and laser combination device (HL) was applied in 12 of the clinical and 2 of the in vitro studies.Additionally, 17 clinical and all the 20 in vitro studies assessed gels of high HP concentration (i.e., HP 30% (w/v) or higher), and only 2 of the included clinical studies examined low concentration gels (i.e., HP below 20% (w/v)).
In a recent systematic review (Maran 2018) investigating whitening efficacy and tooth sensitivity (TS) in office vital bleaching, with or without light application, the authors concluded that bleaching efficacy and TS are not influenced by light activation [8].The authors pointed out that this was an overall comparison without considering variations of protocols or the type of light applied.Specifically, out of the 21 included studies, ten applied a LED/laser combination, and only four applied a laser device to activate the bleaching gel, while the remaining seven applied another source of light activation.In our analysis, the focus was instead based on laser-activated protocols, and only such studies with at least one laser or hybrid LED/laser-activated group serving as the test group were included.Amongst the included 19 clinical and 20 in vitro studies, only three [54,56,71] and three respectively [81,89,91] revealed significantly worse results for the test groups.In contrast, nine clinical [53,55,57,60,63,65,66,69,70] and ten in vitro studies [72,[76][77][78][79]82,84,86,88,90] could, at least in principle, provide significantly improved results for the laser and hybrid LED-laser groups.Nevertheless, the statistical analysis performed did not show a significant difference between these.
In another systematic review (Maran 2019) exploring the differences in efficacy (color outcome) between light-activated protocols and light-free in-office bleaching protocols, no significant difference in color change was found regardless of the HP concentration [92].Amongst the 28 studies incorporated in the above analysis, a laser activation source was utilized in only four of them, while a LED/laser device was employed in 13 of the studies.In our study, through statistical analysis, it was revealed that the conventional (nonirradiated) control group requires a H2O2 (HP) dose level which is, on average, 24.15% (w/w) higher in order to achieve a color status which is equivalent to that of the laseractivated bleaching group.This difference was highly significant (p = 0.0032).
A previous systematic review (He 2012) [93] concluded that light increases the risk of tooth sensitivity during in-office bleaching, and light may not improve the bleaching effect when high concentrations of HP (25-35% (w/v)) are employed, but with lower concentrations the effect is significantly favorable for the activated groups [93].This review and meta-analysis included 11 studies, of which three applied a hybrid LED/laser system, and only one study used a laser as the activation source.All other studies applied LED or halogen lamps.As stated above, in our analysis the control group would require a 24.15% higher HP level to achieve results similar to the laser-activated group.Additionally, and in contrast to the review of He [93], there was no significant change in sensitivity arising from laser activation.
Again, Maran et al. in 2020 conducted another systematic review which compared high and low hydrogen peroxide (HP) concentration bleaching gels [94].Notwithstanding, it was highlighted that various aspects of the bleaching procedure were not taken into account in the analysis.These aspects included: (1) the duration of gel contact with dental tissue, (2) the number and frequency of bleaching applications, (3) the duration of each whitening session, (4) the utilization of light activation, (5) the age of the study population, (6) the pH of the products, and (7) the inclusion of additional substances in the whitening gels [94].Hence, in view of this high level of heterogeneity, results arising therefrom cannot be compared to the present study which also showed similar variations.Nevertheless, here the only two significant factors influencing the outcome significantly were HP concentration and the bleaching gel contact time.
A further meta-analysis by SoutoMaior including 21 articles (out of which 11 used a LED/laser combination, only two used a laser in the test groups, and 14 used LED or halogen lamps amongst the test groups) concluded that light activation did not increase the bleaching efficacy in immediate, short, or medium terms of application [95].Unfortunately, the analysis did not provide any distinction between the types of light applied in the studies included.Once again, a high heterogeneity of data was recorded concerning activation sources and exposure time to the bleaching agent.Sensitivity incidence was not significantly different between light vs. non-light activated procedures.However, in terms of sensitivity intensity, light-activated methods demonstrated a significantly lower intensity of tooth sensitivity than that of the non-light systems.The authors a ributed this finding to the use of the LED/laser HL (hybrid light), and by its potential analgesic and anti-inflammatory effects.Additionally, the reduced contact time was found to play a contributory role towards this favorable result.In our study, no significant differences in color outcome or sensitivity were found between irradiated vs. non-irradiated groups, but similarly to SoutoMaior [95], a significantly reduced contact time (of ca. 10 min) was revealed for the test groups.

Limitations of the Study:
One limitation of the study conducted in the current paper is the overall small number of "qualifying" studies that could be subjected to statistical analysis, both parametric and non-parametric.Indeed, for the clinical and in vitro studies explored, only n = 16/19 and 10/20 of these could be included, respectively, in the final statistical analysis conducted for color outcome (a), whereas for option (b) (tooth sensitivity and morphological alterations for clinical and in vitro studies respectively), only 13/19 and 8/20 could be assessed, respectively.These restrictions were applied in view of the availability of variable parameters and criteria for these evaluations, a sizeable proportion of which were lacking, most especially for the in vitro investigations.

Diffusion of Hydrogen Peroxide
Previous research has assessed the diffusion of various concentrations of HP through dentin.These results have revealed that within a span of 15 min, HP was able to permeate a dentin width of 0.5 mm.This diffusion resulted in concentrations of HP that could potentially damage fibroblast cultures [18,96].Moreover, Trindade et al. [97] affirmed that the degree of cytotoxicity is higher when increasing the concentration and application time of the bleaching agent.Our findings, however, indicated that the laser groups had notably shorter contact times compared to the non-irradiated groups.Typically, the contact time was divided into 3-4 irradiation cycles, involving the removal and rinsing of the gel, followed by its re-application.

Pulp Response to Dental Bleaching
The pulpal response to dental bleaching has undergone extensive investigation [18,19,[98][99][100]. Indeed, in a systematic review conducted by Briso [13], the influence of different light sources on pulp tissue response during dental bleaching procedures was examined.The review findings suggested that the application of different light parameters during dental bleaching can yield distinct effects on pulp tissue.Remarkably, it was observed that PBM, when implemented with specific parameters (10 J/cm 2 on cells [19]), offers a promising potential for mitigating the detrimental impact on pulp tissue caused by bleaching.However, these authors noted that the establishment of a safe and effective protocol for the clinical application of PBM in dental practice remains an ongoing pursuit.
Clinically, pulp health can be examined through its sensitivity [53,55,56,[58][59][60][61][62][63][64][65]67,69,70], but also through the analysis of DNA-damage biomarkers, or proteolytic and/or ROS activities within the dentin-pulp complex [57,66].In the current study, the laser-activated groups did not demonstrate significantly higher sensitivity.Similarly, other investigation methods (e.g., analysis of biomarkers and ROS activities) did not yield negative results for the test groups.To be specific, out of the 14 investigations related to sensitivity, five reported positive effects [53,55,63,65,70], eight showed no significant difference [58][59][60][61][62]64,67,69], and only one had a negative outcome [56].Both studies examining DNA damage biomarkers and proteolytic activities within the dentin-pulp complex found no significant difference between the laser-activated and control groups [57,66].While these outcomes appear encouraging, we were unable to identify any statistically significant impact of contact time, wavelength, fluence, handpiece type, or gel thickness on tooth sensitivity in the analysis conducted on the studies included in this systematic review.
Presently, there is a growing trend towards performing minimally invasive procedures, and hence efforts have been made to decrease the concentrations of HP in bleaching gels, including the incorporation of catalytic nanoparticles, as well as adjusting the duration and frequency of treatment sessions [101][102][103].Although these modified approaches do not achieve the same level of effectiveness as traditionally applied higher concentrations [104], there is a consensus that prioritizing biological safety is preferable over the speed of the procedure [71].As noted herein, our statistical analysis showed that the laser-activated groups required lower HP levels to achieve results similar to those of the control groups.

Pulp Temperature Dynamics and Fluence Considerations in Dental Bleaching Procedures
The duration of a temperature increase and the retention of heat that can potentially cause pulp damage are crucial factors to consider [91].Indeed, a 5 °C increase with a duration of only 1 min is a critical threshold for maintaining pulp health [30,105].However, heat dissipation occurs rapidly according to the concept of Tissue Relaxation Time (TRT), which is directly proportional to the square of the diameter of the irradiated tissue, and inversely proportional to its diffusivity [15].Although the heat transfer through blood flow within the pulp is very low [32], the flow within the periodontal ligament may play a crucial role [106].Furthermore, a recent spectrophotometric investigation examined the light a enuation of various bleaching gels [15].This study integrated theoretical calculations based on Kreisler's research [107], and included calculations of tissue relaxation time [108].The findings from this investigation demonstrated that for a 1 mm layer of bleaching gel with light a enuation of approximately 80% and a 4 mm target area (calculated mean of pulp area) [109], the maximum acceptable fluence was determined to be 93.5 J/cm².
Additionally, based on prior estimations of approximately 80% light a enuation within the bleaching gels, and the established threshold of 5-10 J/cm 2 required for photobiomodulation (PBM) [110], and specifically for pulp tissue exposed to HP [13,19,98], we may suggest that a fluence value of 25-50 J/cm² would be the preferable choice for dental bleaching.Under these conditions, concurrent PBM may potentially occur, which could lead to advantageous effects on the dental pulp.Nevertheless, this is a value which should be confirmed by further research.
In the present systematic review, the fluences employed in the studies featured varied between 12.5 and 180 J/cm 2 .Only one clinical and one in vitro study reached the upper limit of this threshold fluence.Importantly, it was evident that studies employing high fluence parameters exhibited more detrimental effects compared to those with lower parameters [54,89].Notably, only four of the included in vitro studies investigated pulp temperature increases, as shown in Table 7 [76,84,89,91].
Gao's study [76] utilized a 532 nm wavelength laser with a fluence of 84 J/cm 2 for 20 s, which resulted in a temperature increase of 5.13 °C.In contrast, Ozyilmaz [91] employed a fluence of 17.7 J/cm 2 at 980 nm for 20 s, leading to a 6.7 °C temperature rise.However, Hahn's research [89] applied a fluence of 180 J/cm² at 980 nm for 30 s, resulting in a substantial 14.06 °C temperature increase.Notwithstanding, Al-Karadaghi's study [84] employed a fluence of 72 J/cm 2 at 980 nm for 30 s, which resulted in a more modest 2.63 °C temperature increase.
The considerable variance in results can be a ributed to various factors, including differences in parameters and methodology [111].For example, exceptionally lower results may arise from the utilization of a water bath, while excessively higher results might be linked to the direct heat absorption by the thermocouple [112].Interestingly, in one study (Al-Karadaghi et al.) [84], the temperature was continuously monitored between two laser activation cycles, and it exhibited a decline back to the initial temperature after a resting period of 2.5 min.

Clinical Considerations
When using laser technology for dental bleaching, certain guidelines should be followed.The laser fluence should not exceed 90 J/cm 2 , and the irradiation duration should be limited to a maximum of 20-30 s per cycle.To ensure an effective decrease in temperature, it is important to observe a minimum resting period of 2.5 min between cycles.Additionally, the contact time per cycle should not exceed 15 min to prevent HP diffusion into the pulp tissue.It should also be noted that a total fluence of approximately 50 J/cm 2 may have a photobiomodulation (PBM) effect on the pulp of the bleached teeth.These recommendations help ensure the safety and effectiveness of the laser bleaching procedure.

Conclusions
In the analysis conducted, laser activation did not significantly improve color compared to non-activated bleaching, but allowed for a significantly shorter gel contact time.Conversely, non-activated bleaching required higher HP doses to achieve similar results.Higher HP concentrations were not correlated to increased sensitivity; sensitivity levels were not notably higher in the non-activated groups, and our analysis did not reveal elevated sensitivities associated with any applied fluence.Prolonged contact times increased morphological changes.Laser and hybrid LED/laser exerted similar results.Laser activation has efficiency benefits with reduced HP usage, and no significant sensitivity concerns.However, treatment duration has to be considered to avoid unwanted changes, and the choice of a researched gel and protocol is crucial for reliable outcomes.Consequently, ongoing research will be seen to be vital for health and safety considerations, and also to provide guidelines in laser-activated bleaching protocols.

Figure 1 .
Figure 1.PRISMA flowchart of selected criteria for the included article reports [50].
(a) Regarding the clinical studies:  Analysis of aims of studies and respective evaluation methods. Outcome assessment. Evaluation of gel and laser application protocols from the studies showing a positive outcome.(b) Regarding the in vitro studies:  Analysis of aims of studies and respective evaluation methods. Outcome assessment. (a) Color change Regarding the color change, only the H2O2 gel concentration can significantly affect this outcome.Therefore, this factor was further analyzed as shown in Figure 4.

3. 5 . 2 .
In Vitro Studies (a) Color change The results from the statistical analysis are shown in Table

Table 1 .
Extrapolated data for the clinical studies.

Table 2 .
Extrapolated data for the in vitro studies.

Table 3 .
Risk of bias assessment results-clinical studies.

Table 4 .
Risk of bias assessment results-in vitro studies.

Table 7 .
Studies examining pulp temperature changes, including information on wavelength, bleaching gel, laser fluence, irradiation time, and temperature.

Table 8 .
The parameters examined and the respective p values found regarding the two treatment outcomes.Abbreviations: ns, not significant (threshold significance level α = 0.05).

Table 9 .
Parameters examined and the respective p values found regarding the color change outcome.Abbreviations: ns, not significant (threshold significance level α = 0.05).

Table 10 .
Parameters examined and the respective p values found regarding the morphology outcome.Abbreviations: ns, not significant (threshold significance level α = 0.05).