Effect of ultra-fast high-intensity light-curing on the properties of a new bulk-fill restorative resin composite system: A Scoping Review

Background This scoping review aims to analyze the impact of rapid high-intensity light-curing on a new bulk-fill resin-based composites (RBCs) designed for this type of polymerization. Material and Methods This scoping review was reported according to the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) and Joanna Briggs Institute Manual of Evidence Synthesis. The methods were registered on the Open Science Framework (). The literature search was conducted in PubMed/MEDLINE, Embase, Web of Science, Scopus and Cochrane Library databases. Eligibility was considered for in vitro and clinical studies evaluating the effects of ultra-fast high-intensity light-curing on a new system of bulk-fill RBCs. Results Of 1.688 articles identified, 27 were included in the qualitative synthesis. All studies were conducted in vitro. A total of 2.432 specimens were evaluated. The studies have shown that shortness light-curing may result in similar properties (stress generated by polymerization shrinkage, marginal integrity, and bond strength to dental interface) for the new bulk-fill RBCs. Conclusions Therefore, the new bulk-fill RBCs can be light-cured with a short exposure time and high intensity, providing a time-saving benefit in clinical practice, with similar results to standard light-curing in conventional composites, although, its use should be approached with caution in the flowable composite. Key words:Resin composites, polymerization, dental materials, review.


Introduction
Dentists commonly use resin composites in direct dental restorations because of their ability to replicate the optical characteristics of natural teeth (1).However, current protocols demand the use of a 2 mm incremental technique, causing prolonged clinical time and light irradiation in several increments, rendering this technique more sensitive and time consuming (2).Dental manufacturers constantly make changes to the formulation of restorative materials to improve properties, such as fluidity and translucency, try new combinations of photoinitiators and change the content and shape of the load, to ensure adequate polymerization and reduce errors during the clinical protocol (3)(4)(5).In the last decade, a new category of composite resins known as bulk fill resins has emerged.These resins have lower polymerization stress compared to conventional resins, which allows them to be effectively light-cured in increments of up to 4 mm (6)(7)(8).Laboratory studies, such as those evaluating the stress generated by polymerization shrinkage (9), have highlighted the promising performance of bulk-fill resin-based composites (RBCs) compared to conventional resins, particularly in terms of marginal discoloration and adaptation (10,11), clinically similar results were observed regarding brightness, color stability, and translucency (12,13).Photopolymerization has been a subject of study over the years, particularly aiming to reduce the photoactivation time of composite resins.One of the most recent proposed measures is the increase in irradiance of light curing units (LCUs), where a 3-second photoactivation becomes similar to activation times of 40 and 60 seconds (14).In response to this, restorative materials are being developed to meet these demands through a new polymerization approach that combines high irradiance with a short exposure time to light (14,15).Changes in the polymerization mechanism were carried out with the intention of shortening the exposure time, through the implementation of reversible addition-fragmentation chain transfer (RAFT) polymerization (16).In this scenario, modified bulk-fill RBCs have emerged in the market, available in both sculptable and flow consistencies.They have been enhanced with RAFT technology and improved characteristics of photoinitiation, translucency, and filler content, enabling ultra-fast photopolymerization for three seconds with a luminous emittance of 3050 mW/cm² (17).This innovative behavior is attributed to the addition of the β-allyl sulfone reagent, which facilitates polymerization in stages with shorter polymer chains and a more homogeneous polymerization (17).In some cases, the only alteration occurred in ensuring high translucency of particles in the pre-cure state, low filler content, and the addition of Ivocerin photoinitiator in their formulation, capable of enhancing light absorption and reducing polymerization time (18,19), proving more effective than Camphorquinone, commonly found in conventional composites (20).
The introduction of this new generation of bulk-fill RBCs has garnered attention from a clinical standpoint (21).While there are concerns regarding increased polymerization shrinkage stress, marginal integrity (22), and the potential for temperature rise due to high light intensity, which could lead to pulp and soft tissue damage (14), it is essential to analyze the behavior of these materials and their potential clinical benefits and hazards (3).This scoping review aims to comprehensively map the scientific literature regarding the impact of rapid high-intensity light-curing on the behavior of newly developed bulk-fill RBCs, specifically engineered for this advanced polymerization technique.By doing so, it addresses the pressing need for a deeper understanding of the clinical implications associated with these innovative materials.Moreover, the findings of this study can provide invaluable guidance to dental professionals in selecting the most suitable materials, fostering evidence-based clinical practice.Additionally, it offers insights for future research endeavors, promoting knowledge sharing and the ongoing improvement of restorative dentistry.

-Protocol and registration
This scoping review was conducted and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (Extension for Scoping Reviews) (23) and the Joanna Briggs Institute Manual of Evidence Synthesis (24).The protocol is registered in the Open Science Framework (<osf.io/pq57t>).As the new generation of Bulk Fill RBCs was recently introduced on the market, the scope review methodology was chosen, as it is suited for mapping more broadly and utilizing various types of evidence, addressing subjects that require deeper understanding, and thus identifying gaps for more specific future studies.
-Eligibility criteria The PCC strategy (population, concept and context) (23) was used (P: resin composites); (C: bulk-fill resin composites); and (C: rapid high-intensity light curing) which also based the research question: "What are the effects of rapid high-intensity light-curing on a new system of bulk fill RBCs?" Inclusion criteria were experimental in vitro and clinical studies (randomized and non-randomized clinical trials) that investigated the effect of ultra-fast light curing (1, 3 and 5s) on the behavior of new bulk-fill RBCs compared to standard light curing (10 and 20s) of conventional resin composites.Exclusion criteria were letters to the editor, literature reviews, clinical case reports and case series, manufacturer documents, conference abstracts and studies that have not evaluated the behavior of new bulk-fill RBCs with the rapid high-intensity light-curing protocol and studies published in languagens other than English.for the inclusion of studies was estimated based on the Kappa Score (25).
-Data collection Two reviewers (SBM and MLAOPA) collected and interpreted the data using a standardized Excel spreadsheet (Microsoft Corporation, Redmond, Washington, USA).
A consensus meeting with a third reviewer (RBEL) confirmed the extracted data.The variables collected from the studies were author's name, year of publication, restorative composites evaluated, sample number, unit of study, intervention, and main results about the new bulk-fill RBCs; in addition to data referring to the light curing unit (LCU), LCU evaluation method, radiant exposure, emission spectrum and irradiation time.
-Data analysis A qualitative and detailed synthesis of the data extracted from studies that met the eligibility criteria was performed.Tables of individualized results were formulated for the included studies and data related to the most relevant characteristics of these studies.Since composite resins polymerized with high intensity and ultra-fast protocol are materials recently launched on the market, all studies published to date were carried out in vitro.Methodological differences between the different studies may have influenced the results, however, consistency was noticed in the results of the different studies that addressed the same characteristics.As this is a scoping review, it aimed at mapping all research available; bias analysis was not conducted.

-Study search and selection
The search of electronic databases resulted in 1.691 articles: PubMed/Medline (480), Embase (354), Web of Science (417), Scopus (402), Cochrane Library ( 35) and other sources (3).The initial screening removed 929 duplicates; title and abstract reviews excluded 728 studies; 34 studies were selected for full reading.Of these studies, five were excluded because they did not evaluate the behavior of new bulk-fill RBCs with the rapid high-intensity light-curing protocol and two were not full-text available, therefore, 27 studies were eligible for the qualitative synthesis of this scoping review.
The agreement between the reviewers in the selection of studies demonstrated "almost perfect agreement" (k=0.871).Figure 1 presents a schematic flowchart summarizing the article selection process.
-Characteristics of the Included Studies The characteristics and relevant information of the 27 included studies are presented in Table 2.All the stu-

Main Results
Algamaiah et al.
(     (16,35), volumetric wear (28), cellular toxicity and viability (16,45), two body wear (32), fracture toughness (32), light transmission (32), monomer elution (36), artificial aging (37), and micro-tensile bond strength (42).Figure 2 summarizes these evaluated properties and the number of studies that addressed them.-Properties evaluation The degree of conversion was the property most investigated by the studies, which was adequate when carrying out ultra-fast light curing of the composites of the 3s PowerCure system (26,37), the high viscosity composite showed similar values (14) and was significantly higher than conventional composites (3,15,18,36), two studies reported significantly lower values (21,39).Microhardness was the second-most evaluated property, with lower values for PowerFlow (22) and PowerFill resin composites (14,32,39), although other studies did not demonstrate significant differences (22,15,29,33).About the polymerization kinetics, significant differences were found for the resin composites of the 3s PowerCure system (14,19).Regarding temperature changes, a greater temperature increase was observed during the light-curing of PowerFill ( 15) and the new bulk-fill RBCs required the shortest time to reach the maximum recorded temperature (44,45), which was considered acceptable (38) and similar to the temperature observed during conventional light curing (44).Regarding cell viability, the results showed no difference between ultrafast high-intensity polymerization and the standard protocol (45).
The ultra-fast protocol provided adequate viscoelastic characteristics and did not show significant difference to the conventional protocol (16,43), this finding applies to the quasi-static behavior, resistance, and flexural modulus ( 14).The linear shrinkage and tension generated by the shrinkage of the new bulk-fill RBCs showed behaviors close to those of conventional composites (19,21). e906 The artificial aging of the two composites resulted in inferior mechanical properties (37).Furthermore, these resin composites differed from each other in terms of linear shrinkage, with higher values for PowerFlow (21).For PowerFill, ultra-fast light curing did not induce cellular toxicity (16), provided moderate viscosity (35), higher light transmission (32), achieved better marginal integrity compared to conventional composites (31), structural integrity improvements (3), comparable microtensile bond strength (42), less polymerization shrinkage (27,36) and tension generated by polymerization shrinkage similar to conventional composites (27).The PowerFlow composite showed an intact internal structure (3), lower values of water sorption and solubility (34) and a depth of cure close to that of conventional light curing (41).
However, ultra-fast light curing impaired water solubility (32), increased porosity (36), color stability was considered clinically unacceptable (29) and caused greater volumetric wear (28) for the PowerFill.For the Power-Flow resin composite, the tension generated by the polymerization shrinkage was significantly higher (27) and the marginal integrity was significantly lower (40) there was a reduction in the bond strength to dentin (42), higher marginal discoloration (40) and this protocol negatively affected the flexural modulus (37).

Discussion
Achieving reliable and long-lasting clinical results depends heavily on the properties of restorative materials, particularly their relationship to the aesthetics and function of restorations (33), inadequate or insufficient light-curing can cause problems such as fractures, marginal deterioration, and recurrence of caries (32).To meet the clinician's needs for performing restorative procedures more quickly and without adverse consequences (22), new bulk-fill RBCs have been developed and formulated specifically for ultra-fast light curing (14).Therefore, this scoping review synthesized scientific evidence, enabling a comprehensive evaluation of the effects of an ultra-fast light-curing protocol on the properties of a new system of bulk-fill RBCs.The composition of the modified bulk-fill RBcs found in this review are in the Table 4.

New Bulk-fill RBCs Composition
Tetric PowerFlow (Ivoclar Vivadent) Bis-GMA, Bis-EMA and UDMA, plus aromatic dimethacrylate and tricyclodocane dimethanol dimethacrylate.The monomer matrix comprises a higher percentage (approx.34%) of the composite.camphorquinone/amine and Ivocerin initiator.Barium aluminium silicate glass, an Isofiller, ytterbium fluoride and a spherical mixed oxide amounting to an overall filler content of approximately 71%.Aessencio Technology: increase opacity that the composite undergoes during polymerization.
The composition of Tetric PowerFill was further optimized by including a (β-allyl sulfone) addition fragmentation chain transfer (AFCT) reagent.
Bis-GMA: Bisphenol A glycidyl methacrylate; BIS-EMA: Bisphenol A Ethoxylate DimethacrylateUDMA: Urethane Dimethacrylate; It is important to consider that all studies included in this scoping review were laboratory experiments.While these studies can yield precise and reliable results, they cannot fully encompass the clinical challenges that restorative materials face under oral conditions.Therefore, it is essential to develop randomized clinical trials to better understand the effect of ultra-rapid light-curing on new bulk-fill resin composites and its impact on the longevity of dental restorations.The modified bulk-fill RBCs identified in the studies were Tetric PowerFill (low viscosity) and Tetric PowerFlow (high viscosity).The PowerFill uses addition fragmentation chain transfer (AFCT) technology, which generates the ß-allyl sulfone radical after the light-curing process, allowing the reaction to continue, whereas the PowerFlow composite resin does not have differentiated molecules in its composition; its shortened polymerization occurs due to its translucency and the reduced amount of particles (18).The 3s PowerCure system showed characteristics similar to those of conventional resin composites (14,16,18,21,27,41,43,44), even with ultra-fast light-curing delivering half the energy of the conventional curing, which demonstrates its suitability for clinical application (43).In addition, researchers observed the benefits of using a polywave LCU with high irradiance only for resin composites with alternative photoinitiators (26), which would explain the inferior properties of a conventional resin composite when being light cured with an ultra-fast protocol (32).Conventional bulk-fill resin composites are specifically formulated for moderate intensity light-curing protocols.Consequently, using the ultra-rapid light-curing protocol on these resins prevents them from adequately achieving their intended properties (32).Depending on the range of composite resins available, and the range of photopolymerization protocols taken into consideration, the choice of restorative material plays a more important role in determining the mechanical properties and degree of conversion than the photopolymerization protocol used (conventional or ultra-fast) (15,22,32,37).In addition, studies that evaluated the mechanical properties such as depth of cure and degree of conversion of new bulk-fill RBCs at different depths e909 have demonstrated that these composites are adequate and presented equivalent results when light-cured either with the ultra-fast protocol or with the conventional protocol (14,29,43).This performance may be related to the presence of benzoyl germanium-based photoinitiators (Ivocerin) as a reinforcement for camphorquinone and the tricyclodecane dimethanol dimethacrylate (DCP) and propoxylated Bisphenol A dimethacrylate particles (43).The bulk-fill RBC Tetric PowerFill showed a degree of conversion equivalent to that of an unmodified bulk-fill RBC regardless of the light-curing mode used, while the Tetric PowerFlow showed superior results with the ISO 4049 protocol (37).
With the aim of adapting these resin composites to ultra-fast light-curing, changes in their compositions were performed (19).The PowerFill uses addition fragmentation chain transfer (AFCT) technology, thus the presence of ß-allyl sulfone polymerization modulator in the organic matrix of this composite allows for the control of its thermal and mechanical properties and is possibly associated with the development of a lower stress generated during polymerization shrinkage (3,27,29), lower loss of fracture toughness (3) and reduced water sorption (34).The ultra-rapid light-curing protocol increased the solubility of the unmodified bulk-fill resin composites (34).The ß-allyl sulfone can enhance the homogeneity of the polymer network, glass transition, and mechanical properties of the new bulk-fill resin composites (27).
The low viscosity composite resin PowerFlow has a lower load content, therefore exhibiting lower strength and durability than PowerFill resin composite (33).This reduced load content also results in greater polymerization shrinkage and a lower modulus of elasticity (27,40).It is possible to speculate that the low bond strength of the PowerFlow resin composite to dentin may be related to the greater polymerization shrinkage and to the ultra-fast protocol, which delivers less energy at greater depths (42), since its translucency is reduced during light-curing because the refractive index of the polymerized matrix changes, and rapidly increases the opacity of the material (18,37).This characteristic also causes a decrease in flexural strength (37).Furthermore, it is believed that shortened light-curing leads to the formation of a larger quantity of residual monomers, resulting in more pronounced discoloration in the new bulk-fill resin composites (29,30).A previous study demonstrated that the bulk-fill RBC Tetric PowerFlow exhibited a higher degree of pigmentation compared to other evaluated bulk-fill RBCs, likely due to differences in the photoinitiators in their compositions (29).
It has been reported that the ultra-fast protocol transiently increases the temperature during light exposure in the dental cavity, possibly causing damage to pulp tissues (38).In the articles that addressed this occurrence, it was observed that the increase in temperature was pro-portional to the increase in light intensity, which could explain the higher temperature values for the PowerFill and PowerFlow resin composites, although these values were considered clinically acceptable (15,38).Therefore, the application of these 4 mm thick composites in cavities with up to 1 mm of remaining dentin would possibly be clinically safe for use (44).A recently published in-vitro study in simulated pulp chambers with 0.5 mm of remaining dentin and Vero cells concluded that the 3s rapid high-intensity light-curing protocol of bulk-fill RBCs caused a temperature increase greater than 10s and showed cell viability similar to and comparable to the standard protocol (45).Clinical trials are needed to confirm these findings.
The less favorable results regarding the marginal integrity of the PowerFlow resin composite can also be credited to the ultra-fast polymerization speed, which may be responsible for causing very rapid shrinkage, and a short pre-gel phase, preventing deformation during polymerization, and increasing the stresses resulting from high polymerization shrinkage, especially in preparations with high C-factor.To solve the problems related to the significantly lower marginal integrity and microhardness of the flowable resin composite using the ultra-fast protocol, the insertion of a cover layer was suggested, which could attenuate its performance in a clinical environment (40), especially in the region of intense masticatory loads (33).However, the findings regarding marginal integrity should be interpreted with caution, as the type of dental cavity (Class V or Class II) used in the studies may influence the behavior of the modified bulk-fill RBCs (40).
Laboratory analyses recommended that the clinical tolerance for the use of resin composites of the 3s Power-Cure system should be limited to an exposure distance of 5 mm, to avoid insufficient polymerization; when this is not possible, an additional light-curing is advised (14), and cases like this, it was suggested to wait at least the same 3s before the second irradiation to allow thermal recovery and avoid heat propagation (44).It is also of utmost relevance to emphasize that clinicians are responsible for the distance and angle of the LCU tip during the light-curing of the restorative material (44); thus, when using an exposure of 1 or 3s, the operator's aim must be excellent because if the light is off-target, there will be a significant reduction in energy, resulting in insufficient light-curing with all the consequences arising from a low degree of conversion (39).In addition to this, it is recommended that clinicians use RBCs formulated specifically for rapid light-curing, as they require less energy (32).The limitations of this review include the level of scientific evidence of the evaluated studies, which included only experimental in vitro studies, therefore, the results should be interpreted with caution, as these studies are e910 conducted under controlled conditions and with calibrated operators; the high methodological heterogeneity of the included studies, which made the quantitative analysis of the reported results difficult, and the lack of standardization of specimens and restorative materials used as controls.With this evidence mapping, we suggest future systematic reviews to address more specific inquiries concerning these new resins, such as the impacts of elevated temperatures on dental elements.More studies using natural teeth would be beneficial for performing laboratory analyses that are closer to clinical reality, and, most importantly, clinical trials.Nevertheless, this scoping review allow to better predict the impact of ultra-fast light-curing on the behavior of new bulk-fill RBCs, as can help the design of future clinical studies to provide new information, clinically validate laboratory results, and standardize safe protocols for use.

Conclusions
The new bulk-fill RBCs can be light-cured with a short exposure time and high intensity, providing a time-saving benefits in clinical practice.This light-curing protocol has shown similar results to standard light-curing in conventional composites.Although, its use be approached with caution concerning flowable resin composites.

Fig. 1 :
Fig. 1: Screening and registration and PRISMA-ScR flow diagram summarizing the article identification and selection processes.

Fig. 2 :
Fig. 2: Properties of resin composites evaluated in the experimental in vitro studies.

Table 1 :
Search strategy for each database.
powerflow) AND (led dental curing-light OR led dental curing-lights OR dental curing light OR dental curing lights OR curing light dental OR curing lights dental OR dental curing OR dental curing lights OR high-power led OR high intensity led OR light curing intensity OR fast highintensity light-curing OR rapid high-intensity light-curing OR ultra-fast light-curing OR high irradiance light polymerization OR shortened light-curing) No filters applied Embase ('composite'/exp OR composite) AND ('resins'/exp OR resins) OR (('composite'/exp OR composite) AND ('resin'/exp OR resin)) OR (('resin'/exp OR resin) AND ('composite'/exp OR composite)) OR (('resin'/exp OR resin) AND ('composites'/exp OR composites)) OR (('resin'/exp OR resin) AND ('restoration'/exp OR restoration)) OR (('resin'/exp OR resin) AND restorations) OR (('composite'/exp OR composite) AND ('restoration'/exp OR restoration)) OR (('composite'/exp OR composite) AND restorations) AND bulk AND fill OR 'bulk fill' OR bulkfill OR ('bulk fill' AND composite) OR (bulk AND fill AND composite) OR bulk OR (tetric AND powerfill) OR (tetric AND powerflow) AND led AND dental AND 'curing light' OR (led AND dental AND 'curing lights') OR (dental AND curing AND light) OR (curing AND light AND dental) OR (curing AND lights AND dental) OR (dental AND curing) OR (dental AND curing AND lights) OR ('high power' AND led) OR (high AND intensity AND led) OR (light AND curing AND intensity) OR (fast AND 'high intensity' AND 'light curing') OR (rapid AND 'high intensity' AND 'light curing') OR ('ultra fast' AND 'light curing') OR (high AND irradiance AND light AND polymerization) OR (shortened AND 'light curing') PowerFlow composites exhibited faster conversion than the controls.PK and the rate of polymerization were influenced by specimen thickness and material viscosity.ΔE00 and VH All BF composites showed clinically unacceptable ΔE00.There was no difference the bottom VH values of PowerFill.

Table 2 :
Characteristics of the included studies.