Flowable Composite Resins as Sole Restorative Materials

1. Introduction

Until recently, the incrementally placed conventional, medium viscosity composite resins proved to be the gold standard in restoring both anterior and posterior teeth. Conservation of tooth structure, adequate shade matching, satisfactory mechanical properties and the lower cost compared to indirect restorative counterparts, represent their beneficial side [1]. However, their poor rheological characteristics lead to marginal defects due to unfavorable adaptability to cavity walls and voids between increments [2]. To confine these drawbacks, the first-generation flowable composites were introduced in the 1990s and have been proposed as liners prior to restoring a cavity with medium-viscosity composites. They included 20-25% less filler loading than medium-viscosity composite resins [3,4]. Their low viscosity equals ease – of – use, great flow, flexibility, good adaptability to cavity walls, preferable handling and wetting properties, and restricted entrapment of air bubbles in the mass of the material. Despite all the above-mentioned benefits, the reduced level of fillers had an undesirable effect on their mechanical and optical properties (inadequate wear resistance, inferior flexural strength, lower fracture toughness, subordinate color, gloss stability, and decreased modulus of elasticity), limiting their indications only as class V restorative materials, liners, pit and fissure sealants and as materials for marginal repair of restorations. Furthermore, the average volumetric polymerization shrinkage rate of 5% constitutes an additional disadvantage for traditional flowable resin composites [2].

Dental material manufacturers respond to clinicians’ demand for simpler, color-adaptive, and durable materials suitable for a wide range of indications, including Class I and II cavities, extensive tooth wear, full-mouth rehabilitations and novel restorative procedures such as the injection moulding technique, by developing novel nanotechnology-driven flowable composites. [5,6,7]. The injection moulding technique, first introduced by Douglas Terry and John Powers in 2014, is a restorative technique that uses transparent silicone molds based on a wax-up to generate the final restorations [5]. The flowable composite is injected into the mold to form the final shape of the restorations. The predictable reproduction of the anatomical contour, the more accurate execution of the procedure, and the use of easy – to – handle flowable resins proved to be factors that gained the interest of dental experts.

Continuous attempts to improve the mechanical properties of flowable composites are made, mainly through modifications in their formulation, such as the increase in filler loading, optimization of filler particle size, incorporation of refined monomers, and treatment of filler particles. Through these alterations, a significant improvement of their physical and mechanical properties is accomplished, maintaining their superior marginal adaptation [8]. The innovative formulation of these flowable composite resins is characterized by a higher filler content ranging from approximately 61 to 71 % by weight, which broadens their indication spectrum since they may now bear high occlusal loads [9,10]. According to a manufacturer’s recommendations, a “new-generation” flowable resin composite shows a more homogeneous dispersion of nano–sized glass filler particles. The developed silane treatment method, generating a favorable adhesion between the filler particles and the organic matrix, corresponds to the materials’ upgrade [11]. Until now, these statements have not been clearly verified. This new era of flowables is described mainly as highly filled flowable resin composites. The commercial terms next-generation flowable composites or injectable composite resins are interchangeably used, but may lead to confusion.

Despite their improved filler content, highly filled flowable resins still present challenges related to wear resistance, color stability, and polymerization shrinkage. This review aims to critically evaluate and shed light on the narrow spectrum of existent knowledge originating from up–to–date laboratory experiments, randomized controlled clinical trials, and case reports referring to highly filled flowable composites. Since these flowables are used as the preferable material for the injection moulding technique, a short reference on clinical cases implementing this novel technique is going to be made, and the clinical performance of the materials used will be discussed.

2. Materials and Methods

The article selection process started with a comprehensive search of the following databases: PubMed, Scopus, and Google Scholar. The search strategy was developed using the keywords presented in Figure 1. Date and language restrictions were applied. Studies in languages other than English are eliminated from the selection process. For this review, we included research published between 2016 and February 2025 to capture the evolution and the recent developments in the field of highly filled flowable composite resins. This data limitation has been adopted since a systematic review and meta-analysis on the clinical performance of flowable composites was published in early 2017 [12]. Two reviewers screened titles and abstracts for eligibility. When there was a conflict, a consensus was reached by discussion. Letters to the editor, patents, review articles, short communications, and conference papers are excluded. The procedure employed in this review is reflected in Figure 1.

3. Results

The first class of results recites clinical cases utilizing flowable materials, emphasizing the injection moulding technique, whereas the second class focuses on the laboratory behavior of highly filled flowable resin composites, as well as on their clinical performance in randomized controlled clinical trials.

3.1. Clinical Case Reports of Injectable Moulding Technique Using Flowable Resin Composites – Interpretation of the Clinical Outcome

Table 1 presents the total of clinical reports available, giving information on treated teeth, patient’s profile, material used, protocol followed and initial outcome of the procedure. Follow-up information is given where applied [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36].

The injection moulding technique is described as a novel treatment modality with a direct/indirect character that translates an analog or digital wax up into a final resin composite restoration, without or with confined tooth preparation [13]. In order for the final restoration to be accomplished one or more transparent silicone indices based on the wax up (analog or digital) are fabricated, which allow the material – mainly flowable composite resins- to be passively inserted and cured, therefore minimizing the possibility of index and restoration distortion. It presents similarities to its ancestral technique named “index technique”, that uses a single transparent index and stamps the resin composite directly on the surface of the single prepared tooth [37,38]. Their main difference is based on the material used for the final restoration. At the “index technique” preheated conventional composite resin is used, whereas the material of choice for the injectable technique is a flowable composite resin. Furthermore, in a study of Kouri et al. in 2023 [39], the injection moulding technique and its modifications move from the diagnostic wax- up towards the final direct resin composite veneers in a more accurate and predictable manner compared to the “index technique”, which uses preheated resin composite. Douglas Terry and John Powers have been the first to present this alternative technique in 2014 and reported its numerous applications, among others the repair of fractured teeth and restorations, the fabrication of transitional restorations and pediatric composite crowns, the resurfacing of occlusal wear on posterior composite restorations, the development of composite prototypes for copy milling, the establishment of an alternated vertical dimension prior to restoring with final restorations and its use as a communication tool between patients and dental practitioners [5,40]. Diastema closure, reshaping crown morphologies, and correction of dental misalignments are additional indications implementing the injection technique in the restorative dentistry toolkit. The restorations generated from this treatment method may be either transitional or definitive [13,20]. Furthermore, this technique is nowadays used to fabricate an ideal shape and core size without additional tooth preparation [41] or even resin bonded fixed dental prosthesis [17,32]. The tremendous development of Digital Dentistry entails the incorporation of digital workflows in the injection moulding procedure by the fabrication of digital wax-ups and 3D-printed molds, rigid or soft indices and rigid holders of the soft transparent index [42].

The compositions of the flowable resin composite materials used in these clinical cases are presented in Table 2.

These materials have been used in a wide range of situations, such as single tooth restorations, teeth recontouring by fabricating four to six veneers on anterior maxillary and mandibular teeth, replacement of missing teeth as direct composite resin-bonded fixed dental prosthesis and full mouth rehabilitation with or without alteration of the vertical dimension. When minimal tooth structure loss is present, these materials proved to perform to the maximum [13,20,22,30]. When evaluating the use of highly filled flowable resin-based materials in full mouth treatments, no defects were present in the 6- month and one-year follow-up periods [21,31]. Staining on the material’s surface and/or in the tooth – material interface, minor chippings and presence of voids are defects related to some clinical cases [16,29]. These observations strengthen the belief that highly filled flowable resin composites may present inferior properties related to resistance to occlusal forces and color stability.

The results of the clinical reports should be interpreted with general reservation for the following reasons:

• Despite the material per se, external factors such as medical record, intraoral temperature and humidity, acid consumption, grinding, poor oral hygiene, parafunctional behaviors, and polishing procedures are additional influential factors affecting clinical outcomes [52].
• Very short follow-up periods are applied. In order to draw clear conclusions on a technique or a material, long-term follow-up periods are essential.
• The initial evaluation of a restoration or restorative procedure and its behavior through time should be based on specific guidelines and criteria and should not be only assessed by the presence of stains and wear. These criteria include esthetic, functional, and biological parameters, such as anatomical form, surface luster, and surface staining, fracture of material, marginal adaptation, postoperative sensitivity, reoccurrence of dental caries, tooth integrity, and the adjacent mucosa evaluation [53,54,55].
• We should keep in mind that although clinical cases belong to the evidence – based scientific pyramid, their quality as well as their amount of evidence is weak [56]. The selection of a dental biomaterial in conjunction with a restorative technique should be made through well-designed randomized controlled clinical trials reinforced by in vitro studies and by the conduction of well–structured systematic reviews and metanalysis.

3.2. In Vitro and Randomized Control Clinical Studies on Highly Filled Flowable Resin Composites

The presence of staining and discoloration on the surfaces of the highly filled flowable resin composites and on the tooth–material interface, in conjunction with the voids and minor wear defects presented in some clinical cases mentioned in 3.1 section, constitute the stimulus for investigating the performance of highly filled flowable materials based on laboratory studies and clinical trials, which are thoroughly described in Table 3 and Table 4 respectively [10,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90].

A total of 31 in vitro studies are included in this review. Each study evaluated single or multiple characterictics. Most studies focus on optical properties, color stability, surface roughness, microhardess, flexural properties and wear. Surface gloss, water sorption/solubility, filler content, handling properties, wettability, cuspal deflection, bonding properties, microleakage and marginal adaptation, microbial adhesion and viability, degree of conversion and polymerization shrinkage have been evaluated to a lesser extent.

Regarding optical properties and color stability, Nair et al. in 2017 demonstrated inferior performance for the highly filled flowable composite resin compared to nanofilled and nanohybrid conventional composite counterparts after coffee immersion and tooth brushing simulation [57]. On the contrary, Lai et al. in 2018 concluded that the color alterations caused by toothbrushing abrasion were acceptable for all materials tested (highly filled flowables, traditional flowables, and self-adhesive flowables) [58]. No statistically significant differences were found in color stability between highly filled flowable composite resins, a bulk fill flowable composite, and a conventional nanofilled composite resin, meaning that highly filled flowable composites’ optical properties after a 144-hour immersion in colorant solutions are relevant to controls [60]. Again, comparing the color change of bulk-fill composite resins and one conventional resin composite to that of a highly filled flowable composite resin, clinically invisible color changes between materials were observed after 60 days of storage either in dry or wet conditions [66]. When hand polished, the highly filled flowable composite resins performed adequately after beverage immersion [69]. Another research assessing optical properties highlighted that the highly filled G-aerial Universal Injectable flowable composite resin presented the highest translucency and opalescence values and the lowest chroma values. This material was compared to another highly filled flowable resin-based material and a conventional microhybrid composite resin originating from the same company [74]. The same conclusion is drawn in the research of Bai et al. in 2024, who also found that G-aenial Universal Injectable by GC presented the highest traslusency values this time compared to other highly filled flowable materials (Beautifil Injectable XSL by Shofu and Filtek Supreme Flowable, Solventum) and a compomer [77]. Interestingly, Bayrakrat et al. in 2024 demonstrated that the conventional composite resins of different companies presented lower fluorescence and color adjustment levels compared to their highly filled flowable formulations in class V restored cavities [82].

Proceeding to surface roughness properties the highly filled flowable resin material (G-aenial Universal Injectable, GC) showed the lowest Ra values among four other traditional flowable composites and a self-adhesive flowable [58]. Niyomsujarit et al. in 2021, revealed that two highly filled flowable restorative materials, did not present any differences in Ra values compared to a conventional, universal composite resin (control). Controversally, in this study, another highly filled injectable flowable material (Beautifil Injectable X, Shofu) presented higher Ra values than the control [63]. The same results were observed by Degirmenci et al. in 2023, Basheer et al. in 2024, and Checchi et al. in 2024, where all the highly filled flowable resin composites presented statistically similar Ra values compared to conventional nanohybrid composites [74,79,84]. In the study of Elsahn et al. in 2023, G-aenial Universal Injectable performed better in terms of Ra values, even compared to a CAD/CAM resin based material, when applied as a thin occlusal veneer [70]. When investigating a highly filled bulk flowable resin composite solely, the conclusion is focused on the fact that both polishing procedure systems and acidic challenges greatly impact the surface roughness of the material [72]. Vulovic et al. both in 2023 and 2024, comparing four highly filled flowable resin composites to each other concluded that the ones with either ultra – fine barium or strontium fillers and a full coverage silane technology (G-aenial Universal Injectable, GC and G-aenial Universal Flo, GC) presented favorable roughness values compared to other flowables (Filtek Supreme Flowable Restorative, 3M ESPE, and Tetric EvoFlow, Ivoclar Vivadent). The variation of polishing procedures and the immersion in acidic media in different time intervals played a pivotal role on this outcome [73,86]

Another vital aspect is surface gloss. G-aenial Universal Injectable by GC performed favorably by presenting the highest surface gloss values (GU) compared to other traditional flowables availble in the market [58]. Not to forget, the surface gloss of a dental material is influenced by polishing procedures, as presented by a research demonstrating that a multi-step polishing system reinforces the surface gloss of the restoration [72].

Microhardness is another multiply investigated crucial factor. In a plethora of in vitro studies included in this review, the highly filled flowable restorative materials presented inferior Vickers microhardness values either compared to conventional microfilled composites, nanofilled composites, bulk–fill composite resins, and CAD/CAM resin-based materials [57,64,66,70,71,79]. Additionally, one study demonstrated superior hardness values for a highly filled flowable restorative material compared to a traditional flowable counterpart [65]. When comparing highly filled flowable resin composites to each other, G-aenial Injectable GC presented the highest surface hardness values [86].

Emphasizing flexural properties, controversial results are to be seen. Although Imai et al., in 2019 concluded that highly filled flowable composite resins present lower elastic modulus compared to microhybrid and nanofilled conventional resin composites [10], Degirmenci et al., in 2022 presented higher flexural strength and elastic modulus values for the highly filled flowable materials compared to microhybrid conventional composite resins [64]. In a more recent study of 2024, no differences in flexural strength values between highly filled flowables and a nanohybrid conventinal resin composite were observed [79]. When comparing traditional flowable, bulk-fill flowable composites, and highly filled flowable composites, the latter exhibit favorable flexural strength and elastic modulus values [62]. G-aerial Universal Injectable by GC and Majesty ES Flow by Kuraray Noritake exhibited the greatest flexural strength and modulus values in comparison with other highly filled flowable and bulk-fill flowable resin products [61]. Lastly, Rajabi et al. in 2024 demonstrated that two highly filled flowable composites presented statistically higher mean flexural strength values than traditional and conventional nanohybrid composite resin [81].

The wear volume loss in conventional resin composites may be lower than that of highly filled flowable composites. This observation comes to agreement with the results of Turk et al. in 2023, stating that conventional nanohybrid and nanofilled resin composites perform better in terms of wear volume loss and maximum depth loss compared to their flowable composite resin counterparts [68]. G-aenial Universal Injectable surprisingly presented lower wear volume loss compared to CAD/CAM milled resin based material when fabricated for 1mm thin occlusal veneers [70]. Finally, some highly filled flowable resin composites (G-aenial Universal Flo by GC and Filtek Supreme Flow by 3M ESPE) and a highly filled bulk injectable composite (G-aenial Bulk Injectable by GC) exhibit less wear than other highly filled flowables, bulk fill flowables and older flowable resins [59]. Newly published studies demonstrate that the wear of highly filled flowable composites is comparable to that of conventional composites [81,84].

Other characteristics commonly investigated in dental biomaterial science are volumetric or linear polymerization shrinkage, polymerization shrinkage stress, depth of cure, and the degree of conversion of resin composites. According to the studies in this review, highly filled flowable resin composites display higher values of linear and volumetric polymerization shrinkage compared to either bulk fill flowables or conventional microfilled or nanofilled composites [62,67,75,80]. Referring to polymerization shrinkage stress, highly filled flowable resin composites present significantly higher values of stresses compared to traditional flowables and bulk fill flowables, but no differences when compared to conventional composite resins [62,80].

Unfortunately, only four randomized clinical trials are presently assessing the clinical performance of highly filled flowable resin-based materials. Three clinical studies investigated the clinical performance of highly filled flowable resin composites in direct posterior restorations [87,89,90], whereas one in non-cavitated cervical lesions [88]. Highly filled flowables performed equally or even better in marginal adaptation and surface gloss than the conventional materials used as controls [88,90].

3.2.1. Interpretation of the Results of the In Vitro Studies and Randomized Controlled Clinical Trials

Delving deeper into the aforementioned research, the results should be interpreted cautiously. Disparities among studies examining the same material, two or more highly filled flowable materials and variances between flowable materials, bulk fill materials and a nanofilled conventional resin composite present a multifactorial etiological pattern in which etiological factors are interdependent.

The mechanical, physical, and optical properties as well as the surface characteristics of an examined material are predominantly related to the material’s distinct composition, including the composition and structure of the organic matrix and inorganic filler particles, the filler-to-resin matrix ratio, and the silanization of the filler components. These characteristics are inextricably linked to other material characteristics such as degree of conversion and water sorption and solubility [10,71,77,91,92]. Despite material composition, additional factors may have a dominant impact on several properties tested. Among others, finishing and polishing procedures, storage conditions, and type of external aggravating stimuli (colorant solutions, acidic and abrasive challenges) modify materials performances.

A typical example is that optical performance is affected by surface roughness, gloss, and hardness [93]. Furthermore, surface roughness is dependent on filler content, filler type, and size, the surface area in percent occupied by filler particles, the degree of conversion, the interaction of filler with the organic matrix, the silane coupling agent, and eventually, the hardness of the material [63,94,95]. Subsequently, the inferior microhardness and color stability of highly filled flowable resin composite after immersion in instant coffee and tooth brushing simulation may be attributed to the type of fillers. Strontium glass fillers in G-aenial Universal Flo are related to inferior physical properties and difficult attachment to the organic matrix, compared to zirconia and silica fillers in the conventional resin composite [57,96]. The uniform pattern of filler content distribution has a profound positive effect on physical and optical properties [71]. The highly filled flowable G-aenial Universal Flo by GC consists of fillers ranging from 16nm to 200nm in size, whereas conventional resin composite’s fillers (Filtek Z350XT) range from 4nm to 20nm [43,97].

The optical performance of a biomaterial is intertwined to translucency, opalescence, chroma values and refractive indices of monomers and fillers [98]. The different filler composition and the different filler type may have an important effect on the translucency between the tested materials [99]. This statement is endorsed by Bai et al., in 2024, who found higher translucency in G-aenial Universal Injectable, GC compared to another highly filled flowable composite (Filtek Supreme Flowable, 3M ESPE), probably due to the zirconia fillers of the later, that have imperfect refractive index and translucency [77]. G –aenial Universal Injectable consisting of barium glass presented the highest translucency and opalescence values when compared to G-aenial Universal Flo consisting of strontium glass and a microhybrid conventional resin composite consisting of strontium glass and lanthanoid fluoride launched by the same company [74]. Furthermore, polishing systems and especially the hardness of the abrasive particles are strongly related to translucency values [100]. Diamond particles exhibit superior abrasive hardness in comparison to aluminum oxide particles, leading to abrasion of the resin matrix and protrusion of fillers causing differences in translucency even at the same material [101,102]. Lastly, the general rule that higher filler fraction may lead to increased strength and wear resistance, but simultaneously to decreased translucency and increased opacity, should not be forgotten [103,104,105].

The filler loading and the filler shape and size could be etiological factors for the differences in the surface roughness and surface gloss values between several resin composites [106,107]. Lower filler loading, irregular shaped fillers, homogeneous composition of filler particles, and greater average particle size result in increased surface roughness and decreased surface gloss. This is proven by the difference in surface gloss and surface roughness between the two highly filled flowables in the study of Miyashita – Kobayashi et al. in 2024 [78]. The one presenting supra – nano spherical fillers and an average particle size of 0.2 μm performed better than the one with the irregular shaped fillers and the average particle size of 0.8 μm. Another typical example is the increased surface roughness value of the highly filled flowable Beautifil Injectable X by Shofu compared to the highly filled flowables G-aenial Universal Injectable and Filtek Z350XT Flowable Restorative [63]. Beautifil Injectable X consists of bioactive surface pre – reacted glass ionomer (S-PRG) fillers with a 0.8μm average particle size, contrary to smaller average particle sizes present in the other two highly filled flowable resin composites. G-aenial Universal Flo and G-aenial Universal Injectable consist of strontium and barium glass fillers respectively and silica particles, forming a homogeneous filler pattern. On the other hand, Tetric EvoFlow and Filtek Supreme Flowable Restorative consist of Ytterbium trifluoride (YBF3) and three to four different, diverse, heterogeneous ingredients in their filler composition, a fact that may hinder their surface roughness values [73]. At this point, it should be highlighted that most of in vitro studies only analyze the Ra value, which is a single height parameter of a surface. Additional spatial, functional or hybrid (e.g., developed interfacial area ratio, Sdr) parameters may give a greater insight into surface texture and may alter the resulting behavior of some already analyzed highly filled flowable resin composite materials, based on surface characteristics [108].

Wear and elastic modulus are dependent not only on type, shape and size of fillers, but also on interfiller spacing, surface treatment of fillers, degree of conversion, hydrolytic degradation and water sorption, and the surface’s finishing and polishing procedures [3,8,10,59,109,110,111,112]. The smaller particle size of some highly filled flowables which proved to lead to lower friction coefficients and subsequently to lower internal shear stresses in the polymer matrix may be a partial explanation on the favorable wear performance of some highly filled flowable resin composites over other highly filled flowables and bulk – fill flowables [8,59,113]. Moreover, the small interparticle space of small-sized fillers may protect the resin matrix from its ongoing abrasion. The lowest volumetric wear of G-aenial Universal Injectable compared to a resin – based CAD CAM material in 1mm thin veneers should be interpreted with caution. Following the manufacturer’s recommendations, the proposed thickness of this CAD/CAM material is approximately 1.5mm [114]. Therefore, the design of this protocol could be considered as restrictive. All highly filled flowable resin materials performed unfavorable in terms of elastic modulus compared to conventional resin composites. The higher filler loading and the higher concentration of Bis-GMA monomers in the mass of conventional composite resins are known to positively affect the mechanical and physical properties of resin materials providing high elastic modulus [115]. This fact partially justifies their better wear performance compared to the majority of highly filled flowable composite resins. In contrast, G-aenial Universal Flo by GC demonstrated flexural strength similar to that of a nanofilled composite resin, which characterizes the theory of filler loading as a singular etiological factor rather incompetent and broadens the spectrum of possible explanations on this field. Different resin monomers have different molecular weights and viscosities; for all we know, they may have a determinable effect on wear, flexural strength and mechanical properties of flowables in general [116].

Dimensional changes, degradation and weakening of the bond between organic and inorganic components are present after exposure in the oral environment [10,117]. It is evident that higher filler content entails lower water sorption through the increased hydrophobic character of the material [118] and increased filler size leads to higher water sorption through the increased surface area available for the development of this phenomenon. Additionally, the type of the monomer and its quantitative allocation in the organic matrix has an impact on the water sorption of the material [119]. Higher Bis-GMA and TEGDMA contents lead to greater hydrolytic degradation than that of UDMA monomers [120,121]. By elevating the nano-sized fillers and creating a homogeneous, dispersed distribution the internal free volume available for water intake is confined. Complementary, each monomer with its unique molecular weight presents its own water sorption pattern [122,123]. TEGDMA absorbs more water than Bis – GMA due to the greater space between its clusters, its greater flexibility and its consequent higher swelling [124]. The sorption of water eventually leads to hydrolysis of the coupling agent, weakening the bond between organic matrix and fillers, deteriorating in that manner the characteristics of dental materials. These facts may explain the preferable water sorption values of a plethora of conventional composite resins compared to highly filled flowables. Discrepancies between different highly filled flowables are present and are mainly caused by the slightly different organic and inorganic composition of the materials. As an example Beautifil Injectable X by Shofu presents greater water sorption and degradation than G – aenial Universal Injectable after acidic challenges. This is speculated to be caused by the greater percentages of Bis – GMA and TEGDMA in its composition and by the fact that the fluorosilicate glass (S-PRG fillers) has a higher susceptibility to degradation by weak acids (higher affinity to water sorption) [63]. Lastly, we should keep in mind that the design of the study (sample size calculation, sample preparation, condition of observation, presented control groups, methodological parameters-errors) and the devices used have a great influence on the outcome of the study.

Concerning the interpretation of the four randomized controlled clinical trials, their ambiguous results should be assessed with caution. The number of randomized controlled clinical trials is predicatively low. The four clinical studies are not comparable, since different restorative procedures were applied; the number of participants is confined and the number of the final evaluated restorations is even more restricted. The clinical evaluation pattern (FDI criteria and modified USPHS criteria) is admissible. The moderate to high risk of bias, the dropouts and the short – term follow up duration are limitations that need to be taken into account before drawing vague conclusions [125].

4. Conclusions and Future Perspectives

Highly filled flowable resin composites are a new addition in the field of Adhesive Dentistry and should be further investigated so that their full potential could be unfolded. Since scarce evidence could be provided by the four available randomized controlled clinical trials, an emphasis on the establishment of well designed clinical trials should be given. The survival and success rates of several types of cavities restored with highly filled flowable composites remain vast and uncharted. It is common ground that the combination of highly filled flowable resin composites with the injectable moulding technique leads to predictable, highly esthetic clinical outcomes in terms of reproducing the anatomical contour. Highly filled flowable composite resins present a generally satisfactory material properties pattern – good optical properties, competent surface roughness and gloss – but their inferior microhardness values and wear resistance compared to conventional nanohybrid and nanofilled composites recommend their further amendment. Dental materials should perform the best possible way in the complex and not constant oral environment, where masticatory forces, occlusal and dietary habits, temperature fluctuations, biofilm formation, enzyme collection and salivary flow are constantly present. The interaction of these factors with dental materials may change their existing favorable laboratory physical, mechanical and optical behavior. It would have been interesting to start assessing the behavior of highly filled flowable resin based materials in oral conditions by conducting in situ studies, which may reveal potential interconnection between optical parameters, mechanical properties, surface characteristics, saliva and the oral microbiome.