A review and current state of autonomic self-healing microcapsules-based dental resin composites

ObjectiveThis study systematically reviews the literature on self-healing microcapsule technology and evaluates the biocompatibility of self-healing microcapsules and the efficiency of crack repair within resin-based dental composites.MethodsAn electronic search was carried out using the following databases: MedLine (PubMed), Embase, the Cochrane Library and Google Scholar. All titles and abstracts of the articles and patents found were analysed and selected according to the eligibility criteria. Only studies published in English were included; the outcomes sought for this review were dental resin composites with self-healing potential. There were no restrictions on the type of self-healing system involved in dental resin composites.ResultsThe search yielded 10 studies and 2 patents involving self-healing approaches to dental resin composites. According to the current literature on self-healing dental resin composites, when a crack or damage occurs to the composite, microcapsules rupture, releasing the healing agent to repair the crack with a self-healing performance ranging from 25% to 80% of the virgin fracture toughness.SignificanceSelf-healing strategies used with resin composite materials have, to date, been bioinspired. So far, self-healing microcapsule systems within dental composites include poly urea-formaldehyde (PUF) or silica microcapsules. The main healing agents used in PUF microcapsules are DCPD monomer and TEGDMA-DHEPT, with other agents also explored. Silica microcapsules use water/polyacid as a healing agent. All self-healing systems have shown promising results for self-repair and crack inhibition, suggesting a prolonged life of dental composite restorations. More investigations and mechanical enhancements should be directed toward self-healing technologies in dental resin composites.     

Nanodentistry: A Paradigm Shift-from Fiction to Reality

Nanodentistry is an emerging field with significant potential to yield new generation of technologically advanced clinical tools and devices for oral healthcare. Nanoscale topology and quantitative biomechanical or biophysical analysis of dental surfaces are of significant interest. In particular, using Atomic force microscopy techniques-diseases such as dental caries, tooth hypersensitivity, and oral cancer can be quantified based on morphological, biophysical and biochemical nanoscale properties of tooth surface itself and dental materials or oral fluids such as saliva. An outlook on future "nanodentistry" developments such as saliva exosomes based diagnostics, designing biocompatible, antimicrobial dental implants and personalized dental healthcare is presented. This article examines current applications of nanotechnology alongside proposed applications in the future and aims to demonstrate that, as well as a good deal of science fiction, there is some tangible science fact emerging from this novel multidisciplinary science.     

Minimally invasive management of dental caries: contemporary teaching of posterior resin-based composite placement in U.S. and Canadian dental schools

BackgroundResin-based composites are an increasingly popular material for restoring posterior teeth, permitting minimally invasive cavity preparations and esthetic restorations. The authors investigated current teaching of the placement of posterior resin-based composites in U.S. and Canadian dental schools.MethodsIn late 2009 and early 2010, the authors, with the assistance of the Consortium of Operative Dentistry Educators (CODE), invited 67 dental schools to participate in an Internet-based survey.ResultsThe response rate was 73 percent. Although all schools taught the placement of resin-based composites in occlusal and most occlusoproximal cavities, eight schools (16 percent) did not teach placement of three-surface occlusoproximal resin-based composite restorations in permanent molars. Resin-based composites accounted for 49 percent of direct posterior restorations placed by dental students in 2009 and 2010, a 30 percent increase from 2005.ConclusionsTeaching placement of posterior resin-based composites continues to increase in dental schools in the United States and Canada, with predoctoral students gaining, on average, an equal amount of experience placing posterior resin-based composites and amalgams in terms of numbers of restorations.Clinical ImplicationsEvidence-based, up-to-date teaching programs, including those in operative dentistry, are needed to best prepare students for careers in dentistry.     

The bulk compressive creep and recovery behavior of human dentine and resin-based dental materials

ObjectiveTo evaluate and compare the viscoelastic properties of dentine and resin-based dental materials by bulk compressive test and the Burgers model.Materials and methodsSound dentine, three resin composites as well as a resin-based cement were prepared into cylindrical specimens (n = 8). A bulk compressive creep test was applied with a constant load of 300 N (23.9 MPa) for 2 h, followed by another 2 h recovery. The maximum strain, creep stain, percentage of recovery and permanent set was measured using a linear variable displacement transducer. The viscoelastic properties were characterized via the Burgers model, and the instantaneous elastic, viscous as well as elastic delayed deformation were separated from the total strain. Data were analysed via ANOVA (or Welch's Test) and Tukey (or Games–Howell Test) with a significance level of 0.05.ResultsSound dentine presented the lowest maximum strain, creep strain, permanent set and the highest percentage of recovery, followed by 3 resin composites with comparable parameters, while the cement showed a significantly higher maximum strain, permanent set and lower percentage of recovery (p < 0.001). The Burgers model presented acceptable fits for characterization viscoelastic processes of both dentine and resin-based dental materials. Viscous and elastic delayed strain of dentine was significantly lower than those for tested materials (p < 0.001) with the highest instantaneous elastic strain percentage. Similar viscous and delayed strain was found among the 4 resin-based materials (p > 0.05).SignificanceSound dentine exhibited superior creep stability compared to resin-based dental materials. The viscous deformation in sound dentine could be ignored when loading parallel to dentine tubules.     

Repair or replacement of defective direct resin-based composite restorations: contemporary teaching in U.S. and Canadian dental schools

BackgroundOpportunities exist to promote minimally invasive dentistry by repairing rather than replacing defective and failing direct resin-based composite restorations. The authors conducted a study to investigate the current teaching of such techniques in U.S. and Canadian dental schools.MethodsIn late 2010, the authors, with the assistance of the Consortium of Operative Dentistry Educators, invited 67 U.S. and Canadian dental schools to participate in an Internet-based survey.ResultsThe response rate was 72 percent. Eighty-eight percent of the dental schools taught repair of defective direct resin-based composite restorations. Of these schools, 79 percent reported providing both didactic and clinical teaching.ConclusionsAlthough teaching repair of defective resin-based composite restorations was included in the didactic curricula of most schools, students in some schools did not gain experience in minimally invasive management of defective resin-based composite restorations by means of performing repair procedures. The American Dental Association's Code on Dental Procedures and Nomenclature does not have a procedure code for resin-based composite restoration repairs, which may limit patients' access to this dental treatment.Clinical ImplicationsTeaching dental students minimally invasive dentistry procedures, including restoration repair, extends the longevity of dental restorations and reduces detrimental effects on teeth induced by invasive procedures, thereby serving the interests of patients.     

Nanotechnology in dentistry: Present and future perspectives on dental nanomaterials

ObjectivesThe number of dental nanomaterials has increased significantly over the past years. A variety of commercial dental nanomaterials are available and researched. Nevertheless, how these nanomaterials work, what makes them special and whether they are superior to traditional dental materials is not always clear to dentists and researchers. The objective of this review paper is, therefore, to give an overview of the principles of nanomaterials and basic research and applications of dental nanomaterials.MethodsThe fundamentals of materials science of nanomaterials as well as their advantages and disadvantages are elaborated. The most important dental nanomaterials are discussed. This is mainly based on a survey of the literature and a review of the most frequently cited scientific papers in the international peer reviewed journal Dental Materials over the past five years. The developments of commercial dental nanomaterials as well as aspects of their clinical use are considered in this review.ResultsNanomaterials have unique structures and properties that distinguish them from other materials. The journal Dental Materials is the journal with the highest numbers of articles and citations on the subject of dental nanomaterials. The most frequently reported dental nanomaterials are nanocomposites, nanoparticles, antimicrobial nanomaterials and bio-mineralization systems. Hallmarks of dental nanomaterials include a set of unique properties and challenges in the preparation of these materials.SignificanceBy understanding the physical principles of dental nanomaterials, their strengths, limitations and their specific benefits will be better appreciated. Dental nanomaterials have potential for the future but currently do not always exhibit superior properties, for example in clinical situations.     

The trends of dental biomaterials research and future directions: A mapping review

ObjectiveThis literature research aimed to compare, contrast and quantify the innovations in the most commonly used dental biomaterials.MethodologyOriginal research articles based on experimental dental biomaterials published between 2007 and 2019 were retrieved and reviewed. A search of electronic databases, PubMed, Scopus, and Web of Science indexed dental/biomaterials journals, has been conducted. The inclusion criteria in this research were: synthesis of experimental dental materials, whereas commercial dental materials, review articles, and clinical trials (case reports) were excluded.ResultsIt was found that the amount of publications related to dental subgingival implants, computer-aided modeling ceramics, aesthetic restorative materials, adhesives cements, ceramics, bioceramics, endodontic materials, bioactive scaffolds, stem cells, and guided-tissue membranes had increased significantly from 2007. At the same time, the number of publications related to dental cements, silver amalgam, and dental alloys has decreased. For characterization of dental materials it was noted that mechanical properties were tested mostly for restorative materials. On the other hand, biological properties were most assessed for dental subgingival implants and endodontic materials, however, physical properties predominantly for bioceramics.ConclusionIt is concluded that to meet clinical demands there was more focus on restorative materials that provided better aesthetics, including resin composites, adhesive resin composites (luting cements), zirconia, and other ceramics. The boost in laboratory and animal research related to bioceramics was attributed to their regenerative potential. This current literature study will help growing researchers to consider and judge the direction to which research might be guided in order to plan prospective research projects.     

Towards the development of biostable dental resin systems - design criteria and constraints beyond ester-free chemistries

ObjectiveThe objective of this review article is to summarize the current literature on dental resin-based restorative (RBR) materials specifically from the perspective of emerging resin technologies, and to provide researchers with structured design criteria enabling the effective screening of new RBR developments.MethodsThe continued failure of newly introduced RBRs to address biostability without compromising function, over the last decade, are presented as a rationale to support different resin-based concepts. Several developments in the field, aimed at addressing the issues facing modern resin-based systems are summarized and their limitations discussed. A design workflow is proposed for evaluating new RBR, considering resource needs.ResultsWhile several alternative resin chemistries have been suggested over the past decade, all have shown serious limitations in replacing MA-based materials, including their limited physical and mechanical properties, and curing kinetics. Additionally, a broad and inconsistent range of laboratory methods have been used to validate these developments, leading to results that are difficult to compare across studies. A design workflow was conceptualized to facilitate the screening of novel RBRs from both a clinical and research perspective.SignificanceWhile several alternative chemistries have shown some degree of potential in emulating material property aspects of MA-based resins, a complete restorative system that is resistant to biochemical reactions in saliva has yet to achieve broad clinical adoption. To further spur development, it would be useful to have a more systematic design workflow, that may be used to easily screen new resin technologies effectively early in the design phase, so as to mitigate potential performance failures in the clinic.     

Application of neurotransmitters and dental stem cells for pulp regeneration: A review

IntroductionAlthough there have been many studies on stem cells, few have investigated how neurotransmitters and stem cell proliferation interact to regenerate dental pulp. Dental pulp regeneration is an innovative procedure for reviving dental pulp, if feasible for the entire tooth. Upon tooth injury, activated platelets release serotonin and dopamine in bulk to mobilize dental pulp stem cells to mediate natural dental repair. This has induced research on the role of neurotransmitters in increasing the proliferation rate of stem cells. This review also covers prospective future treatments for dental pulp regeneration.MethodsA literature search was performed via PubMed and ScienceDirect from 2001 to 2022, using the keywords “neurotransmitter,” “stem cell,” “tooth regeneration,” “tooth repair,” “regenerative dentistry,” and “dental pulp.” Different inclusion/exclusion criteria were used, and the search was restricted to English articles.ResultsNine publications reporting neurotransmitter interactions with stem cells for tooth and pulp regeneration were selected.ConclusionNeurotransmitters were found to interact with dental stem cells. Evidence pointing to neurotransmitters as a factor in the increased proliferation of stem cells was found. This review thus gives hope for tooth pulp regeneration and repair.     

A review on Vat Photopolymerization 3D-printing processes for dental application

ObjectiveTo give a critical and realistic state-of-the-art overview of research conducted in the field of Vat Photopolymerization for use in the dental field, with recommendations for further research.MethodsThe search of articles published for the period 2005–2022 was performed using Scopus and PubMed databases using keywords. Apart from that, a search for patents in the Patentscope database for the period 2000–2021 was also conducted.ResultsBased on the found articles, insight on the publication’s activity and discussion of some possible areas of future examination, on the mechanical properties of 3D-printed resin-based dental parts, their biocompatibility, antimicrobial properties, accuracy, and surface properties are presented. After reviewing all relevant articles, it was found that there is a need for more research in order to elevate this field to the next level.SignificanceVat Photopolymerization technologies are evidently more or less incorporated into everyday dental practice. In the future, a complete transition to digital workflow is expected. To enable this, it is necessary to examine in detail the materials used in this field, in order not only to ensure patient safety, but also to reduce the cost of dental services. However, not many studies have been published in the searched databases, and more research is needed, the results of which can be compared with previously conducted studies.     

Risk aspects of dental restoratives: From amalgam to tooth-colored materials

Dental materials’ choice of patients has considerably changed. Whereas cast gold and amalgam have been the predominant biomaterials for decades, today tooth-colored materials like resin-based composites and ceramics are more and more successful. However, are we going to replace a good but biologically questionable material (amalgam) with an equal material (resin composite) being more esthetic but also biologically questionable For amalgam, long-term clinical studies reported some significant hints that in single cases amalgam may be a health hazard for patients, finally Norway banned amalgam completely. The main advantage of a resin-based composite over amalgam is its tooth-like appearance and more or less absence of extensive preparation rules. For many years it was believed that resin-based composites may cause pulpal injury. However, pulpal injury associated with the use of resin-based composites is not correlated with their cytotoxic properties. Nevertheless, resin-based composites and other dental materials require rigorous safety evaluation and continuous monitoring to prevent adverse events similar like with amalgam. Because of non-biocompatible pulp responses to resin-based composites and amalgam, they should not be placed in direct contact with the dental pulp. The less dentin remaining in the floor of preparations between resin-based composites or other dental materials is more likely to cause pulpitis. Percentage of patients and dental practitioners who display allergic reactions is between 0.7% and 2%. The release of cytotoxic monomers from resin-based materials is highest after polymerization and much lower after 1 wk. Substances released from resin-based composites have been shown to be toxic in cytotoxicity tests. Nevertheless, in vitro cytotoxicity assays have shown that amalgam has greater toxic effects than resin-based composites, sometime 100-700-fold higher. Altogether, the risk of side-effects is low, but not zero, especially for dental personnel.     

Dynamic covalent chemistry (DCC) in dental restorative materials: Implementation of a DCC-based adaptive interface (AI) at the resin–filler interface for improved performance

ObjectiveDental restorative composites have been extensively studied with a goal to improve material performance. However, stress induced microcracks from polymerization shrinkage, thermal and other stresses along with the low fracture toughness of methacrylate-based composites remain significant problems. Herein, the study focuses on applying a dynamic covalent chemistry (DCC)-based adaptive interface to conventional BisGMA/TEGDMA (70:30) dental resins by coupling moieties capable of thiol–thioester (TTE) DCC to the resin–filler interface as a means to induce interfacial stress relaxation and promote interfacial healing.MethodsSilica nanoparticles (SNP) are functionalized with TTE-functionalized silanes to covalently bond the interface to the network while simultaneously facilitating relaxation of the filler–matrix interface via DCC. The functionalized particles were incorporated into the otherwise static conventional BisGMA/TEGDMA (70:30) dental resins. The role of interfacial bond exchange to enhance dental composite performance in response to shrinkage and other stresses, flexural modulus and toughness was investigated. Shrinkage stress was monitored with a tensometer coupled with FTIR spectroscopy. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine.ResultsA reduction of 30% in shrinkage stress was achieved when interfacial TTE bond exchange was activated while not only maintaining but also enhancing mechanical properties of the composite. These enhancements include a 60% increase in Young’s modulus, 33% increase in flexural strength and 35% increase in the toughness, relative to composites unable to undergo DCC but otherwise identical in composition. Furthermore, by combining interfacial DCC with resin-based DCC, an 80% reduction of shrinkage-induced stress is observed in a thiol–ene system “equipped” with both types of DCC mechanisms relative to the composite without DCC in either the resin or at the resin–filler interface.SignificanceThis behavior highlights the advantages of utilizing the DCC at the resin–filler interface as a stress-relieving mechanism that is compatible with current and future developments in the field of dental restorative materials, nearly independent of the type of resin improvements and types that will be used, as it can dramatically enhance their mechanical performance by reducing both polymerization and mechanically applied stresses throughout the composite lifetime.     

Development of novel dental restorative composites with dibasic calcium phosphate loaded chitosan fillers

The incorporation of antimicrobial agents in restorative dental composites has the potential to slow the development of carious lesions.ObjectiveThe objectives of the present study were to develop experimental composite resins with chitosan or chitosan loaded with dibasic calcium phosphate anhydrous (DCPA) particles and to demonstrate their antimicrobial potential without loss of mechanical properties or biocompatibility.MethodsChitosan and chitosan/DCPA particles were synthetized by the electrospray method. Experimental composites were formulated by adding 0, 0.5, or 1.0 wt% particles into a resin matrix along with 60 wt% barium glass. The degree of conversion and mechanical properties were measured after 1 and 90 days of aging in water after photoactivation. Cytotoxicity and genotoxicity were evaluated using fibroblasts from dental pulp in conditioned medium. The antimicrobial activity against Streptococcus mutans was assessed by crystal violet biofilm assay.ResultsThe experimental restorative composites were not found to be cytotoxic or genotoxic, with cell viability of 93.1 ± 8.0% (p = 0.328) and 3.0 ± 0.8% micronucleus per group (p = 0.1078), respectively. The antimicrobial results showed that all composites with approximately 20% less biofilm (p < 0.001) relative to the control. No chitosan release was detected from the composites, suggesting direct contact of the bacteria with exposed chitosan particles on the surface was responsible for the observed antimicrobial effect. The addition of the chitosan and chitosan/DCPA submicrometer (<250 nm average diameter) particles to restorative composites did not change the degree of conversion, flexural strength, elastic modulus and fracture toughness compared to the control group after 90 days aging in water.SignificanceIt can be concluded that the addition of chitosan or chitosan/DCPA particles in the restorative composites induced antimicrobial activity without compromising the mechanical properties or biocompatibility of the composites.     

Resin composite restorative materials

This paper surveys the most important developments in resin-based dental composites and focuses on the deficits (e.g. polymerization shrinkage) and strengths of the materials and their clinical implications. Moreover, differences between composite categories, such as hybrid, nanohybrid, microfilled, packable, ormocer-based, silorane-based, polyacid-modified composites (compomers) and flowable composites are highlighted, especially in view of their mechanical behaviour. In addition to the classical dimethacrylate-based composites, special attention is given to alternative monomers, such as siloranes, ormocers or high-molecular-weight dimethacrylate monomers (e.g. dimer acid-based dimethacrylates and tricyclodecane (TCD)-urethane), analysing their advantages, behaviour and abilities. Finally, the paper attempts to establish the needs and wishes of clinicians for further development of resin-based composites.     

The implications and applications of nanotechnology in dentistry: A review

The emerging science of nanotechnology, especially within the dental and medical fields, sparked a research interest in their potential applications and benefits in comparison to conventional materials used. Therefore, a better understanding of the science behind nanotechnology is essential to appreciate how these materials can be utilised in our daily practice. The present paper will help the reader understand nanoscience, and the benefits and limitations of nanotechnology by addressing its ethical, social, and health implications. Additionally, nano-applications in dental diagnostics, dental prevention, and in dental materials will be addressed, with examples of commercially available products and evidence on their clinical performance.     

Highly translucent dental resin composites through refractive index adaption using zirconium dioxide nanoparticles and organic functionalization

ObjectivesFor dental resin composites, high translucency is important. Therefore, the aim of the study was to create a biocompatible and highly translucent resin-based composite, and to investigate the effect of material thickness on translucency.MethodsA biocompatible ORMOCER® resin matrix was reinforced with dental glass powder as fillers. To reach a high translucency, refractive index matching of the matrix and fillers was done in the two ways: (1) Highly refractive ZrO₂ nanoparticles were incorporated into the resin. (2) The resin was modified via addition of 4-Methylthiophenol. The corresponding refractive indices were acquired on an Abbe refractometer (n = 5). In both cases, the dental glass powder was added and translucency of the resulting minifilled and nanohybrid composites were measured using spectral photometry (n = 5). Additionally, the translucency of the experimental composites was determined as a function of specimen thickness in the range 10 μm–2 mm (n = 5). One-way ANOVA was performed to determine the significant differences in various optical parameters among different amounts of modifications and thicknesses at α = 0.05. Furthermore, cytotoxicity tests (extract and direct contact tests) were conducted according to ISO 10993 to classify the biocompatibility of the composites (n = 6).ResultsThe translucency values of the composites with 47 wt.-% dental glass powder and a specimen thickness of 2 mm, could be increased from 26% up to 71% by increasing the refractive index of the matrix through incorporating ZrO₂ nanoparticles. Moreover, it can also be increased to 67% via addition of 4-Methylthiophenol. Further results showed that the translucency significantly depended on the sample thickness following an exponential function. The effect of all tested parameters was significant among the materials (p < 0.001). The composites did not show any cytotoxic effect.SignificanceHighly translucent and biocompatible resin composites were developed. They show attractive properties for the use as dental enamel material in direct and indirect restorations.     

Polymerization shrinkage of resin-based composites for dental restorations: A digital volume correlation study

ObjectiveResin-based composites are widely used in dental restorations; however, their volumetric shrinkage during polymerization leads to several issues that reduce the restoration survival rates. For overcoming this problem, a deep study of shrinkage phenomena is necessary.MethodsIn this study, micro-tomography (μ-CT) is combined with digital volume correlation (DVC) to investigate the effect of several factors on the polymerization strain of dental composites in model cavities: the presence/absence of an adhesive, the use of transparent/blackened cavities, and irradiation times between 1 and 40 s.ResultsThe results indicate that the presence of an adhesive at the interface between the cavity and composite does not reduce the total strain but instead limits it to a preferential direction. In addition, regardless of the conditions, the main strain is generated along the axis parallel to the polymerization irradiation (the vertical axis). Finally, the total strain appears to occur in the first 5 s of irradiation, with no further evolution observed for longer irradiation times.SignificanceThis work provides new insight into resin-based composite shrinkage and demonstrates the benefit of coupling DVC and μ-CT to better understand the degradation mechanisms of these materials.     

The color differences of direct esthetic restorative materials after setting and compared with a shade guide

BackgroundThe authors conducted a study to evaluate esthetic restorative materials’ color differences after setting and color matching between set materials and a shade guide.Materials and MethodsThe authors evaluated 13 resin-based composites, one silorane-based composite, two polyacid-modified resin composites and one conventional glass ionomer cement. They measured the color parameters of the samples, which were 8 millimeters in diameter and 1.5 mm in thickness, before and after they were set according to the Commission International de l’Eclairage (CIE) L*a*b* color scale relative to standard illumination against a white background by means of a dental colorimeter. They also compared the final colors of the restorative materials with a shade guide.ResultsColor difference values for each restorative material ranged from 3.25 to 14.04. With the exception of Fuji IX (GC, Tokyo), Filtek P60 (3M ESPE) and Te-Econom (Ivoclar Vivadent), the restorative materials exhibited a perceptible color change after setting. Color difference values between the set materials and the shade guide tabs ranged from 1.86 to 11.83. With the exception of Filtek Supreme XT (3M ESPE) and Fuji IX, the materials exhibited a perceptible difference.ConclusionMost of the materials tested exhibited a significant color change after polymerization and did not match the shade guide tab after undergoing light curing.     

Mechanical properties of experimental dental composites containing a combination of mesoporous and nonporous spherical silica as fillers

ObjectivesMesoporous fillers have been investigated for use in dental composites because of their potential for creating micromechanical filler/resin matrix interphase bonding. Such a micromechanical bonding could eliminate the need for the silane treatment of fillers for interfacial chemical bonding that is prone to hydrolysis in the oral environment. In the case of micromechanical bonding, dental polymer chains are threaded mechanically (like a “necklace”) through nanosized channels in the fillers.MethodsA combination of mesoporous silica, which was synthesized using the non-surfactant templating method, and nonporous spherical silica (500 nm) was used to prepare experimental dental composites. The porous silica used in this study contained interconnected pores and channels as opposed to porous fillers containing surface pores. The compressive strength, compressive modulus, flexural modulus, and flexural strength of these composites were evaluated.ResultsThe results showed that composites containing a combination of mesoporous and nonporous fillers have better mechanical properties than the composites having either of these fillers alone.SignificanceThe results showed that a combination of mesoporous and nonporous materials can be used to prepare stronger dental materials that may resist hydrolysis and wear.     

R-curve behavior and toughening mechanisms of resin-based dental composites: Effects of hydration and post-cure heat treatment

ObjectivesTo test the hypothesis that the fracture resistance of two different particulate resin composites degrade after water hydration and improve after post-cure heat treatment, and to correlate those changes with salient failure micromechanisms.MethodsTwo composites with different filler morphology were selected, denoted microhybrid (Filtek? Z250) and nanofill (Filtek? Supreme plus). Following initial light curing, hydrated samples were aged in water for 60 days at room temperature while post-cured samples were heat treated at 120 °C for 90 min. Fracture resistance was assessed using fracture resistance curves (R-curves) utilizing pre-cracked compact tension, C(T), specimens. The flexural strength of the hydrated composites also was evaluated in four-point bending using unnotched beams. Scanning electron microscopy (SEM) of crack paths and fracture surfaces was performed to determine the micromechanisms of fracture and toughening. The results were compared by two-way ANOVA and Tukey's multiple comparison test (p ≤ 0.05).ResultsSEM observations revealed a predominantly interparticle matrix crack path for all cases except the hydrated nanofill composite, which showed evidence of particle matrix debonding. Hydration lowered the strength for both composites and the peak toughness for the nanofill composite. The strength decrease was attributed to resin matrix plasticization and hydrolytic degradation in both cases, with additional interfacial degradation causing a larger strength decline and concomitant peak toughness decrease in the nanofill composite. The post-cure heat treatment noticeably changed the R-curve shape causing the peak toughness to be reached after shorter amounts of crack extension. Such changes help explain the increases in strength reported in other studies and is attributed to improved resin matrix properties.SignificanceResults from this study provide new insight into the micromechanisms of fracture in resin-based dental composites which should aid the future development and improvement of these materials.