Biomechanics, Bioactive and Biomimetic Philosophy in Restorative Dentistry ̶ Quo vadis?

INTRODUCTION: In recent years, restorative dentistry has embraced various techniques, including direct, semi-direct, and indirect restorations, to address the replacement of lost tooth tissue. The focus has been on integrating the principles of Biomechanics, Bioactivity, and Biomimicry (3-Bio) as key drivers behind these innovations. METHODS: Three enthusiastic researchers in dental materials have come together to collaborate on writing a concise opinion article addressing the three aspects of restorative dental materials: Biomechanics, Bioactivity, and Biomimetic principles. Their aim is to provide readers with relevant information on the 3-Bio concept, offering insights into the innovative approaches shaping modern restorative dentistry. RESULTS: Developing restorative materials with interactive properties aligned with the 3-Bio concept poses a significant challenge. Currently, dentistry lacks a comprehensive system in this regard. Though, the development of dental materials based on the 3-Bio concept could potentially elicit positive mechanical and biological responses in targeted tooth tissues. CONCLUSION: Assessing several parameters through a battery of in vitro and in silico assays could help in tailoring the different aspects of the 3-Bio concept, spanning from bioactivity to biomimetics via biomechanics. This approach could allow the prediction and translation of the clinical performance of the assessed restorative materials. CLINICAL SIGNIFICANCE: The findings of this opinion article highlight that the development of restorative materials aligned with the 3-Bio concept could enhance the management of dental defects and extend the longevity of bonded restorations, thereby improving patient care through tissue preservation. The authors believe that more collective efforts between clinicians, researchers, and even industrial partners are required to fully understand the correlation between bioactive behaviour, biomechanical limitations, and biomimetics to provide suitable restorative materials for specific clinical applications.

Assessing the Efficacy of Novel Fiber-Reinforced Dual-Cure Luting Resins.

Background: Dual-cure resin-based luting materials are increasingly favored in clinical applications due to their capacity to establish a strong bond with natural tooth structure and restorations. This study aimed to examine certain physical and handling characteristics of newly developed experimental dual-cure luting resins reinforced with short fibers (SFRCs) and compare them with commercially available dual-cure luting resins. Material and Methods: Seven dual-cure luting materials were tested (Relyx Ultimate, Duo-Link, eCEMENT, Variolink Esthetic, G-CEM LinkForce, experimental SFRC1, experimental SFRC2). Fourier transform infrared spectroscopy (FTIR) was utilized to determine the degree of monomer conversion (DC%) in the self and light-curing protocol. A rotating disk rheometer measured viscosity at room temperature (22°C) and simulated mouth temperature (35°C). Fracture toughness, flexural strength, and flexural modulus were evaluated using a 3-point bending test. Each luting resin was subjected to the examination of its surface microstructure using scanning electron microscopy (SEM). Analysis of variance (ANOVA) at a significance level of (p = 0.05) was conducted to analyze data. Results: It was revealed that DC% of the tested dual-cure resins was significantly (p< 0.05) affected by the curing mode, the dual-cure SFRC2 having the highest and Relyx having the lowest DC (64%, and 41% respectively). The viscosity of all tested materials decreased with increasing temperature. SFRC2 demonstrated the highest fracture toughness (2.3 MPa m1/2), while Relyx Ultimate, Duo-Link, and eCEMENT exhibited the lowest values (≈ 1 MPa m1/2)(p< 0.05). Both SFRCs and G-CEM link-force exhibited the highest flexural strength values, and SFRCs resulted in the highest flexural modulus values (p<0.05). Conclusions: The experimental fiber-reinforced dual-cure luting resins exhibited superior DC%, fracture toughness, and flexural properties, yet, SFRC2 showed the highest viscosity at elevated temperature. These results highlight the capability of short fiber reinforcement to enhance the mechanical properties of dual-cured resin-based luting materials without compromising handling characteristics. Key words:Dual-cure luting resin; short fibers; degree of conversion; viscosity; fracture toughness; flexural properties.

Mechanical Performance of Extensive Restorations Made with Short Fiber-Reinforced Composites without Coverage: A Systematic Review of In Vitro Studies.

In recent years, composite resin materials have been the most frequently used materials for direct restorations of posterior teeth. These materials have some clinically relevant limitations due to their lack of fracture toughness, especially when used in larger cavities with high volume factors or when utilized as direct or indirect overlays or crown restorations. Recently, short-fiber-reinforced composite materials have been used in bi-structure restorations as a dentine substituting material due to their superior mechanical properties; however, there is no scientific consensus as to whether they can be used as full restorations. The aim of our review was to examine the available literature and gather scientific evidence on this matter. Two independent authors performed a thorough literature search using PubMed and ScienceDirect up until December 2023. This study followed the PRISMA guidelines, and the risk of bias was assessed using the QUIN tool. The authors selected in vitro studies that used short-fiber-reinforced composite materials as complete restorations, with a conventional composite material as a comparison group. Out of 2079 potentially relevant articles, 16 met our inclusion criteria. All of the included studies reported that the usage of short-fiber-reinforced composites improved the restoration's load-bearing capacity. Fifteen of the included publications examined the fracture pattern, and thirteen of them reported a more favorable fracture outcome for the short-fiber-reinforced group. Only one article reported a more favorable fracture pattern for the control group; however, the difference between groups was not significant. Within the limitations of this review, the evidence suggests that short-fiber-reinforced composites can be used effectively as complete restorations to reinforce structurally compromised teeth.

Mechanical Properties and Ion Release from Fibre-Reinforced Glass Ionomer Cement.

The aim of this study was to compare the mechanical properties and ion release from a commercially available resin-modified glass ionomer cement to a formulation reinforced by the addition of short glass fibres at various percentages. Methods: Three experimental groups were prepared by adding a mass ratio of 10%, 15% and 20% of short glass fibres to the powder portion of the cement from a capsule (GC Fuji II LC), while the control group contained no fibres. Microhardness (n = 12), fracture toughness, and flexural, compressive and diametral tensile strength (n = 8) were evaluated. To study ion release, readings were obtained utilising fluoro-selective and calcium-selective electrodes after 24 h, 7 days and 30 days (n = 12). The spatial distribution of fibres within the material was evaluated through scanning electron microscopy. The data were analysed using one-way ANOVA with a Bonferroni adjustment. Results: The findings suggest that elevating fibre weight ratios to 20 wt% results in improved mechanical properties (p < 0.05) in microhardness, flexural strength, diametral tensile strength and fracture toughness. In terms of ion release, a statistically significant difference (p < 0.001) was observed between the groups at the conclusion of 24 h and 7 days, when the fluoride release was much higher in the control group. However, after 30 days, no significant distinction among the groups was identified (p > 0.05). Regarding calcium release, no statistically significant differences were observed among the groups at any of the evaluated time points (p > 0.05). SEM showed the fibres were homogeneously incorporated into the cement in all experimental groups. Conclusions: Resin-modified glass ionomer enhanced with short glass fibres at a weight loading of 20% showcased the most favourable mechanical properties while concurrently maintaining the ability to release fluoride and calcium after a 30-day period.

Polymerization shrinkage of contemporary dental resin composites: Comparison of three measurement methods with correlation analysis.

The aim of this study was to evaluate a novel approach for measuring the polymerization shrinkage of dental resin composites - measurement of sample depth variation. This new method was compared with two testing methodologies used for assessing the polymerization shrinkage (buoyancy and strain gauge methods). Eleven commercial resin composites were investigated (EverX Posterior; EverX Flow Bulk & Dentin; G-aenial Anterior, Posterior, A'chord & Universal Injectable; Filtek One Bulk Fill & Universal Restorative; SDR + Flow and Aura Bulk Fill). In addition, filler content (wt. %), flexural modulus, and the degree of conversion were evaluated. Shrinkage values, obtained by the buoyancy method, are greater than shrinkage evaluated by the strain gauge. There are significant differences in polymerization shrinkage among the tested resin composite materials. There is a strong correlation between the newly proposed method of shrinkage measurement and the buoyancy method (r2 = 0.8; p < 0.01). There is no correlation between volumetric shrinkage measurement (depth changes and buoyancy method) and linear strain measurement. Volumetric filler amount correlates with shrinkage values evaluated by all three methods. The degree of conversion for the tested resin composites ranges from 36 % to 52 %. There are some differences (around 10 %) between the filler content (wt. %) measured by the ashing-in-air method and the data given by the manufacturers. The highest flexural modulus is 14.8 GPa and the lowest 6.6 GPa. New formulations may introduce unknown relationships between the fundamental properties of dental resin composites.

Clinical evaluation of posterior flowable short fiber-reinforced composite restorations without proximal surface coverage.

The purpose of this clinical trail was to assess the clinical behavior of posterior composite restorations supported by a substantial foundation of flowable short fiber-reinforced composite SFRC (everX Flow, GC, Japan) used without proximal surface coverage with particulate filler resin composite (PFC). Seventy patients (20 males, 50 females; mean age: 30 ± 10 years) were randomly enrolled in this trial. Patients received direct restorations of either SFRC covered only on the occlusal surface (1-2 mm) by conventional PFC composite (G-ænial Posterior, GC), or plain conventional PFC composite without fiber-reinforcement, in Class II cavities in premolar and molar vital teeth. One operator made all restorations using one-step, self-etch bonding agent (G-ænial Bond, GC) according to manufacturers' recommendations. Two blinded trained operators evaluated the restorations at baseline, at 6, 12 and 18 months using modified USPHS criteria. Results indicated that, in both groups and at different follow-up intervals, according to evaluated criteria, restorations were rated mostly with best score (Alpha) (p > 0.05). For the marginal integrity after 6 months, a single case in the intervention [increased to 3 (8.8%) after 18 months] and 3 (9.7%) cases of the control group [increased to 4 (12.9%) after 18 months] had Bravo score but with no significant difference (p > 0.05). For color match measured after 6 and 18 months, three (8.8%) cases had Bravo score in the intervention group. The use of flowable SFRC composite without any PFC surface coverage proximally in Class II restorations demonstrated satisfactory clinical outcome throughout the 18-month follow-up.

Polymerization shrinkage stress of contemporary dental composites: Comparison of two measurement methods.

The aim of this study was to compare two testing methodologies employed for assessing the polymerization shrinkage stress of dental resin composites. Ten commercial resin composites were investigated (EverX Posterior & Flow; G-ænial Anterior, Posterior, A'CHORD & Universal Injectable; Filtek One Bulk Fill & Universal Restorative; SDR flow+ and Aura Bulk Fill). Photoelastic and contraction forces measurement methods were performed. The slope of the linear trendline and C-factor of specimens were calculated. The shrinkage stress values (range between: 6.4-13.4 MPa) obtained by the photoelastic method were higher for all resin composites than the values obtained by contraction forces measurements (range between 1.2-4.8 MPa). However, there was a strong linear correlation between these methods (r=0.8). The use of both investigated methods revealed important information about the shrinkage behavior of the restorative resin composites.

Effect of different surface treatments on shear bond strength of autopolymerizing repair resin to denture base materials processed with different technologies.

PURPOSE: To evaluate the effect of chemical, mechanical, and combination surface treatments on the shear bond strength (SBS) of autopolymerizing repair resins to conventional heat-cured, computer aided design (CAD)-computer aided manufacturing (CAM) milled, and three-dimensionally (3D) printed denture base materials. METHODS: Specimens were fabricated and divided according to the surface treatment as follows: no surface treatment (control group), monomer treatment (monomer group), resin remover treatment (resin remover group), roughening with 180 FEPA grit abrasive paper followed by monomer treatment (180-grit plus monomer group), and air particle abrasion (air abrasion group). Autopolymerizing resin cylinders were attached before accelerated aging of the specimens in water at 100 °C for 16 h. The SBS was tested using a universal testing machine. Surface roughness was evaluated using a 3D optical profilometer. Scanning electron microscopy (SEM) and stereomicroscopy were used for surface analysis. Data was collected and analyzed using analysis of variance (ANOVA) and Kruskall-Wallis tests (α = 0.05). RESULTS: The denture base material and surface treatment significantly affected the SBS. The milled Temp Basic Tissue demonstrated the highest SBS values across all surface treatments, whereas the two 3D-printed denture base materials exhibited the lowest SBS values. CONCLUSIONS: The bond strength of CAD-CAM-milled denture base resins to autopolymerizing repair resins is comparable to that of heat-cured resins. Surface roughening using air particle abrasion or 180-grit carbide paper can enhance the bond strength of the autopolymerizing repair resin to 3D-printed denture base materials.

Effect of ethylene oxide unit number in bis-EMA on the physical properties of additive-manufactured occlusal splint material.

PURPOSE: To investigate the effects of the number of ethylene oxide units in bis-EMA on the physical properties of additively manufactured occlusal splints. METHODS: Seven experimental materials containing bis-EMAs with three and 10 ethylene oxide units (BE3 and BE10, respectively) were prepared at different BE10 content rates (BE10-0%, -20%, -30%, -40%, -50%, -60%, and -80%). Half the specimens of each material were aged in boiling water. Flexural strength (FS), flexural modulus (FM), fracture toughness (FT), microwear depth (MD), degree of conversion (DC), water sorption (WSP), water solubility (WSL), color difference between non-aged and aged series (ΔE), and translucency (TP) were evaluated. All the evaluated properties other than FS and MD were analyzed by 1-way ANOVA and Tukey's post hoc analysis, while FS and MD were analyzed by Kruskal-Wallis's test and Bonferroni correction (α=0.05). RESULTS: BE10-80% revealed the lowest FS (P < 0.01 for BE10-0%, -20%, and -30%) and FM (P < 0.01, for all), while revealing the highest DC, WSP, WSL (P < 0.01 for all) and TP (P < 0.01 for all other than BE10-60%). BE10-50% showed the highest FT (P < 0.01 for all). BE10-50%, -60%, and -80% revealed significantly lower ΔE than others (P < 0.01) and lower MD than BE10-0% (P < 0.05). Regardless of the BE10 content, FS, FM, and FT decreased with aging. CONCLUSIONS: The number of ethylene oxide units affects the physical properties of additively manufactured occlusal splints. The higher number of ethylene oxide units in bis-EMA enhanced the microwear resistance, DC, WSP, WSL, color stability, and translucency, whereas it deteriorated the FS and FM.

Evaluation of microhardness of short fiber-reinforced composites inside the root canal after different light curing methods - An in vitro study.

OBJECTIVES: Short fiber-reinforced composite (SFRC) materials make it possible to reinforce root canal treated teeth with individualized, directly layered intraradicular posts (the Bioblock technique). The question arises, however, as to whether the photopolymerization of the material is sufficient deep within the root canal space and if it can be improved through different light-conducting options. Our study aimed to investigate the hardness of intraradicular SFRC material applied using the Bioblock technique and cured with various illumination methods, as measured through nanoindentation. MATERIALS AND METHODS: For this investigation, thirty plastic artificial teeth that had undergone root canal treatment were selected. These teeth were randomly divided into six study groups (Group 1-6; each group consisting of 5 teeth). The restoration procedures involved the use of SFRC or conventional composite materials, placed 6 mm apically from the root canal orifice. In Group 1 and 2, a conventional composite was used, whereas in Group 3-6, SFRC was employed for interradicular reinforcement (with a layered technique in Group 3 and 4 and a bulk-fill technique in Group 5 and 6). A modified light source was utilized for photopolymerization in Group 2, 4, and 6, whereas in Group 3 and 5, the polymerization light was directed through a prefabricated glass fiber posts. The control group (Group 1) utilized conventional composite material with a standard light-curing method. Following embedding and sectioning, the hardness of the composite materials was measured at 2 mm intervals within the root canal (1st, 2nd, 3rd measurements, in the coronal to apical direction). RESULTS: During the 1st measurement, light curing conducted through the glass fiber posts (Group 3 and 5) led to markedly higher hardness levels compared to the groups restored with conventional composite (control group with p = 0.002, p = 0.001, and Group 2 with p = 0.043, p = 0.034, respectively). In the 2nd measurement, only Group 5 demonstrated significantly greater hardness in comparison to the control group (p = 0.003) and Group 2 (p = 0.015). However, in the 3rd measurement, no statistically significant differences were observed among the groups. CONCLUSION: light curing through the glass fiber post provides outstanding hardness for the SFRC material in the apical layer in the root canal.

A comparative assessment of color stability among various commercial resin composites.

OBJECTIVES: The aim was to evaluate the color stability of six commercial restorative resin composites after being exposed to commonly consumed beverages. Repolishing impact on the stained composite was also assessed. METHODS: One-hundred and fifty disc specimens (8 mm diameter & 3 mm thickness) were prepared from Filtek™ Universal Restorative, SDR flow+, everX Flow, G-ænial A'CHORD, G-ænial Universal Flo and G-ænial Universal Injectable. To assess the color stability in five various beverages, 25 specimens from each material were randomly distributed into five groups (n = 5), according to the utilized staining solution. Group 1: distilled water, Group 2: coffee, Group 3: red wine, Group 4: energy drink, Group 5: coke. The color changes (∆E) for all materials were measured using spectrophotometer at the baseline, after 84 days of staining and after repolishing. Data was collected and analyzed using ANOVA (α = 0.05). RESULTS: Both material type and staining solution had a significant effect on the color stability of specimens (p < 0.05). Compared to other beverages, the color value of the specimens submerged in coffee and wine showed the most statistically significant (p < 0.05) mean ∆E. SDR flow + in coffee and wine presented the highest ∆E when compared to other tested materials (p < 0.05). After staining of the composites, repolishing was successful in lowering the ∆E value. CONCLUSIONS: All the beverages had an impact on the color stability of the tested resin composites, with coffee and wine demonstrating the most significant effects. The variations in color stability varied depending on the specific material utilized. Dentists should possess awareness regarding the chemical interactions that occur between different beverages and various types of resin composites. Additionally, repolishing serves as an effective technique for eliminating surface discoloration in composite restorations.

Evaluation of flexible three-dimensionally printed occlusal splint materials: An in vitro study.

OBJECTIVE: To evaluate and compare the mechanical properties, water sorption, water solubility, and degree of double bond conversion of three different commercially available three-dimensional (3D) printing resins used for the fabrication of flexible occlusal splints. METHODS: A digital printer was used to generate specimens from the evaluated splint materials (KeySplint Soft, IMPRIMO LC Splint flex, and V-Print splint comfort). The specimens were equally divided and tested either dry or after water storage at 37 °C for 30 days. A three-point bending test was used to assess flexural strength, elastic modulus, and fracture toughness. A two-body wear test was performed using a dual-axis chewing simulator. Water sorption and water solubility were measured after 30 days. The degree of double bond conversion was determined by FTIR-spectrometry. All data for the evaluated properties were collected and statistically analyzed. RESULTS: Both material and storage conditions had a significant effect on the flexural strength (P < 0.001), elastic modulus (P < 0.001), fracture toughness (P < 0.001), and wear (P < 0.001). The highest water sorption was noticed with IMPRIMO LC Splint flex (1.9 ± 0.0 %), while V-Print splint comfort displayed the lowest water solubility (0.2 ± 0.0 %). For the degree of conversion, it was statistically non-significant among the different materials (P = 0.087). SIGNIFICANCE: Different flexible 3D-printed splints available in the market displayed variations in the evaluated properties and clinicians should consider these differences when choosing occlusal device materials. Among the tested flexible splint materials, KeySplint Soft had the greatest flexural strength, elastic modulus, fracture toughness, wear resistance, and degree of conversion. It also showed the lowest water sorption.

Load-bearing capacity and wear characteristics of short fiber-reinforced composite and glass ceramic fixed partial dentures.

The aim of this study was to evaluate load-bearing capacity and wear performance of experimental short fiber-reinforced composite (SFRC) and conventional lithium-disilicate CAD/CAM fabricated fixed partial dentures (FPDs). Two groups (n = 12/group) of three-unit CAD/CAM fabricated posterior FPDs were made. The first group used experimental SFRC blocks, and the second group fabricated from lithium-disilicate (IPS e.max CAD). All FPDs were luted on a zirconia testing jig with dual-curing resin cement. Half of FPDs per group were quasi-statically loaded until fracture. The other half experienced cyclic fatigue aging (100.000 cycles, Fmax = 500 N) before loading quasi-statically until fracture. Fracture mode was examined using SEM. Wear test was performed using 15,000 loading cycles. Both material type and aging had a significant effect on the load-bearing capacity of FPDs. Experimental SFRC CAD without fatigue aging had significantly the highest load-bearing capacity (2096 ± 149N). Cyclic fatigue aging decreased the load-bearing capacity of the SFRC group (1709 ± 188N) but increased it for the lithium-disilicate group (1546 ± 155N). Wear depth values of SFRC CAD (29.3µm) were significantly lower compared to lithium-disilicate (54.2µm). Experimental SFRC CAD demonstrated the highest load-bearing capacity before and after cyclic fatigue aging, and superior wear behavior compared to the control material.

Adhesion of individually formed fiber post adhesively luted with flowable short fiber composite.

This laboratory study aimed to measure the push-out bond strength of individually formed fiber-reinforced composite (FRC) post luted with flowable short fiber-reinforced composite (SFRC) and to evaluate the influence of post coating with light-cured adhesive. Post spaces (Ø 1.7 mm) were drilled into 20 single-rooted decoronated premolar teeth. Post spaces were etched and treated with light-cured universal adhesive (G-Premio Bond). Individually formed FRC posts (Ø 1.5 mm, everStick) were luted either with light-cured SFRC (everX Flow) or conventional particulate-filled (PFC) dual-cure luting cement (G-CEM LinkForce). Half of the posts from each group were treated with dimethacrylate adhesive resin (Stick Resin) for 5 min before luting. After storage in water for two days, the roots were sectioned into 2 mm thick disks (n = 10/per group). Then, a push-out test-setup was used in a universal testing machine to measure the bond strength between post and dentin. The interface between post and SFRC was inspected using optical and scanning electron microscopy (SEM). Data were statistically analyzed using analysis of variance ANOVA (p = .05). Higher bond strength values (p < .05) were obtained when flowable SFRC was used as a post luting material. Resin coating of a post showed no significant effect (p > .05) on bond strength values. Light microscope images showed the ability of discontinuous short fibers in SFRC to penetrate into FRC posts. The use of flowable SFRC as luting material with individually formed FRC posts proved to be a promising method to improve the interface adhesion.

Tailoring the monomers to overcome the shortcomings of current dental resin composites - review.

Dental resin composites (DRCs) have become the first choice among different restorative materials for direct anterior and posterior restorations in the clinic. Though the properties of DRCs have been improved greatly in recent years, they still have several shortcomings, such as volumetric shrinkage and shrinkage stress, biofilm development, lack of radio-opacity for some specific DRCs, and estrogenicity, which need to be overcome. The resin matrix, composed of different monomers, constitutes the continuous phase and determine the performance of DRCs. Thus, the chemical structure of the monomers plays an important role in modifying the properties of DRCs. Numerous researchers have taken to design and develop novel monomers with specific functions for the purpose of fulfilling the needs in dentistry. In this review, the development of monomers in DRCs were highlighted, especially focusing on strategies aimed at reducing volumetric shrinkage and shrinkage stress, endowing bacteriocidal and antibacterial adhesion activities as well as protein-repelling activity, increasing radio-opacity, and replacing Bis-GMA. The influences of these novel monomers on the properties of DRCs were also discussed.

Effect of 3D printing system and post-curing atmosphere on micro- and nano-wear of additive-manufactured occlusal splint materials.

Although additive manufacturing has been widely applied for occlusal splint (OS) fabrication, it is still unclear whether 3D printing system and post-curing atmosphere would play a role in the wear resistance of additive-manufactured OS. Therefore, the aim of this study was to evaluate the effect of 3D printing system (liquid crystal display (LCD) and digital light processing (DLP)) and post-curing atmosphere (air and nitrogen gas (N2)) on the wear resistance of hard and soft OS materials for additive-manufactured OSs (KeySplint® Hard and Soft). The evaluated properties were microwear (by two-body wear test) and nano-wear resistances (by nanoindentation wear test) as well as flexural strength and flexural modulus (by three-point bending test), surface microhardness (by Vickers hardness test), and nanoscale elastic modulus (reduced elastic modulus) and nano surface hardness (by nanoindentation test). For the hard material, the surface microhardness, microwear resistance, reduced elastic modulus, nano surface hardness, and nano-wear resistance were significantly affected by the printing system (p < 0.05), while all the evaluated properties except flexural modulus were significantly affected by the post-curing atmosphere (p < 0.05). Meanwhile, both the printing system and post-curing atmosphere significantly affected all the evaluated properties (p < 0.05). The specimens additive-manufactured by DLP printer tended to show higher wear resistance in the hard material groups and lower wear resistance in the soft material groups when compared to those by LCD printer. The post-curing at N2 atmosphere significantly enhanced the microwear resistance of hard material groups additive-manufactured by the DLP printer (p < 0.05) and soft material groups additive-manufactured by the LCD printer (p < 0.01), while it significantly enhanced the nano-wear resistance of both hard and soft material groups regardless of the printing system (p < 0.01). It can be concluded that 3D printing system and post-curing atmosphere affect the micro- and nano-wear resistance of tested additively manufactured OS materials. In addition, it can be also concluded that the optical printing system providing higher wear resistance depends on the material type, and using nitrogen gas as a protection gas during post-curing enhances the wear resistance of tested materials.

Fatigue performance of endodontically treated molars reinforced with different fiber systems.

OBJECTIVE: The aim was to investigate the fatigue performance of root canal-treated (RCT) molars restored with different direct restorations utilizing discontinuous and continuous fiber-reinforced composite (FRC) systems. The impact of direct cuspal coverage was also evaluated. MATERIALS AND METHODS: One hundred and twenty intact third molars extracted for periodontal or orthodontic reasons were randomly divided into six groups (n=20). Standardized MOD, regular cavities for direct restorations were prepared in all specimens, and subsequently, root canal treatment and root canal obturation was carried out. After the endodontic treatment, the cavities were restored with different fiber-reinforced direct restorations as follows: SFC group (control), discontinuous short fiber-reinforced composite (SFC) without cuspal coverage (CC); SFC+CC group, SFC with cuspal coverage; PFRC group, transcoronal fixation with continuous polyethylene fibers without CC; PFRC+CC group, transcoronal fixation with continuous polyethylene fibers with CC; GFRC group, continuous glass FRC post without CC; and GFRC+CC, continuous glass FRC post with CC. All specimens underwent a fatigue survival test in a cyclic loading machine until fracture occurred or 40,000 cycles were completed. The Kaplan-Meier survival analysis was conducted, followed by pairwise log-rank post hoc comparisons between the individual groups (Mantel-Cox). RESULTS: The PFRC+CC group was characterized by significantly higher survival compared to all the groups (p < 0.05), except for the control group (p = 0.317). In contrast, the GFRC group showed significantly lower survival compared to all the groups (p < 0.05), except for the SFC+CC group (p = 0.118). The control group (SFC) showed statistically higher survival than the SFRC+CC group (p < 0.05) and GFRC group (p < 0.05), but it did not differ significantly from the rest of the groups in terms of survival. CONCLUSIONS: Direct restorations utilizing continuous FRC systems (in the form of polyethylene fibers or FRC post) to restore RCT molar MOD cavities performed better in terms of fatigue resistance when CC was performed compared to the same FRC restorations without CC. On the contrary, teeth restored with SFC restorations performed better without CC compared to the ones where SFC was covered. CLINICAL RELEVANCE: In the case of fiber-reinforced direct restorations for MOD cavities in RCT molars, direct CC is recommended when utilizing long continuous fibers for reinforcement, however, should be avoided when only SFC is used for their reinforcement.

Fatigue Resistance of Dissected Lower First Molars Restored with Direct Fiber-Reinforced Bridges-An In Vitro Pilot Study.

The aim of this research was to evaluate the mechanical impact of utilizing different fiber-reinforced composite (FRC) systems to reinforce inlay-retained bridges in dissected lower molars with different levels of periodontal support. A total of 24 lower first molars and 24 lower second premolars were included in this study. The distal canal of all molars received endodontic treatment. After root canal treatment, the teeth were dissected, and only the distal halves were kept. Standardized class II occluso-distal (OD) (premolars) and mesio-occlusal (MO) (dissected molars) cavities were prepared in all teeth, and premolar-molar units were created. The units were randomly distributed among four groups (n = six/group). With the aid of a transparent silicone index, direct inlay-retained composite bridges were fabricated. In Groups 1 and 2, both discontinuous (everX Flow) and continuous (everStick C&B) fibers were used for reinforcement, while in Groups 3 and 4, only discontinuous fibers (everX Flow) were used. The restored units were embedded in methacrylate resin, simulating either physiological periodontal conditions or furcation involvement. Subsequently, all units underwent fatigue survival testing in a cyclic loading machine until fracture, or a total of 40,000 cycles. Kaplan-Meyer survival analyses were conducted, followed by pairwise log-rank post hoc comparisons. Fracture patterns were evaluated visually and with scanning electron microscopy. In terms of survival, Group 2 performed significantly better than Groups 3 and 4 (p < 0.05), while there was no significant difference between the other groups. In the case of impaired periodontal support, a combination of both continuous and discontinuous short FRC systems increased the fatigue resistance of direct inlay-retained composite bridges compared to bridges that only contained short fibers. Such a difference was not found in the case of sound periodontal support between the two different bridges.

Wear behavior at margins of direct composite with CAD/CAM composite and enamel.

OBJECTIVES: The aim was to investigate the two-body wear at the marginal area between direct filling composites and substrate of CAD/CAM composites or enamel. MATERIALS AND METHODS: Flat specimens were prepared from CAD/CAM composites (CERASMART 270 and SFRC CAD) and bovine enamel. A box-shaped cavity cut into CAD/CAM composites and enamel surfaces was made. The prepared cavity in CAD/CAM composites was treated with a primer, while in enamel, the cavity was treated with an adhesive. Three conventional composites (Universal Injectable, G-aenial A'Chord, and Filtek Bulk Fill) and one short fiber composite (everX Flow) were placed and cured in the prepared cavities. A two-body wear test was conducted with 15,000 chewing cycles using a dual-axis chewing simulator. The specimens (n = 5/per group) were positioned to produce wear (load = 20 N) across the marginal area between filling composites and substrates. The wear depth was analyzed using a 3D optical profilometer. SEM was used to evaluate the wear behavior and margins between the filling and substrate materials. RESULTS: All composites used displayed different wear behavior (20-39 µm) (p < 0.05). The highest wear values were recorded for A'Chord and Filtek, while the lowest values were for Injectable and CERASMART 270. The data analysis showed that the wear behavior of substrate materials depends on the filling materials used at margins (p < 0.05). The marginal breakdown was seen only between bovine enamel and filling composites. CONCLUSIONS: The use of the two-body wear simulation method revealed important information about the behavior of the filling composites at the marginal area with CAD/CAM composites or bovine enamel substrates. CLINICAL RELEVANCE: The marginal breakdown related to the material combination at the bonding region.

Crack propensity of different direct restorative procedures in deep MOD cavities.

OBJECTIVE: The purpose was to evaluate the crack formation associated with different direct restorative procedures of the utilized resin composites (RC) right after and 1 week later of the restoration. MATERIALS AND METHODS: Eighty intact, crack-free third molars with standard MOD cavities were included in this in vitro study and randomly divided into four groups of 20 each. After adhesive treatment, the cavities were restored either with bulk (group 1) or layered (group 2) short-fiber-reinforced resin composites (SFRC); bulk-fill RC (group 3); and layered conventional RC (control). Right after the polymerization and a week later, crack evaluation on the outer surface of the remaining cavity walls was performed with a transillumination method utilizing the D-Light Pro (GC Europe) with the "detection mode." Between- and within-groups comparisons Kruskal-Wallis and Wilcoxon tests were used, respectively. RESULTS: Post-polymerization crack evaluation showed significantly lower crack formation in SFRC groups compared to the control (p<0.001). There was no significant difference within SFRC groups and non-SFRC groups (p=1.00 and p=0.11, respectively). Within group comparison revealed significantly higher number of cracks in all groups after 1 week (p≤0.001), however, only the control group differed significantly from all the other groups (p≤0.003). CONCLUSIONS: Post-polymerization shrinkage induced further crack formation in the tooth 1 week after the restoration. SFRC was less prone to shrinkage-related crack formation during the restorative procedure; however, after 1 week, besides SFRC, bulk-fill RC also showed less prone to polymerization shrinkage-related crack formation than layered composite fillings. CLINICAL RELEVANCE: SRFC can decrease the shrinkage stress-induced crack formation in MOD cavities.