Influence of different curing modes on flexural properties, fracture toughness, and wear behavior of dual-cure provisional resin-based composites

3D printed, subtractive, and conventional acrylic resins: Evaluation of monotonic versus fatigue behavior and surface characteristics

This study assessed the mechanical properties and surface characteristics of dental prosthetic acrylic resin fabricated by 3D printing, comparing it with subtractive, pressing, and molding techniques. Bar-shaped specimens (N= 90; 65 × 10 × 3.3 mm; ISO:207951) were prepared and assigned into six groups: PRINT (3D printing vis stereolithography with PriZma 3D Bio Denture, Makertech Labs); SUB (subtractive manufacturing with Vipiblock Trilux, Vipi); PRESS Base (pressing using muffle with Thermo Vipi Wave, Vipi for base); PRESS Tooth (pressing with Onda-cryl, Clássico for tooth); MOLD Base (molding using addition silicone with Vipi Flash, Vipi for base); and MOLD Tooth (molding with Dencor, Clássico for tooth). Monotonic flexural strength (FS) and elastic modulus (E) were measured using a three-point bending approach (n= 5) on a universal testing machine at a crosshead speed of 5 mm/min. Fatigue testing (n= 10) followed similar geometry and settings, with a frequency of 2 Hz, initial stress level at 20 MPa, and stress increments of 5 MPa every 2,500 cycles. Surface roughness (n= 10) was assessed through profilometry, and fractographic and topographic analyses were conducted. Statistical analyses included One-Way ANOVA for monotonic FS, roughness, and E, along with Kaplan-Meier with Mantel-Cox post-hoc and Weibull analysis for fatigue strength. PRINT showed lower monotonic FS than the SUB and PRESS Tooth but comparable fatigue strength to these groups and superior to PRESS Base and MOLD (Base and Tooth) groups. All groups had similar Weibull moduli. Surface roughness of the PRINT group was comparable to most techniques but higher than the PRESS Tooth group. Fractographic analysis revealed fractures originating from surface defects under tensile stress, with SEM showing scratch patterns in all groups except PRINT, which had a more uniform surface. Despite its lower monotonic strength, 3D printed resin demonstrated comparable fatigue strength to subtractive and pressing methods and similar surface roughness to most methods, indicating its potential as a viable option for dental prosthesis.

Influence of occlusal thickness on the fracture resistance of chairside milled lithium disilicate posterior full-coverage single-unit prostheses containing virgilite: A comparative in vitro study

PURPOSE

To evaluate the fracture resistance of chairside computer-aided design and computer-aided manufacturing (CAD-CAM) lithium disilicate mandibular posterior crowns with virgilite of different occlusal thicknesses and compare them to traditional lithium disilicate crowns.

MATERIALS AND METHODS

Seventy-five chairside CAD-CAM crowns were fabricated for mandibular right first molars, 60 from novel lithium disilicate with virgilite (CEREC Tessera, Dentsply Sirona), and 15 from traditional lithium disilicate (e.max CAD, Ivoclar Vivadent). These crowns were distributed across five groups based on occlusal thickness and material: Group 1 featured CEREC Tessera crowns with 0.8 mm thickness, Group 2 had 1.0 mm thickness, Group 3 had 1.2 mm thickness, Group 4 with 1.5 mm thickness, and Group 5 included e.max CAD crowns with 1.0 mm thickness. These crowns were luted onto 3D-printed resin dies using Multilink Automix resin cement (Ivoclar Vivadent). Subsequently, they underwent cyclic loading (2,000,000 cycles at 1 Hz with a 275 N force) and loading until fracture. Scanning electron microscopy (SEM) assessed the fractured specimens. Statistical analysis involved one-way ANOVA and the Kruskal-Wallis Test (α = 0.05).

RESULTS

Fracture resistance varied significantly (<0.001) across mandibular molar crowns fabricated from chairside CAD-CAM lithium disilicate containing virgilite, particularly between crowns with 0.8 mm and those with 1.2 and 1.5 mm occlusal thickness. However, no significant differences were found when comparing crowns with 1, 1.2, and 1.5 mm thicknesses. CEREC Tessera crowns with 1.5 mm thickness exhibited the highest resistance (2119 N/mm2), followed by those with 1.2 mm (1982 N/mm2), 1.0 mm (1763 N/mm2), and 0.8 mm (1144 N/mm2) thickness, whereas e.max CAD crowns with 1.0 mm occlusal thickness displayed the lowest resistance (814 N/mm2).

CONCLUSIONS

The relationship between thickness and fracture resistance in the virgilite lithium disilicate full-coverage crowns was directly proportional, indicating that increased thickness corresponded to higher fracture resistance. No significant differences were noted among crowns with thicknesses ranging from 1 to 1.5 mm. This novel ceramic exhibited superior fracture resistance compared to traditional lithium disilicate.

Changes in enamel hardness, wear resistance, surface texture, and surface crystal structure with glass ionomer cement containing BioUnion fillers

This study investigated the potential of BioUnion filler containing glass ionomer cement (GIC) to enhance the properties of enamel surrounding restorations, with a specific focus on the effect on hardness. The hardness of the bovine enamel immersed in the cement was measured using Vickers hardness numbers. Following sliding and impact wear simulations, the enamel facets were examined using confocal-laser-scanning microscopy and scanning-electron microscopy. Surface properties were further analyzed using energy-dispersive X-ray spectroscopy and X-ray diffraction (XRD). A significant increase in Vickers hardness numbers was observed in the BioUnion filler GIC after 2 days. Furthermore, the mean depth of enamel facets treated with BioUnion filler GIC was significantly less than that of untreated facets. Characteristic XRD peaks indicating the presence of hydroxyapatite were also observed. Our findings imply that GIC with BioUnion fillers enhances the mechanical properties of the tooth surface adjacent to the cement.

Influence of CAD/CAM Milling and 3D-Printing Fabrication Methods on the Mechanical Properties of 3-Unit Interim Fixed Dental Prosthesis after Thermo-Mechanical Aging Process

This study assessed the influence of CAD/CAM milling and 3D-printing fabrication methods on mechanical properties of 3-unit interim fixed dental prosthesis (IFDPs) after thermo-mechanical aging. Forty 3-unit IFDPs were fabricated on a mandibular right second premolar and second molar of a typodont cast. Samples were fabricated from the following materials; auto-polymerized polymethyl methacrylate (conventional resin), CAD/CAM PMMA (milled resin) and two different CAD/CAM 3D-printed composite resins; digital light processing Asiga (DLP AS) and stereolithography NextDent (SLA ND). Mechanical properties were compared between the studied materials using Kruskal−Wallis test, followed by multiple pairwise comparisons using Bonferroni adjusted significance. There was a significant difference in flexural strength and microhardness between the studied materials (p < 0.001), with the highest mean ± SD reported in the milled IFDPs (174.42 ± 3.39, 27.13 ± 0.52), and the lowest in the conventional IFDPs (98.02 ± 6.1, 15.77 ± 0.32). Flexural strengths differed significantly between the conventional IFDPs and all materials except DLP AS. The highest elastic modulus was recorded in the milled group, and the lowest in the SLA ND group (p = 0.02). In conclusion, superior flexural strength, elastic modulus, and hardness were reported for milled IFDPs. SLA ND printed IFDPs showed comparable mechanical properties to milled ones except for the elastic modulus.

Depth of cure of 10 resin-based composites light-activated using a laser diode, multi-peak, and single-peak light-emitting diode curing lights

OBJECTIVES

To evaluate the depth of cure (DOC) of ten contemporary resin-based composites (RBCs), light-cured using different LCUs and exposure times.

METHODS

The power, radiant emittance, irradiance, radiant exposure (RE), and beam profiles from a laser (M, Monet), a multi-peak (V, Valo Grand), and single-peak (S, SmartLite Pro) LCU were measured. The DOC was measured using a 6-mm diameter metal mold and a solvent dissolution method to remove the uncured RBC. The length of the remaining RBC was divided by 2. The exposure times were: 1s and 3s for M, 10s and 20s for V, and 10s and 20s for S. Data were analyzed using: Bland-Altman distribution, Pearson's Correlation, and an artificial neural network (ANN) to establish the relative importance of the factors on the DOC (α=0.05; β=0.2).

RESULTS

Significant differences were found in the DOC of the different LCUs and composites. The laser LCU emitted the highest power, radiant emittance, and irradiance. However, this LCU used for 1 s delivered the lowest RE and produced the shortest DOC in all ten RBCs. The ANN demonstrated that the RE is the most critical factor for the DOC. Bland-Altman comparisons showed that the DOCs achieved with the laser LCU used for 1s were between 17 - 34 % shorter than the other conditions.

CONCLUSIONS

Although the laser LCU cured all 10 RBCs when used for 1s, it produced the shallowest DOC, and some RBCs did not achieve the minimum DOC threshold. The RE and not the irradiance was the most important factor in determining the DOC of RBCs.

CLINICAL SIGNIFICANCE

Despite delivering high power and irradiance, the laser used for l s delivered a lower radiant exposure than the conventional LCUs used for 10 s. This resulted in a shorter DOC.

A study on the characteristics of resin composites for provisional restorations

In this study, we evaluated the characteristics of five commercial resin composites used for provisional restorations. The inorganic filler contents of the resins were measured, and three-point bending, wear, surface hardness, water absorption, and staining tests were performed. The specimens underwent additional three-point bending tests after water storage and undergoing thermal stresses at 5°C and 55°C (10,000 cycles). Data were analyzed using one- or two-way analysis of variance and Bonferroni post-hoc tests. Pearson's correlation coefficient was used for pairwise comparisons. Each resin composite presented with different mechanical properties, based on variations in the inorganic filler content. The flexural strength of each resin composite was significantly decreased after water storage. There has a positive correlation between flexural strength and dynamic hardness but a negative correlation between flexural strength and maximum wear depth. The types and contents of the inorganic fillers, the composition of the monomer in the resin matrix, and the addition of plasticizers can affect the properties of the material.

Comparative evaluation of the effect of thermocycling on the mechanical properties of conventionally polymerized, CAD-CAM milled, and 3D-printed interim materials

STATEMENT OF PROBLEM

Studies on the energy absorption characteristics by means of elastic and plastic material deformation of interim materials are lacking.

PURPOSE

The purpose of this in vitro study was to compare the effect of different thermocycling periods on the flexural strength (σ_fs), resilience (U_r), and toughness (U_T) of conventionally polymerized, computer-aided design and computer-aided manufacturing (CAD-CAM) milled, and 3-dimensionally (3D) printed interim materials.

MATERIAL AND METHODS

Rectangular specimens (n=30 for each material) were fabricated from autopolymerized polymethyl methacrylate (PMMA), bis-acryl resin (Bis-acryl), CAD-CAM polymethyl methacrylate-based polymer (CAD-CAM/Milled), and 3D-printed composite resin (3D-Printed). Each material was divided into 3 groups (n=10) according to the applied thermocycling (5 °C to 55 °C) procedure: control (0 cycles), 2500, and 10 000 cycles. Parameters of the materials such as σ_fs, U_r, and U_T were tested in a 3-point bend test according to International Organization for Standardization (ISO) 10477. Data were statistically analyzed with the Shapiro-Wilk test followed by Kruskal-Wallis test, the Mann-Whitney U test, the Friedman test, and Wilcoxon signed-rank test (α=.05).

RESULTS

The tested material and thermocycling had a statistically significant influence on the σ_fs, U_r, and U_T values (P<.05). PMMA showed the lowest mean σ_fs, U_r, and U_T values (P<.05), and CAD-CAM/Milled showed σ_fs values similar to those of 3D-Printed at all thermocycling periods. CAD-CAM/Milled showed the highest U_r values at 10 000 cycles and the highest U_T values at all thermocycles. No significant differences were found in the mean change of σ_fs and U_r of CAD-CAM/Milled among different thermocycling periods.

CONCLUSIONS

The results suggested that digitally fabricated interim materials had better mechanical properties than conventionally polymerized materials and that milled materials had the highest stability in maintaining their initial capacity to absorb energy.

Influence of masticating cycles and chewing patterns on inadvertent enamel wear caused by zirconia brackets

Little information is available about enamel wear caused by zirconia brackets, an inadvertent side effect of orthodontic treatment. The purpose of this study was to examine potential enamel damage induced by contact with zirconia brackets. Sliding and impact wear simulations were performed using bovine enamel specimens positioned at a 25° slant to a zirconium ball to determine wear behaviour. Different chewing patterns, tapping and grinding, were simulated. Specimens were profiled using confocal laser scanning microscopy, and the mean maximum depth and surface roughness were measured. Scanning electron microscopy was also performed. The mean maximum depth of wear values differed according to the number of mastication cycles, with a higher number of cycles producing higher depths of wear. The facet wear depth was significantly greater with the tapping pattern than with the grinding pattern. Scanning electron microscopic observation of the wear facets revealed that surface textures at the edges were rougher than those at the centre of all facets. The results of this study indicated that enamel wear was induced by contact with zirconia brackets during the early period of mastication, and that the patterns and number of cycles of mastication affected the wear progression of enamel.

Handling and Mechanical Properties of Low-viscosity Bulk-fill Resin Composites

This study aimed to evaluate the filler contents (FCs), flexural properties, depth of cure (DOC), wear resistance, and handling properties of different low-viscosity bulk-fill resin composites (LVBRCs) and to determine the correlations between the tested parameters. Six LVBRCs, Beautifil-Bulk (BBF), Bulk Base Hard (BBH), Bulk Base Medium (BBM), Filtek Bulk-Fill Flowable Restorative (FBF), G-ænial Bulk Injectable (GBI), and SDR flow+ Bulk-Fill Flowable (SDR) were used. The DOC and flexural property tests were conducted according to the ISO 4049 specifications. The flexural strength, elastic modulus, and resilience were determined in 12 specimens that were obtained from each of the 6 materials. Sliding-impact-wear testing was conducted by evaluating the wear facets of the specimens using a noncontact profilometer and by confocal laser scanning microscopy. The handling properties of the LVBRC was assessed via extrusion force and thread formation measurements. The DOC for the majority of the LVBRCs was approximately 4 mm. Although the FCs and mechanical properties were material dependent, some LVBRCs exhibited excellent flexural properties and wear resistance. The LVBRCs demonstrated a wide range of extrusion force and thread formation. Regarding the correlations between the tested parameters, extremely strong negative and positive correlations were observed for the DOC versus extrusion force, flexural strength versus elastic modulus, maximum depth versus volume loss, and maximum depth versus thread formation. In addition, strong correlations between FCs and DOC, resilience, wear resistance, and extrusion force were observed.

Effects of Different Amine Activators on the Monomer Conversion, Biaxial Flexural Strength, and Color Stability of Experimental Provisional Dental Restorations

Objective  The aim was to assess the effect of different amine activators including N, N-dimethyl-p-toluidine (DMPT) or Na-N-tolyglycine glycidyl methacrylate (NTGGMA) on chemical-activated monomer conversion, biaxial flexural strength (BFS), and color stability of composites for provisional dental restorations.

Materials and Methods  Two formulations of composites containing either DMPT (D-temp) or NTGGMA (N-temp) were prepared. The degree of monomer conversion was assessed. The BFS of the materials was tested using the ball-on-ring testing jig. The color difference (∆E ₀₀ ) of the materials after immersion in water was also determined. The commercial comparisons were Unifast (UF), Protemp (PT), Luxacrown, and Luxatemp (LT).

Results  The monomer conversion of D-temp (57.4 ± 1.3%) was comparable to that of N-temp (59.0 ± 1.3%). The conversion of both D-temp and N-temp were higher than that of PT (48.1 ± 3.4%) and LT (48.0 ± 1.6%). BFS of both D-temp (164.2 ± 18.1 MPa) and N-temp (168.6 ± 8.9 MPa) were comparable but higher than that of UF (119.8 ± 13.6 MPa). ∆E ₀₀ of D-temp (2.7 ± 0.7) and N-temp (2.5 ± 0.8) were comparable but higher than that of other commercial materials (0.6–1.2).

Conclusion  The use of DMPT or NTGGMA showed negligible effect on monomer conversion, BFS, and color stability of the experimental provisional restorations. The conversion and BFS of the experimental materials were in the range of that obtained from commercial bis-acryl-based materials. However, the color stability of the experimental materials was lower than that of commercial materials.

Temperature Changes in Composite Materials during Photopolymerization

During polymerization, composite materials cause a temperature rise which may lead to irreversible changes in the dental pulp. The mechanical properties of composite materials depend on a number of factors, such as the composition of the material, the type of polymerization unit, the polymerization mode, and the duration of polymerization. The objective of this study was to assess the temperature rise values and flexural strength of composite materials, as obtained using different modes and times of polymerization. A total of six composite materials were used in the study. Samples of each of the materials were cured using seven polymerization protocols. A CMP-401 digital meter (Sonel, Świdnica, Poland), complete with a type K thermocouple (NiCr-Ni), was used to record the temperature increases during the light curing of the resin composites. Temperature rises were recorded beneath the composite disc in an acrylic matrix. The specimens were tested for flexural strength using a Cometech QC-508M2 testing machine. The lowest results for the increased mean temperature were obtained for Fast-Cure 3 s (39.0 °C), while the highest results were obtained for Fast-Cure 20 s (45.8 °C). The highest average temperature values for all tested protocols were recorded for the Z550 Filtek material. Mean flexural strengths as measured in each test group were higher than the minimum value for composite materials as per the ISO:4049 standard. In the case of deep caries with a thin layer of dentin separating the filling from pulp, a base layer or a short polymerization duration mode is recommended to protect pulp from thermal injury.

Surface Treatment Of Nanozirconia Fillers To Strengthen Dental Bisphenol A-Glycidyl Methacrylate-Based Resin Composites

Purpose

To investigate the effects of nanozirconia fillers conditioned with 10-methacryloyloxydecyl dihydrogen phosphate (MDP) with or without zirconium hydroxide precoating on bending strength, Vickers hardness, and translucence of dental resin composites.

Methods

We obtained nanozirconia fillers coated with different concentrations of Zr(OH)₄ using wet-chemical synthesis. We analyzed coating quality by observing electron-diffraction patterns using transmission electron microscopy. We conditioned zirconia fillers, with or without prior Zr(OH)₄-coating, using MDP-containing primers and evaluated the formation of chemical bonds using nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). We then performed three-point bending-strength tests, Weibull analysis, Vickers hardness, and translucence-parameter analysis with or without addition of different concentrations of zirconia using untreated zirconia fillers as controls.

Results

We achieved desirable Zr(OH)₄ coating using 5 mmol/L zirconium chloride. NMR and XPS analysis detected stronger Zr–O–P peaks on MDP-conditioned zirconia fillers with prior Zr(OH)₄-coating compared with MDP-conditioned fillers alone, suggesting that MDP bonding with zirconia was enhanced by zirconium hydroxide. Our three-point bending-strength tests revealed that increasing levels of untreated zirconia fillers decreased the three-point bending strength of the resin composites, while MDP-conditioned zirconia fillers with or without prior Zr(OH)₄ coating improved three-point bending strengths. Adding 5 wt% and 7.5 wt% MDP-conditioned zirconia fillers with prior Zr(OH)₄ coating achieved the highest three-point bending strength. Furthermore, addition of zirconia fillers decreased the translucence of silica-based resin composites.

Conclusion

MDP conditioning with prior Zr(OH)₄ coating is recommended for treating nanozirconia fillers of resin composites.