Fatigue resistance of composite restorations: effect of filler content

Teeth Restored with Bulk–Fill Composites and Conventional Resin Composites; Investigation of Stress Distribution and Fracture Lifespan on Enamel, Dentin, and Restorative Materials via Three-Dimensional Finite Element Analysis

Objectives: the aim of this study was to examine the stress distribution of enamel, dentin, and restorative materials in sound first molar teeth with restored cavities with conventional resin composites and bulk–fill composites, as well as to determine their fracture lifetimes by using the three-dimensional finite element stress analysis method. Materials and Methods: an extracted sound number 26 tooth was scanned with a dental tomography device and recorded. Images were obtained as dicom files, and these files were transferred to the Mimics 12.00 program. In this program, different masks were created for each tooth tissue, and the density thresholds were adjusted manually to create a three-dimensional image of the tooth, and these were converted to a STL file. The obtained STL files were transferred to the Geomagic Design X program, and some necessary adjustments, such as smoothing, were made, and STP files were created. Cavity preparation and adhesive material layers were created by transferring STP files to the Solidworks program. Finally, a FE model was created in the ABAQUS program, and stress distributions were analyzed. Results: when the bulk–fill composite and conventional resin composite materials were used in the restoration of the cavity, the structures that were exposed to the most stress as a result of occlusal forces on the tooth were enamel, dentin, restorative material, and adhesive material. When the bulk–fill composite material was used in restoration, while the restorative material had the longest fracture life as a result of stresses, the enamel tissue had the shortest fracture life. When the conventional resin composite material was used as the restorative material, it had the longest fracture life, followed by dentin and enamel. Conclusion: when the bulk–fill composite material was used instead of the conventional resin composite material in the cavity, the stress values on enamel, dentin, and adhesive material increased as a result of occlusal forces, while the amount of stress on the restorative material decreased. In the fracture analysis, when the bulk–fill composite material was used instead of the conventional resin composite material, a decrease in the number of cycles required for the fracture of enamel, dentin, and restorative materials was observed as a result of the forces generated in the oral cavity.

Edge chipping resistance of veneering composite resins

OBJECTIVES

To investigate the edge chipping resistance (ECR) of six veneering composite resins after different treatment protocols.

MATERIALS AND METHODS

Rectangular bar specimens were manufactured from Ceramage Incisal (CER; Shofu), dialog Vario Occlusal (DIA; Schütz Dental), Gradia Plus Heavy Body Enamel (GRA; GC Europe), in:joy incisal (INJ; Dentsply Sirona Deutschland), SR Nexco Paste Incisal (SRN; Ivoclar Vivadent), and Signum composite enamel (SIG; Kulzer). ECR was determined after five treatment protocols: (1) no treatment, (2) after storage in distilled water at 37 °C for 7 days, (3) storage in distilled water with an additional 10 000 thermal cycles (5 °C/55 °C), and hydrothermal treatment at 134 °C at a water vapor pressure of 0.2 MPa for a duration of (4) 3.5 min or (5) 23.5 min. Force was applied with the universal testing machine ZHU 0.2 (Zwick Roell) mounted with a Vickers diamond indenter until the chip fractured off the specimen and ECR values were computed by dividing the applied maximum force by the distance to the center of the applied force. Fracture analysis was performed employing light microscope imaging. Univariate and one-way ANOVA, Scheffé and Tukey-B post hoc, and partial eta squared (ƞ_p²) were computed (p < 0.05).

RESULTS

DIA presented consistently high ECR values, while CER showed low results. For some groups, seven days' storage in water and hydrothermal treatment for 3.5 min led to higher ECR results than observed in the initial state, while an additional 10 000 thermal cycles and hydrothermal treatment for 23.5 min resulted in lower ECR values.

CONCLUSIONS

The examined veneering composite resins differed in regard to their mechanical properties, with DIA possessing the highest resistance to chipping. While post-processing can initially increase a material's edge chipping resistance, intensified treatment protocols reduced the mechanical properties of veneering composite resins.

Mechanical and hydrolytic degradation of an Ormocer®-based Bis-GMA-free resin composite

OBJECTIVES

The aim of the study was to evaluate the mechanical stability of bisphenol A-glycidyl methacrylate (Bis-GMA) and Ormocer-based resin composites before and after water absorption and to examine water saturation.

MATERIAL AND METHODS

Disc-shaped specimens of the Bis-GMA (Grandio SO, Voco) and the Ormocer-based (Admira Fusion, Voco) dental resin composites were produced, stored in water, and weighed after pre-determined times to measure the absorbed water. Bend bars were produced and stored for 24 h in dry conditions as well as in distilled water for 14 days or 60 days at 37 °C. The initial flexural strength (FS) under quasi-static loading and flexural fatigue strength (FFS) under cyclic loading were determined under 4-point bending. Fracture toughness (K_Ic) of both composites was measured using the single-edge-V-notch-beam (SEVNB) technique after the same storage conditions under 3-point bending.

RESULTS

Within the first 14 days, storage conditions did not affect the initial FS of Grandio SO, while a significant drop in initial FS was observed for Admira Fusion after 2 weeks in water and most of the water was absorbed within this time. FFS for the Bis-GMA composite was not reduced before 2 months in water, whereas for the Ormocer®-based composite, there has been a significant decrease in strength after cyclic fatigue already at 2 weeks of water storage. K_Ic of Admira Fusion decreased significantly after both storage periods, while K_Ic of Grandio SO decreased only significantly after 2 weeks of water storage.

CONCLUSION

All mechanical properties of the Bis-GMA composite were superior to those of the Ormocer®-based material, except water sorption.

CLINICAL SIGNIFICANCE

Water storage seems to have a much more pronounced effect on the mechanical properties of Ormocer®-based dental composites in comparison to Bis-GMA-based composites.

The synthesis, modification, and application of nanosilica in polymethyl methacrylate denture base

This study aimed to investigate amount of γ-methacryloxypropyltrimethoxysilane (MPS) silanized on experimental nanosilica particles (NPs), amount of NP and amount of MPS silanized NP on flexural strength (FS), flexural modulus (FM), and fracture toughness (FT) of NP reinforced polymethyl methacrylate (PMMA). The chemisorbed amount of MPS was determined using elemental analysis. Six groups (n=8) were prepared with chemisorbed amount and mixed with PMMA-monomer to make 0.25, 0.5, 1, 5, 10 and 15% (w/w) of NP reinforced PMMA. PMMA without NP served as control. Seven groups (n=8) were prepared with 1% of NP silanized with 0, 0.061, 0.123, 0.246, 0.493, 0.987, and 1.974 g_MPS/g_silica and mixed with PMMA-monomer to make NP reinforced PMMA. FS, FM, and FT were determined using 3-point bending test. One-way ANOVA and multiple comparisons showed that 0.246 g_MPS/g_silica of 1% amount of silanized NP group was significantly highest in FS, FM, and FT compared to the others (p<0.05).

Multi-scale analysis of the effect of nano-filler particle diameter on the physical properties of CAD/CAM composite resin blocks

The objective of this study was to assess the effect of silica nano-filler particle diameters in a computer-aided design/manufacturing (CAD/CAM) composite resin (CR) block on physical properties at the multi-scale in silico. CAD/CAM CR blocks were modeled, consisting of silica nano-filler particles (20, 40, 60, 80, and 100 nm) and matrix (Bis-GMA/TEGDMA), with filler volume contents of 55.161%. Calculation of Young's moduli and Poisson's ratios for the block at macro-scale were analyzed by homogenization. Macro-scale CAD/CAM CR blocks (3 × 3 × 3 mm) were modeled and compressive strengths were defined when the fracture loads exceeded 6075 N. MPS values of the nano-scale models were compared by localization analysis. As the filler size decreased, Young's moduli and compressive strength increased, while Poisson's ratios and MPS decreased. All parameters were significantly correlated with the diameters of the filler particles (Pearson's correlation test, r = -0.949, 0.943, -0.951, 0.976, p < 0.05). The in silico multi-scale model established in this study demonstrates that the Young's moduli, Poisson's ratios, and compressive strengths of CAD/CAM CR blocks can be enhanced by loading silica nanofiller particles of smaller diameter. CAD/CAM CR blocks by using smaller silica nano-filler particles have a potential to increase fracture resistance.

Mechanical performance of novel bioactive glass containing dental restorative composites

OBJECTIVES

Bioactive glass (BAG) is known to possess antimicrobial properties and release ions needed for remineralization of tooth tissue, and therefore may be a strategic additive for dental restorative materials. The objective of this study was to develop BAG containing dental restorative composites with adequate mechanical properties comparable to successful commercially available composites, and to confirm the stability of these materials when exposed to a biologically challenging environment.

METHODS

Composites with 72 wt% total filler content were prepared while substituting 0-15% of the filler with ground BAG. Flexural strength, fracture toughness, and fatigue crack growth tests were performed after several different soaking treatments: 24h in DI water (all experiments), two months in brain-heart infusion (BHI) media+Streptococcus mutans bacteria (all experiments) and two months in BHI media (only for flexural strength). Mechanical properties of new BAG composites were compared along with the commercial composite Heliomolar by two-way ANOVA and Tukey's multiple comparison test (p≤0.05).

RESULTS

Flexural strength, fracture toughness, and fatigue crack growth resistance for the BAG containing composites were unaffected by increasing BAG content up to 15% and were superior to Heliomolar after all post cure treatments. The flexural strength of the BAG composites was unaffected by two months exposure to aqueous media and a bacterial challenge, while some decreases in fracture toughness and fatigue resistance were observed. The favorable mechanical properties compared to Heliomolar were attributed to higher filler content and a microstructure morphology that better promoted the toughening mechanisms of crack deflection and bridging.

SIGNIFICANCE

Overall, the BAG containing composites developed in this study demonstrated adequate and stable mechanical properties relative to three successful commercial composites.

Non-destructive characterization of voids in six flowable composites using swept-source optical coherence tomography

OBJECTIVE

The aim of this study was to evaluate the void frequency (V(F)) and void volume (V(V)) in different flowable composites using swept-source optical coherence tomography (SS-OCT).

METHODS

Standard class I cavities were prepared and filled with six different flowable composites: Clearfil Majesty LV (MJ; Kuraray), MI Flow (MW; GC), MI Fil (ML; GC), Beautifil flow plus (BF; Shofu), Palfique Estelite low flow (EL; Tokuyama) and Surefil SDR flow (SF; Dentsply). The restorations were scanned under OCT. The OCT tomograms were analyzed and average V(F) and V(V) per restoration for each composite were calculated. Scanning electron microscope (SEM) was used to observe the structure of each composite. In addition, the flowability of the materials was evaluated measuring the displacement of each material placed up-right on a glass slide. V(F) and V(V) obtained by OCT were also compared to those calculated using micro-computed tomography (micro-CT).

RESULTS

Kruskal-Wallis ANOVA and Mann-Whitney U tests revealed significantly different V(F) and V(V) values (p<0.05) among the composites. Voids ranging from 35 to 785 μm in diameter were detected in OCT tomograms. MJ showed highest V(F) and V(V) values followed by MW, but ML, BF, EL and SF showed no significant difference. Filler volume in composites showed a positive correlation with void formation, but flowability did not show a specific trend. Micro-CT evaluation validated the V(F) and V(V) calculation by OCT, with a significant correlation in void size (p<0.001, r=0.94).

CONCLUSION

The results of this study indicate the reliability of SS-OCT for real-time void characterization of composite materials and restorations. Void formation in flowable composites is material dependent.

In vitro investigation of the performance of different restorative materials under cast circumferential clasps for removable dental prostheses

OBJECTIVES

The objective of this in vitro study was to investigate the behavior of different composite restorative materials under the load of cast circumferential clasps for removable dental prostheses (RDPs).

METHODS

In 60 human molars, standardized mesial-occlusal-distal cavities were prepared. The cavities were restored with the following materials: Definite, Tetric Ceram, SureFil, Heliomolar RO, Ariston pHc, and Oralloy, and provided with a rest seat. The rest seats were subjected to 5,000 cycles of thermal cycling and 1,200,000 masticatory cycles in a mastication simulator via cobalt-chromium circumferential clasps cast to standardized frameworks in a laboratory model designed to simulate the biomechanics of a free-end denture base. Fracture analysis of the restorations was performed by light microscopy. Before and after loading, material wear was measured with a 3D-laser scanner, and an analysis of the marginal quality was performed in an SEM at ×200 applying the replica technique.

RESULTS

No significant differences in the fracture behavior among the composite materials were found; the amalgam control group showed a significantly higher fracture resistance. Regarding the wear of the materials, the composites Definite and SureFil exhibited a behavior similar to that of amalgam. The other composites demonstrated higher wear rates. The initial marginal quality was significantly worse for Ariston pHc. The marginal adaptation decreased significantly after thermal and mechanical loading for Definite and Ariston pHc.

CONCLUSIONS

In terms of the investigated aspects of mechanical performance, the tested composites seemed to be inferior to amalgam. Further clinical studies are needed to evaluate the ability of composite restorations to provide support for RDP clasps.

CLINICAL RELEVANCE

The use of composites as direct restoration materials should be avoided in teeth, which serve as abutments for clasp-retained RDPs.

Fracture resistance of endodontically treated premolars with direct composite restorations

Purpose:

To determine the fatigue resistance and failure mode of endodontically treated premolars using direct composite resin restorations.

Materials and Methods:

Eighty-four human premolars were divided into seven groups of 12, prepared as follows: Intact teeth used in Group 1 as control, the second group covers the endodontically treated teeth, restored with direct onlays using Z250 composite resin, the next two groups (i.e. 3,4) were similar to the second group, but subjected to 1 and 2 million fatigue load cycles, respectively. Groups 5, 6, 7 were similar to groups 2, 3, 4, however, in these groups Tetric Ceram was used as the restorative material. All specimens were loaded using a Universal Testing Machine until fracture occurred. One-way Anova andTukey's HSD tests were used to analyze the data of onlay groups.

Results:

All specimens withstood the masticating simulation. The mean fracture strength for Goups 1 to 7 was: 1276.92, 1373.47, 1269/70, 486/35, 484/12, 1130/49, 1113/79 Newton, respectively. No statistically significant differences were found between the groups in fracture strength and failure mode.

Conclusions:

No statistically significant differences in fracture strength were found between sound teeth and composite onlays that were subjected to 1 and 2 million fatigue load cycles.

Microleakage of class V cavities with different adhesive systems prepared by a diamond instrument and different parameters of Er:YAG laser irradiation

OBJECTIVE

The aim of this study was to evaluate the microleakage of composite resin restorations using two different dentine adhesive systems prepared with a diamond instrument and different parameters of Er:YAG laser irradiation.

BACKGROUND DATA

Information on this topic with regard to preparing class V cavities with different parameters of Er:YAG laser irradiation and adhesive systems is scarce.

MATERIALS AND METHODS

Two hundred class V cavities were assigned to ten groups (n = 20 each): group 1: Er:YAG laser (5 Hz, 600 mJ) + phosphoric acid (PA) + Adper Single Bond 2 (ASB2); group 2: Er:YAG laser (10 Hz, 300 mJ) + PA + ASB2; group 3: Er:YAG laser (15 Hz, 200 mJ) + PA + ASB2; group 4: Er:YAG laser (20 Hz, 150 mJ) + PA + ASB2; group 5: diamond instrument + PA + ASB2; group 6: Er:YAG laser (5 Hz, 600 mJ) + Adper Prompt L-Pop (APLP); group 7: Er:YAG laser (10 Hz, 300 mJ) + APLP; group 8: Er:YAG laser (15 Hz, 200 mJ) + APLP; group 9: Er:YAG laser (20 Hz, 150 mJ) + APLP; and group 10: diamond instrument + APLP. Cavities were restored with a nanofill composite (Filtek Supreme XT Body). After thermocycling, the specimens were stained with 0.5% aqueous basic fuchsin dye and sectioned bucco-lingually. Dye penetration was then scored. The data were analyzed using the Kruskal-Wallis and Mann-Whitney U tests with Bonferroni correction. The Wilcoxon signed ranks test was used to compare occlusal and gingival scores.

RESULTS

Leakage was seen in all groups at both the occlusal and gingival margins. The Kruskal-Wallis test showed statistically significant differences among the 10 groups (p < 0.001). The gingival margins had more microleakage than the occlusal margins (p < 0.001). Pairwise analysis by the Mann-Whitney U test showed that statistically significant differences (p < 0.05) in microleakage were found between groups 3 and 5 (3 > 5), 5 and 7 (7 > 5), and 7 and 8 (7 > 8) at the gingival margin, and between groups 3 and 6 (6 > 3), 3 and 7 (7 > 3), 4 and 6 (6 > 4), and 4 and 7 (7 > 4) at the occlusal margin.

CONCLUSION

We concluded that for all groups, microleakage values were higher at the gingival margins. The use of the Er:YAG laser for cavity preparation with different parameters and different dentine adhesive systems influenced the marginal sealing of composite resin restorations.

Compressive fatigue behavior of dental restorative composites

The purpose of this study was to investigate the compressive fatigue behavior of five dental composites. Cylindrical specimens of 8 mm length and 4 mm diameter were made according to manufacturers' recommendations and stored for two weeks in distilled water at 37 degrees C. Compressive fracture strength was measured, and subsequently fatigue tests at 10 Hz frequency were carried out in distilled water. Compressive fatigue strength was thereby obtained using the staircase method for 10(5) cycles (n = 17) under sinusoidal loading. Acquired data for compressive fracture strength were analyzed using ANOVA and Weibull statistics. Among the dental composites examined, Filtek Z250 exhibited the highest fatigue strength. This seemed to be due to the superior matrix properties coupled with a specific filler type at the highest weight%/volume% ratio. In addition, fracture mechanisms of the composites were examined.

Fatigue of packable dental composites

OBJECTIVE

The purpose of the study was to measure the fatigue properties of four dental resin composites using a dynamic mechanical analysis and to relate the results with viscoelastic properties.

METHODS

Dynamic torsional loading was conducted at resonance at 30-50Hz. Specimens were thoroughly cured and tested dry at 21 degrees C.

RESULTS

All of the specimens showed a loss of strength following repeated stress, due to material fatigue. The material with the highest shear modulus had the lowest damping and the highest fatigue strength.

SIGNIFICANCE

Dental composites exhibit a modest loss of strength due to fatigue. Since mastication involves many cycles of stress during the life of a restoration, fatigue properties should be taken into account in restoration design.

Compressive strength and compressive fatigue limit of conventional and high viscosity posterior resin composites

The purpose of this study was to compare the compressive strengths and compressive fatigue limits of three posterior composite resins (Filtek P-60, Surefil and Prodigy Condensable) and a universal restorative composite (Z-100). Cylindrical specimens (8 mm in length x 4 mm in diameter) were used. The dynamic test was performed using the staircase method, and the ratio between compressive fatigue limit and compressive resistance was also calculated (n = 15). The compressive strength and compressive fatigue limit data were analyzed by Anova and Tukeys test. The Z-100 composite demonstrated higher compression strength (307.20 MPa) than Surefil (266.93 MPa) and Prodigy Condensable (222.08 MPa). The resistance of Filtek P-60 (270.44 MPa) was similar to the resistances of Z-100 and Surefil, while Prodigy Condensable presented the lowest compressive strength. In the compressive fatigue limit tests, Filtek P-60 demonstrated a higher value (184.20 MPa) than Prodigy Condensable (155.50 MPa). Surefil (165.74 MPa) and Z-100 (161.22 MPa) presented limits similar to those of Filtek P-60 and Prodigy Condensable. The compressive fatigue limit/compressive strength ratio was 70.01% for Prodigy Condensable, 68.11% for Filtek P-60, 62.09% for Surefil and 52.48% for Z-100. It was concluded that the Z-100 universal composite was more sensitive to the dynamic test than the high viscosity materials.

Wear and fatigue behavior of nano-structured dental resin composites

Theoretically, nano-structured dental resin composites are purported to have increased wear and fatigue resistance compared with microfill composites and may favor the achievement of restoratives with better long-term performance. This study sought to assess the behavior of nano-structured composites resulting from either abrasion and fatigue loading. Ten specimens (12 x 5 x 2.5 mm) were prepared from each of five composites: Ceram-X mono, Filtek Supreme, Grandio, Premise, and Heliomolar (serving as the microfill control). A surface profile was recorded using a three-dimensional profiling system, and the specimens were subjected to 10(5) cycles of three-body abrasion in the new OHSU oral wear simulator. A second profile was generated and the before and after profiles were fit and analyzed. The volume loss and maximum depth of the wear facet on each specimen were calculated. Another 30 specimens (25 x 2 x 2 mm) were tested for flexural fatigue limit (FFL) in four-point bending via the staircase method. The test was carried out until 10(4) cycles were completed or until fracturing the specimen. One-way ANOVA and Tukey's test demonstrated greater volumetric loss for Grandio and Ceram-X than that observed for the remaining composites. Kruskal-Wallis and the least significant difference test ascertained that Heliomolar, Grandio, and Supreme showed significantly higher FFL than Ceram-X and Premise. In terms of wear and fatigue resistance, nano-structured composites may perform either similarly or comparatively worse than a microfilled composite.

A comparison of fatigue resistance of three materials for cusp-replacing adhesive restorations

OBJECTIVES

To investigate the fatigue resistance and failure behaviour of cusp-replacing restorations in premolars using different types of adhesive restorative materials.

METHODS

A class 2 cavity was prepared and the buccal cusp was removed in an extracted sound human upper premolar. By using a copy-milling machine this preparation was copied to 60 human upper premolars. In groups of 20 premolars each, direct resin composite restorations, indirect resin composite restorations and ceramic restorations were made. All restorations were cusp replacements made in standardized shape and with adhesive techniques. Cyclic load (5 Hz) was applied starting with a load of 200 N (10,000 cycles) followed by stages of 400, 600, 800 and 1000 N at a maximum of 50,000 cycles each. Samples were loaded until fracture or to 2,10,000 cycles maximum. In case of fracture, the failure mode was recorded.

RESULTS

No differences were seen in fracture strength between the three groups (Wilcoxon P = 0.16). No differences were observed with regard to failure mode above or below the cemento enamel junctions (chi2 P = 0.63). The indirect resin composite and ceramic restorations showed significantly more combined cohesive and adhesive fractures than the direct resin composite restorations, which showed more adhesive fractures (chi2 P = 0.03 and 0.002).

CONCLUSIONS

The results of this study suggest that ceramic, indirect resin composite and direct resin composite restorations provide comparable fatigue resistance and exhibit comparable failure modes in case of fracture, although the indirect restorations tend to fracture more cohesively than the direct restorations.

Micro-rotary fatigue of tooth–biomaterial interfaces

The bonding effectiveness of restorative materials to tooth tissue is typically measured statically. Clinically tooth/composite bonds are however subjected to cyclic sub-critical loads. Therefore, in vitro fatigue testing of dental adhesives should predict better the in vivo performance of adhesives. The objective of this study was to determine the fatigue resistance of two representative adhesives, a self-etch and an etch&rinse adhesive, bonded to enamel and dentin. Therefore, tooth/composite interfaces were cyclically loaded using a miniaturized version of a rotating beam fatigue testing device. Subsequently, the load at which 50% of the specimens fail after 10(5) cycles, was determined as the median micro-rotary fatigue resistance (microRFR). For both adhesives, the microRFR was about 30-40% lower than the corresponding micro-tensile bond strength (microTBS) to both enamel and dentin. Analysis of the fracture surfaces by Feg-SEM revealed typical fatigue fracture patterns. It is concluded that resin/tooth interfaces are vulnerable to progressive damage by sub-critical loads, with the 3-step etch&rinse adhesive being more resistant to fatigue than the 2-step self-etch adhesive.

Strength and fatigue performance versus filler fraction of different types of direct dental restoratives

The aim of this study was to evaluate the mechanical properties, such as Young's moduli, fracture strengths (FS), and flexural fatigue limits of todays resin composite dental restoratives. All materials have been subdivided into flowable, aesthetic hybrid and nano-filled hybrid composites as marketed by dental manufacturers and analyzed in terms of the actual filler configurations. Specimen bars have been manufactured in reference to ISO 4049 standard, light-cured for 20 s, and stored in distilled water before testing. The elastic moduli (EM), FS, and flexural fatigue limits (FFL) were measured after 14 days storage by using the four-point bending test. The FFL was determined for 10(4) cycles. The fatigue data were analyzed by using the "staircase" approach and statistically treated by ANOVA analysis. Flowable materials with a reduced filler content exhibited the lowest Young's moduli, compared with those measured for higher filled materials. A linear relationship has been found between elastic moduli and filler loading (r(2) = 0.798). Correlations of FS and fatigue data to different filler fractions could not be proved. FS ranged between 61.3 and 124.9 MPa. After 10(4) cycles of fatigue loading, the FS suffered from a decrease between 45.2 and 61.7%. However, materials providing high initial strengths do not obviously reveal the best fatigue resistance. A marketing-based grouping of direct restorative materials has no meaning toward laboratory testing of mechanical properties.

Fracture toughness of resin-modified glass ionomer restorative materials: effect of powder/liquid ratio and powder particle size reduction on fracture toughness

OBJECTIVES

The objectives of the current investigation were two-fold: (1) to examine the effect of different powder/liquid (P/L) ratios on the fracture toughness of commercial resin-modified glass ionomer cement and conventional glass ionomer cement, and (2) to evaluate the effect of powder size reduction on the fracture toughness of experimental resin-modified glass ionomers in order to improve their physical properties.

METHODS

The P/L ratios of the glass ionomer and resin-modified glass ionomers were varied from the manufacturer's recommended ratio to 2.0 and 1.0 by weight. The powder particle sizes for the experimental resin-modified glass ionomers tested were 2, 5, 10 and 25 micro m in diameter. Fracture toughness was determined on ring-shaped specimens with a fatigued pre-crack.

RESULTS

The fracture toughness of the resin-modified glass ionomers was significantly higher (p<0.005) than that of the glass ionomer and was not greatly influenced by the P/L ratio. For the experimental resin-modified glass ionomers, it was observed that fracture toughness gradually decreased as the powder particle size became finer.

SIGNIFICANCE

The resin components in the liquid play an important role in the improvement of the physical properties of the resin-modified glass ionomer. A reduction in the powder particle size of up to 10 micro m, which resulted in a smoother surface, can maintain high fracture toughness. The high fracture toughness values of the resin-modified glass ionomer may be one of the factors contributing to a favorable clinical outcome in high stress-bearing areas.

Mechanical properties and three-body wear of veneering composites and their matrices

Fatigue as one of the major factors affecting three-body wear of resin composites is influenced by mechanical properties of the resin matrix. The aim of this in vitro study was to determine three-body wear (ACTA methodology), fracture strength, and Young's modulus of four veneering composites (Artglass old and new formula, Vita Zeta LC Composite, Targis) and one direct restorative composite (Z 100). Furthermore, three-body wear of the pure matrices of the materials was tested. The wear results were compared to Amalgam as reference material. It should be computed whether there exists a correlation between the wear results of resin composite and matrix alone. Wear of the veneering composites was significantly higher than of Z100 (13 microm) and Amalgam (14 microm; p < 0.01, Mann-Whitney U-test). Mean fracture strengths of indirect composites ranged from 127.5 MPa (Targis) to 71.6 MPa (Vita Zeta LC). The elastic moduli of the composites were between 2.9 and 12.8 GPa. The matrix wear rates did not differ significantly. Three-body wear results of complex resin composites are highly influenced by their filler content, filler particle size distribution, kind of filler particles, shape, and their silanization to the matrix. Due to this fact, three-body wear testing is an essential testing method and cannot be replaced by testing single material components.

In vitro contact wear of dental composites

OBJECTIVE

The aim of this study is to determine the in vitro two-body contact wear mechanisms of three medium filled composites and compare these with a highly filled composite previously investigated.

MATERIALS AND METHODS

Three commercial dental composites with filler mass fraction loading of 75-76% were evaluated. Two of the composites contained Ba-B-Al-silicate glass fillers and fumed silica with different particle sizes and distributions. One of these composites contained a fairly uniform distribution of filler particles ranging in size from 1 to 5 microm, whereas the particle size distribution in the second composite was bimodal consisting of small (less than 1 microm) and large (about 10 microm) particles. The third composite contained Ba-Al-silicate glass and silica with a filler particle size of approximately 1 microm. The composite disks were tested for wear against harder alumina counterfaces. Wear tests were conducted in distilled water using a pin-on-disk tribometer under conditions that represented typical oral conditions (sliding speed of 2.5 mm/s and contact loads ranging from 1 to 20 N). The wear tracks were analyzed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy to elucidate the wear mechanisms. The chemical composition of the water solution collected after the tests was determined using an inductively coupled plasma-mass spectrometer (ICP-MS) to detect possible chemical changes, e.g. dissolution of trace elements due to submersion or wear. The wear results were compared with those reported in an earlier study on a highly filled composite containing predominately alumino-silicate glass fillers and alumina at a filler loading of 92%.

RESULTS

The differences in two-body wear rates between the three medium filled composites were not statistically significant (p<0.05) indicating that the variations in filler particle size and slight differences in chemical composition of the glass fillers do not affect the in vitro wear rates of these composites. Wear rates of these medium filled composites, however, were significantly lower than the highly filled composite (p<0.05). SEM, FTIR and ICP-MS analyses suggested that wear in the medium filled composites occurs by a complex set of processes involving tribochemical reactions between filler particles and water, formation of surface films containing a mixture of filler fragments and reaction products, and film delamination, as well as dissolution of the reaction products.

SIGNIFICANCE

This study reveals that subtle changes in the filler particle size and small differences in filler composition do not significantly affect the two-body wear behavior of medium filled composites. However, the chemistry of filler particles plays an important role in altering the wear performance of composites when significant changes are made in the chemical composition of the fillers and when the filler loading is increased.

Effect of filler fraction on strength, viscosity and porosity of experimental compomer materials

OBJECTIVE

The primary goal is to develop a self-cured polyacid-modified resin composite with good mechanical and rheological properties. To achieve such a goal, the aim of this study is to determine how volume filler fraction (VFF) affects mechanical properties and viscosities of such materials containing different filler volumes.

METHODS

A series of self-cured polyacid-modified composites made from polyacid modified resins and TEGDMA, mixed with filler particles, were evaluated regarding compressive strength (CS), diametral compressive strength (DCS) and viscosity. The maximum filler content, which could be incorporated into the materials, was calculated from CS tests as well as from viscosity measurements using Mooney's equation. Porosity contents were also determined in an attempt to explain different failure behaviours.

RESULTS

The CS values peaked at 18.9 vol.% filler particles and declined afterwards for self-cured polyacid-modified resin composites cured in air. Using photopolymerisation and barium filler in the polyacid-modified resin composites resulted in the highest CS and DCS values. The viscosity increased continuously with increased VFF. VFF results determined experimentally and with Mooney's equation at shear rates of 0.01, 0.1, 1.0, and 10.0 s(-1) revealed that the maximal filler fraction values were 54.9+/-1.8, 55.9+/-1.3, 56.3+/-0.9, and 56.8+/-0.8 vol.%, respectively. The largest porosity content occurred at a VFF value of 53 vol.%

CONCLUSIONS

We conclude that an increase in filler fraction of the investigated experimental polyacid-modified resin composite materials above a certain value (20-30 vol.%) does not result in improved mechanical properties.

Influence of filler loading on the two-body wear of a dental composite

The purpose of the study was to explore the fundamental wear behaviour of a dental composite with different filler loadings under two-body wear conditions. The parent resin and filler components were mixed according to different weight ratios to produce experimental composites with filler loadings ranging from 20 to 87.5% by weight. A two-body wear test was conducted on the experimental composites using a wear-testing machine. The machine was designed to simulate the impact of the direct cyclic masticatory loading that occurs in the occlusal contact area in vivo. The results showed that there was little increase in the rate of wear with filler loadings below 60 wt%, but a sharp increase between 80 and 87.5 wt% in filler loading. Wide striations and bulk loss of material were apparent on the wear surfaces at higher filler loadings. Coefficients of friction increased with filler loading and followed the increase in rate of wear loss closely. It was concluded that, under two-body wear conditions, addition of high levels of filler particles into the resin matrix could reduce the wear resistance of dental composites. This finding may help when designing future dental composites for use in particular clinical settings.

Flexural fatigue behavior of resin composite dental restoratives

OBJECTIVE

The aim of this study was to evaluate the mechanical properties of resin composite dental restoratives under quasi-static and cyclic loading.

METHODS

Four-point-bending bars of 10 different resin composite materials were manufactured according to ISO standard and stored for two weeks in distilled water. The fracture strength (FS) was measured with the four-point-bending test in an universal testing machine. The flexural fatigue limits (FFL) for 10(5) cycles were determined under equivalent loading. All specimens were tested and fatigued in water at 37 degrees C. The data were analyzed using ANOVA, Weibull statistics of FS and the 'staircase' approach of FFL. Fractographic analysis was performed using SEM.

RESULTS

The initial flexural strength values for the resin composite materials varied from 55.4 MPa for Solitaire up to 105.2 MPa for Filtek Z250. The mean flexural fatigue limit for 10(5) cycles ranged between 37 and 67% of the initial strength. SEM analysis of the fractured surfaces suggests two kinds of failure mechanisms for initial and fatigue fracture.

SIGNIFICANCE

The fatigue behavior of resin composite materials does not correlate with initial strength values. Materials providing high initial strengths do not obviously reveal the best fatigue resistance. Flexural fatigue measurement of resin composite materials should be viewed as a useful tool to evaluate long term mechanical properties.

The volumetric fraction of inorganic particles and the flexural strength of composites for posterior teeth

OBJECTIVE

To evaluate the content of inorganic particles and the flexural strength of new condensable composites for posterior teeth in comparison to hybrid conventional composites.

METHOD

The determination of the content of inorganic particles was performed by mass weighing of a polymerized composite before and after the elimination of the organic phase. The volumetric particle content was determined by a practical method based on Archimedes' principle, which calculates the volume of the composite and their particles by differential mass measured in the air and in water. The flexural strength of three points was evaluated according to the norm ISO 4049:1988.

RESULTS

The results showed the following filler content: Alert, 67.26%; Z-100, 65.27%; Filtek P 60, 62.34%; Ariston pHc, 64.07%; Tetric Ceram, 57.22%; Definite, 54.42%; Solitaire, 47.76%. In the flexural strength test, the materials presented the following decreasing order of resistance: Filtek P 60 (170.02 MPa)>Z-100 (151.34 MPa)>Tetric Ceram (126.14 MPa)=Alert (124.89 MPa)>Ariston pHc (102.00 MPa)=Definite (93.63 MPa)>Solitaire (56.71 MPa).

CONCLUSION

New condensable composites for posterior teeth present a concentration of inorganic particles similar to those of hybrid composites but do not necessarily present higher flexural strength.

Monotonic flexure and fatigue strength of composites for provisional and definitive restorations

STATEMENT OF PROBLEM

Ordinarily, the mechanical strength of composites is characterized by their flexural strength. Information as to the material's fatigue strength is seldom provided.

PURPOSE

The purpose of this study was to compare the flexural strength and the resistance to fatigue loading of composites and an acrylic resin for provisional and definitive restorations.

MATERIAL AND METHODS

Artglass, Colombus, and Targis (composites) and Jet, Protemp II, Protemp Garant, and Provipont DC (provisional restorations) were subjected to mechanical tests. Fatigue tests (MPa) (n = 30 specimens/group) were conducted with the rotating-bending cantilever design. Monotonic flexural strength (MPa) (n = 10) was determined in 3-point bending tests. Fatigue resistance was analyzed via the staircase procedure, and flexural strength was examined by use of the 2-parameter Weibull distribution (confidence intervals at 95%).

RESULTS

The mean fatigue resistances (S(50)) in MPa +/- SD were: Targis, 62.1 +/- 7.0; Artglass, 58.5 +/- 3.7; Colombus, 54.6 +/- 6.2; Provipont DC, 29.5 +/- 3.2; Protemp II, 23.1 +/- 5.3; Jet, 22.8 +/- 8.3; Protemp Garant, 19.6 +/- 4.6. The flexure strengths (Weibull's S(0)) in MPa and their shape parameters (m) were: Colombus, 145.2 (13.1); Targis, 110.3 (7.8); Artglass, 5.9 (5.4); Jet, 150.9 (17.3); Provipont DC, 97.3 (23.8); Protemp II, 57.9 (6.4); Protemp Garant, 54.2 (12.8). The S(50) of Targis was significantly higher than that of Colombus but not different from Artglass. In flexion, the S(0) of Colombus was significantly higher than that of Artglass and Targis. The S(50) ranged between 40% and 60% of the S(0) for the composites and between 15% and 30% for the provisional restorative materials.

CONCLUSIONS

Correlations between monotonic flexure strength and resistance to fatigue loading were weak. Because fatigue tests are considered more pertinent than monotonic tests as to their predictive value, it is concluded that flexure strength data alone may not provide relevant information for long-term clinical performance. The material's resistance to fatigue loading should also be determined.

Fatigue of restorative materials

Failure due to fatigue manifests itself in dental prostheses and restorations as wear, fractured margins, delaminated coatings, and bulk fracture. Mechanisms responsible for fatigue-induced failure depend on material ductility: Brittle materials are susceptible to catastrophic failure, while ductile materials utilize their plasticity to reduce stress concentrations at the crack tip. Because of the expense associated with the replacement of failed restorations, there is a strong desire on the part of basic scientists and clinicians to evaluate the resistance of materials to fatigue in laboratory tests. Test variables include fatigue-loading mode and test environment, such as soaking in water. The outcome variable is typically fracture strength, and these data typically fit the Weibull distribution. Analysis of fatigue data permits predictive inferences to be made concerning the survival of structures fabricated from restorative materials under specified loading conditions. Although many dental-restorative materials are routinely evaluated, only limited use has been made of fatigue data collected in vitro: Wear of materials and the survival of porcelain restorations has been modeled by both fracture mechanics and probabilistic approaches. A need still exists for a clinical failure database and for the development of valid test methods for the evaluation of composite materials.

Whisker-reinforced dental core buildup composites: effect of filler level on mechanical properties

The strength and toughness of dental core buildup composites in large stress-bearing restorations need to be improved to reduce the incidence of fracture due to stresses from chewing and clenching. The aims of the present study were to develop novel core buildup composites reinforced with ceramic whiskers, to examine the effect of filler level, and to investigate the reinforcement mechanisms. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whisker surface for improved retention in the matrix. Filler level was varied from 0 to 70%. Flexural strength, compressive strength, and fracture toughness of the composites were measured. A nano-indentation system was used to measure elastic modulus and hardness. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of specimens. Whisker filler level had significant effects on composite properties. The flexural strength in MPa (mean +/- SD; n = 6) increased from (95+/-15) for the unfilled resin to (193+/- 8) for the composite with 50% filler level, then slightly decreased to (176+/-12) at 70% filler level. The compressive strength increased from (149+/-33) for the unfilled resin to (282+/-48) at 10% filler level, and remained equivalent from 10 to 70% filler level. Both the modulus and hardness increased monotonically with filler level. In conclusion, silica particle-fused ceramic single-crystalline whiskers significantly reinforced dental core buildup composites. The reinforcement mechanisms appeared to be crack deflection and bridging by the whiskers. Whisker filler level had significant effects on the flexural strength, compressive strength, elastic modulus, and hardness of composites.

Effects of whisker-to-silica ratio on the reinforcement of dental resin composites with silica-fused whiskers

Resin composites need to be strengthened to improve their performance in large stress-bearing restorations. This study aimed to reinforce composites with whiskers and to investigate the effects of the whisker:silica ratio. It was hypothesized that changing the whisker-silica ratio would affect the whisker-matrix bonding and the filler's distribution, and hence alter the composite properties. Silica particles and whiskers were mixed at various whisker:silica mass ratios, thermally fused, and combined with a dental resin at filler mass fractions of 0-65%. Whisker:silica ratio and filler level had significant effects on composite properties. At 60% filler level, the silica composite (whisker:silica = 0:1) had a flexural strength (mean +/- SD; n = 6) of 104 +/- 21 MPa; that at a whisker:silica ratio of 1:0 was 74 +/- 36 MPa. However, that of the silica-fused whisker composite (whisker:silica = 5:1) was 210 +/- 14 MPa, compared with 109 +/- 23 MPa and 114 +/- 18 MPa of two prosthetic controls. Mixing silica with whiskers minimized whisker entanglement, improved filler distribution in the matrix, and facilitated whisker silanization and bonding to the matrix, thus resulting in substantially stronger composites.

Indentation modulus and hardness of whisker-reinforced heat-cured dental resin composites

OBJECTIVES

Recent studies showed that ceramic whisker reinforcement imparted a two-fold increase in the strength of dental composites. The aim of this study was to investigate the indentation response and measure the elastic modulus, hardness, and brittleness of whisker-reinforced heat-cured resin composites as a function of filler level, heat-cure temperature, and heat-cure duration.

METHODS

Silica particles were fused onto silicon nitride whiskers to facilitate silanization and to roughen the whiskers for improved retention in matrix. Whisker filler mass fractions of 0, 20, 40, 60, 70, 74 and 79% were tested. Heat-cure temperature ranged from 100 to 180 degrees C, and duration from 10 min to 24 h. A nano-indentation system enabled the measurement of elastic modulus. Fracture toughness was measured and composite brittleness index was calculated. An inlay/onlay composite and a prosthetic composite were tested as controls.

RESULTS

Whisker filler level and heat-cure duration had significant effects on composite properties, while heat-cure temperature had non-significant effects. The whisker composite with 79% filler level had a modulus in GPa (mean (SD); n = 6) of 26.9 (1.0), significantly higher than 15.1 (0.2) of an inlay/onlay control, and 16.1 (0.3) of a prosthetic control (Tukey's multiple comparison test; family confidence coefficient = 0.95). The fracture toughness in MPa.m1/2 was 2.22 (0.26) for the whisker composite, higher than 0.95 (0.11) for inlay/onlay control, and (1.13 +/- 0.19) for prosthetic control. The brittleness index was (0.49 +/- 0.07) for whisker composite, lower than (1.02 +/- 0.12) for inlay/onlay control and (0.63 +/- 0.13) for prosthetic control.

SIGNIFICANCE

Whisker filler level had a profound influence, heat-cure duration had significant effects, while temperature did not have significant effects, on the properties of whisker composite. The whisker composite had significantly higher elastic modulus and fracture toughness, and lower brittleness than the inlay/onlay and prosthetic controls.

Whisker-reinforced heat-cured dental resin composites: effects of filler level and heat-cure temperature and time

Currently available dental resin composites are inadequate for use in large stress-bearing crown and multiple-unit restorations. The aim of this study was to reinforce heat-cured composites with ceramic whiskers. It was hypothesized that whiskers substantially strengthen heat-cured composites. It was further hypothesized that whisker filler level and heat-cure temperature and time significantly influence composite properties. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whiskers for improved retention in the matrix. The whisker filler mass fraction was varied from 0% to 79%, the heat-cure temperature from 80 degrees C to 180 degrees C, and cure time from 10 min to 24 hrs. Flexural strength, work-of-fracture, and fracture toughness of the composites were measured, and specimen fracture surfaces were examined with scanning electron microscopy. Filler level had a significant effect on composite properties. The whisker composite with 70% filler level had a flexural strength in MPa (mean +/- SD; n = 6) of 248 +/- 23, significantly higher than 120 +/- 16 of an inlay/onlay composite control and 123 +/- 21 of a prosthetic composite control (Tukey's multiple comparison test; family confidence coefficient = 0.95). Heat-cure time also played a significant role. At 120 degrees C, the strength of composite cured for 10 min was 178 +/- 17, lower than 236 +/- 14 of composite cured for 3 hrs. The strength of whisker composite did not degrade after water-aging for 100 d. In conclusion, heat-cured composites were substantially reinforced with whiskers. The reinforcement mechanisms appeared to be whiskers bridging and resisting cracks. The strength and fracture toughness of whisker composite were nearly twice those of currently available inlay/onlay and prosthetic composites.

Dental composite resins containing silica-fused ceramic single-crystalline whiskers with various filler levels

Currently available direct-filling composite resins are susceptible to fracture and hence are not recommended for use in large stress-bearing posterior restorations involving cusps. The glass fillers in composites provide only limited reinforcement because of the brittleness and low strength of glass. The aim of the present study was to use ceramic single-crystalline whiskers as fillers to reinforce composites, and to investigate the effect of whisker filler level on composite properties. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whiskers, thereby improving retention in the matrix. The composite flexural strength, elastic modulus, hardness, and degree of polymerization conversion were measured as a function of whisker filler mass fraction, which ranged from 0% to 70%. Selected composites were polished simulating clinical procedures, and the surface roughness was measured with profilometry. The whisker composite with a filler mass fraction of 55% had a flexural strength (mean +/- SD; n = 6) of 196+/-10 MPa, significantly higher than 83+/-14 MPa of a microfill and 120+/-16 MPa of a hybrid composite control (family confidence coefficient = 0.95; Tukey's multiple comparison). The composite modulus and hardness increased monotonically with filler level. The flexural strength first increased, then plateaued with increasing filler level. The degree of conversion decreased with increasing filler level. The whisker composite had a polished surface roughness similar to that of a conventional hybrid composite (p>0.1; Student's t). To conclude, ceramic whisker reinforcement can significantly improve the mechanical properties of composite resins; the whisker filler level plays a key role in determining composite properties; and the reinforcement mechanisms appear to be crack pinning by whiskers and friction from whisker pullout resisting crack propagation.

Mechanical properties of resin composites with filler particles aligned by an electric field

OBJECTIVES

To explore possible enhancement of the mechanical properties of resin composites by aligning the filler particles.

METHODS

The resin for the composites consisted of urethane dimethacrylate (UDMA) and 1,6-hexanediol dimethacrylate (HDDMA) mixed in the ratio of 90 to 10; the filler was silica-zirconia in two particle sizes, 1.7 microns (P50), which was mixed with the resin in the volume fractions of 29 and 48 vol%, and 0.7 micron (Z100), which was mixed with the resin in the volume fractions of 37 and 57 vol%. Particle alignment was obtained by applying a 60 Hz AC electric field across the composite before the resin was photopolymerized. The stress-strain behavior and the elastic modulus of the hardened composite were measured along the alignment axis in compression.

RESULTS

The elastic moduli of the aligned composites increased by as much as 20%, and the maximum stress sustainable in compression before significant deformation occurred was elevated. An electric field strength of the order of 1 kV mm-1 was required to obtain sufficient alignment.

SIGNIFICANCE

Besides conferring a structure to resin composites that is more like that of the natural tooth, particle alignment increased some of the mechanical properties and may have improved durability.

Cyclic fatigue testing of five endodontic post designs supported by four core materials

PURPOSE

This pilot study examined the cyclic fatigue of five endodontic post systems (AccessPost, Flexi-Flange, Flexi-Post, ParaPost, and Vlock) with four core materials (Tytin silver amalgam, Ti-Core, Ketac-Silver and G-C Miracle Mix).

MATERIAL AND METHODS

In vitro cyclic fatigue was performed with a machine designed to simulate masticatory fatigue forces. An instantaneous force of 22.2 N (5 pounds) was applied to each post and core combination for a test configuration of 4,000,000 repetitions, or until failure occurred. The type of failure and number of repetitions at failure was recorded for each sample tested. Two-way analysis of variance (ANOVA) was used to compare groups.

RESULTS

All posts/core samples with Ti-Core composite and Tytin silver amalgam completed the test with no failures. All posts/core samples with Ketac-Silver material failed before the 4,000,000 test cycle configuration and all failures were core failures. All posts/core samples with G-C Miracle Mix material failed in a similar manner. Newman-Keuls multiple comparison test illustrated that, with this simulated fatigue test, Ti-Core material and Tytin silver amalgam were superior to both G-C Miracle Mix and Ketac-Silver materials.

Dental materials: 1995 literature review

This critical review of the published literature on dental materials for the year 1995 has been compiled by the Dental Materials Panel of the United Kingdom. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, resin composites, dentine bonding, dental amalgam, endodontic materials, casting alloys, investment materials, resin-bonded bridges and ceramo-metallic restorations, all ceramic restorations, denture base and soft lining materials, impression materials, dental implants, orthodontic materials and biomechanics). Three hundred and thirty articles published in 68 titles have been reviewed.