Shear bond strength vs interfacial fracture toughness — Adherence to CAD/CAM blocks

ObjectiveTo compare shear bond strength (SBS) and interfacial fracture toughness (IK_IC) results when assessing the effect of surface roughness and thermocycling on the adherence of a resin composite luting agent (RCLA) to a CAD/CAM resin composite block (RCB).MethodsTetric CAD HT along with the recommended bonding system, Adhese Universal and Variolink Esthetic LC, were used. Surface roughness was achieved with 600/320/60 grit SiC papers. Samples were stored 24 h in 37 °C water or thermocycled 10000× (5 °C–55 °C) prior to testing. Results were analyzed by univariate ANOVA and Scheffé modified t-tests (α = 0.05). Fractured specimens were viewed with a stereo microscope and selected specimens with a scanning electron microscope.ResultsSBS results showed a significant difference between the 60 grit group and the other groups, both after 24 h and thermocycling. A large number of SBS samples showed cohesive fracture or subsurface damage in RCB. Thermocycling led to a significant decrease in SBS in all groups. IK_IC results showed no significant differences due to surface preparation after 24 h storage in 37 °C. After thermocycling, there was a significant difference between the 60 and the 600 grit groups. All K_IC samples fractured adhesively at the RCB surface. K_IC of the RCLA was significantly higher than IK_IC of all groups.SignificanceThe results endorse the use of fracture mechanics methodology for the assessment and characterization of adherence, while identifying difficulties in its implementation. The results suggest also that adherence to CAD/CAM RCB may be limited by the strength of the resin composite block — adhesive interface.     

Hardness and fracture toughness of resin-composite materials with and without fibers

ObjectiveTo investigate the surface micro-hardness (VHN) and fracture toughness (K_IC) of resin-composites, with and without incorporated short fibers, after solvent storage.MethodsThree resin-composites incorporating fibers, additional to particle reinforcement, were examined: everX™, NovoPro Fill™ and NovoPro Flow™. Four composites were used as controls, with only particle reinforcement: Filtek bulk Fill™, Filtek bulk one™, Filtek XTE™, and Filtek Flow XTE™. For hardness measurement, materials were cured in 2 mm thick molds for 20 s by a LED source of average irradiance 1.2 W/cm². Specimens (n = 6/group) were stored dry for 1 h and then in either water or 75% ethanol/water for 1 h, 1 day and 30 days at 37 ± 1 °C. Vickers hardness was measured under a load of 300 g for 15 s. For fracture toughness (K_IC) measurements, single-edge-notched specimens (n = 6/group) were prepared: (32 × 6 × 3 mm) for 3-point bending and stored for 1 and 7 days in water at 37 °C. Fractured surfaces of fiber-reinforced composite were examined by scanning electron microscopy (SEM). VHN data were analyzed using three-way ANOVA, one-way ANOVA and the Tukey post hoc test (p ≤ 0.05). K_IC data were analyzed by two-way ANOVA and one-way ANOVA and the Tukey post hoc test (p ≤ 0.05). An independent t-test was used to detect differences (α = 0.05) in K_IC between stored groups for each material.ResultsVHN decreased for all composites with storage time in both solvents, but more appreciably in 75% ethanol/water (an average of 20%). K_IC ranged from 2.14 (everX Posterior) to 0.96 NovoPro Flow) MPa. m^0.5. The longer storage period (7 days) had no significant effect on this property relative to 1 day storage.SignificanceReinforcement with short fibers, and possibly matrix compositional differences, significantly enhanced the fracture toughness of EVX. However, for nano-fiber containing composites, there were no evident beneficial effects upon either their fracture toughness or hardness compared to a range of control composites. Water storage for 7 days of all these resin-composites produced no significant change in their K_IC values, relative to 1 day storage.     

Translucency,flexural strength,fracture toughness,fracture load of 3-unit FDPs,Martens hardness parameter and grain size of 3Y-TZP materials

ObjectiveThis investigation tested pre-shaded 3Y-TZP materials on optical, mechanical and structural properties and calculated correlations between these properties.MethodsSeven A2-shaded 3Y-TZP zirconia materials were investigated on translucency (T) via UV–vis-spectrophotometer, fracture load of 3-unit FDPs (FL), biaxial flexural strength (FS), Chevron-Notch Beam (CNB), fracture toughness (K_IC) and Martens parameter (hardness: HM and indentation modulus: E_IT). FL, FS and K_IC were measured in a universal testing machine. The grain size was evaluated by scanning electron microscopy (SEM). Data was analyzed using one-way ANOVA followed by post hoc Scheffé, Kruskal–Wallis-, Mann–Whitney-U- and Pearson-test (p < 0.05).ResultsFor translucency, negative correlations were found with results of facture load (R = −0.444, p < 0.001) and K_IC (R = −0.503, p < 0.001). While a positive correlation was found between translucency and flexural strength (R = 0.238, p = 0.019), between fracture load and E_IT (R = 0.227, p < 0.029), between fracture load and K_IC (R = 0.362, p < 0.001) as well as between fracture load and the grain size (R = 0.598, p = 0.007). While the grain size positively correlated with E_IT (R = 0.534, p = 0.017) as well as E_IT with HM (R = 0.720, p < 0.001).SignificanceDespite of being based on the same raw material, tested zirconia materials significantly differed regarding optical, mechanical (except biaxial flexural strength and Martens hardness) and structural properties. Materials with highest optical properties were those with lowest mechanical properties (CER, COP).     

Effect of cooling protocol on mechanical properties and microstructure of dental veneering ceramics

ObjectivesUnderstand how cooling protocols control the microstructure and mechanical properties of veneering porcelains.MethodsTwo porcelain powders were selected, one used to veneer metallic frameworks (VM13) and one for zirconia frameworks (VM9). After the last firing cycle, the monolithic specimens were subjected to two cooling protocols: slow and fast. Flexural strength (FS) was evaluated by three-point beam bending and fracture toughness (K_IC) was evaluated by the single-edge V-notch beam (SEVNB) method. Scanning electron microscopy (SEM) was performed to determine the leucite crystal volume fraction (%), particle size, and matrix microcrack density. The results were compared by analysis of variances (ANOVA) and Tukey’s multiple comparison test.ResultsThe mechanical properties were significantly (p < 0.05) higher for the VM13 porcelain (FS = 111.0 MPa, K_IC = 1.01 MPa.√m) compared to VM9 (FS = 79.6 MPa, K_IC =0.87 MPa.√m) regardless of cooling protocol due to ∼250% higher volume fraction of leucite crystals. The slow cooled VM13 and fast cooled VM9 resulted in the highest and lowest mechanical properties, respectively, while the VM9 slow cooled properties were similar to the VM13 fast cooled. The SEM revealed that the slow cooling significantly increased the volume fraction of leucite crystals by 33–41 %. Across both porcelains, a significant linear correlation between both mechanical properties (strength and toughness) and leucite crystal content was found. Slow cooling was also associated with increased crystal growth resulting in more matrix microcracking.SignificanceControlled crystallization using slow cooling can be applied as a means of strengthening dental porcelains. However, the benefits of slow cooling may be partially offset by increasing the microcrack density in the glass matrix. To achieve the maximum benefit of slow cooling, it is recommending to develop heat treatments to produce porcelain with fine-grained and homogenously dispersed leucite crystals to achieve minimal glass matrix microcracking.     

Influence of microcapsule parameters and initiator concentration on the self-healing capacity of resin-based dental composites

ObjectiveFracture is one of the main causes for failure of resin-based composite restorations. To overcome this drawback, self-healing resin-based composites have been designed by incorporation of microcapsules. However, the relationship between their self-healing capacity and microcapsule and resin parameters is still poorly understood. Therefore, the objective of this study was to systematically investigate the effect of initiator concentration (in the resin) and microcapsule size and concentration on the self-healing performance of commercially available flowable resin-based composites.MethodsPoly(urea-formaldehyde) (PUF) microcapsules containing acrylic healing liquid were synthesized in small (33 ± 8 μm), medium (68 ± 21 μm) and large sizes (198 ± 43 μm) and characterized. Subsequently, these microcapsules were incorporated into a conventional flowable resin-based composite (Majesty Flow ES2, Kuraray) at different contents (5–15 wt%) and benzoyl peroxide (BPO) initiator concentrations (0.5–2.0 wt%). Fracture toughness (K_IC) of test specimens was tested using a single edge V-notched beam method. Immediately after complete fracture (K_IC-initial), the two fractured parts were held together for 72 h to allow for healing. Subsequently, fracture toughness of the healed resin-based composites (K_IC-healed) was tested as well.ResultsThe fracture toughness of healed dental composites significantly increased with increasing microcapsule size and concentration (2 wt% BPO, p < 0.05). The highest self-healing efficiencies (up to 76%) were obtained with microcapsules sized 198 ± 43 um.Significancecommercially available resin-based composites can be rendered self-healing most efficiently by incorporation of large microcapsules (198 ± 43 μm). However, long-term tests on fatigue and wear behavior are needed to confirm the clinical efficacy.     

Mechanistic aspects of fatigue crack growth behavior in resin based dental restorative composites

ObjectiveTo test the hypothesis that a commercial microhybrid resin based composite (Filtek? Z250) has superior fatigue resistance to a nanofill composite (Filtek? Supreme Plus) and to determine the related micromechanisms involved in the fatigue process.MethodsAfter 60 days of water hydration, the fatigue crack growth resistance of two different resin composites, one microhybrid (Filtek? Z250) and one nanofill (Filtek? Supreme Plus), was measured in wet conditions using compact-tension, C(T), specimens at a load ratio of 0.1 and frequency of 2 Hz. Cyclic fatigue behavior was quantified in terms of the fatigue crack growth rate, da/dN, as a function of the stress intensity range, ΔK.ResultsA sigmoidal da/dN-ΔK curve with three different fatigue crack growth regimes was identified for both composites. In general, fatigue crack growth ranged from ～10^?9 to 10^?5 m/cycle over ΔK of 0.54–0.63 MPa√m for the Z250 composite and ΔK of 0.41–0.67 MPa√m for the Supreme Plus composite. The Supreme Plus composite showed a lower fatigue threshold, ΔK_th, by ～0.13 MPa√m compared to the Z250 composite, while also showing a plateau in the fatigue crack growth curve that is likely related to environmental attack. SEM observations of the fatigue crack paths and fracture surfaces revealed an interparticle crack path and extrinsic toughening mechanisms of crack deflection and crack bridging. No fatigue degradation of reinforcing particles or clusters was found, but cluster–matrix debonding was evident in the Supreme Plus composite, also indicative of environmental attack due to water.SignificanceThis study increases the understanding of both the fatigue behavior and the micromechanisms of fatigue in resin based dental composites.     

Provisional crown and fixed partial denture materials: Mechanical properties and degree of conversion

OBJECTIVES: This study aimed to investigate the flexural strength (FS) and flexural modulus (FM) of temporary crown and bridge materials (t-c&b) at different storage times and to identify possible correlations between the mechanical properties and the degree of conversion (DC). METHODS: FS and FM of four proprietary di-methacrylate-based t-c&bs were tested in a 3-point bending test according to EN ISO 4049:2000 at various storage times after mixing (37 degrees C dry/water) including thermocycling (5000x, 5-55 degrees C). DC was determined by calculating the percentage of reacted CC double bonds using FTIR analysis (baseline method). Mean values of all measurements were calculated and subjected to the Games-Howell test for statistical analysis (p=0.05) as well as a logarithmic regression analysis. RESULTS: FS and FM were very low 10min after mixing for all materials tested (FS: 14.5-24.5MPa; FM: 96.1-211.2MPa). A very high correlation was observed between FS and FM on the one hand and storage time on the other. The DC was on a high level already 10min after mixing (57.7-69.8%) for all materials except for Structur Premium (42.2%). Structur Premium showed a significantly higher FS and FM (p<0.05) compared to all other materials tested though a significantly lower DC (p<0.05). SIGNIFICANCE: FS and FM of t-c&bs significantly depend on the time after mixing. Dentists should be aware of the fact that the mechanical stability of temporary crowns is comparably low in the first hours after fabrication. The DC does not allow drawing conclusions about the mechanical stability of a t-c&b.     

Stability of bonds made to superficial vs. deep dentin,before and after thermocycling

ObjectivesBonding stability of resinous adhesives to dentin is still problematic and may involve regional variations in dentin composition. This study is to evaluate the effect of dentin depth on the stability of resin-dentin bonds under thermocycling challenge.MethodsDentin slabs with two flat surfaces parallel to the tooth axis were obtained from extracted human third molars. The slabs were randomized into eight groups according to the location of dentin [deep dentin (DD) or superficial dentin (SD)], the adhesive treatment (Single Bond 2 or Clearfil S³ Bond), and the storage treatment (thermocycling for 5000 times vs. no). After the adhesive treatment and composite buildup on the dentin slabs, the micro-shear bond strength (μSBS) of each group was detected. The concentrations of cross-linked carboxyterminal telopeptide of type I collagen (ICTP) were also evaluated using an immunoassay to detect the degree of collagen degradation in each group.ResultsDentin depth, adhesive treatment and storage treatment all showed significant effects on both the μSBSs and the ICTP values (P < 0.05). Regardless of the adhesive type, thermocycling decreased the μSBSs and increased the ICTP values (P < 0.05). The DD groups showed significantly lower μSBSs and higher ICTP values than SD groups after thermocycling aging (P < 0.05). The treatment with Single Bond 2 significantly increased the ICTP values (P < 0.05), whereas Clearfil S³ Bond showed no effect on the ICTP values (P > 0.05).SignificanceDeep dentin showed significantly more bond degradation after thermocycling than did superficial dentin.     

Development and characterisation of dental composites containing anisotropic fluorapatite bundles and rods

ObjectivesTo develop dental composites incorporating fluorapatite (FA) crystals as a secondary filler and to characterise degree of conversion, key mechanical properties and fluoride release.MethodsFA rod-like crystals and bundles were hydrothermally synthesised and characterised by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and ¹⁹F MAS-NMR. Composites were formulated containing BisGMA/TEGDMA/BisEMA and barium-aluminium-silicate glass (0FA). FA crystals were incorporated at 10 (10FA), 20 (20FA), 30 (30FA) and 40 wt% (40FA) maintaining a filler content of 80 wt% (63–67 vol%). Degree of conversion (DC), flexural strength (FS), flexural modulus (FM), fracture toughness (K_1C), Vickers hardness (HV) and 2-body wear were measured. Fluoride release was measured in neutral and acidic buffers.ResultsXRD and ¹⁹F MAS-NMR confirmed that only FA was formed, whilst SEM revealed the presence of single rods and bundles of nano-rods. DC ranged between 56–60% (p > 0.05). FA composites showed lower FM and lower FS (p < 0.05), but comparable wear resistance and HV (p > 0.05) to 0FA. 30FA and 40FA showed similar K_1C to 0FA (p > 0.05), with SEM showing evidence of toughening mechanisms, whereas 10FA and 20FA showed lower K_1C (p < 0.05). FA containing composites released fluoride that was proportional to the amount of FA incorporated (p < 0.05) but only under acidic conditions.SignificanceThe addition of FA to the experimental composites reduced strength and stiffness but not the DC, hardness or wear rate. 30FA and 40FA had a higher K_1C compared to other FA groups. Fluoride release occurred under an accelerated acidic regime, suggesting potential as a bioactive ‘smart’ composite.     

Evaluation of test protocol variables for dental implant fatigue research

ObjectivesThis work begins to explore the influence of cycling rate and environment on fatigue testing of dental implants according to the ISO protocol 14801.MethodsTwenty-four Straumann implants (4.1 mm × 12 mm) were tested up to five million cycles per ISO 14801: loaded at either 2 or 30 Hz in room air at 25 °C or normal saline at 37 °C (n = 6 per group). Implant displacements/cycle were captured during all testing. Fracture-surface features were examined using scanning electron microscopy (n = 12). Two complimentary methods were developed to estimate fatigue crack growth rates.ResultsFailures (bulk fracture) were found to be bi-modally distributed, either <350,000 cycles or >1.5 million cycles at both cycling rates. Following initial crack formation, fatigue crack growth required merely 1100–4200 cycles to failure. Initial crack pop-in was statistically more likely under 2 Hz than 30 Hz (χ², p < 0.05) but testing in air and normal saline were equivalent in terms of likelihood of fracture versus runout (χ², p > 0.6). On a microscopic level, fatigue crack growth rates appears to be similar at 2 and 30 Hz, but may be slower in the presence of saline versus dry at 2 Hz.SignificanceImplant failure under fatigue conditions involved “classic” damage mechanisms. Failure appears more likely at 2 Hz than 30 Hz for reasons that remain to be elucidated. Saline may enable chemically assisted crack growth involving grain boundaries during the stage of fatigue crack growth, but did not influence likelihood of failure.     

Residual stress in glass: Indentation crack and fractography approaches

ObjectiveTo test the hypothesis that the indentation crack technique can determine surface residual stresses that are not statistically significantly different from those determined from the analytical procedure using surface cracks, the four-point flexure test, and fracture surface analysis.MethodsSoda-lime–silica glass bar specimens (4 mm × 2.3 mm × 28 mm) were prepared and annealed at 650 °C for 30 min before testing. The fracture toughness values of the glass bars were determined from 12 specimens based on induced surface cracks, four-point flexure, and fractographic analysis. To determine the residual stress from the indentation technique, 18 specimens were indented under 19.6 N load using a Vickers microhardness indenter. Crack lengths were measured within 1 min and 24 h after indentation, and the measured crack lengths were compared with the mean crack lengths of annealed specimens. Residual stress was calculated from an equation developed for the indentation technique. All specimens were fractured in a four-point flexure fixture and the residual stress was calculated from the strength and measured crack sizes on the fracture surfaces.ResultsThe results show that there was no significant difference between the residual stresses calculated from the two techniques. However, the differences in mean residual stresses calculated within 1 min compared with those calculated after 24 h were statistically significant (p = 0.003).SignificanceThis study compared the indentation technique with the fractographic analysis method for determining the residual stress in the surface of soda-lime–silica glass. The indentation method may be useful for estimating residual stress in glass.     

In vitro comparison of the surface roughness of polymethyl methacrylate and bis-acrylic resins for interim restorations before and after polishing

Statement of problemPolymethyl methacrylate and bis-acrylic based resins are widely used for interim restorations. Their initial surface roughness is important because it determines their aesthetic properties and the potential for biofilm adhesion.PurposeThe purpose of this in vitro study was to assess the surface roughness and morphology of 6 bis-acrylic and 2 polymethyl methacrylate resins widely used for interim dental restorations, both before and after polishing.Material and methodsSpecimens made of different bis-acrylic resins (Protemp 4, Luxatemp Star, Systemp, Telio, Structur Premium, Structur 3) or of polymethyl methacrylate (Unifast Trad, Unifast 3) were polished using a 2-step polishing system (Diatech). The average surface roughness before and after polishing (10 seconds at low speed in dry conditions) was measured by optical profilometry. Atomic force microscopy and scanning electron microscopy were used to analyze surface morphology. The Mann-Whitney and Kruskal-Wallis tests were used to evaluate the differences in roughness among specimens (α=.05), and the Pearson r correlation was computed to assess the relationship between fillers and average surface roughness.ResultsIn the 8 groups evaluated, the roughness significantly increased (P<.001) for Protemp 4 (from 0.12 to 0.50 μm), Luxatemp Star (0.17 to 1.19 μm), Unifast 3 (0.40 to 1.00 μm), Systemp (0.46 to 1.51 μm), Structur 3 (0.85 to 1.06 μm), Structur Premium (1.00 to 1.74 μm), and Telio (1.13 to 1.21 μm), except for Unifast Trad (9.20 to 2.59 μm). Pairwise multiple comparisons identified Protemp 4 as having the smoothest surface before and after polishing, while Unifast Trad was the roughest in both. Atomic force microscopy and scanning electron microscopy observations showed that the surface roughness of bis-acrylic resins was related to their surface morphology and average filler sizes. A positive relation between fillers and roughness was assessed (r=0.345, P<.001).ConclusionsFor the bis-acrylic interim resins, the surface roughness after polishing was correlated to the material used and its filler sizes. Nanofiller-based resins showed the smoothest surfaces. For the polymethyl methacrylate–based resins, the recently marketed Unifast 3 had the lowest overall roughness values.     

Compressive strength of two newly developed glass-ionomer materials for use with the Atraumatic Restorative Treatment (ART) approach in class II cavities

ObjectivesThe null-hypotheses tested were that no difference in compressive strength of ART class II cavities exists between those restored with (1) glass-carbomer and a commonly used glass-ionomer; (2) KMEM and the commonly used glass-ionomer and; (3) glass-carbomer and KMEM.Methods100 molar teeth, stratified by size, were randomly allocated to the four test groups. Large ART class II cavities were drilled and restored with Clearfil photoposterior (negative control), Fuji IX (positive control), Glass-carbomer and Ketac? Molar Easymix (KMEM) (experimental groups). Half of the samples in each test group were 5000 times thermocycled between 5 °C and 55 °C, with a 30 s dwell time in each bath and a transfer time of 10 s. The restorations were statically tested at the marginal ridge until failure, using a rounded rectangular testing rod at crosshead speed of 1.0 mm/min. ANOVA and Student's t-test were applied to test for differences between the dependent variable (compressive strength at the final breaking point) and the independent variables (thermocycling and restorative material).ResultsRestorations of Clearfil photoposterior had a statistically significant higher mean compressive strength value at final breaking point than those of the three glass-ionomers tested (p = 0.0001). No thermocycling effect was observed (p = 0.19). ANOVA between the three glass-ionomer materials and mean compressive strength at final breaking point showed no statistically significant difference (p = 0.09).SignificanceClass II ART cavities restored with the newly launched Glass-carbomer and Ketac? Molar Easymix were not significantly more fracture resistant than comparable restorations using the conventional glass-ionomer Fuji IX.     

Fracture strength and fatigue resistance of dental resin-based composites

ObjectivesThe aim of this study was to evaluate in vitro the influence of fiber-reinforcement on the fracture strength and fatigue resistance of resin-based composites.MethodsOne hundred rectangular bar-shaped specimens (2 mm × 2 mm × 25 mm) made of resin-based composite were prepared in a stainless steel split-mould: (i) thirty specimens of particulate filler composite (PFC) (Filtek Z100, 3 M ESPE, St Paul, MN, USA), (ii) thirty specimens of fiber-reinforced composite (FRC) (Everstick C&B, Sticktech Ltd., Turku, Finland) and (iii) forty specimens of PFC and FRC combined in two longitudinal layers of equal thickness. Each specimen was trimmed into a cylindrical hourglass shape. The fracture strength (cantilever beam test, n = 10) and the fatigue resistance (rotating cantilever beam test; staircase method: 10⁴ cycles, 1.2 Hz, n = 20) were determined. Fracture strength, fatigue resistance and work-of-fracture were calculated. The fracture surfaces of failed specimens were analyzed with SEM. Data was analyzed by logistic regression, one-way ANOVA followed by Tukey's post hoc test and, a Student's t-test.ResultsANOVA revealed that fiber-reinforcement had significant effect (P < 0.001) on fracture strength, fatigue resistance, and work-of-fracture. Student's t-test showed significant differences (P < 0.001) in fatigue resistance compared to fracture strength.ConclusionsWithin the limitations of this study, the following conclusions can be drawn (i) the fatigue resistance of resin-based composites is lower than their fracture strength and (ii) FRC are more fatigue resistant than PFC or combinations of FRC and PFC.     

Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics

ObjectiveTo investigate the processing induced particle alignment on fracture behavior of four multiphase dental ceramics (one porcelain, two glass–ceramics and a glass-infiltrated–alumina composite).MethodsDisks (Ø12 mm × 1.1 mm-thick) and bars (3 mm × 4 mm × 20 mm) of each material were processed according to manufacturer instructions, machined and polished. Fracture toughness (K_Ic) was determined by the indentation strength method using 3-point bending and biaxial flexure fixtures for the fracture of bars and disks, respectively. Microstructural and fractographic analyses were performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction.ResultsThe isotropic microstructure of the porcelain and the leucite-based glass–ceramic resulted in similar fracture toughness values regardless of the specimen geometry. On the other hand, materials containing second-phase particles with high aspect ratio (lithium disilicate glass–ceramic and glass-infiltrated–alumina composite) showed lower fracture toughness for disk specimens compared to bars. For the lithium disilicate glass–ceramic disks, it was demonstrated that the occurrence of particle alignment during the heat-pressing procedure resulted in an unfavorable pattern that created weak microstructural paths during the biaxial test. For the glass-infiltrated–alumina composite, the microstructural analysis showed that the large alumina platelets tended to align their large surfaces perpendicularly to the direction of particle deposition during slip casting of green preforms.SignificanceThe fracture toughness of dental ceramics with anisotropic microstructure should be determined by means of biaxial testing, since it results in lower values.     

Effect of ion exchange on strength and slow crack growth of a dental porcelain

ObjectivesTo determine the effect of ion exchange on slow crack growth (SCG) parameters (n, stress corrosion susceptibility coefficient, and σ_f0, scaling parameter) and Weibull parameters (m, Weibull modulus, and σ₀, characteristic strength) of a dental porcelain.Methods160 porcelain discs were fabricated according to manufacturer's instructions, polished through 1 μm and divided into two groups: GC (control) and GI (submitted to an ion exchange procedure using a KNO₃ paste at 470 °C for 15 min). SCG parameters were determined by biaxial flexural strength test in artificial saliva at 37 °C using five constant stress rates (n = 10). 20 specimens of each group were tested at 1 MPa/s to determine Weibull parameters. The SPT diagram was constructed using the least-squares fit of the strength data versus probability of failure.ResultsMean values of m and σ₀ (95% confidence interval), n and σ_f0 (standard deviation) were, respectively: 13.8 (10.1 ? 18.8) and 60.4 (58.5 ? 62.2), 24.1 (2.5) and 58.1 (0.01) for GC and 7.4 (5.3 ? 10.0) and 136.8 (129.1 ? 144.7), 36.7 (7.3) and 127.9 (0.01) for GI. Fracture stresses (MPa) calculated using the SPT diagram for lifetimes of 1 day, 1 year and 10 years (at a 5% failure probability) were, respectively, 31.8, 24.9 and 22.7 for GC and 71.2, 60.6 and 56.9 for GI.SignificanceFor the porcelain tested, the ion exchange process improved strength and resistance to SCG, however, the material's reliability decreased. The predicted fracture stress at 5% failure probability for a lifetime of 10 years was also higher for the ion treated group.     

Bis-GMA co-polymerizations: Influence on conversion,flexural properties,fracture toughness and susceptibility to ethanol degradation of experimental composites

ObjectivesThe aim of this study was to evaluate the influence of monomer content on fracture toughness (K_Ic) before and after ethanol solution storage, flexural properties and degree of conversion (DC) of bisphenol A glycidyl methacrylate (Bis-GMA) co-polymers.MethodsFive formulations were tested, containing Bis-GMA (B) combined with TEGDMA (T), UDMA (U) or Bis-EMA (E), as follows (in mol%): 30B:70T; 30B:35T:35U; 30B:70U; 30B:35T:35E; 30B:70E. Bimodal filler was introduced at 80 wt%. Single-edge notched beams for fracture toughness (FT, 25 mm × 5 mm × 2.5 mm, a/w = 0.5, n = 20) and 10 mm × 2 mm × 1 mm beams for flexural strength (FS) and modulus (FM) determination (10 mm × 2 mm × 1 mm, n = 10) were built and then stored in distilled water for 24 h at 37 °C. All FS/FM beams and half of the FT specimens were immediately submitted to three-point bending test. The remaining FT specimens were stored in a 75%ethanol/25%water (v/v) solution for 3 months prior to testing. DC was determined with FT-Raman spectroscopy in fragments of both FT and FS/FM specimens at 24 h. Data were submitted to one-way ANOVA/Tukey test (α = 5%).ResultsThe 30B:70T composite presented the highest K_Ic value (in MPa m^1/2) at 24 h (1.3 ± 0.4), statistically similar to 30B:35T:35U and 30B:70U, while 30B:70E presented the lowest value (0.5 ± 0.1). After ethanol storage, reductions in K_Ic ranged from 33 to 72%. The 30B:70E material presented the lowest reduction in FT and 30B:70U, the highest. DC was similar among groups (69–73%), except for 30B:70U (52 ± 4%, p < 0.001). 30B:70U and 30B:35T:35U presented the highest FS (125 ± 21 and 122 ± 14 MPa, respectively), statistically different from 30B:70T or 30B:70E (92 ± 20 and 94 ± 16 MPa, respectively). Composites containing UDMA or Bis-EMA associated with Bis-GMA presented similar FM, statistically lower than 30B:35T:35U.SignificanceComposites formulated with Bis-GMA:TEGDMA:UDMA presented the best compromise between conversion and mechanical properties.     

Contributions of stress corrosion and cyclic fatigue to subcritical crack growth in a dental glass-ceramic

ObjectiveThe objective of this study was to test the following hypotheses: (1) both cyclic degradation and stress-corrosion mechanisms result in subcritical crack growth (SCG) in a fluorapatite glass-ceramic (IPS e.max ZirPress, Ivoclar-Vivadent) and (2) there is an interactive effect of stress corrosion and cyclic fatigue to accelerate subcritical crack growth.MethodsRectangular beam specimens were fabricated using the lost-wax process. Two groups of specimens (N = 30/group) with polished (15 μm) or air-abraded surface were tested under rapid monotonic loading. Additional polished specimens were subjected to cyclic loading at two frequencies, 2 Hz (N = 44) and 10 Hz (N = 36), and at various stress amplitudes. All tests were performed using a fully articulated four-point flexure fixture in deionized water at 37 °C. The SCG parameters were determined using the ratio of inert strength Weibull modulus to lifetime Weibull modulus. A general log-linear model was fit to the fatigue lifetime data including time to failure, frequency, peak stress, and the product of frequency and logarithm of stress in ALTA PRO software.ResultsSCG parameters determined were n = 21.7 and A = 4.99 × 10⁻⁵ for 2 Hz, and n = 19.1 and A = 7.39 × 10⁻⁶ for 10 Hz. After fitting the general log-linear model to cyclic fatigue data, the coefficients of the frequency term (α₁), the stress term (α₂), and the interaction term (α₃) had estimates and 95% confidence intervals of α₁ = −3.16 (−15.1, 6.30), α₂ = −21.2 (−34.9, −9.73), and α₃ = 0.820 (−1.59, 4.02). Only α₂ was significantly different from zero.Significance(1) Cyclic fatigue does not have a significant effect on SCG in the fluorapatite glass-ceramic evaluated and (2) there was no interactive effect between cyclic degradation and stress corrosion for this material.     

Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach

ObjectiveThe aim of this study was to analyze and to compare the fracture type and the stress at failure of clinically fractured zirconia-based all ceramic restorations with that of morphologically similar replicas tested in a laboratory setup.MethodsReplicas of the same shape and dimensions were made for 19 crowns and 17 fixed partial dentures, all made of veneered zirconia frameworks, which fractured during intra-oral service. The replicas were statically loaded by applying axial load in a universal testing machine. The principles of fractography were used to identify the location and the dimensions of the critical crack and to estimate the stress at failure. Failure was classified according to origin and type (P < 0.05 was considered significant).ResultsClinically fractured restorations failed due to either: delamination of the veneer ceramic (28.2 ± 9 MPa), defects at core veneer interface (27.7 ± 6 MPa), the generation of Hoop stresses (884.3 ± 266 MPa), radial cracking (831 MPa), or fracture of the connector (971 ± 343 MPa). The replicas failed by mainly by cone cracking of the veneer ceramic (52.4 ± 34.8 MPa) or by fracture of the connector (1098.9 ± 259 MPa). The estimated stress at failure was significantly higher for the replicas compared to the clinically fractured restorations (F = 6.8, P < 0.01).SignificanceWithin limitations of this study, careful design of fracture strength test would lead to more clinically relevant data. The performance of zirconia veneered restorations could be further improved with careful design considerations.