Mean Biaxial Flexural Strength Research Articles

Biaxial Flexural Strength of Printed Splint Materials.

One therapeutical alternative in the treatment of functional disorders is the use of printed oral splints. The mechanical properties of these materials are highly essential to their clinical effectiveness, and their performance may vary depending on factors such as cleaning, post-polymerization, or their orientation during construction. The objective of this in vitro investigation is to evaluate the effectiveness of the selected materials in terms of their biaxial flexural strength in relation to the criteria listed above. Splint materials were used in the printing of 720 discs. The printing process was carried out in different orientations in relation to the building platform. Either an automatic or manual cleaning process was performed on the samples. For post-polymerization, either an LED or Xenon light was utilized. A piston-on-three-ball test was used to measure the biaxial flexural strength (BFS) of the materials after they were stored in water for either 24 h or 60 days. The homogeneity of the data was controlled by employing the Levene method, and the differences between the groups were analyzed using the ANOVA and Bonferroni methods. After being stored for twenty-four hours, the mean BFS ranged anywhere from 79 MPa to 157 MPa. Following a period of sixty hours, the BFS exhibited a substantial drop and revealed values that ranged from 72 to 127 MPa. There was no significant difference that could be identified between the materials or between the various cleaning processes. The results of post-polymerization showed that the LED light produced higher means than the Xenon light did. In terms of position, the mean values varied greatly, with 0°'s mean value being 101 MPa, 45°'s mean value being 102 MPa, and 90°'s mean value being 115 MPa. The use of a build orientation of 90° and post-polymerization with LED light resulted in significantly increased biaxial flexural strength. According to this study, this design should be implemented in order to ensure that splint materials have the highest possible strength.

Comparison of properties and cost efficiency of zirconia processed by DIW printing, casting and CAD/CAM-milling.

The aim of this study was to evaluate the mechanical properties and cost efficiency of direct ink writing (DIW) printing of two different zirconia inks compared to casting and subtractive manufacturing. Zirconia disks were manufactured by DIW printing and the casting process and divided into six subgroups (n=20) according to sintering temperatures (1350°C, 1450°C and 1550°C) and two different ink compositions (Ink 1, Ink 2). A CAD/CAM-milled high strength zirconia (3Y-TZP) was added as reference group. The biaxial flexural strength (BFS) was measured using the piston-on-three-balls test. X-ray-diffraction (XRD) was used for microstructural analysis. The cost efficiency was compared for DIW printing and subtractive manufacturing by calculation of the manufacturing costs of one dental crown. Using XRD, monoclinic and tetragonal phases were detected for Ink 1, for all other groups no monoclinic phase was detected. The CAD/CAM-milled ceramic showed a significantly higher BFS than all other groups. The BFS of Ink 2 was significantly higher than the BFS of Ink 1. At a sintering temperature of 1550°C the mean BFS of the printed Ink 2 was 822±174MPa. The BFS of the cast materials did not show a significantly higher BFS than the corresponding printed group for any tested parameter-set. The manufacturing costs of DIW printed crowns are lower than the manufacturing costs of CAD/CAM-milled crowns. DIW has a high potential to replace subtractive processes for dental applications, as it shows promising mechanical properties for appropriate ink compositions and facilitates a highly cost effective production.

Pre-cementation treatment of glass-ceramics with vacuum impregnated resin coatings.

The study aimed to investigate the effectiveness of a vacuum impregnation process to eliminate the porosity at the ceramic-resin interface to optimize the reinforcement of a glass-ceramic by resin cementation. 100 leucite glass-ceramic disks (1.0±0.1mm thickness) were air-abraded, etched with 9.6% HF acid, and silanated. Specimens were randomly allocated to 5 groups (n=20). Group A received no further treatment (uncoated control). Groups B and D were resin-coated under atmospheric pressure, whereas groups C and E were resin-coated using vacuum impregnation. The polymerized resin-coating surfaces of specimens in groups B and C were polished to achieve a resin thickness of 100±10µm, while in groups D and E no resin-coating modification was performed prior to bi-axial flexure strength (BFS) determination. Optical microscopy was undertaken on the fracture fragments to identify the failure mode and origin. Comparisons of BFS group means were made by a one-way analysis of variance (ANOVA) and post-hoc Tukey test at α=0.05. All resin-coated sample groups (B-E) showed a statistically significant increase in mean BFS compared with the uncoated control (p<0.01). There was a significant difference in BFS between the ambient and vacuum impregnated unpolished groups (D and E) (p<0.01), with the greatest strengthening achieved using a vacuum impregnation technique. Results highlight the opportunity to further develop processes to apply thin conformal resin coatings, applied as a pre-cementation step to strengthen dental glass-ceramics.

Bi-axial flexural strength of resin based dental composites – Influence and reliability of the testing method configuration

ABSTRACT This study aimed to determine the effect of the testing method configuration on the bi-axial flexural strength (BFS) of three resin-based dental composites (RDCs). A total of 180 disc-shaped samples (12 mm diameter x 1 mm thickness) for each RDC were fabricated using split back nylon mould. The BFS was determined for specimens made in three commercial RDCs; microhybrid (MH), nanofilled paste (NFP) and nanofilled flowable (NFF) using the ring-on-ring (RoR; n = 90 for each RDC specimens) and ball-on-ring (BoR; n = 90 for each RDC specimens) configurations at a cross-head speed of 1 mm/min. RDC specimens of MH and NFP tested in the BoR configuration showed higher mean BFS values compared to those tested in the RoR configuration. (p < 0.001). The effect of test configuration on BFS values was insignificant for the specimens belonging to the NFF group (p = 0.207). Moreover, the Weibull modulus of NFF in the RoR configuration was statistically lower compared to those for NFF in the BoR configuration as the 95% confidence interval of Weibull modulus did not overlap. The specimen support and testing configuration significantly impacted the BFS value of the RDCs. The Weibull statistics appeared to be inappropriate for all kinds of RDCs.

Strength-limiting damage and its mitigation in CAD-CAM zirconia-reinforced lithium-silicate ceramics machined in a fully crystallized state

ObjectivesThe objective was to explore how clinically relevant machining process and heat treatment influence damage accumulation and strength degradation in lithium silicate-based glass ceramics machined in the fully crystallized state. MethodsA commercial zirconia-reinforced lithium silicate (ZLS) glass ceramic with a fully developed microstructure (Celtra® Duo) was studied. Disk-shaped specimens (nominal 10 mm diameter and 1 mm thickness) were fabricated either using a CAD-CAM process, creating a clinically relevant dental restoration surface, or were sectioned from water-jet cut cylindrical blocks with their critical surfaces consistently polished. Bi-axial flexure strength (BFS) was determined in a ball-on-ring configuration, and fractographic analysis was performed on failed specimens. XRD, AFM and SEM measurements were conducted before and after heat treatment. For each sample group, BFS was correlated with surface roughness. A two-way ANOVA and post-hoc Tukey tests were used to determine differences in BFS between machining and heat treatment groups (ɑ = 0.05). ResultsA two-way ANOVA demonstrated that BFS was influenced by fabrication route (p < 0.01) with CAD-CAM specimens exhibiting significantly lower mean BFS. A factorial interaction was observed between heat treatment and machining route (p < 0.01), where a significant strengthening effect of post-manufacture heat treatment was noted for CAD-CAM specimens but not sectioned and polished samples. CAD-CAM specimens exhibited sub-surface lateral cracks alongside radial cracks near fracture origin which were not observed for polished specimens. BFS did not correlate with surface roughness for polished specimens, and no change in microstructure was detectable by XRD following heat treatment. SignificanceThe mechanical properties of the ZLS ceramic material studied were highly sensitive to the initial surface defect integral associated with manufacturing route and order of operations. CAD-CAM manufacturing procedures result in significant strength-limiting damage which is likely to influence restoration performance; however, this can be partially mitigated by post-machining heat treatment.

Effect of time period on biaxial strength for different Y-TZP veneering porcelains.

To assess the influence of time period on strength for different conventional Y-TZP porcelains. Four types of porcelain were used. Porcelain powders were pressed in a steel die and fired in a furnace as recommended by the manufacturer. The blocks were sectioned into 2 mm-thick discs. Seventy specimens were fabricated from each type of porcelain. Of six jars, three contained 40 mL of dH2 O and three were dry. Ten specimens were placed in each jar and stored in an incubator at 37°C. A biaxial flexural strength test was performed for all specimens at the baseline and each succeeding year for all groups, for 3 years, using an Instron with a crosshead speed of 0.5 mm/min. The data were analyzed by ANOVA and Tukey at α = .05. At the baseline, the biaxial flexural strength of VitaVM9 was significantly higher than that of NobelRondo; however, there were no significant differences among Vita VM9 and Noritake CZR or NobelRondo, Lava Ceram, and Noritake CZR. There were significant reductions in the biaxial flexural strengths of all porcelains. After 3 years, all porcelain groups showed a significant decrease in biaxial flexural strength. The medium used is found to have no significant effects on the mean biaxial flexural strength. Certain commercially available Y-TZP porcelains are susceptible to decrease in strength over time, which might impact the integrity of veneered zirconia restorations. This reduction in strength is among the most common reasons for the failure of veneered zirconia as reported in several studies.

The adverse effects of tungsten carbide grinding on the strength of dental zirconia

ObjectivesThis study investigated the effects of tungsten-carbide grinding on the surface characteristics and mechanical strength of dental 3 mol% yttria-stabilized zirconia (3Y-TZP). MethodsTwo types of tungsten-carbide burs (TC), 6-blade (TC1) and 8-blade (TC2) were used to grind 3Y-TZP, in a dental air-turbine handpiece with water-cooling and were also subjected to air-particle abrasion (APA): TC1 + APA and TC2 + APA; and rubber polishing (RP): TC1 + RP and TC2 + RP; one group received only rubber-polishing RP. The control group received no treatment. Surface characterization was examined by surface roughness (Ra) and atomic force microscopy. Specimens were also observed with scanning electron microscopy (SEM) and X-ray-diffraction (XRD) for microstructure and crystalline phases. A piston-on-three-balls biaxial-flexural strength (BFS) test was performed with 15 samples-per-group and the broken specimen were observed under SEM to investigate the fracture origin pattern. One-way ANOVA, Kruskal–Wallis test and Weibull analysis were performed at α = 0.05. ResultsGroups TC1 and TC2 had the lowest mean BFS (p < 0.05) with up to 74 % reduction in strength. APA and RP both significantly increased the mean BFS after tungsten-carbide grinding but was still less than the control (p < 0.05). Compared to the control, the mean BFS was significantly reduced for all groups except for the RP group (p < 0.05). APA and rubber-polishing following TC2 grinding had significant higher mean BFS than those following TC1 grinding respectively (p < 0.05). SEM revealed distinct micro-cracks after tungsten-carbide grinding. SignificanceTungsten-carbide burs (6- and 8-blade) are not recommended for zirconia grinding due to the significant reduction of biaxial-flexural strength and observed micro-structural surface and subsurface damage.

Quartz‐containing glass‐ceramics in the SiO2–Li2O–K2O–MgO–CaO–Al2O3–P2O5 system

AbstractVolume crystallization of quartz, triggered by adding P2O5, was investigated in the multicomponent SiO2–Li2O–K2O–MgO–CaO–Al2O3–P2O5 glass system. Glass‐ceramics comprising quartz as the main crystal phase besides lithium disilicate (Li2Si2O5) as a minor crystal phase were produced by controlled crystallization from the parent glass. According to quantitative crystal phase analysis by means of Rietveld refinement the mass fraction of the minor Li2Si2O5 phase decreased with increasing P2O5 content, while the fraction of quartz increased. Scanning electron microscopy revealed lath‐like Li2Si2O5 embedded in a matrix of rather globular quartz crystals forming an interlocking microstructure. Well machinable glass‐ceramics with a mean biaxial flexural strength of about 300 MPa and a fracture toughness of about 2.0 MPa × m0.5 could be realized. With mean values ≥13.6 × 10−6 K−1 the glass‐ceramics coefficient of thermal expansion is close to the one reported for low‐quartz crystals. The ease of production based on conventional glass melting and fabrication technology together with the good mechanical, optical, and machining properties enables the application of these materials in prosthodontics.

Strength-limiting damage in lithium silicate glass-ceramics associated with CAD–CAM

ObjectiveThe fabrication of all-ceramic restorations using Computer Aided Design and Computer Aided Manufacturing (CAD–CAM) most commonly involves subtractive machining which results in strength-limiting, surface and sub-surface damage in the resultant prosthesis. The objective was to explore how clinically relevant machining-process variables, and material variables, affect damage accumulation in lithium silicate glass-ceramics. MethodsThree commercial lithium silicate glass-ceramics (IPS e.max® CAD, Celtra® Duo and Vita Suprinity®) were selected. For each material, two groups of disk-shaped specimens were fabricated (n=15), using a CAD–CAM process, creating surfaces equivalent to those generated for a dental restoration, or alternatively, using a highly controlled laboratory process generating disk-shaped test specimens with a consistent polished surface. Bi-axial flexure strength (BFS) was determined in a ball-on-ring configuration and fractographic analyses performed. For each material BFS was correlated with machining sequence and with surface roughness. ResultsBFS was significantly influenced by material substrate (p<0.01) and by fabrication route (p<0.01). A significant factorial interaction (p<0.01) identified that the magnitude of changes in BFS when comparing the two specimen fabrication routes, was dependent on substrate type. The polished control specimens exhibited a significantly increased BFS when compared with the CAD–CAM counterparts for all materials. IPS e.max® CAD and Celtra® Duo showed a 44 and 46% reduction in mean BFS for the CAD–CAM specimens when compared with the polished counterparts, respectively. In contrast, Vita Suprinity® showed the least disparity in mean BFS (21%) but the greatest variance in BFS data. SignificanceAll CAD–CAM specimens showed evidence of machining introduced damage in the form of median and radial cracks at sites either coincident with, or peripheral to the failure origin. Subtractive machining introduced significant strength limiting damage that is not eliminated by heat treatments applied for either microstructure development (IPS e.max® CAD and Vita Suprinity®) or annealing/crack blunting (Celtra® Duo).

Effect of sandblasting, etching and resin bonding on the flexural strength/bonding of novel glass-ceramics

ObjectivesTo process novel leucite glass-ceramics and test the effects of surface treatment and resin bonding on the biaxial flexural strength (BFS) and shear bond strength (SBS). MethodsAlumino–silicate glasses were ball-milled, and heat treated to form leucite glass-ceramics (LG-C, OLG-C), then sintered into ingots. Ingots were heat extruded into a refractory mould to form disc specimens (1.3×14mm diameter). IPS e.max® was used as a commercial comparison. Glass-ceramic test groups were sandblasted (Groups. 1, 4, 6), sandblasted, etched and adhesively bonded (Groups. 2, 5, 7) or lapped, etched and adhesively bonded (Groups. 3, 8). Specimens were adhesively bonded with Monobond S, followed by the application of Variolink II® cement and light curing. BFS testing was at 1mm/min and SBS testing at 0.5mm/min. Samples were characterised using XRD, SEM and profilometry. ResultsXRD confirmed tetragonal leucite in LG-C/OLG-C and lithium disilicate/lithium orthophosphate in IPS e.max®. Mean BFS (MPa (SD)) were: Gp1 LG-C; 193.1 (13.9), Gp2 LG-C; 217.7 (23.0), Gp3 LG-C; 273.6 (26.7), Gp4 OLG-C; 255.9 (31); Gp5 OLG-C; 288.6 (37.4), Gp6 IPS e.max®; 258.6 (20.7), Gp7 IPS e.max®; 322.3 (23.4) and Gp8 IPS e.max®; 416.4 (52.6). The Median SBS (MPa) were Gp1 LG-C; 14.2, Gp2 LG-C (10s etch); 10.6 and Gp3 IPS e.max®; 10.8. Mean surface roughness was 5–5.1μm (IPS e.max®) and 2.6μm (LG-C). SignificanceNovel leucite glass-ceramics with reduced flaw size and fine microstructures produced enhanced BFS and SBS by resin bonding. These properties may be useful for the fabrication of minimally invasive aesthetic and fracture resistant restorations.

The effect of surface treatments on dental zirconia: An analysis of biaxial flexural strength, surface roughness and phase transformation

ObjectivesThis study investigates the effect of selected surface finishing techniques on the biaxial flexural strength, surface roughness and phase transformation of a zirconia dental restorative material. Materials and methodsFully-sintered zirconia discs (⌀19.5 mm × 0.85 mm) were treated on one side with a single or a combination of the following treatments: diamond and/or tungsten-carbide burs without water coolant in an air-turbine handpiece, air-particle abrasion, rubber-point polishing in a contra-angle handpiece, or no treatment (control). Biaxial flexural strength (BFS) (eleven groups, n = 10) was determined using a universal testing machine and surface roughness (thirteen groups, n = 6) was assessed using a profilometer. Results were analysed using one-way ANOVA and Student-Newman-Keuls Post-hoc test (α = 0.05) with Bonferroni correction. Specimens were observed under scanning electron microscopy (SEM) and x-ray diffraction (XRD) for their microstructure and crystalline phases respectively. ResultsGrinding with diamond burs did not weaken zirconia (p > 0.0045) but produced rougher surfaces than the control group (p < 0.0038). Tungsten-carbide burs smoothened diamond ground specimens (p < 0.0038) for both grits of diamond. Specimens ground by tungsten-carbide burs have significantly reduced mean BFS (p < 0.0045) by up to two-thirds and SEM revealed fine surface cracks. Air-particle abrasion restored the mean BFS of tungsten-carbide ground specimens to control levels (p > 0.0045) and surface cracks were not observed. Phase transformation was not detected by XRD. ConclusionsDental zirconia ground dry with tungsten-carbide burs has a significantly reduced BFS and a smooth but defective surface. These defects may be removed and BFS restored by air-particle abrasion. Clinical significanceThe use of tungsten-carbide burs for grinding dental zirconia should be cautioned. Diamond grinding does not weaken zirconia but requires further polishing.

Effect of Surface Modification on In-Depth Transformations and Flexural Strength of Zirconia Ceramics.

Chairside surface adjustments of zirconia dental restorations enhance the toughening stress-induced tetragonal-to-monoclinic phase transformation and domain reorientation by ferro-elastic domain switching (FDS), but also trigger subsurface damage, which could compromise long-term clinical performance. The purpose of this study was to assess the depth of phase transformation, associated FDS, and flexural strength of dental zirconia (BruxZir HT 2.0), after chairside surface treatments. Square specimens were sectioned from CAD/CAM blocks and sintered according to manufacturer's recommendations (n = 30). They were left as-sintered (AS; control), air abraded with fine (AAF) or coarse (AAC) alumina particles, ground (G) or ground and polished (GP). Roughness was measured by profilometry. Crystalline phases were investigated by grazing incidence X-ray diffraction (GIXRD) (n = 3). GIXRD data were fit using semi-log regression protocols to assess transformation depth and extent of FDS. The mean biaxial flexural strength was measured according to ISO 6872. Subsurface damage was assessed from SEM images using a bonded polished interface configuration. Flaw distribution was assessed by Weibull analysis. Results were analyzed by Kruskal-Wallis with Tukey's adjustment for multiple comparisons (p < 0.05). Air-abraded and ground groups exhibited higher mean surface roughness than control. AAF group exhibited the highest flexural strength (1662.6 ± 202.6 MPa) with flaw size (5.9 ± 1.8 μm) smaller than transformation (14.5 ± 1.2 μm) or FDS depth (19.3 ± 1.1 μm), followed by GP group (1567.2 ± 209.7 MPa) with smallest FDS depth (9.3 ± 2.0 μm) and flaw size (2.6 ± 1.8 μm), but without m-phase. AAC group (1371.4 ± 147.6 MPa) had the largest flaw size (40.3 ± 20.3 μm), transformation depth (47.2 ± 3.0 μm) and FDS depth (41.2 ± 2.2 μm). G group (1357.0 ± 196.7 MPa) had the smallest transformation depth (8.6 ± 1.5 μm), and mean FDS depth (19.8 ± 3.7 μm) and flaw size (18.6 ± 3.1 μm). AAC and AAF exhibited the highest Weibull modulus (11.2 ± 0.4 and 9.8 ± 0.3 μm, respectively). Variations in mean biaxial flexural strength were explained by the balance between the depth of toughening mechanisms (phase transformation and FDS) and subsurface damage. AAF and GP groups were the most efficient surface adjustments in promoting the highest mean biaxial flexural strength.

The Effect of in vitro Aging and Fatigue on the Flexural Strength of Monolithic High-translucency Zirconia Restorations

The aim of this in vitro study was to evaluate the effect of accelerated artificial aging (AAA) and fatigue on the biaxial flexural strength (BFS) of three types of monolithic high-translucency zirconia restorations compared with conventional low-translucency zirconia restorations. Four groups of 20 disc-shaped specimens (10 × 1.2 mm) were made from the following computer-aided design and computer-aided manufacturing (CAD/CAM) zirconia blocks: Low-translucency zirconia (Ceramill ZI-LT) as a control, and three brands of high-translucent zirconia (Lava Plus, Ceramill Zolid White, and Copran Monolithic HT). Ten discs from each group were subjected to the BFS test using the universal testing machine. The other 10 discs from each group were subjected to AAA (thermocycling, 3,500 cycles) and fatigue (250,000 cycles) before the fracture test. The definitive fracture load was recorded, and the BFS was calculated in accordance with International Organization for Standardization (ISO) 6872. The data were analyzed with one-way analysis of variance (ANOVA), Scheffe post hoc, and Mann-Whitney U test. Data analyses were evaluated at a significance level of p < 0.05. Significant differences were detected in the BFS among the four groups before AAA and fatigue. The mean BFS was highest with Ceramill ZI (935.3 ± 47.1 MPa), and least in Ceramill Zolid White (685.7 ± 32.6 MPa). After AAA and fatigue, significant differences were reported where the mean of BFS was highest with Copran Zr-i Monolithic HT (777.5 ± 21.2 MPa), and least in Ceramill Zolid White (576.0 ± 36.3 MPa). Furthermore, Mann-Whitney U test showed that AAA and fatigue significantly affect the BFS of each material individually. The AAA and fatigue significantly affected the BFS of the monolithic high-translucency zirconia restorations. Although monolithic high-translucency zirconia had significantly lower BFS than conventional zirconia tested in this study, they still have sufficient strength for clinical use.

Mechanical and optical properties of monolithic CAD-CAM restorative materials

Statement of problemAchieving natural tooth appearance with sufficient mechanical strength is one of the most challenging issues of computer-assisted design and computer-assisted manufacturing (CAD-CAM) materials. However, limited evidence is available regarding their optical and mechanical properties for proper and evidence-based material selection in clinical practice. PurposeThe purpose of this in vitro study was to assess and compare the translucency and biaxial flexural strength of 5 monolithic CAD-CAM restorative materials. Material and methodsDisk-shaped specimens (n=30) of each material (Lava Ultimate [LU], Vita Enamic [VE], Vitablocs Mark II [VMII], Vita Suprinity [VS], and IPS e.max CAD [IPS]) with a diameter of 12 mm and a thickness of 1.2 ±0.05 mm were prepared. A spectrophotometer was used to measure the translucency parameter. The specimens were then subjected to a biaxial flexure test using 3 balls and loaded with a piston in a universal testing machine at a cross-head speed of 0.5 mm/min until failure occurred (International Organization for Standardization standard 6872). Weibull statistics were used to evaluate the characteristic strength and reliability of each material. Chemical compositions were analyzed using an energy dispersive spectrometer, and microstructural analysis was conducted using scanning electron microscopy. Data were analyzed using 1-way ANOVA and the Tukey honest significant difference test (α=.05). ResultsSignificant differences were found among the materials concerning translucency and biaxial flexural strength (P<.05). The highest mean transparency value was obtained in the VS group, whereas the lowest mean value was obtained in the VE group. The VS group produced the highest mean biaxial flexural strength, followed by the IPS, LU, VE, and VMII groups. ConclusionsBased on the results of the present study, zirconia-reinforced glass-ceramic revealed higher mean translucency and biaxial flexural strength than resin nanoceramic, feldspathic ceramic, lithium disilicate ceramic, and dual-network ceramic.

TRANSLUCENCY AND BIAXIAL FLEXURAL STRENGTH OF MONOLITHIC ZIRCONIA AS AFFECTED BY SINTERING SPEEDS

Statement of the problem: Approaches towards reducing fixed prosthetic restoration fabrication times include; (CAD/CAM) chair side systems in addition to rapid sintering cycles, which can be carried out in minutes rather than hours in order to deliver zirconia-based restorations in one visit .However, limited information is available regarding the effect of rapid sintering on properties of monolithic translucent zirconia.Aim of the study: The aim of this study was to evaluate the effect of changing the sintering speed (classic, speed and super speed cycles) on translucency and biaxial flexural strength of monolithic translucent zirconia before and after thermodynamic aging.Materials & Methods: 36 monolithic translucent zirconia discs (10mm diameter×0.8mm thickness) were CAD/CAM fabricated from translucent zirconia blocks(In Coris TZI) and randomly divided into three main groups (n=12 each) according to the sintering speed; group I: samples sintered by classic sintering cycle (sintering at 1510°C for 120 min holding time & 8 hours total cycle time), group II: speed sintering (sintering at 1540°C for 25 min holding time & 2 hours total cycle time) and group III: super speed sintering (sintering at 1580°C for 10 min holding time representing the total firing cycle). In each group, half of the samples were evaluated in the non-aged condition (subgroup1, n=6) while the other half were evaluated after thermodynamic aging (subgroup2, n=6) in a chewing simulator. Translucency parameter (TP) and contrast ratio (CR) were measured using a digital reflective spectrophotometer. Biaxial flexural strength (MPa) was tested using piston on three ball technique in a universal testing machine. Representative sample from each subgroup was analyzed by Scanning electron microscope. Statistical analysis was performed using Two-way ANOVA and Tukey’s post-hoc tests ( P ≤ 0.05).Results: Two-way ANOVA revealed that sintering speed and thermodynamic aging had a statistically significant effect on mean (TP), (CR).Either before or after aging, classic cycle showed the statistically significantly highest mean (TP) & lowest (CR) values. On the other hand, biaxial flexural strength was statistically significantly affected by the sintering speed only and not by thermodynamic aging; the super speed cycle registered the lowest mean biaxial flexural strength before and after thermodynamic aging.

Effect of Sandblasting Angle and Distance on Biaxial Flexural Strength of Zirconia-based Ceramics.

Surface treatment is necessarily required for bonding of zirconia to the veneering porcelain and luting cements. Sandblasting is the most common and probably the most efficient surface treatment method. Sandblasting roughens the surface and may affect the flexural strength of zirconia. Different sandblasting protocols may yield variable results. This study sought to assess the effect of sandblasting angle and distance on the biaxial flexural strength of zirconia-based ceramics. This in vitro experimental study was conducted on 50 zirconia discs measuring 1.2 ± 0.2 mm in thickness and 15 ± 0.2 mm in diameter, which were randomly divided into five groups (n = 10) of one control and four experimental groups subjected to sandblasting with 110 μm aluminum oxide particles under 2 bar pressure for 10 seconds at 15 and 25 mm distances and 45 and 90° angles (between the nozzle head and zirconia surface). Surface roughness was measured by a roughness tester and samples were subjected to thermocycling followed by biaxial flexural strength testing according to ISO6872. The data were analyzed using one-way analysis of variance (p < 0.05). No statistically significant difference was noted in the mean biaxial flexural strength of the five groups (p = 0.40). Different sandblasting protocols yielded significantly different surface roughness values (p < 0.001). The highest and the lowest mean surface roughness belonged to 15 mm/90° (0.51 μm) and control (0.001 μm) groups respectively. Change in sandblasting angle and distance had no significant effect on the biaxial flexural strength of zirconia-based ceramic, but surface roughness was significantly different in the study groups. Clinical significances: Regardless of sandblasting angle, increasing distance to 25 mm significantly decreases surface roughness that may negatively affect zirconia bond strength.

Effects of core-to-dentin thickness ratio on the biaxial flexural strength, reliability, and fracture mode of bilayered materials of zirconia core (Y-TZP) and veneer indirect composite resins

Statement of problemIndirect composite resins (ICR) are promising alternatives as veneering materials for zirconia frameworks. The effects of core-to-dentin thickness ratio (C/Dtr) on the mechanical property of bilayered veneer ICR/yttria-tetragonal zirconia polycrystalline (Y-TZP) core disks have not been previously studied. PurposeThe purpose of this in vitro study was to assess the effects of C/Dtr on the biaxial flexural strength, reliability, and fracture mode of bilayered veneer ICR/ Y-TZP core disks. Material and methodsA total of 180 bilayered 0.6-mm-thick composite resin disks in core material and C/Dtr of 2:1, 1:1, and 1:2 were tested with either core material placed up or placed down for piston-on-3-ball biaxial flexural strength. The mean biaxial flexural strength, Weibull modulus, and fracture mode were measured to evaluate the variation trend of the biaxial flexural strength, reliability, and fracture mode of the bilayered disks with various C/Dtr. One-way analysis of variance (ANOVA) and chi-square tests were used to evaluate the variation tendency of fracture mode with the C/Dtr or material placed down during testing (α=.05). Light microscopy was used to identify the fracture mode. ResultsThe mean biaxial flexural strength and reliability improved with the increase in C/Dtr when specimens were tested with the core material either up and down, and depended on the materials that were placed down during testing. The rates of delamination, Hertzian cone cracks, subcritical radial cracks, and number of fracture fragments partially depended on the C/Dtr and the materials that were placed down during testing. ConclusionThe biaxial flexural strength, reliability, and fracture mode in bilayered structures of Y-TZP core and veneer ICR depend on both the C/Dtr and the material that was placed down during testing.

The impact of endodontic access on the biaxial flexure strength of dentine-bonded crown substrates - an invitro study.

To investigate how preparation of a simulated access cavity into ceramic materials suitable for the manufacture of dentine-bonded crowns (DBCs) impacted on biaxial flexural strength (BFS) determined as a monolithic structure and in a more clinically representative resin-cemented form. One hundred and twenty feldspathic and 120 leucite-reinforced ceramic disc-shaped specimens were divided into eight groups (n=30). All groups received 'fit' surface treatments representative of pre-cementation modifications and of cementation prior to preparation of a representative endodontic access cavity through the sample. BFS was determined for both 'intact' and the 'annular' disc-shaped specimens which had received simulated endodontic access. Newly reported analytical solutions were used to calculate BFS of the 'annular' specimens. Statistical analysis included two-way anovas (α =0.05) and Weibull analysis. Fractographic examination provided insight into the fracture mechanisms. A two-way anova identified a significant impact of material (P<0.01) and of resin coating (P<0.01) on the mean BFS of intact specimens. For the annular ceramic specimens, the substrate material significantly impacted on mean BFS (P<0.01), but the effect of resin coating was dependent on the substrate type (P<0.01). Endodontic access cavity preparation modified the critical defect population of the all-ceramic restorative materials investigated. The strength of a predominantly glassy ceramic following endodontic access can be maintained if adhesive cementation was used; however, the beneficial effects of adhesive cementation on ceramic reinforcement were lost on leucite-reinforced ceramics following access cavity preparation. Replacement restoration for these materials would be recommended clinically following endodontic access as opposed to repair of the access cavity using a direct restorative material.

Evaluation of a novel multiple phase veneering ceramic

ObjectivesTo produce a new veneering ceramic based on the production of a multiple phase glass–ceramic with improved performance in terms of strength and toughness. MethodsA composition of 60% leucite, 20% diopside and 20% feldspathic glass was prepared, blended and a heat treatment schedule of 930°C for 5min was derived from differential thermal analysis (DTA) of the glasses. X-ray diffraction (XRD) and SEM analysis determined the crystalline phases and microstructure. Chemical solubility, biaxial flexural strength (BFS), fracture toughness, hardness, total transmittance and coefficient of thermal expansion (CTE) were all measured in comparison to a commercial veneering ceramic (VITA VM9). Thermal shock resistance of the leucite–diopside and VITA VM9 veneered onto a commercial high strength zirconia (Vita In-Ceram YZ) was also assessed. Statistical analysis was undertaken using Independent Samples t-test. Weibull analysis was employed to examine the reliability of the strength data. ResultsThe mean chemical solubility was 6μg/cm2 for both ceramics (P=1.00). The mean BFS was 109±8MPa for leucite–diopside ceramic and 79±11MPa for VITA VM9 ceramic (P=0.01). Similarly, the leucite–diopside ceramic demonstrated a significantly higher fracture toughness and hardness. The average total transmittance was 46.3% for leucite–diopside ceramic and 39.8% for VITA VM9 (P=0.01). The leucite–diopside outperformed the VITA VM9 in terms of thermal shock resistance.Significance This novel veneering ceramic exhibits significant improvements in terms of mechanical properties, yet retains a high translucency and is the most appropriate choice as a veneering ceramic for a zirconia base core material.

Laminated ceramics with elastic interfaces: A mechanical advantage?

ObjectivesAs CAD/CAM technologies improve we question whether adhesive lamination of ceramic materials could offer mechanical advantages over monolithic structures and improve clinical outcomes. The aim was to identify whether an adhesive interface (a chemically cured resin–cement) would influence the biaxial flexure strength (BFS) and slow-crack growth in a machinable dental ceramic. MethodsMonolithic and adhesively laminated (with a chemically cured dimethacrylate resin–cement) feldspathic ceramic discs of identical dimensions were fabricated. BFS testing was performed on the Group A monolithic specimens (n=20), on Group B laminated specimens with the adhesive interface positioned below the neutral bending axis (n=20) and Group C laminated specimens with the adhesive interface positioned above the neutral bending axis (n=20). To study subcritical crack growth additional laminated specimens received controlled indentations and were exposed to thermo-mechanical fatigue. BFS data was analysed using parametric statistics (α=0.05). Fractographic analyses were qualitatively assessed. ResultsNo significant differences between the mean BFS data of Groups A and B were observed (p=0.92) but the mean BFS of Group C was slightly reduced (p<0.01). Lamination reduced the stiffness of the structure and fractographic analysis demonstrated that energy consuming crack deflection occurred. Thermo-mechanical fatigue caused subcritical extension of radial cracks associated with indentations adjacent to the adhesive interface. Crack growth was limited to parallel to the interface and was arrested or deflected in a direction normal to the interface. ConclusionsCeramic lamination increased the damage tolerance of the structure and could limit or arrest subcritical crack growth at regions near the ‘interlayer’. Clinical SignificanceLamination of a dental ceramic with a polymeric ‘interlayer’ could offer toughening effects which could potentially delay or arrest sub-critical crack growth at regions near the interface and thereby improve restoration longevity.