Impact of thermal misfit on shear strength of veneering ceramic/zirconia composites

ObjectivesThermal misfit is discussed as one reason for chipping of veneered zirconia restorations. The aim of the investigation was to assess the effect of thermal misfit on the shear strength of zirconia/veneering ceramic composites.MethodsShear strengths of 12 different veneering ceramic/zirconia composites were measured (n = 10). The veneering ceramics were fired onto polished Y-TZP. In order to create a strong thermal mismatch, one of the veneering ceramics was intended for use on alumina and one for the metal–ceramic technique. The glass transition temperatures of the veneering ceramics and the coefficients of thermal expansion of all ceramics were measured (n = 6). Statistical analysis was performed with one-way ANOVA and a post hoc Bonferroni test (p < 0.05).ResultsShear strength ranged from 21.9 ± 6.2 to 31.0 ± 7.1 MPa. The ceramic for the metal–ceramic technique showed spontaneous debonding. The differences in the coefficients of thermal expansion of core and veneer (Δα) were calculated. In addition the differences between glass transition temperatures of the veneering ceramics and room temperature (ΔT) as the effective temperature range for stress formation were calculated. Highest shear strength was observed when ΔαΔT ≈ 1000 × 10^?6.ConclusionsThermal expansion and glass transition temperature of the veneering ceramic have an impact on the shear strength of veneer/zirconia composites.     

Thermo and mechanical cycling and veneering method do not influence Y-TZP core/veneer interface bond strength

ObjectivesThe purpose of this study was to evaluate the influence of thermal and mechanical cycling and veneering technique on the shear bond strength of Y-TZP (yttrium oxide partially stabilized tetragonal zirconia polycrystal) core–veneer interfaces.Materials and methodsCylindrical Y-TZP specimens were veneered either by layering (n = 20) or by pressing technique (n = 20). A metal ceramic group (CoCr) was used as control (n = 20). Ten specimens for each group were thermal and mechanical cycled and then all samples were subjected to shear bond strength in a universal testing machine with a 0.5 mm/min crosshead speed. Mean shear bond strength (MPa) was analysed with a 2-way analysis of variance and Tukey's test (p < 0.05). Failure mode was determined using stereomicroscopy and scanning electron microscopy (SEM).ResultsThermal and mechanical cycling had no influence on the shear bond strength for all groups. The CoCr group presented the highest bond strength value (p < 0.05) (34.72 ± 7.05 MPa). There was no significant difference between Y-TZP veneered by layering (22.46 ± 2.08 MPa) or pressing (23.58 ± 2.1 MPa) technique. Failure modes were predominantly adhesive for CoCr group, and cohesive within veneer for Y-TZP groups.ConclusionsThermal and mechanical cycling, as well as the veneering technique does not affect Y-TZP core–veneer bond strength.Clinical significanceDifferent methods of veneering Y-TZP restorations would not influence the clinical performance of the core/veneer interfaces.     

Making yttria-stabilized tetragonal zirconia translucent

ObjectiveThe aim of this study was to provide a design guideline for developing tetragonal yttria-stabilized zirconia with improved translucency.MethodsThe translucency, the in-line transmission in particular, of 3 mol.% yttria-stabilized tetragonal zirconia (3Y-TZP) has been examined using the Rayleigh scattering model. The theory predicts that the in-line transmission of 3Y-TZP can be related to its thickness with grain size and birefringence the governing parameters. To achieve a threshold value of translucency, the critical grain size of 3Y-TZP was predicted for various thicknesses (0.3–2.0 mm). The threshold value was defined by a measured average in-line transmission value of a suite of dental porcelains with a common thickness of 1 mm. Our theoretical predictions were calibrated with one of the very few experimental data available in the literature.ResultsFor a dense, high-purity zirconia, its in-line transmission increased with decreasing grain size and thickness. To achieve a translucency similar to that of dental porcelains, a nanocyrstalline 3Y-TZP structure was necessitated, due primarily to its large birefringence and high refractive index. Such a grain size dependence became more pronounced as the 3Y-TZP thickness increased. For example, at a thickness of 1.3 mm, the mean grain size of a translucent 3Y-TZP should be 82 nm. At 1.5 mm and 2 mm thicknesses, the mean grain size needed to be 77 nm and 70 nm, respectively.SignificanceA promising future for zirconia restorations, with combined translucency and mechanical properties, can be realized by reducing its grain size.     

The mechanical properties of nanofilled resin-based composites: Characterizing discrete filler particles and agglomerates using a micromanipulation technique

ObjectiveTo assess the mechanical properties of discrete filler particles representative of several inorganic fillers in modern dental resin-based composites (RBCs) and to assess the validity of a novel micromanipulation technique.MethodRBCs with microhybrid (Filtek? Z250), ‘nanohybrid’ (Grandio) and ‘nanofilled’ (Filtek? Supreme), filler particle morphologies were investigated. Filler particles were provided by the manufacturer or separated from the unpolymerized resin using a dissolution technique. Filler particles (n = 30) were subjected to compression using a micromanipulation technique between a descending glass probe and a glass slide. The number of distinct fractures particles underwent was determined from force/displacement and stress/deformation curves and the force at fracture and pseudo-modulus of stress was calculated.ResultsAgglomerated fillers (‘nanoclusters’) exhibited up to four distinct fractures, while spheroidal and irregular particles underwent either a single fracture or did not fracture following micromanipulation. Z-tests highlighted failure of nanoclusters to be significant compared with spheroidal and irregular particles (P < 0.05). The mean force at first fracture of the nanoclusters was greater (1702 ± 909 μN) than spheroidal and irregular particles (1389 ± 1342 and 1356 ± 1093 μN, respectively). Likewise, the initial pseudo-modulus of stress of nanoclusters (797 ± 555 MPa) was also greater than spheroidal (587 ± 439 MPa) or irregular (552 ± 275 MPa) fillers.SignificanceThe validity of employing the micromanipulation technique to determine the mechanical properties of filler particulates was established. The ‘nanoclusters’ exhibited a greater tendency to multiple fractures compared with conventional fillers and possessed a comparatively higher variability of pseudo-modulus and load prior to and at fracture, which may modify the damage tolerance of the overall RBC system.     

Impact of high-speed sintering,layer thickness and artificial aging on the fracture load and two-body wear of zirconia crowns

ObjectiveTo investigate the impact of high-speed sintering, layer thickness and artificial aging in a chewing simulator on the fracture load (FL) and two-body wear (2BW) of 4Y-TZP crowns.Methods4Y-TZP crowns (Ceramill Zolid HT+, Amann Girrbach AG) in three different layer thicknesses (0.5, 1.0, 1.5; N = 192, n = 64/group) were manufactured using CAD/CAM technology and sintered at 1580 °C (high-speed sintering) or 1450 °C (control group). Specimens were polished in two-steps and bonded to standardized CoCr abutments with Multilink Automix (Ivoclar Vivadent). 2BW after 6000 thermo- and 1,200,000 chewing-cycles employing enamel antagonists was determined using best fit machining. FL was tested before and after artificial aging. Univariate ANOVAs, post hoc Scheffé, unpaired t-, Kruskal–Wallis- and Mann–Whitney-U-test were computed (p < 0.05).ResultsHigh-speed sintering resulted in less 2BW of the zirconia than the control group (p = 0.013). High-speed sintering (p = 0.001–0.006) and an increase in layer thickness (p < 0.001–0.012) resulted in higher FL values, while artificial aging led to a reduction of FL (p < 0.001).SignificanceAs high-speed sintering resulted in less two-body wear of the zirconia and comparable or even higher fracture load results than the control group, this cost- and time efficient alternative presents promising mechanical results.     

Effect of thermal expansion mismatch on the Y-TZP/veneer interfacial adhesion determined by strain energy release rate

PurposesThe aim of this study was to assess the effect of differences in the thermal expansion behaviour of veneering ceramics on the adhesion to Y-TZP, using a fracture mechanics approach.MethodsSeven veneering ceramics (VM7, VM9, VM13, Lava Ceram, Zirox, Triceram, Allux) and one Y-TZP ceramic were investigated. Thermal expansion coefficients and glass transition temperatures were determined to calculate residual stresses (σ_R, MPa) between core and veneer. Subsequently, the veneering ceramics were fired onto rectangular shaped zirconia specimens, ground flat and notched on the veneering porcelain side. Then specimens were loaded in a four-point bending test and load-displacement curves were recorded. The critical load to induce stable crack extension at the adhesion interface was evaluated to calculate the strain energy release rate (G, J/m²) for each system.ResultsResidual stresses ranged from ?48.3 ± 1.5 MPa (VM7) to 36.1 ± 4.8 MPa (VM13) with significant differences between all groups (p < 0.05). The strain energy release rate of the Y-TZP/veneer specimens ranged from 8.2 ± 1.7 J/m² (Lava Ceram) to 17.1 ± 2.8 J/m² (VM9). Values for G could not be obtained with the VM7, Allux and VM13 specimens, due to spontaneous debonding or unstable crack growth. Except for Triceram and Zirox specimens, strain energy release rate was significantly different between all groups (p < 0.05).ConclusionThermal residual stresses and strain energy release rates were correlated. Slight compressive stresses in the region of ?20 MPa were beneficial for the Y-TZP/veneer interfacial adhesion. Stresses higher or lower than this value exhibited decreased adhesion.     

Effect of sintering time on biaxial strength of zirconium dioxide

ObjectivesAim of this study was to evaluate effect of sintering time on mechanical properties of yttrium partially stabilized zirconia (Y-TZP) (ICE Zirkon, ZirkonZahn, Italy).MethodsFifty-six zirconia discs were divided into two groups. Discs of the first group were sintered in sintering oven (ZirkonZahn) at 20–1500 °C temperature using rise time of 3 h and kept at 1500 °C for 2 h. Discs of the second group were sintered at 20–1500 °C using rise time of 1 h 40 min and kept at 1500 °C for 1 h. Half of the discs (n = 15) from both groups were thermocycled in distilled water for 20 000 cycles (5–55 °C). Biaxial flexural strength of the discs (diameter 19.0 mm, thickness 1.6 mm) were measured dry at room temperature. Surface microhardness (VHN) was also measured. X-Ray diffraction analysis was performed to evaluate the ratio of tetragonal to monoclinic phase in thermocycled discs. The data was calculated using Weibull and ANOVA analysis.ResultsNo statistically significant difference (p > 0.05) was found between the groups in terms of sintering time or thermocycling. The biaxial flexural strength of the groups varied from 995 MPa to 1127 MPa. Surface microhardness varied from 1478 to 1532. The relative amount of the monoclinic phase was higher when zirconia was thermocycled and stored in water compared to control discs, which had no monoclinic phase at all.SignificanceVariation in the sintering time from 1.6 h to 3.0 h did not influence mechanical properties of Y-TZP zirconia.     

Off-axis sliding contact reliability and failure modes of veneered alumina and zirconia

ObjectiveAll-ceramic dental crowns are popular because of their esthetics and biocompatibility. However, they often chip or fracture when subjected to repeated occlusal loading. Considerable efforts to improve the materials are being done through the study of fatigue and failure modes. The vast majority of fatigue studies have been conducted with uniaxial loading and no sliding action. We hypothesized different failure modes for porcelain veneered Y-TZP and that the reliability of porcelain veneered Y-TZP is higher than that of porcelain veneered alumina when subjected to fatigue under 30° off-axis sliding Y-TZP and alumina plates were porcelain veneered and cemented to aged composite blocks as a model for an all-ceramic crown on dentin.MethodsSpecimens (n = 21 per group) were fatigue at 30° off-axis with a hard sphere sliding contact in water, by means of a mouth-motion simulator apparatus.ResultsAlthough no difference between groups was found, the failure modes differed and there was a tendency to higher reliability for Y-TZP compared to alumina for a mission of 50,000 cycles at 150 N load.SignificanceFailure modes for alumina specimens were deep penetrating partial cone cracks and cementation internal surface radial cracks. Y-TZP specimens showed only surface damage with deep penetrating partial cone cracks extending to the veneer core interface, with no cementation surface radial cracking, which overall agrees with clinical finding. Angled sliding contact appears to better simulate oral function.     

Effect of finishing/polishing techniques and low temperature degradation on the surface topography,phase transformation and flexural strength of ultra-translucent ZrO2 ceramic

ObjectiveTo investigate the effect of different surface finishing and polishing regimes and low temperature degradation on flexural strength, phase transformation and surface topography of ultra-translucent ZrO₂ ceramic.Methods300 (n = 15/group) of conventional zirconia (Z: Ice Zirkon Transluzent) and ultra-translucent zirconia (UT: Prettau Anterior) bar-specimens were made and divided according to the "Finishing/Polishing" - (C — Control, B — diamond rubber polishers, P — adjusting with burs, PB — adjusting with burs + diamond polishers, PG — adjusting with burs + glaze), "Low temperature Degradation (LTD)" (with or without a treatment at 127 °C, 1.7 bar/24 h). Then, a 3-point mini flexural test was performed in a universal testing machine (1 mm/min, 500 kgf load cell). SEM, EDS, XDR, AFM, optical profilometry and Weibull analysis were performed. Data were analyzed by 3-way ANOVA and Tukey’s post-test (5%).ResultsGroups ZPB_D (1670 ± 253 MPa), ZB_D (1664 ± 217 MPa), and ZB (1655 ± 3678 MPa) showed significantly higher flexural strength than the UTPG group (372 ± 56 MPa). The Weibull modulus was significantly higher for the ZP_D group compared to the UB, UC_D, UP_D and UPB_D, while UTB, UTC_D and UTP_D had the lowest value. Monoclinic phases were observed only in the conventional zirconia groups and were more evident after LTD. Diamond rubber polishers presented less roughness for both zirconias.SignificanceThe use of diamond rubber polishers is the most suitable finishing/polishing method for zirconia ceramic restorations and that final glazing reduces the fracture resistance of these materials.     

Fatigue testing of two porcelain–zirconia all-ceramic crown systems

ObjectiveTo evaluate the mouth-motion step-stress fatigue behavior of two porcelain–zirconia all-ceramic crown systems.MethodsThe average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modeled by reducing proximal walls by 1.5 mm and occlusal surface by 2.0 mm. The CAD-based tooth preparation was made by rapid prototyping and used as a master die to fabricate all-ceramic crowns with 1.0 mm porcelain veneered on 0.5 mm Y-TZP cores (LAVA veneer + LAVA frame, 3M/ESPE, and Vita veneer + CERCON frame, Dentsply). Crowns were cemented on aged (60 days in water) composite (Z100, 3M/ESPE) reproductions of the die. Three crowns from the LAVA group were subjected to single cycle load to failure for stress profile design; remainder subjected to step-stress mouth-motion fatigue (three step-stress profiles). All mechanical testing was performed by sliding a WC indenter of 6.25 mm diameter 0.7 mm lingually down the mesio-distal cusp. Master Weibull curves and reliability for missions of 50,000 cycles at 200 N load were calculated (Alta Pro 7, Reliasoft).ResultsSingle load to failure showed fractures through the zirconia core. Reliability for a 200 N × 50K cycle mission was not significantly different between systems. In fatigue, failure occurred by formation of large chips within the veneer originating from the contact area without core exposure.ConclusionsLAVA and CERCON ceramic systems present similar fatigue behavior; fatigue loading of both systems reproduces clinically observed failure modes.     

Fracture strength of CAD/CAM composite and composite-ceramic occlusal veneers

PurposeTo determine the effect of material type and restoration thickness on the fracture strength of posterior occlusal veneers made from computer-milled composite (Paradigm MZ100) and composite-ceramic (Lava Ultimate) materials.Methods60 maxillary molars were prepared and restored with CAD/CAM occlusal veneer restorations fabricated from either Paradigm MZ100 or Lava Ultimate blocks at minimal occlusal thicknesses of 0.3, 0.6, and 1.0 mm. Restorations were adhesively bonded and subjected to vertical compressive loading. The maximum force at fracture and mode of failure were recorded. 2-Way ANOVA was used to identify any statistically significant relationships between fracture strength and material type or thickness. Spearman's rank correlation coefficient was used to analyze mode of failure with regard to fracture strength.ResultsThe average maximum loads (N) at fracture for the Paradigm MZ100 groups were 1620 ± 433, 1830 ± 501, and 2027 ± 704 for the material thicknesses of 0.3, 0.6, and 1.0 mm, respectively. The Lava Ultimate groups fractured at slightly higher loads (N) of 2078 ± 605, 2141 ± 473, and 2115 ± 462 at the respective 0.3, 0.6, and 1.0 mm thickness.Statistical analyses revealed that, while no significant difference existed among the various restoration thicknesses in terms of fracture strength (P > 0.05), the material type was found to be influential (P = 0.04). The maximum load at fracture (N) for Lava Ultimate averaged over all thicknesses (2111 ± 500) was significantly higher than that of the Paradigm MZ100 (1826 ± 564). No correlation between mode of failure and fracture strength was found.ConclusionsUnder the conditions of this study, the maximal loads at fracture for these “non-ceramic” occlusal veneer restorations were found to be higher than human masticatory forces. Occlusal veneers made from the two materials tested are likely to survive occlusal forces regardless of restoration thickness, with those fabricated from the composite-ceramic hybrid material being more likely to survive heavier loads.     

Effect of loading conditions on the fracture toughness of zirconia

PurposeA Vickers hardness indenter was pressed into yttria-stabilized zirconia (Y-TZP) by the indentation fracture method (IF method).MethodsThe effect on the calculated Vickers hardness, fracture toughness values, and indentation fracture load (9.8, 49, 98, 196, and 294 N) was examined to deduce the optimum conditions of the IF method. Calculated Vickers hardness and fracture toughness values were analyzed with one-way analysis of variance and then multiple comparisons (Scheffe). The appearance of on indentation and cracks was also evaluated using a scanning electron microscopy (SEM).ResultsIndentation of Y-TZP was generated by 9.8 and 49 N of indentation fracture load, however cracks could not be confirmed with the microscope attached to the Vickers hardness tester. Both indentation and cracks were observed at 98, 196 and 294 N of indentation fracture load obtained values of 7.1 and 6.8 MPam^1/2. Cracks noted at the 98 N were not clear, whereas the 196 and 294 N showed especially clear cracks. Due to the hardness of zirconia and the light loads, fracture toughness values for 9.8, 49, and 98 N could not be calculated. There was no significant difference between 196 and 294 N, when calculated fracture toughness values were analyzed with multiple comparisons. SEM revealed clear indentation and cracks, that extended linearly, but no chips or fractures were observed. Surface changes were observed at 196 and 294 N that are presumed to be accompanied by phase transition around the cracks.ConclusionsOptimum experimental conditions of the indentation fracture load in the IF method were determined as 196 and 294 N.     

Surface characterisation and bonding of Y-TZP following non-thermal plasma treatment

Objectives(1) To chemically characterise Y-TZP surface via X-ray photoelectron spectroscopy (XPS) and evaluate the surface energy levels (SE) after non-thermal plasma (NTP). (2) To test the microtensile bond strength (MTBS) of Y-TZP bonded to cured composite disks, after a combination of different surface conditioning methods.MethodsTwenty-four Y-TZP discs (13.5 mm × 4 mm) were obtained from the manufacturer and composite resin (Z-100) discs with similar dimensions were prepared. All discs were polished to 600 grit and divided into 8 groups (n = 3 disks each), four control (non-NTP treated) and four experimental (NTP treated for 10 s) groups. All groups received one of the four following treatments prior to cementation with Rely × Unicem cement: sand-blasting (SB), a Clearfil ceramic primer (MDP), sand-blasting + MDP (SBMDP), or baseline (B), no treatment. SE readings and surface roughness parameters were statistically analysed (ANOVA, Tukey's, p < 0.05). Mixed model and paired samples t-tests were used to compare groups on MTBS.ResultsXPS showed increase in O and decrease in C elements after NTP. The polar component increased for BP (42.20 mN/m) and SBP (43.77 mN/m). MTBS values for groups BP (21.3 MPa), SBP (31 MPa), MDPP (30.1 MPa) and SBMDPP (32.3 MPa) were significantly higher in specimens treated with NTP than their untreated counterparts B (9.1 MPa), SB (14.4 MPa), MDP (17.8 MPa) and SBMDP (24.1 MPa).Conclusions(1) Increase of O and decrease of C led to higher surface energy levels dictated by the polar component after NTP; (2) NTP application increased MTBS values of Y-TZP surfaces.     

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.     

Sliding contact wear and subsurface damage of CAD/CAM materials against zirconia

ObjectiveMost previous work conducted on the wear behavior of dental materials has focused on wear rates and surface damage. There is, however, scarce information regarding the subsurface damage arising from sliding contact fatigue. The aim of this study was to elucidate the wear mechanisms and the subsurface damage generated during sliding contact fatigue in 5 contemporary CAD/CAM materials against a zirconia indenter.MethodsForty discs (Ø12 mm, 1.55 mm thick) were cut out of IPS e.max CAD (e.CAD), Suprinity PC (SUP), Enamic (ENA), Vitablocs Mark II (VMII) and Lava Ultimate (LU) blocks and mirror polished. After cementation onto a dentin-like composite, off-axis mouth-motion cycling was conducted with a spherical zirconia indenter (r = 3.18 mm) in water (200 N load, 2 Hz frequency) for 5 different cycling periods (10², 10³, 10⁴, 10⁵, 10⁶ cycles, n = 8). Analysis of the wear scars was conducted using light-microscopy, scanning-electron-microscopy and optical profilometry. Subsurface damage was assessed using sagittal and transverse sections of the samples.ResultsFatigue wear mechanisms predominated in glassy materials (e.CAD, SUP, VMII), accompanied by extensive subsurface damage, whereas abrasive wear mechanisms were responsible for the large wear craters in the resin composite (LU) with an absolute absence of subsurface fracture. A combination of both mechanisms was observed in the polymer-infiltrated reinforced-glass (ENA), displaying large wear craters and severe subsurface damage.SignificanceWell-controlled laboratory simulation can identify wear and subsurface damage susceptibility of various classes of restorative materials. Both wear and subsurface fracture are determining factors for the long-term success of restorations.     

Effect of in vitro aging on the flexural strength and probability to fracture of Y-TZP zirconia ceramics for all-ceramic restorations

ObjectivesDental zirconia restorations should present long-term clinical survival and be in service within the oral environment for many years. However, low temperature degradation could affect their mechanical properties and survival. The aim of this study was to investigate the effect of in vitro aging on the flexural strength of yttrium-stabilized (Y-TZP) zirconia ceramics for ceramic restorations.MethodsOne hundred twenty bar-shaped specimens were prepared from two ceramics (ZENO Zr (WI) and IPS e.max^® ZirCAD (IV)), and loaded until fracture according to ISO 6872. The specimens from each ceramic (n_x = 60) were divided in three groups (control, aged for 5 h, aged for 10 h). One-way ANOVA was used to assess statistically significant differences among flexural strength values (P < 0.05). The variability of the flexural strength values was analyzed using the two-parameter Weibull distribution function, which was applied for the estimation of Weibull modulus (m) and characteristic strength (σ₀). The crystalline phase polymorphs of the materials (tetragonal, t, and monoclinic, m, zirconia) were investigated by X-ray diffraction (XRD) analysis, Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy.ResultsA slight increase of the flexural strength after 5 h, and a decrease after 10 h of aging, was recorded for both ceramics, however statistically significant was for the WI group (P < 0.05). Both ceramics presented a t → m phase transformation, with the m-phase increasing from 4 to 5% at 5 h to around 15% after 10 h.SignificanceThe significant reduction of the flexural strength after 10 h of in vitro aging, suggests high fracture probability for one of the zirconia ceramics tested.     

Effects of aging procedures on the topographic surface,structural stability,and mechanical strength of a ZrO2-based dental ceramic

ObjectiveTo determine the effects of different aging methods on the degradation and flexural strength of yttria-stabilized tetragonal zirconia (Y-TZP)MethodsSixty disc-shaped specimens (∅, 12 mm; thickness, 1.6 mm) of zirconia (Vita InCeram 2000 YZ Cubes, VITA Zahnfabrik) were prepared (ISO 6872) and randomly divided into five groups, according to the aging procedures (n = 10): (C) control; (M) mechanical cycling (15,000,000 cycles/3.8 Hz/200 N); (T) thermal cycling (6,000 cycles/5–55 °C/30 s); (TM) thermomechanical cycling (1,200,000 cycles/3.8 Hz/200 N with temperature range from 5 °C to 55 °C for 60 s each); (AUT) 12 h in autoclave at 134 °C/2 bars; and (STO) storage in distilled water (37 °C/400 days). After the aging procedures, the monoclinic phase percentages were evaluated by X-ray diffraction (XRD), and topographic surface analysis was performed by 3D profilometry. The specimens were then subjected to biaxial flexure testing (1 mm/min, load 100 kgf, in water). The biaxial flexural strength data (MPa) were analyzed by 1-way ANOVA and Tukey's test (α = 0.05). The data for monoclinic phase percentage and profilometry (Ra) were analyzed by Kruskal–Wallis and Dunn's tests.ResultsANOVA revealed that flexural strength was affected by the aging procedures (p = 0.002). The M (781.6 MPa) and TM (771.3 MPa) groups presented lower values of flexural strength than did C (955 MPa), AUT (955.8 MPa), T (960.8 MPa) and STO (910.4 MPa). The monoclinic phase percentage was significantly higher only for STO (12.22%) and AUT (29.97%) when compared with that of the control group (Kruskal–Wallis test, p = 0.004). In addition, the surface roughnesses were similar among the groups (p = 0.165).SignificanceWater storage for 400 days and autoclave aging procedures induced higher phase transformation from tetragonal to monoclinic; however, they did not affect the flexural strength of Y-TZP ceramic, which decreased only after mechanical and thermomechanical cycling.     

Only severe malocclusion correlates with mastication deficiency

ObjectivesThe relation between level of dentofacial deformity and extent of masticatory deficiency was studied.DesignThree groups of human young adults were formed: (i) subjects needing orthodontics plus orthognathic surgery (SevDFD, n = 18), (ii) subjects needing orthodontic treatment only (ModDFD, n = 12), and (iii) subjects needing no treatment (NoDFD, n = 12). For mastication tests, carrot boluses were collected at the deglutition time. Bolus particle size range was expressed as d50 value, which was compared with the Masticatory Normative Indicator (MNI). Index of treatment need (IOTN), global oral health assessment index (GOHAI) and chewing kinematic characteristics were also recorded. We used a general linear model univariate procedure followed by a Student-Newman-Keuls test.ResultsAll the SevDFD subjects showed impaired mastication with MNI above the normal limit (d50 mean = 7.23 mm). All the ModDFD subjects but one were below this limit (d50 mean = 2.54 mm), and so could adapt to a low level of masticatory impairment as also indicated by kinematics. IOTN indicated a treatment need for ModDFD (3.7 ± 0.5) and SevDFD (4.3 ± 0.6) groups, while GOHAI values were unsatisfactory only for SevDFD (42.6 ± 9.2 vs. 55.3 ± 1.9).ConclusionsOur findings emphasize the need for an objective evaluation of masticatory function to discern truly deficient mastication from mild impairment allowing satisfactory adaptation of the function. However, malocclusions are known to worsen with time justifying thus their corrections as early as possible.     

High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings — A new fabrication mode for all-ceramic restorations

ObjectivesWith this in vitro study the fracture strength of zirconia-based crown copings being veneered with a CAD/CAM generated high-strength ceramic cap by sintering is compared with anatomically identical zirconia-based crowns, which were either overpressed or veneered by the layering technique for completion.MethodsA 1.2 mm, 360° chamfer preparation was performed on a second maxillary molar and was dublicated 15 times in a cobalt–chromium-alloy. A sample of 45 zirconia copings was produced and divided into three groups. In the first group (VT) zirconia copings received conventional veneering in layering technique, in the second group the veneering porcelain was pressed over the zirconia coping (PT), and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented conventionally onto their dies and tested in the universal testing machine until clinical failure. The fracture load data were compared by a one-way analysis of variance and a multiple comparison posthoc test (α < 0.05).ResultsSpecimens from group VT showed a mean (S.D.) fracture load of 3700.39 (1238.72) N, group OT 3523.73 (1181.11) N and group ST 6262.67 (2257.42) N. The difference between groups VT/OT and ST were statistically significant (P < 0.001).SignificanceThe new CAD/CAM-fabricated bilayered restorations (ST) were superior to the present techniques (VT and OT) in terms of fracture load and offer the possibility to produce cost-effective crowns and fixed partial dentures with a potential lower risk of chippings.     

Ultrashort-pulse laser as a surface treatment for bonding between zirconia and resin cement

ObjectivesTo evaluate ultrashort-pulse laser (UPL) as a surface treatment for improved bond strength to Yttria-tetragonal zirconia polycrystalline (Y-TZP).MethodsFully-sintered Y-TZP samples received either no treatment (CTL), or were treated by alumina blasting (ALB), tribochemical silica coating (SIL), or one of two UPL patterns: multiple pulses laser surface dots with 2.5 μm spacing (8 mJ, 10 kHz)(LSD); or single pulse laser surface lines with 2.5 μm spacing (4 mJ, 6.7 kHz)(LSL). Surface roughness, wettability (contact angle), and quantification of crystalline phases were evaluated for each group (n = 3/group). Y-TZP treated slabs were cemented to resin composite slabs using silane and 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing adhesive. Beams from the Y-TZP/resin blocks were microtensile tested (n = 5/group) after 48 h water incubation (37 °C) with or without subsequent thermocycling (5–55 °C, 5000 cycles).ResultsAll surface treatments increased surface roughness values versus control (P < 0.001). Contact angles were lowest for SIL (6.57 ± 2.37°) and highest for control (50.97 ± 6.30°). LSL and LSD were the only treatments that did not increase the relative monoclinic phase. All surface treatments significantly increased microtensile bond strengths (μTBS) compared with the control group (P < 0.001), with highest values for UPL (LSD: 35.40 ± 4.53 MPa > LSL: 31.84 ± 8.46 MPa > SIL: 19.95 ± 3.99 MPa = ALB: 19.51 ± 2.55 MPa > CTL: 14.51 ± 2.23 MPa). Thermocycling significantly reduced bond strength for all treatments in a surface treatment-dependent manner.SignificanceThe ability of UPL to alter Y-TZP surface morphology, increase wettability and μTBS without increasing the monoclinic content suggests its potential to improve bonding to the underlying resin cement and tooth without compromising the strength of the restoration.