Effect of glass-ceramic coating versus alumina air-abrasion on the bond strength and residual stress of zirconia.

OBJECTIVES: To assess the effect of glass-ceramic coated zirconia versus alumina air-abraded zirconia on the shear bond strength (SBS) of resin cement and investigate the residual stresses present on both mechanically pre-treated surfaces. MATERIALS AND METHODS: A total of 180 zirconia disks, with diameters of 10 mm and 5 mm, were divided into two groups: DCMhotbond glass-ceramic coated, followed by hydrofluoric acid etching (DCM), and alumina air-abraded (AB). All mechanically pre-treated disks were conditioned with G-Multi Primer and bonded using G-Cem Linkforce Cement. Ninety specimens were immersed in distilled water for 24 h and subsequently allocated into three groups based on aging conditions (n = 15/subgroups): immediate testing, 5000 thermal cycles, and 10,000 thermal cycles. Then, the shear bond strength was assessed, and the obtained data were subjected to analysis using a two-way ANOVA, followed by a one-way ANOVA and Tukey's HSD post hoc test (α = 0.05). The residual stresses present on both mechanically pre-treated surfaces were examined using X-ray diffraction analysis. RESULTS: The mean SBS values of the DCM and AB groups showed no significant difference under each aging condition. The SBS of DCM groups was not affected by thermal cycles, whereas the SBS of AB groups exhibited a significant decrease following thermal cycles. Glass-ceramic coated surfaces exhibited higher compressive stresses than alumina air-abrasion. CONCLUSIONS: The DCMhotbond glass-ceramic coated zirconia showed comparable bond strength to the alumina air-abrasion technique. CLINICAL RELEVANCE: The DCMhotbond glass-ceramic coating technique is a promising alternative for zirconia surface pre-treatment. However, further investigations are needed before suggesting its clinical use.

Effective luting agents for glass-fiber posts: A network meta-analysis.

OBJECTIVES: The aims of this study were to systematically review the literature and compare the relative effects of various luting agents on bonding between glass-fiber posts and root canal dentin in short- and long-term aging conditions. DATA/SOURCES: The literature was electronically searched in PubMed, Embase, and Scopus. A manual search was performed by scanning the reference lists of the included studies. STUDY SELECTION: Two reviewers independently conducted the selection of studies, data extraction, and risk of bias assessment. Pairwise meta-analyses were based on random effect models. Network meta-analyses were conducted within a frequentist framework with a multivariable random effects approach. The standardized mean difference and 95% confidence interval was calculated. RESULTS: One hundred and eighteen studies were included and assessed the effects of five luting agents. For pairwise meta-analyses, in short-term aging conditions, a significantly higher bond strength of self-adhesive resin cement (SARC) compared to etch-and-rinse adhesive composite resin core material (ERCM) in the total, coronal, and middle regions. In long-term aging conditions, a significantly higher bond strength of ERCM compared to etch-and-rinse adhesive resin cement (ERRC) in all regions. Furthermore, SARC showed a significantly higher bond strength compared to self-etch adhesive composite resin core material (SECM) in the total, middle, and apical regions. For network meta-analyses, in the apical region, a significantly higher bond strength of SARC compared to ERRC in both aging conditions. CONCLUSIONS: The SARC tended to be the most effective luting agent in bonding between glass-fiber posts and root canal dentin in short- and long-term aging conditions.

Influence of a Novel Lithium Disilicate Coating on Composite-Zirconia Bonding and Bond Characterization.

PURPOSE: To investigate microtensile bond strength and characterization with the novel lithium disilicate coating technique compared to conventional air abrasion. MATERIALS AND METHODS: Eight zirconia blocks were fabricated and assigned to two groups (n = 4 each): (1) Lithium disilicate coating followed by hydrofluoric acid etching and Monobond N Primer (LiDi group); and (2) alumina air abrasion (MUL group). For each group, two identically pretreated zirconia blocks were bonded together with Multilink Speed Cement and cut into 30 stick-shaped specimens (1 × 1 × 9 mm3). The 120 specimens were stored in water for 24 hours and assigned to one of three groups (n = 20/group): (1) short-term storage for 24 hours; (2) thermocycling for 5,000 cycles; and (3) thermocycling for 10,000 cycles. A microtensile bond strength test was performed and evaluated. The bond strength results were analyzed using two-way ANOVA followed by one-way ANOVA and Tukey HSD (α = .05). Energy-dispersive x-ray spectroscopy (EDS), Fourier-transform infrared (FTIR), x-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and scanning electron microscopy (SEM) were used for chemical, crystalline phase, and failure mode analyses. RESULTS: The MUL groups recorded higher bond strength than the LiDi groups. Thermocycling significantly decreased the bond strength in both groups. Chemical analyses suggested that the lithium disilicate layer underwent hydrolysis, which compromised long-term bond strength. CONCLUSION: The bond between composite cement and alumina-abraded zirconia performed better than that with the lithium disilicate coating technique. Int J Prosthodont 2023;36:172-180. doi: 10.11607/ijp.6744.

Cleaning methods of contaminated zirconia: A systematic review and meta-analysis.

OBJECTIVES: The aims of this study were to systematically review the literature and statistically analyze the effectiveness of different cleaning methods on the bond strength of resin cement to zirconia in short- and long-term aging conditions. DATA/SOURCES: The literature was electronically searched in PubMed (MEDLINE), EMBASE, Wiley, Scopus, and Open Access Theses and Dissertations databases to select relevant articles that evaluated the bond strength between contaminated zirconia and resin cements. A manual search was performed by scanning the reference lists of included studies. STUDY SELECTION: All articles were published online before April 2022 and in English. Meta-analyses were conducted using random effects models to calculate standardized mean differences (SMD) between uncontaminated zirconia and various cleaning methods in two aging conditions (short- and long-term). Statistical heterogeneity was assessed using I-square statistics. The risk of bias of all included studies was assessed. All statistical analyses were conducted using STATA (StataCorp, College Station, Texas). RESULTS: Of the 1181 studies, 25 studies met the inclusion criteria for qualitative analyses. In short-term aging condition, cleaning contaminated zirconia with water, alcohol, or acid etching reported significantly lower bond strength than uncontaminated zirconia. For long-term aging condition, cleaning contaminated zirconia with water, cleaning agents, alcohol, or acid etching reported significantly lower bond strength than uncontaminated zirconia. Alumina air-abrasion or cleaning with sodium hypochlorite were comparable to uncontaminated zirconia for both short- and long-term aging conditions. CONCLUSIONS: This meta-analysis appeared to indicate that the cleaning methods of contaminated zirconia restoration have an effect on zirconia bonding.

Comparison of different artificial landmarks and scanning patterns on the complete-arch implant intraoral digital scans.

OBJECTIVES: The purpose of this study was to compare the effects of four types of artificial landmarks and three different scanning patterns on the accuracy of complete-arch implant intraoral digital scans. METHODS: An edentulous mandibular model with 4 dental implants (Osstem) was prepared as the master reference model (MRM) and scanned with laboratory scanner (E4 Lab Scanner®). Then, the model was modified with four artificial landmarks: (i) CON- unmodified MRM, (ii) PIP- pressure-indicating paste brushed over the edentulous ridge, (iii) LD- liquid dam markers placed on the edentulous ridge, and (iv) FL- floss tied with pattern resin between the scan bodies. In each group, the modified model was scanned with three different scanning patterns: (i) LB- linguo-buccal pattern, (ii) SS- s-shaped pattern, and (iii) QU- quadrant pattern (n = 10/subgroup) using an intraoral scanner (Trios®4). Scans in STL format were exported and superimposed with MRM file using an inspection software (Geomagic Control X). Accuracy (trueness and precision) was evaluated by calculating the deviation, root mean square (RMS). Results were analyzed using two-way ANOVA, one-way ANOVA and Games-Howell (α = 0.05). RESULTS: Significant differences in accuracy values were found across different artificial landmarks and scanning patterns as the LD artificial landmark with QU pattern showed the highest accuracy. The lowest accuracy was recorded in CON with LB pattern, PIP artificial landmark with LB pattern, and FL artificial landmark with SS pattern. CONCLUSIONS: The artificial landmarks and scanning patterns had a significant effect on the accuracy of the complete-arch implant intraoral digital scans. CLINICAL SIGNIFICANCE: When performing complete-arch digital scans with four dental implants, clinicians should select proper artificial landmark and scanning pattern, as the artificial landmark and scanning pattern significantly affect the accuracy of the scan when using an intraoral scanner. ®.

Influence of ceramic-coating techniques on composite-zirconia bonding: Strain energy release rate evaluation.

OBJECTIVES: To compare ceramic-coating techniques versus conventional techniques on bonding between composite cements and zirconia by means of strain energy release rate (Gc, J/m2). METHODS: Two sizes of zirconia bars (30 mm × 8 mm × 1.5 mm and 14.8 mm × 8 mm × 1.5 mm) were fabricated. Two smaller bars were treated and cemented to the surface of a large bar using one of the following methods: (i) AlN-nano-structured alumina coating with RelyX Unicem 2; (ii) HOT-DCM hotbond coating with G-Multi Primer and G-Cem Linkforce; (iii) LiDi-lithium disilicate glass-ceramic coating with Monobond N Primer and Multilink Speed; (iv) COJ-tribochemical silica treatment with RelyX Ceramic Primer and RelyX Unicem 2; (v) GCEM-alumina grit blasting with G-Multi Primer and G-Cem LinkForce; (vi) MUL-alumina grit blasting with Multilink Speed; and (vii) PAN-alumina grit blasting with Clearfil Ceramic Primer and Panavia F2.0. A total of 30 bilayered specimens in each group were stored in distilled water at 37 °C for 24 h and assigned to three subgroups (n = 10/test group): short-term test, thermocycling for 5000 cycles, and thermocycling for 10,000 cycles and tested in 4-point bending configuration. Results were analysed using two-way ANOVA, followed by one-way ANOVA and Games-Howell (α = 0.05). Failure mode and surfaces were analysed using optical microscopy and SEM. RESULTS: The bonding (J/m2) of COJ and MUL groups was significantly higher than the other groups among all aging conditions. Thermocycling affected the bonding in COJ and GCEM groups. SIGNIFICANCE: Surface pre-treatments and artificial aging affect the bonding between composite cements and zirconia.

Shear Bond Strength of Composite Cement to Lithium-Disilicate Glass-coated Zirconia Versus Alumina Air-abraded Zirconia.

PURPOSE: To compare the shear bond strength of composite cement to lithium-disilicate glass-ceramic coated zirconia vs to alumina air-abraded zirconia and to analyze the residual stresses on both of lithium-disilicate glass-ceramic coated zirconia vs alumina air-abraded zirconia specimens. MATERIALS AND METHODS: One hundred eighty zirconia disks (diameters 10 mm and 5 mm, 4.5 mm thick) were divided into two groups: lithium-disilicate glass-ceramic coating followed by hydrofluoric acid etching and Monobond N Primer (LiDi) or alumina air-abrasion (AA). For each group, two different sizes of identically pre-treated zirconia specimens were bonded with Multilink Speed Cement. A total of 90 specimens were stored in distilled water at 37°C for 24 h and then assigned to three subgroups (n = 15/test group): 1. short-term test; 2. thermocycling for 5000 cycles; 3. thermocycling for 10,000 cycles. Bond strength was tested in shear mode and results were analyzed using two-way ANOVA, followed by one-way ANOVA and Tukey's HSD (α = 0.05). Failure mode and surfaces were analyzed with optical and scanning electron microscopy. X-ray diffraction was used to analyze t-m phase transformation and residual stresses on mechanically pre-treated LiDi and AA surfaces. RESULTS: The LiDi groups recorded higher mean bond strength than AA groups after thermocycling (p < 0.05). Thermocycling did not affect the bond strength of either LiDi or AA groups (p > 0.05). Most of specimens in AA groups exhibited mixed failure. Alumina air-abraded surfaces exhibited higher residual compressive stresses than did surfaces with a lithium-disilicate glass-ceramic coating. CONCLUSION: Following thermocycling, composite-zirconia bond strength of specimens with a lithium-disilicate glass-ceramic coating was greater than that of alumina air-abraded specimens.

Two-body wear test of enamel against laboratory polished and clinically adjusted zirconia.

AIM: A two-body wear test experiment was performed on human enamel, in simulated chewing motion, against non-veneered zirconia ceramic. Aim-1 was to ascertain the effect of zirconia roughness on enamel wear. Aim-2 was to ascertain the relative enamel wear between enamel-zirconia wear pair and enamel-enamel control pair. MATERIALS: Six molar and premolar human enamel cusps per group were used for a dental wear test against laboratory polished (LP) zirconia and laboratory polished and clinically adjusted (LP + CA) zirconia. Enamel antagonists were tested against incisor teeth as a control group to demonstrate laboratory enamel wear. METHODOLOGY: Two-body wear tests were conducted in a dual-axis biomimetic dental wear simulator. 49N loading force was used for 120,000 cycles with 1 mm lateral movement of the test specimen at 1.6Hz frequency, under constant ambient temperature water flow. Surface roughness before testing was determined using 3D profilometry. Loss of enamel height and volume i.e. vertical wear and volumetric wear respectively, were measured by superimposition of before and after testing scans by 3D laser scanning. Scanning electron microscopy was used for surface morphology assessment. One-way ANOVA and Post Hoc Multiple Comparisons with Bonferroni corrections were used at the 5% significance level to determine whether surface finish affected volumetric and vertical enamel loss. The relationship between volumetric and vertical loss of enamel was assessed using Pearson's correlation test. RESULTS: No significant difference was found between LP and LP + CA zirconia in vertical and volumetric enamel wear results. Control enamel had significantly higher vertical and volumetric enamel wear than LP and LP + CA zirconia. Pearson correlation revealed a strong relationship between vertical wear and volumetric wear of enamel. CONCLUSION: Within the constraints of the test method in this experiment, zirconia irrespective of surface preparation, was found to cause less vertical and volumetric enamel wear compared to control enamel. No statistically significant difference was seen between LP zirconia and LP + CA zirconia.

Shear bond strength of composite cement to alumina-coated versus tribochemical silica-treated zirconia.

OBJECTIVES: To compare the shear bond strength of composite-resin cement to nano-structured alumina-coated versus to tribochemical silica-treated zirconia, and to analyze the residual stresses on both of nano-structured alumina-coated versus tribochemical silica-treated zirconia specimens. METHODS: One hundred and eighty zirconia disks (10 mm and 5 mm in diameter, 5 mm thickness) were divided into two groups: nano-structured alumina coating (H.C.Starck, AlN) and tribochemical silica treatment (CoJet) followed by RelyX Ceramic Primer (COJ). For each group, two different sizes of identically pre-treated zirconia specimens were bonded together with RelyX Unicem 2 Cement. A total of 90 specimens were stored in distilled water at 37 °C for 24 h and then further assigned to three groups (n = 15/test group): short-term test, thermocycling for 5000 cycles, and thermocycling for 10,000 cycles. Bond strength was tested in shear mode and results were analyzed using two-way ANOVA, followed by one-way ANOVA and Tukey's HSD (α = 0.05). Failure mode and surfaces were analyzed with optical microscopy and SEM. X-ray diffraction (XRD) were used for t-m phase transformation and residual stress analysis on mechanically pre-treated AlN and COJ surfaces. RESULTS: The mean bond strengths of AlN and COJ groups were not statistically different after thermocycling (p > 0.05). However, when compare to 24 h only the bond strength of the COJ groups decreased significantly after thermocycling (p < 0.05). Most of specimens in both AlN and COJ groups exhibited adhesive failure. Compressive stresses were detected on both mechanically pre-treated AlN and COJ surfaces, with significant differences in stress values. CONCLUSION: Following thermocycling, composite-zirconia bond strength of nano-structured alumina coating was comparable to that of tribochemical silica treatment.

Effect of different implant configurations on biomechanical behavior of full-arch implant-supported mandibular monolithic zirconia fixed prostheses.

Mechanical failure of zirconia-based full-arch implant-supported fixed dental prostheses (FAFDPs) remains a critical issue in prosthetic dentistry. The option of full-arch implant treatment and the biomechanical behaviour within a sophisticated screw-retained prosthetic structure have stimulated considerable interest in fundamental and clinical research. This study aimed to analyse the biomechanical responses of zirconia-based FAFDPs with different implant configurations (numbers and distributions), thereby predicting the possible failure sites and the optimum configuration from biomechanical aspect by using finite element method (FEM). Five 3D finite element (FE) models were constructed with patient-specific heterogeneous material properties of mandibular bone. The results were reported using volume-averaged von-Mises stresses (σVMVA) to eliminate numerical singularities. It was found that wider placement of multi-unit copings was preferred as it reduces the cantilever effect on denture. Within the limited areas of implant insertion, the adoption of angled multi-unit abutments allowed the insertion of oblique implants in the bone and wider distribution of the multi-unit copings in the prosthesis, leading to lower stress concentration on both mandibular bone and prosthetic components. Increasing the number of supporting implants in a FAFDPs reduced loading on each implant, although it may not necessarily reduce the stress concentration in the most posterior locations significantly. Overall, the 6-implant configuration was a preferable configuration as it provided the most balanced mechanical performance in this patient-specific case.

Influence of glass-ceramic coating on composite zirconia bonding and its characterization.

OBJECTIVE: The aims of this study were to compare micro-tensile bond strength and characterize the bond of ceramic-coated versus air-abraded and chemically treated zirconia specimens. METHODS: Eight zirconia blocks were fabricated and assigned to two groups as follows: AA-alumina air-abrasion; and CC-DCMhotbond coating followed by alumina air-abrasion and hydrofluoric acid etching. For each group, two identically pre-treated zirconia blocks were applied G-Multi Primer, cemented together with G-Cem Linkforce cement and cut into 30 stick-shaped specimens (1×1×9mm3). A total of 120 specimens were stored in distilled water for 24h and then assigned to three groups: (i) short-term test, (ii) thermocycling for 5000, and (iii) thermocycling for 10,000 cycles. The specimens were tested in tensile mode. The bond strength results were analyzed using two-way ANOVA, followed by one-way ANOVA and Dunnett T3 (α=0.05). Failure mode and surfaces were analyzed with optical microscopy and SEM. The EDS, FTIR, XRD, and FIB-SEM were used for chemical, crystalline phase analyses. RESULTS: The AA groups recorded higher mean bond strength than the CC groups in all aging conditions. Thermocycling did not affect the bond strength of the AA groups, whereas the bond strength of the CC groups decreased significantly after aging. The MDP monomer and silane in G-Multi Primer chemically reacted with mechanically pre-treated AA and CC surfaces via the absorption of P-O and Si-O groups. SIGNIFICANCE: The bond strength of a conventional protocol involving alumina air-abrasion was greater than ceramic coating technique.

Influence of Nano-structured Alumina Coating on Composite-Zirconia Bonding and its Characterization.

PURPOSE: To compare microtensile bond strength and characterize the bond of nano-structured alumina-coated vs tribochemically silica-treated zirconia specimens. MATERIALS AND METHODS: Eight zirconia blocks were assigned to two groups: nano-structured alumina coating (AlN) and tribochemical silica treatment (CoJet) followed by RelyX Ceramic Primer (COJ). For each group, two identically pre-treated zirconia blocks were bonded with RelyX Unicem 2 Cement and cut into 30 stick-shaped specimens (1 x 1 x 9 mm3). A total of 120 specimens were stored in distilled water at 37°C for 24 h and then assigned to three groups (n = 20/test group): short-term test, thermocycling 5000 cycles, and thermocycling 10,000 cycles. The specimens were tested in tensile mode. The bond strength results were analyzed using two-way ANOVA, followed by one-way ANOVA and Tukey's HSD (α = 0.05). Failure mode and surfaces were analyzed with optical microscopy and SEM. FTIR and EDS were used for chemical analyses on primer-, mechanically and/or chemically pre-treated surfaces. RESULTS: The mean bond strengths of AlN and COJ groups were not statistically significantly different in all aging conditions (p > 0.05). Thermocycling significantly decreased the bond strength of both groups (p < 0.01). The AlN groups exhibited predominantly either adhesive or mixed failure, whereas the specimens in the COJ groups mainly presented either mixed or cohesive failure in composite cement. Silane chemically reacted with mechanically pre-treated COJ surface via the absorption of Si-O group. CONCLUSION: The composite-zirconia bond strength after application of a nano-structured alumina coating was comparable to that after tribochemical silica treatment.

Bonding of composite cements to zirconia: A systematic review and meta-analysis of in vitro studies.

OBJECTIVES: The aim of this study was to systematically review the literature and statistically analyze bond strength data to identify the influence that composite cements, type of test methodology, chemical and mechanical pre-treatments have on the bond strength of composite cements to zirconia in three different artificial aging conditions. METHODS: The literature was electronically searched in MEDLINE, PUBMED, EMBASE, and SCOPUS to select relevant articles that evaluated the bond strength between zirconia and composite cements. A manual search was performed by scanning the reference lists of included studies. All articles were published online before December 2016 and in English. From electronic database and manual searches, 444 studies were identified; 161 articles with 1632 test results met the inclusion criteria. Test results were assigned into 3 aging conditions: non-aged, intermediate-aged and aged groups. Generalized estimating equations (GEE) were used to explore actual mean bond strengths. As the bond strength is a non-negative value, lognormal distribution was used. RESULTS: In non-aged condition, data showed statistically significant interactions between cement type and type of test. There was no statistically significant interaction between mechanical and chemical pre-treatments. In intermediate-aged and aged conditions, data showed no statistically significant interactions between mechanical and chemical pre-treatments and between cement type and type of test. CONCLUSIONS: This meta-analysis appeared to indicate that mechanical pre-treatments, and in particular ceramic coating, combined with methacryloyloxydecyl dihydrogen phosphate (MDP) containing primers yielded the highest long-term bond strength (aged-condition). However, data are limited and caution should be exercised before applying these results to clinical situations.

Clinical and laboratory surface finishing procedures for zirconia on opposing human enamel wear: A laboratory study.

AIM: To investigate the effect of laboratory and clinical finishing procedures for zirconia on antagonistic enamel wear. MATERIALS AND METHODS: Forty-eight yttria-tetragonal partially stabilised zirconia (Y-TZP) specimens were prepared and divided into four groups according to their surface preparation: laboratory polished (LP); laboratory polished and glazed (G); clinically adjusted (CA); and clinically adjusted and repolished (CAR). Enamel opposing enamel was used as a control. Pre-testing surface roughness for each group was determined using contact profilometry. Two-body wear resistance tests were conducted using a masticatory simulator. Enamel specimens were subjected to 120,000 cycles in distilled water (frequency 1.6 Hz, loading force of 49 N). Volumetric and vertical enamel losses were measured by superimposition of pre- and post-testing images using a three-dimensional laser scanner and software analysis. Scanning electron microscopy was used for qualitative surface analysis of pre- and post-testing zirconia and enamel surfaces. One-way ANOVA and multiple comparisons with Bonferroni corrections were used for statistical analysis at a significance level of α=0.05. RESULTS: There was no statistical difference in volumetric and vertical enamel loss between CAR, G and LP. CAR produced statistically significantly less volumetric enamel loss compared with CA and control, and statistically significantly less vertical enamel loss compared with CA. Volumetric and vertical enamel loss were highly correlated in all groups. CONCLUSIONS: Enamel wear by clinically ground zirconia is comparable to that of opposing enamel surfaces and greater than clinically repolished zirconia. Repolishing of zirconia restorations following clinical adjustment with diamond burs is effective in reducing antagonistic enamel wear.

Surface roughness of restoration margin preparations: a comparative analysis of finishing techniques.

This study compared the margin profile and surface roughness created by the tips of four different finishing instruments: fine diamond, dura white stone, tungsten carbide, and ultrasonic diamond-coated tips (UDTs). The aim was to determine which of these instruments produced the smoothest finish and created the most evenly contoured margin characteristics. It was hypothesized that UDTs would produce a rougher dentin surface than a fine diamond bur, that a tungsten carbide bur would provide a smoother finish than a fine diamond, and that the dura white stone would produce an intermediate finish. Forty extracted premolars were divided into two groups. For the first group, a 1.5 x 3.0-mm dentin slot was prepared in 30 teeth using a control 50-µm diamond bur, followed by one of the four finishing instruments. The surface roughness (Ra) was then measured using a surface profilometer and a one-way analysis of variance followed by a post hoc Bonferroni test to assess whether any statistical difference existed among the Ra values. For the second group, shoulder margins were prepared in 10 teeth. They were then refined with one of the four finishing instruments and examined with scanning electron microscopy (SEM). The fine diamond bur created a significantly smoother surface than the control diamond (P < .001), UDTs (P < .007), and tungsten carbide bur (P < .010). The fine diamond was not found to be significantly smoother than the dura white stone. SEM images of the fine diamond showed divoting on the margin floor. The dura stone showed a well-defined, undamaged margin. The tungsten carbide bur created frequent chipping in enamel margins. The UDT specimens showed an inconsistent finish and discrete patches of open dentinal tubules. The fine diamond created the lowest Ra values; however, the dura stone offered efficient finishing and less damage to the margin profile.

Thermally induced fracture for core-veneered dental ceramic structures.

Effective and reliable clinical uses of dental ceramics necessitate an insightful analysis of the fracture behaviour under critical conditions. To better understand failure characteristics of porcelain veneered to zirconia core ceramic structures, thermally induced cracking during the cooling phase of fabrication is studied here by using the extended finite element method (XFEM). In this study, a transient thermal analysis of cooling is conducted first to determine the temperature distributions. The time-dependent temperature field is then imported to the XFEM model for viscoelastic thermomechanical analysis, which predicts thermally induced damage and cracking at different time steps. Temperature-dependent material properties are used in both transient thermal and thermomechanical analyses. Three typical ceramic structures are considered in this paper, namely bi-layered spheres, squat cylinders and dental crowns with thickness ratios of either 1:2 or 1:1. The XFEM fracture patterns exhibit good agreement with clinical observation and the in vitro experimental results obtained from scanning electron microscopy characterization. The study reveals that fast cooling can lead to thermal fracture of these different bi-layered ceramic structures, and cooling rate (in terms of heat transfer coefficient) plays a critical role in crack initiation and propagation. By exploring different cooling rates, the heat transfer coefficient thresholds of fracture are determined for different structures, which are of clear clinical implication.

Influence of surface and heat treatments on the flexural strength of a glass-infiltrated alumina/zirconia-reinforced dental ceramic.

OBJECTIVES: The purpose of this study was to investigate the influence of sandblasting, grinding, grinding orientation and polishing before and after heat treatment, on the flexural strength of a glass-infiltrated alumina/zirconia-reinforced dental ceramic (In-Ceram Zirconia). METHODS: The uniaxial flexural strength was calculated on 160 bar-shaped specimens (20 x 4 x 1.2mm) divided equally into eight groups as follows: sandblasted; sandblasted heated; polished; polished heated; ground parallel to the tensile axis; ground parallel heated; ground perpendicular; and ground perpendicular heated. Data were analyzed with multiple regression analysis, one-way ANOVA and Tukey's pairwise multiple comparisons and Weibull analysis. The treated and fractured surfaces were observed with SEM. The relative content of the monoclinic phase was quantified with an X-ray diffraction analysis. RESULTS: A thin layer of glass was present on the surface of the specimens after heat treatment and contributed to an improvement of the flexural strength. Surface treatment (not followed by heat treatment) generated phase transformation which, however, was not sufficient to avoid strength degradation caused by the flaws introduced with the surface treatments. Sandblasting caused the most marked strength degradation. Polishing alone increased the reliability, but did not improve the strength. The orientation of grinding in respect of the direction of the tensile stresses did not influence the ultimate tensile strength. SIGNIFICANCE: The present study suggests that any surface treatment performed on In-Ceram Zirconia should always be followed by heat treatment to avoid strength degradation.

Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic.

OBJECTIVE: The aim of the investigation was to assess the influence of sandblasting, grinding, grinding orientation, polishing and heat treatment on the flexural strength of a yittria stabilized tetragonal zirconia polycrystals ceramic (Y-TPZ). METHODS: The specimens (160 beams) were equally divided into four groups according to the surface treatment (sandblasted, polished, ground parallel to the tensile axis, ground perpendicular). Twenty specimens from each group underwent heat treatment under the firing conditions used to fire a layer of porcelain and glaze. After treatment, the three-point flexure test was used to calculate the flexural strength and X-ray diffraction analysis was used to estimate the relative amount of monoclinic phase. The reliability of strength was assessed through the Weibull distribution. Statistical analysis was conducted with multiple regression analysis, one-way ANOVA and Tukey's pairwise multiple comparisons. Treated and fractured surfaces were observed with SEM. RESULTS: The following values of strength and relative content of monoclinic phase of zirconia were measured for each group: sandblasted (1540MPa; 9.5%); ground parallel (1330MPa; 8.3%); ground perpendicular (1525MPa; 8.3%); ground parallel and heated (1225MPa; monoclinic content not detectable); ground perpendicular and heated (1185MPa; monoclinic content not detectable); polished and heated (1165MPa; monoclinic content not detectable); polished (1095MPa; 0.8%); sandblasted and heated (955MPa; 0.3%). CONCLUSIONS: The present study suggests that sandblasting and grinding may be recommended to increase the strength of dental Y-TZP, provided they are not followed by heat treatment. Fine polishing may remove the layer of compressive stresses and therefore, lower the mean flexural strength.

Influence of hot pressing on the microstructure and fracture toughness of two pressable dental glass-ceramics.

Empress 1 and Empress 2 are well-known pressable all-ceramic dental materials that have generated substantial interest for many clinicians and patients. These two materials are reputed to benefit from heat pressing during the laboratory fabrication procedures, leading to better crystal distribution within a glass matrix, and hence an improved strength. The present study aimed to evaluate the effect of heat pressing on fracture toughness, microstructural features, and porosity. Results showed that Empress 1 had similar fracture toughness values before the pressing procedure, after it, and after the repressing procedure. The microstructural features were also similar among these specimens, but a more uniform distribution of leucite crystals was observed following the pressing and repressing procedures. Empress 2 demonstrated two different fracture toughness values. This was associated with the alignment of lithium disilicate crystals that occurred after the pressing and repressing procedures, which led to different indentation induced crack lengths, depending upon whether cracks propagated parallel to or perpendicular to the aligned crystals, the former having lower toughness than those that propagated in the perpendicular direction. Porosity, in terms of both the size and number of pores, was found to decrease after the pressing and repressing procedures for both materials. Repressing resulted in significant growth of the lithium disilicate crystals in Empress 2, but there was no change for the leucite crystals in Empress 1. The change in the lithium disilicate crystals' size did not have a noticeable effect on the fracture toughness of Empress 2. It was concluded that heat pressing did not significantly affect the fracture toughness of Empress 1, but resulted in two different values for Empress 2. It also decreased the size and number of pores for both materials, which could contribute to the strength improvement found after heat pressing, which has been reported in previous studies.

Biaxial flexural strength and microstructure changes of two recycled pressable glass ceramics.

PURPOSE: This study evaluated the biaxial flexural strength and identified the crystalline phases and the microstructural features of pressed and repressed materials of the glass ceramics, Empress 1 and Empress 2. MATERIALS AND METHODS: Twenty pressed and 20 repressed disc specimens measuring 14 mm x 1 mm per material were prepared following the manufacturers' recommendations. Biaxial flexure (piston on 3-ball method) was used to assess strength. X-ray diffraction was performed to identify the crystalline phases, and a scanning electron microscope was used to disclose microstructural features. RESULTS: Biaxial flexural strength, for the pressed and repressed specimens, respectively, were E1 [148 (SD 18) and 149 (SD 35)] and E2 [340 (SD 40), 325 (SD 60)] MPa. There was no significant difference in strength between the pressed and the repressed groups of either material, Empress 1 and Empress 2 (p > 0.05). Weibull modulus values results were E1: (8, 4.7) and E2: (9, 5.8) for the same groups, respectively. X-ray diffraction revealed that leucite was the main crystalline phase for Empress 1 groups, and lithium disilicate for Empress 2 groups. No further peaks were observed in the X-ray diffraction patterns of either material after repressing. Dispersed leucite crystals and cracks within the leucite crystals and glass matrix were features observed in Empress 1 for pressed and repressed samples. Similar microstructure features--dense lithium disilicate crystals within a glass matrix--were observed in Empress 2 pressed and repressed materials. However, the repressed material showed larger lithium disilicate crystals than the singly pressed material. CONCLUSIONS: Second pressing had no significant effect on the biaxial flexural strength of Empress 1 or Empress 2; however, higher strength variations among the repressed samples of the materials may indicate less reliability of these materials after second pressing.