A Thoroughbred racehorse with a unicortical palmar lateral condylar fracture returned to training 14 days after surgery: a hypothesis on the role of a single bone screw on crack propagation.

A 2-year-old Thoroughbred racehorse had LF lameness that began post high-speed exercise and persisted for two days before the horse once again became sound. Diagnostic analgesia localized the lameness to the LF distal metacarpus, and a standing MRI identified a unicortical condylar fracture. A single 5.5 mm cortical screw was placed in lag fashion. The horse began hand walking at 14 days, racetrack jogging at 30 days, and racing at 5 months after the day of surgery. Placement of a single lag screw ahead of the tip of the crack in unicortical condylar fracture may be useful for reducing the recovery period for horses returning to training and racing.

Relationship between fracture toughness and fractal dimensional increment in two types of dental glass-ceramics with different fracture surface roughness.

OBJECTIVES: Previous studies have reported the fractal dimensional increment of glass-ceramic fracture surfaces. The objective of this study was to determine the relationship between fracture toughness and fractal dimensional increment of two dental glass-ceramics with different volume fraction of crystals and different fracture surface roughness. METHODS: Bar-shaped specimens were prepared from lithium disilicate (LDS) and nanofluorapatite (NFA) glass-ceramics. One face of each specimen was indented using a Knoop diamond at 25 N (LDS) or 10 N (NFA) followed by loading in 4-point, or 3-point flexure, respectively, until failure. Fracture toughness (Kc) was calculated using the surface crack in flexure (SCF) technique (ASTM C1421). Epoxy replicas of the fracture surfaces were scanned using the atomic force microscope (AFM) followed by noise filtering. The FRACTALS software was used to determine the fractal dimensional increment (D*) by the Minkowski cover algorithm. RESULTS: Median (25%, 75% quartiles) fracture toughness of LDS bars were 1.62 (1.59, 1.69) MPa m1/2 and NFA bars were 0.68 (0.66, 0.74) MPa m1/2, respectively. The median fractal dimension (D) value (25%, 75% quartiles) before noise filtering were 2.16 (2.15, 2.17) and after noise filtering were 2.14 (2.14, 2.15) for LDS and before noise filtering were 2.29 (2.21, 2.38) and after noise filtering were 2.17 (2.17, 2.18) for NFA. Median (25%, 75% quartiles) surface roughness (Ra) before noise filtering were 139 (119, 188) nm and after noise filtering were 137 (118, 187) nm for LDS and before noise filtering were 7 (6, 15) nm and after noise filtering were 7 (6, 15) nm for NFA. SIGNIFICANCE: Noise filtering successfully eliminated noise from the material with smooth fracture surfaces (NFA), decreasing the measured fractal dimension. The NFA data fit a Kc vs. D*1/2 statistical model for fused silica previously tested using a similar technique. The equation relating fracture toughness to the fractal dimension was modified, accounting for the toughening mechanisms. Fractal analysis with noise filtering can be used to estimate the fracture toughness of dental glass-ceramics that do not exhibit crack bridging.

Fracture of Lithia Disilicate Ceramics under Different Environmental Conditions.

The objective of this research was to quantify the effect of surface degradation and abrasion separately and in combination on the flexural strength of lithia disilicate ceramics. Lithia disilicate disks were fabricated using the lost wax technique and pressing in vacuum. The eight groups in this pilot experiment were (i) reference, hydrated in distilled water for 24 h prior to fracture; (ii) reference, non-hydrated group; (iii) 28-day pH cycling group; (iv) 125K chewing cycle group; (v) combined pH cycling + 125K chewing cycle; (vi) constant pH 2 solution for 28 days; (vii) constant pH 7 solution for 28 days; and (viii) constant pH 10 solution for 28 days. pH cycling is a method that alternates between pH 2, 7 and 10 over 28 days. A total of 15 disks were used for each group. All the groups were tested using the biaxial piston and a three-ball flexural strength test to obtain their biaxial flexural strength. pH 2 constant immersion demonstrated the highest fracture strength and was significantly greater than all other groups (p < 0.0001). Chewing and pH cycling + chewing groups exhibited the lowest fracture strengths and were significantly lower than all other groups (p < 0.0001). The damage observed from the chewing simulator does not represent apparent clinical fractures.

Forensic and reliability analyses of fixed dental prostheses.

This article describes the protocol for determining the cause of failure for retrieved failed implant supported fixed dental prostheses (FDPs) in a clinical study of three-unit bridges. The results of loading of flexure bars of different veneer compositions at different stress rates were presented for two veneer materials (leucite reinforced and fluorapatite glass-ceramic veneers) and a Y-TZP core zirconia ceramic used in the clinical study. From these results, the strengths of the fast loading conditions were used to determine the fracture toughness of these materials. Fractal dimension measurements of the flexure bars and selected FDPs of the same materials demonstrated that the values were the same for both the bars and the FDPs. This allowed the use of fracture toughness values from the flexure bars to determine the strengths of the FDPs. The failure analysis of clinically obtained FDP replicates to determine the size of the fracture initiating cracks was then performed. Using the information from the flexure bars and the size of the fracture initiating cracks for the failed FDPs, the strengths of the FDPs were determined. The clinical failures were determined to be most likely the result of repeated crack growth due to initial overload and continuous use after initial cracking.

Quantitative fractography as a novel approach to measure fracture toughness of direct resin composites.

Dental resin composites generally fail from small cracks. Large crack techniques are not representative of in vivo failures. Quantitative fractography relies on the observation of small "natural flaws". This study investigates the effect of flaws that occur from fabrication and handling on the measurement of fracture toughness for four different direct resin composites. Microtensile dog-bone shaped specimens for each of four composites were fractured to measure the strength. In addition, we measured crack sizes of the fracture initiating cracks and determined the fracture toughness from these measurements using quantitative fractographic analysis on selected specimens. The characteristic strengths (28-51 MPa) and Weibull moduli (4.9-7.8) were also determined and related to the toughness values (0.5-0.9 MPa m1/2) obtained. The elastic moduli (5-10 GPa) were measured using an indentation technique that has not been used before for direct resin composites. The indentation technique offers an alternative method for small specimens. Quantitative fractographic analysis offers a useful technique to assess fracture toughness of resin composites from cracks of the same size as observed in practice. The toughness of the direct resin composites results from a balance between the size, number and shape of filler particles and the viscosity of the resin matrix.

Demonstration of a SiC Protective Coating for Titanium Implants.

To mitigate the corrosion of titanium implants and improve implant longevity, we investigated the capability to coat titanium implants with SiC and determined if the coating could remain intact after simulated implant placement. Titanium disks and titanium implants were coated with SiC using plasma-enhanced chemical vapor deposition (PECVD) and were examined for interface quality, chemical composition, and coating robustness. SiC-coated titanium implants were torqued into a Poly(methyl methacrylate) (PMMA) block to simulate clinical implant placement followed by energy dispersive spectroscopy to determine if the coating remained intact. After torquing, the atomic concentration of the detectable elements (silicon, carbon, oxygen, titanium, and aluminum) remained relatively unchanged, with the variation staying within the detection limits of the Energy Dispersive Spectroscopy (EDS) tool. In conclusion, plasma-enhanced chemical vapor deposited SiC was shown to conformably coat titanium implant surfaces and remain intact after torquing the coated implants into a material with a similar hardness to human bone mass.

Fracture toughness and work of fracture of hydrated, dehydrated, and ashed bovine bone.

Bone, a tri-phase composite, consists of nano-sized apatite minerals, an organic component, and water. Heat-treated bovine cortical bone has been proposed as a candidate for void-filling bone substitute. However, the toughness of heat-treated bone is not yet fully studied. Fracture toughness (K(c)) and work of fracture (W(f)) of hydrated, dehydrated, and ashed bovine bone were estimated using a single-edge V-notched beam method. Thermal gravimetric analysis and differential thermal analysis were used to determine the temperature at which the organics and water were removed. Dehydrated specimens were obtained by placing the samples in a 60 degrees C vacuum oven for 24h or a 110 degrees C furnace for 2h. Ashed specimens were obtained by heat-treating samples at 600 degrees C for 24h. K(c) of bovine specimens decreased from 5.5MPa.m(1/2) for hydrated bone, to 3.8MPa.m(1/2) for dehydrated specimens, and to 0.36MPa.m(1/2) for ashed specimens. W(f) decreased from 7.1 to 1.1kJ/m(2) for dehydrated specimens, and to 0.04kJ/m(2) for ashed specimens. The main reasons for the significant decreases in K(c) and W(f) may be attributed to water's ability in stabilizing collagen structure and to the organics' ability in making bone more ductile. Because of the large decrease in fracture toughness and work of fracture, we suggest that ashed bone is not appropriate for load-bearing bone substitute in areas where bone experiences loadings in flexure.

Evaluation of fracture toughness of human dentin using elastic-plastic fracture mechanics.

Dentin, the mineralized tissue forming the bulk of the tooth, lies between the enamel and the pulp chamber. It is a rich source of inspiration for designing novel synthetic materials due to its unique microstructure. Most of the previous studies investigating the fracture toughness of dentin have used linear-elastic fracture mechanics (LEFM) that ignores plastic deformation and could underestimate the toughness of dentin. With the presence of collagen (approximately 30% by volume) aiding the toughening mechanisms in dentin, we hypothesize that there is a significant difference between the fracture toughness estimated using LEFM (Kc) and elastic-plastic fracture mechanics (EPFM) (KJc). Single-edge notched beam specimens with in-plane (n=10) and anti-plane (n=10) parallel fractures were prepared following ASTM standard E1820 and tested in three-point flexure. KJc of the in-plane parallel and anti-plane parallel specimens were found to be 3.1 and 3.4 MPa m 1/2 and Kc were 2.4 and 2.5 MPa m 1/2, respectively. The fracture toughness estimated based on KJc is significantly greater than that estimated based on Kc (32.5% on average; p<0.001). In addition, KJc of anti-plane parallel specimens is significantly greater than that of in-plane parallel specimens. We suggest that, in order to critically evaluate the fracture toughness of human dentin, EPFM should be employed.

Fractographic analyses of zirconia-based fixed partial dentures.

OBJECTIVES: Advances in ceramic processing techniques enable clinicians and ceramists to fabricate all-ceramic fixed partial dentures (FPDs) for posterior regions using high-strength yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). However, failures occur in ceramic FPDs due to their design. The objectives of this study were to determine the site of crack initiation and the causes of fracture in clinically failed zirconia-based ceramic FPDs. METHODS: Five clinically failed four-unit Y-TZP-based FPDs (Cercon ceramics, DeguDent GmbH, Hanau, Germany) were retrieved and analyzed. The fragments containing the fracture origins in the veneers (Cercon Ceram S Veneering Ceramic, DeguDent GmbH, Hanau, Germany) of two samples were missing but the rest of veneer structures were present. The other three samples had their veneers intact. Fracture surfaces were examined using fractographic techniques, utilizing both optical and scanning electron microscopes (SEM). Quantitative fractography and fracture mechanics principles were used to estimate the stresses at failure. RESULTS: Primary fractures initiated from the gingival surfaces of connectors at veneer surfaces in four out of the five samples. However, critical flaw sizes could be measured in three of the five cases since fracture origins were lost in the remaining two due to local fragmentation at the crack initiation site. Delaminations between glass veneer and zirconia core were observed in Y-TZP-based FPDs and a secondary fracture initiated from the zirconia core. Secondary fracture controlled the ultimate failure. Failure stresses of the fixed partial dentures that failed due to zirconia fracture ranged from 379 to 501 MPa. Fractures that had origins on the glass veneer surface had failure stresses between 31 and 38 MPa. SIGNIFICANCE: Primary fractures in clinically failed Y-TZP-based FPDs initiated from the veneer surfaces. Interfacial delamination in glass veneer/zirconia core bilayer dental ceramic structures controlled the secondary fracture initiation sites and failure stresses in Y-TZP-based fixed partial dentures.

Characterization of glass-infiltrated alumina-based ceramics.

OBJECTIVE: Characterize the microstructure, composition, and important properties of glass-infiltrated alumina-based ceramics similar to the In-Ceram system. METHODS: Materials used were: IA, In-Ceram Alumina (Vita); IAE, IA electrophoretically deposited (Vita); AEM, IA using a vacuum driven method (Vita); VC, Vitro-Ceram (Angelus); TC, Turkom-Cera (Turkom-Ceramic); CC, Ceramcap (Foto-Ceram); and AG, Alglass (EDG). Ceramic specimens were fabricated following manufacturers' instructions and ISO6872 standard and polished successively through 1 microm alumina abrasive. Semi-quantitative and qualitative analyses were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and stereology (Vv). The elastic modulus (E) and Poisson's ratio (nu) were determined using time-of-flight data measured in an ultrasonic pulser/receiver and the density (rho) was determined using a helium pycnometer. Vicker's indentation was used to calculate hardness (H). Bar specimens (25 mm x 4 mm x 1.2 mm) were loaded in three-point bending to fracture using a universal testing machine with cross-head speed of 1mm/min. Flexural strength (sigma(3P)) was calculated and statistically analyzed using ANOVA, Tukey (alpha = 0.05) and Weibull (m = modulus, sigma(0) = characteristic strength). RESULTS: SEM and EDS analyses revealed similar microstructure for all-ceramics, except for a lead-based matrix in CC and a zirconia phase in VC. TC, AG and CC showed significantly lower mean sigma(3P) values than the other ceramics (p< or =0.05). AEM showed the greatest m (16). CONCLUSION: Despite few differences in microstructure and composition, the IA, IAE, AEM and VC ceramics have similar properties. SIGNIFICANCE: The glass-infiltrated alumina-based ceramics from different manufacturers presented distinct characteristics. It is necessary to characterize new commercially available materials to understand their properties.

Analysis of subcritical crack growth in dental ceramics using fracture mechanics and fractography.

OBJECTIVES: The aim of this study was to test the hypothesis that the flexural strengths and critical flaw sizes of dental ceramic specimens will be affected by the testing environment and stressing rate even though their fracture toughness values will remain the same. METHODS: Ceramic specimens were prepared from an aluminous porcelain (Vitadur Alpha; VITA Zahnfabrik, Bad Säckingen, Germany) and an alumina-zirconia-glass composite (In-Ceram Zirconia; VITA Zahnfabrik). Three hundred uniaxial flexure specimens (150 of each material) were fabricated to dimensions of 25 mmx4 mmx1.2 mm according to the ISO 6872 standard. Each group of 30 specimens was fractured in water using one of four different target stressing rates ranging on a logarithmic scale from 0.1 to 100 MPa/s for Vitadur Alpha and from 0.01 to 10 MPa/s for In-Ceram Zirconia. The fifth group was tested in inert environment (oil) with a target stressing rate of 100 MPa/s for Vitadur Alpha and 1000 MPa/s for In-Ceram Zirconia. The effects of stressing rate and environment on flexural strength, critical flaw size, and fracture toughness were analyzed statistically by Kruskal-Wallis one-way ANOVA on ranks followed by post hoc comparisons using Dunn's test (alpha=0.05). In addition, 20 Vitadur Alpha specimens were fabricated with controlled flaws to simplify fractography. Half of these specimens were fracture tested in water and half in oil at a target stressing rate of 100 MPa/s, and the results were compared using Mann-Whitney rank sum tests (alpha=0.05). A logarithmic regression model was used to determine the fatigue parameters for each material. RESULTS: For each ceramic composition, specimens tested in oil had significantly higher strength (P0.05). Specimens tested at faster stressing rates had significantly higher strength (P0.05). Regarding critical flaw size, stressing rate had a significant effect for In-Ceram Zirconia specimens (P0.05). Fatigue parameters, n and lnB, were 38.4 and -12.7 for Vitadur Alpha and were 13.1 and 10.4 for In-Ceram Zirconia. SIGNIFICANCE: Moisture assisted subcritical crack growth had a more deleterious effect on In-Ceram Zirconia core ceramic than on Vitadur Alpha porcelain. Fracture surface analysis identified fracture surface features that can potentially mislead investigators into misidentifying the critical flaw.

The effects of viscoelastic parameters on residual stress development in a zirconia/glass bilayer dental ceramic.

OBJECTIVES: The aim of this study was to test the hypothesis that the residual stresses in a zirconia-based bilayer dental composite system can be tailored through heat treatment above and below the glass transition temperature of glass veneers. METHODS: Ceramic bilayer disc specimens were prepared from a zirconia core and a glass veneer. Each bilayer ceramic group was heat treated 40 degrees C below, 20 degrees C and 40 degrees C above and at the glass transition temperature of the glass veneer, and cooled using a fast or a slow cooling rate. Specimens were tested for flexure strength using a biaxial bending fixture. Residual stresses were calculated using a fracture mechanics approach. RESULTS: Heat treatments produced significant differences (p < or =0.05) between the mean flexural strengths of the heat treatment groups when the specimens were cooled using a fast cooling rate. However, there was not a significant difference (p >0.05) between the mean flexural strengths of the heat treatment groups when a slow cooling rate was used. Fractures initiated from the veneer surfaces of the specimens. SIGNIFICANCE: Heat treatment above and below the glass transition temperature of the veneer layer, and the cooling rate have a significant effect on the flexural strength of the bilayer ceramic laminates. The existence of residual compressive stress is the most likely reason for the observed strength increases. Residual stresses can be modified using the elastic-viscoelastic relaxation behavior of a glass veneer.

How tough is bone? Application of elastic-plastic fracture mechanics to bone.

Bone, with a hierarchical structure that spans from the nano-scale to the macro-scale and a composite design composed of nano-sized mineral crystals embedded in an organic matrix, has been shown to have several toughening mechanisms that increases its toughness. These mechanisms can stop, slow, or deflect crack propagation and cause bone to have a moderate amount of apparent plastic deformation before fracture. In addition, bone contains a high volumetric percentage of organics and water that makes it behave nonlinearly before fracture. Many researchers used strength or critical stress intensity factor (fracture toughness) to characterize the mechanical property of bone. However, these parameters do not account for the energy spent in plastic deformation before bone fracture. To accurately describe the mechanical characteristics of bone, we applied elastic-plastic fracture mechanics to study bone's fracture toughness. The J integral, a parameter that estimates both the energies consumed in the elastic and plastic deformations, was used to quantify the total energy spent before bone fracture. Twenty cortical bone specimens were cut from the mid-diaphysis of bovine femurs. Ten of them were prepared to undergo transverse fracture and the other 10 were prepared to undergo longitudinal fracture. The specimens were prepared following the apparatus suggested in ASTM E1820 and tested in distilled water at 37 degrees C. The average J integral of the transverse-fractured specimens was found to be 6.6 kPa m, which is 187% greater than that of longitudinal-fractured specimens (2.3 kPa m). The energy spent in the plastic deformation of the longitudinal-fractured and transverse-fractured bovine specimens was found to be 3.6-4.1 times the energy spent in the elastic deformation. This study shows that the toughness of bone estimated using the J integral is much greater than the toughness measured using the critical stress intensity factor. We suggest that the J integral method is a better technique in estimating the toughness of bone.

Effect of temperature on the fracture toughness of compact bone.

Bone is a composite composed mainly of organics, minerals, and water. Many researchers have studied effects such as crack velocity, density, orientation, storage media, porosity, and age on the fracture toughness (K(C), also called critical stress intensity factor) of compact bone. Most of these studies were conducted at room temperature. Considering that the body temperature of animals is greater than room temperature, and that bone has a large volumetric percentage of organics and water (generally, 55-65%), it is hypothesized that temperature has a significant effect on the fracture toughness of compact bone. Single-edge V-notched (SEVN) specimens were prepared to measure the fracture toughness of bovine femur and manatee rib in water at 0, 10, 23, 37, and 50 degrees C in four-point flexure. The fracture toughness values of bovine femur and manatee rib were found to decrease from 7.0 to 4.3MPam(1/2) and from 5.5 to 4.0MPam(1/2), respectively, as temperature increased over a temperature range of 50 degrees C. The results support the hypothesis that temperature has a significant effect on the fracture toughness of compact bone. Therefore, we suggest that study on fracture toughness of bone should be done at physiologically relevant temperatures.

Characterization and surface treatment effects on topography of a glass-infiltrated alumina/zirconia-reinforced ceramic.

OBJECTIVE: Characterize the microstructure, composition and some physical properties of a glass-infiltrated alumina/zirconia-reinforced ceramic (IZ) and the effect of surface treatment on topography. METHODS: IZ ceramic specimens were fabricated according to ISO6872 instructions and polished through 1 microm alumina abrasive. Quantitative and qualitative analyses were performed using scanning electron microscopy (SEM), backscattered imaging (BSI), electron dispersive spectroscopy (EDS) and stereology. The elastic modulus (E) and Poisson's ratio (nu) were determined using ultrasonic waves, and the density (rho) using a helium pycnometer. The following ceramic surface treatments were used: AP-as-polished; HF-etching with 9.5% hydrofluoric acid for 90 s; SB-sandblasting with 25 microm aluminum oxide particles for 15s and SC-blasting with 30 microm aluminum oxide particles modified by silica (silica coating) for 15s. An optical profilometer was used to examine the surface roughness (Ra) and SEM-EDS were used to measure the amount of silica after all treatments. RESULTS: The IZ mean property values were as follows: rho=4.45+/-0.01 g/cm(3); nu=0.26 and E=245 GPa. Mean Ra values were similar for AP- and HF-treated IZ but significantly increased after either SC or SB treatment (p

Fracture toughness of manatee rib and bovine femur using a chevron-notched beam test.

Bone is an anisotropic material with a hierarchical structure consisting of organic matrix, minerals and water. Fracture toughness (K(C)) has been shown to be a good index to assess the mechanical performance of bone. A chevron-notched (CN) beam test, a standard fracture mechanics test successfully applied to many other materials, was used to determine the transverse-direction fracture toughness in manatee rib and bovine femur cortical bone. Although human and bovine bone has been well studied, there is virtually no information on the toughness of manatee rib bone. As a biological material, manatee rib is interesting for study in that it is a highly mineralized bone. Three major advantages of the CN specimen test are: (1) it is easier to reach plane strain condition; (2) there is no fatigue-precracking needed; and (3) it is relatively easy to produce stable crack propagation before catastrophic fracture. The fracture toughness values of manatee rib and bovine femur were measured to be 4.5 +/- 0.5 MPa m(1/2) and 5.8 +/- 0.5 MPa m(1/2), respectively. Based on the microstructures shown in SEM images, two features that contributed to the greater fracture toughness of bovine femur were identified as greater osteon density and lesser porosity.

Residual stresses in bilayer dental ceramics.

It is clinically observed that lithia-disilicate-based all-ceramic fixed partial dentures (FPD) can fail because of the fragmentation of the veneering material. The hypothesis of this study is that the global residual stresses within the surface of those veneered FPDs may be responsible for partial fragmentation of the veneering ceramic. Bilayer and monolithic ceramic composites were prepared using a lithia disilicate based (Li2OSiO2) glass-ceramic core and a glass veneer. A four-step fracture mechanics approach was used to analyze residual stress in bilayered all-ceramic FPDs. We found a statistically significant increase in the mean flexural strengths of bilayer specimens compared with monolithic glass specimens (p < or = 0.05). There was a statistically significant difference between the mean longitudinal and transverse indentation-induced crack sizes in bilayer specimens (p < or = 0.05), which indicates the existence of residual stress. Global residual stresses in the veneer layer, calculated using a fracture mechanics equation, were determined to be responsible for the increased strength and observed chipping, i.e., spallation in bilayer ceramic composites.

Bioactivity of tape cast and sintered bioactive glass-ceramic in simulated body fluid.

A common ceramic processing technique, tape casting, was used to produce thin, flexible sheets of bioactive glass (Bioglass 45S5) particulate in an organic matrix. Tape casting offers the possibility of producing three-dimensional shapes, as the final material is built up layer by layer. Bioactive glass tapes were sintered together to form small discs for in vitro bioactivity testing in simulated body fluid (SBF). Four different sintering schedules were investigated: 800, 900, and 1000 degrees C for 3 h; and 1000 degrees C for 6 h. Each schedule produced a crystalline material of major phase Na2Ca2Si3O9. Tape cast and sintered bioactive glass-ceramic processed at 1000 degrees C formed crystalline hydroxyapatite layers after 20-24 h in SBF as indicated by Fourier transform infrared spectroscopy, Scanning electron microscopy, and EDS data. FTIR revealed that the greatest amount of hydroxyapatite formation after 2 h was observed for samples sintered at 900 degrees C. The differences in bioactive response were likely caused by the variation in the extent of sintering and, consequently, the amount of surface area available for reaction with SBF.

Role of investment interaction layer on strength and toughness of ceramic laminates.

OBJECTIVES: The aim of this study was to test the hypothesis that the interaction of a core ceramic with investment material can significantly reduce the flexural strength and the fracture toughness of core/veneer ceramic laminates. METHODS: Ceramic composites were prepared from experimental core and experimental veneer and Empress 2 core and Empress 2 veneer ceramics. Four divesting techniques were used for each bilayer ceramic group. Core surfaces were etched with 1% HF solution for 15 or 30 min and grit blasted with 100 microm Al2O3 particles for 15 or 30 s. The effect of treatment on strength was analyzed statistically by means of two-way ANOVA. A linear regression graph was made for each group to analyze the relationship between flexural strength and the dimensions of critical cracks. RESULTS: The four surface divesting treatments produced no significant differences (p > 0.05) between the mean flexural strengths and the mean fracture toughnesses. However, groups with different core/veneer combinations showed statistically significant differences (p < or = 0.05) between the mean flexural strengths and between the mean fracture toughnesses. The Empress 2 core/experimental veneer combination exhibited the greatest fracture toughness values. The Empress 2 core/Empress 2 veneer combination exhibited the lowest mean fracture toughness and lowest mean flexural strength. SIGNIFICANCE: The investment interaction layer does not have a significant effect on the flexural strength and fracture toughness of the bilayer ceramic laminates for interfaces that are coherent and well bonded. However, the core/veneer combination of materials does affect the strength of bilayer ceramic laminates. The existence of global residual stress is the most likely reason for the observed strength increases.

Fracture behavior of lithia disilicate- and leucite-based ceramics.

OBJECTIVE: This study was designed to characterize the fracture behavior of ceramics and test the hypothesis that variation in strength is associated with a variation in fracture toughness. METHODS: The following four groups of 20 bar specimens (25 x 4 x 1.2 mm) were fabricated (ISO standard 6872): E1, a hot-pressed leucite-based core ceramic (IPS Empress); E2, a hot-pressed lithia-based core ceramic (IPS Empress 2); ES, a hot-pressed lithia-based core ceramic (Experimental); and GV, a glass veneer (IPS Empress2 body). Specimens were subjected to four-point flexure loading in 37 degrees C distilled water. Fractographic analysis was performed to determine the fracture origin (c) for calculation of fracture toughness (KIC). Weibull analysis of flexure strength (sigma) data was also performed. RESULTS: Differences in mean sigma and KIC were statistically significant for E1 and GV (p<0.05). These differences are associated with processing effects and composition. SIGNIFICANCE: The higher mean sigma and KIC values of E2 and ES core ceramics suggest potentially improved structural performance compared with E1 although the Weibull moduli of E1 and E2 are the same.