Evaluating dental zirconia.

Objectives: The aims of the study were to process a ceramic material with a fine leucite particle size using hot pressing techniques, to increase the flexural strength, reliability and ease of use.Methods: A starting glass composition of wt%; 64.2% SiO2, 16.1% Al2O3, 10.9% K2O, 4.3% Na2O, 1.7% CaO, 0.5% LiO and 0.4% TiO2 was used to produce a leucite reinforced ceramic material. Twenty-one porcelain discs were produced by sintering the ceramic frit (group 1) and sixty-three discs by heat pressing the frit (groups 2, 3 and 4). Twenty-one Empress 1 ceramic discs were also heat pressed (group 5). Disc specimens were tested using the biaxial flexure test at a crosshead speed of 0.15 mm/min and the data analysed using the Scheffe F multiple comparison test and Weibull statistics. Specimens were characterised using X-ray diffraction (XRD), secondary electron imaging and energy dispersive X-ray analysis where applicable.Results: The heat pressed groups (2, 3 and 4) had higher mean biaxial flexural strengths and characteristic strength values than groups 1 and 5 (p < 0.05). XRD revealed the presence of tetragonal leucite in all lest groups. Fine leucite crystals, tabular platelets and minimal matrix microcracking were found in the microstructure of test groups (1-4) with a more uniform leucite distribution in the heat pressed specimen groups (2, 3 and 4), which were associated with a significant increase in the biaxial flexural strength and reliability.Significance: Optimisation of the microstructure by producing a fine microstructure and controlling the distribution via the correct pressing parameters may be extremely advantageous in these systems. (C) 2001 Academy of Dental Materials. published by Elsevier Science Ltd. All rights reserved.

Flexural strength optimisation of a leucite reinforced glass ceramic

Statement of problem The effect of anatomic contour design in all or parts of zirconia crowns is uncertain regarding clinical reliability and survival rates. Purpose The purpose of this in vitro study was to compare the load at fracture of monolithic, anatomic contour zirconia crowns with bilayered crowns with and without a cervical zirconia collar. Material and methods Thirty zirconia crowns were fabricated for a shallow chamfer molar preparation, 10 with a normal core-veneer design, 10 with a core-veneer design with an additional cervical collar of zirconia, and 10 with a monolithic, anatomic contour design. Veneering ceramic was applied to the first 20 specimens to create an anatomic form. All crowns were cemented to epoxy abutments and loaded until complete fracture with a clinically relevant test method. The fracture modes and load at fracture were recorded. Results Statistically significant differences were found in the load at fracture and fracture modes among the test groups ( P Conclusions Monolithic, anatomical contour design gave higher loads at fracture than traditional core-veneer design. Crowns with a cervical zirconia collar had higher load at fracture than the core-veneer design, but lower than the monolithic crowns.

/pdf/load-at-fracture-of-monolithic-and-bilayered-zirconia-crowns-26fpx8tghi.pdf

Load at fracture of monolithic and bilayered zirconia crowns with and without a cervical zirconia collar

Statement of problem Access channels for retrieving ceramic implant-supported screw-retained crowns may decrease their fracture resistance. Purpose The purpose of this in vitro study was to evaluate the effect of screw-access channels on 3 types of ceramic implant-supported crowns. Material and methods Sixty computer-aided designed and computer-aided manufactured (CAD-CAM) ceramic implant-supported screw-retained maxillary premolar crowns were fabricated, 30 with an occlusal screw-access channel and 30 without access channels. Each group was further divided into the following 3 subgroups of 10 specimens each: monolithic zirconia, veneered zirconia, and lithium disilicate. Identical milled titanium implant abutments were fabricated. Crowns were fabricated with standardized thicknesses and subjected to cyclic loading until failure occurred. Data analysis was performed using 1-way analysis of variance test of significance followed by Tukey honest significant difference (HSD) test (α=.05). Results No significant differences in fracture resistance were found between access channel groups and corresponding groups without access channels ( P >.05). Among the subgroups, monolithic zirconia recorded the highest fatigue failure mean load values (2047.8 ±83.2 N for crowns with access channels and 2028.7 ±104.5 N for crowns without access channels), which was significantly higher ( P Conclusions Screw-access channels did not affect the fatigue failure load of monolithic zirconia, monolithic lithium disilicate, or veneered zirconia ceramic crowns. Monolithic zirconia crowns recorded significantly higher fatigue failure load among the 3 types of crowns tested.

Effect of screw-access channels on the fracture resistance of 3 types of ceramic implant-supported crowns

Objective This study aimed to evaluate the effect of the cavity preparation and ceramic type on the stress distribution, tooth strain, fracture resistance and fracture mode of human molar teeth restored with onlays. Material and Methods Forty-eight molars were divided into four groups (n=12) with assorted combinations of two study factors: BL- conventional onlay preparation with boxes made from leucite ceramic (IPS-Empress CAD, Ivoclar Vivadent); NBL- conservative onlay preparation without boxes made from leucite ceramic; BD- conventional onlay preparation with boxes made from lithium disilicate glass ceramic (IPS e.max CAD, Ivoclar Vivadent); NBL- conservative onlay preparation with boxes made from lithium disilicate glass ceramic cuspal deformation (µS) was measured at 100 N and at maximum fracture load using strain gauge. Fracture resistance (N) was measured using a compression test, and the fracture mode was recorded. Finite element analysis was used to evaluate the stress distribution by modified von Mises stress criteria. The tooth strain and fracture resistance data were analyzed using the Tukey test and two-way ANOVA, and the fracture mode was analyzed by the chi-square test (α=0.05). Results The leucite ceramic resulted in higher tooth deformation at 100 N and lower tooth deformation at the maximum fracture load than the lithium disilicate ceramic (P<0.001). The lithium disilicate ceramic exhibited higher fracture resistance than the leucite ceramic (P<0.001). The conservative onlay resulted in higher fracture strength for lithium disilicate ceramic. Finite element analysis results showed the conventional cavity preparation resulted in higher stress concentration in the ceramic restoration and remaining tooth than the conservative onlay preparation. The conservative onlays exhibited increased fracture resistance, reduced stress concentration and more favorable fracture modes. Conclusion Molars restored with lithium disilicate CAD-CAM ceramic onlays exhibited higher fracture resistance than molars restored with leucite CAD-CAM ceramic onlays.

/pdf/effect-of-cavity-preparation-design-and-ceramic-type-on-the-2c0mu7fl8a.pdf

Effect of cavity preparation design and ceramic type on the stress distribution, strain and fracture resistance of CAD/CAM onlays in molars

STATEMENT OF PROBLEM
The outcomes from load-to-failure tests may not be applicable to clinical situations.


PURPOSE
The purpose of this study was to critically evaluate the efficacy of load-to-failure tests in the investigation of the fracture load and pattern of metal-free crowns.


MATERIAL AND METHODS
Four groups were formed from 128 bovine roots restored with metal posts, resin cores, and feldspathic, leucite, or lithium disilicate ceramic systems or polymer crowns. Each group was divided into 4 (n=8) according to the cement: zinc phosphate, self-adhesive resin, autopolymerizing resin, and glass ionomer. Mean fracture loads from compressive tests were submitted to ANOVA and Tukey HSD test. Finite element and fractographic analyses were performed and associated with the fracture load and pattern.


RESULTS
Significantly higher fracture load values were obtained for the lithium disilicate ceramic, but finite element and fractographic analyses showed that the cement effect could not be determined. The finite element analysis showed the cement likely affected the fracture pattern, confirmed that stresses in the cements were little affected by the crown materials, and found that the stressed conditions were lowest in the lithium disilicate compared with other crowns for all cement combinations. The stressed conditions in the crowns depended more on the adhesive properties than on the elastic modulus of the cement materials. The level of the stressed condition in the crowns at the occlusal surface was about the same or higher than along their cement interface, consistent with the fractography, which indicated fractures starting at the load point. Higher stress levels in the crowns corresponded with a lower number of catastrophic fractures, and higher stresses in the cements seemed to reduce the number of catastrophic fracture patterns. The highest stressed conditions occurred along the occlusal surface for crown materials with a low elastic modulus or in combination with adhesive cements.


CONCLUSIONS
The method used was not appropriate either for investigating the crowns' fracture load and pattern or for stating the role of the cements within the crown-cement-tooth interaction.

O Osmir Batista de

Papers

Crown Fracture: Failure Load, Stress Distribution, And Fractographic Analysis