Kenneth J. Anusavice

Structural reliability of alumina-, feldspar-, leucite-, mica- and zirconia-based ceramics

OBJECTIVES The objective of this study was to test the hypothesis that industrially manufactured ceramic materials, such as Cerec Mark II and Zirconia-TZP, have a smaller range of fracture strength variation and therefore greater structural reliability than laboratory-processed dental ceramic materials. METHODS Thirty bar specimens per material were prepared and tested. The four-point bend test was used to determine the flexure strength of all ceramic materials. The fracture stress values were analyzed by Weibull analysis to determine the Weibull modulus values (m) and the 1 and 5% probabilities of failure. RESULTS The mean strength and standard deviation values for these ceramics are as follows: (MPa+/-SD) were: Cerec Mark II, 86.3+/-4.3; Dicor, 70.3+/-12.2; In-Ceram Alumina, 429. 3+/-87.2; IPS Empress, 83.9+/-11.3; Vitadur Alpha Core, 131.0+/-9.5; Vitadur Alpha Dentin, 60.7+/-6.8; Vita VMK 68, 82.7+/-10.0; and Zirconia-TZP, 913.0+/-50.2. There was no statistically significant difference among the flexure strength of Cerec Mark II, Dicor, IPS Empress, Vitadur Alpha Dentin, and Vita VMK 68 ceramics (p>0.05). The highest Weibull moduli were associated with Cerec Mark II and Zirconia-TZP ceramics (23.6 and 18.4). Dicor glass-ceramic and In-Ceram Alumina had the lowest m values (5.5 and 5.7), whereas intermediate values were observed for IPS-Empress, Vita VMK 68, Vitadur Alpha Dentin and Vitadur Alpha Core ceramics (8.6, 8.9, 10.0 and 13.0, respectively). CONCLUSIONS Except for In-Ceram Alumina, Vitadur Alpha and Zirconia-TZP core ceramics, most of the investigated ceramic materials fabricated under the condition of a dental laboratory were not stronger or more structurally reliable than Vita VMK 68 veneering porcelain. Only Cerec Mark II and Zirconia-TZP specimens, which were prepared from an industrially optimized ceramic material, exhibited m values greater than 18. Hence, we conclude that industrially prepared ceramics are more structurally reliable materials for dental applications although CAD-CAM procedures may induce surface and subsurface flaws that may adversely affect this property.

Fracture Surface Characterization of Clinically Failed All-ceramic Crowns

The goal of this study was to establish a protocol for the retrieval and fractographic analysis of failed restorations, and to compare the fracture surface features of clinically failed ceramic restorations and with those of controlled laboratory test specimens fabricated from the same materials. Ten fractured Dicor® crowns and 12 fractured Cerestore® crowns were retrieved and analyzed. Optical microscopy of the failed crowns revealed that the critical segments of nine of the 10 (90%) Dicor® crowns and nine of the 12 (75%) Cerestore® crowns were acceptable for fractographic analysis. Twelve disks of each material were fabricated as controls and fractured by bi-axial flexure for analysis of the similarities and/or differences between the fractographic features of fractured clinical crowns and the disks. Each of the 10 Dicor® crowns was observed to fail along the internal surface. For 78% of the Cerestore® crowns, failure initiation occurred at the porcelain/core interface or inside the core material. Critical flaw sizes of the failed Dicor® crowns ranged from 127 to 272 μm. Failure stresses of the Dicor® crowns, estimated by fractographic techniques and fracture mechanics relationships, ranged from 65 to 94 MPa. Estimated failure stresses for two of the Cerestore® crowns which had failure initiation sites in the porcelain layer were 15 and 68 MPa. It is concluded that the fracture initiation sites of dental ceramics are controlled primarily by the location and size of the critical flaw, and not by specimen thickness.

Three-dimensional finite element analysis of the shear bond test

OBJECTIVES The purpose of this study was to use finite element analyses to model the planar shear bond test and to evaluate the effects of modulus values, bonding agent thickness, and loading conditions on the stress distribution in the dentin adjacent to the bonding agent-dentin interface. METHODS All calculations were performed with the ANSYS finite element program. The planar shear bond test was modeled as a cylinder of resin-based composite bonded to a cylindrical dentin substrate. The effects of material, geometry and loading variables were determined primarily by use of a three-dimensional structural element. Several runs were also made using an axisymmetric element with harmonic loading and a plane strain element to determine whether two-dimensional analyses yield valid results. RESULTS Stress calculations using three-dimensional finite element analyses confirmed the presence of large stress concentration effects for all stress components at the bonding agent-dentin interface near the application of the load. The maximum vertical shear stress generally occurs approximately 0.3 mm below the loading site and then decreases sharply in all directions. The stresses reach relatively uniform conditions within about 0.5 mm of the loading site and then increase again as the lower region of the interface is approached. Calculations using various loading conditions indicated that a wire-loop method of loading leads to smaller stress concentration effects, but a shear bond strength determined by dividing a failure load by the cross-sectional area grossly underestimates the true interfacial bond strength. SIGNIFICANCE Most dental researchers are using tensile and shear bond tests to predict the effects of process and material variables on the clinical performance of bonding systems but no evidence has yet shown that bond strength is relevant to clinical performance. A critical factor in assessing the usefulness of bond tests is a thorough understanding of the stress states that cause failure in the bond test and then to assess whether these stress states also exist in the clinical situation. Finite element analyses can help to answer this question but much additional work is needed to identify the failure modes in service and to relate these failures to particular loading conditions. The present study represents only a first step in understanding the stress states in the planar shear bond test.

Materials Science Comparative Evaluation of Ceramic-metal Bond Tests Using Finite Element Stress Analysis

Eleven porcelain-fused-to-metal bond tests were analyzed for interfacial shear stress distribution using finite-element stress analysis. Stress concentration effects are significant in ten of the 11 tests. A high probability of tensile failure within porcelain or the interfacial region was found in eight of the 11 tests analyzed.Eleven porcelain-fused-to-metal bond tests were analyzed for interfacial shear stress distribution using finite-element stress analysis. Stress concentration effects are significant in ten of the 11 tests. A high probability of tensile failure within porcelain or the interfacial region was found in eight of the 11 tests analyzed.

Weibull analysis and flexural strength of hot-pressed core and veneered ceramic structures

OBJECTIVE To test the hypothesis that the Weibull moduli of single- and multilayer ceramics are controlled primarily by the structural reliability of the core ceramic.Methods. Seven groups of 20 bar specimens (25 x 4 x 1.2 mm) were made from the following materials: (1) IPS Empress--a hot-pressed (HP) leucite-based core ceramic; (2) IPS Empress2--a HP lithia-based core ceramic; (3 and 7) Evision--a HP lithia-based core ceramic (ES); (4) IPS Empress2 body--a glass veneer; (5) ES (1.1 mm thick) plus a glaze layer (0.1 mm); and (6) ES (0.8 mm thick) plus veneer (0.3 mm) and glaze (0.1 mm). Each specimen was subjected to four-point flexure loading at a cross-head speed of 0.5 mm/min while immersed in distilled water at 37 degrees C, except for Group 7 that was tested in a dry environment. Failure loads were recorded and the fracture surfaces were examined using SEM. ANOVA and Duncans multiple range test were used for statistical analysis. RESULTS No significant differences were found between the mean flexural strength values of Groups 2, 3, 5, and 6 or between Groups 1 and 4 (p>0.05). However, significant differences were found for dry (Group 7) and wet (Groups 1-6) conditions. Glazing had no significant effect on the flexural strength or Weibull modulus. The strength and Weibull modulus of the ES ceramic were similar to those of Groups 5 and 6. SIGNIFICANCE The structural reliability of veneered core ceramic is controlled primarily by that of the core ceramic.

Three-dimensional finite element analysis of glass-ceramic dental crowns

Because of the improved esthetic potential of glass-ceramic crowns as dental restorations, they are sometimes preferred over metal-ceramic crowns for restoration of anterior teeth. Because of their relatively high strength, these ceramic crowns are also frequently used for restoration of posterior teeth. However, due to the larger magnitude of biting forces on posterior teeth, intraoral fracture of all-ceramic crowns tends to occur more frequently in posterior crowns (Moffa, 1988). The objective of this study was to determine the relative influence of load orientation and the occlusal thickness of posterior ceramic crowns on the stress distribution which develops under these loading and design conditions. Three-dimensional finite element models for a molar crown were developed to determine the stress distribution under simulated applied loads. Glass-ceramic crowns with occlusal thicknesses of 0.5, 1.5, and 3.0 mm were considered. The largest principal tensile stresses induced in ceramic due to a distributed load of 600 N applied in a cuspal region were approximately 12 and 182 MPa for vertical and horizontal loading orientations, respectively. Stresses which developed in the facial and lingual marginal regions were primarily compressive under vertical loads. However, tensile stresses developed when the load was applied horizontally. Differences in stress distribution within crowns with the three occlusal thicknesses occurred only near the site of loading. Because of the relatively large failure rates of ceramic crowns in the posterior regions, these restorations should be strengthened by improvement in design, composition, and thermal processing conditions. Before any significant progress is made in these areas, these restorations should be used for the anterior teeth. The results of this study suggest that orientation of the applied load has a more important effect on development of large tensile stresses than the occlusal thickness of ceramic.

Failure analysis of resin composite bonded to ceramic

OBJECTIVE To use fractographic principles to classify the mode of failure of resin composite bonded to ceramic specimens after microtensile testing. METHODS A leucite-based ceramic (IPS Empress)-E1) and a lithia disilicate-based ceramic (IPS Empress2)-E2) were selected for the study. Fifteen blocks of E1 and E2 were polished through 1 microm alumina abrasive. The following ceramic surface treatments were applied to three blocks of each ceramic: (1) 9.5% hydrofluoric acid (HF) for 2 min; (2) 4% acidulated phosphate fluoride (APF) for 2 min; (3) Silane coating (S); (4) HF+S; (5) APF+S. An adhesive resin and a resin composite were applied to all treated surfaces and light cured. Twenty bar specimens for each group were prepared from the composite-ceramic blocks and stored in 37 degrees C distilled water for 30 days before loading to failure under tension in an Instron testing machine. Fracture surfaces were examined using scanning electron microscopy and X-ray dot mapping. Statistical analysis was performed using one-way ANOVA, Duncans multiple range test, and Weibull analyses. RESULTS Similar surface treatments were associated with significantly different bond strengths and modes of failures for E1 and E2. All fractures occurred within the adhesion zone. The microstructural difference between etched E1 and E2 ceramics was a major controlling factor on adhesion. SIGNIFICANCE The quality of the bond should not be assessed based on bond strength data alone. Mode of failure and fractographic analyses should provide important information leading to predictions of clinical performance limits.

Degradability of Dental Ceramics

The degradation of dental ceramics generally occurs because of mechanical forces or chemical attack. The possible physiological side-effects of ceramics are their tendency to abrade opposing dental structures, the emission of radiation from radioactive components, the roughening of their surfaces by chemical attack with a corresponding increase in plaque retention, and the release of potentially unsafe concentrations of elements as a result of abrasion and dissolution. The chemical durability of dental ceramics is excellent. With the exception of the excessive exposure to acidulated fluoride, ammonium bifluoride, or hydrofluoric acid, there is little risk of surface degradation of virtually all current dental ceramics. Extensive exposure to acidulated fluoride is a possible problem for individuals with head and/or neck cancer who have received large doses of radiation. Such fluoride treatment is necessary to minimize tooth demineralization when saliva flow rates have been reduced because of radiation exposure to salivary glands. Porcelain surface stains are also lost occasionally when abraded by prophylaxis pastes and/or acidulated fluoride. In each case, the solutes are usually not ingested. Further research that uses standardized testing procedures is needed on the chemical durability of dental ceramics. Accelerated durability tests are desirable to minimize the time required for such measurements. The influence of chemical durability on surface roughness and the subsequent effect of roughness on wear of the ceramic restorations as well as of opposing structures should also be explored on a standardized basis.

Work of adhesion of resin on treated lithia disilicate-based ceramic.

OBJECTIVE This study is to test the hypothesis that chemical etching and silane coating of a ceramic surface will influence the work of adhesion (WA) of adhesive resin to dental ceramic. METHODS A hot-pressed lithia disilicate-based ceramic was used as a model material to investigate the influence of probing media and surface treatments on WA using a dynamic contact angle analyzer. Eighty ceramic specimens were randomly divided into eight experimental groups and treated as follows: (1 and 3) as polished; (2 and 4) etched with 9.5% hydrofluoric acid (HF) for 1 min; (5) etched with 4% acidulated phosphate fluoride (APF) for 2 min; (6) silane coated; (7) etched with HF for 1 min and silane coated; (8) etched with APF for 2 min and silane coated. Advancing and receding contact angles (theta(a) and theta(r)) were measured using high purity water (gamma = 72.6 mN/m) for groups 1 and 2, and a liquid resin (gamma = 39.7) for groups 3-8 as probing liquids. RESULTS The liquid resin medium yielded a lower WA than water. Silanization produced a significantly lower WA (p < 0.001) than non-silanated surfaces. Etching alone consistently yielded a greater WA for all surface treatments (p < 0.001). SIGNIFICANCE The silanated ceramic surface exhibited a lower surface energy and did not enhance bonding to the liquid resin by work of adhesion.

Influence of Connector Design on Fracture Probability of Ceramic Fixed-partial Dentures

Fracture of ceramic fixed-partial dentures (FPDs) tends to occur in the connector area because of stress concentrations. The objective of this study was to test the hypothesis that the radius of curvature at the gingival embrasure of the FPD connector significantly affects the fracture resistance of three-unit FPDs. Two three-dimensional finite element models (FEMs), representing two FPD connector designs, were created in a manner corresponding to that described in a previous experimental study (Oh, 2002). We performed fractographic analysis and FEM analyses based on CARES (NASA) post-processing software to determine the crack initiation site as well as to predict the characteristic strength, the location of peak stress concentrations, and the risk-of-rupture intensities. A good correlation was found between the experimentally measured failure loads and those predicted by FEM simulation analyses. Fractography revealed fracture initiation at the gingival embrasure, which confirms the numerically predicted fracture initiation site. For the designs tested, the radius of curvature at the gingival embrasure strongly affects the fracture resistance of FPDs.