Massimiliano Guazzato

Fracture toughness and hardness evaluation of three pressable all-ceramic dental materials

OBJECTIVES This study evaluates the fracture toughness and hardness of three pressable all-ceramic materials: IPS-Empress, Empress 2 and an experimental ceramic material. METHODS Fifteen discs and 15 bars per material were prepared. Fracture toughness was measured with two different techniques: indentation fracture and indentation strength. During the indentation fracture tests the hardness of each material was also measured. Statistical significance among groups of population was studied using one-way Anova and Tukeys multiple comparison tests. RESULTS Fracture toughness results using the indentation strength technique (with three-point bending and biaxial flexure tests) were: IPS-Empress (1.39 (SD 0.3) and 1.32 (SD 0.3)); Empress 2 (3.14 (SD 0.5) and 2.50 (SD 0.3)) MPa x m(1/2); and the experimental ceramic (3.32 (SD 0.6) and 2.43 (SD 0.3)) MPa x m(1/2). The indentation fracture technique generated orthogonal cracks of different lengths for Empress 2 and the experimental ceramic, whether perpendicular or parallel to the lithium disilicate elongated crystals. Thus, two values were reported: Empress 2 (1.5 (SD 0.2) and 1.16 (SD 0.2)) MPa x am(1/2) and the experimental ceramic (1.67 (SD 0.3) and 1.15 (SD 0.15)) MPa x m(1/2). The IPS-Empress indentation fracture result was 1.26 (SD 0.1). The hardness results were: 6.6, 5.3 and 5.5 GPa for IPS-Empress, Empress 2 and the experimental ceramic, respectively. CONCLUSIONS No significant differences in fracture toughness and hardness results were found between Empress 2 and the experimental ceramic (P>0.05 ANOVA). Both materials exhibited fracture toughness anisotropy following pressing. They demonstrated improved fracture toughness and reduced hardness compared with IPS-Empress P<0.05(ANOVA), which should be beneficial for clinical applications.

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.

Microstructure of Alumina- and Alumina/Zirconia-Glass Infiltrated Dental Ceramics

The microstructure and the fracture behavior of four dental cerami cs In-Ceram Alumina and In-Ceram Zirconia (slip and block) were investigated with SEM in order to study the differences existing among the four materials and correlate them with a pparently contradictory previous mechanical studies. Microstructural observations indicate, In-Ceram Alu ina slip and block to be two different materials and a dissimilar mechanical behavior is also expected. It is postulated that the presence of a low modulus glassy phase and a significant porosity, counteract the transformation toughening potential of the tetragonal zirconia in In-Ceram Zirconia. Introduction In-Ceram Alumina (IA) (Vita Zahnfabrik, Bad Säckingen, and Germa ny) is a partially-sintered dental ceramic, which achieves its ultimate strength after infiltra t on with lanthanum silicate glass. The ceramic is partially sintered to avoid undesirable shrinkage and the molt n glass fills the pores resulting from the incomplete sintering process. In order to improve the mechani cal properties of IA, Vita has developed a new ceramic by adding 33vol% of CeO 2-PSZ to the original material. The new compound is known as In-Ceram Zirconia (IZ). Both materials are commer cially available as an aqueous based slip, which is used to shape the dental coping and then sintered by the dental technicia n, or alternatively as block already sintered which is milled before glass-infiltration. Processing of the block by the manufacturer should result in a mor e homogeneous material with better mechanical properties. Besides, IZ should be significantly stronger and tougher than IA due to the tetragonal to monoclinic phase transformation ( m-t) of Zirconia. However, studies regarding the mechanical properties of these materials do not consistently support such assumptions, suggesting that some other factors are also important in determining the performance of IA and Z [1-3]. The aim of this study was to investigate the microstructure of IA and IZ (slip and block) and find a rationale to this apparent incongruence between theoretical expectation and r eported values. Materials and Methods Several specimens for each material were polished to 1 μm and carbon coated for SEM observation (XL 30, Philips, Windhaven, Holland). Some other samples were polished to 1 μm, etched with ions beam coated with platinum and observed with Field Emission SEM (JSM 6000 FSEM, Joel, Tokyo, Japan). Indentations with a Vickers indenter were also made to inves tigat the crack propagation. Image analysis of SEM micrographs was used to appraise the volume fraction of each phase and porosity. Shape of the grains particle size and features related to the crack propagation were also studied. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 879-882 doi:10.4028/www.scientific.net/KEM.240-242.879

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.

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.

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.

Fracture Toughness, Microstructure and Toughening Mechanism of Leucite and Lithium Disilicate Glass Ceramics

Introduction In recent years the use of all-ceramic materials for dental prosthesis has increased rapidly. All ceramic materials offer better optical properties and biocompati bility as compared with the traditional porcelain-fused-to metallic-substrates. The main dra wback of these materials has been their brittle nature and the extreme sensitivity to flaws and d efects [1]. One class among these systems requires hot pressing by means of a special furnace to produce the required shape (pressable materials). Empress 1 (E1) (leucite reinforced glass-ceramic) and Empress 2 (E2) (lithium disilicate glass-ceramic) (Ivocalr-Vivadent, Schaan, Liechtenstein) are well known pressable materials, which have generated considerable interest b cause of their ease of fabrication (lost waxing technique) and excellent esthetic features. The fi rst material, E1, is recommended for restoring single units, and preferably in low stress bearing are as. E2 was developed in 1998s for the purpose of restoring 3-unit fixed partial dentures up to the second pre molar. Another pressable material (experimental) (E3) (also a lithium disilicate gl ass-ceramic) is about to be introduced into the market by the same manufacturer. This material is cla med to offer better mechanical and optical properties than Empress 2 and is also suitable for the fabri cation of 3-unit fixed partial dentures. The aim of the present study was to evaluate the fract ure toughness of the 3 abovementioned all-ceramic materials, using the indentation fracture (IF) and t he indentation strength (IS) techniques, and to elucidate the toughening mechanism involved in each system.