A systematic review of all-ceramic crowns: clinical fracture rates in relation to restored tooth type.

PURPOSE: The objective of this systematic review was to evaluate the clinical fracture incidence of tooth-supported all-ceramic crowns according to restored tooth type. MATERIALS AND METHODS: An electronic search of clinical trials published in English and Chinese was performed using four databases (Medline/PubMed, EMBASE, Cochrane Library, and the Chinese Biomedical Literature Database) from 1990 to 2011 and complemented by an additional manual search. The annual core and veneer fracture rates of various tooth types were estimated and compared using Poisson regression. Moreover, the 5-year cumulative incidence was calculated. RESULTS: Of 5,600 titles and abstracts retrieved, 37 publications were included, with a follow-up period that ranged from 36 to 97 months. Based on the calculated results, all-ceramic crowns demonstrated an acceptable overall 5-year fracture rate of 4.4% irrespective of the materials used. Molar crowns (8.1%) showed a significantly higher 5-year fracture rate than premolar crowns (3.0%), and the difference between anterior (3.0%) and posterior crowns (5.4%) also achieved significance. Fractures were classified as either core or veneer fractures. Core fracture rates were calculated as having a 5-year incidence of 2.5%, and a significantly higher core fracture rate was found in the posterior region (3.9%). The overall 5-year incidence of veneer fracture was 3.0%, and no clear difference was found between restored tooth types, with incidences of 2.0%, 2.5%, 1.0%, and 3.0% for incisor, canine, premolar, and molar crowns, respectively. CONCLUSIONS: Within the limitations of this study, current dental ceramic materials demonstrated acceptable 5-year core and veneer fracture incidences when used for tooth-supported single crowns in both anterior and posterior segments. A higher fracture tendency for posterior crowns was the trend for all-ceramic crowns, while molar crowns showed a significantly higher fracture rate than premolar crowns. Moreover, it is recommended that randomized controlled trials with large sample sizes be undertaken to obtain more definitive results.

Influence of hydration on nanoindentation induced energy expenditure of dentin.

Improved understanding of the effects of hydration and drying in mineralized tissues is highly desirable, particularly for physiologically hydrated biological materials such as dentin. We investigated the influence of hydration on the nanomechanical properties of healthy dentin and hypothesized that drying leads to an increase in indentation induced energy expenditure and hardness. Hydrated and dry dentin were tested with a UMIS set up with a Berkovich indenter at a maximum load of 50 mN. Values representative of the energy expenditure behavior were presented as dissipated energy, U(d), recovered energy, U(e), normalized energy expenditure index, ψ, and hardness, H. Energy expenditure index results, which normalize the energy expenditure for each test and describe the relative energy dissipation-recovery behavior of a material, suggested that, for the relatively severe contact strains about a sharp Berkovich indenter, dissipation dominates the mechanical response of both the hydrated and dry dentin. In support of our initial hypothesis, dry dentin presented a significantly higher energy expenditure index than hydrated dentin (p<0.0001). These results were primarily associated with a lower U(e) that was found upon drying. Hydration also decreased H significantly (p<0.0001). In summary, this study presents the first direct measurements of the energy expenditure behavior of hydrated and dry dentin using instrumented nanoindentation.

Insights into the structure and composition of the peritubular dentin organic matrix and the lamina limitans.

Dentin is a mineralized dental tissue underlying the outer enamel that has a peculiar micro morphology. It is composed of micrometer sized tubules that are surrounded by a highly mineralized structure, called peritubular dentin (PTD), and embedded in a collagen-rich matrix, named intertubular dentin. The PTD has been thought to be composed of a highly mineralized collagen-free organic matrix with unknown composition. Here we tested the hypothesis that proteoglycans and glycosaminoglycans, two important organic structural features found in dentin, are key participants in the microstructure and composition of the PTD. To test this hypothesis dentin blocks were demineralized with 10 vol% citric acid for 2 min and either digested with 1mg/ml TPCK-treated trypsin with 0.2 ammonium bicarbonate at pH 7.9 (TRY) or 0.1 U/mL C-ABC with 50mM Tris, 60mM sodium acetate and 0.02% bovine serum albumin at pH 8.0 (C-ABC). TRY is known to cleave the protein core of dentin proteoglycans, whereas C-ABC is expected to selectively remove glycosaminoglycans. All specimens were digested for 48 h in 37°C, dehydrated in ascending grades of acetone, immersed in HMDS, platinum coated and imaged using an FE-SEM. Images of demineralized dentin revealed a meshwork of noncollagenous fibrils protruding towards the tubule lumen following removal of the peritubular mineral and confirmed the lack of collagen in the peritubular matrix. Further, images revealed that the peritubular organic network originates from a sheet-like membrane covering the entire visible length of tubule, called lamina limitans. Confirming our initial hypothesis, after the digestion with C-ABC the organic network appeared to vanish, while the lamina limitans was preserved. This suggests that glycosaminoglycans are the main component of the PTD organic network. Following digestion with TRY, both the organic network and the lamina limitans disappeared, thus suggesting that the lamina limitans may be primarily composed of proteoglycan protein cores. In summary, our results provide novel evidence that (1) PTD lacks collagen fibrils, (2) PTD contains an organic scaffold embedded with mineral and (3) the PTD organic matrix is manly composed of glycosaminoglycans, whereas the lamina limitans is primarily made of proteoglycans protein cores.

Pressed ceramics onto zirconia. Part 1: Comparison of crystalline phases present, adhesion to a zirconia system and flexural strength.

OBJECTIVES: To compare the crystalline phases present, quantify the adhesion to zirconia and measure the mechanical properties of four commercially available pressed ceramics suitable for zirconia substructures. MATERIALS AND METHODS: This study compares the X-ray diffraction response and the mechanical properties of four different pressed ceramics (Noritake CZR Press, Vita PM9, Wieland PressXzr and IPS e.max ZirPress) to Vita In-Ceram YZ zirconia substrate. The adhesion was determined using the interfacial strain energy release rate fracture mechanics approach; in addition biaxial flexural strength values of each material was determined. RESULTS: X-ray diffraction analysis revealed that Noritake CZR Press and Vita PM9 contain leucite whereas IPS e.max ZirPress and Wieland PressXzr are non-leucite amorphous materials. The strain energy release rate results revealed that the pressed ceramics with leucite have better adhesion than non-leucite ceramics to zirconia. Differences were observed between biaxial strength results for the pressed ceramics from bilayer compared with monolayer specimens. CONCLUSIONS: Pressed ceramics compatible with zirconia tested in this study were of two types; leucite containing and non-leucite containing essentially glass ceramics. Leucite containing pressable ceramics appears to have better adhesion to zirconia.

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.

Strength, reliability, and mode of fracture of bilayered porcelain/core ceramics.

PURPOSE: The aim of this investigation was to compare the biaxial flexural strength, its reliability, and the mode of fracture of bilayered disks made of two core materials (In-Ceram Alumina and In-Ceram Zirconia), both veneered with conventional feldspathic porcelain (Vita Alpha). MATERIALS AND METHODS: One hundred forty specimens (monolithic and bilayered) of In-Ceram Alumina, In-Ceram Zirconia, and Vita Alpha were made and tested with the biaxial flexural test. Finite element analysis was used to estimate the maximum tensile stress at fracture. Data were analyzed with one-way ANOVA, Tukey HSD, and Weibull distribution. SEM was used to identify the initial crack and characterize the fracture mode. RESULTS: All specimens with the core material on the bottom surface were statistically significantly stronger and more reliable than those with the porcelain on the bottom surface. Among them, In-Ceram Zirconia was stronger than In-Ceram Alumina. There was no statistically significant difference among groups when the porcelain underwent tension. Two different modes of fracture were observed in the bilayered samples according to which material was on the bottom surface. CONCLUSION: The material that underwent tensile stress dictated the strength, reliability, and fracture mode of the specimens. The design of the restorations and the actual distribution of the tensile stresses must be taken into account; otherwise, the significant contribution of stronger and tougher core materials to the performance of all-ceramic restorations may be offset by the weaker veneering porcelain.

Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics.

The aim of this study was to investigate the biaxial flexural strength, reliability and the mode of fracture of bilayered porcelain/zirconia (Y-TZP) disks. For this purpose, 80 specimens were made from conventional dental porcelain and Y-TZP core ceramic, and equally divided into four groups as follows: monolithic specimens of porcelain; monolithic specimens of core material; bilayered specimens with the porcelain on top (facing the loading piston during testing); bilayered specimens with core material on top. The maximum load at fracture was calculated with a biaxial flexural test and finite element analysis was used to estimate the maximum tensile stress at fracture. Results were analyzed with one-way ANOVA, Tukey HSD. The reliability of strength was analyzed with the Weibull distribution. SEM was used to identify the initial crack and characterize the fracture mode. Monolithic core specimens and bilayered sample with the core material on the bottom were statistically significantly stronger than monolithic porcelain disks and bilayered samples with the porcelain on the bottom. The study, which was conducted with sample configurations that reproduce the clinical situation of crowns and fixed partial dentures, indicates that the material which lies on the bottom surface dictates the strength, reliability and fracture mode of the specimens. The contribution of strong and tough core materials to the performance of all-ceramics restorations may be offset by the weaker veneering porcelain if the actual distribution of the tensile stresses within the restoration is not taken into consideration.

Effect of sandblasting, grinding, polishing and glazing on the flexural strength of two pressable all-ceramic dental materials.

OBJECTIVES: During laboratory fabrication procedures and/or clinical adjustments, pressable materials: IPS Empress and Empress 2, may be ground, polished or sandblasted. These treatments may affect their strength by introducing microscopic flaws and defects. This study investigates the effect of these procedures on the mean flexural strength of these materials. METHODS: One hundred and forty disc specimens (14mmx1 mm) of IPS Empress and Empress 2 were prepared, and divided into seven groups of 20 specimens for each material. Groups were untreated, polished, polished and glazed, ground, ground and glazed, sandblasted, sandblasted and glazed. Surface roughness, mean biaxial flexural strength and Weibull modulus were appraised, and a scanning electron microscope was used to describe surface features. Statistical significance among groups of population was analysed using one-way Anova and Tukey's multiple comparison tests. RESULTS: Untreated and sandblasted groups showed significantly the highest roughness values, and polished the lowest for each material (p<0.05). Ground groups showed significantly lower roughness values than the sandblasted groups, and significantly higher roughness than the polished groups for each material (p<0.05). Polished groups for each material demonstrated significantly the highest mean flexural strength values (p<0.05). No significant difference in the mean strength values was found between untreated, sandblasted and ground groups for each material (p>0.05). Heat treatment had no effect on roughness or strength values of all treated groups of both materials. The Weibull modulus values for both materials varied with different treatments. They showed higher values for polished and untreated groups, and lower values for ground and sandblasted groups. CONCLUSIONS: Surface roughness may not be the only feature that determines strength. Other issues such as porosity, microstructural residual stresses, surface and bulk defects may also be pertinent.

Influence of grinding, sandblasting, polishing and heat treatment on the flexural strength of a glass-infiltrated alumina-reinforced dental ceramic.

The influence of processing-introduced flaws and heat treatment on the strength degradation of the dental core material has recently been observed. However, there are insufficient studies which investigate the role of grinding, grinding orientation, sandblasting, polishing and heat treatment on the strength of In-Ceram Alumina (IA), one of the most used glass-infiltrated alumina-reinforced dental core ceramics. To address these issues, the uniaxial flexural strength and reliability of eight groups of specimens (sandblasted, ground parallel to the tensile axis, ground perpendicular, and polished with and without heat treatment) were assessed. Statistical analyses indicate that heat treatment significantly improved the flexural strength of the material regardless of the surface treatment. Conversely, any surface treatment caused strength degradation, if it was not followed by heat treatment. Sandblasting caused the most marked strength degradation. Polishing alone (without heat treatment) did not strengthen the ceramic. The orientation of grinding in respect of the direction of the tensile stresses did not influence the ultimate tensile strength. The present study suggests that, in the case of IA, sandblasting, grinding and polishing should always be followed by heat treatment in order to avoid strength degradation of the material.

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 Tukey's 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.

Biaxial flexural strength, elastic moduli, and x-ray diffraction characterization of three pressable all-ceramic materials.

STATEMENT OF PROBLEM: Before the release of an advanced ceramic material, independent assessment of its strength, elastic modulus, and phase composition is necessary for comparison with peer materials. PURPOSE: This study compared the biaxial flexural strength, elastic moduli, and crystalline phases of IPS Empress and Empress 2 with a new experimental ceramic. MATERIAL AND METHODS: Twenty standardized disc specimens (14 x 1.1 mm) per material were used to measure the biaxial strength. With a universal testing machine, each specimen was supported on 3 balls and loaded with a piston at a crosshead speed of 0.5 mm/min until fracture. Three standardized bars (30 x 12.75 x 1.1 mm) for each material were prepared and excited with an impulse tool. The resonant frequencies (Hz) of the bars were used to calculate the elastic moduli with the equation suggested by the standard ASTM (C 1259-94). X-ray diffraction with Cu Kalpha at a diffraction angle from 20 to 40 degrees was used to identify the crystalline phases by means of a diffractometer attached to computer software. The data were analyzed with 1-way analysis of variance followed by pairwise t tests (P<.05). RESULTS: Mean biaxial strengths were 175 +/- 32, 407 +/- 45, and 440 +/- 55 MPa for IPS Empress, Empress 2, and the experimental ceramic, respectively. Elastic modulus results were 65, 103, and 91 GPa for the same materials, respectively. There was no significant difference in strength and elastic modulus between Empress 2 and the experimental ceramic. Both materials demonstrated a significantly higher elastic modulus and strength than IPS Empress. X-ray diffraction revealed leucite as the main crystalline phase for IPS Empress and lithium disilicate for both Empress 2 and the experimental ceramic. CONCLUSION: Within the limitations of this study, the improved mechanical properties of Empress 2 and experimental ceramic over those of IPS Empress were attributed to the nature and amount of their crystalline content lithium disilicate.

Mechanical properties of In-Ceram Alumina and In-Ceram Zirconia.

PURPOSE: This study compared the mechanical properties of In-Ceram Zirconia and In-Ceram Alumina. MATERIALS AND METHODS: Ninety-four disks and six bars were prepared with the slip-casting technique. The disks were used to assess biaxial flexural strength (piston on three ball), Weibull modulus, hardness, and fracture toughness with two methods: indentation fracture and indentation strength. The bars were used to measure elastic moduli (Young's modulus and Poisson's ratio). X-ray diffraction analysis of the specimens was carried out upon every step of the specimen preparation and of the fractured surfaces. RESULTS: Mean biaxial flexure strengths of In-Ceram Alumina and In-Ceram Zirconia were 600 MPa (SD 60) and 620 MPa (SD 61), respectively. Mean fracture toughness measured according to indentation strength was 3.2 MPa.m1/2 (SD 0.34) for in-Ceram Alumina and 4.0 MPa.m1/2 (SD 0.43) for In-Ceram Zirconia. Mean fracture toughnesses of In-Ceram Alumina and In-Ceram Zirconia measured according to indentation fracture were 2.7 MPa.m1/2 (SD 0.34) and 3.0 MPa.m1/2 (SD 0.48), respectively. X-ray diffraction analysis showed that little phase transformation from tetragonal to monoclinic occurred when the specimens were fractured, supporting the existence of a modest difference of fracture toughness between the two ceramics. CONCLUSION: No statistically significant difference was found in strength. In-Ceram Zirconia was tougher (P < .01) than In-Ceram Alumina when tested according to indentation strength. However, no significant difference was found in the fracture toughness when tested with the indentation fracture technique.