Julie A. Holloway

Ceramics for Dental Applications: A Review

Over the past forty years, the technological evolution of ceramics for dental applications has been remarkable, as new materials and processing techniques are steadily being introduced. The improvement in both strength and toughness has made it possible to expand the range of indications to long-span fixed partial prostheses, implant abutments and implants. The present review provides a state of the art of ceramics for dental applications.

Effect of coloring with various metal oxides on the microstructure, color, and flexural strength of 3Y-TZP.

The purpose of this study was to investigate the effect of cerium and bismuth coloring salts solutions on the microstructure, color, flexural strength, and aging resistance of tetragonal zirconia for dental applications (3Y-TZP). Cylindrical blanks were sectioned into disks (2-mm thick, 25-mm in diameter) and colored by immersion in cerium acetate (CA), cerium chloride (CC), or bismuth chloride (BC) solutions at 1, 5, or 10 wt %. The density, elastic constants, and biaxial flexural strength were determined after sintering at 1350 degrees C. The crystalline phases were analyzed by X-ray diffraction before and after aging in autoclave for 10 h. The results showed that the mean density of the colored groups was comparable with that of the control group (6.072 +/- 0.008 g/cm(3)). XRD confirmed the presence of tetragonal zirconia with a slight increase in lattice parameters for the colored groups. A perceptible color difference was obtained for all groups (DeltaE* = 2.57 +/- 0.48 to 14.22 +/- 0.98), compared with the control. The mean grain size increased significantly for the groups colored with CC or CA at 10 wt %, compared with the control group (0.318 +/- 0.029 mm). The mean biaxial strength of CA1%, CA5%, and BC1% groups was not significantly different from that of the control group (1087.5 +/- 173.3 MPa). The flexural strength of all other groups decreased linearly with increasing concentration for both cerium salts (860.7 +/- 172 to 274.4 +/- 67.3 MPa). The resistance to low temperature degradation was not affected by the coloring process. Coloring with cerium or bismuth salts produced perceptible color differences even at the lowest concentrations. A decrease in flexural strength at the higher concentrations was attributed to an increase in open porosity.

Effect of Cubic Leucite Stabilization on the Flexural Strength of Feldspathic Dental Porcelain

Previous studies (Mackert and Evans, 1993) have shown that, when feldspathic dental porcelain is cooled, leucite undergoes a transformation from cubic to tetragonal, associated with a 1.2% volume contraction. This contraction leads to the formation of microcracks in and around the crystals and the development of tangential compressive stresses around the crystals. Our aim was to stabilize increasing amounts of the cubic form of leucite in a leucite-reinforced dental porcelain, evaluate its effect on the flexural strength, and characterize its microstructure. The hypothesis was that in the absence of crystallographic transformation, the contraction of the leucite crystals would be lower, thereby limiting the formation of microcracks and minimizing the development of tangential compressive stresses around the leucite particles. We prepared 8 porcelain compositions by mixing increasing amounts of either leucite (KAlSi 2O6) or pollucite (CsAlSi2O6) with Optec HSP porcelain (Jeneric/Pentron Inc., Wallingford, CT). Porcelain disks were made from each composition (n = 10 per group). X-ray diffraction analyses showed that the amount of stabilized leucite increased with the amount of pollucite added. The microstructure of the specimens containing tetragonal leucite was characterized by twinned leucite crystals, whereas no twinning was observed in the specimens containing cubic leucite. The evaluation of crack deflection showed that significantly less deflection occurred in the specimens containing cubic leucite. The mean biaxial flexural strength for the group corresponding to 22.2 wt% added pollucite, fired at 1038°C, was significantly lower than that for the control group. The group corresponding to 22.2 wt% added leucite fired at 1150°C exhibited a mean biaxial flexural strength significantly higher than that of all other groups that were not significantly different from the control group. Overall, the stabilization of cubic leucite reduced the flexural strength and the number of crack deflections in leucite-reinforced porcelain. Apparently, the development of tangential compressive stresses around the leucite crystals when cooled is responsible for a significant amount of strengthening of feldspathic dental porcelain.

3D and 2D Marginal Fit of Pressed and CAD/CAM Lithium Disilicate Crowns Made from Digital and Conventional Impressions

PURPOSE This in vitro study evaluated the 3D and 2D marginal fit of pressed and computer-aided-designed/computer-aided-manufactured (CAD/CAM) all-ceramic crowns made from digital and conventional impressions. MATERIALS AND METHODS A dentoform tooth (#30) was prepared for an all-ceramic crown (master die). Thirty type IV definitive casts were made from 30 polyvinyl siloxane (PVS) impressions. Thirty resin models were produced from thirty Lava Chairside Oral Scanner impressions. Thirty crowns were pressed in lithium disilicate (IPS e.max Press; 15/impression technique). Thirty crowns were milled from lithium disilicate blocks (IPS e.max CAD; 15/impression technique) using the E4D scanner and milling engine. The master die and the intaglio of the crowns were digitized using a 3D laser coordinate measurement machine with accuracy of ±0.00898 mm. For each specimen a separate data set was created for the Qualify 2012 software. The digital master die and the digital intaglio of each crown were merged using best-fitting alignment. An area above the margin with 0.75 mm occlusal-gingival width circumferentially was defined. The 3D marginal fit of each specimen was an average of all 3D gap values on that area. For the 2D measurements, the marginal gap was measured at two standardized points (on the margin and at 0.75 mm above the margin), from standardized facial-lingual and mesial-distal digitized sections. One-way ANOVA with post hoc Tukeys honestly significant difference and two-way ANOVA tests were used, separately, for statistical analysis of the 3D and 2D marginal data (alpha = 0.05). RESULTS One-way ANOVA revealed that both 3D and 2D mean marginal gap for group A: PVS impression/IPS e.max Press (0.048 mm ± 0.009 and 0.040 mm ± 0.009) were significantly smaller than those obtained from the other three groups (p < 0.0001), while no significant differences were found among groups B: PVS impression/IPS e.max CAD (0.088 mm ± 0.024 and 0.076 mm ± 0.023), C: digital impression/IPS e.max Press (0.089 mm ± 0.020 and 0.075 mm ± 0.015) and D: digital impression/IPS e.max CAD (0.084 mm ± 0.021 and 0.074 mm ± 0.026). The results of two-way ANOVA revealed a significant interaction between impression techniques and crown fabrication methods for both 3D and 2D measurements. CONCLUSIONS The combination of PVS impression method and press fabrication technique produced the most accurate 3D and 2D marginal fits.

Effect of heat treatment after accelerated aging on phase transformation in 3Y-TZP

Our purpose was to investigate the effect of heat treatment on the reversibility of the tetragonal to monoclinic transformation in 3Y-TZP, and associated surface roughness. The goals were to determine the onset temperature of the reverse transformation, and characterize surface roughness after accelerated aging, and after aging followed by heat treatment. 3Y-TZP disc-shaped specimens were sintered at temperatures from 1300 to 1550 degrees C. The reversibility of the transformation was investigated by X-ray diffraction (XRD) after accelerated aging, followed by heat treatment at temperatures from 350 degrees C up to 850 degrees C. The surface roughness (R(rms)) was measured by atomic force microscopy after polishing, after accelerated aging for 1 or 10 h, and after aging followed by heat treatment. XRD showed that the fraction of m-phase increased linearly with grain size after aging for 10 h (1.0-29.8%). The transformation was reversed for all groups after heat treatment at 850 degrees C/min., with only trace amounts of m-phase remaining for the group sintered at 1550 degrees C. A significant increase in mean surface roughness was observed after accelerated aging (1.59-7.45 nm), compared to polished groups (0.83-1.0 nm). However, the mean surface roughness after accelerated aging for either 1 or 10 h, followed by heat treatment at 850 degrees C/min. (1.18-2.1 nm), was not significantly different from that of the polished groups. This was attributed to the reverse transformation. XRD revealed that the monoclinic to tetragonal transformation, was complete after heat treatment at 500 degrees C for 1 min, for specimens sintered at 1550 degrees C and aged 10 h.

Effect of heat treatment on microcrack healing behavior of a machinable dental ceramic.

The purpose of this study was to evaluate the effect of annealing in air on the crack healing behavior of a machinable dental ceramic (Vita Mark II). The glass transition temperature and the softening point were determined by dilatometry. Polished Vita Mark II blocks (n = 12) were indented with a Vickers indenter under a 9.8 N load. Optical micrographs were taken immediately after indentation, and the crack lengths were measured. The specimens were annealed at 900 degrees C for either 30 min, 1 h, or 2 h. One group was indented and heat treated at 400 degrees C for 2 h as control. The ratio of crack length after annealing to crack length before annealing treatment was calculated for each indentation. Scanning electron microscopy was performed before and after annealing at 900 degrees C for 1 h. Scanning electron microscopy revealed shortening and blunting of the cracks after annealing. Annealing in air at 900 degrees C for either 30 min, 1 or 2 h significantly reduced the mean crack length of an indented machinable dental ceramic compared to the control group. However, the mean biaxial flexural strength was not significantly affected by an annealing treatment.

Enhanced Chemical Strengthening of Feldspathic Dental Porcelain

Previous studies on ion exchange of dental ceramics have shown that the biaxial flexural strength can be improved by exchanging potassium for sodium ions at temperatures below the strain point. The rubidium ion is bigger than the potassium ion and can also be considered as a candidate for replacing smaller ions, i.e., sodium or potassium, although it has not been used for dental ceramics. The double-step method uses the exchange of a small ion for a large ion (Li for Na) above the strain point, and the further exchange of a large ion for a small ion (K for Li) below the strain point. The purpose of this study was to compare the effect of rubidium-for-potassium ion exchange with that of potassium-for-sodium exchange (Tuf-Coat, G-C International Corp., Japan) on the flexural strength of a feldspathic dental porcelain and to test the hypothesis that a double-step ion exchange can lead to greater strengthening than potassium ion exchange alone. Weight measurements were performed before and after treatment. Qualitative chemical analyses allowed the rubidium, potassium, and sodium concentration profiles to be determined along cross-sections of the specimens. The maximum biaxial stresses were calculated after specimens were fractured in water on a ball-on-ring fixture at 0.5 mm/min. Relative to the untreated control group, the flexural strength of the potassium-exchanged groups was significantly increased, except for those treated at 400 or 500°C. All the groups treated with RbNO3 exhibited a significant increase in flexural strength, with a maximum for the group treated at 450°C. The group submitted to a double-step exchange showed a statistically significant increase in the mean flexural strength compared with the group treated with Tuf-Coat at 450°C.

Phase transformations in a leucite-reinforced pressable dental ceramic

The objectives of this study were to determine the conditions for the formation of sanidine in a pressable dental ceramic (OPC; Jeneric/Pentron). Ceramic discs (16 mm in diameter, 1.3 mm thick; n = 60) were pressed according to the manufacturers recommendations. One group was left as pressed as the control; the other groups were further heat-treated at temperatures ranging from 700 degrees C to 1100 degrees C for times ranging from 10 min to 24 h. X-ray diffraction was performed on powdered specimens. The microstructure was investigated by scanning electron microscopy (SEM). X-ray diffraction showed that sanidine was the only crystalline phase present after heat treatment at 900 degrees C for 24 h, whereas a mix of tetragonal leucite and sanidine was found in the specimens heat-treated at 900 degrees C for 12 h and 800 degrees C for 24 h. Tetragonal leucite was the only phase present in the control specimens after heat treatment at 950 degrees C and higher, and in the specimens heat-treated up to 1 h at 850 degrees C or 900 degrees C. Tetragonal and cubic leucite were found in the specimens treated at 750 degrees C or 800 degrees C for 30, 60, and 180 min and 700 degrees C for 24 h. SEM confirmed the presence of sanidine in the specimens heat-treated at 900 degrees C for 24 h. All specimens treated for up to 20 min at temperatures ranging from 700 degrees C to 950 degrees CC contained only tetragonal leucite. The clinical significance of this study is that the formation of sanidine is unlikely to occur at the temperature and time ranges needed for the staining or veneering of this leucite-reinforced pressable dental ceramic.

Effect of magnesium content on the microstructure and crystalline phases of fluoramphibole glass-ceramics

The purpose of this study was to evaluate the effect of magnesium content on the microstructure and crystalline phases of glass-ceramics in the system SiO(2)-MgO-CaO-Na(2)O-K(2)O-F. Four glass compositions were prepared with magnesium content varying from 12-18 wt%. The compositions were melted at 1400 degrees C for 2 h and cast into 30 x 8 mm ingots. Differential thermal analyses were performed on the powdered glasses at a heating rate of 20 degrees C/minute up to 1400 degrees C. Bars (4 x 8 x 25 mm) were cut from the ingots with a low-speed diamond saw, nucleated in the temperature range 600-650 degrees C for 1-2 h and crystallized in the temperature range 900-1000 degrees C for 4-6 h. The crystalline phases were analyzed by X-ray diffraction. The microstructure was investigated by scanning electron microscopy. The Vickers hardness was determined after indentation under a 9.81 N load. Differential thermal analyses revealed that crystallization occurred in the temperature range 700-800 degrees C. X-ray diffraction showed the presence of fluorrichterite as major crystalline phase regardless of the magnesium content in the parent glass or heat treatment temperature. The microstructure consisted of interlocked acicular crystals (5-10 micrometers). The highest magnesium content led to the coexistence of both a mica phase and fluorrichterite. This microstructure promoted crack deflections and arrest.

Differentiation of human mesenchymal stem cells on niobium-doped fluorapatite glass-ceramics

OBJECTIVE Our goal was to characterize the response of human mesenchymal stem cells (hMSCs) to a niobium-doped fluorapatite-based glass-ceramic (FAp). METHODS The glass was prepared by twice melting at 1525 °C for 3 h, and cast into cylindrical ingots later sectioned into discs and heat-treated to promote crystallization of fluorapatite submicrometer crystals. Tissue culture polystyrene (TCP) was used as control. The surface of the FAp discs was either left as-heat treated, ground or etched. Initial cell attachment was assessed at 3 h. Proliferation and alkaline phosphatase (ALP) expression data were collected at days 1, 4, and 8. Cell morphology was examined using SEM, at days 2 and 4. Mineralization was evaluated by Alizarin Red staining and SEM. RESULTS Initial cell attachment on as heat-treated, etched, or ground surfaces was similar to that of the positive control group (p>0.05). The percentage of area covered by living cells increased significantly on as heat-treated, etched, or ground surfaces between days 1 and 8 (p<0.05). There was no significant difference among groups in cell coverage at day 8, compared to TCP control. SEM revealed well spread polygonal cells with numerous filopodia, either attached to the ceramic surface or connected to neighboring cells. ALP expression at day 8 was significantly higher in osteogenic media compared to growth media on both FAp and control. FAp discs stained positively with Alizarin Red and calcium-rich mineralized granules associated with fibrils were observed by SEM at day 35. SIGNIFICANCE hMSCs displayed excellent attachment, proliferation, and differentiation on niobium-doped FAp glass-ceramic.