Why All-Ceramic Crowns?
In recent years there has been an observable movement in the profession toward ceramic
crowns without metal understructures: all-ceramic, or AC, crowns. Some practitioners are
confused about the indications for AC crowns and the characteristics of these crowns relative to porcelain-fused-to-metal, or
PFM, crowns. Often, claims about AC crowns made by clinicians, manufacturers and laboratories
are overly optimistic, causing disappointment and frustration for practitioners and
patients when failure occurs. What is the state of the art for AC crowns? This article presents
the most popular brandsof AC crowns, their advantages and disadvantages, indications and contraindications, and suggestions
for the future.
Marginal fit of leucite-glass pressable ceramic restorations and ceramic-pressed-to-metal restorations
Statement of problem
Fabricating a feldspathic porcelain margin on a metal-ceramic restoration with a clinically acceptable marginal fit has proven to be a technique-sensitive procedure. Pressable ceramics are advocated to solve this problem.
The purpose of this in vitro study was to compare the marginal adaptation of a pressable ceramic system when used with both all-ceramic and metal-ceramic crowns, with a traditional metal-ceramic restoration.
Material and methods
A 1.5-mm, 360-degree chamfer margin was prepared on a typodont maxillary central incisor. Polyether impressions were made and poured in a Type IV dental stone, and the following crowns were fabricated on individual dies: 15 metal ceramic restorations (MCR) (Ceramco II, Ceramco, and Argelite 60), 15 pressed-to-metal restorations (PTM) (CPC-MK, and Argelite 60), and 15 pressed ceramic restorations (PCR) (CPC-MK). The marginal fit of the crowns was evaluated every 90 degrees around the crown margin circumference under a microscope at original magnification × 45. A 1-way analysis of variance (ANOVA) was used to compare data (α=.05).
The mean marginal discrepancy for MCRs was 94 ± 41 μm, for PTMs, 88 ± 29 μm, and for PCRs, 81 ± 25 μm. The 1-way ANOVA showed no significant difference between groups (P=.568).
The marginal fit of pressed-to-metal (PTMs) and pressed all-ceramic crowns (PCRs) was similar to that of traditional metal-ceramic crowns (MCRs).
IPS Empress Crown System: Three-Year Clinical Trial Results
The IPS Empress system is a highly esthetic hot pressed glass ceramic material for fabrication of single crowns. Adhesive cementation of the system not only contributes to the esthetics but is necessary for increased strength of the crown. The purpose of this prospective clinical trial was to evaluate the longevity of 75 adhesively cemented Empress full crowns. An additional aim was to assess the adhesive cementation methodology and potential side effects.
At the three-year point, one molar crown fractured for a 1.3 percent failure rate. The resin cementation technique that was employed exhibited a low incidence of microleakage with few clinical side effects. There was a 5.6 percent incidence of postcementation sensitivity, with all symptoms subsiding by eight weeks. None of the crowns in the study required endodontic therapy.
Materials Design of Ceramic-based Layer Structures for Crowns
Radial cracking has been identified as the primary mode of failure in all-ceramic crowns. This study investigates the hypothesis that critical loads for radial cracking in crown-like layers vary explicitly as the square of ceramic layer thickness. Experimental data from tests with spherical indenters on model flat laminates of selected dental ceramics bonded to clear polycarbonate bases (simulating crown/dentin structures) are presented. Damage initiation events are video-recorded in situ during applied loading, and critical loads are measured. The results demonstrate an increase in the resistance to radial cracking for zirconia relative to alumina and for alumina relative to porcelain. The study provides simple a priori predictions of failure in prospective ceramic/substrate bilayers and ranks ceramic materials for best clinical performance.
Factorial analysis of variables influencing stress in all-ceramic crowns
The objective of this investigation was to evaluate the relative contribution of variables in the crown–cement–tooth system that can influence magnitude of maximum principal stress in all-ceramic crowns.
Factorial analysis was performed to calculate the coefficients of main and interactive effects of seven variables on maximum stress distribution in a series of finite element models of an axisymmetric stylized ceramic crown–cement–tooth system. Variables investigated at two levels were selected for their clinical relevance and included those relating to crown material and geometry (thickness and cuspal incline), cement modulus and thickness, supporting tooth core, and position at which the occlusal load was applied.
The average principal stress for all combinations of all variables was 135.1 MPa but stress magnitude ranged from 73.7 to 214.0 MPa. Crown material and thickness are of primary importance in stress magnitude but other variables (cement modulus, load position, and supporting tooth core) also contribute to the stress magnitude. Interactions between these variables can have an important influence, particularly since the stress in the crown is not necessarily sensitive to the same factors for all crown material systems.
Comprehensive evaluation of stress in a crown of a crown–cement–tooth system must integrate the influence of single variables and their interactions.
Which All-Ceramic System Is Optimal for Anterior Esthetics?
Background. As ceramic materials for dentistry evolve and patients’ demand for esthetic restorations increases, practitioners must keep up with the science as well as the demand. The authors offer guidance to the practitioner in selecting the appropriate all-ceramic systems for crowns when faced with different esthetic demands.
Conclusions. Clinicians should reserve dental ceramics with high translucency for clinical applications in which high-level esthetics are required and the restoration can be bonded to tooth structure. Ceramics with high strength tend to be more opaque and pose a challenge when trying to match natural tooth color, but they can mask discoloration when present.
Practice Implications. Knowledge of the optical properties of available ceramic systems enable the clinician to make appropriate choices when faced with various esthetic challenges.
Microtensile bond strength of different components of core veneered all-ceramic restorations Part II: Zirconia veneering ceramics
Objectives. The purpose of this research was to investigate microtensile bond strength
between one type of zirconia substrate and various commercial veneer ceramics. The effect
of an optional liner material between the core and veneer was also evaluated where applicable.
Methods. Bilayered zirconia veneer discswere fabricated from five layering and two pressable
veneer ceramics. Additionally, discs from each veneer ceramic were prepared. The discs
were cut into microbars of 6mmin length and 1mmin cross-section. The microtensile bond
strength was tested in a universal testing machine. The fracture surfaces of the microbars
were examined with scanning electron microscopy (SEM) and EDAX.
Results. The microtensile strength of Rondo Dentine and Lava Dentine veneer ceramics were
significantly higher than the other tested veneer ceramics. Furthermore, the layered systems
Rondo Dentine and Ceram Express were significantly stronger than the other tested
core–veneer ceramics. The application of liner material dramatically affected the bond
strength and failure mode, which was also material dependent. SEM analysis showed that
two pressable veneers and one type of layering veneer ceramic failed entirely cohesively in
veneer side while the remaining test groups had higher percentage of interfacial failure.
Significance. Selection of stronger veneer ceramics which have good bond strength with zirconia
can reduce the chances of chipping and delamination under function. The liner material
should only be used with some layering veneers but not in combination with pressable
veneers as it will result in weakening of the microtensile bond strength.
Fracture resistance of yttrium oxide partially-stabilized zirconia all-ceramic bridges after veneering and mechanical fatigue testing
Yttria-stabilized zirconia (Y-TZP) ceramic is a high-performance material with excellent mechanical properties suitable for fixed partial dentures. The purpose of this study was to evaluate after fatigue testing, the effect of heat-treatment and veneering on the fracture resistance of frameworks manufactured using sintered and subsequently hot isostatic pressed yttrium oxide partially-stabilized zirconia (Denzir).
The specimens were subjected to dynamic loading in water. Thereafter, using a universal testing machine, the fracture resistance of three-unit fixed partial dentures was determined; (i) of the frameworks as delivered after machining, (ii) after the frameworks had been subjected to heat-treatment similar to veneering with a glass–ceramic (Eris) or a feldspar-based ceramic (Vita D) and (iii) after the frameworks had been veneered with these ceramics. In addition, the fracture resistance of frameworks as delivered after machining not subjected to dynamic loading in water was determined.
Cyclic loading in water did not significantly (p>0.05) affect the fracture resistance. The load necessary to fracture the frameworks as delivered after machining was significantly (p<0.001) higher than for the heat-treated and veneered specimens. No significant difference was seen between the specimens veneered with Eris and those veneered with Vita D (p>0.05). For all but three specimens the fractures were located in the loading point and through one of the connectors.
Heat-treatment and veneering reduce fracture resistance of hot isostatic pressed zirconia. Nevertheless, the results obtained indicate that it is an interesting material for potential in all-ceramic restorations.
Clinical performance and wear characteristics of veneered lithia-disilicate-based ceramic crowns
The objectives of this study were to characterize the clinical performance and wear characteristics of lithia-disilicate-based ceramic crowns.
Thirty posterior crowns were made using the heat-pressing technique and lithia-disilicate-based core ceramic. Subjects were recalled annually. The quality of crowns and adjacent gingival tissues were examined using nine criteria for acceptability. All crowns were examined and ranked from 4 (Excellent) to 1 (Unacceptable) for each criterion. Impressions were made for replica models at each appointment. Wear characteristics of dental ceramic and enamel were obtained by comparing the surface of the original model with the follow-up model using a laser scanner.
Twenty nine subjects returned for the one-year recall examination. The maximum clenching force for the 30 subjects ranged from 125 to 815 N. All clinical criteria were ranked good to excellent at the one-year recall exam and no fractures were observed. The mean occlusal wear volumes for the ceramic crowns after one year were 0.19 (0.065) mm3 for premolar sites and 0.34 (0.08) mm3 for molar sites. The mean occlusal wear volumes of opposing enamel after one year were 0.21 (0.06) mm3 for premolar teeth and 0.50 (0.22) mm3 for molar teeth. The mean occlusal wear volume of ceramic molar crowns was significantly lower than the volume of enamel wear of the opposing teeth (p≤0.05).
The quality of the overall prostheses and the gingival tissues were acceptable after one year. The mean occlusal wear volume of ceramic molar crowns was significantly lower than the enamel wear volume of the opposing teeth.
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