Alteration of the intaglio surface of lithium disilicate glass-ceramic.

STATEMENT OF PROBLEM: Clinical adjustment of a lithium disilicate glass-ceramic (LDGC) restoration may necessitate its return to the laboratory for additional firing. Evidence of how the intaglio surface should be re-etched after internal adjustment, or after refiring, is lacking. PURPOSE: The purpose of this in vitro study was to investigate the effects of different sequences of etching, refiring, diamond rotary instrument adjustment, airborne-particle abrasion, and re-etching on the microstructure and surface roughness of the intaglio surface of heat-pressed LDGCs. MATERIAL AND METHODS: Heat-pressed LDGC specimens were ground with abrasive paper to produce a uniformly flat surface. The groups (n=3) were subjected to different combinations of etching, refiring, diamond rotary instrument adjustment, airborne-particle abrasion, and re-etching. X-ray diffraction was used to characterize the crystalline phases. Scanning electron microscopy and surface profilometry were used to characterize the microstructure and surface roughness. RESULTS: Qualitative differences were observed in the surface texture of specimens etched for different periods. Excessive etching revealed more of the underlying lithium disilicate crystallites and caused surface pitting for the longest etching period studied. Refiring altered the surface condition but did not completely remove the texture created by the original etching. Diamond rotary instrument adjustment resulted in appreciable surface damage and a higher mean value of measured surface roughness (with or without re-etching) than the other groups. Airborne-particle abrasion caused embedding of particles in the specimen surface, likely corresponding to the abrasion media, although this process resulted in qualitatively less surface damage than diamond rotary instrument adjustment. CONCLUSIONS: Excessive etching, refiring, and adjustment by airborne-particle abrasion or diamond rotary instrument result in qualitative changes in surface condition. Adjustment by diamond rotary instrument results in appreciable surface damage.

The effect of endodontic access on all-ceramic crowns: A systematic review of in vitro studies.

OBJECTIVES: The aim of this systematic review was to identify from in vitro studies the effect of endodontic access on the fracture resistance and damage around the access cavity of all-ceramic crowns. DATA: The articles identified were screened by two reviewers according to inclusion and exclusion criteria. The reference lists of articles advanced to second round screening were hand searched to identify additional potential articles. The risk of bias for the articles was independently performed by two reviewers. SOURCES: An electronic search was conducted on PubMed/Medline, Web of Science, Scopus and Embase databases with no limitations. STUDY SELECTION: 383 articles were identified, of which, eight met the inclusion criteria and formed the basis of this systematic review. Factors investigated in the selected articles included the, presence of microcracks at the access cavity, repair protocol, ceramic type, crown fabrication method, luting agent and grit size of the diamond bur. The risk of bias was deemed to be high for three, medium for two and low for three of the reviewed studies. The high level of heterogeneity across the studies precluded meta-analyses. CONCLUSION: Based on the currently available scientific evidence, a 'best practice' protocol with regard to improving the fracture resistance of endodontically accessed and repaired all-ceramic crowns cannot be conclusively identified. However, some key factors which potentially impact on the fracture resistance of endodontically accessed and repaired all-ceramic crowns have been isolated. Cautious clinical interpretation of these factors is concluded for the maintenance of the crown as a permanent restoration. CLINICAL SIGNIFICANCE: Key factors which impact on the fracture resistance of endodontically accessed and repaired all-ceramic crowns have been isolated from in vitro studies. Cautious clinical interpretation of these factors is advised for the maintenance of the crown as a permanent restoration.

Effects of repeated processing on the strength and microstructure of a heat-pressed dental ceramic.

STATEMENT OF PROBLEM: The excess material produced after heat pressing a lithium disilicate glass ceramic restoration can be either discarded or reused. The reuse of this material requires that any degradation of the material quality be investigated. PURPOSE: The purpose of this study was to investigate the number of times that leftover lithium disilicate material can be re-pressed and to determine the effect that repeated use has on material properties. MATERIAL AND METHODS: A large (6.1 g) lithium disilicate ingot (A3.5) was heat pressed to yield a ceramic disk (15 × 1.5 mm) for testing. The leftover material was reused to produce a further 3 disks, with the number of pressings increasing for each specimen. An additional unpressed group was included to investigate the properties before pressing so that, in total, 5 groups were established. Specimens were tested for biaxial flexural strength, Vickers hardness, and fracture toughness. X-ray diffraction was used to characterize the crystalline phase, scanning electron microscopy for the microstructure, and differential scanning calorimetry for the thermal properties. RESULTS: No significant difference was found in the biaxial flexural strength of the groups. The hardness of the material decreased, and no significant difference was seen in fracture toughness with repeated pressings. An increase in grain size was observed with increased pressings. By using x-ray diffraction analysis, lithium disilicate was identified as the main crystal phase, and no difference in crystalline composition was found with repeated processing. CONCLUSION: This material can be reused while maintaining good mechanical properties and without significantly altering the chemical or crystalline composition in an adverse manner.

Endodontic access cavity simulation in ceramic dental crowns.

OBJECTIVES: It is proposed that a non-uniform rational B-spline (NURBS) based solid geometric model of a ceramic crown would be a flexible and quick approach to virtually simulate root canal access cavities. The computation of strain components orthogonal to surface flaws generated during the drilling would be an appropriate way of comparing different access cavity configurations. METHODS: A µCT scan is used to develop a full 3D NURBS geometric solid model of a ceramic crown. Three different access cavity configurations are created virtually in the geometric model and there are then imported into proprietary finite element software. A linear analysis of the each crown is carried out under appropriate in vivo loading and the results are post-processed to carry out a quantitative comparison of the three configurations RESULTS: The geometric model is shown to be a flexible and quick way of simulation access cavities. Preliminary indications are that post processed strain results from the finite element analysis are good comparators of competing access cavity configurations. SIGNIFICANCE: The generation of geometric solid models of dental crowns from µCT scans is a flexible and efficient methodology to simulate a number of access cavity configurations. Furthermore, advanced post-processing of the primary finite element analysis results is worthwhile as preliminary results indicate that improved quantitative comparisons between different access cavity configurations are possible.

Bonding auto-polymerising acrylic resin to acrylic denture teeth.

This study investigated the effect of surface treatments on the shear bond strength of an auto-polymerising acrylic resin cured to acrylic denture teeth. The surface treatments included a combination of grit-blasting and/or wetting the surface with monomer. Samples were prepared and then stored in water prior to shear testing. The results indicated that the application of monomer to the surface prior to bonding did not influence the bond strength. Grit blasting was found to significantly increase the bond strength.

Fabrication of a duplicate denture using visible light-polymerized resin as an interim denture base.

This article describes a technique for producing a duplicate denture using a visible light-polymerized (VLP) denture base for support prior to processing. A 2-part mold of the original denture was made, and then a sheet of VLP resin was reduced to a thickness of 0.5 mm and adapted to the fitting surface of the mold to create a denture base. The base was polymerized and the remaining features, such as the teeth and polished surfaces of the denture, were reproduced in wax. This technique may be helpful when performing subsequent clinical and laboratory procedures.

Heat-pressed ionomer glass-ceramics. Part II. Mechanical property evaluation.

OBJECTIVES: A series of ionomer glasses based on the formula: 4.5SiO2-1.5P2O5-(X)Al2O3-4.5CaO-0.5CaF2, were investigated where X was varied from 3.0 to 1.5 in order to develop heat pressable dental ceramics. METHODS: The glasses were heat-pressed and then subjected to different heat-treatment cycles. The mechanical properties of the glass-ceramics were investigated, specimens were tested for hardness, fracture toughness (indentation method) and flexural strength (biaxial method). RESULTS: Good mechanical properties were obtained for heat-treatments at lower temperatures (i.e. 1150 degrees C). At intermediate heat-treatment temperatures the glass-ceramics were highly crystalline which did not favor the mechanical properties. There appears to be an inverse relationship between fracture toughness and flexural strength. High fracture toughness values of 2.7 (0.4) MPam0.5 were produced for the X = 2.8 glass heat-treated for 8 h at 1150 degrees C, the flexural strength was lowest for this heat-treatment. High flexural strengths of 194.4 (75.0) MPa were obtained by heat-treating the same glass for 1 h at 1150 degrees C. Increasing the hold time increases crystal size thereby increasing the extent of microcracking in the glass-ceramic thus lowering the flexural strength. Microcracks appear to increase the fracture toughness of the glass-ceramics probably by a crack termination mechanism. SIGNIFICANCE: Good flexural strength and high fracture toughness are attainable in this system, but appear to be mutually exclusive in the materials studied. With further investigation this system could provide clinically useful materials.

Heat-pressed ionomer glass-ceramics. Part I: an investigation of flow and microstructure.

OBJECTIVES: This study investigated a series of ionomer glasses based on the formula: 4.5SiO(2)-1.5P(2)O(5-)(X)Al(2)O(3)-4.5CaO-0.5CaF(2), where X was varied from 3.0 to 1.5. The possibility of processing ionomer glasses using a heat-pressing method for dental restorations was investigated. METHODS: A simple flow test was designed to measure the amount of flow the glasses underwent as a result of heat-pressing at 1150 degrees C for different times. Heat-pressed samples of the X=3.0, 2.8, 2.4 and 2.0 glass were further heat-treated for 1 and 4 h at 1150, 1200 and 1250 degrees C to promote crystal growth. Scanning electron microscopy was used to investigate the microstructure of the glass-ceramics. X-ray diffraction was used to identify the crystalline phases in the glass-ceramics. RESULTS: The ionomer glasses exhibited excellent flow ability. Crystallization could not be suppressed during heat-pressing. Very fine scale fluorapatite crystals were present in all of the samples after heat-pressing. Mullite and/or anorthite formed as a second crystal phase. On further heat-treatment of the samples, changes in crystal phases took place. SIGNIFICANCE: Apatite was the main crystalline phase produced in the glass-ceramics; this factor is of clinical significance. In conclusion these glass-ceramics could be suitable for all-ceramic dental restorations.