Evaluation of machinability and flexural strength of a novel dental machinable glass-ceramic

The fracture resistance of pulpotomized primary molars restored with zirconia crowns, lithium disilicate or resin based ceramic endocrowns

BACKGROUND

Endocrown in pediatric dentistry was rooted in the fundamental principles of preserving healthy dental tissues, leveraging contemporary adhesive methodologies.

AIM

This research aimed on assessing and comparing the fracture resistance of pulpotomized primary molars when rehabilitated with zirconia crowns and two distinct types of endocrowns, namely E-Max and Brilliant Crios.

METHODS

The study involved thirty, anonymized, freshly extracted second primary molars that underwent pulpotomy. These teeth were then evenly divided into three groups, each consisting of ten specimens: the zirconia crown, the E-Max endocrown, and the Brilliant Crios endocrown groups. Post-pulpotomy, the teeth were prepared for their respective restorations. Subsequent to this preparation, the zirconia crowns, E-Max endocrowns, and Brilliant Crios endocrowns were secured. To evaluate the fracture resistance using a computer-controlled testing machine (Instron), a progressively increasing load was applied to each group until fracture occurred. The gathered data were then analyzed for outliers and subjected to normality testing using the Shapiro-Wilk and/or Kolmogorov-Smirnov tests, with a significance threshold set at 0.05.

RESULTS

There was no statistically significant difference in fracture resistance of pulpotomized primary molars among lithium disilicate (E-Max) group (mean=1367.59N), Brilliant Crios group (mean=1349.73N) and zirconia group (mean=1240.82N).

CONCLUSION

Endocrowns can be considered a promising restoration for pulpotomized primary molars.

Recent advances in glass-ceramics: Performance and toughening mechanisms in restorative dentistry

The use of glass-ceramics in the medical field has grown significantly since the 1980s. With excellent aesthetic properties, semi-translucency, outstanding mechanical properties, corrosion resistance, wear resistance and great biocompatibility and workability glass-ceramics is one of the most commonly used materials in restorative dentistry and is widely used in veneers, inlays, onlays, all-ceramic crowns, and implant abutments. This review provides an overview of the research progress of glass-ceramics in restorative dentistry, focusing on the classification, performance requirements, toughening mechanisms and their association with clinical performance, as well as the manufacturing and fabrication of glass-ceramics in restorative dentistry. Finally, the developments and prospects of glass-ceramics in restorative dentistry are summarized and discussed.

Investigation of Spark Plasma Sintering on Microstructure-Properties of Zirconia Reinforced Fluormica Glass for Dental Restorations

BACKGROUND

Conventional sintering methods of dental ceramics have limitations of high temperature and slow cooling rates with requirements of additional heat treatment for crystallization. Spark plasma sintering (SPS) is an emerging technique that has the potential to process dental restorations with dense microstructures and tailor-made clinically relevant properties with optimized processing parameters. This study explored the potential of the SPS of zirconia-reinforced fluormica glass (FM) for dental restorative materials.

METHODS

FM glass frit was obtained through the melt-quench technique (44.5 SiO2-16.7 Al2O3-9.5 K2O-14.5 MgO-8.5 B2O3-6.3 F (wt.%)). The glass frit was ball-milled with 20 wt.% of 3 mol% yttria-stabilized zirconia (FMZ) for enhanced fracture toughness. The mixtures were SPS sintered at a pressure of 50 MPa and a heating rate of 100 °C/min for 5 min with an increase in temperature from 650-750 °C-850 °C-950 °C. Phase analysis was carried out using XRD and microstructural characterization with SEM. Micro-hardness, nano-indentation, porosity, density, indentation fracture toughness, and genotoxicity were assessed.

CONCLUSIONS

The increase in the SPS temperature of FMZ influenced its microstructure and resulted in reduced porosity, improved density, and optimal mechanical properties with the absence of genotoxicity on human gingival fibroblast cells.

Influence of Occlusal Thickness and Radicular Extension on the Fracture Resistance of Premolar Endocrowns from Different All-Ceramic Materials

Endocrowns are primarily recommended in a molar region with a standardized preparation design. The aim of the study was to evaluate the effect of different occlusal preparation depths, pulp chamber-radicular extension, and all-ceramic materials on the fracture resistance of premolar endocrowns. Ninety human premolar teeth were root canal treated, randomly divided into three main groups according to all-ceramic material used for fabrication as Lithium Disilicate (LD) ceramic, Polymer infiltrated ceramic (PIC) and High translucency zirconia (HTZ). They were further subdivided into three subgroups (n = 10) according to preparation design of 2 mm occlusal reduction, 4.5 mm occlusal reduction and 4.5 mm occlusal reduction with 2 mm radicular extension. The endocrowns from respective restorative materials were fabricated, surface conditioned, and cemented with self-adhesive resin cement. All samples were thermocycled for 5000 cycles and subjected to compressive static load at 45° angluation with the cross-head speed of 0.5 mm/minute until the fracture. The mean fracture resistance of LD ceramic at 2 mm, 4.5 mm thickness and radicular extension was 62.55 MPa, 45.80 MPa, 74.27 MPa respectively. The corresponding values for the PIC and HTZ ceramics were 26.30 MPa, 21.65 MPa, 25.66 Mpa and 23.47 MPa, 27.30 MPa, 37.29 MPa respectively. The LD ceramic and greater extension inside the pulp chamber had higher fracture resistance.

Zirconia toughened mica glass ceramics for dental restorations: Wear, thermal, optical and cytocompatibility properties

BACKGROUND

In an effort to design novel zirconia reinforced mica glass ceramics for dental restorations, clinically relevant properties such as wear, coefficient of thermal expansion, optical transmittance, and cytocompatibility with human gingival fibroblast cell lines were investigated in the present study.

MATERIALS & METHODS

Microstructure analysis of two body wear of heat treated mica glass ceramic ceramics (47.2 SiO2-16.7 Al2O3-9.5 K2O-14.5 MgO-8.5 B2O3-6.3F wt.%) reinforced with 20wt.% YSZ, were evaluated against a steatite antagonist in a chewing simulator following Willytec Munich method. In addition, Coefficient of thermal expansion (CTE), total transmittance, scattering coefficient and cytocompatibility on human gingival fibroblast cell lines were performed and compared to the commercially available dental ceramic systems.

RESULTS

The experimental mica glass ceramic demonstrate micro-ploughing, pull out and debris formation along the cutting surface, indicating abrasive wear mechanism. Thermal expansion of mica glass ceramic composite was recorded as 5×10-6/°C, which is lower than the thermal expansion of commercially available core and veneering ceramics. Further, significant differences of transmittance and scattering coefficient of mica glass ceramics with 20wt.% YSZ with commercial dental ceramics was found and extensive fibroblast cell spreading with filopodial extension, cell-to-cell bridges and proliferation with human gingival fibroblast cell lines.

CONCLUSION

With acceptable cytocompatibility with human gingival fibroblast cells and better wear properties with respect to commercial IPS emax Press, the mica glass ceramic composites (47.2 SiO2-16.7Al2O3-9.5 K2O-14.5 MgO-8.5 B2O3-6.3F wt.%) with 20wt.% YSZ have the potential for dental restorative applications as machinable veneering ceramics.

Fracture resistance of teeth restored with endocrowns: An in vitro study

OBJECTIVE

The objective of this study is to evaluate the fracture resistance and failure mode of endodontically treated teeth restored with lithium disilicate endocrowns of different crown thicknesses.

MATERIALS AND METHODS

A total of 30 endodontically treated permanent mandibular first molars were sectioned horizontally 2 mm above the highest point of the cemento-enamel junction. The specimens were divided into three groups, and each group was restored with lithium disilicate (IPS e-max press) endocrowns of different crown thicknesses (3, 4.5, and 6 mm, respectively). After cementation, specimens were stored in room temperature for 72 hours, followed by subjecting them to compressive strength testing until failure. The fracture loads and the failure mode were recorded. Statistically analysis was performed using one-way analysis of variance.

RESULTS

A statistically significant difference was found in the fracture resistance between the three groups with the highest fracture resistance in the 3 mm group, followed by the 4.5 mm group, and the least in the 6 mm group (P < .05). Most of the failures were accompanied with tooth fracture (90% in 3 mm group, 100% in 4.5 mm group, and 80% in 6 mm group).

CONCLUSIONS

Increasing the crown thickness of the endocrowns reduced the fracture resistance of restored teeth. Minimum fracture loads for all teeth restored with endocrowns were significantly higher than the maximum occlusal forces reported in the literature.

CLINICAL SIGNIFICANCE

Fracture resistance and mode of failure of lithium disilicate (IPS e-max press) endocrowns varies widely between crown thicknesses. Clinicians should be cautious with crown thickness for endocrown restorations.

Zirconia toughened mica glass ceramics for dental restorations

OBJECTIVE

The objective of the present study is to understand the role of yttria stabilized zirconia (YSZ) in achieving the desired spectrum of clinically relevant mechanical properties (hardness, elastic modulus, fracture toughness and brittleness index) and chemical solubility of mica glass ceramics.

METHODS

The glass-zirconia mixtures with varying amounts of YSZ (0, 5, 10, 15 and 20wt.%) were ball milled, compacted and sintered to obtain pellets of glass ceramic-YSZ composites. Phase analysis was carried out using X-ray diffraction and microstructural characterization with SEM revealed the crystal morphology of the composites. Mechanical properties such as Vickers hardness, elastic modulus, indentation fracture toughness and chemical solubility were assessed.

RESULTS

Phase analysis of sintered pellets of glass ceramic-YSZ composites revealed the characteristic peaks of fluorophlogopite (FPP) and tetragonal zirconia. Microstructural investigation showed plate and lath-like interlocking mica crystals with embedded zirconia. Vickers hardness of 9.2GPa, elastic modulus of 125GPa, indentation toughness of 3.6MPa·m^1/2, and chemical solubility of 30μg/cm² (well below the permissible limit) were recorded with mica glass ceramics containing 20wt.% YSZ.

SIGNIFICANCE

An increase in hardness and toughness of the glass ceramic-YSZ composites with no compromise on their brittleness index and chemical solubility has been observed. Such spectrum of properties can be utilised for developing a machinable ceramic for low stress bearing inlays, onlays and veneers.

Bioactive and inert dental glass-ceramics

The global market for dental materials is predicted to exceed 10 billion dollars by 2020. The main drivers for this growth are easing the workflow of dentists and increasing the comfort of patients. Therefore, remarkable research projects have been conducted and are currently underway to develop improved or new dental materials with enhanced properties or that can be processed using advanced technologies, such as CAD/CAM or 3D printing. Among these materials, zirconia, glass or polymer-infiltrated ceramics, and glass-ceramics (GCs) are of great importance. Dental glass-ceramics are highly attractive because they are easy to process and have outstanding esthetics, translucency, low thermal conductivity, high strength, chemical durability, biocompatibility, wear resistance, and hardness similar to that of natural teeth, and, in certain cases, these materials are bioactive. In this review article, we divide dental GCs into the following two groups: restorative and bioactive. Most restorative dental glass-ceramics (RDGCs) are inert and biocompatible and are used in the restoration and reconstruction of teeth. Bioactive dental glass-ceramics (BDGCs) display bone-bonding ability and stimulate positive biological reactions at the material/tissue interface. BDGCs are suggested for dentin hypersensitivity treatment, implant coating, bone regeneration and periodontal therapy. Throughout this paper, we elaborate on the history, processing, properties and applications of RDGCs and BDGCs. We also report on selected papers that address promising types of dental glass-ceramics. Finally, we include trends and guidance on relevant open issues and research possibilities. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 619-639, 2017.

CAD/CAM glass ceramics for single-tooth implant crowns: a finite element analysis

PURPOSE

To evaluate the load distribution of CAD/CAM mono-ceramic crowns supported with single-tooth implants in functional area.

MATERIALS AND METHODS

A 3-dimensional numerical model of a soft tissue-level implant was constructed with cement-retained abutment to support glass ceramic machinable crown. Implant-abutment complex and the retained crown were embedded in a Ø 1.5 × 1.5 cm geometric matrix for evaluation of mechanical behavior of mono-ceramic CAD/CAM aluminosilicate and leucite glass crown materials. Laterally positioned axial load of 300 N was applied on the crowns. Resulting principal stresses in the mono-ceramic crowns were evaluated in relation to different glass ceramic materials.

RESULTS

The highest compressive stresses were observed at the cervical region of the buccal aspect of the crowns and were 89.98 and 89.99 MPa, for aluminosilicate and leucite glass ceramics, respectively. The highest tensile stresses were observed at the collar of the lingual part of the crowns and were 24.54 and 25.39 MPa, respectively.

CONCLUSION

Stresses induced upon 300 N static loading of CAD/CAM aluminosalicate and leucite glass ceramics are below the compressive strength of the materials. Impact loads may actuate the progress to end failure of mono-ceramic crowns supported by metallic implant abutments.

Colorless and high strength MgO/Al2 O3 /SiO2 glass-ceramic dental material using zirconia as nucleating agent

Glasses in the system of MgO/Al2 O3 /SiO2 with different concentrations of zirconia as nucleating agent, some of them additionally doped with ZnO or P2 O5 , were annealed in a temperature range from 950 to 1150°C. The use of zirconia led to colorless glass-ceramics, which were transparent to opaque. In all studied compositions, α-/β-quartz-solid-solutions, zirconia as well as spinel or gahnite (ZnAl2 O4 )/spinel-solid-solution precipitated. The highest bending strength of 475 MPa was obtained after annealing at 1000°C for 3 h. The increase of the annealing temperature or an increase in the zirconia concentration resulted in an increase of the microhardness up to 13.3 GPa and of the fracture toughness up to 2.7 MPa m(1/2) . The addition of ZnO results in an increase of the hardness up to 12.5 GPa. The addition of ZnO or P2 O5 led to a fracture toughness of 2 MPa m(1/2) . The described physical properties had to be highly advantageous for the preparation of colorless high strength dental glass-ceramics.

Fatigue resistance of 2 different CAD/CAM glass-ceramic materials used for single-tooth implant crowns

PURPOSE

To evaluate the fatigue resistance of 2 different CAD/CAM in-office monoceramic materials with single-tooth implant-supported crowns in functional area.

MATERIALS AND METHODS

A metal experimental model with a dental implant was designed to receive in-office CAD/CAM-generated monoceramic crowns. Laterally positioned axial dynamic loading of 300 N at 2 Hz was applied to implant-supported crowns machined from 2 different glass materials for 100,000 cycle. Failures in terms of fracture, crack formation, and chipping were macroscopically recorded and microscopically evaluated.

RESULTS

Four of 10 aluminasilicate glass-ceramic crowns fractured at early loading cycles, the rest completed loading with a visible crack formation. Crack formation was recorded for 2 of 10 leucite glass-ceramic crowns. Others completed test without visible damage but fractured upon removal.

DISCUSSION

Lack in chemical adhesion between titanium abutment and dental cement likely reduces the fatigue resistance of machinable glass-ceramic materials. However, relatively better fractural strength of leucite glass-ceramics could be taken into consideration. Accordingly, progress on developmental changes in filler composition of glass-ceramics may be promising.

CONCLUSION

Machinable glass-ceramics do not possess sufficient fatigue resistance for single-tooth implant crowns in functional area.

Machinable glass-ceramics forming as a restorative dental material

MgO, SiO(2), Al(2)O(3), MgF(2), CaF(2), CaCO(3), SrCO(3), and P(2)O(5) were used to prepare glass-ceramics for restorative dental materials. Thermal properties, phases, microstructures and hardness were characterized by DTA, XRD, SEM and Vickers microhardness. Three-point bending strength and fracture toughness were applied by UTM according to ISO 6872: 1997(E). XRD showed that the glass crystallized at 892°C (second crystallization temperature+20°C) for 3 hrs consisted mainly of calcium-mica and fluorapatite crystalline phases. Average hardness (3.70 GPa) closely matched human enamel (3.20 GPa). The higher fracture toughness (2.04 MPa√m) combined with the hardness to give a lower brittleness index (1.81 µm(-1/2)) which indicates that they have exceptional machinability. Bending strength results (176.61 MPa) were analyzed by Weibull analysis to determine modulus value (m=17.80). Machinability of the calcium mica-fluorapatite glass-ceramic was demonstrated by fabricating with CAD/CAM.

Novel glass-ceramics for dental restorations

BACKGROUND

There are many different ceramic systems available on the market for dental restorations. Glass-ceramics are a popular choice due to their excellent esthetics and ability to bond to tooth structure allowing a more conservative approach. However, at present, these materials have insufficient strength to be used reliably in posterior regions of the mouth.

PURPOSE

The aim of this review article is to discuss the types of novel glass-ceramic currently be investigated including composition, microstructure and properties.

CONCLUSION

Current research in glass-ceramics focuses on the quest for a highly esthetic material along with sufficient strength to enable crowns and bridgework to be reliably placed in these areas.

CLINICAL SIGNIFICANCE

There is a gap in the market for a machinable resin bonded glass-ceramic with sufficient strength as well as excellent esthetics.