Med-Tech Innovation Materials
biocompatibility, chemical inertness and low thermal conductivity. However, the possible applications for all- ceramic dental restorations are potentially limited due to their hard, brittle nature, sensitivity to flaws and their poor fracture toughness. Currently, the industry standard ceramics such as feldspar can be used only for anterior and premolar crowns and are not sufficiently strong for use in posterior (molar) restorations. This limitation presents a real challenge for materials scientists, but further challenges are also posed by technological changes at the heart of the dental profession.
Figure 1: Block of S82 fluorcanasite ceramic ready for CAD/CAM processing
Dentistry and the digital revolution Dentistry is undergoing a revolution. Traditionally, a patient requiring a ceramic crown or bridge requires two or more visits to the dentist. On the first visit, the underlying tooth is prepared, an impression is taken and the colour/shade of the desired restoration is recorded. On the second, one to two weeks later, a custom-made restoration is bonded into place by the dentist using an appropriate resin adhesive.
CEREC inLab bench top milling machine used to process the ceramic into a finished restoration (ceramic block in centre)
During the one to two week gap, the specification for the restoration is sent to a dental laboratory, where highly skilled technicians fabricate the restoration by hand, a process that is expensive and time consuming. Recent advances in CAD/CAM manufacturing and digital imaging mean that the days of hand-made crowns and bridges are numbered. Advanced, integrated systems are now available that combine digital intra-oral scanners, 3D modelling software applications and rapid milling machines. These systems eliminate the need for physical impressions and highly skilled
Material
Table I: Different materials for indirect dental restorations Disadvantages
Advantages Gold or nickel alloy
Feldspar/leucite reinforced feldspar
Lithium disilicate ceramics Fluorcanasite
Excellent durability, excellent fracture toughness
Excellent aesthetics
Unnatural appearance, conductivity of heat/cold
Brittle nature, sensitivity to flaws, low tensile strength and fracture toughness
Low chemical solubility
Excellent aesthetics, improved fracture toughness and tensile strength
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Requires a veneer for optimum aesthetics. Higher chemical solubility, requires optimising for CAD/CAM processing
technicians. They will enable the 21st century dentist to design, create and mount an all-ceramic restoration, potentially during one patient visit. With the hardware in place, dentists will rely on a steady supply of ceramic blocks with the appropriate mechanical properties from which their crowns and bridges can be milled. The availability of a material that meets both the aesthetic demands of the patient and the functional demands of the oral environment continues to be a challenge, which means the dream of the 21st century dentist is close, but not yet a reality.
A dental ceramic for CAD/CAM processing Fracture toughness is an important criterion in ceramics that are intended for use as dental restorations, particularly for posterior (molar) crowns and bridges. Their mechanical properties are intrinsically linked to the microstructure of the material. Previous research by George Beall in the 1970s had identified that glass-ceramics based on modified chain silicate compositions such as canasite Ca5Na4K2Si12O30(OH,F)4 have particularly high fracture toughness (>5 MPa m1/2
) and bending strength
(200–300 MPa). Synthetic variants of canasite, known as fluorcanasites, also display a combination of high flexural strength and fracture toughness and compare favourably with commercially available resin-bonded glass-ceramic restorative systems, for example, lithium disilicate. However, these materials suffered from unacceptably high chemical solubility.1, 2 The challenge, therefore, was to identify a fluorcanasite composition with improved chemical durability, whilst maintaining a high fracture toughness and flexural strength, and then to assess the potential of this material as an indirect restorative material produced by CAD/CAM technology.
The work at the University of Sheffield’s School of Clinical Dentistry initially focused on producing a chemically durable formulation of fluorcanasite using variations of the composition, 60SiO2-10Na2O-5K2O- 15CaO-10CaF2. It was found that a high silica content fluorcanasite with zirconia additions exhibited improved mechanical properties whilst maintaining the chemical durability.
During the research, the team also found that reducing the CaF2 content of the fluorcanasite resulted in a reduction in chemical solubility together with substantially improved mechanical properties. One composition in particular, the S82 formulation (Figure 1), proved to have the highest fracture toughness, biaxial flexural strength and hardness (Table II). Most notably, the fracture toughness of this glass-ceramic was 4.2 ± 0.3 MPa m1/2
, which was significantly higher than the commercial
standard, lithium disilicate glass-ceramic (3.3 ± 0.8 MPa m1/2
). The optical properties were found to be significantly
improved over the commercial material: a transmittance of 72.5%, which is more optically similar to the natural tooth, compared with the commercial standard’s 37.3%. The material was found to be comparable to the commercial material in a number of other areas, including the strength of resin bonding to underlying tooth (dentine) material.
April 2011 ¦ 13
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