A New Dental Ceramic
The challenge was to create an improved fluorcanasite composition that is suitable for CAD/CAM processing. Dr James Lapworth, Dr Sarah Pollington and Professor Ric van Noort of the University of Sheffield and Dr Andrew Tingey of Fusion IP outline their innovative development.
Increasing demands are being placed on dental restorations, with patients requiring increased longevity and better, more natural aesthetics from their crowns and bridges. This demand is driving the development of new ceramic materials that must also conform to the challenging requirements of rapid computer aided design (CAD)/computer aided manufacturing (CAM) processing techniques. This article focuses on the development of a new dental ceramic and its progress to market.
Recent years have seen a boom in dental restorations, driven by an ageing population, whereby people are living and working longer and remaining active socially. In addition, Western society and an emerging middle class in developing economies have an ever increasing interest in personal appearance and improving their image.
This increasing demand for natural-looking restorations has prompted the development of new materials (Table I). The traditional "black" amalgam fillings, which are a mixture of approximately 50% mercury and powdered alloys consisting of silver, tin, zinc and copper, are being replaced by "white" composite resins; and demand for crowns and bridges made from gold or nickel alloy is declining in favour of more natural-looking ceramic materials.
In addition to good aesthetics, ceramics exhibit many other desirable material properties, including good 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.
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.
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 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).
Figure 1: Block of S82 fluorcanasite
ceramic ready for CAD/CAM processing
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.
CEREC inLab bench top milling machine
used to process the ceramic into a
finished restoration (ceramic block in centre)
Although the chemical durability of the new composition could not be improved to the levels of lithium disilicate, it was significantly improved over previous fluorcanasite formulations and fell well within
the accepted ISO standard for use as a core dental restorative material (EN ISO 6872:1999 for chemical stability).
On seeing these results, it was clear to Ric van Noort, Professor of Dental Materials Science, and Dr Sarah Pollington, Clinical Lecturer at the University of Sheffield, that the new material had the potential to rival the dental ceramics currently available on the market.
Professor van Noort recalled, "I think many members of the dental materials research community thought what we tried to do could not be done. To produce a fluorcanasite glass-ceramic that had a chemical solubility reduced from in excess of 6000 µg/cm
to one with a solubility of less than 1000 µg/cm2
was a major achievement."
Having achieved this technical success and proven that the material could be milled on a laboratory scale, the next challenge was to identify a suitable route to market.
Finished crown as a completed restoration
The road to market
Commercial opportunities at the University of Sheffield are managed in partnership with Fusion IP, an AIM-listed company that works with Sheffield and Cardiff Universities to commercialise intellectual property. Working with Fusion IP, the research team secured funding from the Yorkshire Proof of Commercial Concept Fund (www.yorkshireconcept.org
) to further develop the ceramic and a patent application was filed in June 2008. Fusion IP then undertook detailed assessments of the dental materials market and supply chain to ascertain the best route to market for the technology.
The supply of CAD/CAM technologies and ceramic consumables is big business. Pictet Research estimated in 2008 that the global market for dental restorations had reached $3 billion/year and was growing at a rate of 25% per annum. With established players marketing branded ceramic preparations, it was decided that the most sensible route to commercialisation was to license the new technology to one of the leading suppliers in the market.
So great was the interest in the material's improved properties that demand for evaluation of the new composition quickly grew. Several months of testing followed, during which time the University's laboratory became a small production facility, manufacturing and shipping samples of the new material to locations across Europe.
Initial feedback from potential licensees has been extremely encouraging. The aesthetic properties of the new material have been particularly well received, and industry partners have provided direct feedback on how the material could be improved to align with their manufacturing processes. The team commercialising fluorcanasite received a further boost in January 2011 when the Food and Drug Administration (FDA) approved the material for marketing in the US.
A bright future
Andrew Tingey, Portfolio Licensing Manager for Fusion IP said, "We are extremely encouraged by the level of industry interest that we have had in this material, and that we have been successful in obtaining FDA approval for it. We think that fluorcanasite has the potential to be a real commercial success story."
The technical team has embarked on another round of development of fluorcanasite to perfect the manufacturing process and balance the material's aesthetic and physical properties. The future for fluorcanasite as a dental material looks very bright.
1. G.H. Beall, "Chain Silicate Glass-Ceramics," J. Non-Cryst Solids, 129, 163-173 (1991).
2. C.W. Stokes, "Canasite Glass-Ceramics for Dental Restorations," PhD Thesis, University of Sheffield (2003).
Dr James Lapworth
is Business Manager for Healthcare Innovations.
Dr Sarah Pollington
is Clinical Lecturer in Restorative Dentistry, and
Professor Ric van Noort is Professor of Dental Materials Science,
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