What are the emerging ceramic-based materials for Dentistry?
I. Denry and J.R. Kelly
Three new ceramic-based materials have recently been introduced in dentistry: monolithic zirconia, zirconia-containing lithium silicate ceramics, and interpenetrating phase composites. These emerging restorative materials stem from very different technological approaches that are likely to lead to further developments.
Purpose of the Review
- To give an overview of a selection of emerging ceramics and issues for dental or biomedical applications, with emphasis on specific challenges associated with full-contour zirconia ceramics, and a brief synopsis on new machinable glass-ceramics and ceramic-based interpenetrating phase composites.
- Selected fabrication techniques relevant to dental or biomedical applications such as microwave sintering, spark plasma sintering, and additive manufacturing are also reviewed.
- Introduction of zirconia ceramics has opened a wide range of all-ceramic applications unthinkable 30 years ago.
- The consensus, however, is on caution in selecting highest product quality and strict respect of manufacturers’ recommendations, with special attention on sintering temperature.
- Interpenetrating phase composites show great promise as excellent attempts at reproducing tooth structure. Remarkable progress has been made in ceramic processing and development over the past few years. It is likely that further breakthroughs will occur in the near future.
Full-contour Zirconia Ceramics
- Zirconia ceramics were introduced in dentistry more than a decade ago (Denry and Kelly, 2008; Kelly and Denry, 2008). Clinical studies have demonstrated their excellent performance despite early issues involving chipping of veneering porcelain (Sailer et al., 2006, 2007).
- Zirconia ceramics owe both clinical popularity and success to their outstanding mechanical properties and ease of machining in the green stage via computer-aided design and computer-aided manufacturing (CAD-CAM) technology.
- It can be argued that the perception of porcelain issues likely contributed to the development of full-contour zirconias.
- However, the use of full-contour zirconias in dental applications, which include single- and multi-unit restorations, full-arch implant supported prostheses, abutments, implants, and orthodontic brackets, raises a set of unique challenges stemming from shade production, tribological behavior, and long-term chemical stability.
- It is well-established that every step of the fabrication process of zirconia ceramics has to be carefully controlled to achieve expected mechanical and chemical properties.
- Blank fabrication is the first step of the fabrication process, in which powder chemical purity, granule characteristics, type of pressing, and pre-sintering treatment all play a critical role in final properties.
- A glassy phase forming a continuous layer (1.5 to 2 nm) at grain boundaries and multiple junctions was present in the material containing the largest amount of impurities, together with a greater grain size.
- The implications are that cubic grains are enriched in yttrium, leaving surrounding tetragonal grains depleted, less stable, and more susceptible to transformation (Chevalier et al., 2004).
- The presintering conditions of blanks directly influence machinability in the green stage, final sharpness, and accuracy from the sizes and shapes of chippings (Filser et al., 2003).
- Sintering temperature and duration determine grain size, amount of cubic phase, and yttrium segregation, which in turn dictate metastability, mechanical properties, and resistance to low-temperature degradation (LTD).
- The sintering temperature will also determine the amount of cubic phase and yttrium distribution (Matsui et al., 2003), which have been shown to directly influence resistance to LTD (Chevalier et al., 2004).
- Although, to the authors’ knowledge, no clinical evidence of LTD has yet been reported for dental zirconias, the combination of lower-grade powders, high sintering temperatures, and direct exposure to oral fluids has the potential to trigger this slow but autocatalytic phenomenon (Keuper et al., 2013).
Tooth Color Reproduction
- Precise tooth color reproduction presents one of the most significant clinical challenges associated with full-contour zirconia.
- A popular means of coloring zirconia restorations is by infiltration of various metal salts at low concentrations (Suttor et al., 2004). However, the infiltration technique has some drawbacks, such as a non-uniform color due to the possible presence of porosity gradients (Shah et al., 2008).
- Esthetics matching with monolithic zirconia also relies on achieving an acceptable degree of translucency. Mean grain size influences translucency through the number of grain boundaries, with smaller grain sizes leading to decreased translucency due to the larger number of grain boundaries.
Final Surface State and Tribological Behavior
- The quality of the final surface state in monolithic zirconia restorations is particularly important, since it will condition both metastability and tribological behavior.
- Chair-side polishing to a mirror finish successfully eliminates the thin layer of monoclinic phase and compressive stresses but may not fully remove deep defects created by grinding, due to grain pullout and formation of microcraters.
- Although wear protocols vary widely, there seems to be a consensus on the fact that glazed zirconia is more abrasive than polished or as-sintered zirconia (Figueiredo-Pina et al., 2013; Janyavula et al., 2013; Kontos et al., 2013).
Dental Implant Abutments (the importance of processing/property relationships and design)
Abutment surfaces are likely to play a key role in long-term clinical performance, with significant differences between machined and as-heat-treated surfaces. Machined surfaces exhibit extensive micro-cracking as well as some grain refinement within deep machining grooves.
Zirconia-containing Lithium Silicate Ceramics (ZLS)
- Lithium silicate-based glass-ceramics were recently introduced as machinable materials (Celtra™, Dentsply; Suprinity®, Vita) for CAD-CAM techniques, with claimed mechanical properties comparable with those of lithium disilicate glass-ceramics (L2S).
- The development of zirconia-containing lithium silicate glass-ceramics illustrates the ongoing quest for ceramic materials that offer adequate translucency combined with superior mechanical properties. These stable ceramics may offer a better reliability than zirconia ceramics but may not represent the endpoint for this quest.
Interpenetrating Phase Composites (IPCs)
- These composites are formed by infiltration of a porous structure (first phase) with a liquid to form the second interpenetrating phase.
- Melt-infiltration of glasses followed by solidification and monomer infiltration followed by thermoset polymerization are common fabrication methods (Wegner and Gibson, 2001).
- IPCs are often tougher and stronger and display a higher damage tolerance (R-curve behavior) than either pure phase.
- Currently, the dental literature on microwave sintering seems limited to work with zirconia (Almazdi et al., 2012; Kim et al., 2013; Marinis et al., 2013).
- It appears that microwave sintering does not alter fracture toughness and permits the use of higher heating rates, leading to increased productivity and reduced energy costs (Almazdi et al., 2012; Marinis et al., 2013).
- Zirconias processed by microwave sintering can have smaller grain sizes and increased translucency compared with those from conventional firing (Kim et al., 2013).
Spark Plasma Sintering (kinetic engineering)
Since this process requires that sintering be done within the confines of a die, complex shapes such as those required for dental restorations are not possible. Therefore, the emphasis is on developing special microstructures, such as carbon nano-tube-reinforced hydroxyapatite (Kim et al., 2014).
Additive Manufacturing (robocasting, 3D printing, selective laser sintering)
- Solid freeform fabrication of complex ceramic parts is mainly being explored by slurry additive manufacturing (“robocasting”), which consists of extrusion of continuous filaments or rods to additively build complex porous scaffolds, mainly for bone tissue engineering.
- One modification of this method, using an inkjet printer, has been explored to fabricate crude prostheses, but the total process remains cumbersome, and significant defect introduction has been reported (Ebert et al., 2009).