Dental Materials Quiz: Ceramics and Aluminosilicates

Ceramic-Based Materials in Dental Applications

• Dental professionals are challenged by the rapid development of ceramics in the field.
• Focus on chemical composition and microstructure instead of product names to understand material differences.
• Different processing techniques influence the properties of dental ceramics.
• Ceramics can be broadly categorized into hybrid, silicate, and oxide ceramics.
• Hybrid ceramics are made by infiltrating glass or zirconia/alumina particles with a polymer or molten glass.
• Silicate ceramics contain a SiO2-rich glass fraction and crystals.
• Oxide ceramics are glass-free, often polycrystalline, and commonly include alumina, zirconia, and composites.

Aluminosilicates

• Aluminosilicates are high-content Al3+ glasses with alkali metal ions (Na+ or K+) in an Al/M ratio of 1.
• They are used in powder veneering, block "porcelains", and CAD-CAM materials.
• Feldspar-reinforced blocks contain 20-40 vol.% crystallinity and exhibit fracture toughness of 1.2 MPa√m.
• Leucite (KAlSi2O6) crystallization in aluminosilicates is induced by nucleation agents, contributing to toughness.
• Aluminosilicates have declined in use due to the popularity of monolithic approaches, alternative veneering materials, and the emergence of lithium disilicate.

Lithium-Based Glass-Ceramics

• Lithium disilicate has become a dominant material in dental ceramics.
• Different fabrication methods impact the properties of lithium disilicate, including hot pressed injectables and machinable analogs.
• Market competition is a significant factor in the evolution of dental materials.
• Lithium disilicate and dental zirconia are prominent materials in the contemporary dental market.
• Legal battles are common among companies seeking to enter the lithium disilicate market due to patent protection.

Compositional Variations

• Lithium disilicate compositions stem back to the 1950s.
• Distinguish between "lithium disilicate" used as a general term and its stoichiometric composition (2SiO2·Li2O).
• Lithium silicate (SiO2·Li2O) is another common stoichiometry in lithium-based glasses.
• Dental lithium-based glass-ceramics are multicomponent, which include additional oxides for properties like crystallization kinetics.
• The term "lithium disilicate" is used informally for materials with predominantly Li2Si2O5 phase.
• "Lithium silicate" describes those with a high Li2SiO3 phase.

### Lithium-Based Glass-Ceramics in Dentistry

• Lithium (di)silicates are dominant in prosthetic dentistry due to their mechanical and optical properties, while other glass-ceramics have declined in popularity.
• Lithium disilicate materials are popular for their superior mechanical performance and esthetics, but machinable analogs are gaining popularity due to ease of processing, cost-effectiveness, and chairside fabrication.
• Natural Selection in dentistry is similar to biology, where market forces determine the survival of materials.
• The current market for dental ceramic materials is dominated by lithium (di)silicates and dental zirconias.
• Lithium Disilicate Patent Litigation has resulted in legal battles and licensing agreements, with companies seeking to produce their own versions of lithium-based glass-ceramics.
• Compositional Variations are common in lithium-based glass-ceramics, and various oxides are added to control properties such as melting temperature, viscosity, solubility, and crystallization rate.
• Stoichiometry refers to the chemical composition of a glass vs. its crystal phase.
• Lithium disilicate (Li2Si2O5) is the primary crystal phase in many dental materials, while lithium silicate (Li2SiO3) is another common crystal phase.
• Nomenclature for these materials is not standardized, and the term "lithium (di)silicate" is often used broadly to encompass both the Li2Si2O5 and Li2SiO3 phases.

Early Development of Lithium-Based Glass-Ceramics

• Fotoceram® was the first synthetic glass-ceramic material produced and was based on a photoetchable lithium disilicate glass.
• Foturan® is a similar product fabricated by Schott AG.
• The development of these materials stemmed from the tailoring of lithium disilicate compositions.

Mechanical Properties of Glass-Ceramics

• Microstructure and Mechanical Properties: The mechanical properties of glass-ceramics are strongly linked to their microstructure. The presence of two distinct phases, glass and crystals, leads to interactions influencing the material's behavior.

• Thermal Compatibility and Stresses:

  • A higher thermal expansion coefficient (TEC) in the crystalline phase induces tensile stresses in the crystal and compressive/tensile stresses in the glass, potentially leading to crack propagation or microcracking during cooling.
  • Microcracking is more prevalent at higher crystalline volume fractions.
  • Dental glass-ceramics with high Li2SiO3 (e.g., Suprinity® PC, Celtra® Duo, Obsidian®) often exhibit microcracking due to Li2SiO3's high TEC (15.4 × 10−6 K−1 ).
  • The Li2Si2O5 phase generally shows negligible thermal mismatch, minimizing stress-induced damage.

• Crystallinity and Fracture Toughness: The degree of crystallization significantly impacts the mechanical properties, especially fracture toughness.

  • Fracture toughness generally increases linearly with the crystallized volume fraction.
  • Dental lithium (di)silicate materials show a similar relationship between fracture toughness and crystallinity, although other factors such as crystal size and residual stresses can influence this relationship.

• Crystal Morphology and Toughening:

  • Elongated crystalline phases (Li2SiO3 or Li2Si2O5) contribute to a more interlocking microstructure, enhancing fracture toughness.
  • Highly elongated crystals promote toughening mechanisms like crack deflection, branching, and bridging, resulting in R-curve behavior.
  • IPS e.max® Press, with its highly elongated crystals, exhibits better fatigue resistance than IPS e.max® CAD due to R-curve behavior.

• Crystal Orientation and Fracture Resistance:

  • Local crystal orientation can enhance fracture resistance.
  • Crystals oriented perpendicular to the crack plane can increase fracture toughness and force the crack into less favorable shear loading modes.

• Machinable vs. Pressable Materials:

  • Machinable glass-ceramics (e.g., those needing heat treatment) often have a lower degree of crystallization to facilitate machining, but this comes at the cost of lower fracture toughness. They are more susceptible to machining damage.
  • Pressable materials, while offering potential for oriented crystals and improved fracture resistance, can suffer from porosity, which weakens their strength.

• Crystallization Firing and Crack Healing:

  • Crystallization firing can heal cracks introduced by machining in two-step materials, primarily through viscous flow of the glass and capillarity forces.
  • Polishing of machined surfaces is still recommended to minimize surface flaws acting as fracture initiation sites, especially for intaglio surfaces.

Test your knowledge on the role of ceramic-based materials in dental applications, focusing on their chemical composition and microstructure. This quiz covers different types of ceramics, including hybrid, silicate, and oxide ceramics, as well as the specifics of aluminosilicates used in dental products.