Ceramic Systems in Dentistry Quiz

Ceramic Systems in Dentistry

• Ceramic-ceramic or all-ceramic systems have become popular due to advancements in materials and technology, offering improved fracture resistance, aesthetics, and CAD-CAM capabilities.
• Glass-ceramics are formed as glass and then heat-treated to partially crystallize, increasing strength and toughness.
• Leucite-based glass-ceramics initially were limited to anterior restorations due to lower flexural strength and fracture toughness. Examples include: IPS Empress, Cerpress SL, Finesse, and IPS e.max CAD.
• Lithia-disilicate glass-ceramics provide higher strength and fracture toughness, making them suitable for both anterior and posterior restorations. Examples include: IPS Empress 2, IPS Eris and Optec OPC 3G.
• Fluorapatite glass-ceramic veneers are compatible with lithia disilicate core materials because of their similar coefficient of thermal expansion.

Zirconia-Based Ceramics

• Zirconia (ZrO2) is a highly corrosion resistant material with unique properties, making it useful in various applications, including dentistry.
• Pure zirconia is susceptible to volume expansion during phase transformations, leading to microcracks and reduced strength.
• Doping zirconia with metallic oxides (MgO, CaO, Y2O3, Ce2O3) stabilizes the tetragonal phase, preventing detrimental phase changes.
• Stabilized zirconia is categorized into fully stabilized (FSZ), partially stabilized (PSZ), and tetragonal zirconia polycrystal (TZP), depending on the stabilizer concentration and microstructure.
• Transformation toughening occurs when metastable tetragonal zirconia crystals near a crack transform to the monoclinic phase under stress, causing volume expansion and arresting crack propagation. This mechanism significantly increases the fracture resistance of zirconia-based ceramics.

Monolithic Zirconia

• Monolithic zirconia crowns eliminate the use of weaker veneering ceramics, resulting in higher fracture resistance.
• Polished monolithic zirconia does not cause excessive wear of opposing enamel, unlike glazed zirconia.
• Monolithic zirconia can be less translucent, requiring adjustments for aesthetics, and can present challenges during occlusal adjustments, crown removal, and endodontic access.
• Translucent zirconia ceramics with higher yttria content, such as 4Y-PSZ and 5Y-PSZ, offer improved aesthetics but have lower strength and fracture toughness.

Zirconia Properties and Considerations

• Hydrothermal degradation may compromise the long-term performance of zirconia through surface roughness, grain fragmentation, and microcracking.
• Grain size in zirconia influences fracture toughness, with larger grain sizes generally resulting in higher toughness.
• Grinding procedures can affect zirconia's properties, potential for surface toughening through phase transformation, and susceptibility to microcracking depending on grit size.

Degradation Process

• The degradation process in ceramics is initiated by a transformation of the surface to the monoclinic phase.
• This monoclinic phase spreads through the surface grains and into adjacent grains due to stresses caused by the transformation.
• OH– groups are believed to be responsible for breaking atomic bonds at the surface, initiating the transformation from tetragonal to monoclinic phase.

Glass-Infiltrated Core Ceramics

• Three types of glass-infiltrated core ceramics have been developed: partially sintered alumina, magnesia-alumina spinel (MgAl2O4), and zirconia-alumina core.
• These ceramics can be infiltrated with lanthanum glass without any significant dimensional change.
• VITA In-Ceram Alumina contains 85% alumina and is produced by a slip-casting process, followed by firing at 1120 °C for 10 hours.
• The porous core ceramic framework is then infused with molten lanthanum glass.
• In-Ceram Spinel (ICS) is a magnesia alumina spinel (MgAl2O4) core ceramic that, after glass infiltration, is more translucent than In-Ceram Alumina or In-Ceram Zirconia, but has a lower mean strength (∼350 MPa vs.∼600 MPa).
• In-Ceram Zirconia, a zirconia-toughened alumina (ZTA) material, consists of 62 wt% alumina, 20 wt% zirconia, and ∼18 wt% infiltrated glass, and is primarily indicated for crown copings and three-unit anterior and posterior frameworks.
• ZTA exhibits transformation toughening with conversion from tetragonal to monoclinic phases in areas of stress, doubling its strength and increasing toughness two to four times.

Alumina Core Ceramic

• Procera AllCeram (Nobel Biocare) is an alumina-core ceramic indicated for anterior and posterior crowns.
• Crowns are produced by dry-pressing pure alumina onto an oversized die, followed by sintering.
• The resulting shrinkage creates the desired size and shape for a feldspathic porcelain veneer to be applied and sintered.
• Procera All-Ceram is more translucent than In-Ceram Zirconia and has comparable strength (620–700 MPa).
• As alumina cannot be acid-etched for micromechanical retention, its surface is sandblasted with silica-coated alumina particles to ensure sufficient resin bonding.