Non-metallic ceramics

Non-metallic ceramics in dentistry

Non-metallic ceramics are dental materials used to manufacture crowns, veneers, inlays, and bridgework. A decade ago, the best material for creating dental prosthetics was considered to be metal-ceramic, which consists of a metal alloy frame (chromium-nickel, cobalt-chromium, gold-platinum) with a thin ceramic layer on top.

Prosthetics based on metal alloys have high strength and durability, but do not fully meet aesthetic requirements. In the area where they meet the gum, a distinct gray-blue stripe is visible, and after a short period of time, the gum sinks below the crown edge. Patients' desire to have teeth that are indistinguishable from natural teeth, that do not change color or transparency for a long time, has spurred the development of new, more advanced technologies and materials.

Main features of non-metallic ceramics

All types of prosthetics in this group used in dentistry are divided into:

• all-ceramic;
• all-zirconia;
• combined (a zirconia frame with a surface made of different ceramic alloys).

The main advantage of materials in this category is their absolute safety for health. Ceramic prosthetics do not cause malfunctions in the digestive system and do not provoke pathological changes in the mucous membrane of the oral cavity and tooth tissues.

Ceramics refract light like natural enamel, so prosthetics look natural in both natural and artificial lighting. Modern technologies allow for the manufacture of non-metallic crowns with high precision and detailed texture.

Types and Subtypes of Non-metallic Ceramics

Several types of materials with different compositions and component ratios are included in the category of non-metallic ceramics used in dentistry:

• Glass ceramics
• Feldspathic ceramics
• Hybrid materials
• E.max (lithium disilicate)
• Zirconia dioxide
• Combinations of ceramics and zirconia

The main characteristics of ceramic materials used in dentistry and criteria for selection in prosthetics are:

• Flexural strength (MPa)
• Appearance
• Field of application (prosthetics of the frontal or lateral row, single crowns, veneers, or bridgework)
• Manufacturing technology (layered firing, pressing, innovative CAD/CAM methods, Cerec)

Ceramic materials can also be classified by firing temperature as high-fired (above 1260 degrees Celsius), medium-fired (1080-1260), low-fired (850-1080), and soft-fired (660-850) materials.

Classification of materials into groups 

Ceramic materials, depending on the composition, structural state, processing method, are classified into groups:

• CL-I (powder/liquid)
• CL-II (glass ceramics)
• CL-III (crystalline materials with increased strength)

Class I (CL-I) - Ceramic materials created from fine particles of silicon dioxide dissolved in a glassy matrix. Materials in this group, which are characterized by high light transmission and relatively high fragility, are used to create single non-metallic crowns and veneers on front teeth if enamel (50% or more) and tooth structure are well preserved. The risk of prosthesis fracture is minimized with anatomically adequate loading on the dental arch. A material thickness of 0.2-0.3 mm is used for restorations.

Class CL-II is glass-ceramic material that is divided into two subgroups, a and b. CL-IIa materials are characterized by a low or moderate content of crystalline substance. They are used for restorations of front teeth, premolars, and less often molars.

Compared to class I compositions, CL-IIa non-metallic ceramic can withstand higher loads and has shown superior clinical results in cases of exposed dentin areas that come into contact with the prosthesis in the fixation area. Materials in this subgroup have a minimum working thickness of 0.8 mm, making them suitable for making volumetric veneers and inlays in the area of the molar teeth.

CL-IIb is a subgroup of materials with medium and high levels of crystalline glass content (the glass matrix surrounds a phase consisting of individual, loosely adjacent crystals). The list of materials in this subgroup includes lithium disilicate - E.max (glass-ceramic based on silicon dioxide, aluminum oxide, potassium oxide, lithium dioxide, and phosphorus compounds). This material, which has high strength and an aesthetic appearance, is used to make all-ceramic crowns for front and back teeth, as well as veneers.

A new type of dental ceramic in this subgroup is lithium disilicate reinforced with zirconia (glass-ceramic reinforced with 10% zirconia dioxide crystals). This innovative all-ceramic material has demonstrated excellent optical properties, structural strength, and wear resistance in the context of single restorations of any localization.

CL-III (crystalline materials of increased strength) with a layer thickness of 1.2 to 1.5 mm, is represented by two subclasses:

• Materials of the CL-IIIa group are strong composite compositions that are varieties of crystalline ceramics consisting of 85% of a crystalline network interwoven with fragments of aluminosilicate glass. Currently, this composition is rarely used, gradually giving way to 100% polycrystalline non-metallic ceramics.
• CL-IIIb is 100% ceramic created on the basis of aluminum oxide and zirconium dioxide. This innovative material is suitable for making prostheses in cases of extensive tooth destruction and total loss of natural tissues, with a high risk of stress and deformation in the tooth structure.

A dentist chooses a material for prosthesis taking into account a complex of individual factors, but the priority criteria are the full restoration of aesthetics and functionality of the teeth.