Ceramic Restorations

WHAT IS A CERAMIC

• Non-metallic inorganic materials - including metal oxides, borides, carbides and nitrides. May encompass glasses, glasses with crystal components and crystalline materials.

• Crystalline Structure - displaying a regular periodic arrangement of the component atoms and may exhibit ionic or covalent bonding.

• Brittle - extremely low plasticity, in which cracks can initiate and propagate without plastic deformation leading to fracture

• Strength - wide ranging

ADVANTAGES MODERN CERAMICS

• Mimic natural teeth with respect to colour and light interaction

• Biocompatible - resistant to corrosion, non-allergic

• Strong - Modern ceramics are very strong

• Acceptable tooth reduction

PROCESSING TECHNIQUES

• Powder-Liquid

• Pressed

• Machined

• Printed

MegaPascal (abbrev. MPa) - A basic unit of pressure or tension measurement

1 MPa = 145 psi, 1 MPa = 1 N/mm2

FELDSPAR-BASED

• Alumino-silicate material containing varying amounts of potassium, sodium, or calcium

• Most abundant group of minerals in the Earth's crust, making up about 50% of all rocks

• Flexural strength values usually range from 60 MPa to 70 MPa (megapascal)

• Applications - All Ceramic Veneers, Veneering materials for metal or ceramic substructure

• If produced as fine-grain machinable blocks for use with CAD/CAM systems, minimum strength value of 100 MPa

• Minimum occlusal thickness value for crowns fabricated from this material is about 2.0 mm.

• Leucite-reinforced - contain 10-25% leucite crystals. Alter the coefficient of thermal expansion (CTE) and improve the material's flexural strength, thereby inhibiting crack propagation,

GLASS CERAMICS

• Made from materials that contain mainly silicon dioxide (aka silica or quartz), which contains various amounts of feldspars

• Prepared by controlled crystallisation of glasses via different processing methods. They contain at least one type of functional crystalline phase and a residual glass

• Glass-ceramics are in widespread use for cookware, missile nose cones, and even heat shields on space vehicles

• Leucite-reinforced - Glass matrix ceramics with leucite content of approximately 50% volume were developed to improve flexural strength

• They may be opaque or translucent depending on the chemical composition and the percentage of crystalline material

• Machinable and pressable systems have much higher fracture resistance than powder-liquid systems and have shown excellent clinical results for posterior inlay and onlay applications and anterior veneer and crown restorations.

GLASS CERAMICS with FILLERS

• Glass ceramic with varying amounts of different types of crystals have either been added or grown in the glassy matrix. The primary crystal types today are leucite, lithium disilicate, or fluoroapatite

• Leucite is created in dental porcelain by increasing the K2O (potassium oxide) content of the alumino-silicate glass

• Lithium-disilicate crystals are created by adding Li2O (lithium oxide) to the alumino-silicate glass. It also acts as a flux, lowering the melting temperature of the material

GLASS CERAMICS with FILLERS - Leucite

• Feldspathic Porcelain - low-to-moderate leucite-containing feldspathic glass –these materials have become called “feldspathic porcelains” by default. Even though other categories have a feldspathic-like glass, this category is what most people mean when they say “feldspathic porcelain”

• Leucite is added to these materials to raise the coefficient of thermal expansion (CTE) of the material so that they can be applied to metals and zirconia. The amount of leucite is adjusted in the glass based on what type core it has and its CTE. These materials are typical powder-liquid materials that are used to veneer core systems and are also the ideal materials for porcelain veneers

GLASS CERAMICS with FILLERS - Lithium Disilicate

• Ivoclar introduced this material initially as Empress II and it is now marketed as IPS e.max pressable and machinable ceramics.

• Alumino-silicate glass has lithium oxide added.

• The crystals that form within this material are needle-like in shape and comprise about 70% of the volume of the glass ceramic. The shape and volume of the crystals contribute to roughly double the flexural strength and fracture toughness of this material,  increasing the flexural strength to approximately 360 - 500 MPa

• The glass phase is composed of mainly SiO2, Li2O, P2O5, ZrO2, ZnO and K2O. The glass matrix surrounds micron-size lithium disilicate crystals with sub-micron lithium orthophosphate crystals in between the lithium disilicate

• This material can be very translucent even with the high crystalline content due to the relatively low refractive index of the lithium-disilicate crystals

• The material is translucent enough that it can be used for full-contour restorations anywhere in the mouth and can be veneered with a matched porcelain.

• The machinable version is provided in a partially crystallised block to enable more rapid machining. After machining, the restoration is subjected to a secondary heat treatment to crystallise the material and obtain the final shade and mechanical properties.

• The minimum occlusal thickness values are 1.0 mm if adhesively cemented or 1.5 mm if cemented with non-adhesive materials such as glass-ionomers.

CRYSTALLINE-BASED SYSTEMS with GLASS FILLERS / INTERPENETRATING PHASE CERAMICS

• Glass-infiltrated, partially sintered alumina was introduced in 1988, and marketed under the name In-Ceram. The system was developed as an alterative to conventional metal-ceramics, and has met with great clinical success. The system uses a sintered crystalline matrix of a high modulus material (85% of the volume), in which there is a junction of the particles in the crystalline phase. This is very different than glass or glass-ceramic materials in that these ceramics consist of a glass matrix with or without a crystalline filler in which there is no junction of particles (crystals). The crystalline phase consists of alumina, alumina/zirconia, or an alumina/magnesia mixture appropriately named “spinell"

• Interpenetrating phase ceramics are characterised by two phases that are each intact three-dimensionally (intertwined) throughout the fully dense material. This material class may have improved fracture resistance relative to the individual components due to the geometrical and physical constraints that are placed on the path that a crack must follow to cause fracture. A tortuous route through at the interface of each phase or through each phase is required in order for a crack to propagate through the entire restoration. Interpenetrating phase materials are generally fabricated by first creating a porous matrix - a ceramic 'sponge'. The pores are then filled by a second-phase material. It is important to note that both the ceramic and second phase are continuous and connected to each other. This is unlike glass-ceramics which have two separate phases - a glass matrix and individual crystals.

POLYCRYSTALLINE CERAMICS - Aluminium Oxide

• One of the first all-ceramic CAD/CAM fabricated crowns was a polycrystalline alumina, called Procera AllCeram (Nobel Biocare, Kloten, Switzerland) with a strength of approximately 600 MPa. An oversized die was machined to compensate for the alumina firing shrinkage of about 20%. Alumina powder was pressed on the die, machined to form the crown form, and then fully sintered to form the final restoration.

POLYCRYSTALLINE CERAMICS - Zirconium Dioxide (Zirconia)

• Zirconia (ZrO2) is the oxidised form of zirconium (Zr)

• Computer-aided design/computer-aided manufacturing (CAD/CAM) has enabled materials to be used that ordinarily cannot be fabricated conventionally. One of the most important of these materials is yttria partially stabilised tetragonal zirconia

• Zirconia exists in three major phases: monoclinic, tetragonal and cubic

• Monoclinic is room-temperature stable. Above 1,170 °C, zirconia transforms into the tetragonal intermediate phase; at 2,370 °C, the material changes into a cubic phase.

• Monoclinic zirconia is the worst in terms of mechanical properties.

• In pure zirconia ceramics, the cubic-to-monoclinic phase transformation occurs during cooling with about a 5% volumetric expansion (causes cracks), which may then fracture zirconia at room temperature.

• The addition of other ceramic components may alter the presence and stability of these phases at room temperature. Zirconia (ZrO2) may exist primarily in the tetragonal phase at room temperature by adding components such as calcia (CaO), magnesia (MgO), yttria (Y2O3) and ceria (CeO2). If the right amount of component is added, then a fully stabilised cubic phase can be created, cubic zirconia 'diamond' jewellery. 

•  Transformation Toughening is the zirconia material’s ability to stop crack propagation by transformation of the crystals around the crack tip from Tetragonal to Monoclinic phase. Although stabilised at room temperature, the tetragonal zirconia phase may change under stress to monoclinic with a subsequent 3% volumetric increase. This dimensional change diverts energy from the crack and creates compressive stress that may stop crack propagation. This helps resist catastrophic failure - even though a crack may exist in the material, the phase change prevents it from proceeding throughout the restoration.

• Yttria tetragonal partially stabilised zirconia (Y-TZP) may be a 'universal' ceramic restorative material in that it has sufficient properties to withstand stresses in all regions of the mouth, as well as the ability to support multiple-unit restorations.

• Most zirconia restorations are fabricated by machining a porous or partially fired block of zirconia. The framework is milled oversized by about 25% and then fired at approximately 1,500 °C to fully densify the zirconia, producing a material with micron and submicron crystals with strength values from 900 MPa to 1,200 MPa.

• Yttria Concentration - Zirconia restorative materials with 3 mol% yttria concentration are very strong but exhibit low translucency. By increasing yttria content to 4 and 5 mol%, translucency increases but mechanical properties decrease from the 1,200 MPa range down to 500-800 MPa

• The transformation toughening property is mostly lost in 4 and 5 mol% zirconia materials as the cubic crystal content increases. These materials become much more susceptible to surface damage with decreased fracture resistance. This is of particular importance when occlusal adjustment of the restoration is needed. The aesthetic quality, particularly translucency, enables these materials to be used anteriorly and even as veneers

• Cementation - Zirconia is most often luted to the tooth structure using glass-ionomer or other type of cement. However, several primers and cements containing the chemical 10 methacryloyloxydecyl dihydrogen phosphate (10-MDP) may aid in creating a chemical bond with zirconia and the corresponding resin cement. Glass-ionomers bond weakly or not at all. Zirconia bonding might improve retention to the tooth structure that is particularly helpful with minimum reduction and short crown preparations. When zirconia restorations are non-adhesively cemented, a low-pressure sandblasting of the internal surface is recommended using about 25-50 psi (2 bar) with 25-50 μm alumina. This provides for some mechanical retention of the cement.

• Hardness - Vickers hardness of zirconia is 1,350 as compared to natural tooth structure of 300-500 or feldspathic ceramics of 400-500.Therefore, if it is rough, the hardness may become the dominant factor resulting in excessive wear. There are numerous polishing systems consisting of silicone or other rubber wheels embedded with diamond particles to polish ceramics. It is important to follow the recommended sequential polishing steps with low speed and light pressure using water to cool the ceramic. Final polishing may be accomplished using a felt wheel with a micron-sized diamond paste. This will provide a smooth and wear-kind surface.

• https://kuraraydental.com/wp-content/uploads/2018/05/zirconia_bond_guide-1.pdf

Giordano II, R. Ceramics overview. Br Dent J 232, 658–663 (2022). https://doi.org/10.1038/s41415-022-4242-6

INDICATIONS FOR LITHIUM DISILICATE

• ANTERIOR CROWN WITH ADEQUATE TOOTH TISSUE TO CREATE SPACE FOR RESTORATION - as often superior aesthetic result to Zirconia and reduced strength needed

• POSTERIOR CROWN WITH ADEQUATE TOOTH TISSUE TO CREATE SPACE FOR RESTORATION - where aesthetics are a concern

• POSTERIOR CROWN WITH LITTLE TOOTH TISSUE BUT SPACE ALREADY PRESENT FOR RESTORATION - don't want to remove any more tissue and little scope for mechanical retention and cementing with RMGIC so would bond with resin cement

• POSTERIOR ONLAY WITH ADEQUATE TOOTH TISSUE AND SPACE - tooth has lots of enamel which a resin cement can bond to

• ANTERIOR VENEER

INDICATIONS FOR ZIRCONIA

• RESTORATION WITH  MINIMAL TOOTH TISSSUE REMOVAL AND AESTHETICS ARE NOT A PRIMARY CONCERN

• ANTERIOR CROWN IN BRUXIST

• POSTERIOR CROWN WITH ADEQUATE TOOTH TISSUE TO CREATE SPACE FOR RESTORATION

• POSTERIOR CROWN WITH LITTLE SPACE FOR RESTORATION - want to remove the minimal amount of tooth tissue

• ANTERIOR RESTORATION SUPER WHITE TEETH - 3Y-TZP bleach shade

Preparation features that can can MAXIMISE retention form of a preparation:

•    Height of the preparation (minimum 3mm in posterior regions)

•    Surface area available for bonding

•    Degree of taper (5-7 degrees, TOC 12-15)

•    Grooves, slots, boxes

•    Diameter