Quartz 4

F2 Ceramics

10 min read

Ceramic Crowns12

Rationale for Ceramic Crowns3

Historical Context

Historically, early ceramics were too weak for widespread use, which led to two primary pathways for strengthening them:

  1. Developing a stronger core material to support a weaker, more aesthetic veneering ceramic.
  2. Creating a monolithic material that is inherently strong enough on its own.

The different properties of these resulting materials dictate their clinical applications.

Limitations of PFM crowns

  • Aggressive reduction
  • Not as aesthetic (opaqueness)
    • PFM crowns often exhibit poor aesthetics due to the need for an opaque porcelain layer to block out the dark color of the underlying metal substructure. This can result in a chalky, unnatural appearance.
  • Potential biocompatibility issues with metal sensitivities /allergies.
  • ==Cost: The fabrication of PFM crowns involves significant lab costs, particularly associated with the price of the noble or high-noble metal alloys (e.g., gold) used for the substructure.==

Understanding Ceramics45

Ceramics vs Porcelain

  • Ceramic is a compound of metallic elements (e.g., aluminum, lithium, magnesium, potassium, sodium, tin, titanium, zirconium) and non-metallic elements (e.g., silicon, fluorine, boron, oxygen)
  • Porcelain is a ceramic consisting of a glass matrix phase and one or more crystalline phases (e.g., leucite).
  • All porcelains are ceramics, but not all ceramics are porcelains.

Glassy vs Crystalline6

  • Glassy phases in ceramics are important for aesthetics.

Clinical Significance of the Glassy Phase

A key clinical property is that the glassy phase can be etched. Hydrofluoric (HF) acid reacts with the silica (SiO₂) in the glass matrix, creating microporosities that allow for a strong micromechanical bond with resin cements.

  • Crystalline ceramics are extremely strong but often lead to poor aesthetics.

[Classification of Dental Ceramics](“Classifying Dental Ceramics: Numerous Materials and Formulations” (Helvey, p. 1))7

There are many classification methods for dental ceramics, primarily based on:

  • Composition
  • Processing manner
graph TD
    A[Ceramics] --> B[Glass-based]
    A --> C[Glass-based with crystalline fillers]
    A --> D[Crystalline-based with glass fillers]
    A --> E[Polycrystalline solids]
    B --> F[Feldpathic]
    C --> G["Low-med leucite (<40%)"]
    C --> H["High leucite (>50%)"]
    C --> I[Lithium disilicate]
    D --> J["Alumina&#47;Zirconia"]
    D --> K["Alumina&#47;Magnesia (Spinel)"]
    E --> L[Zirconia]
    E --> M[Alumina]
    A--> P[+resin matrix ceramics]

Classification by Composition8

Glass-Based Ceramics

  • Feldspathic ceramics: Mainly composed of feldspar and also quartz, potash (K₂O) and various metal oxides.
  • Very aesthetic due to mainly glass based ceramic, however often strengthened with some leucite.
  • Used as monolithic ceramic for single-unit anterior prostheses, veneers, inlays or onlays. However can use to veneer other ceramics.
  • Slurry method.
  • Adhesively cemented due to lower flexural strength.
  • Flexural strength:
    • Pure feldspathic: 50–80 MPa
    • Leucite reinforce: ~160 MPa
  • CAN ETCH

Glass-Based (with crystalline fillers)9

  • Leucite reinforced glass ceramics:

    • Leucite is a crystalline mineral formed when feldspar is melted.
    • Good aesthetic properties.
    • They generally possess good aesthetic properties because the refractive index of the leucite crystals is similar to that of the surrounding glass matrix, allowing for natural light transmission.
    • Leucite based ceramics are usually processed via “hot pressing”.
    • Dispersion strengthening: process by which the dispersed phase of a different material (in this case it is leucite in a glass matrix) is used to stop crack propagation, since these crystalline phases are more difficult to penetrate by cracks. (Eg. IPS Empress CAD)
    • This concept is analogous to the role of filler particles in dental composites.

  • Lithium disilicate reinforced glass ceramics:

    • It also formed an intertwined structure after heat pressing, which also aids in increase of fracture strength. Alternatively it can be milled whilst in an intermediate crystalline phase and then tempered at high temperatures to harden it fully. (Eg. IPS Emax CAD)
    • Suited for single unit anterior crowns due to flexural strength of 360–400 MPa.

  • Zirconia reinforced lithium silicate:

    • Similar to above but also has zirconia dispersed into the glassy matrix. This gives it a higher flexural strength of 460 MPa (Eg. VITA Suprinity)

  • **CAN BE ETCHED **

Glass-Infiltrated Ceramics10

  • Requires a porous crystalline “skeleton” that is infiltrated with a glassy phase.
  • Associated with slip casting.
  • Eg (VITA In ceram Alumina, VITA In ceram Spinell, VITA In ceram Zirconia)
  • However very hard to make, and thus has lost popularity.
  • CANT ETCH

Polycrystalline Ceramics11

  • Alumina: High flexural strength 500–700 MPa
  • Zirconia (covered in next slide)
  • Cannot be etched.
  • Zirconia can be used for a wide variety of indirect restorations such as crowns, bridges etc.
  • Can be found as monolithic zirconia crowns, or veneered zirconia crowns.
  • Initially zirconia was very opaque and not suited for anterior crowns. However high translucency zirconia also available now.

Zirconia12

  • Scientifically termed zirconia dioxide, it has fantastic chemical and stability, mechanical strength and elastic modulus similar to stainless steel.
  • The flexural strength ranges from 900–1200 MPa.
  • Zirconia has a unique feature called “transformation toughening”. -
    • Zirconia may have 3 forms (monoclinic at room temp, tetragonal at 1200°C and cubic at 2370°C). Yttira is used to stabilize zirconia so it can maintain the tetragonal form at room temp. When a crack forms zirconia phase changes from tetragonal to monoclinic. During transformation to monoclinic form there is an associated 3–5% volumetric increase, generating compressive forces around the crack, countering the tensile forces at the tip of the crack line.

Summary of Material Properties13

Resin Matrix Ceramics1415

  • Eg Lava Ultimate, VITA Enamic
  • Hybrid ceramics: Nanoceramic particles in a high crosslinked polymeric matrix.
  • Due to the resin polymeric matrix the material is not brittle and has good shock absorbing characteristics.
  • Lava is not recommended for crowns, but can be used for inlays/onlays. Enamic can be used for posterior and anterior crowns too.
  • No need to fire or sinter like other ceramics.

Classification by Processing Manner16

Methods include:

  • Powder/Liquid building
  • Slip casting
  • Hot ceramic pressing
  • CADCAM

Powder/Liquid Building17

  • Uses a ceramic powder and liquid to form a “slurry”. This can then be painted onto a metal or ceramic layer. This is very technically demanding for the dental technicians and poor techniques can result in weakening of the restoration if porosities/voids are created. The ceramic coated crown is fired (known as sintering) to solidify and fuse the ceramic particle together.
  • Usually used for feldspathic.
  • Advantages: Highly translucent and aesthetic. Allows for good control over the shades and can be customized to a large degree.
  • Disadvantages: Weak, shrinkage during firing, porosities from technique sensitivity. Requires a skilled technician!

Slip Casting1819

  • Utilised a porous die is dipped into a slurry made of fine ceramic particles. After absorption of the water particles by the porous die, a thin layer of the slurry material remains on the die. This is then fired, and due to shrinkage of the die, the porous core is able to be easily removed. This structure is then infiltrated by molten glass.
  • Used in manufacturing of: In ceram family

Hot Ceramic Pressing20

  • The dental technician creates the restoration in wax, and using the lost wax technique, an investment mold is created. A plasticized ceramic ingot is pressed into the heated investment mold to create the final restoration.
  • Usually used for leucite based glass or lithium disilicate.
  • Advantages:
    • Improved mechanical strength: Produces restorations strong enough for some short-span bridges.
    • Reduced shrinkage: Offers better dimensional stability compared to other methods.
    • Aesthetics: Capable of producing highly aesthetic restorations.
    • Marginal fit: Can potentially achieve a better marginal fit than techniques like slip casting.
  • Disadvantages:
    • Requires specialized, additional equipment.
    • The process can be time-consuming.

CAD/CAM212223

  • Computer aided design and computer aided manufacture. Additive CADMCAD machines exist, but mostly subtractive machines are used (such as here in OHCWA). Ceramic blocks are milled into appropriate shapes, then fired in the oven. This is especially required for zirconia blocks. Due to the high strength of zirconia, “green” pre-sintered zirconia blocks with porosities are milled whilst they are still softer than “white” fully hardened zirconia. They are then fired at high temps (1300°C+) to densify the zirconia to the crowns we receive from the lab. Due to shrinkage the zirconia crowns need to be milled larger than the final restoration dimensions. It is also possible to mill “white” hardened zirconia, but very time consuming and causes a lot of wear to the milling burs.

Advantages of CAD/CAM24

  • Simplified fabrication method
  • More durable material application
  • Homogeneous material
  • Superior accuracy
  • Chair-side option is available

Cementation and Bonding2526

Take Home Message

  • Some ceramics CAN be etched with hydrofluoric acid (~9%) to increase retention (Suprinity, Emax, Enamic) and cemented adhesively with resin-based cements. These ceramics are predominantly glassy ceramics.
  • Some ceramics CANNOT be etched with hydrofluoric acid (zirconia) because they do not contain glassy phases (they are crystalline). They will require luting cements in conjunction with internal sandblasting, or other chemical agents such as MDP containing cements.

Resin Cements27

  • We will talk more about cements in the later lectures.
  • Briefly:
    • Resin cements strengthen the ceramic material
    • Prevent internal crack propagation
    • Increase restoration interface strength
  • Silane
    • Establishes covalent bond between ceramic surface and composite resin
    • Improves wetting of ceramic by composite resin cement

Adhesive Cementation – Resin cements

A. Etched and silane on ceramic B. Bonding agent C. Composite resin cement D. Etched enamel

Clinical Considerations for Ceramic Crowns28

Advantages and Disadvantages

Advantages

  • Superior aesthetics (some better than others)
  • Biocompatible
  • Chemically inert
  • High hardness and other mechanical properties (certain ceramics)

Disadvantages

  • Some ceramics have poor mechanical properties
  • Brittle if unsupported (not to exceed 2 mm)

Causes of Fracture

Fractures in ceramic restorations can occur for several reasons:

  • Functional stresses: Normal or parafunctional occlusal forces during use.
  • Occlusal adjustments: Iatrogenic damage or introduction of surface flaws during adjustment with dental burs.
  • Fabrication defects: Internal voids or porosity introduced during the firing or processing stages can act as stress concentrators and points of fracture initiation.
  • Technically demanding
  • Possibly more tooth structure reduction required

Indications and Contraindications29

Indications

  • High aesthetic demand
  • Metal allergy
  • More conservative than PFM

Contra-indications3031

  • Heavy bruxers
  • Heavy occlusal forces
  • Parafunctional habits
  • Overly short clinical crowns

Footnotes

  1. Original PDF page 1: F2 Ceramics, p.1 ↩

  2. Original PDF page 2: F2 Ceramics, p.2 ↩

  3. Original PDF page 3: F2 Ceramics, p.3 ↩

  4. Original PDF page 4: F2 Ceramics, p.4 ↩

  5. Original PDF page 5: F2 Ceramics, p.5 ↩

  6. Original PDF page 6: F2 Ceramics, p.6 ↩

  7. Original PDF page 7: F2 Ceramics, p.7 ↩

  8. Original PDF page 8: F2 Ceramics, p.8 ↩

  9. Original PDF page 9: F2 Ceramics, p.9 ↩

  10. Original PDF page 10: F2 Ceramics, p.10 ↩

  11. Original PDF page 11: F2 Ceramics, p.11 ↩

  12. Original PDF page 12: F2 Ceramics, p.12 ↩

  13. Original PDF page 13: F2 Ceramics, p.13 ↩

  14. Original PDF page 14: F2 Ceramics, p.14 ↩

  15. Original PDF page 15: F2 Ceramics, p.15 ↩

  16. Original PDF page 16: F2 Ceramics, p.16 ↩

  17. Original PDF page 17: F2 Ceramics, p.17 ↩

  18. Original PDF page 18: F2 Ceramics, p.18 ↩

  19. Original PDF page 19: F2 Ceramics, p.19 ↩

  20. Original PDF page 20: F2 Ceramics, p.20 ↩

  21. Original PDF page 21: F2 Ceramics, p.21 ↩

  22. Original PDF page 22: F2 Ceramics, p.22 ↩

  23. Original PDF page 23: F2 Ceramics, p.23 ↩

  24. Original PDF page 24: F2 Ceramics, p.24 ↩

  25. Original PDF page 25: F2 Ceramics, p.25 ↩

  26. Original PDF page 26: F2 Ceramics, p.26 ↩

  27. Original PDF page 27: F2 Ceramics, p.27 ↩

  28. Original PDF page 28: F2 Ceramics, p.28 ↩

  29. Original PDF page 29: F2 Ceramics, p.29 ↩

  30. Original PDF page 30: F2 Ceramics, p.30 ↩

  31. Original PDF page 31: F2 Ceramics, p.31 ↩

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