Types Of All-Ceramic Crowns

Types Of All-Ceramic Crowns

Write a short note on all-ceramic restoration.
Answer:

All ceramic restorations are also known as metal-free ceramics. Metal-free ceramic restorations are made without a metallic core or sub-structure. This makes them esthetically superior to metal-ceramic restoration. Metal-free ceramic restorations had lower strength, thus,  metal-ceramics continued to be the restoration of choice.

Continued research has led to improved systems with greater strength and fracture resistance. Manufacturers today claim new generation all-ceramic materials are capable of producing not only single crowns but anterior and even posterior ceramic crowns and bridges as well. Long-span fixed partial dentures have also been attempted.

Classification of Metal Free Ceramic:

All ceramic restorations are grouped according to their type and method of fabrication.

1. Condensed Sintered
   • Traditional feldspathic porcelain jacket crown
   • Porcelain jacket crown with aluminous core (hi–ceram)
   • Ceramic jacket crown with leucite reinforced core (optic HSP)
2. Cast glass ceramics (dicor).
3. Injection molded (leucite reinforced) glass ceramic (IPS Empress).
4. Slip-cast glass infiltrated ceramics
   • Glass infiltrated aluminous core restorations (in-ceram)
   • Glass infiltrated spinal core restorations (in-ceram Spinell)
   • Glass infiltrated zirconia core (in-ceramic zirconia).
5. Milled ceramic restoration or cores:

• • CAD/CAM restorations
  • Copy-milled restorations.

Porcelain Jacket Crown:

These are made up completely of feldspathic porcelain. Their construction is done on a platinum foil matrix which is subsequently removed.

They are of three types, i.e.

1. Porcelain jacket crown (traditional).
2. Porcelain jacket crown with aluminous core.
3. Porcelain jacket crown with Leucite reinforced core (OptekHSP).

1. Porcelain Jacket Crown Traditional:
   • The all-porcelain jacket crown has been around for a century (early 1900s).
   • These early crowns are also referred to as traditional or conventional PJCs.
   • They were made from conventional high-fusing feldspathic porcelains.
   • These were very brittle and fractured easily. The marginal adaptation was also quite poor.
   • Because of these problems they gradually lost popularity and are no longer used presently.
2.  Porcelain Jacket Crown with Aluminous Core:
   • Problems associated with traditional porcelain jacket crowns led to the development of porcelain jacket crowns with an alumina-reinforced core.
   • The increased content of alumina crystals (40 to 50%) in the core strengthened the porcelain by interrupting of crack propagation.
   • In spite of the increased strength they were still brittle and therefore not indicated for posterior teeth and their use was restricted to anterior teeth.
3. Leucite Reinforced Porcelain (Optec HSP):
   • Optec HSP is a feldspathic porcelain with a higher leucite crystal content (leucite reinforced).
   • Its manipulation, condensation, and firing is quite similar to the alumina-reinforced porcelain jacket crowns (using a platinum foil matrix).

Uses of Metal-Free Ceramic:

Inlays, onlays, veneers, and low-stress crowns.

Advantages of Metal Free Ceramic:

• They are more esthetic because the core is less opaque (more translucent) when compared to aluminous porcelain.
• Higher strength.
• No need for special laboratory equipment.

Disadvantages of Metal-Free Ceramic:

• Fit is not as good as metal-ceramic crowns.
• Potential marginal inaccuracy.
• Not strong enough for posterior use.

Castable Glass Ceramic:

• It is the first commercially available castable glass ceramic for dental use made by Corning Glass Works named it as Dicor.
• It consists of 55 vol% of tetra silicic Formica crystals inside a glass matrix.
• The ceramic prosthesis is fabricated the same as the lost wax casting technique used for metals.
• Casted glass core should be gently sandblasted and coated by a protective material and is subjected to heat treatment.
• This causes the growth of microscopic plate-like crystals of mica inside the glass matrix. This is known as cramming.
• Now cerammed glass coping is coated with veneering porcelain to fabricate prostheses.
• The advantages of cramming are increased toughness and strength, increased abrasion as well as thermal resistance, and chemical durability.
• Disadvantages of castable glass ceramic are, it has inadequate strength for posterior use and internal characterization is not possible
• It is available as CAD/CAM material in the form of blanks and ingots.

Heat Pressed Ceramics:

• This ceramic material has a unique way of fabrication, i.e. by injection molding.
• It is a pre-crammed glass having a high concentration of reinforcing crystals.
• The material supplied in the form of an ingot is softened under high temperatures and forced into a mold created by a lost wax process.
• This pressed ceramic is then contoured and subsequently stained and glazed for the final finish of the restoration.
• As the ceramic is cast from a single ingot, restoration is monochromatic, though staining can be done to alter the color.
• Heat-pressed ceramics are used for inlays, onlays, single crowns, and veneers.
• Advantages are better fit; and better esthetics because of the absence of metal or an opaque core.
• Disadvantages are the need of costly equipment and the potential of fracture in posterior areas.

Microstructure of Heat-Pressed Ceramics:

• IPS Empress: It consists of 35 to 40% volume of leucite crystals
• IPS Empress 2: It consists of 65 to 70% by volume of interlocked elongated lithia disilicate crystals. Crystal size varies from 0.5 to 4 µm in length.

Glass Infitered of Heat Pressed Ceramics:

They are specialized core ceramics that are reinforced by a unique glass infiltration process. They are also known as split-cast ceramics.

Types of Glass Infitered:

They are of three types based on the core material used:

1. Glass infitrated alumina core (In-ceram alumina)
2. Glass infitrated spinell core (In-ceram spinell)
3. Glass infiltrated zirconia core ( In-ceramic zirconia)

1. Glass Infitrated Alumina Core (In-Ceram Alumina)

This has a unique glass infiltration process and first of its kind for anterior fixed partial denture fabrication.

Composition:

• Alumina powder:
  • Al₂O₃ – 99.7%
  • MGO – 0.03%
• Infiltration glass powder:
  • La₂O₃ – 49.6
  • SiO₂ – 19.1
  • TiO₂ – 6.16
  • CaO – 3.14
  • Others–2.0

Properties Of  Alumina Core :

• Flexural strength: 256 to 500 MPa
•  Fracture toughness: 4.4 to 4.8 MPa.m^1/2

 Indications Of Alumina Core:

• Single anterior and posterior crowns
• Short span anterior 3 unit fixed partial denture.

Advantages Of Alumina Core :

• Good fi and marginal adaptation
• Good strength as compared to earlier all ceramic crowns
• Minimal fie shrinkage
• Use to cover dark teeth or post and core
• Wearing of opposite teeth is less.

Disadvantages Of Alumina Core :

• Less aesthetic because of the opacity of the alumina core.
• Quite tedious to fabricate.
• Insufficient long-term results.

2. Glass Infitrated Spinell Core (In-Ceram Spinell):

• It is an offhoot of in-ceram alumina.
• These ceramics use magnesium aluminate crystals instead of alumina crystals for strength.
• It is more translucent and is more aesthetic as compared to the in-ceram alumina core.
• As strength is lower, its use is limited to low-stress situations.
• Indication: Because of its high translucency it is the material of choice for crown and restoration in esthetic and stress-free zones.
• Contraindication: In underlying tooth which is severely discolored; in posterior teeth and as fixed partial dentures.

Glass Infitrated Zirconia (In-Ceram Zirconia):

• It is the mixture of zirconium oxide/aluminum oxide in the framework material.
• It is the strongest of all three glass-infiltrated core materials.
• The final core of ceramic zirconia consists of 30% wt% zirconia and 70 wt% aluminas.
• It can be used in posterior fixed partial dentures, and cores for anterior crowns to mask discoloration.

CAD/CAM Ceramics:

Constructing a dental ceramic restoration is technique sensitive, labor-intensive, and time-consuming. CAD/CAM ceramics were introduced to overcome some of these problems. They are also known as milled or machined ceramics.

Machinable ceramic systems can be divided into two categories, which are as follows:

1. CAD/CAM systems
2. Copy milled systems

1. CAD/CAM Systems:

These are systems that can design and produce restorations out of blocks or blanks of ceramics with the aid of a computer. CAD/CAM is an acronym for computer-aided design/computer-aided manufacturing.

Materials for CAD/CAM:

The fabrication process is system and material-specific. The prepared tooth or teeth is scanned directly from the mouth or from a model made from a regular impression. Next, the restoration or sub-structure is designed on the computer. The blank is attached to the milling station and the bar code.

The time taken for milling depends on the size and complexity of the restoration as well as the material used. For example, pre-sintered zirconia is easier to mill than sintered zirconia. It also reduces the wear of the milling tools. After milling, the structure is separated from the blank using water-cooled cutting and grinding discs or burs.

Subsequent processing procedures are then initiated depending on the material and system used.

• Feldspathic blanks:
  • Feldspathic restorations can be milled to full contour. The restoration is glazed after milling.
  • Optional processing includes veneering and staining. It is indicated for inlays, laminates, and anterior crowns.
• Leucite reinforced:
  • These blanks can be milled to full contour. The restoration is glazed after milling.
  • Optional processing includes veneering and staining.
  • It is indicated for inlays, onlays, laminates, and anterior crowns.
• Lithium disilicate:
  • The ceramic is machined in an intermediate crystalline state in which the material shows its characteristic bluish shade.
  • In this stage, the material is easier to shape and can be tried in the mouth.
  • This is followed by a simple, quick crystallization process (30 minutes) in the conventional ceramic oven in which it reaches its final strength and the desired esthetic properties such as tooth color, translucence, and brightness.
  • Optional processing includes veneering and staining. Uses—inlays, onlays, and anterior and posterior crowns.
• Glass infiltrated ceramics:
  • These are usually machined as cores or FDP substructures.
  • Subsequent processing includes glass infiltration, veneering, and glazing.
  • It is indicated that in-ceram Spinell is recommended for anterior single crown copings.
  • In-ceram alumina is indicated for anterior and posterior crowns and 3-unit anterior FDP substructures.
  • In-ceram zirconia can be used for anterior and posterior crowns and 3-unit FDP substructures.
• Presented zirconia:
  • Fully dense zirconia is extremely difficult to machine, taking up to 2 hours just to fabricate a single unit.
  • Therefore, most restorations with zirconia frameworks are fabricated by machining a porous or partially fied block of zirconia known as pre-sintered zirconia.
  • These are usually used as cores for crowns or FPDs.
  • In pre-sintered condition, they are usually softer and easier to mill.
• Sintered zirconia:
  • Since these materials are already fully sintered, post-sintering is not required of this material.
  • It is milled in 1:1 ratio as no shrinkage is expected.
  • Because of its extreme hardness milling takes more time and causes more wear on the milling tool.
  • Subsequent processing includes build-up with compatible veneering ceramics.
  • It is indicated for core construction of crowns and long-span anterior and posterior fixed partial dentures.

2. Copy Milled Systems:

They use a copy-milling technique to produce ceramic cores or substructures for fixed partial dentures. In copy milling a wax pattern of restoration is scanned and a replica is milled out of the ceramic blank. Examples are:

• Clay
• Cercon
• Ceramill system.