The Science Behind All-Ceramic Crowns: Castable Ceramics In Dentistry

The Science Behind All-Ceramic Crowns: Castable Ceramics In Dentistry

Write different types of castable ceramics in detail.
Answer:

Castable ceramics are also known as glass ceramics

Types of Castable Ceramics:

There are four types of castable ceramics, i.e.

1. Dicor
2. Apatite glass ceramic—cera pearl
3. Lithia based
4. Calcium phosphate

1. Dicor:

The first commercially available castable ceramic material for dental usage is Dicor which was named on the names of its manufacturers, i.e. Dentsply International and Corning Glass Works, it is supplied as silicon glass plate ingot containing MgF₂

• Its properties are closer to that of glass and its construction is quite different.
• This is the only porcelain crown made by a centrifugal casting technique.
• The ‘cramming’ process is also quite unique to this porcelain.

Composition Of Dicor:

Dicor contains 55 vol% of tetra silicic Formica crystals in the glass matrix.

Fabrication of a Dicor Crown:

• The wax crown is first constructed in wax and then invested in investment material like a regular cast metal crown.
• After burning out the wax, nuggets of Dicor glass are melted and casts into the mold in a centrifugal machine.
• The glass casting is carefully recovered from the investment by sandblasting and the spores are gently cut away.
• The glass restoration is then covered with a protective ‘embedment material’ to prepare it for the next stage called cramming.
• Cramming is a heat treatment process by which the glass is strengthened. It also reduces the transparency of the glass making it more opaque and less glass-like.
• The cramped glass is now built up with enamel and dentin, condensed, and fixed to complete the restoration.

Advantages of a Dicor Crown:

• Ease of fabrication
• Esthetics is good
• Marginal is good
• Improved strength and fracture toughness
• Processing shrinkage is very low
• Opposing teeth undergo low abrasion.
• Inherent resistance to bacterial plaque and is more biocompatible.
• Thermal conductivity is low
• Radiographic density is similar to dentin.

Uses of a Dicor Crown:

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

Disadvantages of a Dicor Crown:

• Strength is inadequate for posterior use.
• Internal characterization is impossible, so it should be stained externally to improve esthetics.
• Require special equipment

2. Apetite glass-ceramic – In-ceram:

It is developed by Sumiya Hobo and Iwata in 1985. Apatite glass-ceramic melts (1460°C) and flows like molten glass and when cast (1510°C) it has an amorphous microstructure.

• The amorphous CaPO4 formed after casting changes into a crystalline oxy apatite on heat treatment (cramming) at 870°C for 1 hour.
• The chemically unstable oxy apatite when exposed to moisture (water) further converts to crystalline hydroxyapatite
• Similar to natural enamel in composition, density, refractive index, coefficient of thermal expansion, and hardness.

Composition Of Appetite glass-ceramic

• CaO − 45%
• Phosphorous − 15%. Aids in glass formation
• Magnesium oxide − 5%. Decreases the viscosity
• Silicon dioxide− 35% ~ Forms the glass matrix
• Other trace elements—nucleating agents.

Properties Of Appetite glass-ceramic:

• Knoop hardness number (KHN) − 350
• Tensile strength − 150 Mpa
• Young’s modulus of elasticity− 103 /15.0 × 106 Gpa
• Compressive strength − 590/0.08 × 106 Mpa.

3. Lithia based:

• They are developed by Uryu.
• Crystals of LiO. Al₂O₃.4SiO₂ is formed after heat treatment.
• It is available as Olympus castable ceramic.

4. Calcium phosphate:

• It is reported by Kihara.
• It is a combination of calcium phosphate and phosphorus pentoxide plus trace elements.
• It is cast at 1050°C in a gypsum investment mold
• Clear cast crown is converted to crystalline ceramic by heat treating at 645°C for 12 hours.