• Glass ceramics are the material that are initially formed as glass, and then transformed into ceramic usually by a controlled heat treatment.
• The heat induces partial devitrification which increases the strength as well as improves aesthetics by making it less transparent and more teeth like.
• The two glass ceramics used in dentistry are Castable glass ceramics and pressable glass ceramics.
• Advantages :- Ease of fabrication, Good aesthetics, improved strength and fracture toughness, Good marginal fit, very low processing shrinkage, low abrasion of opposing teeth.
• Disadvantages :- Inadequate strength for posterior teeth.
Castable glass ceramics
•It’s properties are more closer to that of glass and it’s construction is quite different. This is only porcelain restoration that is made by centrifuging casting technique.
• The subsequent Ceramming process is also quite unique to this porcelain Ceramming will enhance the growth of mica crystals within the ceramic.
• It is first commercially available glass ceramic for dental use was Dicor.
• Composition :- Dicor glass ceramic contains 55% volume of Tetrasilicic fluoromica crystals.
• features :- The dicor glass ceramic crowns are very aesthetic. This is because of greater translucency (unlike the other porcelain which have more opaque core). It also picks up some of the color from adjacent teeth (chaemelon effect) as well as from underlying cement.
Uses:- inlays , onlays , veneers
Fabrication of a dicor crown:-
• wax pattern is constructed first and then invested with a refractory material.
• After burnout of the wax ,nuggets of the dicor glass are melted and cast into the mould in centrifuging casting machine.
• The glass casting is carefully recovered from the investment by sandblasting and the sprues are gently cut away.
•The glass restoration is then covered with embedment material to prepare it for next stage that is Ceramming.
• Ceramming is the heat treatment process by which the glass is strengthened. Ceramming results in the development of microscopic crystals of mica which
– improves the strength and toughness of glass
– improves the aesthetics of the restoration ( it reduces the transparency of the glass making it more opaque and less glass like)
• The cerammed glass can be built up with special veneering porcelain and fired to complete the restoration. Surface stains may be applied to improve the aesthetics.
Reference :- Manapalil text book of dental materials