**2. Classification based on processing techniques**

For ease of understanding, classification according to processing techniques is advocated. Generally, there are


This system is more relevant clinically. In spite of same chemistry and microstructure, the processing history determines the properties relevant to clinical performance. Specifically, machined blocks of materials have performed better than powder/liquid versions of the same material.

1. Powder/liquid systems

These can be conventional or slip casted. The conventional construction of a porcelain crown involves compaction, ring and glazing. Briefly compaction involves mixing of powder with water and binder to form a paste by spatulation, brush application, whipping or vibrating, that is aimed at compaction, which is painted over the die that is previously coated with platinum foil. This paste is made from different porcelain powders to mimic the esthetics of natural teeth. Usually, an opaque shade (to mask metal core), a dentin shade and then enamel shade is used. The enamel shade is selected from shade guide matched to patients actual tooth shade.

The science of braces – called Orthodontics, describes the corrective procedure as unaesthetic metamorphosis to an esthetic result. The aim of ceramic braces is to reverse the above description. The primary use of these brackets is esthetics. Ceramic braces as such do not experience masticatory forces, But are subjected to the orthodontic forces like sliding over wires, torquing etc. Due to its high wear coefficient, when wrongly placed can cause attrition of opposing teeth. They are basically polycrystalline alumina or

Fig. 21. Ceramic braces, colour codes showing the correct tooth to be cemented to.

For ease of understanding, classification according to processing techniques is advocated.

This system is more relevant clinically. In spite of same chemistry and microstructure, the processing history determines the properties relevant to clinical performance. Specifically, machined blocks of materials have performed better than powder/liquid versions of the

These can be conventional or slip casted. The conventional construction of a porcelain crown involves compaction, ring and glazing. Briefly compaction involves mixing of powder with water and binder to form a paste by spatulation, brush application, whipping or vibrating, that is aimed at compaction, which is painted over the die that is previously coated with platinum foil. This paste is made from different porcelain powders to mimic the esthetics of natural teeth. Usually, an opaque shade (to mask metal core), a dentin shade and then enamel shade is used. The enamel shade is selected from shade guide matched to

**2. Classification based on processing techniques** 

**1.5 Braces** 

Zirconia.

Generally, there are

same material.

3. CAD/CAM systems

1. Powder/liquid systems

patients actual tooth shade.

1. Powder/liquid glass-based systems 2. Pressable blocks of glass-based systems The objective of condensation techniques is to remove water, resulting in a more compact arrangement with high density of particles that reduces the ring shrinkage. The particle shape and size affect the handling characteristics of the powder and have influence on firing shrinkage. The binder helps to hold the fragile particles together in this so-called green state.

Firing initially involves slow heating of crown in the open entrance to the furnace, to drive off excess water before it forms steam that cracks the ceramic. Dried compact is placed in the furnace and the binders are burnt out. Some contraction occurs in this stage. When the porcelain begins to fuse, continuity is achieved at contact points between the powder particles. The material is still porous, and is usually referred to as being at the low bisque stage. As the higher temperature prevails for longer time, more fusion takes place as the molten glass ows between the particles, resulting in more compaction and lling the voids. A large contraction takes place during this phase (~20%), and the resultant material is apparently non-porous. The high shrinkage is caused by fusion of the particles during sintering, and resultant close contact between particles. Longer sintering will lead to pyroplastic flow and loss of form and will become highly glazed. A very slow cooling rate is employed to avoid cracking or crazing.

The furnaces can be programmed to automate these procedures. Vacuum-ring produces a denser porcelain than air ring, as air is withdrawn during the ring process, resulting in fewer voids and a stronger crown and more predictable shade. Areas of porosity in air red porcelain alter the translucency of the crown, as they cause light to scatter. Also, air voids become exposed on grinding of the supercial layer, compromising esthetics by giving a rough surface nish.

Glazing is done to eliminate residual surface porosity that might encourage bacterial colonization and its sequel. Glazing results in surface that is smooth, shiny and impervious. To accomplish this, either low fusing glasses are applied to crown after construction and fused, or final firing is done under controlled condition that fuses the superficial layer to make it impervious.

With regard to slip casting, the "slip" is a homogenous dispersion of ceramic powder in water. The water pH adjustment creates a charge on the ceramic particles, which are coated with a polymer to cause the fine suspension in water. In the case of In-Ceram, the slip is applied on a gypsum die to form the underlying core for the ceramic tooth. The water is absorbed by porous gypsum, leading to packing of particles into a rigid network. The alumina core is then slightly sintered in a furnace to create an interconnected porous network. The lanthanum glass powder is placed on the core and glass becomes molten and flows into the pores by capillary action to produce the interpenetrating network. The last step in the fabrication involves application of aluminous porcelain on the core to produce the final form of the restoration. Other powder dispersions, such as those created with zirconia, may be poured into a gypsum mold that removes the water and leads to formation of homogeneous block of zirconia.

2. Pressable ceramics

Pressed ceramic restorations are fabricated using a method described previously, similar to injection molding. Empress restorations and other materials with a similar leucite/glass

Ceramics in Dentistry 223

Berg NG, Derand T. A 5-year evaluation of ceramic inlays (CEREC). Swed Dent J.

Brochu JF, El-Mowafy O. Longevity and clinical performance of IPS-Empress ceramic restorations–a literature review. J Can Dent Assoc. 2002;68(4):233-237.

Della Bona A, Mecholsky JJ Jr, Anusavice KJ. Fracture behavior of Lithia disilicate and

Dental Ceramics. Chap 26 in Phillip's Science of dental Materials. 10/ed. Edited by

Giordano R, Pelletier L, Campbell S, et al. Flexural strength of alumina and glass

Guazzato M, Albakry M, Ringer SP, et al. Strength, fracture toughness and microstructure of

Hegenbarth EA. Procera aluminum oxide ceramics: a new way to achieve stability,

Hoeland W, Schweiger M, Frank M, et al. A comparison of the microstructure and

Ironside JG and Swain MV. A review and critical issues of dental ceramics. Journal of the

Kingery WD, Bowen HK, Uhlmann DR. Introduction to Ceramics. 2nd ed. New York, NY:

Kraemer N, Frankenberger R. Clinical performance of bonded leucite-reinforced glass ceramic inlays and onlays after 8 years. Dent Mater. 2005;21(3):262-271. Marc A Rosenblum & Allan Schulman. A review of All-Ceramic restorations; JADA - 1997 ;

McLaren EA, Giordano RA, Pober R, et al. Material testing and layering techniques of a new

McLaren EA, Giordano RA. Zirconia-based ceramics: material properties, esthetics, and

McLaren EA, White SN. Survival of In-Ceram crowns in a private practice: a prospective

two phase all glass veneering porcelain for bonded porcelain and high alumina

layering techniques of a new veneering porcelain, VM9. Quintessence Dent

a selection of all-ceramic materials. Part I. Pressable and alumina glass-infiltrated

precision, and esthetics in all-ceramic restorations. Quintessence Dent Technol.

properties of the IPS Empress 2 and the IPS Empress glass ceramics. J Biomed


Clarke D. Interpenetrating phase composites. J Am Ceram Soc. 1992;75:739-759.

leucite based ceramics. Dent Mater. 2004;20(10):956-962.

components of In-Ceram. J Dent Res. 1992;71:253.

Australasian Ceramic Society, 1998; 34(2): 78-91.

frameworks. Quintessence Dent Technol. 2003;26:69-81.

clinical trial. J Pros Dent. 2000;83(2):216-222.

Anusavice, 1998. WB Saunders Press.

ceramics. Dent Mater. 2004;20(5):441-448.

Mater Res. 2000;53(4):297-303.

John Wiley and Sons;1976:1-19.

**4. Acknowledgements** 

**5. References** 


1997;21(4):121-127.

1996;20:21-34.

128 ; 297 - 305.

Technol. 2005;28:99-111.

structure are fabricated in this manner. Pressables may be used for inlays, onlays, veneers, and single-unit crowns.

#### 3. CAD/CAM

Numerous ceramics have found their way into this system, due to its short processing time. Some of them are described here. **Glass/Crystal ceramics** are made from fine-grain powders, producing pore-free ceramics. This was the first material specifically produced for the CEREC system. It has an excellent history of clinical success for inlays, onlays, and anterior and posterior crowns. These blocks are available as monochromatic, polychromatic with stacked shades as in a layered cake, and in a form replicating the hand-fabricated crowns whereas an enamel porcelain is layered on top of dentin porcelain. **Glass/Leucite** is a feldspathic glass with approximately 45% leucite crystal component. **Lithium disilicate** is not initially fully crystallized, which improves milling time and decreases chipping risk from milling. The milled restoration is then heat-treated for 20 - 30 mins to crystallize the glass and produce the final shade and mechanical properties of the restoration. This crystallization changes the restoration from blue to a tooth shade. The microstructural and chemical composition is essentially the same. **Framework** Alumina are fabricated by pressing the alumina-based powder into a block shape. These blocks are only fired to about 75% density. After milling, these blocks are then infused with a glass in different shades to produce a 100% dense material, which is then veneered with porcelain. **Porous Alumina** frameworks may be fabricated from porous blocks of material. Pressing the alumina powder with a binder into molds produces the blocks. The blocks may be partially sintered to improve resistance to machining damage or used as pressed in a fully "green" state (unfired, with binder). The frameworks are milled from the blocks and then sintered to full density at approximately 1500°C for 4 to 6 hours. The alumina has a fine particle size of about 1µm and strength of approximately 600 MPa and is designed for anterior and posterior single units, as well as anterior three-unit bridges. **Porous Zirconia** frameworks milled from porous blocks are fabricated similarly to alumina blocks. As is the case with the alumina block, the milled zirconia framework shrinks about 25% after a 4 - 6 hours cycle at approximately 1300°C to 1500°C. The particle size is about 0.1 µm to 0.5 µm.
