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Ceramics in dental applications.

Introduction

The word Ceramic can be traced back to the Greek term keramos, meaning "a potter" or "pottery." Keramos in turn is related to an older Sanskrit root meaning "to burn." According to Gilman in 1967, a ceramic is an earthy material usually of silicate nature and may be defined as a combination of one or more metals with a non-metallic element usually oxygen. The American Ceramic Society had defined ceramics as inorganic, nonmetallic materials, which are typically crystalline in nature, and are compounds formed between metallic and nonmetallic elements such as aluminum & oxygen (alumina-[Al.sub.2][O.sub.3]), calcium & oxygen (calcia--CaO), silicon & nitrogen (nitride-[Si.sub.3][N.sub.4]).

In Dental science ceramics are referred to as nonmetallic, inorganic structures primarily containing compounds of oxygen with one or more metallic or semi-metallic elements like aluminum, calcium, lithium, magnesium, phosphorus, potassium, silicon, sodium, zirconium & titanium. Ceramics encompass such a vast array of materials that a concise definition is almost impossible. Being an omnipotent material, its applications are innumerable wherein a definite boundary cannot be established.

History

Archeologists have uncovered human-made ceramics that date back to at least 24,000 BC. These ceramics were originally found in Czechoslovakia and were in the form of animal and human figurines, slabs, and balls. The first use of functional pottery vessels is thought to be in 9,000 BC. These vessels were most likely used to hold and store grain and other foods. The ancient glass manufacturing process, which flourished in Upper Egypt about 8,000 BC, is closely related to making of pottery.

A French dentist De Chemant patented the first porcelain tooth material in 1789. In 1808 Fonzi, an Italian dentist invented a "terrometallic" porcelain tooth that was held in place by a platinum pin or frame. Ash developed an improved version of the platinum tooth in 1837. Dr. Charles Land patented the first Ceramic crowns in 1903. In 1963, Vita Zahnfabrik introduced the first commercial porcelain.

Ceramic Segments

The broad categories or segments that make up the ceramic industry can be classified as follows:

Structural clay products, whitewares, refractories, glasses, abrasives, cements and advanced ceramics. Dental applications of ceramics encompass most of the segments in the ceramic industry, which includes:

Investment materials--refractories optical modifiers--glasses, Diamond cutting tools--abrasives, Therapeutic & esthetic dental cements--cements and Gadgets in dental applications.

Structure

The structure of ceramic materials is dictated by the type of atoms present, the type of bonding between the atoms, and the way the atoms are packed together. The atoms in ceramic materials are held together by a chemical bond and the two most common chemical bonds for ceramic materials are covalent and ionic. For metals, the chemical bond is called metallic bond. The bonding of atoms together is much stronger in covalent and ionic than in metallic bonding. That is why, generally speaking, metals are ductile and ceramics are brittle.

Many dental ceramics contain a crystal phase and a glass phase based on the silica structure. This structure is characterized by a silica tetrahedran in which a [Si.sup.4+] cation is positioned at the center of a tetrahedron with [O.sup.-] anions at each of the four corners. The resulting structure is not closely packed and has both covalent and ionic characteristics. The regular dental porcelain, being glassy in nature, is largely non-crystalline, and exhibits only a short-range order in atomic arrangement, which is referred to as dental glass ceramics. The only true crystalline ceramic used at present in restorative dentistry is Alumina ([Al.sub.2][O.sub.3]), which is the hardest and strongest oxide known.

Ceramic structures composed of single element are rare. Diamond is a major ceramic of this type and the unit cell consists of carbon atoms, each one sharing an electron with each of four surrounding carbon atoms--hardest natural material used to cut tooth enamel.

Properties

The properties of most ceramics are enumerated below :

hard, wear-resistant, brittle, refractory, thermal insulators, electrical insulators, nonmagnetic, oxidation resistant, prone to thermal shock & chemically stable.

However, certain ceramics do not fall into any of these categories. Exceptions are Borosilicate glasses (glasses that contain silica and boron as major ingredients) and certain glass ceramics (glasses that contain a crystalline phase), which are highly resistant to thermal shock. Some ceramics are excellent electrical conductors and an entire commercial market is based on the fact that certain ceramics (ferrites) are magnetic.

Ceramics in Medical Applications

Ceramics are employed in a wide range in the medical specialty. Surgeons use bioceramic materials for repair and replacement of human hips, knees, and other body parts. They are also employed to replace diseased heart valves. The applications are based on the fact that when used as implants or even as coatings to metal replacements, ceramic materials can stimulate bone growth, promote tissue formation and provide protection from the immune system.

Moreover, modern ceramic materials play an important role in gadgets used for medical diagnosis including both ultrasonic and X-ray computed tomography (CT) systems. Transducers utilizing lead zirconate titanate (PZT) based piezoelectric ceramics are the heart of ultrasonic systems. These transducers generate the ultrasonic acoustic waves and detect the reflected signals to form the image.

Gadgets for Dental Applications

Ceramics play a vital role in the manufacture and function of various gadgets used in dental science. Various recently introduced diagnostic and working tools of which ceramics play an integral part include:

Radio Visio Graphy (RVG) Pulp tester Apex locators 1st generation--resistance based. 2nd generation--impedance based 3rd generation--frequency based.

Piezo Ceramics

Piezoelectricity can be defined as pressure electricity which is a property of certain classes of crystalline materials including natural crystals of Quartz, Rochelle salt and Tourmaline plus manufactured ceramics such as Barium Titanate and Lead Zirconate Titanates (PZT). When mechanical pressure is applied to one of these materials, the crystalline structure produces a voltage proportional to the pressure. Conversely, when an electric field is applied, the structure changes shape producing dimensional changes in the material.

The piezoelectric materials use polycrystalline ceramics instead of natural piezoelectric crystals. They are more versatile with physical, chemical and piezoelectric characteristics able to be tailored to specific applications. The hard, dense ceramics can be manufactured in almost any given shape or size, which are chemically inert and immune to moisture and other atmospheric conditions.

Therapeutic Ceramics

Silicate cement

Silicates constitute the first dental cement to use glass as its component. The cement powder is a glass consisting silica, alumina and fluoride compounds. The liquid, on the other hand, is an aqueous solution of phosphoric acid with buffer salts. The cement powder and liquid are mixed together resulting in an acid-base reaction. Fluoride ions are leached out from the set cement, which is responsible for the anti-cariogenic property exhibited.

Glass Ionomer Cement (GIC)

Glass ionomer cement represents a logical step in the evolution of therapeutic cements. They constitute an improved version of the silicate cement, in which the liquid is replaced by carboxylic acids with glass remaining as the powder. It is the most popular dental cement that is used in various aspects. The highlight of this material is demonstrated by its superior biocompatibility and anti-cariogenic property. Modifications of glass ionomer cement include the high density Glass ionomers, packable ionomers for use in Atraumatic Restorative Treatment (ART). Resin modified Glass ionomer cements incorporate resins in their powder component for better strength.

Bioceramics

Bioceramics are a group of ceramics, which are biologically active materials rich in calcium and phosphate. Hydroxyapatite and tricalcium phosphate are similar in composition to bone and teeth and can be used for augmentation of alveolar ridges and filling bony defects. They are manufactured and are available in block, granular and injectable forms. These bioactive materials are packed in the required site providing a scaffold for new bone growth and are Osseo-inductive in nature. The various forms of bioceramics are Single crystals (Sapphire), Polycrystalline (Hydroxyapatite) Glass (Bioactive glass) Glass ceramics (Ceravital) Composites (Stainless steel reinforced Bioglass)

Types of Bioceramics

There are about four types of bioceramics:

INERT: Attached by compact morphological fixation. e.g, Alumina, Carbon

POROUS: Attached by vascularization through pores. e.g, Porous Alumina.

SURFACE ACTIVE: Directly attach by chemical bonding with bone.e.g, Bioglass, Hydroxyapatite

RESORBABLE Designed to be slowly replaced by bone.e.g, Tricalcium Phosphate

Ceramics In Esthetic Dentistry

Composite Resins

The evolution of composites has opened up new vistas in the art and science of Esthetic dentistry. Composites constitute an array of materials composed of Bisphenol A Glycidyl dimethacrylate resin, Organosilane coupling agents, Silica fillers, Glass modifiers & Metallic oxides. The advent of composite resins has virtually replaced most of the restorative materials in dentistry.

Classification Of Dental Ceramics

Dental ceramics can be classified in a variety of ways.

Based on Composition

Silicate ceramics- characterized by amorphous glass phase. Main component is silica--Si[O.sub.2]. Oxide ceramics contain a principal crystalline phase like Alumina. Oxides of Zirconia has very high fracture toughness. Non-oxide ceramics not used in dentistry; they possess high processing temp, complex processing methods and high degree of opacity eg. Carbides nitrides. Glass ceramics are type of ceramics that contains a glass matrix phase & at least one crystal phase.

Based on Type

Feldspathic porcelain. Leucite--reinforced porcelain., Aluminous, porcelain. Glass infiltrated, alumina.Glass, infiltrated zirconia. and Glass ceramics.

Based on firing temperature

Ultra-low fusing< 850[degrees]C (1562[degrees]F) Low fusing 850[degrees]C-1100[degrees]C (1562[degrees]F-2012[degrees]F) Medium Fusing 1101[degrees]C-1300[degrees]C (2013[degrees]F-2072[degrees]F) High fusing 1300[degrees]C (2372[degrees]F)

Based on sub-structure metal

Cast Metal, Swaged metal, Glass ceramics Sintered core ceramics and CAD-CAM porcelain

Based on use or indications

Denture teeth fixed partial dentures, Full crowns Veneers, Inlays Post & Cores. The most practically applied classification includes:--Metal Ceramics (Porcelain fused to metal) Metal free Ceramics (All Ceramics). The various types of metals in metal ceramics include Gold alloys,Gold alloys + base metals like iron, indium & tin. Pure metals like commercially pure Titanium, Platinum, gold and palladium alloys and Base metal alloys (Nickel, chromium).

Four types of process for producing a metal coping, Electrodeposition of gold or other metal on a duplicate die, Burnishing and heat treating metal foils on a die. CAD-CAM processing of a metal ingot. Casting of a pure metal or an alloy (predominantly base metal) through the lost wax process.

Feldspathic porcelain

Feldspars are used in the preparation of many dental types of porcelain designed for Porcelain fused to metal (PFM) restorations. Potassium and sodium feldspar are naturally occurring minerals composed primarily of Potash and soda. The most important property of feldspar is its tendency to form crystalline mineral leucite when melted. Leucite is nothing but potassium aluminium silicate mineral with large co-efficient of thermal expansion compared with glasses. The property of Feldspar to form Leucite is taken advantage in the manufacture of porcelains for metal bonding.

Advantages of Metal ceramics

High strength. Utilizes the sub-structure metal coping to withstand stresses. Thermal compatibility. Less crack propagation. High resistance to fracture.

Drawbacks

Inadequate structure for ceramics--thickness of metal coping. More occlusal clearance required. Transparent metallic hue--anterior teeth. Metal exposed in case of gingival recession. Patients allergy to metals. Casting procedural errors with metals. Bonding failures at porcelain-metal interface due to oxide layer production.

All ceramics

With a view to bring-in closer shade match, enhanced esthetics through better diffusion and transmission of light compared to metal-ceramics, all Ceramics were developed. Natural teeth always permit diffuse transmission and regular transmission.

Artificial tooth must possess a depth of translucency to simulate natural teeth, which is made a reality in All-Ceramic restorations.

Types of All ceramic restorations

Aluminous porcelain. Glass Ceramics Castable, Machinable and Pressable Glass infiltrated, CAD-CAM and Cercon Zirconia system.

Aluminous porcelain is composed of a glass matrix phase and at least 35 vol% of Alumina. It is one of the commonly used core ceramic and utilizes a thin platinum foil when employed with all ceramic restorations. Aluminous core is stronger than feldspathic porcelain when used in metal ceramic restorations.

Glass ceramic consists of a glass matrix phase and at least one crystal phase that is produced by the controlled crystallization of glass. It is available is Castable, machinable, pressable and infiltrated forms which is used in all ceramic restorations. The first commercially available castable glass ceramic is Dicor. These ceramics are formed into the desired shape by the lost wax casting process followed by coating with veneering porcelain. The significant aspect of this ceramics is the Chameleon effect in which a part of color is picked up from adjacent tooth.

The Machinable glass ceramic is a high quality product that is crystallized by the manufacturer and provided as CAD-CAM blanks or ingots. They are more precise than castable glass ceramic as the errors involved in the casting process are eliminated. They possess mechanical properties, which are similar to castable forms but are less translucent than them. The Pressable glass ceramic is one, which involves pressure molding in the manufacture. A piston is used to force a heated ceramic ingot through a heated tube into a mold, where the ceramic form cools and hardens to the shape of the mold. When the object is solidified, the refractory mold is broken apart and ceramic piece is removed. Hot pressing occurs over a 45 min period at a high temperature to produce the ceramic sub-structure. Core structure is then stained, glazed or coated by veneering porcelain, which results in translucent ceramic core, moderately high flexural strength, excellent fit & excellent esthetics. Eg. IPS Empress 1 & IPS Empress 2.

Glass infiltrated ceramic is used as one of the 3 core ceramics namely, In-Ceram Spinell, In-Ceram Alumina and In-Ceram Zirconia. They utilize the technique of slip-cast on a porous refractory die and heated in a furnace to produce a partially sintered coping or framework which is infiltrated with glass at 1100[degrees]C for 4 hrs to strengthen the slip-cast core. They possess relatively high flexural strength and ability to be successfully cemented using any cement. CAD-CAM involves a technique wherein the internal surface is ground with diamond discs to the dimensions obtained from a scanned image of the preparation.

Cerec CAD-CAM unit are ceramics supplied in small blocks that can be ground to the desired restorative pattern by computer driven CAD-CAM system. In Cercon ceramic systems, following tooth preparation impression is made and wax pattern is fabricated. The wax pattern is anchored on the left side of the scanning and milling unit called as Cercon Brain and a pre-sintered zirconia blank is attached to the right side of the Brain unit. The blank has an attached barcode, which contains the enlargement factor and other parameters for milling procedure.

The Future Of Dental Ceramics

The proven track record of dental ceramics in all areas of direct and indirect restorative dentistry would soon offer the dental operatory the extra power needed to produce more predictable and perfect all round biofunctional and bioesthetic restoratives.

References

[1.] Science of Dental materials by Kenneth J Anusavice; 11th edition; pg

[2.] Ceramic whitewares--their technologies & applications by Sudhir Sen; 1992; pg 48-62.

[3.] Ceramics--Physical & chemical fundamentals by Hermann Salmang; 1961; pg 165-184.

[4.] Introduction to the principles of Ceramic processing by James S Reed; 2001; pg 55-73.

[5.] Evolution of dental ceramics in the twentieth century. John McLean & Podont; Jour of Prosthet Dent 2001; 85; 61-66.

[6.] The science & art of dental ceramics by John W McLean; volume I & II; 2000; pg 165-304.

[7.] Recent developments in Restorative dental ceramics by Kenneth J Anusavice; JADA--1993; 124; 72-81.

[8.] A review of All-Ceramic restorations by Marc A Rosenblum & Allan Schulman; JADA--1997; 128; 297-305.

[9.] The strength of dental ceramics by Messer, Piddock & Lloyd; Jour of Prosthet Dent 1991; 19; 51-55.

V.G. Sukumaran and Narasimha Bharadwaj

Dept. of Conservative Dentistry & Endodontics

Sree Balaji Dental College & Hospitals

Pallikaranai, Chennai 601 302
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Author:Sukumaran V.G.; Bharadwaj, Narasimha
Publication:Trends in Biomaterials and Artificial Organs
Geographic Code:9INDI
Date:Jul 1, 2006
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