Сraig. Dental Materials

(Courtesy lvoclar Inc., Amherst, NY,1995.)

tems are maximized with high density and small crystal size.

Leucite-Based Leucite-based ceramics are available for heat-pressing. Leucite (KA1Si206 or K20 . A1203. 4Si0,) is used as a reinforcing phase in amounts varying from 35% to 55%. Ceramic ingots are pressed between 1150" and 1180" C (under a pressure of 0.3 to 0.4 MPa) into the refractory mold made by the lost-wax technique (Fig. 18-9). This temperature is held for about 20 minutes in a specially designed automatic press furnace. The ceramic ingots are available in different shades. The final microstructure of these heat-pressed ceramics consists of leucite crystals, 1 to 5 micrometers in size and dispersed in a glassy matrix (Fig. 18-10,A). Two techniques are available: a staining technique or a layering technique involving the application of veneering porcelain. The two techniques lead to comparable mean flexural strength values for the resulting porcelain composite. To ensure compatibility with the thermal expansion coefficient of the veneering porcelain, the thermal expansion co-

efficient of the material for the veneering technique (14.9 x C) is lower than that of the material for the staining technique (18 x lop6/ " C). The flexure strength of these ceramics (120 MPa) is about double that of conventional feldspathic porcelains. The main disadvantages are the initial cost of the equipment and relatively low strength compared with other all-ceramic systems.

Lithium Disilicate-Based In recent years, new all-ceramic materials for heat-pressing have become available. These materials contain lithium disilicate (Li,Si20j) as a major crystalline phase. They are heat-pressed in the 890" to 920" C temperature range, using the same equipment as for the leucite-based ceramics. The heatpressed restoration is later layered with glasses of matching thermal expansion. The final microstructure consists of about 60%elongated lithiumdisilicate crystals (0.5 to 5 micrometers long) dispersed in a glassy matrix (Fig. 18-10, B). The main advantage of the lithium disilicate-contain- ing ceramics is their superior flexural strength

564 Chapter 18 CERAMICS

(350 MPa) and fracture toughness (3.2 MPa . mO.'), which extend their range of applications. The fabrication of fixed partial dentures is theoretically possible with these materials. However, no long-term clinical data is presently available.

The advantages of heat-pressed ceramics include good esthetics for the leucite-reinforced materials, high strength (but higher opacity) for the lithium disilicate-based materials and ability to use the well known lost-wax technique. Processing times are short and margin accuracy is within an acceptable range.

SLIP-CAST ALL-CERAMIC MATERIALS

Slip-casting involves the condensation of an aqueous porcelain slip on a refractory die. The porosity of the refractory die helps condensation by absorbing the water from the slip by capillary action. The piece is then fired at high temperature on the refractory die. Usually the refractory shrinks more than the condensed slip so that the piece can be separated easily after firing. The fired porous core is later glass-infiltrated, a unique process in which molten glass is drawn into the pores by capillary action at high temperature. Materials processed by slip-castingtend to exhibit reduced porosity, fewer defects from processing, and higher toughness than conventional feldspathic porcelains.

Alumina-Based An alumina-based slip is applied to a gypsum refractory die designed to shrink during firing. The alumina content of the slip is more than 90%, with a particle size between 0.5 and 3.5 ym. After firing for 4 hours at 1100•‹C, the porous alumina coping is shaped and infiltrated with a lanthanum-containing glass during a second firing at 1150' C for 4 hours. After removal of the excess glass, the restoration is veneered using matched-expansion veneer porcelain. This processing technique is unique in dentistry and leads to a high-strength material due to the presence of densely packed alumina particles and the reduction of porosity. The microstructure of this material is shown in Fig. 18-10, C. The flexural strength of this slip-cast

alumina material is around 450 MPa. Because of the high strength of the core, short span anterior fixed partial dentures can be made using this process.

Spinel- and Zirconia-Based Two modified ceramic compositions for this technique have been recently introduced. One contains a magnesium spinel (MgAl,O,) as the major crystalline phase with traces of alpha-alumina, which improves the translucency of the final restoration. The second material contains tetragonal zirconia and alumina. The spinel-based material has a lower modulus of rupture than the aluminabased material, whereas the zirconia-based material has a reported flexural strength neighboring 600 MPa.

The main advantage of slip-cast ceramics is their high strength; disadvantages include high opacity (with the exception of the spinel-based materials) and long processing times.

MACHINABLE ALL-CERAMIC MATERIALS

Machinable ceramics can be milled to form inlays, onlays, and veneers using special equipment. One system uses CAD/CAM (computer assisted design/computer assisted machining) technology to produce restorations in one office visit. After the tooth is prepared, the preparation is optically scanned and the image is computerized. The restoration is designed with the aid of a computer, as shown in Fig. 18-11. The restoration is then machined from ceramic blocks by a computer-controlled milling machine. The milling process takes only a few minutes. Although convenient, the CAD/CAM system is very expensive and its marginal accuracy is poor, with values of 100 to 150 pm. Bonding of the restorations with resin cements may help compensate for some of the problems of poor marginal fit.

Another system for machining ceramics is to form inlays, onlays, and veneers using copy milling. In this system, a hard resin pattern is made on a traditional stone die. This handmade pattern is then copied and machined from a ceramic

Fig. 18-11 The Cerec chairside CADICAM system for porcelain inlay fabrication.

(Courtesy Siemens Corp., Bad Soligman, Germany, 1995.)

block using a pantographic device similar in principle to those used for duplicating house keys. Again, marginal accuracy is a concern and there are high equipment costs.

Several machinable ceramics are presently available for use with these systems. One contains a potassium feldspar as a major crystalline phase dispersed in a glassy matrix (Fig. 18-10,D) It is available in several shades. Its flexural strength can be ranked as moderate (105 MPa).

Mica-based glass-ceramics are also available with this system. Their microstructure consists of mica crystals (50% to 70% by volume) dispersed in a glassy matrix. The mica crystals are elongated and randomly oriented (Fig. 18-10, E). Because of the crystalline structure of the mica crystals, cracks are deflected along the crystals, as

shown in Fig. 18-10, F. This microstructure and the cleavage properties of mica crystals gives this type of glass-ceramic its good machinability and a flexural strength around 230 MPa.

Pre-sintered slip-cast alumina blocks can be machined using the copy-milling system to generate copings for crowns and fixed partial dentures. The alumina copings are further infiltrated with glass, resulting in a final marginal accuracy within 50 pm.

A more recent system involves an industrial CAD/CAM process to produce crowns. The die is mechanically scanned by the technician and the data is sent to a workstation where an enlarged die is milled using a computer-controlled milling machine. This enlargement is necessary to compensate for the sintering shrinkage. Aluminum oxide powder is then compacted onto the die and the coping is milled before sintering at very high temperature (>1550•‹C). The coping is further veneered with an aluminous ceramic with matched expansion.

As mentioned earlier, the main advantage of the machining process in making dental ceramic restorations is the ability to accomplish the restoration in one office visit. This is not true for the last system reviewed in which the restoration is fabricated in a laboratory. High equipment costs, poor marginal accuracy (compared to gold restorations) of the machining process, and the high opacity of the ceramic materials available constitute the main disadvantages of this process. Except for the alumina-based systems, strength remains a concern.

Ceramics are probably the best material available for matching the esthetics of a complex human tooth. They are used for ceramic-metal crowns, fixed partial dentures, all-ceramic restorations, and to fabricate denture teeth. However, ceramics are brittle and fragile in tension, and the quality of the final product is very techniquesensitive, in terms of strength and esthetics.

CERAMIC-METAL CROWNS AND FIXED PARTIAL DENTURES

Ceramic is widely used as the veneering material in ceramic-metal crowns and fixed partial dentures. This development was the result of successfully matching the coefficients of thermal expansion of porcelain with alloys and achieving a proper bond. The glazed surface produces a restoration that is color-stable, tissue-friendly, biocompatible, chemically durable and has low thermal diffusivity. Ceramic-metal restorations are still widely used; their failure rate at 10 years is considerably lower than most all-ceramic crown systems. It should be noted, however, that no survival data are available for some of the newest all-ceramic systems.

I I

Fig. 18-12Diagram of porcelain veneer bonded to facial surface of tooth.

(From Craig RG, Powers JM, Wataha JC: Dental materials: properties and manipulation, ed. 7, St Louis, 2000, Mosby.)

ALL-CERAMIC CROWNS, INLAYS, ONLAYS, AND VENEERS

Ceramics have been used to fabricate jacket crowns since the early 1900s. At that time feldspathic porcelain was used in the fabrication. Alumina-reinforced ceramics were later introduced to improve the mechanical properties. In the past 20 years, novel materials and techniques for the fabricating all-ceramicrestorations have been introduced. They include slip-cast, machined, and heat-pressed all-ceramic materials. These new materials and techniques have widened the range of applications of all-ceramic materials, and in some cases made their processing easier.

Ceramic inlays and onlays are becoming increasingly popular as an alternative to posterior composite resins. They have better abrasion resistance than posterior composite resins and therefore are more durable. However, occlusal adjustments are more difficult and can lead to subsequent wear of the opposing tooth if not properly polished. The marginal gap is greater than with gold inlays or onlays. These restorations are not indicated when high occlusal loads are expected.

A porcelain esthetic veneer (laminate veneer) is a layer of porcelain bonded to the facial surface of a prepared tooth to cover an unsightly area (Fig. 18-12). Porcelain veneers are custom made

and are fabricated in a dental laboratory. Initially, porcelain veneers were made of feldspathic porcelain and sintered. Currently, most porcelain veneers are fabricated by heatpressing, using either a leucite-reinforced or lithium-disilicate ceramic.To obtain sufficient adhesion, the tooth enamel is etched with phosphoric acid and the bonding surface of the porcelain is etched with diluted hydrofluoric acid and treated with a silane coupling agent. Resin composites specifically formulated for bonding to ceramic are used as the adhesive.

The properties of dental ceramics depend on their composition, microstructure, and flaw population. The nature and amount of reinforcing crystalline phase present dictate the material's strength and resistance to crack propagation and its optical properties.

Ceramics are brittle and contain at least two populations of flaws: fabrication defects and surface cracks.

Fabrication defects are created during processing and consist of inclusions at the condensation stage or voids generated during sintering.

Inclusions are often linked to improper cleaning of the metal framework or use of unclean instruments. Porosity on the internal side of clinically failed glass-ceramic restorations has been identified as the fracture initiation site. Microcracks also develop upon cooling in feldspathic porcelains and can be due to thermal contraction mismatch between the leucite crystals and the glassy matrix, or to thermal shock if the porcelain is cooled too rapidly.

Surface cracks are induced by machining or grinding. The average natural flaw size varies from 20 to 50 pm. Usually, failure of the ceramic originates from the most severe flaw.

Dental ceramics are subjected to repeated (cyclic) loading in a humid environment (chewing), conditions that are ideal for the extension of the preexisting defects or cracks. This phenomenon, called slow crack growth, can contribute to a severe reduction of the survival probability of ceramic restorations.

ness, contact zone at loading, homogeneity and porosity of the material, and loading rate. For this reason, discrepancies exist amongst the published values of mechanical properties for a given material.

Sometimes, researchers use devices that try to simulate dental morphology. However, the experimental variables can become extremely complex and difficult to reproduce in this type of testing. Finite element analysis (FEA) constitutes another approach to the simulation of clinical conditions. Failure predictions of ceramic inlays by the FEA technique have successfully matched fractographic analyses of clinically failed restorations.

Fractography is well established as a failureanalysis technique for glasses and ceramics. It has been recognized as a powerful analytical tool in dentistry. The in vivo failure stress of clinically failed all-ceramic crowns can be determined using fractography.

TEST METHODS

Numerous test methods are available to evaluate the mechanical properties of ceramics. Studies of the influence of test method on the failure stress of brittle dental materials have shown that important test parameters are the specimen thick-

COMPARATIVE DATA

The flexural strengths of dental ceramics are summarized in Table 18-3. Feldspathic porcelains for ceramic-metal restorations have a mean flexural strength of about 70 MPa. This value is lower than those listed for all-ceramic materials;

568 Chapter I 8 C E R W C S

however, because ceramic-metal restorations are supported by a metallic framework, their probability of survival is usually higher.

Among the currently available all-ceramic materials, slip-cast ceramics exhibit the highest values (378 to 604 MPa), followed by lithiumdisilicate heat-pressed ceramics (350 MPa). The flexural strength of leucite-reinforced heatpressed ceramics is around 100 MPa. Among the machinable ceramic materials, mica-based material has the highest flexural strength value (229 MPa). As mentioned previously, the nature and amount of the crystalline phase present in the ceramic material strongly influences the mechanical properties of the final product.

The flexural strength of feldspathic porcelain is between 62 and 90 MPa, the shear strength is 110 MPa, and the diametral tensile strength is lower at 34 MPa. The compressive strength is about 172 MPa, and the Knoop hardness is 460 kg/mm2.

Fracture toughness is also an important property of ceramics; it is a measure of the energy absorbed by the material as it fails. The fracture toughness of conventional feldspathic porcelains is very similar to that of soda lime glass (0.78 MPa . m0.5). Leucite-reinforced and micabased ceramics have a fracture toughness about double that of soda-lime glass and four times that of glass for lithium-disilicate ceramics.

The elastic constants of dental ceramics are of interest as they are needed in the calculations of both flexural strength and fracture toughness values. Poisson's ratio lies between 0.21 and 0.26 for dental ceramics. The modulus of elasticity is 69 GPa for feldspathic porcelain, varies from 62 to 7 2 GPa for machinable ceramics, and reaches 110 GPa for lithium-disilicate heatpressed ceramics.

The amount of shrinkage in ceramic bodies depends on the average pore size, which is directly related to the particle size distribution. The smaller pores obtained from a broader particle size distribution generate higher capillary pressure, leading to more shrinkage. The linear firing shrinkage of feldspathic porcelains has been reported to be approximately 14% for low-

fusing porcelain (ceramic-metals) and 11.5% for high-fusing porcelain (denture teeth). Overglazed porcelain has a greater percentage of shrinkage for both the lowand high-fusing types, and the volumetric shrinkage shows even greater differences of approximately 8%. Several studies have shown the low-fusing type to have a volumetric shrinkage of from 32% to 37% and the high-fusing porcelain to shrink as much as 28% to 34%. The medium-fusing porcelain has shrinkage values between the highand lowfusing types. As pointed out earlier in this chapter, precise control of the condensation and firing technique is required to compensate for such shrinkage values during the construction of the porcelain restoration. The greatest dimensional change will occur where porcelain is thickest.

Shrinkage remains an issue for all ceramic materials with the exception of machined ceramics from fully sintered ceramic blocks and heatpressed ceramics. Shrinkage of the veneering ceramics applied on all-ceramic cores has to be carefully compensated for during porcelain buildup.

The density of fully sintered feldspathic porcelain is around 2.45 g'cm3 and will vary with the porosity of the material. The density of allceramic materials, which depends on the amount of crystalline phase present, is between 2.4 and 2.5 &'cm3.

The thermal properties of feldspathic porcelain include a conductivity of 0.0030 cal/sec/cm2 (O C/cm), a diffusivity of 0.64 mm2/sec, and a linear thermal coefficient of expansion of 12.0 x C between 25 and 500" C. This coefficient is about 10 x C for aluminous ceramics

and lithium-disilicate ceramics, and 14 to 18 x C for leucite-reinforced ceramics.

Color matching is a critical problem in replacing portions of natural teeth. Porcelain, being partially amorphous in structure, does not resemble crystalline enamel completely. As a result, vari-

ous kinds of light are reflected and absorbed in different manners by the tooth tissue and porcelain, and restorations viewed from an angle may not appear the same as they do when viewed from the front. The cementing medium is an important factor in the final appearance of an all-ceramic restoration. Because of its opacity, an aluminous all-ceramic restoration may be cemented with a wide range of luting agents, but not with resin-modified glass ionorner cements (associated with fracture). However, a more translucent all-ceramic restoration such as a leucite-reinforced heat-pressed crown or veneer, or a machined inlay or veneer, usually requires the use of translucent resin luting agents that are available in different shades.

The colors of commercial premixed dental porcelain powders are in the yellow to yellowred range. Because the range of colors of natural teeth is much greater than the range available in a kit of premixed porcelains, modifier porcelains are also supplied for adjustments. These modifiers are strongly pigmented porcelains usually supplied in blue, yellow, pink, orange, brown, and gray. The dental technician may add the modifier porcelain to the opaque and body porcelains during the building of the crown. Extrinsic surface staining, another way of changing the color of a dental porcelain crown, involves the application of highly pigmented glazes. The main disadvantages of surface staining are a lowered durability (a result of solubility) and the reduction of translucency.

Translucency is another critical property of dental porcelains. The translucency of opaque,

dentin (body), and enamel (incisal) porcelains differs considerably. Opaque porcelains have very low translucency, allowing them to mask metal substructure surfaces. Dentin porcelain translucency values range between 18% and 38%, as seen in Table 18-4. Enamel porcelains have the highest values of translucency, ranging between 45% and 50%. The translucency of materials for all-ceramic restorations varies with the nature of the reinforcing crystalline phase. Alumina-based systems are opaque, whereas leucite-reinforced systems are more translucent. The translucency of spinel-based systems is comparable with that of lithium disilicate-based systems and intermediate between alumina-based and leucite-reinforced systems.

Because dental enamel is fluorescent under ultraviolet light, uranium oxide had been added to produce fluorescence with porcelain. However, because of the low but detectable radioactivity of uranium, newer formulations contain rare earth oxides (such as cerium oxide) to produce fluorescence.

Because the outer layers of a porcelain crown are translucent, the apparent color is affected by reflectance from the inner opaque or core porcelain. Color mixing results from combining the light reflected from the inner, opaque porcelain surface and the light transmitted through the body porcelain. The thickness of the body porcelain layer determines the color obtained with a given opaque porcelain. This thickness effect may be minimized if the body porcelain and the opaque porcelain are the same color as that of some commercial systems

Porcelain denture teeth are commercially manufactured and their composition and properties set them apart from ceramics for fixed restorations. Individual or complete sets of teeth of excellent quality are available in a wide range of shapes and shades from a number of manufacturers. They have been used for complete dentures. The anterior teeth have one or two gold-covered pins to provide retention to the denture base. The posterior teeth have diatoric holes located centrally in the underside of the teeth for retention to the denture base. A typical set of porcelain teeth is shown in Fig. 18-13.

The ceramic composition of porcelain denture teeth belongs to the triaxial porcelain compositions (quartz, clay, kaolin). The teeth are made in split molds, fired under vacuum and slowly cooled to prevent crazing.

The main advantages of porcelain denture teeth are their superior esthetics, resistance to

abrasion, and excellent shade stability. One disadvantage is the difficulty of polishing the surface after occlusal adjustment, resulting in significant wear of the opposing teeth.

The physical properties of tooth porcelain are perhaps best shown when compared with the properties of plastic teeth and, when possible, with those of natural teeth. These properties, however, are often so radically different that they cannot be measured with the same equipment or compared quantitatively.The hardness values of porcelain compared with plastic and natural tooth structures are shown in Table 4-16.

It is evident from these values that the two standard tooth-replacement materials are quite different in hardness. Porcelain is harder than enamel, whereas the best plastic teeth are softer than dentin.

In regard to abrasion resistance, dental porcelain has been clinically evaluated as being equal to or slightly more wear resistant than natural tooth structure. At present, clinical data represent the only measure of abrasion believed to be of real significance. On this basis, porcelain has

been estimated to have from 10 to 20 times the abrasion resistance of plastic teeth.

Porcelain is resistant to the action of solvents, with only hydrofluoric acid known to have any significant effect on it. The cross-linked plastics are relatively craze resistant and are immune to reasonable amounts of ordinary solvents. They may, however, be softened to some extent by a number of organic solvents.

Water bleaching and sunlight have no effect on porcelain, but repeated cycles of drying and water immersion may cause whitening and loss of color in plastic teeth. Continued exposure to ultraviolet light may cause a slight yellowing. When plastic is softened by solvents, organic dyes may penetrate the outer layer and cause discoloration.

The flexural strength of dental plastics is unquestionably superior to that of the dental porcelains. The pin anchorage and the strength of the porcelain that surrounds it are the determining factors in the basic strength of a porcelain tooth. Retention is adequate for most service conditions.

The high impact strength of plastic gives it a definite advantage. Vacuum-firing has improved the fracture resistance of dental porcelain by about 50%, but this does not preclude the possibility of fracture from sudden shock.

In dimensional stability or permanence of

form, plastic is less acceptable than porcelain. The superior strength of plastic is indicated by its ability to cushion the impact blow of mastication and avoid fracture caused by brittleness. Unfortunately, plastic does not return to its original form each time it yields, and there is cumulative loss of dimension; this loss is described as $ow. Porcelain, like tooth enamel, is incapable of cold flow.

It is often necessary to "spot" grind porcelain denture teeth or to heat them in a flame momentarily during the waxing of a denture. Overheating during these procedures may cause a small area to expand and become too large to be compatible with the remainder of the tooth. This may result in immediate breakage, or perhaps only in cracking the tooth, so that failure occurs in service. It is therefore advisable to keep a porcelain tooth wet during all grinding operations and to avoid rapid heating or cooling.

Both porcelain and plastic withstand heating well enough for the usual dental procedures. Plastic teeth will withstand temperatures of approximately 200' C, and porcelain is unaffected by temperatures in excess of 1100" C if the heat is applied slowly.

A direct and more complete comparison of the characteristic properties of porcelain and plastic teeth is discussed under "Properties of Dental Plastics" in Chapter 21.

I SELECTED PROBLEMS

Problem 1

A porcelain that normally glazes at 982' C

was found to require 1010' C for glazing.

Explain.

Solution a

The pyrometer of the porcelain furnace requires periodic calibration, usually with silver

disks that are supplied by the furnace manufacturer.

Solution b

If the particle size is changed, the sintering and glazing temperatures change also. It is important to shake the porcelain before using it to counteract any settling of particles.