4
New CAD/CAM systems and tech- nologies are being introduced to the den- tal marketplace at an ever-increasing rate. Most dental companies that are involved with restorative dentistry are already in the market or are in the process of develop- ing a system or material for the CAD/CAM market. To give the reader an idea of how industry or business views the dental mar- ket, TDS, whose parent company makes Nike™ shoes, recently has entered the CAD/CAM market. Because of the growing confusion as to what these technologies are, what they will do, and, more impor- tantly, what the clinical and laboratory guidelines for using these materials and techniques are, this article will address the important points of these topics. CAD/CAM TECHNOLOGIES AND MATERIALS Advances in dental ceramic materials and processing techniques, specifically CAD/ CAM and milling technology, have facil- itated the development and application of superior dental ceramics. CAD/CAM al- lows the use of materials that cannot be used by conventional dental processing techniques. Tightly controlled industrial ceramic processing can produce increased micro-structural uniformity, higher den- sity, lower porosity, and decreased resid- ual stresses. Such improvements have the potential to improve clinical predictabil- ity. CAD/CAM has become somewhat synonymous with zirconia, but systems are available that can machine any type of ceramics, ie, glass ceramics, interpenetrat- ing (infiltration ceramics) materials, and solid-sintered monophase ceramics such as zirconia. Problems and complaints with early systems focused on the machining accu- racy of a particular system and not with the materials used with the system. Initial CAD/CAM systems produced restorations that had poor marginal fidelity with a general lack of internal adaption to the die as a result of low-resolution scanning de- vices and inadequate computing power. Technological advances in new systems and software development, coupled with specific clinical and laboratory techniques, have minimized or eliminated these prob- lems so that marginal integrity and in- ternal adaptation can be excellent. 1 Recently, several strategies using differ- ent CAD/CAM processes and different materials that can manufacture all types of all-ceramic restorations—from inlays to onlays and from veneers to crowns to fixed partial dentures—have been developed. CAD/CAM TECHNOLOGIES AND GLASS CERAMIC MATERIALS Glass ceramic materials are primarily a glass matrix material that has a fine, even- ly distributed crystalline phase. These mat- erials generally have translucency very similar to tooth structure, which makes them ideally suited for inlay and onlay ap- plications (Figure 1 and Figure 2). They can be fabricated with a conventional power liquid technique or they can be industri- ally processed into a dense machineable block. Two companies fabricate materi- als in block form for the CEREC® inLab systems (Sirona Dental Systems, Charlotte, NC); VITA® (Vident™, Brea, CA) makes the Vitablocs® Mark II and VITA Bloc Esthetic Line materials and Ivoclar Viv- adent® (Amherst, NY) makes the ProCAD and IPS e.max CAD materials. Glass cer- amic materials are ideally suited for more conservative restorations (ie, inlays, onlays, and veneers). The only commercial system for generating inlays, onlays, and veneers is the CEREC inLab system (Figure 3). This system is an evolution from the den- tist-based CEREC III (Sirona Dental Sys- tems) system that is used for generating one-visit inlays and onlays. The system is a self-contained scanning and milling unit, and it is also designed to fabricate single copings and 3-unit fixed partial den- ture frameworks using interpenetrating phase compounds and zirconia. The den- tist prepares inlays and onlays by standard techniques and sends the impression to the laboratory. The author’s experience with the system has shown that overflar- ing or beveling the preparation should be avoided. In the laboratory a die is gener- ated and placed in the system and optical- ly scanned (Figure 4). A virtual die is then displayed on the monitor and the rest- oration, with occlusion, is designed by the software. The die is then replaced with a glass-ceramic block of the desired materi- al and the restoration is machined. The system will give excellent margins that are clean, confluent, noncorrugated, and non- beveled and, ideally, a close to butt-joint margin preparation. For veneers, the same glass ceramic materials are used. Dies are generated and scanned as in inlay/onlay applications. The veneers are machined from the appropri- ate shade and translucent material. For the best esthetics, the machined veneer is cut back in the laboratory in the incisal one third to one half and enamel or translu- cent porcelain is built up (or stacked) and then fired (Figure 5 and Figure 6). The 98 INSIDE DENTISTRYNOVEMBER/DECEMBER 2006 LAB ta L k Laboratory perspectives from the inside out. CAD/CAM Update: Technologies and Materials and Clinical Perspectives Edward A. McLaren, DDS, MDC, and Lu Hyo, MDC Edward A. McLaren, DDS, MDC Adjunct Associate Professor Director UCLA Center for Esthetic Dentistry Founder and Director UCLA Master Dental Ceramist Program UCLA School of Dentistry Los Angeles, California Private Practice limited to Prosthodontics and Esthetic Dentistry Los Angeles, California Lu Hyo Master Dental Ceramist Assistant Director UCLA Master Dental Ceramist Program UCLA School of Dentistry Los Angeles, California Figure 1 Image of an onlay machined from a Vita Mark II glass ceramic block. Figure 2 Cementation of a glass ceramic onlay and inlay after staining, glazing, and polishing. Figure 3 The CEREC inLab system. Figure 4 Prepared die placed in the system ready to be scanned. Figure 5 A glass ceramic veneer that has incisal one third “cut back” and ready for translu- cent porcelains to be applied. Figure 6 Applying translucent porcelains to the incisal one third to create a more natural effect.

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Page 1: CAD/CAM Update: Technologies and Materials and Clinical Perspectives - Dental … · 2010. 6. 24. · and veneers).The only commercial system for generating inlays, onlays, and veneers

New CAD/CAM systems and tech-nologies are being introduced to the den-tal marketplace at an ever-increasing rate.Most dental companies that are involvedwith restorative dentistry are already in themarket or are in the process of develop-ing a system or material for the CAD/CAMmarket. To give the reader an idea of howindustry or business views the dental mar-ket, TDS, whose parent company makesNike™ shoes, recently has entered theCAD/CAM market. Because of the growingconfusion as to what these technologiesare, what they will do, and, more impor-tantly, what the clinical and laboratoryguidelines for using these materials andtechniques are, this article will addressthe important points of these topics.

CAD/CAM TECHNOLOGIESAND MATERIALSAdvances in dental ceramic materials andprocessing techniques, specifically CAD/CAM and milling technology, have facil-itated the development and applicationof superior dental ceramics. CAD/CAM al-lows the use of materials that cannot beused by conventional dental processingtechniques. Tightly controlled industrialceramic processing can produce increasedmicro-structural uniformity, higher den-sity, lower porosity, and decreased resid-ual stresses. Such improvements have thepotential to improve clinical predictabil-ity. CAD/CAM has become somewhatsynonymous with zirconia, but systemsare available that can machine any type ofceramics, ie, glass ceramics, interpenetrat-ing (infiltration ceramics) materials, andsolid-sintered monophase ceramics suchas zirconia.

Problems and complaints with earlysystems focused on the machining accu-racy of a particular system and not with

the materials used with the system. InitialCAD/CAM systems produced restorationsthat had poor marginal fidelity with ageneral lack of internal adaption to the dieas a result of low-resolution scanning de-vices and inadequate computing power.Technological advances in new systemsand software development, coupled withspecific clinical and laboratory techniques,have minimized or eliminated these prob-lems so that marginal integrity and in-ternal adaptation can be excellent.1

Recently, several strategies using differ-ent CAD/CAM processes and differentmaterials that can manufacture all types ofall-ceramic restorations—from inlays toonlays and from veneers to crowns to fixedpartial dentures—have been developed.

CAD/CAMTECHNOLOGIES AND GLASSCERAMIC MATERIALS Glass ceramic materials are primarily aglass matrix material that has a fine, even-ly distributed crystalline phase. These mat-erials generally have translucency verysimilar to tooth structure, which makesthem ideally suited for inlay and onlay ap-plications (Figure 1 and Figure 2). They canbe fabricated with a conventional powerliquid technique or they can be industri-ally processed into a dense machineableblock. Two companies fabricate materi-als in block form for the CEREC® inLabsystems (Sirona Dental Systems, Charlotte,NC); VITA® (Vident™, Brea, CA) makesthe Vitablocs® Mark II and VITA BlocEsthetic Line materials and Ivoclar Viv-adent® (Amherst, NY) makes the ProCADand IPS e.max CAD materials. Glass cer-amic materials are ideally suited for moreconservative restorations (ie, inlays, onlays,and veneers). The only commercial systemfor generating inlays, onlays, and veneers

is the CEREC inLab system (Figure 3).This system is an evolution from the den-tist-based CEREC III (Sirona Dental Sys-tems) system that is used for generatingone-visit inlays and onlays. The system isa self-contained scanning and millingunit, and it is also designed to fabricatesingle copings and 3-unit fixed partial den-ture frameworks using interpenetratingphase compounds and zirconia. The den-tist prepares inlays and onlays by standardtechniques and sends the impression tothe laboratory. The author’s experiencewith the system has shown that overflar-ing or beveling the preparation should beavoided. In the laboratory a die is gener-ated and placed in the system and optical-ly scanned (Figure 4). A virtual die is thendisplayed on the monitor and the rest-oration, with occlusion, is designed by thesoftware. The die is then replaced with aglass-ceramic block of the desired materi-al and the restoration is machined. Thesystem will give excellent margins that areclean, confluent, noncorrugated, and non-beveled and, ideally, a close to butt-jointmargin preparation.

For veneers, the same glass ceramicmaterials are used. Dies are generated andscanned as in inlay/onlay applications. Theveneers are machined from the appropri-ate shade and translucent material. For the

best esthetics, the machined veneer is cutback in the laboratory in the incisal onethird to one half and enamel or translu-cent porcelain is built up (or stacked) andthen fired (Figure 5 and Figure 6). The

98 INSIDE DENTISTRY—NOVEMBER/DECEMBER 2006

LABtaLkLaboratory perspectives from the inside out.

CAD/CAM Update: Technologies andMaterials and Clinical PerspectivesEdward A. McLaren, DDS, MDC, and Lu Hyo, MDC

Edward A. McLaren, DDS, MDCAdjunct Associate Professor

DirectorUCLA Center for Esthetic Dentistry

Founder and DirectorUCLA Master Dental Ceramist Program

UCLA School of DentistryLos Angeles, California

Private Practice limited to Prosthodontics andEsthetic Dentistry

Los Angeles, California

Lu HyoMaster Dental Ceramist

Assistant DirectorUCLA Master Dental Ceramist Program

UCLA School of DentistryLos Angeles, California

Figure 1 Image of an onlay machined from aVita Mark II glass ceramic block.

Figure 2 Cementation of a glass ceramic onlayand inlay after staining, glazing, and polishing.

Figure 3 The CEREC inLab system. Figure 4 Prepared die placed in the systemready to be scanned.

Figure 5 A glass ceramic veneer that hasincisal one third “cut back” and ready for translu-cent porcelains to be applied.

Figure 6 Applying translucent porcelains tothe incisal one third to create a more naturaleffect.

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benefit of veneers fabricated by this tech-nique is that it is much safer to try them inintraorally because the ceramic is muchtougher than veneers fabricated by con-ventional porcelain techniques. Also, ifnecessary, the restorations can be easily re-fired just like a conventional porcelain-fused-to-metal (PFM) or high-strength all-ceramic crown. The disadvantage of thistechnique is that the veneers need to beabout 0.9 mm or thicker for optimal esth-etics. The author uses veneers with thistechnique; the veneer thicknesses are about0.9 mm or more. The author uses conven-tional veneers for veneers thinner than0.9 mm (Figure 7). It is important to notethat the clinical situation should dictatethe material and technique selected (ie, ifthinner, more conservative veneers arethe desired clinical result, then teeth shouldnot be over-prepared to use machined or

pressed glass ceramic materials). The sys-tem does not lend itself to knife-edge orno-preparation margin techniques. Thebest results for these situations are obtainedwith a medium chamfer preparation.

CAD/CAM TECHNOLOGIESAND INTERPENETRATING(INFILTRATION CERAMICS)PHASE MATERIALSThis class of materials uses a partially sin-tered crystalline matrix of a high modulus(stiff) material, in which there is a junc-tion of the particles in the crystalline phase(Figure 8). The crystalline phase consistsof alumina, an alumina/magnesia mixture,or an alumina/zirconia mixture. The mate-rial is supplied in block form (Figure 9)and only supplied for the CEREC inLabsystem. The framework made from any ofthe three materials is then infiltrated with

a low-viscosity lanthanum glass at hightemperature (Figure 10). The ceramic phaseand the glassy phase form a continuousinterconnecting meshwork. The final struc-ture is about 85% crystalline and 15%glass. The materials are virtually the sameas the VITA In-Ceram® (Vident) material.The big difference is the material is ma-chined from a block rather than a slurrymixture of powder and liquid being builtup with a brush. The alumina/magnesiamixture is a very translucent core mate-rial and, in the author’s opinion, is ideal forwhen anterior crowns are indicated andmaximum translucency is desired (Figure11 and Figure 12). The material has shownexcellent clinical results for single poste-rior crowns of either the alumina or zir-conia-infiltrated core; thus, it can berecommended for single posterior crowns.Because of the better physical propertiesof the solid sintered zirconia, the authoruses cores from this material for all-cer-amic posterior crowns. There are a cou-ple of systems on the market that useelectroplating technology (one exampleis WolCeram, MicroDental Laboratories,Dublin, CA) to “plate” the alumina ma-terial to a master die. It is then infiltratedwith the same glass as in the convention-al In-Ceram technique or the CAD/CAMtechnique. These systems should performas well as the conventional In-Ceram mate-rial because it has very similar physicalproperties; thus, it can be recommendedfor single posterior crowns. The system

has been recommended by some for pos-terior bridge applications, but there areno published clinical studies supportingthese plating systems for use in posteriorbridges. The In-Ceram alumina materialis not recommended for posterior fixedpartial dentures (bridges) by the manu-facturer. The author’s personal experi-ence would agree with that, as over 30%of the posterior bridges fabricated withthe alumina material failed by 7 years.The system has proved to be safe and ef-fective for 3-unit anterior bridges.2

Preparations ideally should be a 360°heavy chamfer to shoulder. Sharp lineangles need to be rounded as none of theCAD/CAM systems machine well intosharp line angles. Sharp line angles canalso concentrate stress and lead to earlierthan normal fracture. Preparations forSpinell anterior restorations should allowfor 1.2 mm of crown thickness for idealesthetics. Because of the greater opacityof the alumina or the zirconia material,1.5 mm of space is necessary for ideal esth-etics, which is the same as a PFM.3

SOLID SINTEREDMONOPHASE CERAMICSSolid sintered ceramics have the highestpotential for strength and toughness but,because of high firing temperatures andsintering shrinkage techniques, they werenot available to use as high-strength frame-works for crowns and fixed partial denturesuntil recently. Solid sintered monophaseceramics are materials that are formedby directly sintering crystals together with-out any intervening matrix to form a dense,air-free, glass-free, polycrystalline struc-ture (Figure 13). There are several differentprocessing techniques that allow the fab-rication of either solid sintered alum-inous oxide or zirconia oxide frameworks.

There are three basic techniques forfabricating solid sintered monophase cer-amic frameworks for porcelain applica-tion. One system, DCS Preciscan (marketedin the United States by Dentsply Austenal)machines from a solid sintered block ofmaterial the final desired framework shape.This system is expensive and has not pro-ven cost effective as a result of the excessivemachining time and manual labor neces-sary to adjust and fit the coping. Secondly,the Procera® system (Nobel Biocare, YorbaLinda, CA) employs an oversized die whereslurry of either aluminous oxide or zirco-nia oxide is applied to the die, subsequent-ly fired, fully sintered, and shrunk to fit thescanned die. The Procera system also hasthe ability to fabricate custom abutmentsfor regular platform Brånemark implants(Figure 14 and Figure 15). The third meth-od machines an oversized coping from apartially sintered block of zirconia oxidematerial, which is then fired to full sin-tering temperature and then shrunk tofit the die. Most of the systems on themarket today use some variation of thistype of technology. Examples of thesesystems are the CEREC InLab (whichuses VITA YZ [Vident] and Ivoclar e.max

100 INSIDE DENTISTRY—NOVEMBER/DECEMBER 2006

Figure 7 Layered glass ceramic veneers in-situ. The veneers are about 1 mm thick.

Figure 8 SEM of In-Ceram showing the partial-ly sintered crystalline phase with the interveningglass phase.

Figure 9 Block of In-Ceram material formachining in the CEREC inLab system.

Figure 10 Glass-infiltrated In-Ceram coping. Figure 11 Intraoral view of a transilluminatedSpinell coping. Note the natural translucency.

Figure 12 Single central on tooth No. 9 madeout of Spinell and Vita VM®7 (Vident).

Figure 13 SEM of the densely sintered zirconia. Figure 14 Image of custom zirconia implantabutment made with the Procera system.

Figure 15 Final Vita YZ coping veneered with VM9.

Figure 16 Figure of the Lava and inLab CAD/CAM systems.

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materials), Lava™ (3M™ESPE™, St. Paul,MN) Cercon (Dentsply, York, PA), Everest(KaVo, Lake Zurich, IL), and TDS systems(Figure 16). These systems scan the pre-pared die, and then the software createsvirtual dies and frameworks. An oversizedframework is created through a CAMprocess, which is then fully sintered in aspecial oven. The VITA YZ and the Lava™systems also allow for internal shading ofthe core material. In the author’s experi-ence, the white zirconia is too reflectiveand thus the final result is too opaque.The recommendation is to use a system

that allows internally colored cores. Thetwo systems that allow internally coloredzirconia cores of different shades are theVITA YZ system machined on the CERECinLab; this system is marketed as in-Vizion™ (Vident), and the Lava™ system(Figure 17 and Figure 18). Some systemsallow the external application of color onthe surface of the white core but this alsohas the potential of being too opaque.

Zirconia oxide, sometimes called zir-con, has unique physical characteristicsthat make it twice as strong and twice astough as alumina-based ceramics. While

the reported values for flexural strengthof this new material range from over 900MPa to 1,100 MPa,4 it is important to notethere is no direct correlation between flex-ural strength (modulus of rupture) andclinical performance. Generally, strongermaterials have performed better but thereare many factors that contribute to a mater-ial’s clinical success along with the flexuralstrength of the material. A more importantphysical property is fracture toughness,which has been reported to be between 8MPa m1/2 and 10 MPa m1/2 for zirconia.3

This is significantly higher than any previ-ously reported ceramic, and roughly twicethe amount reported for the aluminamaterials. Fracture toughness is a measureof a material’s ability to resist crack growth(ie, a measure of the amount of energynecessary to cause crack growth). Clin-ically, restorations are not loaded to fail-ure as is done in a flexural strength test;instead, millions of subcritical loads (chew-ing) are applied. Materials ultimately failbecause of this cyclic fatigue by crackpropagation. Thus, materials with higherfracture toughness are more ideal clini-cally as it takes more energy to cause crackgrowth. Other factors such as stress corro-sion (chemically assisted crack growth)and residual flaws in the material greatlyaffect the final strength of a finished mat-erial and are discussed elsewhere.5

CLINICAL INDICATIONS There have been relatively few clinicalstudies reporting on CAD/CAM-gen-erated, all-ceramic, zirconia-core–basedcrowns that have included large samplesizes or long follow-up periods. Ideally,follow-up periods would be at least 5 yearswith sample sizes of several hundred

units. Long follow-up periods are necessarybecause it takes several years for fatigue tocause failure. The author has placed over300 VITA YZ and Lava™ restorationsover the last 36 months with excellentsuccess. To date there have been no zirco-nia core fractures observed; there havebeen a few cases of porcelain delaminatedfrom some of the early crowns. It wasdetermined that inappropriate firing ofthe porcelain to the core was the cause.Proper firing of a bonding layer of porcel-ain to the core is necessary to create a sta-ble porcelain/zirconia core interface. Thelaboratory technique for doing this iscovered elsewhere by the author.3 Thus,based on this early success, zirconia corerestorations can be recommended for sin-gle units anywhere in the mouth.

For fixed partial dentures, reportedclinical data has been short term but prom-ising. This and many anecdotal reports ofsuccess would indicate that high-strengthceramic frameworks subsequently ven-eered with porcelain should perform toclinically acceptable standards for 3-unitanterior and posterior fixed partial den-tures as long as accepted guidelines aremaintained. It is important to note thatwhile several studies are being initiatedor are already ongoing (including here atUCLA) using CAD/CAM technology forthe fabrication of posterior fixed partialdentures, no large-sample, long-term datayet exists to justify their ubiquitous use.Early results look extremely promisingbut the effects of fatigue and chemicalcorrosion take time to manifest theireffects. Clinical use of these materials forposterior fixed partial dentures shouldstill be considered experimental at thispoint and patients should be fully infor-med of possible effects.

CLINICAL GUIDELINES

PreparationsThe correct reduction for the room nec-essary for the esthetic fabrication of azirconia all-ceramic crown is the same asfor esthetic PFM restorations. Evaluationof restorations in which the author per-formed all clinical and ceramic proce-dures has led to the determination that1.5 mm of labial overall crown thicknesswas the minimum ideal dimension forpredictable esthetics and shade repro-duction. The core can be thinned to 0.3mm on the facial, which leaves room for1.2 mm of porcelain. A minimum of 1mm of crown thickness is required forthe lingual walls. Incisal edge thicknesscan be as little as 1.5 mm, but 2 mm isideal esthetically. Posteriorly, it is neces-sary to have 2.5 mm of occlusal reduc-tion for both esthetic all-ceramic andmetal-ceramic restorations, especially ifnatural, unworn occlusal anatomy is de-sired in the final restoration. The best aidthe author has found to accomplish thisreduction is the 2-mm Reduction Guidefrom Kerr Corporation (Orange, CA); ifthe 2-mm guide passes with only slight

Figure 17 Image of a single central (tooth No.8) inVizion crown (inVizion is the trade name forthe VITA YZ material and the CEREC inLabCAD/CAM system).

Figure 18 Image of a single central fabricatedwith a Lava core.

Figure 19 Using the Kerr 2-mm ReductionGuide to judge occlusal reduction.

Figure 20 Image of ideal preparation for scan-ning and machining. Note the very roundedocclusal line angles and generally flatter occlusalpreparation than what might have been for a PFM.

102 INSIDE DENTISTRY—NOVEMBER/DECEMBER 2006

(Circle 80 on Reader Service Card)

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binding through the occluded opposingarches then there is close to 2.5 mm of inte-rocclusal space (Figure 19). As with thepreviously mentioned machinable ma-terials, ideally preparations should be a360° heavy chamfer to shoulder andsharp line angles should be avoided. Prep-aration line angles need to be rounded assharper line angles are not easily milled(Figure 20).

Lab ConsiderationsOne of the main benefits of zirconia isusing the same ceramic or veneer mate-rial for all clinical situations. Porcelainsfor zirconia frameworks can be used forporcelain veneers (Figure 21 and Figure22), crowns, and fixed partial dentures. Theonly other porcelain a laboratory wouldneed is a metal-ceramic material.

There are some tricks to working withthe zirconia cores to get the best estheticresults. Zirconia cores are slightly moreopaque than dentin, so it is ideal to designthe framework to allow for a more trans-lucent porcelain margin material to beplaced. There is a misconception that themargin material should have the sametranslucency as dentin. If the marginalarea were at all visible it would not benoticeable unless the margin materialalso had the exact same chroma and hue asthe surrounding tooth structure. It is actu-ally ideal to have the marginal material to

be slightly more translucent than the sur-rounding tooth structure so that it blendsin by picking up some color from thetooth—the so-called contact lens orchameleon effect. So, just as with metalor more opacious ceramic cores, a porce-lain margin is mandatory for ideal esth-etics. The benefit over metal–ceramics isthat the framework only needs to beshortened slightly to allow enough lightthrough to illuminate the gingival area tocreate a natural effect (Figure 23).

REFERENCES1. McLaren EA, Terry DA. CAD/CAM systems,

materials, and clinical guidelines for all-

ceramic crowns and fixed partial dentures.

Compend Contin Educ Dent. 2002;23(7):

637-654.

2. Sorensen JA, Kang SK, Torres TJ, Knode H.

In-Ceram fixed partial dentures: three-year

clinical trial results. J Calif Dent Assoc. 1998;

26(3):207-214.

3. McLaren EA, Giordano RA. Zirconia based

ceramics: material properties, esthetics,

and layering techniques of a new porcelain,

VM9. Quintessence Dental Technol. 2005;

28:99-111.

4. White SN, Miklus VG, McLaren EA, et al.

Flexural strength of a layered zirconia and

porcelain dental all-ceramic system. J Prosthet

Dent. 2005;94(2):125-131.

5. Sorensen JA, Berge HX, Edelhoff D. Effect

of storage media and fatigue loading on

ceramic strength [abstract]. J Dent Res.

2000;79:271. Abstract 1017.

Figure 21 Layering zirconia porcelain on arefractory die for porcelain veneer fabrication.

Figure 22 Clinical case of veneers on teethNos. 6 through 11 made with VM®9 using therefractory technique.

Figure 23 A zirconia framework on the diedemonstrating a 0.5-mm facial cutback of thecore for a porcelain margin.

INSIDE DENTISTRY—NOVEMBER/DECEMBER 2006 103

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