Operative Dentistry

The new creativity in ceramic restorations: Dental CAD-CIMWerner H. Mormann*/Andreas Bindl**

Abstract The efficiency of the Cerec 2 system, introduced in September 1994. is brieflypresented. The system provides itnproved accuracy of fit, an automaticallygenerated occlusion, free (unrestricted) cavity design, unlimited anatomic veneerpreparation with incisai edge coverage, and automated and simplified operation.(Quintessence Int !996:27:82¡-S28.)

Clinical relevance

Computer-generated ceramic restorations are areality in the everyday practice of more than 3,000dental practitioners worldwide. In 10 years ofclinical use, more than 2 million CAD-CIM restora-tions have been placed.

Introduction

Ceramic inlays, onlays, and veneers have increasinglybecome part of the clinical routine in dental offices.'The adhesive technique permits preparations thatpreset^e the dentai hard tissues and widens the rangeof ceramic restorations.^ The key features of dentalceramics are excellent biocompatibility, good machin-ability, high abrasion resistance, and durable colorstability, as well as enamel-like modulus of elasticityand thermal conductivity. •*•* To date computer-aideddesign-computer-integrdted manufacturing (CAD-CIM)restorations have shown more than 6 years of goodclinical performance and have thus gained scientific

' Professor and Director, Division of Aesthetic and Compuler Restora-tions, Clinic of Preventive Dentistry, Peri odontology, and Cariology.University of Zurich, School of Dentistry, Zurich, Switzerland.

" Research Associate, Division of Aesthetic and Computer Restora-tions, Ciinic of Prevenlive Dentistry, Periodontotogy, and Cariology,University of Zurich, Sctioot of Dentistry, Zurich, Switieriand

Reprint requests: Dr W. H. Mormann, Professor and Director, Divisionof Aesthetic and Computer Restorations, Ctinic of Preventive Dentistry,Period onto iogy, and Cariology, Llniyersity of Zuricti. School of Den-tistry, Plattcnstrasse tl, CH-8028 Zurich, Switzertand.

acceptance. For computer-generated inlays, practi-cally pore-free industtial ceramics that do not requireglazing are used, and the inlays offer excellent marginalseals at both the enamel and dentinal interfaces whencemented with resin cements.^

The Cerec 2 unit (Siemens) is based on the processdeveloped by Mormann and Brandestini' and is theresult of constant tiirther development yia differentgenerations of Cerec units (Figs 1 to 3). W\\henlargement of the grinding unit from three to six axesand upgrading of the software for the occlusion and thecomplex machining of the floor parts, the previouslimitations have been eliminated. Because of its evenmore sophisticated technology, occlusal surfaces cannow be machined with the Cerec 2 unit. On the onehand, the Cerec 2 unit enables the dentist to generateceramic restorations entirely independently of dentallaboratories, and, on the other hand, its ñilly auto-mated grinding process makes it attractive for labora-tory use too.

Technical innovations

The Cerec 2 camera: Optical impression

The Cerec 2 camera has been given a new design and iseasy to handle (Fig 4), To tnaintain good accessibilityto the oral cavity, the size of the intraoral frontal part ofthe camera has not been modified. An importanthygienic feature is the detachable cover, which can besterilized by dry heat in case of exposure to a higherrisk of infection. Usually, the cover is removed, a plugis inserted in its opening, and it is then washed in athermal disinfector. The camera can also be wiped

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Fig 1 Cerec 1. UnJtof the first series1985 (Brains). This series wasplanned tor 25 units. Cerec Opera-ting System 1.0 (COS 1.0) and tur-bine drive.

Fig 2 Cerec 1 Unit ot tine secondgeneration 1988 (Siemens). InitialEottware COS 1.0 and turbine drive.Improved camera. From 1991 withCOS 2 0, from 1992 with electricgrinding drive ¡third generation), andtrom 1993 with veneer software 1.0(fourth generation).

Fig 3 Cerec 2, September 1994(Siemens) According to the sameprinciples as Cerec 1, but a fullyrevised unit.

Fig 4 Cerec camera with the newdesign, in a typical recording posi-tion.

Table I Three-dimensional camera and optical im-pression (as of 1994/1995)

Feature

Active areaActive pixelVoxel patternFrame frequencyScanning timeComputing timeCamera window

Beam exit angleBeam pathDepth of field

Cerec 1

14 X 14 mm65,00054 X 54 X 54 \xm50 Hz- 180 msec-- 130 msecCold & coated

11°Asymmetric8 mm

Cerec 2

17 X 14 mm320,00025 X 25 X 29 |im60 Hz-- 133 msec- 400 msecPreheated &scratch resistant17=Symmetric10 mm

clean with a dispensable cloth moistened with a liquiddisinfectant.The further development of the intraoral three-dimensional camera has been carried out in accor-dance with the original Cerec process.^'" Pixel size(picture element) has been reduced from 54 x 54 |im

to 25 X 29 |im (Table 1). Thus, in the pixel imagesystem, the voxel (volume element) pattern has cometo 25 X 25 X 29 \im in the pixel image system. Becauseof the optimization of the optical beam path by meansof symmetric beam geometry, major measurementerrors in the measuring voiume of a typical inlay have

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Table 2 Image processing board and central proces-sing unit (CPU) board (as of 1994/1995)

Feature

[mage processingboardProcessor

50MhzRAM

Display

CPU board

Disk drive

Cerec 1

Withoutgraphics processorwith CPU support

128 kbyte RAM

Black and white

MC 680007 MHz16 bit, 380 kbyteRAM1 MB unformatted

Cerec 2

Withgraphics processorTITMS 34010

0,5 MB RAM1,75 MBVideo RAMBlack and whiteand colorMC 6802020 MHz32 bit. 4 Mb RAMand coprocessor4 MB unformatted

Fig 5 Opticai impression on theCerec 2 monitor, user interface withicon bar. The last step in the designot-an oniay, the drawing of thefissure line tor the so-calied extrapo-lation occiusion.

been brought down to less than + 25 [im, A moreaccurate control of the projected measuring patternand the particularly low-noise level processing ofthevideo signal has resulted in a distinct reduction ofthespurious components in the measuring data.

Because of the smaller pixel size and the higheraccuracy in the depth measuring, the resolution oftheoptical impression has been doubled compared to thatofthe Cerec 1 unit. Thus, measuring accuracy oftheCerec 2 camera lies within the range of + 25 im, 'For the optical impression, the lens window of thecamera is preheated to prevent fogging. As a result ofIhe enlarged range of depth of field (see Table 1), indifftcult situations, the front edge of the Cerec 2camera may be rested on the distally adjacent tooth toprovide a reliable, completely unblurred recording.

Image processing

Data representation of a typical mesio-occlusodistalinlay in the image memory has been increased from 4milhon voxel in Cerec 1 to 32 million voxel in Cerec2," Accordingly, the amount of data to be processedhas grown by the factor 8, As a result ofthe six timesmore efficient computing capacity (Table 2), thesurface operations can be carried out in about the sametitne and the line operations are about three timesfaster. Main memory (ID 4) overloads no longer occur.

User interface

On the 14 x 17-cm color monitor, the preparation isrepresented at xl2 magnification (Fig 5). Thus,compared to the x8 magnification in Cerec 1, theaccurate drawing of the construction lines has beenfacilitated. Proven elements, such as selecting thetooth to be treated by clicking at it in the tooth arch,confirmation ofthe procedure steps by clicking at theicons, and alternative options in the interactive win-dows, have been maintained, extended, and at thesame time simplified by automation (Table 3), Ac-cordingly, camera calibration and the adjust procedureof the depth profile data are now fully automated. Inconjunction with the automatic adjust, the tneasuringrange has been extended to the complete range ofdepth of ftcld of 10 mm. Thus, even very deep cavitiesno longer present clinical litnitations of any kind.

Another useftil element is the automatic propositionofthe proximal contact lines. Although, for the timebeing, these still have to be aligned to tbe approximalsurfaces ofthe adjacent teeth by editing to establish thedesired close contacts, input errors in defining thestarting and end points of the proximal contact lineshave been eliminated. All of these improvementscontribute to a more secure ftt.

In the Cerec 2 Operating System 4.20 (COS 4.20),handling the projection and cut windows for checkingand correcting (editing) the occlusal margins and for

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Fig 6 Automatic generation oí theocclusai and laterai surtaces of tineonlay with display for the caicuiationoí Ihe floor data for the extendedmachining mode.

Table 3 Software for camera and image processing,design of occlusion and tloor design, and type ofmachining

Feature

Cameracalibration

Adjust

Measuring depthBottom lines

Proximal contactlines

Fissure linesOcclusal design

Occlusalmachining

Floor part designRoor part

machining

Cerec 1

ManualManual

7 mmStart anddirection free

Manual

NoneLinear

Linear

Linear

Linear

Cerec 2

AutomaticManual andautomatic10 mmStart: Cervical lineangle

Proposal andeditingYes, manuallyLinear extra-polation, correlation

Linear and contour.simultaneousComplex contours

Complex shapes

cuspal design is generally more comfortable, faster,and safer than in the Cerec 1 operating systems.

User elements

The monitor can be swiveled and tilted, thus faciU-tating visual control of the video image in the search

mode during the taking of the optical impression.Furthermore, this arrangement allows the patient towatch the design and the program procedure. Duringthe taking of the optical impression, the camera isactivated by a newly arranged foot switch. Unlike theusual operational technique, the foot switch is acti-vated not by pressing but by lifting the foot. This setuphas hygienic and ertgineering advantages, for whichreason this solution has been chosen. Operators soonget accustomed to it.

In drawing, as well as during the entire process ofdesign, entries are made using the trackball and a newenter key, placed at an index finger-thumb distance onthe top side of the Cerec unit. They can easily andquickly be operated in conjunction. The mobility of thetrackball (which can be removed for disinfection) isgood. The translation into a sufficient cursor speed forcovering large distances on the monitor, on the onehand, and the fine tuning in the positioning accurate tothe pixel, on the other hand, are excellent. Operatorsrequire some time to get used to the arrangement ofthesoft keys for the pixel control ofthe cursor, the deletekey, and the level mode key at the lower front frame ofthe monitor. However, this arrangement has advan-tages for hygiene and engineerirtg.

Extrapolation occlusion.- Simultaneous grinding

Three different programs for design are available:Extrapolation, Correlation, and Veneer. There aretlirce choices in designing the occlusion; (/) anatomi-cally adapted (Extrapolation), (2) correlated to aftinctionally generated path (Correlation), and (3)buccolingually flat (linear), as in the past (Table 3).The first studies have now shown that, for practicaluse, the interactive design technique, in conjunctionwith the extrapolation program for inlays and onlays,is the method of choice,'- An essenrial point of thismethod is the tracing ofthe mesiodistal main fissureline (Figs 5 and 6). Fissure depth is determined ininlays by the height ofthe adjacent cavosurface marginline, and in onlays also by the marginal ridge-cusp line.For every supporting point ofthe fissure line, the nextpoint on the cavosurface mai^in line or on themarginal ridge-cusp line, respectively, is searched forin buccolingual direction. The z-value of this point isreduced by 1.0 mm, providing fissure depth.

To ensure sufficient porcelain thickness of at leastLO mm, a sufficient preparation depth in the fissuresection has to be secured. Through triangulation oftheocclusal surface, triangles are calculated, each being

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Fig 7 Simullaneous grinding The floor and the wallprotiles are shaped by the grinding disk, while Ihe ocolusalmorphoiogy is shaped by the cyiindtioai diamond.

Fig 8 Apart from the inner surface, the lateral view showsparts of the extrapoiation occlusion.

Fig 9 Inner surface of the Cereconlay. The reproduction of the com-plex shape IS easily distinguishable.

Fig 10 The triai fitting ot the onlay reveals good anatomioadaptation and an exceiient acouracy of fit.

defined by its edge points and two inclination vectorsper triangle side." The triangle surfaces are inter-polated and form a continuous surface (Fig 6).Grinding is performed by a cylindrical diatnond(diatneter of 2 mm, particle size of 64 pm, 77,000 rpm,atid cutting speed of about 8 m/s), working simultan-eously with the radial infeed grinding ofthe grindingdisk (64-pm panicle size, 18,000 rpm, and cuttingspeed of about 38 m/s) (Fig 7).

Cerec onlays: Extended machining

Wth the extended machining option ofthe Cerec 2system, complex floor shapes in inlays and onlays canhe ground with the cylindrical diamonds (Figs 8 and9). Cuspal coverage, circular margins, and buecalmargins with different levels are possible. Occlusal

floor sections with differing levels can also be de-signed; the grinding disk-dependent orientation ofpulpoaxial box walls no longer exists. However, to befeasible, the preparation shapes have to be locatedwithin the 2-mm resolution range of the cylindricaldiamonds for the time being. Generally, this does notpresent any limitations in practical use. Experiences sofar have shown that onlays placed in combination witha carefully applied adhesive technology in many casesoffer a dental hard tissue-preserving alternative tocomplete crowns that is generally met with a positiveresponse from patients. An anatomically adaptedocclusion is created {Figs 7, 8, and 10) and, with littleadditional work, can be morphologically finished witha 40-|im contouring diamond and an S-pm finishingdiamond and polished with flexible disks (Fig 11),

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Fig 11 Cerec 2 onlay, finished witfisuperfine diamonds and polisiiedwith flexible disks.

Fig 12 Cerec veneer with biconcave contour on Ihe innerside after grinding with tine program option of extendedmachining The design and grinding technique permitcustom anatomic preparations without any restrictions

Fig 13a Anterior aspect of the maxillary dentition of a34-year-old woman who asked for an esthetic correction.She disliked the position of the two lateral incisors and theirincisai edges.

Fig 13b Esthetic correction of the two lateral incisors withCerec 2 veneers. Realignment of the facial surfaces of teeth7 (12) and 10 (22). The maxillary anterior teeth exactly metthe patient's expectations.

Veneers: incisai edge coverage

The Cerec system is the only method in dentistry topermit the direct maciiining of ceramic yeneers,'-' Inthe United States, where the standards of estheticrequirements are high, more than 20% of Cerec usersregularly produce veneers in one appointment andplace them directly with an adhesive technique. ''* Wththe Cerec 1 utiit and the extended computer software.Veneer 1.0, yeneers and onlays can easily be designedand manufacttired,'^ The new Cerec 2 software, COS

4,20, permits custom yeneer preparations with anykind of anatomic reduction, as well as the easy designof the veneer and its direct manufacturing.'* Ciass IVsituations combined with incisai edge coverage maybedesigned directly, without using a wax template for theoptical impression.'^ With the extended machiningoption, any three-dimensional shape can be manu-factured directly (Pig 12).

Patients tend to prefer Cerec ceramic yeneers as analternative to anterior complete ceramic crowns (Figs13a and 13b) because of the hard tissue-presening

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preparation and the rapid treatment, which can becompleted in a single appointment. If the need forrepetition, correction, or any further reconstructionshould arise, this can be carried out without complica-tions (simple reentry). The veneer technique may alsobe used for fixing or replacing veneers in existingcrowns and fixed partial dentures.'^

Grinding precision and acctiracy of fit

Grinding precision, the final element in the CAD-CIM manufacturing process, is crucial to an accuratefit. The result to be achieved should be influenced aslittle as possible by the state of abrasion of the grindingtools, by the type of material used, and the amount ofmaterial to be removed. The optimized calibrationprocess compensates for wear of the tools. Thereduction of the bearing tolerances and the reinforce-ment of the axes have increased the rigidity of theCerec 2 grinding unit as a whole and of the grindingdrive in particular. These modifications also reducedthe deformations occurring during the service life of agrinding disk as a result of increasing contact pres-stire."'

The precision of the Cerec 2 grinding unit has beenfound to be 2.4 times higher than that of the Cerec 1unit, regardless of the type of ceramic used. ' The samestudy found that, with one disk, 17 Dicor MGCspecimens and 26 Vita Mark II specimens could bemanufactured, which corresponds to previous exper-iences regarding the service life of a grinding disk. Inan SEM examination, marginal interfaces of adhesivelyplaced mesio-distocclusal inlays were 84 ± 38 |j.mwith Cerec 1 and 56 ± 27 \im with Cerec 2, revealingthat marginal widths ofCerec 2, in all data points, wereconsiderably smaller than those in Cerec 1 " (Fig i4).

Conclusion

With the features described above, the Cerec 2 systemmeets the demand for a manufacturing system toproduce complete ceramic restorations of variouskinds in a single step, by means of a fiiUy automatedgrinding process, within a short period of time. Indentai offices, in the dental office laboratory, and inindependent dental laboratories, the Cerec 2 systemhas shown good performance.

AcknowledgmentsTliE authors express their gratitude lo Drs Mario Besek and AlexGougotilaùs for their clinical help and Drs Joachtm Pfeiffer and GünterSauger for their technicai assistance.

Fig 14 Widths of marginal interlaces ot Cerec 2 ¡niays.'''Occlusally, marginal widths are less than 60 mm. Comparedto Cerec 1. a further dJstincl improvement in accuracy oí fitwas achieved.

DisclosureDr W. Mörmannls one of the initiai developers of ttie Cerec 1 system. DrA Bindl IS a coworker of Dr MÖrmann. The Cerec 2 project is supportedin part by the Foundation of Computer-Assisted Dentistry. Zurich,Switieriand, and by Siemens Den tal systems, Inc, Bensheim, Germany.

References

s in restorative dentistry In:sthetic Dentistry. Chicago:

1. Crispirt BJ The significance of esthetiMörmann W (ed). CAD/CIM in AQuintessence. 1996:55-71.

2. Nathan sun D Factors in optimi2 ing the strength of bonded ceramicrestorations. In: Mörmann W (ed). Intemational Symposium onComputer Restorations. Chicago: Quintessence. 1991:51-60.

3. Grossniann DG. Structure and physical properties of Dicor-MGCglass-ceramic. In' Mörmann W (ed]. Intemational Symposium onComputer Restorations. Chicago: Quintessence. 1991; 103-116.

4. Thompson JV, tlayne SC, Heyman HO. Mechanicai properties ofMGC-F machinable glass-ceramic for CAD/CAM restorations(abstract IO44|. J Dent Res 1996:75:148.

5. Pallesen U. Clitiical evaluation ofCAD/CAM ceramic restorations:6-year report. In: Mormann W (ed). CAD/CIM in AestheticDentistry. Chicago: Quintessence, 1996:241-253,

6. Walther W. Reiss B. Six year survival analysis of CEREC reslora-lions in a pnvate practice. In: Mormann W (ed). CAD/CIM inAesthetic Dentistry. Chicago: Quintessence, i996;199-204.

7. Liu PR. Essig ME. Isenberg BP Clinical evaluations of CAD/CIMceramic veneers [abstract 1045]. J Dent Res 1996;75:148.

8. LoPresti JT, David S, Calamia JR. Microleakage of CAD-CAMporcelain restorations. Am J Dent l996;9:37-39.

9. Mörmann W. Brandestijji M. The CEREC Computer Reconstruc-tion: Inlays, Onlays and Veneers. Berlin: Quintessence. 1989.

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10, Pfeiffer J. The chataeler of CEREC 2. tii; Mörniaiiii W (ed),CAD/CItil in Aesltietic Dentistry. Chicago; Qu iniesse nee. 1996:2Í5-:S6,

11, Satiger G. Designing a CEREC crown. In; Mörmann W (ed).CAD/CtM in Aesthetic Dentistry, Chicago: Quintessence. 1996'427-440.

t2, Mörmann W. Mattiola A. Computer-generated occlusion of CEREC2 tnlays and overlays, tn: Mörmann W(edI. CAD/CtM in AeithelitDenttstry, Chtcago: Quintessence, 1996;.19l-40M

t3. Akao T. Takahasiit J. Sohmura T. Nakamura T, Moruyama T. Adevetopment of compiilerized laminate veneer restoration syslem. JOsaka Univ Dent School 199.1:33:76-84,

14, Catamia JR, Advances in computer-aided design and computermanufacture tectuiotogy, Curr Sei t994;63:67-73,

15, Jedynakiewicz N, Martin N, CAD-CAM in restorative dentistry;The CEtJ^C Method, Liyerpoot. England: Liverpool UniversityPress, t993,

16, Wiedhahn K. Covering the incisai edge with CEREC veneers, tn:Mörmann W (ed), CAD/CtM in Aesthetic Dentistry, Chicago:Quintessence, 1996:t6l-172.

17, Mörmann W, Schug J, Grinding precision and accuracy of ñt ofCEREC 2 CAD/CIM inlasi. In: Mörmann W (ed), CAD/CtM inAesthetic Dentistr\', Chicago: (Juin¡essence, 1996:335-346, D

828

CAD/CIM in AestheticDentistry: CEREC10-Year AnniversarySymposiumEdited by W. H. Mörmann

The CEREC method has been used in

clinical dentistry for ten years; today

more than 3,000 practitioners use it worlcj-

wide. For those interested in CAD/CIM

(computer-aided design/computer-integrat-

ed machining) in general and CEREC in

particular, this compilation with oral pre-

sentations, poster presentations, and other

contributions by fifty professionals will be

of value.

663 pp (softcover); 415 illus (81 coior);US $68 (8994)

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