Operative Dentistry

Where is the gap? Machinable ceramie systems and conventionallaboratory restorations at a glanceSandro Siervo* / Ambra Pampalone*^" / Paolo Siervo '̂ / Raffaele Siervo**

Seanning electron microscopy was used to compare the marginal gaps of restorations milledby machinabie ceramic .systems to the marginal gaps of conventional laboratory-sinteredceramic restorations. For occlusal surfaces, the average marginal gap was 80 ßm for bothlaboratory- and Celay-produced inlays. The mean gap was 200 ¡im and 170 p.m. respectively,for Cerec T (turbine motor] and Cerec EM (electricmotor) inlays. For approximal boxes, theaverage marginal gap was 100 t^m for inlays produced with conventional laboratory-sinteringteehniques. 80 ¡im for Celay restorations, and 280 ¡xm for the Cerec T restorations, and260 ij.ni for Cerec FM-machined inlays. The ceramics used, as well as the different systemsthemselves, can influence the results and the clinicai outcome ofthe restorations.(Quintessence Int 1994;25:773-779.)

Introduction

Ceramic is increasingly used as a dental tnaterial,' Pa-tients and dentists like this material for different rea-sons. Patients are generally concerned about esthetics'and prefer tooth-colored dental restorations. Ceramicobviously meets this demand. Dentists choose ceramicfor the physical properties of the material.""* Its bio-logic and physicochemical characteristics are wellknown: it does not irritate soft tissues and it resists de-position of dental plaque,""' Moreover, its color isstable and the restoration is resistant to wear, thereforeproviding both esthetics and function over titne. This iswhy dentists {and their patients) face daily an ava-lanche of new ceramic products entering the market.

Among the latest "hits" are machtnable ceramtcsystems (MCS). Some of these systems use CAD-CAM

* Universit)' of Milan. Cotlege of Dentistry. Milan, Itaty,** University of Pisa. College ofDentistry. Pisa, Italy,Address all correspondence to Dr Sandro Siervo, Viale Tunisia 43.2(1124 Milan. Itaty.

technology while others do not. For the time being, twosystems are currently available on the market. Celay(Mikrona)^ and Cerec (Siemens),^ The latter has twoversions, the old one with a turbine {Cerec T) and thenew one with an electric motor (Cerec EM),

Marginal gaps of Celay, Cerec T, and Cerec EM res-torations were measured and compared to the margi-nal gaps of sintered conventional lahoratoty-fabricat-ed restorations. This study was aimed at providing acritical comparison, discussing the advantages and dis-advantages of currently available MCSs,

Method and materials

Cavity preparation

Standard mesio-occlusodistal cavities were pte-pared"*" in extracted impacted human third molars.All cavity preparations were performed by one clini-cian (RS) in a blind fashion: ie. at the time of cavitypreparation, the operator did not know which methodwould be used for tooth restoration. This approach wasused to standardize and optimize the cavities. Tliepreparations were created with identical rotating dia-mond burs with decreasing grits. The teeth were thenstored at 4 ̂ C in a balanced salt solution until needed.

Oriinfessence International Volume 25, Number 11/1994 773

operative Dentistry

Impressiori

Cerec T. Five teeth were cast and lined with a suitablelight-reflecting agent, and the '"optical impression" wastaken, in accordance with the manufacturer's instruc-tions. Data were next processed with the COS 1.4 soft-ware version, and the restoration was milled. All datawere collected and processed by a single operator tostandardise the procedure.

Cerec EM. The same steps described for the Cerec Twere followed, except that data had to be processedwith the COS 2.1 software version.

Celay. In the present study only the direct methodwas used to take the impression. Five teeth were castand the '"proinlay" was modeled with a suitable elasto-mer (Celaytech, ESPE). After perfect adaptation wasconfirmed, the proinlay was light cured (60 seconds),extracted from the tooth, and then reinserted to checkthat seating was correct. After a second bout of lightcuring, the proinlay was inserted in the machine, andthe inlay was milled according to the manufacturer'sadvice. All proinlays were obtained, and all millingsteps were carried out. by a single person.

Laboratory-fabricated restorations Five teeth werecast and the impression was taken with an elastomer(Provil. P. Soft, Bayer Dental). Low-expansion secon-dary casts with removable dies were obtained. Theceramic laboratory inlay was fabricated with the Bio-dent inlay ceramic system (De Trey/Dentsply) in accor-dance with the manufacturer's instructions. All labora-tory restorations were processed by the same person(MSJ).

Adhesive cementation

All four groups of ceramic inlays were checked for cor-rect adaptation to the margins of the cavity. Next the in-lays were washed with deioni7,ed water, air dried,etched with hydrofluoric acid (Vita ceramics etch. VitaZahnfabrik) washed and alcohol dried, silanized (Sili-coup, Kulzer) washed and air dried again, and finallylined with a thin film of resin bonding agent (Helio-bond, Vivadent). Teeth were washed with saline, airdried, etched with phosphoric acid (Gluma etchant,Bayer Dental), washed with deionized water, air dried,coated with a thin layer of bonding agent, and filledwith composite resin (Heliomolar, Vivadent), Tlie in-lays were correctly seated and thoroughly light cured.Specimens were then recontoured with rotating dia-monds and polished with Sof-Lex disks (3M Dental)and diamond paste. Teeth were stored at 4°C in salineuntil examination.

Scanning electron microscopic analysis

Each tooth was analyzed in two ways. First the occlusaland approximal marginal gaps were measured (Fig la,dotted line and crosses). Twelve occlusal points andseven approximal points were selected for analysis (Figlb). Next each tooth was cut in four pieces followingplanes A and B. On line A the marginal gap ofthe ap-proximal box could be measured in cross section, whileon line B the marginal gap at the occlusal surface wasmeasured in cross section. Therefore for each singletooth, 18 single points for the approximal areas and 16single points for the occlusal surface were measured.The measurements were done both directly on screenand from the photographs.

Before analysis, the specimens were coated in a vacu-um chamber with a thin gold layer and then the speci-mens were viewed under a scanning electron micro-scope (Stereoscope 250 MK3. Cambridge Instru-ments). The magnification used was always of xlOO.The so-called tilting effect was regularly checked andthe possible measurement error was kept below 3 |im.All specimens were tested by a single person at a non-profit institution (Eidgenössisches Materialprüfungs-und Forschungsanstalt). Data were plotted with thesoftware program Smartware 11 (Informix).

Results

Marginal gap

Data for fhe approximal boxes are plotted in Fig 2,Every single measured point was plotted to obtain thecurves, which represent the sum of all collected pointson the same and on different teeth. Laboratory inlayshad a mean gap of 100 (j,m, Celay inlays a mean gap of80 |i,m, Cerec T inlays a mean gap of 280 |j.m, and CerecEM inlays a mean gap of 260 \i.m.

Data for the occlusal surfaces are plotted in Fig 3.The curves were obtained in the same way as describedabove. The mean gap of both laboratory-fabricated andCelay inlays was 80 [im, the mean gap was 200 jxm forCerec T and 170 (i,m for Cerec EM inlays.

Ceramic structure

The texture of ceramic employed by MCSs is more ho-mogenous than that of sintered ceramic. Industrial ce-ramic does not exhibit the holes and gross microscopicdefects that are always present in sintered ceramic. Thestructural defects of laboratory ceramic are shown in

774 Quintessence International Volume 25, Number 11/1994

Operative Dentistry

B ,

\A

nr

\/

V

VTJ

Fig la Cutting planes A and B. Dotted and crossed unesshow areas where points were sampled for measurements.

Fig 1 b Cross section of the prepared specimens, showingareas where measurements were taken.

Fig 2 Approximal marginal gap.

APPROXIMAL MARGINAL GAP

\r '

• s .

1

^ s n S S ^ i p f i

¿ 6 1 i ! & !"f S S S p 3 S'̂ . -- ^ ÍJ r- rj

MARGIN IN MICROMETER

I CELIY

' SINTEHED • CEHEC WITH E-MOTOR

Qtjinlessence International Volume 25, Number 11/1994 775

Operative Dentistry

OCCLUSAL MARGINAL GAP

• SINTERED FROM HBOtWTOflv

Fig 3 Occlusal marginal gap.

Fig 4. They are virtually absent in MCS ceramics (Figs 5to 7).

Discussion

Pioneers in esthetic dentistry have met the challenge tobuild MCSs for 10 years now.'-'-* These appliances millesthetic restorations, such as inlays, onlays, veneers,crowns, and even fixed partial dentures, trom industrialceratnic blocks. Computer-driven devices appeared onthe market in the late i9S0s, bringing a new wave in res-lorative as well as conservative dentistry.''-

Two of these systems. Cerec and Celay, are currentlyemployed ¡n clinical trials and in everyday practice.Tliey are designed to be used by the dentist or by asmall laboratory. The two apparatuses exhibit majordifferences both in their clinical use as well as in theirend products. In this study, they were carefully com-pared with conventional laboratory-sintered ceramicpreparations. The results are helpful to understandingthe current status of MCSs.

Cerec works with an "optical impression," a comput-er-aided design device, and an automatic milling appa-ratus with three degrees of freedom.*̂ It manufacturesinlays, onlays, and veneers''""' on a routine basis. Theocclusal surface is easily modeled by the dentist withrotating diamond burs. Anatomic tooth shaping rarely

reaches the ideal standard, as is true for other chairsiderestorations, amalgams, posterior composite resins,etc."* The marginal gap of Cerec restorations, reportedin Figs 2 and 3 and in Table I. represents the Aehilles'heel of this device. The EM version, which is a second-generation machine, shows a little improvement overthe T version, especially in the interproximal areas (seeFig 2). This appliance has built-in constraints that needoptimization to improve marginal adaptation. The op-tical impression system is still not perfect, with report-ed marginal gaps of 250 \Lxn and more." This is con-firmed by the present experimental findings. Perfectadaptation of Cerec restorations is influenced by otherfactors as well. At sites with sudden height variations,incorrect measurements are often performed; more-over the milling device of the Cerec is not stable withregard to mean and standard deviation"* (Table 1).These reasons account for the experimental data pre-sented here, in terms of marginal gap sizes obtainedwith both Cerec systems.

On the other hand, the ceramic employed by thisMCS is of a very high standard. The textnre of industri-al ceramic, its physicochemical properties, and its resis-tance to stress are known and standardized.'^"'' Thistype of ceramic will not be influenced by the skills ofthe laboratory technicians. An advantage of Cerec isthat the whole procedure takes place in a single session.

776 Quintessence International Volume25, Number 1 t/t994

Operative Dentistry

Fig 4 Reference laboratory restoration, occlusai marginalgap in section ¡point B, Fig Ib). Note the "holes" in theceramic texture. (C) Ceramic; (CP) composite resin; (E)enamel.

Fig 5 Approximai box of a Celay restoration, surface view(dotted iinein Fig la}. fCJ Ceramic; fCP) composite resin; fëjenamel.

Fig 6 Approximai box of a Cerec T restoration, surfaceview (dotted line in Fig la). (C) Ceramio; (CP) compositeresin; fÇ enamei.

Fig 7 Approximai box of a Cereo EM restoration, surfaceview (dotted iine in Fig la). (C) Ceramic; (CP) compositeresin; (E) enamel.

Table 1 Margina! gap (|J.m) of restorations

Celay Cerec T Cerec £M

Value

MinimumMaximumMeanSD

A

014067.812S,55

O

021074.6735.63

A

70550275.63P3.20

0

20800265.71152.83

O

Laboratory

A 0

100 90600 820258.39 282.44110.60 177.05

I) 0420 430152.97 102.9672.63 83.40

A = approximal; O = occiusaL

Quintessence International Voiume25, Number 11/1994 777

Operative Dentistry

thus lowering patient's costs and troubles. For this rea-son. Cerec is considered to be a "superdirect" restora-tive method.

Celay has two possibilities for impression taking.Both ways lead to the construction of the proinlay. anaecurate pattern of the final restoration. The directmethod is meant for dentists who want to treat patientsin a single session and who run this MCS by themselves.Ahernatively, the indirect method {with conventionalelastomer impression) is conceived for a small- to me-dium-sized laboratory. The patient needs two sessionsto be treated and nearly always requires a provisionalrestoration.

Celay employs no computer-aided design and ma-chining device. Through a pantograph cutter, the pro-inlay is copied to the real inlay (onlay, veneer, etc) witha manual milling apparatus. The milling appliance haseight degrees of freedom^ and therefore is extremelyversatile. It manufactures inlays, onlays, veneers, andall-ceramic three-quarters crowns on a routine basis.Restorations that can be only experimentally per-formed with Cerec, such as closure of diastemas withveneers and complex onlays, can be easily achievedwith Ce lay. "̂

The occlusal shaping of these restorations is accom-plished in different ways depending on the method em-ployed (direct or indirect). The indirect method per-mits the nearly ideal occlusal shaping achieved for con-ventional laboratory products. The marginal adapta-tion of the restoration is also extremely satisfactory:the marginal gap is smaller than that of Cerec inlays(see Figs 2 and 3).

Results obtained with this recently introduced MCSare encouraging because medium-term success is guar-anteed. Because the materials employed are similar tothose of the Cerec kit (both the ceramic and the adhe-sive cementation techniques adopted), good clinical re-sults, similar to the outcome of Cerec restorations, canbe expected. As stated for Cerec, industrial ceramicemployed with the MCSs is of a higher standard thanconventional laboratory-sintered ceramic. This en-ables use of these all-ceramic reconstructions in theclinic with a good margin of safety.

Laboratory inlays were used as a reference standardfor this experimental work. Therefore, the descriptionsof the known conventional laboratory procedures werenot given in detail. However, there were two notewor-thy points. First, une ofthe two available MCSs (Celay)yielded high-quahty results. This is promising news be-cause the constant efforts of different researchers andinstitutions will produce MCSs of a higher quality.

These appliances will enable the fabrication of restora-tions that have not been feasible so far.

The second pointisthat the marginal gap of Celay in-lays at the interproximal areas was even smaller thanthat of conventional laboratory restorations. The im-provement resulted from the different ceramic tech-nology employed. Conventional ceramic tends to hevery brittle and to crack in areas where it does not havesufficient thickness,''' This is normally the case in theapproximal boxes. When the operator seats the inlay(onlay, etc) in the correct position, microfractures oc-cur in the ceramic, resulting in a wider marginal gap.The overall resistance to compression is extremely sim-ilar for laboratory and industrial ceramic (around 150MPa) as is the shear strength (around 25 Mpa), The dif-ferenee is the homogenous structure of the industrialceramic. The possible defects (eg, condensation) catialter the properties and performances of laboratory-sintered ceramic. On the other hand, the virtually cotn-plete absence of voids or condensation defects in MCSceramic guarantees that the mechanical and physico-chemical properties of the restoration are identicalthroughout the piece. The presence of any defects in la-boratory-sintered ceramic at specific sites, eg, in thinareas as is often true for approximal surfaces, drastical-ly reduces the mechanical properties of the restoration.One of the possible outcomes of these defects is to in-crease the marginal gap because of micro frac turcs.Thus, in this context, different materials can influencethe marginal gap of the final restoration,

Fromachnicalpoint of view, the relevance ofthe dif-ferences measured between the analyzed MCSs is notknown. Theoretically, a correct anatomic occlusalshape and a close-to-zero marginal gap are expected tosecure long-term success,-' These are not, however, ce-mented cast gold restorations. The assumption is yet tobe proved that the better adhesively cemented restora-tions fit the cavity, the longer they last. This assumptionstems from collected data on the fit of metal restora-tions to teeth, in this latter case the retention betweetithe restoration and the tooth is guaranteed solely bymechanical means. Therefore the best fit among thetwo parts {eg, inlay-tooth), provides optimal clinicallongevity. Adhesive cementation, however, impliesboth a chemical bond and a mechanical bond.-'' On apurely theoretical basis, this different and combinediink(chemicalandmechanical).between the tooth andthe restoration could tolerate a wider tnarginal gap.

For the time being, nobody really knows the range ofthe ideal marginal gap for adhesiveiy cemented resto-rations. A close-to-zero marginal gap may, in fact, turn

778 Quintessence International Volume 25, Number 11/1994

operative Dentistry

out to be a detriment for the longevity of these restora-tions. A minimum thickness of the "glue" is probablynecessary for optimal performance. We have some pre-liminary clinical and experimental findings that pointin this direction. Therefore we started a long-term clin-ical trial, in which comparable teeth are restored withCerec or Celay restorations simultaneously in each sin-gle patient. In vivo testing will define long-term perfor-

Conclusiun

Machinable ceramic systems are now available, and thefuture will see their expansion. For the time being, theprototype of these systems, Cerec, remains the onlysuperdirect device. It has its advantages, despite itspresent hmitations. The Celay, a second-generationappliance, exhibits some improvement over Cerec andconventional laboratory-sintered restorations with re-spect to the parameters under review (marginal gapand occlusal shaping). Third-generation MCSs willsoon join the group. The constant innovations and im-proving technology will further optimize tooth-coloredrestorations.

Acknowledgmentstlie authors thank Profs M. Bandcuini, F, Santoro. G. C. Secchi, andM. Trentatancia. as wdl as Drs E. CerTÍ, L. Secchi, G. Signorelti, andG, Valenti for many helpful suggestions bolti during the work ¿mdduring the critical reading ofthe manuscnpl. The tecnical help of N.Coiaianni, M. Colombo, M. Loviso. M, Samarali, M. San Juan, andA, Tomaseth has been escellenl. as weil as ttie secretarial assistanceof M. T. Quaglia. All SEM data were obtained through the courtesyof Mrs C. Schenk. The final editing of the English version is courte-sy of Dr R.Petrillo.

References

1. Friedman MJ. Augmenting restorative denlislrywilh porcelainveneers. J Am Dent Assoc 1991;122:29-34.

2. PhitlipsRW. Era of new biomaterials in esthetic denlisiry.J AmDenl Assoc 1987;(special issue) :7E-12E.

3. Claus H. Werkstoffkundliche Grundlagen der Demalkeramik.Dental Labor tyS0;28:l 743-1752.

Strub JR. Vollkeramischc Sysleme. Dtsch Zahnärztl Z ty92;47:566-571.

Hench LL, Bioceramics, from concepl lo clinic, J Am CeramSoct99l;74:t487-14y3.

Vogel W, Holand W. Zur Entwicklung von Bioglaskeramikcnfür die Mediiin. Angew Chem 1987;39;54I-55Ü.Hahn R, Netüschil L, Löst C Initiale Plaquebesiedelung kera-mischer Restauralionsmalerialien. Otsch Zahnärztt Z 1992;47:330-338.

EidenbenzS. European patent,EP0480209 AI, 1992,Mörmann W, Brandestini M, Verfahren zur Herstellung medi-zinischer und zahn technischer alloplastischer, endo- und exo-prolhetischcr Paßkörper. Patent (CH t980), EP-A-(MS47R.S1981.

Roulet J-F. Herder S. Bonded Ceramic Inlays. Chicago: Quin-tessence, 1991

Lussi A, Gygax M, Hotz P. Die Mini prepara ti on en approxi-mater Kavi taten. Schweiz Monatsschr Zahnmed 1987-97:186-198.

Rekow DE. Dentat CAD-CAM Systems. What is (he stale ofthe an? J Am Dent Assoc 1991:122:43-48.Durel F, Hlouin JL, Duret B. CAD-CAM in dentistry. J AmDent Assoc 1988;It7:715-720.

Siervo S, Pampalone A. Valenii G, Bandetlini B, Siervo R. Por-celain CAD-CAM veneers, Some new uses explored J AmDem Assoc t992;123:63-67.

Mörmann W, Brandestini M. Lut?. F, Barbakow F, Goísch T.CAD-CAM ceramic inlays and onlays: A case report alter 3years in place, J Am Dent Assac 1990; 120:517-520,Schmid O, Krejci I, Lut/ F Ausarbeitung von adhasiven zahn-farbenen inlays aus Komposil und Keramik. Schweiz Mo-natsschr Zahnmed 1991 ;101:l 77-1K4.

Benz C, Schwan,: P. Wie genau ist der optische Cerec-Abdruck?Dtsch Zahnarïil Z 1991;46:632-634.

Kunzelmann K, Hickei R. Zur Präzision des Ceree-System: dieSchleifeinheit. Dtsch Zahnärztl Z 1990:45:277-280.Hahn R. Löst C Konventionelle Dentalporzellane versusbruchzähe Hochleistungskeramiken. Dtsch Zahnärztl Z 1992-47:659-664.

Morena R, Lockwood PE, Fairhurst CW. Fracture toughness ofcommercial denial porcelains. Denl Maler 1986;2:58-62.Schmid M, Fischer J, Salk M. Strub 1 Microgefuge leucitver-stärkler Glaskeramiken. Schweiz Monatsschr Zahnmed 1992-102:1046-1053,

Siervo S, Pampalone A, Siervo P, Bandettini B. Marchelii C,Siervo R. Il Sistema Celay: Nuovi orizzonli délia cerámicafresata. Dentista Moderno 1992; 19:16t 7-1624.Christensen GJ. A look at the state of the art. Tooth colored in-lays and onlays. J Am Dem Assoc 1992; 123:66-70.AdairPJ, Grossman D G The cast able ceramic crown. Int J Pe-riodont Rest Dent :984;4(2):32-^5. n

Quintessence International Volume 25, Number 11/t994 779