Marginal Accuracy andFracture Strength of

Conventional and Copy-Milled All-Ceramic Crowns

Sven Rinke, Dr Med DentLecturer and Instructor

Alfuns Hüls, Prof Dr Med, Or Med DentChairman

Leon fahn, ZAInstructor

Department of FrosthodonticsDental SchoolGeorg-August- UniversityGöttingen, Germany

In this comparative in vitro study, the marginal accuracy and fracturestrength of conventional and copy-mi iled In-Ceram crowns were examined.Anterior crowns with medians of 32.5 pm for the conventional techniqueand 38 (jm for the copy-milled units had significantly smaller gaps thanpremolar crowns that had a median marginai gap of 45 |jm for bothtechniques. There was a significant difference in fracture strength oniy foranterior crowns. The study indicates that copy-milied In-Ceram crowns haveclinically acceptable margins and fracture strength and reduce laboratoryfabrication time, int I Prosthodont i995;8:303-3!0.

There has been increasing interest' in all-ceramicrestorations as more techniques have become

available. While patients are primarily concernedwith improved esthetics, dentists are also interestedin the marginal accuracy and fracture strength ofrestorations to ensure clinical success.

Previous studies-"^ have shown that anterior sin-gle crowns and fixed partial dentures fabricatedusing the In-Ceram system (Vita Zahnfabrik, BadSäckingen, Germany) are capable of meeting thesedemands. The In-Ceram technique uses a slip-castalumina core infused witii glass. The Celay system(Mikrona AG, Spreitenbach, Switzerland), intro-duced in 1992, was designed for the production ofall-ceramic inlays, partial crowns, and veneers.^"^By combining elements of both techniques it isnow possible to produce alumina cores for all-ceramic crowns by copy-milling. A resin pattern ofthe coping is traced for fhe manually guided copy-milling process. Similar fo the conventional in-Ceram technique, the completed alumina core isveneered with aluminous porcelain (Vitadtjr alpha.Vita Zahnfabrik).

A previous study has proven that it is possible toproduce such a structure with a clinically accept-

Reprint requests: Dr Sven Rinke, Department of Prosthodontiesii. Dental School, Georg-August-University, Rohert-Koch-Str.40, 37075 Göttingen, Germany.

able marginal fit.^ However, accuracy is signifi-cantly influenced by the preparation design. Ourprevious results with the production of copy-milledIn-Ceram copings indicate preference should begiven to a shoulder with a rounded internal anglerather than to a chamfer preparation.''

The purpose of this study was to compare themarginal adaptation and fracture resistance ofcopy-milled and conventional In-Ceram crowns.

Materials and Methods

Die Production and Crown Fabrication

For this study, two models, a maxillary centralincisor (master die A) and premolar (master die B),each having a shoulder preparation and roundedaxiogingival internal line angle, were producedusing a cobalt-chromium-molybdenum alloy(Remanium CD, Dentaurum, ispringen, Germany).

The dimensions of the specimens were similar tothese of natural maxillary central incisors and sec-ond premolars. The shoulder width was 1.0 mm inboth preparation designs, at a 6-degree conver-gence angle. To consider all possible sources oferror in the process of manufacturing (impression,production of wisrking diesi, the study followedclinical procedure. The two master dies were dupli-cated with addition silicone material (Coltene

Numbei 4, t995 3 0 3 The International lournal of Proslliodontîcs

Jiginal Accuracv ant) Fr.ictiire Strength ol Ail-Ceriimic Crown, Winkt- et

Conventional In-Ceram technique Copy-miiimg technique Celay/in-Ceram

Manufacture of ffie master model

iApplication of die spacer

iDuplication ol tfie dies

ifvlanufacture of working dies with

special piasferI

Application ot the powder slip

iSintering (10 hours)

i

Trimming of ffie sintered sybsfruoture

*

Glass infiltration (4 hours)

*

Removal of excess glass

i

Veneering wifh aluminous porcelain

Manufacture of fhe master model

i

Application of die spacer

Modeling of a protofyperesin ooping as a paftern

Copy-miiiing of fheindustriaiiy s

Trimming of the m

structure from anntered biank

lied substructure

*

Giass infiltration (40 mm)

*

Removai of excess glass

Veneenng with aluminous porcelain

Fig 1 Flow chart of the twofabricafion techniques.

President, Colfene AC, Alsf,:iffen, Switzerland! byusing the simultaneous mixing technique to form40 working dies, 20 of each form. After 2 hours, theimpressions were poured into a type IV dental stone(Silky Rock, Whip Mix, Louisvilie, KY), Ten incisorand 10 premolar dies were used to produce con-ventional In-Ceram crowns. The remaining modelswere used for copy-milled all-ceramic crowns.Accordingly, the 40 working dies were evenly dis-tributed amon^ the four following groups:

1, Celay A: 10 premolar crowns with copy-milledaluminium-oxide structures

2, In-Ceram A: 10 premolar crowns (conventionaltechnique)

3, Celay B: 10 incisor crowns with copy-milledaluminium-oxide structures

4, In-Ceram B: 10 incisor crowns (convenfionaltechnique)

The convent ional In-Ceram crowns wereprocessed according to the manufacturer's direc-tions." After placing one layer of die spacer, theworking dies were duplicated with an addition-type sil icone (Provil, Bayer AG, Leverkusen,

Germany) to produce the plaster models (In-CeramSpecial Plaster, Vita Zahnfabrik) needed to compen-sate for the sintering shrinkage of the slip casting.After covering the models with the slip-casting mate-rial to form the crown structures, they were sinteredfor 10 hours in a special furnace (In-Ceramat, VitaZahnfabrik) (Fig 1), All axial surfaces of the sinteredunits were 0,5 mm thick, and the incisal/occlusalsurface was 0.7 mm thick prior to glass infiltration.The infiltration firing was 4 hours at 1,100'C,

For the production of copy-milled aluminacrown frameworks with the Celay system, a modi-fication oí the milling machine by using the Celay-Crown-Kit (Mikrona AG) was necessary. In addi-tion to an altered metallic vise, this kit contains animproved set of scanning and milling instrumentstogether with a basic instrument set for the In-Ceram technique. For the production of copy-milled crown structures, two layers of die spacerwere applied on the working dies. The prototyperesin copings were directly modeled on these diesby using light-activated resin (Celay Tech, Espe,Seefeld, Germany), The thickness of these resinpatterns was equivalent to the measurements of theconventionally produced alumina cores (occlusal/

304

uJ Fracture Strenslh of All-Ce

Fig 2a Manuai scanning device ot themodified Celay system with a mountedresin pattern.

Fig 2b Gross form of the coping isproduced using coarse diamond inslru-menls.

Fig 3c Finai contour is establishedusing finishing diamond instruments[gnt size = 64 jjin).

incisai: 0,7 tnm; axial walls: 0,5 mrn). The structuresIFig 2a¡ were scanned using the modified Celay sys-tem atid iiimultaneousiy milled from an industriallysintered aiuminium-uxide blank (Vita CelayAlumina Blank, Vila Zahnfabrik) (Figs 2b and 2c),One operator milled all the units following a stan-dardized routine that specified the scanning direc-tion and the order of the instruments used to avoidindividually produced defects as far as possible.

Subsequent to the milling process, specimenswere removed from the vise and finished. No sin-tering was necessary, but as with the conventionaltechnique, the units were glass-infiltrated (Figs 1and .Í], Because of the higher capillary effect of thealumina blanks, the glass infiltration was shortenedto 40 minutes and was performed prior to veneer-ing in a conventional ceramic furnace. The excessglass was removed for all specimens by grindingand airborne particle abrading (aluminium oxide,50 \>m grain/2 bar).

Fallowing glass infiltration the roping.'; wereveneered with aluminous porcelain (Vitadur alpha.Vita Zahnfabrik) for both techniques. To obtainlargely identical dimensions, the specimens wereveneered by filling the aluminous porcelain in divis-ible metallic casts. Thus, the dimensions shown inFig 4 could be reached with three ceramic firingcycles. The thickness was controlled to an accuracyof 0.1 mm,

Anaiysis of Marginal Accuracy

To measure marginal accuracy, the finishedcrowns were placed on their respective metallic

Fig 3 Process of manufacture for copy-miiled crown cop-ings. (Left to Right) Modeled resin prototype coping, industri-aiiy sintered alumina blank, crown coping direcliy after themilling process, finished and adapted coping, g lass-infiltratedcoping.

master die in a fixation device and exposed to aconstant pressure of JO N applied by a metallicweight. The crowns were not cemented. The gapbetween the external edge of the structure and thepreparation limit was defined as the standard formarginal accuracy. Readings were made parallel tothe crown axes on 54 circularly staggered points ofmeasurement per unit. The specimens were exam-ined using a light microscope (Stemi SV 11. CarlZeiss, Oberkochen, Cermany). A videocamera ¡CF15, Kappa Messtechnik, Cöttingen, Cermanylreproduced a 180X magnification on a high-resolu-tion computer monitor. Thus a video image of themarginal gap could be examined using special soft-ware (Ouanticap, release 1,0, E. Kunzelmann,Friangen, Germany], This procedure provided anincreased depth of field and improved resolution as

;8 , Number 4, 1995 305 The Internat i or a I tournai of Prosthodoniics

inal Accuracy and Fracture Strergrii oiAII-Cersmic Cro«

Master die A

f ,0 mm 1.0 mm.

Fig 4 Schematic presentationof Ihe Iwo metallic master diesand the dimensions of H'^crown forms (core thickness fo'ail specimens: incisal/occtusal:0.7 mm; axial walls: 0,5 mm]

compared with the light-microscope evakialion.Following several preliminary ¡experiments with acalibrating standard, a 180x magnification guaran-tees a precision of measurement of 3 (jm.

Loading of tbe Specimens

After the analysis of the marginal accuracy, fhecrowns were luted to the master metal die usingzinc-phosphate cement (Harvard-Zement, Richterand Hoffmann, Berlin). A constant pressure of 30N was applied for 30 minutes by a hydraulic pressand the die was stored at room temperature for 24hours. Fracture strength testing was carried outusing a universal testing machine (Zwick 1446,Zwick GmbH, Ulm, Germany) at a crossheadspeed of 1 mm/min. All crowns were loaded until

catastrophic failure occurred. The testing machineautomatically recorded the fracture force in new-tons, and the type of fracture was noted. The pre-molar crowns were loaded vertically. A loadingpoint in the shape of an opposing masticatory sur-face simulated a three-point contact in the centralfossa and on the stamp cusp of the crowns.

The incisors were loaded using a chisel-shapedinstrument 2 mm in diameter. The extra-axial loadwas applied at an angle of 30 degrees 1 mm cervi-cally from the incisai edge. To distribute the force,a 1-mm-thick tin foil was placed between thecrown and the load point. The fracture surfaces ofthe crowns were then examined using a scanningelectron microscope (SEM) (DSM 950, Carl Zeiss,Oberkochen, Germany) to analyze the cause offailure. The SEM was also used to determine the

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Sinai Accuracy and Fracture Strenglli of AI I-Cera m ir Cn

wear of the mil l ing instruments after differentworking periods. This study design allowed ananalysis of possible effects of the production tech-nique and the crown design on the accuracy andfracture strength.

Statistical Analysis

The measurement of marginal accuracies gener-ally leads to a nonsymmetric distribution of data. Acharacteristic accumulation can be seen near thezero point. Because the left side of the scale waslimited by the natural zero point, a positive skewdistribution of the data results. The use of distribu-tion-free so-called nonparametric procedures isrecommended when there is an obvious nonsym-metric distr ibution.'" Therefore, instead of themean, the median was used as a measure of loca-tion, and the interquartile range as a measure ofvariation replaced the standard deviation. For astatistical analysis of the fitting accuracy, theapplied two-stage nonparametric procedure con-sisted of an anaiysis of variance using the Kruskal-Wallis test and multiple comparisons using ranksums to identify significant pair differences accord-ing to Dunn."

No normal distribution of the data of fracturestrength was assumed so that a similar two-stageanalysis procedure was used. An analysis of vari-ance (Kruskal-Wallis test) was performed as anoverall test. Statistical evaluation was completedwith multiple comparison testing (Wilcoxon testadjusted according to Bonferroni). in this specialcase, the Dunn test could not be applied, becauseit requires a large number of random samples.^- Alldata were graphically presented by box plotsaccording to Tu key.

Results

Results for the Marginal Accuracy

Over the 2,160 measurements, the marginal gapranged from 1 to 154 pm. The median of the premo-lar crowns was 45 pm for both techniques (Fig 51,with the conventional technique ranging from 1 to153 (jm and the copy-milling technique rangingfrom 6 to 153 \im. However, the lower extension ofthe box (50% of the data are found in the box) andthe vertical bars prove that the data from the con-ventional technique have a smaller range (see Fig 5).Marginal gaps smaller than 100 fim were measuredin 95.9% of crowns fabricated with the conven-tional technique, while 93.8% of the copy-milled

Fig 5 Box plots of the measurements for the marginal adap-tation. The medians are indicated by the horizontal line withinthe box: the box represents the interquartile range: ' and oindicate outside and far oulside values, respectiveiy. (CeiayA: premolar crowns wilh copy-milied coping, in-Ceram A: con-ventionally produced premolar crowns; Celay B: incisorcrowns with copy-miiled oopings; In-Ceram B: conventionallyproduced incisor crowns].

premolar structures measured less than 100 nm. Thedifferences in the results from the two techniquesproved to be statistically significant (P < .05).

In both groups of anterior crowns, the mediangap was smaller than for the premolars; 33.5 pmwith the conventional technique and 38.0 pm withthe Celay system. The range for the conventionalcrowns was smaller than that of the copy-milledunits (see Fig 5¡. The maximum gap for the con-ventional technique was 122 \im. Only three of the540 measurements were greater than 100 |im{99.4% of the data < 100 ^m), and 78.8% of themeasurements of marginal fit were less than 5(] (im(57.7% of the copy-milled crownsi. Neither theanterior nor the premolar crowns had valuessmaller than 6 pm for copy-milled specimens,while the minimum gap for the conventional tech-nique was 1 |jm.

A significant technique influence on the anteriorcrowns was demonstrated by the Dunn test (P <

• - 8 . Number 4, 1995 307 The Internat ron a I loiirnal of Proslhodonics

linai Accuraty ,ind Fracture Strengtii of All-Ceramic Cri

2500 [ I 1—I 1 r

2000

1500

1000

500

J I i U

Fig 6 Box piots of the recorijeû values lor the tracturestrength. (Celay A and in-Ceram A: axial load application; CelayB and in-Ceram B: 30 degrees extra-axiai load appiication).

.01 ). The multiple comparison testing according toDunn also proyed tbe influence of the crown formon tbe marginal accuracy in both techniques (In-Ceram A/in-Ceram B; P< .001; Celay A/Celay B: P

Results of the Fracture Strength Testing

With a median of 1,825.5 N, tbe conyentionallyfabricated premoiar crowns had a significantlyhigher fracture strength than the nonaxially loadedanterior crowns (In-Ceram B median: 1,307 N) (Fig6). Seven of the 20 conventionally producedcrowns indicated a two-phase failure pattern: acrack initiation followed by catastrophic failure.

The fracture strength of the copy-milled crownssbowed a lower variation of measurement, and fiyecrowns had a two-phase rupture. The median ofthe premolar crowns (Celay A: 1,872.6 N) was sig-nificantly greater than that of the nonaxially loadedanterior crowns (Celay B: 1,532 N). A difference mmedians of 225 N was found tor tbe two fabrica-tion techniques (see Fig 6). Nevertheless, the sig-

Fig 7 Homogenous structure ot the core matenai and tightjunction to the veneer porcelain (scanning electron microscope,original magnitication x 5,000, aipha = veneer porcelain).

Fig 8 Fracture surface with multiple porosities at the inter-face between core materiai and veneer porcelain (scanningelectron microscope, griginai magnitication x 100, crown withlowest fracture strength in group Celay B. 1,196.8 N).

nificant difference could only be detected at the10% level (Celay B/ln-Ceram 0: P= .074). For thepremolar crowns, no significant difference in frac-ture strength could be determined (Celay A/In-Ceram A: P= .508).

The SEM studies revealed a homogenous struc-ture of the alumina cores and a tight junctionbetween the core material and the veneeringporcelain (Fig 7). In all four groups, specimenswith lower fracture strength had porosities in theveneering porcelain. These defects were mainlylocated near the interface of tbe core material andthe veneering porcelain (Fig 8).

al of Pro5ihodOiiii( 308 Volume B, Numb-

Marginül Awuraty dnö Fracture SlrenRlh urAII-CeramJc Crowns

Discussion

The marginal adaptation of the conventional In-Ceram crowns, wifh median values of 45 pm forpremolar crowns and 32,5 |jm for the incisorcrowns, is comparable witb the results of otherrecent studies, Kappert and Altvater- found anaverage marginal gap of 38 pm, Wifh an averagevalue of 24 |jm |SD = 25,5 |jml, the findings ofSorensen et aP showed slightly better results forthe marginal gap. Although caution should beapplied when comparing different studies becauseof different study designs and methods of measure-ment, the tit of conventionally produced In-Ceramcrowns favorably resembled other studies of all-ceramic restorations, Vahidi et al'^ found meanvalues of 30 \im for the marginal adaptation ofDicor crowns (Denfsply International, York, PA)and 37 |jm for Renaissance crowns (Williams GoldRefining, Buffalo, NY). Holmes et al'"' reported anaverage value of 48 [im for the marginal fit ofDicor crowns.

Copy-milled units had greater variation in mar-ginal gap than did the conventional in-Ceramcrowns, indicating less reproducibility of marginalaccuracy with this technique. Nevertheless, themeasurements of fit with median values of 45 [jmfor premolar and 38 [jm for antedor crowns indicatethat copy-milled, all-ceramic crowns can be pro-duced with a clinically suftlcient fitting accuracy.

The method of fabrication was significant forboth anterior and posterior forms, although the dif-ferences were greater for the anterior units. Thisdifference could be ascribed to the difficulty oftracing the internal incisai area of the anterior pat-tern. Extremely tapered incisai edges cannot betraced or reproduced exactly, and an adequate,sufficiently die-spaced pattern is essential.

Further deveiopment of the tracing and millinginstruments should lead to improved marginalfidelity. The wear of the milling instruments duringmilling is a source of error. During the milling pro-cedure diamond grains are plucked out, changingthe radius of the instruments and reducing millingprecision (Fig 9), To control this factor, a change ofthe milling instruments at regular intervals is highlyrecommended.

The fracture force of conventional In-Ceramcrowns was in accordance with previous in vitrostudies, and demonstrated the good mechanicalproperties of this material,^'^^ Deviations in thereported fracture forces may be explained by dif-ferent crown designs, a different wall thickness offhe alumina cores, and the method of cementation.

The statistical analysis demonstrates a significant

Fig 9 Diamond wheel after milling of six coprngs. Loss otseuerai diamond grains at fhe outer edge is evident (scanningeiecfron microscope, originai magnification x 100),

influence of the load direction on the fractureStrength, For both fabrication techniques, the verti-cal loading leads to higher fracture forces than theextra-axial loading. The findings of Hölsch andKappert'^ were identical for In-Ceram and otherall-ceramic crowns. The high values for axiallyloaded posterior crowns results because the com-pressive strength of the veneering porcelain ishigher than its flexural strength.

Compared with the convenfional In-Ceram tech-nique, using an industrially sintered alumina corematerial leads to a lower variation of the fractureforces and thus to a better prognosis for long-termclinical success. This result can be explained bythe fact that processing variables during the adap-tation of the slip cast are eliminated by using anindustrially prefabricated material.

The application of an axial load leads to similarmedian values for the fracture strength in both pro-duction techniques. From these results, fhe conclu-sion could be drawn fhat fhe use of an industriallysintered alumina core is not significant for the frac-ture force of a crown if an axial load is applied.

There was an obvious difference in the fracturestrength of conventional and copy-miiled unitsunder a nonaxial load. During nonaxial loading,the shear component of the total load is muchgreater^ = and the breaking strength of the crowns isinfluenced more by the core material flexuralstrength than in vertical loading.

The greater fracture resistance of the copy-milledanterior crowns results from the glass-infiltratedCelay alumina blanks having a 10% higher flexuralstrength (500 MPAI than the conventional corematerial.

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Marginal Accuracy anct Fracture Stren|;tii ol All-Ceranilc Crowi

These results support the conclusion that theindustrially prefabricated alumina material leads toan increased fracture strength if the shear compo-nent of the lotal load is substantial, as is lypicallyfound witb anterior teeth.

Previous in vitro examinations''^'' and SEManalyses have indicated a correlation between thehomogeneity and tbe thickness of the veneerporcelain and the fracture strength of all-ceramiccrowns. Porosities in the veneer porcelain pro-moted initial cracks, and crack growth led to areduced fracture resistance,' A potential reinforce-ment of the core material was neutralized byimperfections of the veneer porcelain. An influ-ence of the milling procedure on the mechanicalproperties of the alumina blanks could not bedetected.

The main advantage of fabricating copy-milledunits is a decreased processing time. Copy-millingobviates the die duplication, the special plasterworking die, as well as tbe manual adaptation ofthe slip-casting material and the 10-hour sinter-fir-ing that are necessary for the conventional tech-nique. Moreover, the infiltration can be performedin a conventional ceramic furnace and the timereduced to 40 minutes.

Thus, it is possible to produce an all-ceramiccrown in 1 day. This production technique allowsan enlarged range of application of the Celay systemat a reasonable financial expenditure. Further inves-tigations on copy-milled all-ceramic restorations,especially concerning tbe production of three-unitbridge frameworks, seem to be appropriate.

Conclusions

Conventionally fabricated In-Ceram cores werecompared to those fabricated using a copy-milling technique. Both a maxillary anterior andpremolar design were tested. Marginal adaptationand fracture strength were evaluated. Within tbedesign of this study, the following conclusionsmay be made:

1, The marginal accuracy of the copy-milled unitsranged from 6 to 153 pm, and that ofthe con-ventionally fabricated units ranged from 1 to153Hm,

2, The median gap of the anterior units (33,5 pmfor tbe conventional technique and 38,0 pm forthe copy-milled technique) was less than that ofthe premolar units (45 pm for both techniques).

3, The fracture strength of the axially loaded pre-molar crowns was greater tban that of the non-axially loaded anterior units,

4. There was less variation in fracture foK.'' forcopy-milled units than for conventionally tabri-cated units.

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