A 3-

cha

Gabriel Bosch, M

aResearch Assistant, Division for CobResearch Assistant, Division for CocHead of Division, Division for Com

Bosch et al

dimensional accuracy analysis of

irside CAD/CAM milling processes

edDent,a Andreas Ender, DrMedDent,b andAlbert Mehl, Prof Dr, DrMedDentc

Center of Dental Medicine, University of Zürich, Zürich, Switzerland

Statement of problem. Milling is a central and important aspect of computer-aided design and computer-aidedmanufacturing (CAD/CAM) technology. High milling accuracy reduces the time needed to adapt the workpiece and providesrestorations with better longevity and esthetic appeal. The influence of different milling processes on the accuracy of milledrestorations has not yet been reviewed.

Purpose. The purpose of this study was to investigate the influence of different milling processes on the accuracy of ceramicrestorations.

Material and methods. Four groups of partial crowns were milled (each n¼17): Three groups in a 4-axial milling unit: (1) 1-stepmode and Step Bur 12S (12S), (2) 1-step mode and Step Bur 12 (1Step), (3) 2-step mode and Step Bur 12 (2Step), and (4) onegroup in a 5-axial milling unit (5axis). The milled occlusal and inner surfaces were scanned and superimposed over the digitaldata sets of calculated restorationswith specialized difference analysis software. The trueness of each restoration and each groupwas measured. One-way ANOVA with a post hoc Tukey test was used to compare the data (a¼.05).

Results. The highest trueness for the inner surface was achieved in group 5axis (trueness, 41 �15 mm, P<.05). The 4-axialmilling unit exhibited trueness at settings ranging from 61 mm (2Step) to 96 mm (12S). For the occlusal surface, the highesttrueness was achieved with group 5axis (trueness, 42 �10 mm). The 4-axial milling unit exhibited trueness at settings rangingfrom 55 mm (1Step) to 76 mm (12S).

Conclusions. Restorations milled with a 5-axial milling unit have a higher trueness than those milled with a 4-axial millingunit. A rotary cutting instrument with a smaller diameter results in a more accurate milling process. The 2-step mode is notsignificantly better than the 1-step mode. (J Prosthet Dent 2014;112:1425-1431)

Clinical Implications

The goal of the milling process is to generate an exact copy of thedigitally calculated restoration. The 5-axial milling unit came closerto the digitally calculated restoration than the 4-axial milling unit.

Dental ceramics have provenlongevity and remain the materials ofchoice for esthetic restorations.1-5

Ceramic restorations can be pro-duced in different ways, one ofwhich involves computer-aided designand computer-aided manufacturing(CAD/CAM). The goal of CAD/CAMtechnology is standardized, reproduc-ible production that is both efficientand accurate.6

mputerizedmputerizedputerized

Clinical long-term success dependson factors such as adequate cementa-tion, restoration design, preparationdesign, and (importantly) the marginaland internal fit of the restoration to thetooth. An important step with regard tothe fit of the restoration is the fabrica-tion process. Ceramic crowns can bedistorted during this process, which cannegatively affect the fit and compromisethe success of the restoration.7-10

Restorative Dentistry.Restorative Dentistry.

Restorative Dentistry.

Important considerations are how ac-curate the milling process is andwhether it damages the restoration.

CAD/CAM allows the dentist towork chairside and is the fastest way toproduce individual ceramic restora-tions. Cerec (Sirona Dental Systems) isa well-known chairside CAD/CAM sys-tem that has proven both reliable andefficient.11-13 The optical scanning sys-tem (Cerec Bluecam; Sirona Dental

Table I. Test groups with different settings (n¼17)

Group Rotary Instrument Milling Unit Milling Option

12S Step Bur 12S inLab MC XL 1-step

1Step Step Bur 12 inLab MC XL 1-step

2Step Step Bur 12 inLab MC XL 2-step

Arctica Arctica Bur Set Arctica 5-axis

1426 Volume 112 Issue 6

Systems) has also been found to pro-duce reliable and highly accurate 3-dimensional digital impressions.14-18

The adaption of the restoration to thepreparation with regard to marginaland internal fit has been found tobe clinically reliable in numerousstudies.19-23 However, marginal gapvalues range from 35 to 246 mm, andinternal gaps range from 17 to 206mm.24 Relatively large internal gaps canresult from the milling process,25 butgaps of up to 150 mm are clinicallytolerable.26 The marginal gap differsamong CAD/CAM systems,27-29 butwith the continual development ofmilling machines, marginal gaps arebecoming progressively smaller.30

The accuracy of fitting to the un-derlying tooth structure is an essentialconsideration and could affect thelongevity of restorations.31,32 Largergaps are associated with acceleratedplaque accumulation, secondary caries,marginal discoloration, exposure of theluting resin, dissolution of the cement,and increased risk of microleakage andmicrocracks.33-35 When the marginalgap is greater than 100 mm, removingexcess cement is more difficult.36 Arestoration with inadequate fit maylead to marginal chipping, and evensmall chips can result in the later clin-ical failure of ceramic restorations.37

With the CAD/CAM technique, high-precision scans are possible, and thesoftware can calculate a restoration withexact control over contact points anddesign. The purposes of this study were(1) to evaluate the trueness of differentmilling processes that use 2 differentmilling units by comparing the milledrestoration with the original digital dataset and (2) to visualize the deviationsand marginal chippings caused by themilling process. The primary null hy-pothesis was that no quantitative dif-ferences would be found between the 2different milling devices investigated ortheir respective milling processes.

MATERIAL AND METHODS

Seventeen clinical ceramic prepara-tions were selected for this study

The Journal of Prosthetic Dentis

(2 inlays with 2 surfaces, 5 inlays with 3surfaces, 6 inlays with 4 surfaces, 4onlays). All preparations were per-formed in accordance with the manu-facturer’s guidelines. The preparationswere digitally copied by using anintraoral scanning system (Cerec Blue-cam; Sirona) and were printed inacrylate polymer (Objet MED610;Objet Geometries GmbH) with a 3-dimensional printer (Objet Eden 260V;Objet Geometries GmbH).

The printed casts were lightlypowdered (Cerec Optispray; SironaDental Systems), and all were scannedwith a digital intraoral scanning system(Cerec Bluecam Connect, v4.03; SironaDental Systems) by the same experi-enced operator. The preparation borderwas defined, and then the data set wastransferred to CAD/CAM software(inLab 4.0, v4.02; Sirona Dental Sys-tems). The restorations were calculatedand sent to the milling preview.

The 4 milling procedures investi-gated are displayed in Table I. Groups12S, 1Step, and 2Step were milled witha 4-axial milling unit (inLab MC XL;Sirona Dental Systems). This millingunit uses 2 instruments for the millingprocess: a stepped bur (Step Bur),milling only the inner surface, and apointed bur (Cylindrical Pointed Bur),milling only the outer surface of therestoration. The Step Bur is available in2 diameters, Step Bur 12S (1.2 mm)and Step Bur 12 (1.0 mm). In group12S, the restorations were milled with aStep Bur 12S and a Cylindrical PointedBur 12S. In group 1Step, the restora-tions were milled with a Step Bur 12and a Cylindrical Pointed Bur 12S. Ingroup 2Step, the restorations weremilled with a Step Bur 12 and a Cylin-drical Pointed Bur 12S by using a

try

so-called 2-step milling procedure. Forthose 3 groups, the parameters forthe restorations were set as follows:Spacer, 80; Marginal-Adhesive Gap, 60;Occlusal-Milling Offset, 0; Proximal-Contacts Strength, �50; Occlusal-Contacts Strength, 25; Minimal Thick-ness (Radial), No; Minimal Thickness(Occlusal), No; Margin Thickness, 20;Consider Instrument Geometry, Yes;Remove Undercuts, Yes.

In groupArctica, the restorationsweremilled with a 5-axial milling unit (Arctica;KaVo). The restorations were exported toSurface Tessellation Language (STL) files(a standard format for CAD/CAM dataexchange) with the manufacturer’s rec-ommended parameters as follows:Spacer, 20; Marginal-Adhesive Gap,0; Occlusal-Milling Offset, 0; Proximal-Contacts Strength, �50; Occlusal-Contacts Strength, 25; MinimalThickness (Radial), No; Minimal Thick-ness (Occlusal),No;MarginThickness, 0;Consider Instrument Geometry, No;Remove Undercuts, Yes.

The STL files were then importedinto the 5-axial CAM software andmilled without any further changes. Forall test groups, the calculated millingsurfaces of the restorations wereexported as STL files for later compari-son with the milled restorations.

After milling, the sprue was removedfrom the restorations, and they wereplaced in the printed casts without anyfurther adjustments. All milled restora-tions were scannedwith a 3-dimensionalscanning system (Bluecam, inLab 4.0,v4.02; Sirona Dental Systems). Bluecamis a scanner with a trueness for single-tooth scans of �19.2 mm.17 The sur-faces were lightly powdered (CerecOptispray; Sirona Dental Systems), therestorations were scanned from the

Bosch et al

Table II. Aspects for visual examination

Aspect Description

Aspect 1 Occlusal relief

Aspect 2 Fine structures in inner surface

Aspect 3 Surfaces with angle close to insertion axis in inner surface

Aspect 4 Marginal areas in inner surface up to 100 mm from outer edge

Table III. Ranking of visual examinations

RankingStep

Bur 12SStep

Bur 12, 1-StepStep

Bur 12, 2-Step Arctica

Aspect 1 4 2 2 1

Aspect 2 4 3 2 1

Aspect 3 4 3 2 1

Aspect 4 4 2 2 1

1 ¼ best results; same number ¼ no difference.

Table IV. Trueness for all test groups (trueness with [90% - 10%]/2 percentile)

Group Process Occlusal TruenessInner SurfaceTrueness

12S MC XL, Bur 12S, 1-step 76 mm �40 mm 96 mm �68 mm

1Step MC XL, Bur 12, 1-step 55 mm �18 mm 67 mm �20 mm

2Step MC XL, Bur 12, 2-step 67 mm �24 mm 61 mm �22 mm

Arctica Arctica 42 mm �10 mm 41 mm �15 mm

MC XL, inLab MC XL (Sirona Dental Systems).

December 2014 1427

occlusal view, and the 3-dimensionaldata sets were exported as STL files(occlusal surface comparison).

The milled restorations were thenfixed to an object plate provided withreference grooves, with the inner sur-faces of the restorations facing upward.The surfaces were lightly powdered(Cerec Optispray) and scanned, andthe 3-dimensional data sets wereexported as STL files (inner surfacecomparison).

Within each group, the scanned sur-faces of the milled restorations werecompared with the calculated millingsurfaces with specialized differenceanalysis software (OraCheck v1.00.10;Cyfex). The software superimposes theSTL files by using a best-fit algorithm forclosest-point matching of both surfaces.The occlusal or inner surfaces wereselected precisely and the restorationssuperimposed. The software calculatedand measured the distance (positive ornegative) from every surface point(approximately 20 000 per surfacematching) from themilled to the originalsurface. The point-by-point differencevalues for each single superimpositionwere exported to an American StandardCode for Information Interchange(ASCII) text file (comma-separatedvalues). All the calculated distances wereimported into statistical software (IBMSPSS v19.0; IBM Corp). For each su-perimposition of 2 surfaces, the 10thand 90th percentiles were calculated.The metric value for the deviation be-tween 2 surfaces was defined as the(90%-10%)/2 percentile (deviationmeasure [DM]). This DM gives the levelby which approximately 80% of thematched area has less negative andpositive deviations. In the next step, themean value and the SD of the DMs werecalculated for each group. These meanvalues describe the trueness of the mill-ing process in terms of the deviationfrom the calculated original data. Alower value indicates a more accuratemilling process. To compare thedifferent groups, 1-way ANOVA with thepost hoc Tukey honestly significant dif-ference test was used as a statistical test(a¼.05).

Bosch et al

All visual examinations of the dif-ferences between the original and themilled restorations were performed bythe same experienced dentist by meansof color-coded difference images withboundary values set from þ100 mmto �100 mm. Separate aspects of theimages were examined, as shown inTable II. Aspect 1 was the occlusal re-lief, aspect 2 was the fine structures inthe inner surface, aspect 3 was thesurfaces with an angle close to theinsertion axis in the inner surface, andaspect 4 was the marginal area in theinner surface up to 100 mm from theouter edge. The different aspects werevisually evaluated separately for eachrestoration in direct comparisons be-tween all groups. For the comparison ofeach restoration (4 images), scoreswere assigned as follows: 1 for the best

result, 4 for the worst result. Rankingwas determined according to theamount of pink, green, and purplepresent. Pink indicates a loss of �100mm of material, green indicates almostno deviation from the CAD data set,and purple indicates differences �100mm larger than the original. The samescore in different groups means no vi-sual difference in quality. A rankingbetween the groups was calculatedbased on the sum of the restorationsfor each group.

RESULTS

The results of the visual examinationare shown in Table III and the truenessresults for all groups are displayed inTable IV. Figure 1 shows graphs of themeans and SDs for trueness. Table V

175

150

125

100

75

50

25

bur12S-1Step bur12-1Step bur12-2Step Arctica

True

ness

of

the

mill

ing

pro

cess

(µm

)

Process

Inner surfaceOcclusal surface

Localization

1 Trueness values and SDs for all test groups.

Table V. All test groups exhibitingstatistically significant differences

Differences P

Inner Surface

Arctica versus Bur 12S, 1Step <.001

Bur 12S versus Bur 12, 2Step .040

Occlusal Surface

Arctica versus Bur 12, 2Step .029

Arctica versus Bur 12S, 1Step .001

1428 Volume 112 Issue 6

shows the groups that were statisticallysignificantly different. For the occlusalarea, df was 3 and F was 5.802. For theinner surface, df was 3 and F was 6.166.For all test groups, the trueness in theocclusal areas ranged from 42 to 76mm; in the inner areas, from 41 to 96mm. Group Arctica exhibited the highesttrueness in both areas, with 42 �10 mmin the occlusal area and 41 �15 mm inthe inner surface. For the inner surface,group Arctica was statistically signifi-cantly better than group 12S (P<.001).For the occlusal surface, group Arcticawas statistically significantly better than

The Journal of Prosthetic Dentis

groups 2Step (P¼.029) and 12S(P¼.001). The difference images fromgroup Arctica showed the most accu-rate results in all 4 aspects (Figs. 2D,3D). Group 2Step yielded a truenessof 67 �24 mm for the occlusal area and61 �22 mm for the inner surface. Inboth areas, group 2Step did not differstatistically significantly from group1Step, but in the inner surface, it wasstatistically significantly better thangroup 12S (P¼.040). The differenceimages show the second-best results inall 4 aspects (see Figs. 2C, 3C).In aspect 1 (the occlusal relief) and inaspect 4 (the marginal areas), the re-sults for groups 1Step and 2Step werethe same. Group 1Step yielded a true-ness of 55 �18 mm in the occlusal areaand 67 �20 mm in the inner surface.The difference images were close to thedifference images of group 2Step (seeFigs. 2B, 3B). However, with regard tothe fine structures in the inner surfacein aspect 2 and to the surfaces withan angle close to the insertion axisin aspect 3, group 2Step was some-what better. Group 12S exhibited the

try

lowest trueness in the occlusal area(76 �40 mm) and in the inner surface(96 �68 mm). The difference imagesshown in Figures 2A and 3A display thelargest deviations compared with theCAD data set in all 4 aspects.

DISCUSSION

The trueness of chairside millingprocesses was examined in this in vitrostudy. On the basis of the results of thisin vitro study, the null hypothesis, thatno quantitative differences would befound between the 2 different millingdevices investigated or their respectivemilling processes, has to be rejected.However, drawing meaningful compar-isons with previously reported researchis problematic. In previous studies,the marginal fit and internal fitwere measured. Conventional methodsmeasure these parameters in only 1 or 2dimensions, and they measure the gapbetween the restoration and die withmicroscopy at 2 to 150 points in 5 to10 specimens per group. Additionally,previous studies have evaluated fit withthe elastomeric putty-wash technique,with low-viscosity silicone to duplicatethe cement space and evaluate itphotometrically or analyze it based onits density and weight.24,25

Achieving clinically relevant resultsmay require between 50 and 230measuring points.38 Modern computer-aided techniques can better evaluatethe fit of the restoration, because theyyield much more extensive and informa-tive data in 3 dimensions.39 Three-dimensional analysis has also provenvalid and reliable.40 In previous studies,the internal andmarginal gaps have beenexamined with 3-dimensional analysis bysuperimposing scans of the die over scansof the inner surface from the ceramiccrown, or by superimposing scans of thedie over scans of the fit-checker on the dieto measure the thickness of the fit-checker, which replicates the cementspace.24,39 With an accurate intraoralscanning system, the fit of the restorationin the oral cavity depends on the millingprocess.18 Thus, this study did not focusdirectly on the marginal and internal fit

Bosch et al

2 Deviations between digital calculated cast and scanned milled cast (trueness).A, Bur 12S, 1-stepmode. B, Bur 12, 1-stepmode. C, Bur 12, 2-stepmode.D, Arctica.Color-coded from �100 mm (blue) to þ100 mm (red).

December 2014 1429

but rather on the milling process. Withbetter trueness in the milling process, abetter fit is facilitated. An inaccuratelymilled restoration results in a poor fit,with numerous occlusal and proximalcontacts, which evidently differs sub-stantially from the contact points pre-cisely determined by the CAD/CAMsoftware. Correcting the contact pointsintraorally may negatively affect theesthetics of the restoration, the contactdistribution, the chairside time, and thelongevity.31,32

In this study, the point-by-pointdifferences were measured betweenthe digitally acquired data and themilled restoration caused by the mill-ing process. With the method re-ported, the inner and outer surfacescould be measured in addition to the

Bosch et al

internal fit. Another advantage of thismethod is that unlike other methods,this one requires neither fit-checkernor several dies, making it easier andmore fail-safe. The difference imagesfacilitate direct visual feedbackencompassing the entire restorationand make it possible to locate themore imprecise areas. Bluecam hasbeen validated as being accurate forsingle-tooth scans, with a trueness of�19.2 mm in this context, and issufficiently accurate for use in thismeasuring method.17

The 4-axial milling unit (inLab MCXL; Sirona Dental Systems) uses theStep Bur for the inner area and theCylinder Pointed Bur for the occlusalarea. During milling, the rotation axisof the material holder is fixed to

ensure that both instruments maintainan appropriate insertion axis. In 2-Stepmode, the restoration is milled in2 cycles. In the first milling cycle, therestoration is milled with an additionalmaterial thickness of 0.3 mm at allsurfaces. The second milling cycleremoves the remaining 0.3 mm ofmaterial to get the final dimensions ofthe restoration. Instruments with alarger diameter (Step Bur 12S, 1.2mm) can withstand more milling cy-cles and have a higher excavation ratethan instruments with a smallerdiameter (Step Bur 12, 1.0 mm).However, a small diameter is neededto ensure the accurate milling out ofsmaller and deeper structures. Impor-tantly in this context, the 5-axial mill-ing unit (Arctica; KaVo) incorporatesseveral different instruments, with in-strument diameters from 3.0 mm to0.5 mm. Additionally, with an addi-tional axis, surfaces with an angleclose to the insertion axis can beprocessed more effectively and accu-rately. The different instruments avail-able within the system make itpossible to generate a more accuraterelief with deeper fissures and moreaccurately milled pointed angles.

Provided that quality is not sacri-ficed, a more rapid milling process forchairside restorations is beneficial forboth the dentist and the patient.Changing to smaller instruments andmilling with more than 2 different in-struments, or milling in several steps,takes time. The milling process with a5-axial milling unit is slower than thatwith a 4-axial milling unit, and the 2-step mode requires more time thanthe 1-step mode. In this context, thefaster milling process results in a lessaccurate restoration, more marginalchipping, reduced longevity, and thepossible need to loop in the restora-tion intraorally.

Future research should investigatethe roughness of the edges of restora-tions milled with different milling stra-tegies (with the scanning electronmicroscope) and the effect of severalmill cycles with the same rotary instru-ment on milled restorations.

3 Deviations between digital calculated casts and scannedmilled casts (trueness).A, Bur 12S, 1-stepmode. B, Bur 12, 1-stepmode. C, Bur 12, 2-stepmode.D, Arctica.Color-coded from �100 mm (blue) to þ100 mm (red).

1430 Volume 112 Issue 6

The limitations of this research werethat only the trueness of the millingprocess was measured (and not theinfluence of the discrepancy on themarginal and internal gap) and thatthe precision of the milling process wasnot investigated with more milling cy-cles reusing the same instrument.Scanning with the need to powder thesurfaces introduces a certain error level.A scan without powder could reducepossible artifacts.

From the scanning process to therestoration design, the CAD/CAM pro-cess has proven accurate under optimalcircumstances. The incorporation ofan optimized milling process with in-struments that have a small diametermay result in similar levels of accuracyand finished restorations with higheresthetics and better longevity.

The Journal of Prosthetic Dentis

CONCLUSIONS

Within the limitations of this in vitrostudy, restorations milled with a 5-axialmilling unit exhibited the best qualityand the highest trueness values. Withregard to the 4-axial milling unit (inLabMC XL; Sirona Dental Systems), the2-step mode was not significantly betterthan the 1-step mode. A rotary instru-ment with a smaller diameter resultedin more accurate milling.

REFERENCES

1. McLaren EA. All-ceramic alternatives toconventional metal-ceramic restorations.Compend Contin Educ Dent 1998;19:307-12.

2. Giordano RA. Dental ceramic restorativesystems. Compend Contin Educ Dent1996;17:779-86.

try

3. Raigrodski AJ. Contemporary all-ceramicfixed partial dentures: a review. Dent ClinNorth Am 2004;48:531-44.

4. Tinschert J, Natt G, Hassenpflug S,Spiekermann H. Status of current CAD/CAMtechnology in dental medicine. Int J ComputDent 2004;7:25-45.

5. Yeo IS, Yang JH, Lee JB. In vitro marginal fit ofthree all-ceramic crown systems. J ProsthetDent 2003;90:459-64.

6. Miyazaki T, Hotta Y, Kunii J, Kuriyama S,Tamaki Y. A review of dental CAD/CAM:current status and future perspectives from20 years of experience. Dent Mater J 2009;28:44-56.

7. Alkumru H, Hullah WR, Marquis PM,WilsonHJ. Factors affecting thefit of porcelainjacket crowns. Br Dent J 1988;164:39-43.

8. Balkaya MC, Cinar A, Pamuk S. Influence offiring cycles on the margin distortion of 3all-ceramic crown systems. J Prosthet Dent2005;93:346-55.

9. Gemalmaz D, Alkumru HN. Marginal fitchanges during porcelain firing cycles.J Prosthet Dent 1995;73:49-54.

10. Mously HA, Finkelman M, Zandparsa R,Hirayama H. Marginal and internal adapta-tion of ceramic crown restorations fabricatedwith CAD/CAM technology and the heat-press technique. J Prosthet Dent 2014;112:249-56.

11. Bindl A, Mormann WH. Clinical and SEMevaluation of all-ceramic chair-side CAD/CAM-generated partial crowns. Eur J Oral Sci2003;111:163-9.

12. Reiss B, Walther W. Clinical long-termresults and 10-year Kaplan-Meier analysisof Cerec restorations. Int J Comput Dent2000;3:9-23.

13. Reiss B. Clinical results of Cerec inlays in adental practice over a period of 18 years. Int JComput Dent 2006;9:11-22.

14. Luthardt RG, Loos R, Quaas S. Accuracy ofintraoral data acquisition in comparison tothe conventional impression. Int J ComputDent 2005;8:283-94.

15. Ziegler M. Digital impression taking withreproducibly high precision. Int J ComputDent 2009;12:159-63.

16. Ender A, Mehl A. Full arch scans: conven-tional versus digital impressionsean in-vitrostudy. Int J Comput Dent 2011;14:11-21.

17. Mehl A, Ender A, Mormann W, Attin T. Ac-curacy testing of a new intraoral 3D camera.Int J Comput Dent 2009;12:11-28.

18. Ender A, Mehl A. Influence of scanningstrategies on the accuracy of digitalintraoral scanning systems. Int J ComputDent 2013;16:11-21.

19. Yuksel E, Zaimoglu A. Influence of marginalfit and cement types on microleakage of all-ceramic crown systems. Braz Oral Res2011;25:261-6.

20. Bindl A, Mormann WH. Marginal and inter-nal fit of all-ceramic CAD/CAM crown-copings on chamfer preparations. J OralRehabil 2005;32:441-7.

21. Nakamura T, Dei N, Kojima T,Wakabayashi K. Marginal and internal fit ofCerec 3 CAD/CAM all-ceramic crowns. Int JProsthodont 2003;16:244-8.

Bosch et al

December 2014 1431

22. Euán R, Figueras-Álvarez O, Cabratosa-Termes J, Oliver-Parra R. Marginal adapta-tion of zirconium dioxide copings: influenceof the CAD/CAM system and the finishline design. J Prosthet Dent 2014;112:155-62.

23. Addi S, Hedayati-Khams A, Poya A,Sjogren G. Interface gap size of manuallyand CAD/CAM-manufactured ceramicinlays/onlays in vitro. J Dent 2002;30:53-8.

24. Schaefer O, Watts DC, Sigusch BW,Kuepper H, Guentsch A. Marginal and in-ternal fit of pressed lithium disilicate partialcrowns in vitro: a three-dimensional analysisof accuracy and reproducibility. Dent Mater2012;28:320-6.

25. Lee KB, Park CW, Kim KH, Kwon TY.Marginal and internal fit of all-ceramiccrowns fabricated with two different CAD/CAM systems. Dental Mater J 2008;27:422-6.

26. Boening KW, Wolf BH, Schmidt AE,Kastner K, Walter MH. Clinical fit of ProceraAllCeram crowns. J Prosthet Dent 2000;84:419-24.

27. Song TJ, Kwon TK, Yang JH, Han JS, Lee JB,Kim SH, et al. Marginal fit of anterior 3-unitfixed partial zirconia restorations usingdifferent CAD/CAM systems. J Adv Prostho-dont 2013;5:219-25.

28. Hamza TA, Ezzat HA, El-Hossary MM,Katamish HA, Shokry TE, Rosenstiel SF. Ac-curacy of ceramic restorations made with twoCAD/CAM systems. J Prosthet Dent2013;109:83-7.

Bosch et al

29. Brawek PK, Wolfart S, Endres L, Kirsten A,Reich S. The clinical accuracy of singlecrowns exclusively fabricated by digitalworkflow-the comparison of two systems.Clin Oral Invest 2013;17:2119-25.

30. Mormann WH, Schug J. Grindingprecision and accuracy of fit of CEREC 2CAD-CIM inlays. J Am Dent Assoc 1997;128:47-53.

31. Sjogren G. Marginal and internal fit of fourdifferent types of ceramic inlays after luting:an in vitro study. Acta Odontol Scand1995;53:24-8.

32. Keshvad A, Hooshmand T, Asefzadeh F,Khalilinejad F, Alihemmati M, VanNoort R. Marginal gap, internal fit, andfracture load of leucite-reinforced ceramicinlays fabricated by CEREC inLab and hot-pressed techniques. J Prosthodont2011;20:535-40.

33. Felden A, Schmalz G, Hiller KA. Retrospectiveclinical study and survival analysis on partialceramic crowns: results up to 7 years. ClinOral Investig 2000;4:199-205.

34. Federlin M, Krifka S, Herpich M, Hiller KA,Schmalz G. Partial ceramic crowns: influenceof ceramic thickness, preparation design andluting material on fracture resistance andmarginal integrity in vitro. Oper Dent2007;32:251-60.

35. Ilie N, Kunzelmann KH, Hickel R. Evaluationof micro-tensile bond strengths of compositematerials in comparison to their polymeriza-tion shrinkage. Dent Mater 2006;22:593-601.

36. Kramer N, Lohbauer U, Frankenberger R.Adhesive luting of indirect restorations. Am JDent 2000;13:60-76.

37. Lohbauer U, Kramer N, Petschelt A,Frankenberger R. Correlation of in vitro fa-tigue data and in vivo clinical performance ofa glass ceramic material. Dent Mater2008;24:39-44.

38. Groten M, Axmann D, Probster L, Weber H.Determination of the minimum number ofmarginal gap measurements required forpractical in-vitro testing. J Prosthet Dent2000;83:40-9.

39. Moldovan O, Luthardt RG, Corcodel N,Rudolph H. Three-dimensional fit of CAD/CAM-made zirconia copings. Dent Mater2011;27:1273-8.

40. Luthardt RG, Kuhmstedt P, Walter MH.A new method for the computer-aidedevaluation of three-dimensional changesin gypsum materials. Dent Mater 2003;19:19-24.

Corresponding author:Gabriel BoschDivision for Computerized Restorative DentistryCenter of Dental MedicineUniversity of ZürichPlattenstrasse 118032 ZürichSWITZERLANDE-mail: gabriel.bosch@zzm.uzh.ch

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.