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Direct mechanical data acquisition of dental impressions for the manufacturing of CAD/CAM restorations

Sebastian Quaas *, Heike Rudolph, Ralph G. Luthardt

University Ulm, Department of Dentistry, Prosthetic Dentistry and Materials Sciences, Albert-Einstein-Allee 11, 89070 Ulm, Germany

a r t i c l e i n f o

Article history:

Received 17 June 2007

Received in revised form

23 August 2007

Accepted 27 August 2007

Keywords:

One-step-impression

Computer aided analysis

Mechanical measurement

Touch-probe-scanner

Digitizing

Data acquisition

Dental restoration

CAD/CAM

a b s t r a c t

Objectives: The basic prerequisite for the production of

dental restorations by means of CAD/ CAM technologies

is the data acquisition (digitization). Currently, two

methods are avail- able, i.e. the extraoral digitization of

master casts and the direct intraoral data acquisition.

However, it seems to be beneficial to immediately

digitize impressions directly at the dental office in order

to combine the high precision of mechanical

digitizing methods and to shorten the production

process.

The aim of this study was to investigate the

measurement uncertainty ( 2s) and the three-

dimensional accuracy of the immediate tactile in-office

digitization of dental impres- sions and of the mechanical

digitizing of ceramic master dies using a high-precision

touch- probe digitizer.

Methods: The experimental set-up consisted of ceramic

904 j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 9 0 3 – 9 0 8

master dies

representing

tooth 13 and 36

as well as their

identical virtual

models (CAD

models). Fifteen

one-step putty-

wash

impressions

were taken

from each tooth.

The

impressions as

well as the

ceramic master

dies were

digitized

applying a

standardized procedure. The datasets were aligned to

the corresponding CAD models; then, a computer-aided

three-dimensional analysis was per- formed.

Results: The digitizing of the dental impressions showed

a measurement uncertainty of 5.8, mean positive

deviations between 27 and 28 mm, and mean negative

deviations between

21 and 31 mm. The digitizing of the ceramic

master dies showed a measurement uncertainty of 2.8,

mean positive deviations between 7.7 and 9.1 mm, and

mean negative deviations between 8.5 and 8.8 mm.

Conclusion: Mechanical digitizers show a very low

measurement uncertainty and a high precision. The

immediate tactile in-office digitization of impressions

cannot be recom- mended as adequate data acquisition

method for CAD/CAM restorations. It is recommend-

able to digitize clinical sites extraorally, i.e. after

taking an impression and fabricating a

model cast thereof.

# 2007 Elsevier Ltd. All rights reserved.

1. Introduction

When using computer-aided technologies for the production

of dental restorations, the minimum requirements are to

digitize the abutment teeth. The digitizing accuracy is a

major

factor, which has an influence on the fit of fixed restorations.1,2

Currently, the data acquisition is either performed directly in

the patient’s mouth (intraoral) or indirectly after taking an

impression and fabricating a master cast (extraoral). Regard-

less of the digitizing mode applied, clinical parameters,

e.g.

* Corresponding author. Tel.: +49 731 500 64245; fax: +49 731 500 64203. E-mail address: sebastian.quaas @ uniklinik-ulm.de (S. Quaas).

0300-5712/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.j d ent.2007.08. 0 08

saliva, blood, movements of the patient, might affect the

reproduction of teeth.3–7

Intraoral digitization allows the dental-care provider to

directly obtain the data from the prepared teeth. Thus,

taking an impression and fabricating a cast model are no

longer necessary.8 Titanium dioxide or magnesium oxide

powder has to be applied to the glossy, lucent tooth

surfaces in order to avoid reflections and to create a

measurable surface. The powder layer applied to the

tooth surface results in an additional thickness of 13–85

mm.9 An in vitro study showed a higher accuracy of the

extraoral digitization with impression taking and model

fabrication than in case of the intraoral digitization.10

Prior to the extraoral digitization, an impression of the

clinical situation has to be taken. The impression technique as

well as the properties of the impression material used

may affect the fitting precision of fixed restorations as

well. The fabrication of a master cast compensates for

volumetric changes of the impression material to a certain

extent.11,12

However, the fabrication of a master cast is a time-

consuming and error-prone method that requires the

presence of or collaboration with a dental laboratory.

Master casts can be digitized extraorally either with optical

or with mechanical tools.13 The optical digitization allows

a fast and wearless high-resolution data acquisition.14 As

a disadvantage, how- ever, shadowing effects limit the use of

the extraoral optical digitization method. Multiple

measurements combined with the alignment of partial

measurements are necessary in order to compensate for

such effects. In principle, mechanical digitizers are mainly

used for the data acquisition of cavities or negative

moulds15,16 such as the silicone impression of a tooth. The

reproduction of small-sized structures is restricted by the

diameter and the design of the probe tip.2 Measurement

errors in case of mechanical digitizing are mainly caused

by the geometry of the measuring objects2,17,18 and were

said to be comparable to those of the extraoral digitization

with optical tools.2,14,17–20 Persson et al. found maximum

shape-related errors of +/ 10 mm with the contact probe

device of the Procera system. It is difficult to mechanically

digitize soft and flexible materials due to possible surface

damages or object deformation caused by the touch of the

probe15 even if the dental-care provider applies only a

gentle contact force. High contact pressures might injure the

surface of the objects to be measured.2

The immediate tactile in-office digitization of impressions

seems to be beneficial as it combines the high precision

of mechanical digitizing methods and a shortened

production process. Studies about the measurement

uncertainty of mechanical digitizers are rare, and studies

investigating the measurement uncertainty of the direct

digitization of dental impressions, check-bites or inner

surfaces of crowns were not even found at all. The aim of

this study was to investigate the three-dimensional (3D)

accuracy and the measurement uncertainty ( 2s) of the

immediate tactile in-office digitization of dental impressions

compared to the mechanical digitiza- tion of ceramic

master dies by means of a high-precision touch-probe

digitizer. In this study, the hypothesis that the 3D accuracy of

immediate tactile in-office digitization of dental

impressions can compete with the accuracy gained by the

mechanical digitizing of ceramic master dies was examined.

2. Materials and methods

The experimental set-up allowing the evaluation of every

step of the CAD/CAM process chain by means of an

established procedure4,12,13,21 consisted of virtual CAD

surface models (reference models) and ceramic master dies

of the prepared teeth 13 (upper right canine) and 36

(lower left first molar) (Fig. 1).12 The ceramic master dies

were made out of alumina– zirconia ceramics (HITK

Hermsdorfer Institut fu¨ r Technische Keramik,

Hermsdorf/Thuringia, Germany). They were digi- tized

mechanically (Triclone 90, Renishaw plc, Gloucester- shire,

GB), and CAD surface models were created as reference

(ce.novation V1.0, ILMCAD, Ilmenau, Germany). In order to

validate the conformance of the CAD surface models with

the ceramic master dies, a second digitizing system was

used. The ceramic master dies were optically digitized with

the hiScanm (IVB GmbH, Jena, Germany; Fraunhofer IOF,

Jena, Germany; Hint-ELs GmbH, Griesheim, Germany).12

In this study, a probe diameter of 1.0 mm and a

contact force of 0.5 N were used for the mechanical

digitization. In preliminary tests, measurement procedures

for the digitiza- tion of positive and negative moulds

were tested. For the comparable digitization of positives

and negatives, a radial measurement procedure was

chosen in order to acquire a sufficient number of

measurement points (approx. 30,000) within an acceptable

time (max. 60 min). In this procedure, the measurement

object was digitized with star-shaped move- ments. Each

ceramic master die was digitized three times applying the

same parameters (step-over: 0.1 mm, scanning speed: 250

mm/min, maximum probe deflection: 0.3 mm, chordal

tolerance: 0.01 mm) in order to determine the

measurement uncertainty.

A temporary crown and bridge material (Luxatemp, DMG,

Hamburg, Germany) was used to create an artificial

gingiva simulating the gingival sulcus (Fig. 2). The width and

depth of this gingival sulcus were measured with a slide

gauge. The measurement results are shown in Table 1.

Fifteen visually perfect impressions each were taken of

the ceramic master dies 13 and 36 with the one-step putty-

wash method (Dimension Penta H/Garant L; 3M ESPE AG,

Seefeld, Germany). In order to create a subgingival

finishing line, artificial gingiva was adapted at the ceramic

master dies. For all impressions, unperforated,

individualized mandibular stock trays were used. Each tray

was prepared with acrylic resin (KALLOCRYL CPGM,

SPEIKO, Muenster, Germany) to define four separate

sections. In general, the impressions were measured

mechanically 4.5 h after having taken the impres- sion

(Triclone 90, Renishaw plc, Gloucestershire, GB).12 After the

calibration of the measurement device with a calibration

gauge, the impressions were fixed on the object table.

Each impression was digitized only once in order to prevent

getting different results caused by volumetric changes

of the impression material or by harming the impression’s

surface during probing.

After transferring all datasets acquired optically

and mechanically (point clouds of the measured surface)

to the CAD analysis software Surfacer (V.10.6, SDRC

Imageware, Ann Arbor, MI, USA), the datasets measured

were processed according to a procedure given by Luthardt

et al.4 and aligned to the CAD surface model. In this context,

alignment means to

905j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 9 0 3 – 9 0 8

Table 1 – Widths and depths of the artificial gingivalsulcus

Tooth Sulcus widths (mm) Sulcus depths (mm)

13 Oral 0.2 Oral 1.2

Vestibular 0.5 Vestibular 1.1

Mesial 1.5 Mesial 0.5

Distal 0.2 Distal 1.0

36 Oral 0.5 Oral 1.0

Vestibular 0.8 Vestibular 2.0

Mesial 0.5 Mesial 0.5

Distal 0.4 Distal 0.8

Fig. 1 – The experimental set-up including the virtual CAD surface model (as reference model) and an identical ceramic

master die of the prepared tooth.

superimpose the 3D datasets in a three-dimensional system of

coordinates according to mathematical correlation calcula-

tions.12,20 For each of such alignments, the root mean square

deviation (RMS error) was calculated,22 and the optimum

coordinate transformation was computed (Surfacer, V.10.6,

SDRC Imageware, Ann Arbor, MI, USA) for the alignment with

the best fit. The measurement points beyond the finishing

line were cut off and, thus, excluded from the analysis. The

three- dimensional deviations between each single point

of the datasets measured and the CAD surface

models were calculated (Surfacer). The results are indicated by

the positive and negative deviations (maximum, mean

and standard deviation) and by a color-coded

representation of the three- dimensional deviations

(qualitative analysis).

Fig. 2 – A sleeve out of temporary crown and bridge

material simulates the gingival sulcus.

The three-dimensional geometric deviations between the

datasets measured and the CAD surface model were

statis- tically analyzed by means of the repeated measure

analysis with one within factor (tooth types: canine, molar)

and a global significance level of 0.05 (SPSS 12.0, SPSS Inc.,

Chicago, IL, USA).

3. Results

The digitization of the ceramic master dies showed a

mean RMS error of 20.7 mm (S.D. 1.8). Table 2 shows the

calculated positive and negative deviations between the

datasets gained by digitizing the ceramic master dies and

the reference CAD surface model as well as the

measurement uncertainty.

The 3D accuracy of the ceramic master dies, which

was tested with an optical digitizing system, showed

mean

Table 4 – P-values of the RMS error and the mean positiveand negative deviations with regard to the tooth shape(canine, molar)

N T-value P-value

RMS error

Variances are equal 15 1.300 0.204

Variances are unequal 15 1.300 0.204

Mean positive deviations

Variances are equal 15 0.843 0.406

Variances are unequal 15 0.843 0.406

Mean negative deviations

Variances are equal 15 10.802 0.000

Variances are unequal 15 10.802 0.000

Table 3 – Deviations between the datasets taken from theimpressions of the ceramic master dies and the reference CAD surface models as well as the calculated measure- ment uncertainty (2s)

Tooth N Digitized impressions

Mean (mm) 2s

Positive deviations

Canine (13) 15 28.1 7.2

Molar (36) 15 27.1 6.0

Negative deviations

Canine (13) 15 30.6 6.2

Molar (36) 15 20.6 3.6

Table 2 – Deviations between the datasets gained fromdigitizing the ceramic master dies and the reference CAD surface models as well as the calculated measurement uncertainty (2s)

Tooth N Ceramic master dies

Mean (mm) 2s

Positive deviations

Canine (13) 3 9.1 3.0

Molar (36) 3 7.7 3.26

Negative deviations

Canine (13) 3 8.5 3.4

Molar (36) 3 8.8 1.6

positive deviations of 5.4 mm (S.D. 7.2) and mean negative

deviations of 5.8 mm (S.D. 7.0) for the canine and mean

positive deviations of 7.5 mm (S.D. 6.9) and mean

negative deviations of 11.2 mm (S.D. 11.2) for the molar.

The RMS error of the digitized impressions resulted in

nearly identical values of the two ceramic master dies,

i.e.

30.7 mm (S.D. 1.2) in case of the canine and 30.0 mm (S.D. 1.4)

in case of the molar. Table 3 illustrates the mean positive

and mean negative deviations between the measured

datasets gathered by digitizing the impressions and their

correspond- ing CAD surface models. Measurement

uncertainties for the digitalization of the impressions

resulted between 3.6 and 7.2. Regarding the digitized

impressions of the canine, the average maximum deviations

amounted to 158.7 mm and 186.0 mm and 17.3 mm and

199.2 mm regarding the digitized impres- sions of the

molar. The statistical analysis showed no significance for

the RMS error (P = 0.204) and the mean positive deviations (P =

0.406) regarding the tooth shape, while the tooth shape

played a significant role for the mean negative deviations (P

0.000) (Table 4).

The qualitative analysis of the deviations between

impression and CAD surface model resulted in reduced

point clouds with circularly enlarged areas of the prepared

teeth. In the color-coded representation of the three-

dimensional deviations, the edges of the point cloud of

the molar appeared reduced. Large negative deviations were

noticed in the areas of the finishing line and of the edges.

Discrepancies of up to 208 mm between the CAD surface

model and the measured dataset of the impressions were

found in these areas (Fig. 3).

4. Discussion

In this investigation, the quality of the alignment of the datasets

was influenced by several process variables (measurement

uncertainty of the digitizer, errors with regard to the impres-

sion); it worsens the better the datasets are matching each

another.14,20 For the quantitative and qualitative analyses of the

Fig. 3 – (a and b) Large discrepancies between the CAD

surface model and the data taken from the impressions

were found in the area of the finishing line. (a) Silicone

impression of the molar 36. The frame indicates the area

with very thin light material without being supported by

the putty. (b) The same area in the color-coded

representation of the three-dimensional deviations in mm;

negative deviations in blue, positive deviations in red.

accuracy of the digitized impressions that, alignment of

datasets measured is required that best fits the reference

CAD surface model. An RMS error of less than 10 mm is

considered as excellent fit, whereas an RMS error of more than

50 mm denotes a poor fit.23 Outliers, data points beyond the

finishing line, and several scattered points were deleted

manually and, thus, remained unconsidered.4 Such points

would have increased the RMS error and induced direction

dependence during the alignment process. In regard to the

handling, digitizing the impressions was more difficult than

digitizing the prepared teeth. A correct orientation of the

impression in the digitizer is necessary to avoid undercuts and

missing data.

Persson et al. reported digitizing errors similar to the

mean positive and negative deviations that were found

in the present study for the digitizing process of prepared

teeth.2,18 If comparing the results indicated in Tables 2

and 3, the measurement uncertainty of the immediate

tactile in-office digitization of dental impressions differs

significantly from the digitization of ceramic master

dies. Therefore, the hypothesis that the 3D accuracy

and the measurement uncertainty ( 2s) of the immediate

tactile in-office digitization of dental impressions can

compete with the mechanical digitizing of ceramic master

dies is falsified. The considerably lower accuracy of the

immediate tactile in-office digitization probably comes from

errors caused by the one-step putty- wash impression as

well as from effects associated with the digitizing of flexible

materials (in this case: polyvinylsiloxane).

Other data found in current literature proved that a mean

deviation of about 10 mm occurs when taking impressions and

fabricating master casts.12,24 Thus, the immediate tactile in-

office digitizing of dental impressions causes an additional

error compared to the conventional procedure.

The mean positive 3D deviations between the two

tooth shapes showed only small differences, whereas the

mean negative deviations between the CAD surface model

and the digitized impressions differed significantly (Table

4). The qualitative analysis of color-coded graphs showed an

elonga- tion and a slightly reduced width of the

impressions of the ceramic master dies. Also other authors

reported of similar three-dimensional changes due to the

one-step putty-wash impression technique.4,5,25,26 Large

discrepancies between the CAD surface model and the data

taken from the impressions were calculated in the areas of

the finishing line and of the occlusal or incisal edges,

respectively. Very thin light material layers, which are not

supported by the putty, were mainly found in the area of

the finishing line (Fig. 3a). The rigid artificial sulcus also

means a limitation since it cannot be treated with a

retraction cord as it is possible in the clinical soft tissue

management. Thus, small tear-offs are unavoidable (Fig.

3a). However, the areas of the finishing line where most of the

tear-offs occurred deformed less during the measurement

process (Fig. 4). In spite of the low contact forces of 0.5 N, the

area of the finishing line showed strong contact

deviations. Compared to the CAD surface model, the

qualitative analysis of the digitized finishing line presented

smaller discrepancies in the area where the artificial sulcus

had its largest width and where it, therefore, was more

resistant to the touch-probe force. Especially in the area of

the finishing line, it seems that the width of the sulcus

and, thus, the thickness of the

Fig. 4 – A thin layer of impression material offers

insufficient resistance against the contact pressure

applied with the probe of the mechanical digitizer.

impression material have an influence on the quality of the

data acquisition when immediately digitizing dental impres-

sions directly at the dental office. A thin layer of impression

material offers insufficient resistance against the contact

pressure of the probe of the mechanical digitizer (Fig. 4).

Edges with a radius that is smaller than the radius of the

probe tip cannot be digitized accurately. Hence, the

deviations at the occlusal and incisal edges probably

occurred due to a mismatch between the probe tip and

the negative shape of the abutment tooth

(mould/impression) to be measured.

5. Conclusions

Mechanical digitizing cannot be recommended for the direct

digitizing of dental impressions. The three-dimensional

geometry changes of the tooth shape would be clinically

acceptable. However, large discrepancies between the CAD

surface model and the data taken from the impressions were

found in the area of the finishing line. Therefore, it is

recommendable to extraorally digitize clinical sites

after taking an impression and fabricating a model cast.

r e f e r e n c e s

1. Ode´ n A, Andersson M, Krystek-Ondracek I, Magnusson D.

Five year clinical evaluation of Procera AllCeram crowns.Journal of Prosthetic Dentistry 1998;80:450–5.

2. Persson M, Andersson M, Bergman B. The accuracy of a high-precision digitizer for CAD/CAM of crowns. Journal of Prosthetic Dentistry 1995;74:223–9.

3. Lee IK, DeLong R, Pintado MR, Malik R. Evaluation of factors affecting the accuracy of impressions using quantitative surface analysis. Operative Dentistry 1995;20:246–52.

4. Luthardt RG, Koch R, Rudolph H, Walter MH. Qualitative computer aided evaluation of dental impressions in vivo. Dental Materials 2006;22:66–76.

5. Rudolph H, Quaas S, Koch M, Preissler J, Koch R, LuthardtRG. Randomized, controlled, clinical trial on timing versus3D accuracy of different impression materials. DeutscheZahnarztliche Zeitschrift 2005;60:695–701.

6. Carrotte PV, Winstanley RB, Green JR. A study of the quality of impressions for anterior crowns received at a commercial laboratory. British Dental Journal 1993;174:235–40.

7. Loos R, Quaas S, Luthardt RG. Accuracy of conventional impression taking compared to intraoral digitising—A randomized controlled trial. Joint Meeting of the Continental European (CED) and Scandinavian (NOF) Divisions of theIADR. 2005.

8. Pfeiffer J. Dental CAD/CAM technologies: the optical impression (I). International Journal of Computerized Dentistry1998;1:29–33.

9. Meyer BJ, Mormann WH, Lutz F. Optimization of the powder application in the Cerec method with environment-friendly propellant systems. Schweizer Monatsschrift Zahnmedizin1990;100:1462–8.

10. Luthardt RG, Loos R, Quaas S. Accuracy of intraoral data acquisition in comparison to the conventional impression. International Journal of Computerized Dentistry 2005;8:283–94.

11. Bloem TJ, Czerniaawski B, Luke J, Lang BR. Determination of the accuracy of three die systems. Journal of Prosthetic Dentistry 1991;65:758–62.

12. Luthardt RG, Ku¨ hmsted P, Walter MH. A new method for the

computer-aided evaluation of three-dimensional changes in gypsum materials. Dental Materials 2003;19:19–24.

13. Luthardt R, Weber A, Rudolph H, Schone C, Quaas S, Walter M. Design and production of dental prosthetic restorations: basic research on dental CAD/CAM technology. International Journal of Computerized Dentistry 2002;5:165–76.

14. Mehl A, Gloger W, Kunzelmann KH, Hickel R. A new optical3-D device for the detection of wear. Journal of DentalResearch 1997;76:1799–807.

15. Hassler T. Reverse engineering with optical and tactile sensors. Quality Engineering 2004;12:38–40.

16. Ku¨ nneth T. Optische 2D-/3D-Messtechnik im Vergleich. 2D/

3D-metrology in comparison. WB Werkstatt und Betrieb2001;134:82–5.

17. Hewlett ER, Orro ME, Clark GT. Accuracy testing of three- dimensional digitizing systems. Dental Materials 1992;8:49–53.

18. Persson A, Andersson M, Oden A, Sandborgh-Englund G. A three-dimensional evaluation of a laser scanner and a touch-probe scanner. Journal of Prosthetic Dentistry2006;95:194–200.

19. Luthardt RG, Sandkuhl O, Herold V, Walter MH. Accuracy of mechanical digitizing with a CAD/CAM system for fixed restorations. International Journal of Prosthodontics2001;14:146–51.

20. Rudolph H, Quaas S, Luthardt RG. Matching point clouds: limits and possibilities. International Journal of Computerized Dentistry 2002;5:155–64.

21. Rudolph H, Luthardt RG, Walter MH. Computer-aided analysis of the influence of digitizing and surfacing on the accuracy in dental CAD/CAM technology. Computers in Biology and Medicine 2006;37:579–87.

22. Kaindl K, Steipe B. Metric properties of the root-mean square deviation of vector sets. Acta Crystallographica1997;53:809.

23. Peters MCRB, DeLong R, Pintado MR, Pallesen U, Qvist V, Douglas WH. Comparsion of two measurement techniques of clinical wear. Journal of Dentistry 1999;27:479–85.

24. Price RB, Gerrow JD, Sutow EJ, MacSween R. The dimensional accuracy of 12 impression material and die stone combinations. International Journal of Prosthodontics1991;4:169–74.

25. Wichmann M, Borchers L, Limmroth E. Bestimmung der Abformgenauigkeit verschiedener Elastomere mit Hilfe einer 3D-Koordinatenmeßmaschine (Teil 1). Deutsche Zahnaerztliche Zeitschrift 1990;45:499–502.

26. Fenske C, Sadat-Khonsari MR, Dade E, Jude HD. Gutschow F.Der Einfluss verschiedener Monophasenabformmaterialien auf die Dimensionstreue von Modellstu¨ mpfen. Zahnarztliche Welt Reform 1998;107:749–52.