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Precision of intraoral digital dental impressions with iTero and extraoral digitization with the iTero and a model scanner Tabea V. Fl ugge, a Stefan Schlager, b Katja Nelson, c Susanne Nahles, d and Marc C. Metzger e Freiburg and Berlin, Germany Introduction: Digital impression devices are used alternatively to conventional impression techniques and materials. The aims of this study were to evaluate the precision of digital intraoral scanning under clinical conditions (iTero; Align Technologies, San Jose, Calif) and to compare it with the precision of extraoral digitization. Methods: One patient received 10 full-arch intraoral scans with the iTero and conventional impressions with a polyether impression material (Impregum Penta; 3M ESPE, Seefeld, Germany). Stone cast models manufactured from the impressions were digitized 10 times with an extraoral scanner (D250; 3Shape, Copenhagen, Denmark) and 10 times with the iTero. Virtual models provided by each method were roughly aligned, and the model edges were trimmed with cutting planes to create common borders (Rapidform XOR; Inus Technologies, Seoul, Korea). A second model alignment was then performed along the closest distances of the surfaces (Artec Studio software; Artec Group, Luxembourg, Luxembourg). To assess precision, deviations between corresponding models were compared. Repeated intraoral scanning was evaluated in group 1, repeated extraoral model scanning with the iTero was assessed in group 2, and repeated model scanning with the D250 was assessed in group 3. Deviations between models were measured and expressed as maximums, means, medians, and root mean square errors for quantitative analysis. Color-coded displays of the deviations allowed qualitative visualization of the deviations. Results: The greatest deviations and therefore the lowest precision were in group 1, with mean deviations of 50 mm, median deviations of 37 mm, and root mean square errors of 73 mm. Group 2 showed a higher precision, with mean deviations of 25 mm, median deviations of 18 mm, and root mean square errors of 51 mm. Scanning with the D250 had the highest precision, with mean deviations of 10 mm, median deviations of 5 mm, and root mean square errors of 20 mm. Intraoral and extraoral scanning with the iTero resulted in de- viations at the facial surfaces of the anterior teeth and the buccal molar surfaces. Conclusions: Scanning with the iTero is less accurate than scanning with the D250. Intraoral scanning with the iTero is less accurate than model scanning with the iTero, suggesting that the intraoral conditions (saliva, limited spacing) contribute to the inaccuracy of a scan. For treatment planning and manufacturing of tooth-supported appliances, virtual models created with the iTero can be used. An extended scanning protocol could improve the scanning results in some regions. (Am J Orthod Dentofacial Orthop 2013;144:471-8) F or the introduction of computer-aided design and computer-aided manufacturing technologies in dentistry, virtual models of teeth are required. Digital processes are applied for prosthetic-driven backward planning of implant surgery, 1,2 orthodontic measurements, and treatment planning 3-6 combined with surgical planning. 7 Data acquired by intraoral scanning, computed tomography, cone-beam computed tomography, and extraoral surface scanning can be fused. 1,2,7 For the acquisition of digital images of teeth, different procedures have been described: digitization of plaster casts, 3,5,8-12 digitization of impressions, 8,13 and intraoral digital impressions. 13,14 The accuracy a Resident, Division of Oral and Maxillofacial Surgery, University Medical Center, Freiburg, Germany. b Research fellow, Biological Anthropology, University Medical Center, Freiburg, Germany. c Professor, Division of Oral and Maxillofacial Surgery, University Medical Center, Freiburg, Germany. d Associate professor, Division of Oral and Maxillofacial Surgery, Charite Campus Virchow, Berlin, Germany. e Associate professor, Division of Oral and Maxillofacial Surgery, University Med- ical Center, Freiburg, Germany. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conicts of Interest, and none were reported. Reprint requests to: Tabea V. Flugge, Division of Oral and Maxillofacial Surgery, University Medical Center Freiburg, Hugstetter Str 55, 79106 Freiburg, Germany; e-mail, tabea.viktoria.[email protected]. Submitted, November 2012; revised and accepted, April 2013. 0889-5406/$36.00 Copyright Ó 2013 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2013.04.017 471 TECHNO BYTES

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TECHNO BYTES

Precision of intraoral digital dental impressionswith iTero and extraoral digitization with theiTero and a model scanner

Tabea V. Fl€ugge,a Stefan Schlager,b Katja Nelson,c Susanne Nahles,d and Marc C. Metzgere

Freiburg and Berlin, Germany

aResidFreibubReseGermcProfeFreibudAssoVirchoeAssoical CAll auPotenReprinUnivee-maiSubm0889-Copyrhttp:/

Introduction: Digital impression devices are used alternatively to conventional impression techniques andmaterials. The aims of this study were to evaluate the precision of digital intraoral scanning under clinicalconditions (iTero; Align Technologies, San Jose, Calif) and to compare it with the precision of extraoraldigitization. Methods: One patient received 10 full-arch intraoral scans with the iTero and conventionalimpressions with a polyether impression material (Impregum Penta; 3M ESPE, Seefeld, Germany). Stonecast models manufactured from the impressions were digitized 10 times with an extraoral scanner(D250; 3Shape, Copenhagen, Denmark) and 10 times with the iTero. Virtual models provided by eachmethod were roughly aligned, and the model edges were trimmed with cutting planes to create commonborders (Rapidform XOR; Inus Technologies, Seoul, Korea). A second model alignment was then performedalong the closest distances of the surfaces (Artec Studio software; Artec Group, Luxembourg, Luxembourg).To assess precision, deviations between corresponding models were compared. Repeated intraoral scanningwas evaluated in group 1, repeated extraoral model scanning with the iTero was assessed in group 2,and repeated model scanning with the D250 was assessed in group 3. Deviations between models weremeasured and expressed as maximums, means, medians, and root mean square errors forquantitative analysis. Color-coded displays of the deviations allowed qualitative visualization of thedeviations. Results: The greatest deviations and therefore the lowest precision were in group 1, with meandeviations of 50 mm, median deviations of 37 mm, and root mean square errors of 73 mm. Group 2 showed ahigher precision, with mean deviations of 25 mm, median deviations of 18 mm, and root mean square errors of51 mm. Scanning with the D250 had the highest precision, with mean deviations of 10 mm, median deviationsof 5 mm, and root mean square errors of 20 mm. Intraoral and extraoral scanning with the iTero resulted in de-viations at the facial surfaces of the anterior teeth and the buccal molar surfaces. Conclusions: Scanningwith the iTero is less accurate than scanning with the D250. Intraoral scanning with the iTero is less accuratethan model scanning with the iTero, suggesting that the intraoral conditions (saliva, limited spacing) contributeto the inaccuracy of a scan. For treatment planning and manufacturing of tooth-supported appliances, virtualmodels created with the iTero can be used. An extended scanning protocol could improve the scanningresults in some regions. (Am J Orthod Dentofacial Orthop 2013;144:471-8)

ent, Division of Oral and Maxillofacial Surgery, University Medical Center,rg, Germany.arch fellow, Biological Anthropology, University Medical Center, Freiburg,any.ssor, Division of Oral and Maxillofacial Surgery, University Medical Center,rg, Germany.ciate professor, Division of Oral and Maxillofacial Surgery, Charite Campusw, Berlin, Germany.ciate professor, Division of Oral and Maxillofacial Surgery, University Med-enter, Freiburg, Germany.thors have completed and submitted the ICMJE Form for Disclosure oftial Conflicts of Interest, and none were reported.t requests to: Tabea V. Fl€ugge, Division of Oral and Maxillofacial Surgery,rsity Medical Center Freiburg, Hugstetter Str 55, 79106 Freiburg, Germany;l, [email protected], November 2012; revised and accepted, April 2013.5406/$36.00ight � 2013 by the American Association of Orthodontists./dx.doi.org/10.1016/j.ajodo.2013.04.017

For the introduction of computer-aided design andcomputer-aided manufacturing technologies indentistry, virtual models of teeth are required.

Digital processes are applied for prosthetic-drivenbackward planning of implant surgery,1,2 orthodonticmeasurements, and treatment planning3-6 combinedwith surgical planning.7 Data acquired by intraoralscanning, computed tomography, cone-beam computedtomography, and extraoral surface scanning can befused.1,2,7

For the acquisition of digital images of teeth,different procedures have been described: digitizationof plaster casts,3,5,8-12 digitization of impressions,8,13

and intraoral digital impressions.13,14 The accuracy

471

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Fig 1. Synopsis of model manufacturing and virtual model generation.

472 Fl€ugge et al

of the different image acquisition methods andsystems has been examined with extraoralreference models.9,12,15-18 However, to date, no studiesconcerning the practical application and precision ofdigital scanning in vivo have been done.

Digital work flow has been proposed to improve treat-ment planning, give higher efficiency, and allow newmethods of production and new treatment concepts.19-21

Data storage and reproducibility are facilitated,9,19 andtreatment documentation and communication betweenprofessionals as well as between dentists and patientshave become more convenient.22

Currently, there are a few major digital impressiondevices: iTero (Align Technologies, San Jose, Calif),Lava COS (3M ESPE, Seefeld, Germany), and Trios(3Shape, Copenhagen, Denmark) for image acquisition;and CEREC AC (Sirona, Bensheim, Germany) and E4D(D4D Technologies, Richardson, Tex) for digital imagingand in-office manufacturing.14 Excluding the iTero andthe Trios, all scanning devices need drying andpowdering of intraoral surfaces (CEREC, E4D, LavaCOS). This limits their practicability and accuracybecause powder application can add to the measuringerror.23 With all devices mentioned, digital impressionsare acquired without contact to the gingival tissues.

The precision of intraoral and extraoral scanningwith the iTero as well as extraoral scanning with a modelscanner was examined in this study.

MATERIAL AND METHODS

Impressions were acquired according to the studyprotocol that was approved by the ethics committee ofthe medical faculty of Freiburg University, after wereceived written consent from the study participant.One participant with a Class I occlusion and full denti-tion was examined.

In-vivo (intraoral) scans (group 1) and ex-vivo(extraoral) scans of 1 patient and the patient’s models

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were made with 2 laser scanners (group 2, iTero; group3, D250; Fig 1).

According to the study protocol, the data acquisitionfor group 1 was based on 10 intraoral scans with theiTero of the maxilla and the mandible of 1 patient(n 5 20).

For groups 2 and 3, a unique model was used. Themodel fabrication for the extraoral scans was performedas follows.

Using the same patient as in group 1, polyether im-pressions of the maxilla and the mandible were taken us-ing a monophase polyether material (Impregum Penta;3M ESPE) and stainless steel impression trays(M1W-Rim-Lock; M1W Dental, B€udingen, Germany).The impressions were disinfected and poured with typeIV stone (picodent U 180; Picodent, Wipperf€urth,Germany) after a setting time of 4 hours. The firstimpression was used for the production of the stonecasts, independently of the subjective assessment ofthe quality. One stone cast of the maxilla and 1 stonecast of the mandible were made.

The stone casts were scanned with the iTero using thesame scanning protocol as for the intraoral scans. Thescans of each stone cast model (maxilla and mandible)were repeated 10 times and produced the data set forgroup 2 (n 5 20).

The virtual models for group 3 (n 5 20) werecollected by repeated scanning (10 times) of the stonecasts with a model scanner (D250).

All scans with the iTero were recorded by the sameexaminer (T.V.F.) in a predetermined order. Scanningstarted with the most distal tooth in the third quadrantcontinuing to the anterior teeth (Figs 2 and 3). Next,the fourth quadrant was scanned, again beginningwith the most distal tooth. Scanning of the maxillastarted with the most distal tooth in the first quadrantand ended at the central incisor. The second quadrantwas recorded starting with the most distal tooth. Eachtooth was scanned from its buccal and lingual sides by

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Fig 2. Intraoral scanning and the iTero-rendered stereolithographic model of the scanned jaw.

Fig 3. Extraoral scanning of a stone cast and the iTero-rendered stereolithographic model of the cast.

Fl€ugge et al 473

placing the camera at an angle of 45� to the tooth axis.Images of each tooth showed neighboring parts of adja-cent teeth. These served to overlap the pictures to createa model of the whole arch from single images. All modelswere exported in a Standard Tesselation Language (STL;3D Systems, Rock Hill, SC) format and were used forevaluation, independently of the subjective assessment.

For digitization, the stone casts were placed intothe D250 scanner next to a laser source and 2 high-resolution cameras. During the scanning process, theplatform moves the model; therefore, the laser reachesthe model from multiple angles. Light planes areprojected onto the model, and the cameras capture theirreflections from the surface (Fig 4).

The principle of triangulation was used for thecreation of a 3-dimensional model, available as a stereo-lithographic data set.

All stereolithographic data sets of 1 dental arch and 1scanning method (n 5 10) were imported in a commoncoordinate system and aligned by a procedure with theclosest distance between 2 surfaces (Rapidform XOR;Inus Technologies, Seoul, South Korea). The modelswere orientated toward the occlusal plane to fit a view

American Journal of Orthodontics and Dentofacial Orthoped

for drawing the cutting planes. A first cutting planerunning through the deepest point of the gingival sulcusof the canines and the second molars in the maxilla andthe canines and the first molars in the mandible wascreated. A second cutting plane was created runningthrough the transverse fissure of both second molars.All surfaces were cut with the common cutting planesto create equal basal and posterior borders (Fig 5).

For quantitative analysis of precision, deviations be-tween the vertices of the surfacesweremeasured. Operatedscan data were imported into Artec Studio software(version0.7.3.39;ArtecGroup, Luxembourg, Luxembourg)to perform a pairwise rigid body registration. Correspond-ing models for each comparison were roughly alignedmanually and then registered onto another using theimplemented surface mapping algorithm.

Deviations between aligned models were analyzedusing the software package Morpho, version 0.25 (basedon R; created by Stefan Schlager, Freiburg, Germany).24

To estimate the differences between the surfaces, eachvertex on the test surface was projected to the closestpoint on the corresponding control surface, and theEuclidean distance was recorded. The model rendered

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Fig 5. Stereolithographic data set obtained from scan-ning with reference planes (reference planes 1 to 3) forcutting, displayed with Rapidform XOR.

Fig 4. Scanning of a plaster cast with the D250 and rendered stereolithographic model of the cast.

Fig 6. Colored presentation of the deviations betweensurfaces in group 1.

474 Fl€ugge et al

from the first scan served as the control surface for theconsecutively acquired models in each group.

Statistical analysis

Further statistical analysis was performed withthe software R.25 For testing differences betweenthe groups’ distributions of averaged distances, theKolmogorov-Smirnov test—a distribution free andnonparametric procedure—was applied. The level ofsignificance was set at 0.05.

For the assessment of error, maximum, mean, andmedian deviations were calculated for each group basedon the averaged errors of each observation. Thedistances between the vertices of the correspondingmodels were displayed with color maps, so that areasof high and low agreement could be identified.

RESULTS

In group 1, the virtual models, rendered from serialintraoral scans of the maxilla and the mandible withthe iTero, were compared. The mean deviation was50 mm (median, 37 mm). Deviations in the maxilla were

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on average 57 mm (median, 43 mm); the mandibledeviated on average 43 mm (median, 31 mm). Maximumdeviations were on average 1.137 mm in the maxilla and717 mm in the mandible. Deviations of the mandiblewere significantly lower than deviations in the maxilla.Deviations between models are displayed in Figure 6.

The highest deviations were observed at the palatalborders, the facial surfaces of the anterior teeth, and themolars on both sides of the maxilla. In the mandible, thehighest deviations were at the buccal side of the molarsand the facial side of the anterior teeth. There were alsodeviations above average in the interdental spaces.

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Fig 7. Colored presentation of the deviations betweensurfaces in group 2.

Fig 8. Colored presentation of the deviations betweensurfaces in group 3.

Fig 9. Colored scale for the deviations shown in Figures 6, 7, and 8 (mm).

Fl€ugge et al 475

In group 2, the virtual models acquired with therepeated scans of the stone casts with the iTero werecompared. Deviations from the test surface were onaverage 25 mm (median, 18 mm). The maxillae had amean deviation of 30 mm (median, 18 mm), whereas themandibles had a mean deviation of 21 mm (median, 17mm). The color-coded deviations are depicted in Figure 7.

The highest deviations in the maxilla were found atthe facial surfaces of the anterior teeth in the left maxillaand at the palatal borders of the models. The maximumdeviation was 1.79 mm in the maxilla. The mandible hadlower deviations and a homogenous distribution withabove-average values in the molar region and thegingival sulcus. The maximum deviation averaged423 mm. Deviations in the mandible were significantlylower than in the maxilla.

In group 3, the deviations between the virtual modelsrendered from repeated model scanning with theD250 were compared. Models deviated on average

American Journal of Orthodontics and Dentofacial Orthoped

10 mm (median, 6 mm). Average deviations were 11 mmin the maxilla and 9 mm in the mandible. The maximumdeviation was 460 mm for the maxilla and the mandible.The color-coded values of the deviations are shown inFigure 8.

Deviations in group 3 were lower than deviations ingroups 1 and 2. The areas of deviations showed similarpatterns in the maxilla and the mandible. The inter-dental spaces had the greatest deviations between thevirtual models. Deviations in the mandible were signif-icantly lower than those in the maxilla. The color scale(Fig 9) shows the deviations in Figures 6, 7, and 8. Themean deviations in all groups are displayed in theTable.

The Kolmogorov-Smirnov test showed that theoverall deviations were significantly different for everymethod.

In Figure 10, the boxplot diagram shows thedistribution of the deviations for all methods.

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Table. Mean deviation of each method (mm)

iTero intraoral iTero extraoral D250Mean deviation 50 25 10

476 Fl€ugge et al

DISCUSSION

The reason for the use of intraoral digital impressionsystems is adequate accuracy and precision comparedwith conventional techniques and extraoral digitizationof stone casts. Ender and Mehl18 defined accuracy as adeviation from the original object and precision as theaccuracy of repeated measurements.

The precision of intraoral scanning was evaluated inthis study and compared with extraoral digitization ofstone casts with the iTero and a model scanner (D250).The accuracy of the different work flows to create a vir-tual model was not our objective. The systematic errorscaused by impression taking and model manufacturingare neither included nor relevant for the present data.

Polyether-based stone casts served as an extraoralreference. All scans were conducted with the samemodel. Scanning with the model scanner D250 has adifferent image acquisition technique compared withintraoral scanners. The model is continuously capturedwith the projection of laser planes and the recording oftheir reflections. With all intraoral scanning techniques,image acquisition is done incrementally. The iTerosystem produces single images of every tooth, whichare assembled for a virtual model of the whole jaw.This process, called stitching, might produce a system-atic error.26 Mehl et al27 found lower accuracy ofquadrant digitization compared to single-toothdigitization with the CEREC system. Because the stitch-ing algorithm of the iTero system is unknown, itscontribution to the error in precision cannot beexplained. However, lower precision of the iTerocompared with the D250 was observed in this study.

Loss of information at model edges, especially in themaxilla, was observed with the iTero system. Thisresulted in high deviation values. According to thescanning protocol, a fixed number of pictures is acquiredof every tooth. The image section of the camera coversthe tooth and, depending on its anatomy, a variableportion of the gingiva. The scans show that the imageacquisition of the marginal gingiva could not beprecisely reproduced in the maxilla. An extendedprotocol, resulting in a longer scanning time, might benecessary to obtain complete information.

The facial surfaces of the anterior teeth wereimprecisely captured with the iTero in intraoral andextraoral digitizations (Figs 6 and 7). Although theyappear to be easily accessible with the scanning wand,

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the steep-angled anterior surface might requireadditional scans from different angles, as Mehl et al27

have already suggested for steep areas.The imprecise digitization of the molar areas with the

iTero in extraoral use was more pronounced for intraoraluse. This might be caused by the complex angledsurfaces of the molars and the undercut surfaces ofthe neighboring teeth. This theory is supported byRudolph et al,15 who used different methods fordigitization of an extraoral reference model to showthat tooth shape was a dominating factor for precisionand that large deviations occurred in areas with strongchanges of curvature. The precision of extraoral modelscanning with the D250 was not lower in areas of highcurvature and undercuts. The continuous imageacquisition with laser planes captures all areas of themodel precisely, except for interdental spaces, thataccounted for the overall imprecision. The deviationsof the group 3 models were on average 10 mm. Thisagrees with the study of Persson et al.28

Despite the identical scanning protocol, the precisionfor extraoral scanning with the iTero (25 mm) was higherthan for intraoral scanning with the iTero (50 mm). Thismight be due to patient movement, limited intraoralspace, intraoral humidity, and saliva flow. Highdeviations in intraoral scans of the molar areas indicatethat patient-related factors had a strong influence onscanning quality. In comparison, the reduced valuesobtained with the extraoral iTero scanning might resultfrom greater freedom of placement of the scanningwand next to the model teeth.

Numerous in-vitro studies have shown that prerequi-sites for clinical use of intraoral and extraoral scanningdevices are met.9,12,15-17 However, the precision ofscanning devices under intraoral conditions has notbeen documented to date. This study shows that theiTero system in vivo and ex vivo can be used tocreate virtual models for diagnostics and treatmentplanning in orthodontics. To manufacture orthodonticappliances on the basis of the virtual models, not onlythe scanning process but also the production processmust be considered. Computer-aided design andcomputer-aided manufacturing on the basis of virtualmodels created with data from the iTero intraoral scansare accompanied by inaccuracies, especially when thedepiction of facial surfaces of anterior teeth and molarsor marginal soft tissues is important for the appliance(eg, aligners, customized brackets). Extraoral scanningtechniques show higher precision and therefore allowhigher accuracy of applicances built with computer-aided design and computer-aided manufacturing. How-ever, the inaccuracies of the impression must be addedto all laser scanning data acquired ex vivo.

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Fig 10. Boxplot of the deviations for every method (outliers are hidden).

Fl€ugge et al 477

We showed that intraoral scanning with the iTero wasless precise than extraoral scanning and digitization withthe D250, which is still the most precise digitizationmethod currently available. The precision of the intraoraliTero scan is similar to the values documented in theliterature with conventional polyether impressions(61.3 6 17.9 mm) for reproduction of the intraoralsituation. The in-vitro precision for the CEREC(30.9 6 7.1 mm) was comparable with that of theiTero under extraoral conditions (25 mm) in our study.18

The in-vitro precision of the Lava-COS system(60.1 6 31.3 mm) was lower than the in-vitro andin-vivo precision of the iTero.18 With CEREC and iTero,the images are recorded in a static position relative tothe tooth, whereas the Lava COS captures images whilethe scanning wand is constantly moving; this mightresult in lower precision for this technique.

Our results indicate that the positions of teeth andtheir surfaces can be reproduced in a virtual model,but soft-tissue reproduction was imprecise.

The image acquisition technique of the iTero doesnot require the application of a scanning powder. Theprecision of the iTero might be higher compared withthe CEREC and the Lava-COS, since powder application,necessary for these systems, produces a layer of variablethickness.23

CONCLUSIONS

Intraoral scanning with the iTero is less precise thanmodel scanning with it. Therefore, patient-relatedfactors influence the scanning process. Scanning of themaxilla is less accurate than scanning of the mandible.An extended scanning protocol might improve thescanning results in the maxilla. Because of its technical

American Journal of Orthodontics and Dentofacial Orthoped

features, extraoral scanning has the highest precision.For treatment planning and manufacturing of tooth-supported appliances, virtual models created with theiTero can be used.

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Journal of Orthodontics and Dentofacial Orthopedics