Accuracy of complete-arch dental impressions: a new method of measuring trueness and precision.

TLDR: The new reference scanner is capable of measuring the precision and trueness of both digital and conventional complete-arch impressions, and the digital impression is less accurate and shows a different pattern of deviation than the conventional impression.

Abstract: STATEMENT OF PROBLEM: A new approach to both 3-dimensional (3D) trueness and precision is necessary to assess the accuracy of intraoral digital impressions and compare them to conventionally acquired impressions. PURPOSE: The purpose of this in vitro study was to evaluate whether a new reference scanner is capable of measuring conventional and digital intraoral complete-arch impressions for 3D accuracy. MATERIAL AND METHODS: A steel reference dentate model was fabricated and measured with a reference scanner (digital reference model). Conventional impressions were made from the reference model, poured with Type IV dental stone, scanned with the reference scanner, and exported as digital models. Additionally, digital impressions of the reference model were made and the digital models were exported. Precision was measured by superimposing the digital models within each group. Superimposing the digital models on the digital reference model assessed the trueness of each impression method. Statistical significance was assessed with an independent sample t test (α=.05). RESULTS: The reference scanner delivered high accuracy over the entire dental arch with a precision of 1.6 ±0.6 µm and a trueness of 5.3 ±1.1 µm. Conventional impressions showed significantly higher precision (12.5 ±2.5 µm) and trueness values (20.4 ±2.2 µm) with small deviations in the second molar region (P<.001). Digital impressions were significantly less accurate with a precision of 32.4 ±9.6 µm and a trueness of 58.6 ±15.8µm (P<.001). More systematic deviations of the digital models were visible across the entire dental arch. CONCLUSIONS: The new reference scanner is capable of measuring the precision and trueness of both digital and conventional complete-arch impressions. The digital impression is less accurate and shows a different pattern of deviation than the conventional impression.

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Year:2013
Accuracyofcomplete-archdentalimpressions:anewmethodofmeasuring
truenessandprecision
Ender,Andreas;Mehl,Albert
Abstract:STATEMENTOFPROBLEM:Anew approachtoboth3-dimensional(3D) truenessand
precisionisnecessarytoassesstheaccuracyofintraoraldigitalimpressionsandcomparethemtocon-
ventionallyacquiredimpressions.PURPOSE:Thepurposeofthisinvitrostudywastoevaluatewhether
anewreferencescanneriscapableofmeasuringconventionalanddigitalintraoralcomplete-archimpres-
sionsfor3Daccuracy.MATERIALANDMETHODS:Asteelreferencedentatemodelwasfabricated
andmeasuredwithareferencescanner(digitalreferencemodel).Conventionalimpressionsweremade
fromthereferencemodel,pouredwithTypeIVdentalstone,scannedwiththereferencescanner,and
exportedasdigitalmodels.Additionally,digitalimpressionsofthereferencemodelweremadeandthe
digitalmodelswereexported. Precisionwasmeasuredbysuperimposingthedigitalmodelswithineach
group.Superimposingthedigitalmodelsonthedigitalreferencemodelassessedthetruenessofeach
impressionmethod.Statisticalsignicancewasassessedwithanindependentsamplettest(฀=.05).RE-
SULTS:Thereferencescannerdeliveredhighaccuracyovertheentiredentalarchwithaprecisionof1.6
±0.6µmandatruenessof5.3±1.1µm.Conventionalimpressionsshowedsignicantlyhigherprecision
(12.5±2.5µm)andtruenessvalues(20.4±2.2µm)withsmalldeviationsinthesecondmolarregion
(P<.001).Digitalimpressionsweresignicantlylessaccuratewithaprecisionof32.4±9.6µmanda
truenessof58.6±15.8µm(P<.001).Moresystematicdeviationsofthedigitalmodelswerevisibleacross
theentiredentalarch.CONCLUSIONS:Thenewreferencescanneriscapableofmeasuringtheprecision
andtruenessofbothdigitalandconventionalcomplete-archimpressions.Thedigitalimpressionisless
accurateandshowsadierentpatternofdeviationthantheconventionalimpression.
DOI:https://doi.org/10.1016/S0022-3913(13)60028-1
PostedattheZurichOpenRepositoryandArchive,UniversityofZurich
ZORAURL:https://doi.org/10.5167/uzh-88978
JournalArticle
AcceptedVersion
Originallypublishedat:
Ender,Andreas;Mehl,Albert(2013).Accuracyofcomplete-archdentalimpressions: anewmethodof
measuringtruenessandprecision.JournalofProstheticDentistry,109(2):121-128.
DOI:https://doi.org/10.1016/S0022-3913(13)60028-1

Dental full arch impression, a new high accurate method of measuring trueness
and precision
A. Ender, A. Mehl
Clinic of Preventive Dentistry, Periodontology and Cariology; Division of Computerized Restor-
ative Dentistry, University of Zurich, Zurich, Switzerland
Abstract
Objectives: With the digital intraoral impression a new class of impression techniques is
introduced. To assess the accuracy of these impressions and compare it to conventio-
nal intraoral impression technique threedimensional trueness and precision measure-
ments are necessary. The aim of this study was to evaluate wether a new reference
scanner is capable to measure these threedimensional deviations occuring with conven-
tional and digital intraoral impression techniques of full arch impressions of an in vitro
model.
Methods: A steel reference model was scanned with the reference scanner to evaluate
precision and trueness. The reference model then was used to perform five conventio-
nal impressions with a polyvinylsiloxanether material (Identium, Kettenbach) in a putty
and wash technique with standard stock trays (ASA Permalock, ASA Dental). The con-
ventional impressions were poured with Type IV stone (CamBase, Dentona) and scan-
ned with the reference scanner. Five digital impressions with a optical intraoral scanning
system (CEREC AC, Sirona) were made. In each group, the models were superimposed
and the differences computed with a signed nearest neighbour method. The 90-10%/2
percentile of the differences from each comparison was taken to compute the mean va-

lue for precision. The trueness of each impression method was assessed through su-
perimposition of the impressions with the refefence scan of the steel reference model.
Results: The reference scanner delivers an accuracy with 1.6±0.6 µm for precision and
5.3±1.1 µm for trueness over a full dental arch scan. The conventional impression me-
thod shows significant higher (p<0.001) precision (12.5±2.5 µm) and trueness (20.4±2.2
µm) with only little amount of larger deviations at the second molar region. The digital
impression method was significantly less accurate (p<0.001) with a precision of
32.4±9.6 µm and trueness of 58.6±15.8 µm. More systematic deviation of the digital
model is visible across the entire dental arch.
Significance:The new reference scanner is capable to measure precision and trueness
of both conventional and digital full arch impressions. The digital intraoral impression is
less accurate and shows a complete different pattern of deviation than the conventional
impression method.

Introduction
Dental impressions are a major step in restorative dentistry. They transfer the intraoral
situation of the preparation, the adjacent teeth, the soft tissue and the antagonist to an
extraoral model. This model is then used to produce the final restoration.
The accuracy of this model influences the fit or the restorations which is a major factor
for longevitiy of the restoration (Wettstein, Sailer, Roos, & Hammerle, 2008)(Perakis,
Belser, & Magne, 2004)(Persson, ODEN, ANDERSSON, & Sandborgh-Englund, 2009).
Todays gold standard impression technique ist the physical impression with elastomeric
impression material and stock trays (CI). This negative form of the patients teeth is then
poured with stone, resulting in a physical gypsum model. Various techniques are
described in literature in order to archieve most accurate results (Piwowarczyk, Ottl,

Buchler, Lauer, & Hoffmann, 2002)(Chandran, D. Jagger, R. Jagger, & Barbour, 2010).
With the development of CAD/CAM systems and especially the use of zirkoniumdioxid
for dental restorations, the digital model becomes more and more important. Parts of the
gypsum model, at least the preparation, had to be digitized with an extraoral scanner to
create a digital threedimensional model. On this digital model, the restoration is desig-
ned on a computer with special design software and then milled out of a material block
in a CNC milling machine (Luthardt, Sandkuhl, Herold, & Walter, 2001). After milling, the
dental technician finishes the restoration at the gypsum model. The latest development
in CAD/CAM dentistry is the digital intraoral impression (DI)(Fasbinder, 2010)(Beuer,
Schweiger, & Edelhoff, 2008). The digital intraoral impression(DI) results in a threedi-
mensional virtual model from scanning the patients teeth directly inside the mouth.
Construction and milling can be carried out without a conventional intraoral impression
(CI) with the gypsum model and the following extraoral digitization. If needed, a physical
model can be by rapid prototyping (SLA, 3D-printing or miling) from the intraoral digital
impression data (Fasbinder, 2010).
A fundamental question, beside the clinical handling of the devices for the digital intra-
oral impression (DI) and the ease of the following steps in the digital workflow, is the ac-
curacy of this new impression technique.
Accuracy consists of precision and trueness (ISO 5725-1). Precision describes, how
close repeated measurements are to each other. The higher the precision, the more
predictable is the measurement. Trueness describes, how far the measurement is from

Frequently asked questions

Q1. What are the contributions in this paper?

PURPOSE: The purpose of this in vitro study was to evaluate whether a new reference scanner is capable of measuring conventional and digital intraoral complete-arch impressions for 3D accuracy.