1 - A comparison of the precision of three-dimensional.pdf

A comparison of the precision of three-dimensional
images acquired by 2 digital intraoral scanners:
effects of tooth irregularity and scanning direction
Objective: The purpose of this study was to compare the precision of three-
dimensional (3D) images acquired using iTero® (Align Technology Inc., San
Jose, CA, USA) and Trios® (3Shape Dental Systems, Copenhagen, Denmark)
digital intraoral scanners, and to evaluate the effects of the severity of tooth
irregularities and scanning sequence on precision. Methods: Dental arch models
were fabricated with differing degrees of tooth irregularity and divided into
2 groups based on scanning sequence. To assess their precision, images were
superimposed and an optimized superimposition algorithm was employed to
measure any 3D deviation. The t-test, paired t-test, and one-way ANOVA were
performed (p < 0.05) for statistical analysis. Results: The iTero® and Trios®
systems showed no statistically significant difference in precision among models
with differing degrees of tooth irregularity. However, there were statistically
significant differences in the precision of the 2 scanners when the starting
points of scanning were different. The iTero® scanner (mean deviation, 29.84 ±
12.08 mm) proved to be less precise than the Trios® scanner (22.17 ± 4.47 mm).
Conclusions: The precision of 3D images differed according to the degree of
tooth irregularity, scanning sequence, and scanner type. However, from a clinical
standpoint, both scanners were highly accurate regardless of the degree of tooth
irregularity.
[Korean J Orthod 2016;46(1):3-12]
Key words: Three-dimensional scanner, Digital models, Dental cast analysis,
Three-dimensional diagnosis and treatment planning
Ji-won Anha
Ji-Man Parkb
Youn-Sic Chuna
Miae Kimc
Minji Kima
a
Department of Orthodontics, Graduate
School of Clinical Dentistry, Ewha
Womans University, Seoul, Korea
b
Department of Prosthodontics and
Dental Research Institute, Seoul
National University Gwanak Dental
Hospital, Seoul, Korea
c
Department of Pediatric Dentistry,
Graduate School of Clinical Dentistry,
Ewha Womans University, Seoul, Korea
Received March 18, 2015; Revised July 17, 2015; Accepted July 22, 2015.
Corresponding author: Minji Kim.
Assistant Professor, Department of Orthodontics, Graduate School of Clinical Dentistry,
Ewha Womans University, 1071 Anyangcheon-ro, Yangcheon-gu, Seoul 07985, Korea.
Tel +82-2-2650-5112 e-mail minjikim@ewha.ac.kr
*This study was supported by research fund from Korean Association of Orthodontist in 2013.
3
© 2016 The Korean Association of Orthodontists.
The authors report no commercial, proprietary, or financial interest in the products or companies
described in this article.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and
reproduction in any medium, provided the original work is properly cited.
THE KOREAN JOURNAL of
ORTHODONTICS
Original Article
pISSN 2234-7518 • eISSN 2005-372X
http://dx.doi.org/10.4041/kjod.2016.46.1.3

Anh et al • The precision of digital intraoral scanners
www.e-kjo.org
4 http://dx.doi.org/10.4041/kjod.2016.46.1.3
INTRODUCTION
The use of plaster casts as representative dental models
poses significant problems, including the possibility
of deformation depending on the type of impression
material, the chance of loss or damage during storage,
and storage space constraints.1-3
To overcome these
shortcomings, an alternative method using an intraoral
scanner has been devised to produce three-dimensional
(3D) digital models. These models offer several
advantages, including easy storage and the possibility of
computerized data acquisition and modeling.4-9
Digital
impression methods performed using intraoral scanners
also completely eliminate the conventional impression-
taking process using impression materials. Additionally,
they reduce the gag reflex of the patient and are more
comfortable, as breathing is freely permitted during the
impression-taking process.10,11
With the introduction of multiple intraoral scanners,
numerous studies on their accuracy have also been
performed. The accuracy of a scanner can be defined
by 2 parameters: trueness and precision (ISO 5725-
1, DIN 55350-13).12
Trueness relates to the ability of
the scanner to reproduce a dental arch as closely to
its true form as possible and without deformation or
distortion, while precision indicates the degree to which
images acquired by repeated scanning under the same
conditions are identical, and has the same meaning
as reproducibility.13,14
Wiranto et al.15
and Naidu and
Freer16
compared differences in tooth widths between
images acquired by intraoral scanning and from a
plaster cast model using a digital caliper to measure
the accuracy of intraoral scanners. Their studies showed
that measurements from the digital image and plaster
cast model were similar, indicating a high level of
accuracy. However, if the lengths of the plaster cast
models are used as the control group, errors in the cast
manufacturing process and inaccuracies originating from
handling the caliper should be taken into consideration.
In addition, it is difficult to set reference points for the
measurement of such lengths.
Recently, to overcome the disadvantages of traditional
measurement methods, the superimposition of digital
images using a software program has been introduced
as a method to determine scanner accuracy. In this
method, the accuracies of images are compared by
calculating deviations from the generated superimposed
images.13,14,17
Nevertheless, to date, studies comparing
the accuracy of 3D images have mostly concerned either
preparation models of single teeth for prostheses,18
the
analysis of partial dental arches,19
and the analysis of
ideal full dental arches.13,14,17
They have not included
cases with irregular tooth alignment, which are
frequently observed in dental clinics. Crowding may
cause undercut areas that appear as holes in the 3D
model, and smoothing these areas with software tools
can lead to errors.
Therefore, this study aimed to investigate whether
tooth alignment and scanning sequence affect the
precision of 3D images by scanning 4 models with
various degrees of dental crowding in 2 directions and
using 2 types of scanners. Moreover, by comparing
sections of superimposed images of the models,
differences in image precision were investigated
depending on location in the arch.
MATERIALS AND METHODS
Dental arch models
Maxillary models were fabricated with resin teeth
(ZYR-7008; Jining Xingxing Medical Instrument, Jining,
China) and a resin base (Ortho-Jet Liquid & Acrylic
powder; Lang Dental Manufacturing Co. Inc., Wheeling,
IL, USA).
Four types of dental arch models were fabricated with
differing arch length discrepancies (ALDs). The degree
of dental crowding of the models was expressed in
terms of ALD, which is normally used for the assessment
of available space in Class I malocclusions. Model C1
represented ideal arch dentition (ALD, 0 mm); model
C2 represented mild crowding (ALD, 3 mm); model C3
represented moderate crowding (ALD, 7 mm); and model
C4 represented severe crowding (ALD, 10 mm) (Figure 1).
To minimize the influence of molar alignment and
differences in the shape and size of the arch, the second
premolar and first and second molars were arranged at
the same positions and widths in all 4 models by using
a putty index to hold the molars in identical positions.
Dental crowding was randomly induced by repositioning
only the first premolars, the canines, and the lateral and
central incisors. Therefore, crowding was expressed only
by anteroposterior and buccolingual displacement, and
did not include deviations in vertical height.
Metal balls 1.5 mm in diameter were attached at 5
locations on the model base as fiducials to determine
scanner accuracy. The posterior fiducials were located
4 mm below the dentogingival junction and below the
mesiobuccal cusp of the first molars on both sides;
the anterior fiducials were located 4 mm below the
dentogingival junction and vertical to the contact point
between the left and right central incisors. The fiducials
in the canine region were located directly below the
occlusal surface at the cusp tips of the canine teeth on
both sides, and at the same height as the fiducials on
the first molars and central incisors (Figure 1).
3D digital intraoral scanners and scanning procedures
Two types of 3D digital intraoral scanners, the iTero®

www.e-kjo.org 5
C1 C2
C3 C4
Figure 1. Dental arch models according to the severity of tooth irregularity expressed as arch length discrepancy (ALD).
C1, Ideal arch dentition (ALD, 0 mm); C2, mildly crowded dentition (ALD, 3 mm); C3, moderately crowded dentition (ALD,
7 mm); C4, severely crowded dentition (ALD, 10 mm). Fiducials with a diameter of 1.5 mm were attached at 5 locations
on the model base located 4 mm below the dentogingival junction. Two posterior fiducials were located below the
mesiobuccal cusp of the first molars and 1 anterior ball marker was located below the contact point between the central
incisors. Two canine fiducials were located directly below the cusp tips of the right and left canine teeth.
Face 3 Face 4
Face 1
Face 2
RM
RP
RA LA
LP
LM
1
1
2
3
4
5
6
3
2
A B C
Figure 2. Scanning sequences and reference faces for model division and the model sections. A, The Group Right sequence
on the iTero®. The right molars were scanned first in the order of the occlusal side ①, buccal side ②, and palatal side
③; subsequently, the left molars were scanned in the same fashion in the order of ④, ⑤, and ⑥. Both scanned images
were merged and connected at the anterior area. The scanning sequence for Group Left first scanned the left molars in
reverse. B, The sequence for Group Right on the Trios®. Scanning started from the occlusal side of the right molars and
proceeded toward the left occlusal side ①, continued along the buccal side in the reverse direction ②, and lastly, from
the palatal side of the right molars, proceeding toward the palatal side of the left molars ③. The scanning sequence for
Group Left first scanned the left side in reverse. C, The reference faces for model division and the model sections. Face 1 is
a plane passing through the posterior fiducials on both sides, Face 2 is a plane passing through the anterior ball marker
and perpendicular to Face 1, and Faces 3 and 4 are planes passing the fiducials in the canine region and the intersection
point of Faces 1 and 2.
The scanned images were divided into 6 sections: RM, right molar; RP, right premolar; RA, right anterior; LM, left molar;
LP, left premolar; LA, left anterior.

www.e-kjo.org
(Align Technology Inc., San Jose, CA, USA) and the
Trios® (3Shape Dental Systems, Copenhagen, Denmark),
were used in this study. Scanning was performed
following the manufacturer’s instructions, and the
models were divided into 2 groups based on scanning
sequence.
For the iTero®, Group Right indicated that the right
molars were scanned first, followed by the left molars,
and the scanned images from both sides were merged
and connected at the anterior area. In Group Left, the
left molars were scanned first, followed by the right
molars, and the scanned images were similarly merged
and connected.
For the Trios®, Group Right indicated that scanning
started from the right molars and proceeded toward
the left occlusal surface, and was completed at the left
lingual surface. In Group Left, the direction of scanning
was reversed; scanning started from the left molars and
proceeded toward the right occlusal surface, and was
completed at the right lingual surface (Figure 2).
The dental models were scanned from both the buccal
and lingual aspects toward the model base; however,
the bottom surface was not scanned. On the basis of
the starting point of the scan, the images were classified
as C1R, C2R, C3R, or C4R when the right molars were
scanned first; and as C1L, C2L, C3L, or C4L when the
left molar were scanned first.
A single researcher with sufficient training in scanner
operation scanned a total of 16 groups (C1R−C4R and
C1L−C4L on the iTero® and Trios®, respectively) 6 times
(n = 6 for each group).
The superimposition of scans and the measurement of
deviations
For the purpose of calculating precision, 6 of the
scanned images were paired, resulting in 15 pairs
of scanned data that were then superimposed. The
superimposition was performed using a best-fit
algorithm in Geomagic VerifyTM
software (3D Systems
Inc., Rock Hill, SC, USA), which is point cloud inspection
software that can align a pair of mesh geometries. This
method is expressed using a color map that shows the
deformed parts of the entire arch. The superimposed
arches were divided into 6 sections, and the precision
was measured in each: RM (right molar), RP (right
premolar), RA (right anterior), LM (left molar), LP (left
premolar), and LA (left anterior), as well as in the entire
arch (Figure 2C).
Statistical analysis
Collected data were analyzed using IBM SPSS
Statistics ver. 19.0 (IBM Co., Armonk, NY, USA). The
precision between groups with varying degrees of tooth
irregularity and the precision between sections within
Table
1.
Comparisons
of
deviations
in
images
from
the
iTero®
and
Trios®
scanners
according
to
severity
of
tooth
irregularity
(units,
mm)
Section
iTero®
Trios®
C1
C2
C3
C4
p
-value
C1
C2
C3
C4
p
-value
RM
35.94
(20.40)
b
40.02
(16.35)
b
37.93
(19.03)
b
46.67
(22.18)
b
0.172
40.64
(19.56)
Bd
31.68
(10.24)
Ac
32.90
(17.37)
ABc
29.45
(13.14)
Ac
0.036*
RP
22.89
(6.98)
a
26.08
(10.12)
a
23.07
(9.12)
a
25.66
(10.40)
a
0.402
23.84
(6.25)
b
21.65
(5.70)
b
19.81
(6.59)
ab
20.60
(6.76)
ab
0.083
RA
23.29
(11.85)
a
20.00
(5.43)
a
20.63
(8.51)
a
23.50
(7.81)
a
0.284
17.23
(4.20)
Ca
15.88
(1.93)
BCa
13.90
(2.81)
Aa
15.11
(1.94)
ABa
0.000*
LA
22.62
(7.79)
a
18.63
(6.03)
a
21.96
(9.02)
a
22.17
(6.18)
a
0.141
18.40
(5.04)
ab
16.82
(4.01)
a
16.11
(2.91)
a
17.20
(3.84)
a
0.170
LP
23.70
(8.04)
a
25.40
(13.15)
a
24.2
7
(13.60)
a
28.04
(11.75)
a
0.500
22.65
(8.43)
ab
24.71
(7.43)
b
24.73
(9.11)
b
24.92
(7.85)
bc
0.678
LM
38.42
(17.47)
b
40.13
(22.50)
b
35.63
(22.28)
b
45.89
(25.42)
b
0.331
34.41
(17.38)
c
37.25
(17.60)
d
38.95
(22.92)
c
37.46
(18.72)
d
0.832
Total
28.16
(10.24)
29.56
(11.75)
28.44
(13.08)
33.20
(12.96)
0.347
23.77
(4.74)
21.87
(3.46)
21.04
(4.66)
22.02
(4.67)
0.115
Values
are
presented
as
mean
(standard
deviation).
iTero®:
Align
Technology
Inc.,
San
Jose,
CA,
USA;
Trios®:
3Shape
Dental
Systems,
Copenhagen,
Denmark.
*Statistically
significant
difference
among
the
models
by
one-way
ANOVA
(
p
<
0.05,
Duncan’s
post
hoc
test:
A
<
B
<
C).
a,b,c,d
Statistically
significant
difference
among
the
sections
(RM,
RP,
RA,
LA,
LP,
and
LM)
of
all
models
by
one-way
ANOVA
(
p
<
0.005,
Duncan’s
post
hoc
test;
a
<
b
<
c
<
d).
C1,
Ideal
arch
dentition;
C2,
mildly
crowded
dentition;
C3,
moderately
crowded
dentition;
C4,
severely
crowded
dentition;
RM,
the
right
molar
region;
RP,
the
right
premolar
region;
RA,
the
right
anterior
region;
LA,
the
left
anterior
region;
LP,
the
left
premolar
region;
LM,
the
left
molar
region;
total,
the
entire
arch.

www.e-kjo.org 7
each group were assessed by one-way ANOVA and
Duncan’s post hoc analysis. The t-test was also used to
compare precision among different scanning sequences
and between scanners. The level of significance was set
at p < 0.05.
RESULTS
Precision comparison according to the severity of tooth
irregularity
For the iTero®, there was no statistically significant
difference in the deviation of any section (RM, RP, RA,
LA, LP, or LM) or the whole arch (total) between models
(p > 0.05). Thus, there was no significant difference in
precision among dentitions with varying amounts of
dental crowding. Additionally, deviations of the molar
regions (LM and RM) were significantly lower than
deviations of the incisal and premolar regions (p < 0.05)
(Table 1). The superimposition of the scanned iTero®
images was expressed as a color map (Figure 3).
For the Trios®, there were statistically significant
differences in the deviations of RM and RA between
models (p < 0.05). Model C1 displayed the greatest
deviations of 40.64 ± 19.56 mm and 17.23 ± 4.20
mm in the RM and RA sections, respectively. However,
there was no statistically significant difference in the
A
B
Figure 3. A color presentation
of the deviations between
surfaces from the iTero® scan
ner. The color map was set to
range from 0 μm to +200 mm.
For the models in upper box
A (C1Right, C2Right, C3Right, and
C4Right), scanning started in
the right molar region and
then continued on the left
side; in lower box B (C1Left,
C2Left, C3Left, and C4Left),
scanning started in the left
molar region and then conti
nued on the right side.
C1, Ideal arch dentition; C2,
mildly crowded dentition; C3,
moderately crowded denti
tion; C4, severely crowded
dentition.

www.e-kjo.org
deviations of the other 4 sections (RP, LA, LP, or LM)
or the entire arch (total) between models (p > 0.05).
All models displayed less precision, and were ranked in
precision in the following descending order: left and
right molars, premolars, and incisors (p < 0.05) (Table 1).
The superimposition of the scanned Trios® images was
also expressed as a color map (Figure 4).
Comparison of precision according to scanning sequence
For the iTero®, the deviations of all sections (RM, RP,
RA, LA, LP, and LM) and the entire arch (total) displayed
statistically significant differences based on scanning
sequence (p < 0.05). In the entire arch, Group Right
showed a significantly greater deviation than Group
Left (Figure 3). All models proved less precise in the left
and right molar regions than in the incisal and premolar
regions (p < 0.05) (Table 2).
For the Trios®, there were significant deviations in 5
sections (RM, RP, LA, LP, and LM) based on scanning
sequence (p < 0.05). In LM and LP, Group Right displayed
greater deviation than Group Left. However, in RM, RP,
and LA regions, Group Left showed greater deviation than
Group Right. In contrast, deviations of the RA region and
the entire arch (total) showed no statistically significant
difference among the groups (p > 0.05) (Figure 4).
In addition, Group Right showed the least precision in
A
B
Figure 4. A color presentation
of the deviations between
surfaces from the Trios® sca
nner. The color map was set
to range from 0 μm to +200
μm. For the models in upper
box A (C1Right, C2Right, C3Right,
and C4Right), scanning started
on the occlusal side of the
right molar and continued
toward the left side; in lower
box B (C1Left, C2Left, C3Left, and
C4Left), scanning started on
the occlusal side of the left
molar and continued toward
the right side.
C1, Ideal arch dentition; C2,
mildly crowded dentition; C3,
moderately crowded denti
tion; C4, severely crowded
dentition.

www.e-kjo.org 9
the left molar region, and Group Left showed the least
precision in the right molar region (p < 0.05) (Table 2).
Comparison of precision between scanners
The mean deviations of the iTero® and Trios® were
29.84 ± 12.08 mm and 22.17 ± 4.47 mm, respectively.
Therefore, the deviation of the iTero® was significantly
greater than that of the Trios®. Deviations in 4 regions
(RM, RP, RA, and LA) and the entire arch (total) showed
statistically significant differences between the 2
scanners. Deviations of LP and LM regions showed no
statistically significant difference between the 2 scanners
(p > 0.05) (Table 3). These can be discerned more clearly
on color maps (Figures 3 and 4). In addition, both
scanners displayed less precision in the left and right
molar regions compared with the anterior and premolar
regions (p < 0.05).
DISCUSSION
As the application of digital models has broadened
into many areas, the accuracy of 3D images acquired
on an intraoral scanner has become an important
topic in research. Hence, this study aimed to evaluate
the accuracy of 2 commonly used intraoral scanner
systems, the iTero® and Trios®. We evaluated the
effects of crowding severity, scanning sequence, and
position in the dental arch on the precision of maxillary
model images obtained using these scanners. Previous
studies have compared the width of teeth from scanned
images with measurements taken from dental models.
This method poses problems resulting from possible
measurement errors and the difficulty of setting a
reference point. However, recently, the superimposition
of 3D images has been used extensively in studies on the
accuracy of scanners.13,14,17
Therefore, “precision” in this
study was evaluated by the deviation revealed through
the superimposition of images scanned repeatedly using
the same scanner.
In previous studies evaluating accuracy, it was shown
that superimposed scanned images were least accurate
Table 2. A comparison of deviations of images from the iTero® and Trios® according to scanning sequence (units, µm)
Section
iTero® Trios®
Group Right Group Left p-value Group Right Group Left p-value
RM 45.15 (21.31)b
35.13 (16.85)b
0.005* 27.28 (9.08)b
40.05 (18.53)e
0.000*
RP 28.08 (10.84)a
20.77 (5.28)a
0.000* 19.08 (3.74)a
23.87 (7.62)c
0.000*
RA 25.24 (10.09)a
18.48 (5.39)a
0.000* 15.88 (3.60)a
15.18 (2.44)a
0.217
LA 24.48 (8.55)a
18.21 (4.31)a
0.000* 16.01 (3.64)a
18.26 (4.16)ab
0.002*
LP 30.64 (13.36)a
20.06 (6.77)a
0.000* 26.93 (9.60)b
21.58 (5.31)bc
0.000*
LM 45.92 (26.02)b
34.11 (15.53)b
0.003* 46.36 (21.59)c
27.68 (9.67)d
0.000*
Total 34.13 (13.66) 25.55 (8.38) 0.000* 22.30 (4.35) 22.05 (4.62) 0.758
Values are presented as mean (standard deviation).
iTero®: Align Technology Inc., San Jose, CA, USA; Trios®: 3Shape Dental Systems, Copenhagen, Denmark.
*Statistically significant difference among the models byt-test (p < 0.05).
a,b,c,d,e
Statistically significant difference among the sections (RM, RP, RA, LA, LP, and LM) of Group Right and Group Left by one-
way ANOVA (p < 0.005, Duncan’spost hoc test; a < b < c < d < e).
Group Right, Scanning started from the right molar region; Group Left, scanning started from the left molar region; RM, the right
molar region; RP, the right premolar region; RA, the right anterior region; LA, the left anterior region; LP, the left premolar
region; LM, the left molar region; total, the entire arch.
Table 3. Comparison of the deviations of scanned images
according to scanner type (units, mm)
Section iTero® Trios® p-value
RM 40.14 (19.78)c
33.67 (15.88)c
0.006*
RP 24.43 (9.25)ab
21.47 (6.44)b
0.005*
RA 21.86 (8.74)ab
15.53 (3.08)a
0.000*
LA 21.35 (7.44)a
17.14 (4.06)a
0.000*
LP 25.35 (11.81)b
24.25 (8.18)b
0.404
LM 40.02 (22.15)c
37.02 (19.11)d
0.263
Total 29.84 (12.08) 22.17 (4.47) 0.000*
Values are presented as mean (standard deviation).
iTero®: Align Technology Inc., San Jose, CA, USA; Trios®:
3Shape Dental Systems, Copenhagen, Denmark.
*Statistically significant difference among the models by
t-test (p < 0.05).
a,b,c,d
Statistically significant difference among the sections
(RM, RP, RA, LA, LP, and LM) of the iTero and Trios by one-
way ANOVA (p < 0.005, Duncan’spost hoc test; a < b < c < d).
RM, the right molar region; RP, the right premolar region;
RA, the right anterior region; LA, the left anterior region; LP,
the left premolar region; LM, the left molar region; total,the
entire arch.

www.e-kjo.org
at the distal surfaces of the molar region, facial surfaces
of the anterior region, and interproximal surfaces in
all sections of the arch.13,14
However, previous studies
evaluating scanner accuracy have been carried out on
models with ideal tooth alignment, and experiments on
teeth with crowding have not been performed. Therefore,
in this study, precision was measured by scanning
models with gradually increasing amounts of crowding.
In particular, the C3 model represented a typical dental
arch of a patient who needs orthodontic treatment as
a result of lingual side displacement of their maxillary
lateral incisors. The impact of interdental overlapping
on the overall scanning results was investigated in all
crowding models. The experimental results suggested
that neither intraoral scanner showed a significant
difference in scanning precision with respect to dental
crowding (Table 1). However, in a model with severe
dental crowding, scanned images of the interproximal
surface could not be obtained because of the limited
accessibility of the scanner tip. Since both the iTero®
and Trios® scanners utilize the confocal principle, they
cannot focus on images if the scanner tip is far from the
target.20,21
In particular, in the iTero®, which has a larger
scanner tip than the Trios®, there were more regions
where images could not be obtained because of the
limited approach of the scanner tip in anterior regions
with crowding. Though there was no overall significant
deviation, a large deviation, especially in interdental
areas, was identified in the anterior region in models
showing severe crowding (Figure 3). Since it is impossible
to completely reproduce all sides of the teeth of patients
with severe dental crowding using an intraoral scanner,
there is the possibility of error, especially in orthodontic
practices such as model measurement and the precise
fabrication of an appliance.
iTero® scanners use a laser light beam based on the
confocal principle, which is called “stitching,” and
this merging process can generate systematic errors.14
In previous studies using a Bluecam scanner (Sirona,
Bensheim, Germany), images were less accurate in an
experiment in which the entire arch was scanned than
in other experiments in which only a quarter of the
arch was scanned.19,22
This implies that an increase in
the image stitching process during data acquisition
may affect image accuracy. In this study, the possibility
of such errors was evaluated by varying the scanning
sequences of both scanners. The scanning protocol
was different between the Trios® and iTero® scanners.
In the case of the iTero®, after half the arch was
scanned separately on the left and right sides, the left
and right scans were merged into a full arch based on
overlapping data in the left and right central incisal
regions. Compared to the Trios®, which scans all regions
continuously, it is more likely that an error originating
in the software occurred in the central incisal region,
and in fact, it was shown that the precision of the
iTero® was lower than that of the Trios® in the anterior
region (Table 3). Additionally, images acquired by iTero®
scanning that started from the right side displayed
overall lower precision than scanned images acquired
in a reverse sequence (Table 2 and Figure 3). It was
likely that an error occurred as a result of changing the
manufacturer’s scan sequence protocol to the reverse
direction. The manufacturers provide guidelines for
the scanning sequence, but there are situations that
make it difficult to follow these guidelines, such as
difficult-to-scan areas because of severe undercuts,
or scanning the more comfortable areas for patients
first and the most uncomfortable areas last. Ender and
Mehl23
also reported that scanning protocols deviating
from those recommended by the manufacturer showed
significantly lower accuracy. In contrast, although the
entire arch is continuously scanned, the Trios® can
generate more errors during the configuration process
in regions scanned later than in those scanned earlier,
as the merging of images is accumulated in the latter
portion of the scanning sequence. When scanning was
started from the right side, it would end on the left
lingual side; therefore, lower precision was observed in
the left molar region than in the right molar region.
Conversely, if scanning started from the left side, the
right molar region displayed lower precision (Table 2 and
Figure 4). In addition, images from both the iTero® and
Trios® showed lower precision closer to the bases of the
models, and in particular, greater deviation was observed
at the boundaries of the inferior region of the rearmost
molar and the boundaries of the palatal region of the
maxillary central incisor. This is consistent with the
results presented as superimposition patterns of images
from intraoral scanners in previous studies.13,14,22
The precision of the Trios® was shown to be greater
(deviation, 22.17 ± 4.47 mm) than that of the iTero®
(deviation, 29.84 ± 12.08 mm) (Table 3). Among
experiments on the precision of intraoral scanners
different from those assessed in this study,13,22
all
deviations shown in such experiments were on the
micrometer scale (less than 0.1 mm), which was difficult
to identify with the naked eye. Therefore, the accuracy
of most clinical intraoral scanners could be considered
very high. Flügge et al.14
also compared deviations
between structural intraoral images scanned directly and
from cast models, and the deviation in scanning the
dental arch was greater than that of model scanning.
Lower precision from direct scanning can be caused by
many factors, such as limited scanner operation because
of the intraoral structure, a limitation of the incident
angle of the scanning laser light, the presence of saliva,
and increase in hand tremor due to difficulty in handling

www.e-kjo.org 11
the equipment. Therefore, it is considered that further
experimental studies measuring accuracy by directly
scanning arches possessing irregular tooth arrangements,
such as in this study, would have significance.
In other studies concerning factors affecting scanner
accuracy, it was suggested that scanning targets
with a slightly rough surface rather than a smooth
surface increased the accuracy of scanning because
light scattering due to reflection can cause inaccurate
images.24
The acrylic resin used in model fabrication
and the metal balls used as fiducials in this experiment
also had the limitation of reflecting light, which caused
more difficulties in scanning than with plaster models.
In particular, since the incisal surface of the anterior
region and the distal side of the rearmost molar were
regions where light scattering readily occurs because
of the sharp bending of the structure, errors occurred
occasionally during the acquisition of images, and
images were subsequently obtained through repeated
scanning.18
Therefore, more attention should be paid
to tooth morphology and prosthesis material when
scanning intraoral structures.
Clinically, both scanners had very high precision,
regardless of the amount of crowding. However, if there
was dental crowding, images of all tooth surfaces within
the arch could not be accurately obtained, especially
of the proximal tooth surface. In addition, even if the
same arch is scanned, the accuracy of an image can
differ depending on the type of scanner and scanning
sequence. This study can provide useful information
for clinicians who intend to use a scanner on patients
in clinical practice by comparing scanner precision
according to dental crowding and by evaluating the
impact of scanning sequence on the accuracy of 3D
images. When clinicians use intraoral scanners, they
should aim to select an appropriate scanner and consider
aspects of the patient’s oral condition that might
increase the possibility of error. In the future, if digital
impression taking using an intraoral scanner is to be
applied to all patients, more studies on the accessibility
and operability of these scanners will be required.
CONCLUSION
Precision according to the degree of tooth irregularity
showed no significant difference for either the iTero®
or the Trios® (p > 0.05). In other words, crowding had
no effect on the precision of 3D models. However,
differences in precision depending on scanning sequence
were evident on both scanners. The iTero® resulted in
less precise images when scanning started from the right
rather than from the left (p < 0.05). The Trios® showed
lower precision in the molar region opposite the location
where scanning started (p < 0.05). Comparing the
accuracy of the 2 scanners, the Trios® showed greater
precision than the iTero®, with mean deviations of 22.17
± 4.47 mm and 29.84 ± 12.08 mm, respectively, for the
entire arch (p < 0.05).
REFERENCES
1. Mah J, Hatcher D. Current status and future needs
in craniofacial imaging. Orthod Craniofac Res
2003;6 Suppl 1:10-6.
2. White AJ, Fallis DW, Vandewalle KS. Analysis of
intra-arch and interarch measurements from digital
models with 2 impression materials and a modeling
process based on cone-beam computed tomography.
Am J Orthod Dentofacial Orthop 2010;137:456.e1-
9.
3. Chandran DT, Jagger DC, Jagger RG, Barbour ME.
Two- and three-dimensional accuracy of dental
impression materials: effects of storage time
and moisture contamination. Biomed Mater Eng
2010;20:243-9.
4. Al Mortadi N, Eggbeer D, Lewis J, Williams RJ.
CAD/CAM/AM applications in the manufacture of
dental appliances. Am J Orthod Dentofacial Orthop
2012;142:727-33.
5. Beuer F, Schweiger J, Edelhoff D. Digital dentistry:
an overview of recent developments for CAD/CAM
generated restorations. Br Dent J 2008;204:505-11.
6. Cha BK, Lee JY, Jost-Brinkmann PG, Yoshida N.
Analysis of tooth movement in extraction cases
using three-dimensional reverse engineering
technology. Eur J Orthod 2007;29:325-31.
7. Hajeer MY, Millett DT, Ayoub AF, Siebert JP.
Applications of 3D imaging in orthodontics: part II.
J Orthod 2004;31:154-62.
8. Patel N. Integrating three-dimensional digital
technologies for comprehensive implant dentistry. J
Am Dent Assoc 2010;141 Suppl 2:20S-4S.
9. Zhang XJ, He L, Guo HM, Tian J, Bai YX, Li S.
Integrated three-dimensional digital assessment of
accuracy of anterior tooth movement using clear
aligners. Korean J Orthod 2015;45:275-81.
10. Watanabe-Kanno GA, Abrão J, Miasiro Junior H,
Sánchez-Ayala A, Lagravère MO. Reproducibility,
reliability and validity of measurements obtained
from Cecile3 digital models. Braz Oral Res 2009;
23:288-95.
11. Yourtee D, Emery J, Smith RE, Hodgson B. Stereo
lithographic models of biopolymers. J Mol Graph
Model 2000;18:26-8, 59-60.
12. Normung DDIf. Accuracy (trueness and precision) of
measurement methods and results - Part 1: General
principles and definitions (ISO 5725-1:1994). Berlin:
Beuth Verlag GmbH; 1997.

www.e-kjo.org
13. Ender A, Mehl A. Accuracy of complete-arch dental
impressions: a new method of measuring trueness
and precision. J Prosthet Dent 2013;109:121-8.
14. Flügge TV, Schlager S, Nelson K, Nahles S, Metzger
MC. Precision of intraoral digital dental impressions
with iTero and extraoral digitization with the iTero
and a model scanner. Am J Orthod Dentofacial
Orthop 2013;144:471-8.
15. Wiranto MG, Engelbrecht WP, Tutein Nolthenius
HE, van der Meer WJ, Ren Y. Validity, reliability, and
reproducibility of linear measurements on digital
models obtained from intraoral and cone-beam
computed tomography scans of alginate impressions.
Am J Orthod Dentofacial Orthop 2013;143:140-7.
16. Naidu D, Freer TJ. Validity, reliability, and repro
ducibility of the iOC intraoral scanner: a comparison
of tooth widths and Bolton ratios. Am J Orthod
Dentofacial Orthop 2013;144:304-10.
17. Patzelt SB, Emmanouilidi A, Stampf S, Strub JR,
Att W. Accuracy of full-arch scans using intraoral
scanners. Clin Oral Investig 2014;18:1687-94.
18. Persson AS, Odén A, Andersson M, Sandborgh-
Englund G. Digitization of simulated clinical dental
impressions: virtual three-dimensional analysis of
exactness. Dent Mater 2009;25:929-36.
19. Mehl A, Ender A, Mörmann W, Attin T. Accuracy
testing of a new intraoral 3D camera. Int J Comput
Dent 2009;12:11-28.
20. Logozzo S, Zanetti EM, Franceschini G, Kilpelä A,
Mäkynen A. Recent advances in dental optics-Part
I: 3D intraoral scanners for restorative dentistry. Opt
Laser Eng 2014;54:203-21.
21. Kusnoto B, Evans CA. Reliability of a 3D surface
laser scanner for orthodontic applications. Am J
Orthod Dentofacial Orthop 2002;122:342-8.
22. Ender A, Mehl A. Full arch scans: conventional
versus digital impressions--an in-vitro study. Int J
Comput Dent 2011;14:11-21.
23. Ender A, Mehl A. Influence of scanning strategies on
the accuracy of digital intraoral scanning systems.
Int J Comput Dent 2013;16:11-21.
24. DeLong R, Pintado MR, Ko CC, Hodges JS, Douglas
WH. Factors influencing optical 3D scanning of vinyl
polysiloxane impression materials. J Prosthodont
2001;10:78-85.

1 - A comparison of the precision of three-dimensional.pdf

More Related Content

Similar to 1 - A comparison of the precision of three-dimensional.pdf (20)

Recently uploaded (20)

1 - A comparison of the precision of three-dimensional.pdf