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EX VIVO DIGITAL COMPARISON OF FOUR IMPRESSION

Original Articles
TECHNIQUES USING AN INDUSTRIAL LASER SCANNER
Bálint Jász1a* , Máté Jász1b , Szandra Körmendi1c , Gellért Joós-Kovács1d , Bálint Vecsei1e ,
Péter Hermann1f , Judit Borbély1g
1
Department of Prosthodontics, Faculty of Dentistry, Semmelweis University, Szentkirályi utca 47, HU-1088 Budapest, Hungary
a
DMD, Clinical Doctor; e-mail: jasz.balint@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0002-5556-137X
b
MD, DMD, Assistant Professor; e-mail: jasz.mate@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0002-6717-5126
c
DMD, Assistant Professor; e-mail: kormendi.szandra@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0003-4694-1342
d
DMD, Assistant Professor; e-mail: joos-kovacs.gellert_levente@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0002-0522-3751
e
DMD, Assistant Professor; e-mail: vecsei.balint@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0001-5748-7276
f
DMD, PhD, MSc, Professor and Head; e-mail: hermann.peter@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0002-9148-0139
g
DMD, PhD, Associate Professor; e-mail: borbely.judit@dent.semmelweis-univ.hu; ORCIDiD: https://orcid.org/0000-0003-3064-8724
ABSTRACT https://doi.org/10.25241/stomaeduj.2022.9(1).art.2
Introduction The aim of the study was to compare different impression techniques used for fixed
prosthodontics.
Methodology A master cast with prepared abutments was created from polymethyl-methacrylate (PMMA).
A high-resolution industrial scanner was used to create a virtual reference model. Four different impressions
were made, three with polyvinyl-siloxane (PVS) (n = 10 for each): one-step putty-wash (1SPW), two-step
putty-wash prepared with an escape channel (2SPW-Ch), two-step putty-wash with a polyethylene spacer
foil (2SPW-Fo), and one with polyether monophasic technique (MP) from the PMMA model and digitized
with an industrial scanner. The stereolithographic (STL) files of the impressions (n = 40) were exported. Each
file was compared to the reference using the Geomagic Verify software. Six points were assigned to enable
virtual calliper measurement of tooth diameters and distances of varying sizes within the arch.
Results In the case of die diameters, the deviation from the mould ranged from 31.84 to 180.64 µm. At the
stump diameter level, the MP and 1SPW techniques showed significantly more minor differences than the
2SPW-Ch, and the MP was significantly more accurate than the 2SPW-Fo. At medium distance, the deviation
ranged from 42.74 to 136.47 µm. Therefore, MP was found to be significantly more accurate than 2SPW-Ch.
When examining the long distance, the difference was between 162.62 and 348.85 µm. The MP and 1SPW
impression techniques proved significantly more accurate than the 2SPW-Ch technique for long distances.
Conclusions With both simultaneous techniques, significantly more true results were achieved than with
the two-step techniques.
KEYWORDS
Dental Impression Technique; Scanning of the Impression; Monophase; One-Step Putty-Wash; Two-Step
Putty-Wash.
1. INTRODUCTION precision refers to the closeness of agreement
between test results [1]. In this study, only the
One of the most critical steps in our dental processes trueness of the impressions was examined. A review
is to make impressions with reasonable accuracy. article published in 2016 defined the still tolerable
The impression allows the dental technician to inaccuracy in the crown’s fit between 50 and 200
have the same condition on the model as in the µm after the turn of the millennium [2]. The article
patient’s mouth. When examining impressions, by McLean and von Fraunhofer from 1972, which is
trueness and precision can be examined, and these still frequently cited, gives the 120-micron deviation
two together constitute accuracy. ISO 5725 uses as an inaccuracy threshold, so this level of accuracy
two terms, trueness and precision, to describe must be aimed at for impressions [3-5].
the accuracy of a measurement method. Trueness According to the ISO standard, which currently
refers to the closeness of agreement between the regulates the properties of elastic impression
arithmetic mean of a large number of test results and materials, the kneadable material with the lowest
the true or accepted reference value. Meanwhile, reproducibility must have a resolution of at least
OPEN ACCESS This is an Open Access article under the CC BY-NC 4.0 license.
Peer-Reviewed Article
Citation: Jász B, Jász M, Körmendi S, Joós-Kovács G, Vecsei B, Hermann P, Borbély J. Ex vivo digital comparison of four impression techniques using an
industrial laser scanner. Stoma Edu J. 2022;9(1):21-26.
Received: January 20, 2021; Revised: January 30, 2022; Accepted: February 15, 2022; Published: February 28, 2022.
*Corresponding author: Dr. Bálint Jász, DMD, Department of Prosthodontics, Faculty of Dentistry, Semmelweis University,
Szentkirályi utca 47, HU-1088 Budapest, Hungary.
Tel: +36-20-497-4232; Fax: +36-1-317-5270; e-mail: jasz.balint@dent.semmelweis-univ.hu
Copyright: © 2022 the Editorial Council for the Stomatology Edu Journal.

Stoma Edu J. 2022;9(1): 21-26 pISSN 2360-2406; eISSN 2502-0285 21
Jasz B, et al.
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Original Articles 75 µm, so there is no material scientific obstacle to polyether impression material (Impregum Penta Soft;
3M ESPE) were used [19]. For the proper adhesion of
achieving this trueness and accuracy [6]. Therefore,
in addition to the spread of digital dentistry, the impression material to the tray, the Polyether
information on the trueness of traditional impression Adhesive (3M ESPE) was applied as required by the
techniques remains of paramount importance. The manufacturer. The mixed impression material was
reason for this is twofold. On the one hand, the filled into the tray, and all the prepared stumps were
injected with an impression syringe.
digitization of conventional impressions or models is
A metal stock tray and ISO-0 putty consistency
one of the possible routes to CAD / CAM techniques;
PVS impression material (Express XT Penta Putty;
on the other hand, the vast majority of dental offices 3M ESPE) were used for the two-time impressions,
still use traditional impressions [7,8]. In addition, followed by ISO-3 consistency PVS wash material
numerous studies and factory data are available (Express XT Light Body; 3M ESPE). Two different
on the impression materials themselves and their techniques were applied to ensure decompression.
accuracy [9]. However, the trueness and accuracy In the first case, escape channels were cut in the
of the various impression techniques are still not impression on both mesial and distal sides of each
widely researched. Most in vitro examinations of prepared tooth while alternating on the oral and
impressions use simplified bodies, such as cones buccal sides of the non-prepared teeth (2SPW-Ch)
instead of teeth or tooth models [10,11]. In many [16]. In the second case, the surface of the putty
studies, only two or three techniques are compared material was covered with a polyethylene spacer foil
in an analogous way or by the evaluation of trained (Impression Separation Wafer GC Europe) to provide
clinicians, but not in a qualitative way [12-14]. In the the proper gap for the wash material (2SPW-Fo) [20].
In all cases, the impression materials were mixed with
present in vitro study using a PMMA master cast
the impression mixing machine as recommended
with prepared stumps, the four most commonly by the manufacturer (Pentamix; 3M ESPE) or with a
used precision impression techniques are compared factory impression gun (Garant; 3M ESPE) following
to explore whether there was a difference in the manufacturer’s instructions [21].
the sampling trueness of these techniques. The A PMMA master model was applied in the study
hypothesis is that one-step impression techniques because its material did not affect the setting of the
are truer than two-step techniques. materials used in any way. As the model is durable,
many impressions could be taken without distortion
2. METHODOLOGY or change. To the best of our knowledge, the model
itself did not affect the results of this study in any
The following four impression techniques are way [7,22]. The impression-taking procedure was
examined in the present study: standardized using a Kaán impression tool [23]. Each
• the one-step putty-wash technique (1SPW), tray was filled with impression material to the edge
• the monophase technique (MP), of the impression tray. The same 3 kg weights were
• the two-step putty-wash technique prepared with used at all impressions. At the Kaán tool, you can
escape channels (2SPW-Ch), and position the cast only in one way accordingly. The
• the two-step putty-wash technique prepared with directions and extent of the force were the same at
a polyethylene spacer foil (2SPW-Fo). all impressions, thus standardizing the impression
Ten impressions were taken from a PMMA master conditions (Fig. 1). The impressions were taken by
model using each technique, i.e., altogether 40 one calibrated person.
impressions were created [15,16]. On the upper
jaw master model, supragingival, chamfer shoulder
preparation of the right first premolar (14), left first
incisor (21), first premolar (24), and second molar
(27) were prepared. In all cases, the impressions
were made considering the manufacturer’s
instructions, the literature recommendations, and
consensus [11,17,18]. Impression materials with
the recommended consistency for the impression
technique were applied in all cases, as stated in
the factory recommendation [18]. For the 1SPW
technique, a metal stock impression tray was used,
with ISO-1 (Express XT Penta H; 3M ESPE), and ISO-
3 consistency (Express XT Light Body; 3M ESPE)
polyvinylsiloxane (PVS) impression material. The
tray was filled with heavy-bodied material, its entire
surface was covered with the wash material, and
on all of the prepared teeth, an impression syringe
was used to inject the wash material [15]. For the
MP technique, a custom-made tray prepared by the
dental technician from Lightplast Baseplate (Dreve-
Dentamid, Unna, Germany) and ISO-2 consistency Figure 1. The Kaán impression tool.

22 Stoma Edu J. 2022;9(1):21-26 pISSN 2360-2406; eISSN 2502-0285
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In the first step, the master model was scanned by

Original Articles
an industrial dot laser scanner (Two Scan; Varinex
Informatikai Zrt., Budapest, Hungary), and then the
forty prints were scanned as well [24]. As previously
recommended, a minimum of 1, a maximum of
24 hours elapsed between the impressions being
completed and the scan [25]. After scanning the
impressions, the resulting .stl files had to be compiled
to allow comparison of the data set with the data set
obtained during the master model scan [26]. Figure 2. Measurement report. The selected distances and the inter-
During the measurement, an incision plane was section plane on the master model are shown in the top right corner.
determined in the virtual space manually, which
The mean deviation for the six stump diameters
intersected all the prepared teeth examined. The
was 31.84 µm for the MP impression technique
reference points were determined automatically. (SD: 28.29), 56.48 µm for the 1SPW (SD 89.07), 74.04
The program selected points that could be used as µm for the 2SPW-Fo (SD 157.51), and in the case of
reference points. Both the plane and the reference 2SPW-Ch, it was 180.64 µm (SD 338.17). Examining
points were determined only once and were the trueness of the stump sampling, we can observe
transferred between each measurement. The the following trend: the truest was the MP technique
distances between measurement points were followed by 1SPW, 2SPW-Fo, and finally the 2SPW-
selected on the master model scan to establish the Ch. In terms of die diameters, MP was significantly
baseline data. Then the corresponding distances on truer than 2SPW-Fo and 2SPW-Ch, while 1SPW
each impression scan were measured and compared was significantly truer than 2SPW-Ch. The other
to the baseline (Software: Geomagic Verify, differences were statistically not significant (Fig. 3).
Geomagic Inc., Morrisville, NC, USA).
The mesiodistal (MD) and bucco-palatal (BP)
diameters of the prepared teeth 14, 21, 24, 27
were measured in the incision plane. Furthermore,
distances between 24 distal (24D) and 27 mesial
(27M) measuring points (20 mm small distance),
between the 24 mesial (24M) and 27 distal (27D)
measurement points (32 mm medium distance),
and 52 mm large distance between 21 mesial (21M)
and 27D were measured [13,27]. GraphPad Prism 5
Software (GraphPad Software Inc., San Diego, CA,
USA) was used for statistical analysis. The Kruskal– Figure 3. The cumulative average deviation for the six stumps. The red
Wallis Test and Dunn’s multiple comparison post- clasp indicates a significant difference (p < 0.05).
hoc test were applied to examine the correlation For the medium distance between 24M and 27D,
between groups, with a significance threshold of the trend was the same as found at the level of the
p < 0.05. stumps. The MP technique proved to be the truest;
the deviation from the sample was 42.74 µm (SD:
3. RESULTS 31.22), followed by 1SPW 71.37 µm (SD: 53.67),
2SPW-Fo 83.46 µm (SD: 57.25), and then 2SPW-Ch
Thus, six stump diameters (14BP, 21MD, 24BP, 136.47 µm (SD: 61.57). A significant difference was
24MD, 27BP, 27MD) and two distances (medium found only between MP and 2SPW-Ch. The other
distance between 24M–27D and large distance differences were not significant statistically (Fig. 4).
between 21M–27D) were analyzed further (Fig. 2).
Since the diameters of the stumps fell in the same
size range, a common average of the six stump
diameters was calculated for better statistical
processability. Furthermore, only six instead of the
originally planned eight diameters and two instead
of three distances were examined. The problem
arose when selecting the incision plane: some of the
measuring points were in an area with undercuts
during the scan. This is a problem, because in the
invisible regions, the computer-calculated data were
displayed partially, which significantly distorted the Figure 4. The average deviation for the medium distance. The red clasp
indicates a significant difference (p < 0.05).
measurement results [28]. Therefore, the study did
not consider the apparently false results between For the large distance between 21M and 27D, the
the 14 MD diameter and the 21 BP diameter and trueness was as follows: the slightest deviation,
24D–27M (short distance) [29]. 162.62 µm (SD: 68.24), was found for 1SPW. This

Stoma Edu J. 2022;9(1): 21-26 pISSN 2360-2406; eISSN 2502-0285 23
Jasz B, et al.
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was followed by MP 188.32 µm (SD: 48.48), 2SPW-Fo below them [17]. Rudolph et al. and Luthardt et al.
Original Articles 286.14 µm (SD: 174.18), and finally 2SPW-Ch 348.85 established the order 1SPW, MP, 2SPW with a spacer
µm (SD: 122.68). For the 2SPW-Ch technique, both cap, while Dugal et al. found the 1SPW technique
1SPW and MP proved significantly truer (Fig. 5). to be more accurate than the 2SPW with spacer
cap [13,18,30]. According to Mann et al., the 2SPW-
Fo impression technique is more accurate than the
2SPW-Ch technique [16]. These findings are all in
accordance with the results published here.
Contrarily, Caputi et al. found a unique 3-phase
impression technique to be the most accurate,
followed by the 1SPW, and finally, the MP techniques
[31]. The discrepancy may be that the MP technique
was used with PVS and a stock tray instead of
polyether and a custom tray. Nissan et al. found the
latter to be the most accurate in both studies when
Figure 5. The average deviation for the long distance. The red clasp comparing techniques using 1SPW, 2SPW-Fo, and
indicates a significant difference (p < 0.05). 2SPW with a spacer cap [11,20]. The third technique,
which was not examined in the present study,
4. DISCUSSION proved to be the most accurate. However, Nissan
et al. did not succeed in detecting a significant
Regarding the sampling trueness of the impressions,
difference between the 1SPW and the 2SPW-Fo
the strict deviation of 50 µm only fits the diameters
techniques they examined, which coincides with the
of the stumps, and the medium distance measured
results presented here. In his study, Nissan used the
between 24M and 27D with the MP impression
same ISO 0, putty consistency impression material
technique [2]. The trueness of 1SPW with 2SPW-
for all three techniques, which is not ideal for the
Fo techniques fell outside this range but is well
1SPW technique. This may be partially responsible
within the most generally accepted trueness of
for the different results. Jamshidi et al. found the
120 µm for both distances mentioned above [3].
2SPW-Ch technique significantly more accurate
The sampling trueness of the 2SPW-Ch technique
than the 1SPW technique. However, in the one-step
shows a difference of more than 120 µm, even in the
technique, the wash material was injected only on
case of the stump diameter. The distance measured
the stumps, they did not cover the kneadable base,
between the stumps increases even more with the
and both techniques had the same ISO 0 consistency
increase of the examined length. Concerning long
[12].
distance, the degree of deviation from the initial
In the most relevant literature, the simultaneous
sample remained below 200 µm in the case of the
techniques proved to be more accurate, i.e., less
1SPW and the MP techniques, while for the 2SPW-
difference can be expected when using them than
Fo technique, it almost reached 300 µm. However, it
does not make sense to use the 120 µm deviation in the case of separate time impression techniques,
for medium and long distances, since the distances except for some less commonly used, individually
examined are larger by orders of magnitude than for modified two-step techniques [17,30]. In almost all
stump diameters. In this case, it is better to express studies examining two-step techniques, using the
the extent of the deviation in proportion to the 2SPW-Fo technique or 2SPW with spacer cap gave
distance examined (Table 1). better results than the 2SPW-Ch method [11,16]. All
Table 1. The average deviation for the medium and long distances in this is in line with the results presented here.
percentage. However, there is no consensus on which simul-
taneous technique provides greater trueness: MP
1SPW 2SPW-Fo 2SPW-Ch MP or 1SPW? One important reason for the discrepancy
medium 0.222% 0.259% 0.425% 0.133% may be that the combinations of impression material
distance and impression tray recommended primarily for the
long 0.311% 0.548% 0.668% 0.360% 1SPW and MP techniques were not appropriately
distance used in all studies [11,12,31]. As is well known, the
1SPW impression technique is primarily based on
Thus, for all the examined distances, except for the ISO 1 consistency (and not ISO 0, as in several of
largest studied length, the MP technique showed the studies cited), and a stock tray is recommended.
the slightest difference, followed by the 1SPW and In contrast, for the MP technique, the use of ISO 2
the 2SPW-Fo methods, and in all cases, the 2SPW- medium consistency material and a custom tray
Ch technique performed the worst. The first and gives the best result [18]. In the latter case, polyether
second places were reversed for the largest distance impression material has an advantage over PVS due
examined. to its thixotropic effect.
Pastoret et al. found that the 1SPW technique According to our study results, it can be said that the
performed best for short distances, while the MP one-step impression techniques performed better.
technique performed best the long distances, but This may be due to the deformation during the
in both cases, the 2SPW-Fo technique remained replacement of the impression for the second time.

24 Stoma Edu J. 2022;9(1):21-26 pISSN 2360-2406; eISSN 2502-0285
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Additionally, the hydraulic distortion, and finally the 5. CONCLUSION

Original Articles
imperfect connection between the two different
consistencies at the two-step impression techniques Within the limitations of this study, the hypothesis
may account for the better performance of one-step was confirmed: it was proved that the MP and the
techniques. 1SPW, i.e., the simultaneous techniques, showed
Within the limitations of this study, it can be significantly better trueness among the examined
concluded that better clinical results can be achieved precision impression techniques. In contrast, the
using one-step impression techniques, either MP or 2SPW-Ch technique proved to be the most inaccurate
1SPW, while preparing fixed prostheses. In the case at all diameters and distances.
of using the two-step putty-wash technique, the
CONFLICT OF INTEREST
decompression with only escape channels might
The authors have no financial interest in any of the companies
be insufficient, leading to hydraulic distortion and whose products are used in this study.
inaccurately sitting fixed prosthesis in the end.
However, it is essential to note that besides the MP AUTHOR CONTRIBUTIONS
and the 1SPW impression techniques, even the BJ, SZK, MJ and JB: concept. SZK, BV, GJK: protocol. BJ, SZK, MJ,
2SPW-Fo method provides adequate results at the BV, GJK: data gathered and analyzed. PH, JB: interpretated and
stump level, only 2SPW-Ch proved to be insufficient critically revised the manuscript. All authors read and approved
this level. the final manuscript.

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Bálint JÁSZ
DMD, Clinical Doctor
Department of Prosthodontics, Faculty of Dentistry
Semmelweis University, Budapest, Hungary

CV
Dr. Bálint Jász, DMD, graduated from the Faculty of Dentistry, Semmelweis University, Budapest, Hungary in 2019. The title of
his thesis was: Comparative evaluation of precisional impression techniques. He is currently attending the residency program in
Prosthodontics at the Department of Prosthodontics, Faculty of Dentistry, Semmelweis University, Budapest, Hungary. He has
been participating in the Digital Dentistry Working Groups since 2017. He has been researching conventional impressions since
2018.

Questions
1. Monophase impression are correctly made with impression material of the following
consistency:
qa. ISO-0;
qb. ISO-1;
qc. ISO-2;
qd. ISO-3.

2. PMMA is ideal for master cast material because:
qa. It does not affect the setting of impression materials;
qb. It has a nice color;
qc. It is sufficiently flexible;
qd. It hardens heat.

3. In our study, the distances between the following teeth were evaluated:
qa. “Small distance”: between 24 distal and 27 mesial points;
qb. “Extra long distance”: between 14 mesial and 27 distal points;
qc. “Medium distance”: between 24 mesial and 27 distal points;
qd. “Horizontal distance”: between 14 mesial and 24 mesial points.

4. On the large distance the most true impression technique in our study is:
qa. One-step putty-wash technique;
qb. Monophasic technique;
qc. Two-step putty-wash technique with spacer foil;
qd. Two-step putty-wash technique with escape chanel.

26 Stoma Edu J. 2022;9(1):21-26 pISSN 2360-2406; eISSN 2502-0285