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Original article
Comparison of the accuracy for three dental impression techniques
and index: An in vitro study
Erica Dorigatti de Avila DDSa,*, Luiz Antonio Borelli Barros DDS, MSc PhDb,Marcelo Antonialli Del’Acqua DDS, MSc, PhDc, Sabrina Maria Castanharo DDS, MSc, PhDa,
Francisco de Assis Mollo Jr. DDS, MSc PhDa
a Department of Dental Materials and Prosthodontics, Araraquara Dental School, Univ Estadual Paulista, Araraquara, SP, Brazilb Department of Social Odontology, Araraquara Dental School, Univ Estadual Paulista, Araraquara, SP, Brazil
c Araraquara University Center – UNIARA, Araraquara, SP, Brazil
Received 21 June 2012; received in revised form 7 May 2013; accepted 15 July 2013
Available online 26 September 2013
Abstract
Objectives: This in vitro study compared the dimensional accuracy of stone index (I) and three impression techniques: tapered impression copings
(T), squared impression copings (S) and modified squared impression copings (MS) for implant-supported prostheses.
Methods: A master cast, with four parallel implant abutment analogs and a passive framework, were fabricated. Vinyl polysiloxane impression
material was used for all impressions with two metal stock trays (open and closed tray). Four groups (I, T, S and MS) were tested (n = 5). A metallic
framework was seated on each of the casts, one abutment screw was tightened, and the gap between the analog of implant and the framework was
measured with a stereomicroscope. The groups’ measurements (80 gap values) were analyzed using software (LeicaQWin – Leica Imaging
Systems Ltd.) that received the images of a video camera coupled to a Leica stereomicroscope at 100� magnification. The results were statistically
analyzed with Kruskal–Wallis One Way ANOVA on Ranks test followed by Dunn’s Method, 0.05.
Results: The mean values of abutment/framework interface gaps were: Master Cast = 32 mm (SD 2); Group I = 45 mm (SD 3); Group T = 78 mm
(SD 25); Group S = 134 mm (SD 30); Group MS = 143 mm (SD 27). No significant difference was detected among Index and Master Cast
(P = .05).
Conclusion: Under the limitations of this study, it could be suggested that a more accurate working cast is possible using tapered impression
copings techniques and stone index.
# 2013 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
Keywords: Dental implant; Impression technique; Transfer coping; Impression material
www.elsevier.com/locate/jpor
Available online at www.sciencedirect.com
Journal of Prosthodontic Research 57 (2013) 268–274
1. Introduction
One of the main interests in implant-supported prosthesis is
the production of structures that show passive fit when connected
with multiples implants. This standard of fit is required because
of the unique quality of the implant–bone relationship [1]. The
natural tooth can move up to 100 mm within its periodontal
ligament, thus compensating for a certain degree of misfit of a
fixed partial denture, whereas an osseointegrated implant has
* Corresponding author at: Departamento de Materiais Odontologicos e
Protese, Faculdade de Odontologia de Araraquara – UNESP, Rua Humaita,
1680, Araraquara, SP 14801-903, Brazil. Tel.: +55 16 3301 6424;
fax: +55 16 3301 6406.
E-mail address: [email protected] (E.D. de Avila).
1883-1958/$ – see front matter # 2013 Japan Prosthodontic Society. Published b
http://dx.doi.org/10.1016/j.jpor.2013.07.001
extremely limited movement in the range of 10 mm [2]. The lack
of intrusion movement of the implant allows the forces
introduced into an implant-supported restoration to produce a
misfit in the prosthesis. If these forces are not relieved, problems
such as screw loosening screw fracture implant fracture, and
occlusal inaccuracy may arise. Thus, this lack of flexibility of the
implants should be compensated for the correct fit between the
prosthetic components obtained by the production of an accurate
impression [3].
An accurate impression affects the accuracy of the definitive
cast, and this is essential to fabricating prosthesis with a good
fit. A successful working cast is dependent on the type of
impression material and implants transfer technique [4–6].
Dullabh and Sykes [7] reported that two of main factors that
appear to be significant for passive fit are: impression material
y Elsevier Ireland. All rights reserved.
Fig. 1. Installation of the four parallel implant analogs with Duraley resin on
master cast: (a) delineator confirming the parallelism between the implants
analogs; (b) analogs fixed with Duraley resin.
E.D. de Avila et al. / Journal of Prosthodontic Research 57 (2013) 268–274 269
and the impression technique [7]. In accordance with McCabe
and Store [8], dimensional alterations occur for various reasons:
loss of alcohol in the condensation of silicones, loss of volatile
substantiate in the polysulfide and water absorption in polyether
[8]. Vinyl polysiloxane impression material transfers the spatial
orientation of implants with equal precision when compared
with polyether materials. This occurs because the vinyl
polysiloxane presents lower modulus of elasticity and reduces
the permanent deformation caused by stress between the
impression material and the copings [9].
Implant transference techniques have a decisive influence in
the manufacturing of casts and the optimal functioning of the
prosthesis. Several studies investigated the variables affecting
the accuracy of transfer procedures in implant prosthodontics.
Among then, it is possible detach: Pick-up technique or transfer
techniques, the use of different impression materials, splinting
or surface treatment of transfer copings, the relative implant
angulations, the die material accuracy, and master cast
manufacturing [1,6,7,9,10].
There is still no consensus among researchers in regards to
the best impression techniques for implants. Thus, the aim of
the present study was analyzed the accuracy of three different
impression techniques: tapered, square and modified squared
impression coping with the master cast (control group) for
prosthesis implant-supported and determines which of the
techniques offer greater passivity with greater clinical viability.
At the same time, this study compared the results of these
techniques with the index because the index is considered to be
the best technique to reproduce the positioning of the implants.
For this, all the procedures were done using manual mixing and
conventional pouring to simulate routine clinical situations.
The null hypothesis was that there would be no significant
difference in the accuracy of casts generated with different
impression techniques.
2. Materials and methods
2.1. Obtaining the master cast
A brass mandibular edentulous cast was fabricated to
simulate a clinically relevant situation (Master Cast – control
group). Four parallel abutment analogs (Micro-Unit Abutment;
Conexao Sistema de Protese, Sao Paulo, Sao Paulo – Brazil)
were installed with Duralay resin to make their removal
possible after making the framework (Fig. 1a and b). A
framework was compost of titanium cylinders and 2 mm
diameter titanium bars (Conexao; Conexao Prosthesis Systems,
Sao Paulo, Sao Paulo – Brazil) using a laser welding technique
for allowing passive fit between the prosthetics components.
After fabrication of the framework, the Duralay resin and the
original analogs were removed and discarded. Four new
analogs were screwed to the framework with the aid of a
calibrated torque wrench (Conexao; Conexao Prothesis
Systems Ltd., Sao Paulo, Sao Paulo – Brazil) limited to
10 N cm and then embedded into the master cast holes with
epoxy resin GY 1109/943 (Huntsman Quımica Brasil Ltda, Sao
Paulo, Sao Paulo – Brasil) [10–13]. The framework was
removed from the master cast only after the polymerization of
the epoxy resin was complete. As a result, the discrepancies due
to the welding procedure were eliminated almost entirely and a
metal master cast with a passively fitting framework was
produced [10–15].
2.2. Study variables
For this study, four groups with sample size of five casts for
each group (n = 5) were evaluated:
� Index
Group I: Index group – Squared Splinted Impression
Copings. The splinting process was initiated by placing
light-polymerized composite resin (Z100; 3 M ESPE)
around the squared copings (Conexao; Conexao Prosthesis
Systems, Sao Paulo, Sao Paulo – Brazil). Performed
composite resin bars with a cross-sectional diameter of
approximately 3 mm were fabricated by the injection of
composite resin into a drinking straw [17]. Appropriate
Fig. 2. Splinted squared impression copings for index technique.
Fig. 3. Tapered impression copings.
Fig. 4. Modified squared impression copings.
Fig. 5. Squared impression copings.
E.D. de Avila et al. / Journal of Prosthodontic Research 57 (2013) 268–274270
lengths of the resin bar were sectioned with a cutting disk
to bridge the spaces between the adjacent impression
copings. The ends of the resin bar were luted to the
impression copings with composite resin (Fig. 2).
� Transfer technique
Group T: Tapered impression coping (Fig. 3)
� Pick-up techniques
Group S: Squared impression copings (Fig. 4)
Group MS: Modified squared impression copings – Pick-
up technique – Squared impression copings with 2 mm
prolongations created with autopolymerizing acrylic resin
(Duralay; Reliance Dental Mfg, Worth, IL) (Fig. 5).
Metal stock trays were used to facilitate and simplify the
impression techniques [16]. The fitting surfaces of all
components were cleaned with isopropyl alcohol before each
impression [17]. All squared impression copings (Micro-Unit
Abutment, Conexao Sistema de Protese, Sao Paulo, Sao Paulo –
Brazil) were adapted to the abutment analogs on the master cast
using 10 N cm torque [18]. Correct seating of the impression
copings was verified visually throughout the impression and
pouring procedures.
2.3. Impression
The impressions were made in a temperature-controlled
environment. Vinyl polysiloxane impression material (Elite
HD+ putty/light body normal setting, Zhermack, Badia
Polesine, Italy) was used according to the manufacturer’s
instructions.
Each impression was produced with equal quantities of
impression material. The impression material was allowed to
set for 10 min after the mixing was started. The manufacturer’s
setting time was doubled to compensate for a delayed
polymerization reaction at temperature environment rather
than at mouth temperature [17,20,21]. For transfer technique,
the impression/matrix set was separated (Fig. 6).
Tapered copings were unscrewed of the analogs and placed
in the mold for all impression.
For Pick-up techniques, after impression material poly-
merization, the impression copings were unscrewed and the
tray was separated from the master cast (Fig. 7). In this case,
when the impression/matrix set are separated, the transfer are
close to the impression.
The squared impression copings and modified squared
impression copings were secured onto the analogs with guide
screws (Fig. 8).
2.4. Confection of stone casts
With the index group, squared splinted impression copings
were unscrewed from the master cast and screwed to the
abutment analogs. A rectangular box for pouring was made with
Fig. 6. Transfer technique.
Fig. 7. Pick-up technique.
Fig. 8. Squared impression copings positioned on the analogs with guide
screws.
Fig. 9. Index – a rectangular box was made with type IV dental stone.
E.D. de Avila et al. / Journal of Prosthodontic Research 57 (2013) 268–274 271
condensation silicone and poured with type IV dental stone
(Fig. 9). The analogs were seated into the stone matrix to,
approximately, half their length. For the Pick-up techniques, four
windows were created in the metal stock tray to expose coping
screws. A box for pouring the impression with dental stone was
made with condensation silicone (Zetaplus/Oranwash; Zermack,
Badia Polesine, Rovigo, Italy). This matrix was used for all the
impressions, allowing the reproduction of the tray position and
standardization of the format of the casts and of the amount of
dental stone. The conventional pouring techniques were used
since these techniques are easier and faster. Two hours after the
impressions; the dental stone type IV (Durone IV – Dentsply
Industria e Comercio Ltda., Petropolis, Rio de Janeiro, Brasil)
was mechanically handled to simulate a routine clinical situation.
All mixes were vibrated into boxed impressions and before and
during the pouring [9]. The 20 casts obtained were stored for two
weeks before measurement [17–23]. The four implant analogs in
the master cast were denoted sequentially A through D from left
to right.
2.5. Registration of misfits vertical (adaptation cervical)
The standard framework was seated on each cast and a
titanium screw was tightened in analog A to 10 N cm using a
torque driver, while measurements of abutment-framework
interface gaps were made in analogs C and D.
This process was repeated for analog D, and the measure-
ments of analogs A and B were noted. These measurements
were analyzed using software (Leica Imaging System, Cam-
bridge, England) that received the image from a video camera
(JVC, 0.5-in. CCD, model TK-C1380, Tokyo, Japan) coupled
to a Leica stereomicroscope (Leica Microsystems, Wetzlar,
Germany) at 100� magnification. Demarcations were made in
the relative center of each abutment between the metallic
structure and the analog implants to standardize the captured
image. For each picture taken, the lineal reading of gap was
carried through in three areas. The arithmetic mean of these
three values determined the value of the gap. The mean gap
value for the master cast was calculated as the average of five
consecutive measurements (20 gap values), and the framework
was screwed before each measurement. The same person
performed all procedures [10,24,16].
2.6. Statistical analysis
All measurements were registers by same blind examiner
calibrated. One person analyzed the same images in two
different occasions to confirm the reliability the calibration of
Table 1
Kruskal–Wallis One Way analysis of variance on ranks.
Group Mean Median 25% 75%
Master Cast 31.63 28.865 20.39 42.87
(I) Index 45.25 43.12 30.625 58.555
(T) Tapered 78.34 71.82 54 99.375
(S) Squared 133.78 129.795 104.31 157.335
(ms) Modified squared 143.15 145.775 117.34 174.045
* H = 72.346 with 4 degrees of freedom. (P = < 0.001).
Table 2
Comparison of all transfer techniques vs. master cast (Dunn’s method).
Comparison Diff. of ranks Q P < 0.05
Index vs. master cast 11.8 1.286 No
Tapered vs. master cast 32.2 3.51 Yes
Squared vs. master cast 58.55 6.382 Yes
Modified squared vs. master cast 62.2 6.78 Yes
E.D. de Avila et al. / Journal of Prosthodontic Research 57 (2013) 268–274272
the examiner, with confidence interval of 95% in two different
occasions. With the aid of SigmaStat version 3.11 (Systat,
Evanston, IL), an appropriate statistical test, for small samples,
was applied. Because it is a non-normal distribution with
unequal variances, a One-Way analysis of variance the
Kruskal–Wallis One Way ANOVA was used. Values of
P < .05 were judged to be significant. The results showed that
there were significant differences among groups. For this
reason, Dunn’s Method posttest was used to perform multiple
comparisons between groups.
3. Results
Four groups with five casts each were formed. The mean
values of abutment/framework interface gaps were: Master
Cast = 32 mm (SD 2); Group I = 45 mm (SD 3); Group
T = 78 mm (SD 25); Group S = 134 mm (SD 30); Group
MS = 143 mm (SD 27). There were significant differences
among groups. Table 1 shows all comparisons among the
groups. The index group was the one that most approached the
Master Cast results (P = .05). Among the impression techni-
ques the Group T presented better results. In contrast, the
Groups S and MS presented the highest gaps determined by the
distance between the framework and the analog. The
differences between S and MS Groups did not produce
significant difference. Table 2 shows, which transfer techni-
ques, are statistically different in comparison with Master Cast.
The results show that only the gap value of Index Group no
present difference in relation to Master Cast.
4. Discussion
The master cast reproduces the intraoral position of the
implants surrounding hard and soft tissues as accurately as
possible, to allow for the fabrication of passively fitting
prostheses and, consequently, eliminate the strain on the
supporting components and around the bone.
If a clinically passive fit is not achieved and the metal
supporting structure is intraorally unstable, the metal frame-
work is usually sectioned, repositioned, and soldered. But this
involves more time and produces a weaker and metallurgically
more complex prosthetic framework [4,21]. A passive fit occurs
when all the surfaces, of the implant and prosthesis, are aligned
without the application of force and when the gap formed
between the metallic framework and implants are within the
limits established by science (111 mm) [25]. However, a gap of
32 mm was still observed between the framework and implant
analogs. This gap is equivalent to the distance between the
analog D and the framework, result of screw tightness on the
set: analog A and framework [10].
A perfect fit occurs when all the matching surfaces of the
implant and prosthesis are in alignment and in contact without
the application of force [5]. In order to identify a passive fit, the
master cast used in this study was fabricated using a previously
completed metal framework.
The scientific literature provides two impression techniques,
which are pick-up and transfer techniques
[1,3,7,10,13,15,17,27,28]. The second technique is performed
with tapered impression copings associated with tray closed.
On the other hand, in the pick-up technique, the square
impression copings are unscrewed of the implant after the
setting of the impression material and removed from the mold,
using a tray opened. The type of tray is directly related with the
dental impression technique used [26]. An ideal impression
technique would require minimal time, be useful, inexpensive,
and comfortable for the patient and, of course, provided more
accurate results [26]. While some authors reported better results
with the pick-up technique [27,28], Humphries and colleagues
found that the transfer technique was more accurate, required
less working time, and was easier for the operator and also more
comfortable for the patient [3]. The feasibility of the transfer
technique enables indication for patients with limited mouth
opening, and patients with an exaggerated gag reflex due the
ease and the speed of removal [5]. In 2009, Rashidan et al. [29]
compared the accuracy of two different impression techniques
with two different coping shapes and showed that was not
statistical significant difference in whole dimensions between
pick-up and transfer impressions techniques.
These results are inconsistent with several other investiga-
tions [22,27], and may be related to the use of different
materials for impression, casting, different prosthetic compo-
nents, mixing time, and type of tooling. However, in the present
study, tapered coping techniques presented gap values higher
than those observed with the index, but lower than other
techniques (squared and modified squared). Thus, the clinician
should choose the technique that requires less time (Group T),
since it is much easier to use. The null hypothesis that the
accuracy of casts would not be affected by the impression
technique was rejected. Significant differences were detected
among T, S and MS groups and Master Cast.
Despite some author’s affirmations that the splinted square
coping technique offers more accurate results [1,11,13,15,30],
others report that the splinted square coping technique showed
no benefit over the square coping technique [17,11]. The aim of
E.D. de Avila et al. / Journal of Prosthodontic Research 57 (2013) 268–274 273
splint the square coping is to stabilize the copings during the
impression to minimize the rotation movement during the
setting time of the impression material. However, this technique
emanates a long time, it is not easy and it is no comfortable for
patient. At the same time, some authors affirm that the choice of
the impression technique must be accompany the impression
material and impression tray specific for that technique.
Considering the clinic practical unviability, this study did not
cite the splinted square coping techniques.
The inaccuracy observed in tapered coping technique may
be correlated with the distortion of the impression material
during removal. Daoudi et al. [31] demonstrated that a large
variation was reported in the anteroposterior and mesiodistal
positions of the coping in the repositioning technique [31].
Similarly, Liou et al. [32] studied the accuracy of repositioning
tapered transfer copings within the impressions elastomeric and
noted that the part cannot be repositioned accurately in all
impressions [32]. Carr noted that the inaccuracy of the transfer
techniques might arise from the apparent deformation caused
by a stiff impression material such as polyether. Thus, a more
elastic impression material could reduce the deformation of the
impression. In this case in question, for impression of the
analogs, the 1-step putty-wash technique using vinyl poly-
siloxane impression material was chosen because of its
convenience, common clinical usage and because the literature
shows more accurate results [26]. Other technique, also
commonly used, is with 2-step putty-wash technique. An
advantage of the two-step impression technique is that the
impression of the teeth can be captured with the wash material.
While the disadvantages are distortions, extra chair side time,
and extra material needed [33].
The squared and modified squared coping technique groups
present the highest gap values, and show the deficiencies of
these types of impressions. In relation to square coping
technique, the results could be justified due to a possible
rotating movement during the impression. These results
contradict some scientific literature that presents satisfactory
results with the use of the square coping techniques [27,28].
However, due to the fact that the casting was done using the
conventional method (without the use of the latex tube) [10],
the expansion of the plaster could have caused distortions in the
impression and the formation of gaps. For this reason, it was
created prolongation with acrylic resin on squared impression
copings to simulate a guide screws. Thus, this extension could
avoid the rotating movement of the square copings during the
impression. However, the results showed the opposite. The
modified squared coping technique group presented a gap of
145,77 mm, determined by the arithmetic mean of three values
referents to the distance between the framework and the analog
determined the value of the gap. The minors gaps observed in
Index Group could be explained by the absence of the
impression material for this technique. For the Index Group, a
small amount of dental stone is employed in the pouring
procedure and this fact could be minimizes the setting
expansion. According to Del’Acqua et al. [10], the index
technique (Group I = 45 mm) was the best technique for
production of implant-supported and fixed restorations with
dimensional accuracy. The clinical advantages in splints
squared copings with light-polymerized composite resin are
minimize problems related to resin polymerization contraction
and avoid this multi-step, time-consuming procedure (time
required for acrylic resin polymerization and the additional step
of sectioning and rejoining the acrylic resin splint). Therefore,
there is improved efficiency, a reduction of chair time and
greater transfer precision due to the splinting stability. If the
final prosthesis is fitted on the index, then, a clinician should
trust that it would most likely fit a patient’s mouth [34]. This
would be advantageous, since passive adaptation of the implant
abutment to the framework is often difficult to achieve and to
interpret in a clinical setting [35].
Further studies evaluating implant impression techniques
simulating partially edentulous casts are necessary. In addition,
other types of impression materials should be also evaluated.
Considering that this study is an in vitro study, and it is
impossible to simulate all clinical conditions, the techniques
evaluated are expected to perform similarly in the oral
environment.
5. Conclusion
Under the limitations of this study, it could be suggested that
a more accurate cast work could be performed using both
techniques: tapered coping technique and the index technique.
Tapered coping technique was not only considered to be
technically easier to work with but also numerically better.
Squared coping and modified squared coping techniques did
not present any clinical advantage and did not improve the
dimensional accuracy of the die stones to interpret a clinical
situation.
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgement
The authors acknowledge with sincere thanks the Mrs. Kim
Kubitza for correcting the English language this article.
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