art-4-1-21
COMPUTERIZED DENTAL PROSTHETICS www.stomaeduj.com
EFFECT OF DIGITAL WORKFLOW ON THE MARGINAL FIT
Original Articles
OF LONG-SPAN IMPLANT-SUPPORTED BARS FOR
KENNEDY II CLASS REMOVABLE PROSTHESES IN VITRO
Aristeidis Villias1a* , Triantafillos Papadopoulos2b , Nick Polychronakis1c , Hercules Karkazis1d , Gregory Polyzois1e
1Department of Prosthodontics, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece
2Department of Biomaterials, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece
a
Clinical Instructor, DDS, MSc, Dr. Med. Dent; e-mail: Aristeidis.Villias@gmail.com; ORCIDiD: https://orcid.org/0000-0003-3561-1955
b
Professor Emeritus, DDS, MSc, PhD; e-mail: trpapad@dent.uoa.gr; ORCIDiD: https://orcid.org/0000-0002-9533-6249
c
Associate Professor, DDS, MSc, Dr. Dent; e-mail: nicpolis@dent.uoa.gr; ORCIDiD: https://orcid.org/0000-0001-7373-3414
d
Professor, DDS, MSc, Dr. Dent; e-mail: hkarkaz@dent.uoa.gr; ORCIDiD: https://orcid.org/0000-0002-9003-2852
e
Professor, DDS, MScD, Dr. Dent; e-mail: grepolyz@dent.uoa.gr; ORCIDiD: https://orcid.org/0000-0003-0032-039X
ABSTRACT https://doi.org/10.25241/stomaeduj.2021.8(1).art. 4
Introduction The production procedures, including impressions, introduce errors affecting the passivity
of fit. A completely digital workflow is possible nowadays because of the intraoral scanners (IOS). This
study aimed to evaluate the effect of the impression technique (conventional versus digital) and the screw
tightening sequence on the marginal discrepancy (MD) of implant-supported bars.
Methodology This laboratory study was conducted on a simulated Kennedy class II edentulous maxilla with
three parallel implants in the edentulous quartile. The closed tray technique with a-silicon (CTM) and the
intraoral scanning with the I-Tero™ system (IOS) were compared and three bars were manufactured from
each technique. Depending on the screw tightening sequence (A11 and A17) 4 groups were created with 6
samples each. The MD was examined implementing 24 negative replicas, which were sectioned and studied
under a stereomicroscope. The Horizontal Discrepancy (BHD), Vertical Discrepancy (BVD) and Conical
Discrepancy (BCD) of the bar were calculated on the means of the measurements of the horizontal, the
vertical and the conical MD respectively. The descriptive statistics, normality tests, one-way ANOVA (a=.05)
and post-hoc Tukey’s tests were run and the graphs were draw with SPSS.
Results There was a significant effect (P<.05) of the impression technique combined with the screw
tightening sequence on all variables. The post-hoc Tukey’s tests revealed significant differences between all
groups except from those of the same impression technique only for the BHD (P<.05).
Conclusion In this study all groups resulted in marginal discrepancies. The closed tray impression technique
gave better results.
KEYWORDS
CAD/CAM; Digital Image Analysis; Implant-Supported Bar; Intraoral Scanner; Marginal Fit.
1. INTRODUCTION usually accompanied by general health issues, which
also affect the dental treatment plan [3]. Therefore,
The preservation of natural teeth is one of the goals the suggested dental treatment plans should be
of modern dentistry, resulting in a progressively realistic, straightforward, versatile, aiming to restore
increasing demand for partial dentures [1]. With the the lost functionality and cover the esthetic needs
propagation of age, replacement of missing teeth of the patient. Removable partial dentures, either
is a common patient need [2]. Elderly patients are traditional or implant-supported are prostheses that
OPEN ACCESS This is an Open Access article under the CC BY-NC 4.0 license.
Peer-Reviewed Article
Citation: Villias A, Papadopoulos T, Polychronakis N, Karkazis H, Polyzois G. Effect of digital workflow on the marginal fit of long-span implant-supported
bars for Kennedy II class removable prostheses in vitro. Stoma Edu J. 2021;8(1):33-44.
Received: January 19, 2021; Revised: February 18, 2021; Accepted: February 23, 2021; Published: February 25, 2021
*Corresponding author: Dr. Aristeidis Villias; Address: Kolokotroni 57-59, 18531 Piraeus, Greece;
Tel.: +304184843; Fax: +304184843; e-mail: Aristeidis.Villias@gmail.com
Copyright: © 2021 the Editorial Council for the Stomatology Edu Journal.
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Original Articles
Figure 1. The pseudo-realistic model (PRM). A simulated partially edentu- Figure 2. The pseudo-realistic model (PRM) with the seated tray placed
lous, Kennedy class II maxilla, including three parallel (±1°), standard, inter- under a constant axial load of 30N at an axial loading device (ALD) for
nal-hex implant analogs embedded in dental stone under 3-4 mm of simu- 12min until the impression material was fully set.
lated mucosa.
meet these requirements. The prosthodontic treat- (IOS) for digital impression facilitated the direct
ment of a Kennedy class II partially edentulous digitization of the oral environment without
maxilla is demanding in terms of biomechanics and impression materials. Furthermore, the already
esthetics. A removable partial denture attached on established implementation of laboratory scanners
an implant-supported bar might be an alternative to digitize the definitive casts affected multiple areas
treatment plan to a traditional partial denture [4]. of dentistry including implantology [21].
For implant supported prostheses, passive fit is consi- The main advantages of digital impressions are
dered an ideal goal, preventing biological and the capability to immediately evaluate the virtual
mechanical complication in the future. The clinically model chair-side, to evaluate the preparation depth,
accepted marginal fit (MF) might even surpass to modify the virtual model, they are producing
200μm [5-7]. Furthermore, the shape of the internal less waste and they are time efficient. However,
connection implant features might affect the the IOS systems require a large investment and
retention and the quality of the connection [8]. are associated with low quality of evidence when
Ideally, discrepancies at the margin ought to be kept the resulting prostheses are compared for the
to a minimum. However, a passive fit and MF with marginal and internal fit with those from traditional
undetectable discrepancies are technically almost impressions [22]. Furthermore, their accuracy can be
impossible to achieve. Additionally, the correlation affected by several parameters, such as the distance
between the degree of MF and the incidence of between implants, their inclination, their depth, the
clinical implications is yet to be defined. Nonetheless, lighting conditions and the user experience [23-
it is clear that the various clinical and laboratory 32]. A fully digital approach in prosthetic dentistry
procedures introduce errors affecting the passivity is nowadays possible, given the fact that Computer
of the fit. Aided Design and Computer Added Manufacturing
The MF can be evaluated clinically with digital dental (CAD/CAM) procedures are widely acceptable over
X-Rays, or with direct view if the margin can be directly dental laboratories [33].
observed [9]. Assessment of MF with an explorer can The introduction of IOS has offered a new way to
be unreliable [7]. Additionally, regarding implant register the implant location. In this direction implant
supported prostheses, the explorer tip could scratch manufacturers have introduced impression copings
the delicate implant surface [10]. In vitro studies for digital procedures, the scan posts [34]. These
have a much wider armamentarium of methods to scan posts are components of standard geometry
examine the MF [11]. The horizontal or the vertical accompanied with their corresponding digital
marginal discrepancy at the restoration margin is design that facilitates the component recognition
reported in previous studies as an indication of the from the IOS software or the CAD software [23,34].
MF [6]. Destructive and non-destructive methods However, the implant system might affect the
can be implemented [12-15]. The quality of the impression accuracy [23].
margin can be assessed with direct observation or A challenge regarding edentulous areas is the lack
through indirect procedures [11,16,17]. MF has been of stable anatomical features on the mucosa that
evaluated with image superimposition methods in hinder accurate stitching of acquired images from
combination with digital techniques as well [12,18]. the intraoral scanners [29,35-39]. Several studies
Until recently impressions were only taken with have compared the accuracy of intraoral digital
impression materials placed in trays and inserted systems for crowns and short-span prostheses
in the patient mouth. The procedure required [20,22]. Scenarios implementing larger prosthesis
a number of expendables, devices, skills and have been run in simplified plaster models as well
experience [19,20]. Introduction of intraoral scanners [39,40]. Furthermore, the introduction of digital
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Original Articles
Figure 4. A screen image of a computer aided design (CAD) of a long-
span, implant-supported, screw-retained at implant level, parallel bar on
Figure 3. Simultaneous capture (red arrows) of the two most distant con- the digitized PRM created by an experienced dental technician with the
secutive scanning posts attached to the underling implant analogs at posi- exocad™ software.
tions 14 and 17 by the field of view of the scanner sensor (yellow high-
lighted area).
Figure 6. A customized Replica Production Device (RPD) utilized for the
non-destructive negative replica technique implemented in this study. The
Figure 5. The parallel screw-retained Co-Cr bars manufactured on one
RPD with three incorporated parallel implant analogs, identical to those
plate by a computer aided manufacturing (CAM) - Selective Laser Melting
utilized in the PRM, placed at the same relevant positions as the PRM and
(SLM) technique utilizing a 3D printer seen after the appropriate stress re-
verified by two interchangeable verification jigs, one produced on the PRM
lieving procedure.
and the other on the RPD.
techniques offer new ways in which the prostheses the dental stone. The analogs were located under
can be produced. However, there is not enough 3-4mm of simulated mucosa, at the locations of
evidence regarding the resulting fit between implant- the lost maxillary right central incisor (11), first
supported bars extending over a quadrant that are right premolar (14) and second right molar (17).
manufactured through a fully digital workflow and The simulated teeth were acrylic and the maxillary
bars manufactured with a partially digital workflow. mucosa was simulated with red pigmented acrylic
The purpose of this in vitro study was to compare resin.
the effect of a conventional impression technique
and a technique implementing an intraoral scanner 2.1. Impression techniques
for digital impression on the marginal fit of implant- Two impression techniques were compared: the
supported, long-span, parallel bars, manufactured closed tray technique with monophase vinyl
with laser sintering technique. The effect of the polysiloxane impression material (CTM) and the
screw tightening sequence was also examined. intraoral digital impression technique (IOS).
The null hypothesis was that the marginal fit of im- 2.1.1. Closed Tray Technique
plant-supported bars would not be affected by Initially three impression copings for the closed
the impression technique or the screw tightening tray technique (Direct impression coping for closed
sequence. tray, internal hex, MD-IT300-SP, MIS Implants
Technologies Ltd, Dentsply Sirona, York, PA, USA-
2. MATERIALS AND METHODS LOT#: W16002796) were tightened with 10Ncm
utilizing a torque ratchet on the implant analogs
This laboratory study was conducted on a simulated of the PRM. The fit of the impression copings was
partially edentulous, Kennedy class II maxilla, verified with digital X-rays (Belmond Phot-X II, Takara
which functioned as a pseudo-realistic model Belmont Corp, Osaka, Japan) captured with the
(PRM). The illustrated PRM (Fig. 1), included three, parallel technique utilizing a sensor (Schick CDR USB
parallel (±1°) standard, internal-hex implant analogs Remote HS, Schick Technologies Inc, NY, USA). Next,
(Implant analog Internal Hex. Seven, MIS Implants the provided plastic rings were firmly seated on the
Technologies Ltd, Lot: W17007917) embedded in impression copings.
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Original Articles Table 1. Summary of experimental groups.
Table 1. Experimental groups
Group I CTM-A11
Group II CTM-A17
Group III IOS-A11
Group IV IOS-A17
Figure 7. The production of negative replicas with a customized Replica Production Device (RPD) through a non-destructive method: A. A parallel bar
before replication on the RPD with the implant analogs placed at the same relevant positions as the PRM. The three specialized copper trays seen in place
surrounding the implant analogs. B. The bar seated and secured with three prosthetic screws on the RPD analogs following a preselected screw tightening
sequence. C. Three negative replicas made simultaneously for each implant-bar connection from low viscosity addition silicone in the specialized cylindri-
cal copper trays. D. The removed bar after polymerization of the A-silicone. E. The three specialized copper trays carefully detached from the RPD utilizing
a modified crown forceps. F. The three removed-from-the-trays, trimmed and marked negative replicas corresponding to each implant-bar connection. G.
The sample intersection device (SID) facilitating the standardized sectioning of each cylindrical negative replica in six 60° sectors. H. One sectioned nega-
tive replica in six standardized marked slices, corresponding to six oriented observations for each of the three implant-bar connections of an implant-
supported bar.
Monophase vinyl polysiloxane (Variotime Dynamix temperature 23±1°C, 50±10% Relative Humidity
Monophase, Heraeus Kulzer GmbH, Germany, (RH) with suitable light conditions (1100 Lux, 5500Κ).
Lot: Κ010109) from an automix device (Variotime After 24h, each impression was used to produce a
Dynamix Monophase, Heraeus Kulzer GmbH, dental stone model. 48h later, three scan posts were
Germany) was used for the closed tray technique. placed on the implant analogs of each model. Then,
6±0.5ml of mixed material was loaded in each of the the three dental stone models were digitized with a
two syringes (Impression Jet, Heraeus Kulzer GmbH, laboratory optical scanner (Identica Blue ColLab scan
Hanau, Germany) and the impression copings were v.2.003, Medit corp, Seoul, Korea) and the Standard
covered with the material. Next, 25±1ml of mixed Tessellation Language (STL) files were saved with a
material was loaded on the perforated commercially code name for later use. The PRM was scanned with
available metal tray within 14±1s. The tray was the same scanner as well.
appropriately seated on the PRM initially with a 2.1.2. Intra-Oral Scanning
finger compression force (15-30N) for 30s. This The Intra-Oral Scanner (IOS) I-tero (I-tero Model
procedure took place within 2min and 30s, which was HDU-E Intra-oral Scanner Optical Impression Device,
the material working time. Next, the PRM with the CADENT® Ltd., Or Yehuda, IL-60212 Israel) was
seated tray was placed under a constant axial load of utilized for a direct digitization of the PRM. Three
30N for 12min at an axial loading device (ALD) until scan posts (Scan Post, int.hex. connection, SP, MIS
the impression material was fully set (Fig. 2). Finally, Implants Technologies Ltd, Dentsply Sirona, York, PA,
the impression was removed from the PRM. Each USA- LOT#: W18002193) were placed on the implant
impression coping was unscrewed from the PRM, analogs of the PRM. The scan posts on analogs 11-
placed on a new implant analog, tightened with 14 and 14-17 were simultaneously captured with the
10Ncm with the Torque ratchet and the assembly IOS (Fig. 3). Their proper seating was checked with
was snapped on the corresponding plastic ring, digital X-rays. Next, the PRM was scanned without
which was embedded in the set impression material. powdering. The scanning took place in standardized
In this manner three impressions were taken at room light conditions within a light chamber with artificial
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Original Articles
Table 2. Summary of descriptive statistics and normality tests.
Shapiro-
Group Variable N Mean SE SD Variance Skewness Kurtosis
Wilk (Sig.)
CTM-A11 BHD 6 57.91 4.048 9.915 98.308 .446 -1.452 .656
BVD 6 186.82 53.411 130.831 17116.719 .110 -2.954 .068
BCD 6 26.39 2.207 5.406 29.225 -.611 -.792 .486
CTM-A17 BHD 6 55.58 4.067 9.962 99.238 -.034 -3.068 .070
BVD 6 180.52 51.932 127.208 16181.761 .081 -3.032 .055
BCD 6 27.72 3.832 9.386 88.099 .982 1.156 .682
IOS-A11 BHD 6 313.59 89.160 218.396 47696.865 -.428 -2.007 .242
BVD 6 469.63 119.117 291.777 85133.759 -.585 -1.895 .111
BCD 6 423.99 147.872 362.212 131197.519 .121 -1.670 .401
IOS-A17 BHD 6 315.90 89.370 218.912 47922.460 -.534 -2.049 .180
BVD 6 484.44 122.869 300.967 90581.294 -.685 -1.998 .061
BCD 6 407.97 136.074 333.313 111097.375 -.099 -1.595 .511
Figure 8. Demonstration of the analysis of the acquired digital photomicrograph. The standardized image shows a negative replica slice presenting the
marginal fit at a sixth of the periphery of one of the three implant-bar connections of an implant-supported bar of this study. The empty space left by the
RPD implant-analog on the right and the implant-supported bar on the left shown as dark areas in the upper part of the photomicrograph. The space
surrounding the analog-bar assembly presented as the purple illuminated area. The gap between the connected components presented as a character-
istic elongated light purple protrusion towards the dark area of the image. The section of the bar surface traced in yellow and the implant analog surface
section traced in black. The dependent variables Bar Horizontal Discrepancy (BHD), Bar Vertical Discrepancy (BVD), Bar Conical Discrepancy (BCD) calcu-
lated on the means of the 18 measurements from each bar corresponding to the blue, orange and green lines shown in the figure respectively.
light (~500 lux). Three consecutive scans were taken, 2.2.1. Manufacturing of the bars
each within 20±2 min. Those scans were used to Six parallel screw-retained bars were manufactured
create 3 STL files which were code-named and sent with the CAM - Selective Laser Melting (SLM)
to the lab by e-mail. technique utilizing a three dimensions (3D) printer
certified for dental use (TruPrint 1000 Multilaser,
2.2. Design of parallel screw-retained bars TRUMPF GmbH, Ditzingen, Germany). The bars were
A CAD of a long-span, implant-supported, screw- produced from a Co-Cr dental alloy powder (Mediloy
retained at implant level, parallel bar on the S-Co™, Bego®, Bremer Goldschlägerei Wilh. Herbst
digitized PRM was created by an experienced dental GmbH, Bremen, Germany, LOT# P180709B). The bars
technician with the exoCAD software (ExoCAD- were manufactured on one plate with the designated
DentalCAD v6136, 2016. Exocad GmbH, Darmstadt printing sequence, followed by the appropriate
Germany) (Fig. 4). stress relieving process (Fig. 5). Finally, the bars were
The designed bar was adjusted so that it would best removed from the base plate, supporting pins were
fit the supporting implants at each of the six acquired trimmed and finishing procedures were applied.
digital models and a unique code-name was given 2.2.2. Marginal fit assessment
to each design. In this way six similar designs were The marginal fit of the bars was evaluated with
created, three for the CTM group and three for the two methods. Initially each bar was examined
IOS group. radiographically on the PRM, implementing the
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Figure 9. Power-Sample size plot created with the G*Power software, in- Figure 10. Bar charts of the mean BHD in μm for every experimental
dicated for the a-priori calculation of adequate sample size for F-tests for a group separately. The confidence interval i-bars are drawn in black lines. All
certain power level given the α-error probability and the effect size. differences are statistically significant (p<0.05) except from those between
groups of the same impression technique.
parallel cone technique. The parallel bars were sources were utilized, increasing the illumination
placed on the PRM and the prosthetic screws at the stage at a mean value of 57000 lux.
were tightened in the sequence 11,17,14 with a In the acquired images one pixel corresponded to
standardized minimal torque 10Ncm. 0.556μm.
Next, the MF was measured through a non- A DIAS was applied for the analysis of the captured
destructive method implementing the negative images [15]. Each image showed the marginal fit
replica (NR) technique and a modification of the at a sixth of the periphery of every implant-bar
digital image analysis sequence (DIAS), which was connection. The dark area in the photomicrograph
previously described and validated [15]. The parallel of the NR slice corresponded to the empty space
bars were seated directly on the three implant left by the analog and the bar assembly and the
analogs and secured with the designated prosthetic purple illuminated area corresponded to the space
screws. For the production of NR, a customized surrounding the assembly. If there was a gap between
Replica Production Device (RPD) was utilized. The the connected components, it was presented as
RPD incorporated three parallel implant analogs a characteristic elongated light purple protrusion
identical to those utilized in the PRM. The analogs towards the dark area of the image (Fig. 8). The image
were placed at the same relevant positions as the processing software Photoshop (Adobe Photoshop
PRM, verified by two interchangeable verification CS 5 V12.0.4 x64, Adobe Systems Inc.) was utilized for
jigs, one produced on the PRM and one on the RPD each image. The outline of the bar was traced and
(Fig. 6). highlighted utilizing a sensitive digitizer (Bamboo
The RPD facilitated the simultaneous production CTH-470/S, Wacom, Toyonodai, Japan). Furthermore,
of three NRs, one for each implant-bar connection. the implant analog outline was superimposed at the
The NRs were made from light viscosity addition best perceived fit position on the acquired image.
silicone (Image PVS Super light body fast, Dental One-pixel thick lines were drawn and the processed
Line Ltd, Piraeus, Greece) in specialized cylindrical images saved without compression.
copper trays. After polymerization of the A-silicon, The standard geometry of the implant analogs in
the bar was removed and the three specialized this experiment facilitated the a-priori drawing of
copper trays were detached from the RPD. The NRs the component outline. The analog outline included
were removed from the trays, trimmed, marked and a marking point at the conical part of the component
sectioned in a sample intersection device (SID). SID and two-line extensions from the inner edge of the
allowed the reproducible section of the cylindrical shallow platform at the margin of the analog. These
NR in six 60° sectors (Fig. 7). One NR was created additions would aid the measurements on a next
for each of the three implant-bar connections and step.
the aforementioned procedure facilitated the assess- 2.3.1. Measurements
ment of the MF at 18 points on every bar. The marginal fit was evaluated on each
photomicrograph through three dependent varia-
2.3. Digital Image Acquisition and Analysis bles: Horizontal discrepancy (HD), Vertical discre-
Following a standardized procedure the NR slices pancy (VD) and Conical discrepancy (CD), measured
were examined under an optical microscope (Digital in μm. VD and HD were defined as the length of the
Microscope Leica DM 4000 B, Leica Microsystems, line segment from the internal edge of the shallow
Mannheim, Germany) with a mounted camera at 320x platform of the utilized implant analog extending
final magnification. Photomicrographs were taken perpendicularly or in parallel towards the outline
and stored in an external hard disc. The microscope of the bar, respectively. The conical discrepancy
settings were adjusted for maximum field of view, was geometrically defined at the conical part of the
minimal depth of field and highest resolution for the internal hexagon of the utilized implants. Following a
selected magnification. External additional lighting standardized procedure, measurements were taken
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Figure11. Bar charts of the mean BVD in μm for every experimental Figure 12. Bar charts of the mean BCD in μm for every experimental
group separately. The confidence interval i-bars are drawn in black lines. group separately. The confidence interval i-bars are drawn in black lines.
None of the differences is statistically significant (ns). None of the differences is statistically significant (ns).
with the open-source software ImageJ (ImageJ 1.52p, The descriptive statistics per group are summarized
National Institutes of Health, Bethesda, MD, 20892 in Table 2 together with the results of the Shapiro-
USA) on the processed image. The measurements Wilk test of normality for relatively small sample
were automatically stored, transferred and organized sizes. The latter examined if the distribution of data
for statistical analysis utilizing programs. The overall differed significantly from the normal distribution.
marginal fit of each bar was quantitatively evaluated No significant differences were found (P>.05ns),
with 3 indices: Bar Horizontal Discrepancy (BHD), Bar hence parametric tests were run.
Vertical Discrepancy (BVD), Bar Conical Discrepancy The one-way ANOVA showed a statistically signifi-
(BCD). These were calculated on the means of the cant difference between groups for the BHD
measurements of the HD, VD and CD respectively. (F(3,20)=5,558, P<,01, ω=,60, η2=0,45, Power (1-β err
2.3.2. Sample size prob)=94%). A significant difference was also found
The marginal fit of the six bars (3 from the CTM and 3 between groups for the BVD (F(3,20)=3,299, P<,05,
from IOS) was also evaluated depending on the screw ω=,15, η2=0.33, Power (1-β err prob)=75%), and
tightening sequence, forming two subgroups: Group between groups for the BCD differences were also
A11 and Group A17. In group A11 the first tightened statistically significant (F(3,20)=4,996, P<,05, ω=,58,
screw was at the most proximal implant analog η2=0.43, Power (1-β err prob)=91%). The Sample Size
(11), followed by the one at the most distal implant -Power analysis diagram regarding the variable BVD
analog (17) and finally at the one in the middle (14). is presented (Fig. 9).
In group A17 the screws were tightened on the Tukey’s post hoc tests were run to reveal the
sequence 17, 11, 14. Deriving from the combination differences by comparing the means of the
of the factors, impression technique and first tight- different groups. The results of these tests are briefly
ened screw, 4 groups were formed and compared summarized next. BHD was significantly different
in this experiment. The NR technique was run twice between groups [CTM-A11] (M= 57,91, SE=4.05)–
for the samples in these groups, doubling the data. [IOS-A11] (M=313,59, SE=89,16) (P<.05), [CTM-A11]
One researcher ran the experiment. The groups are (M= 57,91, SE=4.05)- [IOS-A17] (M=315,90, SE=89,37)
summarized in Table 1. (P<.05), [CTM-A17] (M=55,58, SE=4.07)– [IOS-A11]
2.3.3. Statistical analysis (M=313,59, SE=89,16) (P<.05) and [CTM-A17]
The data were statistically analyzed and graphs were (M=55,58, SE=4.07)-[IOS-A17] (M=315,90, SE=89,37)
drawn with the SPSS software (SPSS for Windows 64- (P<.05).
bit edition, V25. IBM Corp.). Descriptive statistics were The implant-supported bars produced after a closed
computed. The normality of the data was examined tray technique with Monophase PVS impression
by Shapiro-Wilk tests, Q-Q plots and corresponding material either with their most mesial screw
histograms. A one-way Analysis of Variance (ANOVA) tightened first or the most distal one had a mean
was conducted for each of the depended variables horizontal discrepancy between the bar and the
and Tukey HSD post hoc tests were utilized to implant, which was significantly smaller from that of
locate the differences (α=0.05). The effect sizes were bars produced after an intraoral scan independedly
computed and a power analysis was conducted post of the screw tightening sequence.
hoc with the open-source software GPower [41,42]. The post hoc tests could not detect the significant
differences for the BVD vertical discrepancies at
3. RESULTS the marginal fit (P>.05). Furthermore, the post hoc
tests could not reveal the significant differences
Four groups with 24 samples each were formed. between groups for the variable BCD. None the
432 digital images were analyzed in total and 1296 less, the [CTM-A11] (M=26,39, SE=2,21)– [IOS-A11]
measurements were taken from those images. (M=423,99, SE=147,872) comparison of BCD means
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although not significant (ns) had P=0.50 (ns) and relatively deep. Additionally, the scan bodies had
Original Articles the [CTM-A17] (M=27.72, SE=3,83) – [IOS-A11] flattened shape and elongated at the proximal-distal
(M=423,99, SE=147,872) comparison had P=0.51 dimension, which facilitated the partial capture in
(ns). These results were graphically drawn as bar a single frame with the IOS wand of the two most
charts with the SPSS software. The mean BHD, BVD distant successive implant analogs 14 and 17 at the
and BCD were drawn as bar charts with the 95% PRM. Nevertheless, the marginal discrepancies at
Confidence interval shown for every group (Fig. 10, the IOS group were not acceptable. Probably there
Fig. 11, Fig. 12). might not have been enough orientation points to
accurately stitch the frames on the edentulous area
3.1. Radiological results of the PRM.
The analysis of the digital X-Rays taken directly at the Huang et al. found in their study that when a scan
PRM facilitated vertical and horizontal discrepancy body includes an extensional structure, the scanning
measurements at the mesial and distal sides of every accuracy is significantly improved [35]. Although
implant-bar connection. The mean values of these several methods are applied for the assessment of
measurements CTM HD=67μm, IOS HD=322μm and the margin, in this study the NR technique was
CTM VD=67μm, IOS VD=689μm are comparable with selected [11,17]. The MF could be assessed with
the BHD and BVD of the [CTM-A11] and [IOS-A11] direct in-situ observation as well [18]. However,
groups, shown in the descriptive statistics table. direct observation relies only on one-dimensional
measurements at the observed gap between exami-
4. DISCUSSION ned components. Alternatively, samples could be
embedded in resin material and following a
This study compared the marginal fit of implant- destructive method, their sections could be exami-
supported, long span, parallel bars, produced after a ned under a microscope [11]. Nevertheless, such a
fully digital versus a partially digital workflow. Within technique would have included several steps and
the limitation of this study the null hypothesis that in case an electron microscope would have been
there will be no differences among groups in all three utilized, it would have been also a time-consuming
marginal fit indices was rejected. Additionally, within approach and costs would have been increased
the limitations of this study, the partially digital as well [16]. In a previous study DIAS, a recently
workflow combining conventional impressions resul- developed and reliable stepwise procedure, has
ted in prostheses with better marginal fit. The fully been implemented for the assessment of the crown
digital workflow implementing an intraoral scanning margin on cemented implant-supported crowns
system for the direct digitization of the maxilla resulted [15]. Because of its non-destructive approach, the
in implant-supported bars that exhibited marginal NR technique could be applied in clinical studies,
discrepancies, which could not be considered as given appropriate modifications are introduced and
clinically acceptable. Digital impression is a key step in ex-vivo studies. It is a feasible tool for the
in the outcome of a completely digital workflow. quality assessment at the numerous stages of the
Complete arch scanning is associated with larger prostheses production [13,14]. The strict and clear
deviations as compared to partial arch scanning [22- criteria applied in this study for the evaluation of MF,
24]. It has been found that the first scanned quadrant minimized observer subjectivity, facilitated reliable
is recorded more accurately in comparison to the measurements and eliminated loss of data.
one that follows [27]. This laboratory study included In this study, the VD found for the CTM group were
complete arch scanning of a simulated partially smaller as compared to the IOS group. This trend was
edentulous maxilla under controlled lighting also reported by Lo Russo et al [6]. The implementation
conditions. It has been found that artificial lighting of the NR technique in this study also facilitated the
can affect the scanning procedure [25]. The scanning evaluation of HD. Such measurements are indicative
initiated from the dentate quadrant, which provided of the quality of the produced prosthesis. In this
orientation points for the stitching of acquired data. study the parameters BHD and BCD were assessed
The quadrant with the implants was scanned later. as overall indication of the mean HD and the mean
Therefore, both deteriorating factors that have been VD at the restoration margin respectively. The shape
associated with lower accuracy have been combined of the internal connection implants like the ones
in this study, which could explain the unacceptable that were implemented in this study presents
outcomes of the completely digital workflow. It complex features like conical internal walls. Such
has been found that by increasing the distance features are common in newer designs with steeper
between implants the intraoral scanning precision is conical walls featuring Morse taper connections.
decreased [29]. Flügge et al. found that the implant Such features might affect the retention and the
system might also affect the impression accuracy quality of the mechanical connection [8]. Therefore,
[23]. The MIS®-Seven™ implant system, however, was an additional parameter was evaluated in this study,
accompanied with scan-bodies with non-reflecting the BCD, as an evaluation of the near margin quality
surface and high enough to notably protrude from of the connection at the conical part of the internal
the mucosa even when the implants were placed connection implants that were used. BCD was the
40 Stoma Edu J. 2021;8(1): 33-44 pISSN 2360-2406; eISSN 2502-0285
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mean discrepancy between the implant analog and antirotation design, which would later be connected
Original Articles
the prosthesis observed 120μm within the inclined with the rest of the metallic frame. The antirotation
internal walls of the implant analog. connection type might also be a reason for the
There is no consensus about the clinically acceptable unacceptable marginal gaps encountered in the
marginal fit. However, most studies report a range intraoral scanner groups in this study. Furthermore,
between 50-200μm. In this study the groups that the older software version of the utilized scanning
included the parallel bars that were manufactured system might also be a reason.
after a traditional impression with the closed tray In this study, the effect of the impression technique
technique presented discrepancies within that on the marginal fit of implant-supported parallel
range. The observed discrepancies might be over- bars was evaluated at implant level. The field of view
estimated since the prosthesis was fixed on the of the scanner sensor facilitated the simultaneous
implant analogs with prosthetic screws tightened capture of two consecutive scanning posts attached
with a minimal torque (10Ncm). An increased torque to the three implant analogs in that region. The posts
on the other hand might also amplify the tension at positions 11 and 14 were simultaneously covered
and therefore compromise the passivity of fit. In this in a larger part of their surface than the posts at
study, the marginal fit of the implant-supported bar positions 14 and 17. This might have contributed to
produced after an impression with the traditional the insertion of errors during scanning, due to lack of
closed tray technique with VPS impression material adequate orientation points. Flügge TV et al, in their
was comparable with the results found by Lin WS et study, measured the distances and the inclinations of
al [30]. In this study the marginal fit of bars created scan posts on the digital models and underlined the
after an intraoral scanning was not acceptable, negative correlation between accuracy of intraoral
exceeding 1000μm gaps in some cases. Andersen scanning systems and the distance between scan
FS et al., in their study using the iTero system for posts [29]. In this study, although the PRM simulated
implant supported mandible complete dentures, the color of the tissues of the oral cavity and it was
found similarly unacceptable marginal fit levels accordingly polished, clinically saliva, blood and
[36]. Their explanation for those results was the lack humidity might be present, posing additional
of anatomical structures which could facilitate the challenges for an accurate scan. Additionally, the
orientation of the images by the scanner software. scanner wand was bulky which probably poses an
Earlier, Patzelt SB et al. found comparable results extra limitation for accessing distal areas clinically.
regarding intraoral scanners used in edentulous Moreover, in this study implant analogs were placed
patients, concluding that such systems should be parallel to each other, which might have favored
avoided for similar cases [37]. On the contrary Kim the results. On the contrary, Gracis S et al. and Lee
SY et al. found the iTero system had similar accuracy HJ et al., found that inclined implants could hinder
as the impression with addition silicone [38]. In an accurate scan [31,32]. Additionally, in this study
their study however they utilized a plaster model implant analogs were used and not actual implants.
of a partially edentulous case, which facilitated the No data could be found for the accuracy of these
orientation of the images by the scanner software. components. Furthermore, the analogs were loca-
Keul et al. (2020) found that the iTero IOS had ted under 3-4mm of simulated mucosa, which is
comparable results as the traditional impression relatively deep. This might have a negative effect
technique. However, as compared to this study they on the accuracy of the impression as well as the
utilized a more recent model with updated software intraoral scan [28]. Especially for the latter, the scan
[39]. It seems that differences in the connection type posts were deeply submerged under the mucosa
in combination with the implant inclination might obscuring a large portion of their surface. Hence,
affect the accuracy of the impressions especially for there were less points of orientation available for the
the internal connection type. That might be explained software to recognize for an accurate placement of
by the increased contact surface of the impression the implant in the CAD software.
coping with the implant making the removal of the The bars in this experiment were manufactured with
impression harder after the polymerization of the a 3D printing technique using Co-Cr alloy powder.
impression material. Additionally, the number and Although the SLM frameworks are produced with
the relative positions of implants in the mesial-distal acceptable accuracy, the surface of the components
orientation might also affect the removal of the is rough. Such quality is not favorable when the
impression after setting [20]. The intraoral scanning components are intended for precision connections
systems seem to have adequate accuracy for single such as telescopic crowns and bars. Additionally,
crowns and short bridges [34,40]. The complete the coarse surface posed a challenge for the edge
digital workflow for constructing implant supported recognition on the negative replica. The marginal
prostheses might raise additional limitations. In this fit was examined in this study with two techniques.
study, it was found that the digital libraries provided One was a direct technique, applicable clinically, by
by MIS only included antirotation design for the taking digital radiographs with the parallel cone
implant-level connection abutments. They suggest technique of the analog-bar assembly. Yet, the
the use of prefabricated components without radiographs revealed the quality of the implant-bar
Stoma Edu J. 2021;8(1): 33-44 pISSN 2360-2406; eISSN 2502-0285 41
Villias A, et al.
www.stomaeduj.com
connection was inadequate for performing accurate applied method. The simulated intraoral scanning
Original Articles measurements, due to a relatively low resolution. with the iTero system in the setup of the present study
Furthermore, only two points at the margin could resulted in unacceptable marginal gaps, visualized
be examined, one distally and one mesially of on replica segments under the optical microscope.
every analog-prosthesis connection. Wahle et al. In this study, the closed tray impression technique
concluded that the marginal fit is not adequately with addition silicon resulted in better marginal fit
evaluated only with radiographs [9]. The other levels, when examined with the applied method,
technique was the negative replica technique in while the screw tightening sequence does not seem
combination with a DIAS [15]. to affect the prosthesis adaptation on the implants.
4.1. Clinical relevance CONFLICT OF INTEREST
The intraoral scanning systems have undoubtedly The authors declare no conflict of interest.
advantages; however, regarding the implementation
of such systems in implant dentistry, one should ACKNOWLEDGMENTS
proceed with caution as not acceptable fit levels
might result. The authors thank Professor Dr. Demetrios Halazonetis for his
Furthermore, the availability of ready-to-use digi- contribution with the iTero IOS system. The authors also thank
tal designs of components needs to be assessed the Dental Technician Ioannis Malindretos for his contribution
in advance since proprietary rights might imp- with the working models and CADs. The authors appreciate the
ose limitations leading to different treatment donation of the 3D printed - Laser Sintering bars from the Dental
approaches. Technician Laboratory Ergastiri 86 Ltd expressing their thanks. The
authors also express their thanks to George Villias Dipl. Ing., MSc
5. CONCLUSION for his assistance with coding the programs “Coding_Input” and
“Data_Table_Generator_V5FX” for the automated data transfer for
The implementation of the negative replica statistical analysis.
technique in combination with the modified
DIAS was a viable, non-destructive method to AUTHOR CONTRIBUTIONS
simultaneously assess the horizontal, vertical and
conical fit of the implant-supported bars on three
TP and AV: the conception and design of the study. AV: acquisition
implants in this study.
of data. AV, TP, NP, HK and GP: analysis and interpretation of data.
Within the limitations of this study, it was concluded
AV, NP, HK: drafting the article. TP, HK and GP: revising it critically
that all the examined combinations of impression
for important intellectual content. AV, TP, NP, HK and GP: final
techniques and screw tightening sequences resulted
approval of the version to be submitted.
in marginal discrepancies, detectable with the
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Villias A, et al.
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Original Articles Aristeidis VILLIAS
DDS, MSc, Dr.Med.Dent., Clinical Instructor
Department of Prosthodontics
School of Dentistry
National and Kapodistrian University of Athens, Athens, Greece
CV
Aristeidis Villias graduated from the School of Dentistry of the National and Kapodistrian University of Athens, Greece in 2008.
In 2015 he obtained his doctorate degree from the University of Cologne, Cologne (NRW), Germany after a scholarship for
postgraduate studies abroad issued (2010) by the State Scholarships Foundation of the Hellenic Republic (IKY). In 2015 he also
started his private practice in Piraeus, Greece. In 2019 he obtained his master’s degree in Dental Biomaterials from the School of
Dentistry of the National and Kapodistrian University of Athens, Greece. Since 2020 he has been Clinical Instructor in Removable
Prosthodontics, School of Dentistry, National and Kapodistrian University of Athens, Greece and in the Section of Dental
Technology, Department of Biomedical Sciences School of Health and Care Sciences, University of West Attica, Athens, Greece.
Questions
1. According to the article, the implant-supported bars manufactured after the
traditional impression technique as compared to those manufactured after
implementation of the intraoral scanner:
qa. Have significantly better horizontal discrepancy at the implant-bar interface;
qb. Present undetectable discrepancies at the implant-bar interface;
qc. Have inferior marginal fit at the implant-bar interface;
qd. Present marginal discrepancies which are not clinically acceptable.
2. The screw tightening sequence when seating the implant supported bar in this study
seems to:
qa. Have no effect on the marginal fit of the bar at implant level;
qb. Affect significantly the marginal fit of the bar at implant level;
qc. Have a significant effect when the most mesial screw is tightened first;
qd. Have a significant effect when the most distal screw is tightened first.
3. In this study the marginal fit of the computer aided manufactured bars
qa. Was better when a fully digital workflow was followed;
qb. Was worse when a fully digital workflow was followed;
qc. Was similar either with a fully digital workflow or with a partially digital workflow;
qd. Was worse when a partially digital workflow was followed.
4. The computer aided design of implant supported bars
qa. Was versatile allowing smooth implementation of the conceived design;
qb. Was versatile allowing implementation of the conceived design after certain auxiliaries had been
purchased;
qc. Was limited by proprietary design concepts;
qd. Was not applicable in this study.
44 Stoma Edu J. 2021;8(1): 33-44 pISSN 2360-2406; eISSN 2502-0285