Download Accuracy of two impression techniques with angulated implants

Accuracy of two impression techniques
with angulated implants
Heather J. Conrad, DMD, MS,a Igor J. Pesun, DMD, MS,b Ralph
DeLong, DDS, MS, PhD,c and James S. Hodges, PhDd
School of Dentistry and Minnesota Dental Research Center for
Biomaterials and Biomechanics, University of Minnesota,
Minneapolis, Minn; Faculty of Dentistry, University of Manitoba,
Winnipeg, Manitoba, Canada
Statement of problem. Accurate recording of implant locations is required so that definitive restorations are properly
supported and do not place additional stress on the implants. Angulated implants may result in inaccurate impressions, and the impression technique may affect the accuracy of the definitive cast.
Purpose. The purpose of this study was to determine the effect the combined interaction of impression technique,
implant angulation, and implant number has on the accuracy of implant definitive casts.
Material and methods. One definitive stone cast was fabricated for each of 6 experimental groups and 1 control
group. All 7 definitive casts had 3 implants arranged in a triangular pattern creating a plane. In the 6 experimental
groups, the center implant was perpendicular to the plane of the cast while the outer implants had 5, 10, or 15 degrees convergence towards or divergence away from the center implant. The control definitive cast had all 3 implants
parallel to each another and perpendicular to the plane of the cast. Five open tray and 5 closed tray addition silicone
impressions were made of each definitive cast. Impressions were poured with type IV dental stone, and a fine tip
measuring stylus was used to record multiple axis (X-Y-Z) coordinates on the top surface of the implant hex and on
the cast base. Computer software was used to align the data sets and vector calculations determined the difference
in degrees between the implant angles in the definitive cast and the duplicate casts. Statistical analysis used repeatedmeasures ANOVA (a=.05) with post-hoc tests of significant interactions.
Results. The angle errors for the closed and open tray impression techniques did not differ significantly (P=.22).
Implant angulations and implant numbers differed in average angle errors but not in any easily interpreted pattern
(P<.001). The combined interaction of impression technique, implant angulation, and implant number had no effect
on the accuracy of the duplicate casts compared to the definitive casts (P=.19).
Conclusions. The average angle errors for the closed and open tray impression techniques did not differ significantly.
There was no interpretable pattern of average angle errors in terms of implant angulation and implant number. The
magnitude of distortion was similar for all combinations of impression technique, implant angulation, and implant
number. (J Prosthet Dent 2007; 97: 349-356)
Supported by the Tylman Grant from The American Academy of Fixed Prosthodontics; awarded first place in Tylman Research
Award competition.
Presented at the 56th Annual Meeting of the American Academy of Fixed Prosthodontics, February 2007, Chicago, Ill.
Assistant Professor, Division of Prosthodontics, Department of Restorative Sciences, School of Dentistry, University of Minnesota.
Associate Professor and Department Head, Department of Restorative Dentistry, Faculty of Dentistry, University of Manitoba.
c
Professor, Chair Department of Restorative Sciences, and Director, Minnesota Dental Research Center for Biomaterials and Biomechanics, University of Minnesota.
d
Associate Professor, Division of Biostatistics, School of Public Health, University of Minnesota.
a
b
Conrad et al
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Volume 97 Issue 6
Clinical Implications
Accurate casts can be made with either the open or closed tray
impression technique for 3 implants angled up to 15 degrees.
Reproducing the intraoral relationship of implants through impression
procedures is the first step in achieving an accurate, passively fitting prosthesis. The critical aspect is to record
the 3-dimensional orientation of the
implant as it occurs intraorally, rather
than reproducing fine surface detail.1-3
Imprecise superstructure fit results in
mechanical and biologic consequences that disrupt the function of dental
implants.4-10 Mechanical complications include loosening, bending, and
fracture of the prosthetic or implant
components. Biologic complications
from loading above physiologic tolerance levels often result in breakdown
of the osseointegrated interface between the implant and the surrounding bone.4,5,10 Loss of integration and
loss of the implant is a potential outcome of either problem.11-14
The difference between implant
prosthodontics and conventional
fixed prosthodontics is that in the
former, it is more critical to record
the 3-dimensional position of the implants as they occur intraorally. Natural teeth have a periodontal ligament
to compensate for minor inaccuracies
of 3-dimensional positioning of the
abutments. However, integrated implants are not mobile, therefore, it is
important to ensure an accurate relationship on the definitive cast.15
The open tray technique involves
fastening an impression coping to
the implant with a screw that projects
above the height of the coping and
through an opening cut in a custom
impression tray. The screw is loosened
when the material is set and the tray
is removed from the mouth with the
impression coping retained within the
impression. An implant analog is fastened to the impression coping using
the same screw.16, 17
The open tray technique allows
for the impression coping to remain
in the impression. This reduces the effect of the implant angulation, the deformation of the impression material
upon recovery from the mouth, and
removes the concern for replacing the
coping back into its respective space
in the impression. Disadvantages of
this technique are that there are more
parts to control when fastening, there
may be some rotational movement of
the impression coping when securing
the implant analog, and blind attachment of the implant analog to the impression coping may result in a misfit
of components.16
There may be clinical situations
which indicate the use of the closed
tray technique, such as when the patient has limited interarch space, a
tendency to gag, or if it is too difficult
to access an implant in the posterior
region of the mouth.18 The closed tray
technique uses a single-piece impression coping that remains attached intraorally to the implant once the impression is removed from the mouth.
The coping is then removed from the
implant, attached to the implant analog, and carefully repositioned with
the correct orientation back into the
impression.16
Supporters of the closed tray technique suggest that it is more reliable
as visual fastening of the analog to
the coping is more accurate. There is
concern that inaccuracies with recovery and subsequent deformation may
be encountered with nonparallel implants. Impression copings must also
be repositioned exactly into their respective positions in the impression,
otherwise, misfits will occur.16
Daoudi et al19 compared the
closed tray technique at the implant
level with the open tray technique at
The Journal of Prosthetic Dentistry
the abutment level for single tooth
implants and found the open tray
technique to be superior and more
predictable. The closed tray technique
had discrepancies in axial rotation
and inclination of the analogs. Several authors have reported the superiority of the open tray technique.20-23
Carr16 compared the open and closed
tray techniques with a 5 implant mandibular cast where the interabutment
divergence angles were all less than 15
degrees. The open tray technique was
found to be superior as it provided
the most accurate working cast. Carr
indicated that the inaccuracy of the
closed tray technique may arise from
nonparallel implants and the apparent deformation of a stiff impression
material such as polyether. In a subsequent paper evaluating a 2 implant
situation, 1 parallel to the long axis
of the teeth and the other with a 15
degree lingual inclination, Carr17 reported that both techniques provided
comparable results. This is in agreement with 2 other studies.24,25
Separately, Daoudi et al26 and Liou
et al18 evaluated the ability of different
operators to reposition impression
coping analog units into an impression and described the technique as
unpredictable. Repositioning the implant impression coping back into the
impression with the analog attached
to it may result in adjusting or remaking the restoration. The copings could
not be consistently and accurately repositioned into the impression. Ortorp et al27 compared the open tray
technique using 2 different impression materials to a 3-dimensional
photogrammetric technique for recording implant positions. Although
all 3 methods studied failed to exactly
reproduce the original orientation of
the implants, the magnitude of dis-
Conrad et al
351
June 2007
tortion was similar in all situations.
Techniques to achieve more accurate impressions for patients with
multiple dental implants are described in the literature. Most of the
research has focused on techniques
to improve accuracy with parallel implants.1,21,22,24,25,28-30 Carr et al16,17,31
evaluated impression methods for
nonparallel implants; however, no
research was found comparing techniques for implant impressions at
varying degrees of divergence or convergence.
The purpose of this study was to
measure the accuracy of the closed
and open tray impression techniques
in vitro, using 7 definitive casts, each
with 3 implants at various angulations. The null hypothesis was that
the combined interaction of impression technique, implant angulation,
and implant number, would have no
effect on the accuracy of the duplicate casts compared to the definitive
casts.
1 Definitive cast for control group with closed tray impression copings.
MATERIAL AND METHODS
Coordinates were recorded on the
surface of the implant and base of the
cast to calculate the difference in the
angulation of the implant in the definitive and duplicate casts. One definitive stone cast was fabricated for each
of 6 experimental groups and 1 control group. All 7 definitive casts had
3 implants positioned in a triangular
pattern to create a plane. The center
implant (implant 2) in each of the 6
experimental groups was perpendicular to the plane of the cast, while the
implants on either side (implants 1
and 3) had 5, 10, or 15 degrees convergence towards or divergence away
from the center implant. The control
definitive cast had all 3 implants positioned parallel to each other and
perpendicular to the plane of the cast
(Figs. 1 through 3).
To create the definitive casts, type
IV dental stone (GC Fujirock EP; GC
Europe N.V., Leuven, Belgium) was
vacuum spatulated using a mechanical spatulator (Whip Mix Combina-
Conrad et al
2 Definitive cast with 5 degree convergence of implants
with open tray impression copings.
3 Definitive cast with 15 degree divergence of implant
with closed tray impression copings.
352
Volume 97 Issue 6
tion Unit; Whip Mix Corp, Louisville,
Ky) and poured into a preformed
mold and allowed to set. The casts
were held in a vertical milling machine
(Alliant Vertical Milling Machine; Alliant, Cincinnati, Ohio), and the holes
were plunge cut to match the depth
and diameter of the implant analogs.
The differing angle holes were cut with
a precision angle block placed in the
vice under the casts. Regular diameter
implant lab analogs (3i Implant Innovations, Palm Beach Gardens, Fla)
were substituted in the definitive casts
for implants due to cost and were secured with cyanoacrylate (910 Metal
Bonding General Purpose; Permabond, Pottstown, Pa). Seven definitive casts were labeled and prepared
for impression procedures. All procedures were completed by the same
operator and intraoperator error was
measured.
Seventy custom trays were made,
allowing 5 open tray and 5 closed tray
impressions for each of the 7 definitive
casts. The trays were made from a visible light-polymerizing material (Triad
TruTray; Dentsply Intl, York, Pa) and
polymerized (Triad 2000 Visible Light
Curing Unit; Dentsply Intl) for a total
of 6 minutes. The trays were trimmed
and perforated for added retention of
the impression material. The custom
open tray had a 3 opening design to
accommodate the guide pins for any
of the definitive casts.
Two-piece square open tray impression copings (Fig. 2) and twist
lock closed tray impression copings
(both of 3i Implant Innovations) with
a 3 groove orientation (Fig. 3) were
used to transfer the position and angulation of the implant. Either 3 open
tray or 3 closed tray impression copings were placed on the 3 implants in
the definitive cast to engage the hex
and were hand tightened.
The custom trays were painted
with vinyl polysiloxane adhesive (VPS
Tray Adhesive; 3M ESPE, St. Paul,
Minn) and allowed to dry for 15 minutes. The custom trays were filled with
heavy body vinyl polysiloxane impression material while the regular body
vinyl polysiloxane impression material
(Imprint II Garant; 3M ESPE) was syringed around the impression copings
on the definitive cast. The custom
trays were seated on the definitive
cast, and any excess material from the
open tray windows was removed with
a finger swipe to expose the guide
pins.
The impression material was allowed to polymerize for 10 minutes
prior to separation. For the open tray
technique, the guide pins were loosened with a hex driver and removed,
the tray was separated from the definitive cast, and the impression copings
remained locked in the impression.
The guide pins were placed back into
the square open tray impression copings from the top, while an implant
analog was connected to the hex on
the bottom, and the guide pins were
tightened with the driver.
Closed tray impression copings remained on the definitive cast after the
impression material had polymerized
when the tray was removed. These impression copings were removed one
at a time from the definitive cast and
attached to an implant analog. The
combined impression coping analog
unit was inserted into the impression
by firmly pushing it into place to full
depth and slightly rotating clockwise
to feel for the antirotational resistance.
This tactile feel indicated that the 3
grooves on the coping were locked
into place and that the implant orientation was accurately transferred.
Impressions were inspected and
repeated when any inaccuracies were
found such as air voids, impression
material between the analog-impression coping interface, impression
material separation from the custom
tray, or nonhomogenous mix of the
2 viscosities of impression material.
Special care was taken to ensure that
all components were properly oriented and completely seated. Type IV
dental stone (GC Fujirock EP; GC Europe N.V.) was prepared according to
the manufacturer’s instructions. The
impressions were boxed and filled to
form a base height of 2-3 centimeters.
The Journal of Prosthetic Dentistry
Casts were separated from the impressions after allowing the stone to
set for 1 hour, followed by trimming
and labeling to prepare for measurements.
One at a time, all 7 definitive casts
and all 70 duplicate casts were secured
to a mounting plate with a clamp. A
fine tip measuring stylus (FaroArm
Silver; Faro Technologies, Lake Mary,
Fla) with a single point accuracy of 76
μm was used to record 3-dimensional
coordinates on the implant analog
and base of the cast (Fig. 4). While
wearing x2.5 magnification, the operator manually positioned the stylus
on the top flat surface of the hex to
capture 6 points near each apex and
3 points on the base at widely separated points. Measurement of these
21 points was repeated 5 times for a
total of 105 measurements per cast
(Fig. 5). The coordinates for each cast
were recorded in a spreadsheet (Excel, Microsoft Office 2003; Microsoft,
Redmond, Wash).
In order to make comparisons
between the definitive and duplicate casts, an alignment was done to
have all the casts in the same coordinate system. Each duplicate cast was
aligned to the appropriate definitive
cast using a least square fitting algorithm (Cumulus Version 0.7, 2003
Regents of the University of Minnesota; Twin Cities, Minnesota Dental
Research Center for Biomaterials and
Biomechanics, Minneapolis, Minn).
For each of the 18 points on the definitive cast implants, the corresponding closest point on the duplicate cast
implants was identified. The duplicate cast was rotated and translated
in 3-dimensions to minimize the distance between corresponding points.
The duplicate cast orientation corresponding to the minimum distance
represented the optimal alignment
with the definitive cast.
Coordinates for the aligned duplicate casts were exported to a spreadsheet (Excel; Microsoft Office 2003;
Microsoft) where vector algebra was
used to calculate a vector parallel to
the long axis of each implant. Since a
Conrad et al
353
June 2007
4 Measuring stylus used to make measurements in multiple
dimensions (X-Y-Z).
their interaction. The within-subject
(split-plot) fixed effects were implant
number and its interactions with impression technique and implant angulation. Combinations of the 7 implant angles, 5 casts produced from
both the closed tray and open tray
impression techniques, and the 3 implants in each cast, made up the 210
angle error measurements. The analysis used the MIXED procedure in the
statistical software (SAS Version 8;
SAS Institute, Cary, NC) with the restricted maximum likelihood estimation method and the slice option for
post-hoc tests of interactions.
RESULTS
3
19
20
21
18
17
Implant #3
16
13
Implant #2 12
15
11
7
2
10
8
6
9
5
14
Implant #1
4
1
5 Order of measurements on casts.
plane is defined by 3 points that are not
in a straight line, recording 6 points
on the top flat surface of the hex allowed for the formation of 2 triangles
by connecting alternating points. Using the coordinates of the equilateral
triangles, a vector parallel to the long
axis of the implant was calculated for
each cast. Angular differences between corresponding definitive and
duplicate cast implant vectors were
calculated in degrees. Intraoperator
measurement error was calculated
using the replicate angle measures of
individual implants relative to the respective cast base. These angles were
Conrad et al
analyzed using a mixed linear model
with fixed effects for impression technique, implant angulation, implant
number and their interactions, and
variance components corresponding
to subjects and measurements.
The primary analysis used was a
repeated measures (split-plot) analysis of variance (ANOVA) (a=.05). The
dependent variable was the angle (degrees) between the long axis of the implants in the definitive and duplicate
casts, and a ‘subject’ was a duplicate
cast. The between-subject (wholeplot) fixed effects were impression
technique, implant angulation, and
Intraoperator measurement error for a single angle measurement
was determined for measurements
of the angle of a given implant relative to the perpendicular of the plane
of the cast. The standard deviation
of this measurement error was estimated to be 0.7 degrees, using the
5 replicate measurements of a given
implant angle. Table I summarizes the
ANOVA results. The main effect for
impression technique was not statistically significant (P=.22); nor were the
interactions of impression technique
with either the implant angulation
(P=.47), implant number (P=.33), or
both the implant angulation and implant number (P=.19).
Statistically significant results were
found for the main effect for implant
angulation (P<.01) and implant number (P<.001), as well as the interaction of implant angulation and implant number (P<.001). Comparing
the implant angulations, with a separate test for each implant number, the
implant angulations differ significantly using a Bonferroni-adjusted significance threshold (P<.05/3=.017) for
implants 1 (P<.001) and 3 (P<.01)
but not for implant 2 (P=.02) (Table
II). For implant 1, 5 degrees divergence differs from all other implant
angulations except 5 degree convergence, while no other pairs of implant
angulations differ. For implant 3, 10
354
Volume 97 Issue 6
degrees convergence differs from 5 degrees divergence, 15 degrees convergence, and 10 degrees divergence, but
no other pairs of implant angulations
differ. If instead the implant numbers
are compared with a separate test
for each implant angulation, the implant numbers differ significantly us-
ing a Bonferroni-adjusted significance
threshold (P<.05/7=.007) for 5 degrees divergence, parallel, and 15 degrees convergence (Table III). For the
parallel and 15 degrees convergence,
implant 3 had significantly larger angle errors than either implants 1 or 2,
which did not differ from each other.
For 5 degrees divergence, implant 1
had larger angle error than implant
2, but no other pairs of implants differed. Interpreted either way, these
interaction results show no interpretable pattern.
Table I. Repeated-measures analysis of variance
Numerator
Degrees of
Freedom
Denominator
Degrees of
Freedom
F
Statistic
P
Value
Main effect for impression technique
1
56
1.57
.22
Main effect for implant angulation
6
56
3.47
<.01
Main effect for implant number
2
112
7.95
<.001
Interaction of implant angulation and
12
112
3.79
<.001
6
56
0.94
.47
2
112
1.13
.33
12
112
1.37
.19
Effect
implant number
Interaction of impression technique and
implant angulation
Interaction of impression technique and
implant number
Interaction of impression technique,
implant angulation, and implant number
Table II. Tests comparing 7 implant angulations separately for each implant number
Numerator
Degrees of
Freedom
Denominator
Degrees of
Freedom
F
Statistic
P
Value
Implant 1
6
112
4.40
<.001
Implant 2
6
112
2.67
.02
Implant 3
6
112
3.74
.01
Implant Number
The Journal of Prosthetic Dentistry
Conrad et al
355
June 2007
Table III. Tests comparing 3 implant numbers separately for each implant angulation
Numerator
Degrees of
Freedom
Denominator
Degrees of
Freedom
F
Statistic
P
Value
15 degrees divergence
2
112
0.43
.65
10 degrees divergence
2
112
4.57
.01
5 degrees divergence
2
112
5.45
.005
Parallel
2
112
5.52
.005
5 degrees convergence
2
112
1.24
.29
10 degrees convergence
2
112
0.37
.69
15 degrees convergence
2
112
13.08
<.0001
Implant Angulation
DISCUSSION
The data support acceptance of
the null hypothesis as the combined
interaction of impression technique,
implant angulation, and implant
number, had no effect on the accuracy of the duplicate casts compared
to the definitive casts (P=.19) (Table
I). Although there was a significant
interaction of implant angulation and
implant number (P<.001), the average angle errors have no interpretable
pattern in either implant angulation
or implant number.
Errors may be introduced during
any of the several steps required, such
as dimensional changes in the materials, inaccurate repositioning of impression copings, and improper connection of components. The clinical
significance of distortion of this magnitude is unknown. The results of this
study are limited to 3 implants and
may not be relevant for impressions
that have higher or lower numbers of
implants.
One assumption made during this
study was that the top surface of the
hex was flat and that it was perpendicular to the long axis of the implant
analog. This is important as the vector calculated for the implant angu-
Conrad et al
lation was based on the plane of the
hex. The other was that although 18
of the 21 points recorded were on
metal components that would not
wear during impression procedures,
the other 3 points were recorded from
the stone base. The assumption was
made that the stone would not wear
significantly over the course of 10 impressions from each definitive cast.
The intraoperator measurement
error of 0.7 degrees can be accounted
for almost entirely by the precision of
the measuring stylus (FaroArm Silver;
Faro Technologies) as it may not be
sensitive enough to detect differences
between the variables. In order to decrease the error and find a statistical
difference, a higher precision instrument would be required. A positioning device for the measuring stylus
would not likely reduce the error as
exact positioning was not critical as
long as it contacted the top surface of
the hex which was the reference plane
for the angulation of the implant.
The results of this study are not in
agreement with several studies which
reported the superiority of the open
tray technique.16,19-23 Other studies in
agreement with this study have stated
that both impression techniques provide comparable results.17,24,25 Given
the assumption that some degree of
error is inherent with either the closed
tray or open tray impression technique, it seems prudent to eliminate
distortion as much as possible during
the impression and transfer procedures.3 The ultimate clinical objective
should be to fabricate an implant superstructure that imparts no stress to
the implants when fully seated.4
Future research is needed to make
further refinements to the impression
and laboratory procedures, to determine the amount of discrepancy tolerable physiologically and mechanically,
and to clinically analyze failures and
complications in implant treatment.
Whether or not two thirds of a degree
in angle error is clinically relevant may
be answered by further studies which
should evaluate the fit of frameworks
on these casts using either an electron microscope or strain gauges. An
electron microscope could be used to
evaluate the fit of the framework by
measuring the size of the gap between
the abutment and the implant. Strain
could be measured and then correlated to misfit or time to failure in the
prosthesis. Developing a method to
test strain clinically would be a valuable objective tool for clinicians to
evaluate framework fit.
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Volume 97 Issue 6
CONCLUSIONS
Within the limitations of this
study, the following conclusions were
drawn:
1. The combined interaction of
impression technique, implant angulation, and implant number, had no
effect on the accuracy of the duplicate
casts compared to the definitive casts
(P=.19).
2. The average angle errors for
the open tray technique were not significantly different from the average
angle errors for the closed tray technique (P=.22).
3. There were significant differences when isolating the main effect
for implant angulation (P<.01) and
implant number (P<.001), as well
as the combined interaction of implant angulation and implant number
(P<.001). The interaction results had
no interpretable pattern.
4. The interaction of impression
technique with either the implant angulation (P=.47) or the implant number (P=.33) was not significant.
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Reprint requests to:
Dr Heather J. Conrad
Division of Prosthodontics, Department of
Restorative Dentistry
University of Minnesota, School of Dentistry
9-450a Moos Tower
515 Delaware St SE
Minneapolis, MN 55455
Fax: 612-626-1496
E-mail: [email protected] or hjconrad@
comcast.net
Acknowledgements
The authors thank the American Academy
of Fixed Prosthodontics and the Stanley D.
Tylman Research Program for supporting this
study through a Tylman Grant. The authors
also thank 3i, a Biomet Co, and Ms Stephanie
Schoenrock for donating implant analogs used
for this study.
0022-3913/$32.00
Copyright © 2007 by the Editorial Council of
The Journal of Prosthetic Dentistry.
Conrad et al