Minami et al.

Int Chin J Dent 2006; 6: 9-16.

Comparison of reproducibility of dual-arch impression withconventional impressions
Hiroyuki Minami, DDS, PhD,1 Shiro Suzuki, DDS, PhD,2 and Takuo Tanaka, DDS,
PhD1
1

Department of Fixed Prosthetic Dentistry, Kagoshima University Graduate School of Medical and2
Dental Sciences, Kagoshima, Japan, and Department of Prosthodontics and Biomaterials, Universityof Alabama at Birmingham, School of Dentistry, Birmingham, AL, USA

Purpose: This study compared the reproducibility of dual-arch impression technique with conventional
impression techniques.
Materials and Methods: Full-crown preparation was completed on posterior teeth of fresh porcine upper jaws.
Dual-arch impression was made immediately after the preparation without gingival retraction cord or triple-tray(experimental group), followed by conventional 1-step and 2-step impressions (control groups). Sub-marginal
impression lengths were measured at the center of mesial, distal, buccal and palatal surfaces and comparedamong three groups. The accuracy was compared by calculating the ratios of the widths among stone casts
generated from each impression for mesial-distal and buccal-palatal measurements. Differences among thegroups were analyzed by Wilcoxon signed-ranks test at 95% confidence level.
Results: Regarding sub-marginal lengths, there were no significant differences among three groups (p>0.05) at
buccal site. However, dual-arch impression was significantly smaller than control groups at mesial site (p<0.05),
smaller than 1-step impression group at distal site (p<0.05), and smaller than 2-step impression group at palatalsite (p<0.05). There were no significant differences among three groups (p>0.05) for mesial-distal widths,
however, buccal-palatal widths of the dual-arch impression was significantly smaller than the control groups(p<0.05); 99.1 % of 1-step group and 99.20 % of 2-step group.
Conclusion: Sub-marginal lengths of dual-arch impression without using retraction cord seemed to be clinicallysufficient. However, the use of triple-tray is recommended when the dual-arch impression techniques are
employed. (Int Chin J Dent 2006; 6: 9-16.)
Key Words: accuracy, dual-arch impression, gingival retraction, impression, triple-tray, sub-marginal area.

Introduction
Polyvinylsiloxane impression materials have been widely used by clinicians, as the materials possess highaccuracy.1,2 A two-step impression technique has been used to compensate for the shrinkage of the impression
material.1 A preliminary impression is usually made with a very high viscosity material (putty) and used as atray, and then the final impression using a low-viscosity impression material is employed. This technique has
the potential to lessen the polymerization shrinkage,3,4 however, it takes longer time, compared to a one-step
impression technique. The one-step impression technique sometimes produces incorrect impressions due torapid polymerization and poor flow,5 although the literature reported no significant difference with the two-step
impression.6,7
Gingival retraction is one of the keys for success of impression whenever the prepared margin is locatedsub-gingivally. However, it also takes some additional chair time that is disadvantageous for both clinicians andpatients. Therefore, it is essential to select an adequate impression system with an acceptable clinical technique.
Dual-arch impression techniques have been introduced a decade ago as a unique technique,8 which enables thedentist to capture an impression of the prepared tooth, the opposing teeth and the occlusal registration in one
procedure.9,10

The efficacy and problems of this technique have been discussed9-12 and compared withcomplete-arch impressions,13-15 however, the use of them is limited to the impression of one or possibly two
restorations.10,16 Besides the limitation of impression size, it revealed that this technique was quite techniquesensitive, as the distortion of impression was provoked due to lack of the rigidity of dual-arch tray especially9Minami et al.

Int Chin J Dent 2006; 6: 9-16.

when patients bit the tray tightly. The distortion of the tray was also provoked by impinging the alveolus orpalate,15,16 and tori.13 Furthermore, some of the double-arch trays are virtually sideless,16 causing insufficient fit,consequently leading to the distortion of the impression. Therefore, it is hypothesized that exclusion of thedual-arch tray during dual-arch impressions improve the accuracy of dual-arch impressions.
The purported advantages of dual-arch impressions are savings in time and material, patient comfort, and easeof use.15 Elimination of gingival retraction, which is most time consuming process during making impression,
will make the most of these advantages.
The purpose of this study was to evaluate the reproducibility of dual-arch impression technique without using
a dual-arch tray, by comparing it with conventional impression techniques. It was also the purpose of this study
to evaluate the ability of reproducing the subgingival area without gingival retraction and the accuracy.

Materials and Methods
Materials, impression techniques, and specimen preparation
Materials used in this study are presented in Table 1. Aquasil putty material was hand-mixed, and all others
were dispensed using an auto-mix cartridge system (Cartridge Dispensing Gun, J. Morita USA Inc., Irvine, CA,
USA). Five fresh porcine jaws were used for the evaluation of reproducibility. The porcine head was dissectedby a veterinarian at the animal facility in the University of Alabama at Birmingham. The periphery of the tongue
was trimmed to obtain impression space between the tongue and palatal surfaces. Full crown preparation was
carried out on maxillary posterior teeth including a premolar and two molars (Fig.1). The preparation wascompleted with full heavy-chamfer margin using high-speed engine with #018 (coarse) and #021 (fine) diamondburs (Shofu Dental Corporation, San Marcos, CA, USA) under a copious irrigation. The prepared margin was
finished 0.5 mm below the gingival margin.
Table 1.

Impression materials used.

Impression method/Type

Product name

Manufacturer

Dual-arch impression
Medium-viscosity
Low-viscosity

Perfectim Systems 30 second Blue Velvet
Perfectim Systems 90 second Flexi-Velvet

J Morita USA, Irvine, CA, USA
J Morita USA

Conventional impression
Putty consistency
Low-viscosity (type I)

Aquasil Easy Mix Putty
Reprosil Cartridge System

Dentsply/Caulk, Milford, DE, USA 011128
Dentsply/Caulk
010104

Batch number

334335
199200Dual-arch impression was made immediately after the tooth preparation as an experimental group, followedby conventional 1-step and 2-step impressions as control groups. Dual-arch impression was completed using
neither a triple tray nor a gingival retraction cord. For the preliminary impression, a medium-viscosity material
was injected onto and peripheries of the prepared teeth, then the upper and lower jaws were occluded. The
cheek was kept by holding using fingers with light pressure. A single extrusion of medium-viscosity material
through the dispensing gun was injected for each quadrant, which provided approximately 5 mm thickness of
preliminary impression. After one minute from the mix the jaw was opened, and the preliminary impression was
detached from the preparations but kept attached to the antagonistic dentition. Then, the low-viscosity material
was injected to both the prepared teeth and preliminary impression. The jaws were closed again for threeminutes from the beginning of mix and the impression was completely removed from the dentition.

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Int Chin J Dent 2006; 6: 9-16.

For both the 1-step and 2-step impression techniques, the gingivae around the prepared teeth were retractedusing a retraction cord (Ultrapack #0, Ultradent Product, Salt Lake, UT, USA). The cord was removed prior toeach impression. For the 1-step impression technique, a low-viscosity (type I) material was injected onto andperipheries of the prepared teeth, and a putty consistency on a stock plastic quadrant-arch tray (Coe Impression
Tray, GC America, Chicago, IL, USA) was placed onto it immediately, and held in place for 12 minutes. For the
2-step impression technique, surgical gauze was placed on the prepared teeth as a spacer. The putty consistency
material was placed on it using the plastic tray and held for 10 minutes. The preliminary impression wasremoved, and the low-viscosity (type I) material was injected onto and around the prepared teeth and thepreliminary impression. The definitive impression was held for 12 minutes. The setting time of each impression
material was twice longer than the manufacturer’s instruction to compensate prolonged setting time due to lower
room temperature.
Each impression was made twice for right and left dentitions of five porcine jaws. One of the two impressions
was used for measuring sub-marginal lengths, and the other was used for generating stone working casts toevaluate the accuracy. Finally, 10 quadrant impressions for measuring sub-marginal lengths and other 10
impressions for generating stone working casts were obtained for each impression methods. All the impressions
were rinsed with tap water for 10 s, then dried and left for one hour, prior to each investigations.

1Fig. 1.
Fig. 2.

2

Prepared posterior teeth of porcine upper jaw with full heavy-chamfer margin.
Segmentation of the impression for measuring sub-marginal impression length at each measuring site.
M, Mesial measuring site; D, Distal measuring site; B, Buccal measuring site; P, Palatal measuring site.

Measurements of sub-marginal impression lengths
The lengths of sub-marginal impression for respective techniques were compared to each other.

Eachimpression was cut into pieces parallel to the tooth axis at the center of the mesial, distal, buccal, and palatal
surfaces using a surgical blade (Fig. 2). The distance from the prepared margin to the tip of the impression
material infiltrated into gingival groove was measured (Fig. 3) on a series of sections. The images of sectioned
areas were captured by a monochrome CCD video camera (XC-77, Sony Co., New York, NY, USA) anddigitized by a computer soft wear (NUBus, Neotech Ltd, Eastleigh, England). The lengths were measured using
image analysis software (Image 1.45, NIH, Bethesda, MD, USA).
Comparisons were performed among three impressions made from an identical tooth. Impressions were madefor 10 dentitions, and each dentition included three teeth. Therefore, 30 values were accumulated for eachmeasuring surface of each impression technique.

Statistical analysis was performed by repeated measure

ANOVA with impression techniques and measuring surfaces as independent factors. Differences among thegroups were analyzed by Wilcoxon signed-ranks test at 95% confidence level.

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Int Chin J Dent 2006; 6: 9-16.

Evaluation of accuracy
The accuracy of each impression technique was evaluated by comparing both mesial-distal and buccal-palatal
widths of stone casts generated from each impression. Type IV improved dental die stone (Vel-Mix, Kerr,
Orange, CA, USA) with a ratio of 10 mL of distilled water to 50 g of powder was hand mixed for 10 s, thenmixed under reduced pressure for 40 s. Each impression was poured with mixed die stone while being vibrated.
All casts were allowed to set for 24 hours before being removed from the impressions. Casts were mounted into
custom-made mold made by the putty silicone, which was fabricated individually for respective porcine
dentitions. Base of the stone casts was adjusted by the mold to give each cast constant and reproducible
arrangement during the measurement. The casts were sectioned using diamond disks to fabricate stone dies ofrespective prepared teeth, then gingival areas around the teeth were trimmed to enable the measurements. Bothbuccal-palatal and mesial-distal dimensions were measured at the prepared marginal areas (Fig. 4).

Acustom-made indicator, which was made for each prepared tooth using a silicone material (Fit Checker Black,
GC America Inc., Alsip, IL, USA), was used to indicate the identical and reproducible measurement portion of
three casts generated by each impression. The measurements were performed using a traveling magnifying
scope assembled with a CCD camera and a 2-dimensional traveling table (Fig. 5), which is capable of measuring
up to 0.001 mm (1 µm). The cross-hair was placed on the starting point and the position (xs, ys) was recorded.
The cross-hair was then moved to the end point and the position (xe, ye) was also recorded. The distance
between these two points was calculated according to following equation; l=[(xe-xs)2+(ye-ys)2]0.5.

Themeasurements were repeated three times for each measuring portion and averaged to determine the value. All
the measurements were completed by single examiner. The intra-examiner variability for all measurements
ranged between 5 and 8 µm, indicating 0.05% to 0.08% of measurement errors.

3
Fig. 3.
Fig. 4.

4

Measurement of the length of sub-marginal impression.
Measuring sites of the stone cast. MD, Maximum mesial-distal width; BP; Maximum buccal-palatal width.

Comparisons were carried out among casts acquired from the identical tooth. Casts were acquired from 10
dentitions, and each dentition included three teeth. Therefore, 30 values were collected for both buccal-palatal
and mesial-distal widths of stone casts generated from each impression technique.

The ratios of thecorresponding values of conventional 1-step impression to conventional 2-step impression (1-step/2-step),dual-arch impression to conventional 1-step (dual-arch/1step), and dual-arch to conventional 2-step(dual-arch/2step) were calculated for both buccal-palatal and mesial-distal widths, and the averages ofcorresponding 30 comparisons were described in percentages. Differences among the groups were analyzed byWilcoxon signed-ranks test at 95% confidence level.

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Int Chin J Dent 2006; 6: 9-16.

Fig. 5. Measuring apparatus.
a, CCD camerab, Stone castc, 2-dimensional traveling tabled, Display

Results
The results of the measurements of sub-marginal impression are presented in Table 2. The results of repeatedmeasure ANOVA demonstrated that there were no significant differences among impression sites (p>0.05), but
significant differences existed among impression methods (p<0.05).

In addition, there was no significant

interaction between the impression site and the impression method (p>0.05). For the sub-marginal lengths of the
buccal site, there were no significant differences among three impression techniques (p>0.05). However, thesub-marginal length of dual-arch impression was significantly smaller than those of the conventional
impressions at mesial site (p<0.05), smaller than those of the conventional 1-step impression at distal site
(p<0.05), and smaller than those of the conventional 2-step impression at palatal site (p<0.05).
Table 2.
MethodResults of sub-marginal impression lengths in mm.
Site

MesialDistalBuccalPalatalN=30

Mean SD

Mean SD

Mean SD

Mean SD

0.52 0.27 a A
0.70 0.38 b B
0.69 0.30 b C

0.61 0.35 c
0.89 0.58 d
0.84 0.58 c d0.60 0.46 e A
0.84 0.43 e B
0.69 0.29 e C

0.59 0.35 f
0.79 0.31 f g
0.75 0.33 g

Dual-arch
Conventional 1-stepConventional 2-step

A
B
C

A
B
C

Values with the identical lower case letters denote no significant difference within the same impression site (p>0.05).
Values with the identical capital letters denote no significant difference within the same impression method (p>0.05).

Table 3.

Summary of the comparison of stone casts generated from respective technique.

Measurements

Combinations for comparison between the impression methods
1-step / 2-stepDual-arch / 1-stepDual-arch / 2-step

Buccal-palatal widths
Mesial-distal widths

100.14%100.12%99.07%
99.91%

99.20%
100.02%Comparisons for the sizes of stone casts generated from each impression technique are listed in Table 3, andresults of statistical analysis are summarized in Table 4. Buccal-palatal length of the group obtained fromdual-arch impression was 99.07 % of 1-step impression group and 99.20 % of 2-step impression group. The
value of the dual-arch impression group was significantly smaller than conventional impression groups (p<0.05).

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Int Chin J Dent 2006; 6: 9-16.

For mesial-distal measurements, there were no significant differences among three impression groups (p>0.05).
Table 4.

Results of statistical analysis for buccal-palatal and mesial-distal measurements.

1-step2-stepDual-arch

Buccal-palatal measurements

Mesial-distal measurements

1-step-----1-step-----2-stepNS-----

Dual-arch
S
S
-----

2-stepNS-----

Dual-arch
NS
NS
-----

NS denotes no significant difference (p≥0.05). S denotes significant difference (p<0.05).

Discussion
The management of gingival tissue is one of the important tasks for making successful impressions ofprepared teeth. Therefore, the gingiva is usually retracted with a retraction cord prior to making impression,
especially for the cases in which the prepared margins are located sub-gingivally. Therefore, the potential formaking sub-marginal impression plays an important role for the clinical success. Sub-marginal lengths obtained
from dual-arch impression without gingival retraction (0.52-0.61 mm) was less than those of conventional
techniques (0.69-0.89 mm).

Although insufficient sub-marginal impression lengths occasionally lead todifficulties while trimming marginal areas of the working cast, all of the cavo-surface margins were successfully
impressed by the dual-arch impression. Aimjirakul et al.17 have studied by use of the sulcus simulating modeland reported that the penetration ability of elastic impression materials was greater with wider sulci, however,the penetration depths during conventional impressions without gingival retraction have never been clarified.
Furthermore, the appropriate sub-marginal impression lengths have no been defined.

It is obvious thatmanipulation of working casts will be easier with greater sub-marginal impression lengths, however, the lengthsof sub-marginal impression obtained from dual-arch impression might be also acceptable for clinical situation.
Therefore, the exclusion of gingival retraction will make the impression time short and facilitate the patients’
comfort.
Although distortion of impression is a three-dimensional problem, only two-dimensional evaluation including
buccal-lingual and mesial-distal dimensions of the gingival margin were chosen in this study. Addition of
occluso-gingival measurements and the inter-abutment distances might have provided more reliable information.
Most of the studies have evaluated the accuracies of impression techniques by use of an in vitro models, which
include standard master dies of machined stainless steel8,12 or prepared plastic teeth in a typodont.10,13 The use ofstandard master dies facilitates the measurements and comparison of stone dies obtained from various
impression techniques. This study evaluated the sub-marginal impression lengths and the accuracy using fresh
porcine jaws. An advantage of this method was that impressions were made under the existence of soft tissue.
Furthermore, some gingival fluid existed instead of bleeding from marginal area, which is one of the mostannoying problems during impression. The size and configurations of porcine teeth used in this study weredifferent from those of human teeth; porcine teeth had a bucco-palatally compressed configuration, andmesio-distal widths of porcine molar were greater than those of human molar.

Furthermore, there is noconsensus cited in literatures regarding the gingival groove depths of porcine jaw. However, judging from theexperience through gingival retraction procedure, the depths seemed similar to those of human gingiva, andporcine gingiva seemed stiffer than human gingiva. Therefore, clinical evaluation might be the best method for

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Int Chin J Dent 2006; 6: 9-16.

the comparison among impression techniques; however, it could only be achieved on the basis of the subjects’
cooperation. Therefore, the method used in this study might be the second best method for the evaluation ofimpression techniques.
The disadvantage of this method was that there were many difficulties in measuring the size of prepared teethon porcine jaws as standard. Therefore, conventional impression techniques were used as control groups andresulted in the comparison using stone dies obtained from each impression technique.
As some studies have already reported,13,15,16 the distortion of dual-arch impression due to the lack of rigidityof triple tray can cause rebound that will result in distortion of impression. Therefore, dual-arch impression was
performed without using a triple tray. The hypothesis of this study was partially rejected, because the results of
buccal-lingual measurements of dual-arch impression were significantly smaller compared to the conventional
impressions. For the comparison of buccal-palatal distances between master die and stone die obtained bydual-arch impression, the values showed an inconsistency. Ceyhem et al. reported that the stone die showed
greater value than the master die,15 however, and a smaller value of the stone die when impression was madeusing metal and plastic trays,12 greater value with plastic tray and smaller value with metal tray,10 have also beenreported. Although it is quite difficult to compare the results between these studies and current study, distortionof triple tray during insertion or removal may not be the main cause of the inaccuracies and unstableness of triple
impression.

As the preliminary impression of dual-arch impression is made without using spacers, thepreliminary impression might be stretched buccal-palatally during definitive impression and any rebound after
the removal of the impression would result in reduced buccal-palatal dimension. Introduction of any kinds ofspacers during preliminary impression, moreover, addition of least amount of low-viscosity material to the
prepared marginal area of preliminary impression, may bring higher accuracy to the dual-arch impression.
Acknowledgment
This study was supported by UAB grant #633177.

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14. Cox JR, Brandt RL, Hughes HJ. A clinical pilot study of the dimensional accuracy of double-arch and complete-arch
impressions. J Prosthet Dent 2002; 87: 510-5.
15. Ceyhan JA, Johnson GH, Lepe X, Philips KM. A clinical study comparing the three-dimensional accuracy of a working die
generated from two duel-arch trays and a complete-arch custom tray. J Prosthet Dent 2003; 90: 228-34.
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Int Chin J Dent 2006; 6: 9-16.

Correspondence to:Dr. Hiroyuki Minami
Department of Fixed Prosthetic Dentistry
Kagoshima University Graduate School of Medical and Dental Sciences
8-35-1, Sakuragaoka, Kagoshima 890-8544, Japan
Fax: +81-99-275-6218 E-mail: minami@dentb.hal.kagoshima-u.ac.jp
Received November 28, 2005. Revised December 22, 2005. Accepted December 31, 2005.
Copyright ©2006 by the International Chinese Journal of Dentistry.

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