| Abstract|| |
Objectives: The dual-arch impression technique is convenient in that it makes the required maxillary and mandibular impressions, as well as the inter-occlusal record in one procedure. The accuracy of inter-abutment distance in dies fabricated from dual-arch impression technique remains in question because there is little information available in the literature.
Materials and Methods: This study was conducted to evaluate the accuracy of inter-abutment distance in dies obtained from full arch dual-arch trays with those obtained from full arch stock metal trays.
Results and Conclusion: The metal dual-arch trays showed better accuracy followed by the plastic dual-arch and stock dentulous trays, respectively, though statistically insignificant. The pouring sequence did not have any effect on the inter-abutment distance statistically, though pouring the non-working side of the dual-arch impression first showed better accuracy.
Keywords: Dual arch, triple tray, quad tray
|How to cite this article:|
Reddy JM, Prashanti E, Kumar G V, Suresh Sajjan M C, Mathew X. A comparative study of inter-abutment distance of dies made from full arch dual-arch impression trays with those made from full arch stock trays: An in vitro study. Indian J Dent Res 2009;20:412-7
A detailed and dimensionally accurate impression is essential for the indirect fabrication of a fixed prosthesis. Accurate registration of oral structures not only requires an accurate impression material but also a rigid impression tray to support the material and a precise impression technique.  Several impression techniques have been reported to improve the accuracy of impressions used in making fixed partial dentures. One such advancement is the introduction of dual-arch impression technique which was first reported by Getz in 1951, who used water cooled trays with a reversible hydrocolloid.  The credit for the introduction of this technique for fabricating indirect restorations goes to Wilson and Werrin.  Since then, the dual-arch impression technique has been in use in routine clinical practice.
|How to cite this URL:|
Reddy JM, Prashanti E, Kumar G V, Suresh Sajjan M C, Mathew X. A comparative study of inter-abutment distance of dies made from full arch dual-arch impression trays with those made from full arch stock trays: An in vitro study. Indian J Dent Res [serial online] 2009 [cited 2013 Jun 20];20:412-7. Available from: http://www.ijdr.in/text.asp?2009/20/4/412/59437
The dual-arch impression technique has several advantages over conventional impression techniques.  Majority of the studies on dual-arch impressions concluded that these impressions can be used for single unit crowns and short span bridges. The use of full arch dual-arch tray would overcome the limitations of quadrant dual-arch trays and may result in the wider application of the dual-arch impression technique in fixed partial dentures. Hence, the purpose of the present study was to evaluate the efficacy of these trays for fixed partial dentures.
The following objectives were considered in the present study:
- To compare the accuracy of inter-abutment distance of dies made from full arch dual-arch impression trays with those made from conventional full-arch stock trays. To measure and compare the accuracy in inter- abutment distance of dies made from two types of full arch dual- arch trays (plastic and metal) and full arch stock metal trays.
- To evaluate the influence of order of pouring the two arches of the dual-arch impression on the inter- abutment distance.
| Materials and Methods|| |
The impressions were grouped into three depending on the type of tray used to make the impression.
Group A: Consisted of impressions made using plastic full-arch dual arch trays with a one step impression technique.
Group B: Consisted of impressions made by using a metal full-arch dual arch tray with a one step impression technique.
Two variables that could affect accuracy of working dies were studied: Type of dual-arch tray used and the order of pouring the two arches of the impression. A balanced design with independent samples was used to study these two variables. A sample size of 10 was used yielding a total of 40 impressions (20 Group A and 20 Group B). 
Group C: Consisted of impressions made using full arch dentulous stock metal tray with a two step putty wash impression technique (10 Group C).
Preparation of the master model
The typhodont teeth were embedded in the maxillary and mandibular Frasaco model bases (Frasaco GmbH, Tettnang, Germany). The right mandibular second premolar was removed to simulate a clinical case for a three unit fixed partial denture. The right mandibular first premolar and first molar were prepared conservatively to receive full veneer retainers for a three unit fixed partial denture. Dimples were made on the occlusal surface of the prepared teeth using a round bur (1 mm diameter) to act as reproducible reference points for the purpose of evaluation  [Figure 1].
Articulating the master models
The models were mounted in maximum intercuspation on a semiadjustable articulator (Hanau® wide vue series; Waterpik technologies, Inc., Fort Collins, CO, USA) using machined solid steel blocks [Figure 2]. A custom-made tray positioning jig was attached to the articulator so that the position of the impression tray was constant and reproducible between the trials  [Figure 2].
Group A impressions
Plastic full-arch dual-arch trays (Alfa triple tray, Premier Dental Products, Plymouth Meeting, U.S.A) were used to make the impressions using one step technique.  The dual-arch trays were assessed to ensure that the typhodont could be closed into the maximum intercuspal position without any interference from the tray.  Tray adhesive (Universal VPS Adhesive, GC America Inc., Chicago, IL) was applied onto the inner portion of the side walls and extending 2 mm onto the outer walls and allowed to dry for 15 min according to manufacturer's instructions.
A dual mix technique was used where heavy and low viscosity materials (polyvinylsiloxane, Examix NDS, GC America Inc., Chicago, IL) were auto mixed simultaneously.  The light bodied material was injected around and over the prepared teeth. Heavy bodied material was dispensed onto both the sides of the tray and the tray was positioned over the posterior mandibular teeth. The articulator was closed. The weight of the machined block used to articulate the maxillary frasaco base applied a constant force and the tray positioning jig maintained the position of the impression tray constant during the set of the impression material  [Figure 3].
Impressions were removed 8 min after the start of mix, twice the manufacturer's recommended setting time, to compensate for the temperature of the extra oral environment. The impressions were rinsed under tap water for 10 s, dried and poured in improved dental stone (type IV) (Kalrock, Kalabhai, Karson-Mumbai, India) 60 min later.  A total of 20 impressions were made in this group. Half the impressions (10) were poured on the working side (WS, i.e. preparation side of the impression) first and then the non-working side (NWS, i.e. opposite to the preparation/counter side) was poured. In the other half of the impressions (10), the NWS was poured first as recommended by the manufacturers of the tray, and also as recommended by Wilson and Werrin.  After an hour the WS of the impression was poured.
Group B impressions
A metal full arch dual-arch tray (custom made in specification laid by Wilson and Werrin)  was used to make the impressions using a one-step technique.  The procedure was similar to that performed in Group A impressions except that a single metal dual-arch tray was used to make all the impressions. And the metal dual-arch tray used a disposable interocclusal insert which needed to be changed after each impression , [Figure 4]. A total of 20 impressions were made in this group.
Group C impressions
Full-arch dentulous stock metal trays (Jabbar and Co, Mumbai, India) were used to make the impressions using a two step putty reline technique in this group.  Metal stock trays gave greater accuracy in the putty/wash impression technique compared with flexible plastic ones for crown and bridge work.  First a putty impression was made (GC EXAFLEX Poly vinyl siloxane Type O putty consistency, GC America Inc., Chicago, IL) on the frasaco model bases (Frasaco GmbH, Tettnang, Germany) with a 1-mm wax spacer. Later the wax spacer was removed and the light bodied material (polyvinylsiloxane, Examix NDS, GC America Inc., Chicago, IL) was injected around and over the prepared teeth and into the putty tray to make the impression. A total of 10 poly vinyl siloxane (PVS) putty reline impressions were made. 
Preparation of the master cast
Once the impressions were made, all the impressions were stored at room temperature for 60 min before being poured. Polyvinyl siloxane impression materials possess good dimensional stability, and investigations have shown that there is no difference in casts produced from impressions poured at 1 h, one day, and even up to one week. , All the impressions were rinsed in tap water for 10 s and air dried. Impressions were poured using type IV dental stone. After pouring the casts, the impression trays were suspended in a tray holder to ensure that the impression was suspended for 60 min allocated for the gypsum to set.
For the dual-arch impressions, one side of the impression was poured first and allowed to set for 1 h before the other side was poured with die stone. The casts that were obtained by pouring the preparation side (i.e. WS) of the impression first and then the counter impression were designated as working side casts (WC). The casts obtained by pouring the counter side (i.e. NWS) first and then the preparation side were called as non-working side casts (NWC). All the casts were allowed to set for 24 h at room temperature before removal from the impressions. 
All the casts were based with dental stone. A custom-made tripoding device attached to Ney surveyor (Paraline; Dentaurum, Ispringen, Germany) was used to base all the casts in order to ensure that reference points for all the casts were located in the same spatial line  [Figure 5]. Once all the casts were based they were labeled as per group and subjected to measurements.
The measurements of the master model (MM) as well as Group A, B, and C casts were done using a profile projector (Nikon, Japan Inc.) [Figure 6] with accuracy of 0.001 mm or 1 µm.
The inter-abutment distance was measured from outer circumference of one dimple to the other. For each cast the same operator recorded the inter-abutment distance three times with accuracy of 0.001-0.003 mm and the mean value was calculated. The measurements of the MM and the stone casts obtained with the three tray types, i.e. Group A, Group B, Group C impressions were tabulated and statistically analyzed.
Descriptive statistics like mean and standard deviation were calculated for all the groups and for differences with MM. One way analysis of variance (ANOVA) was used for multiple group comparisons. Differences from the MM were analyzed by paired 't' test and intergroup comparison was done by unpaired 't' test. A P value of 0.05 or less was considered for statistical significance.
| Results|| |
The inter-abutment distance in the MM and in the casts obtained from all the three impression groups were tabulated [Table 1]. The inter-abutment distance in the MM was found to be 16.528 mm to which the three groups were compared. The mean difference from the MM in Group A when WS was poured first indicated a decrease in the inter-abutment distance of 58 µm. The percentage difference was found to be (-) 0.4% and was statistically not significant (P > 0.05) [Table 2].
The mean inter-abutment distance when the NWS of the impression was poured first was 23 µm. However, the percentage difference was found to be (-) 0.1% and was statistically not significant (P > 0.05) [Table 2]. The intra group comparison was done with unpaired 't' test and showed no statistical significance (P > 0.05) indicating that the sequence of pouring the two arches of the impressions had least effect on the accuracy of inter-abutment distance statistically [Table 2].
For Group B casts (obtained from metal full arch dual-arch trays) the mean difference in inter-abutment distance when the WS was poured first was 21 µm. The percentage difference was calculated to be (-) 0.1% and was found to be statistically not significant (P > 0.05).
The mean difference in inter-abutment distance when the NWS was poured first for group B impressions was 1 µm. The percentage difference was found to be (-) 0.01% and had no statistical significance. The intragroup comparison using unpaired 't' test showed no significance statistically (P > 0.05) indicating that the sequence of pouring had least effect on the accuracy of inter abutment distance.
For Group C casts, obtained from full arch stock metal trays using two step putty reline technique, the mean difference from MM indicated a decrease in the inter-abutment distance of 71 µm and was not statistically significant (P > 0.05).
Intergroup comparisons between Group A and Group B, Group A and Group C, and Group B and Group C showed no statistical significance (P > 0.05) indicating that there was no significant difference between the three groups. Multiple group comparisons between Group A, Group B, and Group C casts using one way ANOVA revealed no statistical significance (P > 0.05) indicating that all the three trays produced similar results with minor variations.
| Discussion|| |
The dual- or double-arch impression technique as described by Wilson and Werrin  is convenient in that it makes the maxillary and mandibular impressions, as well as the interocclusal record in one procedure. 
The dual-arch impression technique allows the impression to be made in closed mouth position. This position provides two benefits: (1) The mandibular flexure that occurs after 28% of maximum opening is eliminated, and (2) teeth are placed near maximum intercuspation. ,,
In the past decade, a number of authors have evaluated the quadrant dual-arch impression trays, and brought attention to their limitations that they can be used only for single crowns. , Because of the absence of contra lateral teeth in the quadrant dual arch impressions, this technique is indicated in patients having existing anterior guidance, without which non-working (balancing) interferences may be introduced in the new restoration. ,
In an attempt to overcome the objections raised for the older metal and plastic quadrant dual-arch trays the concept of full- arch dual-arch tray was introduced.  Since then no scientific data has been published regarding the accuracy of these full-arch dual arch trays in reproducing the inter-abutment distance so that they can be used for making the impressions for fixed partial dentures. There is a lack of consensus about which impression tray (plastic or metal) should be used, and which side of the dual- arch impression should be poured first to minimize distortion. Investigation of these parameters would also help in planning an anticipated clinical trial.
In the present study, comparison was made between dual-arch and putty wash impression technique since the most commonly practiced technique for fixed partial denture impressions is two-step putty wash impression technique and multiple mix technique when using dual-arch trays. ,
Group A impressions were made using a full-arch plastic dual arch tray (Alfa triple tray, Premier Dental Products). This tray has been designed to overcome the limitations of the earlier plastic quadrant trays in terms of coverage and rigidity. Manufacturers have recommended the use of this tray with both PVS impression materials and with irreversible hydrocolloids.
Group B impressions were made using a full-arch metal dual-arch tray which was fabricated based on the design principles and guidelines given by Getz et al. 
The common observation in all the three groups was that the inter-abutment distance decreased though there was no statistical significance. The altered dimension might be attributed to the polymerization shrinkage in the PVS impression material. The PVS impression material shrinks toward the center of mass during polymerization. ,, The use of a tray adhesive would redirect this shrinkage toward the walls of the tray, , resulting in an increase in the bucco-lingual dimension. Because there is no tray adhesive interproximally, the decreased inter-abutment distance may be in response to the material being stretched buccolingually, much like stretching a rubber band.  These findings are in agreement with the results obtained by Breeding and Dixon  and Ceyhan et al.  in that the bucco-lingual dimension increased and mesio-distal dimension decreased.
In Group A and B it can be observed that though the sequence of pouring did not have statistical significance, the mean difference indicates that pouring the NWS first resulted in more accurate casts than pouring the WS first. This can be due to the compensation of polymerization shrinkage by the die stone expansion. When the NWS is poured first and allowed to set for an hour, a considerable amount of expansion of the die stone compensates for the polymerization shrinkage of the impression material.  After an hour, when the WS is poured, the compensation of shrinkage which has already occurred might also have been resulted in a more accurate inter-abutment distance of the dies.
The inter-abutment distance of dies made from the metal and plastic dual arch trays did not show any statistical significance (P > 0.05). However, in between the metal and plastic dual arch trays, the metal trays showed more accuracy in comparison to the plastic dual arch trays. The full-arch stock metal trays with putty reline technique showed much less accuracy compared to both dual arch groups.
The variation between plastic and metal dual-arch groups could be attributed to the relative flexibility of the plastic dual arch trays in comparison to the metal dual arch trays. Previous studies also indicated that none of the plastic trays are as rigid as metal trays. , Another reason which could have led to the less accuracy in the plastic dual arch trays may be the application of tray adhesive which was more when compared to the metal trays due to the tray design. In the plastic trays, there were additional mechanical retention plugs that increase the surface area of adhesive application. This could have further resulted in redirection of polymerization shrinkage toward the tray walls resulting in decreased inter-abutment distance in Group A casts compared to Group B casts. Ceyhan et al.  have quoted similar results in the studies done by Davis and Schwartz.
The stock metal trays showed much less accuracy compared to both dual arch groups. This may be due to the use of a two step putty reline technique with the stock trays wherein the amount of wash was 1 mm which would have resulted in more polymerization shrinkage within the material.  In case of dual-arch trays, a single step multiple mix technique was used in which the amount of wash (light bodied) material is substantially less compared to putty reline method. This would have resulted in more accurate casts in full arch dual arch impressions in comparison with full arch putty/reline impressions. ,,
Measurements made on stone casts are potentially affected not only by the impression material and tray type, but also by the expansion of the dental stone used. In this investigation, a low expansion improved dental stone with a reported expansion of 0.1% was used. A higher expansion stone such as Hard- Rock (Whip mix), with a reported expansion of 0.28%, would have increased the measurements approximately 30 µm. This would have had a positive effect on the slightly undersized dimensions produced by the three trays used in this study. The magnitude of this difference (0.011-0.071 µm), however, would be insignificant clinically because it could be masked by one to two coats of die spacer.  The thickness of one coat of die spacer can vary from 8 to 40 µm.  In order to compensate for the narrower dimension mesio-distally, it may be advisable to include an extra coat of die spacer on the interproximal surfaces.
Dual-arch impression technique can yield predictably accurate impressions that require a fraction of the impression material and chair side time required by conventional impression and bite-registration procedures. When used properly, it can yield castings with minimum occlusal adjustment.  However, to achieve the best results, the dentist must carefully evaluate the patient before choosing the impression procedure and must select an appropriately sized impression tray that fits the arch and does not impinge on any anatomic structures that may produce a deflection of the tray wall.  The dentist must also select the impression material and a dual-arch impression technique (one step or two step) that are appropriate for each case and will provide the greatest chance of success.
Dual-arch impression procedures are effective and dependable. Problems arise because of operator error and poor case selection, not as a result of any inherent deficiencies in the technique or materials themselves.
| Conclusions|| |
Under the conditions of this investigation and based on the study results the following were obtained.
This study investigates only one aspect of linear relationship of the same side of the arch. Cross arch relationship and also inciso-cervical aspect should be further investigated for which further clinical trials are recommended. This can widen the vistas of application for the dual-arch impression techniques.
- The inter-abutment distance for all the three groups showed a decrease in comparison to the MM though the results were not statistically significant. The least amount of variation was noted with impressions made with full arch metal dual-arch trays followed by the plastic dual- arch trays and stock metal trays, respectively.
- The sequence of pouring the dual-arch impression, i.e. WS first or NWS first did not show significance statistically though pouring the NWS first resulted in a more accurate inter-abutment distance.
In Group C, the distance between the dies was decreased by 71 ìm in comparison to the MM, in Group B the distance was decreased only by 1 ìm, and 23 ìm in Group A. Even though there was no statistical significance between the groups, the results were clinically significant, since a few micron variations can lead to misfit of the castings clinically.
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Jagan Mohan Reddy
Department of Prosthodontics and Oral Implantology, Vydehi Institute of Dental Sciences and Research Centre Whitefield, Bangalore - 560 066
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2]