The dual-arch or double-arch impression technique is extremely accurate and a viable alternative to full-arch impressions. It involves the use of special impression trays to capture the prepared teeth, centric occlusal relationship, and the opposing dentition, all the information needed to fabricate a prosthesis accurately. The technique has been in use for over 40 years but requires some attention to detail to ensure accuracy.
An independent research group1 found that more than half the clinical time was saved with the dual-arch impression procedure compared with the standard impression technique. The mounting of the casts was accurate, fast, and easy, saving laboratory time. If the casts were mounted correctly, occlusal discrepancies were reduced to nearly zero. This group of researchers has fabricated over 10,000 units using the dual-arch impression technique with fewer than 1% remakes. More recent findings from this group concluded that the double-arch impression technique is predictable, and saves time and money.2
|Figure 1. Temrex Bite Relator 2000 dual-arch trays.|
There are a number of tray systems currently on the market that can be used for the dual-arch impression technique. They fall into three distinctive groups: plastic trays for either quadrants or full arches; sideless metal or plastic trays for posterior quadrants; and metal trays with disposable inserts for posterior quadrants. Metal trays are rigid and will not deform during the impression procedure. Plastic trays are flexible and will deform, depending on the thickness of the side walls. The sideless trays rely upon the dense impression material to provide support during the impression procedure and while pouring the stone models. The choice of tray is dependent on the arch form, position of teeth, and retromolar pads. Care must be taken in selecting a tray to ensure that the alveolus and soft tissue do not impinge on the tray, causing a distortion.3 In my practice, I prefer using the trays with walls to help support and contain the impression material (Figure 1). In the posterior quadrants I use metal-walled trays (Temrex Bite Relator 2000) for additional rigidity, and have found that my remakes are less than 0.5%.
Dual-arch systems are also more comfortable for patients.4 We find that the close-mouth posture reduces episodes of gagging. Using quadrant impressions reduces the amount of impression material. Research has shown that using high-viscosity impression materials with dual-arch trays reduces the chance of foreign body aspiration.5
A REVIEW OF THE CLINICAL RESEARCH
Two clinical variables can be controlled with dual-arch impressions. First, the natural shifting of the teeth to assume a maximum interdigitation can be registered with this technique.6 Second, the physical deformation of the mandible during opening is eliminated with the closed, unstrained mandibular position used in taking the impression.7-9 The literature has reported that the mandible decreases in width up to 0.8 mm during opening and protrusion.10,11 No change in arch width occurs until the patient reaches 28% of maximum opening. Controlling these variables increases the likelihood of an accurate impression (see Figure 2).
|Figure 2. Comparison of accuracy of different impression techniques.|
Davis and Schwartz12 compared the accuracy in three dimensions of casts made from complete-arch custom acrylic resin trays, plastic dual-arch trays (posterior trays with side walls), and metal-walled trays. Under conditions of this in vitro investigation, both the metal-walled and the plastic dual-arch trays were at least as accurate as acrylic resin custom trays. The metal-walled tray was more accurate than the plastic trays or custom tray in the inter-tooth measurements. This indicates that the crown width was reproduced very accurately, especially by the metal-walled tray (Figure 2).
These investigators also studied the accuracy of second pour casts using the dual-arch impression.13 Their study found that when a dimensionally stable impression material is used, then all three types of trays—acrylic custom trays, plastic dual-arch trays, and metal-walled trays produced accurate second pour models. The metal-walled trays produced more accurate models than the acrylic custom tray and the plastic trays in the inter-tooth measurements.
Davis et al14 examined the marginal adaptation of castings made with dual-arch and custom trays. They found no differences in the marginal openings of castings made with full-arch acrylic custom trays, plastic dual-arch trays, or metal-walled trays. Each system achieved excellent marginal adaptation.
Breeding and Dixon15 compared the accuracy of stone casts prepared with metal-walled trays and plastic dual-arch trays. The metal-walled trays produced dies that were slightly smaller than the tooth, while the plastic trays produced dies that were larger. They concluded that the metal tray was more likely to produce a full-coverage casting with acceptable marginal integrity compared with the plastic dual-arch tray.
|Figure 3. Radiograph of the bicuspid showing lack of clinical crown.||Figure 4. Occlusal view of the healing cap in place over the Bicon Implant.|
In this example, the patient had an endodontically treated maxillary right first bicuspid that had lost the entire clinical crown (Figure 3). This would not have provided adequate support for any type of post-supported crown. There also appeared to be an apical radiolucency still present around one of the roots. The remaining tooth root was removed, and a bone graft was placed to create an adequate ridge for the placement of an implant. A Bicon Implant (4 X 11 mm) was placed 6 months later, with healing cap exposed (Figure 4). The patient opted not to have any temporary restoration in the area until the second stage of the implant was placed.
|Figure 5. Buccal view of the second-stage post on the Bicon Implant.|
The implant was allowed to integrate without loading for 6 months. The second stage was then placed along with a temporary crown. Once the gingival tissue had matured in the area, we were able to lower the buccal margin of the second stage below the gingival crest (Figure 5).
In this situation, we selected the Temrex Wide Bite Relator 2000 for taking the impression. This tray has metal walls that help provide support for the material and cover a large surface area. We used the Imprint II Quick Start Putty and Wash System from 3M ESPE. We felt that the combination of a rigid wall and rigid material would ensure that the material did not distort during setting or pouring of the models.
We reviewed with the patient how the impression system worked, emphasizing the need to bite into the material and hold the position steady until the impression material had set. The tray was selected and tried in to ensure that it did not impinge on any of the soft tissue. We practiced having the patient close into the tray, and noted which teeth on the opposite side of the arch occluded when they were in maximum intercuspation. To ensure the correct occlusal position, a shim stock can be inserted into these contact points.16 Dual-arch impressions are ideal for patients with a canine rise and incisal guidance. Group function occlusion cases are less successful because the relationship of the teeth on the other side of the mouth is not recorded.1
|Figure 6. Impression of the post using the Temrex Bite Relator 2000.|
Once the impression tray was filled, we slowly seated the tray onto the upper arch in the correct position, and had the patient occlude slowly into the tray. The contralateral contacts were observed to ensure that the tray was in the correct position. If necessary, we could have gently guided the patient into the correct occlusal relationship. Once the material had set, we stabilized the tray against the maxillary arch and had the patient open, gently lifting the teeth out of the tray. We then gently removed the Bite Relator from the maxillary arch. Figure 6 shows the impression of the implant, and includes both the tuberosity area and lateral incisor. We checked the impression to ensure that there were no signs of soft tissue impingement.
|Figure 7. Soft tissue model of the implant.|
The objective of the dual-arch impression technique is to have the laboratory technician use the bite registration as well as the impressions recorded on both sides of the Bite Relator 2000 (Figure 7). If the models are poured separately and an attempt is made to reuse the impression to articulate the casts, mounting errors can occur. Occlusal errors will also occur if the master cast is separated from the impression to be pinned and then re-inserted into the impression.16 The laboratory typically follows these steps in pouring and mounting the models:
(1) The preparation side is poured first, dropping a dowel pin into the stone, in alignment with the preparation. Drop in one or two more dowel pins in adjacent areas of the impression in case those parts of the model need to be separated.
(2) Once the first pour is set, inspect the impression surface of the opposing arch to ensure that no stone has seeped through the Rite Bite trays. Remove any excess stone.
(3) After the first pour is set, pour the opposing side and, with the same mix of stone, mount the poured impression of the opposing cast in an articulator. Typically a very simple plastic disposable hinge articulator is used for the fabrication of a single prosthesis.
(4) When the model of the opposing arch and the mounting is set, close the articulator over the pins and set the pins with stone.
(5) Once the stone is set, you have a fully articulated set of models.
When first using this impression technique, we suggest that you provide your laboratory technician with a check bite so he can confirm the correct occlusal relationship. Once you both gain experience, this step can be eliminated.
CEMENTATION OF THE CROWN
|Figure 8. Finished crown mounted on the model.|
We always have the laboratory return the original impression and the finished prosthesis mounted on the articulator so we can evaluate the articulation (Figure 8). We first inspect the occlusion by evaluating the relative positions of the cusps and incisal edges of the opposing model with the articulator closed, using articulating paper to mark the contacts. If there is a significant discrepancy in the expected orientation of the cusp, then we would suspect a mounting error, and either adjust the occlusion or remake the prosthesis.17
|Figure 9. Buccal view of the crown on the first bicuspid. Note the occlusal relationship.|
Figure 9 shows the crown in place 4 weeks after cementation. In these situations, with the Bicon second- stage post having a very accurate fit, we cemented the crown with ZOE Paste (Temrex Corporation). This zinc oxide eugenol formulation provides adequate retention. The crown required no occlusal adjustment on insertion.
There are a number of criteria for selecting the appropriate cases for the dual-arch impression technique:
•The technique should be used in cases with class I or class II occlusion, if the occlusal scheme is acceptable.
•Canine guidance is the ideal occlusal scheme. When working with a group function occlusion, supply a lateral check bite.
•The opposing teeth must have intact occlusal surfaces.
•Adjacent teeth must have acceptable morphology.
•The patient must be able to close into maximum intercuspation with no interference.
•The tray must not impinge on tissue.
•The impression must be poured and mounted before separating. Do not pour both sides and then try to articulate them using the impression.
•Hand articulation creates errors and destroys the occlusal information that is captured with the technique.
The dual-arch impression technique creates very accurate impressions. This impression system will result in casts that are less affected by technique and clinical variables associated with impression taking. When mounted correctly, a more accurate representation of the patient’s maximum intercuspation is obtained compared with full-arch impressions mounted with either hand articulation or interocclusal records.18 The technique is easy for patients, saves time and materials, and helps to eliminate a number of potential errors that may be captured by conventional impression techniques. Using the appropriate trays and material, one can create restorations that possess excellent marginal adaption and good occlusal relationship.
The author would like to thank his partner, Dr. Warren Hellen, who has always provided a willing “ear” to help with case planning and management; Dr. Edward Reinish for his involvement in planning and placing the Bicon Implant; and Mr. Ron Klausz of Klausz Dental Laboratories, who provided the laboratory services for this case.
1. CRA Newsletter. Double Arch Impression Technique Update. 1986; December. Clinical Research Associates.
2. CRA Newsletter. Posterior Double-Arch Impression Trays. Clinical Research Associates. 2000;July:2-3.
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4. Werrin SR. The 2-minute impression technique. Quintessence Int. 1996;27:179-181.
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12. Davis RD, Schwartz RS. Dual-arch and custom tray impression accuracy. Am J Dent. 1991;4:89-92.
13. Schwartz RS, Davis RD. Accuracy of second pour casts using dual-arch impressions. Am J Dent. 1992;5:192-194.
14. Davis R, Schwartz R, Hilton T. Marginal adaptation of castings made with dual-arch and custom trays. Am J Dent. 1992;5:253-254.
15. Breeding LC, Dixon DL. Accuracy of casts generated from dual-arch impressions. J Prosthet Dent. 2000;84:403-407.
16. Kaplowitz GJ. Trouble-shooting dual arch impressions. J Am Dent Assoc. 1996;127:234-240.
17. Kaplowitz GJ.Trouble-shooting dual arch impressions II. J Am Dent Assoc. 1997;128:1277-1281.
18. Cox JR, Brandt RL, Hughes HJ. The double arch impression technique: a solution to prevent supraocclusion in the indirect restoration. Gen Dent. 2000;January-February:86-91.
Dr. Abrams is the founder of Four Cell Consulting, Toronto, Ontario, Canada, which provides consulting services to dental companies in the area of new product development and promotion, and he maintains a private practice in Scarborough, Ontario. He is a fellow of the Pierre Fauchard Academy and the Academy of Dentistry International, and has published numerous articles in various international publications. Dr. Abrams recently received the Barnabus Day Award from the Ontario Dental Association for 20 years of service to the dental profession.He can be contacted at (416) 265-1400.
Disclosure: Dr. Abrams is the chief clinical consultant for Temrex Corporation.