The triple tray technique is a very efficient method for taking impressions. The impression taken prior to preparing the teeth can be utilized to fabricate both the temporary restoration and then the final impression, which includes the opposing occlusion. The trays are especially suited to impressions of single crowns, provided the tray extends to teeth both anterior and posterior to the preparation. The trays are limited, however, in situations where full arch impressions are required; for example, multiple crowns or bridgework with or without posterior occlusion. Another shortcoming is the length of the tray: often the tray length is inadequate to fully cover the teeth adjacent to those being prepared.
The full arch technique overcomes these limitations by combining two or three quadrant trays to form a continuous arch. A posterior metal triple tray (Figure 1) is placed on the study model and then an anterior plastic sextant tray is adjusted to approximately abut the metal tray (Figure 2). The ability of the patient to occlude properly with the tray in place should be carefully assessed prior to taking the impression. On initial pre-prep insertion, the trays are loaded with impression material and the posterior tray is inserted first, followed by the sextant tray (Figure 3). While the tray is held in place, additional impression material can be added to fortify the area where the trays join by simply lifting the lip and adding to the joint area on the labial. The original bite impression is then used to fabricate both the temporary restorations – with a material such as Provipont by Viadent, or Structure 2 by Voco (Figure 4), and the final impression. Figures 5 through 7 show an example of a full arch impression designed to capture bilateral crown preparations.
The recommended triple trays for this technique are the metal Bite Relator trays by Temrex for posteriors, and plastic Check Bite trays by G. C. America for anteriors. A recent article reports that Dr. Christiansen’s recommendations contradict the use of trays with sides on them because of the possibility of impinging on the tissue, thereby creating pressure areas that could distort the impression. He advocates using only rigid trays. In order to prevent possible impingement metal trays are recommended for the posterior and rigid plastic trays in the anterior. In addition, to decrease the chance of tray walls affecting the final impression, I suggest taking a #1157 bur and removing a collar of heavy body material in the gingival one-third of each prepared tooth captured in the original bite impression (Figure 8). This allows for a bulk of light body material to encompass each prepared tooth.
For the final impression a modified Laminar technique is used. This involves cutting access holes through the sides of the impression tray into the interproximal spaces of each tooth to be prepared. The holes are drilled through the tray using a standard 3/32 utility drill bit in a slowspeed handpiece (Figure 9). With metal trays the holes can be made using the #1157 bur with the highspeed (Figure 10). The impression is seated and light body material is injected through the holes into the preps (Figure 11). Several modifications to the technique serve to decrease crevicular seepage and improve the flow of the material being injected. The first modification as previously noted is to use a #1157 bur to cut away heavy body at the gingival aspect of the original bite impression. This greatly improves flow and prevents creases from developing in the impression material due to sluggish flow. A second modification is to use two retraction cords with all impressions: when the second cord is removed, light body is immediately injected into the gingival crevice and around the entire tooth, preventing moisture and crevicular contamination. The original impression is then seated and the second application of light body material is injected through all the holes leading to the interproximal areas of the preps. Figures 12-13 demonstrate a post, core and bridge preparation captured using just two trays. The cemented units are shown in Figure 14.
This impression technique offers a dependable multi-crown or bridge full arch impression procedure ideal for the general dentist.
Dr. Ronald Porth maintains his practice in Abbotsford, BC following 20 years in practice in North Vancouver, BC. In March, 2001 he will present the first clinical trials of his new laser endodontic intracanal mineraltrioxide aggregate technique to the American Laser Dental Conference in Arizona.
Oral Health welcomes this original article.