Capturing Open Tray Impressions for Implant-Supported Prostheses

Because implants do not have the periodontal ligament that natural teeth have, slight discrepancies created in an impression between the implant fixtures can prevent passive fit of the completed prosthesis. Or in cases involving single implant restorations, inaccurate capture of the implant's connector orientation may lead to incomplete mating of the prosthesis to the implant. A lack of passive fit creates stresses on implants that can lead to crestal bone loss as well as results in strains on the components that can potentially lead to fracture of the prosthetic screw and other failures.⁴

Although the use of digital scanning is increasingly replacing the use of traditional impression materials in the restoration of both teeth and implants, a significant number of practitioners are still utilizing physical impressions, particularly in situations involving multiple implants on an edentulous arch, which can be challenging to scan. With that in mind, this article will address improving the accuracy of physical implant impressions and simplifying the process.

Impressions for implant-supported prostheses, regardless of whether they are single or multiple units or whether they are to be restored by being cemented to custom abutments or using a screw-retained approach, can be acquired using impression copings placed upon the fixtures. Once impression copings are placed upon the individual fixtures, they can be captured by an impression using either a closed or open tray technique.

However, these two techniques present both advantages and drawbacks, and there are reasons to favor an open tray technique over a closed one clinically. Although the closed tray impression technique requires less clinical expertise to perform, it does require reorientation of the impression copings back into the impression upon removal from the mouth. When this is done, if the impression copings are not oriented correctly with respect to the horizontal axis (ie, placed into the impression in their correct rotational positions), it can lead to problems at their connectors and prevent the implants from accurately mating with the prosthesis. When this occurs with nonsplinted restorations (ie, single units), the resulting restorations will be slightly rotated in relation to their desired position. If impression copings are not completely reinserted into a closed tray impression with respect to the vertical axis, the resulting restoration will not be in the desired occlusal relationship and will not fully seat at the connector or at all. In addition, because less rigid impression materials are used in closed tray techniques to permit the reinsertion of impression copings back into the impression, discrepancies can be created between the individual fixtures. Overall, research has shown that open tray impressions are more accurate than closed tray impressions.^5-7

Verification stents can improve the accuracy of the impression by locking the impression copings to each other and capturing them in relation to each other and the implants.⁸ Verification stents can be used during the impression phase but cannot be used in a closed tray impression technique because trying to remove a closed tray impression with the stent in place would destroy the impression in the process. Therefore, if a verification stent is to be used at the impression phase, an open tray impression is required. If the use of a verification stent to verify the master cast is desired, this requires a separate try-in appointment and may require alteration of the master cast by the laboratory prior to fabrication of the prosthesis. For this reason, open tray impressions offer clear benefits to clinicians and laboratories. Beyond allowing more rigid impression materials to be utilized to ensure that the fixtures are captured in relationship to each other in proper orientation, when a verification stent is desired, it can be fabricated at the time of impression making, eliminating the need for a separate appointment to verify the master cast. In addition, because the impression copings are embedded within the impression when it is removed intraorally, any issues associated with reinsertion of the impression copings back into the impression are eliminated as well. When a stiff-setting vinyl polysiloxane (VPS) impression material is used with an open tray technique, the use of a verification stent may not be required because the stiffness of the set material prevents any movement of the impression copings in the impression upon removal from the mouth.

Traditionally, open tray impressions were acquired either by using a custom tray with openings to accommodate the impression copings (Figure 1) or by using a stock closed impression tray that was modified with a handpiece to create openings for the impression copings (Figure 2). The chairside challenge with either of these options is orienting the tray during insertion to get the long pins to emerge through the holes once the tray is filled with impression material. This becomes more challenging when there is a greater number of fixtures in the arch and when fixtures are present in the second molar region. Furthermore, when a custom tray is used, it requires a preliminary impression and a poured model to fabricate the tray in the laboratory or in the office, which adds additional time and costs.

Innovations in the designs of some currently produced stock open impression trays can help mitigate some of these issues. For example, a tray that features an open occlusal surface covered by clear foil (MiraTray^® Implant Advanced, Hager Worldwide) can allow for easy identification of the heads of the pins intraorally. Mechanical retention of the impression material in clear foil-covered trays is provided by retention slots and an internal rim. Should a practitioner choose to supplement the retention with a VPS adhesive, it is recommended that it not be applied to the foil surface because this may obscure visualization of the pins when inserting the tray.

To capture an impression with a clear foil-covered tray, the tray itself should be filled with an appropriate stiff-setting impression material, such as a heavy body, tray, or putty VPS material. Next, a light body or medium body (ie, monophase) VPS material is intraorally injected around the gingival aspects of the impression heads and any natural teeth present but not over the coronal two thirds of the open tray impression copings. The tray is then inserted intraorally over the impression copings and pressed down crestally until the top of the impression pins are visible through the transparent foil. The practitioner then presses the tray further until the pins puncture through the foil and the tray is completely seated. It is then held in that position until the impression material sets. Upon setting, the pins are rotated in a counterclockwise fashion and removed from the impression, and the impression is removed from the mouth. When restoring single units, it is critical to capture the connector of the implant in the correct rotational orientation to ensure that the fabricated prosthesis will allow for full mating of the implant components.

Partially Edentulous Arches

As with single units, when adjacent implants are to be restored with a nonsplinted prosthesis, rotational orientation is important to ensure full seating of the prosthetics at the connector. When adjacent implants will be splinted or receive a fixed bridge, capturing them in their proper position ensures that the prosthesis will seat passively and that no stress will be placed on the implants that may lead to bone loss over time related to a lack of passive fit (Figure 3 and Figure 4).

Fully Edentulous Arches

Achieving passive fit becomes more critical when more implants are being connected in the arch. After open tray impression copings are placed on each of the implants, the selected tray should be tried in to verify that the entire arch can be captured. An appropriately sized tray will ensure that the arch and all implants sit in the middle of the tray as visualized through the clear foil (Figure 5). As with the prior examples, a light body or medium body VPS is injected onto the gingival aspect of the open tray impression heads, and then the tray is filled with a heavy body, tray, or putty VPS material and inserted until the long pins perforate the clear foil (Figure 6). Upon setting, the long pins are removed from the impression, the impression is removed from the mouth, and implant analogs are attached to each of the impression copings before the open tray impression is sent to the laboratory for fabrication of the prosthesis.

Capturing accurate implant impressions is essential to the proper fabrication of implant-supported prostheses. Any inaccuracies introduced during impression making are carried through the process and translate into inaccuracies in the final prosthesis, which can result in the fabrication of a restoration that fails to passively seat and places excessive stress on the implant components and surrounding bone. To avoid such failures and the need for remakes, practitioners need to be able to capture the most accurate impressions possible. The use of open tray impression techniques and the selection of impression trays that provide advantages to help ensure accuracy can improve the resulting impressions for implant-supported prostheses.

Gregori M. Kurtzman, DDS
Master
Academy of General Dentistry
Diplomate
International Congress of Oral Implantologists
Private Practice
Silver Spring, Maryland