Impression procedures and digital impressions/intra-oral scanning
- Understand the difference between fixture level and abutment level.
- Understand the difference between open and closed tray impression techniques.
- Digital impression techniques are also available.
- There are inaccuracies in both techniques.
Impression procedures and digital impressions, intra-oral scanning
The replication of the precise position of the implant body (rotation, depth and angulation) relative to the other oral structures (teeth and/or implants) is critical to the success of the restoration. The ability to accurately capture the hard and soft relationships enables the restorative dentist and dental laboratory to satisfy the esthetic and functional needs of the patient.
There are two types of impression procedures: abutment level or fixture level (whether they are closed tray, open tray, or digital) . The choice of which impression to take is determined by multiple factors inclusive of soft tissue height, choice of final abutment, position in the arch, type of soft tissue, inter arch space, relationship to other teeth or implants, type of implant connection, splinting or not, and others.
Fixture and Abutment Level Impression
Impressions can be made a either the fixture level (implant platform level) of the abutment level. The abutment level impression can either be of a stock abutment or of a custom abutment. The stock abutment can either be a replica of a tooth preparation or a multi-unit abutment (MUA), or other screw type options. In the case of a custom abutment it is exactly the same procedure as impressing a tooth. Soft tissue retraction can be utilized and the margin of the desired end point for the crown can be visualized. The approach many time is used when cementing the restoration. With a multi unit abutment (MUA) a transfer coping is attached to the abutment and then the impression taken. This can be either open or closed tray.
Open Tray Impression
An open tray approach allows for direct visualization of the transfer coping through the tray. transfer coping is unscrewed from the implant and the removal of the tray and the transfer coping are done simultaneously. Therefore the impression coping and tray are essentially one unit. Open tray impressions are typically indicated for multiple-implant restorations, however can be used for single-unit case also.
Closed Tray Impression
The closed tray technique involves the placement of the transfer coping, the final impression material is placed in the tray and then removed upon complete setting. The transfer coping is then unscrewed from the implant, attached to an implant analog, and repositioned back in the tray. Of note, the material for the impression is at the discretion of the provider and should be one that is used for conventional crown and bridge procedures
With the increasing utilization of digital impressions implant systems provide scan bodies that allow for capture of the relationship of the implant connection to adjacent structures. This negates the need for conventional impression materials which provide greater comfort for the patient.
One should consider doing either a metal try-in or bisque try-in of the final restoration prior to going to completion. This minimizes remakes and assures a well fitting restoration, especially when one first gets involved with dental implant restorations. It is essential when multiple implants are in the same splinted restoration. When multiple implants are to be impressed and incorporated in a splinted restoration it is essential that some form of verification procedure be performed prior to commencing the fabrication of the final prosthesis. It is highly recommended that this be carried out in single tooth implant retained restorations also.
When tapered impression copings are used, the long axis of the implant and impression coping are sufficiently parallel to the natural teeth that the impression can be removed from the mouth after the impression material polymerizes. This type of impression involves a traditional type of impression tray without a hole or opening in the tray, and it has become known as a “closed-tray impression” technique.