In restorative dentistry, accuracy is important. This is especially true with respect to our impressions and the casts fabricated from those impressions. Inaccuracy introduced to an implant case at the impression stage has the potential to be propagated, and potentially magnified, as the case progresses. Ultimately, inaccurate impressions may have a negative impact on the definitive outcome if not managed appropriately.
The degree to which accuracy is important is based on how the resulting cast will be used. An impression for an implant-supported restoration that will connect the implants in the final prosthetic design requires a higher level of accuracy than a design in which the restorations will be fabricated as individual units.
Individual implant restorations fabricated from an inaccurate cast may require longer insertion appointments, frequently requiring the clinician to alter the proximal or occlusal contacts.
Closed-tray impression techniques
Many clinicians favor closed-tray impression techniques for their individual implant restorations. The closed-tray technique can be very efficient, especially if natural teeth have also been prepared and need to be captured in the same impression.
One factor that frequently results in inaccurate casts using the closed-tray impression technique is related to inaccurate transfer of the impression coping/analog complex into the impression. For this reason, it is a good idea for the clinician to connect the closed-tray impression abutment and the implant analog and then carefully reseat the impression abutment prior to sending the impression to the lab. Following this approach allows the clinician to verify that the impression abutment is fully seated and replicates the clinical reality.
An aspect of the closed-tray impression technique that isn't often discussed is how to best manage the head of the closed-tray impression abutment screw. The head of the screw could easily be recorded in the impression and, unless the relationship of the abutment screw and the abutment is exactly duplicated (which is virtually impossible), the abutment may not be able to seat fully back into the impression.
In order to prevent this kind of interference to reseating, the Straumann closed-tray impression abutments are shipped with a polymer cap that prevents impression material from flowing into the screw head. The polymer cap is picked up in the impression and allows for positive seating to the abutment.
A close look at this system reveals a groove in the impression abutment and a corresponding ridge inside the polymer cap. As the cap seats onto the abutment, the operator will notice a tactile click.
Clinicians should be aware that, since the ridge is plastic, it could become deformed with repeated use, potentially preventing the abutment from seating fully. As a result, these caps should really only be used once and then replaced.
What about impressions for implant systems without this type of polymer cap? The image below shows a different clinical situation in which I wanted to make an impression for fabrication of acrylic provisional restorations. I wanted to make a closed-tray impression, but polymer caps aren't available for this system (Nobel Biocare Replace).
In this case, I used a soft wax (Disclosing Wax, Kerr) to block out the impression abutment screw heads. The wax prevented the impression material from flowing into the screw heads and created space in the impression for the abutments to be reseated. This is an effective technique but, for maximum accuracy, the wax should be completely removed from the impression abutment prior to reseating.
An alternative approach, of course, is to use an open-tray impression technique. In this case, the final restorations are going to be connected and screw-retained, demanding a highly accurate impression and working cast.
To meet these requirements in the definitive impression, I connected the impression abutments using low-shrinkage pattern resin (Pattern Resin, GC).
(Click this link to read more dentistry articles by Dr. Darin Dichter.)
Darin Dichter, D.M.D., Spear Faculty and Contributing Author
Papaspyridakos P. Accuracy of implant impressions for partially and completely edentulous patients: A systematic review. IJOMI 2014:836-45.