At O’Brien Dental Lab, a common mistake we see with implant impressions is the incorrect use of impression copings which can lead to inaccurate models. In this article, I’ll explain the error so that inaccuracies can be avoided.
Implant impressions can be taken using the closed-tray or open-tray technique and a specific type of impression coping is required to pair with each application. Something we often observe is open-tray impressions taken with closed-tray impression copings and vice versa.
Closed-tray impression copings are designed to be easily removed from the impression after it has set up. They have shallow retention features with soft edges that allow the coping to softly snap in and out of the impression without tearing the impression material.
They are also designed with flat sides or “indexes” so that it’s easy to re-insert them back in the correct alignment. Some impression copings are designed so that they can only be re-inserted in one specific orientation while others can be re-inserted in several rotations that are all accurate.
One thing that almost all closed-tray copings have in common is the reliance on the bottom of the tray as a vertical stop. If a hole is created in the tray, that vertical stop is removed, and we lose vertical accuracy.
Conversely, open-tray impression copings are designed so that they are not (NOT) easily removed from the impression. They have hard edges and deep features that provide retention and allow the impression material to lock them into place.
If an open-tray impression coping is used with a closed-tray technique, the coping will rip through the impression material when the impression is pulled. This reduces the accuracy of the impression when the lab tries to re-insert the impression coping to pour up the model. Additionally, many open-tray copings are designed in a way that makes it possible to re-insert the impression coping in the wrong orientation.
An example is the Astra EV impression copings that have identical anti-rotation slots all around them and yet there is only one correct rotational Astra EV orientation. Once these are removed Impression coping from the impression, it is impossible to determine which way they should be re-inserted.
And so, the bottom line is, always pair the correct impression coping with the correct tray technique to achieve the most precise and predictable results.