Alginate impressions for full upper dentures

 I was having a chat with one of my new graduate colleagues about one of his full denture cases. He was taking a jaw registration but I noted that his upper model had a deep horizontal divot running across the hard palate area. I was wondering how that had happened but when I saw the ZOE impression, it was apparent that he had not seated the impression fully in the palatal area, tried to reline the impression and again not seating it fully leading to a step in the model. 

When using ZOE as an impression material for full upper dentures, the tray is made without spacing so while border moulding with greenstick, the tray is in intimate adaptation to the soft tissues. If the tray is then seated without sufficient pressure, there will be a significant thickness of impression material set onto the tray rather than a thin wash. This makes the previous border moulding step almost redundant and can fail to capture all areas of soft tissue, it will also lead to differential thickness of impression material at different areas of the tray. When I seat the impression material for ZOE, I seat it lightly whilst holding the buccal and labial soft tissues out of the way to allow material flow and to avoid air entrapment. I then put my fingers under the tray in the premolar area, standing behind the patient and lean backwards with my body weight to seat the tray. This results in a firm compression of the tissues by the impression material and a thin wash on the tray. One thing I intend to start doing is putting 3 blobs of greenstick on the tray, 2 posteriorly on the ridge and 1 anteriorly to form a tripod. These will be adapted to the soft tissues to provide tissue stops and should show through the impression material if the tray is seated well. The alternative may be show through of the tray if seating pressure is not even and greenstick will be more accurate than tray material.

So I had a discussion with this colleague on the topic of what is acceptable. He was of the opinion that he would continue with the treatment and if the wax rim wasn't retentive then he would think about redoing the impression. I told him to look at the impression with the ditch in the middle of the palate and explained that the fitting surface will be replicated in the final and there will be a step in the acrylic as well. I asked him if he thought this was an acceptable fitting surface of a denture. This seemed to make more sense. 

I suggested a few solutions:

1. Clean the ZOE off the tray and redo the impression: This is the most ideal option. If it isn't right, start again. Don't accept anything other than your best work or else what you deliver to the patient will be suboptimal. It doesn't take 5 minutes to decide if an impression is good enough. It won't suddenly become better if you stare at it long enough. Decide on the spot if it will work or else do it again.

2. Do a light body wash on the wax rim and get the lab to pour this up as a new secondary impression. This is less ideal as the wax was less rigid than the tray material which can lead to distoriton, there is no handle to manipulate the tray with, the wax rims may be improperly extended and border moulding is not possible. Also, I would imagine the PVS adhesive will not bond to the wax and you risk the impression peeling off during removal.

3. If he wanted to try the model out and see if the denture would be retentive he could order an acrylic baseplate for the jaw registration after. This would make reimpressing as per my second suggestion easier due to the material's rigidity and ability to bond to PVS adhesive.

As an alternative impression material, alginate works very well with full upper impressions. 

-    If any mistakes are made, it is easier to start again as the setting time is shorter.

-    Alginate is easier to load onto the upper impression tray, the more material that has to be loaded, the higher the risk of bubbles and it is very easy to incorporate bubbles into ZOE.

-The flow of alginate is better than ZOE so it is more easily displaced during border moulding to avoid overextension past the attached mucosa. ZOE is advantagenous for lower impressions to slightly displace the tissues to increase the denture bearing area. The better flow of alginate will allow it to flow around any tissue under cuts, to the hamular area and allow seating the posterior before the anterior to stop impression material going down the patient's throat. PVS will get drags if this is attempted.

-The setting time for alginate is faster which is good for gaggers especially during upper impressions.