Border moulding when using zinc oxide eugenol as an impression material

 I have recently started using alginate for my upper full denture impressions and zinc oxide eugenol for my lower full denture impressions as per Dr. Findlay Sutton's lectures. I find the alginate superbly accurate as it is hydrophillic and flows very well as well allowing good border moulding at the flanges. 

A full lower impression I took today with Zinc oxide eugenol had a large deficiency on the labial flange area as I had insufficiently border moulded with greenstick. This would normally not be an issue as on seating the tray, the excess material would extrude out and fill the sulcus area, however, I had also requested tissue stops which the lab had placed at the 34 and 44 area. This prevented over seating of the tray and actually worked quite well to control the level of thickness of ZOE and to control placement of the tray. However, compared to alginate, as ZOE is much thicker and flows more poorly, my inadequate control of flange length meant that insufficient material could be placed in the tray to fill the sulcus area. This highlights the importance of putting enough greenstick at the flanges to allow enough material to fit the tray. On Findlay's suggestion, I avoided greenstick at the retromolar area to minimise pressure on the retromolar pads during the impression. However, as I realised after, the tray was underextended in this area (as is usually the case) and so material did not flow in this area and I could not capture the retromolar pads in the impression.

To remedy this, I added ZOE in the deficient areas and captured them in another impression. It is more simple to do this when the deficiency is at a peripheral area e.g retromolar pad or flange. If it is a bubble in the middle of the denture surface, it is near impossible to put just the right amount of material. You will tend to put too much and cause overlapping onto the surrounding areas or put too little and leave a deficiency. If you are just going to add to the one spot, make sure that you seat the impression very firmly to try and spread the excess as thinly as possible. To be a tad more predictable, I would put a thin wash over the whole impression surface. to avoid the junction of old and new impression material.

What I would do differently in the lower impression would be to: 

-   Adequately add greenstick to the flange area to allow ZOE to flow to the depth of the sulcus. Remember that ZOE won't flow far past the tray edge and won't usually be stiff enough to avoid distortion if not supported by hard tray material/greenstick.

-Alternatively, take an initial impression with greenstick filling the tray. This ensures that the ZOE is a thin wash and very little is needed to take the definitive impression.

-If the tray area is under extended to the retromolar pads, I have to place greenstick at this area to support the impression material. Ensure that the tray covers the retromolar pad when instructing the lab.

-The tissue stops worked quite well but ensure that they are below the finger rests in the 34, 44 area. if the finger rests aren't above the tissue stops, downwards pressure when seating the tray will cause tipping of the tray and impression distortion. I would consider placing greenstick on the tissue stops to make them more accurate as the tray material may not adapt well enough to the stone and can cause sharp show-through through the impression material which will translate to sharp spots in the final denture.

-I have quite a bit of difficulty loading the tray with zinc oxide eugenol. It is best to place it in one piece and carefully spread it around the tray moving the spatula in one direction as changing the direction will fold parts of the material over and incorporate bubbles. This can be achieved in the upper impression by holding the posterior part of the tray against the mixing pad surface and scraping the material up the tray like dragging it up a ramp. The problem with the lower is that the posterior part of the tray has two separate sides and where the two sections of impression material join in the midline may incorporate bubbles. What I would like to try next time is to bring the mixing pad to the edge of the bench. Place the anterior part of the tray on the mixing pad surface with the handle hanging off the bench and scraping the impression material up the tray starting in the anterior midline.