I got a call from my lab a while back telling me that my models didn’t look right. He sent over some images and the models looked like they had some kind of scaly disease (see the photo below). My impression material for this model was vinyl polysiloxane and is one I had used consistently for several years, never having this happen before.

Dental impression problems

I had the patient back, removed the provisionals and redid the impression. The new model was fine so my assumption was that it was a bad batch of impression material.

Fast forward to another veneer case shortly afterward. When I pulled the impression from the mouth and examined it under magnification, parts of the material looked unset and pulled from the teeth. You can see the “gooey-ness” in the image below, mostly on the palatal surface (see the image below).

Dental impression problems image 002

As I reviewed my technique, I realized that the only thing that was different was the timing of fabricating provisionals. In this case, I wanted to measure the thickness of the provisionals as another way to double check the facial reduction of these veneers. I fabricated them and then took the impression.

That’s when I called the manufacturer and discovered that the provisional material I use, a bis-acrylic, has an air-inhibited layer that can retard the set of vinyl polysiloxane impression materials if it is not completely removed prior to making the impression. The air-inhibited layer was gone at the time of the second impression and I also pumiced the teeth thoroughly to remove the temp adhesive. That is what allowed for a successful second impression.

This is a rather simple problem to solve. It can be handled in two ways:

  1. Make the provisionals after the final impressions are completed and avoid contact of the VPS with the air-inhibited layer left behind.
  1. Fabricate the provisionals first, then:
    1. Clean the teeth thoroughly with a prep scrub
    2. Follow the scrub with an additional flour of pumice cleansing with a prophy cup
    3. Alternatively, air abrade the preps with 50-micron aluminum oxide after the prep scrub
    4. If the provisionals are made with retraction cord in place, remove and replace the cord. It can retain enough of the air inhibited layer to alter the set of the marginal impression material
    5. Follow with normal impression procedures

PVS set can also be retarded through contamination by latex gloves and glove powder. If you or your assistant must touch the surface of the impression material or if you are mixing a putty for the impression be sure to use nitrile gloves.

PVS appears to be the only material that has this issue; polyethers are not affected in the same way.

Dental impression problems image 003

Vinyl polysiloxane impressions may also result in a model like the one in the image above. Note the tiny bubbles covering the surface of the impression. Also note that bubbles are more uniform in size and distinct and separate.

VPS materials need to sit for at least 30 minutes prior to being poured so that hydrogen can be completely “outgassed” prior to pouring the impression. Be sure to read the instructions from the manufacturer of your VPS material since some require a longer outgassing time.

(If you enjoyed this article, check out more by Dr. Steve Ratcliff.)

Steve Ratcliff, D.D.S., M.S., Spear Faculty and Contributing Author