Impression Troubleshooting by Nelson J. Gendusa, DDS

Vinyl polysiloxane silicones (also called "addition silicones," "polyvinyls" or "vinyls" for short) are the current state-of-the-art for crown-and-bridge impressions. They provide extraordinary accuracy and are so stable that definitive models can be poured even weeks after the impressions were taken. They can be sterilized in conventional gluteraldehyde solutions. Vinyls are now America's most-used impression material for fixed prosthetics. If you're new to vinyls, especially products made by Parkell, this short guide will help you avoid some of the most common mistakes in handling the material.

Problem #1: The wash material occasionally peels off the primary impression
Only those of you who take two-step impressions encounter this problem. In this technique, a preliminary impression is taken (The material used may be a putty–a heavy-body material–or even the same material you use as the wash.) The first impression is removed after it sets to serve as a custom tray for the wash material.

Fans of the 2-step impression like the hydraulic drive that the primary impression creates when the wash is seated. For many dentists there's another benefit, because if they take this primary impression before they prep the teeth, they can use the impression as a mold to fabricate the provisional crown.

Here are some suggestions if you're experiencing peeling (See Illustration 1).

Problem 1: Wash delamination. In this particular case, the delamination occurred because the dentist waited too long between the primary impression and the wash step. Fortunately, the delamination occurred at the periphery, so it did not compromise the model.

• Try harder to eliminate contamination If you use your primary impression to fabricate the provisional crown, surface residue left by the temporary C&B material may be preventing an effective bonding between putty and wash. To remove it, you should: Wash the mold of the preparation in running water then clean it with acetone and finally wash it again and dry it thoroughly. To complete, cut away the impression material around the prep using your handpiece. This is a good procedure to follow even if you don't make your temps using the impression. Dried mucin can also be a bond killer.

• Don't wait too long before you take the wash impression.
If you wait too long to seat the wash material, the primary impression may have set so completely that it's virtually inert ... without enough remaining free bonds to grab the wash material. This scenario is a real possibility if you take the primary impression in a fast-setting material then delay the wash step by making the temps using the primary impression. Think about it. After you take the primary impression you have to (1) prepare the teeth, (2) fabricate the provisional crowns, and (3) clean the primary impression before you can take the wash impression. So this approach can delay the wash step by 20-40 minutes. Suggestion: If you make your temps using the primary impression and have problems with delamination, try a slower setting material like Cinch AutoMix putty or Cinch-Platinum. Or alternately you can try the following:

• Add mechanical retention.
I know one dentist who claims he hasn't had a single delamination since he started adding potholes to the impression. After he cleans and cuts away the mold of the preparation, he uses his hand-piece to create several undercuts in the buccal and labial walls to grab the wash material. Might help. Can't hurt.

• Try the Saran® wrap technique
This golden oldie isn't used nearly enough today. Stick a sheet of Saran wrap over the putty impression before you seat it. Then pull it off before you apply the wash. The plastic (1) prevents contamination, (2) relieves the impression and (3) creates folds to trap the wash material. This technique works particularly well if you use Blu-Moose as the putty. Not only does it prevent delamination, but it also keeps the Blu-Moose primary impression from locking into undercuts in the mouth.)

• Consider switching to a wet/wet technique
Seating a tray of unset putty over the unset wash virtually assures an excellent bond, because the two materials intermingle at the interface. True, you won't get the same hydraulic drive when you seat a wet/wet impression as you do in a 2-step impression... but if you use good retraction, you really don't need to jam the wash into the sulcus.

• Problem #2: Mushy margins or tacky teeth
Though this problem can appear in a number of forms, the critical clue in the diagnosis is that the material in the offending area remains tacky to the touch. Once set, vinyl impressions are about as inert as anything in dentistry. So you can do pretty much whatever you want to them. Though we recommend gluta-raldehyde, we don't know of any disinfectants that will damage them. Transportation in the hottest lab pick-up van won't distort them. You can trim them and pour virtually any dye material into them.
But before they set, all vinyls are vulnerable to contamination. These addition silicones require a very small amount of catalyst (generally a platinum compound) to trigger the setting reaction. If something interferes with that catalyst, a thorough cross-linking reaction won't occur, and the surface of the impression will remain tacky... almost like paint that hasn't completely dried.

Now the chief culprit in platinum contamination is sulphur or sulphur compounds. And sulphur shows up in the darndest places. For example, it's a natural component of latex (That means rubber dams and examination gloves!) Even miniscule amounts of sulphur are enough to interfere with setting. So if you wear latex gloves you should avoid touching:
• the unset impression material
• the teeth and adjacent gingival
• the interior of the tray
• the spatula blade or mixing pad
• the end of the mixing tip
• the retraction cord


Problem 2: Margins from hell.
Though the sulcular area is a disaster, notice that the adjacent teeth are perfect. The problem was localized contamination, probably transferred from the dentist's latex gloves when the retraction cord was packed.


Same problem, from the model's point of view
Residual sulfur on the tooth prevented the impression from setting around the preparation, and pieces of unset material actually stuck to the stone.

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Problem 2: Just to illustrate how potent latex contamination can be, we lightly touched the facial surface of this central with a gloved hand before we syringed the impression material. Notice the small rough area where the material remained tacky.

Though we haven't actually seen it ourselves, another manufacturer warns that the vapor given off by a package of polysulfide impression material may be enough to cause contamination. While we can't really vouch for that, we wouldn't store our vinyl impression material next to a box of Permlastic®.

The easiest way to avoid latex contamination is to slip cheap polyethylene gloves over your examination gloves during the impression procedure. Though it's the most common source of contamination, latex certainly isn't the only one.

Another source is the oxygen-inhibited layer that remains on the surface of all resin materials immediately after curing. Because of this thin, invisible layer, impressions may remain tacky around new composite cores. And if you employ a two-step (putty-wash) technique and use the primary impression as a mold for the temporary crown, you must carefully clean the impression with alcohol or acetone to remove any remaining inhibited resin. Otherwise, the wash material won't set in that area.

We've also received reports that certain hemostatic agents inhibit vinyl though Astringedent and old-fashioned sodium hypochlorite don't seem to be the culprits.

Problem #3: Impression sets too fast
Two things can cause a vinyl to set faster than it's supposed to... the current temp-erature and the past temperature.

Possible cause #1: The current temperature
Vinyl materials are thermally sensitive. The warmer the environment, the faster they set. That's why you can pull a fast setting vinyl impression from a warm mouth after just 30 seconds, yet the material takes 2-2½ minutes to set on a relatively cool mixing pad. (Some dentists use this thermal sensitivity to adjust the speed of their impressions. When they want to slow down a vinyl, they cool the material slightly in the refrigerator.

Possible cause #2: The past temperature
If a vinyl impression material has been seriously overheated, it may not completely recover to its normal setting time ... even after it cools. That's why we suggest you store your impression materials in a cool location, away from the sun ... preferably the refrigerator. (A closet is okay, but not during the summer if your air conditioner is on a timer.) To preserve the proper setting time, store the material in a cool spot where it won't bake.

Problem #4: Impression sets too slowly

Possible cause #1: The material is too cold.
Remember, vinyls are thermally sensitive. Heat them, and they set faster. Cool them, and they slow down. If you keep your impression material in the refrigerator, let it warm up to room temperature before you use it. Otherwise, it will take a lot longer to set than you expect. (Since chilled vinyl impression material is thicker than warm material, it will also be more difficult to express through an automix cartridge.)

Possible cause #2: The material is old.
With time, the platinum catalyst that triggers setting can weaken, and the setting reaction will slow down. At room temperature, all Parkell impression materials offer a shelf-life of at least a year and at least a year and a half in the refrigerator. However, heat will dramatically speed up the aging process. So if you store the material in a warm closet or in a cabinet over a radiator, you may significantly shorten its useful life.

To avoid using out-dated impression material: 1.) Rotate the boxes in your closet so you always grab the oldest stock first. 2.) Store the material in a cool spot. 3.) And no matter how attractive the quantity discount may be, never order more material than you can reasonably use before it expires.

Possible cause #3: Base and catalyst weren't properly mixed. Vinyls require a one-to-one ratio of base and catalyst. If you spatulate, be sure the two ribbons of material on the pad are the same length. Then spatulate until the mixture is a consistent color. If you're using an automix cartridge, extrude about 1/4" of material onto a pad before you attach the mixing tip. Double-check that both the colored base and white catalyst are being expressed evenly. Then attach the mixing tip and proceed with the impression.

(Incidentally, some of the cheaper mixing-tips available in bulk from mail-order houses don't create uniform 1/1 mixes.)

Problem #5: Difficulty in expressing material from the automix cartridge

Possible cause #1: Your impression material is too cool.
When they're cold, all vinyl polysiloxane materials thicken dramatically. For example, at room temperature, Green-Mousse by Parkell, has the light consistency of Cool-Whip®. But right out of the refrigerator it's more like thick custard. This can make it difficult to extrude from a cartridge. If you normally keep your impression materials in the refrigerator, let them warm to room temperature before using them.

Possible cause #2: Small plugs of hardened material can form at the openings of an automix cartridge.
If you don't remove these plugs, they'll clog the system. (Never, ever use excessive force to express material. It may damage the gun or cartridge.) Before you attach a mixing tip to the automix cartridge, always clean any hardened material from the cartridge openings using a hand instrument. Then check that the material is flowing smoothly by expressing a short 1/4" bead directly from the cartridge onto the mixing pad. The material should express easily and you should see equal amounts of base and catalyst.


After a 23-year career as a practicing dentist, Dr. Gendusa agreed to join Parkell as Director of Research. He brought the company a unique, priceless view of dentistry "in the trenches," while becoming an integral member of their staff. He continues to provide a realistic evaluation of the company's products and how real dentists will use them. If you have a question about any of Parkell's products, he is the person to ask for an honest, no-nonsense response you'll understand. Have a technical question best answered by a fellow Dentist? E-mail your questions directly to our Dr. Gendusa at: asknelson@parkell.com. Please feel free to e-mail him with your clinical questions or suggestions. He'll respond promptly. You can also fax him at: (631) 249-1242. For additional information about Parkell products, call: 1-800-243-7446. If you'd like more information on impression troubleshooting, visit Parkell's website at: www.parkell.com. There are many special reports and technique articles available online including Special Report #45 on the laminar impression by Townie, Rod Kurthy, DDS. This technique is also discussed at length on the message boards at: www.dentaltown.com, under the Cosmetic Dentistry forum, Rod's Mach 2 Impression Technique.