When you’re sure you did everything right, but the impression
came out all wrong ...
by Nelson J. Gendusa, DDS
Director of Research
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, this short guide will help you avoid some
of the most common mistakes in handling the material. We’ve included
pictures, so if you have trouble with an impression, you can identify
the problem quickly.
By the way, we had some genuine reservations about writing this Special
Report. Several people in our marketing department questioned the wisdom
of devoting a major article like this to discussing potential problems
you might encounter using our products. Therefore, we want to make it
clear that we’re discussing a category of products here ... not our
products in particular. This troubleshooting guide applies just as well
to other good vinyls like 3-M’s Express® or Caulk’s Reprosil®.
So even if you never use a Parkell impression material (Cinch® Vinyls,
Green-Mousse®, or Blu-Mousse® ), you may find this guide helpful.
Problem #1:
The wash material occasionally peels off the primary impression.
This problem is encountered only by those of you who take two-step impressions.
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).
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• 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,
- clean it with acetone,
- wash it again and dry it thoroughly,
- then cut away the impression material around
the prep using your handpiece.
In fact, 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 like Blu-Mousse SuperFast or Cinch-90, and
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 ...
• 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-Mousse as the
putty. Not only does it prevent delamination, but it also keeps the
Blu-Mousse 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
glutaraldehyde, 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 die 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 gingiva
• the interior of the tray
• the spatula blade or mixing pad
• the end of the mixing tip
• the retraction cord
 
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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. 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.
Polyether impression materials (Impregum®) and polysulfides (Permlastic®)
leaves the mouth coated with a chemical film (hence, that yucky taste!)
that inhibits vinyl. Occasionally a dentist will take an impression with
Impregum ... then decide to take a second impression using a vinyl material.
The second impression won’t set properly.
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 temperature 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 Blu-Mousse Super-Fast
impression from a warm mouth after just 30 seconds, yet the material takes
2-21ž2 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 like Blu-Mousse or Green-Mousse, 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.)
So 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:
Small bubbles in the stone.
Possible cause #1: Hydrogen out-gassing.
Traditional vinyl impression materials such as Express burp hydrogen gas
for several hours after impressing. If you pour too soon, the stone will
capture these bubbles and produce a model covered with tiny pits.
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Problem 5: Tiny pits all over the model. Traditional impression materials like Express (left) emit Hydrogen for several hours after setting. Pour too soon, and the model will capture these gas bubbles. New-formula Cinch materials (right) eliminate outgassing ... so they can be poured almost immediately. |
The solution is either to wait longer before making the model ... or change your brand of impression material. Starting September 1, 1999 all Cinch materials feature a proprietary component that completely eliminates hydrogen bubbles. You can pour the model just 5 minutes after you remove the impression from the mouth.
Possible
cause #2: If you spatulate, you may be inadvertently trapping air.
It’s virtually impossible to mix base and catalyst on a pad without stirring
at least some air into the material. This air creates voids in the impression
... bubbles.
All of our work here at Parkell suggests that automix cartridges produce
impressions with significantly fewer bubbles. Not just with Parkell materials,
but with everybody’s. As you extrude, the mixing tip automatically blends
base and catalyst without including air.
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Problem 5: Bubbles in the impression can occur when you spatulate because it's almost impossible to avoid trapping air. Here we cut apart two samples of impression material. The one on the left was mixed by hand. The one on the right came from an automix cartridge. As you can see, hand-spatulation created significantly more bubbles |
So if bubbles in the impression
seem to be a significant problem, you might want to give the automix cartridges
a try.
Problem #6:
Small bubbles in the preps.
Possible cause #1: Saliva or blood on the teeth.
To reduce the chance of sulcular hemorrhage, wet the retraction cords
before you remove them. Then wash and dry the teeth thoroughly. If you
encounter significant bleeding, reschedule the patient to allow time for
tissue healing before you take the impression.
(It’s amazing what several days of healing can mean to the quality
of an impression. After the tissue has had a chance to conform to the
provisional crown, the wide open sulcus practically sucks up the impression
material.)
Possible cause #2: Loss of contact between the syringe tip and tooth.
When you syringe around the preparation, try to fill the sulcus in one
smooth circumferential motion. If you pick up the syringe tip or stop
expressing material before the sulcus is filled, there’s a good chance
you’ll trap some air right at the critical margin when you start
syringing again.
If you use automix cartridges, Parkell offers a tiny intraoral tip that
snaps into the mixing tip. These allow you to express directly from the
cartridge into the mouth with excellent access around the tooth.
Whether you use a hand syringe or the impression gun, try to push the
tip over the wash material as you express it. It feels more natural to
drag the tip away from the material as you express it, but if you push
it into the wash, the tip will squeegee it down into the sulcus.
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Problem 6: Large bubbles around the preps. If you push the syringe tip over the bead of impression material as you express it (instead of drawing it away), the tip will act like a squeegee and force the material down into the sulcus, reducing the chance you'll catch a bubble. |
Problem #7:
Narrow castings from double-arch impressions.
Though double-arch trays have earned a bad reputation in some circles,
I personally believe that with proper care, these inexpensive disposable
trays can produce fast, easy, and highly accurate models for 1 and 2-unit
castings. Like most things in dentistry, however, they require careful
attention to technique.
Disposable double-arch trays are extremely flexible, so if there is any
interference at all when the patient occludes, the plastic will distort
and then spring back to its original shape when you remove it.
The result will be a narrow casting.
Spring-back distortion can be a particular problem in the posterior, where
it’s hard to be certain that the molars and mucosal tissue are not
touching the tray.
To avoid spring-back distortion ...
- Spend a few seconds having the patient close into centric with the
empty tray in position. Check to see that neither teeth nor ridge are
touching the plastic.
- Use Blu-Mousse as the primary impression material. It sets so hard
that it structurally reinforces the tray, almost like an I-beam running
from buccal to lingual. If there is subtle distortion when the patient
closes, the stiff Blu-Mousse will prevent the tray from springing back.
Conventional putties are not a good substitute for Blu-Mousse in the double-arch technique. Putty is so viscous that it exaggerates tray distortion when the patient occludes. And most putties don’t set hard enough to resist spring-back.
Green-Mousse, Cinch-Platinum (and most other vinyls) are fine for the wash step ... but they are far too flexible to be the primary Cinch-Light impression material in a double-arch tray.
- After removing the tray from the mouth, examine the impression to be certain that the plastic didn’t impinge on any hard or soft tissue. If it did, RETAKE THE IMPRESSION.
Problem #8:
Folds in the impression.
Occasionally impression material folds over on itself when the tray is
seated and creates a crease-like artifact in the final impression. These
folds don’t normally jeopardize the impression, because they tend
to occur away from the syringed wash, in areas where the tray material
has to flow around irregular shapes. (That is, in noncritical areas.)
Some folding is normal even in the most carefully-taken impression. However,
excessive folding suggests that your impression material may have started
to set before the tray was seated.
The obvious solution is to get the tray to the mouth within the material’s
recommended working time. If you’re uncomfortable operating that
fast, switch to a slower-setting material. For example, you should seat
a Green-Mousse impression within 11ž2 minutes of starting the mix. Cinch-Platinum,
however, allows a full 3 minutes. If you’re moving fast, and still
getting a lot of folds, the impression material may be too warm ... which,
as we mentioned earlier, will speed setting. Try cooling your material
slightly in the refrigerator.
Problem #9:
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 our Green-Mousse 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.
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Problem 9: To prevent clogging, always remove plugs of set material from the cartridge, and check that the material flows properly by expressing 1/4&” onto the mixing pad before putting on the mixing tip. |
| After you’ve
used the cartridge, do not remove the mixing tip and replace the original
cap. This will cause unnecessary mixing of base and catalyst and virtually
assure plug formation. The easiest way to minimize plug formation is to simply leave the mixing tip in place. The hardened impression material in the tip will act as a cap. Possible cause #3: You’re using the wrong impression gun. For years 3M® sold an impression gun with a metal handle and plungers with a diameter just slightly larger than the industry standard. As a result, if you use the 3M gun with a Parkell®, Kerr® or Caulk® cartridge, the plungers tended to stick, making it very difficult to express the material. (And once you’d finished, removing the plungers from the cartridge could be a horrible struggle.) Possible cause #4: You’re using the wrong impression material. There is a huge variation in the force necessary to express different brands of materials. For example, 3M’s bite registration material requires more than twice the hand pressure of Blu-Mousse. |
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Impression pulls out of the tray when you remove it from the mouth.
Though all vinyl manufacturers offer tray adhesive, none of these adhesives really stick to vinyl as well as we’d like. Fact is, adhesive alone isn’t enough to hold the material securely in the tray. So use trays with good mechanical retention.
Custom trays should be perforated. If you still get loosening, try using both adhesive and perforations. Better yet, roughen the tray surface with sandpaper, perforate the tray AND use an adhesive.
If you’d like to learn more about alternative impression techniques, ask for our Special Report discussing the laminar impression and Special Report #45..
Trademarks mentioned in this article: Blu-Mousse, 3M, Aquasil, Caulk, Cinch, Cool-Whip, Green-Mousse, Jet Bite, Kerr, Parkell, Reprosil, Saran, Express, Reprosil
Have a technical question best answered by a fellow Dentist? E-mail your questions directly to our Director of Clinical Research, Dr. Nelson Gendusa, DDS. (Click here) for more information.