A good impression
is the key to an accurately fitting crown and bridge restoration. When
the impressioning process is carried out correctly, final seating of
the restoration is carried out with minimal time and adjustment. The
following checklist outlines key points to be considered for consistently
successful simultaneous impressions:
- Use a rigid tray
with the stability to prevent distortion.
- Ensure there is
a smooth putty/wash junction without ledges. These ledges are difficult
to see in the impression but will become apparent in the stone model.
Occurrence of ledges can be lessened by preparing a deep indentation
in the putty (extending to the two adjacent teeth) opposite the preparation
site. Fill the indentation with syringeable material and slowly, but
firmly, seat and immobilize the tray.
- Ensure margins
are captured in syringeable material and one free of voids and tears.
- Make sure you have
a very accurate impression of the adjacent and contra lateral teeth,
especially on the facial surface. Ceramists cannot match surface texture
and contours of the contra lateral teeth if they can't see them.
- Ensure there is
a good bond between putty and wash prior to removal from the mouth.
This bond is the last to reach sufficient strength. Premature removal
of the impression can lead to tearing of the syringeable material from
the putty. Tearing at the margins can be minimized by better retraction
of the sulcus and leaving the impression in the mouth for the designated
set time.
- Avoid tray burn-through.
If the impression tray is not properly seated and immobilized until
sufficiently set, the preparation may contact the tray. With weaker
plastic trays, this can cause an expansion of the tray that will rebound
upon removal and create a tight-fitting crown.
- Record accurate
detail in putty. The best impressions with putty are obtained by seating
the putty within one minute of mixing. If not seated when fluid, the
putty impression will capture less detail.
- Work in dry field.
Rounded cervical areas or margins full of bubbles are frequently caused
by moisture in the sulcus. Be sure the prepared tooth, sulcus and surrounding
tissues are thoroughly dry prior to placing the syringeable material.
A dry sulcus is important. When using cords, proper packing is necessary.
If you can't see the margins, neither can the impression material.
- Ensure there are
no bubbles or pulls in the impression.
- Proper tissue management
is critical for a good impression. Prepare the initial finish line to
the gingival margin. Then pack the retraction cord and allow three to
five minutes for the gingival tissue to retract. Remove the cord and
finish the preparation in an open and dry sulcus. After prepping, place
a large piece of retraction cord in the sulcus and below the preparation
line. Place a thicker cord on top of the first. When ready to take the
impression, remove the top cord, leaving the bottom one in place and
inject the impression material. Remove the remaining cord after removing
the set impression from the mouth.
- To avoid surface
porosity in the model, avoid pouring polyvinyl siloxane impressions
for at least 15 minutes.
- Block out undercuts
on prepared teeth to prevent the impression material from tearing.
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