impression tech in fpd
TRANSCRIPT
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PRESENTED BY-: GAGANDEEP SINGH
IMPRESSIONS IN FIXED PARTIAL DENTURES
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Introduction
To obtain an impression is the first step necessary for the
indirect fabrication of a prosthesis.
An acceptable impression must be an exact negative replica
of the prepared tooth . The impression should reproduce the
critical area where tooth and restoration meet so that the
restoration can be correctly contoured. All teeth or tissues
immediately surrounding the prepartion must be reproduced
in the impression.
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An impression made for Fixed partial denture should meet the following requirements
1. It should be a an exact duplication of the prepared tooth, including all of the preparation and enough uncut tooth surface beyond the preparation to allow the dentist and the technician to be certain of the location and configuration of the finish line.
2. Other teeth and tissue adjacent to the prepared tooth must be accurately reproduced to permit proper articulation of the casts and contouring of the restoration.
3. It must be free of bubbles especially in the area of the finish line and occlusal surfaces of other teeth in the arch.
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Impression techniques
1.Stock tray technique
double mix
single mix
2.Custom tray technique
single mix
3.Closed bite double arch technique
4.Copper band technique
5.Reversible hydrocolloid technique
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Putty wash Method
Double mix and single mix.
Stock trays can also be used with medium and heavy bodied elastomers normally used with custom trays.
Advantages:
1)Eliminates time and expense of fabricating custom trays.
2) Metal stock trays are rigid and are not susceptible to distortion.
Disadvantages:
1) More impression material is required
2) Metal tray must be sterilized
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Technique:
A trays is selected based on shape and size of the patients
arch.
Coat the tray evenly with adhesive on the inside.
Mix the high viscosity putty impression material according
to manufactures instruction.
Roll putty into elongated cylinder. Insert into the stock
impression tray.
Cover putty with the spacer (a sheet of polyethylene) .
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Insert and seat the tray with a rocking type of motion.
Hold and wait until initial set (approximately 2min).
Remove from the mouth with minimal side ward movement
Wait and test for final set.
When the impression rebound completely.
Peel of the spaces.
Remove excess impression material with a sharp knife.
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Putty placed on the tray
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Placing the spacer
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Removal of the spacer from the tray
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Making the final impression
Mix the low viscosity impression material according to the
manufactures instructions .
Load the low viscosity impression material on to a syringe .
Syringe the inaccessible area first eg disto lingual line angle
For the stock tray double mix insert the low viscosity
impression material into the tray without overfilling it.
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For stock tray single mix technique the unset high viscosity
material should already be in the tray and the preparation
syringed with low viscosity impression material.
Seat the tray .
Wait for the final set.
Remove the tray parallel to the preparation path of
withdrawal.
The impression is evaluated for the finish lines and for any
distortion and tear.
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Putty impression with polyethene spacer sheet and then wash
Scalpel to create space
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Final impression
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One stage or one step putty wash impression
(Simultaneous technique / squash technique / twin mix technique)
Disadvantage :
Difficult to control thickness of wash materialHigher viscosity material displaces the wash material
Setting distortion of the putty add to over all distortion of impression
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Custom Tray Fabrication For Elastomeric
Impression
Custom tray is made from autopolymerizing acrylic resin or
vacuum formed thermoplastic resin.
Tray rigidity is important for even slight flexing of the tray will
lead to distorted impressions.
Resin thickness of 2 or 3mm is needed to give adequate rigidity.
Clearance between the tray and the teeth should be 2 to 3mm
however greater clearance is needed for the more rigid polyether
material.
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Procedure:
1)Mark the border of the tray on the diagnostic cast with a pencil approximately 5mm apically to crest of the free gingiva.
2)Adapt a wax space to the diagnostic cast two layers for 2.5mm thickness approximately.
3)Apply a layer of tinfoil over the wax.
4)Mix autopolymerizing acrylic resin according to manufacturers recommendation
5)Adapt the resin to the cast when in dough stage.
6)After the material has polymerized ,remove it from the cast and trim with an acrylic trimming bur.
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Custom tray method
Mixing method-single mix
Advantages .
Less impression material is required than stock tray.
The tray is used only once so sterilization is not a problem.
Less distortion of the impression .
Disadvantages
Construction of the custom tray is time consuming .
The tray should age for 24 hours to minimize distortion
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Making the final impression.
Medium or high viscosity is used in the tray and low viscosity in
the syringe.
The low viscosity material is mixed first then the high viscosity
material.
The syringe is loaded with the low viscosity material and
syringed in the prepared site.
The high viscosity material is placed in the custom tray and
seated in the mouth.
The impression is removed and evaluated.
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Removal of the tray using the wings
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Close bite double arch method .
synonyms :dual quad tray, double arch, triple arch, accu -
bite, closed mouth impression.
Requirements
The articulator should provide for a positive stop or there should
be sufficient teeth to maintain the vertical dimension.
There should be sufficient space distal to the terminal tooth in
the arch to allow tray approximation.
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AdvantagesThe physical deformation of the mandible is minimal
The shifting of teeth occurring during maximum intercuspation
is captured.
Less impression material is needed.
Less gagging may occur.
DisadvantagesThe tray is not rigid .
Limited to one casting per quadrant .
The distribution of impression material is not uniform
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Technique
Evaluate the fit of the tray in the patients mouth.
Observe the complete bilateral closure and the patients comfort.
Practice till the patient is familiar with the task.
Making the final impression
Mix the low viscosity material and load the syringe
The high viscosity material is used in the tray.
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After the low viscosity material is syringed the tray is
positioned on the arch.
Instruct the patient to close the mouth and observe for the
interdigitation on the opposite arch.
Wait for the material to set .
Instruct the patient to open the mouth remove the tray by
applying equal pressure bilaterally.
Evaluate the impression.
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Closed mouth impression
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Copper band or tube methodThe copper band is used to salvage an impression of
multiple preparation where there is only vague margins on
one or two preparation that are not adequately replicated in
the impression.
Fitting of the copper band to preparationA copper band is selected and annealed by heating on a
flame and quenching in alcohol.
Mark the finish line with the explorer and round off the
edges. Also evaluate the fit and cut orientation hole in top
one fifth of the facial surface .
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Copper band with orientation holes
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Copper band contoured
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Evaluating the fit of the copper band
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Making the compound plugThe red stick compound is heated over the Bunsen flame .
Insert the warm compound mass to fill approximately the
top third of the copper band.
Seat and orient on to preparation and compress excess in to
the band.
The compound should just touch the occlusal surface .
Remove and evaluate the impression, only the occlusal
surface should be impressed.
Remove 0.2mm of the compound to create space for the
heavy body poly vinyl silicone.
Drill a hole through the centre of the compound plug.
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Making the impression.
Make 4 to 5 holes evenly distributed above the bottom
of the copper tube for the retention of the impression
material.
Coat the internal surface sparingly with adhesive .
Clean and isolate the preparation.
Mix heavy viscosity material and inject in to the band
and position it on the tooth.
Wait the material to set remove the band and inspect the
impression
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Copper band with the final impression
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Reversible hydrocolloid technique
This impression technique requires a special conditioning
unit which consists of 3 thermostatically controlled water
baths.
1. A liquefaction bath .
2.A storage bath .
3.A tempering bath.
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Hydrocolloid conditioner
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Rim lock trays with stops
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ProcedureSelect the correct size of water cooled impression trays.
Place small modeling compound or prefabricated stops
in the tray to prevent over seating .
Fill the impression tray with heavy bodied material
from the storage bath and place it in the tempering bath
Load the syringe material in the syringe and replace it in
the storage bath.
Carefully remove the retraction cord and flood the tooth
with water.
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Inject the syringe material on the tooth cover the entire
tooth.
Remove the impression tray from the bath wipe with
the gauze and place it in the mouth.
After seating cold water is circulated through the tray
until the impression material is set.
Remove the impression with a rapid motion, wash with
cold water ,evaluate for accuracy.
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Removal of the retraction cord
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Injecting hydrocolloid into the sulcus
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Final impression in reversible hydrocolloid
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HYDROCOLLOID LAMINATE TECHNIQUE
ALGINATE – AGAR METHOD
Suggested by Schwartz in (1951)
Prepared tooth surfaces
Injection of reversible hydrocolloid
Stock tray with chilled alginate
Combined alginate –agar impression
Master cast
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Critical to success bond between agar and alginate
Advantages :• Satisfactory surface reproduction • Good wettability (smooth, dense model)• Simple conditioning (no tempering bath / water cooled trays)• More economical
Disadvantages :
• The bond between agar and alginate is not sound
• Higher viscosity alginate displaces agar hydrocolloid
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Summary
A good impression is an exact replica of each prepared tooth
and should include adequate amount of the unprepared
surfaces adjacent to the margin.
Impression technique should be selected on the basis of
biologic factor dictated by the anatomy and the physiology
of the mouth dictated by the orofacial tissues. Even though
there are innumerable techniques and procedure for
impression making ,it is the responsibility of the dentist to
select the best possible procedure for achieving the best
possible result for the patient.