Impression material and impression procedure for partial denture
Impression material:
any substance or combination of substances used for making
an impression or negative reproduction of the intraoral tissue
1. Plaster of paris:
This material are composed of calcium sulfate hemihydrate (plaster of paris) to
which modifier have been added to regulate the setting time and setting expansion.
It is used for complete denture but now are rarely used due to no elasticity, not
used for partial denture due to low elasticity so break when removed
2.Zinc oxide eugenol
Biological effects:
Some patients experience burning sensation in the mouth due to eugenol and can
also cause tissue irritation.
Advantages:
The advantages of zinc oxide-eugenol include:
1-high accuracy of soft tissue impression due to its low viscosity.
2-the material is stable after setting .
3-has good surface detail reproduction.
4-unexpensive.
Disadvantages:
1-burning sensation due to eugenol.
2-messy to work with.
3-sticks to skin and instruments (difficult to clean)
4-rigid so tends to break off in areas of undercut.
Uses:
Secondary impression material for complete dentures
Impression for extension base edentulous ridge areas of removable partial denture
Impression material for relining distal extension denture bases.
3-Impression compound:
Is a mucocompressive thermoplastic material(is soft when heated and hardens
when cooled). Use forMaking primary impression for edentulous ridge and
Border molding of special trays for kennedy Cl I and Cl II R.P.D bases.
Reversible Hydrocolloids
Reversible (agar-agar) hydrocolloids, which are fluid at higher temperatures and
gel on reduction in temperature, are used primarily as impression materials for
fixed restorations. They
demonstrate acceptable accuracy when properly used; however, the reversible
hydrocolloid impression materials offer few advantages over the irreversible
(alginate) hydrocolloids.
Irreversible Hydrocolloids
Irreversible hydrocolloids are used for making diagnostic casts, orthodontic
treatment casts, and master casts for removable partial denture procedures.
Because they are made of colloid materials, neither reversible nor irreversible
hydrocolloid impressions can be stored for any length of time but must be poured
immediately.
These materials have low tear strength, provide less
surface detail than other materials (e.g., mercaptan rubber base) and are not as
dimensionally stable as other materials. They can, however, be used in the
presence of moisture (saliva); are hydrophilic; pour well with stone; have a
pleasant taste and odor; and are nontoxic, nonstaining , and inexpensive. The
combination reversibleirreversible
hydrocolloids have demonstrated a tendency
to separate and should be used with that understanding. The hydrocolloids can be
acceptably disinfected with a spray solution of 2% acid glutaraldehyde, stored in
100% humidity, and poured within 1 hour.
Polyether Impression Materials
Polyether impression material is an elastic-type material, as are the polysulfide and
silicone materials. These materials have demonstrated good accuracy in clinical
evaluations and are thixotropic, which provides good surface detail and makes
them useful as a border molding material. It should
be noted, however, that these materials are not compatible with the addition
reaction silicone impression materials and should not be used to border mold
custom trays when the silicone impression materials are to be used as the final
impression material. The polyethers are also hydrophilic,which produces good
wettability for easy cast forming.
The polyethers have low to moderate tear strength and much shorter working and
setting times, which can limit the usefulness of the material. The flow
characteristics and flexibility of the polyether materials are the lowest of any of the
elastic materials.
Silicone Impression Materials
The silicone impression materials are more accurate and easier to use than the
other elastic impression materials. The condensation silicones have a moderate (5
to 7 minutes) working time that can be altered by adjusting the amount of the
accelerator. They have a pleasant odor, moderately high tear strength, and
excellent recovery from deformation.
These materials can be used with a compatible putty material to form fit a custom
tray. Silicone impression materials are hydrophobic, which can make cast
formation a problem.
These materials can be disinfected in any of the disinfecting solutions with no
alteration in accuracy. Ideally, these materials should be poured within 1 hour.
Polyether Impression Materials
Polyether impression material is an elastic-type material, as are the polysulfide and
silicone materials. These materials have demonstrated good accuracy in clinical
evaluations and are thixotropic, which provides good surface detail and makes
them useful as a border molding material. It should be noted, however, that these
materials are not compatible with the addition reaction silicone impression
materials and should not be used to border mold custom trays when the silicone
impression materials are to be used as the final impression material. The polyethers
are also hydrophilic, which produces good wettability for easy cast forming. The
polyethers have low to moderate tear strength and much shorter working and
setting times, which can limit the usefulness of the material. The flow
characteristics and flexibility of the polyether materials are the lowest of any of the
elastic materials.
Polysulfide:
Composition: supplied in two tubes as base and catalyst ,equal length
are mixed Have light, regular and heavy viscosity
Uses:
1-impression of complete or partial edentulous patients
.
2-secondary altered
cast impression of R.P.D
.
Also other type of impression like crown and bridge
Advantages:
1-high tearing resistance. 2-accuracy improves if impression is poured within
30min
Disadvantages:
1-very unpleasant test and odor
.
2-can be irritant to oral mucosa
The step-by-step procedure and important points to observe in the making of a
hydrocolloid impression are as follows:
1. Select a suitable, sterilized, perforated or rim-lock impression tray that is large enough to
provide a 4 to 5mm thickness of the impression material between the teeth and tissues and the
tray.
2. Build up the palatal portion of the maxillary impression tray with wax or modeling plastic to
ensure even distribution of the impression material and to prevent the material from slumping
away from the palatal surface .At this time, it is also helpful to pack the palate with gauze that
has been sprayed with a topical anesthetic.
3. The lingual flange of the mandibular tray may need to be lengthened with wax in the
retromylohyoid area or to be extended posteriorly, but it rarely ever needs to be lengthened
elsewhere.
4. Place the patient in an upright position, with the arch to be impressed nearly parallel to the
floor.
5. When irreversible hydrocolloid is used, place the measured amount of water (at 70° F) in a
clean, dry, rubber mixing bowl (600-mL capacity). Add the correct measure of powder. Spatulate
rapidly against the side of the bowl with a short, stiff spatula.
6. In placing the material in the tray, avoid entrapping air. Have the first layer of material lock
through the perforations of the tray or rim-lock to prevent any possible dislodgment after
gelation.
7. After loading the tray, remove the gauze with the topical anesthetic and quickly place (rub)
some of the impression material on any critical areas using your finger (areas such as rest
preparations and abutment teeth).
8. Use a mouth mirror or index finger to retract the cheek on the side away from you as the tray
is rotated into the mouth from the near side.
9. Seat the tray first on the side away from you, next on the anterior area, while reflecting the lip,
and then on the near side, with the mouth mirror or finger for cheek retraction. Finally, make
sure that the lip is draping naturally over the tray.
10. Be careful not to seat the tray too deeply, leaving room for a thickness of material over the
occlusal and incisal surfaces.
11. Hold the tray immobile for 3 minutes with light finger pressure over the left and right
premolar areas.
12. After releasing the surface tension, remove the impression quickly in line with the long axis
of the teeth to avoid tearing or other distortion.
13. Rinse the impression free of saliva with slurry water, or dust it with plaster, and rinse gently;
then examine it critically.
.