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(Impression trays)-(Boxing in the
impressions and making the casts)
)(Post damming)-(Relief Areas)
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics
Impression trays
Impression tray:
is an instrument used for carrying the impression material in
to the mouth for maintaining it in position during setting or
hardening and supporting it during removal from the
mouth and when pouring the model.
The impression trays consist of a floor and flanges.
The difference between the upper and lower tray is that the
upper has a palatal portion while the lower has lingual
flanges
Requirement of impression tray
1- They should be strong and rigid, and can be sterilizer and
polish
2-They should be clean and smooth.
3-They should allow for equal thickness of impression
material over the entire fitting surface.
4- The flanges of the tray must reach the functional position
of the sulci and frena and yet not displace them.
5-The tray should hold the impression material in correct
position in the mouth and consequently must cover the
whole areas of the jaw, which are required for the
impression .They must prevent distortion of the material
during setting and removal from the mouth
Stock tray
1- Stock trays are ready made trays.
2- Supplied in various shapes and sizes.
3- Trays are usually made from nickel silver,
stainless steel, aluminum or tin,
plastics
4- in edentulous cases:
A- Stock trays having round floor.
B- Short flange
C- The handles staggered (inclined) as to clear
the lip and avoid its distortion
5-in dentulous cases:
A-stock trays having flat floor
B-vertical high flanges
C-the handles can be made straight by extending it
straight across the floor of the trays
6- in partially edentulous cases:
Tray with combined round and flat floor.
7- If it is too large it will distort the tissue around the
borders of the impression and will pull the soft
tissues under the impression away from the bone
8- If it is small, the border tissue will collapse inward on to
the residual ridge (this will reduce the support for the
denture and prevent the proper support of the lips by the
denture flange.
Special (individual) tray
"Custom tray"
To produce accurate impression and avoid variation
in transmitted pressure there must be an equal
thickness of impression material over the entire
surface; also the flanges of the tray must reach the
functional position of the sulci and frena and yet
not displace them. It is unusual for a stock tray to
full these requirements, and therefore, special tray
should be construction for each patient.
Before making a special tray, the best impression
possible with a stock tray is made, and model
(cast) is made for this impression.
The special tray materials vary according to
the type of impression technique selected,
the more common being:
A-non metallic (plastic) individual tray:
1-shellac base plate
2-compound impression
3-acrylic resin (most commonly used)
4-old dentures.
B- Metallic special tray
A- Non metallic (plastic) individual
tray
1- Shellac base plate:
These materials are normally used when the
impression is to be made of plaster of Paris or
alginate impression materials.
To provide space in the tray for the impression
material, the model must be covered with two
layers of wax and the tray adapted to this surface.
This layer of wax is called shim or spacer. This
shim can also be made of shellac. It can also be
produced by immersing the model in the water for
10 minutes then dipping it in molten wax .
Three dips are usually enough to produce the shim. The tray should be
perforated if alginate impression is used. A special tray with stops may
be preferred for mucostatic impression. These stops can be made by
perforating the shim.
Four stops, two in the lateral incisor areas and two in the molar regions are
usually made. These stops should touch the oral mucosa during
impression making and should be lightly scraped lateral before pouring
the cast.
When the shim is used it should be dusted over with talcum powder to
prevent the tray material from sticking to it. The shellac base plate
material is softened uniformly by passing it to and for over a flam and
then adapted to the shim. If necessary, resoft to complete the process.
Trim the edges with a sharp knife or scissors and smoothen with a file
and sandpaper, leaving a rounded periphery to the tray. If alginate
impression material is to be used, the tray should be perforated over its
entire surface by drilling holes of approximately one sixteenth of an
inch diameter space about one quarter to three eight of an inch apart. A
handle of any desired shape can be made of shellac base plate or iron
wire.
2- Compound impression
Sometimes compound impression are used as
special tray after scraping 2-3 mm. from the
fitting surface of the impression to provide
space for the plaster impression( plaster
wash impression) . Perforation are required
if alginate is used for making the
impression.
3- Acrylic resin
Certain impression techniques call for the use of a
close fitting tray (when zinc oxide and eugenol
past is used, the shim is not required and the tray
is directly adapted to the model) in particular zinc
oxide eugenol pastes for edentulous cases. This
type of tray can be made in either heat curing or
autocuring acrylic resin. If undercuts are present
on the cast, they should be blocked out with
plaster and separating medium is then applied to
the cast.
A wax base of the required thickness is made on the cast and the edges are
seated down. The wax base is flasked in the same manner as
recommended for a complete denture. After separation of the flask, the
wax is boiled out, the flask is cooled, the cast is coated with a suitable
separating medium and packing is carried out in the usual manner.
Deflasking, finishing and polishing is carried out as described for
complete denture process.
A quicker method of making an acrylic resin tray is by the use of
autopolymerizing acrylic resin material.
Undercuts are blocked out with plaster or wax.
After covering the model with a separating medium, the powder and
liquid of the acrylic resin should be mixed together according to the
manufacture's instruction.
When the material is dough stage in consistency it is rolled flat to an even
thickness and spread rapidly over the cast, using the fingers to adapt.
Polymerization will take place in a few minutes at room temperature
and if required can be accelerated further by placing the cast in warm
water. When the heat or cold curing acrylic resin is hard, the periphery
is trimmed with a stone or bur and the tray is ready for use in final
impression making.
Advantages of the acrylic impression
trays
1- easier to make
2-rigid
3-light in weight
4-can be easily cut down, if over sized.
Constructing the auto polymerizing
acrylic resin impression tray having
a spacer
On the cast obtained from the preliminary
impression the periphery can be outlined
with a disposable indelible marker wax
spacer is placed with in the outlined border
to provide space in the individual
impression tray for the final impression
material.
The posterior palatal seal area on the cast is not covered with
the wax spacer. Thus the completed final impression tray
will contact the upper residual ridge across the posterior
palatal seal. Base plate wax approximately 1mm thick is
placed on the cast as designated by the previously drawn
outline. A self curing acrylic resin tray material mixed and
uniformly adapted over the cast so the tray will be 2-3mm
thick. A resin handle is attached in the anterior of the tray
to facilitate removable of the final impression. The handle
is placed in the position of the upper anterior teeth so it
will not distort the upper lip when the tray is in the mouth.
The acrylic resin tray is removed from the preliminary
cast; the labial and buccal flanges of the impression tray
are marked and reduced until they are about 2mm shorter
than the reflections.
Spacer constructing the lower final
impression tray having
The cast is outlined for a wax spacer, which
will provide space in the individual tray for
the final impression material. a wax spacer
about 1mm thick is placed over the crest
and slopes of the residual ridge.
The buccal shelf on each side and the
retromylohyoid spaces on the cast are left
uncovered.
. Thus the completed final impression tray will contact the
mucosa in the region of the buccal shelves to place
additional pressure in this primary-stress bearing area
when the final impression is made. Extra wax can be
placed over the lingual slopes of the cast to provide
additional space for the action mylohyoid muscles when
the final impression is made self-curing (cold curing or
auto polymerizing) acrylic resin tray material is mixed and
uniformly distributed over the cast so the final impression
tray will be approximately 2-3mm. thick. An anterior resin
handle is centered over the labial flange. The flange of the
tray should be contoured. The buccal, labial and lingual
flanges of tray are reduced until the borders are short of the
limiting anatomic structures.
4- Old dentures as a final impression
tray
The existing denture may be used as a special
tray as in case of making zinc oxide eugenol
impression for relining of the dentures.
B – Metallic special trays
This type can be used for any impression materials,
but it is required only when compound is to be
used. Although swaging nickel silver between dies
and counter die can make metal trays, casting is
the method generally used. The material for
casting tray is alloy of tin two parts, lead one part,
or tin alone. The production of metallic special
tray is time consuming and expensive for this
reason it is not normally used.
Relief Areas
Relief: elimination of the pressure.
1- Hard areas
A-owing to the varying thickness of the
mucosa membrane on which the denture
rests it is frequently necessary to relieve the
denture over areas of thin mucosa in order
to avoid pain \or rocking of the denture and
the commonest position requiring such
relief is the midline raphe of the hard palate.
B- All areas to be relieved should be
determined by careful palpation and their
outlined shown on the model used for
constructing the special tray. The depth of
relief's dependent on the compressibility of
the areas of thick mucosa membrane and
should be sufficient to prevent the denture
from pressing on the areas of thin mucosal
coverage when full masticatory loads are
imposed.
C- Relief areas on dentures should always
merge in to the surrounding fitting surface
and should never have aclearly defined
outline. The median palatine raphe and the
torus palatinus and torus mandibularis are
examples of the hard areas that need relief.
Methods of relief
1- Automatic relief:
This type of relief can be obtained by using mucocompressing (mucofunctional) impression
technique.
2- Direct relief:
a- a plaster impression may be scraped to added of
1mm or less depending on the effect desired, over
area corresponding to the hard parts in the mouth.
This method is not applicable to hydrocolloids or
zinc oxide and eugenol, or rubber base past
impression materials.
b- Another method, witch can produce the
same effect but more conveniently and
accurately, is by attaching one ore more
layers of tin foil to the cast. An outline of
the area to be covered should be marked by
pencil on the cast. Foil of 0.05-inch
thickness is then cut into shape and
burnished by a blunt instrument into close
contact and cemented to the cast before the
trial denture is made.
2- Sensitive area
Relief of pressure over certain structures is required
because they are sensitive to pressure.
a- incisive papilla in the anterior part of the hard
palate.
b- The rugae are usually rough, resistant to friction
and insensitive, yet they sometimes become
flattened and deformed by pressure of an
unrelieved.
c- Over prominent tuberosities or bony
nodules in any location especially if they
are covered with a thin tightly stretched
mucosa membrane.
d- Mental foramen in the premolar region of
the mandible.
e-over the crest of thin lower ridge
f- Sharp mylohyoid ridge.
Shape of the relief
1- Upper cast: the relief area will normally be
pear-shaped with the broadest part
anteriorly; it should not encroach on the
crest of the ridge except the incisive papilla.
2- Lower cast: rarely requires relief; but foil
may be necessary over the areas that need
relief.