Impression for RPD
Transcript of Impression for RPD
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Impression Procedures
for Partially Dentate Patients
SDS 321
McCrackens Removable Partial Prosthodontics.
Chapter 15.
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An impression,
of a partially dentate arch must record accurately theanatomic form of the teeth and surrounding tissues so that ..
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.. a definite path of insertion
can be selected,
and
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.. support, retention and stability can be planned.
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Materials
that could permanently deform on
removal from the undercuts
should not be used,e.g., rigidand thermoplastic.
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Impression Materials(A review)
Rigid: Impression plasterMetallic oxide (ZnO Euginol) paste
Thermoplastic:
Modeling compound
Impression waxes
Elastic: Reversible Hydrocolloid
Irreversible Hydrocolloid
Rubber base (Marcaptan, Silicon, Polyether)
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Hydrocolloid Materials
(A reminder)
These materials are elastic enough to be withdrawn from the
undercuts without permanent deformation or distortion.
One piece impression is possible.
No separating medium is needed for pouring.
Extremely accurate in tissue details when handled properly.
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Reversible Hydrocolloid
(AgarAgar)
The gelation of Agar is primarily a physical change in which alatticework of fibrils forms as the temperature is lowered.
The gel can be dispersed by heating. It is liquefied at atemperature compatible to oral tissues and then solidified to afirmyet elastic gel at about 100 F (38C).
The gelation is accomplished by means of special water cooledimpression trays.
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Irreversible Hydrocolloid
(Alginate)
These are not thermally reversible. The gelation is induced bya chemical reaction.
Powder: Sodium alginate & Calcium sulphate.Add water forreaction to form a latticework of fibrils of insoluble Calciumalginate.
Alginate is dimensionally stable for a brief period after itsremoval from the mouth, hence should be poured immediately
or stored for up to 15 minutesin a saturated atmosphere.
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Procedure of Impression Making
1. Select a suitable size perforated tray for obtaining an
initial impression.
2. Add wax or modeling compound in the tuberosity &palatal area for maxillary, or in the lingual flanges for a
mandibular impression.The edentulous areas may also need
modifications of the tray with impression compound.
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For the Final Impression
A better extended custom tray is normally fabricated
on the diagnostic cast.
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Why to have Tissue stops
The custom tray is fabricated to have tissue stops for accurate
relocation of the trayand for achieving even thickness of the
impression material.These stops are placed on the teeth otherthan abutments and on the edentulous ridge distally.
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Tissue stops can be added intra-orally
by applying green stick compoundto the chosen location on the tray
and then seating it in the mouth
under gentle pressure.
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For obtaining an Impression
Seat the patient in an upright position and check the tray
border extensions.For Class I & II it should cover the
retromolar pad or maxillary tuberosity.Any overextensions
should be relieved by trimming the tray, especially in the
frenum areas.
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What to do !
Over-extended Under-extended
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Border molding
is carried out in the areas of under extension of the tray and
for the distal extension saddles.The commonly used bordermolding material is the tracing (green stick)compound.
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Choice of an Impression material
Any of the elastic material (alginate, agar, polysulphide,polyether, or silicone) can be used.However,
in the presence of deep undercuts(bilateral lingually tiltedpremolars)alginate should not be usedas it may be stressedbeyond its elastic limit on removal.
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Before making the impression
Remove the excessive saliva from the mouth with an ejector
and dry the rest seat preparations and dimpling (if any) with anair syringe.An excessive drying should be avoided as the
material may adhere to the dried tooth surface.
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.. also
open inter-dental spaces should be occluded with soft waxprior to inserting the loaded tray,
otherwise the impression material will flow beneath thecontact points and lock the impression in place and distortionwill occur upon removal.
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Mix the impression material
By adding alginate powder to the desired amount of water
spatulate rapidly against the walls of the bowl for one minute,
or
By mixing the catalyst and base components of the rubber
base material uniformly.
Place the material in the tray, try to avoid entrapping air and
lock the alginate in the perforationsfor rubber base
materials,an adhesive should be applied to the tray beforemixing the material.Do not overload the tray as the excess
material may initiate the gag reflex.
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For a bubblefree impression
Alginate should be wiped over the teeth and into the deep sulciwith a finger immediately prior to inserting the loaded tray.
The rubber base materials can be poured over the teeth using a
syringe.
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Inserting the tray in the mouth
Stand behind(for maxillary)and in front of the patient(formandibular)impression.Retract the cheek with the tray and
with your free handand rotate the tray in the mouth.
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Then,
Retract the lip (upper or lower)and seat the tray anteriorly,
centering it according to the frenum and the sulcus, and then
posteriorly.Let the lip drape naturally and complete the bordermolding.For lower impressionask the patient to elevate the
floor of the mouth, e.g., by licking his lips.
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Muscle trimming of the borders
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While waiting for the material to set
Hold the tray immobile for at least 3 minutes with
a light finger pressure applied over the left andright premolar areas.Any movement of the tray
can cause an inaccurate impression.
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Once the material is set
Release the surface tension by lifting the lip (up or down) away
from the tray.
Remove the impression quickly (snap removal) in line with the
long axis of the teeth.Rinse it thoroughly under running tap waterfree it from saliva
or blood.
Examine the impression carefully for acceptance and disinfect it
before pouring.
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An acceptable impression
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Another acceptable one !
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Edentulous Vs Distal extension
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Now the unacceptable ones
Incorrect seating of the impression tray laterally.
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Not Acceptable
In-complete seating of the impression tray
unsupported impression at the borders.
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Repeat the Impression
Under extended traythin edge of unsupported
impression of the posterior lingual sulcus.
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Re-do border molding also
Over-extended border moldingresulting in the distorted
sulcus reflection in the impression.
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Finally, an acceptable impression
A maxillary impression taken withcombinationof impression
compound in the distal extension areas and overall alginateimpression.Alginate has been cut off on one side to show its thickness.
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Management of Gag reflex
Exaggerated gag reflex can frustrate both the patient and thedentist. It can also compromise the treatment plan.
Its effective management
is based on experience with combinations of clinical techniques,prosthodontic management,medication,and
psychologist referral,ora specialist referral.
Some simple measures that usually works:
Reassurance and kind handling.
Upright position of the patient.
Avoid overloading the tray.
Distract patients mind talking, raising one foot etc.
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Care of Alginate Impression
Should not be exposed to air (dehydration).
Should not be immersed in water (imbibition).
Should be stored in a humid atmosphere by wrapping in adamp paper towel or in 100% relative humidity (humidor).
Should be poured immediately, or immersed in a solution ofaccelerator (Pot. Sulphate) to avoidsyneresis -giving off amucinous exudate that has retarding effect on gypsumandresults in a soft or chalky cast surface.
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Procedure of Forming a Cast
(Without Boxing)
1. Remove the alginate impression from the damp paper towel
and shake out any moisture.
2. Add 100 gm of stone (powder) into 28 ml of water and
spatulate for one minute.Place the bowl on the vibrator to
escape the entrapped air.
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3. Hold the impression tray against thevibrator and add a small amount of mixedstone to the distal area.Let it vibratearound the arch - from tooth to tooth.Continue to add small increments untilimpressions of the teeth have been filled.
4. Continue to add stone in larger portionsuntil the impression is completely filled.
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5. Let the stone reach its initial set.
6. The base of the cast can then beformed with a new mix of stone.
The base should be at least 15mm thick.
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Forming the Cast(With Boxing)
Apply beading wax to theperiphery of the impression.It should be placed 2-3 mm
from the borders of theimpressionand should be4 mm wide.
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Alternately, beading can be
done with a 5050 mix of
plaster and pumice.
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Box the Impressionwith Boxing wax sheet
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Hold the boxed impression on a
vibratorand add mixed dental
stone in small increments until
completely filled.
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After the stone has completely set & the exo-therm completed,peal off the boxing wax and remove the beading wax.
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Remove the impression gently
off the cast with a plaster
knife.For the border molded
impression,the cast should be
dipped in warm water for easy
removal.
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Trimming the Cast
The sides of the cast are trimmed to be parallel- any stoneblebs or nodules are carefully removed.
The base can be trimmed for either orthodontic specifications
(for a record cast) or to remove excess stone only ( for amaster or a working cast).
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Land area of the cast
should be trimmed with a sharp knife to have uniform
height of the sulcus reflection ( 2-3 mm ).
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and
it should be 3-4 mm wide to provide
necessary strength to the cast.
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