impression for rpd

8/12/2019 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These materials are elastic enough to be withdrawn from the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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impression for rpd

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