IMPRESSION IMPRESSION TECHNIQUES FOR TECHNIQUES FOR

REMOVABLE PARTIAL REMOVABLE PARTIAL DENTURESDENTURES

Impression defined as “ An imprint or negative likeness of the teeth and / or edentulous areas where the teeth have been removed, made in a plastic material which becomes relatively hard or set while in contact with these tissues. Impressions may be made of full complement of teeth , of areas where some teeth have been removed, or in months from which all teeth have been removed”- GPT..

IMPESSION TECHNIQUES FOR KENNEDY’S IMPESSION TECHNIQUES FOR KENNEDY’S TOOTH SUPPORTED PARTIAL DENTURESTOOTH SUPPORTED PARTIAL DENTURES

Kennedy’s Class III & Class IV arches are considered as tooth supported partial denture. Since maximum support is obtained from abutment teeth, a preliminary (anatomic ) impression is generally only that is required followed by making of master cast.

If extensive edentulous spaces exist, this is followed

by making a primary cast, on which an individual acrylic resin tray is made. Then a final impression is taken which is followed by making of master cast.

Hence, following two impressions are taken for tooth supported partial dentures:

(A) Primary Impression (Preliminary Impression/Anatomic Impression)

(B) Final Anatomic Impression

(A) (A) MAKING A PRIMARY IMPRESSIONMAKING A PRIMARY IMPRESSION

IMPRESSION MATERIALIMPRESSION MATERIAL : : Alginate is the material of choice for recording primary

impression although elastomers such as polysulphide, polyether, silicone rubber or reversible hydrocolloid such as agar can also be used.

COMPOSITION : Sodium or Alginate (soluble alginate) : 15% Calcium Sulphate (reactor) : 16% Zinc Oxide (filler particle ) : 04% Potassium Titanium Flouride (accelerator) : 03% Diatomaeceous Earth (filler particle) : 60% Sodium Phosphate : ( retarder) : 02% WATER : POWDER RATIO : 40ml of water : 15g of powder GELATION TIME : 3-4 min

ADVANTAGESADVANTAGES : :1.Used in presence of moisture (saliva).2.Hydrophillic3.Pour well with stone

4.Pleasant taste & odour.5. Non-Toxic, Non-staining, Inexpensive..

DISADVANTAGEDISADVANTAGE : :1.Low tear strength2.Not as dimensionally stable as other materials.3.Cannot be stored for any length of time.

TRAY SELECTIONTRAY SELECTION A perforated stock tray is used for taking primary

impression A tray must be sufficiently rigid to prevent distortion

during the impression and cast forming procedures. To assess the size of an upper tray, place it in the

mouth so that it is position in the molar region but depressed anteriorly .Check whether it encloses the tuberosity region with sufficient clearance for the impression material. Now check for correct anterio-posterior dimension by swinging the tray handle upwards. Check for correct extension into buccal and labial sulcus.

Lower trays are judged by same principles. It should fit the mouth with about 4 to 5 clearance for

the impression material without interfering with teeth or bordering tissue.

It may sometimes be necessary to modify the stock tray by addition of compound in in areas of upper tuberosity and vault if palate of upper tray when patient has an unusally high palate.

MIXING ALGINATE AGAINST THE SIDE OF THE MIXING ALGINATE AGAINST THE SIDE OF THE BOWL.BOWL.

TECHNIQUE TECHNIQUE

Select a suitable, sterilized, perforated impression tray.

Place the patient in the upright position with the arch to be impressed nearly parallel to the floor.

Place the measured amount of water in a clean dry rubber mixing bowl. Add power of correct measure. Stir rapidly against the side of bowl with a short, stiff spatula. This should be accomplished in < 1 min.

Load the material on a tray and avoid air entrapment while placing material..

Quickly place (rub) some of impression material on any critical areas using your finger (areas such as rest preparation and abutment teeth). If maxillary impression is being made, place material in the highest aspect of palate and over the rugae.

Use a mouth mirror or index finger to retract the check on the side away from you as tray is rotated into the mouth from the near side

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 mouth mirror or index finger for cheek retraction. Finally make sure the lip is drapping naturally over the tray.

Be carefully not to seat too deeply, leaving room for a thickness of material over the occlusal & incisal surfaces.

Hold the tray immobile for 3min with light finger pressure over the left & right premolar areas. Do not remove the impression mouth until the impression material is completely set.

Rinse the impression free of saliva with slurry water, or dust it with plaster & rinse gently. Then examine it critically. Spray the impression thoroughly with suitable disinfectant & cover it immediately with damp paper towel.

After releasing surface tension remove the impression quickly on line with long axis of the teeth to prevent tearing or other distortion.

A cast should be poured immediately into a disinfected hydrocolloid impression to prevent

dimensional changes.

DIAGRAMMATIC ILLUSTRATION OF ALTERNATE COMPRESSION & DIAGRAMMATIC ILLUSTRATION OF ALTERNATE COMPRESSION & TENSION OF ALGINATE IF REMOVAL FROM MOUTH IS TENSION OF ALGINATE IF REMOVAL FROM MOUTH IS

ATTEMPTED BY UPWADS & DOWNWARDS MOVEMENT OF THE ATTEMPTED BY UPWADS & DOWNWARDS MOVEMENT OF THE TRAY HANDLE.TRAY HANDLE.

PRECAUTIONS FOR HANDLING OF PRECAUTIONS FOR HANDLING OF HYDROCOLLOID IMPRESSIONHYDROCOLLOID IMPRESSION

Impression should not be exposed to air as this will inevitably result in some dehydration will occur and result in shrinkage.

Impression should not be immersed in water or as some imbibition will inevitably result with an accompaning expansion.

Impression should be protected from dehydration by placing it in a humid atmosphere or wrapping it in a damp paper towel until a cast can be poured. To prevent volume change ,it should be dine within 15 min after removal of the impression from the mouth.

Exudate from hydrocolloid has a retarding effect on the chemical reaction of gypsum products & results in chalky cast surface. This can be prevented by pouring the cast immediately.

(B) (B) MAKING FINAL ANATOMIC MAKING FINAL ANATOMIC IMPRESSIONIMPRESSION..

After a primary cast is made, an individual special tray is fabricated. This is then used to take a final anatomic impression. Final impression can be made with alginate, mercaptan rubber or silicone impression material.

Special tray are made of aluminium filler shellac composition. They can also made of acrylic resin. They have advantage of extra strength and are preferred with the elastomeric materials & not essential with alginate.

INDICATIONS FOR SILICONE INDICATIONS FOR SILICONE IMPRESSIONSIMPRESSIONS

In cases where deep undercuts are present.

Patients who exhibit a marked tendency of retching although in these cases extra acceleration is necessary to shorten to a minimum the time required for impression to be held in the mouth.

Patients who are unable to open the mouths fully & yet require a partial denture.

SILICON IMPRESSION MATERIALSSILICON IMPRESSION MATERIALS

Consist of partially polymerized dimethyl siloxane mixed with filler such as zinc oxide, chalk, asbestos, flour, dextrin, starch or cellulose to form a paste. In using he materials this paste is mixed with liquid activator which converts low molecular weight fluid to high molecular weight elastic solid. The activator employed may be stannous octate. The proportions of activator to paste is indicated by manufacturer. Ideally, should be poured in 1hr.

ADVANTAGES :1.More accurate & easier than other elastic impression material.2.Moderate working time.3.Pleasant odour4. Moderately high tear strength.5.Excellent recovery from deformation.DISADVANTAGE : Hydrophobic & can make cast formation a problem.

TECHNIQUETECHNIQUE

Tray peripheries should be reduced to be slightly short of the sulcus depth.

For most Impressions, but particularly for free-end saddle cases it is advisable to place three compound ‘stops”.

Silicon putty should be adapted in a thin roll to the tray peripheries having first applied a silicon adhesive to the appropriate areas.

The tray is now seated firmly in position on the ‘stops’ & patient instructed to carry out muscular movements of tongue, cheeks, & lops to mould the silicon putty to functional shape of the periphery.

Proportioning of powder to liquid is done. Mix on glass slab. A broad bladed, slightly flexible spatula is used to thoroughly mix the constituents.

PERIPHERY OF IMPRESSION DEFINED IN SILICON PERIPHERY OF IMPRESSION DEFINED IN SILICON PUTTY. PUTTY.

COMPOUND STOPSCOMPOUND STOPS

The tray should be loaded & inserted into the mouth as soon as possible after mixing since the viscosity of the mixed material increases as polymerization proceeds & reaction commences as soon as mixing is begun.

General principles of tray positioning & tissue manipulation should be followed as for alginate.

When the silicon material have lost their stickiness to touch & have reached as initial set they can still be moulded to a limited degree. At this stage, however, they also show elastic properties & ,if moulded, a distorted impression results. Consequently, silicon impression should be held in mouth for further period (say 2min),after they appear to set.

At least 5min should be allowed en all from commencement of mixing to removal from mouth.

A slower rate of removal is more comfortable for patient than removal with a sudden jerk as done in case of alginate.

COMPLETE PARTIAL LOWER SILICON IMPRESSION. COMPLETE PARTIAL LOWER SILICON IMPRESSION.

IMPRESSION TECHNIQUES FOR IMPRESSION TECHNIQUES FOR DISTAL EXTENSION DENTURE DISTAL EXTENSION DENTURE

BASESBASES

DUAL IMPRESSION TECHNIQUESDUAL IMPRESSION TECHNIQUES

In this technique both functional and anatomic impressions are taken. We take the functional impression of the saddle area. The remaining areas are recorded with an anatomic impression using a stock tray.

These techniques are indicated for all tissue-tooth supported partial dentures. Kennedy’s Class I and Class II fall under this category.

Dual impression techniques are broadly Dual impression techniques are broadly classified as :classified as :

(A) Physiological or Functional Dual Impression (A) Physiological or Functional Dual Impression TechniquesTechniques

(B) Selective Pressure Functional Dual Impression (B) Selective Pressure Functional Dual Impression Techniques Techniques

(A) PHYSIOLOGICAL OR FUNCTIONAL (A) PHYSIOLOGICAL OR FUNCTIONAL DUAL IMPRESSION TECHNIQUES.DUAL IMPRESSION TECHNIQUES.

In this technique one anatomic impression is made of the entire ridge & one functional impression is made only of the edentulous portion by applying occlusal load on the impression tray while making the impression. The common techniques employed to record a functional dual impression are:

1. McLean’s Technique. 2. Hindle’s modification of McLean’s

technique 3.Functional relining method 4. Fluid wax

1. McLEAN’S PHYSIOLOGICAL TECHNIQUE1. McLEAN’S PHYSIOLOGICAL TECHNIQUE PRINCIPLEPRINCIPLE : : A functional impression of the edentulous

ridge is made. The second impression is made over the functional impression & it records the structures in their anatomic form. The second impression is known as ‘pick up ‘ impression as it covers’& pucks up the functional impression along with itself.

PROCEDUREPROCEDURE : : A custom-made impression tray is fabricated over the

edentulous areas of the preliminary cast. A spacer is not adapted as we intend to record only the supporting tissues with this tray.

Occlusal rims are made on the custom tray. Occlusal rims are required for the patient to close (bite) on while making the impression.

The tray loaded with the impression material is inserted into the patient’s mouth & the patient is asked to close on the occlusal rims built over the tray.

After making the impression, the custom tray should not be removed from the mouth.

An alginate over-impression ( this impression is made over the existing impression) is made using a large stock tray.

When the over-impression is removed, the functional master impression comes along with it. The alginate over impression is known as ‘pick up’ impression.

While making the over-impression’ finger pressure should be applied on the stock tray so that the custom tray under it is pushed towards the tissue making the over-impression.

A cast is poured into the impression. This cast reproduces the teeth in the anatomical form and the tissues in the functional form.

PRESSURE SHOULD BE APPLIED OVER THE STOCK PRESSURE SHOULD BE APPLIED OVER THE STOCK TRAY WHILE MAKING THE PICK UP IMPRESSION.TRAY WHILE MAKING THE PICK UP IMPRESSION.

INVERTED ‘PICK UP’ IMPRESSION .INVERTED ‘PICK UP’ IMPRESSION .

DISADVANTAGES:DISADVANTAGES:

Finger pressure used to settle the functional impression while making the over-impression, is not equal to the biting force used while making the functional impression. Hence, the supporting tissues may not be compressed as they were while making the functional impression. This leads to errors.

There will be a small quantity of alginate between the occlusal rim of the custom tray tray & the over-impression stock tray. This alginate acts like a buffer & prevent the transfer of the entire load (finger pressure) applied on the stock tray.

2. HINDLE’S MODIFICATION OF THE 2. HINDLE’S MODIFICATION OF THE McLEAN’S TECHNIQUEMcLEAN’S TECHNIQUE

PRINCIPLEPRINCIPLE : :

It is similar to McLean’s technique. Hindle modified McLean’s technique to overcome the disadvantages.

PROCEDUREPROCEDURE : : A special tray with an occlusal rim is fabricated using

the primary cast. The special tray should have stoppers to avoid excessive pressure on the tissues The stoppers should be placed on the tray extending over the stress-bearing areas.

The special tray is used to record the supporting tissues under rest (anatomical impression).

A special stock tray with large holes is used to make the over-impression. While making the over-impression the clinician should place his finger into the holes of the tray & apply steady constant pressure on the occlusal rim built on the special tray. Pressure should be held till the alginate sets completely.

The finger pressure pushes the special tray against the edentulous ridge to give a pseudo-functional stress( similar to the functional load). The over-impression is a functional impression.

The stoppers present on the tissue side of the custom side of the custom tray help to avoid excessive tissue compression.tray help to avoid excessive tissue compression.

MAKING THE PICK UP IMPRESSION USING HINDLE’S MAKING THE PICK UP IMPRESSION USING HINDLE’S MODIFIED STOCK TRAY. MODIFIED STOCK TRAY.

ILL- EFFECTS PRODUCED BY McLEAN & HINDLE’S METHODS :

The denture fabricated by such technique has an inherent property to compress the tissues even when there is no occlusal load.

Since the tissues are constantly compressed there will be excessive bone resorption which occurs because:

- constant pressure stimulates the osteoprogenitor cells to form osteoclasts which resorb bone.

- constant pressure decreases the blood suply to the bone which again though a series of chemical mediators stimulate osteoclasts.

If retentive clasps do not hold the denture base properly, the partial denture will be slightly occlusal to the normal position. Since the dentures are occlusally displaced, they will be the first to contact the opposing teeth during occlusion. This will produce premature contacts.

3. FUNCTIONAL RELINING METHOD3. FUNCTIONAL RELINING METHOD

PRINCIPLEPRINCIPLE : :

In this method, an anatomical master impression of all the oral structures is made. Two casts are made from this impression, one is the master cast & the other is the refractory cast. (The refractory cast is made by duplicating he mastre cast). The framework of the partial denture is fabricated using the refractory cast.

This framework is verified in the patient’s mouth & is directly used to record the functional impression. The master cast made from the anatomical is altered according to the functional impression made on the tissue surface of the framework.

PROCEDUREPROCEDURE : : The partial framework is constructed on the cast made

from a single anatomic impression (usually made with alginate). This is the first master impression.

A master cast is made from the anatomical impression. The master cast is duplicated & a refractory cast is made.

A framework is fabricated using the refractory cast. Ash no. 7 soft metal spacer is adapted on the cast

before constructing the framework to provide space for the impression material. The spacer should be removed before making the impression.

Next, the framework is tried in the patient’s mouth. Once the fit of the framework is verified, a functional impression is made on the tissue surface of the framework. This functional impression is the second master impression. Hence this technique is categorized as dual impression.

The functional impression is made with low fusing modelling plastic. The material is added in a flowing consistency onto the tissue surface of the framework. It is tempered & placed within the mouth.

Sufficient pressure is applied during impression making to ensure compression of the tissue. The function of the modelling plastic is to act like a tray or receptacle to make the final impression with zinc oxide eugenol impression.

The modelling plastic at the borders of the framework are re-softened to do border moulding. Border moulding is done by manipulating the cheeks & tongue.

After recording the tissues with modelling plastic, it is trimmed to provide space for zinc oxide eugenol impression paste.

After reducing the modelling plastic, the final impression is made with zinc oxide eugenol impression paste. If undercuts are present, light bodied polysulfide or silicone rubber can be used

The amount of relief given to the modelling plastic controls the amount of soft tissue displacement.

Patient should keep his mouth in a partially opened position during impression making as:

- in this position the movements of the cheek & the tongue are in best control.

- The relationship between the partial denture framework & the teeth can be observed.

ADVANTAGESADVANTAGES : : It improves the fit of the denture after bone

resorption. The tissue surface of the metal framework can

be relined after insertion.

DISADVANTAGESDISADVANTAGES : : It is difficult to maintain the relationship of the

framework to the relationship to the abutment teeth while making the impression.

Occlusion is usually affected due to the addition of a new layer to the tissue surface of the denture base.

4. FLUID WAX IMPRESSION4. FLUID WAX IMPRESSION PRINCIPLEPRINCIPLE : : In this technique a framework is fabricated using the

anatomical impression. The framework is modified into a special tray. A fluid wax functional impression is made using the special tray.This technique differs from the functional relining in that here the frame work is converted into a special tray to record functional impression, whereas in functional relining framework is directly used to record the functional impression.

USESUSES : : Reline the tissue surface of the existing RPD. Correct the distal extension edentulous ridge of the

original master cast.

OBJECTIVES : To obtain maximum extension of the peripheral borders

of the denture without interfering the movable tissues. To record the stress- bearing areas in the functional

form. To record the non-stress bearing areas in anatomic

form.

MATERILAS USED : The waxes used are rigid or firm at room temperature.

They have adequate flow at mouth temperature. The waxes used here include:

Iowa wax by Dr. Smith Korrecta Wax by OC & SG Applegate ( no.4 has

more fluidity than Iowa wax) The relief between the tray & the ridge should be

atleast 1-2mm. After insertion, the tray should be left undisturbed for 5-7 minutes to cool the wax.

PROCEDUREPROCEDURE : : The metal framework is fabricated using a refractory cast

made from the anatomical master impression, The framework is modified into a special tray to record the functional impression.

(A) FABRICATING THE SPECIAL TRAY : After try-in of the framework it is positioned on the

master cast. The outline of the tray is drawn in the master cast made

from from the anatomical impression. The cast is coated with a separating medium or tin foil

substitute. A spacer is adapted over the crest of the edentulous ridge

using a single layer of base plate wax. The spacer should be gently adapted without damaging the separating medium.

The framework is placed in position over the spacer.

Auto-polymerizing resin is mixed is mixed to dough & is adapted & contour over the framework along the length of the ridge.

Excess material is removed with a sharp knife before the resin hardens.

The borders of the cured resin tray should be trimmed according to the outline.

The borders of the tray should not be more than 2mm short of movable tissues because the fluid wax to make the impression does not have sufficient strength to support itself.

Relief holes can be prepared along the crest of the ridge (on the temporary denture base) & the retromolar pad to allow the escape of excess impression material.

IMPRESSION TAKING : Wax is softened in a bath at 51-54°C. The softened wax is painted evenly on the tissue

surface of the impression tray with a brush. The wax is painted in layers till the sufficient thickness is obtained. The should be painted in excess near the borders to records the sulcus.

The tray is seated & held in position with three fingers.Two are placed posteriorly over the primary abutments &the other one is placed on the tray anteriorly.

After seating the tray , the cheeks are pulled over the borders of the tray to record the buccal vestibule. The patient is asked to force his tongue against the cheeks to record the sub-lingual borders & against the anterior teeth to record the distolingual extensions. The patient is asked to open the mouth widly to rcord the distal limit of the impression.

The patient should keep his mouth half open for at least The patient should keep his mouth half open for at least 5 minutes to ensure cooling & hardening of the wax.5 minutes to ensure cooling & hardening of the wax.

The framework special tray is removed &the impression The framework special tray is removed &the impression is examined.is examined.

The wax surface that contacted the tissues appear glossy The wax surface that contacted the tissues appear glossy & the other areas that did not contact the tissues will & the other areas that did not contact the tissues will appear dull.appear dull.

Additional wax is painted over the dull areas 7 the Additional wax is painted over the dull areas 7 the procedure is repeated until glossy borders are obtained. procedure is repeated until glossy borders are obtained. Hard wax can be applied to increase the thickness of the Hard wax can be applied to increase the thickness of the wax in the borders.wax in the borders.

Each time the wax impression is inserted into the mouth, Each time the wax impression is inserted into the mouth, the operator must wait for atleast 5 minutes before the operator must wait for atleast 5 minutes before removing the impression.removing the impression.

The impression should be placed in the mouth finally for The impression should be placed in the mouth finally for 12 min. The cast is then poured using altered cast 12 min. The cast is then poured using altered cast technique.technique.

(B) SELECTIVE PRESSURE FUNCTIONAL DUAL (B) SELECTIVE PRESSURE FUNCTIONAL DUAL IMPRESSION TECHNIQUEIMPRESSION TECHNIQUE

In this technique, one anatomical impression& one In this technique, one anatomical impression& one functional impression is made. A master cast is prepared functional impression is made. A master cast is prepared from the anatomical impression & the later is altered from the anatomical impression & the later is altered according to the selective pressure functional according to the selective pressure functional impression. This technique differs from other technique impression. This technique differs from other technique in that the second functional master impression is a in that the second functional master impression is a selective pressure functional impression.selective pressure functional impression.

Here the tissue surface of the special tray lying over Here the tissue surface of the special tray lying over the relieving areas is reduced. Reduction is done such the relieving areas is reduced. Reduction is done such that adequate space is available for the impression that adequate space is available for the impression material to not exert any pressure in those areas. Thus, material to not exert any pressure in those areas. Thus, the impression tray contacts the tissues only over the the impression tray contacts the tissues only over the areas that were not reduced (stress-bearing areas) areas that were not reduced (stress-bearing areas) during impression making. Consequently only these during impression making. Consequently only these areas are compressed( recorded functionally).areas are compressed( recorded functionally).

As the pressure is applied only to the selected areas, As the pressure is applied only to the selected areas, while making functional impression, this technique is while making functional impression, this technique is known as selective pressure impression technique.known as selective pressure impression technique.

PROCEDURE :PROCEDURE : The special tray is fabricated on the master cast The special tray is fabricated on the master cast

made from an anatomical impression. The tray is made from an anatomical impression. The tray is fabricated without a wax spacer.fabricated without a wax spacer.

The tissue surface of the special tray is trimmed with The tissue surface of the special tray is trimmed with burs to provide adequate relief.burs to provide adequate relief.

The impression material (preferably zinc oxide The impression material (preferably zinc oxide eugenol) is loaded on the prepared special tray & eugenol) is loaded on the prepared special tray & inserted into the patient’s mouth.inserted into the patient’s mouth.

The patient is advised to keep his mouth open. The The patient is advised to keep his mouth open. The impression is recorded under finger pressure. Only impression is recorded under finger pressure. Only the stress bearing areas will be compressed during the stress bearing areas will be compressed during impression making.impression making.

ADVANTAGE :

It equalizes the stress acting on the It equalizes the stress acting on the abutment teeth & the soft tissues. The abutment teeth & the soft tissues. The rate of ridge resorption is reduced rate of ridge resorption is reduced because relieving areas that cannot because relieving areas that cannot withstand any load are not stressed.withstand any load are not stressed.