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Good Morning. Impression techniques used in fabrication of removable partial denture. WHAT IS AN IMPRESSION?.
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Impression techniques used in fabrication ofremovable partial denture
WHAT IS AN IMPRESSION? • Impression is the negative replica /imprint of an oral structure which is used to produce the positive replica( cast) of the stucture to be used as a permanent record for the production of a dental prosthesis. • Types of impression - primary impression secondary impression anatomical impression functional impression
IMPRESSION MATERIALS • That could permanently deform on removal from an undercut should never be used i.e.,rigid and thermoplastic. • Types of impression materials:- Rigid: Impression plaster Metallic oxide (ZnO Euginol) paste Thermoplastic: Modeling compound Impression waxes Elastic: Reversible Hydrocolloid Irreversible Hydrocolloid Rubber base (Marcaptan, Silicon, Polyether)
Hydrocolloid i.e., agar what are the commonly used Impression materials in partially edentulous patients? Materials Irreversible Reversible hydrocolloid i.e., agar Hydrocolloid i.e., alginate
IMPRESSION TECHNIQUES USING ALGINATE (IRREVERSIBLE HYDROCOLLOID) ALGINATE :- • it is an aqueous elastomer,developed as a substitute for agar • It is the most commonly used impression material specially for partially edentulous patients • Its main components are - # sodium/potassium triethanolamine alginate(chief ingredient) # calcium sulphate(reactor) # diatomaceous earth(filler) #potassium titanium fluoride(accelerator) # trisodium phosphate(retarder) • Alginate records good surface details • Is elastic enough to be drawn over the undercuts 6. Is cheap, requires less equipments for manipulation.
Then pour the cast using two pour technique :-# pour the impression with dental stone wait for it to set then invert the poured impression onto base stone. This provides superior surface strength.
Secondary impression • Anatomical impressions: • anatomical form is the surface contour of the ridge which is not under occlusal load. • Recorded with:ZOE,PLASTER OF PARIS,etc • Disadvantage:dentures prepared using anatomic impressions compress the tissue under occlusal load and rotate around terminal abutment axis. • Note-generally anatomic impressions are preferred for tooth supported partially edentulous arches(kennedy’s class III and class IV cases. • Contraindication :For distal extension cases.
Functional impressions: • It records the ridge portion by placing an occlusal load on the impression tray as the impression is being made. The underlying supporting tissue will be displaced because displacement will normally occur under function. • In this soft tissues are recorded in compressed form. • The denture will not exert additional stress on the abutment teeth during functional loading. • Functional impressions are required only to record edentulous saddles. • Note-these impression are indicated for tooth tissue supported partial dentures.
Impression making for Kennedy’s tooth supported partial denture. • Conventional anatomic impression are made. • The procedure for impression making is similar to procedures described for making a diagnostic impression. • A small quantity of alginate is spread over the prepared tooth surfaces before placing the tray material. • After making the impression,it is gently wash under water to clear the saliva and debris. • The cast should be poured within 12 min. after impression making.
Impression making for distal extension bases: • Support for distal extension denture is obtained from both the teeth and the tissues. • Factors influencing the support of a distal extension base: • Contour and quality of residual ridge. • Extent of residual ridge coverage by denture base. • Type and accuracy of the impression. • Accuracy and fit of the denture base. • Design of the partial denture frame work. • Total occlusal load applied.
Dual impression technique: • We record the ridge using two impression i.e., one portion in the functional form and other in the anatomical form. • We limit the functional impression to the saddle area. • Dual impressions are indicated for all tooth tissue supported partial denture(Kennedy’s class I and class II). There are various dual impression techniques available to record a distal extension denture base. • The two impression are used together to fabricate the denture with the help of following procedures: • the functional and anatomical impression are fused together (pick-up impression) . • A cast is poured from the anatomical impression and later altered according to the functional impression (altered cast technique).
Dual impessions are broadly classified as: • Physiological or functional dual impression. • Selective pressure functional dual impression technique
Physiological or functional dual impressions • One anatomical impression is made of the entire ridge and one physiological or functional impression is made on the edentulous portion. • The common techniques employed to record a physiological dual impression are: • McLean’s technique. • Hindle’s modification of Mclean’s technique. • Functional relining method. • Fluid wax technique.
McLean’s physiologic impression • principle:Two impressions are made in this procedure.A functional impression of the edentulous ridge is made .The second impression is made over the functional impression and its records the structures in their anatomic form .The second impression is also known as the pick up impression because it covers,and pick up the functional impression(first impression)alongwith itself. • Procedure:A custom made impression tray is fabricated over the edentulous areas of the preliminary cast. • Occlusal rims are made on the custom tray. • The tray loaded with the impression material is inserted into a patient mouth and the patient is asked to close on the occlusal rims built over the tray.when patient occludes on rims,the tissues under the tray are compressed and impression is recorded.
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 trays. • When over-impression is removed, the functional master impression comes along with it. Since the alginate over impression carries the functional impression along with it, it is called pick up impression. • A cast is poured into the impression. This will reproduce the teeth in the anatomical form and the tissues in functional form. • Disadvantage:-The supporting tissues may not be as compressed as they were while making the functional impression,this can lead to errors. • Small quantity of alginate b/w the occlusal rim of the custom tray and the over-impression stock tray.This alginate may act like a buffer and prevent the transfer of the entire load(finger pressure)applied on the stock tray to the special tray.
Hindle’s modification of McLean’s technique • Method • Special tray with an occlusal rim is fabricated using the primary cast. The special tray should have stoppers to avoid excessive pressure on tissues. • It records the supporting tissues under rest. • Special tray with the impression is left untouched in the patient’s mouth. • A special stock tray with large holes is used to make the over impression.Over impression is a functional impression. • The stoppers present on the tissue side of the custom tray to avoid excessive tissue compression.
Ill effects produced by McLean and Hindle’s methods • Dentures made using functional impressions will constantly pressurise the soft tissues. • Constant pressure from the denture will stimulate osteoprogenitor cells to form osteoclast which resorb bone. • Constant pressure from the denture can produce ischemia. Ischemia can produce bone resorption via various chemical mediators that stimulate osteoclasts. • Dentures made using functional impressions get occlusally displaced at rest due to tissue rebounce. This may lead to premature contacts.
Functional relining method • Principle • In this method, an anatomical master impression of all the oral structures. Two casts are made from this anatomical impression, one is the master cast and the other is the refractory cast. The framework of the partial denture is fabricated using the refractory cast.
Advantages • Improves the fit of the denture after bone resorption. • The tissue surface of the metal framework can be relined after insertion. • Disadvantages • Difficult to maintain the relationship of the framework 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
Procedure • Partial denture framework is constructed on the cast made from a single anatomic impression: first master impression. • Master cast is made from the anatomical impression. The master cast is duplicated and a refractory cast is made. • A framework is fabricated using the refractory cast. • Next the framework is tried in the patient’s mouth. A functional impression is made on the tissue surface of the framework: second master impression. Hence this technique is categorized as a dual impression. • The functional impression is made with low fusing modelling plastic. • The final impression after border moulding is made with zinc oxide eugenol impression paste.
Fluid wax functional impression • Principle • 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. • Uses • Reline the tissue surface of an 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. • To record stress bearing areas in functional form. • To record the non stress bearing areas in anatomic form.
Materials used: • Iowa wax • Korrecta wax • Procedure • Fabricating the special tray • Framework is positioned on the master cast. • Outline of tray is drawn in the master cast made from the anatomical impression. • Cast coated with separating medium. • A spacer is adapted over the crest of the edentulous ridge using a single layer of the base plate wax. • Framework is placed over the spacer. • Autopolymerising resin is adapted over the framework along the length of the framework.
Borders of the cured resin tray should be trimmed according to the outline. • Relief holes can be prepared along the crest of the ridge and retromolar pad to allow the escape of excess impression material. • Impression making • Wax is softened in a water bath at 51-54C. It is then painted evenly on the tissue surface of impression tray with a brush. Wax should be painted in excess near the borders to record the sulcus. • Tray is seated and held with three fingers. • The cheeks are pulled over the borders of the tray to record the buccal vestibule. • When the wax hardens, the framework special tray is removed and impression is examined. • The wax surface that contacted the tissues appear glossy and the other areas will appear dull. • Additional wax is painted over the dull areas and the procedure is repeated until glossy borders are obtained.
Selective pressure functional dual impression technique • In this technique one anatomical impression and one selective pressure functional impression is made. A master cast is prepared from the anatomical impression and is later altered according to the selective pressure functional impression. • Advantage • It equalizes the stress acting on the abutment teeth and the soft tissues. The rate of ridge resorption is reduced because relieving areas that cannot withstand any load are not stressed.
Procedure • Free end saddles are liable to be displaced under occlusal pressure (antero-posterior rocking around the abutment tooth, which acts as a pivot). This is as a result of the displaceability of the mucosa. • The altered cast technique is employed to try and prevent this by taking an impression of the mucosa under controlled pressure. • The metal framework is constructed on a cast produced by a mucostatic impression material, usually alginate. Baseplates are then constructed in self-cured acrylics on the framework in the saddle areas, these are close fitting.
Impression paste or a medium viscosity silicone paste is then applied to the fitting surface of the self-cured acrylic. The denture is then inserted in place, held in place by the framework only, no finger pressure is applied as this would lead to over displacement of the mucosa. • Border moulding is then carried out as the is impression material is setting. • In the laboratory, the free end saddle areas on the master cast are sectioned off. The denture is then positioned on the model and the new saddle areas are poured. The resulting model represents the free end saddle areas under conditions, which mimic functional load. Denture construction then continues as normal. The distribution of loading of the free end saddles is improved and denture is more stable.
