for different removable prostheses

Dr. Ahmad Rabah

For the production of an accurate master cast the impression technique far

outweighs the selection of the impression material. No available

knowledge of the person making the impression material will produce

results greater than the skill and knowledge of the person making the

impression. Good technique will indeed result in better treatment and

improved patient care.

All major classes of impression materials irreversible hydrocolloid,

reversible hydrocolloid, polysulflde rubber, and silicone rubber are capable

of producing accurate results if respect is shown for the properties of the

particular material.

Final impression techniques have been classified in different ways, either

depending on the degree of mouth opening (closed or open mouth) or on the

amount of pressure exerted on the tissues during impression taking.

No single impression material can be used for all impressions. The operator

must be in complete control of all aspects of the impression procedure: the

position and intraoral condition of the patient, the size and position of the

tray, and the selection of material and technique.

1. Minimal pressure (muco-static) impression technique

2. Definite pressure (muco-compressive) impression technique

3. Selective pressure impression technique

4. Functional impression technique

Also called no pressure, passive or open mouth impression technique.

Static means at rest, so the theory of this technique is to record oral

tissues in its static condition. In order to achieve that a highly flowable

(low viscosity) material is used like plaster of paris or light body

elastomeric material is used under minimal pressure and the oral tissue

are recorded with minimal displacement.

It’s based on Pascal’s law which states “Pressure in a confined liquid will be

transmitted throughout the liquid in all directions”, according to this, the

mucosa which contains more than 80% water will react like a liquid in a

closed vessel and thus can not be compressed.

of course this is not true as the mucosa is not a closed vessel and the

tissue fluids can easily escape under the denture borders.

Requirements 1. Needs metal base tray rather than dimensionally unstable acrylic 2. Needs highly fluid material to record the mucosa without distortion so

the finished denture will would fit at all times 3. Requires a special tray with definite stops with holes to allow excess

material to escape

Advantages 1. Better operator visibility due to it’s an open mouth technique 2. The technique of choice in cases with flabby ridge (increased thickness of

mucosa covering the ridge)

Disadvantages 1. Dimensional changes of impression material and/or casts render the care

used worthless 2. Mucosal typography is not static 24 hrs 3. That technique neglects the value of masticatory force distribution 4. Considered the interfacial surface tension as the only contributing

retentive mechanism

Also called closed mouth technique. Usually this technique is carried out

under controlled load to the denture bearing area.

The theory is to record oral tissues under forces resembling forces during

mastication, as a result, the denture will be more stable under occlusal

loads. Such impression would provide an equalized distribution of forces

to the supporting tissue during function.

It presumes that occlusal loads during impression making is comparable

to that during function.

Requirements 1. Needs a relatively longer setting time material to allow for functional

movements of border tissues 2. Should not be easy flowing under pressure in order to maintain pressure

against tissues, like Zinc-Oxide paste

Advantages 1. Ensures maximum retention of the denture during chewing due to

maximum contact with tissues 2. The ability to form sufficiently the lingual borders as a result of tongue

movements

Disadvantages 1. Dentures made from such technique will not have the same fit during

rest because of the rebound tendency of the distorted tissues 2. Will the long abused tissues (under maximum pressure) maintain the

shape that they assumed to on the impression day? 3. Potentially increased bone resorption due to interference with blood

supply

A combination technique in which firm areas are recorded with definite

pressure while resilient areas are recorded with little pressure

Due to the variability in displaceability of oral mucosa, some areas are

believed to withstand more load than other areas like the mucosa

covering the ridge and the primary stress bearing areas, so these areas

should be recorded under heavy force while other softer areas and those

require protection like mid palatal area should be recorded with little

forces; this is the principle of this technique

It aims to construct a denture base that selectively loads the oral tissues

during function, optimizing the stability and retention of the prosthesis

It depends on recording the tissues “in function”, thus a provisional

denture is delivered to the patient with its fitting surface relieved all over,

by the aid of a slow set material like tissue conditioner, and after few days

of use, the oral tissues are recorded functionally in various conditions.

Then this layer in converted to hard reline material through laboratory

procedure.

It’s the shaping of the border areas of an impression tray by functional or manual manipulation of tissues adjacent to the borders to duplicate the contour and size of the vestibules

Techniques and Materials used 1. Sectional technique: a thermoplastic material is used like green stick

compound. It has the advantage of allowing the tray border to be progressively developed until they are correct. Modifications can be made anytime, but it’s more time consuming technique since it’s made in sections.

2. One-step technique: using elastomers and acrylic resin for this purpose. They have irreversible chemical reaction so the molding is done in one shot approach. It has the disadvantage of lack of modification

• Many impression materials are available for use as final impression

material.

• These includes:

Plaster of paris Zinc Oxide paste

Polysulphides Polyethers

Condensation silicone Addition silicone

• Selection of which depends on the objectives desired by the operator

i.e. the impression technique followed.

1. Physiologic or functional impression technique

a) Functional Relining method

b) Mc Lean’s and Hindel’s methods

c) Fluid Wax method

2. Selected Pressure technique

3. Altered Cast technique

• Here a new surface is added into the inner, or tissue side of the

denture base. The partial denture is made from a cast made

from impression made with alginate

• A space is provided by adapting a metal spacer over the ridge on

the cast before processing the denture base.

• A functional impression of the edentulous area is made using the

cast partial denture framework.

• The patient must maintain the mouth in a partially opened

position. Border molding is carried out.

• Then a low fusing modeling plastic/green stick compound is

allowed to flow over the tissue side of the denture base.

1.A FUNCTIONAL RELINING METHOD

• It is tempered in water bath & seated in patient’s mouth.

• To provide space for the impression material, modeling plastic is

scraped to a depth of 1mm

• The modeling plastic serves a s a tray material for the secondary

impression material

• The final impression is made with a Zinc Oxide Eugenol

impression paste

• If undercuts are present, light bodied rubber based impression

materials can be used

Procedure

• A custom impression tray is constructed over a preliminary cast

• Functional impression of distal extension ridge is made.

• Patient applies some biting force with occlusion rims

• Then an overall Alginate impression is made with the impression

held in it’s functional position with finger pressure

1.B.1 McLEAN’S PHYSIOLOGIC IMPRESSION

• The main difference of this with McLean’s is that impression of

edentulous ridge is not made under pressure but is an anatomic

impression made at rest with ZOE paste.

• As the hydrocolloid impression was being made, finger pressure

was applied through holes in the tray to the anatomic impression.

1.b.1 HINDEL’S MODIFICATION

1. Constantly compressed residual ridge is prone to excessive bone

resorption.

2. If the clasp do not hold the partial denture, the denture will be

pushed slightly occlusally by the tissue causing premature contacts

(TISSUE REBOUND)

Disadvantages of these methods

• Make an anatomic impression of the arch using alginate

• Fabricate a refractory cast using this impression

• Fabricate the partial denture framework over the refractory cast

• Acrylic custom tray of the dentulous area is made and relieved by

1-2 mm

• Fluid wax (51-54) °C is painted inside the tissue side of the tray and

inserted in patient’s mouth for 4-5 mins

• The tray is then removed, trimmed and inspected

• The final wax set stage is for 12 min intra-orally

1.C FLUID WAX FUNCTIONAL IMPRESSION

2. Selective pressure impression

technique • More force are applied to areas that can absorb stress without

adverse response & protect that areas that is least able to absorb

force

• Stress bearing areas are the buccal shelf area & the lingual slopes of

residual ridge stress bearing areas

• The denture base made from this impression will be closely adapted

to & in firm contact with the tissues in buccal shelf area

• Impression material is loaded on the prepared (relieved) special tray

and inserted into the patient’s mouth

• Impression is made with the patient with his mouth open under finger

pressure

• Only the stress bearing areas will be compressed during impression

making

3. Altered Cast Technique

(Corrected, Split,

Destroyeed)

• It is mainly a modification of functional impression through

laboratory procedure.

• Functional impression for this technique can be made using any of

the above mentioned material and techniques

Reasons for rejecting impression

1. Bubbles or voids in and around rest preparations

2. Contact of cusps with the tray, especially when the teeth are

involved in the framework design

3. Show through between teeth and modeling plastic or modeling

plastic and hard palate if the tray has been modified for an alginate

impression

4. Voids or bubbles in palatal vault when palatal major connectors are

to be constructed

5. Peripheral underextension when a denture base has been designed

and a corrected cast impression is not planned

6. Interproximal tearing of the impression material when coverage of

those teeth has been designed

7. Lack of detail on the impression surface

8. Any doubt as to the accuracy of the impression