IMPRESSION TECHNIQUES AND THEORIES OF IMPRESSION

MAKING

AMJATH KFINAL YEAR PART 11MALABAR DENTAL College

CONTENTS

• INTRODUCTION

• DEFINITIONS

• CLASSIFICATION OF IMPRESSION TECHNIQUES

• PRIMARY IMPRESSION

• MATERIALS USED

• SPACER DESIGNS

• BORDER MOULDING

• FINAL IMPRESSION MATERIALS

• CONCLUSION

• REFERENCES

•Introduction

•Impression forms a important virtue for the success of compete denture treatment and hence the concepts of impression should be properly understood.

• The need to make accurate impression is fundamental to the practice of Prosthodontics.

• This necessitates the dental clinicians to make careful assessment of the tissues to be recorded in the impression, type of impression trays, impression material and the techniques to be used.

Definitions

IMPRESSION

A negative replica or copy in reverse of the surface of an object , an imprint of the teeth and adjacent structures for use in dentistry.

– gpt 8

BASIC REQUIREMENTS FOR IMPRESSION MAKING

• Knowledge of Basic anatomy

• Knowledge of basic reliable technique

• Knowledge and understanding of impression materials

• Skill

• Patient management

IMPRESSION TECHNIQUES

• MUCOCOMPRESSIVE TECHNIQUE

• MUCOSTATIC TECHNIQUE

• SELECTIVE PRESSURE TECHNIQUE

• MUCO-SEAL TECHNIQUE

• IMPRESSION BY THE USE OF SUB-ATMOSPHERIC PRESSURE

• IMPRESSION TECHNIQUES IN COMPROMISED PATIENTS

• IMPRESSION TECHNIQUES FOR SEVERLY RESORBED MANDIBULAR RIDGE

Pressure theory or mucocompressive theory:

• This theory was proposed on the assumption thattissues recorded under functional pressure providedbetter support and retention for the denture.

• Greene Brothers in 1896 gave this concept

• Pressure manually applied using high viscositymaterials.

• Impression compound• Irreversible hydrocolloids

Demerits of the theory

1. Excess pressure could lead to increase alveolarbone resorption.

2. Excess pressure was often applied to the peripheraltissues and the palate.

3. Dentures which fit well during mastication tend torebound when the tissue resume their normalresting state.

4. Pressure on sharp bony ridges results in pain.

• The technique tells that border tissues are recordedin their functional positions and denture cannot bedislodged during functional movements of jaws.

• Usually this technique is used for preliminaryimpression making as it gives a positive peripheralseal and tissues are recorded in function. Amount ofpressure applied is for short duration and the areascan be relieved during the final impression.

Minimal pressure or mucostatic theory –

The main advantage of this technique is its high regardfor tissue health & preservation.

• 1946 Page gave the concept of mucostatic based onPascal’s law.

• Oral mucosa recorded in resting position• Impression made with oversized tray with spacer.• Border moulding not done,hence flanges are shorter

than other techniques• Impression material of choice is impression plaster.

•Hary L. Page in 1946 stated that all soft tissues were cheifly fluid and 80% or more of the tissues are composed of water.

• According to pascal’s law which states that any pressure applied to a confined fluid is transmitted undiminished and equally in all directions. Page contended that since the soft tissues are confined under a denture, any pressure applied will be transmitted in all directions.

• The short denture borders are readily accessible to thetongue which might provoke irritation.

• The lack of border molding reduces effective peripheralseal.

• The short flanges may reduce support for the face.

• The shorter flanges prevent the wider distribution ofmasticatory stresses.

• The shorter flange would mean less lateral stability.

Selective pressure theory

• Advocated by Boucher in 1950 it combines the principlesof both pressure and minimal pressure technique.

• In this technique idea of tissue preservation is combinedwith mechanical factor of achieving retention, throughminimum pressure which is within physiologic limits oftissue tolerance.

• This theory is based on a thorough understanding of theanatomy and physiology of basal seat and surroundingareas.

•The philosophy of the selective pressure technique is that certainareas of the maxilla and mandible are by nature better adapted forwithstanding extra loads from the forces of mastication.

•These tissues are recorded under slight placement of pressure whileother tissues are recorded at rest or relieved with minimal pressure ina position that will offer maximum coverage with the least possibleinterference with the health of surrounding tissues.

•Here an equilibrium between the resilient and the non resilient tissues iscreated.

•This is achieved by use of a custom tray where relief areas are relieved andonly stress bearing areas contact the tray.

Cannot be used in flabby ridges

• Inspite of some of its apparent drawbacks all theimpression techniques based on the selectivepressure technique are still popular.

• Final impressions using this technique are madewhere relief areas are provided and pressure isdistributed on the stress bearing areas.

MUCOSEAL TECHNIQUE

• Stated by pryor in 1946

• Variation of mucostatic technique.

• Here posterior lingual border covers just the retromolar area and anterior lingual border is moulded by the floor of mouth with tongue in repose.

• The tray is extended horizontally backwards over the sublingual glands towards the tongue to effect a border seal.

Impression by use of controlled sub atmospheric pressure

• Milo v kubalek & dert c buffington

• Based on principle of vacustatics,describes atmospheric pressure as a significant factor in that technique.

• The object of vacustatics technique is to reduce the stress on any given tissues by increasing the load bearing area.

• An impression tray specially built with rubber hoses is maintained in patients mouth without direct mechanical support of any kind.

• The difference between the sub atm pressure inside tray and atmospheric tray outside the tray is all that needed to centre the tray over the ridges in static position

• As vaccum develops between soft tissue and tray a recording material in a fluid state flows from border region to evacuated spaces and envelops the basal tissues.

Impression techniques in compromised patients

• 1.restricted access to oral cavity

• Walter described the technique of sectional stock trays

• Impressions of each side of jaw was made and joined

2.Patients with hyperplastic or flabby tissues

• Impression technique for patients with unsupported movable tissue is done by recording unsupported tissues with minimal displacement and rest of tissues with selective pressure.

• William H Filler described a technique using two trays

• Second tray keyed over first tray. Light bodied material is used in the initial tray as a corrective wash material and plastogumis painted over areas not covered by first impression in the second tray and impression is made

• Two trays are held together lightly until material sets and impression is removed as a single unit

• Hobrik used a technique where only a single tray is used

• Border molding is done in usual manner and impression made with heavy bodied addition silicone

• The area of movable tissue is cut out and relief holes are made and a wash impression is made with light bodied impression material.

• Zafrulla Khan's Window technique: In this case the primary impression was made with mucostatic material, i.e. alginate

• A close fitting custom tray was prepared and border molding was done followed by final impression of the entire denture-bearing area with zinc oxide eugenol impression paste.

• The displaceable tissue was marked intraorally with indelible pencil and this marking was transferred on to the final impression .

• A window was prepared and tray was placed intraorally . An impression of the displaceable tissue was recorded

Impression technique for patients with severly resorbedmandibular ridge

• Flange technique by Lott and Levin

• Involves making impression of soft tissues of the mouth adjacent to the buccal ,labial and palatal surfaces and incorporating the resulting extensions or flanges in denture.

• Flange is extended from retromolar pad region to sublingual region,large enough to restore the diameter of estimated resorption and patient is asked to forcefully perform functions of swallowing,etc to give border extension which cover maximum surface area.

Impression for combination syndrome patients

• Charecteristic feature of edentulous maxilla opposed by natural mandibular anterior teeth.

• These include loss of bone from anterior portion of maxillary ridge,overgrowth of tuberosities,papillary hyperplasia of hardpalatal mucosa,extension of mandibular anterior teeth and loss of alveolar bone and ridge height beneath mandibular RPD

• The impression should record edentulous ridge with minimum distortion

• This may be achieved by additional relief in custom tray,verified clinically with pressure indicating paste,placement of additional vent holes to minimize hydraulic tissue displacement and the subsequent use of highly fluid impression material

• Flat (atrophic) mandibular ridge covered with atrophic mucosa

• These ridges equate to Atwood's ridge orders v and vi and may be complicated by folds of atrophic and/or non-keratinised tissue lying on the ridge.

• McCord and Tyson described this technique which is specific for this clinical situation. The philosophy is that a viscous admix of impression compound and tracing compound

• Removes any soft tissue folds and smoothes them over the mandibular bone;

• This reduces the potential for discomfort arising from the 'atrophic sandwich', ie the creased mucosa lying between the denture base and the mandibular bone.

• The impression medium here is an admix of 3 parts by weight of (red) impression compound to 7 parts by weight of greenstick;

• The admix is created by placing the constituents into hot water and kneading with vaselined, gloved fingers.

• Using a standard impression technique, the lower impression is recorded. The working time of this admix is 1–2 minutes and this enables the clinician to mould the peri-tray tissues to give good peripheral moulding

Preliminary impressions

• objective: to fabricate a custom tray.

• An impression material with high viscosity is used to compensate for any deficiencies in tray.

• Materials commonly used

• 1.impression compound

• 2.Irreversible hydrocolloids

• 3.Silicone putty

• IMPRESSION COMPOUND

• Can be reused• Simple to use• Corrected by reinsertion• Can be boxed for casting

• Disadvantages

• Not suitable if severe undercuts present• Not elastic• Can distort

CHECK TRAY EXTENSION NON PERFORATEDTRAY

60’C

• Alginate

• Elasticity-used even if undercuts present• Reproduce details accurately

• Disadvantages

• Shrinkage and expansion• Casted immediately• Boxing is difficult • Do not adhere to the tray surface effect the hardness of

stone cast.

Custom Impression Tray Design:

2-3mm thickness.Border extensions should be 2-3mm short of thedepth of the vestibule when the tissues are atrest.

Requirements

• It should include all denture bearing area.

• Periphery should be such that impression material should flow without displacement of soft tissues.

• Appropriately releived and spaced if necessary.

• Allow free movements of muscle attachments.

• Custom tray fabricated according to condition of ridges.

• 1.custom tray with spacer

• Ideal ridge where uniform pressure can be applied.• Impression made of tissues in undistorted form.• Function of spacer is to provide space for final impression material.

• 2.CUSTOM TRAY WITH RELEIF

• Close fitting tray

• Areas such as incisive papillae,midpalatine suture need to be releived in some cases.

ROY MAC GREGOR

Spacer designs

• ROY MAC GREGOR RECOMENDS PLACEMENT OF METAL FOIL IN REGION OF INCISIVE PAPILLA AND MID PALATINE RAPHAE

• HE ALSO SAYS THAT OTHER AREAS THAT REQUIRE RELIEF ARE RUGAE,AREAS OF MUCOSAL DAMAGE,BUCCAL SURFACE OF PROMINENT TUBEROSITIES.

• FINALY HE CONCLUDED THAT RELEIF SHOULD NOT BE USED ROUTINELY IN DENTURES.

NEILS DESIGN

NEIL RECOMENDS ADAPTATION OF 0.9MM CASTING WAX ALL OVER EXCEPT PPS.

SHARRY

1MM BASE PLATE WAX ALL OVER INCLUDING PPS

4 TISSUE STOPS 2MM WIDE IN MOLAR AND CUSPID AREA

EXTENDED FROM PALATAL ASPECT TO MUCOBUCCAL FOLD

BOUCHERS

• 1MM BASEPLATE WAX EXCEPT PPS IN MAXILLA

• EXCEPT BUCCAL SHELF AND RETROMOLAR PAD IN MANDIBLE

• HE SAYS THAT PPS ACT AS A GUIDING STOP TO POSITION THE TRAY

• HE ALSO SAYS PLACEMENT OF ESCAPE HOLES USING ROUND BUR NO 6 ON PALATE

MORROW,RUDD AND RHOADS

• BLOCK OUT UNDERCUTS WITH WAX

• FULL WAX SPACER 2MM SHORT OF BORDERS

• 3 TISSUE STOPS 4*4MM EQUIDISTANT FROM EACH OTHER

BERNARD LEVIN

I LAYER OF PINK BASE PLATE WAX 2MM THICK ALL OVER RIDGES EXCEPT PPS AND BUCCAL SHELF

DALE E SMITH HALPRINS

• Tissue Stops: Strategically placed

• Tissue stops provide even thickness of impression material in custom impression trays.

• Placement of four tissue stops of 2mm width in cuspid and molar regions

• extends from palatal aspect of ridge to the muco buccal fold are usually recommended in completely edentulous cases

• Tissue stops are made by removing wax at an angle of 45’ to the occlusal surface on, that have a tripod or quadrangular arrangement in the arch.

• This provides stability to the tray and the 45’ angulated stops will help centre the tray during insertion.

Methods to manipulate peripheral tissues

• Active method

• Passive method

• Combination of above

Techniques of border moulding

• Incremental or sectional border moulding

• In this method sections of periphery of tray are refined individually,according to the anatomical landmark in that area

• This is a better method for a beginner as it allows each section to be recorded verified and refined.

• Material of choice is lowfusing impression compound or green stick compound.

• Heavy body elastomeric material can be used

Method of adapting green stick

compound

Soften till it begins to droop

Placed on the border of trayRotate and pull to prevent long strings forming

Tempered in warm water(50’c)

Formed to appropriate shape

insert

chill trim

Maxillary border moulding

1

2

3

4

1 labial flange 2 buccal flange 3 distobuccal4 pps

• Labial flange

• passive :lips elevated and extended outwards downwards and inwards

• Active:pucker lips and suck on dentists finger

Frenal area Lip movement near labial frenum is vertical → labial notch long & narrow.

• Buccal flange

• Buccal frenum area

• passive:cheek elevated and pulled outward,downward and inward and moved backward and forward.

• patient asked to pucker lips and smile

Muscle movements around buccal frenum are horizontal & vertical → buccal frenum wide V shaped. Anterior movement should be recorded by pursing the lips

• Buccal vestibular area

• If border molding in the buccal space is inadequate, the denture will lose its seal because of the ingress of air under the denture base when the buccal vestibule is opened such as during laughing

• Distobuccal area

• passive:cheek pulled outwards downwards,inwards.

• active:patient asked to open mouth wide open and move mandible from side to side.

• Opening the mouth delineates the depth and width of distobuccal flange as governed by the muscle attachments,while moving the mandible from side to side,accomodates the movement for coronoid process.

• Posterior palatal seal area

• Active :patient asked to say ‘ah’ in short bursts.

Mandibular tray border moulding

• 1.labial flange

• The lip is slightly elevated outward,upward and inward

• When the lip is pulled too much during border molding, the vestibule becomes shallow as the mentalis muscle attachment is higher than the base of the labial vestibule.

• On biting the operator fingers, the masticatory muscles become tense and the lips become relaxed as a reflex, then the impression is made in this siuation.

• Disto buccal area

• Developed bilaterally

• passive:cheek is pulled buccally to ensure it is not caught in the tray and then moved upward and inward

• The masseteric notch is recorded by asking the patient to close while dentist exerts a downward pressure on tray.

• A)buccal frenum area

• Can be developed unilaterally

• passive :cheek lifted outward,upward and inward and moved backward and forward

• Active :patient asked to pucker and smile

Buccal flange

• Anterior lingual flange

• patient is asked to protrude the tongue and then push the tongue against the anterior part of palate.

• This develops the length and thickness of flange in that area.

• The border of the impression in this area is mainly influenced by the lingual frenum & the genioglossus muscle.

• To provide adequate clearance in this area the patient is instructed to lick the lower lip by moving tip of the tongue from side to side

• Middle portion of lingual flange

• Patient is asked to protrude the tongue and lick upper lip from side to side

• Distolingual flange

• Developed bilaterally

• Patient is asked to protrude the tongue and then place the tongue in distal part of palate in the right and left vestibule.

Single step or simultaneous border moulding

• Entire periphery of tray refined in a single step

• Putty or heavy body elastomeric materials are ideal for this method.

• Advantages

• Number of insertions reduced

• Error in one segment will not propagate the mistakes to other segments

• Materials used

• Elastomers• Zinc oxide Eugenol• Impression plaster

FINAL IMPRESSION

• Escape holes: After removing waxspacer from inside of the tray, a series of holes are prepared, about 12.5 mm apart in the center of alveolar groove and in the retro molar fossa with no. 6 round bur.

• The relief holes provide escape way for the final wash impression material and relieve pressure over crest of the residual ridge and in the retro molar pads when the final impression is made.

• For good adhesion between impression material and custom impression trays, use of tray adhesives and escape holes both should be encouraged because this provides a chemical -mechanical type of adhesion.

• Zinc oxide/eugenol impression pastes

• These materials are normally used to record the final impressions of edentulous arches.

• These materials are normally supplied as two pastes which are mixed together on a paper pad or glass slab.

• The zinc oxide paste, typically, being white and the eugenolpaste, a reddish brown colour.

• This enables thorough mixing to be achieved as indicated by a homogeneous colour, free of streaks, in the mixed material.

• The pastes are normally dispensed from toothpaste- like tubes and are mixed in equal volumes.

• The proportioning is achieved, simply, by expression equal lengths of each paste onto the mixing pad or slab.

• Accurate reproduction of all details

• Sticks to dry surface of tray

• Minimum tissue distortion

• Faults can be corrected

• Cannot be used if undercuts present

• Burning sensation

• Setting time to be controlled

• Difficult to clean

• Impression plaster

• Impression plaster used to record final impression for completely edentulous arch.

• Impression plaster is similar in composition to the dental plaster used to construct models and dies.

• The fluid mix is required to enable fine detail to be recorded in the impression and to give the material mucostatic properties.

• Freshly mixed plaster is too fluid to be used in a stock impression tray and is normally used in a special tray, constructed using a 1–1.5 mm spacer.

• Before casting a plaster model in a plaster impression, the impression must be coated with a separating agent (soap), otherwise separation is impossible.

• The mixed impression material is initially very fluid and is capable of recording soft tissues in the uncompressed state.

• In addition, the hemihydrate particles are capable of absorbing moisture from the surface of the oral soft tissues, allowing very intimate contact between the impression material and the tissues.

• The fluidity of the material, combined with the ability to remove moisture from tissues and a minimal dimensional change on setting, results in a very accurate impression which may be difficult to remove.

• The water-absorbing nature of these materials often causes patients to complain about a very dry sensation after having impressions recorded.

• The material is not suitable for use in any undercut situations

• Elastomers

• Four types of elastomers are in general use:

• • Polysulphides.•

• • Silicone rubbers (condensation curing type).

• • Silicone rubbers (addition curing type).

• • Polyethers.

Polysulfides

• These materials are generally supplied as two pastes which are dispensed from tubes One paste is normally labelled base paste whilst the other is labelled catalyst paste.

• Setting times of 10 minutes or more particularly for light-bodied materials.

• The polysulphide elastomers have very good tear resistance.

• It has an unpleasant odour. • The colour contrast between the two pastes is an aid to efficient mixing,

which is continued until a homogeneous colour, with no streaks, is achieved.•

• An adhesive is used to promote adhesion between the impression material and tray.

• Condensation silicon

• Developed as alternative to Polysulfides

– Has more desirable qualities in comparison: Easy mix

– Better taste and odorless

– Shorter setting time (5-7 minutes)

• Addition silicon – Desirable clinical qualities: Dimensional stability – Accuracy – Clean – Easy to mix – No foul odor or taste

• However, they are among the most expensive

conclusion

• There is no evidence to indicate that one technique provides better long term result than other.

• The choice is made by dentist on the basis of oral conditions,conceptof the function of tissues surrounding dentures and the ability to handle the available impression materials.

• Complete denture impression techniques seem to be based upon various philosophies and personal preferences and our heads often whirl in an effort to choose the alternatives

• Nobody is satisfied with any one particular method or philosophy.• Good impressions are basic to the fabrication of a well fitting

denture.

• An impression should fulfill MM Devan’s dictum. “It is perpetual preservation of what already exists and not the meticulous replacement of what is missing”.

• Ideal impression must be in the mind of the dentist before it is in his hand. He must literally make the impression rather than take it”.

References• Sheldon winkler• Syllabus of complete denture Heartwell• Complete dentures and implant supported prosthesis -Zarb

bolender• Text book of prosthodontics.v Rangarajan TV Padmanaban

• Internet

• Indian journal of dental sciences 2012• Indian journal of dental advancements• Jaypee journals• British dental journal