impression techniques of complete denture

IMPRESSION TECHNIQUES IN COMPLETE DENTURE

Presented by:AKANKSHA ARYA

CONTENTS• INTRODUCTION• DEFINITIONS• PRINCIPLES OF IMPRESSION MAKING• CLASSIFICATION OF IMPRESSIONS• IMPRESSION TECHNIQUES• IMPRESSION PROCEDURES• IMPRESSION TECHNIQUES IN COMPROMISED

SITUATIONS• SUMMARY• BIBLIOGRAPHY.

INTRODUCTION

IMPRESSION A negative replica or copy in reverse of the surface of

an object . – gpt 8

• An impression can also be defined as an imprint of the teeth and adjacent structures for use in dentistry. - gpt 4

• COMPLETE DENTURE IMPRESSION A complete denture impression is a negative registration of

the entire denture bearing, stabilizing and border seal areas present in the edentulous mouth

• PRELIMINARY IMPRESSION A preliminary impression is an impression made for the

purpose of diagnosis or for the construction of a tray

BASIC REQUIREMENTS FOR IMPRESSION MAKING

• Knowledge of Basic anatomy• Knowledge of basic reliable technique• Knowledge and understanding of impression

materials • Skill• Patient management

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OBJECTIVES OF IMPRESSION MAKING

1) RETENTION2) STABILITY3) SUPPORT4) ESTHETICS5) PRESERVATION OF REMAINING

STRUCTURES

RETENTION Retention is defined as the ability of denture to resist

the displacement against vertical forces Retention resists the adhesiveness of food, the force

of gravity, & the forces associated with the opening of jaws.

Retention begins with the impression. It depends upon factors that produce attachment of the denture to the mucosa.

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Factors affecting Retention

Anatomical factors Physiological factors Physical factors Mechanical factors Muscular factors

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Anatomical factors

Size of the denture bearing area

Quality of the denture bearing area.


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Physiological factors

Saliva and its quality


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Physical factors

Adhesion Cohesion Interfacial surface tension Capallarity and capillary attraction Atmospheric pressure and peripheral seal


Adhesion :- It is the physical attraction of unlike molecules • It acts when saliva sticks to the denture base & to

the mucous membrane of basal seat .

• Adhesion is achievied by ionic forces between charged salivary glycoproteins & surface epithelium or acrylic resin.

• Quality of adhesion depends on :-

Close adaption of denture

Size of denture bearing area Type of saliva

• The most adhesive saliva is thin serous but contains some mucous components.

• Thick & ropy saliva is very adhesive but tends to build up so that it is too thick in palatal area & interferes with oral adaptation .

• In this situation patient should rinse out the ropy saliva every two to three hours

• The amount of retention provided by adhesion is directly proportional to the area covered by denture.

• Mandibular dentures cover less surface area than maxillary prosthesis & therefore are subject to a lower magnitude of adhesive retentive forces.

• Similarly patients with small jaws or very flat alveolar ridges cannot expect retention to be as great as can patients with large jaws or prominent alveoli.

Cohesion:-it is the physical attraction of like molecules for each other .

• it occurs within the layer of fluid (usually saliva ) that is present between the denture base & the mucosa.

• Normal saliva is not very cohesive , therefore most of the retentive forces of denture –mucosa interface comes from adhesive & interfacial surface tension factors.

Interfacial surface tension :-it is the resistance to separation of two parallel surfaces that is imparted by a film of liquid between them .

• It is dependent on the ability of the fluid to wet the rigid surrounding material .

• If the surrounding material has low surface tension ,

as oral mucosa does ,fluid will maximize its contact

with the material, thereby wetting it readily &

spreading out in a thin film.

• If the material has high surface tension ,fluid will

minimize its contact with the material , resulting in

formation of beads on the material surface.

• All denture base material have higher surface tension than oral mucosa ,but once coated by salivary pellicle ,their surface tension is reduced ,which promotes maximizing the surface area between liquid & base.

• Role of surface tension is through capillary attraction or capillarity.

• When the adaptation of denture base to mucosa is sufficiently close ,the space filled with a thin film of saliva act like a capillary tube in that the liquid seeks to increase its contact with both denture & mucosal surface.

• It plays a major role in retention of maxillary denture. It is totally dependent on presence of air at the margin of liquid & solid contact (liquid air interface).

• As there is excess saliva along the lower border of mandibular denture, Surface tension is lost in mandibular denture due to loss of liquid air interface at denture border .

Mucostatics dismiss adhesion and cohesion as factors in retention, the entire phenomenon being attributed to interfacial surface tension.

But an analysis has proved that if it was not for the forces of adhesion and cohesion, the forces of interfacial surface tension wont exist. Attachment of a denture is possible because both tissue and denture base material can become wet which means its molecule will adhere to water molecules.

Oral & facial musculature :-supplement retentive forces , provided :-

a)Teeth are positioned in “neutral zone “between the cheeks & tongue

b)The polished surface of the denture are properly shaped.

• If the buccal flange of maxillary denture slope up & out of occlusal surface of teeth & the buccal flange of mandibular denture slope down & out from the occlusal plane, the contraction of buccinator will tend to retain both denture on basal seat.

Atmospheric pressure:-

• Act to resist dislodging forces applied to the denture ,if the denture have an effective seal around their borders.

• Retention due to atmospheric pressure is directly proportional to the area covered by the denture base.

In function, atmospheric pressure is superior to interfacial surface tension as a retentive force, for forces horizontal as well as parallel to the mean of mucosal plane are resisted.

Interfacial surface tension will resist only forces perpendicular to the axis of surface tension forces.

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Mechanical factors

Undercuts Retentive springs Magnetic forces Denture adhesive Suction chambers and

suction discs

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Muscular factors

The muscles apply supplementary retentive

forces on the denture.

It is most effective in the neutral zone.


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STABILITY

The quality of a denture to be firm, steady, or constant, to resist displacement by functional stresses and not to be subject to change of position when force is applied. It is the ability of the denture to withstand horizontal forces.

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Vertical height of the residual ridge.

Quality of soft tissue covering the ridge.

Occlusal plane

Quality of the impression.

Teeth arrangement.

Contour of the polished surfaces.

Factors Affecting Stability

SUPPORT

• It is the resistance to vertical forces of mastication & to occlusal or other forces applied in a direction toward the basal seat .

• When the natural teeth are missing ,the alveolar ridge & their covering of mucosal tissue become the supporting elements.

• Unfortunately , they were never meant to endure the forces of mastication & other constant occlusal pressure that result from swallowing , clenching ,or bruxing.

• To make the best of bad situation , it is necessary to enhance the available support by utilizing maximum coverage of all usable ridge bearing areas.

Areas of support are divided into:-

slight

pimary

secondary

Primary support area:- area of edentulous ridge that are at right angle to occlusal forces & usually do not resorb easily .

• Maxillary:- a)posterior ridge b)

flat areas of the palate

• Mandibular:- a)buccal shelf area b)Posterior ridge c)pear shaped pad

Secondary supporting area:- area of edentulous ridge that are greater than at right angle to occlusal forces ; also the area of dentulous ridge that are at right angle to occlusal forces but tend to resorb under load.

• Maxillary :- anterior ridge ,rugae & all ridge slopes

• Mandibular:- anterior ridge & all ridge slopes

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ESTHETICS

The thickness of the denture flanges is one of the important factors that govern esthetics.

Thicker denture flanges are preferred in long-term edentulous patients to give required labial fullness.

Impression should perfectly reproduce the width and height of the entire sulcus for the proper fabrication of the flanges.

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PRESERVATION OF REMAINING STRUCTURES

De Van (1952) stated that, “the preservation of that which remains is of utmost importance and not the meticulous replacement of that which has been lost.

Impressions should record the details of the basal seat and peripheral structures in an appropriate form to prevent injury to the oral tissues.

IMPRESSIONS

CLASSIFICATION

classification

Depending on the theories of impression

making.

Depending on the

technique

Depending on the tray

type

Depending on the purpose of the impression

Depending on the

material used

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Depending on theories of impression making

Mucostatic

Mucocompressive

Selective pressure

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Mucostatic or Passive Impression

First proposed by Richardson and later popularised by

Harry Page.

The impression is made with the oral mucous membrane

and the jaws in a normal, relaxed condition. Border

moulding is not done here.

The impression is made with an oversized tray.

Impression material of choice is impression plaster.

Retention is mainly due to interfacial surface tension.

The mucostatic technique results in a denture, which

is closely adapted to the mucosa of the denture-

bearing area but has poor peripheral seal.

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Mucocompressive Impression (Carole Jones)

Records the oral tissues in a functional and displaced form. The materials used for this technique include impression compound, waxes and soft liners.

The oral soft tissues are resilient and thus tend to return to their anatomical position once the forces are relieved. Dentures made by this technique tend to get displaced due to the tissue rebound at rest. During function, the constant pressure exerted onto the soft tissues limit the blood circulation leading to residual ridge resorption.

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Selective Pressure Impression (Boucher)

In this technique, the impression is made to extend over as much denture-bearing area as possible without interfering with the limiting structures at function and rest.

The selective pressure technique makes it possible to confine the forces acting on the denture to the stress-bearing areas. This is achieved through the design of the special tray in which the non stress-bearing areas are relieved and the stress-bearing areas are allowed to come in contact with the tray.

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Depending on the technique

Open-mouth

Closed-mouth

Open mouth impressions

The open mouth impression is built in a tray which

carries the impression material of choice into the

desired contact with the supporting tissues and into an

approximate relation to the peripheral tissues when

the mouth is opened and without applied pressure.

The rationale behind this method is that the dentures

do not dislodge when subjected to biting force.

The open mouth methods provide clearance for the

tissues that are pulled over the edges of the dentures

as in function of speech.

It develops a contour of impression surface which is in

harmony with the relaxed supporting tissues, and

which may be out of perfect adaptation with these

tissues when the denture is subjected to occlusal

loading.

Closed mouth impression technique

These require wax occlusal rims to be fabricated on the preliminary cast .

The patient is made to close on these rims and a

generous clearance is made for the various frenula so that the patient can manipulate his tissues by closing, grimacing, sucking and swallowing to form peripheral borders.

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Depending on the tray type

Stock tray

Custom tray

Type of tray

Some dentists use a stock tray and an impression

material such as alginate , impression plaster or

impression compound is used .However such

impressions are generally overextended and serve as

primary impressions.

Edentulous stock trays

On casts made from these primary impressions,

special/custom trays are fabricated. The tray is tried

in the mouth and modified and the final impressions

are made using zinc oxide eugenol or other such

materials.

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Depending on the purpose of the impression

Diagnostic

PrimarySecondary

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Diagnostic Impression The negative replica of the oral tissues used to prepare a

diagnostic cast.

Used for study purposes like measuring the undercuts, locating

the path of insertion.

Is made as a part of treatment plan and to estimate the

amount of pre-prosthetic surgery.

Articulate the casts on tentative jaw relation and evaluate the

inter-arch space.

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Primary Impression(PRELIMINARY IMPRESSION)

An impression made for the purpose of diagnosis or for the construction of a tray.

There should be at least 5mm clearance between the stock tray and the ridge.

The tray should extend over hamular notch and maxillary tuberosity. Mandibular tray should cover retromolar pad.

Tray can be extended using modelling wax.

Impression compound, Alginate, Impression plaster

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Secondary Impression(WASH IMPRESSION)

Involve:

Fabriction of custom tray.

Border molding.

Developing the posterior palatal seal.

Making the wash impression.

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Depending on the material used

Reversible hydrocolloid impression.

Irreversible hydrocolloid impression.

Modeling plastic impression.

Plaster impression.

Wax impression.

Silicone impression.

Thiokol rubber impression. (Polysulphide)

IMPRESSION TECHNIQUES

Impression techniques may be classified depending on:

a) Amount of pressure used1. Pressure technique2. Minimal pressure technique3. Selective pressure technique

b) Based on the position of the mouth while making impression

1. Open mouth 2. Close mouth

c) Based on the method of manipulation for border molding.

1. Hand manipulation2. Functional movements

Pressure theory or mucocompressive theory:

• This theory was proposed on the assumption that tissues recorded under functional pressure provided better support and retention for the denture.

• Greene in 1896 gave this concept

Primary impression made with impression compound

Special tray made using shellac base plate.

Second Impression is made in this tray using compound

Bite rims with uniform occlusal surfaces are then made.Areas to be relieved are softened and the impression is inserted in mouth and held under biting pressure for one or two minutes.

Borders are molded by asking the patient to perform functional movements.

Demerits of the theory

1. Excess pressure could lead to increase alveolar bone resorption.

2. Excess pressure was often applied to the peripheral tissues and the palate.

3. Dentures which fit well during mastication tend to rebound when the tissue resume their normal resting state.

4. Pressure on sharp bony ridges results in pain.

Applied aspects:

• The technique tells that border tissues are recorded in their functional positions and denture cannot be dislodged during functional movements of jaws.

• The pressure applied is more and directed towards the palate and peripheral tissues. So the retention will be for short time and will be lost as soon as the bone undergoes resorption.

• Usually this technique is used for preliminary impression making as it gives a positive peripheral seal and tissues are recorded in function. Amount of pressure applied is for short duration and the areas can be relieved during the final impression.

Minimal pressure or mucostatic theory –

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

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

Technique

• A compound impression is made.

• A baseplate wax space is adapted.

• A special tray is adapted over the wax spacer.

• Spacer is removed and an impression is made with a free flowing material with little pressure.

• Escape holes are made for relief.

Demerits

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

• The lack of border molding reduces effective peripheral seal.

• The short flanges may reduce support for the face.

• The shorter flanges prevent the wider distribution of masticatory stresses.

• The shorter flange would mean less lateral stability.

Applied aspect:

• The technique holds good in the sense it helps in preservation of tissue health.

• In practice with short flanges the oral musculature is non supported and stresses are not widely distributed.

• Food can slip beneath the denture and tongue can readily access the denture borders.

• This technique is useful in impressions of flabby and sharp or thin ridges.

Selective pressure theory

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

• In this technique idea of tissue preservation is combined with mechanical factor of achieving retention, through minimum pressure which is within physiologic limits of tissue tolerance.

• This theory is based on a thorough understanding of the anatomy and physiology of basal seat and surrounding areas.

Demerits

• Some feel that It is impossible to record areas with varying pressure.

• Some areas still recorded under functional load, the dentures still faces the potential danger of rebounding and loosing retention.

Applied aspect:

• Inspite of some of its apparent drawbacks all the impression techniques based on the selective pressure technique are still popular.

• Final impressions using this technique are made where relief areas are provided and pressure is distributed on the stress bearing areas.

Open mouth technique

Made with tray held by dentist and mouth open

Muscle movements may be emphasized and can be seen by the operator

Closed mouth technique

The rationale behind this technique is that the supporting tissues are recorded in a functional relationship.

Requires occlusal rims to be made

Border molding done and final impressions made

Hand manipulation

Dentist uses hand manipulation for movements of lips and cheeks

Functional movements

Patient makes functional movements such as sucking, swallowing, licking or grinning

STEPS IN MAKING AN IMPRESSION

Preliminary examination of the patientSeating the patientSelection of the tray Selection of the materialMaking impression-primary border molding secondary

Preliminary examination of the patient

• A complete case history and thorough clinical examination is done.

• Factors that can complicate impression making are identified.

• Patient education.

Seating of the patientPosition of the operator for maxillary impression

Position of the operator for mandibular impression

Selection of tray:

• The beginning of good impression starts with the selection of the correct stock tray.

• Tray is a device that is used to carry, confine and control impression material while making an impression.

• The space available in the mouth for upper impression is studied carefully by observation of the width and height of the vestibular spaces with mouth partly open.

• And in the lower the general form and size of basal seat is studied.

IMPRESSION PROCEDURES• First technique:- border- molded special tray:

Preliminary impression:

An edentulous stock metal tray that is approximately 6mm larger than the outside surface of the residual ridge is selected.

The borders of the stock tray are lined with a strip of soft boxing wax so a rim is created to help confine the alginate material.

The objective is to obtain a preliminary impression that is slightly overextended around the borders.

The tissue surface and borders of the tray, including the rim of wax, are painted with an adhesive material.

The loaded tray is positioned in the mouth.

The tray is left in the mouth for 1 minute after the initial set. The impression is removed and inspected to ensure all basal seat is included.

The impression is poured in artificial stone.

Primary impression making

• With alginate (Maxillary)

(Mandibular impression with alginate)

A wax spacer is placed within the outlined border to provide space in the tray for final impression material.

A custom tray made using self- curing acrylic resin.

• Preparing the final impression tray: Border molding is the process by which the shape of

the borders of the tray is made to conform accurately to the contours of the buccal and labial vestibules.

It begins with manipulation of the border tissues against a moldable impression material that is properly supported and controlled by tray.

Border molding

Mandibular border molding

Stick modeling compound is added in sections to the shortened borders of the resin tray and molded to a form that will be in harmony with the physiologic action of the limiting anatomic structures.

The final impression material is mixed according to manufacturer’s directions and uniformly distributed within the tray.

Secondary impression

Mandibular secondary impression

• Second technique:- one- step border- molded tray:

• A material that will allow simultaneous molding of all borders has two general advantages:

1. The number of insertions of the tray for maxillary and mandibular border molding is reduced.

2. Developing all borders simultaneously avoids propagation of errors caused by a mistake in one section affecting the border contours in another.

• The requirements of such a material are that it should:

1. Have sufficient body to allow it to remain in position on the borders during loading of the tray.

2. Allow some preshaping of the form of the borders without adhering to the fingers.

3. Have a setting time of 3 to 5 min4. Retain adequate flow while the tray is seated in the

mouth 5. Allow finger placement of the material into

deficient parts after the tray is seated

• Not cause excessive displacement of the tissues of the vestibule.

• Be readily trimmed & shaped so excess material can be carved & the borders shaped before the final impression is made.

• The following procedure utilizes polyether impression materials for border molding.

1. Place adhesive for polyether impressions on the borders of tray.

2. Express a 3- inch strip of polyether material from large tube onto a mixing pad. Next express 2.5 inches of catalyst to provide sufficient working time to complete border molding.

3. Thoroughly mix material for 30 to 45 seconds using a metal spatula.

4. Position the polyether material on the borders, making certain that a minimum width of 6 mm exists on inner portion.

5. Quickly preshape material to proper contours with fingers moistened in cold water

6. Place the impression tray in the mouth .

7. Inspect all borders to be sure that impression material is present in the vestibule

8. Border molding is done

9. Remove tray when impression material is set.

10. Examine border molding to determine that it is adequate.

• Preparing the tray to secure the final impression:

1. Reduce the borders on the tray that protrude through the polyether.

2. Remove any material that extends internally within the tray more than 6mm.

3. Remove the relief wax.4. Reduce the thickness of labial flange to

approximately 2.5 to 3mm from one buccal frenum to another.

5. Make the final impression in silicone, metallic oxide paste, or rubber base.

• Third technique:- custom tray design based on previously worn denture:

1. The denture is treated like a standard impression, and a stone cast is poured.

2. An acrylic resin tray is made on the cast over a wax spacer that is outlined just short of the borders of the impression.

3. The tray is tried in the mouth and checked for overextensions.

4. The spacer is removed, relief holes prepared, an adhesive is applied and an impression is made in the preferred material.

impression techniques of complete denture

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