impression making IN COMPLETE

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impression making IN COMPLETE

DENTURE

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INTRODUCTION

IMPRESSION

A negative replica or copy in reverse of the surface of an object -GPT.

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

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• 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

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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) ^RETENTION 2) STABILITY

3) SUPPORT

4) ESTHETICS

5) PRESERVATION OF REMAINING STRUCTURES

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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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Factors affecting Retention Anatomical factors

The various anatomical factors that affect retention, are:

 Size of the denture bearing area:Retention increases with increase in size of the denture bearing area.

1. Size of maxillary denture bearing area 24 cmxcm 2. Size of maxillary denture bearing area 14 cmxcm

 Quality of the denture bearing area: The displace-ability of the tissues influences the retention of the denture. Tissues displaced during impression making will lead to tissue rebound during

denture use, leading to loss of retention.

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Factors affecting Retention

Physiological factors

Saliva and its quality:

1) Normal resting salivary flow is about 1ml/min. A flow of medium viscosity at this rate lubricates the mucosa and assists retention of complete dentures.

2) The viscosity of saliva determines retention. Thick and ropy saliva gets accumulated between the tissue surface of the denture and the palate leading to loss of retention.

3) Thin and watery saliva can also lead to compromised retention.

4) Patients with xerostomia, dentures can produce soreness and irritation.

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Factors affecting Retention

Physical factors: The various physical factors which affect retention, are:

 Adhesion

 Cohesion

 Interfacial surface tension

 Capillarity and capillary attraction

 Atmospheric pressure and peripheral seal

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Adhesion :-

• “

It is the physical attraction of unlike molecules to one another “GPT

• The role of saliva is very important for adhesion

• It acts when saliva sticks to the denture base & to the mucous membrane of basal seat

.

• In patients with xerostomia , adhesion does not play a

major role.

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• Adhesion is achieved 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

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• 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.

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Cohesion:-

1)“It is the physical attraction of like molecules for each other” .

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

3)Normal saliva is not very cohesive , therefore most of the retentive forces of denture –mucosa interface

comes from adhesive & interfacial surface tension factors.

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Interfacial surface tension :-

 “It is the resistance to separation of two

parallel surfaces that is possessed by a film of liquid between two well- adapted surfaces” – GPT

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

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 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.

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• 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 .

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To obtain maximum interfacial surface tension:

• Saliva should be thin and even.

• Perfect adaptation should be present between the tissues and the denture base .

• The denture base should cover a large area.

• There should be good adhesive and cohesive

forces, which aid to enhance interfacial surface

tension.

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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.

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Capillarity and capillary attraction

Defined as, “that quality or state ,because of

surface tension causes elevation or depression of the surface of a liquid that is in contact with a

solid ”

A liquid tends to rise in a capillary tube by maximizing its contact along the walls of the tube at the interface

between the liquid and glass.

Factors that aid to improve capillary attraction:

1. Closeness of adaptation of denture base to soft tissue.

2. Greater surface of the denture bearing area 3. Thin film of saliva should be present.

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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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• Most effective in retention when:

 Denture has a perfect seal around its entire border

 Proper border molding with physiological,

selective pressure techniques is carried out

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Factors affecting Retention

Mechanical Factors Are:

 Undercuts

 Retentive springs

 Magnetic forces

 Denture adhesive

 Suction chambers and suction discs

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Undercuts:

Unilateral undercuts aid in retention while bilateral undercuts will interface with denture insertion and required surgical correction

Magnetic forces

^:

Intramucosal magnets aid in increasing retention of highly resorbed ridges.

Denture adhesives

^:

They are available as creams or gels or powders they should be coated on the tissue surface before wearing the denture .

Suction chambers and suction discs

^:

In the past suction chambers in the maxillary dentures were used to aid in retention. The suction chamber creates an area of negative pressure, which increases retention.

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Factors affecting Retention

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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Factors Affecting Stability

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.

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Vertical height of the residual ridge:

The residual ridge should have sufficient vertical height to obtain good stability. Highly resorbed ridges offer the least stability.

Quality of soft tissue covering the ridge:

The ridge should provide a firm soft tissue base with

adequate submucosa to offer good stability. flabby tissue with excessive submucosa offer poor stability.

Occlusal plane

^:

The occlusal plane should be oriented parallel to the ridge. If the occlusal plan is inclined, then the sliding forces may act on the denture, reduce its stability. The occlusal plan should divide the interarch space equally.

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Quality of the impression:

• An impression should be as accurate as possible.

• The impression surface should be smooth and duplicate all the details accurately.

• The impression should be dimensionally stable and the cast should be poured as soon as possible.

Teeth arrangement^:

The teeth in the denture should be arranged in the natural zone.

Natural zone is defined as, “the potential space between the lips and cheeks on one side and tongue on the other. Natural or artificial teeth in this zone are subject to equal and opposite forces from the surrounding musculature” –GPT

Contour of the polished surfaces:

The polished surface of the denture should be harmonious with the oral structures. They should not interfere with the action of the oral musculature

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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 .

• 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.

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Areas of support are divided into:-

1. Primary

2. Secondary

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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

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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

 Muller 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.

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CLASSIFICATION of impressions

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

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Mucostatic

Mucocompressive

Selective pressure

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Mucostatic or Passive Impression

• First proposed by Richardson and later popularised by Henry 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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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 or take out the excessive material to come out from the hole

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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.

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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.

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Mucocompressive Impression

(Carole Jones)

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

 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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Technique

• 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.

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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.

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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.

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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.

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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.

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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.

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Depending on the technique

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Open- mouth Closed-

mouth

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• 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.

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• 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.

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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

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Stock tray Custom

tray

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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.

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Edentulous stock trays

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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

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Diagnostic

Primary Secondary

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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 60

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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

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Reversible hydrocolloid impression.

Irreversible hydrocolloid impression.

Modeling plastic impression.

Plaster impression.

Wax impression.

Silicone impression.

Thiokol rubber impression.

(Polysulphide)

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STEPS IN MAKING AN IMPRESSION

 Examination of the patient

 Seating the patient

 Selection of the tray

 Selection of the material

 Making impression-primary -secondary

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Selection of tray

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Primary impression making

• With alginate (Maxillary)

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(Mandibular impression with alginate)

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Border molding

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Mandibular border molding

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Secondary impression

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Mandibular secondary impression

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impression making IN COMPLETE