basic principles impression making

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BASIC PRINCIPLES IN IMPRESSION MAKING JPD 2005 93:503-8;M.M.Devan Resolution of the problem : The problem is due to the fact that the mouth is lined by displaceable tissue ; it varies in degree of displaceability according to :- Thickness Rigidity Point,magnitude,direction of forces applied to it Definition of impression : A complete denture impression is a negative registration of the entire denture bearing , bracing, stabilizing and border seal areas present in the edentulous mouths. THE IMPRESSION AREA ARE:- The vault ridge area The flange heel areas Theories of impression making Based on amount of pressure used : Mucostatic

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Page 1: Basic Principles Impression Making

BASIC PRINCIPLES IN IMPRESSION MAKING

JPD 2005 93:503-8;M.M.Devan

Resolution of the problem :

The problem is due to the fact that the mouth is lined by displaceable tissue ; it varies in degree of displaceability according to :-

Thickness

Rigidity

Point,magnitude,direction of forces applied to it

Definition of impression:

A complete denture impression is a negative registration of the entire denture bearing , bracing, stabilizing and border seal areas present in the edentulous mouths.

THE IMPRESSION AREA ARE:-

The vault ridge area

The flange heel areas

Theories of impression making

Based on amount of pressure used :

Mucostatic

Mucocompressive

Selective pressure

Depending on the technique used:

Open mouth ; Closed mouth

Page 2: Basic Principles Impression Making

Depending on the tray type

Stock tray

Custom tray

Depending on the purpose of the impression

Diagnostic

Primary

final

Hand manipulation:

The contour of denture borders obtained by the dentist with the manual manipulations of the lips & cheeks.

Functional movements:

Denture borders obtained by asking the patient to perform functional movements such as sucking; grinning; licking; swallowing.

Mucostatic or non pressure impression

First introduced 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. The impression is made with an oversized tray and impression material

of choice was impression plaster. Retention is mainly due to surfacetension. The mucostatic technique results in a denture which is closely adapted

to the mucosa of denture bearing area but has poor peripheral seal.

FACTS :

Law of hydrostatistics tells us that water is incompressable. The mucoperioteum is a semi solid with over 80% of water.

Page 3: Basic Principles Impression Making

While muceriosteum cannot be compressed it may be displaced in absence of walls.

Any substance no matter how fluid when enclosed in a rigid container taken same rigidity as that container.

Tissue is elastic and will not remain passive in a displaced position . Merits :

Maintains tissue in healthy conditions. Useful in sharp & flabby ridge

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.

Disadvantages :

Displaced tissues attempts to return to its normal unstrained position so will move the denture out of its intended position resulting in deflective occlusal contacts.

This technique does not allow for adequate muscle trimming of periphery.

It is unable to obtain desired pressure so tend to create excessive pressure.

This technique doesnot allow adequate muscle imming of periphery.

Selective pressure technique

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

Page 4: Basic Principles Impression Making

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.

Primary stress bearing areas: Maxilla : Crest of alveolar ridge & horizontal plate of palatine bone Mandible :Buccal shelf area

Secondary stress bearing areas : Maxilla : Rugae area & all ridge slopes Mandible : Retromolar pad area & all ridge slopes

Relief areas :

Maxilla : Incisive papilla , Mid palatine suture , tori Mandible : Crest of the residual ridge

Retromolar pad is an area of support so if denture is not extended over this area a typical boat shaped resorption is seen.

In maxilla posterior palatal seal is more readily displaced for maintenance of peripheral seal

De merits :

Cannot be used for flabby ridge cases

This technique is based on principle of both biologic as well as physical factors of impression procedure it is widely preffered for well formed healthy ridges.

spacers

Selective pressure can be achieved either by scraping of the primary impression in selected areas or by fabrication of custom tray with a proper spacer design & escape holes.

Different spacer design:

Page 5: Basic Principles Impression Making

Roy Mac gregor : placement of metal foil in the region of incisive papilla & mid palatine raphae.

Heartwell mentions 2 techniques

1. Scraping in selected areas of impression

2. Placement of 5 relief holes . 3 in rugae area & 2 in glandular region.

Boucher :placement of one mm base plate wax on the cast except posterior palatal seal area.

Morrow, rudd, Rhoads: three tissue stops equidistant from each other. Sharry : A layer of base plate wax over the whole area outlined for

tray even in pps area. Bernard : Attach 2mm thick baseplate wax over areas of cast that

usually have softer tissue.

wax spacer is placed all around except posterior part of palate.

The Basic principles in impression making

1. Preservation of the alveolar ridge

2. Support

3. Retention

4. Stability.

5. Esthetics

Preservation of the alveolar ridges : M.M.Devans dictum, it is more important to preserve what already exists than to replace what is missing.

Support :

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

The support is enhanced by utilizing maximum coverage of ridge areas.

Page 6: Basic Principles Impression Making

Areas of support :

Primary :

Secondary

slight

Primary: Areas of edentulous ridge at right angles to occlusal forces & donot resorb easily.

Maxillary: posterior ridges & flat areas of palate

Mandibular: Buccal shelf area ,posterior ridges ; pear shaped pad

Secondary: Areas of edentulous ridge greater than at right angle or parallel to occlusal forces. The areas of edentulous ridge that are at right angles to occlusal forces but tend to resorb under load

Maxillary: Anterior ridge & all ridge slopes Mandibular: Anterior ridge & all ridge slopes

Slight: areas of displaceable tissues like vestibular areas.

Importance of covering buccal shelf area

It is a intact cortical plate and not to resorb due to stimulation and attachment of buccinator muscle.

Importance of covering pear shaped pad:

It rarely resorbs because large active temporalis tendon attached to alveolar bone distal end of this pad.

SNOW SHOE EFFECT : All the masticatory forces are distributed across the resilient tissue. This is termed as snow shoe effect.

PALATAL SUPPORT

u-shape: Excellent support

Page 7: Basic Principles Impression Making

high or v-shape: secondary support

ROOFLESS DENTURES

o Better taste perception & good phonation.

o Used when maxillary ridge is good & opposing a complete mandibular denture.

Improving support :

Surgical removal of pendulous tissue

Use of tissue conditioning materials

Surgical reduction of sharp ridge

Surgical enlargement of ridge

implants

Retention

It is defined as that quality inherent in the prosthesis which resists forces of gravity ,adhesiveness of foods &forces associated with the opening of the jaws-GPT

The factors that affect retention are:

1. Anatomical factors

2. Physiological factors

3. Physical factors

4. Mechanical

5. muscular

Anatomical factors

Page 8: Basic Principles Impression Making

size of the denture bearing area : Retention increases with increase in size of denture bearing area.The size of maxillary denture bearing area is about 24 cm & that of mandible is about 14 cm

Quality of the denture bearing area : Tissues displaced during impression making will lead to tissue rebound during denture use, leading to loss of retention.

Physiological factors

Saliva :

The viscosity of saliva determines retention. Cases with Ptyalism can lead to gagging & in patients with xerostomia

dentures produce soreness & irritation of mucosa.

Physical factors

Adhesion Cohesion Interfacial surfacetension Capillarity Atmospheric pressure

Adhesion :

It is the physical attraction of unlike molecules.

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

o The quality of adhesion depends on:

1. Close adaptation of denture

2. The size of denture bearing area

3 The type of saliva

Page 9: Basic Principles Impression Making

o The most adhesive saliva is thin but it is not as effective due to its inability to draw up column of saliva.

cohesion :

• It is the physical attraction between like molecules.

• In order to be effective the salivary layer should be thin.

• Retentive effect is in direct proportion to the amount of area covered.

Interfacial surfacetension :

It is resistance to separation by the film of liquid between the denture base and supporting tissues.

It combines action of adhesion and cohesion and it is similar to capillary action.

It is totally dependent on the presence of air between liquid and solid contact.

It is most effective when the salivary film is thin and even. For retention this factor is more importanrt. Surface tension is inversely proportional to the thickness of the moisture layer

between the two surfaces This indicates that the better the complementary fit; the more attraction the

surfaces will have for each other. The application of this principle can also be used to explain, in part, why a

denture, originally tight, becomes a floater Stress impressions often reduce the fluid layer over the tissues either by

forcibly squeezing out the film or by their setting actions. A thin and consequently very powerful film remains.

As the tissues resorb, the space between the denture base and oral tissues increases causing the liquid film to increase in depth, thus losing surface tension and affording a better opportunity for the oral fluids to destroy the meniscus.

To obtain maximum interfacial surface tension:

Page 10: Basic Principles Impression Making

Saliva should be thin and even Perfect adaptation should be present between the tissues and

denturebase The denture base should cover large area. There should be good adhesive and cohesive forces which aid to

enhance interfacial surface tension.

Capillarity :

It is 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 & glass.

When there is close adaptation between the denture and mucosa the film of saliva tends to flow & increase its contact there by increasing the retention.

Peripheral seal & atmospheric pressure :

peripheral seal is a positive contact of the denture base to the resilient tissues that outline the basalseat.

Peripheral seal prevents air entry between denture surface & soft tissue . Hence low pressure is maintained.

Peripheral seal & atmospheric pressure are most important & effective factors for retention.

Atmospheric pressure plays a roll in retention when relief is present & not filled with saliva.

Atmospheric pressure can be utilized when the denture base is partially unseated without breaking the seal

Mechanical factors :

Undercuts

Retentive springs

Page 11: Basic Principles Impression Making

Magnetic forces

Denture adhesives

Suction chambers & suction discs

Denture adhesives: These are available as creams or gels or powders.

They should be created on the tissue surface before wearing the denture

.Suction chambers : creates negative pressure & enhances retention

Oral & Facial Musculature

Oral & facial muscle action Can significantly contribute to retention but if the polished surfaces of denture are correctly positioned.

To get maximum retention for mandibular denture; the maximum buccal extension of the mandibular denture in the buccinator attachment zone; utilization of retromylohyoid space & the sub lingual crescent areas with in physiologic limits will augment retention & stability

Forces of retention:

Retention begins with impression. It depends primarily on forces that produce attachment of the

denture to the mucosa. These forces are adhesion; cohesion; surfacetension Mucostatics dissmisses adhesion cohesion as factors in retention entire

phenomenon being attributed to surfacetension. Attachment of denture to mucosa is possible only when these

molecules adhere to water.

So forces of cohesion in between water molecules not there then there is no adhesion.

Stability :

Page 12: Basic Principles Impression Making

It is the ability of a denture to remain securely in place when it is subjected to horizontal movements.

To be stable a denture requires:

Good retention

Non interfering occlusion

Proper tooth arrangement

Proper form & contour of polished surfaces

Proper orientation of occlusal plane

Good control & coordination of patient musculature

Vertical height & residual ridge

occlusion

Ideally the denture should be fabricated so that all the posterior teeth have simultaneous contact in eccentric positions as well as in centric relation i.e the dentures should have bilateral balanced occlusion.

• Vertical height of residual ridge:

The residual ridge should have sufficient height to obtain good stability.

• proper tooth arrangement:

The best denture stability will be obtained if teeth are placed on or lingual to crest of the ridge.

Occlusal plane

It is mainly related to esthetics & occlusion.

Ideally best stability is obtained when occlusal

Page 13: Basic Principles Impression Making

plane is parallel to & anatomically oriented to ridges.

Patient muscle control & coordination

A well coordinated patient will usually manage if the dentures are inadequate.

Esthetics

The role of esthetics in improving impression making refers to the development of labial & buccal borders so that they are not only retentive but also support lips & cheek.

ReLation of denture to mucosa

o When the force is directed toward the base ; the base is used to support the object

A base to support an object must b e hard & unyielding possesing compressive strength.

When the force is directed away from base ; the base is used to suspend the object.

A base from which to suspend an object must be tough , possessing tensile strength.

The dead weight of the lower denture is supported by lower base where as upper is held in suspension by the upper base.

Strategy of using soft tissue

A denture may be supported , suspended or sustained by the mucosal base in one of 3 ways

A denture is supported when the force is base wise& perpendicular resulting in compressive loads.

Page 14: Basic Principles Impression Making

A denture is suspended when the force is counterwise resulting in tensile loads.

A denture is sustained when the force is base wise in one area & simultaneously counterwise in another area.

Means of attachement

The means of attaching a denture to the mucoperiosteum is by interfacial surface tension ; atmospheric pressure or both.

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

Interfacial surfacetension will resist only forces perpendicular to axes of surfacetension forces.

Attachment through atmospheric pressure is usually transient for air chambers & reliefs tend to fill in with tissue or saliva.

Mucosa contacted but not contained

The mouth tissues including the mucosa may be contacted but it can hardly be said that they can be contained

The pascals law would apply namely

Any substance no matter how fluid when enclosed in a rigid container takes on the same rigidity as that container

Pressure applied to a confined liquid is transmitted undiminished to all parts & acts in all directions.

Mucostatic school

The pages announcement in this:

Page 15: Basic Principles Impression Making

Soft tissues should be registered in an impression in unstrained rest position & any other position lead to dislodgement of denture.

A denture may move with mucosa without being detached from it.

Free movement of tissue are limited by 2 factors namely thickness & rigidity.

These are in a constant state of flux depending on fluid content & stresses applied to it .

When the denture moves under masticatory loads the function of border tissue & roll is to enable the border tissues to follow the movement & thus keep out saliva as well as air.

If adhesion fails that is if denture is detached & dislodged it is probably due to extreme torque & failure of border tissues to maintain contact.

In presence of mucosal torque a relief in the palatal area helpful in retention.

Palatal relief prevents vault from acting as a fulcrum & permits the denture to shift bodily the movement being more translatory than rotatory.

Rotation puts more strain on retention than traslation

When denture shifts bodily that is horizontally the cheek on working side comes to rescue & help to stop movement

A Mathematical approach

Rest registration of ridge areas is important in attaining stability

The horizontal stability is impossible in the presence of horizontal forces

According to Synge, the increasing in displaceability of an incompressible membrane placed between two rigid bodies is directly proportional to the cube of increase in thickness or cube of decrease in rigidity

Page 16: Basic Principles Impression Making

Maxillary & Mandibular buccal regions

The buccinator is the principle muscle of cheek.

It originates in 3 sets of fibers.

The middle fibers arise posteriorly on the pterygomandibular raphae with those of the opposing constrictor pharynges superior

They are joined in the molar regions by the upper fibers from above maxillary molars & lower fibers from below mandibular molars & all 3 sets of fibers converse as they proceed to angle of the mouth.

.

Movement of angle of mouth

Backward : middle fibers

Upward : upper fibers

Downward : lower fibers

When the maxillary tuberosity is prominent, there is but little space for the buccinator muscle to pass between the denture base and the tuberosity.

A closing movement will squeeze an impression material against the tuberosity and often mold the posterior border of the buccal flange to a more or less sharp edge.

This is interesting and important because many dentures have been shaped with too thick a disto buccal margin and a swing of the mandible to the opposite side has caused pain or dislodgment of the denture or both

Conclusion:

For achieving adequate retention, stability and support of a complete denture proper impression making is must.

Page 17: Basic Principles Impression Making

For making proper impression the clinician should have an idea about basic principles in impression making.

References

Bhanna, h.m.,: A critical analysis of the mucostatic principle. JPD 4:232, 1954.

Friedman, S. : Edentulous impression procedures for the maximum retention & stability. JPD 7:14,1957

LOTT,F., & LEVIN, B.: Flange technique: An anatomic & Physiologic approach to increased retention, function, comfort, & appearance of dentures. JPD

16:394 ,1966

o STAMOULIS,S.: physical factors affecting the retention of complete dentures

o Bernard Levin

o Principles involved in impression making Theodare E.logan DMD .

o Complete denture impression Henry A.Collett DDS jacksonville, Fla

o A Minimum pressure Complete denture impression technique Glen E. Tilton DDS

o Physical considerations in impression making . Charles H. Moser DDS MSC