impression making-theories and techniques in complete denture

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IMPRESSION MAKING-THEORIES AND TECHNIQUES

Dr.Priyanka MakkarP.G. 2nd Year

Dept of Prosthodontics

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INTRODUCTION

• Complete denture impression procedures are perhaps one phase on which much has been spoken about. The literature on the subject shows a persistent disagreement ever since 1850.

• Much of this confusion results from the fact that many impression procedures have been developed on empirical basis.

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• Many have used the available knowledge of functional and histological anatomy for the development of their procedures, but the variation in these techniques indicate a wide difference in interpretation of the foundation of dentures.

• Whatever the method used it is generally agreed that good impressions are basic for the construction of a good denture.

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“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”

- M.M. Devan

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DEFINITIONS

IMPRESSION A negative likeness 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

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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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FINAL IMPRESSION: A final impression is an impression for making the

master cast .

IMPRESSION MATERIAL: Any substance or combination of substances used for

making an impression or negative reproduction. -gpt 8

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LITERATURE REVIEW

• Before the middle of the 18th century, no method was available for producing an impression of the alveolar ridge. A widely used method at that time was the painting of the ridge with a dye , and the pressing of a block of ivory or bone against the dyed surfaces.

• Areas of contacts were scraped away from the block until the best fit for the prosthesis was achieved

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• 1756 – beeswax was apparently first used in making impressions in the mouth.

Philip Pfaff(Berlin) made a sectional wax impression of half of an arch at a time.

• 1782 -William Rae said that “he got the measurement of the jaws in a piece of wax pushed into the gum, afterwards making a cast of it with plaster of paris”

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• 1840 - Charles De loude (london) made one of the earliest refrences to impression trays . He wrote

“for impressions, I use wax in tin cups or shapes, the whole size of the upper and lower jaws, right or left, half jaws and fronts.”

• 1842- Montgomery discovered gutta percha. It was obtained from various sapotaceous trees in Malaysia. It was introduced as an impression material in 1848 by Colburn.

Colburn said it should be thoroughly soaked in boiling water,

kneaded and moulded in the same way as wax and …. immediately place in the mouth, and firmly pressed to the jaws.

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• 1844- Plaster of paris• Wescott, Dwinelle and Dunning have been credited

with this discovery.

• 1847 - Desirabode referred to an impression tray as“ we place wax in a box, a kind of semi elliptical gutter of

tin or silver, upon the anterior part of which is a shaft which forms a handle. The walls of this receptacle offering some resistance, opposite the deformation of the wax”.

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• 1862 Franklin described the first correct impression . He used

wax for the preliminary impression followed by a plaster wash.

• 1870 Wescott described a similar technique using oversized

wax trays made by scooping out primary impressions

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• 1874 - Use of impression compounds dates from contributions of J.W.Greene, P.T.Greene and of Rubert Hall.

• The Greene brothers (about 1900) introduced a modeling plastic , a method to manipulate it , and a technique that is said to have been the first to utilize all the surfaces of a mouth to advantage for denture retention. They were probably the first to describe the closed mouth all modeling plastic technique in detail.

Furthermore, they were the first to use the word “ post dam” in describing posterior palatal seal.

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• 1915 - Rupert hall , perfected the first moderate heat modeling plastic for making individual trays and introduced the correctable modeling plastic-plaster technique that became a standard method for making an impression.

• Hall used a specially prepared hard black modeling plastic for making a custom tray in which a very thin mix of impression plaster was placed for correction.

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• 1922- Everett described an early wax technique around this time. He used fluid wax compound of three consistencies: hard, medium and soft.

He said “ in every way possible represents the three general tissues of the mouth on the bone”

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• 1925- Alphous Poller (Vienna) described his elastic material for “molding articles of all kinds, more particularly, parts of a living body.

• He was most likely the first to suggest the use of agar for dental impressions.

• Booth, however, described complete denture impression technique using agar but found it necessary to build custom water cooled trays and to pre-medicate the patient with a drug to reduce salivation.

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• During late 1920’s the idea became widely held that uniform tissue support may be of value. It was believed that this would be attained by controlled placement of soft tissues.

• During this period, the first true functional impression wax was developed. The waxes before this (beeswax and parafinn wax) were far from ideal because they were hard, flowed too slowly, or were crumbly.

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• 1930 - According to Applegate a series of true physiologic waxes was developed by the cooperative effort of G.C.Bowles , S.G.Applegate and himself and was made available in 1935.

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• Early 1930’sFirst real impetus in use of zinc oxide eugenol for

impression materials came From A.W.Ward and E.B.Kelly.

Ward’s preparation was intended more so for a surgical pack , but he also said that it could also be used as lining for dentures as an impression material .

Kelly’s preparation was primarily intended as an impression material

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1938Harry.L.Page introduced the mucostatic concept.

The advocates of this concept such as Page, Albinson, Dykin and Addison thought the universally accepted concept involving compression of soft tissues and relief of the hard areas was in error because hydrostatics proved that human tissues was not amenable to either of this condition.

They believed that the impression should be an absolute accurate negative of the ridge tissues at rest.

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• 1939 Dirksen reported the findings of his research in IOWA

which resulted in the development of still another physiologic impression wax. Over the years functional waxes have grown steadily in popularity and many clinicians have suggested methods for their use.– Applegate for immediate dentures– Mc. Cracken, borkin, and faber for mandibular

complete dentures – Hardy , ostrem and schultz for complete denture

reline procedures

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• 1939 Trapozzano described one of the early techniques

using Zinc oxide eugenol paste. Compound preliminary impressions were made in stock trays and plaster of paris casts were poured. Vulcanite or shellac bases were constructed, on which occlusion rims of wax or compound were placed. After a tentative vertical and centric relation was established , the final corrective impression was made using closed mouth technique.

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• 1942- Pendleton suggested a fluid wax technique using asiatic or indian paraffin fro the final mandibular impression

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• Wright and Denen suggested using alginate in a border molded perforated customized acrylic tray.

• Collett described an alginate technique for the maxillary impression using the material as a wash in a modeling paste preliminary impression .

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Middle 1950’sElastomeric impression materials were introduced.

They were of two chemical types.– Polysulfides– Silicone base

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• Pierson in 1955 reported on a new elastic material of a polysulfide base (thiokol).

Shortly there after silicone base materials were introduced

elastomeric impression materials were intended primarily for making impressions for

• inlays • Crown • And for fixed partial dentures

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• Chase in 1961 first described the moldable acrylic material used for tissue conditioning and for functional(dynamic) impression for complete dentures.

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According to Emmett Beckley in 1973 , the first moldable acrylic material consisting of an ethyl methacrylate and an ethinol liquid was developed by Clark Smith and he (Beckley) performed the first practical research with this material in complete denture construction.

MUCOUS MEMBRANE

• The bones of the upper and lower edentulous jaws are covered with soft tissue, and the oral cavity is lined with soft tissue known as mucous membrane.

• The denture bases rest on the mucous membrane, which serves as a cushion between the bases and the supporting bone.

Structure of Oral Mucosa

• The mucous membrane is composed of two layers

– Mucosa

– Submucosa

The mucosa is formed by the stratified squamous epithelium and a subjacent layer of connective tissue known as the lamina propria.

• The submucosa is formed by connective tissue.

It may contain glandular , fat , or muscle cells and

transmits the blood and nerve supply to mucosa.

• The thickness and consistency of submucosa are largely responsible for the support that the soft tissue affords the denture, since in most instances the submucosa makes up the bulk of the mucous membrane.

• In a healthy mouth the submucosa is firmly attached to the periosteum of the underlying bone of the residual ridge and will usually successfully withstand the pressure of the denture.

CLASSIFICATION OF ORAL MUCOSA:

•The oral mucosa is divided in three catogories depending on its location in the mouth

•Masticatory mucosa-25%•Lining mucosa-60%•Specialized mucosa-15%

The Masticatory mucosa covers the crest of the ridge

The residual attached gingiva firmly adherent to the

supporting bone

• Hard palate

It is characterized by a well defined keratinized layer on its

outermost surface subject to changes in thickness

The specialized mucosa covers the dorsal surface of the tongue. This mucosal covering is keratinized

The Lining mucosa - nonkeratinized layer

Vestibular spaces

Alveolingual sulcus

Soft palate

Ventral surface of the tongue

Unattached gingiva found on slopes of residual

ridge.

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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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MUCOCOMPRESSIVE IMPRESSION TECHNIQUE

• Also known as definite pressure impressions.

• Because denture retention is tested most during mastication, many dentists formerly considered it essential for the tissues to remain in contact with the denture during chewing.

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• It was logical to them to make impressions that would press the tissues in the same manner as chewing forces, thus ensuring contact during chewing stroke.

• However, dentures made from such impressions did not fit well at rest, because tissues so distorted tend to rebound.

• Furthermore, these abused tissues will not be able to long maintain the shape that they assumed on the day of impression.

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• Many of the proponents of pressure impressions advocate the use of closed mouth techniques.

• But closed mouth technique do not allow for adequate muscle trimming of the periphery.

• Very often dentures made with closed mouth technique are over-extended and must be arbitrarily trimmed.

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• The materials used for this technique include impression compound, waxes and soft liners.

Mucocompressive Technique

• ADVANTAGE: Good retention during function.


• The type of the sub mucosa & the relation of the supporting bone to the denture bases show best to record the soft tissues.

• The oral mucosa with a tightly attached sub mucosa covers the crest & slopes of the residual alveolar ridges & anterior 2/3rd of the palate. When this type of mucosa is displaced in an impression & a denture is constructed on a cast made from this impression & the denture is seated, the tissues will attempt to return to its undisplaced position.


• This effort of the tissues to return to its undisplaced position creates objectionable forces that produce pressure to the supporting bone & dislodging pressure against the denture.

It is not desirable to record this type of tissue in a displaced position.

Disadvantages of Mucocompressive Technique

• 1 : Dentures made from such impression do not fit well at rest ‘coz tissues so distorted tend to rebound to its former contour. Dentures will fit well during mastication, and will lift up at rest due to tissue rebound. This results in premature contacts.


• 2 : Pressure is sufficient to interfere with the blood supply to the tissues of basal seat & eventually cause resorption of the residual ridge.

Due to constant pressure on the tissues, mucosal tissue reaction is seen.


• 3 : Dentures are in occlusal contact for only a relatively short period of time & the constant pressure even at rest , even if equal may overstress the tissues.

The total time during 24 hours associated with directs functional occlusal force application to periodontal tissue is 17.5 minutes .


• 4 : Closed mouth technique does not allow for adequate muscle trimming of the periphery. Dentures made are often overextended & must be arbitrarily trimmed.

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MUCOSTATIC IMPRESSION TECHNIQUE

• Also known as minimal pressure impressions.

• Addison in 1944 described this technique and attributed this to Page.

• The main point of the mucostatic principle concerned Pascal’s law, which states that which states that pressure on a confined liquid will be transmitted through the liquid in all directions.

Pascal’s Law• The pressure

applied on the confined liquid will be equally transmitted undiminished throughout the liquid in all directions.

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• According to this concept, the mucosa being more than 80 percent water, will react like a liquid in a closed vessel and thus cannot be compressed.

• According to the principle of mucostatics, the impression material should record without distortion, every detail of the mucosa so that the completed denture would fit all minute elevations and depressions.

• Mucostatics further demanded that a metal base be used rather than the dimensionally stable scrylics.

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• Most important of all, the mucosal topography is not static over a 24 hour period.

• There is a difference between the mucosal contours just after rising in morning, and that which exist after 12 hours in the upright position.

• So it would appear that all the infinite details achieved in the impression would be altered by the time the denture was finished.

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• The adherents of the mucostatic principle considered interfacial surface tension as the only important retentive mechanism in complete dentures.

• The mucostatic principle ignores the value of dissipating masticatory forces over the large possible basal seat area.

• If for eg, patient could develop the masticatory force of 30lb, it is evident that larger the basal seat area ,the less force will be exerted on each sq millimeter of underlying mucosa.

Mucostatic ImpressionTechnique

• ADVANTAGES:

1:Tissue health is preserved and maintained.

2: Suitable to areas where the residual ridges are sharp, thin & flat flabby ridges.

Disadvantages Of Mucostatic Technique

• 1: Inadequate support : Ridge tissues are not uniformly displaceable & a base made from a mucostatic impression will result in the firmer areas bear greatest part of pressure & the more displaceable areas giving little (less) support. This condition is undesirable from viewpoint of bone preservation & comfort.

Disadvantages of Mucostatic Technique

• 2 : Lack of Peripheral Seal: The impressions made by mucostatic technique does not displaces even the soft tissues at the borders. This theory would eliminate any possibility of border seal & result in absence of secondary retention which in many instances is even greater than the primary retention.

Disadvantages of Mucostatic Technique

• 3 : SHORT FLANGE LENGTH : The impressions made with non-pressure technique were significantly under extended. The flanges of the dentures are shorter. Short flanges do not support the lips and cheeks.

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Selective pressure impressions• It is an impression technique that combines pressure over

certain areas and little pressure over others.

• The technique utilizes a preliminary compound impression that is generously relieved over the midline and incisive papilla areas.

• The final impression is taken in plaster , which acts as a wash and also records the relieved areas with minimal pressure while the ridge areas are undergoing considerable presssure.

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• Thus the papilla and midline sections of the denture will not make contact with the mucosa when the denture is not in function, but by the same token, they will not bear heavily when the patient is chewing.

• This principle of impression making is based on the belief that the mucosa over the ridge is best able to withstand pressure ,whereas covering the midline is thin and contains very little submucosal tissue.(Boucher,1951)

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.

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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.

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Advantage

Preferred because the operator can see whether muscle trimming is done properly

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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Advantage• Saving of time

Disadvantage• Appointment time may fatiguing the dentist and

patient • Tendency for overextensions• Problem of limited space between the tuberosity

and pear shaped pad• No control over the amount of pressure during the

final impressions• Soft tissues – displaced- rebound • bone resoption

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Depending on the tray type

Stock tray

Custom tray

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Stock Trays:-

Beginning of good impression starts with the selection of the correct stock tray.

Trays used for primary impression making are called stock trays. These are factory made and available in various sizes.

They are made of metal or plastic, can be perforated or non perforated.

An appropriate stock tray should be selected for each patient.

Various Trays For Making Impression

Points to be considered during Tray Selection

There should be at least 2 – 3 mm clearance between the stock tray and the ridge. It should have 5 – 6 mm clearance for impression compound.

With the stock tray in position in the mouth, the handle of the tray is tilted downwards and the posterior border of the tray is observed. The tray should extend over the tuberosity and the hamular notch.

Points to be considered during Tray Selection

• The tray should be neither too large nor too small. In both cases a distorted impression will result.

• If the tray is too large, it will distort the border tissues by pulling them away from the bone.

• If the tray is too small, the border tissues will collapse inwards towards the residual ridge thus reducing support for the denture.

Selection of Tray for Maxillary Arch

• After examining the alveolar ridges & palate for shape & size, suitable upper stock tray is chosen & inserted in the mouth.

• The posterior border of the tray is raised to make contact with the anterior part of the soft palate. It must cover the maxillary tuberosity

• The tray is then slowly raised anteriorly & lateral flanges watched for clearance of the alveolar ridges.

PRIMARY ( PRELIMINARY ) IMPRESSION

The ideal relationship of a stock tray to the sulcus and the denture bearing mucosa

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CUSTOM TRAYS

• They are of two types:

Spaced trays

Close fitting trays

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Spaced Tray

• In order to avoid permanent distortion of an elastic impression material as it is withdrawn from the undercut areas, an adequate thickness of the material is required.

• The special tray should be constructed on the preliminary cast after a spacer of appropriate thickness for the planned impression material has been adopted to it.

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• Alginate is the most commonly used elastic impression material for edentulous patients and this requires the spacer of about 3mm.

• The elastomers have a better elastic recovery than alginate and so require less spacing of the special tray.

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Close-fitting Trays

• Are used with impression materials that are used in the section such as zinc oxide-eugenol impression paste and light-bodied elastomers.

• It is an advantage if a lower acrylic close-fitting tray has vertical pillars in the premolar regions to act as finger rests.

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• These rests keep the fingers, which stabilise the tray and support the impression, well clear of critical border areas of the impression when it sets.

• If this is not done, inaccuracies will result from fingers restricting the border molding movements of the soft tissues.

• They can also displace excess material into the sulci.

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• The anterior stub handle is for holding and manipulating the tray.

• Its shape avoids interference with the lower lip which otherwise can make placement of the tray difficult and can hinder border trimming of the impression in that area.

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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)

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, conform or configure 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.

SUMMARY• Most of the difficulties encountered when making

impressions can be traced to the operator’s lack of attention to details of technique, and especially the acceptance of a poor stock tray impression.

• It is of extreme importance that the preliminary impression records the entire possible denture-bearing surface but, at the same time, does not encroach on movable muscular tissues.

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impression making-theories and techniques in complete denture

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