IMPRESSION PROCEDURES FOR DISTAL EXTENSION BASES
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Contents INTRODUCTION DEFINITIONS RPD IMPRESSION Vs C D IMPRESSION PRIMARY IMPRESSION FINAL IMPRESSION METHODS Mc LEAN’S TECHNIQUE HINDEL’S TECHNIQUE SELECTIVE PRESSURE TECHNIQUE FUNCTIONAL RELINING TECHNIQUE FLUID WAX TECHNIQUE ALTERED CAST TECHNIQUE MODIFICATION REVIEW OF LITERATURE CONCLUSION REFERENCES
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Definitions Impression A negative likeness or copy in reverse of the surface of an object ; imprint of teeth and adjacent structures for use in dentistry. GPT – 8
Partial denture impression A negative likeness of a part or all of a partially
edentulous arch - GPT – 8
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An impression of partially edentulous arch
must record accurately the anatomic form of teeth and surrounding tissues.
Unless the cast upon which the prosthesis
is to be constructed is an exact replica of mouth, the prosthesis can‘t be expected to fit properly and accurate cast can be obtained only from an accurate impression.
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Impression trays
A receptacle in to which suitable impression material is placed to make negative likeness OR A device that is used to carry, confine and control impression material while making an impression.
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Impression trays can be classified broadly in to stock trays and custom trays Stock Trays Stock trays for partially edentulous patients may be perforated to retain the impression material or they may be constructed with a rimlock for this purpose. Another type of stock tray designed for the reversible type of hydrocolloid is water cooled trays. It contains tubes through which water can be circulated for purpose of cooling the tray. www.indiandentalacademy.com
Disadvantages: STOCK TRAY a. The peripheral borders cannot be accurately recorded. b. Considerably more bulkier than a custom tray.
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Custom impression trays: a. Peripheral borders can be precisely recorded in the impression b. Thickness of impression material can be controlled.
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C. Well fitted tray will better support the impression in the palate, then avoiding even present danger of material slumping in vital areas. Custom trays are sometimes needed for mouths that are abnormally or of unusual configuration.
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Plaster Non-elastic
Compound
Impression Materials
Waxes Impression Materials
ZnO - Eugenol
Aqueous Hydrocolloids Elastic
Agar (reversible) Alginate (irreversible) Polysulfide
Non-aqueous Elastomers
Silicones Polyether
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Condensation Addition
O’Brien Dental Materials & their Selection 1997
RPD IMPRESSION Vs
o
COMPLETE DENTURE
partial denture impression records relative soft yielding tissues (the oral mucosa) as well as a hard unyielding substance (the remaining teeth).
IMPRESSION
ď‚— The complete denture
impression records the edentulous mucosa with underlying bone only
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Removable partial denture impression need to record the teeth that are irregular in contour as well as varying in their vertical relations to occlusal plane. The chosen impression material must be capable of recording the tissue contours as accurately as possible without distortion, which occurs as impression is withdrawn.
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PRIMARY IMPRESSION Objective: To obtain an impression of all the standing teeth and denture - supporting tissues of each jaw from which study casts may be prepared.
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The purpose of the study casts are:
To enable special trays and occlusion rims to be constructed if necessary. To examine the occlusion in detail on an articulator. By use of a surveyor, to plan the path of insertion of the proposed denture, arrive at a tentative design and plan any mouth preparation.
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Factors Influencing Support of Distal Extension Base
Quality of Soft tissue covering edentulous ridge Type of bone making up denture-bearing area Design of partial denture Amount of tissue coverage of denture base Amount of occlusal forces Denture bearing area Fit of the denture base
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Impression Methods:
There are basically two dual impression techniques. The physiologic impression techniques that discussed are as
follows:
Mc Lean’s and Hindel’s methods, the functional relining method, and the fluid wax method.
Selected pressure impression
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The need for physiologic impressions was first recognized by
McLean
For this dual impression a custom impression tray was
constructed over a preliminary cast of the arch
A function impression of the distal extension ridge was
made, and then hydrocolloid impression was made with the first impression held in its functional position with finger pressure
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o The greatest weakness of the technique was that finger
pressure could not produce the same functional displacement of the tissue that biting force produced.
o Many variations of this technique have been developed and
advocated, but all require some form of finger loading pressure as the second impression is made.
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• Hindels and other developed irreversible
hydrocolloid trays for the second impression that were provided with holes so that finger pressure could be applied through the tray as the hydrocolloid impression was made.
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o The main change that Hindels introduced
to McLean ‘s original technique was that
o The impression of the edentulous ridge was
not made under pressure but was an anatomic impression of the ridge at rest made with a free flowing zinc oxide eugenol paste.
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o As the hydrocolloid second impression was being
made, finger pressure was applied through the holes in the tray to the anatomic impression.
o The pressure had to be maintained until the
alginate was completely set.
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The main purpose of these techniques was to relate an impression of the edentulous ridge to the teeth under a form of functional loading.
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o A disadvantage of these techniques was that if the action of
the retentive clasps of the partial denture is sufficient to maintain the denture base in relation to the soft tissues in the displaced or functional form,
o Interruption of blood circulation would ensue, with possible
adverse soft tissue reaction and resorption of the underlying bone.
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o
Most methods of obtaining a physiologic impression for support of a distal extension denture base accomplish the impression procedure before completion of the denture, usually following the construction of the framework.
o
It is possible, however, to obtain the same results after the partial denture has been completed.
o
The technique is referred to as a functional reline. It consists of adding a new surface to the inner, or tissue, side of the denture base.
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o The procedure may be accomplished before the insertion of
the partial denture, or it may be done at a later date because of bone resorption, the denture base no longer fits the ridge adequately.
o Although the functional reline has many advantages, and for
correcting the fit of denture base that has been worn for a period of time is essential, it does present many difficulties.
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o The main problems that arise are caused by failure to
maintain the correct relationship between the framework and the abutment teeth during the impression procedure and failure to maintain accurate occlusal contact following the reline.
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o To allow room for the impression material between the denture
base and the ridge, space must be provided.
o One of the most accurate methods of ensuring uniform space
for the impression is to adapt a soft metal spacer over the ridge on the cast before processing the denture base.
o After processing, the metal is removed leaving an even space
between the base and the edentulous ridge.
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The patient must maintain the mouth in a partially open position while the border molding and impression are being accomplished because: 1.The border tissues, cheek, and tongue are thus best controlled and
2.The relationship between the partial denture frame work and th teeth must be observed.
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The functional reline method has the advantage that the amount of soft tissue displacement can be controlled by the amount of relief given to the modeling plastic before the final impression is made. The greater the relief the less will be the tissue displacement.
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The fluid wax impression may be used to make a reline impression for an existing partial denture or to correct the distal extension edentulous ridge portion of the original master cast.
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OBJECTIVES To obtain maximum extension of the peripheral borders of the denture base while not interfering with the function of movable border tissues. To record the stress bearing areas of the ridges in their functional form. To record non pressure bearing areas in their anatomic form.
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The fluid wax impression is made with the open mouth technique so that there is less danger of over displacement of ridge tissue by occlusal or vertical forces.
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The term fluid wax is used to denote waxes that are firm at room temperature and have the ability to flow at mouth temperature.
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The most frequently used fluid waxes are Iowa wax, developed by Dr.Smith at the University of Iowa, and Korrecta Wax No. 4, developed by Dr. 0. C. Applegate and S. G Applegate at the Universities of Michigan and Detroit, respectively. Korrecta wax no. 4 is slightly more fluid than Iowa wax.
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The key to the use of fluid wax lies in two areas: space and time. Space refers to the amount of relief provided between the impression tray and the edentulous ridge. :1 to 2 mm is desired. Each time the tray is introduced into the mouth, it must remain in place 5 to 7 minutes to allow the wax to flow and to prevent buildup of pressure under the tray with resulting distortion or displacement of the tissue.
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o The clinical technique for the use of the fluid wax calls
for the water bath maintained at 51째 to 54째 C into which a container of the wax is placed.
o At this temperature the wax becomes fluid. The wax is
painted on the tissue side of the impression tray with a brush.
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The borders must be short of all movable tissue, but not more than 2 mm short because the fluid wax does not have sufficient strength to support itself beyond that distance.
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Inaccuracies will develop if the wax is extended beyond that length. Originally a harder wax, Korrecta Wax no:1 was used to support the softer No.4 wax if extension beyond that length was needed. The no.1 wax however, is no longer available.
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o The wax is painted on the surface of the tray to a depth
slightly greater than the amount of relief provided. The tray is seated in the mouth.
o The patients must remain with the mouth approximately half
open for about 5 minutes.
o The tray is removed, and the wax examined for evidence of
tissue contact. Where tissue contact is present the wax surface will be dull.
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o
If needed additional wax is painted on those areas not in contact with the tissue. The tray must remain in the mouth a minimum of 5 minutes after each addition of wax.
o The peripheral extensions are developed by tissue movements
by the patient.
o For the buccal and distobuccal extension in a mandibular
impression the patient must move to a wide- open-mouth position.
o This will activate the buccinator muscle and pterygomandibular
raphe and produce the desired border anatomy.
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o For the proper lingual extension for a mandibular
impression the patient must thrust the tongue into the cheek opposite the side of the arch being border molded.
o The distolingual extension is obtained by having the patient
press the tongue forward against the lingual surface of the anterior teeth.
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These movements must be repeated a number of times after the impression has been in the mouth long enough for the wax to have softened sufficiently to flow.
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o When the impression evidences complete tissue
contact and when the anatomy of the limiting border structure is evident, the impression should be replaced in the mouth for 12 minutes.
o This final time to be certain that the wax has
completely flowed and released any pressure that may be present.
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The finished impression must be handled carefully and the new cast poured as soon as possible because the wax is fragile and subject to distortion.
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The fluid wax impression technique can produce an accurate impression if the technique is properly executed The procedure is time consuming, but if the time periods are not followed accurately, an impression with excessive tissue displacement will result.
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Selective Pressure technique
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ď‚— This impression technique attempts to direct more
force to those portions of the ridge able to absorb the stress without adverse response
ď‚— Tissue surface of the tray is selectively relieved .
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Impression materials ď‚— Zinc-oxide Eugenol paste ď‚— Rubber base materials
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Impression technique Border molding Making impression with the ZOE or Rubber base
materials The critical point is to determine visually that all rest and indirect retainers are completely seated
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Corrected cast
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Technique 1. Fashioning custom acrylic resin impression tray to retention lattice work of removable partial denture. 2. Developing denture base impression on these trays. 3. Removing edentulous ridge from master cast. 4. Securing framework with developed bases to master cast. 5. Pouring the impression with dental stone.
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