Impression for distal extension bases /certified fixed orthodontic courses by Indian dental academy

IMPRESSION PROCEDURES
FOR DISTAL EXTENSION
BASES

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INDIAN DENTAL ACADEMY
Leader in continuing dental education


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


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


 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.


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.


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.

Disadvantages: STOCK TRAY
a. The peripheral borders cannot be
accurately recorded.
b. Considerably more bulkier than a custom
tray.


Custom impression trays:
a. Peripheral borders can be precisely recorded in
the impression
b. Thickness of impression material can be
controlled.


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.


Plaster
Non-elastic

Compound

Impression
Materials

Waxes
Impression
Materials

ZnO - Eugenol

Aqueous
Hydrocolloids
Elastic

Agar (reversible)
Alginate (irreversible)
Polysulfide

Non-aqueous
Elastomers

Silicones
Polyether


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

The complete denture

impression records the
edentulous mucosa with
underlying bone only


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.


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.


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.


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


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


 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


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.


• 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.


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.


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.


The main purpose of these techniques was to relate an
impression of the edentulous ridge to the teeth under
a form of functional loading.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


The term fluid wax is used to denote waxes that are
firm at room temperature and have the ability to flow
at mouth temperature.


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.


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.


o The clinical technique for the use of the fluid wax calls

for the water bath maintained at 51° to 54° 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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


Selective Pressure technique


This impression technique attempts to direct more

force to those portions of the ridge able to absorb the
stress without adverse response

Tissue surface of the tray is selectively relieved .


Impression materials
Zinc-oxide Eugenol paste
Rubber base materials


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


Corrected cast


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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Impression for distal extension bases /certified fixed orthodontic courses by Indian dental academy

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