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Posterior palatal seal
(post damming)
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics
Posterior palatal seal
(post damming)
1- The posterior palatal seal (post dam) of the upper denture
should be placed on non-movable tissue of the soft palate
just behind the hard palate.
2- The vibrating line of the soft palate , normally used as a
guide to the ideal posterior border of the denture, is usually
located slightly posterior to the foveae palatine.
3- The posterior palatal seal is formed through both humular
(pterygomaxillary) notch and across the palate over the
vibrating line.
4- The vibrating line is observed in patient's mouth as the
patient says a series of short "ah" and the hamular notches
are palpated.
Techniques of post-damming
There are several established for the
placement of the posterior palatal seal:
1- Conventional method
2- Scraping of master cast
3- Post damming during try-in stage
1- Conventional method
A strip of low fusing compound is traced on the
impression over the vibrating line and through the
hamular notches. The compound is heated with
alcohol torch, tempered and seated in the mouth
under pressure. The added material will spread out
on both sides of the vibrating line and form a
raised strip across the distal end of the impression.
The final impression with the posterior border seal
is carefully boxed and poured in stone.
2- Scraping of master cast
A –this technique is the least accurate and leaves the
most to chance of tissue compressibility of the
insertion of the denture.
B – This technique is almost as unphysiologically
correct as the technician's attempt to place the
posterior palatal seal. So post damming must be
done at the impression stage rather than by
attempting the scrap the cast.
3- Post damming during try-in stage
The trial denture base is inserted so the
indelible pencil line marked on vibrating
line of the soft palate will be transferred
from the soft palate to the trial denture base.
And the excess base plate is reduced to this
line. The trial denture base is placed on the
cast and a knife or pencil is used to mark a
line following the posterior limits of the
base plate.
Dimensions of the post dam
1- The post dam extends from the humular
notch on one side to the other humular
notch of the other side. It should be wide
enough to avoid cutting or irritating the soft
tissues, but not so wide to compress too
large an area and thus prevent the denture
from sealing properly.
2- The post dam is usually narrow in it's central part,
wider as it extends laterally on each side, and
narrow again as it approaches the hamular notch to
fade out behind the tuberosity. It is sometimes
called butterfly (Cupid's bow) post dam
3- The post dam should be about 3-4mm wide in its
widest part. The depth or thickness of the post
dam should vary in different individuals,
according to compressibility and softness of the
tissue. Its depth should also vary in different parts
of the same mouth in exactly the same manner as
its width. The average thickness is 1mm.
Function of post damming
1- during impression making:
The rational for the placement of a post dam in the
impression tray is:
A – To establish contact posteriorly to prevent the
final impression material from sliding down in to
the pharynx.
B – To serve a guide for positioning the impression
tray, especially if a shim has been used within the
tray to establish the borders.
2- during jaw relationship:
1- Posterior palatal seal is incorporated into
the trial denture base for added retention.
2- To determine if adequate retention and seal
of the potential denture border is present.
3-Post dam with finished denture:
1-it slightly displaced the soft tissue of the
distal end of the denture to inhance the
posterior border seal:
A- Increase retention of the denture by
atmospheric pressure.
B –it prevents air and food from getting under
the denture.
2- it makes the thickness of the base less
conspicuous to the tongue and diminishes
reflex irritation.
Boxing-in the impressions and
making the casts
Def:
Boxing in an impression is the process of
building up of vertical walls, usually in wax
developing around the final impression for
complete dentures to produce the desired
size and form of the base of the cast, and to
preserve certain landmarks of the
impressions.
Technique
1- A strip of boxing wax is attached all the way
around the outside of the impression
approximately 1 to 2mm below the border and
seated to it with a spatula.
2- The strip must be at its full width, particularly at
the distal ends of the impression, to hold the
vertical wall of the boxing away from the
impressions and provide space for adequate
thickness of the cast in these regions.
3-the vertical walls of the boxing are made of
sheets of beeswax.
4- The tongue space in the lower impression is
with a sheet of beeswax or plasticine that is
fitted and attached on the superior surface
of the boxing wax. The beeswax tongue
space filler is seated to the boxing wax. And
located just below the lingual border.
5- A thin sheet of wax is used for making the vertical walls of
the boxing. This may be special boxing wax. Or half sheet
of beeswax may be cut lengthwise and used as a boxing
wax. It is attached around the outside of the boxing strip so
as not to alter the borders of the impression.
It
should extend 9 to 15mm above the impression so the base
of the cast at its narrowest point will be of this thickness.
6- The sheet of boxing wax should extend
completely around the impression and be seated to the
boxing wax strip to prevent the escape of stone when this
is poured in to the impression. Sufficient space must be
available posteriorly between the impression and boxing to
provide for adequate thickness of casts distal to the
impression.
7- A stone is mixed carefully and vibrated, and
then sufficient stone is poured in to the boxed
impression that the base of the cast will be 9 to
15mm thick. The stone is allowed to harden for at
least 30 minutes before impression. After the final
impression is separated from the cast, the borders
of the cast are trimmed to leave a ledge of about
3mm posteriorly and little anteriorly. The cast
must be shaped to maintain the form of the
borders of the impression and yet be easily
accessible for adaptation of the materials used in
making the record blockes.
Advantages of boxing –in
1- The correct width of the periphery of the
impression is preserved.
2-the mixed stone can be vibrated, the cast will not
contain air bubbles and a stronger cast will be
produced.
3- The thickness of the cast may be more
accurately judged.
4- boxing-in is time saving, because trimming may
be not be required.
5- Material is economized.
Recording bases and occlusion rim
Def: the recording base (trial denture base) is
a temporary form that closely resembles the
final base of the denture under construction.
It is used for recording maxillo-mandibular
jaw relationships and for setting the
artificial teeth.
Function of recording base
1- Establishment of the rest and occlusal
vertical dimensions.
2- Determination and recording of centric
relation
3- The transfer of the accurate jaw
relationships to an articulator
4- Enabling the setting of artificial teeth for
the trial denture.
Requirements for recording base
1- Well adapted and accurately formed to the final cast.
2- Stable, both on the cast and in the mouth.
3- Rigid and strong.
4- Smooth and rounded to be comfortable to the
patient.
5- Easily contoured and polished.
6- Fabricated from materials that are dimensionally stable.
7-the materials used should be easy to manipulate.
8- Base plate materials should be non-irritant to the mouth
tissues.
Materials for recording bases
I-Temporary recording bases: the materials that are most frequently used are:
1- Shellac base plate wax.
2- cold-curing acrylic resin.
3-vacuum formed vinyl or polystyrene.
4-base plate wax.
5-swaged tin base plate.
N.b these materials are used during the various technical procedures, but discarded
at the time the denture is processed in to its permanent form.
II-permanent denture bases:
Is one, which eventually becomes the base of the finished denture:
1- Processed (heat-cured) acrylic resin.
2-cold
3-chromium – cobalt alloys.
4-chromium – nickel alloys.
The material used should meet the
following criteria as much as
possible
1- Readily adapted to the required shape and
contours with minimum of time, expense
technical skill.
and
2-rigid and strong in relatively thin sections.
3-not exhibit flow at mouth temperature.
4-not warp or distort appreciably during the
procedures required for denture fabrication.
5-exhibit a color that will not distract from viewing
the arrangement of the teeth of the trial denture, as
they will appear in the complete denture.
I-Temporary recording bases
1- Shellac recording base material:
1- Shellac is commonly used material for
recording base.
2-it is supplied in forms shaped to correspond
to the general shapes of the maxillary and
mandibular arches.
3-if adapted accurately strengthened and handled
carefully, it can be effectively utilized both for
maxillary and mandibular recording base. If not
adequately strengthened, shellac tends to warp
when subjected to repeated changes in
temperature.
Wires should be used to increase strength rigidity
and thus reduce distortion of shellac base. For the
maxillary cast, the wire is placed across the
posterior palatal seal area, while for the
mandibular cast it is adapted within the lingual
flange.
Fabrication of shellac recording
base
1- The proper shellac form, corresponding to
either the maxillary or the mandibular arch,
is selected for the final cast.
2-all undercut must be blocked out prior to
adapting the base.
3-to prevent the base material from sticking to the
cast,
A- It is first dusted with talcum powder
B-soaked in water for a short period of time until
the surface of the cast is moist: immersion of the
cast in water for a prolonged period of time
many damage the surface of the cast.
C-tin foil (0.001 inch) can be adapted to the cast.
4-soften the shellac until it appears shiny and
applied with wet fingers to accurately adapted the
shellac to the palatal portion of the maxillary cast
or to the lingual surface of mandibular cast.
5-the material is then reheated and adapted over the crest of
the ridge and into the reflections. Care must be taken to
avoid trapping air between the shellac form and cast
surface.
6-while the material is still warm and soft; it is removed from
the cast and trimmed with scissors, leaving approximately
5mm beyond the edge of the cast.
7-the shellac is repositioned and reheated, and then
carefully readapted. The trimmed edges are heated,
elevated from the cast, and folded onto themselves and
burnished with wax spatula to form a smooth rounded
border.
8-care must be taken not to overheat, the shellac overheating
will cause the molten shellac to adhere to the surface of the
cast on cooling attempt to remove the shellac base can
result in a fracture of the cast surface.
Advantages of shellac recording base:
1- It can be easily and quickly adapted
2-inexpensive
Disadvantages:
Shellac being a brittle material, it is subjected
to breakage.
2- autopolymerizing resin recording
base materials
Fabrication of autopolymerizing resin recording
base: three basic techniques are used to fabricate
recording base utilizing chemically activated
resins:
A-no-flasking method:
1-a layer of tin foil (o.oo1 inch) or a thin layer of
petrolatum is applied to the prepared final cast.
2-all undercuts are blocked out with wax or and the
suitable plastic material.
3-The autopolymerizing resin is mixed, when it
reaches the doughy stage, it is rolled to the
desired thickness 2 to 3mm.
4-a thin film of petrolatum must be applied to
prevent the resin from adhering. A thin
application of petrolatum to the fingers will
prevent the resin from sticking to the hands.
Keeping the fingers wet with water during
handling will also prevent this difficulty.
5-the resin sheet is transferred to the cast, adapted
to the hard plate area first or to the lingual surface
of the mandibular cast and then onto the crest of
the ridge and into the reflection area.
6-a sharp instrument is used to trim the excess resin
while it is in the soft, moldable state.
7-after polymerization has been completed; the
resin base is removed and trimmed. Soaking the
cast in water will help removable of the base if
difficulty is encounted.
8-the borders are further adjusted with burs, and the
external surface can be polished with wet pumic.
The thickness on the facial slope of the ridge and
extending over the crest of the ridge is reduced to
approximately 1mm.
B- Alternating application of coldcuring powder
(sprinkle-on technique).
1-well-adapted bases can be formed using this
technique.
2-undercuts are blocked out and tin foil or
petrolatum is applied to the cast.
3-polymerization shrinkage is kept to a minimum.
A thin layer of powder (polymer) is dusted over a
small surface area of the cast and sufficiently
wetted with liquid (monomer) to produce a
slight flow.
4-alternative applications of powder and liquid are
made until a thickness of 2 to 3mm has been
developed
5-the complete base is then removed, trimmed and
polished. The polymerization reaction takes
approximately 20 to 30 minutes; the bases should
not be removed during this period in order to
prevent distortion.
C-flasking method
1-recording bases produced from this method
are accurate and stable. Thy do, however,
require considerable time for fabrication
and therefore are most costly them bases
formed using shellac or autopolymerizing
resin.
2-since breakage of the master cast is possible with
this technique, it is advisable to duplicate the cast
and to construct the recording base on the
duplicated model.
3-An autopolymerizing resin is mixed in glass jar
and then covered .when the resin reaches the
doughy stage ,its placed into the model , the
flask is closed, and the resin is allowed to
polymerize for 20 to 30 minutes . The base is
removed from the flask, trimmed, and
polished. If there are a number of undercuts
present that will, interfere with seating the
base on the master cast, they must be
relieved before seating is attempted
Advantage of resin recording base:
1-acrylic resin base plates are excellent for making
maxillo-mandibular relation records.
2-They fit accurately and are not easily distorted
3-any type of occlusal rims can be mounted on it
Disadvantages:
1- They may take up space needed for setting the
teeth, necessitating some grinding of the resin
base in required areas.
2-they may be loose because of the necessary block
out of undercuts in the cast
3-vacuum-formed bases
1-the vacuum method provides a fast and efficient
means of forming rigid, accurate fitting recording
base. Depending on the material used, temporary
or permanent bases can be formed. Permanent
bases formed using this method have not gained
wide acceptaptance.
2-the cast is prepared by blocking out the existing
undercuts with a suitable material (wax or any
other material that melts during heating should
no be used
3-a sheet of base plate resin is inserted in the frame
located below the electric heater coil, and the
heater is activated. Heating is contained until the
resin sheet begins to sag approximately 1\2 inch.
At this time, the sagging sheet of softened resin is
lowerd onto the cast by means of the supporting
farm and vacuum is turned on.
4-the sheet of soften resin is drawn in to close
adaptation to the cast. The heater is then turned
off and the base allowed to cool for one minute.
After removal from the cast the base is trimmed
and finished.
Other recording base materials
1-base plate wax:
This type of base plate is used in conjunction with
wax rim so forming the all wax-recording blocks.
Talcum powder is applied to the cast .an
alternative the cast may be immersed in water for
a short period of the time until moist. The wax is
then softened and adapted. Excess wax is removed
with a sharp instrument and the borders rounded
and smooth.
Advantages:
Base plate wax recording bases are
inexpensive, easily formed and esthetic.
Disadvantages:
1-base plate wax recording base softens
readily at body temperature that leads to
distortion during the maxillo-mandibular
jaw relations recording
2-it does not withstand the pressure to witch it
is subjected during jaw relationship
recording.
2-swaged tin base plate
Three pieces of 5-gauge tin are swaged down
on top of other, on to a model die made
from the master cast and trimmed similar in
out line to a cast or swaged metal base. The
layers of tin may be cemented together with
a thin film of hard wax and the complete
until re-swaged for final adaptation to the
cast.
Advantage:
1- it does not soften at mouth temperature
2-it has a reasonable accurate fit
3-it gives a uniform thickness for the palate of
the finished denture.
4-it reproduces the rugae to some extend.
Disadvantage:
1-time consuming
2-expensive
II-Permanent denture base
A-processed resin (heat-curing acrylic
resin):
1-fabrication of heat-processed resin
recording bases result in the destruction of
the final casts.
The recording bases that are formed are
permanent and become part of the final
denture.
2-undercuts are not blocked out. A wax form of the desired
shape and dimensions is adapted onto the cast. The wax
pattern is invested in applied, and a denture resin is mixed
and packed into the model.The resin is processed
according to the manufacture's instruction.The processed
base is recovered and finished. Undercuts on the tissue
side of processed base are blocked out with a plastic
material, and dental plaster or stone is poured in to the base
to provide mounting casts for the transfer of jaw records to
the articulator.
3-the artificial teeth are set in wax, which is attached to the
processed base, to provide the trial denture. When
satisfactory, the trial denture is flasked, processed and
finished. Either cold-cured resin or heat-cured resin may be
used to attach the teeth to the processed base.
Advantages of processed resin-recording base:
1- Accurate base plates are produced
2-no warpage during recording jaw relationship.
3-the bond between the wax rim and base is strong.
4-it also enables a check on the accuracy of the
final impression to be made at an early stage in
denture construction, since their retention and
stability should be equal to that of the finished
denture.
Disadvantage:
1- Time consuming
2-owing to the warpage that always occurs when
acrylic resin is subjected to reprocessing; it is not
advisable to finish the denture on these bases.
B-cast alloys
Cast recording bases are more costly than other types
of bases, since they require more time to
fabrication. They become part of the final
prosthesis and are therefore called permanent
bases. Cast recording base rigid, accurate, and
dimensionally stable.
They have several advantages over the other
materials they add more weight to mandibular
dentures and more thermal conductivity to
maxillary dentures.
. Refractory casts are prepared from the final cast. A
wax pattern is formed on the refractory model (a
cast made of material that withstand high
temperatures without disintegrating called
investment cast), which is sprued, invested in a
suitable investment, the wax is burned out, and the
molten alloy is cast into the model cavity. on
cooling, the casting is removed from the
investment ,finished and polished, and then
retained to then final cast. The artificial teeth are
set in wax on the metal base. Once satisfactory, the
teeth are attached to the metal base in the same
manner as for the processed base.
Occlusion rims
An occlusion rim is a wax form used to
establish:
1-the proper lip and cheek support (fullness of
the lips and cheeks)
2-the arch form, which is related to the
activity of the lips, cheeks and tongue.
3-the level of the occlusal plane.
4-accurate maxillo-mandibular jaw relations i.e. Help to determine:
A-vertical dimension and an estimate of the interocclusal distance.
B-horizontal jaw relations (centric occluding relation and condylar path)
5-occlusion rims help to determine the length and width of the artificial
teeth:
A-high and low lip lines are used for determining the length of the
artificial teeth.
B-canine line i.e. corners of the mouth; the distance between the canine
lines determines the width of the six anterior teeth.
C-the distance between the canine line and the posterior end of the
occlusion rim determines the mesiodistal width of the posterior teeth.
6-position of midline of the arch for the correct placement of the central
incisors.
7-arrangement of the artificial teeth to the trial denture (setting up of the
teeth).
Base plate wax is the most commonly used
occluasl rim material. Compound and a
mixture of the plaster and pumice are
sometimes used for constructing occlusal
rims. There are four basic factors that
should be considered in the proper
fabrication of occlusion rims, to assist the
dentist and the dental laboratory technician
throughout the many phases of denture
construction.
These factors are:
1-relationship of natural teeth to alveolar bone:
Since the final goal in the treatment of the edentulous patient is to provide
a functional and esthetic prosthesis, the relationship of the natural teeth
to the alveolar bone must be understood. The fabrication of successful
replacements can be accomplished in most cases only if the artificial
teeth are placed in the same position that was occupied by the natural
teeth they are replacing.
The natural maxillary anterior teeth are inclined slightly forward of the
alveolar bone. They contribute to the support of the upper lip
With the canines providing support for the corners of the mouth. The
mandibular incisors are also inclined forward and tend to support the
lower lip. The maxillary posterior teeth are positioned slightly buccal
to the alveolar ridge, when occluding with the mandibular molars; the
maxillary buccal cusps usually project 2 to 3 mm. beyond the buccal
cusps of the mandibular teeth. The crowns of the lower posterior teeth
are inclined inward
2-relationship of occlusion rims to edentulous
ridges:
The location and dimensions of the occlusion rims in
relation to the edentulous ridge are basically the
same as those for the crowns of the natural teeth
that are to be replaced in their relation to the
alveolar ridge. The occlusion rims simply replace
the natural teeth both in dimension and in their
relationship to anatomic structures. These
relationships should be re-established by the
occlusion rims even if resorption of the residual
ridge has occurred following the removal of the
natural teeth.
3-fabrication technique and
dimensions of occlusion rim:
1-base plate wax (modeling wax):
A sheet of base plat wax is heated over
approximately one half its length until the wax is
soft and pliable. The soft wax is rolled to point to
a point just short of unheated area. The wax is
again heated and rolled until a soft roll has been
formed. The soft wax roll is adapted to a bead of
sticky wax that was previously applied to the
recording base. The roll is further seated to the
base. The roll is further seated to the base with
spatula, with additional molten wax.
The edges of the roll are extended along the lateral surfaces to the border
of the recording base. Additional wax is added to fill any voids in the
contour of the rims. A heated broad bladed knife or plaster spatula is,
used to quickly shape the labial surface of occlusion rim. The anterior
surface should be inclined outward while the posterior surface is
sloped slightly inward.
A hot wax spatula is used to smooth the lingual surface and form a rim
approximately 5mm wide in the anterior area and approximately 810mm in the posterior area.
These basic dimensions are subjected to final chair side changes, since the
dentist uses the rims to determine the proper vertical dimension,
occlusal plane, facial support, as well as the midline of the arch, the
length and width of the anterior teeth, the buccal eminence, the smiling
(high) lip line, and the speaking(low) lip line.
The occlusal surface of the occlusal rim must be smooth and flat. All
surfaces of the rim should be smooth.
2-the composition
(compound impression)
1-May be used and softend in warm water, molded
into a back of the necessary size and placed into
position.
2-a hot wax knife is used to adapt the edge, the
surface is best smoothed with a sand paper.
3-the use of compound is indicated when it is
desired to obtain more than one record of the
occlusion, as with any type of fully adjustable
articulator, or when gothic arch tracing is to be
taken.
3-plaster and pumice
When a functional recording of mandibular
movements is to be made by the patient chewing
on bite rims, these should be made of a mixture of
plaster and pumic. In this technique the patient
goes through the movement of mastication with
the bite blocks in occlusion and so produces the
occlusal plane conforming to those movements.
An acrylic base is the most satisfactory for this
technique. The plaster is mixed with pumic equal
parts of each, to facilate the grinding down in the
mouth.
This plaster-pumic combination is mixed with water
into a thick consistency and a roll of it is placed on
to the base whilst the initial set is taking place. It
can be smoothed with moistened fingers before
setting is completed. A preliminary recording of
vertical dimension is desirable which should be
taken 3mm greater than finally required to allow
for the reduction in grinding down in the mouth.
These plaster rim should be made less than 24 hours
before they are required , otherwise the patient
will have difficulty in grinding them down owing
to the increased hardness of the plaster.