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Posterior palatal seal

(post damming)

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics


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Posterior palatal seal

(post damming)1- The posterior palatal seal (post dam) of the upper dentureshould 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 aguide to the ideal posterior border of the denture, is usuallylocated slightly posterior to the foveae palatine.

3- The posterior palatal seal is formed through both humular(pterygomaxillary) notch and across the palate over thevibrating line.

4- The vibrating line is observed in patient's mouth as thepatient says a series of short "ah" and the hamular notchesare palpated.


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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


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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 withalcohol 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.


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2- Scraping of master cast

Athis 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 byattempting the scrap the cast.


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3- Post damming during try-in stage

The trial denture base is inserted so theindelible pencil line marked on vibrating

line of the soft palate will be transferredfrom the soft palate to the trial denture base.And the excess base plate is reduced to thisline. The trial denture base is placed on the

cast and a knife or pencil is used to mark aline following the posterior limits of thebase plate.


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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 wideenough to avoid cutting or irritating the soft

tissues, but not so wide to compress too

large an area and thus prevent the denturefrom sealing properly.


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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 tofade 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 itswidest 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 partsof the same mouth in exactly the same manner as

its width. The average thickness is 1mm.


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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 thefinal impression material from sliding down in tothe pharynx.

B

To serve a guide for positioning the impressiontray, especially if a shim has been used within thetray to establish the borders.


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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 sealof the potential denture border is present.


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3-Post dam with finished denture:

1-it slightly displaced the soft tissue of thedistal end of the denture to inhance theposterior border seal:

A- Increase retention of the denture by

atmospheric pressure.Bit prevents air and food from getting underthe denture.

2- it makes the thickness of the base lessconspicuous to the tongue and diminishesreflex irritation.


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Boxing-in the impressions and

making the castsDef:

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 topreserve certain landmarks of the

impressions.


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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 andseated 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 theimpressions and provide space for adequate

thickness of the cast in these regions.


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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.


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5- A thin sheet of wax is used for making the vertical walls ofthe boxing. This may be special boxing wax. Or half sheetof beeswax may be cut lengthwise and used as a boxingwax. It is attached around the outside of the boxing strip soas 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 thisthickness.

6- The sheet of boxing wax should extendcompletely around the impression and be seated to the

boxing wax strip to prevent the escape of stone when thisis poured in to the impression. Sufficient space must be

available posteriorly between the impression and boxing toprovide for adequate thickness of casts distal to theimpression.


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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 to15mm 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 easilyaccessible for adaptation of the materials used in

making the record blockes.


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Advantages of boxingin

1- The correct width of the periphery of theimpression is preserved.

2-the mixed stone can be vibrated, the cast will not

contain air bubbles and a stronger cast will beproduced.

3- The thickness of the cast may be moreaccurately judged.

4- boxing-in is time saving, because trimming maybe not be required.

5- Material is economized.


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Recording bases and occlusion rim

the recording base (trial denture base) isDef:

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.


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Function of recording base

1- Establishment of the rest and occlusalvertical dimensions.

2- Determination and recording of centricrelation

3- The transfer of the accurate jawrelationships to an articulator

4- Enabling the setting of artificial teeth forthe trial denture.


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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 thepatient.

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 mouthtissues.


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Materials for recording bases

Temporary recording bases: the materials that are most f requently used are:-I

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 discardedat the time the denture is processed in to its permanent form.

permanent dentur e bases:-I I

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.


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The material used should meet the

following criteria as much as

possible1- Readily adapted to the required shape andcontours with minimum of time, expense

andtechnical skill.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 viewingthe arrangement of the teeth of the trial denture, asthey will appear in the complete denture.


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I-Temporary recording bases

Shellac recording base mater ial:-1

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.


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3-if adapted accurately strengthened and handled

carefully, it can be effectively utilized both for

maxillary and mandibular recording base. If notadequately 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 themandibular cast it is adapted within the lingual

flange.


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Fabrication of shellac recording

base1- 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.


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3-to prevent the base material from sticking to thecast,

A- It is first dusted with talcum powderB-soaked in water for a short period of time untilthe surface of the cast is moist: immersion of thecast 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 andapplied with wet fingers to accurately adapted theshellac to the palatal portion of the maxillary castor to the lingual surface of mandibular cast.


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5-the material is then reheated and adapted over the crest ofthe ridge and into the reflections. Care must be taken toavoid trapping air between the shellac form and cast

surface.6-while the material is still warm and soft; it is removed fromthe cast and trimmed with scissors, leaving approximately5mm 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 andburnished with wax spatula to form a smooth roundedborder.

8-care must be taken not to overheat, the shellac overheating

will cause the molten shellac to adhere to the surface of thecast on cooling attempt to remove the shellac base canresult in a fracture of the cast surface.


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Advantages of shellac recording base:

1- It can be easily and quickly adapted

2-inexpensive

Disadvantages:

Shellac being a brittle material, it is subjectedto breakage.


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2- autopolymer izing resin recording

base mater ialsFabrication of autopolymerizing resin recording

base: three basic techniques are used to fabricaterecording base utilizing chemically activated

resins:A-no-flasking method:

1-a layer of tin foil (o.oo1 inch) or a thin layer ofpetrolatum is applied to the prepared final cast.

2-all undercuts are blocked out with wax or and thesuitable plastic material.


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3-The autopolymerizing resin is mixed, when itreaches 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 thinapplication of petrolatum to the fingers will

prevent the resin from sticking to the hands.Keeping the fingers wet with water duringhandling 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 surfaceof the mandibular cast and then onto the crest ofthe ridge and into the reflection area.


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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; theresin 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 toapproximately 1mm.


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B- Alternating application of cold-

curing 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.


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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 sufficientlywetted with liquid (monomer) to produce aslight flow.

4-alternative applications of powder and liquid are

made until a thickness of 2 to 3mm has beendeveloped

5-the complete base is then removed, trimmed andpolished. The polymerization reaction takes

approximately 20 to 30 minutes; the bases shouldnot be removed during this period in order to

prevent distortion.


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C-flasking method

1-recording bases produced from this method

are accurate and stable. Thy do, however,

require considerable time for fabricationand therefore are most costly them bases

formed using shellac or autopolymerizing

resin.


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2-since breakage of the master cast is possible withthis technique, it is advisable to duplicate the castand to construct the recording base on theduplicated model.

3-An autopolymerizing resin is mixed in glass jarand then covered .when the resin reaches thedoughy stage ,its placed into the model , the

flask is closed, and the resin is allowed topolymerize for 20 to 30 minutes . The base isremoved from the flask, trimmed, and

polished. If there are a number of undercutspresent that will, interfere with seating the

base on the master cast, they must berelieved before seating is attempted


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: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


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3-vacuum-formed bases

1-the vacuum method provides a fast and efficientmeans of forming rigid, accurate fitting recording

base. Depending on the material used, temporary

or permanent bases can be formed. Permanentbases formed using this method have not gainedwide acceptaptance.

2-the cast is prepared by blocking out the existing

undercuts with a suitable material (wax or anyother material that melts during heating shouldno be used


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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 theresin 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.


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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 todistortion during the maxillo-mandibularjaw relations recording

2-it does not withstand the pressure to witch itis subjected during jaw relationshiprecording.


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:Advantage

1- it does not soften at mouth temperature2-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 consuming2-expensive


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II-Permanent denture base

A-processed resin (heat-curing acrylicresin):

1-fabrication of heat-processed resinrecording bases result in the destruction ofthe final casts.

The recording bases that are formed arepermanent and become part of the finaldenture.


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2-undercuts are not blocked out. A wax form of the desiredshape 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 processedaccording to the manufacture's instruction.The processed

base is recovered and finished. Undercuts on the tissueside of processed base are blocked out with a plasticmaterial, and dental plaster or stone is poured in to the base

to provide mounting casts for the transfer of jaw records tothe articulator.

3-the artificial teeth are set in wax, which is attached to theprocessed base, to provide the trial denture. When

satisfactory, the trial denture is flasked, processed andfinished. Either cold-cured resin or heat-cured resin may beused to attach the teeth to the processed base.


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B-cast alloys

Cast recording bases are more costly than other typesof bases, since they require more time tofabrication. They become part of the final

prosthesis and are therefore called permanentbases. Cast recording base rigid, accurate, anddimensionally stable.

They have several advantages over the other

materials they add more weight to mandibulardentures and more thermal conductivity tomaxillary dentures.


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. Refractory casts are prepared from the final cast. Awax pattern is formed on the refractory model (a

cast made of material that withstand hightemperatures without disintegrating calledinvestment cast), which is sprued, invested in asuitable investment, the wax is burned out, and themolten alloy is cast into the model cavity. oncooling, the casting is removed from theinvestment ,finished and polished, and thenretained to then final cast. The artificial teeth areset in wax on the metal base. Once satisfactory,

the teeth are attached to the metal base in the samemanner as for the processed base.


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Occlusion rims

An occlusion rim is a wax form used to

establish:

1-the proper lip and cheek support (fullness ofthe 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.


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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 artificialteeth:

A-high and low lip lines are used for determining the length of theartificial teeth.

B-canine line i.e. corners of the mouth; the distance between the caninelines determines the width of the six anterior teeth.C-the distance between the canine line and the posterior end of theocclusion rim determines the mesiodistal width of the posterior teeth.6-position of midline of the arch for the correct placement of the centralincisors.7-arrangement of the artificial teeth to the trial denture (setting up of theteeth).


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Base plate wax is the most commonly used

occluasl rim material. Compound and a

mixture of the plaster and pumice aresometimes used for constructing occlusal

rims. There are four basic factors that

should be considered in the properfabrication of occlusion rims, to assist the

dentist and the dental laboratory technician

throughout the many phases of dentureconstruction.


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These factors are:

relationship of natural teeth to alveolar bone:-1

Since the final goal in the treatment of the edentulous patient is to providea functional and esthetic prosthesis, the relationship of the natural teethto the alveolar bone must be understood. The fabrication of successfulreplacements can be accomplished in most cases only if the artificialteeth are placed in the same position that was occupied by the naturalteeth they are replacing.

The natural maxillary anterior teeth are inclined slightly forward of thealveolar bone. They contribute to the support of the upper lip

With the canines providing support for the corners of the mouth. Themandibular incisors are also inclined forward and tend to support thelower lip. The maxillary posterior teeth are positioned slightly buccalto the alveolar ridge, when occluding with the mandibular molars; themaxillary buccal cusps usually project 2 to 3 mm. beyond the buccalcusps of the mandibular teeth. The crowns of the lower posterior teeth

are inclined inward


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relationship of occlusion r ims to edentulous-2

ridges:

The location and dimensions of the occlusion rims inrelation to the edentulous ridge are basically thesame as those for the crowns of the natural teeththat are to be replaced in their relation to the

alveolar ridge. The occlusion rims simply replacethe natural teeth both in dimension and in theirrelationship to anatomic structures. Theserelationships should be re-established by theocclusion rims even if resorption of the residualridge has occurred following the removal of thenatural teeth.


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3-fabrication technique and

dimensions of occlusion rim:1-base plate wax (modeling wax):

A sheet of base plat wax is heated overapproximately one half its length until the wax is

soft and pliable. The soft wax is rolled to point toa point just short of unheated area. The wax isagain heated and rolled until a soft roll has beenformed. The soft wax roll is adapted to a bead ofsticky wax that was previously applied to the

recording base. The roll is further seated to thebase. The roll is further seated to the base withspatula, with additional molten wax.


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The edges of the roll are extended along the lateral surfaces to the borderof the recording base. Additional wax is added to fill any voids in thecontour 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 issloped slightly inward.A hot wax spatula is used to smooth the lingual surface and form a rimapproximately 5mm wide in the anterior area and approximately 8-10mm in the posterior area.

These basic dimensions are subjected to final chair side changes, since thedentist uses the rims to determine the proper vertical dimension,occlusal plane, facial support, as well as the midline of the arch, thelength 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.


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2-the composition

(compound impression)1-May be used and softend in warm water, moldedinto a back of the necessary size and placed into

position.

2-a hot wax knife is used to adapt the edge, thesurface is best smoothed with a sand paper.

3-the use of compound is indicated when it isdesired to obtain more than one record of theocclusion, as with any type of fully adjustablearticulator, or when gothic arch tracing is to betaken.


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3-plaster and pumice

When a functional recording of mandibularmovements is to be made by the patient chewingon bite rims, these should be made of a mixture of

plaster and pumic. In this technique the patientgoes through the movement of mastication withthe bite blocks in occlusion and so produces theocclusal plane conforming to those movements.An acrylic base is the most satisfactory for this

technique. The plaster is mixed with pumic equalparts of each, to facilate the grinding down in themouth.


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This plaster-pumic combination is mixed with waterinto a thick consistency and a roll of it is placed on

to the base whilst the initial set is taking place. Itcan be smoothed with moistened fingers beforesetting is completed. A preliminary recording ofvertical dimension is desirable which should be

taken 3mm greater than finally required to allowfor the reduction in grinding down in the mouth.

These plaster rim should be made less than 24 hoursbefore they are required , otherwise the patient

will have difficulty in grinding them down owingto the increased hardness of the plaster.

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