Tooth Center

Among the most annoying and difficult problems in prosthetic dentistry are those related to prolonging the useful life of dentures

As the denture foundations change, the impression surfaces of the dentures cease to fit the tissues properly
The loss of the foundations for dentures causes loss of their retention and stability

If the occlusal vertical dimension is enough and if the dentures didn’t lose their orientation to their basal seat, the dentures may be corrected in the short working time allowed by the available impression materials.
Complete dentures are rebased by supplying an entirely new denture base when it is refitted to the oral tissues and the opposing denture

Dentures are relined by adding new denture base material to an existing denture base as it is being refitted
Both procedures involve making of a new impression in the existing denture

The broken edges are cleaned of debris and other interferences so that the two parts will fit together well

The two halves are held together by means of an old bur, which is luted to the teeth and resin surfaces by means of sticky wax

No wax is placed over the fracture so that the tissue and palatal sides of the fracture can be examined to see that they are in correct apposition

Plaster is vibrated gently onto the palatal side to prevent air bubles and is then set on the balance of the mix to form cast

Repairs using cold-curing resin

Replace the cast
Paint the acrylic resin monomer on the cut surfaces
Place a cold-curing repair resin in the break
Fill the area to be repaired withe alternate applications of monomer and polymer
Slightly overfill the area to allow for finishing
Adapte a tinfoil to the basal surface of the denture

Pour plaster inside the denture

When setting time of the plaster was overed, remove the cast from the denture

Cut avay and beveled the resin on both sides of the break

Curing Under Air Pressure

Pressure can be maintained on the cold-curing resin while it cures by placing the denture in a pressure cooker after the resin has been forced into place

Use a pressure which will condense the repair resin

This curing method can be used when the repair resin is added in small increments of powder (polymer) and liquid (monomer)

Without this pressure, the resin added in this manner is less dense than resin cured under pressure

The repair of dentures can be a difficult part of prosthesis construction

The dentist must be aware of the hazards in repairing a denture

Many repairs are difficult to assemble correctly, which will cause the dentures to be ill-fitting
If a second process of curing by heat is necessairy, changes will occur because the old resin resoftens during processing

These changes may cause a change in occlusion as well as in the fit of the denture over the basal seat
For this reason, it may be advisable to make a new centric relation record, mount the denture, and regrind it after the repair has been completed
The availability of autopolimerizing or cold-curing resins for repairs simplifies the repair procedure

It is not necessary to flask the denture to cure the new resin
Breakage of individual teeth, as well as breakage of the entire denture, is often caused by a change in occlusal relations, and remounting and regrinding would then be essential to prevent breakage from occuring again

Caused by a lack of adequate pressure during the polymerization or a by a definite lack of dough or gel in the mould at the time of the final closure

The bubles are not spherical

In this type of porosity the resin appear white

Lack of homogenity in the dough or gel at the time of the polymerization

It may be difficult or even impossible to obtain complete homogeneity in powder-liquid dough masses
It is probable than some regions will contain more monomer than others

These regions will shrink more during polymerization than the adjacent regions and such a lokalized shrinkage will tend to produce voids
The occurence of a such a type of porosity can be minimised by insuring that the gratest possible homogeneity of resin is attained

Use of proper P/L ratios and the most favourable mixing procedures will aid this connection
Be carefull

Pack the resin until the dough stage is reached

There are a number of causes of porosity

If the porosity occur on the surface of the denture:

-proper cleansing of the denture will be difficult

-the appearence of the denture will be unsightly
Even tough the porosity may be internal, the denture base will be weakened

Since each internal pore is an area of stress concentration, the denture may warp as the stress relax
Internal porosity may develop in the portion of a denture base as a result of the vaporisation of the monomer or of the
low molecular weight poymers,
when the temperature of the resin increases above the boiling point of these phases
Making mistake at the processing process cause internal porosity

The polimerization reaction is an exotermic reaction
The monomer boil at 100.3º C
The benzole peroxide break into at 65ºC and form free radical
If the flask are placed into the hot water and  the water boil up in a short time, the extra monomer which left in the flask is vaporize so quickly and big spheric porose area occur especially at the thick surface of the denture. This porose area can be spread to the whole surface.
If the flask are placed into the hot water and  the water boil up in a short time, the extra monomer which left in the flask is vaporize so quickly and big spheric porose area occur especially at the thick surface of the denture. This porose area can be spread to the whole surface.

The dentures are removed from the artificial stone casts

The feather edges of the denture base material are removed with files, scrapers and burs
The feather edges around the gingival line of the teeth are cut down by means of burs and chisels to conform with the desired contour

Any difficulty encountered in polishing the dentures is caused by the fact that they are not properly prepared for polishing
With burs, stones, chisels and sharp scrapers, the surface is shaped until it presents a smooth, clean surface

No plaster and no deep scratches should remain after the preparation for polishing

It is impossible to retain the desired contour of the dentures if abrasives such as pumice are used for all the finishing
A rag wheel and felt cone with pumice are used to finish the palatal portion of the upper denture

A single-row brush wheel and a rag wheel about 6 mm in widht are used with pumice to smooth the labial and buccal surfaces of the denture without destroying the contour

A final high polish is given all the surfaces with a rag wheel and polishing material (tripoli, tin oxide and water, or Shure Shine)

Attention
Resin teeth must be covered or protected during pumice and rag wheel smoothing: otherwise they appear too yellow because of the loss of the outer labial enamel shade from the denture inadvertent reduction while smoothing the facial side of the denture
The incisals can easily become knife-edged quicly while polishing the palatal surface
Gingival crevices should be polished with a brush wheel and an abundance of wet pumice.
Felt cones and dry rag wheels generate lots of heat, the former sometimes burning the resin surface of the reflections of the final cast and latter warping the base

Sould be accomplish with considerable care

Never pound on the flask with a metal hammer

That’s it

Conversion of the monomer to the polymer when a mixture of the two is subjected to heat

Methods:

1- The long polmerization cycle:
Immerse the flasks in the compress in cold water
Slowly bring the temperature of the water to 160°F constant temperature
Leave them for about 8 hours, after which the water is brought to boiling and the assembly is boiled for 30 minutes

2- The short polmerization cycle:
Immerse the flasks in water at 160°F constant temperature for 1,5 hours, then transfer it to boiling water and leave for 30 minutes

The long polymerization cycle is the method of choice if time permits

All polymerization cycles should end with a boiling period to remove the excess monomer
After polymerization, allow the flasks to cool to room temperature before deflasking the dentures

They should cool in the water in which they were polymerized

The powder-liquid mixture should be packed into the flask at the dough consistency for several reasons
If it is packed at the sandy or stringy stages, too much monomer will be present between the polymer particles and the material will be of too low a viscosity to pack well and will flow out of the flask too easily
Packing early may also result in porosity in the final denture base
If at the rubber to the stiff stage, the material will be too viscous to flow under the available pressure of the flask press, and metal-to-metal contact of the flask halves will not to be obtained.

Delayed packing will result  in loss of detail in the denture, movement or fracture of the teeth and an increase in the contact vertical dimension of the denture
The plastic dough should not be manipulated excessively with bare hands
The monomer is a good solvent for body oils and may pick up dirt from the hands, resulting in a no aesthetic denture.

Monomer may also enter the blood stream through the skin
Pack the material in the upper half of the flask, being sure to press it well to the area around the teeth.

Use the index finger covered with cellophane

To avoid trapping air between the material and the mould pack in one direction , use enough material to insure over packing on the first closure
Place two pieces of wet cellophane between the folds of a towel to remove the excess water

Place the cellophane over the acrylic resin

Put the lower half of the flask in position and press the flask together, using hand pressure

Place the flask in a press and close it very slowly to give the acrylic resin plenty of time to flow.

Finger pressure in adequate until the final closure

Remove the flask from the press, open the flask carefully and trim off the excess acrylic resin with a sharp scalpel
Add a small amount of acrylic resin in three or four places, using new cellophane

Close the press slowly and again remove any excess material as before

Test pack until all excess material has been removed and the metal edges of the parts of the flask are in complete contact
Before final closure, the separating film is removed and discarded

The final closure of the flask, or metal-to-metal contact of the flask halves, is then completed in the press

The flask are placed in a flask press, which maintains the pressure and the denture is processed