The Altered Cast Technique, Processing and Polishing.
For an ideal denture, we need a denture base that rests on tissues that are able to support it, doesnít contact other tissues which cant support it, and doesnít overly displace those it does contact as that would cause inflammation, irritation, bone resorption, etc.
We know that different areas of the arches are responsible for being the primary or secondary stress bearing areas, and so to make an ideal denture base we need to first record these areas perfectly by an impression material, so we can then transfer the information to a cast. However, the problem lies in that there is no perfect impression material that will record very accurately both the soft tissues and anatomic the form of the teeth at the same time - which is why we have the Ďaltered cast technique.í You can use this whenever youíre working a denture with a metal framework.
Firstly, you take your metal framework which has already been tested in the patients mouth and been seen to fit. Now youíre going to make an impression tray using these steps:.
1. Take your cast and place wax on all the edentulous areas, to create space for the impression material.
2. Heat the metal framework and place it in position on the wax and cast.
3. Paint tinfoil substitute over all the areas where youíre going to put acrylic (which is going to form the tray).
4. Add on self cure acrylic with finger pressure.
5. When set, remove tray off the cast and trim it so that it is in line with the edges of the wax spacer (borders will be 2mm away from vestibules). It should cover the retromolar pad.
6. Make holes over ridge and retromolar pads for escapeways.
1. On the individualized tray that you have made, do border moulding, using either polyether, impression compound, or soft tissue impression wax.
2. Shorten borders by 1-1.5mm, and reduce whole inside of impression by 1mm except over the buccal shelf region.
3. Take final impression with condensation silicone.
4. Cut the edentulous areas of the cast and put the new impression of the soft tissue areas taken with the framework there. Stick in place with wax.
You now have your new and accurate cast on which you can make your denture base.
Stable record bases are required to correctly orient diagnostic casts representative of distal extension removable partial denture situations. They should possess accuracy and so are generally made of autopolymerising acrylic resin. The technique is as follows:
1) The extent of the record bases is outlined in pencil. It should include the cingula of the anterior teeth and the distal surface of terminal abutments.
2) Undercut areas such as interproximal areas, rugae are blocked out with wax.
3) Utility wax is adapted according to the outline of the cast to confine the acrylic to the edentulous areas. It should be 2 mm in thickness to allow of thickness of the record base borders.
4) 16 gauge wire is adapted to the lingual surface of the mandibular cast to strengthen and eliminate flexibility of the base.
5) Two thin coats of tinfoil substitute (Alcote) are painted onto the cast to act as a separator.
6) Acrylic resin is sprinkled onto the cast, with alternating application of the monomer and polymer and left in a closed glass dish for polymerization. Slow polymerization occurs so that overall shrinkage is negligible and accuracy is assured. Should be left overnight.
7) Trim to establish outlines.
8) Occlusal rims can then be added on. They should be as wide as the occlusal surfaces of the remaining teeth so that natural arch form is maintained.
Similarly, acrylic resin bases may be processed directly to the master cast and directly onto the framework for distal extension removable partial denture.
Occlusal rims can be made of several materials, the most commonly used is the hard baseplate wax rim. To enhance its accuracy, some soft materials that set to a rigid state such as impression paste or bite registration paste can be used. Modelling plastic can be used which has several advantages over baseplate wax such as it can be softened uniformly by flaming.
Occlusal rims should not be made too broad or else:
Ě√) They alter the shape of the palate and arch form of the mandibular arch
Ě») Crowd the patients tongue
ĚŐ) And offer more resistance to jaw relation recording media
Occlusal rims should be trimmed short of contact at the vertical dimension of occlusion and bite registration paste should be interposed for the final record. They may be used to support intraoral tracing devices.
ESTABLISHING FUNCTIONAL OCCLUSAL RECORDS
Inlay waxes such as Pecks purple hard inlay wax can be used to record functional occlusal patterns. It comes in the form of sticks. The technique is as follows:
1) A layer of hard sticky wax is first flowed onto the surface of the denture base. Two sticks of the inlay wax are placed parallel to the longitudinal center of the denture base and secured with a hot spatula.
2) At the chair side a hot spatula is inserted between the two sticks making the central portion fluid. Transfer of heat results in the rims becoming uniformly softened.
3) The patient is asked to close on this wax rim till the natural teeth are in contact thus establishing both height and width of the occlusal rim.
4) The patient is asked to make lateral excursions.
5) Excess wax is then removed.
6) Wax is added to increase the vertical dimension to allow for
ii)changes in jaw relation brought about by re-establishment of posterior support
iii)and carving in all mandibular excursions
7) After sufficient height and width has been established the patient is given instructions for chewing records.
Posterior tooth forms:
∑ Acrylic - better as more easily modified, e.g. you can more easily add grooves, narrow the crown bucco-lingualy, grind it to fit connectors. However, the occlusal surfaces have to be duplicated in gold so they donít get worn out.
In complete dentures, it was up to the dentist to decide what the best occlusion would be. In partial dentures, the occlusion has to harmonize and fit in with an already existing occlusal pattern so you canít just choose any manufactured teeth and put them in.
You choose a tooth accurately to satisfy:
2. Form - and even then you most likely will have to modify it.
Most important considerations here are esthetics and incising function. We have several options:
∑ Porcelain or acrylic teeth attached to metal framework with acrylic.
∑ ĎPress on method,í- when readymade acrylic teeth are processed directly onto retentive elements of the metal framework. These are hollowed out lingually to help attachment.
∑ Acrylic resin teeth that you first design in wax, then try in patientís mouth for esthetics and occlusion, then process in acrylic.
∑ Porcelain or acrylic facings. Could be tried in mouth first on a baseplate wax base. These have greater strength and are easily replaced but are inferior estethically.
∑ Acrylic veneers - to be used on hollowed out anterior teeth. If you already have a previously cast metal base, you can cast these veneers separately and then solder them onto the metal.
WAXING THE PARTIAL DENTURE BASE
∑ Waxing should be done to and around exposed part of the metal framework.
∑ Undercut finishing line should be provided when necessary.
∑ To avoid voids and to prevent fracture of the investing material waxing should be overdone and acrylic resin should be finished to the metal finishing line with finish burs.
∑ Avoid abrasive disks and wheels since they can cut the metal and burn the acrylic resin.
∑ For this purpose pumice should be used because theyíll cut the acrylic resin more than the metal.
∑ Thick wax is left to provide bulk to the acrylic resin at the its junction with polished metal.
∑ Wax gingival form in a way to avoid entrapment of food particle.
∑ Artificial teeth should remain uncovered to expose their anatomical crown.
∑ As a guide, the contours around natural teeth can be helpful to reproduce gingival contours on prosthesis.
∑ Interproximal spaces are almost filled.
∑ Shape the interdental papilla so that it will be self-cleaning. If properly formed inter dental papilla enhances the natural appearance.
∑ Denture should be carefully waxed and carved.
INVESTING THE PARTIAL DENTURES.
When investing partial denture for processing its important to remove the denture from the flask intact for remounting. Teeth should be only cut off from the cast if an existing denture base is relined and the cast with exposed connectors and retainers are than placed in the upper half of the flask.
∑ To establish a harmonious occlusion relation with opposing teeth: the denture framework is settled into supporting contact with abutment teeth.
∑ Acrylic denture resin is placed in a in a mold in fluid state: minimum changes in vertical dimension
∑ Use of autopolymerizing acrylic resin: to reduce dimensional changes during relining and avoid thermal expansion.
∑ If two partial dentures are being made concurrently :it is not necessary to remount the first denture after processing whereas in other situations where natural teeth occur remounting should be done for correction of errors in occlusion.
∑ To prevent voids the base of cast is covered with 0.001 inch tin foil before investing cast and denture for processing.this will help in recovering of the base cast free of investing stone and will return the original stone mounting on articulator for occlusion. Petroleum jelly can be used too.
STEPS FOR INVESTMENT:
1. Invest the entire cast except for wax and teeth in the lower half of the flask.
2. Another method is to invest cast unto top of tinfoil and applying a separator.
3. Pour a second layer of investment covering the exposed denture frame and anatomic portion of cast.
4. Before pouring second half of the flask everything should be smooth and free of undercuts.
5. To apply tinted acrylic resin make sure the border of wax are not embedded in the lower half of the flask and tinting can be carried all the way to the border and removal from flask will not affect the tinted areas.
STEPS FOR TRIAL PACKING AND FINAL FLASK CLOSURE:
1. This is done by placing two sheets of cellophane between the two layers of resin dough.
2. If not done it will result in difficulty in opening of the flask and could result in pulling of resin from metal framework or the teeth.
3. After placing of the dough in each half the flask is closed for trial packing.
4. Open the flask, remove the sheets, and flush the excess. Repeat trial procedure till no excess is visible.
5. Final closure is carried out without using cellophane.
1. These materials mixed in the same manner as other acrylic and can be poured in the mold or place in the mold in soft state.
2. There is no resistance during the closure of flask.
3. They are slightly used in excess and excess escapes from between the halves.
4. To avoid formation of thin film landscape can be used. The space helps in accommodating the excess and helps in escape of acrylic when flask is closed.
1. If the flask is not to be poured completely with the stone then stone cap can be used which prevents tilting of teeth.
2. Before pouring stone cap separator should be applied and plaster from surfaces of teeth should be removed.
3. Let the stone harden before filling the flask with plaster.
4. Removing of stone cap easily carries out deflasking
BOILOUT IS CARRIED AFTER SETTING OF INVESTMENT MATERIAL
1. Use clean hot water to eliminate all residues.
2. Immerse the flask for 5 minutes in boiling water.
3. Flush the remaining wax by opening the flask.
4. After complete elimination of wax the denture is flushed with grease-dissolving detergent.
5. The mold is then painted with a thin film of tinfoil and allowed to dry.
6. Then apply second coat of tin foil allow it to dry and pack the mold immediately.
7. Before packing of the denture resin paint the distal end of framework with autopolymerizing acrylic resin. It provides support for the distal extension of framework during flask closure.
PROCESSING THE DENTURE.
∑ If denture base is visible in mouth tinting should be carried out before packing.
∑ Characterization for posterior acrylic resin is not very important since its not really visible.
∑ Ideal requirements for acrylic resin base material:
a. Used without packing for trial packing.
b. To have shade compatible with surrounding tissue
c. Dimensionally stable and accurate.
d. Dense and lends to polishing.
∑ Use of tinfoil-substitute is universal. It creates undesirable film around the gingival margin which results in microscopic separation of teeth and acrylic resin.
∑ For the adaptation of acrylic resin around cervical portions of porcelain and acrylic teeth layered silicone method is used.
∑ It is applied to the wax surface of denture and teeth.
∑ Mixing should be carried out according to the manufacturer before addition of outer investment stone. Boilout should be completed and there is ease of deflasking.
∑ Fit of the denture depends on the accuracy, material of resin and technique used
1. Greater dimensional accuracy.
4. Better appearance.
5. Elimination of trial packing.
6. To avoid mold expansion by using autopolymerizing acrylic resin.to accurately record natural tissue tones.
7. Eliminate lengthy processing procedures.
Polishing should consist primarily of trimming away the flash, stippling polished surfaces when desired, and polishing lightly with brush wheels and pumice followed by final polishing with a soft brush wheel and a non-abrasive shining agent such as whiting.
The areas to be considered in the polishing of a partial denture are as follows:
1) Denture borders: It should consist of removal of flash and artifactual blebs with the exception of certain areas such as the disto-lingual aspect of the mandibular denture base to avoid interference with the tongue and the distobuccal area of the maxillary denture base to avoid interference with the coronoid process of the mandible.
2) Facial Surfaces: The facial surfaces of the denture base are those polished surfaces lying between the buccal borders and the supplied teeth. Polishing of concave surfaces is always more difficult than polishing flat and convex surfaces.
3) Gingival and interproximal areas: Finishing should consist only of trimming around the teeth and the interdental papillae with small round burs to create a more natural stimulation of living tissue, plus light stippling with an off-center round bur for the same reason. Polishing should consist of light buffing with brush wheels and pumice and then with a special non-abrasive polishing agent.