Removable Die

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

Definition:

It is an accurate positive reproduction of the prepared abutment tooth. The die made to be removable to adjust and establish correct proximal

relationship .

Selection criteria

The materials used for construction of cast-and-die system should:

1 )Have high stability and excellent dimensionally accuracy.

2 )Have high mechanical strength properties to withstand handling without being fractured or destroyed.

3 -Have high surface hardness to resist scratching and abrasion during wax pattern construction.

4 -Reported surface detail accurately.

5 -Be easily sectioned section able and easily to trim with the routinely available equipment.

6 -Be available in a color that contrasts with the wax used, so the preparation margin can be seen.

7 -Be compatible with separating agent that will be used, so the wax pattern does not stick.

8 -Be easy wet table by the wax.9 -Be compatible with the impression

materials .NB. The type of the restoration needs to be considered because certain procedures ( e.g a porcelain jacket crown) require the strength of metal or epoxy resin and cannot

be as fabricated on weaker stone die .

Materials

Resin

Resin are used as a die material to overcome the abrasion resistance of die stone. Available resin die materials are epoxy resins and polyurethane. Epoxy resin can be cured at room temperature without expensive or complicated equipment, and it yield a form that is reasonably stable dimensionally. Its abrasion resistance is many times greater than that of gypsum products. However, it is more expensive than gypsum and undergoes some shrinkage during polymerization. ( prosthesis fabricated on resin dies will fit more tightly than those made on gypsum).

Certain impression materials, polysulphide and hydrocolloid are not compatible with resin, however, good result are achieved with silicone and polyether.

Electroplated dies

Electroplating can be used to overcome the poor abrasion resistance of gypsum. This technique involves the deposition of a coat of pure silver or copper on the impression. The areas to be electroplated are first coated with finely powder silver or graphite to make then conduct electricity, and the impression is then placed in an electroplating bath. A layer of pure metal is deposited on the impression and is supported with type IV stone or resin.

Some problems remain this technique, variable degrees of distortion commonly occur, and the technique must be performed slowly, lest distortion in the metal subsequently stress the impression. The time required to produce cohesive film of metal ( 8 hrs) is ample for the development of dimensional changes in the impression. However, when done properly, an electroplated die can be as accurate as die stone.

Not all impression materials are compatible with electroplating, silicone impression materials have low surface energy, are difficult to be electroplated, polyether impressions, because of their hydrophilic nature, imbibe water and become distorted . Polysulphide polymers can be silver plated, but it is much more difficult to copper plate them. The main drawback of silver plating is that a cyanide solution is utilized and this requires special precautions because of its extreme toxicity.

Techniques of removable die construction

1 -Solid cast with individual die.2 -Di -lock tray technique.

3 -Dowel pin technique4 -Pindex system technique.

5 -DVA model system.6 -Zeiser model system.

Requirements of good removable die

1-Should allow no movement in any direction.

2-Should maintain good relationship to the adjacent teeth for establishing correct:

a) Marginal adaptation.

b) Contact area.

3-Should be constructed from a material that is dimensionally stable.

Solid cast and individual die

The solid cast and individual die is the simplest means of fabricating a working cast and die, since no procedures are required to create a die other than making a sectional cast and a full-arch cast. In addition to ease of fabrication, it keeps the relationship between the abutments fixed and immovable because the gingival tissues around the prepaired teeth left intact, they can be used as a

guide when contouring the restorations .

Disadvantages: of the solid cast and separate die system are that:

1.It may be difficult to transfer complex or fragilewax patterns from cast to die.2.

Seating the pattern on the master cast may beproblematic because the second pour ofmany impression materials is slightly largerthan the first; therefore, it may be necessaryto relieve the stone slightly to seat the patternbefore occlusal evaluation.

3 .The technique can be used only with elastomericimpression materials (if reversiblehydrocolloid is used, separate impressions

are needed for master cast and die.)

Technique

When the impression has been judged satisfactory, it is poured in type IV stone in the area of the preparations only. When set, it is separated and a second pour is made of entire arch.

The first pour, which the most accurate, is trimmed into a die with a handle of sufficient length (similar to a tooth root) .the complete arch cast (second pour) is mounted on an articulator.

1-Di – lock technique:

Done by using Di -lock tray which consist of 3 parts:

1-A base that has an elevated middle portion.

2-The surrounding side portion.

3-The key portion, which connects the above 2 portions posteriorly.

Procedure:

1-Poure a full arch impression producing a u-shaped cast with an open lingual area. It must be trimmed to fit a Di-lock tray.

2-An undercut is done in the base of the cast using a carborundum disc. This is to lock the cast into the new stone base that will be formed by the tray.

3-The cast is then soaked in water for few minutes (as a separating medium). Fill 2\3 of the assembled di-lock tray with stone of a different colour and seat the cast in the locked di –lock.

4-The working cast is now invested in a layer of stone within the tray.

5-After setting of the stone, unlock the tray. A master cast of both stone pouring is produced.

6-Two vertical cuts are done using a saw blade on each side of the prepared tooth.

They must be:

a) Adjacent to the prepared teeth.

b) Extend to 3\4 the depth of the stone base leaving 3-4mm un sawed.

7-Using finger pressure fracture the remaining part of the base. Thus you get a removable die. Repeat the process for all prepared teeth.

8-The fractured pieces can be reassembled using the aid of:

a) Fracture lines of the base of the cast

b) Ribs of the di-lock tray.

Wax pattern is then done on the dies after trimming them .

Dowel pin technique

1-Surround the impression by boxing wax.

2-Poure the impression with stone covering the cervical margin by 1.5 mm

3-Befor hardening of the stone insert the following:

a) Dowel pin: in the prepared teeth should be flat from one side, to prevent rotation.

Parallel to the long axis of the tooth.

Its end should be covered by a small ball of wax.

b) Wire loops:

1-To engage the 2nd pouring of the stone place them in between and parallel to the dowel pins.

2-Pour the rest of the impression with 2nd stone of different colour.

3-After complete setting of 2nd stone pour, remove the impression and trim the base of the cast.

The die can be made removable by:

a) Making 2 vertical cuts adjacent to the prepared teeth and to the depth of 2nd stone using a saw blade.

b) Removing the ball of wax at the end of the pin and using gentle tapping with a blunt instrument, the die can be made removed.

The die can be reassembled again by the flat side of the dowel pin.

Wax pattern is then carved on the die.

5 -Pindex system technique:

A recent technique which allows pin placement after the master cast is separated from the impression.

Advantages:

a) Final impression is poured without delay.

b) Removable sections are planned with the finished master cast rather than the impression.

c) All pins are made parallel to each other.

d) Double parallel pins for each die improve accuracy and stability.

Technique:

1 -Produce a horse-shoe stone of 15-18mm buccolingual width and 15mm base thickness.

2 -Flatten the base of the produced cast.

3 -Drill 2 holes for each removable section to ensure die stability.

4 -Cement pins in the prepared holes with cyanoacrylate. Long pins bucally and short pins lingually.

5 -Wax the pin extensions, box the cast, paint it with separator and pour a secondary stone base.

6 -Dies are sectioned with a die saw, short of the base.

DVA model system

Zeiser model system

Single die

Single copper band impression may be done using either impression compound or rubber base. The type of the single die material depends on the type of the impression material used.

Classification of the single die according to the die materials

1 -Stone die.

2 -Amalgam die.

3 -Acrylic die.

4 -Refractory die.

5 -Electroplated die

a) Silver plated die

b) Copper plated die

1 )Stone die

It can be either with impression compound or rubber base. The stone used as a die material should have minimum amount of setting and thermal expansion and maximum amount of surface hardness and smoothness.

Technique

1 )Warp the impression band by molding wax extending 10mm beyond the cervical margin.

2 )Following the manufactures instruction, the stone is mixed at a proper water/powder ratio.

3 )Pour the mixed stone into the waxed band, increment, maintaining proper vibration to avoid air bubbles.

4 )Insert a dowel pin after the impression has been filled with stone.

5 )After complete setting of the stone, remove the die from the rubber impression material, while incase of impression compound, the band is placed in hot water bath till the compound is softened and the die is carefully removed.

Amalgam die

It is indicated only for copper with impression compound, because the elastic property of the rubber base impression cannot withstand the forces of amalgam condensation.Advantages:

It can produce a harder die than the stone die.Disadvantage :

1 )Done only with impression compound.2 )Long setting time.

3 )Much setting expansion .

Technique

1 -Warp the impression band by modeling wax extending 10mm beyond the cervical margin of the root portion.

2 -The waxed band is centralized inside a casting ring and investment material is poured around and in between the band and the casting ring this is to give better handling and condensation of amalgam.

3 -Trituration amalgam according to the manufactures instruction and packed properly to fill the impression and the root portion.

4 -After complete setting of amalgam, remove the die and trim the root portion so it can be

seated in the working cast .

Acrylic and epoxy resin dies

Pour either self-curing acrylic resin or epoxy into the copper band with impression compressive strength than that the stone

die .

Refractory die (ceramic die)

A special type of ceramic material can be used for the construction of the single die. It is supplied as a powder and liquid that mixed and fired according to the manufacture’s instruction “fired after the condensed porcelain is removed from the impression.