working cast and dies part 2

how can the classification of working casts and dies be done according to design and die system?

1. single die

2. working cast with separated dies

3. working cast with removable dies (dowel pin system, straight dowel pin and curved dowel pin)

4. pindex system

5. di-lok tray

6. DVA model system

7. zeiser model system

what is a single die?

an old system that was used with single copper band. the impression was made either of compound or rubber base impression materials

what is the simplest method of fabricating working cast and die?

working cast with separated dies

what is the method of working cast with separated dies?

working cast (full arch cast) and the separated sectional dies can be obtained by:

• separate impressions of the cast and dies

• doing a double pour of the same impression. the impression is poured twice, with the first cast sectioned and used as a die and the second cast remaining unsectioned and used as the full arch working cast

the double pour method in working cast with separated dies is limited to what?

rubber base impression because they have enough elasticity to allow removal and repositioning of the cast without tearing or distortion.

what are the advantages of working cast with separated dies?

1. ease of fabrication

2. keeps the relationship between the abutments fixed and immovable which is necessary for fabrication of bridges, minimizing need for post-casting adjustment

3. gingival tissue and other landmarks are intact so it is easier to obtain harmonious contour of the wax pattern (working cast)

what are the disadvantages of working cast with separated dies?

1. when the wax pattern is transferred from the separated die back to the working cast, it may become distorted, which can affect the internal adaptation (fit) to the final restoration

2. proximal margins tend to get over contoured

3. needs elastomeric impression material because hydrocolloid distorts too much to be used for an accurate second pour

what is the technique of working cast with separated dies?

the impression should be first washed under cold tap water to remove any mucosa and saliva prior to disinfecting it. type IV and V hard improved stone should be used for the fabrication of the die and cast to prevent surface abrasion when wax pattern is made. the manufacture’s instructions for correct W/P should be followed. a small instrument should be used to carry stone to the impression of prepared teeth, with a small amount placed on the side of the impression above the preparation. the impression is then vibrated until the stone reaches the occlusal surface of the preparation. the impression is tilted so that the stone flows slowly across the bottom of the preparation. stone should be added in small increments. the stone is built up to a heigh of about 1 inch (2-5cm) over the preparation to allow bulk for an adequate handle on the die. when the first pour hast set, the cast is separated and repoured, while the first pour is sectioned into individual dies.

why should type IV or V gypsum (hard improved stone) be used?

to resist abrasion

why should the manufacturer’s instructions be followed for correct water-powder ratio?

it can affect setting time, porosity, setting expansion and ultimate strength

what happens if stone is added in large amounts of increments?

air will be trapped and voids in the cast form as a result

what should be done to pour a full arch impression?

stone is added and placed on a tray on a vibrator until teeth in the arch are filled

how is the stone built in working casts with separated dies?

1 inch or 2-5 cm over the preparation to allow bulk for an adequate handle on the die

how many times is the impression poured in working casts with separated dies?

twice or triple, with a full arch done for proximal contact and occlusion and section casts (separate dies) for wax coping and margins

what is the objective of working cast with removable dies?

to eliminate discrepancies (differences or inaccuracies) between the working cast and the separate die, which can occur in working cast with separated die due to impression distortion, deterioration between pours or by a cast and die made from separate impressions that are not identical

what are types of working cast with removable die?

1. dowel pin system (straight or curved)

2. pindex system

3. Di-Lok tray

4. DVA model system

5. ZEISER model system

how is a working cast with a straight dowel pin done?

a dowel pin is positioned over each prepared tooth in the impression using bobby pins and sticky wax. the die stone is then poured into the impression, filling the impression of the teeth, covering the knurled end (constriction) of the dowel pin. the paper clips and retentive pins are set into stone before it sets to provide retention for the base which will be placed later. a soft ball of wax is placed on each dowel and a V-shaped facio-lingual orientation groove is cut to aid in reseating the die completely and accurately during use. a separating media is applied over the first pour

how is a dowel pin positioned in working casts with straight dowel pins?

over each prepared tooth in the impression, placed parallel to the long axis of the tooth, not impinging on the margins

why are dowel pins flat from one side?

to prevent rotation

what are bobby pins used for?

positioning of dowel pins, which is placed between the arms of the bobby pins. they are positioned bucco-lingually across the impression and the dowel pins must not touch the impression

why must the first pour of stone completely cover the dowel head but not the shaft?

to allow dowel stability

why are paper clips put into the stone before it sets?

to provide retention for the base which will be placed later. the straight and bobby pins are removed

why is wax placed around the tips of the dowel pins?

to protect them from the plaster contamination

how is the stone cut in straight dowel pins?

two vertical cuts adjacent to the abutments and to the depth of the 2nd stone pour. parallel saw cuts or converging towards the base, with caution to avoid damaging margin or proximal contact areaas

what happens if saw cuts are made diverging towards the base of the cast?

the die becomes locked in the cast and cannot be removed properly.

why must sawing of removable dies be parallel or converging towards the base?

to allow easy insertion and removal of the die and avoid exposing the dowel pin

what is a curved dowel pin?

pins that are used so the tail protrudes from the facial surface of the cast, allowing easy removal and repositioning of the die while the cast is mounted on the articulator.

what is the pindex system?

a reverse drill press used to create a master cast with dies that can be removed and replaced repeatedly with great precision. the impression is poured without positioning and attaching dowel pins as the machine accurately drills parallel holes from the underside of the trimmed cast

how is the pindex system done?

• pour impression with correct w/p ratio of extra hard stone

• direct the light indicator centralized over prepared teeth

• drill from the other side, or towards the base, and drill a hole in the center of the die

• repeat drilling for each prepared tooth, and all holes will be parallel to each other

• pins long and short are inserted into their sleeves

• a dowel pin is attached to a hole using cyano-acrylate cement

• long and short pins are placed facially and lingually and glued for better access

• petroleum applied as separating medium and plastic sleeves are positioned

• box and seal cast

• second pour done with yellow stone

• mesially and distally sawed to each removable die, converging or parallel

what does the long pin face in pindex system?

facially

what does the short pin face in pindex system?

lingual (glue first for better access)

why is petroleum applied on the pins in pindex system?

it acts a separating medium and allows for easy separation

why are the two dowel pins, long buccally and short lingually, placed like that?

to stabilized the die and provide single insertion direction

what is DVA model system?

a model consisting of an underside base plate that has alignment fixture for impression and a top clear plate which have another fixture opposing those on the bottom base plate. dowel pins are attached to the clear plate so that when the clear plate is positioned over the impression, the dowels are suspended in their selected location

what is the zeiser model system?

similar to DVA but instead of visual determination of dowel pin position, a drill is used with a pointer to locate pin position in an attaching plate with the hole being drilled

what is a more recent technique?

virtual casts, which are done by digital impressions that may be acquired from optical scanners, with the data being obtained from intraoral scanners or solid casts scanned in the lab. digital data can be used to generate solid master cast (additive process by incremental layers of resin which is polymerized by the laser)