Working casts and Dies in fixed dental prosthesis

The working (or master) cast is the replica of the prepared teeth, ridge areas, and other parts of the dental arch.
The die is a model of the individual prepared tooth on which the margins of the wax pattern are finished.
The accuracy of a cast and die is a function of the completeness and accuracy of the impression.

1. WORKING CASTS AND DIES DR. ATHIRA KRISHNA ASST PROFESSOR DEPT OF PROSTHODONTICS SREE ANJANEYA INSTITUTE OF DENTAL SCIENCES

2. Introduction The working (or master) cast is the replica of the prepared teeth, ridge areas, and other parts of the dental arch. The die is a model of the individual prepared tooth on which the margins of the wax pattern are finished. The accuracy of a cast and die is a function of the completeness and accuracy of the impression.

3. Ideal requirements for a die/cast material 1. Should reproduce the tissue details accurately i.e. both the hard and the soft tissues. 2. Should be dimensionally stable under normal conditions of use and storage. 3.Dimensional variations in response to changes in temperature should be minimum . 4. Should be strong and durable to withstand the subsequent manipulative procedures without abrasion or fracturing.

4. 5.Accuracy of the impression procedure. 6. Accuracy of the cast/die system. 7. Inherent setting expansions of the dental stones used. 8. Precise repositioning of the removable die

5. Selection criteria for cast-die systems 1. The material should allow a dimensionally accurate cast and should be strong and resistant to abrasion. 2. It should be easily sectionable and easy to trim with routinely available materials. 3. It should be compatible with the separating agent so that the wax pattern doesn’t stick. 4. It should reproduce surface details accurately

6. 5. It should be available in a colour that contrasts with the wax used so that the preparations margins can be seen. 6. It should be easily wettable by the wax. In addition, it must be compatible with the impression material. 7. Finally, the type of restoration needs to be considered, because certain procedures (e.g. some all ceramic crowns) require the strength of metal or epoxy resin and cannot be fabricated on weaker stone dies.

7. Sawing the die out of the cast Difficulty also may be encountered in sawing the die out of the cast. Interproximal margins can easily be damaged during this procedure, particularly if clearance between a proximal preparation margin and the adjacent tooth is minimal. Careful model trimming of the initially poured cast is necessary before drilling the holes for the pins. If done properly, highly accurate and stable removable dies results.

8. CLASSIFICATION OF DIES According to the material used for fabrication of the die: Non metallic • Gypsum dies • Resin dies • Silicophosphate dies • Ceramic dies • Flexible dies Metallic • Electroplated dies • Low fusing alloys • Amalgam dies • Metal sprayed dies.

9. MATERIALS SCIENCE

10. Gypsum Dental gypsum products are available in five forms (ADA Types 1 to V)-defined as impression plaster; model plaster; dental stone; high-strength dental stone; and high-strength, high-expansion stone. Surface detail reproduction is acceptable with Type IV and Type V gypsum products.

11. DENTAL STONE, HIGH STRENGTH (TYPE IV) The principal requisites for a die material are strength, hardness , and minimal setting expansion : To obtain these properties , modified α-hemihydrate is used. The cube-shaped particles and the reduced surface area produce such properties without undue thickening of the mix. This material is also called die stone. , Improved stone (modified α-hemihydrate); the cube-shaped particles have a reduced surface area that improves the flow of the mixture.

12. A hard surface is necessary for a die stone because the tooth preparation is covered with wax and carved flush with the margins of the die. A sharp instrument is used for this purpose; therefore, the stone must be resistant to abrasion. Gypsum hardening solutions, silver plating, coating with cyanoacrylate adhesive, and other methods of increasing the abrasion resistance .

13. The surface hardness increases more rapidly than the compressive strength because the surface dries more rapidly. This is a real advantage in that the surface resists abrasion, whereas the core of the die is tough and less subject to accidental breakage. The average dry surface hardness of the Type IV stones is approximately 92 (Rockwell hardness) and that of Type III stone is 82. Even though the surface of the Type IV stone is harder, care should be taken when the pattern is being carved

14. DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION (TYPE V) This gypsum product exhibits an even higher compressive strength than the Type IV dental stone. The improved strength is attained by making it possible to lower the W/P ratio even further than that used for Type IV stone. T he setting expansion has been increased from a maximum of 0.10 % to 0.30% . The rationale for increasing setting expansion is that certain newer alloys, such as base metal , have a greater casting shrinkage than do the traditional noble metal alloys . Thus, higher expansion is required in the stone die to aid in compensating for the alloy solidification shrinkage.

15. Disadvantage Relatively poor resistance to abrasion. Attempts to overcome this have included the use of so-called "gypsum hardeners. " Although these materials (e.g., colloidal silica) have relatively little effect on the hardness of the stone, they improve abrasion resistance (some by as much as 100%). Their use is accompanied by a slight increase in setting expansion, but it is probably not clinically significant. An alternative approach' is to impregnate the surface of the die with a low-viscosity resin such as cyanoacrylate. As abrasion resistance is the physical property most improved by this technique.

16. RESIN Resins are used as a die material to overcome the low strength and abrasion resistance of die stone Most available resin die materials are epoxy resins, but polyurethane is also used Epoxy resin is well known as a household and industrial adhesive. It can be cured at room temperature without expensive or complicated equipment, and it yields a form that is reasonably stable dimensionally. It shows good properties of strength, hardness, abrasion, resistance though with slight shrinkage occurring during polymerisation .

17. Disadvantages It is more expensive than gypsum, and it undergoes some shrinkage during polymerization .

18. ELECTROPLATED DIES It involves the deposition of a coat of pure silver or copper on the impression. The areas to be plated are first coated with finely powdered silver or graphite to make them 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. The main drawback of silver plating is the use of a cyanide solution, which requires special precautions because of its extreme toxicity. Its abrasion resistance is many times greater than that of gypsum product. Used in the construction of ceramic and PFM restorations

19. Metal sprayed dies A bismuth tin alloy, which melts at 138°C can be sprayed directly on to an impression to form a metal shell, which can then be filled with dental stone.

20. FLEXIBLE DIES Flexible die materials are similar to heavy body silicones or polyether impression materials and have been used to make provisional restorations or indirect composite resin inlays or onlays chair side.

21. DVP (divestment phosphate) Combined die investment stone and die stone for making resin retained prosthesis The development of DVP (divestment phosphate) has made possible the development of a phosphate-bonded refractory cast and framework pattern than can be invested and cast without removal from the DVP investment cast.

22. Ceramic die Two ceramic die materials are available : A material for the production of dies on which porcelain restorations are to be fabricated. To form the dies heating to over 1000°C is necessary. A ceramic material, supplied as powder and liquid, and mixed to a putty like consistency. After 1 hour material is removed from the impression and fired at 600°C for 8 minutes to produce a hard strong die.

23. DIE SYSTEMS There are 2 basic working casts and die systems: Working cast with a separate die Working cast with a removable die. TYPES : Removable Dies Solid Cast with Individual Die . Pindex system The Di- Lok system DVA Model System Zeiser model system

24. Working cast with a separate die : This is the simplest means of fabricating a working cast and die Procedure Impression is poured using type IV die stone or type V Hold the cast by the base while cutting it down to form a handle on the die, handle of die should be slightly larger in diameter than the preparation and octagonal in cross section . A pear shaped diamond bur is used to trim the die apical to finish line of the preparation approximating the contour of the root to facilitate good axial contours in the finished preparation.

25. Relief should be applied to preparation areas to provide space for cements. A die hardening agent like cyanoacrylate or acrylic resin lacquer can be applied to finish line area of the die to prevent abrasion from carving instruments.

26. The multiple pour intact cast technique This technique involves the use of a separate intact casts in addition to individual dies . It may be used with either 1 or 2 sets of dies if 2 sets are used; the dies from first pour are used for the final readaptation of wax pattern and as an emergency backup in case a second casting is needed after an unsatisfactory casting.

27. Advantages 1 . Ease of fabrication of wax patterns on separate dies. 2. The accuracy in development of proximal contacts. 3. The accuracy with which a fixed prosthesis can be assembled on an intact cast.

28. Working cast with a removable die. In a removable die system, the die is an integral component of the master cast and can be lifted from the cast to facilitate access. Precise relocation of the die in the master cast is critical to this system's success and is usually accomplished with brass pins or dowels. The cast is made in two pours of Type IV or V stone of contrasting colours: the first forms the teeth, and the second forms the base of the cast.

29. The area to be removed is coated with a separating agent before the second layer is poured. In other areas, undercuts are provided to prevent unwanted separation. The location and orientation of the dowels are critical; if they are improperly placed, the dowels will not allow the die of the prepared teeth to be withdrawn from the cast.

30. Types of dowels Flat sided single dowel pin Single curved dowel pin Double dowel pins with single head Two separate parallel dowels Pindex system

31. Several methods can be employed to reposition the die accurately in its working casts: Most of these dies can be oriented in the impression before it is poured - Pre Pour Technique. 2. They can be attached to the cast that has already been poured - Post Pour Technique

32. Die Systems : Wet pinning system • Straight dowel pin • Curved dowel pin Dry pinning system • Pindex system • Dilok system • DVA system • Zeiser system • Accutrac system

33. Wet pinning system Straight dowel pin : This means of orienting dies has been in use for a number of years and most of the dowel systems are modifications of this. The brass dowel pin is one of the most accurate in terms of resisting horizontal displacement and the second lowest in vertical deviation among the mentioned die systems. b) Curved dowel pin : Curved dowels can be incorporated in a working cast by fixing the dowels to the impression before it is poured, or by cementing the dowels into holes drilled in a previously poured cast.

34. Dry pinning system a ) Optimum location of the pins to the removable die. b) Parallelism of die shafts. c) Uniformity of length of die pins in relation to the base of the cast. d) Smoothness of the die base at the interface with cast base material. e) Guidance to prevent rotation and allow precise repositioning of die. f) Stabilization of die pins during setting of the primary pour ( during wet pinning technique ).

35. Pindex system The Pindex system was developed by Whale dent International. A similar system, developed by APM - Sterngold and named the Accudrill , is described as a dowel pin press. A reverse drill press is used to create a master cast with dies that can be removed and replaced repeatedly with great precision.

36. The Di- Lok system It uses a special tray for precise assembly of a sectioned cast. The impression is poured and the cast made into horseshoe configuration , till it fits into the special tray. Then tray is filled with second mix and cast is seated. After setting saw cuts are made on cast ,resulting preparation of dies. Cast and die is assembled and then mounted on articulator.

37. Accutrak System Can be used for fabrication of veneers (similar procedure ). It is a modification of plastic tray with internal orientation grooves and notches.

38. Zeiser Model System Advantages Allows advantage of allowing for the expansion of stone which is relieved by saw cuts . Disadvantages Use a precision drill & special baseplates that are aligned & drilled to provide die removal

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