Сraig. Dental Materials

Kohn DH, Ducheyne P: Microstructural refinement of beta-sintered and porous coated Ti-6A1-4V by temporary alloying with hydrogen, J Mater Sci 26:534, 1991.

Kohn DH, Ducheyne P: Tensile and fatigue strength of hydrogen treated Ti-6A1-4V alloy, J Mater Sci 26:328, 1991.

Lutjering G , Gysler A: Critical review-fatigue. In Lutjering G , Zwicker U, Bunk W, editors: Titanium, science and technology, Oferursel, West Germany, 1985, Deutsche Gesellschaft Fur Metallkunde.

Margolin H, Williams JC, Chesnutt JC et al: A review of the fracture and fatigue behavior of Ti alloys. In Moser JB, Lin JHC,

Taira M et al: Development of dental Pd-Ti alloys, Dent Mater 1:37, 1985.

Okabe T, Hero H: The use of titanium in dentistry, Cells and Mater 5:211, 1995.

Peters M, Gysler A, Lutjering G: Influence of microstmcture on the fatigue behavior of Ti-6A1-4V. In Kimura H, Izumi 0, editors: Ti- tanium '80 science and technology, Warrendale, Penn, 1980, The Metallurgical Society of AIME.

Szurgot KC, Marker BC, Moser JB et al: The casting of titanium for removable partial dentures, Dent Mater Sci QDT Yearbook: 171, 1988.

Taira M, Moser JB, Greener EH: Studies of Ti alloys for dental castings, Dent Mater 5:45, 1989.

Voitik AJ: Titanium dental castings, cold worked titanium restorations-yes or no?

Trends and Techniques 8(10):23, Dec, 1991. Waterstrat RM: Comments on casting of Ti-

13Cu-4.5Nialloy, Pub No (NIH) 77-1227, DHEW, p 224, 1977.

Yamauchi M, Sakai M, Kawano J: Clinical application of pure titanium for cast plate dentures, Dent Mater J 7:39, 1988.

Implants

Adell R, Lekholm U, Rockler B et al: A 15-year study of osseointegrated implants in the treatment of the edentulous jaw, Int J Oral Surg 10:387, 1981.

For periodic updates,

Albrektsson T, Brinemark PI, Hansson HA et al: The interface zone of inorganic implants in vivo: titanium implants in bone,

Ann Biomed Engr 11:1, 1983.

Brinemark PI, Zarb GA, Albrektsson T: Tissueintegrated prostheses-osseointegration in clinical dentisty, Chicago, 1987, Quintessence.

Brhemark PI, Hansson BO, Adell R et al: Osseointegrated implants in the treatment of the edentulous jaw: experience from a 10-year period, Scand J Plast Reconstr Surg ll(supp1 16):l-132, 1977.

Brunski JB, Hipp JA: In vivo forces on endosteal implants: a measurement system and biomechanical considerations, J Prosthet Dent 51232, 1984.

Brunski JB, Moccia AF, Pollack SR et al: The influence of functional use of endosseous dental implants on the tissue-implant interface. I. Histological aspects, J Dent Res 58:1953, 1979.

Cook SD, Thomas KA, Kay JF et al: Hydroxyapatite-coated porous titanium for use as an orthopedic biologic attachment system, Clin Orthop 230:303, 1988.

Deporter DA, Friedland B, Watson PA et al: A clinical and radiographic assessment of

a porous-surfaced, titanium alloy dental implant system in dogs, J Dent Res

65:1071, 1986.

Ducheyne P: Bioceramics: material characteristics versus in vivo behavior, J Biomed Mater Res; Appl Biomater 2l(suppl A2):219,

Aug 1987.

Ducheyne P, Hench LL, Kagan A et al: The effect of hydroxyapatite impregnation on skeletal bonding of porous coated implants,

J Biomed Mater Res 14:225, 1980.

Healy KE, Ducheyne P: The mechanisms of passive dissolution of titanium in a model physiological environment, J Biomed Mater Res 26:319, 1992.

Hench LL, Ethridge EC: Biomaterials: a n interfacial approach, New York, 1982, Academic Press.

visit www.mosby.com

Hench LL, Splinter RJ, Allen WC et al: Bonding mechanisms at the interface of ceramic prosthetic materials, J Biomed Mater Res Symp

2:117, 1972.

Kasemo B: Biocompatibility of titanium implants: surface science aspects, J Prosthet Dent 492332, 1983.

Koeneman J, Lemons J, Ducheyne P et al: Workshop on characterization of cal-

cium phosphate materials, J Appl Biomater 1:79, 1990.

Kohn DH: Overview of factors important in implant design, J Oral Implant01 18:204, 1992.

Kohn DH: Structure-property relations of biomaterials for hard tissue replacement, in Wise DL, editor, Encyclopedia of bionzaterials and bioengineering, Matawan, NJ,

83, 1995, Marcel Dekker.

Lemons JE: Dental implant retrieval analyses, J Dent Ed 52:748, 1988.

Luthy H, Strub JR, Scharer P: Analysis of plasma flame-sprayed coatings on endosseous oral titanium implants exfoliated in man: preliminary results, Int J O ~ aMaxillofac Imp 2:197, 1987.

Maniatopoulos C, Pilliar RM,Smith DC: Threaded versus porous-surfaced designs for implant stabilization in bone-endodontic implant model, J Biomed Mater Res

20:1309, 1986.

National Institutes of Health Consensus Development Conference Statement on Dental Implants, June 13-15, 1988,J Dent Ed

522324, 1988.

Schnitman PA, Schulman LB: Dental implants: benefit and risk. In US Department Health and Human Services, publication no 81-1531, 1980.

C asting is the process by which a wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to

make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures. Because castings must meet stringent dimensional requirements, the casting process is extremely demanding. In dentistry,virtually all casting is done using some form or adaptation of the lost-wax technique. The lost-wax technique has been used for centuries, but its use in dentistry was not common until 1907,when W.H. Taggart introduced his technique with the casting machine.

Soldering is a method of joining two or more cast or wrought pieces using another alloy called a solder. Like casting, soldering in dentistry must ensure that the dimensions of soldered pieces are maintained to a high degree of accuracy. Casting and soldering techniques use many of the same laboratory equipment and materials.

This chapter will discuss casting and soldering techniques. These techniques use several materials that are discussed in detail in other chapters. Alloys for casting have been previously discussed in Chapters 15 and 16. Die materials and investments are discussed in Chapter 13, and waxes in Chapter 14.This chapter will focus on techniques used to work with these materials, beginning with an overview of the lost-wax casting process, then reviewing each step in this process: waxing, spruing, investing, burnout, and casting. Finally, some general considerations for dental soldering will be presented. The techniques of casting and soldering are complex and vary considerably for different alloy types, and this chapter is not intended to be a comprehensive discussion of these techniques. For further details, the reader is referred to books on casting and soldering techniques in the dental and metallurgical literature.

LOST-WAX TECHNIQUE

The lost-wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out

("lost") to create a space into which molten metal is placed or cast. The entire lost-wax casting process is diagrammed in Fig. 17-1. A wax pattern is first formed on a die of the tooth to be restored or, occasionally, directly on the tooth. All aspects of the final restoration are incorporated into the wax pattern, including the occlusion, proximal contacts, and marginal fit. Once the wax pattern is completed, a sprue is attached, which serves as a channel for the molten metal to pass from the crucible into the restoration. Next, the pattern and sprue are invested in a ceramic material, and the invested pattern is heated until all remnants of the wax are burned away. After burnout, molten metal is cast into the void created by the wax pattern and sprue. Once the investment is broken away, the rough casting is pickled to removed oxides. Finally, the sprue is removed and the casting is polished and delivered to the patient. If all steps have been done well, the final restoration will require minimal modification during cementation into the patient's mouth.

Dimensional Changes in the Lost-Wax Technique If materials used during the casting process didn't shrink or expand, the size of the final cast restoration would be the same as the original wax pattern. However, dimensional changes occur in most of the steps in Fig. 17-1 and, in practice, the final restoration may not be exactly the same size as the pattern. The management of these dimensional changes is complex, but can be summarized by the equation:

wax shrinkage + metal shrinkage =

wax expansion + setting expansion + hygroscopic expansion + thermal expansion

This equation balances the shrinkage (left side of equation) against the expansion (right side of equation) that occurs during the casting process. If the final restoration is to fit the die, the shrinkage and expansion during the casting process must be equal.

Shrinkage forces in the casting process come from two sources: wax and metal. Although the die restricts the wax from shrinking to a large degree while the pattern is on the die, residual

Wax pattern

I

Spruing

Investing

wax

Investment

Void

Casting

f

Breakout casting from investment

Pickle casting

Remove sprue and polish

C) Deliver to patient

Oxidized metal

Pickled metal (oxides removed)

Polished metal

518 Chapter 17 CASTING AND SOLDERING PROCEDURES

stresses may be incorporated into the pattern and released during investing, when the pattern is removed from the die. Furthermore, if the investing is done at a temperature lower than that at which the wax pattern was formed, the wax will shrink significantly because of the high coefficient of thermal expansion of waxes (see Chapter 14). Metal shrinkage occurs when the molten metal solidifies, but this shrinkage is nortnally compensated by introducing more metal as the casting solidifies. However, once the entire casting has reached the solidus temperature of the alloy, shrinkage will occur as the casting cools to room temperature. As for wax, the metallic shrinkage that occurs below the solidus is caused by the coefficient of thermal expansion for the alloy. Cooling shrinkage may reach 2.5% for an alloy that cools from a high solidus temperature (1300" to 1400' C), depending on the coefficient of thermal expansion of the alloy. A typical

2.5%. Furthermore, because the casting is solid at this point, the only possible compensation mechanism is to start with a void space that is 1.25% to 2.5% too large. Thus, shrinkage of wax and metal must be compensated with expansion in the investment if the casting is to have the appropriate dimensions. Fig. 17-2 shows a casting that was too small because of inadequate compensation of the solidification shrinkage of the alloy.

Sources for expansion of the investment are listed in the equation previously presented and come from two sources: expansion of the wax pattern or expansion of the investment itself. Investment expansion may be setting, hygroscopic, or thermal; these phenomena are discussed in detail in Chapter 13. The relative contributions of these three expansions are not important as long as the total expansion is sufficient to balance the shrinkage of metal previously discussed. Expansion of the wax pattern before investing is theoretically possible, but impractical because of the release of residual stresses that distort the pattern (see Chapter 14). Thus, in the shrinkage-expansion equation, the primary shrinkage comes from cooling of the solidified casting, and this shrinkage must be

Fig. 17-2 A picture of a casting that is too small and will not seat on the tooth because there was inadequate compensation of solidification shrinkage of

the alloy. As the picture shows, the degree of misfit can be substantial.

(Courtesy Dr. Carl W. Fairhurst, Medical College of Georgia School of Dentistry.)

con~pensatedby an appropriate total expansion of the investment.

Accuracy of the Lost-Wax Technique

A casting should be as accurate as possible, although a tolerance of rt0.05% for an inlay casting is acceptable. If the linear dimension of an average dental inlay casting is assumed to be 4 mm, +0.05% of this value is equal to only +2 ym, which suggests that if two castings made for the same tooth have a variation of 4 ym, the difference may not be noticeable. To visualize this dimension, recall that the thickness of an average human hair is about 40 ym. Therefore the tolerance limits of a dental casting are approximately one-tenth the thickness of a human hair. To obtain castings with such small tolerance

limits, rigid requirements must be placed not only on the investment material but also on the impression materials, waxes, and die materials. Naturally, technical procedures and the proper handling of these materials are equally important. The values for the setting, hygroscopic, and thermal expansions of investment materials may vary from one product to another, and slightly different techniques may be used with different investments. In each case, the values obtained for any one property should be reproducible from one batch to another and from one casting to another.

FORMATION OF THE WAX PATTERN

The fabrication of a restoration in wax is convenient because it involves few materials and is inexpensive, fast, reversible, and customizable. However, the properties of wax, such as its high coefficient of thermal expansion and tendency to flow and harbor residual stresses, must be kept in mind. These properties are reviewed in detail in Chapter 14. Failure to accommodate for these properties will result in inaccurate castings. There are two fundamental ways to prepare a wax pattern for a dental restoration. In the direct method, the pattern is prepared on the tooth in the mouth. This method can only be used for small inlay restorations. In the indirect method, a model (die) of the tooth is first made, and the pattern is made on the die. The indirect method is used for all types of restorations. Each of these techniques will be discussed at length.

Direct Wax Patterns As mentioned in Chapter 14,the wax for a direct wax pattern must be heated sufficientlyto have adequate flow and plasticity under compression to reproduce all details of the cavity walls. Adequate compression of the wax is required in forming direct wax patterns. Also, overheating of the wax should be avoided because of possible tissue damage and discomfort to the patient, as well as the difficulty encountered in compression of very fluid wax when it is overheated.

When wax is heated to the proper working

temperature of approximately 50' to 52" C for a short period, the previously induced stress from manufacturing and handling tends to be dissipated. A stress-free piece of wax at the proper consistency should be obtained so that the pattern, when formed under pressure, remains relatively stress free. Thus minimal distortion results when the pattern is subsequently removed from the tooth. This heating and annealing of the wax before insertion into the cavity preparation is accomplished most easily and effectively in a small dry-heat oven. A nonuniform consistency and possible volatilization of some of the wax mass tends to result form excessively heating the wax over a Bunsen burner flame. Although wax may be annealed in water that is at a proper temperature, it should not be stored for long periods under these conditions. Prolonged heating of the wax in water, especially at high temperatures, may result in a crumbly mass.

Because wax has a rather low thermal conductivity, cooling from the working temperature to mouth temperature occurs slowly, and ample time for cooling should be allowed. The decrease in temperature of the wax to mouth temperature results in a contraction. This contraction is offset to some degree when the pattern is held under pressure until mouth temperature is reached, because the compression stresses tend to be released, to some degree, when the wax is removed from the prepared tooth. However, the degree and distribution of these stresses vary from one pattern to another. Although these induced stresses are undesirable, their presence is unavoidable.

Because carving a wax pattern directly in the mouth demands a high degree of dexterity, any property that makes manipulation easier is desirable. Therefore ANSVADA Specification No. 4 (IS0 1561) for inlay wax states that the wax color should contrast with the hard and soft tissues of the mouth; the wax should soften without becoming flaky; and the wax should not show appreciable chipping or flaking when trimmed to a fine margin. Carving instruments that have been sufficiently warmed are desirable to soften, but not melt, the wax as the marginal

adaptation and contour are developed. The warm instrument brings the portion of the wax that is being manipulated to its proper working temperature, so that less stress is induced in these areas. A cool carving instrument burnishing over or cutting through the wax introduces tensile and compressive stresses into the pattern, which are detrimental to the ultimate fit of the casting.

Indirect Wax Patterns When a wax pattern is formed by the indirect method, a metal or stone die is used, which is the positive replica of the prepared tooth and at least some of its surrounding structure. This tooth replica permits the pattern to be formed outside the mouth. Forming the wax pattern on a die permits a change in the type of wax and certain manipulative procedures that are necessary for the direct technique. The convenience provided by the indirect method makes the property of wax flow less critical, because the pattern may be removed from the die at a lower temperature and with greater ease.

Some laboratories will coat the die with a die spacer on stone dies to allow space for the cement in the final restoration. The spacer is brushed onto the die as a viscous liquid, then allowed to dry before applying wax lubricant and wax. The thickness of the spacer, which can be difficult to control, ranges from 10 to 30 ym and is a function of the manipulative technique. Die spacers should not be used to compensate for improper manipulation of the other materials in the casting process, nor should they be used on the margins of the restoration.

When adapting wax to stone or some metal dies, some form of lubricant must be used to release the wax pattern from the die. In the mouth no such lubricant is required because a thin film of saliva or dentinal fluid serves as a lubricant. A variety of fluids is currently available to prevent the attachment of the wax to the die. These fluids produce a separator film of minimum thickness. An excess of separator is to be avoided because it leads to inaccuracies in the wax pattern and a poor surface of the cast alloy.

The wax may be adapted to the die either by flowing small melted increments from a spatula

to build up the desired contour or by the compression method, as is suggested in the direct technique. By either method the temperature of a stone die is of little concern in wax adaptation because the stone is a poor thermal conductor. Likewise, the temperature of a metal die is not critical when compression of the wax is used to form the pattern. However, this temperature is critical when molten increments are used to build up the pattern because the manner of solidification of the wax depends on the temperature of the metal die.

When molten wax flows onto a cool metal die, the wax immediately adjacent to the die solidifies rapidly because the heat from the molten wax is rapidly dissipated. The wax adjacent to the air stays molten for a period; as it solidifies and contracts, it pulls the previously congealed wax away from the metal, as shown in Fig. 17-3, A . Conversely, if the metal die is warmed throughout to near body temperature, the wax solidifies more evenly throughout its mass, resulting in better adaptation, as shown in 17-3, B. The die can be warmed by placing it under an electric lamp or with the carving instruments on an elec-

Fig. 17-3 Improper manipulation of the wax pattern can cause the pattern to distort significantly. A, The wax was cooled too quickly and residual stresses have been released, causing the pattern to pull away from the die. B, A proper cooling rate and manipulation results in a pattern that remains adapted to the die.

tric heating pad at a suitable temperature. The indirect wax pattern is carved with a warm instrument, as is the direct pattern, again to minimize the formation of stresses in the wax.

Because of the basic physical nature of wax, distortion of the pattern is a continual hazard. Not only does wax have one of the highest coefficients of thermal expansion of any dental material, it also possesses a relatively low softening temperature, which may cause stress release or flow to occur. Stresses are easily induced in forming any pattern; in fact, the formation of a completely stress-free wax pattern would appear improbable. However, with knowledge of the physical characteristics of the wax, an operator can minimize the stresses in the pattern by applying the appropriate manipulative procedures.

SPRUNG THE WAX PATTERN

The purpose of the sprue is to provide a conduit for the molten metal to reach the void formed by the wax pattern after burnout. Proper spruing technique and sprue design is critical to the successful casting of the restoration. Before spruing, the wax pattern is generally briefly removed from the die. After the die is cleaned and relubricated, the pattern is replaced on the die and the margins of the restoration are finalized. The type, number, location of attachment, diameter, length, and direction of the sprues are all important to the success of the casting.

Several types of materials are used for sprues, depending on the type of restoration being cast. For small inlays, a hollow-metal sprue may be used. The metal is stronger than a comparably sized wax sprue. The core of the hollow pin should be filled with sticky wax to preclude sucking of the pattern wax into the sprue when attaching the sprue to the pattern. Of course, the metallic sprue cannot be burned out, but must be carefully removed after investing of the pattern. Round wax is a commonly used sprue material for many restorations of all sizes. Wax has the advantages of being inexpensive, easy to manipulate, easy to burn out, and available in a variety of diameters. Wax sprues can also be easily designed for complex castings that require mul-

tiple sprues and vents. Plastic sprues have also been used for casting. Plastic has the rigidity of metal, an advantage, but can still be burned out, although longer burnout times may be required than with wax.

For most common crowns and inlays, a single sprue is sufficient. However, bridges and removable partial dentures may require multiple sprues in a complex configuration (Fig. 17-4). Sprues of small diameter may also be attached to the pattern to act as vents to enhance the displacement of gases from the mold during the entry of the molten metal. Two sprues in the configuration of a "Y" may also be used on some relatively small restorations to prevent warpage of the wax pattern susceptible to distortion. The Y-sprue design is often used on MOD inlay restorations.

Fig, 17-4 Picture of a complex sprue design for a 3-unit fixed partial denture. The restoration is on the bottom (lighter colored wax). Each unit has a sprue feeding to it from a horizontal feeder bar. Two largediameter sprues feed into the feeder bar; to encourage turbulence of the molten metal, they are purposely not aligned with the unit sprues. The use of complex arrangements such as this ensures that the molten metal will cause the sprue bar area to become hotter and freeze last. Note that the attachments of

the sprues to the individual units are broad and flared.

(CourtesyCarl W. Fairhurst, Medical College of Georgia School of Dentistry.)