Wax Patterns

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A large percentage of time and effort spent in fabricating fixed prostheses is devoted to producing a very accurate wax pattern. From this pattern, the finished cast restoration is duplicated by using the lost-wax process as part of the indirect procedure.

This technique consists of obtaining an accurate impression of the prepared tooth (Fig. 18-1, A) and making a cast from the impression (Fig. 18-1, B) on which a wax pattern that resembles the shape of the final restoration is shaped (Fig. 18-1, C). A mold is then made around the wax pattern with a refractory investment material (Fig. 18-1, D). When the investment has set, the wax is vaporized in an electric furnace. The hollow mold is then filled with molten casting alloy, reproducing every detail of the wax pattern (Fig. 18-1, E). The metal casting is retrieved, excess metal is removed, and after polishing, the cast restoration is ready for clinical evaluation (Fig. 18-1, F). As

Wax Pattern
Fig. 18-1. Most dental castings are made indirectly by the lost-wax process. A, Impression. B, Cast. C, Wax pattern on die. D, The pattern is attached with sprue to a rubber crucible former and invested. E, Casting. F, Cemented restoration.

the solidifying metal (casting) cools to room temperature, it shrinks. Dimensional accuracy of the casting is achieved by balancing this shrinkage against precisely controlled expansion of the mold (see Chapter 22). Wax is used to make the patterns because it can be conveniently manipulated and precisely shaped. By heating, it can be completely eliminated from the mold after investing.

The lost-wax technique is widely used in industrial and jewelry manufacturing. The first bronze castings reportedly were made in the third millennium bce with beeswax and clay refractory materials. Ancient lost-wax castings such as Chinese bronzes, Egyptian deities, and Greek statues have withstood the centuries, yielding information about ancient societies and cultures. The lost-wax method may have been used in Sumeria as early as the Second Early Dynastic Period for figurines and even larger body parts.'

In dentistry, successful results depend on careful handling of the wax. The practitioner must understand that every defect or void in the wax will appear in the casting. Most defects can be corrected easily in wax, but not in a metal casting. More often than not, compensating for an error in waxing technique is impossible once the metal casting has been formed. Careful evaluation of the pattern, preferably under magnification, is critical to obtaining a good casting.

This chapter approaches the waxing procedure in a logical sequence. As with most aspects of fixed prosthodontics, a successful restoration is possible only if each step is carefully followed and evaluated before moving to the next.

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