Marking The Margins

The technician's awareness of the cavosurface margin's location is very important. By marking it with colored pencil, the technician can pinpoint this location (Fig. 18-5). The color should contrast with that of the wax that will be used (e.g., a red pencil can be used for a green wax). An ordinary lead pencil is not recommended, because it can abrade the die, its darker color can interfere with efforts to verify that the wax was properly adapted at the margin, and traces of the graphite (an antiflux) can prevent complete casting of the margins. The marked margins can be coated with low-viscosity cyanoacrylate resin and immediately blown dry. If performed properly, this procedure will add no more than a micrometer"' to the die. Although removing the excess with acetone is sometimes possible, care must be taken not to create a thick layer of cyanoacrylate, which can result in an unacceptable fit of the final cast restoration. For this reason, higher-viscosity resins should be avoided.

^MATERIALS SCIENCE

M.H. Reisbick

Inlay casting wax (the name given all wax used in forming the pattern for cast restorations) is actually composed of several waxes. Paraffin is usually the main constituent (40% to 60%). The remaining balance consists of dammar resin (to reduce flaking) plus carnauba, ceresin, or candelilla wax (to raise the melting temperature), or beeswax. Sometimes a synthetic wax is substituted for the natural material. Dyes are added to provide color contrasts. Exact formulations are trade secrets, but Coleman" has published the formula for an experimental compound.

The American National Standards Institute (ANSI) and the American Dental Association (ADA) 12 have categorized waxes into two types:

1. Type I-a medium wax (generally used with the direct technique for making patterns in the oral cavity)

2. Type Il-a softer wax (generally used for the indirect fabrication of castings)

Waxes used with direct techniques must not flow appreciably at mouth temperature. Those used with indirect techniques must resist flow at room temperature to maintain their newly shaped forms.

Specifications of the ANSI and ADA govern the important properties of residue, flow, and expansion. Because the mold must burn out cleanly to allow the escape of gases and the complete entry of molten alloy, there can be no residual ash. However, the specifications allow a 0.1% residue, which apparently is effectively negligible. Flow requirements, as previously stated, are necessary to control the stability of the wax once it has reached the temperature at which it is carved, burnished, and polished (37° C [99" F] for direct-type, 25C [77° F] for indirect-type waxes). In addition, the wax must flow well at typical forming temperatures. Curves of temperature plotted against percentage flow (Fig. 18-6) are furnished by reputable manufacturers and should be consulted when choosing a casting wax. All waxes expand or contract when heated or cooled. Manufacturers' curves of percentage expansion and contraction at various working temperatures (Fig. 18-7) are helpful when considering methods to use in the investing and casting process. For example, a wax that solidifies at a higher temperature will shrink more and will therefore require more compensation to control fit than a wax that solidifies at a lower temperature (a reason for not interchanging Type I and Type 11 waxes within an established technique). These properties can be adversely affected by repeated heating of the wax, which will drive off the more volatile components.13 When selecting waxes for optimal casting accuracy, the use of waxes with different properties for the margin and occlusal portions may be necessary." If a casting is to be accurate, the wax pattern must not become significantly distorted. One cause of distortion is that wax has "memory," which means that it exhibits some elasticity unless it is thoroughly liquefied. This problem can be overcome by applying the initial layer of wax in melted increments or drops. As an alternative, the initial coping can be made by dipping the die into thoroughly melted wax.

Graph Thermal Expansion Dental Wax

Fig. 18-7. Wax expansion curve.

20 25 30 35 40 45 Temperature (°Q

Fig. 18-7. Wax expansion curve.

However, a serious problem exists when the added wax incorporates strain within the pattern as each increment solidifies. This strain tends to be released with time and subsequently distorts the wax pattern. The rate of wax change is temperature dependent, which means that it increases at higher ambient temperatures. Because wax has a relatively high coefficient of thermal expansion and changes dimension subject to air temperature changes, and because the pattern will tend to release its incorporated strain, the margins must be remelted, readapted, and resmoothed immediately before investing. The internal fit of the remelted portion will then be closer to the prepared surface of the tooth than the rest of the casting and therefore may help obtain the necessary space for the luting agent.

^TECHNIQUE

A step-by-step waxing technique is recommended. Each step is evaluated before proceeding to the next, which allows corrections and minimizes extra work. The finished wax patterns should be an accurately shaped anatomic replica of the original teeth. Information needed to shape the restoration correctly is derived from the contours of the unprepared tooth surface, adjacent tooth surfaces, and the opposing occlusal surfaces; however, additional input is needed. This stems from a thorough knowledge of tooth anatomy and the ability to copy three-dimensional structures accurately.

When making a drawing or painting, artists constantly refer to the real-life scene they are trying to reproduce. Similarly, when waxing a restoration, the dentist or technician should refer to a suitable model (e.g., diagnostic casts, unworn extracted teeth, a contralateral tooth) or casts of the unworn natural teeth. It is unwise to copy reproductions of natural teeth (plastic teeth or casts of restored mouths), no matter how skillfully they are made. This would be like an artist trying to render a scene from another artist's painting, rather than from real life.

Evaluating a three-dimensional shape is difficult. The finished wax pattern for a tooth may be too bulbous or too flat. Although it appears "wrong," pinpointing and correcting the exact problem is a skill achieved only after in-depth study of what constitutes "normal" anatomic form. When evaluating oc-clusal morphology, breaking down the complex surfaces into individual components is helpful. When evaluating axial contours, the practitioner should assess a series of two-dimensional outlines by rotating the wax pattern. These can easily be compared to an appropriate model, and any aberrations can be corrected (Fig. 18-8).

Armamentarium (Fig. 18-9)

Axial Contour
Fig. 18-8. A, incorrect midfacial contour is difficult to determine by looking directly at a three-dimensional object. B, It is more easily seen by sequential evaluation of the profile of the pattern as it is rotated.
Fig. 18-9. Waxing armamentarium.

Waxing instruments (C)

Cotton cleaning cloth (D)

Sharp colored pencil (contrasting color to wax)

Separating liquid (E)

Occlusal indicator powder (zinc stearate* or powdered wax) (F)

*NOTE: Zinc stearate may present a health hazard if it is inhaled. Powdered wax is a safer alternative.

Pkt Carving Instrument
Fig. 18-10. A to F, PKT waxing instruments (A, Nos. 1 to 5. B, Nos. 1 and 2. C, No. 3. D and E, No. 4. F, No. 5) G and H, The no. 7 waxing spatula.

• Soft toothbrush

Waxing Instruments. Waxing instruments can be categorized by the intent of their design: wax addition, carving, or burnishing. Of the popular PKTs (Fig. 18-10, A to F) (designed by Dr. Peter K. Thomas specifically for the additive waxing technique), no. 1 and no. 2 are wax addition instruments, no. 3 is a burnisher for refining occlusal anatomy, and nos. 4 and 5 are wax carvers.

Wax is added by heating the instrument in the Bunsen flame, touching it to the wax, and quickly reheating its shank in the flame. Wax flows away from the hottest part of the instrument, so that if the shank is heated, a bead of wax will flow off the tip (Fig. 18-11). However, if the tip is heated, the wax will flow up the shank of the instrument (to the considerable annoyance of inexperienced operators). The PKT no. 1 instrument is used for large increments; the smaller no. 2 is used for lesser additions.

Fig. 18-11. Always heat the shank of the instrument so wax will flow off its tip.

A no. 7 or 7A waxing spatula (see Fig. 18-10, G and H) is useful for adding large amounts of wax, particularly in forming the initial coping or thimblelike layer of wax that covers all prepared surfaces. Electric waxing instruments (Fig. 18-12) are preferred by some technicians because they allow precise temperature control of the wax, which is important for

Fig. 18-12. Electric waxing instrument.
Fig. 18-13. Wax carvers. No. 2 Ward and nos./, and 3 Hollenback.

proper manipulation. Another advantage is that carbon buildup can be kept to a minimum, which easily results from overheating a waxing instrument in a Bunsen flame.

Wax carvers should be kept sharp and should never be heated. In addition to the PKT instruments, the nos. 1/z and 3 Hollenback and the no. 2 Ward carvers (Fig. 18-13) are popular. When carving wax, light pressure should be used to obtain the desired smooth surface.

Burnishing is an alternative to carving for obtaining a smooth wax pattern of the desired contour. Burnishing consists of slightly warming a blunt instrument and rubbing the wax. The instrument should not be so hot that it melts the wax surface. The PKT no. 3 instrument is useful for bur-

nishing the occlusal surfaces. The PKT nos. 1 and 2 can be used for burnishing as well as for wax addition. Another popular burnisher is the Darby-Perry trimmer (DPT) no. 6 (Fig. 18-14). For removing wax, burnishing is less effective than carving, but it is probably easier to control and leaves a smoother surface, which can be particularly important when trimming excess wax near the margin. Careless (excessive) carving in this area can result in abrasion of the die, creating a ledge around the finished casting.

0 0

Post a comment