29: FINISHING THE CAST RESTORATION

29 FINISHING THE CAST RESTORATIONimage

KEY TERMS

A cast metal restoration is not ready for evaluation and cementation merely because it has been stripped of its investment. The unpolished surface is relatively rough, and a series of finishing procedures are needed to produce highly polished axial surfaces. Such surfaces limit the accumulation1,2 and retention3 of plaque and facilitate maintenance of health of the supporting periodontal tissues. The sprue needs to be removed, and the area of its attachment needs to be recontoured. Any nodules or other minor irregularities remaining on the cast surface must be eliminated.

Metal finishing for metal-ceramic restorations is similar to that for cast metal. The discussion in this chapter is applicable to both restoration types. In practice, the final polishing of metal-ceramic restorations is not done until after characterization and glazing (see Chapter 30).

OBJECTIVES AND PROCEDURES

The objectives and procedures for finishing are different for each part of the cast restoration. The following discussion is sequentially divided into corresponding phases; each is identified as a zone (Fig. 29-1).

Zone 2: Internal Surface (Intaglio)

Procedure

Under normal circumstances, a casting’s internal surface does not require finishing. It should, however, be examined for nodules (Fig. 29-3) before the restoration is seated on the die. Nodules can be removed with a small round carbide bur, which can be time consuming because it may need to be repeated several times. If the internal surface needs to be adjusted more than occasionally, the investing procedure should be reexamined for flaws.

Even a very small nodule can result in significant increase of the marginal gap width (Fig. 29-4). A binocular microscope is especially helpful in identifying nodules. High-quality loupes can also be used. Great care should be exercised when seating a casting on its die. Any significant force will abrade or chip the die so that the casting will seat on the die but will not seat fully on the prepared tooth. Overlooking this at the cementation appointment will result in a restoration with open margins and a poor prognosis. If a casting does not seat, a nodule may have been overlooked and may have scratched the die, or a little stone may have been picked up in the process. Close examination of the internal surface of the casting or the axial walls of the die (Fig. 29-5) will reveal this. Corrective action is often relatively simple, and the casting may be acceptable. Care must be taken not to seat a faulty casting repeatedly, thereby abrading the die. After a die has been abraded by a casting, it should not be used for rewaxing a restoration. If rewaxing is necessary, a new impression is required.

When a nodule is removed from the internal aspect of a casting, deliberately removing a slightly greater amount of alloy in the area is recommended. Once the casting has been adjusted, determining the exact location of the nodule is no longer possible. Therefore, the nodule should be removed entirely in one step, rather than through sequential relief of the internal surface (Fig. 29-6).

Indiscriminately removing material from the internal aspect of any casting is not an acceptable alternative. This results in excessive loss of retention and resistance form, and the restoration must be remade.

Marking agents

Several agents are commercially available to facilitate identification of the seating interference between the casting and the die. These include water-soluble dies (e.g., Liqua-Mark*), solvent-based dies (e.g., AccuFilm IV), and powdered sprays (e.g., Occlude). A suspension of rouge in turpentine or an elastomeric detection paste (e.g., Fit Checker, Fit Checker II§) can also be used as an alternative.5,6 These agents should be applied as a thin film to the internal surface of the casting. High magnification of the casting after seating reveals initial contact for grinding (Fig. 29-7). Regardless of the method used, the internal surface of the casting should always be thoroughly cleaned before the luting procedure (see Chapter 31).

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on 29: FINISHING THE CAST RESTORATION
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