Objective

To minimize any dissolution of the luting agent, a 1-mm-wide band of metal must be obtained that is closely adapted to the tooth surface.⁴ A defect within this zone can significantly reduce a restoration’s longevity. Good adaptation is obtained by carefully reflowing the wax pattern (Fig. 29-2). With careful standardization of technique, the dentist can achieve predictable and consistent results.

Fig. 29-2 Reflowing the wax pattern. The objective is to create a well-adapted 1-mm zone to prevent cement dissolution. Proper reflowing before investing is a prerequisite to accomplish this.

Procedure

If a defect occurs in the marginal area, the restoration must be remade. This may require an additional patient visit to make the new impression. Defects can be prevented or minimized by paying particular attention to reflowing the margins of the wax pattern and through careful investing (see Chapter 22).

Even small nodules can prevent a casting from seating completely. Careful examination under ample magnification helps identify interferences. Small nodules, if far enough away from the margin itself, can be removed under a binocular microscope with exceptionally cautious use of very small rotary instruments (e.g., a No. ¼ round bur). However, great care is needed to avoid damage to the margin and annoying remakes.

Objective

No contact should exist between the die and the internal surface (intaglio) of the casting. A uniform space of 25 to 35 μm is necessary for the luting agent to spread evenly. Any contacts must be identified and relieved by careful selective grinding of the internal surface.

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.

Fig. 29-3 Nodules on a casting have resulted from improper investing. To enable complete seating of the casting, even small ones such as these must be removed entirely.

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.

Fig. 29-4 A relatively small nodule (arrow) results in a substantial open margin.

Fig. 29-5 A, Internal surface of a casting. Note the stone (arrow) adhering where the die has been abraded by the casting. B, A suitable marking agent (e.g., rouge and turpentine) can be used to detect areas that must be relieved to allow complete seating.

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).

Fig. 29-6 When removing a nodule, remove slightly more than the defect to ensure complete seating of the restoration.

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).

Fig. 29-7 Liquid marking agents can be helpful if the internal surface of a casting has a nodule. A, Incomplete seating. B, Liquid marking agent. C, A thin coat is applied to the internal surface and air dried. D, Casting gently returned to the die. E, Area of interference identified. F, Nodules are best removed with a small round bur. G, Seated casting.

Objective

To reestablish proper coronal structure and function, the sprue must be sectioned, and the casting must be recontoured in the area of its attachment.

Procedure

Once the fit of the casting has been verified on the die and it has been found to be acceptable, the sprue is sectioned, and the area of its attachment to the casting (Fig. 29-8) is reshaped.

Fig. 29-8 A, The button is most effectively removed by cutting around the sprue and then twisting it off (B). C, With multiple castings made simultaneously, access is more difficult. When it is necessary to sever a sprue completely, care must be taken not to damage the margin inadvertently. D, Disks and stones are used for gross recontouring.

A carborundum separating disk is used to cut through the sprue. Cutting should be performed circumferentially, with a small area maintained in the center of the sprue. This last connection is broken by twisting and separating it from the casting. Wire cutters are not recommended, because they may lead to distortion of the casting. Any excess in the area of the sprue attachment is removed with the disk, and the area is refined with stones and sandpaper disks.

Objective

The proximal contact areas are adjusted in the laboratory so that they will be correct (or slightly too tight) when the casting is evaluated in the mouth.

Procedure

Special care is needed to prevent the finishing procedures from producing an overreduced and, consequently, inadequate proximal contact. Although this can be corrected with solder (see p. 889), it is a time-consuming and unnecessary procedure.

A slightly excessive contact, however, may be corrected easily during clinical evaluation. The proximal contacts on the stone cast can be minimally relieved by careful scraping with a scalpel (Fig. 29-9). The casting is then adjusted until it just seats. When adjacent castings are made, they should not be simultaneously adjusted to seat on the definitive cast. Under these circumstances, the proximal contacts should be left slightly too tight in the dental laboratory. For such multiple castings, clinical evaluation is done sequentially and on an individual basis. Adjustments are made for each casting independently.

Fig. 29-9 Rather than risk a deficient proximal contact at evaluation, the technician may reduce the cast slightly by scraping the adjacent teeth with a blade.

When proximal contacts are adjusted, placing a thin Mylar articulating film between adjacent castings or between the casting and the adjacent tooth is helpful (Fig. 29-10). Doing this allows the areas where binding contact occurs to be adjusted through selective adjustment where markings result.

Fig. 29-10 A, Thin articulating film interposed between a metal-ceramic restoration and the adjacent tooth. B, The area of contact that prevents complete seating is readily apparent. C to E, Articulating film is used to detect the location of an excessive proximal contact on cast metal.

Connectors

When a partial fixed dental prosthesis is being finished, the connectors require special attention. Unless they are properly contoured and highly polished, periodontal health is invariably adversely affected, even in the presence of the most meticulous oral hygiene. Mesiodistally, a properly finished connector has a parabolic configuration (Fig. 29-11). Rotary instruments such as rubber wheels, which allow access to the cervical aspect of the connector for finishing while not jeopardizing the margin, are essential in these situations. In cases of root proximity between adjacent teeth, this can be quite problematic. After preliminary finishing with rubber wheels, a piece of twine can be used to impart the final polish to the cervical aspect of the connector (Fig. 29-12).

Fig. 29-11 Cross-section depicting properly finished connectors.

Fig. 29-12 Polishing connector areas. A and B, Twine impregnated with polishing compound is an efficient way to polish this hard-to-reach area.

Objective

Occlusal contacts are reestablished in static and dynamic relationships to the opposing arch. Obtaining accurate and stable contacts does not require highly polished metal occlusal surfaces. A satin finish is acceptable. Occlusal structure must ensure positional stability and satisfy all functional requirements (see Chapter 4).

Procedure

The occlusal contacts are checked with thin Mylar articulating film (Fig. 29-13) to ensure that they match the design in the waxing stage. If they do not, the occlusion must be adjusted. Wax is subject to elastic recovery. If an occlusal contact is heavy in wax, it springs back slightly when the articulator is opened and produces an occlusal prematurity in the casting (Fig. 29-14). If “pinpoint” contacts are established carefully during the waxing phase, significant occlusal adjustment should not be necessary.

Fig. 29-13 After complete seating is verified, the initial point of contact is marked.

Fig. 29-14 A and B, Occlusal prematurities are generally the result of excessively heavy contact on the wax pattern.

Occlusal adjustments can be performed with flame-shaped finishing burs or diamonds (Fig. 29-15). A large stone creates unwanted concavities in the occlusal surface. The correct technique for occlusal adjustment is to redevelop the anatomy of the entire ridge or cusp rather than grinding only the point of interference. Simultaneously, any nodules can be removed, and grooves can be defined with a finishing bur or a small round bur.

Fig. 29-15 A, Occlusal adjustment is readily accomplished with a pointed diamond or carbide bur. B, The grooves and fissures are concurrently refined.

Before starting any adjustment, the practitioner should use a thickness gauge on the metal. If only minimum clearance was established at the tooth-preparation stage, indiscriminate adjustment leads to inadequate thickness of the casting (Fig. 29-16) and possible perforation. Although soldering such a hole in a casting is possible, the occurrence of this complication usually indicates an earlier error that requires correction (e.g., inadequate clearance necessitates additional reduction of the tooth preparation).

Fig. 29-16 As occlusal adjustments are made (A), the residual thickness is continually monitored with an appropriately designed thickness gauge (B). For structural durability, less than 1.0 mm is inadequate metal thickness and is the result of insufficient occlusal reduction.

After the occlusal contacts have been refined, they must not be altered by extensive polishing. A high polish may be essential for plaque control on axial surfaces (Zones 6 and 7), but its benefit is questionable on the occlusal surface of metal castings. In fact, an accurate occlusion so painstakingly established in wax can be rapidly destroyed by overzealousness to make a casting “look pretty.”

If the wax pattern has been carefully finished, a smooth casting results, and removing surface oxides with a soft wire brush wheel is sufficient. The surface can then be polished with rouge on a soft brush wheel (which removes only 5 μm from the surface of the casting⁷) (see Fig. 29-19).

Fig. 29-19 Finishing and polishing. A, Initially a wire brush is used on the occlusal surfaces. B, A fine-grit sandpaper disk is applied for removing pits and irregularities from the axial walls. Note that the margin is not touched at this time. C and D, Rubber points and small carbides for selective finishing of the occlusal structure. E, A rubber wheel is then used on the axial walls. F, Castings, after polishing with buffing compound, immediately before clinical evaluation. G, When the fit has been verified clinically, the margins are polished. H and I, The completed castings immediately before cementation. J, Scanning electron micrograph of a gold alloy in the “as-cast” state. K, The same casting after finishing and polishing with a series of abrasives culminating in rouge.

(J K, Courtesy of Dr. J. L. Sandrik.)

Some authorities⁸ recommend producing a matte finish on the occlusal surfaces to aid in the initial identification of wear facets during function, which show up as shiny marks on an otherwise dull surface. This type of finish is usually achieved with an airborne particle abrasion unit and 25- to 50- μm Al₂O₃ (alumina) particles. However, a 5-second blast with 50-μm alumina at 0.5 MPa (73 psi) pressure removes about 20 μm of metal from the air-abraded surfaces⁹; therefore, the margins should be protected.¹⁰ An exposure of about 1 second usually produces a smooth satin finish. If this cannot be accomplished, it is likely that the preparation before this step was deficient, and further refinement is necessary.

Objective

When axial wall finishing is completed, the walls should be smoothly contoured and highly polished, enabling the patient to carry out optimum plaque control.

Procedure

Surface defects are removed by grinding with abrasive particles bound into a grinding stone or rubber wheel, on a paper disk, or applied as an abrasive paste (Fig. 29-17). Each particle acts as a cutting tool on the metal surface.

Fig. 29-17 Abrasives for finishing. A sequence of progressively finer grades is used to attain the desired surface. Carborundum disks and stones of varying degrees of coarseness (A) are typically used first; these are followed by garnet paper and sandpaper disks (B), rubber points and white Arkansas stones (C), and rubber wheels and points along with small carbide burs for removing nodules (D).

The most efficient method of polishing¹¹ is a sequence of progressively finer abrasives (Fig. 29-18), each removing the scratches made by the previous grade. Time is wasted if the progression to a finer grade abrasive is too rapid, because the coarser grits remove material much more efficiently.

Fig. 29-18 Finishing armamentarium. A, Assorted abrasives, sandpaper disks, rubber points, and polishing wheels. B, Instruments used range from small carbides (for removing nodules) and a steel wire brush (for occlusal surface smoothing) to buffing wheels and compounds. C, A coarse wheel is used to true and thin the edge of a rubber wheel. D, Buffing compounds applied on a felt wheel or bristle brush.

Light pressure is applied when abrasives are used, and the instrument must be kept rotating; otherwise, the surface of the casting is ground into a series of facets that ultimately impede plaque control. When all surface irregularities have been removed and the progression through the series of abrasives has left a finish with only minute scratches, the axial surfaces of the restoration are polished. Jeweler’s rouge rapidly produces a high polish on a well-prepared surface of a dense casting (Fig. 29-19K). This is carried on a wheel or brush, with heavier pressures and higher rotational speeds than were used in finishing (see Fig. 29-19).

Objective

Margin finishing is crucial for a restoration’s longevity and therefore merits special attention. The objective of all cast restoration finishing is a highly polished metal surface without ledges or steps as the transition is made from restoration to unprepared tooth. Failure to accomplish this leads to compromised plaque control.

Procedure

Where access allows, cavosurface margins should be finished directly on the tooth (see Fig. 30-10). Unfortunately, the areas where access for finishing is restricted (e.g., proximally or subgingivally) are precisely where plaque control presents the most problems. Therefore, only the least crucial areas can be finished intraorally. An advantage of partial-coverage restorations over complete crowns is that they allow better access for finishing margins and for subsequent plaque control.

The parts of the margin that cannot be finished on the tooth are finished on the die (Fig. 29-20). Care must be taken not to remove more metal than is strictly necessary. Excessive finishing creates problems similar to those caused by incomplete polishing. This raises the issue of how much material can be removed from the surface of a casting without compromising the ultimate fit and emergence profile of the finished restoration.

Fig. 29-20 When subgingival margins do not allow access, final finishing is performed on the die. A to C, During final polishing, the margin is carefully supported with a finger.

A stone die from a polysulfide impression is approximately 25 μm wider than the tooth because of polymerization and thermal shrinkage of the impression material and expansion of the gypsum.¹² In theory, therefore, if 12.5 μm is removed during finishing, the casting will be flush with the tooth surface. Although these values cannot be measured on a day-to-day basis in a dental office, they illustrate the tolerances of, and restrictions imposed by, the materials that are currently in use.

The edge of the margin must not be distorted during finishing, although carefully rubbing a smooth instrument along the length of the margin (burnishing) (see Fig. 29-20A) may improve the margin,^13,14* but only when softer alloys are used.¹⁵

Finishing should be performed by gently brushing a fine-grit stone over the surface to remove casting roughness (see Fig. 29-20B). This is followed by a soft rubber wheel or point (see Fig. 29-20C) and finally by rouge on a brush. The margin should be supported with a finger during final polishing.

When the casting is smooth on all critical surfaces, any remaining polishing compound can be removed with a soft toothbrush, by ultrasonic cleaning in an appropriate solution, or by steam cleaning.