30: EVALUATION, CHARACTERIZATION, AND GLAZING

30 EVALUATION, CHARACTERIZATION, AND GLAZINGimage

KEY TERMS

EVALUATION

When the laboratory procedures have been completed, the restoration is ready to be evaluated in the patient’s mouth before final finishing and cementation. The completed prosthesis is cleaned either ultrasonically or with a steam cleaner to remove any residual polishing compound and then disinfected. Metal castings need to be evaluated in terms of proximal contacts, margin integrity, stability, internal fit, external contours, occlusion, and surface finish.

Metal-ceramic restorations often require two evaluations: a metal evaluation stage, followed by reevaluation after the esthetic veneer has been applied. At the metal evaluation appointment, the margin integrity, stability, occlusion, and substructure design are evaluated. Especially important at this appointment is the assessment of the cut-back area. Minor adjustments can be made: for instance, by extending the veneering surface slightly interproximally to enhance the appearance of the completed prosthesis. During the subsequent bisque evaluation, marginal integrity and stability are reassessed to determine whether any distortion has occurred during porcelain firing. Proximal contacts are also evaluated during this stage, as are porcelain contours, stability, and the shade, texture, and glaze. For fixed dental prostheses (FDPs), tissue contact of the pontics and the location and shape of connectors need careful assessment; otherwise, tissue irritation may occur. Primarily because of the inevitable inaccuracies that result during the indirect technique and the high degree of precision needed for a successful fixed prosthesis, only rarely does the restoration not require some chairside adjustment.

Interim Restoration and Luting Agent

The interim restoration is removed by gently positioning hemostats or a Backhaus towel clamp on the buccal and lingual surfaces and rocking it gently in a buccolingual direction to break the seal of the interim luting agent. Special band removers (Fig. 30-1) may also be used. Most of the luting agent or interim cement adheres to the interim restoration when it is taken out of the mouth. Any remaining cement should be loosened from the prepared tooth surface with an explorer, followed by careful application of a water-pumice mixture* in a prophylaxis cup. Slow speed and relatively light pressure are essential. Polishing the preparation is undesirable because it may lessen retention. The preparations are rinsed with water and air spray, and after drying, the area is inspected. All residual luting agent must have been removed, because even a very small particle of interim cement can prevent a casting from seating completely.

Proximal Contacts

The location, size, and tightness of a restoration’s proximal contacts should resemble those of the natural teeth. Typically, textbooks refer to contacts that allow unwaxed floss to “snap” through “relatively easily.” Although this is not a very scientific definition, the use of floss is a convenient method for comparing the contacts with other contacts between adjacent teeth in the arch. If the floss does not pass, the contact is excessively tight; if it goes through too easily, food impaction may result (Fig. 30-2). The use of shim stock (thin Mylar film) is probably a more reliable indicator of proximal contact. The ideal contact should allow for positional stability of the abutments and adjacent teeth, as well as easy maintenance of the supporting structures. Most patients give reliable information as to a tight proximal contact when asked whether they “feel as though they have a seed between their teeth,” provided that a local anesthetic has not been administered. A deficient contact is easily overlooked but invariably results in discomfort as food becomes impacted.

Deficiency

Margin Integrity

The completed restoration should go into place without binding of its internal aspect against the occlusal surface or the axial walls of the tooth preparation; in other words, the best adaptation should be at the margins. If the indirect procedure is handled properly, there should be no noticeable difference between the fit of a restoration on the die and that in the mouth.

Several techniques have been used to detect where a casting binds against an occlusal or axial wall, including disclosing waxes, a suspension of rouge in turpentine or acetic acid, air abrasion to form a matte finish surface, powdered sprays, water-soluble marking agents (Fig. 30-7), and special elastomeric detection pastes. However, none has proved to be entirely satisfactory. Most techniques are rather messy and time consuming and should not be needed on a routine basis.

Nevertheless, elastomeric paste (Fig. 30-8) has some advantages. The material is similar to a silicone impression material and is obtained as a two-paste system. Its viscosity is similar to that of the final luting agents, and so it can be used not only to identify unwanted internal contacts but also to assess adequate marginal fit. The degree of clinically acceptable marginal opening (i.e., the discrepancy unlikely to have an adverse effect on the prognosis) is hard to define. Margin integrity has been the subject of many laboratory and clinical evaluations. Obviously, to limit dissolution of the luting agent, the thickness of the cement film at the margins should be kept minimal. Through careful technique, a marginal gap width of less than 30 μm can be obtained consistently.1,2

Occlusion

After the restoration has been seated and the margin integrity and stability are acceptable, the occlusal contact with the opposing teeth is carefully checked. The criteria for these relationships, both static and dynamic, have been discussed in Chapters 4 and 18. Any undesirable eccentric contacts as well as centric interferences must be identified. Minor adjustment of eccentric contacts may be needed if a semiadjustable (as opposed to a fully adjustable) articulator is used.

Evaluation and adjustment

Step-by-step procedure (Fig. 30-11)

An alternative technique requires the use of an airborne particle abrasion unit with aluminum oxide (Fig. 30-13). A matte finish is obtained on the occlusal surfaces of the casting in question, and the patient is asked to close. Where shiny marks appear, an adjustment is made. This technique, however, presents the following disadvantages:

Gross occlusal adjustment involving dental porcelain is better done in the bisque stage, because interferences are more easily marked on a bisque surface than on glazed porcelain. Minor adjustments are needed after glazing because of the pyroplastic flow of the porcelain.7 After adjustment, the porcelain can be polished with silicone wheels or diamond-polishing paste.

Remount

If there is a need for significant occlusal adjustment, a remount procedure8 may be recommended. It is typically used when extensive restorative dentistry has been performed, and it serves to convey the relationships of the restorations and teeth to the dental laboratory (Fig. 30-14). Detailed adjustments can then be made in an organized manner. Any inaccuracy (e.g., slight tooth movement, previous mounting discrepancies, or small dimensional change inherent with the indirect process) can be compensated for relatively easily, thus reducing the amount of chair time needed for precementation adjustment.

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on 30: EVALUATION, CHARACTERIZATION, AND GLAZING
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