Introduction
When providing removable partial dentures (RPDs), an accurate definitive cast of the arch after tooth preparation is essential for fabricating the framework.1 Definitive casts are conventionally obtained from irreversible hydrocolloid impressions and poured with gypsum. 1, 2 However, fractured teeth in the definitive cast can occur, especially when the abutment teeth are associated with periodontal disease, have retentive undercuts, or are lone standing. 3
In these situations, the conventional technique of obtaining definitive casts can become a challenge because the gypsum could fracture during removal of the tray from the set gypsum. 4 Therefore, techniques have been described for avoiding that situation, including the use of pins as additional reinforcement structures. 5 However, their placement adds an additional step and may interfere with critical areas in the cast. This report presents an alternative technique by coating the tray with melted wax before the impression to help obtain a precise definitive cast. The technique is straightforward, reproducible, and effective.
Figure 1
A: Clinical situation with long and retentive abutment teeth; B: Stock tray coated with even layer of melted wax.
Technique
The technique is illustrated in a patient with long and retentive abutment teeth (Figure 1).
Coat the stock tray with melted modeling wax (Cera 7; Lysanda Produtos Odontológicos) (Figure 2).
Make an impression with irreversible hydrocolloid impression material (Hydrogum; Zhermack GmbH Deutschland).
Remove the impression after its geleification by applying displacement force on the impression material at the depth of the vestibule to avoid displacing the impression material from the tray (Figure 3).
Pour the impression in a mold with a Type IV gypsum (Durone IV; Dentsply) and, after it has fully set, immerse it in hot water (Figure 4A) to soften the wax and allow displacement of the tray (Figure 4B).
Carefully trim away the impression material with a scalpel blade to expose the definitive cast.
Discussion
A fractured definitive cast may be repaired with an adhesive, but the result is an imprecise cast, risking a poorly fitting RPD framework.2 The technique described uses melted modeling wax in a tray to minimize the risk of fracturing the definitive cast, especially with long, retentive, or lone-standing teeth. Studies are needed to evaluate the clinical benefits of this approach and whether the accuracy of the impression might be adversely affected.
