Since the 1920s,2 practitioners have recognized that a space should exist between the internal surface of the casting and the prepared surface of the tooth everywhere except immediately adjacent to the margin. The space provides room for the luting agent and allows complete seating of the restoration during cementation (see Chapters 7 and 31). At the preparation margin, there should be a band of close adaptation (about 1 mm wide) to prevent dissolution of the luting agent. The ideal dimension 3-5 for the cement space has been suggested at 20 to 40 um for each wall, which implies that a complete crown should have an internal diameter between 40 and 80 um larger than the diameter of the prepared tooth. By using available techniques in an appropriately standardized manner, such a degree of casting adaptation can routinely be obtained, independent of the geometry of the finish-line -6'7
If the cement space is too narrow, the casting will not seat properly during cementation because of hydraulic pressure that develops when the viscous mass of luting agent cannot escape through the narrow gap between crown and preparation as the restoration is seated. Conversely, if the cement space is too wide, the casting will be loose on the tooth, resistance form (see Chapter 7) will be reduced, and the position of the casting will be difficult to maintain accurately during evaluation and occlusal adjustment. In addition, the risk of the crown loosening during function increases considerably, and its longevity is adversely affected. The precise amount of cement space obtained depends on the materials and techniques used in the indirect process, particularly the choice of impression material (see Chapter 14), die material (see Chapter 17), investment (see Chapter 22), and casting alloy (see Chapters 19 and 22 and Fig. 18-1). These factors directly affect the size of the cement space.
Increasing the Cement Space. A number of factors increase the cement space for a complete crown:
1. Thermal and polymerization shrinkage of the impression material (see Chapter 14)
2. Use of a solid cast with individual stone dies (see Chapter 17)
3. Use of an internal (initial) layer of soft wax
4. Use of die spacers
5. Increased expansion of the investment mold (see Chapter 22)
6. Removal of metal from the fitting surface by grinding, airborne-particle abrasion, etching with aqua regia, or electrochemical milling
All factors being equal, the factors just mentioned result in an increased distance between the internal surface of the casting and the surface of the prepared tooth. Although the dentist has little control over the polymerization shrinkage of impression materials, die system selection has a direct influence on the size of the wax pattern. Using a multiple pour system for fabrication of a solid master cast and a separate die will yield a die that is slightly larger with some impression materials, effectively stretching the pattern, which results in a proportionally larger casting. An internal layer of soft wax is subject to slightly more compression by the setting refractory investment material, leading to a looser fit. Spacers enlarge the die by coating the oc-clusal surface and vertical axial walls with a thin layer of rapidly drying paint. The expansion of the investment mold can be increased by heating the mold to a slightly higher temperature during the wax elimination phase, and metal can be removed from the internal surface of a cast crown through air-abrasion, etching, or milling procedures.
Reduction of the Cement Space. A number of factors reduce the cement space:
1. Use of resin or electroplated dies
2. Use of alloys with a higher melting range
Resin and electroplated dies are slightly smaller than stone dies and will therefore result in a smaller casting. As alloys cool over a larger temperature range, the additional shrinkage that takes place will cause the same. If the investment is mixed with an adjusted water/powder ratio resulting in less setting expansion, the size of the resulting casting is again reduced. When problems routinely surface with castings that are either too loose or too tight, any of the previously mentioned variables may be altered, leading to more predictable results.
Problems with fitting castings become apparent at two stages of the indirect procedure: when the casting is tried on the die and when it is cemented. Recognizing problems at each stage and correcting them before proceeding is crucial. Difficulty with seating the casting on the die is generally due to wax distortion, the presence of flash extending cervical to the preparation margin (excess wax that was not removed before the investing and casting procedure), improper investment expansion (underexpansion), or a casting nodule (Fig. 18-3). Modification of the investing and casting protocol will solve these problems (see Chapter 22). Consistent problems with castings that do not seat completely when tried on the prepared tooth may be corrected by changing just one variable in the protocol. Although most practitioners
advocate the routine use of die spacer, this is just one of many options to influence the size of the resulting cement space.
Die Spacer (Fig. 18-4). This material (similar to model airplane paint8) is applied to the die to increase the cement space between axial walls of the prepared tooth and the restoration. It is formulated to maintain constant thickness when painted on the die. However, it should not coat the entire preparation. For adequate marginal adaptation, a band of
about 1 mm must be left unpainted. 3 Thinner is provided to replace the solvent, which tends to evaporate, resulting in an excessive thickness of spacer.
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