Even a very small undercut on the die of a tooth preparation will result in an inability to remove the wax pattern. There may be small dimples in the die (resulting from caries removal or loss of a previous restoration) that are undercut relative to the path of placement of the new restoration. Normally such areas are blocked out intraorally with glass ionomer or restored with amalgam or another suitable foundation material as part of the mouth preparation phase (see Chapter 6). Occasionally, however, blocking them out on the working die may be more practical and convenient, as long as the defect does not extend to within 1 mm of the cavity margin. Zinc phosphate cement is a suitable material, but other commercial products (e.g., resin) are available for this purpose (Fig. 18-2).
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