PubMed · 2392633
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W A Rogers. 1990. Evaluation for crown and bridge working models.. https://pubmed.ncbi.nlm.nih.gov/2392633/
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STATEMENT OF PROBLEM: Numerous materials and methods have been used for complete crown fabrication. Conventional investing and casting procedures for phosphate-bonded investments require a 2- to 4-hour process before completion. Accelerated laboratory techniques have been used, but may not result in castings with equal marginal accuracy. PURPOSE: This study measured the marginal gap and determined the clinical acceptability of single castings invested in a phosphate-bonded investment with the use of conventional and accelerated methods. MATERIAL AND METHODS: Forty-four individual stone casts were poured from impressions made from a master die. Conventional and accelerated methods of investing and casting were followed in the fabrication of 44 single-unit castings. Twenty-two casts were used in each of the 2 groups. Each casting and its respective stone die were examined with a microscope at 4 predetermined sites. Perpendicular and 25-degree tilted measurements of marginal gap were documented for each. Evidence of marginal gap was then evaluated by t test. RESULTS: Measurements recorded on the perpendicular and on a 25-degree tilt showed no statistically significant difference between conventional and accelerated groups. All gap measurements except one were within the range of clinical acceptability. The measurements revealed that conventional and accelerated perpendicular gap means were 13.2 and 13.6 microm, respectively, and the average tilted gap means were 31.6 and 32.2 microm, respectively. CONCLUSIONS: A phosphate-bonded investment (Ceramigold) selected for an accelerated casting technique produced single castings within 30 minutes with marginal gaps comparable to those found that used conventional methods.
When a tooth prepared for a crown does not comply with the general principles of retention and resistance form, and it is not possible to correct it without damage to the biologic structures of the tooth, consideration must be given to the purpose of the crown, such as whether it is a single crown, a retainer for a fixed restoration, or a retainer for a fixed removable prosthesis with precision attachments. The crown prepared with receptacles for internal attachments will undoubtedly require more retention and resistance to dislodgment than a single crown because of forces that will be applied to it through the removable portion of the restoration. The purpose of this article is to present an alternative procedure that uses the intracrown ring, which provides additional retention for the abutment crown because of the compressible ring. The compressible ring is placed in the groove prepared in the lower third of the abutment during the crown preparation.
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