The accuracy of occlusal cusp height in cast crowns

Our plain-English summary of The accuracy of occlusal cusp height in cast crowns, by J. Takahashi, M. Okazaki, H. Kimura, et al (The Journal of Prosthetic Dentistry, 1983).

The study

• Authors: J. Takahashi, M. Okazaki, H. Kimura, K. Hiraiwa, Y. Iwakawa, S. Mori, K. Joshin, Y. Moriwaki, Y. Doi
• Journal: The Journal of Prosthetic Dentistry
• Published: 1983;50(3):392-8
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Despite a good fit at the finish line, the occlusal cusp height decreased about 80 micron in gypsum-bonded cristobalite investments and increased about 40 micron in phosphate-bonded investments. Disks, tested as simulated occlusal portions of cast crowns, produced similar dimensional changes. This finding suggests that the occlusal cusp height is closely related to the thickness of the disk. An asbestos liner used with a gypsum-bonded investment failed to furnish adequate cushioning. The Kaowool liner provided good accuracy to the occlusal portion and finish line of cast complete crowns.

In plain English

This 1983 laboratory study came from dental schools in Japan. It concerned how accurately metal dental crowns are cast and did not study human health. Asbestos appeared only as a material: a dry asbestos sheet 0.8 mm thick lined the metal rings in which wax crown patterns were set in casting material (investment) before metal was cast. The authors made wax crowns on artificial stone models of a lower first molar, enlarged one and a half times. They cast them in two gypsum-bonded cristobalite investments and five phosphate-bonded investments, using a copper-zinc-silver alloy and a gold alloy. Although the crowns fit well at the edge, cusp height fell about 80 micrometres with the gypsum-bonded investments and rose about 40 micrometres with the phosphate-bonded ones. Metal disks cast from wax disk patterns showed matching changes in thickness, becoming thinner than their wax patterns in a gypsum-bonded investment. The authors found that a liner called Kaowool cushioned better than asbestos. They concluded that an asbestos liner used with a gypsum-bonded investment failed to provide adequate cushioning, while the Kaowool liner gave good accuracy to cast crowns.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) dental materials experiment on the accuracy of cast crowns and cast disks; not a health study
• Who was studied: No people were studied. Wax crown patterns were made on artificial dental stone models of a lower (mandibular) first molar, enlarged one and a half times, and wax disks of different sizes were also used. The work was done at Osaka University Dental School, Japan. Figures 7 and 8 report four and seven examples, and six and four examples, for the two alloys in two of the investments.
• Exposure: Not a health exposure. Dry asbestos 0.8 mm thick (fiber type not stated) was used as a liner in metal casting rings. Other liners were also tested, including Kaowool, which is made from SiO2 and Al2O3 fibers. Castings used gypsum-bonded cristobalite and phosphate-bonded investments with a Cu-Zn-Ag alloy and a type IV gold alloy.
• Main results:
  • Despite a good visual fit at the finish line, occlusal cusp height decreased about 80 micrometres with gypsum-bonded cristobalite investments and increased about 40 micrometres with phosphate-bonded investments; the difference between the two kinds of investment was reported as significant.
  • There were no significant differences among the five cusps or between the two alloys.
  • The change in cusp height was related to the change in thickness of the occlusal plate of the crown, not to the gap between casting and die.
  • Metal disks cast from wax disk patterns became thinner than their wax patterns in a gypsum-bonded investment.
  • The agreement between the change in cusp height and the change in disk thickness was also seen with phosphate-bonded investments.
  • In cast disks, the change from wax pattern to casting was greater for small-diameter disks than for large-diameter disks.
  • Kaowool had a higher cushioning effect than asbestos. A Kaowool liner 1 or 2 mm thick, or an asbestos liner 1.6 mm thick, gave more accurate disk thickness.
  • A crown cast in a gypsum-bonded cristobalite investment with a 2 mm Kaowool liner fit well at the finish line and had an accurate cusp height.
• The authors' conclusion: The authors concluded that an asbestos liner used with a gypsum-bonded investment failed to provide adequate cushioning, and that the Kaowool liner gave good accuracy to the occlusal portion and finish line of cast complete crowns.
• Limitations the authors state:
  • The authors stated that the difference in dimensional change between the two kinds of investment may relate to expansion and high-temperature strength but could not be clearly explained.

Related studies

• The fit of gold-alloy full-crown castings made with pre-wetted casting ring liners — E.F. Morey, R. Earnshaw (1992)
• Full-crown castings made with water-saturated and dry casting ring liners — F. Keyf, G. Erman (2000)
• Dimensional accuracy of small gold alloy castings. Part 4. The casting ring and ring liners — E.F. Morey (1992)
• A procedure for restoring proximal contact surfaces of cast gold restorations with solder — P.M. Kimondollo (1991)
• Dimensional accuracy of small gold alloy castings. Part 1. A brief history and the behaviour of inlay waxes — E.F. Morey (1991)
• The effect of casting ring liners on the potential expansion of a gypsum-bonded investment — R. Earnshaw (1988)

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