Dimensional accuracy of small gold alloy castings. Part 4. The casting ring and ring liners

Our plain-English summary of Dimensional accuracy of small gold alloy castings. Part 4. The casting ring and ring liners, by E.F. Morey (Australian Dental Journal, 1992).

The study

• Author: E.F. Morey
• Journal: Australian Dental Journal
• Published: 1992;37(2):91-7
• Type of article: Review
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The role of the casting ring and its asbestos liner is discussed. Asbestos as a liner has now largely been replaced by two alternative materials, one based on cellulose and the other on ceramic fibres. The limited literature on the effect of these newer materials on casting accuracy is also reviewed as their introduction may require significant changes in the traditional technology of dental casting.

In plain English

This article, the fourth in a series by the author, was a review of published literature on the metal rings used to cast small gold alloy dental restorations and on the liners placed inside them. It traced how wet asbestos paper came to be used as a soft liner, and summarized laboratory studies which, the author wrote, all confirmed the need for a liner or cushion so the mould material could expand. The review noted that reports had stated asbestos fibres in casting ring liners can cause asbestosis, bronchogenic lung cancer or mesothelioma. It added that it had been claimed exposure limits can be considerably exceeded when castings are recovered from asbestos-lined rings. It also cited a statement attributed to Selikoff that there had been no reports of disease or death among dentists or dental technicians from asbestos used as a ring liner. The author suggested that silica in investment powders was likely a greater health risk. Asbestos had largely been replaced by cellulose and ceramic fibre liners. The author concluded that the limited literature suggested either could be acceptable if their different characteristics were recognized.

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: Narrative literature review (part 4 of a series on the dimensional accuracy of small gold alloy castings); no new study was carried out
• Who was studied: No people were studied. The article reviewed published dental literature and laboratory studies on casting rings and ring liners, from Taggart's 1907 casting technique to studies of alternative liners in the 1970s and 1980s.
• Exposure: Wet asbestos paper used as a casting ring liner in dental laboratories (the type of asbestos in liners is not specified), and the replacement liners: absorbent cellulose and non-absorbent ceramic liners made from fibres of an alumino-silicate glass derived from kaolin
• Main results:
  • The author wrote that, although the reviewed laboratory studies used widely varying methods, they all confirmed that a liner or cushion is needed in the casting ring because unlined rigid rings restrict expansion of the investment.
  • The review reported that other sources had stated asbestos fibres in casting ring liners can cause asbestosis, bronchogenic lung cancer or mesothelioma.
  • The review noted a claim from another study that the airborne asbestos threshold limit value can be considerably exceeded when dental castings are recovered from asbestos-lined casting rings.
  • The review cited Selikoff, as quoted by Davis, as pointing out that there had been no reports of diseases or deaths of dentists or dental technicians from asbestosis, mesothelioma or bronchogenic lung cancer induced by asbestos used as a ring liner for over fifty years.
  • The author's own view was that silica in dental investment powders is likely a greater health risk than either asbestos or ceramic ring liners.
  • For ceramic liner fibres, which some dental literature suggested could pose a health risk, the review noted that experimental studies reviewed by Davis produced little evidence that glassy fibres can produce pulmonary fibrosis or pulmonary carcinomas.
  • Several reviewed studies of casting accuracy found no statistically significant difference between castings made with asbestos liners and those made with ceramic liners (Steinbock and Asgar; Khatchaturian and Caputo; Warfield, Lipson and Priest).
  • In the study by Gil, Moore and Dykema, dry ceramic liners gave very inconsistent castings, which those authors linked to vacuum investing. In that study a single layer of wet asbestos gave the best overall fit.
  • In the study by Yli-Urpo, Oilo and Syverud, ceramic liners gave the most uniformly accurate castings with a gypsum-bonded investment, and cellulose gave the least accurate and most variable castings with both investments tested.
• The authors' conclusion: The author concluded that the limited literature suggests either cellulose or ceramic liners may be acceptable replacements for asbestos if their different characteristics are recognized. The author added that vacuum investing should apparently be avoided with ceramic liners, and that an investment with greater potential expansion would be desirable with them.
• Limitations the authors state:
  • The author noted that, because cellulose and ceramic liners were recently introduced, there were only limited data on their effects on investment expansion and casting accuracy.

Related studies

• The fit of gold-alloy full-crown castings made with pre-wetted casting ring liners — E.F. Morey, R. Earnshaw (1992)
• 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)
• An investigation of the potential biohazards associated with asbestos-substitute casting ring liners — G.J. Tulachka, W.A. Brantley, et al. (1987)
• Release of asbestos fibers during casting ring liner manipulation — D.R. Davis (1995)
• Full-crown castings made with water-saturated and dry casting ring liners — F. Keyf, G. Erman (2000)

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