READERS ROUND TABLE

porcelain. It is obvious, therefore, that Figs. 17 to 19 do not represent the true interface but a section through the porcelain away from the bonded interface. These surfaces would be porous as would be any fractured porcelain surface. Because the authors have not looked at the true interface, it appears that the conclusion regarding the amount of porosity at the interface of the bonded crown is speculation.

Furthermore, the porosity at the interface has been attrib- uted to tin oxide, which, according to Munoz et al., may have resulted in incomplete wetting of the tin-plated surface. Such a conclusion is in direct conflict with the authors’ observation that there was a bond between the core porcelain and the tin-plated platinum matrix in crowns built by the twin foil technique as opposed to no bonding achieved in conventional technique. It is inconceivable to us how chemical bonding between the porcelain and the tin-plated platinum can be accomplished unless the porcelain wets the tin-plated sub- strate.

These inconsistencies should be brought to the attention of the readers so that they can make informed decisions regard- ing the merits and demerits of the twin foil technique.

NIKHIL SARKAR, Ph.D. EDMUND E. JEANSONNE, D.D.S. LOUISIANA STATE UNIVERSITY SCHOLL OF DENTISTRY

NEW ORLEANS, LA 70119

REFERENCES

1. hlclean, J. W., and Seed, I. R.: The bonded alumina crown. I. The bonding of platinum to aluminous dental porcelain using tin oxide coatings. Aust Dent J 21:119, 1976.

2. Seed, I. R.. McLean, J. W., and Hotz, P.: The strengthening of aiuminous porcelain with bonded platinum foils. J Dent Res 56:1067, 1977.

3. Sarkar, N. I<., and Jeansonne, E. E.: Strengthening mechanism 01 bonded alumina crowns. J PROSTHET DENT 4595, 1981.

Reply

To THE EDITOR: The authors of the article “A Comparative Study of the

Strength of Aluminous Porcelain Jacket Crowns Constructed With the Conventional and Twin Foil Technique” (J PROSTHET DENT 48:271, 1982) offer the following clarifica- tion in response to Dr. McLean’s comments.

The article reports on a laboratory investigation of the comparative strengths of simulated clinical porcelain jacket crowns constructed by the two techniques. The objective of the study was to test the premise that the twin foil technique produces stronger aluminous porcelain crowns. Nowhere do the authors state or infer that based on the results of this study twin foil crowns are clinically inferior to those con- structed by conventional techniques. On the other hand, to

the authors’ knowledge there do not exist published data from a controlled clinical study that compare the clinical perfor- mance of jacket crowns constructed by the two methods. The only other known (laboratory) study that measured compar- ative strengths of simulated crowns is that of Minassian. Unfortunately, the results from his thesis have not been published, and thus a detailed comparison of the methodology could not be made to compare his data with the results of the authors’ article.

In reference to questions from McLean about underfiring of the core porcelain in this study, the authors would like to emphasize that the technique practiced was that prescribed in the literature and in detail by UNITEK, the supplier in the United States of materials used in the twin foil procedure. If McLean has data to support higher firing temperatures and/or longer firing times than those previously described to produce improved twin foil crowns? the authors would encourage publication of such data along with appropriate technique modifications. The authors feel comfortable that they have adequate clinical experience with porcelain jacket crowns.

The responses from Dr. McLean and Drs. Sarkar and Jeansonne imply that if a bond is achieved. there is little chance of having porosity. A bond was found with the twin foil crown but there was also porosity. Metal-ceramic crowns are known to have good bonding, but examination of the interface reveals porosity. Porcelain does wet the tin-plated surface and a bond is achieved, but porcelain seems to wet a nonplated matrix better, even though there ic not a bond.

The authors are in complete agreement wir h McLean and the published literature that glass-ionomer cements gain strength more slowly over a longer period when compared to silicophosphate cements and are in fact likely to be weaker 24 hours after cementation. The clinician who chooses to use the twin foil crown should be aware of this behavior and of the poor internal adaptation of the twin foii crown when selecting a luting cement.

In summary, the ultimate test of any new procedure is a carefully controlled clinical trial. Until such data become available, the clinician must weigh factors such as laboratory data, clinical experience, ease of fabrication. c>sthetics, and fit when deciding whether to use a new material or technique. We would urge Dr. Simmons to publish data from a controlled clinical study comparing the ainminous crown with the bonded alumina crown.

CARLOS MUNOZ, D.D.S., M.S.D. CHARLKS J. GOODACRE, D.D.S., M.S.D. B. KEITH MOORE, Ph.D. ROLAND W. DYKEMA, D.D.S., MAD. Iimmr\ ~JNIVEKSIT~

SCH~L OF DENTISTRY

I 171 W. MICHIGAN ST. IPil%9NAPOl IP. IN Ib’OZ

THE JOURNAL OF PROSTHETIC DENTISTRY 867