Seventy sticky years: Adhesion in dentistry

Coffee has been used to measure the weakest link in restorations - Adhesion

For more than seventy years researchers and clinicians have been seeking the solution to the weak-link in restorative dentistry: Adhesive bonding of the replacement material to a natural tooth.

This search has taken many interesting turns over the years and many a promising lead has often turned into a dead end road.

The story, however, is not just about adhesives.

The basic and obvious goal of restorative dentistry is that the repair or replacement should keep its size, shape and utility long after the work is completed.

In the 1942 edition of The Science of Dental Materials, Eugene Skinner wrote of silicate cement restorations: “Tooth structure can be imitated with complete satisfaction for the first few months after the restoration is placed, but almost invariably the material discolors and gradually disintegrates in the mouth.”

This was certainly not a situation to achieve a good outcome for the patient, nor did it well serve the reputation of the practitioner.

From the end of WWII to about the late 1950s, materials utilized for dental restorations were generally limited to gold foil, cast gold, amalgam, fused porcelain; in certain cases, silicate cement might be used instead.

During this same time period the primary resin filling materials used were methyl methacrylates and even these materials experienced high shrinkage during polymerization.

The long term results were not much better than they had been in earlier years.

By 1967 silicate cement restorative replacements had been developed. Even with its superior esthetic qualities and insolubility properties, acrylic resin did not resolve all of the earlier issues.

The sixth edition of The Science of Dental Materials, E. Skinner and R. W. Phillips discussed the problem of percolation, which the authors described as the alternative imbibing and extruding of liquids.

In other published works, the definition of percolation has been given as the expansion and contraction of the filling material due to thermal changes forming space between the filling and the tooth.

Some fifteen years earlier, Fluid exchange at the margins of dental restorations appeared in the Journal of the American Dental Association [1952;44(3)]. In this article the authors, R. J. Nelsen, R. B. Wolcott and G. C. Paffenbarger described their study of the volumetric changes of acrylic filling materials when subjected to thermal changes in the mouth.

The test subjects drank extremely hot coffee followed by very cold soft drinks, creating thermal cycling.
After accounting for known variables, the authors came to the conclusion that the space which developed between the restoration and the tooth surface allowed for a constant fluid exchange.
This, they said, explained the cause of secondary decay around the margins of existing restorations and, at the end of the article, noted the need for further investigation into dealing with the percolation problem.


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Image courtesyof: Stuart Miles

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