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QUESTION
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What, if anything, can I do to control the directional shrinkage of resin-based composites?
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ANSWER
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Resin-based composites, as introduced to the dental profession more than 30 years ago, self-polymerized by means of chemical activation. Less than five years later, the concept of polymerization by ultraviolet light was introduced. This innovative approach offered a definite advantage over the original process: specifically, the restoration could be cured in seconds rather than minutes.
Shortly thereafter, visible light radiation was offered as a better method of curing resin-based composites. Rather than effecting a depth of only 1.5 millimeters, the newer system effectively generated a depth of cure of 2.5 to 3 mm in the same time or less. Also, by comparison, the visible-light–curing units could initiate polymerization immediately, whereas the ultraviolet systems required up to one minute to warm up. Interestingly, the light-curing process added another dimension. Unlike the chemical self-curing process, the newer mode of curing provided the operator with an indefinite working time prior to initiation of curing.
In the last several years, the concept of self-cured resin-based composites has been reintroduced. It has been suggested that this type of composite shrinks toward the center of the mass.1 Comparatively, light-cured composite shrinks toward the light source.1
On the basis of these principles, numerous clinical techniques have been proposed. For example, many clinicians have recommended placing the light-cured resin against the walls within the preparation. The curing light then is directed to the external surface of the prepared tooth. It is postulated that owing to the above-described concept of curing shrinkage, the light being transmitted through the cusps actually draws the resin-based composite toward the prepared wall as it undergoes polymerization.
Some clinicians also have recommended using self-curing resin to control the direction of shrinkage. It has been suggested that if the composite is placed against an internal surface of the preparation, the relatively higher temperature of the tooth structure as compared with the ambient temperature within the preparation (between the cusps) will initiate the curing process. This, in turn, supposedly causes the resin-based composite to shrink toward the prepared surface.
It has been proposed that both of these techniques can improve composite’s marginal adaptation. A study conducted at the University of Minnesota strongly suggested that this, in fact, may not be the case.2 Specifically, the investigators demonstrated that the direction in which the resin-based composite shrinks during cutting is best controlled by the proper use of dentin-bonding agents.
In the Minnesota study, the authors investigated the directions in which resin-based composites shrank under various clinical conditions. Their study included both a self-cured and a light-cured resin-based composite. One-third of the preparations generated in posterior teeth were restored without acid etching and bonding. In these preparations, both the self-cured and the light-cured resin-based composites shrank toward the center of the mass.
On the other hand, when acid etching and bonding procedures were used, both the self-cured and the light-cured composites shrank toward the walls of the prepared cavity. To further demonstrate the effect of bonding on the direction of shrinkage, the researchers included a second group of conditions in which the enamel walls were etched and bonded, but the dentinal surfaces were not. Surprisingly, in the enamel component of the preparation, the etching and bonding resulted in the composite’s shrinkage toward the surface of the preparation. In the dentin component of the preparation, which had not undergone etching and bonding, the material pulled in a direction away from the walls of the preparation. Most interestingly, the directions of shrinkage were no different, regardless of the method used to cure the restorative material.
On the basis of this interesting study, the following conclusions can be made:
- – The method of cure has little influence on the direction of shrinkage.
- – Both the light-curing and self-curing methods tend to cause shrinkage toward the center of the restoration when etching and bonding procedures are not used.
- – Total etching and bonding cause resin-based composite to pull toward the walls of the preparation.
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